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TEXT-BOOK  OF  GEOLOGY 

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BY 


SIR  ARCHIBALD  GJIIKIE,  F.R.S. 

D.Sc.  Oaiib.,  Dubl.  :  LL.D.  Edin.,  St.  And. 

MBfX/nill-UEIIKRAL  OF  THK  OKOUKIK'AL  8UKVBY  Or  OIUUT  BRITAIN  AND  IRELAMD 

CORRKSPOXDCrr  or  THE  INHTITtrrE  OP  PRANCE 

CORBI^PONDKNT  OP  THE  ROTAL  ACADEMY  OF  HCIKXCE8  OP  BBRLIK 

Ac,  &r. 


THIRD  EDITION,  REVISED  AND  ENLARGED 


i^onlion 
MACMILLAN  AND  CO. 

AND    NEW    YORK 
1893 

t 
-I" 

AU  righU  natrvtd 


2ii 183 


Firtt  Edition,  i88a  ;  Second,  1885  ;  Third,  1893. 


,   •  •  •  • 


PREFACE  TO  THE  THIRD  EDITION 

The  present  edition  of  this  Text:book  has  been  entirely  revised,  and  in 
some  portions  recast  or  rewritten,  so  as  to  bring  it  abreast  of  the 
continuous  advance  of  Geological  Science.  The  additions  made  to  the 
text,  which  extend  to  every  branch  of  the  subject,  increase  the  volume 
by  about  150  pages.  Care  has  been  taken  to  preserve  a  characteristic 
feature  of  former  editions  by  inserting  references  to  the  more  important 
memoirs  and  papers,  where  the  student  will  find  fuller  information  than 
can  be  given  in  a  Text-book, 

While  the  book  was  passing  through  the  press  I  received  from  my 
friend  Prof.  Zirkel  the  first  volume  of  the  new  edition  of  his  great  text- 
book of  Petrography,  but  too  late  to  avail  myself  of  its  assistance.  I  can 
only  now  recommend  it  as  an  indispensable  part  of  the  outfit  of  every 
serious  student  of  the  petrographical  section  of  Geology. 

In  the  revision  of  the  stratigraphical  portion  of  this  work  I  have 
been  assisted  with  suggestions  and  information  by  my  colleagues,  Mr. 
Topley,  Mr.  H.  B.  Woodward,  Mr.  E  T.  Newton,  and  Mr.  C.  Reid,  to 
whom  my  best  thanks  are  due. 

Museum,  Jermyn  Street, 
1^  August  1893. 


FROM  THE  PREFACE  TO  THE  FIRST  EDITION 


The  method  of  treatment  adopted  in  this  Text-book  is  one  which,  while 
conducting  the  class  of  Greology  in  the  University  of  Edinburgh,  I  have 
found  to  afford  the  student  a  good  grasp  of  the  general  principles  of  the 
science,  and  at  the  same  time  a  familiarity  with  and  interest  in  details  of 
which  he  is  enabled  to  see  the  bearing  in  the  general  system  of  know- 
ledge. A  portion  of  the  volume  appeared  in  the  autumn  of  1879  as  the 
article  "  G^logy  "  in  the  Encydopxdia  Britannica,  My  leisure  since  that 
date  has  been  chiefly  devoted  to  expanding  those  sections  of  the  treatise 
which  could  not  be  adequately  developed  in  the  pages  of  a  general  work 
of  reference. 

While  the  book  will  not,  I  hope,  repel  the  general  reader  who  cares 
to  know  somewhat  in  detail  the  facts  and  principles  of  one  of  the  most 
fascinating  branches  of  natural  history,  it  is  intended  primarily  for 
students,  and  is  therefore  adapted  specially  for  their  use.  The  digest 
given  of  each  subject  will  be  found  to  be  accompanied  by  references  to 
memoirs  where  a  fuller  statement  may  be  sought.  It  has  long  been  a 
charge  against  the  geologists  of  Great  Britain  that,  like  their  countrymen 
in  general,  they  are  apt  to  be  somewhat  insular  in  their  conceptions,  even 
in  regard  to  their  own  branch  of  science.  Of  course,  specialists  who  have 
devoted  themselves  to  the  investigation  of  certain  geological  formations 
or  of  a  certain  group  of  fossil  animals,  have  made  themselves  familiar 
with  what  has  been  written  upon  their  subject  in  other  countries.  But  I 
am  afraid  there  is  still  not  a  little  truth  in  the  charge,  that  the  general 
body  of  geologists  here  is  but  vaguely  acquainted  with  geological  types 
and  illustrations  other  than  such  as  have  been  drawn  from  the  area  of  the 
British  Isles.  More  particularly  is  the  accusation  true  in  regard  to 
American  geology.  Comparatively  few  of  us  have  any  adequate  concep- 
tion of  the  simplicity  and  grandeur  of  the  examples  by  which  the  principles 
of  the  science  have  been  enforced  on  the  other  side  of  the  Atlantic. 

Fully  sensible  of  this  natural  tendency,  I  have  tried  to  keep  it  in 
constant  view  as  a  danger  to  be  avoided  as  far  as  the  conditions  of  my 
task  would  allow.  In  a  text-book  designed  for  use  in  Britain,  the  illustra- 
tions must  obviously  be  in  the  first  place  British.     A  truth  can  be  enforced 


viii  TEXT-BOOK  OF  GEOLOGY 

much  more  vividly  by  an  example  culled  from  familiar  ground  than  by 
one  taken  from  a  distance.  But  I  have  striven  to  widen  the  vision  of  the 
student  by  indicating  to  him  that  while  the  general  principles  of  the 
science  remain  uniform,  they  receive  sometimes  a  clearer,  sometimes  a 
somewhat  diflferent,  light  from  the  rocks  of  other  countries  than  our  own. 
If  from  these  references  he  is  induced  to  turn  to  the  labours  of  our  fellow- 
workers  on  the  Continent)  and  to  share  my  respect  and  admiration  for 
them,  a  large  part  of  my  design  will  have  beei^  accomplished.  If,  further, 
he  is  led  to  study  with  interest  the  work  of  our  brethren  across  the 
Atlantic,  and  to  join  in  my  hearty  regard  for  it  and  for  them,  another 
important  section  of  my  task  will  have  been  fulfilled.  And  if  in  perusing 
these  pages  he  should  find  in  them  any  stimulus  to  explore  nature  for 
himself,  to  wander  with  the  enthusiasm  of  a  true  geologist  over  the  length 
and  breadth  of  his  own  country,  and,  where  opportunity  offers,  to  extend 
his  experience  and  widen  his  sympathies  by  exploring  the  rocks  of  other 
lands,  the  remaining  and  chief  part  of  my  aim  would  be  attained. 

The  illustrations  of  Fossils  in  Book  VI.  have  been  chiefly  drawn  by 
Mr.  George  Sharman ;  a  few  by  Mr.  B.  N.  Peach,  and  one  or  two  by  Dr. 
R.  H.  Traquair,  F.R.S.,  to  all  of  whom  my  best  thanks  are  due.  The 
publishers  having  become  possessed  of  the  wood-blocks  of  Sir  Henry  de 
la  Beche's  *  Geological  Observer,'  I  gladly  made  use  of  them  as  far  as  they 
could  be  employed  in  Books  III.  and  IV.  Sir  Henry's  sketches  were 
always  both  clear  and  artistic,  and  I  hope  that  students  will  not  be  sorry 
to  see  some  of  them  revived.  They  are  indicated  by  the  letter  {B),  The 
engravings  of  the  microscopic  structure  of  rocks  are  from  my  own  draw- 
ings, and  I  have  also  availed  myself  of  materials  from  my  sketch-books. 
The  frontispiece  is  a  reduction  of  a  drawing  by  Mr.  W.  H.  Holmes,  whose 
pictures  of  the  scenery  in  the  Far  West  of  the  United  States  are  by  far 
the  most  remarkable  examples  yet  attained  of  the  union  of  artistic 
effectiveness  with  almost  diagrammatic  geological  distinctness  and  accuracy. 
Captain  Dutton,  of  the  Geological  Survey  of  the  United  States,  furnished 
me  with  this  .drawing,  and  also  requested  Mr.  Holmes  to  make  for  me 
the  caiion-sections  given  in  Book  VII.  To  both  of  these  kind  friends  I 
desire  to  acknowledge  my  indebtedness. 


CONTENTS 


PAGE 

INTRODUCTION 1 

BOOK   L 
CosMiCAL  Aspects  of  Geology,  7. 

I.  Relations  of  the  Earth  in  the  Solar  System  ...  8 

II.  Form  and  Size  of  the  Earth       ......        13 

III.  Movements  of  the  Earth  in  their  Geological  Relations  15 

1.  Rotation,  lb— 2.  Revolution,  16—3.  Precession  of  the  Equinoxes,  16 — 4.  Change 
in  the  obliquity  of  the  Ecliptic,  17—5.  Stability  of  the  Earth's  Axis,  17—4). 
Changes  of  the  Earth's  Centre  of  Gravity,  20—7.  Results  of  the  Attractive 
Influence  of  3un  and  Moon  on  the  Geological  Condition  of  the  Earth,  21—8. 
Climate  in  ito  Geological  Relations,  28. 


BOOK   II. 

Geognosy — An  Investigation  of  the  Materials  of  the 

Eariu's  Substance. 

Part  I. — A  General  Description  op  the  Parts  of  the  Earth. 
I.  The  Envelopes — Atmosphere  and  Hydrosphere  .  .31 

1.  The  Atmosphere,  81—2.  The  Oceans,  83. 

II.  The  Solid  Glohe  or  Lithosphere  .....        38 

1.  The  Outer  Surface,  38—2.  The  Crust,  46—3.  The  Interior  or  Nucleus,  47; 
Evi<lence  of  Internal  Heat,  48 ;  Irregularities  In  the  downward  Increment  of 
Heat,  51 ;  Proljable  Condition  of  the  Earth's  Interior,  5»— 4.  Age  of  the  Earth 
and  Measures  of  Geological  Time,  58. 

Part  II. — An  Account  op  the  Composition  op  the  Earth's  Crust — 

Minerals  and  Rocks. 

I.  General  Chemical  Constitution  of  the  Crust  .  .60 

II.  Rock-forming  Minerals       .......        64 

III.  Determination  of  Rocks    .  .80 

I.  Megascopic  Examination,  81— ii.  Chemical  Analysis,  87— iii.  Chemical  Synthesis, 
89—  iv.  Microscopic  Investigation,  80. 

IV.  General  Outward  or  Megascopic  Characters  of  Rocks  .  .        96 

1.  structure,  96—2.  Composition,  104—3.  State  of  Aggregation,  105 — 4.  Colour  and 
Lustre,  106—6.  Feel  and  Smell,  107— <5.  Si)ecitlc  Gravity,  108—7.  Magnetism,  108. 


TEXT-BOOK  OF  GEOLOGY 


PAOE 

V.  Microscopic  Characters  of  Rocks  .....      108 

1.  Microscopic  Elements  of  Rocks,  109 — 2.  Microscopic  Structures,  117. 

VI.  Classification  of  Rooks    .......      123 

VII.  A  Description  of  the  more  important  Rocks  of  the  Earth*s  Crust. 
i.  Sedimentary 


A.  FragmetUal  {Clastic) 
1.*  Gravel  and  Sand  Rocks  (Psammites) 

2.  Clay  Rocks  (Pelites)     . 

3.  Volcanic  Fraginental  Rocks  (Tuffs) 

4.  Fragmental  Rocks  of  Organic  Origin 

(1)  Calcareous,  188— <2)  Siliceous,  141— (8)  Phospliatic,  141— (4)  Carbonaceous, 
142— (6)  Ferruginous,  140. 


126 

126 
127 
132 
135 

138 


B.  Crystalline^  including  Bocks  formed  from  Chemical  Precipitation      148 
ii.  Massive,  Eruptive,  Igneous         .  .  .  .  .  .154 

i.  Acid  Series,  156— ii.  Intermediate  Series,  163— iii.  Basic  Series,  169. 

iii.  Schistose  (Metamorphic)    .  .  .  .  .  .  .175 

1.  Argillites,  179—2.  Quartz-Rocks,  179—8.  Pyroxene-Rocks,  181 — 1.  Hornblende- 
Rocks,  182—5.  Garnet-Rocks,  182—6.  Epidote-Rocks,  188—7.  Chlorite-RockM, 
183—8.  Talc-Rocks,  IKS— 9  OH  vine-Rocks,  or  Peridotites,  183—10.  Felsitoid- 
Rocks,  183—11.  Quartz-  and  Tourmaline- Rocks,  184 — 12.  Quart-z-  and  Mica- 
Rocks,  184 — 13.  Quartz-  and  Felspar-Rocks,  185 — 14.  Quartz-,  Felspar-,  and 
Mica-Rocks,  185 — 15.  Quartz-,  Felspar-,  and  Oamet-Ro<'.ks,  186 — 16.  Felsiwr- 
and  Mica- Rocks,  187 — Composition  of  some  Schistose  Rocks,  188. 


BOOK  III. 

Dynamical  Geology,  189. 

Part  I. — Hypoqene  Action — ^An  Inquiry  into  the  Geological  Changes 
IN  Progress  beneath  the  Surface  of  the  Earth,  190. 

I.  Volcanoes  and  Volcanic  Action. 

1.  Volcanic  Products  ........       191 

I.  Oases  and  Vajwurs,  193—2.  Water,  197—3.  Lava,  198—4.  Fragmentary  Mate- 
rials, 199. 

2.  Volcanic  Action       ........       202 

Active,  Dormant,  and  Extinct  Phases,  202 — Sites  of  Volcanic  Action,  203 — 
Onlinary  Phase  of  an  Active  Volcano,  204 — Ck>nditions  of  Eruption,  205 — 
Periodicity  of  Eruptions,  206 — General  Sequence  of  Events  in  an  Eruption, 
207— Fissures,  208— Explosions.  211— Showers  of  Dust  and  Stones,  218— Lava- 
streams,  217— Elevation  and  Subsidence,  231— Torrents  of  Water  and  Mud,  282 
—Effects  of  the  closing  of  a  Volcanic  Chimney— Sills  and  Dykes,  238— Ex- 
halations of  Vapours  and  Gases,  233— Geysers,  235— Mud  Volcanoes,  238. 

3.  Structure  of  Volcanoes        .  .  .....       239 

i.  Volcanic  Cones,  240  —  Submarine  Volcanoes,  and  Volcanic  Islands,  249 — ii. 
Fissure  (Massive)  Eruptions,  255. 

4.  Geographical  and  Geological  Distribution  of  Volcanoes  .  259 

5.  Causes  of  Volcanic  Action  .......       263 

II.  Earthquakes  ........      270 

Amplitude  of  Earth  -  Movements,  271— Velocity,  272— Duration,  273— Modifying 
Influence  of  Geological  Structure,  273— Extent  of  Country  affected,  274— Depth 
of  ijource,  275— Geological  Effects,  276— Distribution,  279— Origin,  280. 


CONTENTS  xi 


PAOB 

III.  Secular  Upheaval  and  Depbebsion  281 

Upheaval,  284— Sabaidence,  28S— Causes  of  Upheaval  and  Snbaidence,  292. 

IV.  Hypooene  Causes  of  Changes   in  the  Texture,   Structure,   and 

Composition  of  Rooks  .......      296 

1.  Effects  of  Heat        .......       297 

Rise  of  Temperature  by  Depression,  297— Rise  of  TempeFatore  by  Chemical  Trans- 
formation, 298— Rise  or  Temperature  by  Rock-cnishing,  298— Rise  of  Tempera- 
ture by  Introsion  of  Erupted  Rock,  299— Expansion,  299— Crystallisation 
(Marble),  800— Production  of  Prismatic  Structure,  800-Dry  Fusion,  800— 
Contraction  of  Rocks  in  passing  fh)m  a  Glassy  to  a  Stony  State,  SOi— Sublima- 
tion, 805. 

2.  Influence  of  Heated  Water  .......       305 

Presence  of  Water  in  all  Rocks,  806— Solvent  Power  of  Water  among  Rocks,  807— 
This  Power  increased  by  Heat,  807— Co-operation  of  Pressure,  807- Aquo- 
igneous  Fusion,  808— Artificial  Production  of  Minerals,  809— Artificial  Altera- 
tion of  Internal  Structures,  809. 

3.  Effects  of  Compression,  Tension,  and  Fracture       .  .  .  .311 

Minor  Ruptures  and  Noises,  811— Consolidation  and  Welding  812— Cleavage,  312— 
Deformation,  814— Plication,  817— Jointing  and  Dislocation,  318. 

4.  The  Metamorphism  of  Rocks  ......       319 

Production  of  Marble  from  Limestone,  820— Dolomitisation,  821— Conversion  of 
Vegetable  Substance  into  Coal,  822— Production  of  new  Minerals,  8^— Pro- 
duction of  the  Schistose  Structure,  328. 


Part  II. — Epioene  or  Surface  Action,  325. 

I.  Air     ...  .  326 

1.  Geological  Work  on  Land  .......       327 

(1)  Destructive  Action,  327— Effects  of  Lightning,  828— Effects  of  Changes  of  Tem- 
perature, 328— Effects  of  Wind,  829— (2)  Reproductive  Action— Growth  of  Dust, 
SSl— Loess,  882— Sand -Hills  or  Dunes,  834— Dust -showers.  Blood-rain,  837 
—Transportation  of  Seeds,  838— Efflorescence  Products,  388. 

2.  Influence  on  Water  ........       338 

Ocean  Currents,  388— Waves,  389— Alteration  of  Water-level,  839. 

IL  Water  .........       339 

1.  Rain  .........       341 

(1)  Chemical  Action,  841 — Chemical  Composition  of  Rain-water,  841— Chemical  and 
Mineralogical  Changes  produced  by  Rain,  843— Weathering,  845— Formation  of 
Soil,  851 -(2)  Mechanical  Action,  853— Removal  and  Renewal  of  Soil,  853— 
Movement  of  Soil-cap,  854 — Unequal  Erosive  Action  of  Rain,  354. 

2.  Underground  Water  .......       35^ 

Springs,  357— (1)  Chemical  Action,  860— Alteration  of  Rocks,  8d4 — Chemical 
Deposits,  865  —  Subterranean  Channels  and  Caverns,  367  —  (2)  Mechanical 
Action,  369. 

3.  Brooks  and  Rivers  ........       371 

i.  Sources  of  Supply,  871— ii.  Discharge,  873— iii.  Flow.  375— iv.  Geological  Action, 
877:  (1)  Chemical,  877;  (2)  Mechanical,  379  —  Transporting  Power,  879— 
Excavating  Power,  384 — Reproductive  Power,  893 — Cones  de  Ejection,  393 — 
River-beds,  894 — Flood-plains,  895— Deposits  in  Lakes,  397— Bars  and  Lagoon- 
barriers,  898— Deltas  in  the  Sea,  400— Sea-borne  Sediment,  408. 

4.  Lakes  .........       404 

Fresh-water,  404— Saline,  408— Deposits  in  Salt  and  Bitter  Lakes,  411. 

5.  Terrestrial  Ic»         ........       413 

Frost,  418— Froren  Rivers  and  Ijikes,  414— Hail,  415— iJnow,  416— Glaciers  and  Ice- 
SheeU,  417— Work  of  Glaciers :  (a)  Transport,  423 ;  (6)  Erosion,  427. 


xii  TEXT-BOOK  OF  GEOLOGY 

PAGE 

6.  Oceanic  Waters        ........       482 

i.  Movements :  (1)  Tides,  488— (2)  Corrents,  4S4— (3)  Waves  and  Groond-Kwell,  436 
— <4)  Ice  on  the  Sea,  488— ii.  Geological  Work  :  (1)  Influence  on  Climate,  440— 
(2)  Erosion:  (a)  Chemical,  441;  (b)  Mechanical,  442— (3)  Transport,  450— 
i4)  Reproduction,  453 — Chemical  Deposits,  453 — Mechanical  Deposits,  454: 
(a)  Land-derived  or  Terrigenous  :  Shore  Deposits,  454;  Inftn-littoral  and  Deeper- 
water  Deposits,  455— <&)  Abysmal  or  Pelagic,  457. 

7.  Denudation  and  Deposition — The  Results  of  the  Action  of  Air  and  Water 

upon  Land  ........       460 

i.  Subaerial  Denudation :  the  general  Lowering  of  Land,  460 — 2.  Subaerial  Denuda- 
tion :  the  unequal  Erosion  of  Land.  465 — 3.  Marine  Denudation,  its  compara- 
tive Rate,  466 — 4.  Marine  Denudation,  its  final  Result,  468—5.  Deposition: 
the  Framework  of  New  Land,  470. 

III.  Life. 

1.  Destructive  Action  of  Plants  and  Animals  .....       471 

2.  Conservative  Action  .  .  .  .475 

3.  Reproductive  Action  .  .  .  .  .  .477 

Sea-weeds,  477— Humus  and  Black  Soils,  477— Peat-Mosses  and  Bogs,  478 — Man- 
grove Swamps,  481 — Diatom  Earth,  481— Chemical  Deposits  formed  by  Plant- 
agency,  482- Chemical  Deposits  formed  by  Animal-agency,  484 — Shell-marl, 
484 — Coral-reefs,  485— Limestone  and  Ooze,  492— Siliceous  Ooze,  493— Phos- 
phatic  Deposits,  494. 

4.  Man  as  a  Geological  Agent .......       495 


BOOK  IV. 

Gkotectonic  (Structural)  Geology,  or  the  Architecture  of 

THE  Earth's  Crust. 

Part  I. — Stratification  and  its  Accompaniments,  498. 

Forms  of  Bedding.  498— False-bedding,  501— Intercalated  Contortion,  502— Irr^u- 
laritics  of  Beading  due  to  Inequalities  of  Deposition  or  of  Erosion,  504 — Sur- 
face-Markings (Ripple-mark,  Sun-cracks,  kc),  507— Concretions,  510 — Alterna- 
tions and  Associations  of  Strata,  513— Relative  Persistence  of  Strata,  515 — 
Influence  of  the  Attenuation  of  Strata  upon  apparent  Dip,  517 — Overlap,  518 — 
Relative  Lapse  of  Time  represented  by  Strata  and  by  the  Intervals  between 
them,  518 — Ternary  Succession  of  Strata,  521— Groups  of  Strata,  522— Order  of 
Superposition :  the  Foundation  of  Geological  Chronology,  523. 

Part  IL — Joints,  523. 

1.  In  Stratified  Rocks,  524—2.  In  Massive  (Igneous)  Rocks,  527—3.  In  Foliated 
(Schistose)  Rocks,  531. 

Part  IIL — Inclination  of  Rocks,  531. 

Dip,  581— Outcrop,  533— Strike,  534. 

Part  IV. — Curvature,  536. 

Monoclines,  638 — Anticlines  and  Synclines,  SSS^Invcrsion,  530— Crumpling,  541— 
Deformation  and  Crushing,  543. 

Part  V. — Cleavage,  545 
Part  VI. — Dislocation,  547. 

Nature  of  Faults,  548— Origin  of  Faults,  550— Normal  Faults,  550— Reversed  Faults, 
550— Thrust-planes,  551— Throw  of  Faults,  551— Dip- Faults  and  Strike-Faults, 
552— Dying  out  of  Faults,  555 — Groups  of  Faults,  556 — Detection  and  Tracing 
of  Faults,  557. 


CONTENTS  xiii 


Part  VIL — Erdptivb  (Igneous)  Rocks  as  Part  op  the  Structure  op  the 

Earth's  Crust,  559. 

PAGE 

I.  Plutonic,  Intrusive,  or  Subsequent  Phase  of  Eruptivity    .  663 

1.  Bosses  .........       564 

Granite-boMes,  565 — Relation  of  Granite  to  Contiguous  Rocks,  567— Connection  of 
Granite  with  Volcanic  Rocks,  569— Diorite,  &c,  571— Enects  on  Contiffuous 
Rocks,  572— Connection  with  Volcanic  Action  and  with  Crystalline  Schists, 
573. 

2.  Sheets,  Sills  ........       573 

General  Characters,  578— Effects  on  Contiguous  Rocks,  676— Connection  with  Vol- 
canic Action,  576. 

3.  Veins  and  Dykes     ........       577 

Eruptive  or  Intrusive,  678— "  Contemporaneous  "  and  other  Veins,  580— Dykes,  582 
— Effects  on  Contiguous  Rocks,  584. 

4.  Necks  .........       584 

II.  Interbedded,  Volcanic,  or  Contemporaneous  Phase  of  Eruptivity. 

1.  Crystalline,  or  Lavas  .......       589 

2.  Fragmental,  or  Tuffs  .......       598 

Part  VIII. — Metamorphism,  Local  and  Regional,  595. 
I.  Local  Metamorphism  (Metamorphism  of  Contact  or  Juxtaposition)      597 

Bleaching,  598— Coloration,  5f)8— Induration,  508— Expulsion  of  Water,  599— Pris- 
matic Structure,  599— Calcination,  Melting,  Coking,  600— Marmarosis,  602— 
Production  of  New  Minerals,  608 — Production  of  Foliation,  604 — Alteration  of 
the  Intrusive  Rock,  608— Summary  of  Facts,  609. 

II.  Regional  (Normal)  Metamorphism,  the  Crystalline  Schisi*8  611 

Introduction :  General  Characters  of  the  Crystalline  Schists,  611— Dispute  regarding 
their  Origin,  618 — Influence  of  Movements  of  the  Earth's  Crust,  614— Nature  of 
the  rock-changes  in  Regional  Metamorphism,  617  —  Illustrative  Examples  : 
Anlennes,  619— Taunus,  620— Scandinavia,  621— The  Alps,  622— Scottish  High- 
lands, 624— Greece,  627— Green  Mountains,  628 — Menominee  and  Marquette 
Regions,  628— Table  showing  the  wide  Range  of  Geological  Systems  affected  by 
Regional  Metamorphism,  628— Summary,  629. 

Part  IX. — Ore  Deposits,  631. 

i.  Mineral- Veins  or  Lodes,  638— Variations  in  Breadth,  OSS— Structure  and  Con- 
tents, 6S4— Successive  infilling,  636— Connection  with  Faults,  686— Relation  of 
Contents  to  Surrounding  Rocks,  688 — Decomposition  and  Recom position,  638. 

ii.  Stocks  and  Stock -works,  6S9— Origin  of  Mineral- Veins,  640. 

Part  X. — Unconformability,  641. 

BOOK  V. 
Pat^ontological  Geology,  645. 

Deflnition  of  the  term  Fossil,  645.  i.  Conditions  for  the  Entombment  of  Organic 
Remains,  646:  on  Land,  646;  in  the  Sea,  648— ii.  Preservation  of  Organic 
Remains  in  Mineral  Masses,  650—1.  Influence  of  Original  Structure  and  Com- 
position, 650—2.  FossllisatiJm,  651— iii.  Relative  Palseontological  Value  of 
Organic  Remains,  652— iv.  Uses  of  Fossils  in  Geology,  658.  They  show  H) 
Changes  in  Physical  Geology,  663 ;  (2)  Geological  Chr(»noli>gy,  656 ;  (S)  Sub- 
divisions  of  the  Geological  Ueconl,  661— v.  Bearing  of  Palteontological  Data 
upon  Evolution,  666— vi.  The  Collecting  of  Fossils,  669. 


XIV 


TEXT-BOOK  OF  GEOLOGY 


BOOK  VI. 


Stratigraphical  Geology. 


General  Principles 


I'AOE 

674 


Table  of  the  Stratified  Formations  constituting  the  Geological  Record — 

TofoM  p.  679 


Part  I. — Pre-Cambrian. 

m 

i.  General  Characters  ........ 

1.  The  lowest  Gneisses  and  Schists  ..... 

2.  Pre-Cambrian  Sedimentary  and  Volcanic  Groups 

ii.  Local  Development  ....... 

Britain.  698— ScaDdinavia,  711— Central  Euroiie,  713— America,  715— India,  717  - 
China,  717— Australasia,  717. 


680 
685 
692 
698 


Part  II. — PALiRozoic,  718. 

L  Cambrian  (Primordial  Silurian). 

1.  General  Characters,  Rocks,  Flora  and  Fauna         .... 

2.  Local  Development ........ 

Britain,  725— Continental  Enrojte,  781 — North  America,  735— South  America,  China, 
India,  Australia,  787. 


719 
725 


n.  Silurian 

1.  General  Characters  . 

2.  Local  Development . 


737 

73. 
746 


Britain,  746— Basin  of  the  Baltic,  Russia  and  i^candinavia,  766— Western  Europe, 
7<W— Central  and  Southern  Europe,  772— North  America,  775— Asia,  776— 
Australasia,  776. 

IIL  Devonian  and  Old  Red  Sandstone  .777 

(i.)  Devonian  Type, 

1.  General  Characters  ........       778 

2.  Local  Development.  .......       783 

Britain,  788— Central  Europe,  785— Russia,  788--Nortli  America,  7S9— Aaia,  790— 
Australasia,  790. 

(ii.)  Old  Bed  Sandstone  Tyjtc. 

1.  General  Characters  ........       791 

2.  Local  Development ........       797 

BriUin,  797— Norway,  Ac,  802— North  America,  803. 


IV.  Carbonikerous. 

1.  General  Characters  . 

2.  Local  Development . 


British  Isles,  824— Franco  and  Belgium,  834— Gennany,  836-SouU\em  Gennany, 
Bohemia,  H37— Alus,  Italy,  838— Russia,  838— Spitzbergeu,  338— Africa,  839— 
Asia,  839— Australasia,  839— North  America,  840. 


804 
824 


CONTENTS 


XV 


V.  Permian  (Dyas). 

1.  General  Characters  . 

2.  Local  Development . 


BriUin,  840— GeniunT,  Ac,  848— Voages,  860— France,  851— Alps,  852— Russia, 
852— Asia,  853— Australia,  854— Africa,  855  — North  America,  855— Spitz- 
bergen,  856. 


PAGE 

841 
846 


Part  IIL — Mesozoic  or  Secondary,  856. 

I.  Triassic. 

1.  General  Characters  ........ 

2.  Local  Development.  ....... 

BriUin,  864— Central  Europe,  868— Scandinavia.  870— Alpine  Trias,  871- Spitz- 
bergen,  876— Asia,  877— Australia,  877— New  Zealand,  878— Africa,  878— North 
America,  878. 

IL  Jurassic. 

1.  General  Characters  ........ 

2.  Local  Development.  ....... 

Britain,  897— France  and  the  Jure,  910— Oennany,  915— Alps,  917— Sweden,  918— 
Russia,  918— North  America,  919— Asia,  919— Austrelasia,  92a 

IIL  Cretaceous. 

1,  General  Characters  ........ 

2.  Local  Development ........ 

Britain,  936— France  and  Belgium,  947— Germany,  953— Switzerland  and  the  Alps, 
954— Basin  of  the  Mediterranean,  956— Russia,  956— India,  957— North  America, 
957— Australasia,  960. 


858 
864 


879 
897 


920 
986 


x' 


Part  IV. — Cainozoic  or  Tertiary,  961. 


I.  EocENi-:. 

1.  General  Characters  ........ 

2.  Local  Development ........ 

Britain,  970— Northern  France  and  Belgium,  975— Southern  Euroi>e,  979— India, 
&c.,  981— North  America,  981— Australasia,  982. 

11.  Olkjocene. 

1.  General  Characters  ........ 

2.  lx)cal  Development.  ....... 

Britain,  986— France,  989— Belgium,  990— Germany,  991— Switzerland,  992— Vienna 
Basin,  992— Italy,  998— North  America,  993. 

111.  Miocene. 

1.  (leneral  Characters  ........ 

2.  Local  Development ........ 

France,  998— Belgium,  998— Germany,  999— Mainz  Basin,  999— Vienna  Basin,  999— 
SwltzerUnd,  1000- Italy,  1001— Greenland,  1001- India,  1002— North  America, 
1002— Australia,  1008— New  Zealand,  1003. 


IV.  Pliocene. 

1.  General  Characters  . 

2.  Local  Development . 


Britain.  1008— Belgium  and  Holland,  1015— France,  1015— lUly,  1016 -Gennany, 
1017— Vienna  Basin,  1018— Greece,  1019— Samos,  1020— India,  1020— North 
America,  1022— Australia,  1022— New  Zealand,  1023. 


964 
970 


983 
986 


993 
998 


1003 
1008 


xvi  TEXT'BOOK  OF  GEOLOGY 

Part  V. — Post-Tertiary  or  Quaternary,  1023. 
I.  Pleistocene  or  Glacial. 

PAGE 

1.  General  Characters  ........     1024 

Pre-fflacial  Land-surfaces,  1025— The  Northern  Ice-sheet,  102G— loe-crumpled 
Rocks,  1031— Detritus  of  the  Ice-sheet,  Boulder-clay,  Till,  1031— Inter-glacial 
beds,  1038— Evidences  of  Submergence,  1036— Second  Glaciation,  Re-elevation, 
Raised  Beaches,  1037. 

2.  Local  Development ........     1042 

Britain,  1042— Scandinavia,  1045— Ocrniany,  1045— France,  1046— Belgium,  1047— 
Alps,  104&— Russia,  1049— North  America,  1050— India,  1054— Australasia,  1065. 

II.  Recent,  Post-glacial  or  Human  Period. 

1.  General  Characters  ........     1055 

Palffiolithic  Alluvia,  1057— Brick-Earths,  1058— Cavern  Deposits,  1058— Calcareous 
Tufks,  1059— Loess,  1059— Pabcolithic  Fauna,  1061— Neolithic,  1063. 

2.  Local  Development ........     1065 

Britain,  1065— France,  1065— Germany,  1066— Smteerland,  106d— Denmark,  1066— 
North  America,  1067— Australasia,  1067. 


BOOK   VII. 
Physiographigal  Geology,  1068. 

1.  Terrestrial  Features  due  more  or  less  directly  to  Disturbance  of  the  Crust,  1071 
—2.  Terrestrial  Futures  due  to  Volcanic  Action,  1079—8.  Terrestrial  Features 
due  to  Denudation,  1079. 


List  of  Authors  quoted  or  referred  to       .  .  1091 

■ 

Index         ....  .....     1103 


*        . 


•    -. 


«  • 


INTRODUCTION. 

Geology  is  the  science  which  investigates  the  history  of  the  Earth.  Its 
object  is  to  trace  the  progress  of  our  planet  from  the  earliest  beginnings 
of  its  separate  existence,  through  its  various  stages  of  growth,  down  to 
the  present  condition  of  things.  It  unravels  the  complicated  processes, 
involving  vast  geographical  revolutions,  by  which  each  continent  and 
country  has  been  built  up,  tracing  out  the  origin  of  their  materials  and 
the  manner  in  which  their  existing  outlines  have  been  determined.  It 
likewise  follows  into  detail  the  varied  sculpture  of  mountain  and  valley, 
crag  and  ravine. 

Nor  does  this  science  confine  itself  merely  to  changes  in  the  inorganic 
world.  Geology  shows  that  the  present  races  of  plants  and  animals  are 
the  descendants  of  other  and  very  different  races  that  once  peopled  the 
earth.  It  teaches  that  there  has  been  a  progress  of  the  inhabitants,  as 
well  as  one  of  the  globe  on  which  they  have  dwelt ;  that  each  successive 
period  in  the  earth^s  history,  since  the  introduction  of  living  things,  has 
been  marked  by  characteristic  types  of  the  animal  and  vegetable  king- 
doms ;  and  that,  how  imperfectly  soever  they  may  have  been  preserved 
or  may  be  deciphered,  materials  exist  for  a  history  of  life  upon  the  planet. 
The  geographical  distribution  of  existing  faunas  and  floras  is  often  made 
clear  and  intelligible  by  geological  evidence ;  and  in  a  similar  way, 
light  is  thrown  upon  some  of  the  remoter  phases  in  the  history  of  man 
himself. 

A  subject  so  comprehensive  as  this  must  require  a  wide  and  varied 
basis  of  evidence.  One  of  the  characteristics  of  geology  is  to  gather 
evidence  from  sources  which,  at  first  sight,  seem  far  removed  from  its 
Bcope,  and  to  seek  aid  from  almost  every  other  leading  branch  of  science. 
Thus,  in  dealing  with  the  earliest  conditions  of  the  planet,  the  geologist 
must  fully  avail  himself  of  the  laboui-s  of  the  astronomer.  Whatever  is 
ascertainable  by  telescope,  si)ectroscope,  or  chemical  analysis,  regarding 
the  constitution  of  other  heavenly  bodies,  has  a  geological  bearing.  The 
experiments  of  the  physicist,  undertaken  to  determine  conditions  of 
matter  and  of  energy,  may  sometimes  be  taken  as  the  starting-point  of 
^ological  investigation.     The  work  of  the  chemical  laboratory  forms  the 

B 


2  ••  INTRODUCTION 


•  •- 

•  •  • 


•  • 


founjisiltioh  of  a  vast  and  increasing  mass  of  geological  inquiry.  To  the 
botanist)  the  zoologist,  even  to  the  unscientific,  if  observant,  traveller  by 
hxi^  or  sea,  the  geologist  turns  for  information  and  assistance. 

'.But  while  thus  culling  freely  from  the  dominions  of  other  sciences, 

geology  claims,  as  its  peculiar  territory,  the  rocky  framework  of  the 

**'glijbe.     In  the  materials  composing  that  framework,  their  composition 

•and  arrangement,  the  processes  of  their  formation,  the  changes  which 

.  they  have  individually  undergone,  and  the  grand  terrestrial  revolutions 

.  to  which  they  bear  witness,  lie  the  main  data  of  geological  history.     It 

is  the  task  of  the  geologist  to  group  these  elements  in  such  a  way  that 

they  may  be  made  to  yield  up  their  evidence  as  to  the  march  of  events 

in  the  evolution  of  the  planet.     He  finds  that  they  have  in  large  measure 

arranged  themselves  in  chronological  sequence, — the  oldest  lying  at  the 

bottom  and  the  newest  at  the  top.     Relics  of  an  ancient  sea-floor  are 

overlain  with  traces   of  a   vanished   land  -  surface  ^    these   are  in  turn 

covered  by  the  deposits  of  a  former  lake,  above  which  once  more  appear 

proofs  of  the  return  of  the  sea.     Among  these  rocky  records,  too,  lie 

the  lavas  and  ashes  of  long-extinct  volcanoes.     The  ripple  left  upon 

a  sandy  beach,  the  cracks  formed  by  the  sun's  heat  upon  the  muddy 

bottom  of  a  dried-up  pool,  the  very  imprint  of  the  drops  of  a  passing 

rain-shower,  have  all  been  accurately  preserved,  and  often  bear  witness 

to  geographical  conditions  widely  different  from  those  that  exist  where 

such  markings  are  now  found. 

But  it  is  mainly  by  the  remains  of  plants  and  animals  imbedded  in 
the  rocks  that  the  geologist  is  guided  in  unravelling  the  chronological 
succession  of  geological  changes.  He  has  found  that  a  certain  order  of 
appearance  characterises  these  organic  remains ;  that  each  successive 
group  of  rocks  is  marked  by  its  own  special  types  of  life ;  that  these 
types  can  be  recognised,  and  the  rocks  in  which  they  occur  can  be  corre- 
lated, even  in  distant  countries,  where  no  other  means  of  comparison  are 
available.  At  one  moment,  he  has  to  deal  with  the  bones  of  some  large 
mammal  scattered  through  a  deposit  of  superficial  gravel,  at  another 
time,  with  the  minute  foraminifers  and  ostracods  of  an  upraised  sea- 
bottom.  Corals  and  crinoids,  crowded  and  crushed  into  a  massive 
limestone  on  the  spot  where  they  lived  and  died,  ferns  and  terrestrial 
plants  matted  together  into  a  bed  of  coal  where  they  originally  grew, 
the  scattered  shells  of  a  submarine  sand-bank,  the  snails  and  lizards  that 
left  their  mouldering  remains  within  a  hollow  tree,  the  insects  that  have 
been  imprisoned  within  the  exuding  resin  of  old  forests,  the  footprints  of 
birds  and  quadrupeds,  or  the  trails  of  worms  left  upon  former  shores — 
these,  and  innumerable  other  pieces  of  evidence,  enable  the  geologist  to 
realise  in  some  measure  what  the  vegetable  and  animal  life  of  successive 
periods  has  been,  and  what  geographical  changes  the  site  of  every  land 
has  undergone. 

It  is  evident  that  to  deal  successfully  with  these  varied  materials,  a 
considerable  acquaintance  with  diff'erent  branches  of  science  is  desirable. 
The  fuller  and  more  accurate  the  knowledge  which  the  geologist  has  of 
kindred  branches  of  inquiry,  the  more  interesting  and  fruitful  will  be 


INTRODUCTION 


his  own  researches.  From  its  very  nature,  geology  demands  on  the 
part  of  its  votaries,  wide  sympathy  with  investigation  in  almost  every 
branch  of  natural  science.  Especially  necessary  is  a  tolerably  large 
acquaintance  with  the  processes  now  at  work  in  changing  the  surface 
of  the  earth,  and  of  at  least  those  forms  of  plant  and  animal  life  whose 
remains  are  apt  to  be  preserved  in  geological  deposits,  or  which,  in  their 
structiu*e  and  habitat,  enable  us  to  realise  what  their  forerunners  vere. 

It  has  often  been  insisted  upon  that  the  Present  is  the  key  to  the 
Past ;  and  in  a  wide  sense  this  assertion  is  eminently  true.  Only  in 
proportion  as  we  understand  the  present,  where  everything  is  open  on  all 
sides  to  the  fullest  investigation,  can  we  expect  to  decipher  the  past, 
where  so  much  is  obscure,  imperfectly  preserved,  or  not  preserved  at  all. 
A  study  of  the  existing  economy  of  nature  ought  evidently  to  be  the 
foundation  of  the  geologist's  training. 

While,  however,  the  present  condition  of  things  is  thus  employed, 
we  must  obviously  be  on  our  guard  against  the  danger  of  imconsciously 
assuming  that  the  phase  of  nature's  operations  which  we  now  witness 
has  been  the  same  in  all  past  time ;  that  geological  changes  have  taken 
place,  in  former  ages,  in  the  manner  and  on  the  scale  which  we  behold 
to-day,  and  that  at  the  present  time  all  the  great  geological  processes, 
which  have  produced  chimges  in  past  eras  of  the  earth's  history,  are  still 
existent  and  active.  Of  course,  we  may  assume  this  uniformity  of  action, 
and  use  the  assumption  as  a  working  hypothesis.  But  it  ought  not  to 
be  allowed  a  firmer  footing,  nor  on  any  account  be  suffered  to  blind  us 
to  the  obvious  truth  that  the  few  centuries,  wherein  man  has  been 
observing  nature,  form  much  too  brief  an  interval  by  which  to  measure 
the  intensity  of  geological  action  in  all  past  time.  For  aught  we  can 
tell,  the  present  is  an  era  of  quietude  and  slow  change,  compared  with 
some  of  the  eras  that  have  preceded  it.  Nor  can  we  be  sure  that  when 
we  have  explored  every  geological  process  now  in  progress,  we  have 
exhausted  all  the  causes  of  change  which,  even  in  comparatively  recent 
times,  have  been  at  work. 

In  dealing  with  the  Geological  Record,  as  the  accessible  solid  part  of 
the  globe  is  called,  we  cannot  too  vividly  realise  that,  at  the  best,  it 
forms  but  an  imperfect  chronicle.  Geological  history  cannot  be  compiled 
from  a  full  and  continuous  series  of  documents.  Owing  to  the  very 
nature  of  its  origin,  the  record  is  necessarily  from  the  first  fragmentary, 
and  it  has  been  further  mutilated  and  obscured  by  the  revolutions  of 
successive  ages.  Even  where  the  chronicle  of  events  is  continuous,  it  is 
of  very  unequal  value  in  different  places.  In  one  case,  for  example,  it 
may  present  us  with  an  unbroken  succession  of  deposits,  many  thousands 
of  feet  in  thickness,  from  which,  however,  only  a  few  meagre  facts  as  to 
geological  history  can  be  gleaned.  In  another  instance,  it  brings  before 
us,  within  the  compass  of  a  few  yards,  the  evidence  of  a  most  varied 
and  complicated  series  of  changes  in  physical  geography,  as  well  as  an 
abundant  and  interesting  suite  of  organic  remains.  These  and  other 
characteristics  of  the  geological  record  will  become  more  apparent  and  in- 
telligible to  the  student  as  he  proceeds  in  the  study  of  the  science. 


INTRODUCTION 


In  the  present  volume  the  subject  will  be  distributed  under  the 
following  leading  divisions. 

1.  The  Cosniical  Aspects  of  Gedogy. — It  is  desirable  to  realise  some 
of  the  more  important  relations  of  the  earth  to  the  other  members  of 
the  solar  system,  of  which  it  forms  a  part,  seeing  that  geological  pheno- 
mena are  largely  the  result  of  these  relations.  The  form  and  motions  of 
the  planet  may  be  briefly  touched  upon,  and  attention  should  be 
directed  to  the  way  in  which  these  planetary  movements  influence 
geological  change.  The  light  cast  upon  the  early  history  of  the  earth  by 
researches  into  the  composition  of  the  sun  and  stars  deserves  notice  here. 

2.  Geognosy — An  Inquiry  into  the  Materials  of  the  EartKs  Svhstance. — 
This  division  describes  the  constituent  parts  of  the  earth,  its  envelopes  of 
air  and  water,  its  solid  crust,  and  the  probable  condition  of  it«  interior. 
Especially,  it  directs  attention  to  the  more  important  minerals  of  the 
crust,  and  the  chief  rocks  of  which  that  crust  is  built  up.  In  this  way, 
it  lays  a  foundation  of  knowledge  regarding  the  nature  of  the  materials 
constituting  the  mass  of  the  globe,  whence  we  may  next  proceed  to 
investigate  the  processes  by  which  these  materials  are  produced  and 
altered. 

3.  Dynamical  Geology  embraces  an  investigation  of  the  operations 
which  lead  to  the  formation,  alteration,  and  disturbance  of  rocks,  and 
calls  in  the  aid  of  physical  and  chemical  experiment  in  elucidation  of 
these  operations.  It  considers  the  nature  and  operation  of  the  processes 
that  have  determined  the  distribution  of  sea  and  land,  and  have  moulded 
the  forms  of  the  terrestrial  ridges  and  depressions.  It  further  investi- 
gates the  geological  changes  which  are  in  progress  over  the  surface  of 
the  land  and  floor  of  the  sea,  whether  these  are  due  to  subterranean 
disturbance,  or  to  the  eflbct  of  operations  above  ground.  Such  an 
inquiry  necessitates  a  careful  study  of  the  existing  economy  of  nature, 
and  forms  a  fitting  introduction  to  the  investigation  of  the  geological 
changes  of  former  periods.  This  and  the  previous  section,  including 
most  of  what  is  embraced  under  Physical  Geography  and  Petrogeny  or 
Geogeny,  will  here  be  discussed  more  in  detail  than  is  usual  in  geological 
treatises. 

4.  GeotectoniCj  or  Structural  Geology — the  Architecture  of  the  Earth, — 
This  section  of  the  investigation,  applying  the  results  arrived  at  in  the 
previous  division,  discusses  the  actual  arrangement  of  the  various 
materials  composing  the  crust  of  the  earth.  It  proves  that  some  have 
been  formed  in  beds  or  strata,  whether  by  the  deposit  of  sediment  on 
the  floor  of  the  sea,  or  by  the  slow  aggregation  of  organic  forms,  that 
others  have  been  poured  out  from  subterranean  sources  in  sheets  of 
molten  rock,  or  in  showers  of  loose  dust,  which  have  been  built  up  into 
mountains  and  plateaux.  It  further  shows  that  rocks  originally  laid 
down  in  almost  horizontal  beds  have  subsequently  been  crumpled, 
contorted,  dislocated,  invaded  by  igneous  masses  from  below,  and  ren- 
dered sometimes  crystalline.  It  teaches,  too,  that  wherever  exposed 
above  sea-level,  they  have  been  incessantly  worn  down,  and  have  often 
been  depressed,  so  that  older  lie  buried  beneath  later  accumulations. 


INTRODUCTION 


5.  Palceonldogiail  Geology. — This  branch  of  the  subject  deals  with  the 
organic  forms  which  are  found  preserved  in  the  rocks  of  the  crust  of  the 
earth.  It  includes  such  questions  as  the  manner  in  which  the  remains 
of  plants  and  animals  are  entombed  in  sedimentary  accumulations,  the 
relations  between  extinct  and  living  types,  the  laws  which  appear  to  have 
governed  the  distribution  of  life  in  time  and  in  space,  the  nature  and  use 
of  the  evidence  from  organic  remains  regarding  former  conditions  of 
physical  geography,  and  the  relative  importance  of  different  genera  of 
animals  and  plants  in  geological  inquiry. 

6.  Stratigraphical  Geology. — This  section  might  be  called  Geological 
History,  or  Historical  Geology.  It  works  out  the  chronological  succession 
of  the  great  formations  of  the  earth's  crust,  and  endeavours  to  trace  the 
sequence  of  events  of  which  they  contain  the  record.  More  particularly, 
it  determines  the  order  of  succession  of  the  various  plants  and  animals 
which  in  past  time  have  peopled  the  earth,  and  thus,  by  ascertaining 
what  has  been  the  grand  march  of  life  upon  the  planet,  seeks  to  imravel 
the  story  of  the  earth  as  made  kno>vn  by  the  rocks  of  the  crust. 
Further,  by  comparing  the  sequence  of  rocks  in  one  country  with  that  of 
those  in  another,  it  furnishes  materials  for  enabling  us  to  picture  the 
successive  stages  in  the  geographical  evolution  of  the  various  portions  of 
the  earth's  surface. 

7.  Physiographical  Geology,  starting  from  the  basis  of  fact  laid  down  by 
stratigraphical  geology  regarding  former  geographical  changes,  embraces 
an  inquiry  into  the  history  of  the  present  features  of  the  earth's  surface 
— continental  ridges  and  ocean  basins,  plains,  valleys,  and  mountains.  It 
investigates  the  structure  of  mountains  and  valleys,  compares  the  moun- 
tains of  diflferent  countries,  and  ascertains  the  relative  geological  dates  of 
their  upheaval.  It  explains  the  causes  on  which  local  differences  of 
scenery  depend,  and  shows  under  what  very  different  circumstances,  and 
at  what  widely  separated  intervals,  the  varied  contours,  even  of  a  single 
country,  have  been  produced. 


BOOK   I. 

COSMICAL  ASPECTS  OF  GEOLOGY. 

Before  geology  had  attained  to  the  position  of  an  inductive  science, 
it  was  customary  to  begin  all  investigations  into  the  history  of  the  earth 
by  propounding  or  adopting  some  more  or  less  fanciful  hypothesis,  in 
explanation  of  the  origin  of  our  planet  or  of  the  universe.  Such  pre- 
liminary notions  were  looked  upon  as  essential  to  a  right  understanding 
of  the  manner  in  which  the  materials  of  the  globe  had  been  put  together. 
To  the  illustrious  James  Hutton  (1785)  geologists  are  indebted,  if 
not  for  originating,  at  least  for  strenuously  upholding  the  doctrine  that 
it  is  no  part  of  the  province  of  geology  to  discuss  the  origin  of  thinjgs. 
He  taught  them  that  in  the  materials  from  which  geological  evidence  is 
to  be  compiled  there  can  be  found  "  no  traces  of  a  beginning,  no  prospect 
of  an  end."  In  England,  mainly  to  the  influence  of  the  school  which  he 
foiuided,  and  to  the  subsequent  rise  of  the  Geological  Society  (1807), 
which  resolved  to  collect  facts  instead  of  fighting  over  hypotheses,  is  due 
the  disappearance  of  the  crude  and  unscientific  cosmologies  of  previous 
centuries. 

But  there  can  now  be  little  doubt  that  in  the  reaction  against  the 
visionary  and  often  grotesque  speculations  of  earlier  writers,  geologists 
were  carried  too  far  in  an  opposite  direction.  In  allowing  themselves  to 
believe  that  geology  had  nothing  to  do  with  questions  of  cosmogony, 
they  gradually  grew  up  in  the  conviction  that  such  questions  could  never 
be  other  than  mere  speculation,  interesting  or  amusing  as  a  theme  for 
the  employment  of  the  fancy,  but  hardly  coming  >vithin  the  domain  of 
sober  and  inductive  science.  Nor  would  they  soon  have  been  awakened 
out  of  this  belief  by  anything  in  their  own  science.  It  is  still  true  that  in 
the  data  with  which  they  are  accustomed  to  deal,  as  comprising  the  sum 
of  geological  evidence,  there  can  be  found  no  trace  of  a  beginning,  though 
there  is  ample  proof  of  constant,  upward  progression  from  some  invisible 
starting-point.  The  oldest  rocks  which  have  been  discovered  on  any 
part  of  the  globe  have  possibly  been  derived  from  other  rocks  older  than 
themselves.  Geology  by  itself  has  not  yet  revealed,  and  is  little  likely 
ever  to  reveal,  a  portion  of  the  first  solid  crust  of  our  globe.     If,  then, 


8  COSMICAL  ASPECTS  OF  GEOLOGY  book  i 

geological  histx)ry  is  to  be  compiled  from  direct  evidence  furnished  by 
the  rocks  of  the  earth,  it  cannot  begin  at  the  beginning  of  things,  but 
must  be  content  to  date  its  first  chapter  from  the  earliest  period  of  which 
any  record  has  been  preserved  among  the  rocks. 

Nevertheless,  though,  in  its  usual  restricted  sense,  geolog}'^  has  been, 
and  must  ever  be,  unable  to  reveal  the  earliest  history  of  our  planet,  it 
no  longer  ignores,  as  mere  speculation,  what  is  attempted  in  this  subject 
by  its  sister  sciences.  Astronomy,  physics  and  chemistry  have  in  late 
years  all  contnbuted  to  cast  much  light  on  the  earliest  stages  of  the 
earth's  existence,  previous  to  the  beginning  of  what  is  commonly  regarded 
as  geological  history.  Whatever  extends  our  knowledge  of  the  former 
conditions  of  our  globe  may  be  legitimately  claimed  as  part  of  the  domain 
of  geological  inquiry.  If  Geology,  therefore,  is  to  continue  worthy  of  its 
name  as  the  science  of  the  earth,  it  must  take  cognisance  of  these  recent 
contributions  from  other  sciences.  It  can  no  longer  be  content  to  begin 
its  annals  with  the  records  of  the  oldest  rocks,  but  must  endeavour  to 
grope  its  way  through  the  ages  which  preceded  the  formation  of  any 
rocks.  Thanks  to  the  results  achieved  with  the  telescope,  the  spectro- 
scope, and  the  chemical  laboratory,  the  story  of  these  earliest  ages  of  our 
earth  is  every  year  becoming  more  definite  and  intelligible. 


1.  Relations  of  the  Earth  in  the  Solar  System. 

• 

As  a  prelude  to  the  study  of  the  structure  and  history  of  the  earth, 
some  of  the  general  relations  of  our  planet  to  the  solar  system  may  here 
be  noticed.  The  investigations  of  recent  years,  showing  the  community 
of  substance  between  the  difierent  members  of  that  system,  have  revived 
and  have  given  a  new  form  and  meaning  to  the  well-known  nebular  hypo- 
thesis of  Kant,  Laplace  and  W.  Herschel,  which  sketched  the  progress  of 
the  system  from  the  state  of  an  original  nebula  to  its  existing  condition 
of  a  central  incandescent  sun  with  surrounding  cool  planetary  bodies. 
According  to  this  hypothesis,  the  nebula,  originally  diffused  at  least 
as  far  as  the  furthest  member  of  the  system,  began  to  condense  towards 
the  centre,  and  in  so  doing  threw  oft*  or  left  behind  successive  rings. 
These,  on  disruption  and  further  condensation,  assumed  the  form  of 
planets,  sometimes  with  a  fiu*ther  formation  of  rings,  which  in  the  case 
of  Saturn  remain,  though  in  other  planets  they  have  broken  up  and 
united  into  satellites. 

Accepting  this  view,  we  should  expect  the  matter  composing  the 
various  members  of  the  solar  system  to  be  everywhere  nearly  the  same. 
The  fact  of  condensation  round  centres,  however,  indicates  probable  diff*er- 
ences  of  density  throughout  the  nebula.  That  the  materials  composing 
the  nebula  may  have  arranged  themselves  according  to  their  respective 
densities,  the  lightest  occupying  the  exterior,  and  the  heaviest  the 
interior  of  the  mass,  is  suggested  by  a  comparison  of  the  densities  of  the 
various  planets.  These  densities  are  usually  estimated  as  in  the  follow- 
ing table,  that  of  the  earth  being  taken  as  the  unit : — 


BOOK  I 

FLANE'i 

VAR 

Y  01 

UGL 

NS 

y 

Density  of  the  Sun 0*25 

,,          Mercury 
5,          Venus  . 

ri2 

103 

Earth   . 

1-00 

,,         Mars     . 

0-70 

,,         Jupiter 
,,          Saturn. 

.     0-24 
013 

,,         Uranus 

.     0-17 

,,         Neptune 

.     0-16 

It  is  to  be  observed,  however,  that  "  the  densities  here  given  are  mean 
densities,  assuming  that  the  apparent  size  of  the  planet  or  sun  is  the  true 
size,  t.d,  making  no  allowance  for  thousands  of  miles  deep  of  cloudy 
atmosphere.  Hence  the  numbers  for  Jupiter,  Saturn,  and  Uranus  are 
certainly  too  small,  that  for  the  sun,  much  too  small."  ^  Taking  the 
figures  as  they  stand,  while  they  do  not  indicate  a  strict  progression  in 
the  diminution  of  density,  they  state  that  the  planets  near  the  sun 
possess  a  density  about  twice  as  great  as  that  of  granite,  but  that  those 
lying  towards  the  outer  limits  of  the  system  are  composed  of  matter  as 
light  as  cork.  Again,  in  some  cases,  a  similar  relation  has  been  observed 
between  the  densities  of  the  satellites  and  their  primaries.  The  moon, 
for  example,  has  a  density  little  more  than  half  that  of  the  earth.  The 
first  satellite  of  Jupiter  is  less  dense,  though  the  other  three  are  said  to 
be  more  dense  than  the  planet.  Further,  in  the  condition  of  the  earth 
itself,  a  very  light  gaseous  atmosphere  forms  the  outer  portion,  beneath 
which  lies  a  heavier  layer  of  water,  while  within  these  two  envelopes  the 
materials  forming  the  solid  substance  of  the  planet  are  so  arranged  that 
the  outer  layer  or  crust  has  only  about  half  the  density  of  the  whole 
globe. 

According  to  the  hypothesis  now  under  consideration  it  is  conceived 
that,  in  the  gradual  condensation  of  the  original  nebula,  each  successive 
mass  left  behind  represented  the  density  of  its  parent  shell,  and  consisted 
of  progressively  heavier  matter.-  The  remoter  planets,  with  their  low 
densities  and  vast  absorbing  atmospheres,  may  be  supposed  to  consist  of 
metalloids,  like  the  outer  parts  of  the  sun's  atmosphere,  while  the  interior 
planets  are  no  doubt  mainly  metallic.  The  rupture  of  each  planetary 
ring  would,  it  is  thought,  raise  the  temperature  of  the  resultant  nebulous 
planet  to  such  a  height  as  to  allow  the  vapours  to  rearrange  themselves 
by  degrees  in  successive  layers,  or  rather  shells,  according  to  densities. 
And  when  the  planet  gave  off  a  satellite,  that  body  might  be  expected  to 
possess  the  composition  and  density  of  the  outer  layers  of  its  primary.^ 


*  Professor  Tait,  MS.  note. 

^  On  the  origin  of  Satellites,  see  the  researches  of  Prof.  G.  H.  Dar>^nn,  Phil.  Trans. 
(1879)  clxx.  p.  535.     Proc.  Boy.  Soc.  xxx.  j).  1. 

'  Lockyer  in  Prestwich's  Inaugural  Lecture,  Oxford,  1875,  and  in  Manchester 
Lectures,  Why  (he  Earth's  Chemistry  is  as  it  is.  Readers  interested  in  the  historical 
tlerelopment  of  geological  opinion  will  find  much  suggestive  matter  bearing  on  the  questions 
diflcnssed  above,  in  De  U  Beche's  'Researches  in  Theoretical  Geology,'  1834,— a  work 
notably  in  advance  of  its  time. 


10  COSMIC AL  ASPECTS  OF  GEOLOGY  book  i 

For  many  years,  the  only  evidence  available  as  to  the  actual  com- 
position of  other  heavenly  bodies  than  our  own  earth  was  furnished  by 
the  meteorites,  or  fallen  stars,  which  from  time  to  time  have  entered  our 
atmosphere  from  planetary  space,  and  have  descended  upon  the  surface 
of  the  globe.^  Subjected  to  chemical  analysis,  these  foreign  bodies  show 
considerable  diversities  of  composition;  but  in  no  case  have  they  yet 
revealed  the  existence  of  any  element  not  already  recognised  among  ter- 
restrial materials.  They  have  been  classified  in  three  groups:  Sideriies, 
composed  chiefly  of  iron ;  Siderolites,  consisting  partly  of  iron  and  partly 
of  various  stony  materials ;  and  Aerolites,  formed  almost  entirely  of  such 
stony  minerals.  Twenty-four  of  our  elements  have  been  detected  in 
meteorites.  Those  most  commonly  found  are  iron,  nickel,  phosphorus, 
sulphur,  carbon,  oxygen,  silicon,  magnesium,  calcium  and  aluminium. 
Less  frequent  or  occurring  in  smaller  quantities  are  hydrogen,  nitrogen, 
chlorine,  lithium,  sodium,  potassium,  titanium,  chromium,  manganese, 
cobalt,  arsenic,  antimony,  tin  and  copper.  These  various  elements  occur 
for  the  most  part  in  a  state  of  combination.  The  iron  exists  as  an  alloy 
with  nickel,  the  phosphorus  is  combined  with  nickel  and  iron,  the  silicon 
is  combined  with  oxygen  and  various  bases.  A  few  of  the  elements  occur 
in  a  free  state.  Thus  hydrogen  and  nitrogen  are  found  as  occluded  gases 
and  carbon  as  graphite,  rarely  as  diamond.  Of  combinations  of  elements 
in  meteorites  some,  not  yet  recognised  among  terrestrial  minerals,  comprise 
alloys  of  iron  and  nickel  and  various  sulphides  and  silicates.  But  others 
have  been  identified  with  well-known  minerals  of  the  earth's  crust, 
including  olivine,  enstatite  and  bronzite,  diopside  and  augite,  hornblende, 
anorthite  and  labradorite,  magnetite  and  chromite,  &c.  There  is  likewise 
a  carbonaceous  group  of  meteorites  containing  carbon,  both  amorphous 
and  as  black  diamond,  also  combined  ^vith  hydrogen  and  oxygen,  and  in 
some  cases  combustible,  with  a  bituminous  smell.  Some  iron  meteorites 
contain  a  large  proportion  of  occluded  hydrogen,  nitrogen,  or  carbonic 
oxide,  occasionally  as  much  as  six  times  the  volume  of  the  meteorite  itsell 

Various  theories  have  been  propounded  as  to  the  origin  or  source  of 
those  bodies  which  come  to  our  planet  from  space.  But  at  present  we 
possess  no  satisfactory  basis  of  fact  on  which  to  speculate.  Whether 
these  stones  belong  to  the  solar  system,  or,  as  seems  more  probable,  reach 
us  from  remoter  space,  they  prove  that  some  at  least  of  the  elements  and 
minerals  with  which  we  are  familiar  extend  beyond  our  planet 

But,  in  recent  years,  a  far  more  precise  and  generally  available  method 

1  On  meteorites  consult  Partsch,  *  Die  Meteoriten,'  Vienna,  1843.  Rose,  Abfiand,  IcGnigL 
Akad.  Berlin,  1863.  Rammelsberg,  *  Die  Chemische  Natur  der  Meteoriten,'  1870-9.  Tscher- 
raak,  Sitjd),  Akad,  Wisaen,  Vienna  (1875),  Izzi.  ;  '  Die  Mikroskopische  Beschaffenheit  der 
Meteoriten,*  Stuttgart,  1885.  Daubr^e,  'Etudes  Synth^tiques  de  Geologie  Exp^rimentale, ' 
1879  ;  C(mpt.  rend.  cvi.  (1888),  1671-1682  (compare  >4»i«r.  Joum,  Set.  xlii.  (1891),  p.  413). 
S.  Meunier,  *Le  Ciel  Geologique,'  1871;  *M^t^orite8,*  1884.  Brezina  und  Cohen,  *Die 
Stnictur  und  Zusammensetzung  der  Meteoreisen,'  Stuttgart,  1886.  W.  Flight,  Oeol.  Mag. 
1875,  Pop.  Set.  Rev.  new  scr.  i.  p.  390.  Proc.  Roy.  Soc,  xxxiii.  p.  343.  A.  W.  Wright, 
Amer.  Joum.  ser.  3,  xi.  p.  253  ;  xii.  p.  165.  L.  Fletcher,  *  An  Introduction  to  the  Study 
of  Meteorites,*  British  Museum  Catalogue,  1886. 


BOOK  I  SPECTROSCOPIC  RESEARCH  11 

of  research  into  the  composition  of  the  heavenly  bodies  has  been  found  in 
the  application  of  the  spectroscope.  By  means  of  this  instrument,  the 
light  emitted  from  self-luminous  bodies  can  be  analysed  in  such  a  way  as 
to  show  what  elements  are  present  in  their  intensely  hot  luminous  vapour. 
When  the  light  of  the  incandescent  vapour  of  a  metal  is  allowed  to 
pass  through  a  properly  arranged  prism,  it  is  seen  to  give  a  spectrum 
consisting  of  transverse  bright  lines  only.  This  is  termed  a  radiaiion- 
spedrunk  Each  element  appears  to  have  its  own  characteristic  arrange- 
ment of  lines,  which  in  general  retain  the  same  relative  position,  intensity 
and  colours.  Moreover,  gases  and  the  vapours  of  solid  bodies  are  found  to 
intercept  those  rays  of  light  which  they  themselves  emit.  The  spectrum 
of  sodium-vapour,  for  example,  shows  among  others  two  bright  orange 
lines.  If  therefore  white  light,  from  some  hotter  light^source,  passes 
through  the  vapour  of  sodium,  these  two  bright  lines  become  dark  lines, 
the  light  being  exactly  cut  off  which  would  have  been  given  out  by  the 
sodium  itself.     This  is  called  an  ahsorption-spectram. 

From  this  method  of  examination,  it  has  been  inferred  that  many  of 
the  elements  of  which  our  earth  is  composed  must  exist  in  the  state  of 
incandescent  vapour  in  the  atmosphere  of  the  sun.  Thirty-two  metals 
have  been  thus  identified,  including  aluminium,  barium,  manganese, 
lead,  calciiun,  cobalt,  potassium,  iron,  zinc,  copper,  nickel,  sodium  and 
magnesium.  These  elements,  or  at  least  substances  which  give  the  same 
groups  of  lines  as  the  terrestrial  elements  with  which  they  have  been 
identified,  do  not  occur  promiscuously  diffused  throughout  the  outer  mass 
of  the  sun.  According  to  Mr.  Lockyer^s  first  observations,  they  appear  to 
succeed  each  other  in  relation  to  their  respective  densities.  Thus  the 
coronal  atmosphere  which,  as  seen  in  total  eclipses,  extends  to  so  prodigious 
a  distance  beyond  the  disc  of  the  sun,  consists  mainly  of  subincandescent 
hydrogen  and  another  element  which  may  be  new.  Beneath  this  external 
vaporous  envelope  lies  the  chromosphere,  where  the  vapours  of  incan- 
descent hydrogen,  calcium  and  magnesium  can  be  detected.  Further 
inward  the  spot-zone  shows  the  presence  of  sodium,  titanium,  &c.  ;  while 
stiU  lower,  a  layer  (the  reversing  layer)  of  intensely  hot  vapours,  lying 
probably  next  to  the  inner  brilliant  photosphere,  gives  spectroscopic 
evidence  of  the  existence  of  incandescent  iron,  manganese,  cobalt,  nickel, 
copper,  and  other  well-known  terrestrial  metals.^ 

It  is  to  be  observed,  however,  that  in  these  spectroscopic  researches  the 

*  On  spectroscopic  research  as  applied  to  the  sun,  see  Kirchhoff  and  Bunsen, 
*  Researches  on  Solar  Spectrum,*  &c.,  1863  ;  Angstrom,  *  Recherches  sur  le  Spectre 
nonnal  du  Soleil' ;  Lockyer,  'Solar  Physics/  1873,  and  'Studies  in  Spectrum  Analysis' 
(International  Series),  1878  ;  Huggins  and  Miller,  Proc.  Roy.  Soc.  xii.,  PhU.  Trans.  1864  ; 
Roscoe's  'Spectrum  Analysis,'  with  authorities  there  cited.  An  ingenious  theory  to 
account  for  the  conservation  of  solar  energy  was  suggested  by  the  late  Sir  C.  W. 
Siemens  {Proc.  Roy.  Soc.  xxxiii.  (1881)  p.  389).  It  requires  the  presence  of  aqueous 
vapour  and  carbon  compounds  in  stellar  space,  which  are  dissociated  and  drawn  into 
the  solar  photosphere,  where  they  burst  into  flame  with  a  large  development  of  heat, 
and  then  passing  into  aqueous  vapour  and  carbonic  anhydride  or  oxide,  flow  to  the  solar 
equator  whence  they  are  projected  into  space. 


12  COSMICAL  ASPECTS  OF  GEOLOGY  book  i 

decomposition  of  the  elements  by  electrical  action  was  not  considered. 
The  conclusions  embodied  in  the  foregoing  paragraph  have  been  founded 
on  the  idea  that  the  lines  seen  in  the  spectrum  of  any  element  are  all  due 
to  the  vibrations  of  the  molecules  of  that  element.  But  Mr.  Lockyer  has 
suggested  that  this  view  may  after  all  be  but  a  rough  approximation 
to  the  truth ;  that  it  may  be  more  accurate  to  say,  as  a  result  of  the 
facts  already  acquired,  that  there  exist  basic  elements  common  to  calciimi, 
iron,  &c.,  and  to  the  solar  atmosphere,  and  that  the  spectrum  of  each  body 
is  a  summation  of  the  spectra  of  various  molecular  complexities  which  can 
exist  at  different  temperatures,  the  simplest  only  being  found  in  the  hottest 
part  of  the  sun.^ 

The  spectroscope  has  likewise  been  successfully  applied  by  Mr.  Huggins 
and  others  to  the  observation  of  the  fixed  stars  and  nebulae,  with  the 
result  of  establishing  a  similarity  of  elements  between  our  own  system 
and  other  bodies  in  sidereal  space.  In  the  radiation  spectra  of  nebulae, 
Mr.  Huggins  finds  the  hydrogen  lines  very  prominent ;  and  he  conceives 
that  they  may  bo  glowing  masses  of  that  element.  Professor  Tait  has 
suggested,  on  the  other  hand,  that  they  are  more  probably  clouds  of 
stones  frequently  colliding  and  thus  giving  off  incandescent  gases.  Sir 
William  Thomson  (now  Lord  Kelvin)  favom^s  this  view,  which  is  fiulber 
amply  supported  by  spectroscopic  observations.  Among  the  fixed  stars, 
absorption-spectra  have  been  recognised,  pointing  to  a  structiwe  resembling 
that  of  our  sun,  viz.,  an  incandescent  nucleus  which  may  be  solid  or  liquid 
or  of  very  highly  compressed  gas,  but  which  gives  a  continuous  spectrum 
and  which  is  surrounded  with  an  atmos[)here  of  glowing  vapour.-  Those 
stars  which  show  the  simplest  spectra  are  believed  to  have  the  highest 
temperature,  and  in  proportion  as  they  cool  their  materials  will  become  more 
and  more  differentiated  into  what  we  call  elements.  The  most  brilliant  or 
hottest  stars  show  in  their  spectra  only  the  lines  of  gases,  as  hydrogen. 
Cooler  stars,  like  our  sun,  give  indications  of  the  presence,  in  addition,  of 
the  metals — magnesium,  sodium,  calcium,  iron.  A  still  lower  temperature 
is  marked  by  the  appearance  of  the  other  metals,  metalloids,  and  compounds.^ 
The  sun  would  thus  be  a  star  .considerably  advanced  in  the  process  of 
differentiation  or  association  of  its  atoms.  It  contains,  so  far  as  we  know, 
no  metalloid  except  carbon,  and  possibly  oxygen,  nor  any  compound, 
while  stars  like  Sirius  show  the  presence  only  of  hydrogen,  with  but  a 
feeble  proportion  of  metallic  vapours ;  and  on  the  other  hand,  the  red 
stars  indicate  by  their  spectra  that  their  metallic  vapours  have  entered 
into  combination,  whence  it  is  inferred  that  their  temperature  is  lower 
than  that  of  our  sun. 

More  recently,  however,  another  view  of  the  evolution  of  stars  has 
been  propounded  by  Mr.  Lockyer.  He  conceives  that  all  self-luminous 
cosmical  bodies   are    composed  either  of  swarms  of   meteorites,  or    of 

'  See  also  the  opposite  views  of  Dewar  and  Liveing,  Pro.  lioy.  Soc.  xxx.  p.  93,  and 
H.  W.  Vogel,  Naiuret  xxvii.  p.  233. 

-  Huggins,  Proc.  Roy.  Soc.  1863-66,  and  Brit.  Assoc.  Lecture  (Nottingham,  1866)  ; 
Huggins  and  Miller,  Phil.  Trans.  1864. 

2  Lockyer,  Comptes  rendm^  Dec.  1873. 


BOOK  r  FORM  AND  SIZE  OF  THE  EARTH  13 


masses  of  vapour  produced  by  collisions  of  meteorites ;  that  stars,  comets 
and  nebulae  are  only  different  phases  of  the  same  series  of  changes  ;  that 
where  the  temperature  of  a  star  is  increasing  the  star  consists  of  a  meteor- 
swarm,  which  by  constant  collision  of  its  individual  meteorites  is  gradually 
being  vapourised  by  heat ;  and  that  after  volatilisation  cooling  sets  in  and 
the  vapour  finally  condenses  into  a  globe.^ 

11.  Form  and  Size  of  the  Earth. 

Further  confirmation  of  some  of  the  foregoing  views  as  to  the  order 
of  planetary  evolution  is  furnished  by  the  form  of  the  earth  and  the 
arrangment  of  its  component  materials. 

That  the  earth  is  an  oblate  spheroid,  and  not  a  perfectly  spherical 
globe,  was  discovered  and  demonstrated  by  Newton.  He  even  calcu- 
lated the  amount  of  ellipticity  long  before  any  measurement  had  con- 
firmed such  a  conclusion.  During  the  present  century  numerous  arcs  of 
the  meridian  have  been  measured,  chiefly  in  the  northern  hemisphere. 
From  a  series  made  by  different  observers  between  the  latitudes  of 
Sweden  and  the  Cape  of  Good  Hope,  Bessel  obtained  the  following  data 
for  the  dimensions  of  the  earth  : — 

Equatorial  diameter        .         .         41,847,192  feet,  or  7925*604  miles. 
Polar  diameter        .         .  41,707,314       „      7899114     ,, 


Amount  of  polar  flattening     .  139,768        ,,  26*471 


i> 


The  equatorial  circumference  is  thus  a  little  less  than  25,000  miles, 
and  the  difference  between  the  polar  and  equatorial  diameters  (neiu'ly 
26^  miles)  amounts  to  about  ^^xr^h  of  the  equatorial  diameter. ^  More 
recently,  however,  it  has  been  shown  that  the  oblate  spheroid  indicated 
by  these  measurements  is  not  a  symmetrical  body,  the  equatorial  circum- 
ference being  an  ellipse  instead  of  a  circle.  The  greater  axis  of  the 
equator  lies  in  long.  8°  15'  W. — a  meridian  passing  through  Ireland, 
Portugal,  and  the  north-west  corner  of  Africa,  and  cutting  off  the  north- 
east corner  of  Asia  in  the  opposite  hemisphere.^ 

The  polar  flattening,  established  by  measurement  and  calculation  as 
that  which  would  necessarily  have  been  assumed  by  an  originally  plastic 
globe  in  obedience  to  the  movement  of  rotation,  has  been  cited  as 
evidence  that  the  earth  was  once  in  a  plastic  condition.  Taken  in 
connection  with  the  analogies  supplied  by  the  sun  and  other  heavenly 
bodies,  this  inference  appeared  to  be  well  grounded.*     More  recently, 

>  *  The  Meteoritic  Hypothesis,' 1890. 

'  Herschel,  'Astronomy,'  p.   139. 

'  A.  R.  Clarke,  PhU,  Mag.  August  1878  ;  Encydopctdia  BriUinnica^  9th  edit.  x.  172. 

^  It  was  opposed  by  Mohr  (*  Geschichte  der  Erde,'  p.  472),  who,  adopting  a  suggestion 
long  ago  made  by  Playfair,  endeavoured  to  show  that  the  polar  flattening  can  be 
accounted  for  by  greater  denudation  of  the  polar  tracts,  exposed  as  these  have  been  by  the 
heaping  up  of  the  oceanic  waters  towards  the  equator  in  consequence  of  rotation.  He  dwelt 
chiefly  on  the  effects  of  glaciers  in  lowering  the  land,  but  as  Pfaff  has  pointed  out,  the  work 
of  eroeion  is  chiefly  performed  by  other  atmospheric  forces  that  operate  rather  towards  the 


14  COSMIC AL  ASPECTS  OF  GEOLOGY  book  i 

however,  it  has  been  contended  that  even  in  a  truly  solid  body  a  polar 
flattening  might  be  developed  under  the  influence  of  rotation.^ 

Though  the  general  spheroidal  form  of  our  planet,  and  probably  the 
general  distribution  of  sea  and  land,  are  referable  to  the  early  effects  of 
rotation  on  a  fluid  or  viscous  mass,  it  is  certain  that  the  present  details 
of  its  surface-contours  are  of  comparatively  recent  date.  Speculations 
have  been  made  as  to  what  may  have  been  the  earliest  character  of  the 
solid  surface,  whether  it  was  smooth  or  rough,  and  particularly  whether 
it  was  marked  by  any  indication  of  the  existing  continental  elevations 
and  oceanic  depressions.  So  far  as  we  can  reason  from  geological 
evidence,  there  is  no  proof  of  any  uniform  superficies  having  ever 
existed.  Most  probably  the  first  formed  crust  broke  up  irregularly, 
and  not  until  after  many  successive  corrugations  did  the  surface 
acquire  stability.  Some  writers  have  imagined  that  at  first  the  ocean 
spread  over  the  whole  surface  of  the  planet.  But  of  this  there  is  not 
only  no  evidence,  but  good  reason  for  believing  that  it  never  could  have 
taken  place.  As  will  be  alluded  to  in  a  later  page,  the  preponderance 
of  water  in  the  southern  hemisphere,  seems  to  indicate  some  excess  of 
density  in  that  hemisphere.  This  excess  can  hardly  have  been  produced 
by  any  change  since  the  materials  of  the  interior  ceased  to  be  mobile ;  it 
must  therefore  be  at  least  as  ancient  as  the  condensation  of  water  on  the 
earth's  surface.  Hence  there  was  probably  from  the  beginning  a  tendency 
in  the  ocean  to  accumulate  in  the  southern  rather  than  in  the  northern 
hemisphere. 

That  land  existed  from  the  earliest  ages  of  which  we  have  any  record 
in  rock-formations,  is  evident  from  the  obvious  fact  that  these  formations 
themselves  consist  in  great  measure  of  materials  derived  from  the  waste 
of  land.  WTien  the  student,  in  a  later  part  of  this  volume,  is  presented 
with  the  proofs  of  the  existence  of  enormous  masses  of  sedimentary 
deposits,  even  among  some  of  the  oldest  geological  systems,  he  will 
perceive  how  important  must  have  been  the  tracts  of  land  that  could 
furnish  such  piles  of  detritus. 

The  tendency  of  modem  research  is  to  give  probability  to  the 
conception,  first  outlined  by  Kant,  that  not  only  in  our  own  solar  system, 
but  throughout  the  regions  of  space,  there  has  been  a  common  plan  of 
evolution,  and  that  the  matter  diffused  through  space  in  nebulaB,  stars, 
and  planets  is  substantially  the  same  as  that  with  which  we  are  familiar. 
Hence  the  study  of  the  structure  and  probable  history  of  the  sun  and 
the  other  heavenly  bodies  comes  to  possess  an  evident  geological  interest, 
seeing  that  it  may  yet  enable  us  to  carry  back  the  story  of  our  planet 
far  beyond  the  domain  of  ordinary  geological  evidence,  and  upon  data  not 
less  trustworthy  than  those  furnished  by  the  rocks  of  the  earth's  crust. 

equator  than  the  poles  (*Allgemeine  Geologie  als  exacte  Wissenschaft/.p.  6).  Compare 
Naumaun,  NeutaJahrh.  1871,  p.  250.  Nevertheless,  Mohr  undoubtedly  recalled  attention 
to  a  conceivable  cause  by  which,  in  spite  of  polar  elevation  or  equatorial  subsidence,  the 
external  form  of  the  planet  might  be  preserved. 

*  See    in    particular    the    papers    by   Mr.   C.    Chree.      PhU.    Mag,    1891,    pp.    233 
and  342. 


BOOK  I  THE  EARTH'S  ROTATION  15 


III.  The  Movements  of  the  Earth  in  their  Geological  Relations. 

We  are  here  concerned  with  the  earth's  motions  in  so  far  only  as  they 
materially  influence  the  progress  of  geological  phenomena. 

§  1.  Rotation. — In  consequence  of  its  angular  momentum  at  its 
original  separation,  the  earth  rotates  on  its  axis.  The  rate  of  rotation 
has  once  been  much  more  rapid  than  it  now  is  (p.  21).  At  present  a 
complete  rotation  is  performed  in  about  twenty-four  hours,  and  to  it  is 
due  the  succession  of  day  and  night.  So  far  as  observation  has  yet  gone, 
this  movement  is  imiform,  though  recent  calculations  of  the  influence  of 
the  tides  in  retarding  rotation  tend  to  show  that  a  very  slow  diminution 
of  the  angular  velocity  is  in  progress.  If  this  be  so,  the  length  of  the 
day  and  night  will  slowly  increase  until  finally  the  duration  of  the  day 
and  that  of  the  year  will  be  equal.  The  earth  will  then  have  reached 
the  condition  into  which  the  moon  has  passed  relatively  to  the  earth,  one 
half  being  in  continual  day,  the  other  in  perpetual  night. 

The  linear  velocity  due  to  rotation  varies  in  different  places,  according 
to  their  position  on  the  surface  of  the  planet  At  each  pole  there  can  be 
no  velocity,  but  from  these  two  points  towards  the  equator  there  is  a 
continually  increasing  rapidity  of  motion,  till  at  the  equator  it  is  equal  to 
a  rate  of  507  yards  in  a  second. 

To  the  rotation  of  the  earth  are  due  certain  remarkable  influences 
upon  currents  of  air  circulating  either  towards  the  equator  or  towards  the 
poles.  Currents  which  move  from  polar  latitudes  travel  from  parts  of 
the  earth's  surface  where  the  velocity  due  to  rotation  is  small,  to  others 
where  it  is  great.  Hence  they  lag  behind,  and  their  course  is  bent  more 
and  more  westward.  An  air  current,  quitting  the  north  polar  or  north 
temperate  regions  as  a  north  wind,  is  deflected  out  of  its  course,  and 
becomes  a  north-east  wind.  On  the  opposite  side  of  the  equator,  a  similar 
current  setting  out  straight  for  the  equator,  is  changed  into  a  south-east 
wind.  Hence,  as  is  well-known,  the  Trade-winds  have  their  characteristic 
westward  deflection.  On  the  other  hand,  a  current  setting  out  north- 
wards or  southwards  from  the  equator,  passes  into  regions  having  a  less 
velocity  due  to  rotation  than  it  possesses  itself,  and  hence  it  travels  on  in 
advance  and  appears  to  be  gradually  deflected  eastward.  The  aerial 
currents,  blowing  steadily  across  the  surface  of  the  ocean  towards  the 
equator,  produce  oceanic  currents  which  unite  to  form  the  westward- 
flowing  Equatorial  current. 

It  has  been  maintained  by  Von  Baer,^  that  a  certain  deflection  is 

*  **Ueber  ein  allgemeines  Gesetz  in  der  Gestaltung  der  Flussbetten."  Bull.  Acad.  St. 
PHerabourgy  ii.  (1860).  See  also  Ferrel  on  the  motion  of  fluids  and  solids  relatively  to  the 
earth's  surface,  Camh.  {Mass.)  Math.  Monthly^  vols.  i.  and  ii.  (1859-60) ;  Dulk,  X.  Deutsch. 
Gtd.  Get.  xxxi.  (1879)  p.  224.  The  River  Irtisch  is  said  in  flowing  northward  to  have  cut 
so  much  into  its-  right  bank  that  villages  are  gradually  driven  eastwards,  Demiansk  having 
been  shifted  about  a  mile  in  240  years  (Nature ^  xv.  p.  207).  But  this  may  be  accounteil 
for  by  local  causes.  See  an  excellent  paper  on  this  subject  with  special  reference  to  the 
regime  of  some  rivers  in  northern  Germany,  by  F.  Klockmann,  Jahrb.  Preuss.  Geoi.  Landes- 
anst.  1882  ;  also  E.  Dunker,  Zeitsch.  fur  die  (jesammten  yaturwissenschaften,  1875,  p.  463  : 
G.  K.  Gilbert,  Amer,  Joum.  Set.  xxvii.  (1884)  p.  427. 


16  COSMIC AL  ASPECTS  OF  GEOLOGY  book  i 

experienced  by  rivers  that  flow  in  a  meridional  direction,  like  the  Volga 
and  Irtisch.  Those  travelling  polewards  are  asserted  to  press  upon  their 
eastern  rather  than  their  western  banks,  while  those  which  run  in 
the  opposite  direction  are  stated  to  be  thrown  more  against  the  western 
than  the  eastern.  When,  however,  we  consider  the  comparatively  small 
volume,  slow  motion,  and  continually  meandering  course  of  rivers,  it  may 
reasonably  be  doubted  whether  this  vera  caum  can  have  had  much  effect 
generally  in  modifying  the  form  of  river-channels. 

j$  2.  Revolution. — Besides  turning  on  its  axis,  the  globe  performs  a 
movement  round  the  sun,  termed  revolution.  This  movement,  accom- 
plished in  rather  more  than  365  days,  determines  for  us  the  length  of 
our  year,  which  is,  in  fact,  merely  the  time  required  for  one  complete 
revolution-  The  path  or  orbit  followed  by  the  earth  round  the  sun  is  not 
a  perfect  circle  but  an  ellipse,  with  the  sun  in  one  of  the  foci,  the  mean 
distance  of  the  earth  from  the  sun  being  92,800,000,  the  present  aphelion 
distance  94,500,000,  and  the  perihelion  distance  91,250,000  miles.  By 
slow  secular  variations,  the  form  of  the  orbit  alternately  approaches  to 
and  recedes  from  that  of  a  circle.  At  the  nearest  possible  approach 
between  the  two  bodies,  owing  to  change  in  the  ellipticity  of  the  orbit, 
the  earth  is  14,368,200  miles  nearer  the  sun  than  when  at  its  greatest 
possible  distance.  These  maxima  and  minima  of  distance  occur  at  vast 
intervals  of  time.^  The  last  considerable  eccentricity  took  place  about 
200,000  years  ago,  and  the  previous  one  more  than  half  a  million  years 
earlier.  Since  the  amount  of  heat  received  by  the  earth  from  the  sun  is 
inversely  as  the  square  of  the  distance,  eccentricity  may  have  had  in  past 
time  much  effect  upon  the  climate  of  the  earth,  as  will  be  pointed  out 
further  on  (§  8). 

,^  3.  Precession  of  the  Equinoxes. — If  the  axis  of  the  earth  were 
perpendicular  to  the  plane  of  its  orbit,  there  would  be  equal  day  and 
night  all  the  year  round.  But  it  is  really  inclined  from  that  ]X)6ition  at 
an  angle  of  23°  27'  21".  Hence  our  hemisphere  is  alternately  presented 
to  and  turned  away  from  the  sun,  and,  in  this  way,  brings  the  familiar 
alternation  of  the  seasons.  Again,  were  the  earth  a  perfect  sphere,  of 
uniform  density  throughout,  the  position  of  its  axis  of  rotation  would 
not  be  changed  by  attractions  of  external  bodies.  But  owing  to  the 
protuberance  along  the  equatorial  regions,  the  attraction  chiefly  of  the 
moon  and  sun  tends  to  pull  the  axis  aside,  or  to  make  it  describe  a 
conical  movement,  like  that  of  the  axis  of  a  top,  round  the  vertical. 
Hence  each  pole  points  successively  to  different  stars.  This  movement, 
called  the  precession  of  the  equinoxes,  in  combination  with  another 
smaller  movement,  due  to  the  attraction  of  the  moon  (called  nutatioti), 
completes  its  cycle  in  21,000  years,  the  annual  total  advance  of  the 
equinox  amounting  to  62".  At  present  the  winter  in  the  northern  hemi- 
sphere coincides  with  the  earth's  nearest  approach  to  the  sun,  or  perihelion. 
In  10,500  years  hence  it  will  take  place  when  the  earth  is  at  the  farthest 
part  of  its  orbit  from  the  sun,  or  in  aphelion.     This  movement  may  have 

^  See  Croll's  'Climate  and  Time,*  chaps,  iv.,  xix. 


BOOK  I  STABILITY  OF  EARTH'S  AXIS  17 


had  great  importance  in  connection  with  former  secular  variations  in  the 
eccentricity  of  the  orbit  (§  8). 

§  4.  Change  in  the  Obliquity  of  the  Ecliptic— The  angle  at  which 
the  axis  of  the  earth  is  inclined  to  the  plane  of  its  orbit  does  not  remain 
strictly  constant.  It  oscillates  through  long  periods  of  time  to  the  extent 
of  about  a  degree  and  a  half,  or  perhaps  a  little  more,  on  either  side  of 
the  mean.  According  to  Dr.  Croll,^  this  oscillation  must  have  consider- 
ably affected  former  conditions  of  climate  on  the  earth,  since,  when  the 
obliquity  is  at  its  maximum,  the  polar  regions  receive  about  eight  and  a 
half  days'  more  of  heat  than  they  do  at  present — that  is,  about  as  much 
heat  as  lat.  76°  enjoys  at  this  day.  This  movement  must  have  augmented 
the  geological  effects  of  precession,  to  which  reference  has  just  been  made, 
and  which  are  described  in  §  8. 

§  5.  Stability  of  the  Earth's  Axis. — That  the  axis  of  the  earth's 
rotation  has  successively  shifted,  and  consequently  that  the  poles  have 
wandered  to  different  points  on  the  surface  of  the  globe,  has  been  main- 
tained by  geologists  as  the  only  possible  explanation  of  certain  remarkable 
conditions  of  climate,  which  can  be  proved  to  have  formerly  obtained 
within  the  Arctic  Circle.  Even  as  far  north  as  lat.  81°  45',  abundant 
remains  of  a  vegetation  indicative  of  a  warm  climate,  and  including  a  bed 
of  coal  25  to  30  feet  thick,  have  been  found  in  situ.-  It  is  contended 
that  when  these  plants  lived,  the  ground  could  not  have  been  permanently 
frozen  or  covered  for  most  of  the  year  with  thick  snow.  In  explanation 
of  the  difficulty,  it  has  been  suggested  that  the  north  pole  did  not  occupy 
its  present  |x>sition,  and  that  the  locality  where  the  plants  occur  lay  in 
more  southerly  latitudes.  Without  at  present  entering  on  the  discussion 
of  the  question  whether  the  geological  evidence  necessarily  requires  so 
important  a  geographical  change,  let  us  consider  how  far  a  shifting  of  the 
axis  of  rotation  has  been  a  possible  cause  of  change  during  that  section 
of  geological  time  for  which  there  are  records  among  the  stratified  rocks. 

From  the  time  of  Laplace,^  astronomers  have  strenuously  denied  the 
possibility  of  any  sensible  change  in  the  position  of  the  axis  of  rotation. 
It  has  been  urged  that,  since  the  planet  acquired  its  present  oblate 
spheroidal  form,  nothing  but  an  utterly  incredible  amount  of  deformation 
could  overcome  the  greater  centrifugal  force  of  the  equatorial  protuber- 
ance. It  is  certain,  however,  that  the  axis  of  rotation  does  not  strictly 
coincide  with  the  principal  axis  of  inertia.  Though  the  angular  difference 
between  them  must  always  have  been  small,  we  can,  without  having 
recourse  to  any  extramundane  influence,  recognise  two  causes  which, 
whether  or  not  they  may  suffice  to  produce  any  change  in  the  position 
of  the  main  axis  of  inertia,  undoubtedly  tend  to  do  so.  In  the  first 
place,  a  widespread  upheaval  or  depression  of  certain  unsymmetrically 
arranged  portions  of  the  surface  to  a  considerable  amount  would  tend 
to  shift  that  axis.  In  the  second  place,  an  analogous  result  might  arise 
from  the  denudation  of  continental  masses  of  land,  and  the  consequent  filling 

*  CroU,  Trans.  Oeoh  Sac.  Glasgow,  ii.  177.     'Climate  and  Time,'  chap.  xxv. 

-  Fielden  and  Heer,  Quart.  Journ.  Geol.  Sac.  Nov.  1877. 

*  *  Mecanique  Celeste,'  tome  v.  p.  14. 

C 


18  COSMIC AL  ASPECTS  OF  GEOLOGY  book  i 

up  of  sea-basins.  Lord  Kelvin  (Sir  William  Thomson)  freely  concedes 
tie  physical  possibility  of  such  changes.  "  We  may  not  merely  admit,''  he 
says,  "  but  assert  as  highly  probable,  that  the  axis  of  maximum  inertia 
and  axis  of  rotation,  always  very  near  one  another,  may  have  been  in 
ancient  times  very  far  from  their  present  geographical  position,  and  may 
have  gradually  shifted  through  10,  20,  30,  40,  or  more  degrees,  without 
at  any  time* any  perceptible  sudden  disturbance  of  either  land  or  water."  ^ 
But  though,  in  the  earlier  ages  of  the  planet's  history,  stupendous 
deformations  may  have  occurred,  and  the  axis  of  rotation  may  have 
often  shifted,  it  is  only  the  alterations  which  can  possibly  have  occurred 
during  the  accumulation  of  the  stratified  rocks,  that  need  to  be  taken 
into  account  in  connection  with  the  evidence  of  changes  of  climate 
during  geological  history.  If  it  can  be  shown,  therefore,  that  the 
geographical  revolutions  necessary  to  shift  the  axis  are  incredibly 
stupendous  in  amount,  improbable  in  their  distribution,  and  not  really 
demanded  by  geological  evidence,  we  may  reasonably  withhold  our 
belief  from  this  alleged  cause  of  the  changes  of  climate  during  the 
periods  of  time  embraced  by  geological  records. 

It  has  been  estimated  by  Lord  Kelvin  "that  an  elevation  of  600 
feet,  over  a  tract  of  the  earth's  surface  1000  miles  square  and  10  miles 
in  thickness,  would  only  alter  the  position  of  the  principal  axis  by 
one- third  of  a  second,  or  34  feet."^  Prof.  George  Darwin  has  shown 
that,  on  the  supposition  of  the  earth's  complete  rigidity,  no  redistribu- 
tion of  matter  in  new  continents  could  ever  shift  the  pole  from  its 
primitive  position  more  than  3*^,  but  that,  if  its  degree  of  rigidity  is 
consistent  with  a  periodical  re-adjustment  to  a  new  form  of  equilibrium, 
the  pole  may  have  wandered  some  10°  or  15°  from  its  primitive  position, 
,  or  have  made  a  smaller  excursion  and  returned  to  near  its  old  place. 
In  order,  however,  that  these  maximum  efiects  should  be  produced, 
it  would  be  necessary  that  each  elevated  area  should  have  an  area 
of  depression  corresponding  in  size  and  diametrically  opposite  to  it, 
that  they  should  lie  on  the  same  complete  meridian,  and  that  they 
should  both  be  situated  in  lat.  45°.  With  all  these  coincident  favourable 
circumstances,  an  effective  elevation  of  -^^  of  the  earth's  surface  to  the 
extent  of  10,000  feet  would  shift  the  pole  11^';  a  similar  elevation 
of  ^^  would  move  it  1°  46^';  of  ^,  3°  17';  and  of  i,  8°  4 J'.  Mr. 
Darwin  admits  these  to  be  superior  limits  to  what  is  possible,  and  that, 
on  the  supposition  of  intumescence  or  contraction  under  the  regions  in 
question,  the  deflection  of  the  pole  might  be  reduced  to  a  quite 
insignificant  amount.^ 

Under  the  most  favourable  conditions,  therefore,  the  possible  amount 
of  deviation  of  the  pole  from  its  first  position  would  appear  to  have  been 
too  small  to  have  seriously  influenced  the  climates  of  the  globe  within 
geological  history.     If  we  grant  that  these  changes  were  cumulative,  and 

*  Brit.  Assoc.  Rep.  (1876),  Sections,  p.  11. 

-  Trans.  Oeol.  Soc.    Glasgow^    iv.    p.    313.     The   situation   of  the   supposed   area   of 
uplieaval  on  the  earth's  surface  is  not  stated. 
8  PhU.  Trans.  Nov.  1876. 


BOOK  I  STABILITY  OF  EARTH'S  AXIS  19 

that  the  superior  limit  of  deflection  was  reached  only  after  a  long  series 
of  concurrent  elevations  and  depressions,  we  must  suppose  that  no  move- 
ments took  place  elsewhere  to  counteract  the  effect  of  those  about  lat.  45*^ 
in  the  two  hemispheres.  But  this  is  hardly  credible.  A  glance  at  a 
geographical  globe  suffices  to  show  how  large  a  mass  of  land  exists  now 
both  to  the  north  and  south  of  that  latitude,  especially  in  the  northern 
hemisphere,  and  that  the  deepest  parts  of  the  ocean  are  not  antipodal  to 
the  greatest  heights  of  the  land.  These  features  of  the  earth's  surface 
are  of  old  standing.  There  seems,  indeed,  to  be  no  geological  evidence  in 
favour  of  any  such  geographical  changes  as  could  have  produced  even 
the  comparatively  small  displacement  of  the  axis  considered  possible  by 
Prof.  Dsiwin. 

In  an  ingenious  suggestion.  Sir  John  Evans  contended  that,  even 
without  any  sensible  change  in  the  position  of  the  axis  of  rotation  of  the 
nucleus  of  the  globe,  there  might  be  very  considerable  changes  of 
latitude  due  to  disturbance  of  the  equilibrium  of  the  outer  portion 
or  shell  by  the  upheaval  or  removal  of  masses  of  land  between  the 
equator  and  the  poles,  and  to  the  consequent  sliding  of  the  shell  over  the 
nucleus  until  the  equilibrium  was  restored.^  Subsequently  he  precisely 
formulated  his  hypothesis  as  a  question  to  be  determined  mathematic- 
ally;^ and  the  solution  of  the  problem  was  worked  out  by  the 
Rev.  J.  F.  Twisden,  who  arrived  at  the  conclusion  that  even  the  large 
amount  of  geographical  change  postulated  by  Dr.  Evans  could  only 
displace  the  earth's  axis  of  figure  to  the  extent  of  less  than  10'  of 
angle,  that  a  displacement  of  as  much  as  10"  or  15°  could  be  effected 
only  if  the  heights  and  depths  of  the  areas  elevated  and  depressed 
exceeded  by  many  times  the  heights  of  the  highest  mountains,  that 
under  no  circumstances  could  a  displacement  of  20""  be  effected  by  a 
transfer  of  matter  of  less  amount  than  about  a  sixth  part  of  the  whole 
equatorial  bulge,  and  that  even  this  extreme  amount  would  not  necessarily 
alter  the  position  of  the  axis  of  figure.^ 

Against  any  h3rpothesis  which  assumes  a  thin  crust  enclosing  a 
liquid  or  viscous  interior,  weighty  and,  indeed,  insuperable  objections 
have  been  urged.  It  has  been  suggested,  however,  that  the  almost 
universal  traces  of  present  or  former  volcanic  action,  the  evidence  from 
the  compressed  strata  in  mountain  regions  that  the  crust  of  the  earth 
must  have  a  capacity  for  slipping  towards  certain  lines,  the  great 
amount  of  horizontal  compression  of  strata  which  can  be  proved  to 
have  been  accomplished,  and  the  secular  changes  of  climate — notably 
the  former  warm  climate  near  the  north  pole — furnish  grounds  for 
inquiry  whether  the  doctrine  of  a  fluid  substratum  over  a  rigid  nucleus, 
which  has  been  urged  by  several  able  ^mters,  would  not  be  compatible 
with  mechanical  considerations,  and  whether,  under  these  circumstances, 
changes  in  latitude  would  not  result   from  unequal   thickening  of   the 

»  Proc  Roy.  Soc.  xv.  (1867),  p.  46.  2  q  j  ^,,^^^   ^^  ^^^^^^  (1876),  p.  62. 

*  g.  J,  Oeol,  Soc.  xxxiv.  (1878),  p.  41.     See  also  E.  Hill,  Geol.  Mag.  v.  (2nd  ser.) 
pp.  262,  479.     0.  Fisher,  op.  cit.  pj).  291,  551. 


20  COSMIC  A  L  ASPECTS  OF  GEOLOGY  book  i 


crust.  ^  This  qiiestiou  of  the  internal  condition  of  the  globe  is  dis- 
cussed at  p.  47. 

§  6.  Changes  of  the  Earth's  Centre  of  Gravity. — If  the  centre 
of  gravity  in  our  planet,  as  pointed  out  by  Herschel,  ]ye  not  coincident 
with  the  centre  of  figure,  but  lies  somewhat  to  the  south  of  it,  any 
variation  in  its  position  will  affect  the  ocean,  which  of  course  adjusts 
itself  in  relation  to  the  earth's  centre  of  gra^dty.  How  far  any  redis- 
tribution of  the  matter  within  the  earth,  in  such  a  way  as  to  affect  the 
present  equilibrium,  is  now  possible,  we  cannot  tell.  But  certain  re- 
volutions at  the  sui'face  may  from  time  to  time  produce  changes  of 
this  kind.  The  accumulation  of  ice  which,  as  will  be  immediately 
ilescribed  (§  8),  is  believed  to  gather  round  one  pole  during  the 
maximum  of  eccentricity,  will  displace  the  centre  of  gravity,  and,  as  the 
result  of  this  change,  will  raise  the  level  of  the  ocean  in  the  glacial 
hemisphere.^  The  late  Dr.  CroU  estimated  that,  if  the  present  mass  of 
ice  in  the  southern  hemisphere  is  taken  at  1000  feet  thick  extending 
down  to  lat.  60°,  the  transference  of  this  mass  to  the  northern  hemi- 
sphere would  raise  the  level  of  the  sea  80  feet  at  the  north  pole.  Other 
methods  of  calculation  give  difi'erent  results.  Mr.  Heath  put  the  rise  at 
128  feet ;  Archdeacon  Pratt  made  it  more  ;  while  the  Rev.  O.  Fisher 
gave  it  at  409  feet.^  Subsequently,  in  returning  to  this  question,  Dr. 
CroU  remarked  "  that  the  removal  of  two  miles  of  ice  from  the  Antarctic 
continent  [and  at  present  the  mass  of  ice  there  is  probably  thicker  than 
that]  would  displace  the  centre  of  gravity  190  feet,  and  the  formation  of 
a  mass  of  ice  equal  to  the  one-half  of  this,  on  the  Arctic  regions,  would 
carry  the  centre  of  gravity  95  feet  farther ;  gi\ing  in  all  a  total  displace- 
ment of  285  feet,  thus  producing  a  rise  of  level  at  the  north  pole  of  285 
feet,  and  in  the  latitude  of  Edinburgh  of  234  feet."  A  very  considerable 
additional  displacement  would  arise  from  the  increment  of  water  to  the 
mass  of  the  ocean  by  the  melting  of  the  ice.  Supposing  half  of  the  two 
miles  of  Antarctic  ice  to  be  replaced  by  an  ice-cap  of  similar  extent  and 
one  mile  thick  in  the  northern  hemisphere,  the  other  half  being  melted 
into  water  and  increasing  the  mass  of  the  ocean,  Dr.  Croll  estimated  that 
from  this  scource  an  extra  rise  of  200  feet  would  take  place  in  the 
general  ocean  level,  so  that  there  would  be  a  rise  of  485  feet  at  the 
north  pole,  and  434  feet  in  the  latitude  of  Edinburgh.*  An  intermittent 
submergence  and  emergence  of  the  low  polar  lands  might  be  due  to  the 
alternate  shifting  of  the  centre  of  gravity. 

To  what  extent  this  cause  has  actually  come  into  opei-ation  in  j>ast 
time  cannot  at  present  be  determined.  It  has  been  suggested  that  the 
"raised  beaches,"  shore-lines  {straiMinien),  or  old  sea-terraces,  so  numerous 

»  0.  Fisher,  Geol.  Mag.  1878,  p.  552,  *  Physics  of  the  Earth's  Crust/  1882  ;  2nd  Edition 
1889.  -  Adhemar,  'Revolutions  de  la  Mer,'  1840. 

»  CVoll,  iu  ReacUr  for  2nd  September,  1865,  and  Phil,  Mag.  April,  1866  ;  Heath, 
Phil.  Mag.  April,  1869;  Pratt,  PhU.  Mag.  March,  1866:  Fisher,  Rea4^r,  10th 
February,  1866. 

■•  Croll,  Geol.  Mag.  new  series,  i.  (1874),  p.  347  ;  *  Climate  and  Time,'  chaps,  xxiii. 
and  xxiv.  and  jw^^fa,  p.  286.    Consult  also  Fisher,  Phil.  Ma^.  xxxiv.  (October,  1892),  p.  337. 


BOOK  I  INFLUENCE  OF  SUN  AND  MOON  ON  EARTH  21 


at  various  heights  in  the  north-west  of  Europe,  might  be  due  to  the 
transference  of  the  oceanic  waters,  and  not  to  any  subterranean  movement, 
as  generally  believed.  Had  they  been  due  to  such  a  general  cause,  they 
ought  to  have  shown  evidence  of  a  gradual  and  uniform  decline  in  elevation 
from  north  to  south,  with  only  such  local  variations  as  might  be  accounted 
for  by  the  influence  of  masses  of  high  land  or  other  local  cause.  No  such 
feature,  however,  has  been  satisfactorily  established.^  On  the  contrary, 
the  levels  of  the  terraces  vary  within  comparatively  short  distances. 
Though  numerous  on  both  sides  of  Scotland,  they  disappear  further  north 
among  the  Orkney  and  Shetland  islands,  although  these  localities  were 
admirably  adapted  for  their  formation  and  preservation.*^  The  conclusion 
may  be  drawn  that  the  "  raised  beaches  "  cannot  be  adduced  as  evidence 
of  changes  of  the  earth's  centre  of  gravity,  but  are  due  to  local  and 
irregularly  acting  causes.  (See  Book  III.  Part  I.  Section  iii.  §  1,  where 
this  subject  is  more  fully  discussed.) 

§  7.  Results  of  .the  Attractive  Influence  of  Sun  and  Moon  on  the 
Geologrical  Condition  of  the  Earth. — Many  speculations  have  been  oflered 
to  account  for  supposed  former  greater  intensity  of  geological  activity  on 
the  surface  of  the  globe.  Two  causes  for  such  greater  intensity  may  be 
adduced.  In  the  first  place,  if  the  earth  has  cooled  down  from  an 
original  molten  condition,  it  has  lost,  in  cooling,  a  vast  amount  of 
potential  geological  energy.  It  does  not  necessarily  follow,  however, 
that  the  geological  phenomena  resulting  from  internal  temperature  have, 
during  the  time  recorded  in  the  accessible  part  of  the  earth's  crust,  been 
steadily  decreasing  in  magnitude.  We  might,  on  the  contrary,  contend 
that  the  increased  resistance  of  a  thickening  cooled  crust  may  rather 
have  hitherto  intensified  the  manifestations  of  subterranean  activity,  by 
augmenting  the  resistance  to  be  overcome.  In  the  second  place,  the 
earth  may  have  been  once  more  powerfully  affected  by  external  causes, 
such  as  the  greater  heat  of  the  sun,  and  the  greater  proximity  of  the 
moon.  That  the  formerly  larger  amount  of  solar  heat  received  by  the 
surface  of  our  planet  must  have  produced  warmer  climates  and  more 
rapid  evaporation,  with  gi*eater  rainfall  and  the  important  chain  of 
geological  changes  which  such  an  increase  would  introduce,  appears  in 
every  way  probable,  though  the  geologist  has  not  yet  been  able  to  observe 
any  indisjmtable  indication  of  such  a  former  intensity  of  superficial 
changes. 

Prof.  Darwin,  in  investigating  the  bodily  tides  of  viscous  spheroids, 
has  brought  forward  some  remarkable  results  bearing  on  the  question 
of  the  possibility  that  geological  operations,  both  internal  and  superficial, 
may  have  been  once  greatly  more  gigantic  and  rapid  than  they  are 
now.^  He  assumes  the  earth  to  be  a  homogeneous  spheroid  and  to  have 
possessed  a  certain  small  \iscosity,*  and  he  calculates  the  internal  tidal 

*  The  student  ought,  however,  to  consult  Prof.  Suess'  Antlitz  tier  Erde  for  the  arguments 
in  favour  of  an  opposite  opinion. 

-  XcUure,  xvi.  (1877),  p.  415.  •'  Phil,  Trans.  1879.  parts  i.  and  ii. 

"*  The  degree  of  viscosity  assumed  is  such  tliat  **thirteeu  and  a  half  tons  to  the 
square  inch  acting  for  twenty -four  hours  on  a  slab  an  inch  thick  displaces  the  upper 


22  COSMICAL  ASPECTS  OF  GEOLOGY  book  i 

friction  in  such  a  mass  exposed  to  the  attraction  of  moon  and  sun,  and 
the  consequences  which  these  bodily  tides  have  produced.  He  finds  that 
the  length  of  our  day  and  month  have  greatly  increased,  that  the 
moon's  distance  has  likewise  augmented,  that  the  obliquity  of  the 
ecliptic  has  diminished,  that  a  large  amount  of  hypogene  heat  has  been 
generated  by  the  internal  tidal  friction,  and  that  these  changes  may  all 
have  transpired  within  comparatively  so  short  a  period  (57,000,000  years) 
as  to  place  them  quite  probably  within  the  limits  of  ordinary  geological 
history.  According  to  his  estimate,  46,300,000  years  ago  the  length  of 
the  sidereal  day  was  fifteen  and  a  half  hours,  the  moon's  distance  in  mean 
radii  of  the  earth  was  46*8  as  compared  with  60*4  at  the  present  time. 
But  56,810,000  years  back,  the  length  of  a  day  was  only  6|^  hours,  or 
less  than  a  quarter  of  its  present  value,  the  moon's  distance  was  only 
nine  earth's  ladii,  while  the  lunar  month  lasted  not  more  than  about  a 
day  and  a  half  (1*58),  or  ^j  of  its  present  duration.  He  arrives  at  the 
deduction  that  the  energy  lost  by  internal  tidal  friction  in  the  earth's 
mass  is  converted  into  heat  at  such  a  rate  that  the  amount  lost  during 
57,000,000  years,  if  it  were  all  applied  at  once,  and  if  the  earth  had  the 
specific  heat  of  iron,  would  raise  the  temperature  of  the  whole  planet's 
mass  1,760^  Fahrenheit,  but  that  the  distribution  of  this  heat-generation 
has  been  such  as  not  to  interfere  with  the  normal  augmentation  of 
temperature  downward  due  to  secular  cooling,  and  the  conclusion  drawn 
therefrom  by  Sir  William  Thomson.  Mr.  Darwin  further  concludes  from 
his  hypothesis  that  the  ellipticity  of  the  earth's  figure  having  been 
continually  diminishing,  "  the  polar  regions  must  have  been  ever  rising 
and  the  equatorial  ones  falling,  though  as  the  ocean  followed  these 
changes,  they  might  quite  well  have  left  no  geological  traces.  The  tides 
must  have  been  very  much  more  frequent  and  larger,  and  accordingly 
the  rate  of  oceanic  denudation  much  accelerated.  The  more  rapid 
alternation  of  day  and  night  ^  would  probably  lead  to  more  sudden  and 
violent  storms,  and  the  increased  rotation  of  the  earth  would  augment 
the  violence  of  the  trade-winds,  which  in  their  turn  would  affect  oceanic 
currents."  ^  As  above  stated,  no  facts  yet  revealed  by  the  geological 
record  compel  the  admission  of  more  violent  superficial  action  in  former 
times  than  now.  But  though  the  facts  do  not  of  themselves  lead  to  such 
an  admission,  it  is  proper  to  enquire  whether  any  of  them  are  hostile  to 
it.  It  will  be  shoAvn  in  Book  VI.  that  even  as  far  back  as  early  Palaeozoic 
times,  that  is,  as  far  into  the  past  as  the  history  of  organised  life  can  be 
traced,  sedimentation  took  place  very  much  as  it  does  now.  Sheets  of 
fine  mud  and  silt  were  pitted  with  rain  drops,  ribbed  with  ripple-marks, 
and  furrowed  by  crawling  worms,  exactly  as  they  now  are  on  the  shores 
of  any  modern    estuary.     These    surfaces   were    quietly    buried   under 

surface  relatively  to  the  lower  through  one -tenth  of  an  inch.  It  is  obvious,"  says  Mr. 
Darwin,  "that  such  a  substance  as  this  would  be  called  a  solid  in  ordinarj'  parlance, 
and  in  the  tidal  problem  this  must  be  regarded  as  a  very  small  viscosity."  Op,  cit. 
p.  531. 

^  According  to  his  calculation,  the  year  57,000,000  of  years  ago  contained  1300  days 
instead  of  365.  ^  Op,  cit.  p.  532. 


BOOK  I  CLIMATE  IN  ITS  GEOLOGICAL  RELATIONS  23 


succeeding  sediment  of  a  similar  kind,  and  this  for  hundreds  and 
thousands  of  feet.  Nothing  indicates  violence ;  all  the  evidence  favours 
tranquil  deposit^  K,  therefore,  Mr.  Darwin's  hypothesis  be  accepted, 
we  must  conclude  either  that  it  does  not  necessarily  involve  such  violent 
superficial  operations  as  he  supposes,  or  that  eveA  the  oldest  sedimentary 
formations  do  not  date  back  to  a  time  when  the  influence  of  increased 
rotation  could  make  itself  evident  in  sedimentation,  that  is  to  say,  on 
Mr.  Darwin's  h3rpothesis,  the  most  ancient  fossiliferous  rocks  cannot  be 
as  much  as  57,000,000  years  old.    * 

§  8.  Climate  in  its  Geolosrical  Relations. — In  subsequent  parts  of 
this  volume  data  will  be  given  from  which  we  learn  that  the  climates  of 
the  earth  have  formerly  been  considerably  different  from  those  which  at 
present  prevail.  A  consideration  of  the  history  of  the  solar  system 
would  of  itself  suggest  the  inference  that,  on  the  whole,  the  climates  of 
early  geological  periods  must  have  been  warmer.  The  sun's  heat  was 
greater,  probably  the  amount  of  it  received  by  the  earth  was  likewise 
greater,  while  there  would  be  for  some  time  a  sensible  influence  of  the 
planet's  own  internal  heat  upon  the  general  temperature  of  the  whole 
globe.^  Although  arguments  based  upon  the  probable  climatal  neces- 
sities of  extinct  species  and  genera  of  plants  and  animals  must  be  used 
with  extreme  caution,  it  may  be  asserted  with  some  confidence  that  from 
the  vast  areas  over  which  Palaeozoic  moUusks  have  been  traced,  alike 
in  the  eastern  and  the  western  hemispheres,  the  climates  of  the  globe 
in  Palaeozoic  time  were  probably  more  uniform  than  they  now  are. 
There  appears  to  have  been  a  gradual  lowering  of  the  general  tempera- 
ture during  past  geological  time,  accompanied  by  a  tendency  towards 
greater  extremes  of  climate.  But  there  are  proofs  also  that  at  longer 
or  shorter  intervals  cold  cycles  have  intervened.  The  Glacial  Period, 
for  example,  preceded  our  own  time,  and  in  successive  geological  forma- 
tions indications,  of  more  or  less  value,  have  been  found  that  suggest  if 
they  do  not  prove  a  former  prevalence  of  ice  in  what  are  now  temperate 
regions.' 

*  Sir  R  Ball  {Nature,  xxv.  1881,  pp.  79,  103),  starting  from  Professor  Darwin's  data, 
pushed  his  conclusions  to  such  an  extreme  as  to  call  in  the  agency  of  tides  more  than  600 
feet  high  in  early  geological  times.  In  repudiating  this  application  of  his  results,  Mr. 
Darwin  {Xaturt,  xxv.  p.  213)  employs  the  argument  I  have  here  used  fh)m  the  absence  of 
any  evidence  of  such  tidal  action  in  the  geological  formations,  and  f^om  the  indication,  on 
the  contrary,  of  tranquil  deposit 

'  Lord  Kelvin  (Sir  William  Thomson)  believes  that  the  hypothesis  that  terrestrial  tempera- 
ture was  formerly  higher  by  reason  of  a  hotter  sun  "  is  rendered  almost  infinitely  probable 
by  independent  physical  evidence  and  mathematical  calculation."  (Trans.  Geol.  Soc. 
Olasffowj  V.  p.  238.)  Profes»or  Tait,  however,  has  suggested,  that  the  former  greater 
heat  of  the  sun  may  have  raised  such  va.st  clouds  of  absorbing  vapour  round  that 
Inminary  as  to  prevent  the  effective  amount  of  radiation  of  heat  to  the  earth's  surface 
from  being  greater  than  at  present ;  while  on  the  other  hand,  a  similar  supposition  may  be 
made  with  reference  to  the  greater  amount  of  vajwur  which  increased  solar  radiation  would 
raise  to  be  condensed  in  the  earth's  atmosphere.  *  Recent  Advances  in  Physical  Science, ' 
1876,  p.  174. 

»  Consult  a  suggestive  paper  by  the  late  Dr.  M.  Neumayr,  Nature,  xlii.  (1890),  p.  148. 


24  COSMICAL  ASPECTS  OF  GEOLOGY  book  i 


Various  theories  have  been  proposed  in  explanation  of  such  alternations 
of  climate.  Some  of  these  have  appealed  to  a  change  in  the  position  of 
the  earth's  axis  relatively  to  the  mass  of  the  planet  (an7e,  §  5).  Others 
have  been  based  on  the  notion  that  the  earth  may  have  passed  through 
hot  and  cold  regions  of  space.  Others,  again,  have  called  in  the  effects 
of  terrestrial  changes,  such  as  the  distribution  of  land  and  sea,  on  the 
assumption  that  elevation  of  land  about  the  poles  must  cool  the  temperature 
of  the  globe,  while  elevation  round  the  equator  would  raise  it.^  But  the 
changes  of  temperature  appear  to  have  affected  the  whole  of  the  earth's 
surface,  while  there  is  not  only  no  proof  of  any  such  enormous  vicissitudes 
in  physical  geography  as  would  be  required,  but  good  grounds  for 
believing  that  the  present  terrestrial  and  oceanic  areas  have  remained, 
on  the  whole,  on  the  same  sites  from  very  early  geological  time.  More- 
over, as  evidence  has  accumulated  in  favour  of  periodic  alternations  of 
climate,  the  conviction  has  been  strengthened  that  no  mere  local  changes 
could  have  sufficed,  but  that  secular  variations  in  climate  must  be  assigned 
to  some  general  and  probably  recurring  cause. 

By  degrees,  geologists  accustomed  themselves  to  the  belief  that  the 
cold  of  the  Glacial  Period  was  not  due  to  mere  terrestrial  changes,  but 
was  to  be  explained  somehow  as  the  result  of  cosmical  causes.  Of  various 
suggestions  as  to  the  probable  nature  and  operation  of  these  causes,  one 
deserves  careful  consideration — change  in  the  eccentricity  of  the  earth's 
orbit.  Sir  John  Herschel  *^  pointed  out  many  years  ago  that  the  direct 
effect  of  a  high  condition  of  eccentricity  is  to  produce  an  unusually  cold 
Avinter,  followed  by  a  correspondingly  hot  summer,  in  the  hemisphere 
whose  winter  occurs  in  aphelion,  while  an  equable  condition  of  climate 
at  the  same  time  prevails  on  the  opposite  hemisphere.  But  both  hemi- 
spheres must  receive  precisely  the  same  amount  of  solar  heat,  because 
the  deficiency  of  heat,  resulting  from  the  sun's  greater  distance  during 
one  part  of  the  year,  is  exactly  com}>ensated  by  the  greater  length  of  that 
season.  Sir  John  Herschel  even  considered  that  the  direct  effects  of 
eccentricity  must  thus  l>e  nearly  neutralised.^  As  a  like  verdict  was 
afterwards  given  by  Arago,  Humboldt,  and  others,  geologists  were  satisfied 
that  no  importiint  change  of  climate  could  be  attributed  to  change  of 
eccentricity. 

The  late  Dr.  James  CroU,  as  far  back  as  the  year  1864,  made  an  im- 
portant suggestion  in  this  matter,  and  subsequently  worked  out  an 
elaborate  development  of  the  whole  subject  of  the  physical  causes  on 
which  climate  depends."*  He  was  good  enough  to  draw  up  the  following 
abstract  of  them  for  former  editions  of  the  present  work. 

"Assuming  tlie  mean  distance  of  the  auu  to  \)e  92,400,000  miles,  then  when  the 
eccentricity  is  at  its  8Uj>erior  limit,   '07776,  the  distance  of  the  sun  from  the  earth, 

'  In  Lyell's  *  Principles  of  Geology,'  this  doctrine  of  the  influence  of  geographical  changes 
is  maintained. 

*  Tnins.  (wed,  Soc.  vol,  iii.  p.  293  (2ud  series). 

^  '  Cabinet  Cyclopaedia,*  sec.  315;  'Outlines  of  Astronomy,'  sec.  368. 

*  Phil.  Mag.  xxviii.  (1864),  p.  121.     His  researches  will  be  found  in  detail  in  his  volume 
'Climate  and  Time,'  1875,  and  his  later  work  'Discussions  on  CMinmte  and  Cosmology.' 


BOOK  I       EFFECTS  OF  ECGENTRIGITY  OF  EARTHS  ORBIT 


25 


when  the  latter  is  in.  the  aphelion  of  its  orbit,  is  no  less  than  99,584,100  miles,  and 
when  in  the  perihelion  it  is  only  85,215,900  miles.  The  cai-th  is,  therefore,  14,368,200 
miles  farther  from  the  snn]in  the  former  than  in  the  latter  position.  ^^The  direct  heat 
of  the  sun  being  inversely  as  the  square  of  the  distance,  it  follows  that  the  amount  of 
heat  received  by  the  earth  in  these  two  positions  will  be  as  19  to  26.  The  present 
eccentricity  being  -0168,  the  earth's  distance  during  our  northern  winter  is  90,847,680 
miles.  Suppose  now  that,  irom  the  precession  of  the  equinoxes,  winter  in  our  northern 
hemisphere  should  happen  when  the  earth  is  in  the  aphelion  of  its  orbit,  at  the  time 
that  Uie  orbit  is  at  its  greatest  eccentricity ;  the  earth  would  then  be  8,736,420  miles 
farther  from  the  sun  in  winter  than  it  is  at  present.  The  direct  heat  of  the  sun  would 
therefore,  during  winter,  be  one-fifth  less  and  during  summer  one-fifth  greater  than  now. 


HP. 


•-P. 


H.P. 


y.  Winter  SoUti4x  in  Aphelion.  N.  Winter  Solslke  in  Perihelion, 

Fig.  1.— Eccentricity  of  the  Eftrtirs  Orbit  in  Relation  to  Climate. 

This  enormous  difference  would  necessarily  affect  tlie  climate  to  a  very  great  extent. 
Were  the  wintera  under  these  circumstances  to  occur  when  the  earth  was  in  the  perihelion 
of  its  orbit,  the  earth  would  then  be  14,368,200  miles  nearer  the  sun  in  winter  than  in 
summer.  In  this  case  the  difference  between  winter  and  sunmier  in  our  latitudes  would 
be  almost  annihilated.  But  as  the  winters  in  the  one  liemi8j)here  correspond  with  the 
summers  in  the  other,  it  follows  that  while  the  one  hemisphere  would  be  enduring  the 
greatest  extremes  of  summer  heat  and  winter  cold,  the  otlier  would  be  enjoying  perpetual 
summer. 

**  It  is  quite  tnie  that,  whatever  may  be  the  eccentricity  of  the  earth's  orbit,  the  two 
hemispheres  must  receive  equal  quantities  of  heat  [)er  annum  ;  for  proximity  to  the  sun 
is  exactly  compensated  by  the  effect  of  swifter  motion.  The  total  amount  of  heat 
received  from  the  sun  between  the  two  equinoxes  is,  therefore,  the  same  in  both  halves 
of  the  year,  whatever  the  eccentricity  of  the  earth's  orbit  may  he.  For  example,  whatever 
extra  heat  the  southern  hemisphere  may  at  present  receive  i)cr  day  from  the  sun  during 
its  summer  months,  owing  to  greater  j)roximity  to  the  sun,  Ls  exactly  com})ensated  by  a 
corresponding  loss  arising  from  the  shortness  of  the  season  ;  and,  on  the  other  hand, 
whatever  deficiency  of  heat  we  in  the  northern  hemisphere  may  at  jiresent  have  i>er  day 
during  our  summer  half-year,  in  consequence  of  the  earth's  distance  from  the  sun,  is  also 
exactly  compensated  by  a  corres|)onding  length  of  season. 

•*  It  is  well  known,  however,  that  those  simple  changes  in  the  summer  and  winter 


26  COSMICAL  ASPECTS  OF  GEOLOGY  book  i 

distances  would  not  alone  produce  a  glacial  epoch,  and  that  physicists,  confining  their 
attention  to  the  purely  astronomical  effects,  were  perfectly  correct  in  affirming  that  no 
increase  of  eccentricity  of  the  earth's  orbit  could  account  for  that  epoch.  But  the  im- 
portant fact  was  overlooked  that,  although  the  glacial  epoch  could  not  result  directly 
from  an  increase  of  eccentricity,  it  might  nevertheless  do  so  indirectly  from  physical 
agents  that  were  brought  into  operation  as  a  result  of  an  increase  of  eccentricity.  The 
following  is  an  outline  of  what  these  physical  agents  were,  how  they  were  brought  into 
operation,  and  tlie  way  in  which  they  may  have  led  to  the  glacial  epoch. 

"  With  the  eccentricity  at  its  superior  limit  and  the  winter  occurring  in  the  aphelion, 
the  earth  would,  as  we  have  seen,  be  8,736,420  miles  farther  from  the  sun  during  that 
season  than  at  present.  The  reduction  in  the  amount  of  heat  received  from  the  sun 
owing  to  his  increased  distance,  would  lower  the  midwinter  temperature  to  an  enormous 
extent.  In  temperate  regions  the  greater  portion  of  the  moisture  of  the  air  is  at  present 
precipitated  in  the  form  of  rain,  and  the  very  small  portion  which  falls  as  snow  disappears 
in  the  course  of  a  few  weeks  at  most.  But  in  the  circumstances  under  consideration, 
the  mean  winter-temperature  would  be  lowered  so  much  below  the  freezing-point  that 
what  now  falls  as  rain  during  that  season,  w^ould  then  fall  as  snow.  This  is  not  all ;  the 
winters  would  then  not  only  be  cooler  than  now,  but  they  would  also  be  much  longer. 
At  present  the  wiutei*s  are  nearly  eight  days  shorter  than  the  summers ;  but  with 
the  eccentricity  at  its  superior  limit  and  the  winter  solstice  in  aphelion,  the  length  of 
the  winters  would  exceed  that  of  the  summers  by  no  fewer  than  thirty-six  days.  The 
lowering  of  the  temperature  and  the  lengthening  of  the  winter  would  both  tend  to  the 
same  effect,  viz.,  to  increase  the  amount  of  snow  accumulated  during  the  winter ;  for, 
other  things  being  equal,  the  longer  the  snow-accumulating  period  the  greater  the 
accumulation.  It  may  be  remarked,  however,  that  the  absolute  quantity  of  heat  received 
during  winter  is  not  affected  by  the  decrease  in  the  sun's  heat,  for  the  additional  length 
of  the  season  compensated  for  this  decrease.^  As  regards  the  absolute  amount  of  heat 
received,  increase  of  the  sun's  distance  and  lengthening  of  the  "winter  are  compensatory, 
but  not  so  in  regard  to  the  amount  of  snow  accumulated.  The  consequence  of  this  state 
of  things  would  be  that,  at  the  commencement  of  the  short  summer,  the  ground  would 
be  covered  with  the  winter's  accumulation  of  snow.  Again,  the  presence  of  so  much 
snow  would  lower  the  summer  tem^ierature,  and  prevent  to  a  great  extent  the  melting  of 
the  snow. 

"There  arc  three  separate  ways  whereby  accimuilated  masses  of  snow  and  ice  tend 
to  lower  the  summer  temperature,  viz.  : — 

**  First,  By  means  of  direct  radiation.  No  matter  what  the  intensity  of  the  sun's 
rays  may  be,  the  temjierature  of  snow  and  ice  can  never  rise  above  32*.  Hence,  the 
presence  of  snow  and  ice  tends  by  direct  radiation  to  lower  the  temperature  of  all 
surrounding  bodies  to  32°.  In  Greenland,  a  country  covered  with  snow  and  ice,  the 
pitch  has  been  se«n  to  melt  on  the  side  of  a  ship  exposed  to  the  direct  rays  of  the  sun, 
while  at  the  same  time,  the  surrounding  air  was  far  l)elow  the  freezing-point ;  a  thermo- 
meter exposed  to  the  dii*ect  radiation  of  the  sun  has  been  observed  to  stand  above  100*, 
while  the  air  surrounding  the  instrument  was  actually  12*  below  the  freezing-point.  A 
similar  experience  has  been  recorded  by  travellers  on  the  snow -fields  of  the  AIjjs.  These 
results,  surjnising  as  they  no  doubt  apjiear,  are  what  we  ought  to  exi)ect  under  the 
circumstances.  Perfectly  dry  air  seems  to  be  nearly  incapable  of  absorbing  radiant  heat. 
The  entire  radiation  jyasses  through  it  almost  without  any  sensible  absorption.  Conse- 
quently the  pitch  on  the  side  of  the  ship  may  be  melted,  or  the  bulb  of  the  thennometer 
raised  to  a  high  temperature  by  the  direct  rays  of  the  sun,  while  the  suiTounding  air 


'  When  the  eccentricity  is  at  it«  superior  limit,  tlie  absolute  quantity  of  heat  received  by 
the  earth  during  the  year  is,  however,  about  one  three-hundredth  part  greater  than  at  present. 
But  this  does  not  affect  the  question  at  issue. 


BOOK  I  CAUSE  OF  GLACIAL  CLIMATES  27 

reniAins  intensely  cold.     The  air  is  cooled  by  conUut  with  the  snow-covered  ground,  but 
is  not  heated  by  the  radiation  from  the  sun. 

'*  When  the  air  is  charged  with  aqueous  vapour,  a  similar  cooling  effect  also  takes 
place,  but  in  a  slightly  different  way.  Air  charged  with  aqueous  vapour  is  a  good 
absorber  of  radiant  heat,  but  it  can  only  absorb  those  rays  which  agree  with  it  in  period. 
It  80  happens  that  rays  from  snow  and  ice  are,  of  all  others,  those  which  it  absorbs  best. 
The  humid  air  will  absorb  the  total  radiation  from  the  snow  and  ice,  but  it  will  allow 
the  greater  part  of,  if  not  nearly  all,  the  sun's  rays  to  pass  unabsorbed.  But  during  the 
day,  when  tiie  sun  is  shining,  the  radiation  from  the  snow  and  ice  to  the  air  is  negative  ; 
that  is,  the  snow  and  ice  cool  the  air  by  radiation.  The  result  is,  the  air  is  cooled  by 
radiation  fix>m  the  snow  and  ice  (or  rather,  we  should  say,  to  the  snow  and  ice)  more 
rapidly  than  it  is  heated  by  the  sun  ;  and  as  a  consequence,  in  a  country  like  Greenland, 
covered  with  an  icy  mantle,  the  temperature  of  the  air,  even  during  summer,  seldom 
rises  above  the  freezing-point.  Snow  is  a  good  reflector,  but  as  simple  reflection  does  not 
change  the  chsffacter  of  the  rays,  they  would  not  be  absorbed  by  the  air,  but  would  pass 
into  stellar  space.  Were  it  not  for  the  ice,  the  summers  of  North  Greenland,  owing  to 
the  continuance  of  the  sim  above  the  horizon,  would  be  as  warm  as  those  of  Ehigland  ; 
but  instead  of  this,  the  Greenland  summers  are  colder  than  our  winters.  Cover  India 
with  an  ice  sheet,  and  its  summers  would  be  colder  than  those  of  England. 

**  Second f  Another  cause  of  the  cooling  effect  is  that  the  ra^-s  which  fall  on  snow  and 
ice  are  to  a  great  extent  reflected  back  into  space.  But  those  that  are  not  reflected,  but 
absorbed,  do  not  raise  the  temperature,  for  they  disappear  in  the  mechanical  work  of 
melting  the  ice.  For  whatsoever  may  be  the  intensity  of  the  sun's  heat,  the  surface  of 
the  ground  will  be  kept  at  32°  so  long  as  the  snow  and  ice  remain  unmelted. 

Third,  Snow  and  ice  lower  the  temperature  by  chilling  the  air  and  condensing  the 
vapour  into  thick  fogs.  The  great  strength  of  the  sun's  rays  during  summer,  due  to  his 
nearness  at  that  season,  would,  in  the  first  place,  tend  to  produce  an  increased  amount  of 
evaporation.  But  the  presence  of  snow-clad  mountains  and  an  ic}'  sea  would  chill  the  atmo- 
sphere and  condense  the  vapour  into  thick  fogs.  The  thick  fogs  and  cloudy  sk}'  would 
effectually  prevent  the  sun's  rays  from  reaching  the  earth,  and  the  snow,  in  consequence, 
would  remain  unmelted  during  the  entire  summer.  In  fact,  we  have  this  very  condition 
of  things  exemplified  in  some  of  the  islands  of  the  Southern  Ocean  at  the  present  da}'. 
Sandwich  Land,  which  is  in  the  same  parallel  of  latitude  as  the  north  of  Scotland,  is 
covered  with  ice  and  snow  the  entire  summer  ;  and  in  the  island  of  South  Georgia,  which 
is  in  the  same  parallel  as  the  centre  of  England,  the  perjwtual  snow  descends  to  the  ver}- 
sea-beach.  Captain  Sir  James  Ross  found  the  perpetual  snow  at  the  sea-level  at  Admir- 
alty Inlet,  South  Shetland,  in  lat  64** ;  and  while  near  this  place  the  thermometer  in 
the  very  middle  of  summer  fell  at  night  to  23"  F.  The  reduction  of  the  sun's  heat  and 
lengthening  of  the  winter,  which  would  take  place  when  the  eccentricit}'  is  near  to  its 
superior  limit  and  the  winter  in  aphelion,  would  in  this  countiy  produce  a  state  of  things 
perhaps  as  bad  as,  if  not  worse  than,  that  wliich  at  present  exists  in  South  Georgia  and 
South  Shetland. 

**  The  cause  which  above  all  others  must  tend  to  produce  great  changes  of  climate,  is 
the  deflection  of  great  ocean  currents.  A  high  condition  of  eccentricity  tends,  we  have 
seen,  to  produce  an  accimiulation  of  snow  and  ice  on  the  hemisphere  whose  wintei-s  occur 
in  aphelion.  The  accumulation  of  snow,  in  turn,  tends  to  lower  the  simimer  temperature. 
cut  off  the  sun's  rays,  and  retard  the  melting  of  the  snow.  In  short,  it  tends  to  produce. 
on  that  hemisphere,  a  state  of  glaciation.  Exactly  opjiosite  effects  take  place  on  the 
other  hemisphere,  which  has  its  winter  in  [perihelion.  There  the  shortness  of  tlic  winters, 
combined  with  the  high  temperature  arising  from  the  nearness  of  the  sun,  tends  to 
prevent  the  accumulation  of  snow.  Tlie  general  result  is  that  the  one  hemisphere  Is 
cooled  and  the  other  heated.  This  state  of  things  now  brings  into  play  the  agencies  which 
lead  to  the  deflection  of  the  Gulf-stream  and  other  great  ocean  euiTents. 

**  Owing  to  the  great  difference  between  the  tem|)erature  of  the  equator  and  the  poles, 


28  COSMIC AL  ASPECTS  OF  GEOLOGY  book  i 


there  is  a  constant  flow  of  air  from  the  yioles  to  the  equator.  It  is  to  this  that  the  trade- 
winds  owe  their  existence.  Now,  as  the  strength  of  these  winds  will,  as  a  general  rule, 
depend  upon  the  difference  of  tenijjerature  that  may  exist  between  the  equator  and 
higher  latitudes,  it  follows  that  the  trades  on  the  cold  hemisphere  will  be  stronger  than 
those  on  the  warm.  When  the  jwlar  and  temyierate  regions  of  the  one  hemisphere  are 
covered  to  a  large  extent  with  snow  and  ice,  the  air,  as  we  have  just  seen,  is  kept 
almost  at  the  freezing-{>oint  during  both  summer  and  winter.  The  trades  on  that  hemi- 
sphere will,  of  necessity,  he  exceedingly  ywwerful  ;  while  on  the  other  hemisphere,  where 
there  is  comiMtrativcly  little  snow  or  ice,  and  the  air  is  warm,  the  trades  will  consequently 
be  weak.  Supjwse  now  the  northern  hemisphere  to  be  the  cold  one.  Tlie  north-east 
trade- winds  of  this  hemisphere  will  far  exceed  in  strength  the  south-east  trade-winds  of 
the  southern  hemisphere.  The  otiedian  line  between  the  tra<les  will  consetjuently  lie  to 
a  very  Gonaidera1>le  distance  to  the  south  of  the  equator.  We  have  a  good  example  of 
this  at  the  present  day.  The  difference  of  temperature  lietween  the  two  hemispheres  at 
present  is  but  trifling  to  what  it  would  be  in  the  ca.se  under  consideration  ;  yet  we  find 
that  the  south-east  trades  of  the  Atlantic  blow  with  gi*eater  force  than  the  north-east 
trades,  sometimes  extending  to  10"  or  Ifi**  X.  lat,,  whereas  the  north-east  trades  seldom 
l)low  south  of  the  equator.  The  effect  of  the  northern  trades  blowing  across  the  equator 
to  a  grcAt  distance  will  be  to  imjiel  the  warm  water  of  the  tropics  over  into  the  Southern 
Ocean.  But  this  is  not  all ;  not  only  would  the  median  line  of  the  trades  be  shifted 
southwards,  but  the  great  equatorial  currents  of  the  globe  would  also  be  shifted  southwards. 

"  Let  us  now  consider  how  this  would  affect  the  Gulf-stream.  The  South  American 
continent  is  shaped  somewhat  in  the  fomi  of  a  triangle,  with  one  of  its  angular  comers, 
called  Cape  St,  Roque,  ]>ointing  eastwards.  Tlie  equatorial  current  of  the  Atlantic 
impinges  against  this  corner  ;  but  as  the  greater  jwrtion  of  the  current  lies  a  little  to  the 
north  of  the  comer,  it  flows  westward  into  the  Gulf  of  Mexico  and  fonns  the  Gulf-stream. 
A  considerable  portion  of  the  water,  however,  strikes  the  land  to  the  south  of  the  cajie, 
and  is  deflected  along  the  shorc  of  Brazil  into  the  Southern  Ocean,  forming  what  is 
known  as  the  Brazilian  current  Now,  it  is  obvious  that  the  shifting  of  the  equatorial 
current  of  the  Atlantic  only  a  few  degi-ees  to  the  south  of  its  i>resent  position — a  thing 
which  would  certainly  take  place  under  the  conditions  which  we  have  heen  detailing — 
would  turn  the  entire  current  into  the  Brazilian  l^ranch,  and  instead  of  flowing  chiefly 
into  the  Gulf  of  Mexico,  as  at  present,  it  would  all  flow  into  the  Southern  Ocean,  and 
the  Gulf-stream  would  consequently  be  stopped.  The  stopjwge  of  the  Gulf-stream, 
combined  with  all  those  causes  which  we  have  just  been  considering,  would  place  Eurojie 
under  a  glacial  condition,  while  at  the  same  time  the  temjierature  of  the  Southern  Ocean 
would,  in  consequence  of  the  enormous  ([uantity  of  warm  water  received,  have  its 
temperatin*e  (alread}^  high  from  other  causes)  raised  enoimously.  And  what  holds  true 
in  regard  to  the  cun'ents  of  the  Atlantic  holds  also  tme,  though  j)erha})S  not  to  the  same 
extent,  of  the  cun-ents  of  the  Pacific. 

"  If  the  breadth  of  the  Gulf-stream  be  taken  at  50  miles,  its  depth  at  1000  feet,  its 
mean  velocity  at  2  statute  miles  an  hour,  the  temperature  of  the  water  when  it  leaves 
the  Gulf  at  65**,  and  the  return  cun*ent  at  40°  F.,^  then,  the  quantity  of  heat  conveyed 
into  the  Atlantic  by  this  stream  is  equal  to  one-fourth  of  all  the  heat  received  from  the 
sun  by  that  ocean  from  the  Tropic  of  Cancer  to  the  Arctic  Circle.-    From  principles 


^  Sir  Wyville  Thomson  states  tliat  in  May,  1873,  the  CfiuUenger  expedition  found 
the  Gulf-stream,  at  the  ])oint  where  it  was  crossed,  to  be  about  sixty  miles  in  width, 
100  fathoms  deep,  and  flowing  at  the  rate  of  three  knots  per  hour.  This  makes  the 
volume  of  the  stream  one-fifth  greater  than  the  above  estimate. 

-  The  quantity  of  heat  conveyed  by  the  Gulf-stream  for  distribution  is  equal  to 
77,479,650,000,000,000,000  foot-pounds  j^er  day.  The  quantity  received  from  the  sun 
by  the  North  Atlantic  is  310,923,000,000,000,000,000  foot  -  i)0unds.  'Climate  and 
Thne,'  chap.  ii. 


30  COSMICAL  ASPECTS  OF  GEOLOGY  book  i 

to  the  deduction  from  this  alternation  or  periodicity  that  they  have 
probably  been  due  to  some  general  or  cosmical  cause.  Dr.  CroU 
ingeniously  showed  that  every  long  cold  period  arising  in  each  hemisphere 
from  the  circumstances  sketched  in  the  preceding  pages,  must  have  been 
interrupted  by  several  shorter  warm  periods. 

"When  the  one  hemisphere,"  he  says,  "is  under  glaciation,  the  other  is  enjoying  a 
warm  and  equable  climate.  But,  owing  to  the  precession  of  the  equinoxes,  the  condition 
of  things  on  the  two  hemispheres  must  be  reversed  every  10,000  years  or  so.  When  the 
solstice  passes  the  aphelion,  a  contrary  process  commences  ;  the  snow  and  ic^  gradually 
begin  to  diminish  on  the  cold  hemisphere  and  to  make  their  appearance  on  the  other 
hemisphere.  The  glaciated  hemisphere  turns  by  degrees  warmer,  and  the  warm  hemi- 
sphere colder,  and  this  continues  to  go  on  for  a  period  of  ten  or  twelve  thousand  years, 
until  the  winter  solstice  reaches  the  perihelion.  By  this  time  the  conditions  of  the  two 
hemispheres  have  been  reversed  ;  the  formerly  glaciated  hemisphere  has  now  become  the 
warm  one,  and  the  warm  hemisphere  the  glaciated.  The  transference  of  the  ice  from 
the  one  hemisphere  to  the  other  continues  as  long  as  the  eccentricity  remains  at  a  high 
value.  It  is  probable  that,  during  the  warm  inter-glacial  periods,  Greenland  and  the 
Arctic  regions  would  be  comparatively  free  from  snow  and  ice,  and  enjoying  a  temperate 
and  equable  climate." 


BOOK  11. 

GEOGNOSY. 

AN  INVESTIGATION  OF  THE  MATERIALS  OF  THE  EARTH'S 

SUBSTANCE. 

Part  I. — A  General  Description  of  the  Parts  of  the  Earth. 

A  discussion  of  the  geological  changes  which  our  planet  has  undergone, 
ought  to  be  preceded  by  a  study  of  the  materials  of  which  the  planet 
consists.     This  latter  branch  of  inquiry  is  termed  Geognosy. 

Viewed  in  a  broad  way,  the  earth  may  be  considered  as  consisting 
of  (1)  two  envelopes, — an  outer  one  of  gas  (atmosphere),  completely  sur- 
rounding the  planet,  and  an  inner  one  of  water  (hydrosphere),  covering 
about  three-fourths  of  the  globe;  and  (2)  a  globe  (lithosphere),  cool  and 
solid  on  its  surface,  but  possessing  a  high  internal  temperature. 

I. — The  Envelopes — Atmosphere  and  Hydrospliere. 

It  is  certain  that  the  present  gaseous  and  liquid  envelopes  of  the 
planet  form  only  a  portion  of  the  original  mass  of  gas  and  water  with 
which  the  globe  was  invested.  Fully  a  half  of  the  outer  shell  or  crust  of 
the  earth  consists  of  oxygen,  which,  there  can  be  no  doubt,  once  existed 
in  the  atmosphere..  The  extent,  likewise,  to  which  water  has  been 
abstracted  by  minerals  is  almost  incredible.  It  has  been  estimated  that 
already  one-third  of  the  whole  mass  of  the  ocean  has  been  thus  absorbed. 
Eventually  the  condition  of  the  planet  will  probably  resemble  that  of 
the  moon — a  globe  without  air,  or  water,  or  life  of  any  kind. 

1.  The  Atmosphere. — The  gaseous  envelope  to  which  the  name  of 
atmosphere  is  given,  extends  to  a  distance  of  perhaps  500  or  600  miles 
from  the  earth's  surface,  possibly  in  a  state  of  extreme  tenuity  to  a 
still  greater  height  But  its  thickness  must  necessarily  vary  with  lati- 
tude and  changes  in  atmospheric  pressure.  The  layer  of  air  lying  over 
the  poles  is  not  so  deep  as  that  which  surrounds  the  equator. 

Many  speculations  have  been  made  regarding  the  chemical  composition 


32  GEOGXOSy  book  u 


of  the  'atmosphere  chiriDg  former  geological  periods.  There  can  indeed 
})e  no  doubt  that  it  must  originally  have  differed  very  greatly  from  its 
present  condition.  Besides  the  abstraction  of  the  oxygen  which  now 
forms  fully  a  half  of  the  outer  crust  of  the  earth,  the  vast  beds  of  coal 
found  all  over  the  world,  in  geological  formations  of  many  different  ages, 
doubtless  represent  so  much  carbon-dioxide  (carbonic  acid)  once  present 
in  the  air.  Acconling  to  Sterry  Hunt,  the  amount  of  carbonic  acid 
absorbed  in  the  process  of  rock-decay,  and  now  represented  in  the  form 
of  carbonates  in  the  earth*s  crust,  probably  equals  two  hundred  times  the 
present  volume  of  the  entire  atmosphere.^  The  chlorides  in  the  sea, 
likewise,  were  probably  carried  down  out  of  the  atmosphere  in  the 
primitive  condensation  of  aqueous  vapour.  It  has  often  been  stated 
that,  during  the  Carboniferous  period,  the  atmosphere  must  have  been 
warmer  and  i^ith  more  aqueous  vapour  and  carbon-dioxide  in  its  com- 
position than  at  the  present  day,  to  admit  of  so  luxuriant  a  flora  as  that 
from  which  the  coal-seams  were  formed.  There  seems,  however,  to  be  at 
present  no  method  of  arriving  at  any  certainty  on  this  subject 

As  now  existing,  the  atmosphere  is  considered  to  be  normally  a 
mechanical  mixtiu-e  of  nearly  4  vohunes  of  nitrogen  and  1  of  oxygen 
(N79'4,  020*6),  i»ith  minute  proportions  of  cai*bon- dioxide  and  ni-ater- 
vapour  and  still  smaller  quantities  of  ammonia  and  the  powerfiU 
oxidising  agent,  ozone.  These  quantities  are  liable  to  some  variation 
according  to  locality.  The  mean  proportion  of  carbon -dioxide  is  about  3 '5 
parts  in  every  10,000  of  air.  In  the  air  of  streets  and  houses  the  pro- 
portion of  oxygen  diminishes,  while  that  of  carbon -dioxide  increases. 
According  to  the  researches  of  Angus  Smith,  very  pure  air  should 
contain  not  less  than  20*99  per  cent,  of  oxygen,  i^-ith  0*030  of  carbon- 
dioxide  ;  but  he  found  impure  air  in  Manchester  to  have  onlj'  20*21  <rf 
oxygen,  while  the  j>roportion  of  carbon-dioxide  in  that  city  during  fog 
was  ascertained  to  rise  sometimes  to  0*0679,  and  in  the  pit  of  the  theatre 
to  the  very  large  amount  of  0*2734.  As  plants  absorb  carbon -dioxide 
during  the  day  and  give  it  off  at  night,  the  quantity  of  this  gas  in  the 
atmosphere  oscillates  between  a  maximiun  at  night  and  a  minimum 
during  the  day.  During  the  part  of  the  year  when  vegetation  is  active, 
it  is  believed  that  there  is  at  least  10  per  cent,  more  carbonic  acid  in 
the  air  of  the  oj)en  country  at  night  than  in  the  day.^  Small  as  the 
normal  percentage  of  this  gas  in  the  air  may  seem,  -yet  the  total  amount 
of  it  in  the  whole  atmosphere  probably  exceeds  what  would  be  disengaged 
if  all  the  vegetable  and  animal  matter  on  the  earth's  surface  were  burnt. 

The  other  sul>stances  in  the  air  are  gases,  vapours,  and  solid  particles. 
Of  these  by  much  the  most  important  is  the  vapour  of  water,  which  is 
always  present,  but  in  very  variable  amount  according  to  temi)erature.* 

*  Brit.  Asgoc.  Hep.  1878,  Sects,  p.  544. 

-  Prof.  G.  F.  Arnistrong.     Proc.  Hot/.  .Swc.  xxx.  (1880),  p.  343. 

'  A  cubic  iiietnj  of  air  at  the  freezing-point  can  hold  only  4*871  grammes  of  water- 
rai>oiir,  but  at  40*^  C.  can  take  up  50*70  grammes.  One  cubic  mile  of  air  saturated  with 
vai>our  at  35''  C.  will,  if  cooleti  to  0*.  deposit  upwards  of  140,000  tons  of  water  as  rrnin. 
Roscoe  and  Schorlemmer's  'Chemistr}','  i.  p.  452. 


PART  1  THE  OCEANS  33 


It  is  this  vapour  which  chiefly  absorbs  radiant  heat.^  It  condenses  into 
dew,  rain,  hail,  and  snow.  In  assuming  a  visible  form,  and  descending 
through  the  atmosphere,  it  takes  up  a  minute  quantity  of  air,  and  of  the 
different  substances  which  the  air  may  contain.  Being  caught  by  the 
nun,  and  held  in  solution  or  suspension,  these  substances  can  be  best 
examined  by  analysing  rain-water.  In  this  way,  the  atmospheric  gases, 
ammonia,  nitric,  sulphurous,  and  sulphuric  acids,  chlorides,  various  salts, 
solid  carbon,  inorganic  dust,  and  organic  matter  have  been  detected.  The 
fine  microscopic  dust  so  abundant  in  the  air  is  no  doubt  for  the  most 
part  due  to  the  action  of  wind  in  lifting  up  the  finer  particles  of  dis- 
integrated rock  on  the  surface  of  the  land.  Volcanic  explosions  sometimes 
supply  prodigious  quantities  of  fine  dust  There  is  probably  also  some 
addition  to  the  solid  particles  in  the  atmosphere  from  the  explosion  and  dis- 
sipation of  meteorites  on  entering  our  atmosphere.  To  the  wide  diffusion 
of  minute  solid  particles  in  the  air  great  importance  in  the  condensation 
of  vapour  has  recently  been  assigned.     (Book  III.  Part  11.  Section  ii.) 

The  comparatively  small,  but  by  no  means  unimportant,  proportions 
of  these  minor  components  of  the  atmosphere  are  much  more  liable  to 
variation  than  those  of  the  more  essential  gases.  Chloride  of  sodium, 
for  instance,  is,  as  might  be  expected,  particularly  abundant  in  the  air 
bordering  the  sea.  Nitric  acid,  ammonia,  and  sulphuric  acid  appear  most 
conspicuously  in  the  air  of  towns.  The  organic  substances  present  in  the 
air  are  sometimes  living  germs,  such  as  probably  often  lead  to  the  pro- 
pagation of  disease,  and  sometimes  mere  fine  particles  of  dust  derived 
from  the  bodies  of  living  or  dead  organisms.  ^ 

As  a  geological  agents  the  atmosphere  effects  changes  by  the  chemical 
reactions  of  its  constituent  gases  and  vapours,  by  its  varying  temperature, 
and  by  its  motions.  Its  functions  in  these  respects  are  described  in 
Book  III.  Part  II.  Section  i. 

2.  The  Oceans. — Rather  less  than  three-fourths  of  the  surface  of  the 
globe  (or  about  144,712,000  square  miles)  are  covered  by  the  irregular 
sheet  of  water  known  as  the  Sea.  Within  the  last  twenty  years,  much 
new  light  has  been  thrown  upon  the  depths,  temperatures,  and  biological 
conditions  of  the  ocean -basins,  more  particularly  by  the  Lightrdng, 
Porcupine,  Challenger^  Tuscarora,  Blake,  Gazelle  and  other  expeditions  fitted 
out  by  the  British,  American,  German,  and  Norwegian  Governments.^     It 

^  See  Tyudall's  researches  which  established  this  important  function  of  the  aqueous 
mpour  of  the  atmosphere,  and  their  confirmation  by  meteorological  observation.  S.  A. 
HilL  Ptoc.  Ray.  Soc,  xxxiii.  216,  435. 

*  The  air  of  towns  is  peculiarly  rich  in  impurities,  especially  in  manufacturing  districts, 
where  much  coal  is  used.  These  impurities,  however,  though  of  serious  consequence  to 
the  towns  in  a  sanitary  point  of  view,  do  not  sensibly  affect  the  general  atmosphere, 
•eeing  that  they  are  probably  in  great  measure  taken  out  of  the  air  by  rain,  even  in  the 
districts  which  produce  them.  They  possess,  nevertheless,  a  special  geological  significance, 
and  in  this  respect,  too,  have  important  economic  bearings.  See  on  this  whole  subject, 
Angus  Smith's  *  Air  and  Rain,'  and  the  account  of  Rain  in  Book  III.  Part  II.  Sect.  ii. 

»  See  Wyville  Thomson,  '  The  Depths  of  the  Sea, '  1873  ;  *  The  Atlantic, '  1877  ;  'Report  of 
Challenger  Expedition,'  especially  the  forthcoming  volumes  giving  a  summary-  of  results  ;  A. 
AgMsiz,  '  Three  Cruises  of  the  Blake,'  1888  :  *  Den  Norske  Nordhavs-Expedition,'  1876-1878. 

D 


34  GEOGNOSY  book  ii 

has  been  ascertained  that  few  parts  of  the  Atlantic  Ocean  exceed  3000 
fathoms,  the  deepest  sounding  obtained  there  being  one  taken  about  100 
miles  north  from  the  island  of  St  Thomas,  which  gave  3875  fathoms,  or 
rather  less  than  4|  miles.  The  Atlantic  appears  to  have  an  average 
depth  in  its  more  open  parts  of  from  2000  to  3000  fathoms,  or  from 
about  2  to  3^  miles.  In  the  Pacific  Ocean  H.M.  Ship  Challenger  got 
soundings  of  3950  and  4475  fathoms,  or  about  4|  and  rather  more  than 
5  miles.  Since  then  the  U.S.  Ship  Tuscarora  obtained  a  still  deeper 
sounding  (4655  fathoms),  to  the  east  of  the  Kurile  Islands.  This  is  the 
deepest  abyss  yet  found  in  any  part  of  the  ocean.  But  these  appear  to 
mark  exceptionally  abysmal  depressions,  the  average  depth  being,  as  in 
the  Atlantic,  between  2000  and  3000  fathoms.  We  may  therefore 
assume,  as  probably  not  far  from  the  truth,  that  the  average  depth  of  the 
sea  is  about  2500  fathoms,  or  nearly  3  miles.  Its  total  cubic  contents 
will  thus  be  about  400  millions  of  cubic  miles. 

With  regard  also  to  the  form  of  the  bottom  of  the  great  oceans,  much 
additional  information  has  recently  been  obtained.  Over  vast  areas 
in  the  central  regions,  the  sea-floor  appears  to  form  great  plains,  with 
comparatively  few  inequalities,  but  with  lines  of  submarine  ridges,  com- 
parable to  chains  of  hills  or  mountains  on  the  land.  Recent  soundings, 
however,  taken  at  short  distances,  have  revealed,  in  parts  of  the  Atlantic 
that  were  supposed  to  be  deep  and  with  a  tolerably  uniform  bottom,  sub- 
marine peaks  rising  to  within  50  fathoms  from  the  surface.^  A  vast  central 
ridge  has  also  been  traced  down  the  length  of  this  ocean,  from  which  a 
few  lonely  peaks  rise  above  sea-level — the  Azores,  St.  Paul,  Ascension,  and 
Tristan  d'Acunha.  In  the  Pacific  Ocean,  the  lines  of  coral-islands  appear 
to  rise  on  submarine  ridges,  having  a  general  north-westerly  and  south- 
easterly trend.  It  is  significant  that  the  islands  which  thus  appear  far 
from  any  large  mass  of  land  are  either  coral-reefs  or  of  volcanic  origin, 
and  contain  none  of  the  granites,  schists  and  other  ordinary  continental 
rocks.  St.  Helena  and  Ascension  in  the  Atlantic,  and  the  Friendly  and 
Sandwich  Islands  in  the  Pacific  Ocean  are  conspicuous  examples. 

Another  important  result  of  recent  deep-sea  research  is  the  determina- 
tion of  the  relation  of  mediterranean  seas  to  the  main  ocean.  These 
basins,  such  as  the  North,  Mediterranean,  and  Black  Seas,  the  Gulf  of 
Mexico,  Caribbean  Sea,  Baffin's  Bay,  Hudson's  Bay,  Sea  of  Okhotsk,  and 
Chinese  Sea,  belong  rather  to  the  continental  than  the  oceanic  areas  of  the 
earth's  surface.  An  elevation  of  a  few  hundred  fathoms  would  convert 
most  of  them  into  land,  with  here  and  there  deep  water-filled  basins. 

A  question  of  high  importance  in  geological  enquiry  is  the  form  of  the 
surface  of  the  sea  or  what  is  usually  called  the  sea-level.  It  has  been 
generally  assumed  that  this  surface  is  stable  and  uniform  and  nearly  that 
of  an  ellipsoid  of  revolution,  owing  its  equilibrium  to  the  force  of  gravity 
on  the  one  hand  and  the  centrifugal  force  of  rotation  on  the  other.  But 
in  recent  years  this  conception  has  been  called  in  question  both  by 
physicists  and  geologists.  Observations  as  well  as  calculations  have 
shown  that  the  attraction  exercised  by  masses  of  land  raises  the  level  of 

^  Times,  7th  Deer.  1883.     [J.  Y.  Buchanan.] 


PART  I  THE  OCEANS  36 

the  adjacent  sea,  and  attempts  have  been  made  to  determine  how  far  the 
deformation  thus  caused  departs  from  the  mean  of  the  theoretical  ellipsoid 
of  revolution.  According  to  Bruns  a  continent  may  cause  a  difference  of 
more  than  3000  feet  between  the  actual  level  of  the  sea  and  that  of 
the  ellipsoid.  But  the  results  of  such  calculations  will  greatly  depend  on 
the  assmnption  on  which  they  start  as  to  the  nature  of  the  earth's  crust. 
R  S.  Woodward  has  calculated  that  if  the  continent  of  Europe  and  Asia 
be  supposed  to  be  simply  a  superficial  aggregation  of  matter  with  a 
density  as  great  as  the  parts  under  the  sea,  the  elevation  of  sea-level  at 
the  centre  of  the  continent  due  to  attraction  would  amount  to  about 
2900  feet,  but  that,  if  the  continental  mass  be  assumed  to  imply  a  defect 
of  density  underneath  it,  the  elevation  of  the  sea  at  the  centre  of  the 
continent  due  to  attraction  would  be  only  about  10  feet.^  This  subject 
is  further  considered  in  Book  IIL  Part  I.  Sect.  iii. 

The  water  of  the  ocean  is  distinguished  from  ordinary  terrestrial 
waters  by  a  higher  specific  gravity,  and  the  presence  of  so  large  a  pro- 
portion of  saline  ingredients  as  to  impart  a  strongly  salt  taste.  The 
average  density  of  sea- water  is  about  1*026,  but  it  varies  slightly  in 
different  parts  even  of  the  same  ocean.  According  to  the  observations 
of  J.  Y.  Buchanan  during  the  Challenger  expedition,  some  of  the 
heaviest  sea -water  occiu^  in  the  pathway  of  the  trade -winds  of  the 
North  Atlantic,  where  evaporation  must  be  comparatively  rapid,  a  density 
of  1 '02781  being  registered.  Where,  however,  large  rivers  enter  the  sea, 
or  where  there  is  much  melting  ice,  the  density  diminishes ;  Buchanan 
found  among  the  broken  ice  of  the  Antarctic  Ocean  that  it  had  sunk  to 
1  •02418.*  A  series  of  soundings  taken  during  the  Fega  expedition  in  the 
Kara  Sea  (lat.  76°  18',  long.  95"  30'  E.)  gave  a  progressive  increase  of 
salinity  from  I'l  at  the  surface  to  3*4  at  30  fathoms,  the  surface  being 
freshened  by  the  water  poured  into  the  sea  by  the  Siberian  rivers.^ 

The  greater  density  of  sea-water  depends,  of  course,  upon  the  salts 
which  it  contains  in  solution.  At  an  early  period  in  the  earth's  history, 
the  water  now  forming  the  ocean,  together  with  the  rivers,  lakes  and 
snowfields  of  the  land,  existed  as  vapour,  in  which  were  mingled  many 
other  gases  and  vapours,  the  whole  forming  a  vast  atmosphere  sur- 
rounding the  still  intensely  hot  globe.  Under  the  enormous  pressure 
of  the  primeval  atmosphere,  the  first  condensed  water  might  have  had  a 
temperature  little  below  the  critical  one.*  In  condensing,  it  would  carry 
down  with  it  many  substances  in  solution.  The  salts  now  present  in 
sea-water  are  to  be  regarded  as  principally  derived  from  the  primeval 
constitution  of  the  sea,  and  thus  we  may  infer  that  the  sea  has  always 
been  salt  It  is  probable,  however,  that,  as  in  the  case  of  the  atmosphere, 
the  composition  of  the  ocean-water  has  acquired  its  present  character 

»  Bruns,  •Die  Figur  der  Erde,'  Berlin,  1876  ;  R.  S.  Woodward,  BuU.  U,  S.  GeU,  Surv, 
Na  48,  p.  85  (1888). 

'  Buchanan,  Proc,  Roy.  Soc,  (1876),  voL  xxiv. 

»  0.  Petterason,  *  Vega-Expeditionens  Vetenskapliga  lakttagelser,*  vol.  ii.  Stockholm,  1888. 
*  Q,  J.  QtoL  Soc  xxxvi.  (1880),  pp.  112,  117.     Fisher,  •  Physics  of  Earth's  Crust,*  2nd 
«dit  p.  148. 


36  GEOGNOSY  book  u 

only  after  many  ages  of  slow  change,  and  the  abstraction  of  much  mineral 
matter  originally  contained  in  it.  There  is  eWdence,  indeed,  among 
the  geological  formations  that  large  quantities  of  lime,  silica,  chlorides 
and  sulphates  have  in  the  course  of  time  been  removed  from  the  sea.^ 

But  it  is  manifest  also  that,  whatever  may  have  been  the  original 
composition  of  the  oceans,  they  have  for  a  vast  section  of  geological  time 
been  constantly  receiving  mineral  matter  in  solution  from  the  land. 
Every  spring,  brook,  and  river  removes  various  salts  from  the  rocks 
over  which  it  moves,  and  these  substances,  thus  dissolved,  eventually 
find  their  way  into  the  sea.  Consequently  sea -water  ought  to  contain 
more  or  less  traceable  proportions  of  every  substance  which  the  terrestrial 
waters  can  remove  from  the  land — in  short,  of  probably  every  element  pre- 
sent in  the  outer  shell  of  the  globe,  for  there  seems  to  be  no  constituent  of 
the  earth  which  may  not,  under  certain  circumstances,  be  held  in  solution 
in  water.  Moreover,  unless  there  be  some  counteracting  process  to  remove 
these  mineral  ingredients,  the  ocean- water  ought  to  be  growing,  insensibly 
perhaps,  Salter,  for  the  supply  of  saline  matter  from  the  land  is  incessant. 
It  has  been  ascertained  indeed,  with  some  approach  to  certainty,  that  the 
salinity  of  the  Baltic  and  Mediterranean  is  gi^adually  increasing.^ 

The  average  proportion  of  saline  constituents  in  the  water  of  the 
great  oceans  far  from  land  is  about  three  and  a  half  parts  in  every 
hundred  of  water.'*  But  in  enclosed  seas,  receiving  much  fresh  water,  it 
is  greatly  reduced,  while  in  those  where  evaporation  predominates  it  is 
correspondingly  augmented.  Thus  the  Baltic  water  contains  from  one- 
seventh  to  nearly  a  half  of  the  ordinary  proportion  in  ocean  water,  while 
the  Mediterranean  contains  sometimes  one-sixth  more  than  that  propor- 
tion. Forchhammer  has  shown  the  presence  of  the  following  twenty - 
seven  elements  in  sea-water :  oxygen,  hydrogen,  chlorine,  bromine, 
iodine,  fluorine,  sulphur,  phosphorus,  nitrogen,  carbon,  silicon,  boron, 
silver,  copper,  lead,  zinc,  cobalt,  nickel,  iron,  manganese,  aluminium, 
magnesium,   calcium,   strontium,  barium,  sodium,  and  potassium.*      To 

^  Sterry  Hunt  supix)8ed  that  the  saline  watent  of  North  America  derive  their  mineral 
ingredients  from  the  sediments  and  precipitates  of  the  sea  in  which  the  Paheozoic  rocks 
were  deposited.      '  Geological  and  Chemical  Essays,'  p.  104. 

^  Paul,  in  Watts's  *  Dictionary  of  Chemistry',*  v.  p.  1020.  For  a  detailed  study  of  the 
Eastern  Meiliterranean,  see  the  Re]x>rts  of  a  Commission,  Denksch.  Aka^.  Wiss.  Vienna^ 
1892,  et.  seq. 

'  Dittmar's  elaborate  researches  on  the  samples  of  ocean  water  collected  by  the  Chal- 
lenger expedition  show  that  the  lowest  percentage  of  salts  obtained  was  8*301,  from  the 
southern  part  of  the  Indian  Ocean,  south  of  lat.  66°,  while  the  highest  was  3*737,  from  the 
middle  of  the  North  Atlantic,  at  about  lat.  23^.  Some  valuable  results  from  observations 
on  the  waters  of  the  North  Atlantic  are  given  by  H.  Tomoe  and  L.  Schmelck  in  the  Report 
of  the  Nonoegian  North- Atlantic  Expedition^  1876-1878.  The  average  proportion  of  salts 
was  found  to  be  from  3*47  to  3*51  per  cent,  the  mean  quantities  of  each  constituent  as 
estimated  being  as  follow  :  CaCOs,  0*002  ;  CaSo4,  0*1395  ;  MgS04,  0*2071  ;  MgClj,  0*3561  ; 
KCl,  0*0747  ;  NaHCOj,  0*0166 ;  NaQ,  2*682. 

*  Forchhammer,  PhU,  Trang.  civ.  p.  205.  According  to  Thorpe  and  Morton  (Chem,  Sac, 
Journ,  zziv.  p.  507),  the  water  of  the  Irish  Sea  contains  in  summer  rather  more  salts  than  in 
winter.     In  1000  grammes  of  the  summer  water  of  the  Irish  Sea  they  found  0*04754 


PART  I 


COMPOSITION  OF  SEA-JVATER 


37 


these  may  be  added  arsenic,  lithium,  caesium,  rubidium,  gold,  and 
probably  most  if  not  all  of  the  other  elements,  though  in  pro- 
portions too  minute  for  detection.  The  chief  constituents  have  been 
determined  by  Dittmar  to  be  present  in  the  proportions  shown  in  the 
first  column  of  the  subjoined  tables.  Assuming  them  to  occur  in  the 
combinations  shown  in  the  second  column,  they  are  present  in  the  average 
ratios  therein  stated^ : — 


I. 


Chlorine 
Bromine 

Sulphuric  acid,  SO, 
Carbonic  acid,  CO2 
Lime,  CaO  . 
Magnesia,  MgO    . 
Potash,  KO . 
Soda,  Na^O . 


Snbtract  Basic  Oxygen  equiva- ) 
lent  to  the  Halogens  J 

Totol  Salts 


55-292 
0-188 
6-410 
0*152 
1-676 
6-209 
1  -332 

41-234 

12-493 
100-000 


II. 

Chloride  of  sodium 
Chloride  of  magnesium 
Sulphate  of  magnesia  . 
Sulphate  of  lime  * 
Sulphate  of  potash 
Bromide  of  magnesium 
Carbonate  of  lime 

Totol  Salts 


77-758 
10-878 
4-787 
3-600 
2-465 
0-217 
0-345 

100-000 


Sea-water  is  appreciably  alkaline,  its  alkalinity  being  due  to  the 
presence  of  carbonates,  of  which  carbonate  of  lime  is  one/^  In  addition 
to  its  salts  it  always  contains  dissolved  atmospheric  gases.  From  the 
researches  conducted  during  the  voyage  of  the  BoniU  in  the  Atlantic 
and  Indian  Oceans,  it  was  estimated  that  the  gases  in  100  volumes  of 
sea- water  ranged  from  1-85  to  3-04,  or  from  two  to  three  per  cent. 
From  observations  made  during  the  Porcupine  cruise  of  1868,  it  was 
ascertained  that  the  proportion  of  oxygen  was  greatest  in  the  surface 
water,  and  least  in  the  bottom  water.  The  dissolved  oxygen  and  nitro- 
gen are  doubtless  absorbed  from  the  atmosphere,  the  proportion  so 
absorbed  being  mainly  regulated  by  temperature.  According  to  Ditt- 
mar's  recent  determinations,  a  litre  of  sea-water  at  0°  C.  will  take  up 
15-60  cubic  centimetres  of  nitrogen  and  8  18  of  oxygen,  while  at  30^  C. 
the  proportions  sink  respectively  to  8*36  and  4' 17.  He  regards  the 
carbonic  acid  as  occurring  chiefly  as  carbonates,  its  presence  in  the  free 
state  being  exceptional.  During  the  voyage  of  the  Challenger^  Buchanan 
ascertained  that  the  proportion  of  carbonic  acid  is  always  nearly  the 
same  for  similar  temperatures,  the  amount  in  the  Atlantic  surface  water, 
between  20°  and  25""  C,  being  00466  gramme  per  litre,  and  in  the 
surface  Pacific  water  0*0268 ;  and  that  sea- water  contains  sometimes  at 
least  thirty  times  as  much  carbonic    acid    as   an    equal    bulk  of   fresh 

grammes  of  carbonate  of  lime,  0-00503  of  ferrous  carbonate  and  traces  of  silicic  acid.  For 
ezhaostive  chemical  investigations  regarding  the  chemistry  of  ocean  water  con8ult  Dittmar 
in  vol.  i.  "Physics  and  Chemistry,"  Rep&rt  of  Voyage  of  the  ChaUengery  1884  ;  also  the 
"Chemistry"  part  of  the  Iteport  of  the  Noncegian  North- Atlantic  Expedition,  1876-1878. 

*  Dittmar,  op.  cii.  p.  203  et  seq.  For  furtlier  reference  to  the  chemistry  of  sea-water, 
e^>ecially  in  connection  with  the  action  of  marine  organisms,  see  postea,  p.  484. 

'  Dittmar,  op.  cit.  p.  206. 


-»  i^y^jri'ti 


'ut  uwsi^ae  nut  ^:\hws  -noe^  tf  ^nif 
njaui    taiUK;  tf   -is*  dasgoftsusiixisft   it   'aic: 

Aji^.rtfit;?   tf   vuk  truucnijfiija  if   aeh 


^'rJiutt  *^  «r.ffli'jiH&ats%'  uif  moudc  flEDttkcia  Ls  laie  imer  solid 
jj**./^.  7«:  ^jfLTv  yjnu'jL  'jt  t:  -pxi-i.  SEizi£  *.":#:•«»  aie  saw  »  Tufale 
V/  u^fc;  «aiC  l'jrxu«   'vid^  "r*  urn.  Lkac.  odcx^ns  z^suet  ukr  tkia  one- 


wrjT,e:wze^  V/  ^Vrjc:  iiAa  iLe  di<:ii£fce  tiervfiEz.  i2ke  <*z'TBicr  and  the 
9/r.f.e.  f//>r,  V^  ^^if^ra^Ji  fa^iyt  ion  res  becB  ksffigrric  icr  tke  present 
c'ijf.r.^rA^jg.  *A  *iji  jac/i.  It  <:k2  be  f^Kivx.  bc<vieT«r.  lisis  partMOs  of 
\i>i  */ju*isj^,^A  ^t  'A  ^fztrecrf:  g*i0ttcoai  unaq^ahr.  lbere>  k  resson  to 
f^;>%^^;  uAf:^^  *XaCv  \tJ:  ynss^ent  t^rressnJ  aj«fts  kftTe  on  the  whde 
*>^9«i«4  Ikiyl,  fjr  lAr^,,  att  ;ead?i.  ZfeT«r  beien  scLmcgyed  l^neBsh  deep  wstcr, 
U'm*  *>h^  iitt^  *A  itA  ^^arikct  ftrmnfied  formA:k<i« :  A2)d  tkit,  on  the 
'AK^r  t^Mjf If  *h^  '^JOkii^yAinia  luiTe  ^v^rs  been  v&s^  atcas  of  depreBsion. 
II;;*  at^jf/j^,-!  vjJJ  >^  di^rUMMad  in  SFubsequent  pases. 

ffi  tl;f4;  N>ir  WVyrld,  *b«  cofitinental  trend  is  apfjcoiinntelT  north 
'4.ft/\  tt^jfiXh ;  tit  %hh  Old  World,  though  less  distinctlr  marked,  it  langes 
on  itt^.  wh^A^.  tsikt  TLtA  wen.  The  intimate  relation  which  mar  be 
oS0if^irs'*^\  ^^weeri  tfai^  general  trend  and  the  direction  of  moontain 
t:\utt0if  u  \As^t  exhibited!  by  the  American  continent.  Eoit^  and 
A1n*'/4.  ut^y  U;  c/^rixidere^l  a«  forming,  with  Asia,  the  vast  continental 
ut'4>M  *A  th<;  0\A  U'orKL  The  existing  severance  of  Africa  and  Eorope 
\h  of  'y/rfjf^ratively  recent  rlate.  On  the  other  hand,  Eorope  and  Asia 
w';f';  %\tA  ulIwhvh  v^i  ^y^ntinuous  as  at  present.  Bat  even  where  the 
t'jfiiUht^ttU  'A  ih<;  (M  World  are  separated  by  sea,  the  interrening 
hollowH,  ihtt%xii\i  uow  cj>\ht(A  by  ocean -water,  must  be  regarded  as 
i:wfii*uuMy  fMrt  of  tho  r^/^ntinental  areas.     Asia  is  linked  with  Australia 

'  /'f/fT,  It/fff.  Mtjc.  x%'\y.  A*'J:ffrdiug  to  Mr.  Tamife  {Xvncegian  Sorik  -  Atlantic 
hrjtrtl'diirn,  \H7fi-7^',  *  *  ^  ^U^ntiihiry  ")  mot  of  the  carbonic  acid  of  sea- water  is  in  combination 
uritli  n4f«\h  mt  Unut\MfttuUt  of  v^^la,  Hee  his  memoir  for  an  estimate  of  the  proportion  of 
fi)r  lu  M*  wtnUrr  ;  hIm/i  J,  V.  Huohanan,  Xalurff  zxv.  p.  386.     Dittmar.  op.  cii.  p.  209. 

»  O//.  /;//.  i*.  2ri. 

'  lf\tffrftd  t:nt\inuU'M  have  l>ecii  made  of  the  proportion  of  organic  matter.  According 
to  th*-  nriurfcrrli^n  of  L.  H<:hfiielck  {Xonreginn  Ncrth- Atlantic  Expedition^  1876-8,  Part  ix. 
|f,  i),  iUtt  pro|r//Kioii  U  ()'0()7Xf  gramme  in  100  c.c.  of  water. 


PART  1  TERRESTRIAL  CONFIGURATION  39 

by  a  chain  of  islands.  The  great  contrast  between  the  Asiatic  and 
Australian  faunas,  however,  affords  good  grounds  for  the  belief  that,  at 
least  for  an  enormous  period  of  time,  Asia  and  Australia  have  been 
divided  by  an  important  barrier  of  sea. 

While  any  good  map  of  the  globe  enables  us  to  see  at  a  glance 
the  relative  positions  and  areas  of  the  continents  and  oceans,  most 
mi^M  fail  to  furnish  any  data  by  which  the  general  height  or  volume 
of  a  continent  may  be  estimated.  As  a  rule,  the  mountain -chains  are 
exaggerated  in  breadth,  and  incorrectly  indicated,  while  no  attempt  is 
made  to  distinguish  between  high  plateaux  and  low  plains.  In  North 
America,  for  example,  a  continuous  shaded  ridge  is  placed  down  the  axis 
of  the  continent,  and  marked  "  Rocky  Mountains,"  while  the  vast  level 
or  gently  rolling  prairies  are  left  with  no  mark  to  distinguish  them  from 
the  maritime  plains  of  the  eastern  and  southern  states.  In  reality 
there  is  no  such  continuous  mountain-chain.  The  so-called  "Rocky 
Mountains"  consist  of  many  independent  and  sometimes  widely  sepa- 
rated ridges,  having  a  general  meridional  trend,  and  rising  above 
a  vast  plateau,  which  is  itself  4000  or  5000  feet  in  elevation.  It 
is  not  these  intermittent  ridges  which  really  form  the  great  mass  of 
the  land  in  that  region,  but  the  widely  extended  lofty  plateau,  or 
rather  succession  of  plateaux,  which  supports  them.  In  £urope,  also, 
the  Alps  form  but  a  subordinate  part  of  the  total  bulk  of  the  land. 
If  their  materials  could  be  spread  out  over  the  continent,  it  has  been 
calculated  that  they  would  not  increase  its  height  more  than  about 
twenty-one  feet.^ 

Attempts  have  been  made  to  estimate  the  probable  average  height 
which  would  be  attained  if  the  various  inequalities  of  the  land  could  be 
levelled  down.  Humboldt  estimated  the  mean  height  of  £urope  to  be 
about  671,  of  Asia  1132,  of  North  America  748,  and  of  South  America 
1151  feet^  Herschel  supposed  the  mean  height  of  Africa  to  be  1800 
feet'  These  figures,  though  based  on  the  best  data  available  at  the  time, 
are  no  doubt  much  under  the  truth.  In  particular,  the  average  height 
assigned  to  North  America  is  evidently  far  less  than  it  should  be ;  for  the 
great  plains  west  of  the  Mississippi  valley  reach  an  altitude  of  about  5000 
feet^  and  serve  as  the  platform  from  which  the  mountain  ranges  rise.  The 
height  of  Asia  also  is  obviously  much  greater  than  this  old  estimate. 
G.  Leipoldt  has  computed  the  mean  height  of  Europe  to  be  296*838 

>  M.  De  Lapparent  (*Traite  de  Geologie,'  3rd  edit.  p.  57)  gives  the  following  estimate  of 
relative  heights  and  areas,  the  area  below  sea-level  being  taken  as  0*6  of  the  whole. 
Zone    I.     (from  sea-level  to  200  metres)  covers  347  per  cent  of  the  terrestrial  surface. 


„  11. 

200,,  600 

21-6 

„  in.   , 

600  „  1000 

21-4 

..  IV.    , 

1000  „  2000 

14-2 

..  V.    , 

2000  „  8000 

3-7 

,.  VI.    , 

3000  „  4000 

21 

„vii.   , 

,    above  ,,  4000 

1-7 
99-4 

»  'Aaie  Centrale,'  torn.  i.  p.  168.  »  *  Physical  Geography,'  p.  119. 


40  GEOGXOSY  book  n 

metres  (973*628  feet).^  Prof.  A.  De  Lapparent  makes  the  mean  height 
of  the  land  of  the  globe  2120  feet«  and  estimates  the  mean  hei^t  of 
Europe  to  be  958  feet,  Asia  2884,  Africa  1975,  North  America  1952,  and 
South  America  1762.-  Dr.  John  Murray  cmnputes  these  heists  as 
follows:  Europe  939,  Asia  3189,  Africa  2021,  North  America  1888, 
South  America  2078,  Australia  805  feet,  general  mean  hei^t  of  land 
2252  feet.^  It  is  of  some  consequence  to  obtain  as  near  an  approxi- 
mation to  the  truth  in  this  matter  as  maj  be  possible,  in  order  to 
furnish  a  means  of  comparison  between  the  relatiTe  bulk  of  different 
continents,  and  the  amount  of  material  on  which  geological  changes  can 
be  effected. 

The  highest  elevation  of  the  surface  of  the  land  is  the  summit  of 
Mount  Everest,  in  the  Himalaya  range  (29,000  feet);  the  deepest 
depression  not  covered  by  water  is  that  of  the  shores  of  the  Dead  Sea 
(1300  feet  below  sea- level).  There  are,  however,  many  subaqueous 
portions  of  the  land  which  sink  to  greater  depths.  The  bottom  of  the 
Caspian  Sea,  for  instance,  lies  about  3000  feet  below  the  general  sea-leveL 
The  vertical  difference  between  the  highest  point  of  the  land  and  the 
maximum  known  depth  of  the  sea  is  56,932  feet  or  nearly  1 1  miles. 

There  are  two  conspicuous  junction-lines  of  the  land  with  its  over- 
lying and  surrounding  envelopes.  First,  with  the  Air,  expressed  by 
the  contours  or  relief  of  the  land.  Second,  with  the  Sea,  expressed  by 
coastrlines. 

(1.)  Contours  or  Relief  of  the  Land. — While  the  surface 
of  the  land  presents  endless  diversities  of  detail,  its  leading  features 
may  be  generalised  as  mountains,  table-lands,  and  plains. 

Mountains. — The  word  "mountain"  is,  properly  speaking,  not  a 
scientific  term.  It  includes  many  forms  of  ground  utterly  different  from 
each  other  in  size,  shape,  structure,  and  origin.  It  is  popularly  applied 
to  any  considerable  eminence  or  range  of  heights,  but  the  height  and  size 
of  the  elevated  ground  so  designated  vary  indefinitely.  In  a  really 
mountainous  country  the  word  would  be  restricted  to  the  loftier  masses 
of  ground,  while  such  a  word  as  hill  would  be  given  to  the  lesser  heights. 
But  in  a  region  of  low  or  gently  undulating  land,  where  any  conspicuous 
eminence  becomes  important,  the  term  mountain  is  lavishly  used.  In 
Flastern  America  this  habit  has  been  indulged  in  to  such  an  extent,  that 
what  are,  so  to  speak,  mere  hummocks  in  the  general  landscape,  are 
dignified  by  the  name  of  mountains. 

It  is  hanlly  ix)ssible  to  give  a  precise  scientific  definition  to  a  term  so 
vaguely  employed  in  ordinary  language.  When  a  geologist  uses  the  word, 
he  must  either  be  content  to  take  it  in  its  familiar  vague  sense,  or  must 
add  some  ])hrase  defining  the  meaning  which  he  attaches  to  it.     He  finds 

'  '  Die  Mittlere  Uiihe  Europas,'  Leipzig,  1874.  lu  tLis  work  the  mean  height  of 
Hwitzerland  is  put  down  as  1299*91  metres  ;  Spanish  peninsula,  700*60  ;  Austria,  517*87  ; 
Italy,  517*17:  Scandinavia,  428  10;  France,  393*84:  Great  BriUiu,  217*70;  German 
Kmpire,  213*66  ;  Russia,  167*09  ;  Belgium,  163*36  ;  Denmark  (exclusive  of  Iceland),  35-20  ; 
the  Netherlands  (exclusive  of  Luxenil>ourg  and  the  tracts  below  sea-level),  9*61. 

2  'Traits,'  p.  56.  »  .Scottish  Gfog.  Mag,  iv.  (1888),  23. 


PART  I  TYPES  OF  MOUNTAINS  41 

that  there  are  three  leading  and  totally  distinct  types  of  elevation  which 
are  all  popularly  termed  mountains.  1.  Single  eminences,  standing  alone 
upon  a  plain  or  table-land.  This  is  essentially  the  volcanic  type.  The 
huge  cones  of  Vesuvius,  £tna,  and  Teneriffe,  as  well  as  the  smaller  ones 
so  abundant  in  volcanic  districts,  are  examples  of  it  There  occiu-, 
however,  occasional  isolated  eminences  that  stand  up  as  remnants  of  once 
extensive  rock -formations.  These  have  no  real  analogy  with  volcanic 
elevations,  but  should  be  classed  luider  the  next  type.  The  remarkable 
huUes  of  Western  America  are  good  illustrations  of  them.  2.  Groups  of 
eminences  connected  at  the  sides  or  base,  often  forming  lines  of  ridge 
between  divergent  valleys,  and  owing  their  essential  forms  not  to  under- 
ground structure  so  much  as  to  superficial  erosion.  Many  of  the  more 
ancient  uplands,  both  in  the  Old  World  and  the  New,  furnish  examples 
of  this  type,  such  as  the  Highlands  of  Scotland,  the  hills  of  Cumberland 
and  Wales,  the  high  grounds  between  Bohemia  and  Bavaria,  the  Lauren- 
tide  Mountains  of  Canada,  and  the  Green  and  White  Mountains  of  New 
England.  3.  Lines  of  lofty  ridge  rising  into  a  succession  of  more  or  less 
distinct  summits,  their  general  external  form  having  relation  to  an  internal 
plication  of  their  component  rocks.  These  linear  elevations,  whose 
existence  and  trend  have  been  determined  immediately  by  subterranean 
movement,  are  the  true  mountain-ranges  of  the  globe.  They  may  be 
looked  upon  as  the  crests  of  the  great  waves  into  which  the  crust  of  the 
earth  has  been  thrown.  All  the  great  mountain-lines  of  the  world  belong 
to  this  type. 

Leaving  the  details  of  mountain -form  to  be  described  in  Book  VIL, 
we  may  confine  our  attention  here  to  a  few  of  the  more  important 
general  features.  In  elevations  of  the  third  or  true  mountain  type, 
there  may  be  either  one  line  or  range  of  heights,  or  a  series  of  parallel 
and  often  coalescent  ranges.  In  the  Western  Territories  of  the  United 
States,  the  vast  plateau  has  been,  as  it  were,  wrinkled  by  the  uprise  of 
long  intermittent  ridges,  with  broad  plains  and  basins  between  them. 
Each  of  these  forms  an  independent  mountain-range.  In  the  heart  of 
Europe,  the  Bernese  Oberland,  the  Pennine,  Lepontine,  Rhaetic,  and 
other  ranges  form  one  great  Alpine  chain  or  system. 

In  a  great  mountain-chain,  such  as  the  Alps,  Himalayas,  or  Andes, 
there  is  one  general  i>ersistent  trend  for  the  successive  ridges.  Here 
and  there,  lateral  offshoots  may  diverge,  but  the  dominant  direction  of 
the  axis  of  the  main  chain  is  generally  observed  by  its  component  ridges 
until  they  disappear.  Yet  while  the  general  parallelism  is  preserved,  no 
single  range  may  be  traceable  for  more  than  a  comparatively  short  dis- 
tance ;  it  may  be  found  to  pass  insensibly  into  another,  while  a  third  may 
be  seen  to  begin  on  a  slightly  different  line,  and  to  continue  with  the 
same  dominant  trend  until  it  in  turn  becomes  confluent.  The  various 
ranges  are  thus  apt  to  assume  an  arrangement  en  Echelon. 

The  ranges  are  separated  by  lon^tudinal  valleys,  that  is,  depressions 
coincident  with  the  general  direction  of  the  chain.  These,  though 
sometimes  of  great  length,  are  relatively  of  narrow  width.  The  valley 
of  the  Rh6ne,  from  the  source  of  the  river  down  to  Martigny,  offers  an 


42  GEOGNOSY  book  ii 

excellent  example.  By  a  second  series  of  valleys  the  ranges  are  trenched, 
often  to  a  great  depth,  and  in  a  direction  transverse  to  the  general  trend. 
The  Rhdne  famishes  also  an  example  of  one  of  these  transverse  valleys, 
in  its  course  from  Martigny  to  the  Lake  of  Greneva.  In  most  mountain 
regions,  the  heads  of  two  adjacent  transverse  valleys  are  often  connected 
by  a  depression  or  pass  (co/,  joch), 

A  large  block  of  mountain  ground,  rising  into  one  or  more  domi- 
nant summits,  and  more  or  less  distinctly  defined  by  longitudinal 
and  traverse  valleys,  is  termed  in  French  a  massif — a  word  for  which 
there  is  no  good  £nglish  equivalent  Thus  in  the  Swiss  Alps  we 
have  the  massifs  of  the  Glamisch,  the  Todi,  the  Matterhom,  the  Jung- 
frau,  &c 

Very  exaggerated  notions  are  common  regarding  the  angle  of 
declivity  in  mountains.  Sections  drawn  across  any  mountain  or 
mountain-chain  on  a  true  scale,  that  is,  with  the  length  and  height  on 
the  same  scale,  bring  out  the  fact  that^  even  in  the  loftiest  mountains, 
the  breadth  of  base  is  always  very  much  greater  than  the  height 
Actual  vertical  precipices  are  less  frequent  than  is  usually  supposed, 
and  even  when  they  do  occur,  generally  form  minor  incidents  in  the 
declivities  of  mountains.  Slopes  of  more  than  30°  in  angle  are  likewise 
far  less  abundant  than  casual  tourists  believe.  £ven  such  steep 
declivities  as  those  of  38^  or  40°  are  most  frequently  found  as 
talus-slopes  at  the  foot  of  crumbling  cliffs,  and  represent  the  angle 
of  repose  of  the  disintegrated  debris.  Here  and  there,  where  the 
blocks  loosened  by  weathering  are  of  large  size,  they  may  accumulate 
upon  each  other  in  such  a  manner  that  for  short  distances  the  average 
angle  of  declivity  may  mount  as  high  as  65°.  But  such  steep  slopes 
are  of  limited  extent.  Declivities  exceeding  40  ,  and  bearing  a  large 
proportion  to  the  total  dimensions  of  hill  or  mountain,  are  always  found 
to  consist  of  naked  solid  rock.  In  estimating  angles  of  inclination  from 
a  distance,  the  student  will  learn  by  practice  how  apt  is  the  eye  to  be 
deceived  by  perspective  and  to  exaggerate  the  true  declivity,  sometimes 
to  ^stake  a  horizontal  for  a  highly  inclined  or  vertical  line.  The 
mountain  outline  shown  in  Fig.  2  presents  a  slope  of  25°  between  a  and 
b,  of  45°  between  b  and  c,  of  17°  between  c  and  ^,  of  40°  between  d  and  e, 
and  of  70°  between  e  and/.  At  a  great  distance,  or  with  bad  conditions 
of  atmosphere,  these  might  be  believed  to  be  the  real  declivities.  Yet  if 
the  same  angles  be  observed  in  another  way  (as  on  a  cottage  roof  at  B), 
we  may  leani  that  an  apparently  inclined  surface  may  really  be 
horizontal  (as  from  a  to  b  and  from  c  to  d),  and  that  by  the  effect 
of  perspective,  slopes  may  be  made  to  appear  much  steeper  than  they 
really  are.^ 

Much  evil  has  resulted  in  geological  research  from  the  use  of 
exaggerated  angles  of  slope  in  sections  and  diagrams.  It  is  therefore 
desirable  that  the  student  should,  from  the  beginning,  'accustom  himself 

'  Mr.  Ruskin  has  well  illustrated  this  point.     See  '  Modem  Painters,'  vol.  iv.  p.  183, 
whence  the  illustrations  in  the  text  are  taken. 


PA«T  I  TABLE-LANDS 

to  the  drawing  of  outlines  as  nearly  as  possible  on  a  true  scale, 
accompanying  section  of  the  Alps  by  De  la  Beche  (Fig.  3) 
is  of   interest  in  this  respect,  as  one  of   the  earliest  iUus-     ^ 
tntions  of  the  advantage  of  constructing  geological  sections 


OD  a  true  scale  as  to  the  relative  proportions  of  height  and 
length.^ 

Xaite-laiub  or  Plateaux  are  elevated  regions  of  flat  or 
undulating  country,  rising  to  heights  of  1000  feet  and  up- 
wards above  the  level  of  the  sea.  They  are  sometimes 
bordered  with  steep  elopes,  which  descend  from  their  edges, 
as  the  table-land  of  the  Spanish  peninsula  does  into  the  sea. 
In  other  cases,  they  gradually  sink  into  the  plains  and  have 
no  definite  boundaries;  thus  the  prairie- land  west  of  the 
Missouri  slowly  and  imperceptibly  ascends  until  it  becomes 
a  vast  plateau  from  4000  to  5000  feet  above  the  sea.  Occa- 
sionally a  high  table-land  is  encircled  with  lofty  mountains,  as 
in  those  of  Quito  and  Titicaca  among  the  Andes,  and  that  of 
the  heart  of  Asia ;  or  it  forms  in  itself  the  platform  on 
which  lines  of  mountains  stand,  as  in  North  America,  where 
the  ranges  included  within  the  Bocky  Mountains  reach 
elevations  of  from  10,000  to  14,000  feet  above  the  sea,  but 
not  more  than  from  5000  to  10,000  feet  above  the  table-land. 
Two  types  of  table-land  structure  may  be  observed.  I. 
Table-lands  consisting  of  level  or  gently  undulated  sheets  of 
rock,  the  general  surface  of  the  country  corresponding  with 
that  of  the  stratification.  The  Rocky  Mountain  plateau 
is   an   example   of    this  type,   which   may   be    called    that 

>  'Sections  and  Views,  HlostratjTe  of  Geological  Phenoniean.'  1830.     Oeo 
V.U6. 


p 


44  GEOGNOSY  book  ii 


of  Deposit,  for  the  flat  strata  have  been  equably  upraised  nearly  in 
the  position  in  which  they  were  deposited.  2.  Table-lands  formed  out 
of  contorted,  crystalline,  or  other  rocks,  which  have  been  planed  down 
by  superficial  agents.  This  type,  where  the  external  form  is  independent 
of  geological  structure,  may  be  termed  that  of  Erosion.  The  fjdds  of 
Norway  are  portions  of  such  a  table-land.  In  proportion  to  its  antiquity, 
a  plateau  is  trenched  by  ninning  water  into  systems  of  valleys,  until  in 
the  end  it  may  lose  its  plateau  character  and  pass  into  the  second  t3rpe 
of  mountain-ground  above  described.  This  change  has  largely  altered 
the  ancient  table-land  of  Scandinavia,  as  will  be  illustrated  in  Book  VII. 

Plains  are  tracts  of  lowland  (under  1000  feet  in  height)  which 
skirt  the  sea-board  of  the  continents  and  stretch  inland  uj)  the  river 
valleys.  The  largest  plain  in  the  world  is  that  which,  beginning  in  the 
centre  of  the  British  Islands,  stretches  across  Europe  and  Asia.  On  the 
west,  it  is  bounded  by  the  ancient  table-lands  of  Scandinavia,  Scotland 
and  Wales  on  the  one  hand,  and  those  of  Spain,  France  and  Germany 
on  the  other.  Most  of  its  southern  boundary  is  formed  by  the  vast  belt 
of  high  ground  which  spreads  from  Asia  Minor  to  the  east  of  Siberia. 
Itfi  northern  margin  sinks  beneath  the  waters  of  the  Arctic  Ocean.  This 
vast  region  is  divided  into  an  eastern  and  western  tract  by  the  low  chain 
of  the  Ural  Mountains,  south  of  which  its  general  level  sinks,  until 
underneath  the  Caspian  Sea  it  reaches  a  depression  of  about  3000  feet 
below  searlevel.  Along  the  eastern  sea-board  of  America  lies  a  broad 
belt  of  low  plains,  which  attain  their  gi-eatest  dimensions  in  the  regions 
watered  by  the  larger  rivers.  Thus  they  cover  thousands  of  square 
miles  on  the  north  side  of  the  Gulf  of  Mexico,  and  extend  for  hundreds 
of  miles  up  the  valley  of  the  Mississippi.  Almost  the  whole  of 
the  valleys  of  the  Orinoco,  Amazon  and  I^a  Plata  is  occupied  with  vast 
plains. 

From  the  evidence  of  upraised  manne  shells,  it  is  certain  that  large 
portions  of  the  great  i)lain  of  the  Old  World  comparatively  recently 
formed  part  of  the  sea-floor.  It  is  likewise  })robable  that  the  beds  of 
some  enclosed  sea-basins,  such  as  that  of  the  North  Sea,  have  formerly 
been  plains  of  the  dry  land.  ^ 

It  is  obvious,  from  their  distribution  along  river-valleys,  and  on  the 
areas  between  the  base  of  high  gi-ounds  and  the  sea,  that  plains  are 
essentially  areas  of  deposit.  They  are  the  tracts  that  have  received  the 
detritus  washed  down  from  the  slopes  above  them,  whether  that  detritus 
has  originally  accumulated  on  the  land  or  below  the  sea.  Their  surface 
presents  everywhere  loose  sandy,  gravelly,  or  clayey  formations,  indica- 
tive of  its  comjyaratively  recent  subjection  to  the  operation  of  running 
water.         y 

(2.)  Coast-lines. — A  mere  inspection  of  a  map  of  the  globe  brings 
before  the  mind  the  striking  differences  which  the  masses  of  land  present 
in  their  line  of  junction  with  the  sea.  As  a  rule,  the  southern  con- 
tinents possess  a  more  uniform  unindented  coast-line  than  the  northern. 
It  has  been  estimated  that  the  ratios  between  area  and  coast-line  among 
the  different  continents,  stand  approximately  as  in  the  following  table : — 


PART  1  COAST-LINES  OF  CONTINENTS  45 

r  Europe  has  1  geographical  mile  of  coast-line  to  143  st^uare  miles  of  surface. 
Korthem. -I  North  America  ,,  ,,  265 

I  Asia,  including  the  islands  ,,  469 


(Africa  „  „  895 


Southern. -<  South  America  ,,  ,,  434  ,, 

V  Australia  ,,  t,  332  ,, 

In  estimating  the  relative  potency  of  the  sea  and  of  the  atmospheric 
agents  of  disintegration,  in  the  task  of  wearing  down  the  land,  it  is 
eyidently  of  great  importance  to  take  into  account  the  amount  of  surface 
respectively  exposed  to  their  operations.  Other  things  being  equal, 
there  is  relatively  more  marine  erosion  in  Europe  than  in  North  America. 
But  we  require  also  to  consider  the  nature  of  the  coastnline,  whether  flat 
and  alluvial,  or  steep  and  rocky,  or  with  some  intermediate  blending  of 
these  two  characters.  By  attending  to  this  point,  we  are  soon  led  to 
observe  such  great  differences  in  the  character  of  coast-lines,  and  such 
an  obvious  relation  to  differences  of  geological  structure,  on  the  one 
hand,  and  to  diversities  in  the  removal  or  deposit  of  material,  on  the 
other,  as  to  suggest  that  the  present  coast-lines  of  the  globe  cannot  be 
aboriginal,  but  must  be  referred  to  the  operation  of  geological  agents  still 
at  work.  This  inference  is  amply  sustained  by  more  detailed  investi- 
gation. While  the  general  distribution  of  land  and  water  must  un- 
doubtedly be  assigned  to  terrestrial  movements  affecting  the  solid  globe, 
the  present  actual  coasts  of  the  land  have  chiefly  been  produced  by 
local  causes.  Headlands  project  from  the  land  because,  for  the  most 
part,  they  consist  of  rock  which  has  been  better  able  to  withstand  the 
shock  of  the  breakera.  Bays  and  creeks,  on  the  other  hand,  have  been  cut 
by  the  waves  out  of  less  durable  materials.  Again,  by  the  sinking  of 
land,  ranges  of  hills  have  become  capes  and  headlands,  while  the  valleys 
have  passed  into  the  condition  of  bays,  inlets,  or  fjords.  By  the  uprise  of 
the  sea-bottom,  tracts  of  low  alluvial  ground  have  been  added  to  the  land. 
Hence,  speculations  as  to  the  history  of  the  elevation  of  the  land, 
based  merely  upon  inferences  from  the  form  of  coast-lines  as  expressed 
upon  ordinary  maps,  to  be  of  real  service,  demand  a  careful  scrutiny 
of  the  actual  coast-lines,  and  an  ^amount  of  geological  investigation 
which  would  require  long  and  patient  toil  for  its  accomplishment. 

Passing  from  the  mere  external  form  of  the  land  to  the  composition 
and  structure  of  its  materials,  we  may  begin  by  considering  the  general 
density  of  the  entire  globe,  computed  from  observations  and  compared 
with  that  of  the  outer  and  accessible  portion  of  the  planet.  Reference 
has  already  been  made  to  the  comparative-  density  of  the  earth  among 
the  other  members  of  the  solar  system.  In  inquiries  regarding  the 
history  of  our  globe,  the  density  of  the  whole  mass  of  the  planet,  as 
compared  with  water — the  standard  to  which  the  specific  gravities  of 
terrestrial  bodies  are  referred — is  a  question  of  prime  importance. 
Various  methods  have  been  employed  for  determining  the  earth's 
density.  The  deflection  of  the  plumb-line  on  either  side  of  a  mountain 
of  known  structure  and  density,  the  time  of  oscillation  of  the  pendulum 


46 


GEOGNOSY 


BOOK  U 


at  great  heights,  at  the  sea-level,  and  in  deep  mines,  and  the  comparative 
force  of  gravitation  as  measured  by  the  torsion  balance,  have  each  been 
tried  with  the  following  various  results  : — 

Plumb-line    ex])erimeiits    on    Schichallien    (Maskelyne    and    Playfair) 

gave  as  the  mean  density  of  the  earth 4-713 

Do.  on  Arthur's  Scat,  Edinburgh  (James) 5-316 

Pendulum  experiments  on  Mont  Cenis  (Carlini  and  Giulio)  .        .  4-960 

Do.  in  Hartou  coal-pit,  Newcastle  (Airy) 6-565 

Torsion  lialance  experiments  (Cavendish,  1798) 5-480 

Do.  do.  (Reich,  1838) 5-49 

Do.  do.  (Baily,  1843) 5-660 

Do.  do.  (Comu  and  Bailie,  1872-3)       .         .         .  5-50-5-56 

Though  these  observations  are  somewhat  discrepant,  we  may  feel 
satisfied  that  the  globe  has  a  mean  density  neither  much  more  nor  much 
less  than  5*5  ;  that  is  to  say,  it  is  five  and  a  half  times  heavier  than  one 
of  the  same  dimensions  formed  of  pure  water.  Now  the  average 
density  of  the  materials  which  compose  the  accessible  portions  of  the 
earth  is  between  2*5  and  3 ;  so  that  the  mean  density  of  the  whole  globe 
is  about  twice  as  much  as  that  of  its  outer  part.  We  might,  therefore, 
infer  that  the  inside  consists  of  much  heavier  materials  than  the  outside, 
and  consequently  that  the  mass  of  the  planet  must  contain  at  least  two 
dissimilar  portions — an  exterior  lighter  crust  or  rind,  and  an  interior 
heavier  nucleus.  But  the  effect  of  pressure  must  necessarily  increase 
the  specific  gravity  of  the  interior,  as  will  be  alluded  to  further  on. 

§  2.  The  Crust. — It  was  formerly  a  prevalent  belief  that  the  exterior 
and  interior  of  the  globe  differed  from  each  other  to  such  an  extent  that, 

while  the  outer  parts  were 
cool  and  solid,  the  vastly  more 
enormous  inner  intensely  hot 
part  was  more  or  less  completely 
liquid.  Hence  the  term  "  crust " 
was  applied  to  the  external  rind 
in  the  usual  sense  of  that  word. 
This  crust  was  variously  com- 
puted to  be  ten,  fifteen,  twenty, 
or  more  miles  in  thickness.  In 
the  accompanying  diagram  (Fig. 
4),  for  example,  the  thick  line 
forming  the  circle  represents  a 
relative  thickness  of  100  miles. 
There  are  so  many  proofs  of 
enormous  and  "wide-spread  cor- 
r..    .    o         .  r^  *  ***.  1^  -*u  ,^«:i    *v.  1      rugation  of  the  materials  of  the 

Fig.  4.— Supposed  Cruat  of  the  Earth,  100  Miles  thick.  ^  ,  ,  ,  _  . 

earth  s  outer  layers,  and  such 
abundant  traces  of  former  volcanic  action,  that  geologists  have  naturally 
regarded  the  doctrine  of  a  thin  crust  over  a  liquid  interior  as  necessary 
for  the  explanation  of  a  large  class  of  terrestrial  phenomena.  For 
reasons  which  will  be  afterwards  given,  however,  this  doctrine  has  been 


PART  I  DISTRIBUTION  OF  TERRESTRIAL  DENSITY  47 


opposed  by  eminent  physicists,  and  is  now  abandoned  by  most  geologists. 
Nevertheless  the  term  "  crust "  continues  to  be  used,  apart  from  all  theory 
regarding  the  nucleus,  as  a  convenient  word  to  denote  those  cool,  upper 
or  outer  layers  of  the  earth's  mass  in  the  structure  and  history  of  which, 
as  the  only  portions  of  the  planet  accessible  to  human  observation,  lie 
the  chief  materials  of  geologic^d  investigation.  The  chemical  and  mineral 
constitution  of  the  crust  is  fully  discussed  in  later  pages  (p.  60  et  seq,) 

§  3.  The  Interiop  op  Nucleus. — Though  the  mere  outside  skin  of 
our  planet  is  all  with  which  direct  acquaintance  can  be  expected,  the 
irregular  distribution  of  materials  beneath  the  crust  may  be  inferred 
from  the  present  distribution  of  land  and  water,  and  the  observed 
differences  in  the  amount  of  deflection  of  the  plumb-line  near  the  sea  and 
near  mountain  chains.  The  fact  that  the  southern  hemisphere  is  almost 
wholly  covered  with  water,  appears  only  explicable,  as  already  remarked, 
on  the  assumption  of  an  excess  of  density  in  the  mass  of  that  half  of  the 
planet.  The  existence  of  such  a  vast  sheet  of  water  as  that  of  the 
Pacific  Ocean  is  to  be  accounted  for,  says  Archdeacon  Pratt,  by  the 
presence  of  "  some  excess  of  matter  in  the  solid  parts  of  the  earth  between 
the  Pacific  Ocean  and  the  earth's  centre,  which  retains  the  water  in  its 
place,  otherwise  the  ocean  would  flow  away  to  the  other  parts  of  the 
earth."  ^  The  same  writer  points  out  that  a  deflection  of  the  plumb-line 
towards  the  sea,  which  has  in  a  number  of  cases  been  observed,  indicates 
that  "  the  density  of  the  crust  beneath  the  mountains  must  be  less  than 
that  below  the  plains,  and  still  less  than  that  below  the  ocean-bed."^ 
Apart,  therefore,  from  the  depressions  of  the  earth's  surface,  in  which  the 
oceans  lie,  we  must  regard  the  internal  density,  whether  of  crust  or 
nucleus,  to  be  somewhat  irregularly  arranged, — there  being  an  excess  of 
heavy  materials  in  the  water-hemisphere,  and  beneath  the  ocean-beds  as 
compared  with  the  continental  masses. 

It  has  been  argued  from  the  difference  between  the  specific  gravity 
of  the  whole  globe  and  that  of  the  crust,  that  the  interior  must  consist 
of  heavier  material,  and  may  be  metallic.  But  the  effect  of  the  enormous 
internal  presisure,  it  might  be  supposed,  should  make  the  density  of  the 
nucleus  much  higher,  even  if  the  interior  consisted  of  matter  which,  on 
the  surface,  would  be  no  heavier  than  that  of  the  cnist.  In  fact,  we 
mighty  on  the  contrary,  argue  for  the  probable  comparative  lightness 
of  the  substance  composing  the  nucleus.  That  the  total  density  of  the 
planet  does  not  greatly  exceed  its  observed  amount,  may  indicate  that 
some  antagonistic  force  counteracts  the  effect  of  pressure.  The  only  force 
we  can  suppose  capable  of  so  acting  is  heat,  though  to  what  extent  this 
counterbalancing  takes  place  is  still  unknown.  It  must  be  admitted 
that  we  are  still  in  ignorance  of  the  law  that  regulates  the  compression 
of  solids  under  such  vast  pressure  as  must  exist  within  the  earth's 
interior.     We  know  that  gases  and  vapours  may  be  compressed  into 

1  'Fignra  of  the  Earth/  4th  edit.  p.  236. 

»  Op.  «<.  p.  200.  See  also  Herschel,  'Phys.  Geog/  §  13  ;  0.  Fisher,  Cambridge  PhU. 
Trans,  xii  part  ii. ;  'Physics  of  the  Earth's  Crust,'  p.  75.  PhU.  Mag.  July,  1886.  Faye, 
CoK^fUi  rtndus,  cii  (1886),  p.  651. 


48  GEOGNOSY  book  ii 

liquids,  sometimes  even  into  solids,  and  that  in  the  liquid  condition 
another  law  of  compressibility  begins.  We  know  also  from  experiment 
that  some  substances  have  their  melting-point  raised  by  pressure.^  It 
may  be  that  the  same  effect  takes  place  within  the  earth  ;  that  pressure 
increasing  inward  to  the  centre  of  the  globe,  while  augmenting  the 
density  of  each  successive  shell,  may  retain  the  whole  in  a  solid  condition, 
yet  at  temperatures  far  above  the  normal  melting-points  at  the  surface. 
Hence,  on  this  view  of  the  matter,  it  is  conceivable  that  the  difference 
between  the  density  of  the  whole  globe  and  that  of  the  crust  may  be  due 
to  pressure,  rather  than  to  any  essential  difference  of  composition.  Laplace 
proposed  the  hypothesis  that  the  increase  of  the. square  of  the  density  is 
proportional  to  the  increase  of  the  pressure,  which  gives  a  density  of  8-23 
at  half  the  terrestrial  radius  and  of  10-74  at  the  centre.  From  another 
law  proposed  by  Prof.  Darwin,  the  density  at  half  the  radius  is  only  7-4 
but  thence  towards  the  centre  increases  rapidly  up  to  infinity.^  Dr.  Pfaff 
believes  that  the  mean  terrestrial  density  of  5*5  is  not  incompatible  with 
the  notion  that  the  whole  globe  consists  of  materials  of  the  same  density 
as  the  rocks  of  the  crust.^  It  is  possible  that  the  gases  dissolved  in  the 
hot  magma  of  the  nucleus,  with  their  very  high  tension,  may  counteract 
the  effects  of  compression  and  thus  reduce  density. 

Analogies  in  the  solar  system,  however,  as  well  as  the  actual  struc- 
ture of  the  rocky  cnist  of  the  globe,  suggest  that  heavier  metallic 
ingredients  possibly  predominate  in  the  nucleus.  If  the  materials  of  the 
globe  were  once,  as  they  are  believed  to  have  been,  in  a  liquid  condition, 
they  would  then  doubtless  be  subject  to  internal  arrangement,  in  accord- 
ance with  their  relative  specific  gravities.  We  may  conceive  that,  as  in 
the  case  of  the  sun,  as  well  as  of  the  solar  system  generally  (ante,  p.  9), 
there  would  be,  so  long  as  internal  mobility  lasted,  a  tendency  in  the 
denser  elements  of  oiu*  planet  to  gravitate  towards  the  centre,  in  tfie 
lighter  to  accumidate  outside.  That  a  distribution  of  this  nature  has 
certainly  taken  place  to  some  extent,  is  evident  from  the  structure  of  the 
envelopes  and  crust.  It  is  what  might  be  expected,  if  the  constitution  of 
the  globe  resembles,  on  a  small  scale,  the  larger  planetary  system  of 
which  it  forms  a  part.  The  existence  even  of  a  metallic  interior  has 
been  inferred  from  the  metalliferous  veins  which  traverse  the  crust,  and 
which  are  commonly  supposed  to  have  been  filled  from  below. 

Evidence  of  Internal  Heat. — In  the  evidence  obtainable  as  to 
the  former  history  of  the  eiirth,  no  fact  is  of  more  importance  than  the 
existence  of  a  high  temperature  beneath  the  crust,  which  has  now  been 
placed  beyond  all  doubt.  This  feature  of  the  planet's  organisation  is 
made  clear  by  the  follo\ving  proofs : — 

(1.)   Volcarwes. — In  many  regions  of  the  earth's  surface,  openings  exist 

^  Under  a  pressure  of  792  atmospheres,  spermaceti  has  its  melting-point  raised  from  51*^ 
to  80*2^,  and  wax  from  64  S*  to  80-2". 

-  See  Fisher  *  Physics  of  Earth's  Crust/  2nd  edit.  chap.  ii.  Legendre  supposed  that  the 
density  being  2'5  at  the  surface,  it  is  8*5  at  half  the  length  of  the  radius  and  11*3  at  the 
centre.   More  recently  K  Roche  calculated  these  densities  to  be  2  '1, 8  '5,  and  10  '6  respectively. 

^  '  Allgemeine  Geologie  als  ezacte  Wissenschaft,'  p.  42. 


PAKT  I  THE  EARTH'S  INTERNAL  HEAT  49 

from  which  steam  and  hot  vapours,  ashes  and  streams  of  molten  rock, 
are  from  time  to  time  emitted.  The  abundance  and  wide  diffusion  of 
these  openings,  inexplicable  by  any  mere  local  causes,  must  be  regarded 
as  indicative  of  a  very  high  internal  temperature.  If  to  the  still  active 
vents  of  eruption,  we  add  those  which  have  formerly  been  the  channels 
of  communication  between  the  interior  and  the  surface,  there  are 
perhaps  few  large  regions  of  the  globe  where  proofs  of  volcanic 
action  cannot  be  found.  Everywhere  we  meet  with  masses  of  molten 
rock  which  have  risen  from  below,  as  if  from  some  general  reservoir. 
The  phenomena  of  active  volcanoes  are  fidly  discussed  in  Book  III.  Part  I. 

(2.)  Hot  Springs, — Where  volcanic  eruptions  have  ceased,  evidence  of  a 
high  internal  temperature  is  still  often  to  be  found  in  springs  of  hot  water 
which  continue  for  centuries  to  maintain  their  heat.  Thermal  springs, 
however,  are  not  confined  to  volcanic  districts.  They  sometimes  rise 
even  in  regions  many  hundreds  of  miles  distant  from  any  active  volcanic 
vent  The  hot  springs  of  Bath  (temp.  120°  Fahr.)  and  Buxton  (temp. 
82°  Fahr.)  in  England  are  fully  900  miles  from  the  Icelandic  volcanoes  on 
the  one  side,  and  1100  miles  from  those  of  Italy  and  Sicily  on  the  other. 

(3.)  Borings,  IVells  and  Mines. — The  influence  of  the  seasonal  changes 
of  temperature  extends  downward  from  the  surface  to  a  depth  which 
\'aries  with  latitude,  with  the  thermal  conductivity  of  soils  and  rocks, 
and  perhaps  with  other  causes.  The  cold  of  winter  and  the  heat  of 
summer  may  be  regarded  as  following  each  other  in  successive  waves 
downward,  until  they  disappear  along  a  limit  at  which  the  temperature 
remains  constant  This  zone  of  invariable  temperature  is  commonly 
believed  to  lie  at  a  depth  of  somewhere  between  60  and  80  feet  in  tem- 
perate regions.  At  Yakutsk  in  Eastern  Siberia  (lat.  62"*  N.),  however, 
as  shown  in  a  well-sinking,  the  soil  is  permanently  frozen  to  a  depth  of 
about  700  feet^  In  Java,  on  the  other  hand,  a  constant  temperature  is 
said  to  be  met  with  at  a  depth  of  only  2  or  3  feet^ 

It  is  a  remarkable  fact,  now  verified  by  observation  all  over  the 
world,  that  below  the  limit  of  the  influence  of  ordinary  seasonal  changes 
the  temperature,  so  far  as  we  yet  know,  is  nowhere  found  to  diminish 
downwards.  It  always  rises ;  and  its  rate  of  increment  never  falls  much 
below  the  average.  The  only  exceptional  cases  occur  under  circum- 
stances not  difficult  of  explanation.  On  the  one  hand,  the  neighbourhood 
of  hot-springs,  of  large  masses  of  lava,  or  of  other  manifestations  of 
volcanic  activity,  may  raise  the  subterranean  temperatiu*e  much  above 
its  normal  condition ;  and  this  augmentation  may  not  disappear  for  many 
thousand  yeara  after  the  volcanic  activity  has  wholly  ceased,  since  the 
cooling  down  of  a  subterranean  mass  of  lava  must  necessarily  be  a  very 
slow  process.  Lord  Kelvin  has  even  proposed  to  estimate  the  age  of  sub- 
terranean masses  of  intrusive  lava  from  their  excess  of  temperature  above 
the  normal  amount  for  their  isogeotherms  (lines  of  equal  earth -tem- 
perature), some  probable  initial  temperature  and  rate  of  cooling  being 
assumed.     On  the  other  hand,  the  spread  of  a  thick  mass  of  snow  and  ice 


1 


Helmcrsen,  Brit,  Assoc.  Rep.  1871,  p.  22.     See  vol.  for  1886,  p.  271. 

2  Junghuhn's  *  Java,'  ii.  p.  771. 
E 


50  GEOGNOSY  book  ii 

over  any  considerable  area  of  the  earth's  surface,  and  its  continuance 
there  for  several  thousand  years,  would  so  depress  the  isogeotherms  that, 
for  many  centuries  afterwards,  there  would  be  a  fall  of  temperature  for  a 
certain  distance  downwards.  At  the  present  day,  in  at  least  the  more 
northerly  parts  of  the  northern  hemisphere,  there  are  such  evidences  of  a 
former  more  rigorous  climate,  as  in  the  well-sinking  at  Yakutsk  just 
referred  to.^  Lord  Kelvin  (Sir  W.  Thomson)  ^  has  calcidated  that  any 
considerable  area  of  the  earth's  surface  covered  for  several  thousand  years 
by  snow  or  ice,  and  retaining,  after  the  disappearance  of  that  frozen 
covering,  an  average  surface  temperature  of  IS""  C,  "would  during  900 
years  show  a  decreasing  temperature  for  some  depth  down  from  the  surface, 
and  3600  years  after  the  clearing  away  of  the  ice  would  still  show  residual 
effect  of  the  ancient  cold,  in  a  half  rate  of  augmentation  of  temperature 
downwards  in  the  upper  strata,  gradually  increasing  to  the  whole  normal 
rate,  which  would  be  sensibly  reached  at  a  depth  of  600  metres." 

Beneath  the  limit  to  which  the  influence  of  the  changes  of  the  seasons 
extends,  observations  all  over  the  globe,  and  at  many  diflferent  elevations, 
give  a  rate  of  increase  of  temperature  downwards,  or  "  temperature  gra- 
dient," which  has  been  usually  taken  to  be  1"  Fahr.  for  every  50  or  60 
feet  of  descent,  this  computation  being  based  especially  on  observations 
in  deep  mines  and  borings.  Professor  Prestwich  concluded  from  a  large 
series  of  observations  collated  by  him,  that  the  average  increment  might 
be  taken  at  1"  Fahr.  for  every  45  feet.^  Observations  taken  in  the 
extraordinarily  deep  boring  at  Schladebach,  near  Diirrenberg,  showed 
that  in  a  depth  of  5736  feet  the  average  rise  of  temperature  was  1° 
Fahr.  for  every  65  feet.*  According  to  data  collected  by  a  Committee 
of  the  British  Association,  the  average  gradient  appears  to  be  1^  Fahr. 
for  every  64  feet,  or  ^^^  of  a  degree  per  foot. 

Isogeotherms  near  the  surface  follow  approximately  the  contours  of 
the  surface,  but  are  flatter  than  these,  and  '*  their  flattening  increases  as 
we  pass  to  lower  ones,  until  at  a  considerable  depth  they  become  sensibly 
horizontal  planes.  The  temperature  gradient  is  consequently  steepest 
beneath  gorges  and  least  steep  beneath  ridges.' 


»5 


^  Professor  Prestwich  {Inaugural  Ledurtt  1875,  p.  45)  has  suggested  that  to  the  more 
rapid  refrigeration  of  the  earth's  surface  during  this  cold  period,  and  to  the  consequent 
depression  of  the  subterranean  isothermal  lines,  the  alleged  present  comparative  quietude 
of  the  volcanic  forces  is  to  be  attiibuted,  the  internal  heat  not  having  yet  recovered  its 
dominion  in  the  outer  crust. 

"^  Brit,  Assoc,  Repcn'ts^  1876,  Sections,  p.  3.  ^  Proc,  Roy,  Soc.  xli.  (1885),  p.  55. 

*  Brit.  Assoc.  1889.     Report  of  Underground  Temperature  Committee. 

*  J.  D.  Everett,  Brit.  Assoc.  1879,  Sections,  p.  345.  Compare  also  the  elaborate 
observations  made  in  the  St  Gothard  Tunnel,  F.  StapfT,  *  Rapports,  Conseil  Fed. 
St.  Gothard,'  vol.  viii.,  and  'Geologische  Durchschnitte  des  Gothard  Tunnels;*  'J^tude  de 
rintiuence  de  la  Clialeur  de  I'lnt^rieur  de  la  Terre,'  &c.,  Becue  Univ.  Mines,  1879-80. 
Min.  Proc*  X.  EngUind  Inst.  Mining -Mechan.  Engin.  xxxii.  (1883),  p.  19.  'Reports 
of  Committee  on  Underground  Temperature,'  Brit.  Assoc.  Hep.  from  1868  onwards,  with 
summary  of  results  in  tlie  volume  for  1882.  A  voluminous  and  valuable  collection  of  data 
bearing  on  this  subject  was  compiled  by  Professor  Prestwich  and  is  published  in  Proc.  Boy. 
Soc.  xli.  (1885),  p.  1. 


PART  I 


THE  EARTH'S  INTERNAL  HEAT 


51 


Irregularities  in  the  Downward  Increment  of  Heat. 
— While  there  is  everywhere  a  progressive  increase  of  temperature 
downwards,  its  rate  is  by  no  means  uniform.  The  more  detailed 
obeervations  which  have  been  made  in  recent  years  have  brought  to 
light  the  important  fact  that  considerable  variations  in  the  rate  of 
increase  take  place,  even  in  the  same  bore.  The  temperatures  obtained 
at  different  depths  in  the  Rose  Bridge  colliery  shaft,  Wigan,  for  instance, 
read  as  in  the  following  columns : — 


Depth  ill 
Yardtt. 

558 

605 

630 

663 

671 

679 

734 


TempeTBture 

(Fahr.) 

78 

Depth  in 
Yards. 
745 

80 

761 

83 

775 

85 

783 

86 

800 

87 

806 

88i 

815 

Temperature 
(Fahr.) 

89 

90i 

914 

92 

93 

93J 

94 


At  La  Chapelle,  in  an  important  well  made  for  the  water-supply  of 
Paris,  observations  have  been  taken  of  the  temperature  at  different 
depths,  as  shown  in  the  subjoined  table  :  ^ — 


Depth  ill 

Temperature 

Depth  in 

Temiwraturt 

Metres. 

(Fahr.) 

Metres. 

(Fahr.) 

100 

59-5 

500 

72-6 

200 

61-8 

600 

75-0 

300 

65-5 

660 

76  0 

400 

69-0 

In  drawing  attention  to  the  foregoing  temperature-observations  at  the 
Ro6e  Bridge  colliery — the  deepest  mine  in  Great  Britain — Professor  Everett 
points  out  that,  assuming  the  surface  temperature  to  be  49°  Fahr.,  in  the 
first  558  yards,  the  rate  of  rise  of  temperature  is  1^  for  57*7  feet;  in  the 
next  257  yards  it  is  1°  in  48*2  feet;  in  the  portion  between  605  and  671 
yards — a  distance  of  only  198  feet — it  is  l""  in  33  feet;  in  the  lowest 
portion  of  432  feet  it  is  1**  in  54  feet.-  When  such  irregularities  occur 
in  the  same  vertical  shaft,  it  is  not  surprising  that  the  average  should 
vary  so  much  in  different  places. 

There  can  be  little  doubt  that  one  main  cause  of  these  variations 
is  to  be  sought  in  the  different  thermal  conductivities  of  the  rocks  of 
the  earth's  crust.  The  first  accurate  measurements  of  the  conductini^ 
powers  of  rocks  were  made  by  the  late  J.  D.  Forbes  at  Edinburgh 
(1837-1845).  He  selected  three  sites  for  his  thermometers,  one  in 
"trap-rock"  (a  porphyrite  of  Lower  Carboniferous  age),  one  in  loose 
sand,  and  one  in  sandstone,  each  set  of  instruments  being  sunk  to  depths 
of  3,  6,  12  and  24  French  feet  from  the  surface.  He  found  that  the 
wave  of  summer  heat  reached  the  bulb  of  the  deepest  instrument  (24 
feet)  on  4th  January  in  the  trap-rock,  on  25th  December  in  the  sand 

»  BrU.  Assoc.  Rep.  1873,  Sectiou8,  p.  254.       ^  Brit,  Assoc.  Hep.  1870,  Sections,  p.  31. 


62  GEOGNOSY  book  ii 

<ind  on  3r(l  November  in  the  sandstone,  the  trap-rock  l)eing  the  worst 
conductor  and  the  solid  sandstone  by  far  the  best* 

As  a  rule,  the  lighter  and  more  porous  rocks  offer  the  greatest 
resistance  to  the  passage  of  heat,  while  the  more  dense  and  crystalline 
offer  the  least  resistance.  The  resistance  of  opaque  white  quartz  is 
expressed  by  the  number  114,  that  of  Itasalt  stands  at  273,  while  that  <A 
cannel  coal  stands  very  much  higher  at  1538,  or  more  than  thirteen 
times  that  of  quartz.^ 

It  is  c^'ident  also,  from  the  texture  and  structure  of  most  rocks,  that 
the  conductivity  must  vary  in  different  directions  through  the  same 
mass,  heat  being  more  easily  conducted  along  than  across  the  "grain," 
the  bedding,  and  the  other  numerous  divisional  surfaces.  Experiments 
have  been  made  to  determine  these  variations  in  a  number  of  rocks. 
Thus,  the  conductivity  in  a  direction  transverse  to  the  divisional  planes 
being  taken  as  unity,  the  conductivity  parallel  with  these  planes  was 
found  in  a  variety  of  magnesian  schist  to  lie  4028.  In  certain  slates 
and  schistose  rocks  from  central  France,  the  ratio  varied  from  1  :  256  to 
1 :  3'9d2.  Hence  in  such  fissile  rocks  as  slate  and  mica-schist,  heat  may 
travel  four  times  more  easily  along  the  planes  of  cleavage  or  foliation 
than  across  them.^ 

In  reasoning  upon  the  discrepancies  in  the  rate  of  increase  of  sub- 
terranean temperatures,  we  must  also  bear  in  mind  that  convection  by 
])ercolating  streams  of  water  must  materially  affect  the  transference  <^ 
heat  from  below.^  Certain  kinds  of  rock  are  more  liable  than  others  to 
1)0  charged  with  water,  and,  in  almost  every  boring  or  shaft,  one  or  mrav 
horizons  of  such  water-bearing  i-ocks  are  met  with.  The  effect  of 
interstitial  water  ta  to  diminish  thermal  resistance.  Dry  red  brick  has 
its  resistance  lowered  from  680  to  405  by  being  thoroughly  soaked  in 
water,  its  conductivity  being  thus  increased  68  per  cent,  A  piece  of 
sandstone  has  i\&  conductivity  heightened  to  the  extent  of  S  per  cent  by 
being  wetted." 

Mallet  contended  that  the  variations  in  the  amount  of  increase 
in  subterranean  temperature  arc  t^X)  great  to  j^ermit  us  to  believe  tham 
to  be  due  merely  to  differences  in  the  transmission  of  the  general 
internal  heat,  and  that  they  point  to  local  accessions  of  heat  arising  from 
transformation  of  the  mechanical  work  of  compression,  which  i>  due  to 
the   constant  cooling  and   contraction   of  the  globe."      But  it  nuj  bs 

'  Tram.  Rvy.  Soc.  Edia,  nvi.  p.  211. 

'  UeTBchel  «inl  Lsbour  (Britisb  Ansociation  Cominittei:  on  Tlieriijnl  Conilin 
Rocks),  Bril.  At»"c  Hep.  1875,  p.  59.     The  fiual  Report  U  in  Ihe  toI.  for  1881. 

^  "  Report  of  Committee  on  Thermal  Cooductivities  of  Biek, "  Brit.  Auoe.  ftrji.  1876, 
!>.  ttl.  JBDnetttti,  BuU.  Soc.  QM.  Frana  (April-June,  1874),  li.  p.  284.  Thb  obstrrer 
haa  carried  out  a  aeries  of  detailed  reBearclies  on  tbe  projiaiiistiun  of  lieat  Uiroiigli  rocVi 
which  will  be  foaiid  in  BuU.  Soc  GM.  Franar,  tomn  i.-ix.  (3r<1  &er'm}. 

*  In  the  great  bore  of  Sperenberg  (4172  feet,  entinlf  iu  rock-utlt,  eic*|it  tbe  Snl  SU 
feet]  there  ia  evidence  thut  the  water  near  the  top  ii  mrnied  4}°  ~  '      ' 
Bril.  A'sv.  1882,  p,  78. 

'  Hentchel  and  Leboiir,  Brit,  Aaoe.  Sep.  IS75,  p.  68. 

'  "Voleanic  Energy,"  Phil.  Tram.  1876. 


^ 


PART  I  THE  EARTH'S  INTERIOR  53 

replied  that  these  variations  are  not  greater  than,  from  the  known  diver- 
gences in  the  conductivities  of  rocks,  they  might  fairly  be  expected  to  be. 
Probable  Condition  of  the  Earth's  Interior. — Various  theo- 
ries have  been  propounded  on  this  subject.  There  are  only  three  which 
merit  serious  consideration.  (1.)  One  of  these  supposes  the  planet  to 
consist  of  a  solid  cnist  and  a  molten  interior.  (2.)  The  second  holds 
thsLty  with  the  exception  of  local  vesicular  spaces,  the  globe  is  solid  and 
rigid  to  the  centre.  (3.)  The  third  contends  that  while  the  mass  of  the 
globe  is  solid,  there  lies  a  liquid  substratum  beneath  the  crust 

1.  The  arguments  in  favour  of  intermtl  liquidity  may  be  summed  up 
as  follows,  (a.)  The  ascertained  rise  of  temperature  inwards  from  the 
surface  is  such  that,  at  a  very  moderate  depth,  the  ordinary  melting- 
point  of  even  the  most  refractory  substances  would  be  reached.  At  20 
miles  the  temperature,  if  it  increases  progressively,  as  it  does  in  the 
depths  accessible  to  observation,  must  be  about  1760°  Fahr.  ;  at  50  miles 
it  must  be  4600°,  or  far  higher  than  the  fusing-point  even  of  so  stubborn 
a  metal  as  platinum,  which  melts  at  3080°  Fahr.^  (b,)  All  over  the  world 
volcanoes  exist  from  which  steam  and  torrents  of  molten  lava  are  from 
time  to  time  erupted.  Abundant  as  are  the  active  volcanic  vents,  they 
form  but  a  small  proportion  of  the  whole  which  have  been  in  operation 
since  early  geological  time.  It  has  been  inferred,  therefore,  that  these 
numerous  funnels  of  commimication  with  the  heated  interior  could  not 
have  existed  and  poured  forth  such  a  vast  amount  of  molten  rock,  unless 
they  drew  their  supplies  from  an  immense  internal  molten  nucleus,  (c.) 
When  the  products  of  volcanic  action  from  different  and  widely-separated 
regions  are  compared  and  analysed,  they  are  found  to  exhibit  a  remark- 
able uniformity  of  character.  Lavas  from  Vesuvius,  from  Hecla,  from 
the  Andes,  from  Japan,  and  from  New  Zealand  present  such  an  agree- 
ment in  essential  particidars  as,  it  is  contended,  can  only  be  accounted 
for  on  the  supposition  that  they  have  all  emanated  from  one  vast 
common  source.*-  (d.)  The  abundant  earthquake -shocks  which  affect 
large  areas  of  the  globe  are  maintained  to  be  inexplicable  unless  on  the 
supposition  of  the  existence  of  a  thin  and  somewhat  flexible  crust. 
These  arguments,  it  will  be  observed,  are  only  of  the  nature  of  inferences 
drawn  from  observations  of  the  present  constitution  of  the  globe.  They 
are  based  on  geological  data,  and  have  been  frequently  urged  by  geo- 
logists as  supporting  the  only  view  of  the  nature  of  the  earth's  interior, 
supposed  by  them  to  be  compatible  with  geological  evidence. 

2.  Th€  argument!^  in  favour  of  the  interned  solidity  of  the  earth  are  based 
on  physical  and  astronomical  considerations  of  the  greatest  importance. 
They  may  be  arranged  as  follows  : — 

((I.)  Argument  from  precession  and  nutation. — The  problem  of  the 
internal  condition  of  the  globe  was  attacked  as  far  back  as  the  year  1839 

*  Bnt  Lord  Kelvin  (Sir  W.  Tliomson)  has  shown  that  if  the  rate  of  increase  of  tempera- 
ture is  taken  to  be  1*  for  every  51  feet  for  the  first  100,000  feet,  it  will  begin  to  diminish 
below  that  limit,  being  only  1*  in  2550  feet  at  800,000  feet,  and  then  rapidly  lessening. 
Trans.  Roy.  Soe.  Edin.  xxiii.  p.  163. 

*  See  D.  Forbes,  Popular  Science  Review^  April,  1869. 


64  GEOGNOSY  book  ii 

by  Hopkins,  who  calculated  how  far  the  planetary  motions  of  precession  and 
nutation  would  be  influenced  by  the  solidity  or  liquidity  of  the  earth's 
interior.  He  found  that  the  precessional  and  nutational  movements 
could  not  possibly  be  as  they  are,  if  the  planet  consisted  of  a  central  core 
of  molten  rock  surrounded  with  a  crust  of  twenty  or  thirty  miles  in 
thickness ;  that  the  least  possible  thickness  of  crust  consistent  with  the 
existing  movements  was  from  800  to  1000  miles;  and  that  the  whole 
might  even  be  solid  to  the  centre,  with  the  exception  of  comparatively 
small  vesicular  spaces  filled  with  melted  rock.^ 

M.  Delaunay  *  threw  doubt  on  Hopkins'  views,  and  suggested  that,  if 
the  interior  were  a  mass  of  suflBcient  viscosity,  it  might  behave  as  if  it 
were  a  solid,  and  thus  the  phenomena  of  precession  and  nutation  might  not 
be  aff'ected.  Lord  Kelvin  (Sir  W.  Thomson),  who  had  already  arrived  at 
the  conclusion  that  the  interior  of  the  globe  must  be  solid,  and  acquiesced 
generally  in  Hopkins'  conclusions,  remarked  that  the  hypothesis  of  a 
viscous  and  quasi-rigid  interior  "  breaks  down  when  tested  by  a  simple 
calculation  of  the  amount  of  tangential  force  required  to  give  to  any 
globular  portion  of  the  interior  mass  the  precessional  and  nutational 
motions  which,  with  other  physical  astronomers,  M.  Delaunay  attributes 
to  the  earth  as  a  whole."  ^  He  held  the  earth's  crust  down  to  depths 
of  hundreds  of  kilometres  to  be  capable  of  resisting  such  a  tangential 
stress  (amounting  to  nearly  y^**^  ^^  ^  gramme  weight  per  square  centi- 
metre) as  would  with  great  rapidity  draw  out  of  shape  any  plastic  sub- 
stance which  could  properly  be  termed  a  viscous  fluid,  and  he  concluded 
"  that  the  rigidity  of  the  earth's  interior  substance  could  not  be  less  than 
a  millionth  of  the  rigidity  of  glass  without  very  sensibly  augmenting  the 
lunar  nineteen-y early  nutation.  "  * 

In  Hopkins'  hypothesis  he  assumed  the  crust  to  be  infinitely  rigid 
and  unyielding,  which  is  not  true  of  any  material  substance.  Lord 
Kelvin  subsequently  returning  to  the  problem,  in  the  light  of  his  own 
researches  in  vortex-motion,  found  that,  while  the  argument  against  a 
thin  crust  and  vast  liquid  interior  is  still  invincible,  the  phenomena  of 
precession  and  nutation  do  not  decisively  settle  the  question  of  internal 
fluidity,  as  Hopkins,  and  others  following  him,  had  believed,  though  the 
solar  semi-annual  and  limar  fortnightly  nutations  absolutely  disprove  the 
existence  of  a  thin  rigid  shell  full  of  liquid.  If  the  inner  surface  of  the 
crust  or  shell  were  rigorously  spherical,  the  interior  mass  of  supposed 
liquid  could  experience  no  precessional  or  nutational  influence,  except  in 
so  far  as,  if  heterogeneous  in  composition,  it  might  suffer  from  external 
attraction  due  to  non-sphericity  of  its  surfaces  of  equal  density.  But 
"  a  very  slight  deviation  of  the  inner  surface  of  the  shell  from  perfect 
sphericity  would  suffice,  in  virtue  of  the  quasi -rigidity  due  to  vortex- 

^  PhU.  Trans.  1839,  p.  381  ;  1840,  p  193  ;  1842,  p.  43 ;  Brit,  Assor.  1847. 

-  In  a  paper  on  the  hypothesis  of  the  interior  fluidity  of  the  globe,  CUymptes  rendus, 
July  13,  1868.  Oeol.  Mag.  v.  p.  507.  See  also  H.  Hennessy,  Compter  rendus,  6  March, 
1871,  Oeol.  Mag.  viii.  p.  216.  Nature,  xv.  p.  78.  0.  Fisher,  *  Physics  of  the  Earth's 
Crust,'  2nd  Etlition,  1889. 

•''  Xature,  February  1, 1872.  •*  Tjk.  cCt.  p.  258. 


56  GEOGNOSY  book  ii 

{('.)  Argument  from  relative  densities  of  melted  and  solid  rock. — 
The  two  preceding  arguments  must  be  considered  decisive  against 
the  hypothesis  of  a  thin  shell  or  crust  covering  a  nucleus  of  molten 
matter.  It  has  been  further  urged,  as  an  objection  to  this  hypothesis, 
that  cold  solid  rock  is  more  dense  than  hot  melted  rock,  and  that  even 
if  a  thin  crust  were  formed  over  the  central  molten  globe  it  would  immedi- 
ately break  up  and  the  fragments  would  sink  towards  the  centre.^  Kecent 
experiments  show  that  diabase  (of  density  3*017)  contracts  nearly  4  i)er 
cent  on  solidification,  and  that  the  resulting  homogeneous  glass  has  a  density 
of  only  2*717.-  As  has  been  already  pointed  out,  the  specific  gravity  of  the 
interior  is  at  least  twice  as  much  as  that  of  the  visible  parts  of  the 
crust.  If  this  difference  be  due,  not  merely  to  the  effect  of  pressure, 
but  to  the  presence  in  the  interior  of  intensely  heated  metallic  8ul>- 
stances,  we  cannot  suppose  that  solidified  portions  of  such  rocks  as 
granite  and  the  various  lavas  could  ever  have  sunk  into  the  centi-e  of  the 
earth,  so  as  to  build  up  there  the  honey-combed  cavernous  mass  which 
might  have  served  as  a  nucleus  in  the  ultimate  solidification  of  the 
whole  planet.  If  the  eai*liest  formed  portions  of  the  comparatively 
light  crust  were  denser  than  the  underlying  liquid,  they  would  no  doubt 
descend  until  they  reached  a  stratum  with  specific  gi*avity  agreeing  with 
their  own,  or  until  they  were  again  melted.-* 

3.  Ihjpothesis  of  a  liquid  mhsiraUnn  between  a  solid  nucleus  and  tht 
crust. — Since  the  early  and  natural  belief  in  the  liquidity  of  the  earth's 
interior  has  been  so  weightily  opposed  by  physical  arguments,  geologists 
have  endeavoured  to  modify  it  in  such  a  way  as,  if  possible,  to  satisfy 
the  requirements  of  physics,  while  at  the  same  time  providing  an 
adequate  explanation  of  the  corrugation  of  the  earth's  crust,  the 
phenomena  of  volcanoes,  iVc*  The  hypothesis  has  })een  proposed  of  "a 
rigid  nucleus  nearly  approaching  the  size  of  the  whole  globe,  covered  by 
a  fluid  substratum  of  no  great  thickness,  compared  with  the  radius,  upon 
which  a  crust  of  lesser  density  floats  in  a  state  of  equilibrium."  The 
nucleus  is  assumed  to  owe  its  solidity  to  "the  enormous  pressure  of 
the  superincumbent  matter,  while  the  crust  owes  its  solidity  to  having 
become  cool.     The  fluid  substratum  is  not  under  sufficient  pressiu-e  to  be 

^  Tliis  objection  lias  been  repeate^lly  urged  by  Lord  Kelvin.  See  Trans,  Hoy.  Soc.  Edin, 
xxiii.  p.  157  ;  and  Brit.  Assoc.  Hep.  1870,  Sections,  p.  7. 

*  C.  Barus,  Pliii.  Mafj.  1893,  p.  174.  It  is  nevertheless  tnie  that,  from  a  cause  merely 
mechanical,  pieces  of  the  original  cold  rock,  though  so  much  denser,  will  float  for  a  time  on 
the  melted  material.     Ih.  p.  189. 

^  See  D.  Forbes,  Oed.  Mag.  vol.  iv.  p.  435.  The  evidence  for  the  internal  solidity  of  the 
earth  is  criticised  by  Dr.  M.  E.  Wadaworth  in  the  American  Xaturaiistt  1884. 

■*  See  Dana  in  Siilinuins  Journal^  iii.  (1847),  p.  147.  Ainer.  Journ.  Science  (1873). 
The  hypothesis  of  a  fluid  substratum  has  been  advocated  by  Shaler.  Proc.  BosL  Nat, 
Hist.  Soc.  xi.  (1868),  p.  8.  Geol.  Mag,  v.  p.  511.  J.  Le  Conte,  Amei',  Jonni,  Set.  1872, 
1873.  O.  Fisher,  Geol.  Mag.  v.  (new  series),  pp.  291  and  551.  '  Physics  of  the  Earth's 
Crust,'  1883.  [This  author  in  his  second  edition  modifies  this  view.]  Hill,  Oeiji.  Mag.  v. 
(new  series),  pp.  262,  479.  The  idea  of  a  viscous  layer  between  the  solidifying  central  mass 
and  the  crust  was  present  in  Hopkins'  mind.     Brit.  Assoc.  1848,  Reports,  ]>.  48. 


58  GEOGNOSY  book  ii 


M' 


%   4.  Age  of  the  Earth  and  Measures  of  Geological  Time. — The 

age  of  our  planet  is  a  problem  which  may  be  attacked  either  from  the 
geological  or  physical  side. 

1.  The  geological  arguments  rest  chiefly  upon  the  observed  rates 
at  which  geological  changes  are  being  effected  at  the  present  time, 
and  is  open  to  the  obvious  preliminary  objection  that  it  assumes 
the  existing  rate  of  change  as  the  measure  of  past  revolutions, — ^an 
assumption,  however,  which  may  be  erroneous,  for  the  present  may  be 
a  period  when  all  geological  events  march  forward  more  slowly  than 
they  used  to  do.  The  argument  proceeds  on  data  partly  of  a  physical 
and  partly  of  an  organic  kind,  (a.)  The  physical  evidence  is  derived 
from  such  facts  as  the  observed  rates  at  which  the  surface  of  a 
country  is  lowered  by  rain  and  streams,  and  new  sedimentary  deposits 
are  formed.  These  facts  will  be  more  particularly  dwelt  upon  in 
later  sections  of  this  volume.  If  we  assume  that  the  land  haJs  been 
worn  away,  and  that  stratified  deposits  have  l)een  laid  down,  nearly 
at  the  same  rate  as  at  present,  then  we  must  admit  that  the  stratified 
portion  of  the  crust  of  the  earth  must  represent  a  very  vast  period 
of  time.^  (6.)  On  the  other  hand,  human  experience,  so  far  as  it 
goes,  warrants  the  belief  that  changes  in  the  organic  world  proceed 
>vith  extreme  slowness.  Yet  in  the  stratified  rocks  of  the  terrestrial 
crust,  we  have  abundant  proof  that  the  whole  fauna  and  flora  of  the 
earth's  surface  have  passed  through  numerous  cycles  of  revolution, — 
species,  genera,  families,  orders,  appearing  and  disappearing  many 
times  in  succession.  On  any  supposition,  it  must  be  admitted  that 
these  vicissitudes  in  the  organic  world  can  only  have  been  effected 
with  the  lapse  of  vast  periods  of  time,  though  no  reliable  standard 
seems  to  be  available  whereby  these  periods  are  to  be  measured. 
The  argument  from  geological  evidence  indicates  an  interval  of 
probably  not  much  less  than  100  million  years  since  the  earliest  forms 
of  life  appeared  upon  the  earth,  and  the  oldest  stratified  rocks  began  to 
be  laid  down. 

2.  The  physical  argument  as  to  the  age  of  our  planet  is  based 
by  Lord  Kelvin  upon  three  kinds  of  evidence: — (1)  the  internal  heat 
and  rate  of  cooling  of  the  earth ;  (2)  the  tidal  retardatidn  of  the  earth 
rotation ;  and  (3)  the  origin  and  age  of  the  sun's  heat. 

^  Dr.  Croll  put  this  j^eriod  at  not  less,  but  possibly  much  more,  than  60  million 
years.  Dr.  Haughton  gives  a  much  more  extemled  i>eriod.  Rstimating  the  present 
rate  of  deposit  of  strata  at  1  foot  in  8616  years,  assuming  the  former  rate  to  have  been 
ten  times  more  rapid,  or  1  foot  in  861*6  years,  and  taking  the  thickness  of  the  stratified 
rocks  of  the  earth's  crust  at  177,200  feet,  he  obtains  a  minimum  of  200,000,000  years 
for  the  whole  duration  of  geological  time :  *  Six  Lectures  on  Physical  Greography,*  1880, 
]).  94.  Dr.  Haughton  has  also  proposed  another  geological  measure  of  past  time, 
based  upon  the  assumed  effects  of  continental  upheaval  {Proc,  Roy.  Soc.  xxvi.  (1877), 
1>.  534).  But  Professor  Darwiu  has  shown  it  to  be  inadmissible.  {Op,  cit.  xxrii. 
(1878),  p.  179.)  For  various  opinions  regarding  geological  measures  of  time  see  J.  Phillips, 
Hnt.  Assoc,  1864  :  C^oll,PAi7.  Mag.  1868  :  T.  M'K.  Hughes,  Proc.  Hoy.  Inst.  Great  BHtain, 
March  24,  1876  :  Dupont,  Bull.  Acad.  Hoy.  Belyique,  viii.  (1884)  :  T.  Mellard  Reade,  Quart. 
Joum.  Oeol.  ,Soc.  1888,  p.  291. 


PART  I  THE  AGE  OF  THE  EARTH  69 

(1.)  Applying  Fourier's  theory  of  thermal  conductivity,  he  pointed 
out  as  far  back  as  the  year  1862,  that  in  the  known  rate  of  increase  of 
temperature  downward  beneath  the  surface,  and  the  rate  of  loss  of  heat 
from  the  earth,  we  have  a  limit  to  the  antiquity  of  the  planet.  He 
showed,  from  the  data  available  at  the  time,  that  the  superficial 
consolidation  of  the  globe  could  not  have  occurred  less  than  20  million 
years  ago>  or  the  underground  heat  would  have  been  greater  than  it 
is ;  nor  more  than  400  million  years  ago,  otherwise  the  underground 
temperature  would  have  shown  no  sensible  increase  downwards.  He 
admitted  that  very  wide  limits  were  necessary.  In  subsequently  discuss- 
ing the  subject,  he  inclined  rather  towards  the  lower  than  the  higher 
antiquity,  but  concluded  that  the  limit,  from  a  consideration  of  all  the 
evidence,  must  be  placed  within  some  such  period  of  past  time  as  100 
millions  of  years.     He  would  now  restrict  the  time  to  about  20  millions.^ 

(2.)  The  reasoning  from  tidal  retardation  proceeds  on  the  admitted 
fact  that,  owing  to  the  friction  of  the  tide-wave,  the  rotation  of  the 
earth  is  retarded,  and  is  therefore  slower  now  than  it  must  have  been 
at  one  time.  Lord  Kelvin  contends  that  had  the  globe  become  solid 
some  10,000  million  years  ago,  or  indeed  any  high  antiquity  beyond  100 
million  years,  the  centrifugal  force  due  to  the  more  rapid  rotation  must 
have  given  the  planet  a  very  much  greater  polar  flattening  than  it 
actually  possesses.  He  admits,  however,  that  though  100  million  years 
a^  that  force  must  have  been  about  3  per  cent  greater  than  now,  yet 
"  nothing  we  know  regarding  the  figure  of  the  earth  and  the  disposition  of 
land  and  water  would  justify  us  in  saying  that  a  body  consolidated  when 
there  was  more  centrifugal  force  by  3  per  cent  than  now,  might  not  now 
be  in  all  respects  like  the  earth,  so  far  as  we  know  it  at  present."  - 

(3.)  The  third  kind  of  evidence  leads  to  results  similar  to  those  derived 
from  the  two  previous  lines  of  reasoning.  It  is  based  upon  calculations 
as  to  the  amount  of  heat  that  would  be  available  by  the  falling  together 
of  masses  from  space,  which  gave  rise  by  their  impact  to  our  sun,  and 
the  rate  at  which  this  heat  has  been  radiated.  Assuming  that  the  sun 
has  been  cooling  at  a  uniform  rate.  Professor  Tait  concludes  that  it  cannot 
have  supplied  the  earth,  even  at  the  present  rate,  for  more  than  about  15 
or  20  million  years.^  Lord  Kelvin  also  believes  that  the  sun's  light  will 
not  last  more  than  5  or  6  millions  of  years  longer.* 

There  can  be  no  doubt  that  the  demands  of  the  earlier  geologists  for 
an  unlimited  duration  of  past  time,  for  the  accomplishment  of  geological 
history,  were  extravagant  and  unnecessary.  But  it  may  be  questioned 
how  far  the  recent  limitation  of  time  proposed  from  physical  consider- 

*  Trans.  Roy.  Soc  Edin.  xxiii.  p.  157.  Trans.  Qeol.  Soc.  Glasgow,  iii.  p.  25.  'Popular 
Lectures  and  Addresses,'  2nd  etlit.  (1891),  p.  397.  Professor  Tait  reduces  the  period  to  10 
or  15  mUlions.     'Recent  Advances  in  Physical  Science,'  p.  167. 

'  Trans.  Geol.  Soc.  Glasgow,  iii.  p.  16.  Professor  Tait,  in  repeating  this  argument 
concludes  that,  taken  in  connection  with  the  previous  one,  **  it  probably  reduces  the  possible 
period  which  can  be  allowetl  to  geologists  to  something  less  than  10  millions  of  years." 
'Recent  Advances,'  p.  174.     Compare  Newcomb,  'Popular  Astronomy,*  p.  505. 

»  Op.  cit.  p.  174.  *  'Popular  Lectures,  etc.,'  p.  397. 


6  0  GEOGNOS  Y  book  ii 

ations  are  really  founded  on  well-established  facts.  The  argument  from 
the  geological  record  in  favour  of  a  much  longer  period  than  physicists 
are  disposed  to  concede  is  so  strong  that  one  is  inclined  to  believe  that 
these  writers  have  overstated  their  case.  The  evidence  from  the  nature 
of  the  sedimentary  rocks,  and  from  the  succession  of  organic  remains  in 
these  rocks,  appears  to  me  to  demand  an  amount  of  time  not  far  short  of 
the  hundred  millions  of  years  originally  granted  by  Lord  Kelvin.^ 


Part  II. — Ax  Account  of  thk  Composition  of  the  PIarth's 

Crust — Minerals  and  Kocks. 

The  earth's  crust  is  composed  of  minei^l  matter  in  various  aggregates 
included  under  the  general  term  Rock.  A  rock  may  be  defined  as  a 
mass  of  matter  composed  of  one  or  more  simple  minerals,  having 
usually  a  variable  chemical  composition,  with  no  necessaiily  symmetrical 
external  form,  and  ranging  in  cohesion  from  mere  loose  debris  up  to 
the  most  compact  stone.  Granite,  lava,  sandstone,  limestone,  gi*avel,  sand, 
mud,  soil,  marl  and  peat,  are  all  recognised  in  a  geological  sense  as 
rocks.  The  study  of  rocks  is  known  jis  Lithology,  Petrogiuphy  or 
Petrology. 

It  will  be  most  convenient  to  treat — Ist^  of  the  general  chemical 
constitution  of  the  crust ;  2nd,  of  the  minerals  of  which  rocks  mainly 
consist ;  3rd,  of  the  methods  employed  for  the  determination  of  rocks ; 
4th,  of  the  external  characters  of  rocks,  5th,  of  the  internal  texture  and 
structure  of  rocks ;  6th,  of  the  classification  of  rocks ;  and  7th,  of  the 
more  important  rocks  occurring  as  constituents  of  the  earth's  crust. 


§  i.  (General  Chemical  Constitution  of  the  Crust. 

Direct  acquaintance  with  the  chemical  constitution  of  the  globe  must 
obviously  be  limited  to  that  of  the  crust,  though  by  inference  we  may 
eventually  reach  highly  probable  conclusions  regarding  the  constitution 
of  the  interior.  Chemical  research  has  discovered  that  some  sixty-four  ^ 
simple  or  as  yet  undecomposable  bodies,  called  elements,  in  various  pro- 
portions and  compounds,  constitute  the  accessible  i^art  of  the  cnist.  Of 
these,  however,  the  great  majority  are  comparatively  of  rare  occurrence. 
The  crust,  so  far  as  we  can  examine  it,  is  mainly  ])uilt  up  of  about  sixteen 
elements,  which  may  be  arranged  in  the  two  following  groups,  the  most 
abundant  bodies  being  placed  first  in  each  list : — 

^  I  have  touched  on  this  question  in  my  Presidential  Addre.^s  to  the  British  Association 
1892.     But  see  a  i^aper  by  Mr.  Clarence  King,  Ain^r.  Journ,  Sci.  xlv.  (1893). 

-  This  number  has  within  the  last  few  years  been  increased  by  the  allege*!  discovery  of 
no  fewer  than  fourteen  new  metals.  Some  of  the.se  Ixxlies,  however,  have  not  yet  been 
satisfactorily  proved  to  be  new.     T.  S.  Humpidge,  yatitre^  xxii.  p.  232. 


62  GEOGNOSY  book  u 

magmas  co- existing  in  the  earth's  crust,  the  one  beneath  the  other, 
according  to  their  specific  gravities.  The  upper  or  outer  shell,  which  he 
termed  the  acid  or  siliceous  magma,  contains  an  excess  of  silica,  and  has 
a  mean  density  of  2*65.  The  lower  or  inner  shell,  which  he  called  the 
basic  magma,  has  from  six  to  eight  times  more  of  the  earthy  bases  and 
iron-oxides,  with  a  mean  density  of  2*96.  To  the  former  he  assigned 
the  early  plutonic  rocks,  granite,  felsite,  &c.,  with  the  more  recent 
trachytes ;  to  the  latter  he  relegated  all  the  heavy  lavas,  basalts,  diorites, 
v^c.  The  ratio  of  silica  is  7  in  the  acid  magma  to  5  in  the  basic 
Though  the  proportion  of  silicic  acid  or  of  the  earthy  and  metallic  bases 
cannot  be  regarded  as  any  certain  evidence  of  the  geological  date  of 
rocks,  nor  of  their  probable  depth  of  origin,  it  is  nevertheless  a  fact  that 
(with  many  important  exceptions)  the  eruptive  rocks  of  the  older  geo- 
logical periods  are  very  generally  super-silicated  and  of  lower  specific 
gravity,  while  those  of  later  time  are  very  frequently  poor  in  silica,  but 
rich  in  the  earthy  bases  and  in  iron  and  manganese,  with  a  consequent 
higher  specific  gravity.  The  latter,  according  to  Durocher,  have  been 
forced  up  from  a  lower  zone  through  the  lighter  siliceous  crust  The 
sequence  of  volcanic  rocks,  as  first  announced  by  Eichthofen,  has  an 
interesting  connection  with  this  speculation.^ 

The  main  mass  of  the  earth's  crust  is  composed  of  a  few  predominant 
compounds.  Of  these  in  every  respect  the  most  abundant  and  important 
is  Silicon-dioxide  or  Silica  (Kieselerde)  SiO^.  As  the  fundamental  in- 
gredient of  the  mineral  kingdom,  it  forms  more  than  one  half  of  the 
known  crust,  which  it  seems  to  bind  firmly  together,  entering  as  a  main 
ingredient  into  the  composition  of  most  crystalline  and  fragmental  rocks 
as  well  as  into  the  veins  that  traverse  them.  It  occurs  in  the  free  state 
as  the  abundant  rock -forming  mineral  quartz,  which  strongly  resists 
ordinary  decay,  and  is  therefore  a  marked  constituent  of  many  of  the 
more  enduring  kinds  of  rock.  As  one  of  the  acid-forming  oxides  (H^SiO^, 
Silicic  acid,  Kieselsaure)  it  forms  combinations  with  alkaline,  earthy,  and 
metallic  bases,  which  appear  as  the  prolific  and  universally  diffused 
family  of  the  silicates.  Moreover,  it  is  present  in  solution  in  terrestrial 
and  oceanic  waters,  from  which  it  is  deposited  in  pores  and  fissures  of 
rocks.  It  is  likewise  secreted  from  these  waters  by  abundantly  diffused 
species  of  plants  and  animals  (diatoms,  radiolarians,  &c.)  It  has  been 
largely  eftective  in  replacing  the  organic  textures  of  former  organisms, 
and  thus  preserving  them  as  fossils. 

Alumina  or  aluminium-oxide  (Thonerde),  AlyO^,  occurs  sparingly  as 
corundum,  which,  however,  according  to  F.  A.  Genth,  was  the  original 
condition  of  many  now  abundant  complex  aluminous  minerals  and  rocks. 
The  most  common  condition  of  aluminium  is  in  union  with  silica.  In 
this  form  it  constitutes  the  basis  of  the  vast  family  of  the  aluminous 
silicates,  of  which  so  large  a  portion  of  the  crystalline  and  fragmental 
rocks  consists.  Exposed  to  the  atmosphere,  these  silicates  lose  some  of 
their  more  soluble  ingredients,  and  the  remainder  forms  an  earth  or  clay 
consisting  chiefly  of  silicate  of  aluminium. 

*  Postea,  Book  III.  Part  I.  Section  i.  §  6. 


PART  n  §  i  COMPOSITION  OF  THE  CRUST  63 

Carbon  is  the  fundamental  element  of  organic  life.  In  combination 
with  hydrogen,  as  well  as  with  oxygen,  nitrogen  and  sulphur,  it  forms 
the  various  kinds  of  coal,  and  thus  takes  rank  as  an  important  rock- 
forming  element  As  carbon-dioxide,  CO,^  it  is  present  in  the  air,  in 
rain,  in  the  sea  and  in  ordinary  terrestrial  waters.  This  oxide  is  soluble 
in  water,^  giving  rise  then  to  a  dibasic  acid  termed  Carbonic  Acid 
(Kohlensaure),  CO(OH)o  or  HgCOg,  which  forms  carbonates,  its  combina- 
tion with  calcium  having  been  instrumental  in  the  formation  of  vast 
masses  of  solid  rock.  Carbon-dioxide  constitutes  a  fifth  part  of  the 
weight  of  ordinary  limestone. 

Sulphur  (Soufre,  Schwefel)  occurs  uncombined  in  occasional  deposits 
like  those  of  Sicily  and  Naples,  to  be  afterwards  described,  also  in  union 
with  iron  and  other  metals  as  sulphides ;  but  its  principal  condition 
as  a  rock -builder  is  in  combination  with  oxygen  as  sulphuric  acid 
(Schwefelsiiure)  H2S0^  which  forms  sulphates  of  lime,  magnesia,  &c. 

Calcium  enters  into  the  composition  of  many  crystalline  rocks  in 
combination  with  silica  and  with  other  silicates.  But  its  most  abundant 
form  is  in  union  with  carbon-dioxide,  when  it  ap|)ears  as  the  mineral, 
calcite  (CaCOg),  or  the  rock,  limestone.  Calcium -carbonate,  being 
soluble  in  water  containing  carbonic  acid,  is  one  of  the  most  universally 
diffused  mineral  ingredients  of  natural  waters.  It  supplies  the  varied 
tribes  of  mollusks,  corals,  and  many  other  invertebrates  with  mineral 
substance  for  the  secretion  of  their  tests  and  skeletons.  Such  too  has 
been  its  office  from  remote  geological  periods,  as  is  shown  by  the  vast 
masses  of  organically-formed  limestone,  which  enter  so  conspicuously  into 
the  structure  of  the  continents.  In  combination  with  sulphuric  acid, 
calcium  forms  important  beds  of  gypsum  and  anhydrite. 

Magnesium,  Potassium,  and  Sodium  play  a  less  conspicuous  but  still 
essential  part  in  the  composition  of  the  earth's  crust.  Magnesium,  in 
combination  with  silica,  forms  a  class  of  silicates  of  prime  importance  in 
the  composition  of  volcanic  and  metamorphic  rocks.  As  a  carbonate,  it 
unites  with  calcium-carbonate  to  form  the  widely  diftused  rock,  dolomite. 
In  union  with  chlorine,  it  takes  a  prominent  place  among  the  salts  of  sea- 
water.  Potassium  or  Sodium,  combined  with  silica,  is  present  in  small 
quantity  in  most  silicates.  In  union  with  chlorine,  as  common  salt, 
sodium  is  the  most  important  mineral  ingredient  of  sea-water,  and  can  be 
detected  in  minute  quantities  in  air,  rain,  and  in  terrestrial  waters.  In 
the  old  chemical  formulae  hitherto  employed  in  mineralogy  the  metals  of 
the  alkalies  and  alkaline  earths  are  represented  as  oxides.  Thus  lime 
(calcium-monoxide),  soda  (sodium-monoxide),  potash  (ix)tassium-monoxide), 
magnesia  (magnesium-oxide),  are  denoted  as  in  union  with  carbonic  acid, 
sulphuric  acid,  silica,  &c.,  forming  carbonates,  sulphates,  silicates  of  lime, 
soda,  &c. 

Iron  and  Manganese  are  the  two  most  common  heavy  metals,  occurring 
both  in  the  form  of  ores,  and  as  constituents  of  rocks.  Iron  is  the  great 
pigment  of  natm*e.     Its  peroxide  or  sesquioxide,  now  known  as  ferric 

*  One  volume  of  water  at  0*  C.  dissolves  1*7967  volumes  of  carbon-dioxide ;  at  15"  C 
the  amount  is  reduced  to  1*0020  volumes. 


64  GEOrrNOSY  wmjh  ii 


oxide,  forms  large  ininera]  masses,  and  together  with  the  protoxide  or 
ferrous  oxide,  occurs  in  smaller  or  larger  proportions  in  the  great  majority 
of  crystalline  rocks.  Iron  (as  sulphate  or  in  combination  with  organic 
acids)  is  removed  in  solution  in  the  water  of  springs,  and  precipitated  as 
a  hydrous  |)eroxide.  Manganese  is  commonly  associate<l  with  iron  in 
minute  pro}>ortion8  in  igneous  i*ocks,  and  being  similarly  removed  in 
solution  in  water,  is  thrown  down  as  bog  manganese  or  wad. 

Silicic  Acid,  Carbonic  Acid,  and  Sulphuric  Acid  are  the  three  acids 
with  which  most  of  the  bases  that  compose  the  earth's  crust  have  been 
combined.  With  these  we  may  connect  the  water  which,  besides  merely 
percolating  through  rocks,  or  existing  enclosed  in  the  vesicles  of  minerals, 
hsis  been  chemically  absorbed  in  the  process  of  hydration,  and  which  thus 
constitutes  more  than  10  or  even  20  per  cent  of  some  rocks  (gypsum). 

Chemical  analysis  has  revealed  the  numerous  combinations  in  which 
the  elements  are  united  to  form  minerals  and  rocks.  Considerable 
additional  light  has  Ixjen  thrown  on  the  subject  by  chemical  synthesis, 
that  is,  by  artificially  producing  the  minerals  and  rocks  which  are  found 
in  nature.  The  experiments  have  been  varied  indefinitely  so  as  to 
imitate  as  far  as  jwssible  the  natural  conditions  of  production.  Further 
reference  to  this  subject  will  be  found  on  pp.  89,  297  et  seq. 

Although  every  mineral  may  be  made  to  yield  data  of  more  or  less 
geological  significiince,  only  those  minerals  need  be  referred  to  here  which 
enter  as  chief  ingredients  into  the  com|)osition  of  rock-masses,  or  which 
are  of  frequent  occurrence  as  accessories,  and  special  note  may  be  taken 
of  those  of  their  characters  which  are  of  main  interest  from  a  geological 
point  of  view,  such  as  their  modes  of  occiu*rence  in  relation  to  the 
genesis  of  rocks,  and  their  weathering  as  indicative  of  the  nature  of 
rock-decom})osition. 

§  ii.  Bock-forminff  Minerals. 

Minerals,  as  constituents  of  rocks,  occur  in  four  conditions,  according 
to  the  circumstances  under  which  they  have  been  produced. 

(1.)  Crystalline,  as  {a)  more  or  less  regularly  defined  crystals,  which, 
exhibiting  the  outlines  proper  to  the  mineral  to  which  they  belong,  are 
said  to  l)e  uliomoiyhk ;  {h)  amorphous  granules,  aggi-egations  or  crystalloids, 
having  an  internal  crystalline  structure,  in  most  cases  eiisily  recognisable 
with  polarized  light,  as  in  the  quartz  of  granite,  and  an  external  form 
which  has  })een  determined  by  contact  with  the  adjacent  mineral  particles  ; 
such  crystiilline  Ixxlies  which  do  not  exhibit  their  proper  crystalline  outlines 
are  said  to  be  aUotrwnuyrphic ;  (c)  "  crystallites  "  or  "  microlites,"  incipient 
forms  of  crystiilliziition,  which  are  described  on  p.  115.  The  crystalline 
condition  may  arise  from  igneous  fusion,  aqueous  solution,  or  sublimation.^ 

(2.)  Glaniiy  or  vitremis^  as  a  natural  glass,  usually  including  either  crystals 
or  crystallites,  or  both.  Minerals  have  assumed  this  condition  from  a 
state  of  fusion,  also  from  solution.  The  glass  may  consist  of  several 
minerals  fused  into  one  homogeneous  substance.     Where  it  has  assumed  a 

^  For  the  microscopic  characters  of  minerals  auil  rockN,  see  p.  lOS. 


66  GEOGNOSY  book  n 

stone  have  formed  part  of  the  rock  ever  since  it  was  accumulated,  and 
are  its  essential  constituents.  Yet  each  of  these  once  formed  part  of  some 
older  rock,  the  destruction  of  which  yielded  materials  for  the  production 
of  the  sandstone.  The  minute  crystals  of  zircon,  rutile,  tourmaline  and 
other  minerals  so  often  found  in  sands,  clays,  sandstones,  shales  and  other 
sedimentary  deposits,  have  been  derived  from  the  degradation  of  older 
crystalline  rocks. 

The  same  mineral  may  occur  both  as  an  original  and  as  a  secondary 
constituent  Quartz,  for  example,  appears  everywhere  in  both  conditions ; 
indeed,  it  may  sometimes  be  found  in  a  twofold  form  even  in  the  same 
rock,  though  there  is  then  usually  some  difference  between  the  original 
and  secondary  quartz.  A  quartz-felsite,  for  instance,  aboimds  in  original 
little  kernels,  or  in  double  pyramids  of  the  mineral,  often  enclosing  fluid 
cavities,  while  the  secondary  or  accidental  forms  usually  occur  in  veins, 
reticulations,  or  other  irregular  aggregates. 

Accessory  minerals  frequently  occur  in  cavities  where  they  have 
had  some  room  to  crystallize  out  from  the  general  mass.  The  "  drusy  " 
cavities,  or  open  spaces  lined  with  well  developed  crystals,  found  in  some 
granites  are  good  examples,  for  it  is  there  that  the  non-essential  minerals 
are  chiefly  to  be  recognised.  The  veins  of  segi-egation  found  in  many 
crystalline  rocks,  particularly  in  those  of  the  granite  series,  are  further 
illustrations  of  the  original  separation  of  mineral  ingredients  from  the 
general  magma  of  a  rock  (see  pp.  578,  580). 

In  some  cases  minerals  assume  a  concretionary  shape,  which  may  be 
observed  chiefly  though  not  entirely  in  rocks  formed  in  water.  Some 
minerals  are  particularly  prone  to  occur  in  concretions.  Siderite  (ferrouA 
carbonate)  is  to  be  found  in  abundant  nodules,  mixed  with  clay  and 
organic  matter  among  consolidated  muddy  deposits.  Calcite  (caJcium- 
carbonate)  is  likewise  abundantly  concretionary.  Silica  in  the  forms  of 
chert  and  flint  appears  in  irregular  concretions,  in  calcareous  formations^ 
composed  mainly  of  the  remains  of  marine  organisms. 

Secondary  minerals  have  been  developed  as  the  result  of  subsequent 
changes  in  rocks,  and  are  almost  invariably  due  to  the  chemical  action  of 
percolating  water,  either  from  above  or  from  below.  Occurring  under 
circumstances  in  which  such  water  could  act  with  eff^ect,  they  are  found 
in  cracks,  joints,  Assures  and  other  divisional  planes  and  cavities  of  rocks^- 
especially  in  the  minute  interspaces  between  the  component  grains  or 
minerals.  Subterranean  channels,  frequently  several  feet  or  even 
yards  wide,  have  been  gradually  filled  up  by  the  deposit  of  mineral 
matter  on  their  sides  (see  the  Section  on  Mineral  Veins).  The  cavitiea 
formed  by  expanding  steam  in  ancient  lavas  (amygdaloids)  have  oflered 
abundant  opportunities  for  deposits  of  this  kind,  and  have  accordingly 
been  in  large  measure  occupied  by  secondary  minerals  (amygdales),  as 
calcite,  chalcedony,  quartz  and  zeolites. 

In  the  subjoined  list  of  the  more  important  rock-forming  minerals, 
attention  is  drawn  mainly  to  those  features  that  are  of  geological 
importance ;  the  physical,  chemical  and  microscopic  characters  of  these 
minerals  will  be  found  in  a  text-book  of  mineralogy  or  petrography. 


PART  n  §  ii  ^    ROCK-FORMING  MINERALS  67 


Reference  is  therefore  made  here  to  features  of  more  special  signifi- 
cance to  the  geologist,  such  as  modes  of  occurrence,  whether  original 
or  secondary ;  modes  of  origin,  whether  igneous,  aqueous,  or  organic ; 
pseudomorphs,  that  is,  the  various  minerals  which  any  given  mineral  has 
replaced,  while  retaining  their  external  forms,  and  likewise  those  which 
are  found  to  have  supplanted  the  mineral  in  question  while  in  the  same 
way  retaining  its  form — a  valuable  clue  to  the  internal  chemical  changes 
which  rocks  undergo  from  the  action  of  percolating  water  (Book  III. 
Part  IL  Section  ii.  §§  1  and  2) ;  and  lastly,  characteristics  or  peculiarities 
of  weathering,  where  any  such  exist  that  deserve  special  mention. 

1.  Nathte  elements  are  comparatively  of  rare  occurrence,  and  only  two  of  them, 
Carbon  and  Sulphur,  occasionally  play  the  part  of  noteworthy  essential  and  accessory 
eonstitaents  of  rocks.  A  few  of  the  native  metals,  more  especially  copper  and  gold,  now 
and  then  appear  in  sufficient  quantity  to  constitute  commercially  important  ingredients 
of  veins  and  rock>masses. 

Qraphiie  is  found  chiefly  in  ancient  crystalline  rocks,  as  gneiss,  mica-schist,  granite, 
kc,  ;  some  of  the  Laurentian  limestones  of  Canada  being  so  full  of  the  diffused  mineral 
as  to  be  profitably  worked  for  it ;  in  rare  instances  coal  has  been  observed  changed  into 
it  by  intrusive  basalt  (Ayrshire).  In  some  cases  graphite  results  from  the  alteration  of 
imbedded  organic  matter,  especially  remains  of  plants ;  but  its  presence,  and  that  of 
diamond,  among  ancient  crystalline  rocks  and  in  meteorites  can  hardly  be  thus 
acooonted  for.  Occasionally  it  is  observed  as  a  |)seudomorph  after  calcite  and  pyrites, 
and  sometimes  enclosing  sphene  and  other  minerals.^ 

Sulphur  occurs  1st,  as  a  product  of  volcanic  action  in  the  vents  and  fissures  of  active 
and  dormant  cones.     Volcanic  sulphur  is  formed  from  the  oxidation  of  the  sulphuretted 
hydrogen,  so  copiously  emitted  with  the  steam  that  issues  from  volcanic  vents,  as  at  the 
Soliatara,  near  Naples.     It  may  also  be  produced  by  the  mutual  decomposition  of  the 
sune  gas  and  anhydrous  sulphuric  acid.     2nd,  in  beds  and  layers,  or  diff'used  particles, 
resulting  from  the  alteration  of  previous  minerals,  |>articularly  sulphates,  or  from  deposit 
in  water  through  decom|)osition  of  sulphuretted  hydrogen.     The  frequent  crystallization 
of  sulphur  shows  that  the  mineral  must  have  been  formed  at  ordinary  temi>eratures,  for 
its  natural  crystals  melt  at  238*1*'  Fahr.     Its  formation  may  be  observed  in  progress  at 
many  sulphureous  springs,  ¥^ere  it  falls  to  the  bottom  as  a  i^ale  mud  through  the 
oxidation  of  the  sulphuretted  hydrogen  in  the  water.     It  occurs  in  Sicily,  Spain  and 
elsewhere,  in  beds  of  bituminous  limestone  and  gypsum.     These  strata,  sometimes  full 
of  remains  of  fresh-water  shells  and  plants,  are  interlaminated  with  sulphur,  the  very 
shells  being  not  infrequently  replaced  by  this  mineral.     Here  the  presence  of  the  sulphur 
may  be  traced  to  the  reduction  of  the  calcium-sulphate  to  the  state  of  sulphide,  through 
the  action  of  the  decomposing  organic  matter,  and  the  subsequent  production  and 
decomposition  of  sulphuretted  hydrogen,  with  consequent  liberation  of  sulphur.^    The 
sulphur  deposits  of  Sicily  furnish  an  excellent  illustration  of  the  alternate  deposit  of 
snlphor  and  limestone.     They  consist  mainly  of  a  marly  limestone,  through  which  the 
sulphur  is  partly  disseminated  and  partly  interstratified  in  thin  laminse  and  thicker 
layers,  some  of  which  are  occasionally  28  feet  deep.     Below  these  deposits  lie  older 
Tertiary  gypseous  formations,  the  decomi)osition  of  which  has  probably  produced  the 
deposits  of  sulphur  in  the  overlying  more  recent  lake  basins.'    The  weathering  of  sulphur 


*  Vom   Rath.    Sitzungsber.  Wien,  Akad.   x.   p.    67  ;    Sullivan  in  Jukes'   *  Manual  of 
Geology,'  3rd  edit  (1872),  p.  56. 

'  Braun,  Bull.  Soe.  Oiol,  France^  Ist  ser.  xii.  p.  171. 
»  Memarie  del  R,  Comitalo  Geologico  d' Italia,  i.  (1871). 


68  GEOGNOSY  book  ii 

is  exemplified  on  a  considerable  scale  at  these  Sicilian  deposits.  The  mineral,  in 
presence  of  limestone,  oxygen  and  moisture,  becomes  sulphuric  acid,  which,  combining 
with  the  limestone,  forms  gyjisum,  a  curious  return  to  what  was  probably  the  original 
substance  from  the  decom|)osition  of  which  the  sulphur  was  derived.  Hence  the  site  of 
the  outcrop  of  the  sul])hur  beds  is  marked  at  the  surface  by  a  white  earthy  rock,  or 
borscale,  which  is  regarded  by  the  miners  in  Sicily  to  be  a  sure  indication  of  snlphur 
underneath,  as  the  gossan  of  Cornwall  is  indicative  of  underlying  metalliferous  veins.* 

Iron,  the  most  imjtortant  of  all  the  metals,  is  found  only  sparingly  in  the  native 
state,  in  blocks  which  have  fallen  as  meteorites,  also  in  grains  or  dust  enclosed  in 
hailstones,  in  snow  of  the  Alps,  Sweden  and  Siberia,  in  the  mud  of  the  ocean  floor  at 
remote  distances  from  land,  and  in  some  eruptive  rocks.  Tliere  can  be  no  doubt  that  a 
small  but  constant  supply  of  native  iron  (cosmic  dust)  is  falling  upon  the  earth's  sur£M)e 
from  outside  the  terrestrial  atmosphere.^  This  iron  is  alloyed  with  nickel,  and  contains 
small  quantities  of  cobalt,  copper  and  other  ingredients.  Dr.  Andrews,  however, 
showed  in  1852  that  native  iron,  in  minute  spicules  or  granules,  exists  in  some  basalts 
and  other  volcanic  rocks, ^  and  Mr.  J.  Y.  Buchanan  has  detected  it  in  appreciable 
quantity  in  the  gabbro  of  the  west  of  Scotland.  It  occurs  also  in  the  basalts  of  Bohemia 
and  Greenland.** 

In  the  great  majority  of  eases  the  Oxides  occur  combined  with  some  acid.  A  few 
uncombined  take  a  prominent  place  as  essential  constituents  or  frequent  ingredients  of 
rocks,  especially  the  oxides  of  silicon  and  iron. 

2.  Silica  (SiOa)  is  found  in  three  chief  forms,  Quartz,  Tridymite,  and  Opal. 

Quartz  is  abundant  as  (1)  an  essential  constituent  of  rocks,  as  in  granite,  gneiss, 
mica-schist,  rhyolite  (quartz-trachyt*,)  quartz-poq)hyry,  sandstone ;  (2)  a  secondaiy 
ingredient,  wholly  or  ])artially  tilling  veins,  joints,  cracks,  and  cavities.     It  has  been 


*  Jotirn.  Soc.  Art^j  1873,  p.  170.  E.  Lcdoux,  Ann.  dcs  MineSy  7™*  s<Jr.  vii.  p.  1.  The 
Sicilian  sulphur  beds  belong  to  the  Oeningen  stage  of  the  Upper  Tertiary  deposits.  They 
coutain  numerous  plants  and  some  iusects.  H.  T.  Geyler,  PulaonUtffraphica,  xxiii.,  Zi-c^. 
9,  p.  317.     Von  Lasaulx,  Xeties  Jahrb,  1879,  p.  490. 

-  See  Ehrenl^erg,  Frorieps  Xotizenj  Feb.  1846  ;  Nordenskiold,  Comptes  renduSj  IxxviL 
1».  463,  Ixxviii.  p.  236.  Tissandier,  op.  cit.  Ixxriii.  p.  821,  Ixxx.  p.  58,  Ixxxi.  p.  676.  See 
Ixxv.  (1872)  p.  683.  Yung,  Ball,  Soc.  Vaudoise  Set.  Nat.  (1876),  xiv.  p.  493.  Ranyard, 
Monthly  Not.  Roy.  Astron.  Soc  xxxix.  (1879)  p.  161.  T.  L.  Phipson,  Comptes  rend. 
Ixxxiii.  p.  364.  A  Committee  of  the  British  Association  was  appointed  in  1880'to  investi- 
gate the  subject  of  cosmic  dust.     See  its  reports  for  1881-83. 

•*  Brit.  Assoc.  Rep.  1852,  postea,  p.  457. 

*  Nordenskiold  describes  fifteen  blocks  of  iron  on  the  island  of  Disco,  Greenland,  the 
weight  of  the  two  largest  })eiug  21,000  and  8000  kilogrammes  (20  and  8  tons,  respectively). 
He  observed  that  at  the  same  locality,  the  underlying  basalt  contains  lenticular  and  disc- 
shax>ed  blocks  of  precisely  similar  iron,  and  inferred  that  the  whole  of  the  blocks  may  belong 
to  a  meteoric  shower  which  fell  during  the  time  (Tertiary)  when  the  basalt  was  poured  ont 
at  the  surface.  He  dismissed  the  suggestion  that  the  iron  could  possibly  be  of  tellnric 
origin  {Ocol.  Mag.  ix.  (1872)  p.  462).  But  the  microscope  reveals  in  this  basalt  the 
presence  of  minute  particles  of  native  iron  which,  associated  with  viridite,  ore  moulded  round 
the  crystals  of  labradorite  and  augite  (Fouquc  and  Michel-Levy,  op.  cit.  p.  443).  Steen- 
strup,  Daubr^e,  and  others  appear  therefore  to  be  justified  in  regarding  this  iron  as  derived 
from  the  inner  metallic  portions  of  the  globe,  which  lie  at  depths  inaccessible  to  our 
ob8t|r\'ations,  but  from  which  the  vast  Greenland  basalt  eruptions  have  brought  up  traces  to 
the  surface  (K.  J.  T.  Steenstrup,  Vid.  Medd.  Nat.  Foren.  Copenhagen  (1875)  No.  16-19, 
p.  284  ;  Zeitsch.  Deutsch.  Ged.  Ges.  xxriii.  (1876)  p.  225  ;  Mineralog.  Mag.  July,  1884. 
F.  Wohler,  Neties  Jahrb.  1879,  p.  832.  Daubree,  Discours  Acad.  Set.  1  March  1880,  p. 
17.     W.  Flight,  Geol.  Mag.  ii.  (2nd  ser.)  p.  152. 


PART  II  §  ii  ROCK-FORMING  MINERALS  69 


produced  from  (a)  igneous  action,  as  in  volcanic  rocks ;  {b)  aquo-igneous  or  plutonic 
action,  as  in  granites,  gneisses,  &c. ;  (c)  solution  in  water,  as  where  it  lines  cavities  or 
replaces  other  minerals.  The  last  mode  of  formation  is  that  of  the  crystallized  quartz 
and  chalcedony  found  as  secondary  ingredients  in  rocks. 

The  study  of  the  endomorphs  and  pseudomorphs  of  quartz  is  of  great  importance  in 
the  investigation  of  the  history  of  rocks.  No  mineral  is  so  conspicuous  for  the  variety 
of  other  minerals  enclosed  within  it.  In  some  secondary  quartz-crystals,  each  prism 
forms  a  small  mineralogical  cabinet  enclosing  a  dozen  or  more  distinct  minerals,  as 
mtile,  haematite,  limonite,  pyrites,  chlorite,  and  many  others.^  Quartz  may  be 
obserred  replacing  calcite,  aragonite,  siderite,  gypsum,  rock-salt,  haematite,  &c.  This 
facility  of  replacement  nuikes  silica  one  of  the  most  valuable  petrifying  agents  in  nature. 
Organic  bodies  which  have  been  silicified  retain,  often  with  the  utmost  i)erfection,  their 
minutest  and  most  delicate  structures. 

Quartz  may  usually  be  identified  by  its  external  character,  and  esjiecially  by  its 
vitreous  lustre  and  hardness.  When  in  the  form  of  minute  blebs  or  crystals,  it  may  1)e 
recognised  in  many  rocks  with  a  good  lens.  Under  the  microscope,  it  presents  a 
characteristic  brilliant  chromatic  jiolarization,  and  in  convergent  light  gives  a  black 
cross.  Where  it  is  an  original  and  essential  constituent  of  a  rock,  quartz  very  commonly 
contains  minute  rounded  or  irregular  cavities  or  pores,  jtartially  filled  with  liquid.  So 
minute  are  these  cavities  that  a  thousand  millions  of  them  may,  when  they  are  closely 
aggregated,  lie  within  a  cubic  inch.  Tlie  liquid  is  chiefly  water,  not  uncommonly 
containing  sodium  chloride  or  other  salt,  sometimes  liquid  carbon -dioxide  and  hydro- 
carbons.'   Chalcedony  exhibits  under  the  microscope  a  minute  radial  fibrous  structure. 

Rock-crystal  and  crystalline  quartz  resist  atmospheric  weathering  with  great  per- 
sistence. Hence  the  quartz-grains  may  usually  be  easily  discovered  in  the  weathered 
cmst  of  a  quartziferous  igneous  rock.  But  corroded  quartz-crystals  have  been  observed 
in  exposed  mountainous  situations,  with  their  edges  rounded  and  eaten  away.'  The 
chalcedonic  and  more  or  less  soluble  forms  of  silica  are  more  easily  affected.  Flint  and 
many  forms  of  coloured  chalcedony  weather  with  a  white  crust.  But  it  is  chiefly  from 
the  weathering  of  silicates  (especially  through  the  action  of  organic  acids)  that  the 
soluble  silica  of  natural  waters  is  derived.     (Book  III.  Part  II.  Section  ii.  §  7). 

Ttidyxnite  has  been  met  with  chiefly  among  volcanic  rocks  (trachytes,  andesiten, 
kc ),  both  as  an  abundant  constituent  of  those  which  have  been  poured  out  in  the  form 
of  lava,  and  also  in  ejected  blocks  (Vesuvius).'* 

Opal,  a  hydrous  condition  of  silica  formed  from  solution  ifi  water,  is  usually 
disseminated  in  veins  and  nests  through  rocks.  Semi-opal  occasionally  replaces  the 
original  substance  of  fossil  wood  (wood-opal).  Several  forms  of  o^^l  are  deposited  by 
geysers,  and  are  known  under  the  general  appellation  of  sinters.  Closely  allied  to  the 
oi«l8  are  the  forms  in  which  hydrous  (soluble)  silica  appears  in  the  organic  world,  where 
it  constitutes  the  frustules  of  diatoms,  the  skeletons  of  radiolaria,  &c.  Tiipoli  powder 
(Kieselgnhr),  randanite,  and  other  similar  earths,  are  composed  mainly  or  wholly  of  the 
remains  of  diatoms,  kc. 

Oomndiim,  aluminium- oxide,  is  found  in  crystalline  rocks,  particularly  in  certain 
serpentines  and  schists,  gneiss,  granite,  dolomite,  and  rocks  of  the  metamorphic  series. 

8.  Iron  Oxides. — Four  minerals,  coniiK>8ed  mainly  of  iron  oxides,  occur  abundantly 

*  See  Sullivan,  in  Jukes'  'Manual  of  Geologj-,'  3rd  edit.  (1872),  p.  61. 

'  See.  Brewster,  Trans,  Roy.  Soc,  Edin.  x.  p.  1.  Sorby,  Quart.  Joum.  Ged.  Soc.  xiv. 
p.  453.  Proc  Roy.  Sac.  xv.  p.  153  ;  xvii.  p.  299.  Zirkel,  *  Mikroskopische  Beschaffenheit 
der  Mineralien  und  Oesteine,'  p.  39.  Rosenbusch,  '  Mikroskopische  Physiographie,'  i.  ]>.  30. 
Hartley,  Joum.  Ckem.  Sac.  February,  1876.  The  occurrence  of  fluid-cavities  in  the  crystals 
of  rocks  is  more  fully  described  in  Part  II.  §  iv.  of  this  Book. 

'  Roth,  Ckem.  Ged.  L  p.  94. 

*  Vom  Rath,  Z.  DeuUch.  OeoL.  Ges.  xxv.  p.  236,  1873. 


70  GEOGNOSY  book  n 

as  essential  and  accessory  ingredients  of  rocks.  Haematite,  Limonite,  Magnetite,  and 
Titanic  iron. 

Hnmatite  (Fer  oligiste,  Rotheisen,  Eisenglanz,  Fe3O3=Fe70,  O30)  in  the  crystal- 
lized form  occurs  in  veins,  as  well  as  lining  cavities  and  fissures  of  rocks.  The 
fibrous  and  more  conmion  form  (which  often  has  portions  of  its  mass  passing  into  the 
crystallizecl  condition)  lies  likewise  in  strings  or  veins  ;  also  in  cavities,  which,  when  of 
large  size,  have  given  opportunity  for  the  deposit  of  great  masses  of  hfematite,  as  in 
cavernous  limestones  (Westmoreland).  It  occurs  with  other  ores  aifd  minerals  as  an 
abundant  component  of  mineral  veins,  likewise  in  beds  interstratified  with  sedimentaxy 
or  schistose  rocks.  Scales  and  specks  of  opaque  or  clear  bright  red  haematite,  of  frequent 
occurrence  in  the  crystals  of  rocks,  give  them  a  reddish  colour  or  i)eculiar  lustre  (perthite, 
stilbite).  Haematite  appears  abundantly  as  a  product  of  sublimation  in  clefts  of  volcanic 
cones  and  lava  streams.  It  is  probably  in  most  cases  a  deposition  from  water,  resulting 
from  the  alteration  of  some  previous  soluble  combination  of  the  metal,  such  as  the 
oxidation  of  the  sulphate,  and  occurs  in  veins  and  beds,  and  as  the  earthy  pigment  that 
gives  a  red  colour  to  sandstones,  clays  and  other  rocks.  It  is  found  pseudomorphons 
after  ferrous  carltonate,  and  this  has  pro1)ably  been  the  origin  of  beds  of  red  ochre  occa- 
sionally intercalated  among  stratified  rocks.  It  likewise  replaces  calcite,  dolomite, 
quartz,  barytes,  pyrites,  magnetite,  rock-salt,  fluor-spar,  &c. 

Limonite  (BroiiTi  iron-ore,  2FesO, -f- SH^O  =  Fe203  85-56,  H2O  14-44)  occurs  in  beds 
among  stratified  formations,  and  may  be  seen  in  the  course  of  deposit,  through  the  action 
of  organic  acids,  on  marsh-land  (l>og-iron-ore)  and  lake-bottoms.  (Book  IV.  Part  XL 
Section  iiL )  In  the  form  of  yellow  ochre,  it  is  precipitated  from  the  waters  of  chalybeate 
springs  containing  green  vitriol  derived  from  the  oxidation  of  iron -sulphides.^  It  is  a 
common  decomposition  product  in  rocks  containing  iron  among  their  constituents.  It  is 
thus  always  a  secondary  or  derivative  substance,  resulting  from  chemical  alteration.  It 
is  the  usual  pigment  which  gives  tints  of  yellow,  orange  and  broiMi  to  rocks.  The 
{iseudomorphous  forms  of  limonite  show  to  what  a  large  extent  combinations  of  iron  aie 
carried  in  solution  through  rocks.  The  mineral  has  been  foimd  replacing  calcite,  siderite, 
dolomite,  haematite,  magnetite,  pyrite,  niarcasite,  galena,  blende,  gypsum,  barytes, 
fluor-sj>ar,  pyroxene,  quartz,  garnet,  beryl,  &c. 

Magnetite  (Fer  oxydule,  Magneteisen,  FezOi)  occurs  abundantly  in  some  schists,  in 
scattered  octohedral  crystals  ;  in  crystalline  massive  rocks  like  granite,  in  diffused  grains 
or  minute  crystals  ;  among  some  schists  and  gneisses  (Non\'ay  and  the  eastern  states  of 
North  America),  in  massive  beds ;  in  basalt  and  other  volcanic  rocks,  as  an  essential 
constituent,  in  minute  octohedral  crystals,  or  in  granules  or  crystallites.  It  is  likewise 
found  as  a  pscudomorphous  secondary  pro<luct,  resulting  from  the  alteration  of  some 
previous  mineral,  as  olivine,  haematite,  j)yrite,  quartz,  hornblende,  augite,  garnet  and 
sj>hpnc.  It  occurs  AN-ith  hapmatite,  Ac,  as  a  product  of  sublimation  at  volcanic  foci,  where 
chlorides  of  the  metals  in  presence  of  st«am  are  resolved  into  hydrochloric  acid  and 
anhydrous  oxides.  It  may  thus  result  from  either  wpieous  or  igneous  operations.  It 
is  liable  to  weather  by  the  reducing  effects  of  <lecomi)08ing  organic  matter,  whereby  it 
heconies  a  carbonate,  and  then  by  ex}K)sure  jwusses  into  the  hydrous  or  anhydrous 
I»eroxide.  Tlie  magnetite  grains  of  basalt-rocks  are  very  generally  oxidized  at  the 
surface,  and  sometimes  even  for  some  depth  inwanl. 

Titanic  Iron  (Titanifcrous  Iron,  !Meuaccanit(^  Ilmenite,  Fer  titan^,  Titaneisen 
(FeTi)203)  occurs  in  scattered  grains,  ]>lates  and  crystals  as  an  abundant  constituent  of 
many  cr^'stalline  ixxiks  (l>asalt-rocks,  diabase,  gabbro  and  other  igneous  masses) ;  also  in 
veins  or  l)e<ls  in  syenite,  serjKmtine  and  metamorphic  rocks  ;  ^  scarcely  to  be  distinguished 
from  magnetite  when  seen  in  small  jjaiticles  under  the  mi(?roscoj)e,  but  i)osses8ing  a 

'  Sullivan,  Jukes'  'Manual  of  Geology,'  p.  63. 

"^  Some  of  the  Canadian  mosses  of  this  mineral  are  90  feet  thick  and  many  yards  in 
length. 


PART  II  §  ii  ROCK-FORMING  MINERALS  71 

brown  aemi-metallic  lustre  with  reflected  light ;  resists  corrosion  by  acids  when  the 
powder  of  a  rock  containing  it  is  exposed  to  their  action,  while  magnetite  is  attacked 
and  diBsolyed.  Titanic  iron  frequently  resists  weathering,  so  that  its  black  glossy 
granules  project  from  a  weathered  surface  of  rock.  In  other  cases,  it  is  decomposed 
either  by  oxidation  of  its  protoxide,  when  the  usual  brown  or  yellowish  colour  of  the 
hydrous  ferric  oxide  appears,  or  by  removal  of  the  iron.  The  latter  is  believed  to  be 
the  origin  of  a  peculiar  milky  white  opaque  substance,  frequently  to  be  observed  under 
the  microscope,  surrounding  and  even  replacing  crystals  of  titanic  iron,  and  named 
Leocoxene  byGiimbeL^  In  other  cases  the  decomposition  has  resulted  in  the  production 
of  sphene. 

Cbromite  (FeCrjOi)  occurs  in  black  opaque  grains  and  crystals  not  infrequently  in 
altered  olivine-rocks. 

SpiiiAU,  a  group  of  minerals,  may  be  taken  here.  Tliey  are  closely  related  to  each 
other,  having  cubic  forms  and  varying  in  composition  from  magnetite  (see  above)  at  the 
one  end  to  spinel  (}igA\fi^)  at  the  other.  They  are  not  infrequent  as  minute  grains  or 
crystals  in  some  igneous  and  metamorphic  rocks.  Between  magnetite  and  spinel  come 
intermediate  varieties,  as  chroniUe  (see  above),  Picotite^  Hercynite  and  Pleonaste. 

4.  Manganese  Oxides  are  frequently  associated  with  those  of  iron  in  ordinary  rock- 
forming  minerals,  but  in  such  minute  proportions  as  to  have  been  generally  neglected  in 
analyses.  Their  presence  in  the  rocks  of  a  district  is  sometimes  show^  by  deposits  of 
the  hydrous  oxide  in  the  forms  of  Psilomelane  (H2Mn04+H20),  and  Wad  (Mn03+ 
MnO  +  H3O).  These  deposits  sometimes  take  place  as  black  or  dark  brown  branching, 
plant-like  or  dendritic  impressions  between  the  divisional  planes  of  close-grained  rocks 
(limestone,  felsite,  &c.),  sometimes  as  accumulations  of  a  black  or  brown  earthy  substance 
in  hollows  of  rocks,  occasionally  as  deposits  in  mai'shy  places,  like  those  of  bog-iron-ore, 
and  abundantly  on  some  parts  of  the  sea-floor.     (See  p.  458.) 

5.  Silicates. — These  embrace  by  far  the  largest  and  most  imiwrtant  series  of  rock- 
forming  minerals.  Their  chief  groups  are  the  anhydrous  aluminous  and  magnesian 
silicates  embracing  the  Felspars,  Hornblendes,  Augites,  Micas,  &c.,  and  the  Iiydrous 
silicates  which  include  the  Zeolites,  Clays,  talc,  chlorite,  ser])entiue,  &e. 

The  family  of  the  Felspars  forms  one  of  the  most  important  of  all  the  constituents 
of  rocks,  seeing  that  its  members  constitute  by  much  the  largest  portion  of  the  plutonic 
and  volcanic  rocks,  are  abundantly  present  among  many  crystalline  schists,  and  by  their 
decay  have  supplied  a  gi-eat  i>art  of  the  clay  out  of  which  argillaceous  sedimentary  forma- 
tions have  been  constructed. 

The  felspars  are  usually  divided  into  two  series.  1st,  The  orthoclastic  or  monoclinic 
felspars,  consisting  of  two  sjtecies  or  varieties,  Orthoclase  and  Sanidine ;  and  2nd,  The 
pUgioclastic  or  triclinic  felspars,  among  which,  as  constituents  of  rocks,  may  be  men- 
tioned the  species  albite,  anorthite,  oligoclase,  andesine,  labradorite,  and  microcline. 

OrthocUM  (K3O  16-89,  ALjO,  18-43,  SiOa  64-68)  occurs  abundantly  as  an  original 
constitnent  of  many  crystalline  r«cks  (granite,  syenite,  felsite,  gneiss,  &c.),  likewise  in 
cavitiefi  and  veinings  in  which  it  has  segregatefl  from  the  surrounding  mass  (pegmatite) ; 
seldom  found  in  unaltered  sedimentary  rocks  except  in  fragments  derived  from  old 
crystalline  masses ;  generally  associated  with  quartz,  and  often  Mith  hornblende,  while 
the  felspars  less  rich  in  silica  more  rarely  accomjtany  free  quartz.  It  is  an  original  con- 
stitnent  of  plutonic  and  old  volcanic  rocks  (granite,  felsite,  &c.),  and  of  gneiss  and  various 
schists.  A  few  examples  have  been  noticed  where  it  has  replaced  other  minerals  (prehnite, 
analcime,    laumontite).     Under  the   microsco[)e  it  is  recognisable   from  quartz  by  its 

*  *  Die  PalSolitiache  Eruptivgesteine  des  Fichtelgebirges,'  1874,  p.  29.  See  Rosenbusch, 
Mik.  Physiog,  iL  p.  836.  De  la  Vallee  Poussin  and  Renanl,  Mem,  Couronnks  Acad.  Roy. 
de  Bdffique^  1876,  xl.  Plate  vi.  pp.  34  and  35.  Fouque  and  Michel-Levy,  •  Mineralogie 
Micrograph,*  p.  426.     See  postea,  p.  618. 


72  GEOGNOSY  book  ii 

eharactoiistic  i-ectaiigular  fonns,  cleavage,  t\«'iiming,  angle  of  extinction,  turbidity,  and 
frequent  alteration.*  Oithoclase  weathers  on  the  whole  with  comparative  rapidity, 
though  durable  vaneties  are  known.  Tlie  alkali  and  some  of  the  silica  are  removed, 
and  the  minei-al  jtasses  into  clay  or  kaolin  (p.  77). 

S  a  n  i  d  i  n  c,  the  clear  glassy  fissured  variety  of  orthoclase  so  conspicuous  in  the  more 
silicated  Tertiary  and  modem  lavas,  occurs  in  some  trachytes  in  large  flat  tables  (hence 
the  name  "sanidine") ;  more  commonly  in  fine  clear  or  grey  crystals  or  crystalline 
gi-anules  ;  an  eminently  volcanic  mineral. 

Plagioclase  (Triclinic)  Felspars. — While  the  different  felsi>ars  which  crystallize  in 
the  triclinic  system  may  be  more  or  less  easily  distinguished  in  large  cr^'stals  or 
crystalline  aggregates,  they  ai-e  difficult  to  se})arate  in  the  minute  forms  in  which  they 
conmionly  occur  as  rock  constituents.  Tliey  have  been  gi*ou])ed  by  ];>etrographers  under 
the  general  name  Plagioclase  (with  oblique  cleavage),  proposed  by  Tschermak,  who 
regards  them  as  mixtures  in  vaiious  projjortions  of  two  fundamental  comi>ounds — albite 
or  so<la-fels[)ar,  and  anorthite  or  lime-felsi>ar. 

They  occur  mostly  in  well-develoj^ed  ciystals,  jwtrtly  in  irregular  crj'stalline  gn&ins, 
crystallites  or  microlites.  On  a  fresh  frat^ture,  their  crystals  often  a]q)ear  as  clear 
glassy  striiKH,  on  which  may  usually  be  detected  a  fine  ]>arallel  lineation  or  ruling, 
indicating  a  characteristic  polysynthetic  twinning  which  never  a])]>ears  in  orthoclase. 
A  felspar  striated  in  this  manner  can  thus  be  at  once  pronounced  to  be  a  triclinic  form, 
though  the  distinction  is  not  invariably  present.  Under  the  microscojie,  the  fme 
] parallel  lamellation  or  stri])ing,  best  seen  with  [solarized  light,  forms  one  of  the  most 
distinctive  features  of  this  group  of  felspars.  The  chief  tiiclinic  felsj^ars  are,  Microcline 
([lotash- felspar,  K^Al-jSi^Oje),  which  occurs  in  gi*anites,  jjaitieularly  as  the  common 
fels{)ar  of  the  graphic  varieties  ;  also  in  some  gneisses,  &c.  ;  Albite  (soda-fels^tar,  NasO, 
11-82,  Al-jOs  18'56,  Si.jO  68'62),  found  in  some  granites,  and  in  several  volcanic  rocks: 
01igo<rlase  (soda-lime  and  lime-soda  felsi)ars,  Na^O  8-2,  CaO  4-8,  AljOs  23-0,  SiOj  62-8) 
occurs  in  many  granites  and  other  eruptive  rocks  ;  Andesine  (NaoO  7*7,  CaO  7*0, 
AloOs  26'6,  Si0.j  60'0)  observed  in  some  syenites,  &c.  ;  Labradoiite  (XajO  4 '6,  CaO 
12-4,  AI2F3  30-2,  SiOo  52'9),  an  essential  constituent  of  many  lavas,  Ac,  abundant  in 
masses  in  the  azoic  rotrks  of  Canada,  &c.  ;  Anorthite  (lime-felsjwir,  CaO  20*10,  AljO, 
36'82,  SiOo  43«08)  found  in  many  volcanic  rocks,  sometimes  in  gi-anites  and  metamori>hic 
rocks. 

Tlie  triclinic  felsi»ars  have  l>een  ])roduced  sometimes  directly  from  igneous  fusion,  as 
can  bo  studied  in  many  lavas,  whei-e  often  one  of  the  fii-st  minei-als  to  apitear  in  the 
devitrification  of  the  original  molten  glass  has  been  the  labradorite  or  other  ]>lagioclas(i. 
In  other  cases,  they  have  resulted  from  the  ojHjration  of  the  jn-ocesses  tx)  which  the  fonuation 
of  the  crystalline  schists  was  due  ;  large  beds  as  well  as  abundant  diffused  strings, 
veinings,  and  ciystals  of  triclinic  felsjiar  (labradorite)  fonu  a  markerl  feature  among  tlie 
ancient  gneisses  of  Eastern  Canada.  The  more  highly  silicated  si>ecies  (albite,  oligoclase) 
(Kcur  with  orthoclase  as  essential  (Constituents  of  many  gi'anites  and  other  plutonic  rocks. 
The  more  basic  forms  (labiwlorite,  anorthite)  are  generally  absent  where  free  silica  is 
present ;  but  occur  in  the  moi-e  basic  igneous  rocks  (basalts,  &c.) 

Considerable  differences  are  pi-esented  by  the  triclinic  fels^virs  in  regard  to  weathering. 
On  an  exposed  face  of  rock  they  lose  their  glassy  lusti^e  and  l»ecome  white  and  opa<|ue. 
Tliis  change,  as  in  orthoclase,  arises  from  loss  of  bases  and  silica,  and  from  hydration. 
Traces  of  carbonates  may  often  b<»  oliserved  in  weathered  crystals.  Tlio  oiiginal  steam 
cavities  of  old  volcanic  rocks  have  generally  been  filled  with  infiltrated  mineiBls,  which 
in  many  cases  have  resulted  from  the  weatheiing  and  decomiKisition  of  tlie  triclinic 
felspars.  Calcite,  pi-elinite,  and  the  fannly  of  zeolites  have  lK»en  abundantly  i^roduced  in 
this  way.     The  student  will  usually  obsei-ve  that  where  these  minerals  aliound  in  the 

*  On  niicroscoi)ic  determination  of  felsjiars,  see  Fouqut*  and  Michel-Levy,  op,  cU.  pp. 
209,  227,  andi;(M/«/,  pp.  94-90. 


PART  n  §  ii  EOCK'FORMIXG  MINERALS  73 


cells  and  crevices  of  a  rock,  the  rock  itself  is  for  the  most  i»art  proi»ortionately  deconi- 
l>osed,  showing  the  relation  that  subsists  between  iutiltration-products  and  the  decom- 
position of  the  surrounding  mass.  Abundance  of  calcite  in  veins  and  cavities  of  a  fel- 
s]iathic  rock  affords  good  ground  for  8usi>ecting  the  ju'esence  in  the  latter  of  a  lime 
felspar.^     (See  under  **Albitization," />oa^a,  p.  618.) 

Saussurite,  formerly  described  as  a  distinct  mineral  S]>ecies,  is  now  found  to  be  the 
result  of  the  decomposition  of  felsiMirs,  which  have  thus  acquired  a  dull  white  asjiect  and 
contain  secondary  crystallizations  (zoisite)  out  of  the  decomiK^sed  substance  of  the 
original  felspar.  Such  saussuritic  felsjiara  occur  in  varieties  of  gabbro  and  diorite.  Under 
the  microscope  they  present  a  confused  aggregate  of  crystalline  needles  and  granules 
imbedded  in  an  amoriihous  matrix.    (See  postca,  p.  618.) 

Lsneite  (K]0  21-53,  AlgO^  23*50,  SiOa  54-97)  is  a  markedly  volcanic  mineral,  occiu-- 
ring  as  an  abundant  constituent  of  many  ancient  and  modem  Italian  lavas,  and  in  some 
varieties  of  basalt.  Under  t^^e  micro8Coi)e,  sections  of  this  mineral  are  eight-sided  or 
nearly  circular,  and  very  commonly  contain  enclosures  of  magnetite,  &c.,  confoiining  in 
arrangement  to  the  external  fonn  of  the  crystal  or  disjwsed  radially. 

Hepheline  (NajO  17-04,  AI2O3  35-26,  KjO  6-46,  SiOa  41-24),  essentially  a  volcanic 
mineral,  l)eing  an  abundant  constituent  of  ])honolite,  of  some  Vesu\nan  lavas,  and  of 
some  forms  of  basalt,  presents  under  the  microscoi*  various  six-sided  and  even  four-sided 
forms,  according  to  the  angles  at  which  the  prisms  are  eut.^  Under  the  name  of  Elmolite 
are  comprised  the  greenish  or  reddish,  dull,  gi-easy-lustred,  conijiact  or  massive  varieties 
of  nepheline,  which  occur  in  some  syenites  and  other  ancient  crystalline  rocks. 

The  Mica  Family  embraces  a  nimil)er  of  minerals,  distinguished  esi»ecially  by 
their  very  perfect  basal  cleavage,  whereby  they  can  lie  split  into  remarkably  thin  elastic 
laminfe,  and  by  a  predominant  si)leiident  })early  lustre.  They  consist  essentially  of 
silicates  of  alumina,  magnesia,  iron  and  alkalies,  and  may  be  conveniently  divided  into 
two  groujis,  the  white  uiicaSy  which  are  silicates  of  alumina  with  alkalies,  iron  and  mag- 
nesia, and  the  black  micas,  in  which  the  magnesia  and  iron  jilay  a  more  con8i)icuous  part. 

MuBCOTite  (Potash-mica,  white  mica,  Glimmer,  Kfi  3-07-12-44,  Na-jO  0-4-10,  FeO 
0-1-16,  FeaO,  0-46-8-80,  MgO  0-37-308,  AI2O3  28.05-3841,  SiOa  43-47-51.73,  HaO  0-98- 
6-22),  abimdant  as  an  original  constituent  of  many  crystalline  rocks  (granite,  &c. ),  and 
as  one  of  the  characteristic  minerals  of  the  crystalline  schists  ;  also  in  many  sandstones, 
where  its  small  [parallel  flakes,  derived,  like  the  suiTounding  quartz  grains,  fiom  older 
crystalline  masses,  im]iart  a  silvery  or  "micaceous"  lustre  and  fissility  to  tlie  stone. ^ 
The  persistence  of  muscoWte  under  exjwsure  to  weather  is  shown  by  the  silver}'  plates  of 
the  mineral,  which  may  lie  detected  on  a  crumbling  surface  ofgrjiniteor  schist  where 
most  of  the  other  minerals,  save  the  quartz,  have  decayed  ;  also  by  the  fre(piency  of  the 
micaceous  lamination  of  sandstones. 

--»  Biolite  (Magnesia-mica,  black  mica,  MgO  10-30  per  cent)  occui-s  abundantly  as  an 
original  constituent  of  many  granites,  gneisses,  and  schists ;  also  sometimes  in  basalt, 
trachyte,  and  as  ejected  fragments  and  crystals  in  tufl".  Tts  small  scales,  when  cut  trans- 
verse to  the  dominant  cleavage,  may  usually  l>e  detected  under  the  microscoi)e  by  their 
remarkably  strong  dichroism,  their  fine  i>arallel  lines  of  cleavage,  and  their  frequently 
frayed  appearance  at  the  ends.  Under  the  action  of  the  weather  it  assumes  a  i>ale,  dull, 
soft  crust,  owing  to  removal  of  its  bases.  The  inineral  rufiellav,  which  occui-s  in  hexa- 
gonal brown  or  red  opaque  inelastic  tables  in  some  basalts  and  other  igneous  rocks,  is 
regarded  as  an  altered  form  of  biotite. 

*  A  valnable  essay  on  the  stages  of  the  weathering  of  triclinic  felspar  as  revealed  by  the 
microtoope  was  published  by  G.  Rose  in  1867.     Zeitsch.  Deutsch,  Ocd,  Oes.  xix.  \\  276. 

*  On  the  microscopic  distinction  between  nepheline  and  apatite,  see  Fouque  and  Michel- 
Levy,  *  Mineral.  Micrograph. '  p.  276. 

*  On  the  microscopic  determination  of  the  micas,  see  Fouqu6  and  Michel-Levy,  op,  cit. 
p.  333. 


74  GEOGNOSY  book  ii 

Phlogopit^  is  another  dark  ferro-magnesiaii  mica  wliich  contains  a  little  fluorine. 
Lepidolitc  (Lithia-mica),  o(*eur8  in  some  granites  aiid  crystalline  schists,  especially  in 
veins.  Daniourite,  merely  a  variety  of  muscovite,  occurs  among  crystalline  schists. 
ScriciU,  a  talc-like  variety  of  museovite,  occurs  in  soft  inelastic  scales  in  many  schists, 
as  a  result  of  the  alteration  of  orthoclase  felsi)ar.^  Margarodite,  a  silvery  talc-like 
hydrous  mica,  is  widely  diffused  as  a  constituent  of  granite  and  other  crystalline  rocks. 
ParayoniU,  a  scaly  micaceous  mineral,  forms  the  main  mass  of  certain  alpine  schists. 

Hornblende  (Monoclinic  Amphibole,  CaOa  10-12,  MgO  11-24,  FeaOj  0-10.  K\fi^ 
5-18,  Si02  40-50  also  usually  with  some  Na^O,  K,0  and  FeO).  Divided  into  two  grou|i6. 
1st.  Non- aluminous,  including  the  white  and  pale  green  or  grey  fibrous  varieties 
(tremolite,  actinolite,  kc, )  2nd.  Aluminous,  embracing  the  more  abundant  dark  green, 
brown,  or  black  varieties.  Under  the  microscope,  hornblende  presents  cleavage-angles 
of  124°  30',  the  definite  cleavage-planes  intersecting  each  other  in  a  well-marked  lattice 
work,  sometimes  ^ith  a  finely  fibrous  character  superadded.  It  also  shows  a  marked 
pleochroism  with  polarized  light,  which,  as  Tschermak  first  pointed  out,  usuaUy 
distinguishes  it  from  augite.'  Hornblende  has  abundantly  resulted  from  the  alteration 
(paramori)hisni)  of  augite  (see  l)elow,  Uralite).  In  many  rocks  the  ferro-magneaian 
silicate  which  is  now  hornblende  was  originally  augite  ;  the  epidiorites,  for  instance, 
were  probably  once  dolerites  or  allied  pyroxenic  rocks.  The  pale  non-aluminous  horn- 
blendes are  found  among  gneisses,  crystalline  limestones,  and  other  metamorphic  rocks. 
The  dark  varieties,  though  also  found  in  similar  situations,  sometimes  even  forming  entire 
masses  of  rock  (amphibolite,  honiblende-rock,  hornblende-schist),  are  the  common  forms 
in  granitic  and  volcanic  rocks  (syenite,  diorite,  honiblende-andesite,  &c.)  The  former 
group  naturally  gives  rise  by  weathering  to  various  hydrous  magnesian  silicates,  notably 
to  serjientine  and  talc.  In  the  weathering  of  the  aluminous  varieties,  silica,  lime, 
magnesia,  and  a  jwrtion  of  the  alkalies  are  removed,  with  convei'sion  of  jiart  of  the 
earths  and  the  iron  into  carbonates.  The  further  oxidation  of  the  ferrous  carbonate  is 
shown  by  the  yellow  and  brown  crust  so  commonly  to  be  seen  on  the  surface  or 
j)enetiuting  cracks  in  the  hornblende.  The  change  proceeds  until  a  mere  internal 
kernel  of  unalterwl  mineral  remains,  or  until  tlie  whole  has  been  converted  into  a 
ferruginous  clay. 

Anthophyllite  (Rhombic  Anijihibole  (MgFe)Si03)  is  a  mineral  which  occurs  in 
bladed,  sometimes  rather  fibrous  fonns,  among  the  more  Imsic  ]>ai-tjs  of  old  gneisses  ;  also 
in  zone^  of  alteration  round  some  of  the  ferro-magnesian  minerals  of  certain  gabbros. 

Soda-amphiboles  resemble  orrlinary  hornblende,  but,  as  their  name  denotes,  they 
contain  a  more  marked  proportion  of  soda.  They  include  a  lilue  variety*  called 
GlaucopJuinCj  which  is  found  abundantly  in  certain  schists  ;  RieheckUe^  which  is  also 
blue  and  occurs  in  some  granites  and  micro-granites  ;  Ar/irdsonitCy  a  dark  greenish  or 
brown  variety. 

Uralite  is  the  name  given  to  a  mineral  which  was  onginally  pyroxene,  but  has  now  by 
a  process  of  j»arani()r})hism  acquired  the  internal  cleavage  and  structure  of  hornblende 
(amphibole).  Under  the  niicroscoi>e  a  still  unchanged  kernel  of  jiyroxene  may  in  some 
sjiecimens  l>e  observed  in  the  centre  of  a  crystal  sniTounded  by  strongly  pleochroio  horn- 
blende, with  its  characteristic  cleavage  and  actinolitic  needles  {postcOy  j).  617).  SmaragdUe 
is  a  beautiful  grass-green  variety  also  resulting  from  the  alteration  of  a  pyroxene. 

Augite  (Monoclinic  Pyroxene,  CaO  12-27-5,  MgO  3-22-5,  FeO  1-34,  FcjO,  0-10, 
AI2O3  0-11  ;  Si0.j  40-57-4).  Divided  like  hornblende  into  two  groups.  1st.  Non- 
aluminous,  >vith  a  j»revalent  green  colour  (malacolite,  coccolite,  dio|>side,  salUite,  Ac) 
2n<l.  Aluminous,  including  generally  the  dark  gi-een  oi*  black  varieties  (common  augite, 
fassaite).     It  would  a]){iear  that  the  substance  of  hornblende  and  augite  is  dimorphous, 

^  On  the  occurrence  of  this  mineral  in  schists,  see  Lessen,  Zeitsch.  Deuiach.  Oecl.  Ot». 
1867,  pp.  546,  661. 

-   U7fw.  Acad.  May  1869.     See  also  Fouque  and  Michel-Levy,  op,  cit,  pp.  349,  865. 


PART  II  §  u  ROCK'FORMING  MINERALS  75 

for  the  experiments  of  Berthier,  MitHcherlich  and  G.  Rose  showed  that  hornblende, 
when  melted  and  allowed  to  cool,  assumed  the  crystalline  form  of  augite  ;  whence  it  has 
been  inferred  that  hornblende  is  the  result  of  slow,  and  augite  of  comparatively  rapid 
cooling.^  Under  the  microscope,  augite  in  thin  slices  is  only  very  feebly  pleochroic,  and 
presents  cleavage  lines  intersecting  at  an  angle  of  87°  5'.  It  is  often  remarkable  for  the 
amount  of  extraneous  materials  enclosed  within  its  crystals.  Like  some  felspars,  augite 
may  be  found  in  basalt  with  merely  an  outer  casing  of  its  own  substance,  the  core  being 
compoeed  of  magnetite,  of  the  ground-mass  of  the  surrounding  rock,  or  of  some  other 
mineral  (Fig.  7).  The  distribution  of  augite  resembles  that  of  hornblende  ;  the  pale, 
non- aluminous  varieties  are  more  specially  found  among  gneisses,  marbles,  and  other 
crystalline,  foliated,  or  metamorphic  rocks ;  the  dark-green  or  black  varieties  enter  as 
essential  constituents  into  many  igneous  rocks  of  all  ages,  from  Paleeozoic  up  to  recent 
times  (diabase,  basalt,  andesite,  kc )  Its  weathering  also  agrees  >vith  that  of  hornblende. 
The  aluminous  varieties,  containing  usually  some  lime,  give  rise  to  calcareous  and 
ferruginous  carbonates,  from  which  the  fine  interstices  and  caWties  of  the  surrounding 
rock  are  eventually  filled  with  threads  and  kernels  of  calcite  and  strings  of  hydrous  ferric 
oxide.  In  basalt  and  dolerite,  for  example,  the  weathered  surface  often  acquires  a  rich 
yellow  colour  from  the  oxidation  and  hydration  of  the  ferrous  oxide. 

OmpkaeiUy  a  granular  variety  of  pyroxene,  grass  green  in  colour,  and  commonly 
associated  with  red  garnet  in  the  rock  known  as  eclogite. 

Diallagf,  a  variety  of  augite,  characterised  by  its  somewhat  metallic  lustre  and 
foliated  aspect,  is  especially  a  constituent  of  gabbro. 

BbomUc-Pjrrozflnss. — Tliere  are  three  rhombic  forms  of  )>}Toxene,  which  occur  as 
important  constituents  of  some  rocks,  Enstatite,  Bronzite  and  Hypersthene.  EnstaiiU 
occurs  in  Iherzolite,  serpentine,  and  other  olivine  rocks  ;  also  in  meteorites.  Brmvdte  is 
found  under  similar  conditions  to  enstatite,  from  which  it  is  with  difficulty  seiiarable. 
It  occurs  in  some  basalts  and  in  seqientiues  ;  also  in  meteorites.  Bronzite  and  enstatite 
weather  into  dull  green  serpentinous  pixxlucts.  BastUe  or  Schiller-siiar  is  a  frequent 
product  of  the  alteration  of  Bronzite  or  Enstatite,  an<i  may  l)e  observed  with  its 
characteristic  pearly  lustre  in  8eri»entine.  Hypersthene  occurs  in  hy]ier8thenite  and 
hypersthene -andesite  ;  also  associated  vnth  other  magnesian  niineralH  among  the 
crystalline  schists. 

OliTiiie  (Peridot,  MgO  32.4-50.5,  FeO  6-29.7,  SiOj  31.6-42-8)  fonns  an  essential 
ingredient  of  basalt,  likevise  the  main  {lart  of  various  so-called  olivine-rocks  or  perido- 
tites  (as  Iherzolite)  and  pikrite),  and  occurs  in  many  gabbros  ;  under  the  microsco])e  with 
polarized  light,  gives,  when  fresh,  bright  colours,  specially  re<l  and  green,  but  it  is  not 
perceptibly  pleochroic.  Its  orthorhombic  outlines  can  sometimes  be  readily  observed, 
but  it  often  occurs  in  irregularly  shaped  granules  or  in  broken  (;rystals,  and  is  liable  to 
be  traversed  by  fine  fissures,  which  are  jwrticularly  developed  transverse  to  the  vertical 
axis.  It  is  remarkably  prone  to  alteration.  The  change  begins  on  the  outer  sui'face  and 
extends  inwards  and  siiecially  along  the  fissures,  until  the  whole  is  converted  either 
into  a  green  granular  or  fibrous  substance,  which  is  probably  in  most  cases  seri)entine 
(Fig.  26),  or  into  a  reddish-yellow  amor|)hous  mass  (limonite). 

HanyiM  (SiOa  84-06,  Al  27-64,  NajO  11.79,  KjO  496,  CaO  10-60,  SO4  11-25)  occurs 
abundantly  in  Italian  lavas,  in  basalt  of  the  Eifel,  and  elsewhere. 

HosMn  (SiO,  33.79,  Al  28-75,  Na^O  26-20,  SO4  11-26),  under  the  niicroscoi>e,  is  one 
of  the  most  readily  recognised  minerals,  showing  a  hexagonal  or  quadrangular  figure, 
with  a  characteristic  broad  dark  bonier  conesiionding  to  the  external  contour  of  the 
crystal,  and  where  weathering  has  not  jnoceeded  too  far,  enclosing  a  clear  colourless 
centre.     It  occurs  in  minute  forms  in  most  j)honolites,  also  in  large  cr^'stals  in  some 


^  The  same  results  have  been  obtained  recently  by  Fouque  anfl  Michel- Levy,  'Synthase 
des  Mineranx  et  des  Roches,'  1882,  p.  78. 


76  GEOGNOSY  book  ii 

sHiiidiiie  volcanic  rocks.      Both  hauvuc  and  nosoan  are  volcanic  minerals  a&M)ciated 
with  the  lavas  of  more  recent  geological  jieriods. 

Epidote  (Pistacite,  CaO  l»-30,  MgO  0-49,  Yefis  7-5-17-24,  AI3O,  14.47-28.9, 
Si0.j  d3*81-57*65)  occurs  in  many  crystalline  rocks,  as  a  i-esult  of  the  alteration  of 
other  silicates  such  as  fels[Mirsand  hornblende  {see  postea^  p.  618) ;  largely  distributed  in 
certain  schists  and  quaitzites,  sometimes  associated  with  beds  of  magnetite  and  haematite. 

Zoisite  is  allied  to  e])idote  but  contains  no  iron.  It  occurs  in  altered  basic  igneous 
ix)cks  and  also  (sometimes  in  large  aggregations)  in  metamorjihic  grou{)8. 

VesuTianite  (Idoci-ase,  CaO  27-7-37-5,  MgO  0-10-6,  FeO  0-16,  AI0O3  10-5-26.1, 
SiOa  35-397,  H2O  0-2*73)  occurs  in  ejected  blocks  of  altei*ed  limestone  at  Somma, 
also  among  crystalline  limestones  and  schists. 

Andalosite  (AI2O3  50-96-62-2,  Fe-jOs  0-5-7,  SiO-j  35.3-40-17).— Found  in  crystal- 
line schists.  The  variety  Chiastolite,  abundant  in  some  dark  clay-slates,  is  dis- 
tinguished by  the  regular  manner  in  which  the  dark  sultstance  of  the  surrounding  matrix 
has  been  endased,  giving  a  cross-like  transverse  section.  These  crystals  have  been 
develojjed  in  the  rock  after  its  formation,  and  are  regarded  as  proofs  of  contact-meta- 
morphism.  (Book  IV.  Part  VIII.)  Silliinanile  or  Fibrolitc  is  the  name  given  to  a 
til)rous  variety  which  is  not  infrequent  among  schistose  rocks. 

Dichroite  (Coitlierite,  lolite,  MgO  8-2-20-45,  FeO  0-11-58,  AI2O3  28.72-33-11, 
SiOo  48-1-50-4,  HuO  0-2-66)  occura  in  gneiss,  sometimes  in  large  amount  (cordierite- 
gneiss)  ;  occasionally  as  an  accessory  ingredient  in  some  gi-anites  ;  also  in  talc-schist. 
Uuilergoes  numerous  alterations,  having  been  found  changed  into  ]»inite,  chloro[)liyllite, 
mica,  &c. 

Scapolites,  a  series  of  minerals  consisting  of  silicates  of  alumina,  lime  and  soda,  with  a 
little  chlorine.  They  are  found  among  the  cavities  of  lavas,  but  more  frec[uently  among 
metamorphic  rocks,  where  they  appear  in  association  with  altered  fels]»ars.  Dipyre,  Cou- 
arranUc  and  Mcionite  are  varieties  of  the  series. 

Kyanite  ( Alj  SiOg)  occurs  in  bladed  aggregates  of  a  beautiful  delicate  blue  colour  among 
scOiistose  rocks  ;  also  in  gi*anular  foims. 

Garnet  (CaO  0-5-78,  MgO  0-10-2,  Fe^Og  0-6-7,  FeO  24-82-39-68,  MnO  0-6-43, 
AI0O3  15-2-21-49,  SiOa  35-75-52-11). — The  common  red  and  Inowni  varieties  occur  as 
essential  constituents  of  edogite,  garnet- rock ;  and  often  as  abundant  accessories  in  mica- 
schist,  gneiss,  granite,  &c.  Under  the  microscope,  garnet  as  a  constituent  of  rocks, 
l>resents  three-sided,  four-sided,  six-sided,  eight-sided  (or  even  rounded)  figures  according 
to  the  angle  at  which  the  indi\ndual  crystals  are  cut ;  it  is  usually  clear,  but  full  of  llavv's 
or  of  cavities  ;  [lassive  in  jM>larized  light. 

Tourmaline  (Schori,  CaO  0-2*2,  AlgO  0-14*89,  Na-^O  0-4*95,  KoO  0-3*59,  FeO  0-12, 
Fe-jOs  0-1308,  AljOg  30*44-44*4,  SiOj  35*2-41*16,  B  3*63-11  78,  F  1*49-2*58),  with 
quartz,  fonns  tounnaline-rock  ;  associated  with  some  granites ;  occurs  also  diffused 
tlirough  many  gneisses  schists,  crystalline  limestones,  and  dolomites,  likewise  iu  sands 
(see  Zireon).     Pleo(;hroism  strongly  marked. 

Zircon  (ZrOa  63*5-67*16,  FcjOj  0-2,  SiOo  32-35*26)  occurs  as  a  chief  ingredient 
in  the  zircon-syenite  of  Southern  Norway  ;  frequent  in  granites,  diorites,  gneisses,  crystal- 
line limestones  and  schists  ;  in  eclogite  ;  as  clear  red  grains  in  some  basalts,  and  also  iii 
ejecte<l  volcanic  blocks  ;  of  common  ocininence  in  san<ls,  clays,  sandstones,  shales  and 
other  sedimentaiy  nxiks  derive<l  from  ciystalline  ma.sses  such  as  graiut<»,  etc. 

Titanito  (Sphene,  CaO  21*76-33,  TiOa  33-43*5,  SiO-j  30-35),  dlsjiersed  in  small 
chaiTurteristically  lozeng<^shai)cd  ciystals  in  many  syenites,  also  in  gi*anite,  gneiss,  and 
in  some  volcanic  rocks  (l>asalt,  trachyte,  j»honolite). 

Zeolites.  — Under  this  name  is  included  a  characteristic  family  of  minerals,  which  have 
resulted  fix>m  the  alteration,  and  jMii-ticularly  from  the  hydration,  of  other  minerals, 
esi)ecially  of  felsjiars.  Secondary  products,  rather  than  original  constituents  of  rocks, 
they  often  occur  in  caWties  lK)th  as  prominent  amygdales  and  veins,  and  in  minute 
interstices  only  ^lereeptible  by  the  microscojw.    In  these  minute  forms  they  ver}'  commonly 


PARTUgii  ROOK-FORMING  MINERALS  77 

present  a  finely  fibrous  divergent  structure.  As  already  remarked,  a  relation  may  often 
be  traced  between  the  containing  rock  and  its  enclosed  zeolites.  Thus  among  the  basalts 
of  the  Inner  Hebrides,  the  dirty  green  decomposed  amygdaloidal  sheets  are  the  chief 
repositories  of  zeolites,  while  the  firm,  compact,  columnar  beds  are  conqiaratively  free 
from  these  alteration  products.*  Among  the  more  common  zeolites  are  Analcinic, 
NcUrciiU,  PrehnUe  and  StUbite, 

Kaolin  (AI3O,  38-6-40-7,  CaO  0-3'5,  KjO  0-19,  SiOj  45 -6-46  53,  H2O  9-14*54) 
results  from  the  alteration  of  {K>tash-  and  soda-felspars  exposed  to  atmospheric  influences. 
Under  the  microscope  the  fine  white  powdery  substance  is  found  to  include  abundant 
minute  six-sided  colourless  plates  and  scales  which  have  been  formed  by  re-crystallization 
of  the  decomposed  substance  of  the  felspar.  The  purest  white  kaolin  is  called  china-clay^ 
from  its  extensive  use  in  the  manufacture  of  porcelain.  Ordinary  clay  is  impui'e  from 
admixture  of  iron,  lime,  and  other  ingredients,  among  which  the  debris  of  the  unde- 
composed  constituents  of  the  original  rock  may  form  a  marked  pro]  tort  ion. 

Talc  (MgO  23-19-35-4,  FeO  0-4-5,  Al,Os  0-5*67,  SiOj  56*62-64*53,  HjO  0-6*65) 
occurs  as  an  essential  constituent  of  talc-schist,  and  as  an  alteration  product  replac- 
ing mica,  honiblende,  augite,  olivine,  diallage,  and  other  minei*als  in  crystalline 
rocks. 

CShlorite  (MgO  24*9-36,  FeO  0-5*9,  FeaOs  0-11*36,  AI2O3  10*5-19*9,  SiOo  30-33*5, 
H5O  11 '5-16),  including  several  varieties  or  si)ecie8,  occurs  in  small  green  hexagonal 
tables  or  scaly  vermicular  or  earthy  aggregates ;  is  an  essential  ingredient  of  chlorite- 
achist,  and  occurs  abundantly  as  an  alteration  product  (of  honiblende,  &c.)  in  fine 
filaments,  incrustations,  and  layers  in  many  crystalline  rocks.  (See  under  '*  Chloritiza- 
tion,*' ;N»^ea,  p.  618.)  Among  the  minerals  grouj)ed  under  the  general  head  of  chlorites 
are  Chlcrophonte^  Cliiwchlorey  Deles^Uc,  Pennine ^  RipidolitCf  and  othera. 

(Htrelite  (Chloritoid,  H2O  (FeMg)  AlaSiOy)  occurs  in  small  lustrous  iron-black  or 
greenish-black  lozenge-shai)ed  or  six-side  plates  in  certain  schists.  It  resembles  chlorite 
Imt  is  at  once  distinguishable  from  that  mineral  by  its  much  greater  hardness. 

Sexpentine  (MgO  28-43,  FeO  1-10*8,  AI2O3  0-5*5,  SiO^  37*5-44*5,  H2O  9*5-14*6)  is  a 
product  of  the  alteration  of  pre-existing  minerals,  and  especially  of  olivine.  It  occurs 
in  nests,  grains,  threads,  and  veins  in  rocks  which  once  contained  olivine  *  (p.  75),  also 
massive  as  a  rock,  in  which  it  has  replaced  olivine,  enstatite  or  some  other  magnesian 
biailicate  (pp.  173,  618).  Under  the  microscope  it  presents,  in  very  thin  slices,  a  i>ale  leek- 
green  or  bluish-green  base,  showing  aggregate  [polarization.  Through  this  base  runs  a 
network  of  dark  opaf^ue  threads  and  veinings.  Sometimes  among  these  veinings,  or 
throogh  the  network  of  green  serpentinous  matter  in  the  base,  the  forms  of  original 
olivine  crystals  may  be  traced  (Figs.  26,  27). 

Olanocmite  (CaO  0-4*9,  MgO  0-5*9,  K^O  0-12*9,  NayO  0-2*5,  FeO  3-25*5, 
FeiQt  0-28*1,  AljO,  1*5-13*3,  Si02  46*5-60*09",  H2O  0-14*7).  Found  in  many  strati- 
fied formations,  particularly  among  sandstones  and  limestones,  where  it  envelopes 
grains  of  sand,  or  fills  and  coats  foraminifera  and  other  organisms,  giving  a  general 
green  tint  to  the  rock.  It  is  at  present  being  formed  on  the  sea-floor  off  the  coasts  of 
Georgia  and  South  Carolina,  where  Pourtaleir  found  it  filling  the  chambers  of  recent 
polythalamia. 

6.  Carbonates.  This  family  of  minerals  furnishes  only  four  which  enter  largely 
into  the  formation  of  rocks,  viz.,  Carbonate  of  Calcium  in  its  two  forms,  Calcite  and 
Aragonite,  Carbonate  of  Magnesium  (and  Calcium)  in  Dolomite,  and  Carbonate  of  Iron  in 
Siderite. 

Calcite  (CaCOs)  occurs  as  (1)  an  original  constituent  of  many  aqueous  rocks  (lime- 
stone, calcareous  shale,  Ac. ),  either  as  a  result  of  chemical  deposition  from  water  (calc- 


'  See  Sullivan  in  Jukes'  *  Manual  of  Geology,'  p.  85. 
*  See  Tschermak,  Wien,  Akad,  Ivi.  1867. 


78  GEOGNOSY  book  n 

sinter,  stalactites,  &c. ),  or  as  a  secretion  by  ])lants  or  animals  ^ ;  or  (2)  as  a  secondary 
product  resulting  from  n^eathering,  when  it  is  found  filling  or  lining  cavities,  or  diffused 
through  the  capillary  interstices  of  minerals  and  rocks.  It  probably  never  occurs  as  an 
original  ingredient  in  the  massive  crystalline  rocks,  such  as  granite,  felsite,  and  lavas. 
Under  the  niicrosco|je,  calcite  is  readily  distinguishable  by  its  intersecting  cleavage  lines, 
by  a  fi-equent  twin  lamellation  (sometimes  giving  interference  colours),  strong  double 
refraction,  weak  or  inappreciable  ])leochroism,  and  characteristic  iridescent  polarization 
tints  of  grey,  rose  and  blue. 

From  the  readiness  with  which  water  absorbs  carbon -dioxide,  from  the  increased 
solvent  power  which  it  thereby  acquires,  and  from  the  abundance  of  calcium  in  various 
forms  among  minerals  and  rocks,  it  is  natural  that  calcite  should  occur  abundantly  as  a 
}iseudomorph  replacing  other  minerals.  Thus,  it  has  been  ob8er>'ed  taking  the  place  of  a 
number  of  silicates,  as  orthoclase,  oligoclase,  garnet,  augite  and  several  zeolites  ;  of  the 
sul[>hate8,  anhydrite,  gypsum,  barytes,  and  celestine  ;  of  the  carbonates,  aragonite, 
dolomite,  cerussite  ;  of  the  fluoride,  fluor-spar  ;  and  of  the  sulphide,  galena.  Moreover, 
in  many  massive  crystalline  rocks  (diorite,  dolerite,  &c.),  which  have  been  long 
exposed  to  atmospheric  influence,  this  mineral  may  be  recogiused  by  the  briak 
effervescence  productni  by  a  drop  of  acid,  and  in  niicroscojnc  sections  it  appears  filling  the 
crevices,  or  sending  minute  veins  among  the  decayed  mineral  constituents.  Calcite  is 
likcM-isc  the  great  i)etrifying  medium  :  the  vast  majority  of  the  animal  remains  found 
in  the  rocky  cnist  of  the  globe  have  been  replaced  by  calcite,  sometimes  with  a  com- 
plete preservation  of  internal  organic  structure,  sometimes  with  a  total  substitution 
of  cr^'stallinc  material  for  that  structure,  the  mere  outer  form  of  the  organism  alone 
surviving. - 

Aragonite  (CaCO^),  harder,  heavier,  and  much  less  abundant  than  calcite,  which  is 
the  more  stable  form  of  calcium-carbonate  ;  occurs  with  l)eds  of  g^'jxsum,  also  in  mineral 
veins,  in  strings  ninning  tlirough  basalt  and  other  igneous  rocks,  and  in  the  alielU  of 
many  mollusca.  It  is  thus  always  a  de|)osit  from  water,  sometimes  from  warm  mineral 
springs,  sometimes  as  the  result  of  the  internal  alteration  of  rocks,  and  sometimes 
through  the  action  of  living  organisms.  Being  more  easily  soluble  than  calcite,  it  has 
no  doubt  in  many  cases  disa])peared  from  limestones  origiually  fonned  mainly  of 
aragonite  shells,  and  has  ])een  replaced  by  the  moi-e  durable  calcite,  with  a  consequent 
destniction  of  the  traces  of  organic  origin.  Hence  what  are  now  thoroughly  crystalline 
limestones  may  have  l>een  formed  by  a  slow  alteration  of  such  shelly  deposits  (p.  484). 

Dolomite  (Bitter-spar  (Ca ;  Mg)C03,  ]i.  151)  occurs  (1)  as  an  original  deposit  in 
massive  IkmIs  (magnesian  limestone),  belonging  to  many  different  geological  formationa ; 
(2)  as  a  product  of  alteration,  esi)ecially  of  oixiinary  limestone  or  of  aragonite  (Dole- 
mitizatiou  j).  321). 

Siderite  (Brown  Ironstone,  Si)athic  Iron,  Chalybite,  Ferrous  Carbonate,  FeCO,) 
occurs  crystallized  in  association  with  metallic  ores,  also  in  beds  and  veins  of  many 
crystalline  rocks,  jwirticularly  with  limestones  ;  the  comi)act  argillaceous  varieties  (clay- 
ironstone)  are  found  in  abundant  nodules  and  beds  in  the  shales  of  Carboniferous  and 
other  formations  where  they  have  been  dejMJsited  from  solution  in  water  in  presence  of 
decaying  organic  matter  (see  pp.  147,  153). 

7.  SuLPHATJiis.  Among  the  sulphates  of  the  mineral  kingdom,  only  two  deserve 
noticiJ  here  as  inii»ortant  comjiounds  in  the  constitution  of  rocks — \'iz.,  calcium-sulphate 


^  Mr.  Sorby  has  investigated  the  condition  in  which  the  calcareous  matter  of  the 
harder  parts  of  invertebrates  exists.  He  finds  that  in  foraminifera,  echinoderms, 
brachiopods,  Crustacea,  and  some  lamellibranchs  and  gasteropods,  it  occurs  as  calcite ; 
that  in  nautilus,  sepia,  most  gasteropods,  many  lamellibranchs,  &c.,  it  is  aragonite  ;  and  that 
in  not  a  few  cases  the  two  forms  occur  tc^ether,  or  that  the  carbonate  of  lime  is  hardened  by 
an  admixture  of  pho8i>hate.     Quart.  Joum.  Oeol.  Soc.  1879.     Address,  p.  61. 

^  See  index  sub  voc.  Calcite. 


PAOT II  §  u  ROCK-FORMING  MINERALS  79 

or  sulphate  of  lime  in  its  two  forms,  Anhydrite  and  Gyjisum  ;  and  bariuni-sul])hate  or 
sulphate  of  baryta  in  Barytes. 

Anhydzite  (CaSOi)  occurs  more  esi>ecially  in  association  witli  beds  of  gy|)simi  and 
rock-salt  (see  p.  152). 

Oypnim  (Selenite,  CaS04  +  2H20).  Abundant  as  an  original  aqueous  de|X)sit  in 
many  sedimentary  formations  (see  p.  152). 

BazytM  (Heavy  Spar,  BaSOJ.  Frequent  in  veins  and  es|iecially  associated  with 
metallic  ores  as  one  of  their  characteristic  vein-stones. 

8.  Phosphates.  The  phosphates  which  occur  most  conspicuously  as  constituents  or 
acceoDory  ingredients  of  rocks  are  the  tricalcic  phosphate  or  Ajiatite,  and  tri ferrous 
phosphate  or  Yivianite. 

Apatite  (3Ca3  (POJ  +  CaF,)  occurs  in  many  igneous  rocks  (granites,  Ijasalts,  &c.),  in 
minute  hexagonal  non-pleochroic  needles,  giving  faint  polarization  tints ;  also  in  large 
crystals  and  massive  beds  associated  with  metamorphic  rocks. 

YiTimaite  (Blue  iron -earth,  FesP^Og,  SH^O)  occurs  crystallized  in  metalliferous 
veins ;  the  earthy  variety  is  not  infrequent  in  2)eat-mosses  where  animal  matter  has 
decayed,  and  is  sometimes  to  be  observed  coating  fossil  fishes  as  a  fine  layer  like  the 
bloom  of  a  plum. 

9.  Fluorides.  The  element  fluorine,  though  widely  diffused  in  nature,  occurs  as  an 
important  constituent  of  comparatively  few  minerals.  Its  most  abundant  com|K)und  is 
with  Calcium  as  the  common  mineral  Fluorite.  It  occui*s  also  with  sodium  and 
almninium  in  the  mineral  Cryolite. 

Fluorite  (Fluor-sjiar,  CaFj)  occurs  generally  in  veuis,  esiH?cialIy  in  association  with 
metallic  ores. 

10.  Chlorides.  Tliere  is  only  one  chloride  of  importance  as  a  constituent  of  rocks 
— sodium-chloride  or  common  salt  (NaCl),  which,  occurring  chiefly  in  beds,  is  described 
among  the  rocks  at  p.  148.  Camallite  (KClMgCl26H20),  a  liydrated  chloride  of 
potassium  and  magnesium,  occurs  in  beds  associated  with  rock-salt,  gy[)sum,  &c.,  in  some 
salt  districts  (p.  149). 

11.  Sulphides.  Sulphur  is  found  united  with  metals  in  tlie  foi-m  of  sulpliides, 
many  of  which  form  common  minerals.  The  sulphides  of  lead,  silver,  copper,  zinc, 
antimony,  Ac.,  are  of  great  commercial  imiwrtance.  Iron-disulphide,  however,  is  the 
only  one  which  merits  consideration  here  as  a  rock-fonuing  substance.  It  is  fonned  at 
the  present  day  by  some  thermal  springs,  and  has  been  develoi»ed  in  many  rocks  as  a 
result  of  the  action  of  infiltrating  water  in  presence  of  deconijwsing  organic  matter  and 
iron  salts.     It  occurs  in  two  forms,  Pyrite  and  Marcasite. 

P3frtte  (Eisenkies,  Schwefelkies,  FeSo)  occurs  disseminated  through  almost  all  kinds 
of  rocks,  often  in  great  abundance,  as  among  dial>ases  and  clay-slates  ;  also  frequent  in 
Teins  or  in  beds.  In  microscopic  sections  of  rocks,  jiyrite  apjiears  in  small  cubical, 
perfectly  opaque  crystals,  which  with  reflected  light  show  the  characteristic  brassy 
histre  of  the  mineral,  and  cannot  thus  be  mistaken  for  the  isometric  magnetite,  of  which 
the  square  sections  exhibit  a  characteristic  blue-black  colom*.  Pyrite  when  free  from 
marcesite  yields  but  slowly  to  weathering.  Hence  its  cubical  crystals  may  be  seen 
projecting  still  fresh  from  slates  which  have  been^  exi)osed  to  the  atmosphere  for  several 
genermtions.^ 

Haicaiite  (Hejiatic  pyrites)  occurs  abundantly  among  sedimentar}'  foimations, 
sometimes  abundantly  diffused  in  minute  pai-ticles  which  inij)art  a  blue-grey  tint,  and 
speedily  weather  yellow  on  exposure  and  oxidation  ;  sometimes  segregated  in  layers,  or 
replacing  the  substance  of  fossil  plants  or  animals  ;  also  in  veins  through  crystalline 
rocks.  This  form  of  the  sulphide  is  especially  characteristic  of  stratified  fossiliferous 
rocks,  and  more  particularly  of  those  of  Secondary  and  Tertiary  date.     It  is  extremely 


*  For  an  elaborate  paper  on  the  decomposition  of  Pyrites,  see  A.  A.  Julien,  Annals 
New  York  Acad,  Set.  vols.  iii.  and  iv. 


^  0  'V  EO*  ixos  y  BOOK  II 


iU'-!«:  t«^  «i»:».-'*ini*r-iii«iii.  H»-n'.-»r  exj-j^uri?  f«.»r  f  vru  a  -hort  linie  to  the  air  causes  it  to 
•«.•■#, rue  bro'Aii :  fr*-»-  -liliiliiiri*;  a^.i'l  i*  |in>liiiTeii.  whii-h  attai.-k->  the  Mirroanding  miDermls, 
-'irii»:tirii«7i  at  oii'.-^  fonuiu;;  '-ulphate^.  at  oth*T  tiiu*^  «i»r<^>m|<iHinjr  aluminous  ailicatm 
ari'i  'li^Vihiij^  t)i*rrii  in  •.-'tii-iileraM<r  tfiiautity.  Ih-.  Sullivan  mentions  that  the  water 
iririii.AlIy  puniii^l  tWmi  one  mine  in  Inrlan<l  >-arriH<l  u|»  !•■  the  Mirface  more  than  a 
;.iirj<lr*^i  ton^  of  t}i:v^Av*r*\  «ili<-ate  of  alumina.^  In'jn  <lLMiI|ihiil«:  i>  thus  an  inifiortant 
ik'^ful  in  etf«-<-tin;;  the  int^-nial  tltf^rfmiifr-xihtu  of  nArkiiw  It  al»o  plays  a  large  |«rt  as  a 
ji^trifyin;;  ni«-<iiiini.  rfplat-in^  t)ir  onranio  matter  itf  plant j^  an<l  animals,  and  leaving 
«a-t->  of  th»*ir  fonn-*.  •ift»-n  with  hri^ht  niftallit.-  lustre.     Sm-h  c-a>ts  when  exixiaied  to  the 

air  f\*r*:'flU\ftr^. 

Pjrrrhotiiia  Maaw-tu-  ]*yrites.  Fe^S^  i>  mui.-h  lr>«  ahun<lant  than  either  of  the  forms 
of  onlinar}'  irin-pyritt;-,  fn>ni  whiirli  it  i.>  di>tin^ii«>he<l  hy  its  inferior  hardness  and  its 
ma^^netif  «;Iianu:ter. 

It  will  lie  observed  that  great  differences  exist  in  the  relative  im- 
IKirtiince  of  the  minerals  aliove  enumerated  as  constituents  of  rocks. 
iVofertsor  Kosenbusch  jxiints  out  that  they  lUiiy  be  naturally  arranged 
in  four  groujis — 1st,  ores  and  accessory  ingredients  (magnetite,  haematite, 
ilmenite,  a{iatite,  zircon,  spinel,  titanito),  2nd,  magnesian  and  ferru- 
ginous silicates  (biotite,  amphibole,  pyroxene,  olivine),  3rd,  felspathic 
constituents  (felspar  [)roj)er,  ne]>heline.  leucite,  melilite,  so<lalite,  hauyne), 
4th,  free  silica.- 

§  Lil.  Determination  of  Rocks. 

Kocks  considered  as  mineral  subsUmces  are  distinguished  from  each 
other  by  certain  external  characters,  such  as  the  size,  form,  and  arrange- 
ment  of  their  component  jmrticles.  These  characters,  readily  perceptible 
to  the  naked  eye,  and  in  the  great  majority  of  cases  observable  in  hand 
sjiccimcns,  are  termed  mf(jascopk  or  inncmsct^pk  (pp.  81-87),  to  distinguish 
them  from  the  more  minute  features  which,  l)eing  only  visible  or  satis- 
factorily oV>8ervable  when  greatly  nuignitied,  are  known  as  microscopic 
(pp.  89-96).  The  larger  (geotectonic)  aspects  of  rock-structure,  which  can 
only  be  proj^rly  examined  in  the  field  and  1)elong  to  the  general 
architecture  of  the  earth's  crust,  are  treated  of  in  Book  IV.^ 

In  the  discrimination  of  rocks,  it  is  not  enough  to  specify  their 
comi>onent  minerals,  for  the  same  minerals  may  constitute  very  distinct 
varieties  of  rock.  For  example,  quartz  and  mica  form  the  massive 
crystalline  rock,  greisen,  the  foliated  crysUilline  rock,  mica-schist,  and 
the  sedimentary  rock,  micaceous  sandstone.  Chalk,  encrinal  limestone, 
stalagmite,  statiuiry  mar1)le  arc  all  comiK)sed  of  calcite.  It  is  needful 
to  take  note  of  the  megascopic  and  microscopic  structure  and  texture, 
the  state  of  aggregation,  colour,  and  other  charactei*s  of  the  several 
masses. 

1  JukeH'  '  Manual  of  Geology,'  j..  65.  '  Xcues  JaJtrb.  1882  (iL)  p.  6. 

*  The  student  who  would  jmreue  physical  geology  by  original  research  in  the  field  and 
abroad  may  consult  Bouc,  'Guide  da  Geologue  Voyageur,' 2  vols.  1835;  £lie  de  Beaumont, 

*  Le<;on8  de  Geologie  pratique/  vol.  1.  1845  ;  Penuing  and  Jukes- Brown,  'Field  Geology,' 
•2nd  edit.  1880  ;  A  Geikie,  '  Outlines  of  Field  Geolog}-,'  4th  edit  1891.     F.  v.  Richthofen. 

*  Fuhrer  fiir  Forschungsreisendc/  1886  ;  Grenville  Cole,  'Aids  in  Practical  Greology,'  1891. 


•ART  II  §  iii  DETERMINATION  OF  ROCKS  81 

Four  methods  of  procedure  are  available  in  the  investigation  and 
le termination  of  rocks  :  1st,  megascopic  (macroscopic)  examination,  either 
>7  the  rough  and  ready,  but  often  sufficient,  appliances  for  use  in  the 
ield,  or  by  those  for  more  careful  work  indoors ;  2nd,  chemical  analysis ; 
Srd,  chemical  synthesis ;  4th,  microscopic  investigation. 

i.  Megascopic  {Macroscopic)  Examination, 

Tefte  in  the  field. — The  instmmeuts  indisi^usable  for  the  investigation  of  rocks  in 
:he  field  are  few  in  number,  and  simple  in  character  and  application.  The  observer 
«rill  be  sufficiently  accoutred  if  he  carries  with  him  a  hammer  of  such  form  and  weight 
iS  will  enable  him  to  break  off  clean,  sharp,  unweathered  chit)S  from  the  edges  of  rock- 
naases,  a  small  lens,  a  [)ocket-knife  of  hard  steel  for  determining  the  hardness  of  rocks 
ind  minerals,  a  magnet  or  a  magnetized  knife-blade,  and  a  small  i>ocket-phial  of  dilute 
lydrochloric  acid,  or  better  still  some  citric  acid  in  powder. 

Should  the  object  be  to  form  a  collection  of  rocks,  a  hammer  of  at  least  three  or  four 
[Monds  in  weight  should  be  carried :  also  one  or  two  chisels  and  a  small  trimming 
lammer,  weighing  about  i  lb.,  for  reducing  the  specimens  to  shape.  A  convenient  size 
>f  specimen  is  4x3x1  inches.  They  should  be  as  nearly  as  possible  uniform  in  size, 
M>  as  to  be  capable  of  orderly  aiTangement  in  the  drawers  or  shelves  of  a  case  or  cabinet. 
/Attention  should  be  jiaid  not  only  to  obtain  a  thoroughly  fresh  fracture  of  a  rock,  but 
ilso  a  weathered  surface,  wherever  there  is  anything  characteristic  in  the  weathering. 
Every  specimen  should  have  affixed  to  it  a  label,  indicating  as  exactly  as  possible  the 
iocadity  from  which  it  was  taken.  This  information  ought  always  to  be  written  down  in 
the  field  at  the  time  of  collecting,  and  should  be  affixed  to  or  wTap]>ed  up  with  the 
specimen,  before  it  is  consigned  to  the  collecting  bag.  If,  however,  the  student  does 
not  purpose  to  form  a  collection,  but  merely  to  obtain  such  chips  as  will  enable  him  to 
judge  of  the  characters  of  i*ocks,  a  hammer  weighing  from  1^  to  2  lbs.,  with  a  square 
fkce  and  tai)ering  to  a  chisel-edge  at  the  opposite  end,  will  be  most  useful.  The  advantage 
if  this  form  is  that  the  hammer  can  be  used  not  only  for  breaking  hard  stones,  but  also 
Tor  splitting  open  shales  and  other  fissile  rocks,  so  that  it  unites  the  uses  of  hammer  and 
shiseL 

It  is,  of  course,  desirable  that  the  learner  should  first  acquire  some  knowledge  of  the 
nomenclature  of  rocks,  by  carefully  studying  a  collection  of  correctly  named  and 
iudiciously  selected  rock-specimens.  Such  collections  may  now  be  purchased  at  small 
M)6t  from  mineral  dealers,  or  may  be  studied  in  the  museums  of  most  towns.  Having 
locustomed  his  eye  to  the  ordinary  external  characters  of  rocks,  and  become  familiar 
idth  their  names,  the  student  may  j>roceed  to  determine  them  for  himself  in  the  field. 

Finding  himself  face  to  face  with  a  rock-mass,  and  after  noting  its  geotectonic 
sharacters  (Book  IV.),  the  observer  will  proceed  to  examine  the  exposed  or  weathered 
mxiiyce.  The  earliest  lesson  he  has  to  learn,  and  that  of  which  perhaps  he  will  in  after 
ife  meet  with  the  most  varied  illustrations  is  the  extent  to  which  weathering  conceals 
he  true  aspect  of  rocks.  From  what  has  been  said  in  previous  pages,  the  nature  of 
lome  of  the  alterations  will  be  understood,  and  fmlher  information  regarding  the 
chemical  processes  at  work  will  be  found  in  Book  III.  The  practical  study  of  rocks  in 
he  field  soon  discloses  the  fact,  that  while,  in  some  cases,  the  weathered  crust  so 
Mnnpletely  obscui-es  the  essential  character  of  a  rock  that  its  true  nature  might  not  be 
inspected,  in  other  instances,  it  is  the  weathered  crust  tliat  best  reveals  the  real 
(tructure  of  the  mass.  Spheroidal  crusts  of  a  decomposing  yellow  ferruginous  earthy 
mbetance,  for  example,  would  hardly  be  identihed  as  a  coniiwet  dark  basalt,  j^et,  on 
woetrating  within  these  crusts,  a  central  core  of  still  undecomposed  basalt  may  not 
infrequently  be  discovered.  Agtiin,  a  block  of  limestone  when  broken  open  may 
iresent  only  a  uniformly  crjstalliue  structure,  yet  if  the  weathered  surface  be  examined 


82  GEOGNOSY  book  ii 

it  may  show  many  projecting  fragments  of  shells,  ]K)lyzoa,  corals,  criuoids,  or  other 
organisms.  The  i-eally  fossiliferous  nature  of  an  ap])arently  unfossiliferous  rock  may 
thus  1)e  revealed  by  weathering.  Many  limestones  also  might,  from  their  fresh  fracture, 
be  set  down  as  tolerably  pure  carbonate  of  lime  ;  but  from  the  thick  crust  of  yellow 
ochre  on  their  weathered  faces  are  seen  to  be  highly  ferruginous.  Among  crystalline 
rocks,  the  weathered  surface  commonly  throws  light  uix>n  the  mineral  constitution  of 
the  mass,  for  some  minerals  decompose  more  rapidly  than  others,  which  are  thus  left 
isolated  and  more  easily  recognisable.  In  this  manner,  the  existence  of  quartz  in  many 
fels|)athic  rocks  may  be  detected.  Its  minute  blebs  or  crystals,  which  to  the  naked  eye 
or  lens  are  lost  among  the  brilliant  facettes  of  the  felsjiars,  stand  out  amid  the  dull  clay 
into  which  these  minerals  are  decomposed. 

The  depth  to  which  weathering  extends  should  be  noted.  The  student  must  not  be 
too  confident  that  he  has  reached  its  limit,  even  when  he  comes  to  the  solid,  more  or  less 
hard,  splinteiy,  and  apiurently  fresh  stone.  Granite  sometimes  decomposes  into  kaolin 
and  sand  to  a  depth  of  twenty  or  thirty  feet  or  more.  Limestones,  on  the  other  hand, 
have  often  a  mere  filni  of  crust,  because  their  substance  is  almost  entirely  dissolved  and 
removed  by  rain  (Book  III.  Part  II.  Section  ii.  §  2). 

With  some  practice,  the  inspection  of  a  weathered  surface  will  frequently  suffice  to 
determine  the  tme  nature  and  name  of  a  rock.  Should  this  ])reliminary  examination, 
and  a  comx)arison  of  weathered  and  unweathered  surfaces,  fail  to  afford  the  information 
sought,  we  proceed  to  apply  some  of  the  simple  and  useful  tests  available  for  field-work. 
The  lens  "will  usually  enable  us  to  decide  whether  the  rock  is  compact  and  apparently 
structureless,  or  cr^'stalline,  or  fragmental.  Having  settled  this  point,  we  proceed  to 
ascertain  the  hardness  and  colour  of  streak,  by  scratching  a  fresh  surface  of  the  stone.. 
A  drop  of  acid  placed  ujion  the  scratched  sui-face  or  on  the  powder  of  the  streak  may 
reveal  the  presence  of  some  carbonate.  By  [iractice,  considerable  facility  can  be  acquired 
in  approximately  estimating  the  si^ecific  gravity  of  i-ocks  merely  by  the  hand.  The 
following  table  may  be  of  assistance,  but  it  must  be  understood  at  the  outset  that  a 
knowledge  of  rocks  can  never  be  gained  from  instructions  givien  in  books,  but  must  be 
actiuired  by  actual  handling  and  study  of  the  rocks  themselves. 

i.  A  fresh  fracture  8how8  the  rock  to  be  close-grained,  daU,  with  no  diitiBet 
Btractnre.^ 

o.  H.  0*5  or  less  up  to  1  ;  soft,  crumbling  or  easily  scratched  with  the  knife,  if  not 
with  the  finger  -  nail ;  emits  an  earthy  smell  when  breathed  upon,  does  not 
effervesce  with  acid  ;  is  dark  grey,  brown,  or  blue,  perhaps  red,  yellow,  or  even 
white  =  probably  some  clay  rock,  such  as  niudstone,  massive  shale,  or  fire-clay 
(p.  132) ;  or  a  decomjwsed  felspar-rock,  like  a  close-grained  felsite  or  orthoclase 
t>orjihyry.  If  the  rock  is  hard  and  fissile  it  may  be  shale  or  clay-slate  {\u 
134). 
/9.  H.  1*5-2.  Occurs  in  Ijeds  or  veins  (perhaps  fibrous),  white,  yellow,  or  reddish. 
Sp.  gr.  2 '2-2 '4.     Docs  not  effervesce  =  probably  gypsum  (pp.  79,  152). 

7.  Friable,  cnimbling,  soils  the  tiugere,  white,  or  yellowish,  brisk  effervescences 

chalk,  marl,  or  some  pulverulent  form  of  limestone  (pp.  139,  149). 

8.  H.  3-4.     Sp.  gr.  2 '5-2 '7  ;  i)ale  to  dark  green  or  reddish,  or  with  blotched  and 

clouded  mixtures  of  these  colours.  Streak  white  ;  feels  soapy  ;  no  effervescence, 
splintery  to  8ul>conchoidal  fracture,  edges  subtranslucent.  See  serpentine 
(p.  173). 
€.  H.  averaging  3.  Sp.  gr.  2 '6-2 '8.  White,  but  more  frequently  bluish-grey,  also 
yellow,  brown  and  l)lack  ;  streak  white  ;  gives  brisk  effervescence  =  some  fomi 
of  limestone  (]>p.  139,  149). 

^  In  this  table,  H.  =  hanlness  ;  Sp.  gr.  =  specific  gravity.  The  scale  of  hardness  usually 
employed  is  1,  Talc ;  2,  Rock-salt  or  gypsum  ;  3,  Calcite  ;  4,  Fluorite  ;  5,  Apatite ;  6» 
Orthoclase  ;  7,  Quartz  ;  8,  Topaz ;  9,  Corundum  ;  10,  Diamond. 


PABT  II  §  iii  DETERMINATION  OF  ROCKS  83 


f.  H.    3 '5-4 '5.      Sp.   gr.    2 '8-2 -95.     Yellowish,   white,  or  pale  brown.     Powder 
slowly  soluble  in  acid  with  feeble  effervescence,  which  becomes  brisker  when 
the  acid  is  heated  with  the  powder  of  the  stone.     See  dolomite  (pp.  78,  151). 
i|.  H.  3-4.     Sp.  gr.  3-8*9.     Dark  brown  to  dull  black,  streak  yellow  to  brown, 
feebly  soluble  in  acid,  which  becomes  yellow ;   occurs  in  nodules  or  beds, 
usually  with  shale  ;  weathers  with  brown  or  blood-red  crust  =  brown  iron -ore. 
See  clay-ironstone  (pp.  147,  163) ;  and  limonite  (pp.  70,  158) ;  if  the  rock  is 
reddish  and  gives  a  cherry-red  streak,  see  haematite  (pp.  70,  153). 
0,  Sp.  gr.  2*55.     White,  grey,  yellowish,  or  bluish,  rings  under  the  hammer,  splits 
into  thin  plates,  does  not  effervesce,  weathered  crust  white  and  distinct = 
perhaps  some  compact  variety  of  phonolite  (p.  166.     See  also  felsite  (p.  161), 
and  porphyrite,  p.  168). 
*•  Sp.   gr.    2 '9-3  "2.      Black  or  dark  green,   weathered  crust  yellow  or  brown = 
probably  some  close-grained  variety  of  basalt  (p.   170),  andesite  (p.    167), 
aphanite  (p.  166),  or  amphibolite  (p.  182). 
K,  H.  6-6*5,  but  less  according  to  decomposition.     Sp.  gr.  2*55-2*7.     Can  with 
difficulty  be  scratched  with  the  knife  when  fresh.     White,  bluish-grey,  yellow, 
lilac,  brown,  red  ;  white  streak  ;  sometimes  with  well  defined  white  weathered 
crust,  no  effervescence = probably  a  felsitic  rock  (p.  161). 
X,  H.  7.     Sp.  gr.  2*5-2*9.     The  knife  leaves  a  metallic  streak  of  steel  upon  the 
resisting  surface.    The  rock  is  white,  reddish,  yellowish,  to  brown  or  black, 
very  finely  granular  or  of  a  homy  texture,  gives  no  reaction  with  acid  = 
probably  silica  in  the  form  of  jasper,  homstone,  flint,  chalcedony,  halleflinta 
(pp.  69,  183),  adinole  (p.  183). 
iL  A  fireih  fracture  showt  the  rock  to  be  glassy. 

Leaving  out  of  account  some  glass-like  but  crystalline  minerals,  such  as  quartz  and 
rock-salt,  the  number  of  vitreous  rocks  is  comparatively  small.  The  true  nature  of  the 
mass  in  question  will  probably  not  be  difficult  to  determine.  It  must  be  one  of  the 
ICaasive  volcanic  rocks  (p.  15A  et  seq.)  If  it  occurs  in  association  with  siliceous  lavas 
(liparites,  trachytes)  it  will  probably  be  obsidian  (p.  162),  or  pitchstone  (p.  163) ;  if  it 
pionfo  into  one  of  the  basalt-rocks,  as  so  commonly  happens  along  the  edges  of  dykes 
and  intrusive  sheets,  it  is  a  glassy  form  of  basalt  (p.  171).  Each  of  the  three  great 
series  of  eruptive  rocks.  Acid,  Intermediate,  and  Basic,  has  its  glassy  varieties  (see 
pp.  162,  163,  171). 

ilL  A  fresh  fracture  shows  the  rock  to  be  crystaUine. 

If  the  component  crystals  are  sufficiently  large  for  determination  in  the  field,  they 
may  suggest  the  name  of  the  rock.  Where,  however,  they  are  too  minute  for  identifi- 
cation even  with  a  good  lens,  the  observer  may  require  to  submit  the  rock  to  more 
precise  investigation  at  home,  before  its  true  character  can  be  ascertained.  For  the 
purposes  of  field-work,  however,  the  following  points  should  be  noted. 
a.  The  rock  can  be  easily  scratched  with  the  knife. 

(a)  Effervesces  briskly  with  acid = limestone. 

(b)  Powder  of  streak  effervesces  in  hot  acid.     See  dolomite  (p.  151). 

{c)  No  effervescence  with  acid :  may  be  granular  crystalline  gypsum  (alabaster) 
or  anhydrite  (pp.  79,  152). 
fi.  The  rock  is  not  easily  scratched.  It  is  almost  certainly  a  silicate.  Its  character 
should  be  sought  among  the  massive  crystalline  rocks  (p.  154).  If  it  be  heavy, 
appear  to  be  composed  of  only  one  mineral,  and  have  a  marked  greenish 
tint,  it  may  be  some  kind  of  amphibolite  (p.  182) ;  if  it  consist  of  some 
white  mineral  (felspar)  and  a  green  mineral  which  gives  it  a  distinct  green 
colour,  while  the  weathered  crust  shows  more  or  less  distinct  effervescence,  it 
may  be  a  fine-grained  diorite  (p.  165),  or  diabase  (p.  170)  ;  if  it  be  grey  and 
granular,  with  striated  felspars  and  dark  crystals  (augite  and  magnetite),  with 
a  yellowish  or  brownish  weathered  crust,  it  is  probably  a  dolerite  (p.  169)  or 


84  GEOGNOSY  book  u 

andcsite  (p.  167) ;  if  it  be  compact,  finely-crystalliiie,  scratched  with  difficulty, 
showing  crystals  of  orthoclase,  and  witli  a  bleaclied  argillaceous  weathered 
crust,  it  is  probably  an  orthoclase-porphyry  (p.  164),  or  quartz-porphyry  (p.  160). 
The  occurrence  of  distinct  blebs  or  crystals  of  quartz  in  the  fresh  fracture 
or  weathered   face  will   suggest  a  place   for  the  rock   in  the  quartziferous 
crystalline  series  (granites,  quartz-porphyries,  rhyolites),  or  among  the  g^eisMS 
and  schists. 
iv.  A  fresh  fracture  shows  the  rock  to  have  a  foliated  structure. 
The  foliated  rocks  are  for  the  most  ]>art  easily  recognisable  by  the  prominence  of 
their  component  minerals  (p.  175).     Where  the  minerals  are  so  intimately  mingled  as 
not  to  be  separable  by  the  use  of  the  lens,  the  following  hints  may  be  of  service  : — 
a.  The  rock  has  an  unctuous  feel,  and  is  easily  scratched.     It  may  be  talc-schist 
(p.   183),  chlorite  -  scliist  (p.   183),  sericitic  mica -schist  (p.  185),  or  foliated 
serpentine  (p.  183). 
/3.  The  rock  emits  an  earthy  smell  when  breathed  on,  is  harder  than  those  included 
in  a,  is  fine -'grained,  dark-gi'ey  in  colour,  splits  with  a  slaty  fracture  and 
contains  i)erhaps  scattered  crystals  of  iron -pyrites  or  some  other  mineral.     It 
is  some  argillaceous-schist  or  clay-slate,  the  varieties  of  which  are  named  from 
the   predominant   enclosed   mineral,    as   chiastolite  -  slate,    andalusite  -  schist, 
ottrelite-schist  Ac.  (p.  179)  ;  if  it  has  a  silky  lustre  it  may  be  phyllite. 
y.  The  rock  is  comjwsed  of  a  mass  of  ray-like  or  fibrous  ci-ystals  matted  together. 
If  the  fibres  are  exceedingly  fine,  silky,  and  easily  separable,  it  is  probably 
asbestos ;    if  they  are    coarser,   greenish   to  white,   glassy,   and  hard,    it  is 
probably  an  actinolite-schist  (p.  182).     Many  seri^en tines  are  seamed  with  veins 
of  the  fine  silky  fibrous  variety  termed  chrysotile,  which  is  easily  scratched. 
8.  The  rock  has  a   hardness  of  nearly  7,  and   splits  with   some  difficulty  along 
micaceous  folia.      It  is  probably  a  quartzose  variety  of  mica-schist,  quartz- 
schist,  or  gneiss  (pp.  179,  184,  185). 
c.  The  rock  shows  on  its  weathered  surface  small  ]>articles  of  quartz  and  folia  of 
mica  in  a  fine  decomposing  base.     It  is  probably  a  fine-grained  variety  of 
mica-schist  or  gneiss. 
y.  A  fresh  fracture  shows  the  rock  to  have  a  frag^ental  (clastic)  structure. 
Where  the  component  fragments  are  large  enough  to  be  seen  by  the  naked  eye  or 
with  a  lens,  there  is  usually  little  difficulty  in  dctennining  the  true  nature  and  proper 
name  of  the  rock.     Two  characters  require  to  Ije  specially  considered — the  component 
fragments  and  the  cementing  jMwte. 

1.  The  FragmetUs. — According  to  the  sliajw,  size,  and  comi)Osition  of  the  fragments, 
different  names  are  assigned  to  clastic  rocks. 

a.  Shape. — If  the  fi-agmeuts  are  chiefly  rounded,  the  rock  may  be  sought  in  the 
sand  and  gravel  series  (p.  127),  while  if  they  are  large  and  angular,  it  may  be  classed 
as  a  breccia  (p.  130).  Some  mineral  substances,  however,  do  not  acquire  rounded 
outlines,  even  after  long-continued  attrition.  Mica,  for  example,  splits  up  into  thin 
laminae,  which  may  bo  broken  into  small  flakes  or  spangles,  but  never  become  rounded 
granules.  Other  minerals,  also,  which  have  a  ready  cleavage,  are  apt  to  break  up 
along  their  cleavage -i)lanes,  and  thus  to  retain  angular  contours.  Calc-spar  is  a 
familiar  example  of  this  tendency.  Organic  i-emains  comj^sed  of  this  mineral  (such  as 
crinoids  and  echinoids)  may  often  be  noticed  in  a  very  fragmentary  condition,  having 
evidently  been  subjected  to  long-continued  comminution.  Yet  angular  outlines  and 
fresh  or  little  worn  cleavage-surfaces  may  be  found  among  them.  Many  limestones 
consist  largely  of  sub-angular  organic  debris.  Angular  inorganic  detritus  is  character- 
istic of  volcanic  breccias  and  tuffs  (p.  135). 

/3.  Size. — Where  the  fragments  are  hai-d,  rounded,  or  sub-angular  quartzose  grains, 
the  size  of  a  pin's  head  or  less,  the  rock  is  probably  some  form  of  sandstone  (p.  131). 
Where  they  range  up  to  the  size  of  a  i)ea,  it  may  be  a  j>ebbly  sandstone,  fine  con- 


PART  II  §  iii  DETERMINATION  OF  ROCKS  85 

glomerate  or  grit ;  where  they  vary  from  the  size  of  a  pea  to  that  of  a  walnut,  it  is  an 
ordinary  gravel  or  conglomerate  ;  where  they  range  up  to  the  size  of  a  man's  head  or 
larger,  it  is  a  coarse  shingle  or  conglomerate.  A  considerable  admixture  of  sub-angular 
stones  makes  it  a  breociated  conglomerate  or  breccia ;  but  where  the  materials  are 
loosely  aggregated,  the  deposit  may  be  some  kind  of  glacial  drift,  such  as  moraine-stuff 
or  boulder-clay  (p.  133).  Large  angular  and  irregular  blocks  are  characteristic  of  coarse 
volcanic  agglomerates  (p.  137). 

y  ,  Composition. — In  the  majority  of  cases,  the  fragments  are  of  quartz,  or  at 
least  of  some  siliceous  and  enduring  mineral.  Sandstones  consist  chiefly  of  rounded 
quartz-grains  (p.  131).  Where  these  are  unmixed  with  other  ingredients,  the  rock  is 
sometimes  distinguished  as  a  quartzose  sandstone.  Such  a  rock  when  indurated 
becomes  quartzite  (p.  180).  Among  the  quartz -grains,  minute  fragments  of  other 
minerals  may  be  observed.  When  any  one  of  these  is  prominent,  it  may  give  a  name  to 
the  variety  of  sandstone,  as  felspathic,  micaceous  (p.  104).  Volcanic  tuffs  and  breccias 
are  characterised  by  the  occurrence  of  lapilli  (veiy  commonly  cellular)  of  the  lavas  from 
the  explosion  of  which  they  have  been  formed.  Among  interbedded  volcanic  rocks,  the 
student  will  meet  with  some  which  he  may  be  at  a  loss  whether  to  class  as  volcanic, 
or  as  formed  of  ordinary  sediment.  They  consist  of  an  intermixture  of  volcanic  detritus 
with  sand  or  mud,  and  pass  on  the  one  side  into  tnie  tuffs,  on  the  other  into  sandstones, 
shales,  limestones,  etc.  If  the  component  fragments  of  a  non-crystalline  rock  give  a 
brisk  effervescence  with  acid,  they  are  calcareous,  and  the  rock  (most  likely  a  limestone, 
or  at  least  of  calcareous  composition)  may  be  searclied  for  traces  of  fossils. 

2.  The  Paste, — It  sometimes  happens  that  the  component  fragments  of  a  clastic 
rock  cohere  merely  from  pressure  and  without  any  discoverable  matrix.  This  is 
occasionally  the  case  mth  sandstone.  Most  commonly,  however,  there  is  some  cementing 
paste.  If  a  drop  of  weak  acid  produces  effervescence  from  between  the  component 
non-calcareous  grains  of  a  rock,  the  paste  is  calcareous.  If  the  grains  are  coated  with  a 
red  crust  which,  on  being  bruised  between  white  i»ai>er,  gives  a  cherry-red  powder,  the 
cementing  material  is  the  anhydrous  peroxide  of  iron.  If  tlie  jMiste  is  yellow  or  brown 
it  is  probably  in  great  part  the  hydrous  peroxide  of  iron.  A  dark  brown  or  black 
matrix  which  can  be  dissipated  by  heating  is  bituminous.  Where  the  comjwnent 
grains  are  so  firmly  cemented  in  an  exceedingly  hard  matrix  that  they  break  across  rather 
than  separate  from  each  other  when  the  stone  is  fractured,  tlie  paste  is  j>robably  siliceous. 

Detennination  of  Specific  Gravity. — The  student  will  find  this  character  of  con- 
siderable advantage  in  enabling  him  to  discriminate  between  rocks.  He  may  acquire 
some  dexterity  in  estimating,  even  with  the  hand,  the  probable  specific  gravity 
of  substances ;  but  he  should  l>egin  by  determining  it  with  a  balance.  Jolly's  spring 
balance  is  a  simple  and  serviceable  instrument  for  this  purpose.  It  consists  of  an 
upright  stem  having  a  graduated  strip  of  mirror  let  into  it,  in  front  of  which  hangs 
a  long  spiral  wire,  with  rests  at  tlie  l>ottom  for  weighing  a  substance  in  air  and  in 
water.  For  most  purposes  it  is  sufficiently  accurate,  and  a  determination  can  be  made 
with  it  in  the  course  of  a  few  minutes.^  Another  and  more  convenient  instrument  has 
been  invented  by  W.  N.  Walker,  consisting  of  a  lever  graduated  into  inches  and  tenths, 
and  resting  on  a  knife-edge  stand,  on  one  side  of  which  is  ]>Iaced  a  movable  weight, 
while  on  the  long  graduated  side  the  substance  to  l>e  weiglied  is  suspended.  This 
instrument  has  the  advantage  of  not  being  so  liable  to  get  out  of  order  as  other 
contrivances.' 

^  Jolly's  spring  balance  can  be  obtained  through  any  optician  or  mineral  dealer  from 
Berberich,  of  Munich,  for  nine  florins  or  27s.  In  the  United  States  it  is  manufactured 
bv  Geo.  Wade  &  Co.,  at  the  Hoboken  Institute. 

*  See  Oeol,  Mag,  1883,  p.  109,  for  a  description  and  drawing  of  this  instrument,  and 
the  manner  of  using  it.  It  may  be  obtained  of  Lowden,  optician,  Dundee,  and  How  &  Co., 
Farriogdon  Street,  London.     Its  price  is  31 «.  6ti. 


86  GEOGNOSY  book  u 

Meohaaioal  AnalyBis. — Much  may  be  learnt  regarding  the  composition  of  a  rock 
by  reducing  it  to  powder.  In  the  case  of  many  sandstones  and  clays  this  reduction  may 
easily  be  effected  by  drying  the  stone  and  crumbling  it  between  the  fingers.  But  when 
the  material  is  too  compact  for  such  treatment  some  fragments  of  it  placed  within  folds 
of  paper  upon  a  surface  of  steel  may  be  reduced  to  |K)wder  by  a  few  smart  blows  of  a 
hammer.  The  powder  can  be  sifted  through  sieves  of  varying  degrees  of  fmeness  and 
the  separate  fragments  may  be  picked  out  with  a  fine  brush  and  examined  with  a  lens. 
If  they  are  dark  in  colour  they  may  be  placed  on  white  paper,  if  light-coloured  they 
are  more  readily  observed  upon  a  black  paper.  Portions  of  this  ix>wder  may  be  carefully 
washed  and  mounted  with  Canada  balsam  on  glass,  as  in  the  way  described  below  for 
microscopic  slices.  In  this  way  the  constituent  minerals  of  many  crystalline  rooks  may 
be  isolated  and  studied  with  great  facility.  For  purposes  of  comparison  specimens  of  the 
rock-forming  minerals  should  be  procured  and  treated  in  a  similar  way.  A  series  of 
typical  preparations  of  the  {K>wder  or  minute  fragments  of  such  minerals  affords  to  the 
student  an  admirable  basis  from  which  to  start  in  his  study  of  the  crystallographic  and 
optical  characters  of  the  minerals  which  he  will  require  to  identify  among  the  con- 
stituents of  rocks. 

Another  method  of  isolating  the  several  components  of  certain  rocks  is  by  washing 
the  triturated  materials  in  water  and  alloiKing  the  sediment  to  subside.  The  finer 
and  lighter  ]>articles  may  be  drawn  off,  while  the  coarser  and  heavier  grains  will  sink 
according  to  their  resi>ective  specific  gravities,  and  may  then  be  separated  and  ooUeoted. 
This  may  be  done  by  means  of  a  ^ide  tube  with  a  stop-c^ck  at  the  bottom,  or  by 
gently  washing  the  powder  with  water  on  an  inclined  surface,  when,  as  in  the  analogons 
treatment  of  veinstones  and  ores  in  mining,  the  particles  arrange  themselves  acoordxng 
to  their  re8i>ective  gravities,  the  lightest  being  swept  away  by  the  current. 

Magnetic  particles  may  be  extracted  with  a  magnet,  the  end  of  which  is  preserved 
from  contact  witli  the  powder  by  being  covered  with  fine  tissue-|>aper.  An  electro- 
magnet will  at  once  ^i-ithdraw  the  particles  of  minerals  which  contain  far  too  little  iron 
to  be  ordinarily  recognised  as  magnetic ;  in  this  way  the  particles  of  a  ferruginous 
magiiesian  mica  may  in  a  few  seconds  be  gathered  out  of  the  powder  of  a  granite.^ 

Where  the  difference  between  the  specific  gravity  of  the  comjwnent  minerals  of  a 
rock  is  slight,  they  may  be  separated  by  means  of  a  solution  of  given  density.  Mr.  £. 
Sonstadt  proposed  the  use  of  a  saturated  solution  of  iodide  of  mercury  in  iodide  of 
potassium,  which  has  a  maximum  density  of  nearly  3 '2.*''  Rohrl)ach's  solution,  consisting 
of  iodide  of  mercury  and  iodide  of  barium,  has  a  density  of  as  much  as  3 '588.'  More 
serviceable  is  the  solution  of  borotungstate  of  cadmium,  with  a  density  of  3*28,  proposed 
by  D.  Klein.^  The  x)owder  of  a  rock  being  introduced  into  one  of  these  liquids,  those 
particles  whose  sjiecific  gravity  exceeds  that  of  the  liquid  will  sink  to  the  bottom,  while 
those  which  are  lighter  will  float.  This  ]>roce8s  allows  of  the  sc|)aration  of  the  felspars 
from  each  other,  and  at  once  eliminates  the  heavy  minerals  such  as  hornblende,  augite, 
and  black  mica.  By  the  addition  of  water  or  other  liquid,  as  the  case  may  be,  the 
si>ecific  gravity  may  l>e  reduced,  and  different  solutions  of  given  density  may  be  emjdoyed 
for  determining  and  isolating  rock-constituents.  This  method  of  analysis  is  important 
in  affording  a  ready  means  of  se])arating  the  quartz  and  fels^Mir  of  a  rock.'^ 

*  Mim.  AccuL.  ties  Sci.  xxxiL  No.  11  ;  Fouque  and  Michel-Levy,  '  Mindralogie  Micro- 
graphique,*  p.  115. 

•-^  Chem,  News,  xxix.  (1874),  p.  128.  »  Xeues  Jahrb.  1888,  p.  186. 

**  Compt.  rend,  xciii.  (1881),  p.  318.  More  recently  R.  Brauns  has  introduced 
methylene  iodide,  which  gives  a  density  of  3*33  and  is  diluted  with  benzole.  Neties  Jahrb, 
1886,  ii.  p.  72.     See  also  J.  W.  Retgers,  op,  cit.  1889,  ii.  p.  185. 

*  Fouqu^  and  Michel-L^vy,  *  Mineralogie  Micrographicjue,'  p.  117.  Tlioulet,  BidL  Soc 
Afin,  France^  ii.  (1879),  p.  17.  A  cheap  form  of  instrument  for  isolating  minerals  by  means 
of  heavy  solutions  is  described  by  Mr.  J.  W.  Evans,  Geol,  Mag,  1891,  p.  67. 


lRT  n  §  iii  DETERMINATION  OF  ROGKS  87 

Hydrofluoric  add  may  be  used  in  separating  the  mineral  constituents  of  rocks.  The 
ck  to  be  studied  is  reduced  to  powder  and  introduced  gently  into  a  platinum  capsule 
ntaining  the  concentrated  acid.  During  the  consequent  effervescence,  the  mixture  is 
ntiouBly  stirred  with  a  platinum  spatula.  Some  minerals  are  converted  into  fluorides, 
hers  into  fluosilicates,  while  some,  particularly  the  iron-magnesia  species,  remain 
idisaolved.  The  thick  jelly  of  silica  and  alumina  is  removed  with  water,  and  the 
ystalline  minerals  lying  at  the  bottom  can  then  be  dried  and  examined.  By  arresting 
le  solution  at  different  stages  the  different  minerals  may  be  isolated.  This  process  is 
Imintbly  adapted  for  collecting  the  pyroxene  of  pyroxenic  rocks.* 

ii.  Chemical  Analysis, 

The  determination  of  the  chemical  composition  of  rocks  by  detailed  analysis  in 
le  wet  way,  demands  an  acquaintance  with  practical  chemistry  which  comparatively 
w  geologists  possess,  and  is  consequently  for  the  most  part  left  in  the  hands  of  chemists, 
lio  are  not  geologists.  But  as  some  theoretical  questions  in  geology  involve  a  consider- 
»le  knowledge  of  chemical  processes,  so  a  satisfactory  analysis  of  rocks  is  best  performed 
r  one  who  understands  the  nature  of  the  geological  problems  on  which  such  an  analysis 
■y  be  expected  to  throw  light.  As  a  rule,  detailed  chemical  analysis  lies  out  of  the 
there  of  a  geologist's  work  :  yet  the  wider  his  knowledge  of  chemical  laws  and  methods 
le  better.  He  should  at  least  be  able  to  employ  with  accuracy  the  simpler  processes  of 
lemical  research. 

TrtatmetU  with  Add. — The  geologist's  accoutrements  for  the  field  should  include  a 
aall  bottle  of  powered  citric  acid,  or  one  with  a  mineral  acid,  and  provided  with  a 
ass  stopper  prolonged  downwards  into  a  point  Dilute  hydrochloric  acid  has  been 
mmonly  employed  ;  but  H.  C.  Bolton  proposed  in  1877  the  use  of  organic  acids  in  place 
'  the  usual  mineral  acids.  Citric  acid  is  |)articularly  serviceable  for  the  purpose,  and  has 
le  advantage  over  the  mineral  acids  that  it  can  be  carried  in  powder,  and  a  strong  solution 
it  in  water  can  be  made  in  such  quantity  and  at  such  time  as  may  be  required.  A 
;tle  of  the  powder  placed  with  the  point  of  a  knife  on  a  surface  of  limestone  and 
oistened  ^^'ith  a  drop  of  water  will  give  the  proper  reaction.* 

When  a  drop  of  acid  gives  effervescence  upon  a  surface  of  rock,  the  reaction  is  caused  by 
le  liberation  of  bubbles  of  carbon  dioxide,  as  this  oxide  is  replaced  by  the  more  power- 
1  acid.  Hence  effervescence  is  an  indication  of  the  ])resence  of  carbonates,  and  when 
isk  is  specially  characteristic  of  calcium-carbonate.  Limestone  and  markedly  cal- 
reoos  rocks  may  thus  at  once  be  detected.  By  the  same  means,  the  decomposition  of 
ch  rocks  as  dolerite  may  be  traced  to  a  considerable  distance  inward  from  the  surface, 
le  original  lime-bearing  silicate  of  the  rock  having  been  decomposed  by  infiltrating 
in-water,  and  partially  converted  into  carbonate  of  lime.  This  carbonate  l)eing  far 
ore  sensitive  to  the  acid-test  than  the  other  carbonates  usually  to  be  met  with  among 
cks,  a  drop  of  weak  cold  acid  suffices  to  produce  abundant  effervescence  even  from  a 
ystalline  face.  But  the  effervescence  becomes  much  more  marked  if  we  apply  the  acid 
I  the  powder  of  the  stone.  For  this  purpose,  a  scratch  may  be  made  and  then  touched 
ith  acid,  when  a  more  or  less  copious  discharge  of  carbonic  acid  may  be  obtained, 
here  otherwise  it  might  ap|>ear  so  feebly  as  perhaps  even  to  esca])e  observation.  Some 
Tbonates,  dolomite  for  example,  are  hardly  affected  by  acid  until  it  is  heated.  This  is 
)De  by  placing  some  fragments  of  the  substance  at  the  bottom  of  a  test-tube,  covering 
lem  with  acid  and  applying  a  flame. 

It  is  a  convenient  method  of  roughly  estimating  the  purity  of  a  limestone,  to  place  a 
agment  of  the  rock  in  acid.  If  there  is  much  impurity  (clay,  sand,  oxide  of  iron,  &c. ), 
lis  will  remain  behin'd  as  an  insoluble  residue,  and  may  then  be  further  tested  chemi- 

*  Fouqu4  and  Michel-L^vy,  op.  cit.  p.  116. 
*  Ann.  N€w  York  Acad.  ScL  i.  (1879)  p.  1.     Chem.  News,  xxxvi..  xxxvii.,  xxxviii.,  xliii. 


88  GEOGNOSY  book  n 

pally,  or  examined  with  the  microscoi^.  In  this  way  many  limestones  among  the 
crystalline  schists  may  l)e  dissolved  in  acetic  acid,  leaving  a  residue  of  pyroxenes,  amphi- 
boles,  micas  or  other  silicates.  Of  course  the  acid,  e8i»ecially  if  strong  mineral  acid  is 
employed,  may  attack  some  of  the  non-calcareous  constituents,  so  that  it  cannot  be 
concluded  that  the  residue  al»solutely  represent*  ever}'thing  present  in  the  rock  except 
the  carbonate  of  lime  ;  but  the  projwrtion  of  non-calcareoiw  matter  so  dissolved  by  the 
acid  will  usually  be  small. 

Further  cheinical  processes. — A  thorough  chemical  analysis  of  a  rock  or  mineral  is 
indispensable  for  the  elucidation  of  its  comi)Osition.  But  there  are  several  i>roces8es  by 
which,  until  that  complete  analysis  has  l>een  made,  the  geologist  may  add  to  his  know- 
ledge of  the  chemical  nature  of  the  objects  of  his  study.  It  is  commonly  the  case  that 
minerals  about  which  he  may  be  doubtful  are  ]>reci8ely  those  which,  from  their  small 
size,  are  most  difficult  of  separation  fi"om  the  rest  of  the  rock  preparatory  to  analytical 
processes.  The  mineral  apatite,  for  exam})le,  occurs  in  minute  hexagonal  prisms,  which 
on  cross-fracture  might  l:)e  mistaken  for  nepheline,  or  even  sometimes  for  quartz.  If, 
however,  a  drop  of  nitric  acid  solution  of  molyMate  of  ammonia  be  placed  upon  one  of 
these  crystals,  a  yellow  precipitate  will  api)ear  if  it  I ►e  aimtit^;.  Xepheline,  which  is 
another  hexagonal  mineral  likewise  abundant  in  some  rocks,  gives  no  yellow  precipitate 
with  the  ammonia  solution,  while  if  a  drop  of  hydrochloric  acid  l)e  jnit  over  it,  crystals 
of  chloride  of  sodiimi  or  common  salt  will  be  obtained.  These  reactions  can  l)e  observed 
even  with  minute  ci-ystals  or  fragments,  by  placing  them  on  a  glass  slide  under  the 
microscoiKJ  and  using  an  exceedingly  attenuated  pipette  for  dropping  the  liquid  on 
the  slide.  ^ 

Two  ingenious  api)lications  of  chemical  processes  to  the  detennination  of  minute 
fragments  of  minerals  are  now  in  use.  In  one  of  these,  devised  by  Boricky,* 
hydrofluosilicic  acid  of  extreme  purity  Is  employed.  This  acid  decomi)ose8  most 
silicates,  and  forms  from  their  bases  hydrofluosilicates.  A  i)article  about  the  size  of  a 
pin's  head  of  the  mineral  to  be  examined  is  fixed  by  its  base  u^jon  a  thin  layer  of  Canada 
balsam  spread  upon  a  slip  of  glass,  and  a  drop  of  the  acid  is  placed  upon  it.  The 
preparation  is  then  set  in  moist  air  nrar  a  saucer  of  water  under  a  bell-glass  for  twenty- 
four  houi*s,  after  which  it  is  enclosed  in  dry  air,  with  chloride  of  calcium.  In  a  few 
hours  the  hydrofluosilicates  ciystallize  out  upon  the  balsam  and  can  be  examined  with 
the  microscope.  Those  of  jiotassium  take  the  form  of  cubes,  of  sodiimi  hexagonal 
prisms,  Ac. 

The  second  process,  devised  by  SzaIk),  consists  in  utilizing  the  colorations  given  to 
the  flame  of  a  Bunsen-burner  by  sodium  and  jjotassium.  An  elongated  splinter  of  the 
mineral  to  be  examined  is  fii-st  placed  in  the  outer  or  oxidizing  part  of  the  flame  near 
the  base,  and  then  in  the  reducing  }»art  fui-ther  up  and  nearer  the  centre.  The  amount 
of  sodium  i>resent  in  the  mineral  is  indicated  by  the  extent  to  which  the  flame  is  coloured 
yellow.  The  potassium  is  similarly  estimated,  but  the  flame  is  then  looked  at  with 
cobalt  glass,  so  as  to  eliminate  the  influence  of  the  sotlium.^ 

Blow-pipe  Testa. — The  chemical  tests  with  the  blow-pipe  are  simple,  easily  applied, 
and  require  only  patience  and  practice  to  give  gioat  assistance  in  the  detennination  of 
minerals.     If  unacquainterl  with  blow-piin^  analysis,  the  student  nmst  refer  to  one  or 

^  An  excellent  treatise  on  the  chemical  examination  of  minerals  under  the  microsco])e  is 
that  by  MM.  Klement  and  Renard,  *  Inactions  microchemiques  fi  cristaux  et  lenr  applica- 
tion en  analyse  qualitative,'  Brussels,  1886.  See  also  H.  Behrens,  Ann.  icoU  Polytechnique 
de  Del/tf  i.  ISSfi  p.  176  ;  Xeucit  Johrh.  vii.  Beilage  Baud.  p.  43.')  ;  Zvitsch./.  Analyt.  Chfmie, 
XXX.  ii.  p.  126-174  (1891). 

'^  Archiv  Natvnmss.  Ltnidesifyrch/nracfntiifj  rou  Bohmen,  iii.  fasc.  3,  1876.  'Elemente 
einer  ueueu  cheniisch-niikroskoi^isclien  Mineral-  und  Gesteinsanalyse.'     Prag,  1877. 

'  Szabo,  *  Ueber  eiue  neue  Methodic  die  Felspathe  audi  in  Gesteinen  zu  bestimmen.' 
Buda-Pest,  1876. 


»ART  II  §  iii  DETERMINATION  OF  ROCKS  89 

•ther  of  the  numerous  text-books  on  the  subject,  some  of  which  are  mentioned  below.  ^ 
•"or  early  practice  the  following  apparatus  will  be  found  sufficient : — 

1.  Blow-pipe. 

2.  Thick-wicked  candle,  or  a  tin  box  filled  with  the  material  of  Child's  night-lights, 
»nd  ftiniished  with  a  piece  of  Freyberg  wick  in  a  metallic  support. 

8.  Platinum-tipped  forceps. 

4.  A  few  pieces  of  platinum  wire  in  lengths  of  three  or  four  inches. 

5.  A  few  pieces  of  platinum  foil. 

6.  Some  pieces  of  charcoal. 

7.  A  number  of  closed  and  open  tubes  of  hard  glass. 

8.  Three  small  stoppered  bottles  containing  sodium -carbonate,  borax,  and  micro- 
cosmic  salt. 

9.  Magnet. 

This  list  can  be  increased  as  experience  is  gained.  The  whole  apparatus  may  easily 
le  packed  into  a  box  which  will  go  into  the  corner  of  a  ^portmanteau. 

iii.  Chemual  Synthesis, 

As  already  remarked  (j).  64),  much  interesting  light  has  been  thrown  on  the  natural 
t>nditions  in  which  minerals  and  rocks  have  been  formed,  by  actual  experiments  in 
rhich  these  bodies  are  reproduced  artificially.  Since  the  classic  experiments  of  Hall 
auch  progress  has  been  made  in  this  subject,  notably  from  the  prolonged  and  admirable 
'esestxihes  carried  on  in  Paris  by  Professor  Daubree  and  by  Messrs.  Fouqu6  and  Michel- 
'j^ry.  To  some  of  the  results  obtained  by  these  observers  reference  will  be  made  in 
look  III.  Part  I.  Sect.  iv.  The  j>roces8es  of  investigation  have  been  grouj)ed 
n  three  classes.  1st.  Those  by  the  *  dry  way '  as  in  fusion  and  sublimation,  sometimes 
imply,  sometimes  with  the  intervention  of  a  mineralizing  agent  such  as  borax,  borates, 
luorides,  chlorides,  &c.  2nd.  Those  by  the  *  wet  way  '  where  water  or  steam  are  used 
is  dissolvauts  either  by  tliemselves  or  with  the  aid  of  some  mineralizing  agent,  and  3rd, 
rhose  where  some  combination  of  the  two  foregoing  methods  is  employed,  that  is,  where 
rater  or  steam  is  made  to  act  at  a  high  tenii^erature  and  imder  great  pressure.* 

iv.  Mi^nrascopic  Investigation.^ 

The  value  of  the  microscoj^  as  an  aid  in  geological  research  is  now  everywhere 
cknowledged.  Some  infonnation  may  here  be  given  as  to  the  methods  of  procedure  in 
nicroscopical  inquiry. 

^  Tlie  great  work  on  the  blow-pipe  Is  Plattner's,  of  which  an  English  translation  has  been 
»iiblishe<l.  Elderhorst's  'Manual  of  Qualitative  Blow -pipe  Analysis  and  Determinative 
fineralog>',*  by  H.  B.  Nason  and  C.  F.  Chandler  (Philadelphia :  N.  S.  Porter  and  Coates), 
I  a  smaller  but  useful  volume  ;  while  still  less  pretending  is  Scheerer's  *  Introduction  to  the 
Jse  of  the  Mouth  Blow-pipe,'  of  which  a  third  edition  by  H.  F.  Blandford  was  published  in 
876  by  F.  Norgate.  An  admirable  work  of  reference  will  be  found  in  Professor  Brush's 
Manual  of  Determinative  Mineralogy '  (New  York  :  J.  Wiley  and  Son).  F.  v.  Kobell's 
Tafein  znr  Bestimmung  der  Mineralieu '  (Munich)  are  useful.  A  valuable  summary  will  be 
oand  in  Prof.  Cole's  'Aids  in  Practical  Geology,'  1891. 

•  See  on  this  subject  Daubree's  great  work  'Geologie  Experiraentale,'  1879  ;  Fouque  et 
€ichcl-Levy,  *Synthese  des  Mineraux  et  des  Roches,'  1882;  Stanislas  Meunier,  *  Les 
l^thodes  de  Syuth^  en  Mineralogie, '  1891  ;  ti\so  postea,  p.  302  et  seq. 

•  On  the  microscopic  investigation  of  rocks  consult  Fouque  and  Michel-Levy,  *  Min^ralogie 
tficrogfaphique,'  2  vols.  Paris,  1879  ;  Michel-Levy,  *  Les  Min«^raux  des  Roches,'  Paris,  1888  ; 
lichcl-L^vyand  Lacroix,  'Tableaux  des  Mineraux  des  Roches,'  1889  ;  Rosenbusch,  *  Mikro- 
kopische  Physiographie  der  Mineralien  und  Gesteine,'  2  vols.,  one  of  which  has  been  trans- 
ited into  English  by  Iddings  and  published  by  Macmillan  and  Co. ;  also  his  *  Hiilfstabellen 


90  GEOGXOSY  book  n 

1.  PreparatiGn  of   mioroBCopic  slides  of   rocks  and  mlnarals. — Tlie  ohserver 

ought  to  l>e  able  to  ]>re))aro  his  own  slices,  and  in  many  cases  will  Hnd  it  of  advantage 
to  do  so,  or  at  least  personally  to  Aui)erintend  their  preparation  by  others.  It  is 
desirable  that  he  should  know  at  the  outset  that  no  costly  or  unwieldy  set  of  apparatus 
is  needful  for  his  purpose.  If  he  is  resident  in  one  place  and  can  accommodate  a  catting 
machine,  such  as  a  lapidary's  lath,  he  Mill  find  the  process  of  preparing  rock-slices 
greatly  facilitated.^  The  thickness  of  each  slice  must  be  mainly  regulated  by  the  nature 
of  the  rock,  the  rule  being  to  make  the  slice  as  thin  as  can  conveniently  be  cut,  so  as  to 
save  labour  in  giindiug  down  afterwards.  Perhaps  the  thickness  of  a  shilling  may  be 
taken  as  a  fair  average.  The  operator,  however,  may  still  further  reduce  tliis  thicknesi 
by  cutting  and  polishing  a  face  of  the  si>ecimen,  cementing  that  on  glass  in  the  way  to 
1>e  immediately  de8crilx^d,  and  then  cutting  as  close  as  possible  to  the  cemented  sur&oe. 
The  thin  slice  thus  left  on  the  glass  can  then  be  ground  down  with  comjiarative  ease. 

Excellent  rock -sections,  however,  may  be  prepared  without  any  machine,  provided 
the  operator  possesses  ordinary  neatness  of  hand  and  patience.  He  must  procure  as  thin 
chii»s  as  ])ossible.  Should  the  rocks  be  accessible  to  him  in  the  field,  he  should  select 
the  freshest  portions  of  them,  and  by  a  dexterous  use  of  the  hammer,  break  off  from  a 
sharp  edge  a  number  of  thin  splinters  or  chips,  out  of  which  he  can  choose  one  or  m<at 
for  rock-slices.  These  chips  may  l>e  about  an  inch  s(|uare.  It  is  well  to  take  several  of 
them,  as  the  first  s])ecimen  may  chance  to  be  s]K)iled  in  the  preparation.  The  geologist 
ought  also  always  to  carry  off  a  ])i(^e  of  the  same  block  from  which  his  chip  is  taken, 
that  he  may  have  a  s|>ecimen  of  the  rock  for  future  reference  and  comparison.  Eveiy 
such  hand-specimen,  as  well  as  the  chips  1)elonging  to  it,  ought  to  be  wrapped  up  in 
I>aper  on  the  s]K>t  where  it  is  obtained,  and  with  it  should  be  ]>laced  a  label  containing 
the  name  of  the  locality  and  any  notes  that  may  l>e  thought  necessary.  It  can  hardly 
be  too  fre^^uently  reiterated  that  all  such  field-notes  ought  as  far  as  }H)ssible  to  be  written 
down  on  the  ground,  when  the  actual  facts  are  before  the  eye  for  examination. 

Having  obtained  hia  thin  slices,  either  by  having  them  slit  with  a  machine  or  by 
detaching  with  a  hammer  as  thin  splinters  as  |)ossible,  the  o])erator  may  proceed  to  the 
prc])aration  of  them  for  the  microscop.  For  this  puriH)8e  the  following  simple  apparatus 
is  all  that  is  absolutely  needful,  though  if  a  grinding-machine  be  added  it  will  save 
time  and  labour. 

List  of  Apparatus  required  in  the  Prcimratian  of  Thin  Slices  of  Rocks  and  AfineraU 

for  Microscopical  Examination. 

1 .  A  cast-iron  plate  \  inch  thick  and  9  inches  square. 

2.  Two  jueccs  of  plate-glass,  9  inches  s<[uare. 

zur  Mikroskopischen  Mineralbestimmung,'  1888,  translated  into  English  by  F.  H.  Hatch 
and  published  by  Swan  Sonnenschein  k  Co. ;  F.  Uutley,  '  Kock-foruiiug  Minerals,'  London, 
1888,  and  Prof.  Cole's  volume  above  cited. 

^  A  machine  well  adapted  for  both  cutting  and  polishing  was  devhted  some  years  ago  by 
Mr.  J.  B.  Jordan,  and  may  be  had  of  Messrs.  Cotton  and  Johnson,  Gerrard  Street,  Soho, 
London,  for  £10,  10^.  Another  slicing  and  polishing  machine,  invented  by  Mr.  F.  6.  Cnttell, 
costs  £6,  105.  These  machines  are  too  un\\ieldy  to  be  carried  about  the  country  by  a  field- 
geologist.  Fuess  of  Berlin  8ui>plies  two  small  and  convenient  hand-instruments,  one  for 
slicing,  the  other  for  grinding  and  polishing.  The  sliciug-machine  Is  not  quite  so  satisfactory 
for  hard  rocks  as  one  of  the  larger,  more  solid  forms  of  apparatus  worked  by  a  treadle.  Bat 
the  grinding-machine  is  useful,  and  might  be  added  to  a  geologist's  outfit  without  material 
inconvenience.  If  a  lapidary  is  within  reach,  much  of  the  more  irksome  part  of  the  work 
may  be  saved  by  getting  him  to  cut  off  the  thin  slices  in  directions  marked  for  him  upon  the 
specimens.  Many  lapidaries  now  undertake  the  whole  labour  of  cutting  and  monntiiig 
microscopic  slides. 


ART  II  §  iii  DETERMINATION  OF  ROCKS  91 

8.  A  Water  of  Ayr  stone,  6  inches  long  by  2}  inches  broad. 

4.  Coarse  emery  (1  lb.  or  so  at  a  time). 

5.  Fine  or  flour-emery  (ditto). 

6.  Putty  powder  (1  oz.) 

7.  Canada  balsam.  (There  is  an  excellent  kind  prepared  by  Rimmington,  Bradford, 
peeially  for  microecopio  preparations,  and  sold  in  shilling  bottles. ) 

8.  A  small  forceps,  and  a  common  sewing-needle  with  its  head  fixed  in  a  cork. 

9.  Some  oblong  pieces  of  common  flat  window-glass  ;  2  x  1  inches  is  a  convenient 
lie. 

10.  Glasses  with  ground  edges  for  mounting  the  slices  upon.  They  may  be  had 
.t  any  chemical  instrument  maker's  in  different  sizes,  the  commonest  in  this  country 
mng  3x1  inches,  though  this  size  is  rather  too  long  for  convenient  handling  on 
k  rotating  stage. 

11.  Thin  covering-glasses,  square  or  round.  These  are  sold  by  the  ounce  ;  4  oz.  will 
m  saiBcient  to  begin  with. 

12.  A  small  bottle  of  spirits  of  wine. 

The  first  part  of  the  process  consists  in  rubbing  down  and  polishing  one  side  of  the 
hip  or  slice,  if  this  has  not  already  been  done  in  cutting  off"  a  slice  affixed  to  glass, 
•  above  mentioned.  We  place  the  chip  upon  the  wheel  of  the  grinding-inachine,  or, 
iOing  that,  upon  the  iron  plate,  with  a  little  coarse  emery  and  water.  If  the  chip  is 
0  shaped  that  it  can  be  conveniently  pressed  by  the  finger  against  the  plate  and  kept 
here  in  regular  horizontal  movement,  we  may  proceed  at  once  to  nib  it  down.  If,  how- 
rer,  we  find  a  difficulty,  from  its  small  size  or  otherwise,  in  holding  the  chip,  one  side 
f  it  may  be  fastened  to  the  end  of  a  bobbin  or  other  convenient  bit  of  wood  by  means  of 
cement  formed  of  three-parts  of  resin  and  one  of  beeswax,  which  is  easily  softened  by 
leating.  A  little  practice  will  show  that  a  slow,  equable  motion  with  a  certain  steady 
ttmaie  is  most  effectual  in  producing  the  desired  flatness  of  surface.  When  all  the 
(nighnesses  have  been  removed,  which  can  be  told  after  the  chip  has  been  dipped  in 
rater  so  as  to  remove  the  mud  and  emery,  we  place  the  specimen  upon  the  square  of 
late-glass,  and  with  flour-emery  and  water  continue  to  rub  it  down  until  all  the  scratches 
itieed  by  the  coarse  emery  have  been  removed  and  a  smooth  polished  surface  has  been 
roduced.^  Care  should  be  taken  to  wash  the  chip  entirely  free  of  any  grains  of  coarse 
mery  before  the  polishing  on  glass  is  begun.  It  is  desirable  also  to  reserve  the  glass  for 
oliahing  only.  The  emery  gets  finer  and  finer  the  longer  it  is  used,  so  that  by  remain- 
ig  on  the  plate  it  may  be  used  many  times  in  succession.  Of  course  the  glass  itself  is 
^om  down,  but  by  using  alternately  every  portion  of  its  surface  and  on  both  sides,  one 
late  may  be  made  to  last  a  considerable  time.  If  after  drying  and  examining  it  carefully, 
'e  find  the  surface  of  the  chip  to  be  polished  and  ftee  from  scratches,  we  may  advance  to 
lie  next  part  of  the  process.  But  it  will  often  ha])pen  that  the  surface  is  still  finely 
sratched.  In  this  case  we  may  place  the  chip  upon  the  Water  of  Ayr  stone  and  with  a 
ttle  water  gently  rub  it  to  and  fro.  It  should  be  held  quite  flat  The  Water  of  Ayr 
^ne,  too,  should  not  be  allowed  to  get  worn  into  a  hollow,  but  should  also  be  kept  quite 
at,  otherwise  we  shall  lose  part  of  the  chip.  Some  soft  rocks,  however,  will  not  take  an 
Dscratched  surface  even  with  the  Water  of  Ayr  stone.  These  may  be  finished  with 
atty  powder,  applied  with  a  bit  of  woollen  rag. 

The  desired  flatness  and  polish  having  been  secured,  and  all  trace  of  scratches  and 
irt  having  been  completely  removed,  we  proceed  to  a  further  stage,  which  consists  in 


1  Exceedingly  impalpable  emery  powder  may  be  obtained  by  stirring  some  of  the  finest 
ncry  in  water,  and  after  the  coarse  particles  have  subsided,  pouring  ofl'  the  liquid  and 
llowing  the  fine  suspended  dust  gradually  to  subside.  Filtered  and  dried,  the  residue  can 
I  kept  for  the  more  delicate  parts  of  the  polishing. 


92  GEOGXOSY  book  ii 

grinding  down  tlie  opposite  side  and  reducing  the  chip  to  the  re(|nisite  degree  of  thin- 
ness. The  first  step  is  now  to  cement  the  ^)olislied  surface  of  the  chip  to  one  of  the 
pieces  of  common  glass.  A  thin  piece  of  iron  (a  common  shovel  does  quite  well)  is 
heated  over  a  lire,  or  is  placed  between  two  supjKjrts  over  a  gas-flame.^  On  this  plate 
must  be  laid  the  piece  of  glass  to  which  the  slice  is  to  be  affixed,  together  with  the  slice 
itself.  A  little  Canada  balsam  is  drop{>ed  on  the  centre  of  the  glass  and  allowed  to 
remain  until  it  has  acquired  the  necessary  consistency.  To  test  this  condition,  the  point 
of  a  knife  should  be  inserted  into  the  balsam,  and  on  being  removed  should  be  rapidly 
cooled  by  being  pressed  against  some  cold  surface.  If  it  soon  becomes  hard  enough  to 
resist  the  pressure  of  the  finger  nail,  it  has  been  sufficiently  heated.  Care,  however, 
must  l)e  observed  not  to  let  it  remain  too  long  on  the  hot  plate  ;  for  it  will  then  become 
brittle  and  start  from  the  glass  at  some  future  stage,  or  at  least  will  break  away  from 
the  edges  of  the  chip  and  lo^ve  them  exposed  to  the  risk  of  being  frayed  off.  The  heat 
should  be  kept  as  moderate  as  {)ossible,  for  if  it  becomes  too  great  it  may  injure  some 
portions  of  the  rock.  Chlorite,  for  example,  is  rendered  quite  opaque  if  the  heat  ia  so 
great  as  to  drive  off  its  water. 

When  the  balsam  is  found  to  be  I'eady,  the  chip,  which  has  l^een  warmed  on  the 
same  plate,  is  lifted  with  the  forceps,  and  laid  gently  down  uiK)n  the  balsam.  It  is 
well  to  let  one  end  touch  the  balsam  fii-st,  and  then  gi*adually  to  lower  the  other,  as  in 
this  way  the  air  is  driven  out.  With  the  jwint  of  a  needle  or  a  knife  the  chip  should 
be  moved  about  a  little,  so  as  to  ex])cl  any  bubbles  of  air  and  promote  a  firm  cohesion 
luitween  the  glass  and  the  stone.  The  glass  is  now  removed  with  the  forceps  from  the 
plate  and  put  upon  the  table,  and  a  lead  weight  or  other  small  heavy  object  is  placed 
upon  the  chip,  so  as  to  keep  it  pressed  down  until  the  balsam  has  cooled  and  hardened. 
If  the  o]>eration  ha.s  l>ecn  successful,  the  slide  ought  to  be  ready  for  further  treatment  as 
soon  as  the  balsam  has  become  cold.  If,  however,  the  balsam  is  still  soft,  the  glass  must 
be  again  placed  on  the  plate  and  gently  heated,  until  on  cooling,  the  balsam  fulfils  the 
condition  of  resisting  the  pressure  of  the  finger-nail. 

Having  now  produced  a  firm  union  of  the  chip  and  the  glass,  we  pi"oceed  to  rub  down 
the  remaining  side  of  the  stone  with  coarse  emery  on  the  iron  plat«  as  before.  If  the 
glass  cannot  be  held  in  the  hand  or  moved  by  the  simple  ju-essui-e  of  the  fingers,  which 
usually  suffices,  it  may  be  fastened  to  the  end  of  tlu;  bobbin  with  the  cement  as  before. 
When  the  chip  has  been  reduced  until  it  is  tolerably  thin  ;  until,  for  example,  light 
Hpi)eai*s  through  it  when  held  l)etween  the  eye  and  the  window,  we  may,  as  l>cfore,  wash 
it  clear  of  the  course  emery  and  continue  the  reduction  of  it  on  the  glass  plate  with  fine 
emery.  Crystalline  rocks,  such  as  granite,  gneiss,  diorite,  dolerite,  and  modem  lavas, 
can  be  thus  re<luced  to  the  re({uired  thinness  on  the  gloss  platt?.  Softer  rocks  may 
re<iuire  gentle  treatment  with  the  Wat4?r  of  Ayr  stone. 

The  last  parts  of  the  process  are  the  most  delicate  of  all.  We  desire  to  make  the 
section  as  thin  as  possible,  and  for  that  j)ur]K)se  continue  rubbing  until  after  one  final 
attempt  we  may  perhajis  find  to  our  dismay  that  gi*eat  iwiit  of  the  slice  has  disappeared. 
The  utmost  caution  should  be  ust^d.  The  slide  should  be  kept  as  flat  as  ])Ossible,  and 
looked  at  fre<iuently,  that  the  first  indications  of  disniption  may  be  detected.  The 
thinness  desirable  or  attainable  depends  in  gi-eat  measure  u})on  the  nature  of  the  rock. 
Transjiarent  minerals  need  not  be  so  much  reduce^l  as  more  opaque  ones.  Some 
minerals,  indeed,  remain  absolutely  oi>a([ue  to  the  last,  like  pyrite,  magnetite,  and 
ilmenite. 

The  slide  is  now  ready  for  the  microscoj)o.  It  ought  always  to  be  examined  with 
that  instrument  at  this  stage.  We  can  thus  see  whether  it  is  thin  enough,  and  if  any 
chemical  tests  are  require<l  they  can  readily  be  applied  to  the  exjwsed  surface  of  the 

^  A  piece  of  wire-gauze  placed  over  the  Hame,  with  an  interval  of  an  inch  or  more 
between  it  and  the  overlying  thin  iron  plate,  tends  to  diffuse  the  heat  and  prevent  the 
balsam  from  being  unequally  heated. 


IT  n  §  iii  DETERMIXA  TION  OF  ROCKS  93 

».  If  the  rock  has  proved  to  be  very  brittle,  and  we  have  only  succeeded  in  procur- 
a  thin  slice  after  much  labour  and  several  failures,  nothing  further  should  be  done 
h  the  preparation,  unless  to  cover  it  with  glass,  as  will  be  immediately  explained, 
eh  not  only  protects  it,  but  adds  to  its  transparency.  But  where  the  slice  is  not  so 
^e,  and  will  bear  removal  from  its  original  rough  scratched  piece  of  glass,  it  should 
trmnsferred  to  one  of  the  glass-slides  (No.  10).  For  this  purpose,  the  preparation  is 
e  more  placed  on  the  warm  iron  plate,  and  close  alongside  of  it  is  ]mt  one  of  the 
of  glass  which  has  been  carefully  cleaned,  and  on  the  middle  of  which  a  little 
balsam  has  been  dropped.  The  heat  gradually  loosens  the  cohesion  of  the  slice, 
ich  is  then  very  gently  pushed  with  the  needle  or  knife  along  to  the  contiguous 
in  slip  of  glass.  Considerable  practice  is  needed  in  this  pai-t  of  the  work,  as  the 
e,  being  so  thin,  is  apt  to  go  to  pieces  in  being  transferred.  A  gentle  inclination  of 
warm  plate,  so  that  a  tendency  may  be  given  to  the  slice  to  slip  downwards  of  itself 
to  the  clean  glass,  may  be  advantageously  given.  We  must  never  attempt  to  lift 
•lice.  All  shifting  of  its  position  should  be  performed  with  the  point  of  the  needle 
>ther  shar|>  instrument.  If  it  goes  to  pieces  we  may  yet  be  able  to  pilot  the  frag- 
its  to  their  resting-place  on  the  balsam  of  the  new  glass,  and  the  resulting  slide  may 
mfficient  for  the  required  ])urpose. 

When  the  slice  has  been  safely  conducted  to  the  centre  of  the  glass  slip,  we  put  a 
le  Canada  balsam  over  it,  and  warm  it  as  before.  Then  taking  one  of  the  thin  cover- 
Mes  with  the  forceps,  we  allow  it  gradually  to  rest  upon  the  slice  by  letting  down 
t  one  side,  and  then  by  degrees  the  whole.  A  few  gentle  circular  movements  of  the 
er-glass  with  the  point  of  the  needle  or  forceps  may  be  needed  to  ensure  the  total 
ippearance  of  air-bubbles.  When  these  do  not  appear,  and  when,  as  before,  we  find 
t  the  balsam  has  acquired  the  proper  degree  of  consistence,  the  slide  containing  the 
e  is  removed,  and  placed  on  the  table  with  a  small  lead  weight  above  it  in  the  same 
f  as  already  described.  On  becoming  quite  cold  and  hard  the  su])erabundant  balsam 
nd  the  edge  of  the  cover-glass  may  be  scraped  off  with  a  knife,  and  any  which  still 
lepcs  to  the  glass  may  be  removed  with  a  little  spirits  of  wine.  Small  labels  should 
kept  ready  for  affixing  to  the  slides  to  mark  localities  and  reference  numbers.  TIius 
elled,  the  slide  may  be  put  away  for  future  study  and  comparison. 
The  whole  process  seems  i)erhaps  a  little  tedious.  But  in  reality  much  of  it  is  so 
ehanical,  that  after  the  mode  of  manipulation  has  been  learnt  by  a  little  experience, 
rubbing-down  may  be  done  while  the  oi)erator  is  reading.  Thus  in  the  evening, 
en  enjoying  a  pleasant  book  after  his  day  in  the  field,  he  may  at  the  same  time,  after 
le  practice,  rub  down  his  rock-chiiw,  and  thus  get  over  the  dnidgery  of  the  oiieration 
lost  unconsciously. 

Boxes,  with  grooved  sides  or  with  flat  trays  for  carrying  microscopic  slides,  are  sold 
iifferent  sizes.     Such  boxes  are  most  convenient  for  a  travelling  equipage,  as  they  go 

0  small  s|>ace,  and  with  the  help  of  a  little  cotton-wool  they  hold  the  glass  slides 
oly  without  risk  of  breakage.  For  a  final  resting-place,  a  case  with  shallow  trays  or 
men  in  which  the  slides  can  lie  flat  is  most  convenient. 

2.  The  Microscope. — Unless  the  observer  proposes  to  enter  into  great  detail  in  the 
estigation   of  the  minuter  parts  of  rock  -  stinicture,    he  does   not  require  a   large 

1  expensive  instrument.  For  most  geological  purposes,  objectives  of  2,  1,  and  I  inch 
al  length  are  sufficient.  But  it  is  desirable  also  for  sjiecial  work,  such  as  the 
estigation  of  crystallites  and  inclusions  of  minerals,  to  have  an  objective  capable  of 
gnif3ring  up  to  200  or  300  diameters.  An  instrument  with  fairly  good  glasses  of 
ise  powers,  according  to  the  arrangement  of  object-glasses  and  eye- pieces,  may  be  had 
some  London  makers  for  £5.  But  for  some  of  the  most  imi>ortant  i>arts  of  the 
sroscopical  study  of  rocks  a  rotating  stage  is  requisite,  the  presence  of  which 
wssarily  adds  to  the  cost  of  the  instrument.  One  of  the  best  microscopes  s]>ecially 
ipted  for  petrographical  research  is  that  devised  by  Mr.  A.  Dick,  and  manufactured  by 
lit  k  Son,  of  81  Tottenham  Court  Road,  London,  price  £18  without  objectives. 


94  GEOGNOSY  book  n 


Among  the  indisjKjnsable  adjuncts  are  two  Nicol-prisms,  one  (iK)lari2er)  to  be  fitted 
below  the  stage,  tlie  other  (analyser)  most  advantageously  placed  over  the  eye-piece.  A 
([uartz-wedgc  is  useful  in  examination  i^ith  ]>olarized  light.  A  nose-piece  for  tvo 
objectives,  screwed  to  the  foot  of  the  tube,  saves  time  and  trouble  by  enabling  the 
observer  at  once  to  pass  from  a  low  to  a  high  |)ower.  The  numerous  pieces  of  apiMratos 
necessary  for  })liysiological  work  are  not  needed  in  the  examination  of  rocks  and 
minerals. 

3.  Methods  of  ETaminatJon. — A  few  hints  may  be  here  given  for  the  guidance  of 
the  student  in  making  his  own  microscopic  obsen'ations,  but  he  must  consult  some  of 
the  s{)ecial  treatises,  mentioned  on  p.  89,  for  full  details. 

Rtflccted  Light. — It  is  not  infrequently  desirable  to  observe  with  the  microscope  the 
characters  of  a  rock  as  an  oiMu^ue  object.  This  cannot  usually  be  done  with  a  broken 
fragment  of  the  stone,  except  of  course  with  very  low  |K)wers.  Hence  one  of  the  most 
useful  preliminary  examinations  of  a  pre]^>arod  slice  is  to  place  it  in  the  field,  and, 
throwing  the  mirror  out  of  gear,  to  converge  as  strong  a  light  uiM)n  it  as  can  be  had, 
short  of  bright  direct  sunlight.  The  observer  can  then  see  some  way  into  the  rock  and 
observe  the  relative  thicknesses  and  forms  of  its  constituents.  The  advantage  of  this 
method  is  particularly  noticeable  in  the  ease  of  o]>a(iue  minerals.  The  sulphides  and 
iron-oxides  so  abundant  in  rocks  appear  as  densely  black  objects  with  transmitted  light, 
and  show  only  their  external  form.  But  by  throwing  a  strong  light  upon  their  surface, 
we  may  often  discover  not  only  their  distinctive  colours,  but  their  characteristic  internal 
structure.  Titaniferoiis  iron  is  an  admirable  example  of  the  advantage  of  this  method. 
Seen  with  transmitted  light,  that  mineral  ap^^ars  in  black,  structureless  grains  or 
oi)at[ue  patches,  though  frc(|ueutly  bounded  by  definite  lines  and  angles.  But  with 
reflected  light,  the  cleavage  and  lines  of  growth  of  the  mineral  can  then  often  be  clearly 
seen,  and  what  seemed  to  be  uniform  black  j^tehes  are  found  in  many  cases  to  enclose 
bright  brassy  kernels  of  pyiite.  Magnetite  also  })rcsents  a  characteristic  blue-black 
colour,  which  distinguishes  it  from  the  other  iron-oxides. 

Transmitted  Light. — It  is,  of  course,  with  the  light  allowed  to  pass  through  prepared 
slices  that  most  of  the  microscopic  examination  of  minerals  and  rocks  is  perfonned.  A 
little  exi>erience  will  show  the  learner  that,  in  viewing  objects  in  this  way,  he  may 
obtain  somewhat  ditterent  results  from  two  slices  of  the  same  rock  according  to  their 
relative  thinness.  In  the  thicker  one,  a  cei-tain  mineral  or  rock,  obsidian  for  example, 
will  ap|>ear  i)erhai)s  brown  or  almost  black,  while  in  the  other  what  is  evidently  the 
same  substance  may  be  pale  yellow,  green,  brown,  or  almost  colourless.  Triclinic 
felsi>ars  seen  in  i»olarizcKi  light  give  only  a  i>ale  milky  light  when  extremely  thin,  but 
present  brij;ht  chromatic  l>ands  when  somewhat  thicker. 

Polarized  Light. — By  means  of  polarized  light,  an  exceedingly  delicate  method  of 
investigation  is  made  available.  We  use  l)oth  the  Nicol- prisms.  If  the  object  be  singly- 
refracting,  such  as  a  piece  of  glass,  or  an  amorphous  body,  or  a  crystal  1)elonging  to 
some  substance  which  crystallizes  in  the  isometric  or  cubic  system  (or  if  it  \)e  a  tetragonal, 
hexagonal  or  rlionibohedral  crystal,  cut  i)ei']»endicular  to  its  princi];»al  axis),  the  light 
will  reach  our  eye  apparently  unaffected  by  the  intervention  of  the  object.  The  field 
will  remain  dark  when  the  axes  of  the  two  prisms  are  at  right  angles  (crossed  Nicols),  in 
the  same  way  as  if  no  intervening  object  were  thei-e.  Such  bodies  are  isotropic^  In 
all  other  cases,  the  substance  is  doubly  •  refracting  and  modifies  the  (solarized  beam 
of  light.  On  rotating  one  of  the  prisms,  we  ]^)erceive  l)ands  or  flashes  of  colour,  and 
numerous  lines  ap{iear  which  before  were  invisible.  The  field  no  longer  remains  dark 
when  the  two  Nicol- prisms  are  crossed.     Such  a  substance  is  anisotropic. 

It  is  evident,  therefore,  that  we  may  readily  tell  by  this  means  whether  or  not  a 
rock  contains  any  glassy  constituent.  If  it  does,  then  that  jiortion  of  its  mass  will 
become  dark  when  the  prisms  are  crossed,  while  thtj  ciystallinc  i»arts  which,  in  the  vast 

'  But  the  effect  of  pressure  may  give  weak  colour-tints  in  glasses  and  in  cubic  crystals. 


ABT  II  §  Hi  DETERMINATION  OF  ROCKS  95 

tugoiity  of  cases,  do  not  belong  to  the  cubic  system,  will  remain  conspicuous  by  their 
frightness.  A  thin  plate  of  quartz  makes  this  separation  of  the  glassy  and  crystalline 
larts  of  a  rock  even  more  satisfactory.  It  is  placed  between  the  Nicol-prisms,  which 
nay  be  so  adjusted  with  reference  to  it  that  the  field  of  the  microscope  appears  uniformly 
riolet.  The  glassy  portion  of  any  rock,  being  singly-refracting  or  isotropic,  placed  on 
he  stage  will  allow  the  yiolet  light  to  pass  through  unchanged,  but  the  crystalline 
portions,  being  doubly-refracting  or  anisotropic,  will  alter  the  violet  light  into  other 
[Hismatic  colours.  The  object  should  be  rotated  in  the  field,  and  the  eye  should  be  kept 
iteadily  fixed  upon  one  portion  of  the  slide  at  a  time,  so  that  any  change  may  be  observed. 
rhiB  is  an  extremely  delicate  test  for  the  presence  of  glassy  and  crystalline  constituents. 

In  searching  for  the  crystallographic  system  to  which  a  mineral  in  a  microscopic 
slide  should  be  referred,  attention  is  given  to  the  directions  in  which  the  mineral  placed 
betwieen  crossed  Nicols  appears  dark,  or  to  what  are  called  the  directions  of  its  extinc- 
tion.  It  is  extinguished  (that  is,  the  normal  darkness  of  the  field  between  the  crossed 
NiooIb  is  restored)  when  two  of  its  axes  of  elasticity  for  vibrations  of  light  coincide  with 
the  principal  sections  of  the  two  prisms.  During  a  complete  rotation  of  the  slide  in  the 
field  of  the  microscope  the  mineral  becomes  dark  in  four  positions  90"*  apart,  each  of 
vrhich  marks  that  coincidence.  When,  on  the  other  hand,  the  prisms  are  placed  parallel 
to  each  other,  the  coincidence  of  their  principal  sections  with  the  axes  of  elasticity  in 
the  mineral  allows  the  maximum  of  light  to  pass  through,  which  likewise  occurs  four 
times  in  a  complete  rotation  of  the  mineral.  The  different  crystallographic  systems  are 
distinguishable  by  the  relation  between  their  crystallographic  axes  and  their  axes  of 
elsaticity.  By  noting  this  relation  in  the  case  of  any  given  mineral  (and  there  are 
nsoally  sections  enough  of  each  mineral  in  the  same  rock-slice  to  furnish  the  required 
data)  its  crystalline  system  may  be  fixed.  But  in  many  cases  it  has  been  found  possible 
to  establish  characteristic  distinctions  for  individual  mineral  species,  by  noting  the 
angle  between  the  direction  of  their  extinction  and  certain  principal  faces. 

The  determination  of  whether  the  component  grains  of  a  rock  belong  to  uniaxial  or 
biaxial  doubly-refracting  minerals  is  a  point  of  much  importance,  which  is  effected  by 
means  of  an  achromatic  condenser  inserted  in  the  apei-ture  of  the  stage  below  the  slide 
and  soitably  adjusted  so  as  to  converge  the  rays  of  light  within  the  grain  or  crystal.  The 
Kiools  having  been  crossed,  the  eye-piece  is  removed,  and  the  eye  when  held  a  little 
distance  from  the  open  end  of  the  tube  will  perceive  a  dark  bar,  ring,  or  cross  move  across 
the  field  as  the  stage  is  rotated,  if  the  mineral  examined  has  been  cut  at  a  favourable 
angle.  By  the  form  and  behaviour  of  these  indications  the  uniaxial  or  biaxial  character 
is  made  evident. 

PUoehraism  (Dichroism). — Some  minerals  show  a  change  of  colour  when  a  Nicol- 
prism  is  rotated  below  them  ;  hornblende,  for  example,  exhibiting  a  gi*adatiou  from  deep 
brown  to  dark  yellow.  A  mineral  presenting  this  change  is  said  to  be  pleochroic 
(polychroic,  dichroic,  trichroic).  To  ascertain  the  pleochroism  of  any  mineral  we  may 
remove  the  upper  polarizing  prism  (analyser)  and  leave  only  the  lower  (polarizer).  If 
as  we  rotate  the  latter,  no  change  of  tint  can  be  observed,  there  is  no  pleochroic  mineral 
present,  or  at  least  none  which  shows  pleochroism  at  the  angle  at  which  it  has  been 
bisected  in  the  slice.  But  in  a  slice  of  any  crystalline  rock,  crystals  may  usually  be 
observed  which  offer  a  change  of  hue  as  the  prism  goes  round.  These  are  examples  of 
pleochroism.  This  behavioiu*  may  be  used  to  detect  the  mineral  constituents  of  rocks. 
Thus  the  two  minerals  hornblende  and  augite,  which  in  so  many  respects  resemble  each 
other,  cannot  always  be  distinguished  by  cleavage  angles,  in  microscopic  slices.  But  as 
Tschermak  pointed  out,  augite  remains  passive  or  nearly  so  as  the  lower  prism  is  rotated  : 
it  is  not  pleochroic,  or  only  very  feebly  so  ;  while  hornblende,  on  the  other  hand, 
especially  in  its  darker  varieties,  is  usually  strongly  pleochroic.  It  is  to  be  observed, 
however,  that  the  same  mineral  is  not  always  er^ually  pleochroic,  and  that  the  absence 
of  this  property  is  therefore  less  reliable  as  a  negative  test,  than  its  presence  is  as  a 
positiye  test. 


9 

96  GEOGNOSY  book  n 


It  would  be  beyond  tlie  scope  of  tliis  volume  to  enter  into  the  complicated  details  of 
the  microscopic  structure  of  minerals  and  rocks.  This  information  must  be  sought  in 
some  of  the  works  specially  <levoted  to  it,  a  few  of  which  are  cited  on  p.  89. 

In  his  examination  of  rocks  with  the  microscope,  the  student  may  find  an  advantage 
in  pro])ounding  to  himself  the  following  questions,  and  i-eferring  to  the  pages  here 
cited. 

1st,  Is  the  rock  entirely  crystalline  (pp.  97,  148,  154),  consisting  solely  of  crystals  of 
different  minerals  interlaced  ;  and  if  so,  what  are  these  mineiuls  ?  2nd,  Is  there  any  trace 
of  a  glassy  ground-mass  or  base  (])p.  100,  114)  If  Should  this  \)e  detected,  the  rock  is 
certainly  of  volcanic  origin  (pp.  162,  171).  3rd,  Can  any  c\'idence  be  found  of  the  devitri- 
tication  of  wliat  may  have  l>een  ut  one  time  the  glassy  basis  of  the  whole  rock  ?  This 
devitrification  might  be  shown  by  the  appearance  of  numerous  microscopic  hairs,  rods, 
bundles  of  feather-like  in-egular  or  gi*anular  aggregations  (p.  115).  4th,  In  what  uitier  did 
the  minerals  crystallize  ?  This  may  often  be  matle  out  with  a  niicrosco])e,  as,  for  instance, 
where  one  mineral  is  enclosed  within  another  (p.  114).*  5th,  What  is  the  nature  of  any 
altei-ation  which  the  roi'k  may  have  undergone  ?  In  a  vast  number  of  cases  the  slices 
show  abundant  evidence  of  such  alteration  :  felsjMir  passing  into  gi-anular  kaolin,  augite 
changing  into  viridite,  olivine  into  serpentine,  while  secondaiy  calcite,  e])idote,  quartz, 
and  zeolites  run  in  minute  veins  or  till  up  interstices  of  the  rock  (p.  345).  6th,  Is  the 
rock  a  fragniental  one  ;  and  if  so,  what  is  the  nature  of  its  comiH>nent  gi*ains  (p.  127)1 
Is  any  trace  of  organic  remains  to  be  detected. 


§  iv. — General  outward  or  Megascopic  (Macroscopic)  Chaxacters  of  Bocks.* 

I 

1.  Structure.^ — The  diifereiit  kinds  of  rock-structures  distinguishable 
by  the  unaided  eye  are  denoted  either  by  ordinary  descriptive  adjectiveSi 
or  by  terms  derived  from  rocks  in  which  the  special  structures  are 
characteristically  developed,  such  as  gi-anitoid,  brecciated,  shaly.  It 
must  be  borne  in  mind,  however,  that  the  external  character  of  a  rock 
does  not  always  supjdy  us  with  its  true  internal  structure,  which  may  be 
gained  only  by  microscopic  examination.    This  is  of  course  more  especially 

*  It  is  i)0ssible,  however,  that  a  crystal  enclosed  within  another  may  sometimes  have 
crystallized  tliere  out  of  a  portion  of  the  surrounding  magma  of  the  rock  which  has  been 
unclosed  within  the  larger  crystal  {^tstea^  p.  303). 

-  The  following  general  text-books  on  rocks  may  l)e  referred  to  :  Macculloch,  *  A 
Geological  Classilication  of  Rocks,*  &c.,  Loudon,  1821.  B.  von.  Cotta,  ' Rocks  Classified 
and  Described,'  translated  by  Lawrence,  Loudon,  1866.  Zirkel,  *  Lehrbuch  der  Petrographie,' 
two  vols.  lionn,  1866.  Senft,  'Classification  der  Felsarten,'  Breslau,  1857;  *.Die 
Krystalliuischen  Felsgeiuengtheile,*  Berlin,  1868.  Kenngott,  *  Eleniente  der  Petrographie,' 
Leipz.  1868.  A.  von  Lasaulx,  'Elemente  der  Petrographie,'  Bonn,  1875.  Bischof, 
*  Chemical  Geology,'  translated  for  Cavendish  Society,  1854-59,  au<l  supplement,  Bonn, 
1871.  Roth,  *  Allgenieine  und  Chemische  Geologic,'  Berlin,  18/9.  Other  works  in  which 
the  niicrascoincal  characters  are  more  si>ecially  treated  of,  are  enumerated  on  p.  108. 

'^  In  the  3rd  e«iition  of  Jukes'  'Student's  Manual  of  Geology*  (1871),  p.  93,  it  wae 
proi>osed  to  reser\'e  the  term  **  Structure  "  for  large  features,  such  as  characterise  rock-blockf, 
and  to  use  the  term  "Texture  "  for  the  minuter  characters,  such  as  can  be  judged  of  in  hand 
specimens.  M.  De  Lai)parent  makes  a  similar  distinction  (Traite,  p.  602,  note).  But  the 
practice  of  using  the  word  structure  as  it  is  employed  above  in  the  text,  has  received  such  a 
support  from  the  petrographers  of  Germany  that  though  I  still  think  it  would  be  preferable 
to  distinguish  1)etweeu  texture  and  stmcturey  I  have  adopted  what  has  now  the  sanction  of 
coimnon  usage. 


ART  II  §  iv         MEGASCOPIC  CHARACTERS  OF  ROCKS  97 

rue  of  the  close-grained  kinds,  where  to  the  naked  eye  no  definite 
tructure  is  discernible.  Some  of  the  definitions  originally  founded  on 
xtemal  appearance  have  been  considerably  modified  by  microscopic 
fivestigation.  Many  compact  rocks,  for  instance,  have  been  proved  to  be 
rhoUy  crystalline. 

The  same  rock-mass  may  show  very  different  structures  and  textures 
n  different  parts  of  its  extent.  This  is  true  alike  of  sedimentary  and 
gneous  materials.  It  may  be  observed  even  in  the  several  portions  of 
►ne  continuous  mass  of  erupted  rock — variations  in  the  rate  of  cooling,  in 
emperature,  and  other  circumstances  have  combined  to  produce  some- 
imes  the  most  extraordinary  textural  and  even  structural,  as  well  as 
hemical  and  mineralogical  contrasts  in  a  boss  or  sheet  of  igneous 
ock.^  Hence  the  student  must  be  on  his  guard  against  concluding  that 
wo  portions  of  rock  strikingly  unlike  each  other  in  outward  appearance 
annot  be  portions  of  one  original  continuous  mass. 

Crystalline  (Phanerocrystalline),  consisting  wholly  or  chiefly 
I  crystalline  particles  or  crystals.^  Where  the  individual  elements  of 
he  rocks  are  of  large  size,  the  structure  is  coarse-crystalline  (granitic),  as  in 
aany  granites.  When  the  particles  are  readily  visible  to  the  naked  eye, 
nd  are  tolerably  uniform  in  size,  as  in  marble,  many  granites  and 
domites,  the  rock  is  said  to  be  granular-crystalline.  Successive  stages 
a  the  diminution  of  the  size  of  the  particles  may  be  traced  until  these 
re  no  longer  recognisable  with  the  naked  eye,  and  the  structure  must 
hen  be  resolved  with  the  microscope  {fine-crystalline^  micro-ci'ystalline,  crypto- 
rydaUine).  Fine-grained  rocks  may  also  be  called  compad,  though  this 
erm  is  likewise  applicable  to  the  more  close-grained  varieties  of  the 
ragmental  series.  The  microscopic  characters  of  such  rocks  should 
Iways  be  ascertained  where  possible.^ 

Many  crystalline  rocks  consist  not  only  of  crystals,  but  of  a  magma 
r  paste,  in  which  the  crystalline  particles  are  seen  by  the  naked  eye  to 
■e  imbedded.  It  is  of  course  impossible,  except  from  analogy,  to  deter- 
line  macroscopically  what  may  be  the  nature  of  this  magma.  It  may 
e  entirely  composed  of  minute  crystals,  or  may  consist  of  various 
rystallitic  products  of  devitrification.  Its  intimate  structure  can  only 
e  ascertained  with  the  microscope.  But  its  existence  is  often  strikingly 
lanifest  even  to  the  unassisted  eye,  for  in  what  are  termed  "  porphyries  " 
;  forms  a  large  part  of  their  mass.  The  term  ^^  ground-rnass  ^^  is 
mployed  to  denote  this  megascopic  matrix.  Microscopic  examination 
hows  that  a  ground-mass  may  consist  of  minute  crystals,  or  crystallites, 
r  granules  and  filaments,  or  glass,  or  combinations  of  these  in  various 
roportions.     (See  pp.  109,  117.) 

Lithoid,  compact  and  stony  in  aspect,  with  no  very  distinct  crystalline 

*  See  postetty  pp.  564,  576  ;  G.  F.  Becker,  Amer.  Joum,  Sci.  xxxiii.  (1887),  p.  50.     J. 
L  L.  Vogt,  Oe^l.  Foren,  Forhand.  Stocklwlniy  xiii.  (1891). 

'  Prof.  Rosenbusch  proposed  the  terni  holocry stall ine  for  rocks  in  which  there  is  uo 
orphous  material  among  the  crystalline  constituents. 

•  On  the  crystaUization  of  igneous  rocks,  see  J.  P.  Idclings,  Bull.  Phil.  Soc.  Washington, 
.  (1889),  p.  71. 

H 


98  GEOGNOSY  book  ii 

structure.  The  term  is  especially  applied  to  the  devitrified  condition  of 
once  glassy  rocks,  such  as  obsidians,  which  have  asaumed  the  character  oE 
perlitos  or  felsites. 

Granitic  (Granitoid),  thoroughly  crystalline,  and  consiating  of 
crystals  approximately  uniform  in  size,  as  in  granite.  This  s^ucture 
in  characteristic  of  many  eruptive  rocks.  Though  usually  distinctly 
recognisable  by  the  naked  eye  ("  macromerito "  of  V<^elsang  '^  it 
sometimes  becomes  very  fine  ("  micronierite "),  and  may  be  only 
recognisable  with  the  microscofw  as  thoroughly  crystalline  (micrognn- 
itic) ;  at  other  times  it  pmfses  into  a  porphyritic  or  porphyroid  chaiwster 
by  the  ap|>earance  of  largo  crystals  di»>persed  through  a  general 
ground-mass. 

Pegmat{tic(Pegmatoid,  Graphic),  exhibiting  the  peculiar  arrange- 
ment of  crystalline  constituents  seen  in  i>egmatite  or  graphic  granite 
(p.  158),  where  the  quartz  and  felspu 
have  crystallized  simultaneously  so 
i\s  to  be  enclosed  within  each  other. 
This  structure  may  bo  seen  on  a  large 
scale  in  many  massive  veins  of  peg- 
matite ;  where  it  takes  an  exceedingly 
minute  form  it  is  known  as  micro- 
pegmatitic  (Fig.  5).  Such  micro- 
scopic intergrowth  of  quartz  and  fel- 
spar is  characteristic  of  large  maaaet 
of  eniptive  rock  (micropegmatite, 
gi-anophyre). 

Aphanilic,  a  name  given  to  tJie 
veiy  close  texture  exhibited  by  some 
igneous  rocks  {diabases,  diorites)  where 
the  component  ingredients  cannot  be 
deteiTuined  except  with  the  microscope. 
Porphyritic  (Porphyroid),  composed  of  a  compact  or  finely 
crystalline  ground-mass,  through  which  larger  crystals  of  earlier  con- 
solidation,- often  of  felspar,  are  dispersed  (Fig.  G).  This  and  the  granitic 
structure  are  the  two  great  structure*  types  of  the  eruptive  rocks. 
By  far  the  largest  number  of  these  rocks  belong  to  the  porphyritic 
type.  Microscopic  research  has  thrown  much  light  on  the  nature  of 
the  ground-mass  of  porphyritic  rocks.  Vogelsang  proposed  to  clasBify 
these  rocks  in  three  divisions  : '  1st,  Grwnophyre,  where  the  ground-mass, 
is  a  microscopic  crystalline  mixture  of  tbe  component  minerals  with 
absence  or  sparing  development  of  an  imperfectly  individualised  magma 
(seep.  118);  2nd,  ft/s(>p/iyre,  having  usually  an  impei-fectly  individualised 
or  felsitic  magma  for  the  ground-mass  (pp.  117,  119);  3rd,  VUrophyre, 
where  the  ground-mass  is  a  glassy  magma  (pp.  114,  120),     The  second 

'  Z.  DaUacJi.  Qtol.  Qea.  iiiv.  p.  534. 

'  PtienocTfstB,  Iddiuga,  Bull.  Phil.  Soc.  Waahinglm,  ii.  (1889),  p.  TS. 

'  Vogelssng,  loc.  eit.     Compnn  the  cluwlficfttion  into  graniloiJ  sod  iradiyloid,  p.  1S5. 


PABT  II  §  iv         MEOASGOPIC  CHARACTERS  OF  ROCKS  99 

sub-division  embraces  most  of  the  porphyries,  and  a  very  large  number 
of  eruptive  rocks  of  all  ages.' 


Klg,  6, 


-Puridi^tic  Strucli 


Segregated. — In  granite  and  other  crystnllino  massive  rocks, 
vein-like  portions,  coarser  (or  finer)  in  texture  than  the  rest  of  the  mass, 
may  be  observed.  These  belong  to  the  last  phase  of  consolidation,  when 
segregations  from  the  original  molten  or  viscous  magma  took  place  along 
certain  lines  or  round  particular  centres,  where  the  individual  minerals 
crystaUized  out  from  the  general  mass.  They  have  been  sometimes 
termed  "segregation,"  or  "exudation"  veins.  They  arc  to  be  dis- 
tinguished from  the  veins,  usually  of  finer  and  more  acid  material,  which 
runify  through  a  mass  of  igneous  rock  and  probably  represent  portions 
of  the  original  molten  magma  which  remained  still  liquid  and  were 
injected  into  rents  of  the  already  consolidated  parts.  These  are  the  true 
"  contemporaneous  veins "  (p.  580). 

Granular — a  somewhat  vague  term  applied  to  rocks  composed  of 
approximately  equal  grains,  which  are  sometimes  worn  fragments,  as  in 
MUidstone,  Bometimes  crystalline  particles,  as  in  granite  and  marble.  This 
texture  may  become  so  fine  as  to  pass  insensibly  into  compact.'  The 
peculiar  granular  structure  found  so  abundantly  among  metamorphic  rocks 
which  have  been  intensely  crushed  and  in  which  there  seems  to  have 
been  a  process  of  re-crystallization  among  the  powdered  particles,  has 
been  termed  granulitic  (p.  119).  This  word,  however,  is  liable  to  the 
objection  that  in  Germany  it  is  applied  to  rocks  bearing  that  structure 
while  in  France  it  is  used  for  a  holocrystalline  granite.' 

'  Aecording  to  Rownbusch  the  porphyritic  mRS«ire  rocke  ue  those  in  which,  during  the 
diflareot  ttign  of  their  producttan,  the  ibidb  minerala  have  been  formed  mare  than  once. 
SnfJaiui.  1SS2(U.},  p.  14. 

'  Ai  epplied  to  miMive  (8nipti?e)  rocki,  Rosenbusch  would  restrict  the  term  granular  to 
tbOH  in  which  och  Indiridiut  constituent  sepanted  out  during  but  one  deSnite  atige  o(  tbe 
prooew  of  rock-baiJdiiig.     Loc.  nil. 

*  Ulchel-Lirjr,  Amt.  da  Mines,  vui.  (1875),  p.  3S7  ;  'Structure  et  Classification  des 
Bocha  BroptiTai,'  1SS9,  p.  14. 


100  GEOGNOSY  book  ii 

Vitroous  or  glassy,  having  a  structure  like  that  of  artificial  glass, 
as  in  obsidian.  Among  tho  crystalline  rocks  there  is  often  present  a 
variable  amount  of  an  amorphous  ground  mass,  which  may  increase  until 
it  forma  the  main  part  of  the  substance.  Tho  nature  of  this  amorphous 
portion  is  described  at  pp.  114,  130.  Its  most  obvious  megascopic  con- 
dition is  that  of  a  volcanic  glass.  Most  vitreous  rocks  present,  even  to 
the  naked  eye,  dispersed  grains,  crystals,  or  other  enclosures.  Under  the 
microscope,  they  arc  found  to  \>e  often  crowded  with  minut«  crystals  and 
imperfect  or  incipient  crystalline  forms  (pp.  109,  115).  Resinous  is  the 
term  applied  to  vitreous  rocks  having  the  lustre  of  pitchstone,  and  to 
others  which  are  still  less  vitreous.  Devitrification  is  the  conversion  of 
tho  vitreous  into  a  crystalline  or  lithoid  structure  (pp.  116,  121). 

Streaked,  arranged  in  streaky  inconstant  lines  (Germ.  Sehlieren), 
cither  parallel  or  convergent,  and  often  undulating.  This  structure, 
conspicuously  shown  by  the  lines  of  flow  in  ritreous  rocks  (flow-Btructure, 
rtuxion-stnicturo,  fluidal- structure)  is  less  marked  where  the  materials 
have  assumed  definite  crystalline  forms.  It  can  be  seen  on  a  minute  scale, 
however,  in  many  crystalline  masses  when  examined  tvith  the  microBcope 
{p.  120). 

Banded,  arranged  in  parallel  bands,  distinguished  from  each  other 
by  colour,  texture,  structure  or  composition  ;  characteristic  of  many 
gneisses,  and  of  jaspers,  flints,  hallefiintas  and  other  flinty  rocks.  This 
term  may  frequently  bo  applied  to  the  flow-stnictiu-e  of  igneous  rocks 
referred  to  in  the  previous  paragraph,  likewise  to  the  segregation  veins 
of  eruptive  bosses  and  sheets,  and  to  the  parallel  arrangement  of  materials 
produced  in  rocka  which  liavo  under  intense  mechanical  pressure  been 
cnishe<I  and  sheared.  With  the  naked  eye  it  is  often  hardly  possible  to 
distinguish  between  the  banded  stnicture  of  deritrified  igneous  rocks  and 
that  resulting  from  intense  mecbaniciil  deformation. 

Mylonitic,  a  terra  inti-oduced  to  denote  the  peculiar  granular 
stmcture  of  rocks  which  liave  luidergone  intense  crushing.  The  materials 
have  been  rctluced  to  minute  grnins  which  have  not  recrystallized  as 
they  have  done  in  the  graiiulitic  structure,  llany  remarkable  examples 
of  this  stnicture  have  been  observed 
among  the  schists  of  the  Scottish  High- 
lands. 

Spherulitic,  eomixised  of,  or  con- 
taining small  globides  or  spherules  which 
may  Ih'  colloid  and  isotropic,  or  more  or 
less  distinctly  crystalline,  [urticularly 
with  an  internal  tibi-ous  divei^ent  struc- 
ture (Figs,  r,  17).  This  structure  occurs 
in  vitreous  mcks,  where  it  is  one  of 
the  stages  of  devitrification  in  obsidian, 
pit<.-hstonc,  etc'  (jx  121). 

The  term  lithophvse  has  been 
.:--!'i.iir™ii;icsmi«i.n-.  i>i.vniar.i.)  ;,j)j,iiod  by  F.  von  Kichthofen  to  brge 
'  Od  th«  coniitlliitioi)  anil  ot^n  of  splietnlit*  in  sciil  «niptiTF  rocka,  wc  Vhitauit. 


UEOASCOPIC  CHARACTERS  OF  BOCKS 


101 


bladder-like  apherulitea  wherein  interspaces  lined  with  crystals  occur 
between  the  auccessive  concentric  internal  layers.^  Many  andont  rhy- 
olites  present  an  aggregate  of  nodular  bodies  (Pyromeride)  duS  originally 
to  devitrification  and  subsequently  more  or  less  altered  esp^ciaily  by 
the  deposition  of  silica  within  them  (poslea,  p.  161). 

Orbicular  structure  is  one  in  which  the  component  minerals  of  an>Ck 
have  crystallized  in  such  a  way  as  to  form  spheroidal  aggregations  'some- 


times with  an  internal  radial  or  concentric  grouping.      It  is  typically  s 
in  the  napoleonite  or  ball-diorite  (kugel- 
diorit«,  orbicular  diorite,  p.  165)  of  Cor- 
sica (Fig.  8),  but  occurs  in  other  rocks, 
sometimes  even  in  granite. 

Perlitic  (Figs.  9  and  20),  having 
the  structure  of  the  rock  formerly  termed 
perlite,  wherein  between  minute  rectili- 
near fissures  the  substance  of  the  mass  ha^ 
assumed,  during  the  contraction  resulting 
from  cooling  a  finely  globular  character, 
not  unlike  the  spheroidal  structure  seen 
in  weathered  basalt  which  is  also  a  phe- 
nomenon of  contraction  during  the  cool- 
ing and  consolidation  of  an  igneous  rock. 

Crom,  Ptiil.  Sic.  Waihingtim,  xi.  p.  Ill  (18S1)  d  J  P.  Iddiugs,  o}>.  cil.  p,  446. 
Qoirtz  ummca  In  «oioe  rocks  (e.g.  baud  J  ntea)  ti  1)  globular  structure  which  was 
developed  before  the  ceBution  of  the  m  t  a  th  t  prod  ed  How.atructure,  and  which, 
according  to  M.  Michel-Levy,  may  be  reg    led  as  t    g  the  colloid  and  cryelalliied 

conditiaoi  of  silica.     Ball.  Soc,  Otot.  Fra  u   (3)        p   140 

'  JaKri.  K.  K.  OeoL  Meickianil,  1880  p   180      See  Idd   g»,  7lh  Ana.  Rep.   U.fi.  Otal. 
Surv.  (18SG-SS),  p.  249.     Amer.  Jmm.  &i  (1887)  p.  38. 


102  ;.  '-■  flEOGNOSY  Booin 

Horny,-(irnty,  having  a  compact,  homogeneous,  dull  texture,  like 
that  of  tfohi  or  flint,  as  in  chalcodony,  jasper,  flint,  and  many  halleflintaa 
and  iol^it^i. 

O^yitrnoMB  (porous),  containing  irregular  cavities  due,  in  moat 
caac^-to  the  abstraction  of  some  of  the  minerals ;  but  occasionally,  as  in 
some  limCBtoneB  (sinters),  dolomites  and  lavas,  forming  part  of  the 
(irigjrial  structure  of  the  rock. 
.'■/-[■Cellnlar. — Many  lavas,  ancient  and  modern,  have  been  saturated 
'^•.'mth  steam  at  the  time  of  their  eruption,  and  in  consequence  of  the 
-segregation  and  expansion  of  this  imprisoned  va]K>ur,  have  had  spherical 
cavities  developed  in  their  mass.  When  this  cellular  structure  is  marked 
by  comparatively  few  and  small  holes,  it  may  be  called  vesicular  j  wh^v 
the  rock  consists  partly  of  a  roughly  cellular,  and  partly  of  a  more 
compact  substance  intermingled,  as  in  the  sli^  of  an  iron  furnace,  it  ia 
said  to  bo  slaggy ;  i>ortions  where  the  colls  occupy  about  as  much  space 
as  the  solid  part,  and  vary  much  in  size  and  shape,  are  called  scoriaceous, 
this  being  the  character  of  the  rough  clinker-like  scoriie  of  recent  lava- 
streams  ;  when  the  cells  are  so  much  more  numerous  than  the  solid  part, 
that  the  stone  would  almost  or  tiuite  float  on  water,  the  structure  is  called 
pumiceouB,  the  term  jntmice  being  applied  to  the   froth-like  part  of 


4  f.  /^^J^fL^  * 


Fig,  lO.-Aiiij^Blui.lnl  Stni.tiiiTs  ;  l'.>r|ili)Titf,  old  K<-.1  Sanastolw,  Armhlre.    (K»t.  ■1»-) 

obsidian.  As  the  cellular  stnicture  can  only  be  developed  while  the  rock 
is  still  liquid,  or  at  least  \-iscid,  and  as,  while  in  this  condition,  the  man 
is  often  still  moving  away  from  its  point  of  emission,  the  cells  are  not 
infrequently  elongated  in  the  direction  of  movement  Subsequently, 
water  infiltrating  through  the  rock,  deposits  various  mineral  substances 
(calcite,  fpiartz,  chalcedony,  zeolites,  etc.)  from  solution,  so  that  the 
flattened  and  elongated  nlmond-shaped  cells  are  eventually  filled  up. 
A  cellular  rock  which  has  undergone  this  change  ia  said  to  be  an 
amygdaloid,  or  amygdaloidal,  and  the  almond-like  kernels  u'e known 
as  amygdalcs  (Fig.  10).  AVhere  the  cells  or  cavernous  spaces  of  a  roA 
are  lined  with  crystals  and  empty  inside  they  are  said  to  be  druses  or 
druay  cavities. 


PART  II  §  iv         MEGASCOPIC  CHARACTERS  OF  ROCKS  103 

Cleaved,  having  a  fissile  structure  superinduced  by  pressure  and 
known  as  cleavage  (see  p.  312,  545).  The  planes  of  cleavage  are  inde- 
pendent of  those  of  bedding,  though  they  may  coincide  with  them.  A 
cleaved  structure  is  best  seen  in  fine-grained  material,  and  is  typically 
developed  in  roofing-slate,  but  it  may  occur  in  any  compact  igneous  rock. 

Foliated,  consisting  of  minerals  that  have  crystallized  in  approxi- 
mately parallel,  lenticular,  and  usually  wavy  layers  or  folia.  Rocks  of 
this  Idnd  commonly  contain  layers  of  mica,  or  of  some  equivalent  readily 
cleavable  mineral,  the  cleavage-planes  of  which  coincide  generally  with  the 
planes  of  foliation.  Gneiss,  mica^chist  and  talc-schist  are  characteristic 
examples.  So  distinctive,  indeed,  is  this  structure  in  schists,  that  it  is 
often  spoken  of  as  schistose.  In  gneiss,  it  attains  its  most  massive 
form ;  in  chlorite-schist  and  some  other  schists,  it  becomes  so  fine  as  to 
pass  into  a  kind  of  minutely  scaly  texture,  often  only  perceptible  with  the 
microscope,  the  rock  having  on  the  whole  a  massive  structure. 

Fibrous,  consisting  of  one  or  more  minerals  composed  of  distinct 
fibres.  Sometimes  the  fibres  are  remarkably  regular  and  parallel,  as  in 
fibrous  gypsum,  and  veins  of  chrysotile,  fibrous  aragonite  or  calcite  (satin- 
spar)  ;  in  other  instances,  they  are  more  tufted  and  irregular,  as  in  asbestos 
and  actinolite-schist. 

Clastic,  fragmental,  composed  of  detritus  (p.  121).  Rocks  possess- 
ing this  character  have,  in  the  great  majority  of  cases,  been  formed  in  water, 
and  their  component  fragments  are  usually  more  or  less  rounded  or  water- 
w<»ni.  Different  names  are  applied,  according  to  the  form  or  size  of  the 
fragments.  Brecciated,  composed,  like  a  breccia,  of  angular  fragments, 
which  may  be  of  any  degree  of  coarseness.  Agglomerated,  consisting 
of  large,  roughly  rounded  and  tumultuously  •  grouped  blocks,  as  in  the 
agglomerate  filling  old  volcanic  funnels.  Conglomerated  (Conglo- 
meratic)^ made  up  of  well-rounded  blocks  or  pebbles ;  rocks  having  this 
character  have  been  formed  by  and  deposited  in  water.  Pebbly, 
containing  dispersed  water-worn  pebbles,  as  in  many  coarse  sandstones, 
which  thus  by  degrees  pass  into  conglomerates.  Psamraitic,  or  sand- 
.stone-Iike,  composed  of  rounded  grains,  as  in  ordinary  sandstone :  when 
the  grains  are  larger  (often  sharp  and  somewhat  angular)  the  rock  is 
gritty,  or  a  grit.  Muddy  (pel i tic),  having  a  texture  like  that  of  dried 
mud.  Cryptoclastic  or  compact,  where  the  grains  are  too  minute  to 
reveal  to  the  naked  eye  the  truly  fragmental  character  of  the  rock,  as  in 
fine  mudstones  and  other  argillaceous  deposits. 

Concretionary,  containing,  or  consisting  of  mineral  matter,  which 
has  been  collected,  either  from  the  surrounding  rock  or  from  without, 
round  some  centre,  so  as  to  form  a  nodule  or  irregularly  shaped  lump. 
This  aggregation  of  material  is  of  frequent  occurrence  among  water-formed 
rocks,  where  it  may  be  often  observed  to  have  taken  place  round  some 
organic  centre,  such  as  a  leaf,  cone,  shell,  fish-bone,  or  other  relic  of  plant 
or  animal.  (Book  IV.  Part  I.)  Among  the  most  frequent  minerals  found 
in  concretionary  forms  as  constituents  of  rocks,  are  calcite,  siderite,  pyrite, 
marcasite,  and  various  forms  of  silica.  In  a  true  concretion,  the  material 
at  the  centre  has  been  deposited  first,  and  has  increased  by  additions  from 


104  GEOGNOSY  book  u 

without,  either  during  the  formation  of  the  enclosing  rock,  or  by 
subsequent  concentration  and  aggregation.  Where,  on  the  other  hand, 
ca\'ities  and  fissures  have  been  filled  up  by  the  deposition  of  materials 
on  their  walls,  and  gradual  growth  inward,  the  result  is  known  as  a 
secretion.  Amygdales  and  the  successive  coatings  of  mineral  veins  arc 
examples  of  the  latter  process. 

Septarian — a  structui*e  often  exhibited  by  concretions  of  limestone 
and  clay -ironstone  which  in  consolidating  have  shrimk  and  cracked 
internally.  These  shrinkagc-ci-acks  radiate  in  an  iiTegular  way  from  the 
middle  towards  the  circumference,  but  die  out  before  reaching  the  latter 
(Fig.  26).  Usually  they  have  been  filled  with  some  subsequently  infil- 
trated mineral,  notably  calcite. 

Oolitic,  a  structure  like  tish-roe,  formed  of  spherical  grains,  each  of 
which  has  an  internal  radiating  and  concentric  structure,  and  often 
possesses  a  central  nucleus  of  some  foreign  l)ody.  This  structure  is 
specially  found  among  limestones  (see  p.  150).  When  the  grains  are 
as  large  as  peas,  the  structure  is  termed  pi  soli  tic. 

Various  structures  which  affect  large  masses  of  rock  rather  than 
hand-specimens  will  be  found  described  in  Book  IV.  But  a  few  of  the 
morg  imix)rtant  may  l)e  included  here. 

Massive,  unstratified,  ha>ang  no  arrangement  in  definite  layers  or 
strata.  Lava,  granite,  and  generally  all  crystalline  rocks  which  have  been 
erupted  to  the  surface,  or  have  solidified  below  from  a  state  of  fusion 
are  massive  rocks. 

Stratified,  bedded,  composed  of  layers  or  beds  lying  parallel  to 
each  other,  as  in  shale,  sandstone,  limestone,  and  other  rocks  w^hich  have 
been  deposited  in  water.  Successive  streams  of  lava,  poured  one  upon 
another,  have  also  a  bedded  arrangement.  Laminated,  consisting  of 
fine,  leaf -like  strata  or  laminae  ;  this  structure  being  characteristically 
exhibited  in  shales,  is  sometimes  also  called  shalv. 

Jointed,  traversed  by  the  divisional  planes  termed  Joints  which  are 
fully  treated  of  in  Book  IV.  Part  II. 

Columnar,  divided  into  prismatic  joints  or  columns.  This  stnicture 
is  typically  represented  among  the  basalts  and  other  basic  lavas  (p.  629 
and  Figs.  230-2),  but  it  may  also  be  observed  as  an  effect  of  contact- 
metamorphism  among  stratified  rocks  which  have  l)een  invaded  by  in- 
trusive masses  (p.  599). 

2.  Composition. — Before  having  recourse  to  chemical  or  microscopic 
analysis,  the  geologist  can  often  pronounce  as  to  the  general  chemical  or 
mineralogical  nature  of  a  rock.  Most  of  the  terms  which  he  employs  to 
express  his  opinion  are  derived  from  the  names  of  minerals,  and  in  almost 
all  cases  are  self-explanatory.  The  following  examples  may  suffice. 
Calcareous,  consisting  of  or  containing  carbonate  of  lime.  Argilla- 
ceous, consisting  of  or  containing  clay.  Fel spathic,  having  some  form 
of  felspar  as  a  main  constituent.  Siliceous,  formed  of  or  containing 
silica ;  usually  applied  to  the  chalcedonic  forms  of  this  cementing  oxide. 
Quart zose,  containing  or  consisting  entirely  of  some  form  of  quartz. 


PART  II  §  iv         MEGASCOPIC  CHARACTERS  OF  ROCKS  105 

Carbonaceous,  containing  coaly  matter,  and  hence  usually  associated 
with  a  dark  colour.  Pyritous,  containing  diffused  disulphide  of  iron. 
Gypseous,  containing  layers,  nodules,  strings  or  crystals  of  calcium- 
sulphate.  Saliferous,  containing  beds  of,  or  impregnated  with  rock- 
salt.     Micaceous,  full  of  layers  of  mica-flakes. 

As  rocks  are  not  definite  chemical  compounds,  but  mixtures  of 
different  minerals  in  varying  proportions,  they  exhibit  many  intermediate 
varieties.  Transitions  of  this  kind  are  denoted  by  such  phrases  as 
**  granitic  gneiss,"  that  is,  a  gneiss  in  which  the  normal  foliated  structure 
is  nearly  merged  into  the  massive  structure  of  granite;  "argillaceous 
limestone " — a  rock  in  which  the  limestone  is  mixed  with  clay ; 
"  calcareous  shale  " — ^a  fissile  rock,  consisting  of  clay  with  a  proportion  of 
lime.  It  is  evident  that  such  rocks  may  graduate  so  insensibly  into  each 
other,  that  no  sharp  line  can  be  drawn  between  them  either  in  the  field 
or  in  their  terminology. 

As  already  alluded  to,  and  as  will  be  more  fully  explained  in  later 
pages,  the  progress  of  research  goes  to  show  that  even  in  the  same  mass  of 
eruptive  rock  considerable  differences  of  chemical  composition  may  be  found. 
These  differences  seem  to  point  to  some  separation  of  the  constituents,  by 
gravity  or  otherwise,  before  consolidation.  Thus  the  picrite  of  Bathgate 
shades  upwards  into  a  rock  in  which  the  heavy  magnesian  silicates  are 
replaced  in  large  measure  by  felspars.^  Mr.  Iddings  has  recently  called 
attention  to  some  remarkable  gradations  of  composition  among  the  vol- 
canic rocks  of  the  Tewar  mountains.  New  Mexico,  where  he  believes  a 
series  of  intermediate  varieties  to  be  traceable  from  obsidian  at  the  one 
end  to  basalt  at  the  other.2  A  remarkable  instance  of  a  similar  kind  is 
described  by  Mr.  Teall  and  Mr.  Dakyns  from  the  Scottish  Highlands. 

3.  State  of  Aggregation. — The  hardness  or  softness  of  a  rock,  in 
other  words,  its  induration,  friability,  or  the  degree  of  aggregation  of  its 
particles,  may  be  either  original  or  acquired.  Some  rocks  (sinters,  for 
example)  are  soft  at  first  and  harden  by  degrees  ;  the  general  effect  of 
exposure,  however,  is  to  loosen  the  cohesion  of  the  particles  of  rocks.  A 
rock  which  can  easily  be  scratched  with  the  nail  is  almost  always  much 
decomposed,  though  some  chloritic  and  talcose  schists  are  soft  enough  to 
be  thus  affected.  Compact  rocks  which  can  easily  be  scratched  >\'ith  the 
knife,  and  are  apparently  not  decomposed,  may  be  fine-grained  limestones, 
dolomites,  ironstones,  mudstones,  or  some  other  simple  rocks.  Crystalline 
rocks,  except  limestone  cannot,  as  a  rule,  be  scratched  with  the  knife 
unless  considerable  force  be  used.  They  are  chiefly  composed  of  hard 
silicates,  so  that  when  an  instance  occurs  where  a  fresh  specimen  can  be 
easily  scratched,  it  will  usually  be  found  to  be  a  limestone  (pp.  82,  139, 
149).  The  ease  with  which  a  rock  may  be  broken  is  the  measure  of  its 
frangibility.  Most  rocks  break  most  easily  in  one  direction ;  attention 
to  this  point  will  sometimes  throw  light  upon  their  internal  structure. 

Fracture  is  the  surface  produced  when  a  rock  is  split  or  broken,  and 

*  Trans.  Roy.  Soc.  Edin.  vol.  xxix.  (1879),  p.  504. 

«  Bull.  U.  S.  Oeol.  Surv,  No.  66  (1890),  Bull.  Phil.  Soc.   Washington,  xi.  (1890),  pp. 
65,  191,  tLndpodeat  pp.  269,  576.     Teall  and  Dakyns,  Qvart.  Joum.  (itd.  Soc.  1892. 


106  GEOGNOSY  book  n 

depends  for  its  character  upon  the  texture  of  the  mass.  Finely  granular, 
compact  rocks  are  apt  to  break  with  a  splintery  fracture  where  wedge- 
shaped  plates  adhere  by  their  thicker  ends  to,  and  lie  parallel  with  the 
general  siu'face.  When  the  rock  breaks  off  into  concave  and  convex 
rounded  shell-like  siuiaces,  the  fracture  is  said  to  be  conchoidal,  as  may 
be  seen  in  obsidian  and  other  vitreous  rocks,  and  in  exceedingly  compact 
limestones.  The  fracture  may  also  be  foliated,  slaty,  or  shaly,  accord- 
ing to  the  stnicture  of  the  rock.  Many  opaque,  compact  rocks  are  trans- 
lucent on  the  thin  edges  of  fracture,  and  afford  there,  with  the  aid  of  a 
lens,  a  glimpse  of  their  internal  composition.  A  rock  is  said  to  be  flinty, 
when  it  is  hard,  close-grained,  and  breaks  "vnth  a  smooth  or  conchoidal 
fracture  like  flint;  friable,  when  it  crumbles  down  like  dry  clay  or 
chalk;  plastic,  when,  like  moist  clay,  it  can  be  worked  into  shapes; 
pulverulent,  when  it  falls  readily  to  powder;  earthy,  when  it  is  de- 
composed into  loam  or  earth  ;  in  coherent  or  loose,  when  its  particles  are 
quite  sepanite,  as  in  dry  blown  sand. 

4.  Colour  and  Lustre. — These  characters  vary  so  much,  even  in  the 
same  rock,  according  to  the  freshness  of  the  surface  examined,  that  they 
possess  but  a  subordinate  value.  Nevertheless,  when  cautiously  used, 
colour  may  be  made  to  afford  valuable  indications  as  to  the  probable 
nature  and  composition  of  rocks.  It  is,  in  this  respect,  always  desirable 
to  compare  a  freshly-broken  with  a  weathered  piece  of  the  rock.^ 

//7i?76  indicates  usually  til e  absence  or  a  compai-atively  small  amount  of 
the  heavy  metallic  oxides,  esj)ecially  iron.  It  may  either  be  the  original 
colour,  as  in  chalk  and  calc-sinter,  or  may  be  developed  by  weathering, 
as  in  the  whitij  crust  on  flints  and  on  many  ix)rphyries.  Grey  is  a  fre- 
quent colour  of  rocks  which,  if  quite  pure,  would  be  white,  but  w^hich 
ac({uire  a  greyish  tint  by  admixture  of  dark  silicates,  organic  matter,  dif- 
fused pyi'ites,  tKrc.  Bhw^  or  Iduiah-grey  is  a  characteristic  tint  of  rocks 
through  which  iron-disulphide  is  diflused  in  extremely  minute  subdivision. 
But  as  a  rule  it  rapidly  disa])pcars  from  such  rocks  on  exposure,  especi- 
ally where  they  contain  organic  matter  also.  The  stiff  blue  clay  of  the 
sea-]>ottoni  which  is  coloured  by  iron-disulphide  becomes  reddish-brown 
when  dried,  and  then  shows  no  trace  of  sulphide.*-  Black  may  ])e  due 
either  to  the  presence  of  carbon  (when  weathering  ^^^ll  not  change  it 
much),  or  to  some  iron-oxide  (magnetite  chiefly),  or  some  silicate  rich 
in  iron  (as  hornblende  and  augit^).  Many. rocks  (ba.salts  and  mela^ 
phyrcs  particularly)  which  look  quite  black  on  a  fresh  surface,  become 
red,  brown  or  yellow  on  exposure,  black  being  comparatively  seldom  a 
weathered  colour.  Ydlow  (or  Onnif/e),  as  a  dull  earthy  colouring  matter, 
almost  always  indicat^^s  the  presence  of  hydrated  peroxide  of  iron.  In 
modern  volcanic  districts  it  may  be  due  to  iron-chloride,  sulphur,  &e. 
Bright,  metiillic,  gold-like  yellow  is  usually  that  of  iron-disulphide.  Brown 
is  the  normal  colour  of  some  carbonaceous  rocks  (lignite),  and  ferruginous 
dei)osits  (bog-iron-ore,  clay-ironstone,  S:c.)    It  very  generally,  on  weathered 

*  Alterations  of  tlie  colours  of  minerals  and  rocks  are  effected  by  lieat  and  even  by  gun- 
light.     See  Jauettaz,  Tiull.  kSoc.  (iwl.  xxix.  (1872),  p.  300. 
'  J.  Y.  Uucliailan,  Brit.  i4.wc.  1881.  p.  584. 


PART  n  §  iv         MEGASCOPIC  CHARACTERS  OF  ROCKS  107 

surfaces,  points  to  the  oxidation  and  hydration  of  minerals  containing 
iron.  Bed^  in  the  vast  majority  of  cases,  is  due  to  the  presence  of 
anhydrous  peroxide  of  iron.  This  mineral  gives  dark  blood-red  to 
pale  flesh-red  tints.  As  it  is  liable,  however,  to  hydration,  these  hues  are 
often  mixed  with  the  brown,  orange  and  yellow  colours  of  limonite.^ 
Green,  as  the  prevailing  tint  of  rocks,  occurs  amongst  schists,  when  its 
presence  is  usually  due  to  some  of  the  hydrous  magnesian  silicates 
(chlorite,  talc,  serpentine).  It  appears  also  among  massive  rocks,  especi- 
ally those  of  older  geological  formations,  where  hornblende,  olivine,  or 
other  silicates  have  been  altered.  Among  the  sedimentary  rocks,  it  is 
principally  due  to  ferrous  silicate  (as  in  glauconite).  Carbonate  of  copper 
colours  some  rocks  emerald-  or  verdigris-green.  The  mottled  character  so 
common  among  many  stratified  rocks  is  frequently  traceable  to  unequal 
weathering,  some  portions  of  the  iron  being  more  oxidized  than  others ; 
while  some,  on  the  other  hand,  become  deoxidized  from  the  reducing  action 
of  decaying  organic  matter,  as  in  the  circular  green  spots  so  often  found 
among  red  strata. 

Lustre,  as  an  external  character  of  rocks,  does  not  possess  the  value 
which  it  has  among  minerals.  In  most  rocks,  the  gi'anular  texture 
prevents  the  appearance  of  any  distinct  lustre.  A  completely  vitreous 
lustre  without  a  granular  texture,  is  characteristic  of  volcanic  glass.  A 
splendent  semi-metallic  lustre  may  often  be  observed  upon  the  foliation 
planes  of  schistose  rocks  and  upon  the  laminae  of  micaceous  sandstones. 
As  this  silvery  lustre  is  almost  invariably  due  to  the  presence  of  mica,  it 
is  commonly  called  distinctively  mimceons.  A  metallic  lustre  is  met  with 
sometimes  in  beds  of  anthracite  ;  more  usually  its  occurrence  among  rocks 
indicates  the  presence  of  metallic  oxides  or  sulphides.  A  rednous  lustre 
is  characteristic  of  many  pitchstoncs.  Lustre-viotiliiKj  is  a  term  applied 
to  the  intemipted  sheen  on  the  cleavage  faces  of  minerals  which  have  en- 
closed much  smaller  crystals  or  grains  of  other  minerals.  It  is  well  seen 
on  the  surfaces  of  some  of  the  constituents  of  serpentine  rocks. 

5.  Feel  and  Smell. — These  minor  characters  are  occasionally  useful. 
By  the  feel  of  a  mineral  or  rock  is  meant  the  sensation  experienced  when 
the  fingers  are  passed  across  its  surface.  Thus  hydrous  magnesian  sili- . 
cates  have  often  a  marked  soapy  or  greasy  feel.  Some  sericitic  mica- 
schists  show  the  same  character.  Trachyte  received  its  name  from  its 
characteristic  rough  or  harsh  feel.  Some  rocks  adhere  to  the  tongue,  a 
quality  indicative  of  their  tendency  to  absorb  water. 

SmelL — Many  rocks,  when  freshly  broken,  emit  distinctive  odours. 
Those  containing  volatile  hydrocarbons  give  sometimes  an  appreciable 
hiiumifums  odour,  as  is  the  case  with  certain  eruptive  rocks,  which  in 
central  Scotland  have  been  intruded  through  coal-seams  and  carbon- 
aceous shales.  Limestones  have  often  a  fetid  odour ;  rocks  full  of 
decomposing  sulphides  are  apt  to  give  a  s^dphurous  odour ;  those  which 
are  highly  siliceous  yield,  on  being  struck,  an  empijreumatic  odour.  It  is 
characteristic  of  argillaceous  rocks  to  emit  a  strong  earthy  smell  when 
breathed  upon. 

1  Sec  I.  C.  Russell,  Bull,  U.  S.  Oed.  Sure.  No.  52  (1889). 


108  GEOGNOSY  book  n 

6.  Specific  Gravity. — This  is  an  important  character  among  rocks  u 
well  as  among  minerals.  It  varies  from  0*6  among  the  hydrocarbon 
compK>unds  to  3*1  among  the  basalts.  As  already  stated,  the  average 
specific  gravity  of  the  rocks  of  the  earth's  crust  may  be  taken  to  be  about 
2*5,  or  from  that  to  3*0.  Instruments  for  taking  the  specific  gravity  of 
rocks  have  been  already  (p.  85)  referred  to. 

7.  Magnetism  is  so  strongly  exhibited  by  some  crystalline  rocks  as 
powerfully  to  aflfect  the  magnetic  needle,  and  to  vitiate  observations  with 
this  instrument  It  is  due  to  the  presence  of  magnetic  iron,  the  existence 
of  which  may  be  shown  by  pulverising  the  rock  in  an  agate  mortar,  wash- 
ing carefully  the  triturated  powder,  and  drying  the  heavy  residue,  from 
which  grains  of  magnetite  or  of  titaniferous  magnetic  iron  may  be  ex- 
tracted with  a  magnet.  This  may  be  done  with  any  basalt  (p.  86).  A 
freely  swinging  magnetic  needle  is  of  service,  as  by  its  attraction  or 
repulsion,  it  affords  a  delicate  test  for  the  presence  of  even  a  small  quantity 
of  magnetic  iron. 

§  V.  MicroBCopic  Characters  of  Bocks. 

No  department  of  Geology  has  been  more  advanced  in  recent  yean 
than  Lithology,  and  this  has  been  mainly  due  to  the  introduction  of  the 
microscope  as  an  instrument  for  investigating  minute  internal  structure. 
As  far  back  as  the  year  1827,  a  method  of  making  thin  ti*anspar^t 
sections  of  fossil  wood,  and  mounting  them  on  glass  with  Canada  balsam, 
had  been  devised  by  William  Nicol  of  Edinburgh,  and  was  employed  by 
Henry  Witham  in  his  *  History  of  Fossil  Vegetables.'  ^ 

It  was  not,  however,  until  1856  that  Mr.  H.  C.  Sorby,  applying  this 
method  to  the  investigation  of  minerals  and  rocks,  showed  how  many 
and  important  were  the  geological  questions  on  which  it  was  calculated 
to  shed  light.  2  Reference  will  be  made  in  subsequent  pages  to  the 
remarkable  results  then  announced  by  him.  To  the  publication  of  his 
memoir  the  subsequent  rapid  development  of  the  microscopic  study  (rf 
rocks  may  be  distinctly  traced.  The  microscopic  method  of  analysis 
is  now  in  use  in  every  country  where  attention  is  paid  to  the  history 
of  rocks.* 

^  Small  4to,  Edinburgh,  1831.  Tliis  work,  though  dedicated  to  Nicol,  does  not  distinctly 
recognise  him  as  the  actual  inventor  of  the  process  of  slicing  mineral  substances  for  micro* 
scopic  investigation.  All  that  was  original  in  Witham's  researches  he  owed  either  directly 
or  indirectly  to  Nicol. 

*  Brit.  Assoc.  1856,  Sect.  p.  78.  Quart.  Jottrn.  Geol.  Soc.  xiv.  1858.  Micr.  Jaunt. 
xvii.  (1877),  p.  113. 

'  Among  the  best  text-books   on  this  subject  the   following  may  be  mentioned : — 

*  Mikroskopische  Beschafienheit  der  Mineralien  und  Gesteine,'  F.  Zirkel,   1   vol.    1873. 

*  Mikroskopische  Physiographie  der  Mineralien  und  Gesteine,'  H.  Rosenbusch,  2  toIb. 
2nd  Edit.  1885-87*  and  the  English  translation  of  the  first  volume  quoted  on  p.  89  ;  likewiae 
the  Tables  translated  by  F.  H.  Hatch  quoted  on  p.  90.  *  Elemente  der  Petrographie,'  A. 
von  Lasaulx,  1875.  *  Mineralogie  micrograph ique :  roches  eruptives  fran^aiaes,'  Fonqn^ 
and  Michel-Levy,  2  vols.  4to,  Paris,  1879.  'Microscopical  Petrography,'  Zirkel,  being  toL 
vi.  of  the  Geol.  Explor.  of  40/A  Parallel j  Washington,  1876.  *  British  Petrography/  J.  J. 
H.  Teall,  London,  1888.     '  Les  Mineraux  des  Roches,'  Michel-L^vy  and  Lacroix,  Paris,  1888. 


PART  II  §  V  MICROSCOPIC  CHARACTERS  OF  ROCKS  109 

In  §  iii.  p.  90  information  has  been  given  regarding  the  preparation  of 
sections  of  rocks  for  microscopical  examination,  the  methods  of  procedure 
in  the  practice  of  this  part  of  geological  research,  and  some  of  the  terms 
employed  in  the  following  pages. 

1.  Microscopic  Elements  of  Rocks. 

Rocks  when  examined  in  thin  sections  with  the  microscope  are  found 
to  be  composed  of  or  to  contain  various  elements,  of  which  the  more 
important  are,  1st,  crystals,  or  crystalline  substances ;  2nd,  glass ;  3rd, 
crystallites ;  4th,  detritus. 

A.  Crystals  or  Crystalline  Substances. — Rock-forming  minerals, 
when  not  amorphous,  may  be  either  crystallized  in  their  proper  crystal- 
lographic  forms  (idiomorphic),  or  while  possessing  a  crystalline  internal 
structure,  may  present  no  definite  external  geometrical  form  (allotriomor- 
phic,  p.  1 18).  The  latter  condition  is  more  prevalent,  seeing  that  minerals 
have  usually  been  developed  round  and  against  each  other,  thus  mutually 
hindering  the  assumption  of  determinate  crystallographic  contours. 
Other  causes  of  imperfection  are  fracture  by  movement  in  the  original 
magma  of  the  rock,  and  partial  solution  in  that  magma  (Fig.  1 2),  as  in  the 
corroded  quartz  of  quartz-i>orphyries  and  rhyolites,  and  the  hornblende 
crystab  of  basalts.  The  ferro-magnesian  minerals  of  earlier  consolidation 
among  basalts  and  andesites,  are  sometimes  surrounded  with  a  dark  shell 
called  the  corrosion-zone.  In  some  rocks,  such  as  granite,  the  thoroughly 
crystalline  character  of  the  component  ingredients  is  well  marked,  yet 
they  less  frequently  present  the  definite  isolated  crystals  so  often  to  be 
obeerved  in  porphyries  and  in  many  old  and  modern  volcanic  rocks. 
Among  thoroughly  crystalline  rocks,  good  crystals  of  the  component 
minerals  may  be  obtained  from  fissures  and  cavities  in  which  there  has 
been  room  for  their  formation.  It  is  in  the  "  dnisy  "  cavities  of  granite, 
for  example,  that  the  well-defined  prisms  of  felsjmr,  quartz,  mica,  topaz, 
beryl  and  other  minerals  are  found.  Successive  stages  in  order  of 
appearance  or  development  can  readily  be  observed  among  the  crystals 
of  rocks.  Some  appear  as  large,  but  frequently  broken,  or  corroded 
forms.  These  have  evidently  been  formed  first.  Others  are  smaller  but 
abundant,  usually  unbroken,  and  often  disposed  in  lines.  Others  have 
been  developed  by  subsequent  alteration  within  the  rock.^ 

A  study  of  the  internal  structure  of  crystals  throws  light  not  merely 

The  Tolnines  for  the  last  fifteen  or  twenty  years  of  the  Quarterly  Journal  of  the  Geohxjicnl 
Society,  Oeoloffieal  Magazine^  Neves  Jahrbuch  fur  Mineralogie,  <{.r.,  Zeitschrift  der  Deutschen 
Oeologischtn  Geadlschqftj  Bulletin  de  la  SocUti  geologique  de  France^  Jahrlmch  der  K.  K. 
fhohgitchen  Reichsanstalt  {Vienna),  contain  numerous  papers  on  the  microscopic  structure 
of  rocka.  Rutley's  *  Study  of  Rocks,'  1879,  and  his  '  Rock-fomiing  Minerals,'  1888  ;  Cole's 
•Aids  in  Practical  Geology,'  1891  ;  and  Hatch's  'Petrology— Igneous  Rocks,'  1891,  are 
useful  handbooks.  The  manual  of  Rosenbusch  and  the  work  of  Fouque  and  Michel- Levy, 
contain  a  tolerably  ample  bibliography  of  the  subject,  to  which  the  student  is  referred.  Tlie 
titles  of  some  of  the  more  important  memoirs  which  have  recently  a])peared  will  be  given  in 
footnotes. 

1  Fooqa^  and  Michel-L^vy,  *  Miu.  Micrograph.'  p.  151. 


1 1 0  GEOGNOS  Y  book  n 


on  their  own  genesis,  but  on  that  of  the  rocks  of  which  thoy  form  part, 
and  is  therefore  well  worthy  of  the  attention  of  the  geologist.  That  many 
apparently  simple  crystals  are  in  reality  compound,  may  not  infrequently 
l)e  detected  by  the  different  condition  of  weatheiing  in  the  two  opposite 
parts  of  a  twin  on  an  exposed  face  of  rock.  The  internal  stinicture  of  a 
crystal  modifies  the  action  of  solvents  on  its  exterior  {e.g,  weathered 
surfaces  of  calcite,  aragonite  and  felsj)iii*s).  Crystals  may  occasionally  be 
observed  built  up  of  rudimentiiry  **  microlites,"  as  if  these  were  the 
simplest  forms  in  which  the  molecules  of  a  mineral  begin  to  api>ear  (p.  115). 

A  microscopic  examination  of  some  rocks  shows  that  a  subsequent  or 
secondary  growth  of  different  minerals  has  taken  place  after  their  original 
crysUdline  form  was  complete.  These  later  additions  are  in  optical  con- 
tinuity >\'ith  the  original  crystal,  and  sometimes  have  taken  place  even 
upon  worn  or  imperfect  forms.  They  may  be  occasionally  detected 
among  the  silicates  of  igneous  rocks,  and  also  even  among  the  sandgrains 
of  sandstones  which  have  thus  had  thoii*  rounded  forms  converted  into 
crystallographic  faces.  ^ 

Crystalline  minerals  are  seldom  free  from  extraneous  inclusions. 
These  are  occasionally  large  enough  to  be  reiulily  seen  by  the  naked 
eye.  But  the  microscope  reveals  them  in  many  minerals  in  almost 
incredible  quiintity.  They  ai*e,  a,  vesicles  containing  gas ;  )8,  vesicles 
containing  liquid ;  -y,  gloJndes  of  glass  or  of  some  litlioid  substance ; 
8,  crystals ;  €,  filaments,  or  other  indefinitely -shaped  pieces,  patches,  or 
streaks  of  mineral  matter. 

a.  Gas-filled  cavities — are  most  frequently  globular  or  elliptical, 
and  appear  to  be  due  to  the  presence  of  gas  or  steam  in  the  crystal  at 
the  time  of  consolidation.  Zirkel  estimates  them  at  360,000,000  in  a 
cubic  millimetre  of  the  hauyne  from  Melfi.^  In  some  instances  the 
cavity  has  a  geometric  form  belonging  to  the  crystalline  system  of  the 
enclosing  mineral.  Such  a  space  defined  by  crystallographic  contours  is 
a  nefjative  crystal,  A  cavity  filled  with  gas  contains  no  bubble,  and  its 
margin  is  marked  liy  a  broad  dark  band.  The  usual  gas  is  nitrogen,  with 
traces  of  oxygen  and  carlwn-dioxide ;  sometimes  it  is  entirely  carbon- 
dioxide  or  hydrogen  and  hydrociU'bons. 

p.  Vesicles  containing  liquid  (and  gas). — As  far  back  as  the 
year  1823,  Brewster  studied  the  nature  of  certain  fluid-bearing  cavities  in 
difterent  minerals.^  The  first  observer  who  showed  their  important 
bearing  on  geological  researches  into  the  origin  of  crystalline  rocks  was 
Mr.  Sorby,  in  whose  piper,  already  cited,  they  occupy  a  prominent  place. 
Vesicles  entirely  filled  with   liquid  are  distinguished  by  their  sharply- 

^  H.  C.  Sorby,  Presidential  Address,  Oeol.  Soc,  1880,  p.  62.  R  D.  Irving  and  C.  R.  Van 
Hise  *0n  secondary  enlargements  of  Mineral  Fragments  in  certain  rocks.*  Bull,  U,  8,  OeoL 
Surt\  No.  8  (1884).     J.  W.  Judd.  Quart.  Journ.  Oeol.  Soc,  xlv.  (1889),  p.  175. 

2  *  Mik.  Beschaff.'  p.  86. 

^  ISdiju  Phil.  Journ.  ix.  p.  94.  Trans.  Roy.  Soc.  Edin.  x.  p.  1.  See  also  W.  Niool, 
Edin.  Nnc  Phil,  Journ.  (1828),  v.  \>.  94  ;  De  la  Vallee  Pous.«»in  and  Renard,  Aead.  Hof. 
Belg.  1876,  p.  41  ;  Hartley,  Journ.  Chem.  Soc  ser.  2,  xiv.  137  ;  ser.  3,  U.  p.  241  ; 
Microscop.  Journ.  xv.  p.  170  ;  BrU,  Assoc,  1877,  Sect.  p.  232. 


PART  II  §  V  MICROSCOPIC  CHARACTERS  OF  ROCKS  111 

defined  and  narrow  black  borders.  Vesicular  spaces  containing  fluid  may 
be  noticed  in  many  artificial  crystals  formed  from  aqueous  solutions 
(crystals  of  common  salt  show  them  well)  and  in  many  minerals  of 
crystalline  rocks.  They  are  exceedingly  various  in  form,  being  branching, 
curved,  oval,  or  spherical,  and  sometimes  assuming  as  negative  crystals 
a  geometric  form,  like  that  characteiistic  of  the  mineral  in  which  they 
occur,  as  cubic  in  rock-salt  and  hexagonal  in  quartz.  They  also  vary 
greatly  in  size.  Occasionally  in  quartz,  sapphire,  and  other  minerals, 
large  cavities  are  readily  observable  with  the  naked  eye.  But  they  may 
be  traced  with  high  magnifying  powers  down  to  less  than  yiriiTiy  ^^  ^'^ 
inch  in  diameter.  Their  proportion  in  any  one  crystal  ranges  within  such 
wide  limits,  that  whereas  in  some  crystals  of  quartz  few  may  be  observed, 
in  others  they  are  so  minute  and  abundant  that  many  millions  must  be 
contained  in  a  cubic  inch.  The  fluid  present  is  usually  water,  frequently 
with  saline  solutions,  particularly  chloride  of  sodium  or  of  potash,  or 
sulphates  of  potash,  soda,  or  lime.  Carbon-dioxide  may  be  present  in 
the  water ;  sometimes  the  cavities  are  partially  occupied  with  it  in  liquid 
form,  and  the  two  fluids,  as  originally  observed  by  Brewster,  may  be  seen 
in  the  same  cavity  unmingled,  the  carbon-dioxide  remaining  as  a  freely 
moving  globule  within  the  carbonated  water.  Cubic  crystals  of  chloride 
of  sodium  may  be  occasionally 
observed  in  the  fluid,  which  must      (^     ^n\ 

in    such    cases   be   a    satui^ated      _  /^ v^ 

solution  of  this  salt  (Fig.  11, 
lowest  figure  in  Colunm  A). 
Usually  each  cavity  contains  a 
small  globule  or  bubble,  some- 
times stationary,  sometimes  mov- 
able from  one  side  or  end  of  the 
cavity  to  the  other,  as  the 
specimen    is    turned.      With    a 

high  magmfying  power,  the  Fjg.n.-Cavitiesln  crystals,  highly  magnified;  a.  Liquid 
minuter  bubbles  may  be  observed  inclusions  ;  b,  Olass  inclusions  -.  c,  cavities  showing 

to  be  in  motion,  sometimes  slowly         ^^e  devitrification  of  the  original  glass  by  the  appear- 

pulaating  from  side  to  side,  or  r/^^I^Lti'p^'uctt  1™^  """''  "^^  ' 
rapidly  vibrating    like  a  living 

organism.  The  cause  of  this  trepidation,  which  resembles  the  so-called 
"Brownian  movements,"  has  been  plausibly  explained  by  the  incessant 
interchange  of  the  mole6ules  from  the  liquid  to  the  vaporous  condition 
along  the  surface  where  vapours  and  liquid  meet — an  interchange  which, 
though  not  visible  on  the  large  bubbles,  makes  itself  apparent  in  the 
minute  examples,  of  which  the  dimensions  are  comparable  to  those  of  the 
intermolecular  spaces.^  The  bubble  may  be  made  to  disappear  by  the 
application  of  heat. 

With  regard  to  the  origin  of  the  bubble,  Sorby  pointed  out  that  it  can 

1  ChArfoonelle  and  Thirion,  Rev.  Quest,  Scientif.  vii.  (1880)  43.  On  the  critical  point  of 
water,  &c.,  in  these  cayities  see  Hartley,  Joum.  Chein,  Soc.  ser.  3,  vol.  ii.  p.  241.  Pop.  Scu 
Rev.  new  aer.  L  p.  119. 


112  GEOGNOSY  Booxn 

be  imitated  in  artificial  crystals,  in  which  he  explained  its  existence  by 
diminution  of  volume  of  the  liquid  owing  to  a  lowering  of  temperature 
after  its  enclosure.  By  a  series  of  experiments  he  ascertained  the  rate  of 
expansion  of  water  and  saline  solutions  up  to  a  temperature  of  200°  G 
(392'  Fahr.),  and  calculated  from  them  the  temperature  at  which  the 
liquid  in  crystals  would  entirely  fill  its  enclosing  cavities.  Thus,  in  Uie 
nepheline  of  the  ejected  blocks  of  Monte  Somma,  he  found  that  the 
relative  size  of  the  vacuities  was  about  *28  of  the  fluid,  and  assuming  the 
pressure  under  which  the  crystals  were  formed  to  have  been  not  much 
greater  than  sufficient  to  counteract  the  elastic  force  of  the  vapour,  he 
concluded  that  the  nepheline  may  have  been  formed  at  a  tempera- 
ture of  about  340°  C.  (644''  Fahr.),  or  a  very  dull  red  heat>  only  just 
visible  in  the  dark.  He  estimated  also  from  the  fluid  cavities  in  the 
quartz  of  granite  that  this  rock  has  probably  consolidated  at  somewhat 
similar  temperatures,  under  a  pressure  sometimes  equal  to  that  of  76,000 
feet  of  rock.^  Zirkel,  however,  has  pointed  out  that  even  in  contiguooi 
cavities,  where  there  is  no  evidence  of  leakage  through  fine  fissures,  the 
relative  size  of  the  vacuole  varies  within  very  wide  limits,  and  in  such  a 
manner  as  to  indicate  no  relation  whatever  to  tlie  dimensions  of  the 
enclosing  ca\dties.  Had  the  vacuole  been  due  merely  to  the  contractioii 
of  the  liquid  on  cooling,  it  ought  to  have  always  been  proportionate  to 
the  size  of  the  cavity.- 

MM.  De  la  Vallee  Poussin  and  Renard,  attiicking  the  question  from 
another  side,  measured  the  relative  dimensions  of  the  vesicle  and  of  its 
enclosed  water  and  cube  of  rock-salt,  as  contained  in  the  quartziferoos 
diorite  of  Qucnast  in  Belgium.  The  temperature  at  which  the  ascertained 
volume  of  water  in  the  cavity  would  dissolve  its  salt  was  found  by  calcub- 
tion  to  be  307''  C.  (520'  Fahr.)  But  as  the  law  of  the  solubility  of 
common  salt  has  not  l>een  experimentally  determined  for  high  tempera- 
tures, this  figure  can  only  be  accepted  provisionally,  though  other 
considerations  go  to  indicate  that  it  is  probably  not  far  from  the  truth. 
Assuming  then  that  this  wa5  the  temperature  at  which  the  vesicle  was 
formed,  these  authors  proceed  to  detennine  the  pressure  necessary  to 
prevent  the  complete  vaporization  of  the  water  at  that  temperature,  and 
obtain,  lus  the  result,  a  pressure  of  87  atmospheres,  equal  to  84  tons  per 
s([uare  foot  of  surface.^  That  many  rocks  were  formed  under  great  pressuro 
is  well  shown  ])y  the  liquid  carbon-dioxide  in  the  pores  of  their  crystals. 

Although,  in  almost  all  cases,  the  liquid  inclusions  are  to  be  referred 
to  the  conditions  under  which  the  mineral  crystallized  out  of  the  original 
magma,  they  may  be  exceptionally  developed  long  subsequently,  either 
in  one  of  the  original  minerals  during  decomposition,  or  in  a  mineral  erf 
secondary  origin,  such  as  quartz  of  subsequent  introduction.* 

1  Sorby,  Q.  J.  Geol.  ^>c.  xiv.  pp.  480,  493.  2  i^ik.  Beschaff.'  p.  46. 

^  *  Memoire  sur  les  Roches  dites  Plutoniennes  de  la  Belgique, '  De  la  Vallee  Pouasin  and 
A.  Reuard,  Aciuf.  Roy.  Belg.  1876,  p.  41.  See  also  Ward,  Q.  J.  Geol.  Soc.  xxxL  p.  668, 
who  believed  that  the  granites  of  Cuniberlaud  consolidated  at  a  maximum  depth  of  22,000 
to  30,000  feet. 

*  See  Whitman  Cross  on  the  development  of  liquid  inclusions  in  plagioclase  dnring  tbe 


PART  II  g  V  MICROSCOPIC  CHARACTERS  OF  ROCKS  113 

Liquid  inclusions  may  ]ye  dispersed  at  random  through  a  crystal,  or 
as  in  the  quartz  of  granite,  gathered  in  intersecting  planes  (which  look 
like  fine  fissures  and  which  may  sometimes  have  become  real  fissures, 
owing  to  the  line  of  weakness  caused  by  the  crowding  of  the  cavities),  or 
disposed  regularly  in  reference  to  the  contour  of  the  crystal.  In  the  last 
case  they  are  sometimes  confined  to  the  centre,  sometimes  arranged  in 
zones  along  the  lines  of  growth  of  the  crystal.^  They  are  specially  con- 
spicuous in  the  quartz  of  granite  and  other  massive  rocks,  as  well  as  of 
gneiss  and  mica -schist ;  also  in  felspars,  topaz,  beryl,  augite,  nepheline, 
olivine,  leucite  and  other  minerals. 

y.  Inclusions  of  glass  or  of  some  lithoid  substance. — In  many 
rocks  which  have  consolidated  from  fusion,  the  component  crystals  contain 
globules  or  irregularly  shaped  enclosures  of  a  vitreous  nature  (Fig.  11, 
Column  B).  These  enclosures  are  analogous  to  the  fluid-inclusions  just 
described.  They  are  portions  of  the  original  glassy  magma  out  of  which 
the  minerals  of  the  rock  crystallized,  as  portions  of  the  mother-liquor  are 
enclosed  in  artificially  formed  crystals  of  common  salt.  That  magma  is 
in  reality  a  liquid  at  high  temperatures,  though  at  ordinary  temperatures 
it  becomes  a  solid.  At  first,  these  glass-vesicles  may  be  confounded  with 
the  true  liquid-cavities,  which  in  some  respects  they  closely  resemble. 
But  they  may  be  distinguished  by  the  immobility  of  their  bubbles,  of 
which  several  are  sometimes  present  in  the  same  cavity ;  by  the  absence 
of  any  diminution  of  the  bubbles  when  heat  is  applied  ;  by  the  elongated 
shape  of  many  of  the  bubbles ;  by  the  occasional  extrusion  of  a  bubble 
almost  beyond  the  walls  of  the  vesicle ;  by  the  usual  pale  greenish  or 
brownish  tint  of  the  substance  filling  the  vesicle,  and  its  identity  with 
that  forming  the  surrounding  base  or  ground-mass  in  which  the  crystals 
are  imbedded ;  and  by  the  complete  passivity  of  the  substance  in  polarized 
light  (see  p.  94). 

Glass  inclusions  occur  abundantly  in  some  minerals,  aggregated  in  the 
centre  of  a  crystal  or  ranged  along  its  zones  of  growth  with  singular 
regularity.  They  appear  in  felspars,  quartz,  leucite,  and  other  crystalline 
ingredients  of  volcanic  rocks,  and  of  course  prove  that  in  such  positions 
these  minerals,  even  the  refractory  quartz,  have  undoubtedly  crystallized 
out  of  molten  solutions. 

In  inclusions  of  a  truly  vitreous  nature,  traces  of  devitrification  may 
not  infrequently  be  seen.  In  particular,  microscopic  crystallites  (p.  115) 
make  their  ap|)earance,  like  those  in  the  ground -mass  of  the  rock. 
Sometimes  the  inclusions,  like  the  general  ground-mass,  have  an  entirely 
stony  character  (Fig.  11,  C).  This  may  be  well  observed  in  those  which 
have  not  been  entirely  separated  from  the  surrounding  ground-mass,  but 
are  connected  with  it  by  a  narrow  neck  at  the  periphery  of  the  enclosing 
crystal.  In  some  granites  and  in  elvans,  the  quartz  by  irregular  contrac- 
tion, while  still  in  a  plastic  state,  appears  to  have  drawn  into  its  substance 

decomposition  of  the  gneiss  of  Brittany.     Tschermak's  Min.  Mittheil.   1880.  p.   369  ;  also 
G.  F.  Becker,  *  Geology  of  Comatock  Lode.'     V.  S.  Geol.  Sun\  1882,  p.  371. 

*  The  way  in  which  vesicles,  enclosed  crystals,  &c..  are  grouped  along  the  zones  of 
growth  of  crystals  is  illustrated  in  Fig.  12. 

I 


of  a  fncluml  lUid  coirodeil  A 
a  dykd,  Crawfortjnhii,  L»D»rki 
lowing  Llnei  of  growth  with  wi 


114  GEOGNOSY  BOOi  n 

portionB  of  the  surrounding  already  lithoid  base ; '  but  this  appeamnce 
may  sometimes  be  due  to  irregular  corrosion  of  the  crystals  by  the  magmiL* 
6.  Crystals  and  crystalline  bodies. — Man;  component 
minerals  of  rocks  contain  other  minerals  (Fig.  13).  These  occur  some- 
times as  perfect  crystals,  more 
usually  as  what  are  termed  micro- 
lit«s  (p.  115).  Like  the  ^bm- 
inclusions,  they  tend  to  range 
themselves  in  Unes  along  t£e 
successive  zones  of  growtJi  in 
the  enclosing  mineraL  Micro- 
lites  are  of  frequent  occurrence 
in  leucite,  garnet,  augite,  horn- 
blende, calcite,fluorite,&c  From 
the  fact  that  microlites  of  the 
easily  fusible  augite  are,  in  the 
Vesuvian  lavaa,  enclosed  wit^ 
the  extremely  refractory  leucite, 
it  was  supposed  that  the  relative 
order  of  fusibility  is  not  always 
followed  in  the   microlites  and 

^ enveloping  crystals.      But  this 

has  been  satisfactorily  explained 
by  Fouquii  and  Miche!-L*vy,  who  have  shown  experimentally  that  leudtfl^ 
when  crystallizing  from  fusion,  tends  to  catch  up  inclusions  of  the  sur- 
rounding glass,  which,  should  the  glass  be  pyroxenic,  may  assume  the 
form  of  augite.' 

(.  Filaments,  streaks,  patches,  discoloration s.  — 
Besides  the  enclosures  already  enumerated,  crystals  likewise  frequently 
enclose  iiTegular  portions  of  mineral  matter,  due  to  alteration  of  the 
original  substance  of  the  minerals  or  rocks.  Thus  tufta  and  vermicular 
aggregates  of  certain  green  ferruginous  silicates  are  of  common  occur- 
rence among  the  crystals  and  cavities  of  old  pyroxenic  volcanic  rocks. 
Orthoclasc  crystals  are  often  mottled  with  patches  of  a  granular  nature, 
due  to  partial  conversion  of  the  mineral  into  kaolin.  The  magnetite,  so 
frequently  enclosed  within  minerals,  is  abundantly  oxidized,  and  haa 
given  rifle  to  brown  and  yellow  patches  and  discol orations.  Care  must 
be  taken  not  to  confound  these  results  of  infiltrating  water  with  the 
original  characters  of  a  rock.  Practice  will  give  the  student  confidence 
in  distinguishing  them,  if  he  familiarises  his  eye  with  decomposition 
products  by  studying  slices  of  weathered  minerals  and  of  the  weathered 
parts  of  rocks. 

B.  Glass.— Even  to  the  unassist«d  eye,  many  volcanic  rocks  consist 
obviously  in  whole  or  in  great  measure  of  glass.*     This  sul^tance  in 

'  J.  A.  Phillips,  Q.  J.  Orol.  Sac.  xxxi.  iJ.  388. 

-  Pquhub  and  Michel-Wvy,  'Miii.  Micrograph.' 

'  'Sj-nthiae  dea  Miniiraui,'  1882,  p.  155. 

*  See  E.  CoheD  an  glasny  Bocks.     iV«w>  JaJirb.  1S80  (ii.),  p.  23. 


iBTii  §  V  MICROSCOPIC  CHARACTERS  OF  ROCKS  116 

lass  is  usually  black  or  dark  green,  but  when  examined  in  thin  sections 
nder  the  microscope,  it  presents  for  the  most  part  a  pale  brown  tint,  or 
\  nearly  colourless.  In  its  purest  condition,  it  is  quite  structureless, 
bat  is,  it  contains  no  crystals,  crystallites,  or  other  distinguishable 
idividualised  bodies.  But  even  in  this  state  it  may  sometimes  be 
bserved  to  be  marked  by  clot-like  patches  or  streaks  of  darker  and 
ighter  tint,  arranged  in  lines  or  eddy-like  curves,  indicative  of  the  flow 
f  the  original  fluid  mass.  Rotated  in  the  dark  field  of  crossed  Nicol- 
orisms,  such  a  natural  glass  remains  dark,  as,  unless  where  it  has  under- 
gone internal  stresses,  it  is  perfectly  inert  in  polarized  light  Being  thus 
wiropiCy  it  may  readily  be  distinguished  from  any  enclosed  crystals  which, 
cting  on  the  light,  are  anisotropic  (p.  94).  Perfectly  homogeneous 
tructnreless  glass,  without  enclosures  of  any  kind,  occurs  for  the  most 
«rt  only  in  limited  patches,  even  in  the  most  thoroughly  vitreous  rocks. 
hriginally  the  structure  of  all  glassy  rocks,  at  the  time  of  most  complete 
ludon,  may  have  been  that  of  perfectly  unindividualised  glass.  But  as 
heee  masses  tended  towards  a  solid  form,  devitrification  of  their  glass 
et  in.  Many  forms  of  incipient  or  imperfect  crystallization,  as  well  as 
lerfect  crystals,  were  developed  in  the  still  fluid  and  moving  mass, 
nd,  together  with  crystals  of  earlier  growth,  were  aiTanged  in  the 
lirection  of  motion.     Devitrification  has  in  frequent  examples  proceeded 

0  far  that  no  trace  remains  of  any  actual  glass.^ 

C.  Crystallites  and  Microlites.^ — Under  these  names  may  be 
oehided  minute  inorganic  bodies  possessing  a  more  or  less  definite  form, 
»at  generally  without  the  geometrical  characters  of  crystals.  They  occur 
Qoet  commonly  in  rocks  which  have  been  formed  from  igneous  fusion, 
lut  are  found  also  in  others  which  have  resulted  from,  or  have  been 
Itered  by,  aqueous  solutions.     They  seem  to  be  early  or  peculiar  forms 

1  crystallization.  They  are  abundantly  developed  in  artificial  slags, 
nd  appear  in  many  modern  and  ancient  vitreous  rocks,  but  the 
onditions  imder  which  they  are  produced  are  not  yet  well  understood.^ 

Crystallites  are  distinguished  by  remaining  isotropic  in  polarized 
ight  The  simplest  are  extremely  minute  drop-like  bodies  or  globuliteSy 
ometimes  crowded  confusedly  through  the  glass,  giving  it  a  dull  or 
omewhat  granular  character,  while  in  other  cases  they  are  arranged  in 
ines  or  groups.  Gradations  can  be  traced  from  spherical  or  spheroidal 
jlobulites  into  other  forms  more  elliptical  in  shape,  but  still  having  a 
oonded  outline  and  sometimes  sharp  ends  (longulites).     There  does  not 

^  Consult  a  paper  on  the  microscopic  character  of  devitritied  glass  and  some  analogous 
9ck-stmctares,  by  D.  Herman  and  F.  Rutley.     Proc,  Roy,  Soc  1885,  p.  87. 

'  The  word  cryatalliU  was  first  used  by  Sir  James  Hall  to  denote  the  lithoid  substance 
bteined  by  him  after  fusing  and  then  slowly  cooling  various  **  whinstones  "  (diabases,  &c.) 
ineo  its  leviTal  in  lithology  it  has  been  applied  to  the  .minuter  bodi&s  above  described. 
iMstadeDt  should  consult  Vogelsang's  '  Philosophie  der  Geologie, '  p.  139;  '  Krystalliteu, ' 
ionii,  8vo,  1875 ;  also  his  descriptions  in  Archives  yeerlandaises,  v.  1870,  vi.  1871. 
orby,  Brxi,  Auoc,  1880. 

*  They  are  well  exhibited  also  in  ordinary  blow-pii)e  beads.  See  Sorby,  Brit.  Assoc, 
880,  or  Otol,  Mag.  1880,  p.  468.  They  have  been  produced  experimentally  in  the 
rtiildal  rocks  fused  by  Messrs.  Fouque  and  Michel-L<'vy. 


ajjpcar  to  be  any  essential  distinction,  save  in  degiee  of  development^ 
between  these  forms  and  the  long  rod-like  or  needle-shaped  bodies  which 
have  been  termed  Monilcs.  Existing  somedmes  as  mere  simple  needles 
or  rods,  these  nioi'c  elongated  crystallites  may  be  traced  into  more 
complex  forms,  cun-cd  or  coiled,  at  one  time  solitary,  at  another  in 
groups.  Ill  most  casus,  crystallites  are  transparent  and  colourless,  or 
slightly  tinted,  but  sometimes  they  are  black  and  opatjue,  from  a  coating 
of  ferruginous  oxide,  or  only  appear  so  as  an  optical  delusion  from  their 
position.  Black,  seemingly  o|Mqiie,  hair-like,  twisted  and  cm-ved  formi^ 
termed  IricliiUn,  occur  abundantly  in  obsidian. 

Microlites  are  other  incipient  forms  of  crystallization  which  differ 
from  crystallites  in  that  they  react  on  ]^)olarized  light.  They  assume  rod- 
like or  needle-shaped  forms  sometimes  occurring  singly,  sometimes  in 
aggregates,  and  even  occasionally  grouped  into  skeleton-crystals.  They 
can  for  the  most  jtart  l>e  identified  as  rudimentary  forms  of  definite 
minerals  such  as  augite,  hornblende,  felsjiar,  olivine,  and  magnetite. 

Good  illiutrations  of  the  general  charLtcter  and  grouping  of  crystallites 
and   microlites   are  shown   in   some   vitreous  basalts.      In    Fig.    13,    for 


— AuRltf  Cr}»t«l  aumiuiulni  hy  Crjii- 
Ullitrw  «n.l  MitrolitvK,  fr.™  thr  vitn.-"i» 
AioU'ilt.^  Kf  Bskdalrnmlr,  iiiajnilHi.<l  SOU 


exaniple,  the  outer  jmrtion  of  the  field  displays  crowded  globulites  and 
longulites,  as  well  as  here  and  thei'e  a  few  belonites  and  some  curved  and 
coiled  trichites-  Round  the  rude  augite  crystal,  these  various  bodies  have 
been  drawn  together  out  of  the  surrounding  glass.  Numerous  rod-like 
microlites  diverge  from  the  crystal,  and  these  are  moi-e  or  less  thickly 
crusted  with  the  simpler  and  smaller  forms.'  In  Fig.  H,  the  remarkably 
1>eautiful  stnicture  of  an  Arran  pitchstone  is  shown ;  the  glassy  base 
Iwing  crowded  with  minute  microlites  of  horrd>lende  which  are  grouped 
in  a  fine  feathery  or  brush-like  arrangement  round  tai>ering  rods.  In 
this  case,  also,  we  see  that  the  glassy  base  has  been  clarified  round  the 
larger  individuals  by  the  altstraction  of  the  crowded  smaller  microliter. 
6,  Plnl»  V.  Vlg.  S.     J.  J.  H.  Teall,  Q.  J.  Oeot. 


PART  11  §  V  MICROSCOPIC  CHARACTERS  OF  ROCKS  117 


By  the  progressive  development  of  crystallites,  microlites,  or  crystals 
during  the  cooling  and  consolidation  of  a  molten  rock,  a  glass  loses  its 
vitreous  character  and  becomes  lithoid ;  in  other  words,  undergoes 
devitrification. 

The  characteristic  amorphous  or  indefinitely  granular  and  fibrous  or 
scaly  matter,  constituting  the  microscopic  base  in  which  the  definite 
crystals  of  felsites  and  porphyries  are  imbedded  (pp.  160,  161),  has 
been  the  subject  of  much  discussion.  Between  crossed  Nicol-prisms  it 
sometimes  behaves  isotropically,  like  a  glass,  but  in  other  cases  allows  a 
mottled  glimmering  light  to  pass  through.  It  is  now  well  understood  to 
be  a  product  of  the  devitrification  of  once  glassy  rocks  wherein  the 
crystallitic  and  microlitic  forms  can  still  be  recognised  or  have  been  more 
or  less  effaced  by  subsequent  alteration  by  infiltrating  water.  ^ 

Every  gradation  in  the  relative  abundance  of  crystallites  may  be 
traced.  In  some  obsidians  and  other  vitreous  rocks,  portions  of  the 
glass  can  be  obtained  with  comparatively  few  of  them ;  but  in  the  same 
rocks  we  may  not  infrequently  observe  adjacent  parts  where  they  have 
been  so  largely  developed  as  to  usurp  the  place  of  the  original  glass,  and 
give  the  rock  in  consequence  a  lithoid  aspect  (Fig.  11,  C  and  pp.  160-3). 

D.  Dktritus. — Many  rocks  are  composed  of  the  detritus  of  pre-exist- 
ing materials.  In  the  great  majority  of  cases  this  can  be  readily  detected, 
even  with  the  naked  eye.  But  where  the  texture  of  such  detrital  or 
fragmental  (clastic)  rocks  becomes  exceedingly  fine,  their  true  nature  may 
require  elucidation  with  the  microscope  (Figs.  21,  22).  An  obvious  dis- 
tinction can  be  drawn  between  a  mass  of  compact  detritus  and  a  crystalline 
or  vitreous  rock.  The  detrital  materials  are  found  to  consist  of  various  and 
irregularly  shaped  grains,  with  more  or  less  of  an  amorphous  and  generally 
granular  paste.  In  some  cases,  the  grains  are  broken  and  angular,  in 
others  they  are  rounded  or  waterworn  (pp.  128,  129).  They  may  consist 
of  minerals  (quartz,  chert,  felspars,  micii,  t^'c),  or  of  rocks  (slate,  limestone, 
basalt,  &c.),  or  of  the  remains  of  plants  or  animals  (spores  of  lycopods, 
fragments  of  shells,  crinoids,  <fcc.)  It  is  evident  therefore  that  though 
some  of  them  may  be  crystalline,  the  rock  of  which  they  now  fonn  part 
is  a  non-crystalline  compound.  Water,  with  carbonate  of  lime  or  other 
mineral  matter  in  solution,  permeating  a  detrital  rock,  has  sometimes 
allowed  its  dissolved  materials  to  crystallize  among  the  interstices  of  the 
detritus,  thus  producing  a  more  or  less  distinctly  crystalline  structure. 
But  the  fundamentally  secondary  or  derivative  nature  of  the  mass  is  not 
always  thereby  effaced. 

2.  Microscopic  Structures  of  Bocks, 

We  have  next  to  consider  the  manner  in  which  the  foregoing 
microscopic  elements  are  associated  in  rocks.  This  inquiry  brings  before 
us  the  minute  structure  or  texture  of  rocks,  and  throws  great  light  upon 
their  origin  and  history.^ 

^  See  Zirkel,  *  Mik.  Beschaff.'  p.  280.     Roseubusch,  vol.  ii.  p.  60. 

'  The  first  broad  classification  of  the  microscopic  structure  of  rocks  was  that  proposed 
by  Zirkel,  which,  with  slight  moditicatiou,  is   here   adopted.     'Mik.  Beschafi*.*  \k  265. 


118  QEOGNOSY  bookh 

Four  types  of  rock-atnicture  arc  revealed  by  the  microBCOpo.  A, 
holocrystalliiie ;  B,  henii-cryatalline  ;  C,  glaeay ;  D,  clastic. 

A.  HoLOCRYSTALLiNE,  conBistiiig  entirely  of  crystala  or  crystalline 
indinduals,  whether  visible  to  the  naked  eye,  or  I'equiring  the  aid  of  ■ 
microscope,  imbedded  in  each  other  without  any  intervening  amorphoiu 
substance.  Rocks  of  this  tj-jtc  are  exemplified  by  granite  (Figs.  15  and  29) 
and  by  other  igneous  rocks.  But  they  occur  also  among  the  cryBtalline 
limestones  and  schists,  as  in  stittuarv  marble,  which  consist*  entirely  of 
crystalline  granules  of  calcite  (Fig.  28), 

According  to  the  classification  proj)osed  by  Prof.  Rosenbusch  Um 
holocrystalline  structure  is  idwmm-phic  or  panidiomorphic  when  each  ot 
the  component  crystals  has  assumed  its  own  crystal lographic  form,  and 
iillntriomtyrphic  when  it  has  its  outlines  determined  by  those  of  its  neighbounL 
When  interspaces  have  been  left  between  the  crystals  or  crystalline  grains 
the  Btnictiire  is  minrolilir  or  saccharoid. 

The  holocrystalline  eruptive  rocks  (p.  164)  are  typically  represented 
by  granite,  hence  the  term  <franitm<l  has  been  adopted  to  express  their 


Bw  trirliDli-  Fclainr :  tho  bnader  mi 
roniii,  illghUr  ahided  in  die  dnvtng,  m 
AUKltr :  the  lilsck  apecka  an  Htgnatlte; 
the  nwlle-Bhaped  rorma  >n  AjuXMk.     <Bm 

microscopic  stnicture.  Varieties  of  this  structure  are  designated 
according  to  the  relations  of  the  component  minerals.  ^V'here  no  one 
mineral  greatly  preponderates,  but  where  they  are  all  confusedly  and 
tolerably  equally  distributed  in  individuals  readily  observable  by  the 
naked  eye,  as  onlinary  granite,  the  structure  is  granUic  (see  gnmviar, 
p.  99).  \\Tiere  a  similar  stnicture  is  so  fine  that  it  can  only  be  re- 
cognised with  the  microsco]>e,  it  has  been  called  micro^-a-nUie  or  tm-Uic. 
Where  the  minerals  arc  gronjicd  in  small,  isolated,  grain-like  individuals, 
'  Bualt^9i|«iQe,'  p.  88.     8™  nlso  Roscnbusoh's  sngRBstive  [Kiper  already  cited,  A'etiet  JaJiti, 


PAST  n  g  T  MICROSCOPIC  CHARACTERS  OF  ROCKS  119 

each  having  its  own  independent  crjBtalline  structure,  so  that  under  the 
microscope  in  polarized  light,  the  rock  presents  the  appearance  of  a 
brilliant  mosaic,  the  structure  has  been  named  granulilic  or  micro^anuHiic 
{ panidicmorphic  ffranvlar  OT  pi»yhyric  oi  RosenhuBch).  Where  the  quartz 
and  felspar  of  a  granitic  rock  have  crystallized  together,  one  within  the 
other,  the  atmcture  is  pe^matUic  (Fig.  31)  where  visihle  to  the  naked  eye, 
and  micropegmalitie  {graaophyrie  of  Rosenbusch)  where  the  help  of  a 
microscope  is  needed  (Fig.  5).^ 

R  Hemi-CRTSTALLINE.' — This  division  probably  comprehends  the 
majority  of  the  massive  eruptive  or  igneous  rocks.  It  is  distinguished 
by  the  occurrence  of  what  appears  to  the  naked  eye  as  a  compact  or 
finely  granular  ground-mass,  through  which  more  or  less  recognisable 
crystals  are  scattered.  Examined  with  the  microscope,  this  ground-mass 
is  found  to  present  considerable  diversity  (Figs.  16,  18,  32).  It  may  be 
(1)  wholly  a  glass,  as  in  some  basalts,  trachytes,  and  other  volcanic 
products;  (2)  partly  devitrified  through  separation  of  peculiar  little 
granules  and  needles  (crystallites  and  microlitcs)  which  appear  in  a  vitreous 
base ;  (3)  still  further  devitrified,  until  it  becomes  an  aggregation  of  such 
little  granulea,  needles,  and  hairs,  between  which  little  or  no  glass-base 
appears  (microerystallitic) ;  or  (4)  "  microfelsitic  "  (petrosillccous),  closely 
related  to  the  two  previous  groups,  and  consisting  of  a  nearly  structureless 
— 'T".  marked  usually  with  indefinite  or  half-effaced  granules  and  filaments, 
bat  behaving  like  a  singly-refracting,  amorphous  body  (p.  1 1 5). 

In  rocks  belonging  to  this  type,  a  spherulitic  structure  has  sometimes 


been  produced   by  the  appearance  of  globular  bodies  composed  of  a 

1  Foaqa^  and  Mlcbel-L^vy,  '  Min.  Micn^rajili.'  The  niicropegniBtite  of  Mii-liel-L^vy 
ii  the  Mmc  u  the  structure  sabstqucatly  named  grBoapliyre  by  RoBeabnsch.  Michel 
Uvy,  'Roches  Ernptlvea.'  p.  19. 

*  Pot  this  rtructure  the  term  "  miierl "  haa  been  proposed,  a.«  being  a  mixture  of  tlie 
cijit^Iiue  and  amorphona  (glaaay)  strnctures.  It  has  becu  deai^ated  by  Fouiiue  and 
Hichd-UTjr  "  tr«chytoid, "  u  being  typically  developed  among  the  trachytes  {potlra  p. 
IM).     It  U  called  "hypocryntalline  "  by  Rosenbusch. 


120  GEOnXOsy  book  n 

crystal! i HO  internally  radiating  substance,  sometimes  with  concentric 
i>he)ls  of  amorphous  material.  In  many  caiieA,  spheiailites  nre  only 
recognisable  with  the  microscojjc,  when  they  each  present  a  bbick  eros* 
between  crossed  Xicol-prisma,  and  thereby  characteristically  reveal  the 
micnmpkeriiliHc  stiiicture  (Figs.  7  and  17),' 

The  term  ophitic  is  applied  to  a  rftnicture  in  which  one  mineral  aft«r 
ciystallizing  has  been  enclost^  within  another  during  the  consoiitlation  of 
an  ifpieouK  ruck  (Fig.  Ir*).  It  is  abundant  in  many  dolerites  and  diabases 
where  some  bisilicate  siich  as  angite  senes  as  a  matrix  in  which  the 
fels|)ars  and  other  crystals  are  enclosed.  The  name  ie  derived  from  tha 
so-called  "  ophites  "  of  the  PyrenoeB.- 

C.  Gla-S-sy. — Composed  of  »  volcanic  glass  such  as  has  already  been 
described.  It  seldom  happens,  however,  that  rocki:  which  seem  to  the  eye 
to  be  tolerably  homogeneous  glass  do  not  contain  abundant  cr^'stallite* 
and  minute  crj'stals.  Hence  truly  vitreous  rocks  tenil  to  graduate  into 
the  second  or  hemi-cni'stalline  tyjx'.  This  gradation  and  the  abundant 
ti-aces  of  a  deritrified  base  or  magma  lictween  the  crystals  of  a  vast 
nnniber  of  eniptive  rocks,  lead  to  the  belief  that  the  glassy  type  was  the 
original  condition  of  most  if  not  all  of  these  rocks.  Enipted  as  molten 
masses,  their  mobility  wonlil  depend  u]H>n  the  fluidity  of  the  glass.  Yet 
even  while  still  deep  within  the  eai'th's  crust,  some  of  their  constituent 
minerals  (felspars,  leucite,  magnetite,  Ac.)  were  often  already  crystallized, 
and  suflered  fracture  and  corrosion  by  su1tse(|Upnt  action  of  the  enclosing 
magma.     This   is  well  shown  by  what   is  termed  the  Jtoitsirueliire  or 


stallites  are  rangeil  in  current-like  lines, 

t  ion  of  these  lines.     Where  a  large  older 

ter  individuals  is  found  to  siveep  round 


J.  Kiiliii.  XalscJi.  IhaUch.  l/fi. 


PART  u  §  V  MICROHGOPIC  CHARACTERS  OF  ROCKS  121 

it  and  to  reunite  on  the  further  side,  or  to  be  diverted  in  an  eddy-like 
course  (Fig.  19).  So  thoroughly  is  this  arrangement  characteristic  of  the 
motion  of  a  somewhat  viscid  liquid,  that  there  cannot  be  any  doubt  that 
such  was  the  condition  of  these  masses  before  their  consolidation.  The 
flow-structure  may  be  detected  in  many  eruptive  rocks,  from  thoroughly 
vitreous  compounds  like  obsidian,  on  the  one  hand,  to  completely  crystal- 
line masses  like  some  dolerites,  on  the  other.  It  occurs  not-  only  in  what 
are  usually  regarded  as  volcanic  rocks,  but  also  in  plutonic  or  deep-seated 
masses  which,  there  is  reason  to  believe,  consolidated  beneath  the  surface, 
as  for  instance  in  the  Bode  vein  of  the  Harz,  among  quartz-porphyries 
associated  with  granites  in  Aberdeenshire,  and  in  felsite  dykes  and  bosses 
in  the  Shetlands,  Skye,  central  Scotland,  and  County  Waterford.  The 
structure,  therefore,  cannot  be  regarded  as  certainly  indicating  that  the 
rock  in  which  it  is  found  ever  flowed  out  at  the  suriface  as  lava. 

Some  glassy  rocks,  in  cooling  and  consolidating,  have  had  spherulites 
developed  in  them  (Fig.  17) ;  also  by  contraction  the  system  of  reticulated 
and  spiral  cracks  known  as  perlitic  structure  (p.  101  and  Figs.  9  and  20). 

The  final  stiffening  of  a  vitreous  mass  into  solid  stone  has  resulted 
(Ist)  from  mere  solidification  of  the  glass :  this  is  well  seen  at  the  edge 
of  dykes  and  intrusive  sheets  of  different  basalt-rocks,  where  the  igneous 
mass,  haling  been  suddenly  congealed  along  its  line  of  contact  with  the 
surrounding  rocks,  remains  there  in  the  condition  of  glass,  though  only 
an  inch  further  inward  from  the  edge  the  vitreous  magma  has  dis- 
appeared, as  represented  in  Fig.  287;  (2nd)  from  the  devitrification  of 
the  glass  by  the  abundant  development  of  microfelsitic  granules  and 
filaments,  as  in  quartz-porphyry,  or  of  crystallites,  microlites  and  crysUils, 
as  in  such  glassy  rocks  as  obsidian  and  tachylite ;  or  (3rd)  from  the 
complete  crystallization  of  the  whole  of  the  onginal  glassy  base,  as  may 
be  obeerved  in  some  dolerites. 

D.  Clastic. — Composed  of  detrital  materials,  such  as  have  been  already 
described  (p.  103  and  Fig.  2 1 ).  Where  these  materials  consist  of  grains  of 
quartz-sand,  they  withstand  almost  any  subsequent  change,  and  hence 
can  be  recognised  even  among  a  highly  metamorphosed  series  of  rocks. 
Quartzite  from  such  a  series  can  sometimes  be  scarcely  distinguished  under 
the  microscope  from  unaltered  quartzose  sandstone.  Where  the  detritus 
has  resulted  from  the  destruction  of  aluminous  or  magnesian  silicates, 
it  is  more  susceptible  of  alteration.  Hence  it  can  be  traced  in  regions 
of  local  metamorphism,  becoming  more  and  more  crystalline,  until  the 
rocks  formed  of  or  containing  it  pass  into  true  crystalline  schists. 

Detritus  derived  from  the  comminution  or  decay  of  organic  remains 
presents  very  different  and  characteristic  structures  ^  (Fig.  22). 
Sometimes  it  is  of  a  siliceous  nature,  as  where  it  has  been  derived  from 
diatoms  and  radiolarians.  But  most  of  the  organically-derived  detritiil 
rocks  are  calcareous,  formed  from  the  remains  of  foraminifera,  corals, 
echinoderms,    i)olyzoa,    cirripedes,    annelides,    mollusks,     Crustacea    and 

*  The  student  who  would  further  investigate  tliis  subject,  will  tin«l  a  suggestive  and 
laininoufl  essay  upon  it  by  Mr.  Sorby  in  liis  Presidential  Address  to  the  Geological  Society. 
Qtuirt.  Journ,  Otol,  Sue.  1879. 


other  iiivertel)rate8,  «-iih  <M:casioiial  traces  of  fishes  or  even  of   higher 
vertebrates.     Distinct  differences  of  microscopic  structure  can  be  detected 


kii'tim',  or  Orguile  OrlglB- 
Htmctun  of  Cluilk  (Sorb]).    Migniflrd  IM 
mttm.    !4W1>.132.)  Diainet«ni.        {Sf<- |i.  140l) 

in  the  hanl  iwrts  of  some  of  the  livn'ng  representatives  of  these  fornu, 
and  similar  diKerences  have  been  detected  in  beds  of  limestone  of  all  age^ 
Mr.  Sorby,  in  the  i>a))er  cited  below,  has  shown  how  characteristic  and 
persistent  are  some  of  these  distinctions,  and  how  they  may  be  mode  to 
indicate  the  origin  of  the  rock  in  which  they  occur.  There  is  ui 
important  difference  between  the  two  forms  in  which  carbonate  of  litiie 
in  made  use  of  by  invertebrate  animals ;  aragonite  being  much  lesi 
durable  than  calcito  (pp.  78,  139).  Hence  while  shells  of  gasteropoda, 
many  lamellibninchB,  corals  and  other  organisms,  formed  largely  w 
wholly  of  aragonite,  crumble  down  into  mere  amorphous  mud,  pass  into 
crystalline  calcite,  or  disappear,  the  fragments  of  those  consisting  of 
calcito  may  remain  (luite  recognisable. 

It  is  evident,  therefore,  that  the  absence  of  all  trace  of  organic 
structure  in  a  limestone  need  not  invalidate  an  inference  from  other 
evidence  that  the  rock  has  been  formed  from  the  remains  of  oi^nienu. 
The  calcareous  organic  debris  of  a  sea-bottom  may  he  disintegrated,  and 
reduced  to  amorphous  detritus,  by  the  mechanical  action  of  wave*  and 
currents,  by  the  solvent  chemical  action  of  the  water,  liy  the  decay  of 
the  binding  matei-ial,  such  as  the  organic  matter  of  shells,  or  by  being 
swallowed  and  digested  by  other  animals  {pnska,  p.  138).' 

Moreover,  in  clastic  calcareous  rocks,  owing  to  their  liability  to  alter- 
ation by  infiltrating  water,  there  is  a  tendency  to  aujuire  an  internal 
crystalline  texture  (p.  366).  At  the  time  of  foi-mation,  little  empty  spocea 
lie  between  the  component  granules  and  frai^ents,  and  according  to  Mr. 
Sorby,  these  interspaces  may  amount  to  alrout  a  quarter  of  the  whole 
mass  of  the  rock.  They  have  very  commonly  bceti  tilled  up  by  calcite 
introtluced  in  solution.  This  infiltrated  calcite  itcquircs  a  crystalline 
'  Sorby,  Preaiiienlinl  Aitdrem,  (i.  J.  Hfol.  .■4ic  187i>.  0.  Rom.  Ablian^.  Amd.  Berlin, 
1858  ;  Oiimbel,  XeilKh.  DeuHch.  f/fni.  Gmellach.  1884,  {i.  386.  Cornish  moA  Kendall,  StU, 
Mag.  1888,  p.  C6. 


PART  n  §  vi  CLASSIFICATION  OF  ROCKS  123 

structure,  like  that  of  ordinary  mineral-veins.  But  the  original  com- 
ponent organic  granules  also  themselves  become  crystalline,  and,  save 
in  so  far  as  their  external  contour  may  reveal  their  original  organic 
source,  they  cannot  be  distinguished  from  mere  mineral-grains.  In  this 
way,  a  cycle  of  geological  change  is  completed.  The  calcium-carbonate 
originally  dissolved  out  of  rocks  by  infiltrating  water,  and  carried  into 
the  sea,  is  secreted  from  the  oceanic  waters  by  corals,  foraminifera, 
echinoderms,  moUusks  and  other  invertebrates.  The  remains  of  these 
cretttares  collected  on  the  sea-bottom  slowly  accumulate  into  beds  of 
detritus,  which  in  after  times  are  upheaved  into  land.  Water  once 
more  percolating  through  the  calcareous  mass,  gradually  imparts  to  it  a 
crystalline  structure,  and  eventually  all  trace  of  organic  forms  may  be 
effaced.  But  at  the  same  time,  the  rock,  once  exposed  to  meteoric 
influences,  is  attacked  by  carbonated  water,  its  molecules  are  carried  in 
solution  into  the  sea,  where  they  will  again  be  built  up  into  the  frame- 
work of  marine  organisms. 

K  Alteration  of  Rocks  by  Meteoric  Water. — An  impoitant 
revelation  of  the  microscope  is  the  extent  to  which  rocks  suffer  from  the 
influence  of  infiltrating  water.  The  nature  of  some  of  these  changes  is 
described  in  subsequent  pages.  (Book  III.  Part  II.  Sect.  ii.  §  2.)  It  may 
be  sufficient  to  note  here  a  few  of  the  more  obvious  proofs  of  alteration. 
Threads  and  kernels  of  calcite  running  through  an  eruptive  rock,  such 
as  diabase,  dolerite,  or  andesite,  are  a  good  index  of  internal  decomposi- 
tion. They  usually  point  to  the  decay  of  some  lime-bearing  mineral  in 
the  rock.  Some  other  minerals  are  likewise  frequent  signs  of  alteration, 
such  as  serpentine  (often  resulting  from  the  alteration  of  olivine  (Figs. 
33,  34),  chlorite,  epidote,  limonite,  chalcedony,  &c.  In  many  cases, 
however,  the  decomposition  products  are  so  indefinite  in  form  and  so 
minute  in  quantity,  as  not  to  permit  of  their  being  satisfactorily  referred 
to  any  known  species  of  mineral.  For  these  indeterminate,  but 
frequently  abundant  substances,  the  following  short  names  were 
proposed  by  Vogelsang  to  save  periphrasis,  until  the  true  nature  of  the 
substance  is  ascertained.  Viridite — green  transparent  or  translucent 
patches,  often  in  scaly  or  fibrous  aggregations,  of  common  occurrence  in 
more  or  less  decomposed  rocks  containing  hornblende,  augite,  or  olivine  : 
{Hx>bably  in  many  cases  serpentine,  in  others  chlorite  or  delessite.  Ferrite 
— yellowish,  reddish,  or  brownish  amorphous  substances,  probably  consist- 
ing of  peroxide  of  iron,  either  hydrous  or  anhydrous,  but  not  certainly 
referable  to  any  mineral,  though  sometimes  pseudomorphous  after 
ferruginous  minerals.  Opaciie — black,  opaque  grains  and  scales  of 
amorphous  earthy  matter,  which  may  in  different  cases  be  magnetite, 
or  some  other  metallic  oxide,  earthy  silicates,  graphite,  <fcc.^ 

§  vi.  Classification  of  Bocks. 

It  is  evident  that  Lithology  may  be  approached  from  two  very 
different  sides.     We  may,  on  the  one  hand,  regard  rocks  chiefly  as  so 

*  VogelsaDg,  Z.  Deutach.  OeoL    Oes.  xxiv.    (1872),  p.   ri29.     Zirkol,    Oeol.  ExpL  iOt?i 
PetraiMf  toL  tL  p.  12. 


1 2  4  GEOGXOS  y  BOOK  II 


many  masses  of  mineral  matter,  presenting  great  'vanety  of  chemical 
comiK)sition  and  marvellous  diversity  of  microscopic  structure.  Or,  on 
the  other  hand,  passing  from  the  details  of  their  chemical  and  mineral- 
ogical  characters,  we  may  look  at  them  rather  as  the  records  of  ancient 
t<>rrestrial  changes.  In  the  former  aspect,  they  present  for  consideration 
problems  of  the  highest  interest  in  inorganic  chemistry  and  mineralogy  ; 
in  the  latter  view,  they  invite  attention  to  the  gi'cat  geological  revolu- 
tions through  which  the  planet  htu*  jmssod.  It  is  evident,  therefore, 
that  two  distinct  systems  of  classification  may  l)e  followed,  the  one 
iKised  on  chemical  and  mineralogical,  the  other  on  geological  con- 
siderations. 

P'rom  a  chemical  point  of  view,  rocks  may  be  gi'ouped  according  to 
their  composition ;  as  Oxides,  exemplified  by  foiTiiations  of  quartz, 
luematite,  or  magnetite ;  Carbonates,  including  the  limestones  and 
clay-ironstones ;  Silicates,  embracing  the  vast  majority  of  rocks,  whether 
eomixosed  of  a  single  mineral,  or  of  more  than  one ;  Phosphates,  such  ae 
guano  and  the  older  bone-beds  and  coprolitic  dej)osits.  A  classification 
of  this  kind,  however,  piys  no  regard  to  the  mode  of  origin  or  conditions 
of  occui-rence  of  the  rocks,  and  is  not  well  suite<l  for  the  purposes  of  the 
geologist.^ 

P^rom  the  mineralogical  side,  rocks  may  be  classified  with  reference 
to  their  prevailing  minei'al  constituent.  Thus  such  subdiAisions  as 
Calcareous  rocks,  Quartzose  rocks,  Orthoclase  rocks,  Plagioclase  rocks, 
Pyroxenic  rocks,  Hornblendic  rocks,  &c.,  may  be  adopted ;  but  these 
terms  are  hardly  less  objectionable  to  the  geologist,  and  are  in  fact 
suited  rather  for  the  arrangement  of  hand-specimens  in  a  museum,  than 
for  the  investigation  of  rocks  in  situ. 

From  the  standpoint  of  geological  inquiry,  rocks  have  been  classified 
according  to  their  mode  of  origin.  In  one  system  they  arc  arranged 
under  three  great  divisions  :  1st,  If/nwus,  embracing  all  which  have  been 
erui>ted  from  the  heated  interior  of  the  earth  ;  2nd,  -  Iqueous  or  SaHment- 
anf,  including  all  which  have  been  laid  down  as  mechanical  or  chemical 
deposits  from  water  or  air,  and  all  which  have  resulted  from  the  growth 
and  decay  of  plants  or  animals :  3rd,  Metn ntorjihic,  those  which  have 
undei'gone  subsequent  change  within  the  crust  of  the  eiuth,  whereby 
their  original  character  has  l>een  so  modified  as  to  be  sometimes  quite 
indeterminable.  Another  geological  arrangement  is  l)ased  upon  the 
geneial  structure  of  the  rocks,  and  consists  of  two  divisions,  1st, 
Sfratifu'fl,  embiucing  all  the  arpieous  and  sedimentary,  with  part  of  the 
less  altered  metumorphic  rocks  :  2nd,  l/nsfrafijiid,  nearly  conterminous 
with  the  term  igneous,  since  it  includes  all  the  erui)tivo  rocks.  Further 
suMivisions  of  this  series  have  been  proi>08cd  according  to  differences  of 
structure  or  texture,  as  jMtrphf/ritiCy  granitic^  Szc.  'i'hese  geological  sulv 
divisions,  however,  ignore  the  chemic^d  and  mineralogical  characters  of 
the  locks,  and  are  based  on  deductions  which  may  not  always  be  sound. 
Thus,  rocks  may  be  included  in  the  igneous  series,  which  further  research 

'  The  eruptive  rook«  are  siisceptilile  of  a   coiivi«iiieiit,    though   not  strictly  accurate, 
cheiiiicul  (.'lassificution  into  acidj  luterim'tliaft',  an«l  basic  (sec  \k  ir)0). 


PART  II  g  vi  CLASSIFICATION  OF  ROCKS  125 


may  show  not  to  be  of  igneous  origin ;  others  may  be  classed  as  meta- 
morphic,  regarding  the  true  origin  of  which  there  may  be  considerable 
uncertainty. 

A  further  system  of  classification,  based  upon  relative  age,  has  been 
applied  to  the  arrangement  of  the  eruptive  rocks,  those  masses  which 
were  erupted  prior  to  Secondary  time  being  classed  as  "older,"  and 
those  of  Tertiary  and  later  date  as  "  younger."  This  system  has  been 
elaborated  in  great  detail  by  Michel-L^vj',  who  maintains  that  the  same 
types  have  been  reproduced  nearly  in  the  same  order  in  the  two  series, 
Uiough  basic  rocks,  often  with  vitreous  characters,  rather  predominate  in 
the  later.  ^  It  must,  indeed,  be  admitted  that  certain  broad  distinctions 
between  the  older  and  the  later  eruptive  rocks  have  been  well  ascertained, 
and  appear  to  hold  generally  over  the  world.  Among  these  distinctions 
may  be  mentioned  as  more  characteristic  of  the  Palaeozoic  rocks  the  presence 
of  microcline,  tiu*bid  orthoclase  in  Carlsbad  twins,  muscovite,  enstatite, 
bronzite,  diallage,  tourmaline,  anatase,  rutile,  cordierite,  and  in  the 
younger  rocks  the  presence  of  sanidine,  tridymite,  leucite,  nosean,  hauyne, 
and  zeolites.  Even  where  the  same  mineral  occurs  in  both  the  older  and 
newer  series,  it  often  presents  a  somewhat  different  aspect  in  each,  as  in 
the  case  of  the  plagioclase  and  augite,  which  in  the  younger  series  are 
distinguished  by  the  occurrence  in  them  of  vitreous  and  gaseous  in- 
clusions which  are  rare  or  absent  in  those  of  the  older  series.^  Throughout 
the  younger  eruptive  rocks,  the  vitreous  condition  is  much  more  frequent 
and  perfectly  developed  than  in  the  older  group,  where,  on  the  other 
hand,  the  granitic  structure  is  characteristically  displayed.  Still,  to  these 
rules  so  many  exceptions  occur  that  it  may  be  doubted  whether  enough 
of  positively  ascertained  data  have  been  collected  regarding  the  relative 
ages  of  eruptive  rocks  to  warrant  the  adoption  of  any  classifi cation  upon 
a  chronological  basis.  There  can  be  no  doubt  that,  making  due  allowance 
for  the  alterations  arising  from  permeation  by  meteoric  water,  there  is  no 
essential  difference  between  some  types  of  volcanic  rock  in  Palaeozoic  and 
in  recent  times.  The  Carboniferous  basalts  and  trachvtes  of  Scotland,  for 
example,  present  the  closest  resemblance  to  those  of  Tertiary  age.^ 

Though  no  classification  which  can  at  present  be  proposed  is  wholly 
satisfactory,  one  which  shall  do  least  violence,  at  once  to  geological  and 
mineralogical  relationships,  is  to  be  preferred.  The  arrangement  which  has 
met  with  the  most  general  acceptance  is  threefold.  1st,  Sedimentary 
Rocks,  including  first  the  rocks  which  have  resulted  from  the  accumulation 

*  See  on  this  subject,  J.  D.  Dana,  Awer.  J.  «Slct.  xvi.  1878,  p.  336.  Michel-Levy, 
BM,  Soc,  OM,  Frafice,  8rd  ser.  iii.  (1874),  p.  199;  vi.  p.  173.  Ann.  ihs  Mines,  viii. 
(1875)  *  Roches  Eruptives,*  1889.  Fouqueand  Michel-Levy,  'Miueralogie  Microgr.'  p.  ir»0. 
Rosenbusch,  *  Mik.  Physiog.'  ii.  Reyer,  'Physikder  Eruptionen,'  1877,  part  iii.  opposes 
the  mdoption  of  relative  age  as  a  basis  of  classification.  On  the  classification  of  compound 
silicated  rocks,  see  Vogelsang,  Z.  Deutsch.  Oeol.  Oes,  xxiv.  p.  507  :  and  for  an  incisive 
criticisni  of  a  too  merely  mineralogical  classification,  Lossen,  oj).  cit.  xxiv.  p.  782.  Consult 
also  O.  Lang,  *  Ober  die  Individaalitiit  der  Gesteine  '  in  Tsch^rmaJca  Min.  Mittheil.  vol.  xi. 
part  6  (1890),  p.  467. 

*  See  J.  Murray  and  A.  Renard,  Proc  Hoy.  Sot-.  Edin.  xi.  p.  669. 
»  See  Xature,  iii.  (1871),  p.  303. 


126  frKOirXOSY  BOOKU 


of  detritus,  either  inorganic  or  organic,  under  water  or  on  land,  and 
secondly  those  which  have  been  deposited  irom  aqueous  solution.  The 
former  are  mechanical,  the  latter  chemical  accumulations ;  but  they  have 
oiten  been  deposited  together.  Certain  rocks  of  mechanical  origin,  such 
as  detrital  limestones,  mav  bv  subseciuent  alteration  be  converted  into 
materials  that  cannot  l)e  distinguished  from  others  of  true  chemical 
origin.  Hence  the  whole  series  is  intimately  linked  together.  2nd, 
Massive,  Eruptive,  or  Intrusive  Kocks,  embiucing  all  those  which 
have  solidified  from  fusion  within  the  earth's  crust,  or  have  been  erupted 
as  lava  to  the  surface.  3rd,  Schistose  Rocks,  and  their  accompani- 
ments, including  the  so-called  Metamorphic  rocks  which  have  reached 
their  pi-esent  condition  as  a  consequence  of  the  alteration  sometimes  of 
sedimentiiry,  sometimes  of  igneous  rocks.  This  group  graduates  into  the 
two  others,  but  it  contains  some  distinctive  masses,  the  origin  of  which 
is  still  involved  in  doul)t. 

It  must  be  kept  in  view  that  in  this  projwsed  system  of  classificatioD, 
and  in  the  following  detailed  description  of  rocks,  many  questions 
regarding  the  origin  and  decom])osition  of  these  mineral  masses  must 
necessai-iiy  be  alluded  to.  The  student,  however,  will  find  these  ques- 
tions discussed  in  later  pages,  and  will  prol)ably  recognise  a  distinct 
advantiige  in  this  unavoidable  ])reliminary  reference  to  them  in  connec- 
tion with  the  rocks  by  which  they  are  suggested. 

§  vii.— A  Description  of  the  more  Important  Rocks  of  the  Earth's  Cruatb 

Full  detiiils  regarding  the  comix>sition,  microscopic  structure,  and 
other  characters  of  rocks  must  be  sought  in  such  general  treatises  and 
sjK'cial  memoirs  as  those  already  cited  (pp.  89,  96,  108).  The  purposes 
of  the  present  text-lx>ok  will  be  served  by  a  succinct  account  of  the  more 
common  or  imi)ortiUit  rocks  which  enter  into  the  composition  of  the 
criLst  of  the  earth. 

I.  Sedimentary. 

A.  Fi:a«;mkntal  (Cla-stic^ 

This  great  series  eml)races  all  rocks  of  a  secondary  or  derivative 
origin  ;  in  other  words,  all  formed  of  fragmentary  materials  which  have 
previously  existed  on  or  l^eneath  the  surface  of  the  eaith  in  another 
form,  and  the  accumulation  and  consolidation  of  which  gives  rise  to  new 
compoiuids.  Some  of  these  materials  have  been  produced  by  the 
mechanical  action  of  wind,  as  in  the  sand-hills  of  sea-coasts  and  inland 
deserts  (-L-Eolian  rocks) ;  othei^s  by  the  operation  of  moving  water,  as  the 
gravel,  sand  and  mud  of  shores  and  river-beds  (Aqueous  sedimentary 
rocks) ;  others  ])y  the  accumulation  of  the  entire  or  fragmentary  remains 
of  once  living  plant*;  and  animals  (Organically-formed  rocks) ;  while  yet 
another  series  has  arisen  from  the  gathering  together  of  the  loose  debris 
thrown  out  by  volcanoes  (Volcanic  tuffs).     It  is  evident  that  in  dealing 


PARTii§vii  FRAGMENT AL  ROCKS  127 

with  these  various  detrital  formations,  the  degree  of  consolidation  is  of 
secondary  importance.  The  soft  sand  and  mud  of  a  modern  lake-bottom 
differ  in  no  essential  respect  from  ancient  lacustrine  strata,  and  may  tell 
their  geological  story  equally  well.  No  line  is  to  be  drawn  between 
what  is  popularly  termed  rock  and  the  loose,  as  yet  uncompacted,  debris 
out  of  which  solid  rocks  may  eventually  be  formed.  Hence  in  the 
following  arrangement,  the  modem  and  the  ancient,  being  one  in  structure 
and  mode  of  formation,  are  classed  together. 

It  will  be  observed  that^  in  several  directions,  we  are  led  by  the  frag- 
mental  rocks  to  crystalline  stratified  deposits,  some  of  which  have  been 
deposited  from  chemical  solution,  while  others  have  resulted  from  the 
gradual  conversion  of  a  detrital  into  a  crystalline  structure.  Both  series 
of  deposits  are  accumulated  simultaneously  and  are  often  interstratified. 
Calcareous  rocks  formed  of  organic  remains  (p.  138)  exhibit  very  clearly 
this  gradual  internal  change,  which  more  or  less  effaces  their  detrital 
origin,  and  gives  them  such  a  crystalline  character  as  to  entitle  them 
to  be  ranked  among  the  crystalline  limestones. 

1.  Gravel  and  Sand  Bocks  (Psammites). 

• 

As  the  deposits  included  in  this  subdivision  are  produced  by  the  disintegration  and 
remoTal  of  rocks  by  the  action  of  the  atmosphere,  rain,  rivers,  frost,  the  sea,  and  other 
saperficial  agencies,  they  are  mere  mechanical  accumulations,  and  necessarily  vary 
indefinitely  in  composition,  according  to  the  nature  of  the  sources  from  which  they  are 
derired.  As  a  rule,  they  consist  of  the  detritus  of  siliceous  rocks,  these  being  among 
the  most  durable  materials.  Quartz,  in  }»articular,  enters  largely  into  the  composition 
of  aendy  and  gravelly  detritus.  Fragmentary  materials  tend  to  group  themselves 
according  to  their  size  and  relative  density.  Hence  they  are  apt  to  occur  in  layers,  and 
to  show  the  characteristic  stratified  arrangement  of  sedimentary  rocks.  They  may 
enclose  the  remains  of  any  plants  or  animals  entombed  on  the  same  sea-floor,  river-bed, 
or  lake-bottom. 

In  the  nu^ority  of  these  rocks,  their  general  mineral  composition  is  obvious  to  the 
naked  eye.  But  the  application  of  the  microsco])e  to  their  investigation  has  thrown 
considerable  light  U{K>n  their  com{)osition,  formation,  and  subsequent  mutations.  Their 
component  materials  are  thus  ascertained  to  be  divisible  into — 1st,  derived  fragments,  of 
which  the  most  abundant  are  qu^lz,  after  which  come  felspar,  mica,  iron-qres,  zircon, 
mtile,  apatite,  tourmaline,  garget,  sphene,  augite,  hornblende,  fragments  of  various 
rocks,  and  clastic  dust ;  2nd,  constituents  which  have  been  deposited  between  the 
paiticlee,  and  which  in  many  cases  serve  as  the  cementing  material  of  the  rock.  Among 
the  more  important  of  these  are  silicic  acid  in  the  form  of  quartz,  chalcedony  and  oj>a) ; 
carbonates  of  lime,  iron  or  magnesia  ;  haematite,  limonite  ;  ])yrite  and  glauconite.^ 

dUMMbria,  Moraine  Stnif— ^angular  rul)bish  disengaged  by  frost  and  ordinary 
atmospheric  waste  from  cliffs,  crags,  and  steep  slopes.  It  slides  down  the  declivities  of 
hilly  regions,  and  accumulates  at  their  base,  until  washed  away  by  rain  or  by  brooks. 
It  forms  talns-sloi>es  of  as  much  as  40*",  though  for  short  distances,  if  the  blocks 
are  laige,  the  general  angle  of  slope  may  be  much  steeper.  It  naturally  dei)ends  for 
its  composition  upon  the  nature  of  the  solid  rocks  from  which  it  is  derived.  Where 
clifT-debriB  iaUs  upon  and  is  borne  along  by  glaciers  it  is  called  ''Moraine-stuff,"  which 


1  G.  Klemm,  ZeUsch,  Deutsch.  Oecl,  Ges,  xxxiv.  (1882),  p.  771.     H.  C.  Sorby,  Quart. 
Jtmrn,  Otol.  Soe,  xxxvi.  (1880).     J.  A.  PhiUips,  op.  cit.  xxxvii.  (1881),  p.  6. 


12H  fiEfffwyosy  BOOK  It 


iiiav  !»♦.•  «I»-jni>it»^i  m-rti-  its  SMiinr.  or  may  \**r  traiisiM>rte«l  f«"»r  many  milcr^  ou  the  :$urfjic<:  of 
til-  i«.f   p.  A'2'-i  . 

Perched  Blocki.  Erratic  Blocki — lai-^-  ma.ss*'>  of  i-o.k.  oftrii  a.s  big  as  a  Look, 
wlii"li  havi."  li^eii  traii>|»«»rtf«l  l»y  glacuT-Lf.  aipl  liavr  l»eeu  I«H.lfp:d  iu  a  promiiienc  |HkjtitioB 
ill  iilmnt-.v  vall*"ys  or  hdvf  Yn-en  s*.dttr!f:Hl  nver  liilU  ami  plaiii>.  An  examination  <rf 
tiif-irmiiit'i-alojirif.'al  •Itaracti.T  iiMrls  to  t)ie  iileiitiHt-ation  of  tlieir  stjurev  ami,  cons^|nentIr, 
t<^i  the  j»ath  taken  hy  tho  tran>iN^)rtiii^:  ice.     •Sr*.*  15<»ok  III.  Part  II.  Section  ii.  §  5.) 

Bain-waah—  a  loam  or  i*arth  whii-li  atraniulati's  «iii  thi'  lower  |iarts  of  ^loiies  or  at 
tlifir  IiasJr,  ail' I  i>»  diif  tn  tii»*  ^iidual  <.h'scent  of  the  finer  |Kirtit'lfs  of  ilisintegrated  rocks 
hy  t)ie  ti-aii»iM>rtin^  artioii  of  rain.  Urirk-earth  is  the  name  given  in  the  south-cast 
of  En;j:Ian<l  to  thick  iiia.snf>  of  Mii.h  hianu  w)iir)i  in  extena^ively  Uifed  for  making 
hiii.ks. 

Soil — the  }iro<hi<'t  of  the  siil^f  rial  de(.-omi»ositiou  of  riM-ks  an(l  of  tlit*  decay  of  jilanD 
anrl  animals.  I'l-imaiily  thr  character  of  t)ie  .soil  is  determinKl  hy  that  of  the  sul^ioiL 
of  which  indeed  it  is  merely  a  further  disintegi^ation.  Aci-ording  to  tlie  nature  of  th* 
ror-k  iin«lerneath.  a  soil  niav  vaiv  from  a  stitt'  elav.  throiii;h  various  davev  and  saudv 
I'Mims  to  nifie  sau'l.     The  formation  «»f  snil  is  treated  *}(  in  Book  HI.  Part   II.  Section 

II.    §  1. 

Subsoil-  -the  hroken-ii]i  {lurt  of  the  n^-ks  immeiliately  under  tht*  soil.  Its  character. 
of  c«»ui>e.  is  fletermined  by  that  of  the  rcK-k  out  of  which  it  is  forme^l  by  subaerial  di«in- 
t<';rratiijn.      B<M)k  III.  l*:irt  U.  Sertiiiii  ii.  §  1. 

Blown  Band  -liMtsi*  siind  usually  urran^'d  in  lines  of  dunes,  fi-rmting  a  asaudy  beach 
or  in  th**  ari<I  int«-iir»r  <»f  a  continent.  It  is  piled  \ip  hy  the  driving  action  of  wind. 
(l>iH)k  III.  Tart  II.  Section  i.  It  varies  in  eonijiosition,  l>eiug  sometimes  entirely 
-Niliceon-*.  as  u|M»n  ^hon-s  wlu-n-  >ilice«ms  rocks  are  exiKiseil  ;  sometimes  calcareous,  where 
fh'rivi'd  from  trituraic<l  shells.  nuIliiMjrcs.  or  other  calraitous  organisms.  The  minute 
giains  fri>m  long-continut'il  mutual  friction  assume  i-emarkably  i-ounded  and  iioliflhed 
fitiiiis.  I^iyei>  of  Hner  and  marser  {uirticles  often  alternate,  as  in  water-formed  sand- 
stone. On  many  fOii.st-lin«-.s  in  Euroiie,  gi^as^es  aiiil  otlier  ]ilant.>  bind  the  surface  of  the 
shifting  sand.  The.s*.*  layi-r'.  of  vegetation  are  apt  to  be  eoverwl  by  fresh  encroachments 
of  the  lo<»se  material,  and  then  ]»y  their  decay  to  give  ris*,*  to  dark  }ieaty  seams  in  the 
sand.  Caleai-e<iu»  blown  sand  is  «>om]>iU-te<l  into  hanl  st<»ne  by  the  a^-tion  of  rain-water, 
which  alternately  dis.so]vcs  a  little  nf  the  linu*,  an<l  re-de}N)sits  it  on  eva[)oration  as  a 
tliin  cnist  cementing  the  grains  of  siind  together.  In  the  llaliamas  and  Bermuda^ 
extensive  masses  of  calcareous  blown  ^alnl  have  In-en  cement***!  in  this  way  into  solid 
stone,  wliich  weathers  into  pi«'tui-esiiue  rrags  and  caves  like  a  limestone  of  older  geological 
datt-.'  At  Ni:w<iuay,  (Cornwall,  blown  Sjind  has  Ikmmi  by  the  decay  of  abundant  land* 
shells  scjliflitied  into  a  mateiial  capable  of  being  used  as  a  building-stone. 

Biver-sand,  Sea-sand. — Wlim  the  rounded  water-worn  detiitiis  is  tinerthan  tliat  tu 
which  the  tfini  gravel  would  be  applied,  it  is  called  sand,  though  there  is  obriously  do 
line  to  be  drawn  between  the  tw<i  kinds  of  de]H.»sit,  which  necessarily  graduate  into  each 
lit  her.  The  ]iaiticles  of  .sand  range  down  to  such  minute  forms  as  can  only  be  distinctly 
di.Merned  with  a  mierof,i'opc.  The  smaller  forms  ai-e  generally  less  well  rounded  than 
those  of  gr«'ater  <liiiiensinns.  no  doubt  l»ei'au^e  their  diminutive  sijce  alloa*s  them  to 
remain  su>] Handed  in  agitated  water,  and  tlius  to  escain*  the  mutual  attrition  to  which 
the  larger  and  heaviiri-  grains  aiv  exi»ost»d  ujhui  the  bottom.  (Yivok  III.  Part  II.  Section 
ii.)  S<t  tar  as  exi^'rieiice  has  yet  gone,  theix'  is  no  metluxl  by  which  inorganic  sea-sand 
can  1m!  distinguishe<l  from  that  of  rivers  or  lakes.     As  a  rule,  sand  consists  lai^ly 


*  For  interesting  aecoinits  of  the  .Et^lian  deposits  of  the  Hahanias  and  Bermudas, 
Nel-on,  V-  •^-  ^'•'"'-  •^"-  ix-  !'•  -00,  Sir  Wyville  Thomson's  *"  Atlantic,"  vol.  i.  ;  also  J. 
J.  Iteiu,  .Sr/.'/.r/'//.  A'«/.  fi»>.>'fhrh.  JWic/it.  1^69-70,  p.  140,  lS72-a,  p.  131.  On  the  Red 
Sands  of  the  .Vrabiaii  De.sert,  svc  J.  A.  Phillips,  y.  J.  <f'tol.  .*h-.  xxxviii.  (1882),  |i.  110,  alw) 
oyi.  rif.  XXX vii.  (ISSl),  p.  I'J. 


PART  11  §  vii  FRAGMENTAL  ROCKi^  129 

(often  wholly)  of  quartz-grains.  The  presence  of  fragments  of  marine  shells  will  of 
course  betray  its  salt-water  origin  ;  but  in  the  trituration  to  which  sand  is  exposed  on 
a  coast-line,  the  shell-fragments  are  in  great  measure  ground  into  calcareous  mud  and 
remove<i, 

^fr.  Sorby  has  shown  that,  by  microscopic  investigation,  nmch  information  may  be 
obtaine<l  regarding  the  history  and  source  of  sedimentaiy  materials.  He  has  studied 
the  minute  structure  of  modern  sand,  and  finds  that  sand-grains  present  the  following 
five  distinct  tyi>es,  which,  however,  graduate  into  each  other. 

1.  Normal,  angular,  fresh- fonned  sand,  such  as  has  been  derived  almost  directly 
from  the  breaking  up  of  granitic  or  schistose  rocks. 

2.  Well-woni  sand  in  rounded  grains,  the  original  angles  being  completely  lost,  and 
the  surfaces  looking  like  fine  ground  glass. 

3.  Sand  mechanically  broken  into  sharp  angidar  chips,  showing  a  glassy  fracture. 

4.  Sand  having  the  grains  chemically  corroded,  so  as  to  produce  a  peculiar  texture  of 
the  surface,  <liffering  from  that  of  worn  gi'ains  or  crystals. 

5.  Sand  in  which  the  grains  have  a  perfectly  crystalline  outline,  in  some  cases  un- 
doubtedly due  to  the  deiwsition  of  quartz  upon  rounded  or  angular  nuclei  of  ordinary 
uon -crystalline  sand.^ 

The  same  acute  observer  jwints  out  that,  as  in  the  familiar  case  of  conglomei*atc 
pebbles,  which  have  sometimes  been  used  over  again  in  conglomerates  of  very  diiferent 
ages,  so  with  the  much  more  minute  grains  of  sand,  we  must  distinguish  between  the 
age  of  the  grains  and  the  age  of  the  deposit  formed  of  them.  An  ancient  sandstone 
may  consist  of  grains  that  had  hardly  been  worn  before  they  were  finally  brought  to 
reut,  while  the  sand  of  a  motlem  beach  may  have  been  ground  down  by  the  waves  of 
many  successive  geological  jieriods. 

Sand  taken  by  Mr.  Sorby  from  the  old  gravel  terraces  of  the  River  Tay,  was  found  to 
be  almost  wholly  angular,  indicating  how  little  wear  and  tear  there  may  be  among 
particles  of  ([uartz  jJxr  of  *ii  mch  in  diameter,  even  though  exi>osed  to  the  drifting 
action  of  a  rapid  river.'  Sand  from  the  boulder  clay  at  Scarborough  was  likewise 
ascertained  to  be  almost  entirely  fresh  and  angular.  On  the  other  hand,  in  geological 
formations,  which  can  be  traced  in  a  given  du'ection  for  several  hundred  miles,  a 
progressively  large  proportion  of  rounded  particles  may  ])e  detected  in  the  sandy  Ijeds, 
as  Mr.  Sorby  luis  found  in  following  the  Greensand  from  Devonshire  to  Kent.  In  wind- 
blown sand  exposed  for  a  long  period  to  drift  to  and  fro  along  the  surface  the  larger 
particles  and  pebbles  ac<[uire  a  remarkably  smoothed  and  polished  surface. 

The  occurrence  of  various  other  minerals  besides  «[uartz  in  ordinary  sand  has  long 
been  recognised,  but  we  owe  to  the  recent  observations  of  Mr.  A.  B.  Dick  the  discovery 
that  among  these  minerals  some  of  the  most  plentiful  and  most  perfectly  jjreserved 
belong  to  species  that  were  not  supposed  to  be  so  widely  dilTused,  such  as  zircon,  rutile, 
and  tourmaline.  He  has  found  that  these  heavy  minerals  constitute  sometimes  as  much 
as  4  per  cent  of  the  Bagshot  sand  of  the  older  Tertiary  series  of  the  London  basin.  ^ 
Felspuv,  micas,  hornblendes,  x>yroxenes,  magnetite,  glauconite  and  other  minerals  may 
likewise  be  recognised.  The  remarkable  perfection  of  some  of  the  crystallographic 
forms  of  the  minuter  mineral  constituents  of  certain  sands  has  been  well  shown  by 
Mr.  Dick. 

Varieties  of  river  or  sea -sand  may  be  distinguished  by  names  referring  to  some 
remarkable  constituent,  e.g.y  magnetic  sand,  iron-sand,  gold-sand,  auriferous  sand,  &c. 

GntTVl,  Shingle — names  applied  to  the  coarser  kinds  of  rounded  water- worn  detritus. 

*  Address,  Q,  J.  Geol.  Soc.  xxxvi.  (1880),  p.  58,  and  Monthly  Microscop.  Journ.  Anniv. 
Address,  1877. 

s  See  Book  UI.  Part  II.  Section  u.  §  iii. 

»  Xaiurt^  xzxvL  (1887),  p.  91,  Mem,  Otd,  Surv.  'Geology  of  London,'  vol.  i.  (1889), 
p.  523.    Teall,  'Microscopic  Petrography,'  Plate  xliv. 

K 


1 30  GEOGNOH  Y  book  n 

In  Gravel,  the  avera^f  size  of  tlie  conipoiieiit  ]H;1>ble.s  rang4.>s  from  that  of  a  small  ]jea  1^) 
to  al)Out  that  of  a  walnut,  thou«(Ii  of  course  many  included  fragments  mill  be  obserred 
which  exceed  these  limits.  In  Shingle,  the  stones  are  coarser,  ran^ng  up  to  blocks  at 
big  as  a  man's  head  or  larger.  Oennan  geologists  distinguish  as  ''sehotter/'  a  shingle 
containing  dis]>erse<i  Ix^uldci's,  and  ''scliotter- conglomerate,"  a  rock  wherein  these 
materials  have  become  consolidated.'  All  these  names  are  apiilie<l  quite  irresiiective  of 
the  comjiosition  of  the  fragments,  which  varies  greatly  from  |X)iiit  to  jtoiut.  As  a  rule, 
the  stones  consist  of  hanl  rocks,  since  these  are  1>est  fitted  to  withstand  the  i»owerfal 
grinding  action  to  which  they  are  expose<l. 

Conglomerate  (Puddiugstone)  -a  roi'k  formed  of  consolidated  gravel  or  shin^e. 
The  com]K)nent  jiebbles  arc  rounded  and  water-worn.  They  may  consist  of  any  kind 
of  rock,  though  usually  of  some  hard  and  durable  sort,  such  as  quartz  or  quartzite: 
A  special  name  may  be  given  according  to  the  nature  of  the  i)ebbles,  as  quartz-eon* 
glomerate,  limcstone'Conglomi>rate,  granite-conglomerate,  &c.,  or  according  to  that  of 
the  paste  or  cementing  matiix,  which  may  consist  of  a  hardened  sand  or  clay,  and 
may  be  siliceous,  calcareous,  argillaceous,  or  femiginous.  In  the  coarser  conglomerates, 
where  the  blocks  may  exceed  six  feet  in  length,  there  is  often  very  little  indication 
of  stratification.  Except  where  the  Hatter  stones  show  by  their  general  parallelism  the 
rude  lines  of  d('i»osit,  it  may  !>«•  only  when  the  mass  of  conglomerate  is  taken  as  ■ 
whole,  in  its  relation  to  the  rocks  l>elow  and  above  it,  that  its  claim  to  be  considered 
a  l)edded  rock  will  Ki  conceded.  ^  The  occun^ence  of  (K'casional  bands  of  conglomerate 
in  a  series  of  arenaceous  strata  is  analogous  probably  to  that  of  a  shingle-bank  w 
gi'avel-beach  on  a  mo<lern  coast-line.  Hut  it  is  not  easy  to  undei-stand  the  circom- 
stances  under  which  some  ancient  conglomerates  accumulated,  such  as  that  of  the 
Old  Red  Samlstone  of  Central  Scotland,  which  attains  a  thickness  of  many  thousand 
feet,  and  consists  of  well-rounded  and  smoothed  blocks  often  several  feet  in  diameter. 

In  many  ol«l  (ronglomerates  (and  even  in  those  of  Miocene  age  in  Switzerland)  tbe 
conqKment  ]K;bbles  may  be  observed  to  have  indented  each  other.  In  such  cases  alio 
they  may  be  found  elongated,  distorted  or  split  and  recementiMl ;  sometimes  the  sanw 
I>ebble  has  l>een  cnished  into  a  number  of  piece>,  which  are  held  together  by  a  retaining 
cement.  These  phenomena  \m\\t  to  great  pressure,  and  some  internal  ivlative  movement 
in  the  rocks.     (Book  III.  Part  I.  Section  iv.  §  3.^ 

Breccia — a  rock  coni]>ost.Ml  of  angular,  instead  of  rounded,  fragments.  It  commonly 
prc*scnts  less  trace  of  stratification  than  conglomerate.  Inteniiediate  stages  between 
these  two  rocks,  where  the  stones  ai-e  |>artly  angular  and  ]tartly  sultangular  and  rounded, 
are  known  as  hrecciatcd  coivjlomrrati'.  Considered  as  a  detrital  deposit  formed  by 
buiKjrficial  waste,  breccia  |K)ints  to  the  dlsint^'gration  of  rocks  by  the  atmosphere,  and 
the  accumulation  of  their  fragment:»  with  little  or  no  intervention  of  numiug  water. 
Thus  it  may  be  formed  at  the  base  of  a  elilf,  eitlK*r  subaerially,  r>r  where  the  debris  of 
the  cliff  falls  at  once  into  a  lake  or  into  deep  sea-water. 

The  term  Breccia  has,  however,  been  applieil  to  i-ocks  formed  in  a  totally  diflforent 
manner.  Angular  bl<K*ks  of  all  sizes  and  shapes  have  lieen  dischargdl  from  volcanic 
orifices,  and,  falling  liack,  have  coiLsoIidated  there  into  masses  of  breeciated  material 
(volcanic  breccia).  Intrusive  igneous  eruptions  have  sometimes  torn  off  fragments  of  the 
rrkcks  through  which  they  have  ascended,  and  these  angular  fragments  have  been 
encIose<l  in  the  liquid  or  }>asty  mass.  Or  the  intrusive  i*ock  has  cooled  and  solidified 
externally  while  still  mobile  within,  and  in  its  ascent  has  caught  uj)  and  involved  some  of 
these  consolidattni  ]>ails  of  its  own  substance.  Again,  where  solid  masses  of  rock  within 
the  crust  of  the  earth  have  giound  against  each  other,  as  in  dislocations,  angalar  frag- 
mentary rubbish  has  been  ]»nxluced,  which  has  subsequently  been  consolidated  by  some 
infiltrating  cement  (Fault-rock).     It  is  evident,  however,  that  breccia  formed  in  one  or 


*  See,  for  exunq.'le,  an  account  of  the  sehotter-conglomerates  of  Northern  Persia  by 
E.  Tietze,  JiUirh.  GeoL  KeichsariAf.  Vienna,  1881,  p.  68. 


PART  n  §  vii  FRA  OMENTA  L  ROCKS  131 

other  of  these  hyx)ogene  ways  will  uot,  as  a  rule,  be  apt  to  be  mistaken  for  the  true 
breccias,  arising  from  superficial  disintegration. 

ftfinilTtinift  (Or^)  ^ — a  rock  composed  of  consolidated  sand.  As  in  ordinary  modern 
sand,  the  integral  gi-ains  of  sandstone  are  chiefly  quartz,  which  must  here  be  regarded 
as  the  residue  left  after  ail  the  less  durable  minerals  of  the  original  rocks  have  hecu 
carried  away  in  solution  or  in  suspension  as  fine  mud.  The  colours  of  sandstones  arise, 
not  so  much  from  that  of  the  quartz,  which  is  commonly  white  or  grey,  as  from  the 
film  or  crust  which  often  coats  the  grains  and  holds  them  together  as  a  cement.  Iron, 
the  great  colouring  ingredient  of  rocks,  gives  rise  to  red,  brown,  yellow,  and  green  hues, 
according  to  its  degree  of  oxidation  and  hydration. 

Like  conglomerates,  sandstones  differ  in  the  nature  of  their  component  grains,  and 
in  that  of  the  cementing  matrix.  Though  consisting  for  the  most  part  of  siliceous 
grains,  they  include  others  of  clay,  felspar,  mica,  zircon,  rutile,  tourmaline,  or  other 
minerals  such  as  occur  in  sand  (p.  129),  and  these  may  increase  in  number  so  as  to  give  a 
special  character  to  the  rock.  Thus,  sandstones  may  be  argillaceous,  felspathic,  mica- 
ceoQB»  calcareous,  kc.  By  an  increase  in  the  argillaceous  constituents,  a  sandstone  may 
pass  into  one  of  the  clay-rocks,  just  as  modern  sand  on  the  sea-floor  shades  imperceptibly 
into  mud.  On  the  other  hand,  by  an  augmentation  in  the  size  and  sharpness  of  the 
grains,  a  sandstone  may  become  a  grit,  and  by  an  inci-easo  in  the  size  and  numWr  of 
pebbles,  may  pass  into  a  pebbly  or  conglomeratic  sandstone,  and  thence  into  a  tine 
conglomerate.  A  piece  of  fine-grained  sandstone,  seen  under  the  microscoxte,  looks  like 
a  coarse  conglomerate,  so  that  the  difference  between  the  two  rocks  is  little  more  than 
one  of  relative  size  of  particle.^. 

The  cementing  material  of  sandstones  may  be  ferruginoua,  as  in  most  oixlinaiy  red 
and  yellow  sandstones,  where  the  anhydrous  or  hydrous  iron -oxide  is  mixed  with  clay 
or  other  impurity — in  red  sandstones  the  grains  are  held  together  by  a  hjematitic,  in 
yeUow  sandstones  by  a  limonitic  cement  ;  argillaaoiiSy  where  the  gi*ain8  arc  united 
by  a  base  of  clay,  recognisable  by  the  earthy  smell  when  breathed  upon  ;  calcareous, 
where  carbonate  of  lime  occurs  either  as  an  amorphous  paste  or  as  a  ciystalline  cement 
between  the  grains ;  siliceous,  where  the  component  particles  are  bound  together  by 
silica,  as  in  the  exposed  blocks  of  Eocene  sandstone  known  as  "  greyweathers "  in 
Wiltshire,  and  which  occur  also  over  the  north  of  France  towards  the  Ardennes. 

Among  the  varieties  of  sandstone  the  following  may  here  Ikj  mentioned.  Flag- 
stone— a  tliin-bedded  sandstone,  capable  of  being  split  along  the  lines  of  stratification 
into  thin  beds  or  flags ;  Micaceous  sandstone  {mica-psammitc) — a  rock  so  full  of 
mica-flakes  that  it  splits  readily  into  thin  lamime,  each  of  which  has  a  lustrous  surface 
from  the  quantity  of  silvery  mica.  This  rock  is  called  "fakes"  in  Scotland.  Free- 
stone— a  sandstone  (the  term  being  apjilied  sometimes  also  to  limestone)  which  can  Ijc 
cat  into  blocks  in  any  direction,  without  a  marked  tendency  to  split  in  any  one  plane 
more  than  in  another.  Though  this  rock  occurs  in  beds,  each  bed  is  not  divided  into 
laminae,  and  it  is  the  absence  of  this  minor  stratification  which  makes  the  stone  so  useful 
for  anshitectural  purix)ses  (Craigleith  and  other  sandstones  at  Edinburgh,  some  of  which 
contain  98  per  cent  of  silica).  Glauconitic  sandstone  (green -sand) — a  sandstone 
containing  kernels  and  dusty  grains  of  glauconite,  which  im[)arts  a  general  greenish  hue 
to  the  rock.  The  glauconite  has  probably  been  deposited  in  association  with  decaying 
organic  matter,  as  where  it  fills  echinus-spines,  foraminifera,  shells  anrl  corals  on  the 
floor  of  the  present  ocean. ^  Buhrstone— a  highly  siliceous,  exceedingly  compact, 
thong^  cellular  rock  (with  Chara  seeds,  &c. ),  found  alternating  with  unaltered  Tertiary 
strata  in  the  Paris  basin,  and  forming  from  its  hardness  and  roughness,  an  excellent 

*  See  J.  A.  Phillips  on  the  constitution  and  history  of  grits  and  sandstones.  Quart. 
Joum.  Gtol.  Soe,  xxxvii.  (1881),  p.  6.  For  analyses  of  some  British  sandstones  used  as 
building  stones,  see  WaUace,  Proc.  Phil.  Sot:  Olasguic,  xiv.  (1883),  p.  22. 

^  Ante,  p.  77 ;  Sollas,  Geol.  Mag.  iii.  2nd  ser.  p.  539. 


132  tfEOf.L\n.<y  DOOKU 


iiiatcrial  f«ir  the  p-irnUtnins  of  tluur-iiiills.  iimy  U*  unriitioiietl  ben*,  tliouj^rh  it  jirobablj 
liiLs  \Htfn  fririii«-d  by  tbtr  prt-i-iiiitatioii  iif  silira  tliroiij;b  tbe  action  of  or^iiisms.  Arkose 
'jranUif  sintd.'*fonr)—a.  io«-k  comjiosc^l  of  ilisiiitt-gratitl  granite,  and  fouDd  in  geolqgifial 
lorinations  of  ditfen^nt  a^^'s,  wbirh  liave  In-eii  di-rivni  fn)m  granitic  rocks.  Crystallized 
sand'itono— an  arena'-fous  r«M;k  in  \vbi<.b  a  d^Hxit  of  rr}'stalUiii.>  quartz  lias  taken  place 
upon  tbe  individual  grains,  cacli  n{  wliirb  lie^'oiues  tlic  nucleus  of  a  more  or  less  perfect 
•juaitz  crystab  Mr.  S«irby  bas  olrsi-rviil  sucb  crystalliziMl  sand  in  de|H)«its  of  variom 
ag»'s  from  tbe  Oolites  down  to  tbe  Old  Ked  Sandstone.  * 

Qreywacke — a  r.-nni{>a<:t  aggregati'  of  roundel  or  sul^ngular  grains  of  quartz,  fel8]iAr, 
slate,  or  otlier  miueral.s  or  rock^,  cem»'nted  by  a  jiaste  wliicb  is  usually  siliceous,  but 
may  1m- argillaceous,  feNjiatbic,  calcareous.  t»r  antbracitic  (Fig.  21).  (^rey,  as  its  name 
denotes,  is  tbe  prevailing  colour :  but  it  {msses  into  brown,  brownisb-]iurple,  and  ifomt- 
times,  wbere  antbi-acite  ]ire<lominates,  into  black.  Tlie  nxrk  is  distinguished  from 
onlinary  sandstone  by  its  darker  Ime,  its  bartlness,  tbe  vanety  of  its  comiionent  graln», 
ami,  above  all.  by  tbe  comjiact  cement  in  wbicb  tbe  grains  are  iml^eddod.  In  many 
varinties,  so  jKjrva'led  is  tbe  vovk  by  tbe  silice<ms  j^ste,  tbat  it  [K>sse.sses  great  toiigbnesi, 
and  its  gi-ains  seeTu  to  graduate  into  eacb  otber  as  well  as  into  tbe  surrounding  matrix. 
Sui-b  rocks  wben  tine-gi*ained,  can  banlly,  at  first  sigbt  or  witb  tbe  unaided  eye,  l>e  di»« 
tingiii»be<l  from  some  eom{iti«.-t  igneous  r<X'ks,  tbougb  a  micms^ropic  examination  at  onoe 
r«^ veals  tbeir  fragmi-nlal  fbai-aj'ter.  In  otber  cas«\s,  wbeiv  tbe  greywacke  has  been  fonned 
mainly  out  of  tbe  debriji  of  granite,  quartz  -  ]X»rpbyry,  or  otber  feLs]tathie  masses,  the 
grains  eonslst  so  largely  of  felsjiar,  and  tbe  jiaste  aUo  is  so  felsjiathic,  that  the  rotk 
migbt  1k!  mistaken  for  sonu*  close-grained  granular  jH^rjibyry.  Greywacke  occurs  exten* 
».ively  among  tbe  Pabcozoie  formations,  in  IkmIs  alternating  witb  shales  and  conglo* 
meratcs.  It  ivpresents  tbe  nniddy  s-uhI  of  some  of  tbe  Palfeozoic  sea-floors,  retaining  often 
its  n]>])le-marks  and  sun-eracks.  Tbe  metaniorpbism  it  bas  undergone  has  generally 
not  IxMMi  great,  and  for  tbe  most  i>art  is  limite<l  to  induration,  ]tartly  by  pressure  and 
I  tartly  by  ]ienneation  of  a  siliceous  cement.  But  wbere  felspatbic  ingredients  pre^'ail, 
tbe  roik  bas  offered  facilities  for  alteration,  and  bas  l>een  here  and  there  clianged  into 
highly  crystalline  mica-scbists  full  of  garnets  and  other  secfuidary  minerals  (contact- 
meiamf»rpbism  at  tbe  granite  of  Xew  Galloway,  Scotland,  jiostea,  j).  606). 

Tbe  mon?  fissile  fnn'-giained  vaiieties  of  this  i"ock  have  been  temied  greywacke-slate 
(p.  l'3r>).  In  these,  a>  well  as  in  givwacke,  organic  remains  occur  among  the  Silurian 
ami  iK'Vonian  formations.  Sometimes  in  the  I^wer  Siluiian  rocks  of  Scotland,  these  strati 
become  black  witb  carl)ona<-eous  matter,  amoug  which  vast  numl>ers  of  graptoUtes  may 
In.'  obs<Mved.  Gradations  into  sandstone  are  termed  Greywacke-sandstone.  In  Nor- 
way tbe  re<ldish  felspatbic  greywacke  or  sindstone  of  tbe  Primordial  itx^ks,  is  called 
Sparagmite  ;  similar  material  forms  much  of  tlnj  Toiridon  sandstone  of  Scotland. 

Quartzite.  -An  altered  siliceous  sandstone  (siMt  p.  180\ 

2.  Clay  Bocks  (Pelites). 

TbcMt  arc  ci>nq»osed  of  line  argillaceous  se<liment  or  mud,  derived  from  the  waste  of 
rocks.  Perfectly  pure  clay  or  kaolin,  bydi-atiMl  silicate  of  alumina,  may  be  obtained  where 
granites  and  other  fels|iar- bearing  rocks  decom^Ktse.  But,  as  a  nile,  the  alliaceous 
materials  are  mixi^d  witb  various  impurities. 

Clay,  Mud. — Tbe  decom|K)sition  of  felsiars  and  allie<l  minerals  gives  rise  to  the 
formation  i>f  hydrous  aluminous  silicatt^s,  which  occurring  usually  in  a  state  of  fine  sub- 
tlivision,  are  (ra)iable  of  lH;ing  held  in  sus]>ension  in  water,  and  of  being  transported  to 


'  <j.  J.  fi'of.  SfH-..  XXX vi.  i».  63.  See  Daubrec,  Ann.  tf'S  Mi/ira,  2nd  ser.  i.  p.  206.  A. 
A.  Young,  A  nun:  J  fit  in.  S:i.  'Srd  ser.  xxiii.  257  ;  xxiv.  47,  and  es))ecially  the  work  of 
Irving  and  Van  Hise  (quoted  on  ]i.  110),  which  gives  some  excellent  figures  of  enlaiiged  quaiti- 
grains. 


PAKT  II  §  vii  FBAGMENTAL  ROCKS  133 

great  distances.  Tliese  substances,  differing  much  in  com^wsitiou,  are  embraced  under 
the  general  term  Clay,  which  may  be  defined  as  a  wliite,  grey,  brown,  red,  or  Iduish 
substance,  which  when  dry  is  soft  and  friable,  adheres  to  the  tongue,  and  shaken  in 
water  makes  it  mechanically  turbid  ;  when  moist  is  plastic,  when  mixed  with  much 
water  becomes  mud.  It  is  evident  that  a  wide  range  is  i>ossi]>le  for  varieties  of  this 
substance.     The  following  are  the  more  imjxirtant. 

Kaolin  (Porcelain -clay,  Chinan-lay)  has  l»een  already  noticed  (]>.  77). 

Pipe-Clay — white,  nearly  pure,  and  free  from  iron. 

Flre-Clay — largely  found  in  connection  with  coal-seams,  contains  little  iron,  and 
is  nearly  free  from  lime  and  alkalies.  Some  of  the  most  typical  fire-clays  are  those  long 
uaed  at  Stourbridge,  Worcestershire,  for  the  manufacture  of  jiotterj'.  The  best  glass- 
house ]iot-clay,  that  is,  the  most  refractory,  and  therefore  used  for  the  constniction  of 
pots  which  have  to  stand  the  intense  heat  of  a  glass-house,  has  the  following  comi>osi- 
tion  :  silica,  73*82  ;  alumina,  15*88  ;  protoxide  of  iron,  2*95  ;  lime,  trace  ;  magnesia, 
tZBce  ;  alkalies,  *90  ;  sulphuric  acid,  trace  ;  chlorine,  trace  :  water,  6*45  ;  sjiecific 
gravity,  2*51. 

Qannister — a  very  siliceous  close-grained  variety,  found  in  the  Lower  Coal  measures 
of  the  North  of  England,  and  now  largely  ground  down  as  a  material  for  the  hearths  of 
iron  fnmaces. 

Brick-day — j>ro])erly  rather  an  industrial  than  a  geological  tenn,  since  it  is  ap])lied 
to  any  clay,  loam,  or  earth,  from  which  bricks  or  coarse  j)otteiy  are  made.  It  is  an  iin- 
\mre  clay,  containing  a  good  deal  of  iron,  with  other  ingi'edients.  An  analysis  gave  the 
following  composition  of  a  brick -clay  :  silica,  49*44  ;  alumina,  34*26  ;  sewpiioxideof  iron, 
7*74  ;  lime,  1*48,  magnesia,  5*14  ;  water,  1*94. 

Fuller*!  Earth  (Terre  a  foulon,  Walkerde) — a  gieenish  or  brownish,  earthy,  soft, 
somewhat  unctuous  substance,  with  a  shining  streak,  which  does  not  iKH'ome  plastic 
with  water,  but  cnimbles  down  into  mud.  It  is  a  hydrous  aluminous  silicate  with  some 
magnesia,  iron-oxide  and  soda.  The  yellow  fuller's  earth  of  Keigate  contains  silica  44, 
alumina  11,  oxide  of  iron  10,  magnesia  2,  lime  5,  soda  5.^  In  England  fuller's  earth 
occurs  in  beds  among  the  Jurassic  and  Cretaceous  formations.  In  Saxony  it  is  found  as 
a  result  of  the  decom}x>sition  of  diaba.se  and  gabbro. 

Wacke — a  dirty -green  to  bro\%'ni8h  -  black,  earthy  or  comi)act,  but  tender  and 
apjiarently  homogeneous  clay,  which  arises  as  the  ultimate  stage  of  the  deconipasition 
of  basalt-rocks  in  situ. 

Loam — an  earthy  mixture  of  clay  and  .sand  with  more  or  less  organic  matter.  The 
black  soils  of  Russia,  India,  &c.  (Tchemosem,  Rcgur),  are  dark  deposits  of  loam  rich  in 
organic  matter,  and  sometimes  upwards  of  twenty  feet  dee]». 

Loan — a  pale,  somewhat  calcareous  clay,  probably  of  wind-drift  origin,  found  in 
some  river- valleys  (Rhine,  Danube,  Mis.si.ssippi,  &c.),  and  over  wide  regions  in  China  and 
elsewhere.     It  is  described  in  Book  III.  Part  II.  Sect.  i.  §  1. 

Laiarite — a  cellular,  reddish,  ferruginous  clay,  found  in  some  tropical  countries  as 
the  result  of  the  subaerial  decomi^sition  of  rocks  ;  it  accpiires  great  hardness  after  being 
c^narried  out  and  dried. 

Till,  Bonldtr-day — a  stiff  sandy  and  stony  clay,  varying  in  colour  and  com|>osition, 
according  to  the  character  of  the  rocks  of  the  district  in  which  it  lies.  It  is  full  of 
worn  stones  of  all  sizes,  up  to  blocks  weighing  .several  tons,  and  often  well -smoothed  and 
striated.  It  is  a  glacial  dejwsit,  and  will  be  described  among  the  fonnations  of  the 
(Uacial  Period. 

Muditone — a  fine,  usually  more  or  less  sandy,  argillaceous  rock,  having  no  fissile 
character,  and  of  somewhat  greater  hardness  than  any  form  of  clay.  The  term  Clay- 
rock  has  lieen  applied  by  some  ^mters  to  an  indurated  clay  recjuiring  to  be  ground  and 
mixed  with  water  l)cfore  it  acquires  plasticity. 

»  Ure's  Diet.  Arts,  &c.  ii.  p.  142. 


1.34  GEOGNOSY  book  u 

Shale  (Schihte,  Sdiieferthon) — a  general  term  to  describe  clay  that  has  assumed  a 
thinly  stratified  <»r  fissile  structure.  Under  this  term  are  included  laminated  and  some- 
what haitlened  ar^pllaeeous  roc-ks,  which  an>  ca]>a1>le  of  being  split  along  the  lines  of 
de^josit  into  thin  leaves.  They  |»n«ent  almost  endle^ss  varieties  of  texture  and  compofd- 
tion,  ])assing,  on  the  one  hand,  into  clays,  or,  where  much  indurated,  into  elates  and 
argillaceous  schists,  on  the  other,  into  flagstones  and  sandstones,  or  again,  throngh  cal- 
careous gradations  iuUt  limestone,  or  through  ferniginous  vaiieties  into  clay-ironstone, 
and  through  bituminous  kinds  into  coal. 

Clay-slate  (Sehiste  ardoise,  Thons^-hiefer). — Under  this  name  are  included  certain 
hanl  fissile  argillaceous  masses,  composed  primarily  of  comjiact  clay,  sometimes  with 
megascopic  and  mierosco])ic  si^ales  of  one  or  more  micaceous  minerals,  granules  of  quartz 
and  cubes  or  concretions  of  ])yritcs,  as  well  as  veins  of  quartz  and  calcite.  The  fissile 
structure  is  sjiecially  characteristic.  In  some  vtaiea  this  structure  coincides  with  tliat  of 
oiiginal  dejKtsit,  as  is  proved  by  the  alternation  of  fissile  beds  with  bands  of  hardened 
sandstone,  conglomerate  or  fossiliferous  limestone.  But  for  the  most  part  as  the  rocks 
have  l)ei>n  much  comxtressed,  the  fissile  structure  of  the  argillaceous  bands  is  inde|)endent 
of  stratification,  and  can  be  seen  traversing  it.  Sorby  has  shown  that  this  superinduced 
fissility  or  "cleavage"  has  resulted  from  an  internal  rearrangement  of  the  particles  in 
l>lanes  ])er})endieular  to  the  direction  in  which  the  rocks  have  Ix^en  compressed  (see  Book 
III.  Part  I.  Section  iv.  §  3).  In  England  the  tenn  "slate  "  or  "clay-slate  '*  is  given  to 
argillaceous,  not  obviously  crystalline  rocks  |K)ssessing  this  cleavagc-stnioture.  Where 
the  micaccims  lustn?  of  the  finel}'  disseminat^-d  superinduced  mica  is  prominent,  the  rocks 
are  f»hyllites. 

Microscopic  examination  shows  that  while  some  argillaceous  rocks  consist  mainly  of 
gi-anular  debris,  many  cleaved  clay-slates  contain  a  large  proportion  of  a  mioaceous 
mineral  in  extremely  minute  flakes,  which  in  the  best  Welsh  slates  have  an  averags 
si/e  of  Yihio  of  an  inch  in  bi*eadth,  and  haVu  o{  an  inch  in  thickness,  together  ¥rith  ¥eiy 
fine  black  hairs  which  may  l>e  magnetite.'  Moveover,  many  clay-slates,  though  to 
outward  ap[)earance  thoroughly  noncrystalline,  and  evidently  of  fragmental  com|K>8ition 
and  serlimentary  origin,  yet  contain,  sometimes  in  remarkable  abundance,  microscopie 
microlites  and  crystals  (»f  different  minerals  placed  with  their  long  axes  iiarallel  with  the 
planes  of  fissility.  These  minute  bodies  include  yellowish -brown  needles  of  rutile, 
greenish  or  yellowish  flakes  of  mica,  scales  of  calcite,  and  probably  other  minerala' 
Small  granules  of  quartz  containing  fluid-cavities,  show  on  their  surfaces  a  distinct 
blending  with  the  substance  of  the  surrounding  rock.  M.  Kenaitl  has  found  that  the 
Belgian  whet-slate  is  full  of  minute  crystals  of  garnet.^  Some  of  the  more  ciystaUine 
varieties  (phylliti')  are  almost  wholly  comi»r»sed  of  minute  crystalline  ])articles  of  mica, 
({uartz,  fels]tar,  chlorite,  and  rutile,  and  form  an  intcnnediate  stage  between  ordinaiy 
clay-slate  and  mica-schist. 

A  distuiction  has  bt?en  drawn  by  sonu<  i>etrographei-s  between  cei'tain  rocks  (phyllitCi 
Urthonschiefer)  which  occur  in  Archaean  regions  or  in  gi'oujw  probably  of  high  antiquity, 
and  others  (ardoise,  Thonschiefer)  which  are  found  in  Palreozoic  and  later  formations. 
But  there  does  not  ap]Myir  to  be  adi^quate  justification  for  this  grouping,  which  luw  prob- 


*  Sorby,  Q.  J.  Geol.  »^i«'.  xxvi.  p.  68. 

'  These  "  clay-slate  needles  *'  were  j^robably  not  crystallized  couteniixjraueously  with  the 
deposit  of  the  original  rock.  In  some  cases  they  may  have  been  de]>osited  with  the  rest  of  the 
sediment  as  part  of  the  debris  of  pre-existing  crystalline  rocks  (see  p.  129)  ;  but  in  general 
they  appear  to  have  Iteen  developed  where  they  now  occur  by  subsequent  actions  (see 
/nostra J  p]i.  312,  545).  For  their  character  see  Zirk el,  *  Mik.  Beschaff.'  p.  490.  Kalkowsky, 
y.  Juhrh.  1870,  p.  382;  A  C'athrein,  np,  dt.  1882  (i.),  p.  169.  F.  Peuck,  Siisb.  Bayer. 
Akfuf,  Math.  PInjs.  1880,  p.  461.     A.  Wichniann.  Q.  J,  Otol.  Stn:  xxxv.  p.  156. 

'^  Aauf.  Roy.  Btlf/iqne^  xli.  (1877).  See  also  his  paper  on  the  composition  and  stractare 
of  the  phyllades  of  the  Ardennes,  BuU.  Mns.  Roy.  Bfly.  iii.  (1884),  p.  231. 


PART  u  §  vii  FRAGMENTAL  ROCKS— VOLCANIC  136 

ably  been  suggested  rather  by  theoretical  exigences  than  by  any  essential  differences 
between  the  rooks  themselves.  That  the  whole  of  the  series  of  argillaceous  rocks,  begin- 
ning with  clay  and  passing  through  shale  into  slate  and  phyllite,  is  of  sedimentary 
origin  is  indicated  by  the  organic  remains,  false  bedding,  ripple-mark,  &c.,  found  in 
those  at  one  end  of  the  series,  and  by  the  insensible  gradation  of  the  mineralogical 
characters  through  increasing  stages  of  metamorphism  to  the  other  end.  Some  micro- 
scopic crystals  may  possibly  have  been  originally  formed  among  the  muddy  sediment  on 
the  sea-floor  (see  p.  459).  Others  may  have  formed  part  of  the  onginal  mechanical 
detritus  that  went  to  make  the  slate!  But,  for  the  most  part,  they  have  been  subsequently 
developed  within  the  rock,  and  represent  early  stages  of  the  process  which  has  culminated 
in  the  production  of  crystalline  schists.  The  development  of  crystals  of  chiastolite  and 
other  minerals  in  clay-slate  is  frequently  to  be  observed  round  bosses  of  granite,  as 
one  of  the  phases  of  contact-metamorphism  (see  pp.  568,  605). 

A  number  of  varieties  of  Clay-slate  are  recognised.  Roofing  slate  (Dachschiefer) 
includes  the  finest,  most  compact,  homogeneous  and  durable  kinds,  suitable  for  roofing 
hooses  or  the  manufacture  of  tables,  chimney-pieces,  writing-slates,  &c.  ;  it  occurs  in  the 
Silnrian  and  Devonian  formations  of  Ontral  and  AVesteni  £uro()e.  Anthracitic- 
slate  (autliracite-phyilite,  alum-slate),  dark  carbonaceous  slate  with  much  irou-disul- 
phide.  Bands  of  this  nature  sometimes  run  through  a  clay-slate  region.  The  carbon- 
aceous material  "arises  from  the  alteration  of  the  remains  of  ])lants  (fucoids)  or  animals 
(freqneutly  graptolites).  The  marcasite  so  abundantly  associated  with  these  organisms 
decomposes  on  exposure,  and  the  sulphuric  acid  produced,  imiting  \rith  the  alumina, 
potash,  and  other  bases  of  the  surrounding  rocks,  gives  rise  to  an  efflorescence  of  alum, 
or  the  decomposition  produces  sulphurous  springs  like  those  of  Moffat.  The  name  Grey- 
wacke- slate  has  been  applied  to  extremely  fine-grained,  hard,  shaly,  more  or  less 
micaceous  and  sandy  bandvS,  associated  with  greywacke  among  the  older  Palaeozoic 
rocks.  Whet-slate,  Novaculite,  Hone-stone,  is  an  exceedingly  hard  fine-grained 
siliceous  rock,  some  varieties  of  which  derive  their  economic  value  from  the  presence  of 
microscopic  crystals  of  garnet.  The  various  fonns  of  altered  clay -slate  are  described  at 
p.  179  among  the  metamorphic  rocks.  * 

Poroellmnite  (Argillite)  or  baked  shale — a  name  applied  to  the  exceedingly  indurated 
sometimes  partially  fused  condition  which  shales  arc  apt  to  assume  in  contact  with 
dykes  and  intrusive  sheets  or  bosses.  For  an  account  of  this  form  of  contact-meta- 
morphism  see  p.  600. 

3.  Volcanic  Fragmental  Bocki — Toffs. 

This  section  comi>rises  all  deposits  which  have  resulted  from  the  comminution  of 
volcanic  rocks.  They  thus  include  (1)  those  which  consist  of  the  fragmentary  materials 
ejected  from  volcanic  foci,  or  the  true  ashes  and  tuffs  ;  and  (2)  some  rocks  derived  from 
the  superficial  disintegration  of  already  erupted  and  consolidated  volcanic  masses. 
ObTioasly  the  second  series  ought  properly  to  be  classed  with  the  sandy  or  clayey  rocks 
aboTC  described,  since  they  have  been  formed  in  the  same  way.  In  practice,  however, 
these  detrital  reconstructed  rocks  cannot  always  be  certainly  distinguished  from  those 
which  have  been  formed  by  the  consolidation  of  true  volcanic  dust  and  sand.  '  Their 
chemical  and  lithological  characters,  both  megascopic  and  microscopic,  are  occasionally 
so  similar,  that  their  respective  modes  of  origin  have  to  be  decided  by  other  considera- 
tions, such  as  the  occurrence  of  lapilli,  bombs,  or  slags  in  the  truly  volcanic  series,  and  of 
well  water-worn  pebbles  of  volcanic  rocks  in  the  other.  Attention  to  these  features, 
however,  usually  enables  the  geologist  to  make  the  distinction,  and  to  perceive  that  the 
number  of  instances  where  he  may  be  in  doubt  is  less  than  might  be  sup})osed.  Only  a 
comparatively  small  number  of  the  rocks  classed  here  are  not  true  volcanic  ejections.^ 

'  For  a  classification  of  tuffs  and  tuffaceous  deposits  see  E.  Reyer,  Jahrh,  K.  K.  deol, 
ReUhaantt,  xxzL  (1881),  p.  57. 


1 3(5  aKOGXOS  y  BOOK  11 


Roferriiig  to  the  account  of  volcanic  action  in  Hook  III.  Part  1.  Sect,  i.,  we  may  here 
merely  define  the  use  of  the  names  hy  which  the  diflerent  kinds  of  ejected  volcanii- 
materials  are  known. 

Volcanic  Blocks — angular,  sub-angular,  round,  or  irregularly  -  shaped  masses  of 
lava,  several  feet  in  diameter,  sometimes  of  uniform  texture  throughout,  as  if  they  were 
large  fragments  dislodged  by  explosion  from  a  previously  eon.soIidated  rock,  sometimes 
compact  in  the  intcnor  and  cellular  or  slaggy  outside. 

Bombs— round,  elliptical,  or  discoidal  i»ieces  of  lava  from  a  few  inches  up  to  one 
or  more  feet  in  diameter.  They  are  freipiently  cellular  internally,  while  the  outer  iiarts 
arc  fine-grained.  Occasionally  they  consist  of  a  mere  shell  of  lava  ^vith  a  hollow 
interior  like  a  bomb-shell,  or  of  a  easing  of  lava  enclosing  a  fragment  of  rock.  Their 
mofle  of  origin  is  explainwl  in  IJook  III.  Pait  I.  Sect.  i.  §  1. 

Lapilli  (rapilli) — ejected  fragments  of  lava,  round,  angidar,  or  indetinite  in  shape, 
var}'iug  in  size  from  a  i^ea  to  a  walnut.  Their  minei-alogical  composition  dejiendfl  ujion 
that  of  the  lava  from  which  they  have  been  thrown  up.  Usually  they  are  (Kirous  or 
finely  vesicular  in  texture. 

Volcanic  Band,  Volcanic  Ash— the  finer  detritus  enipted  from  volcanic  orifices, 
consisting  jMii-tly  of  rounded  an<l  angular  fragments  up  to  alwut  the  size  of  a  ]ieft 
derived  from  the  explosion  of  lava  within  eruptive  vents,  |>artly  of  vast  quantities 
of  microlitcs  and  ciystals  of  some  of  the  minerals  of  the  lava.  The  finest  dust  w  in  a 
state  of  extremely  minute  sulidi vision.  When  examined  un<ler  the  niicrosco])e,  it  ii 
somethnes  found  to  consist  not  only  of  minute  ciystals  and  mici'olites,  but  of  volcanic 
glass,  which  niaj-  be  ob.servcd  adhering  to  the  microlites  or  ciystals  round  which  it 
flowed  when  still  ^Mirt  of  the  fluid  lava.  The  i)resence  of  minutely  cellular  fragments  is 
characteristic  of  most  vok-anic  fragmental  rocks,  ami  this  structure  may  commonly  be 
ol.)scr\*ed  in  the  mii-roseopic  fragments  and  filaments  of  gla.'ts. 

"When  these  various  materials  are  allowed  to  accumulate,  they  become  consolidated 
and  receive  distinctive  names.  In  cases  where  they  fall  into  the  sea  or  into  lakes,  they 
are  liable  at  the  outer  margin  of  their  area  to  be  mingled  with,  and  insensibly  to  {«■ 
into  onlinary  non-volcanic  sediment.  Hence  we  may  ex])ect  to  find  transitional  varieties 
between  rocks  fornie«l  directly  from  the  results  of  volcanic  ex})losion  and  ordinary  sedi- 
mentary de]>osits. 

Volcanic  Conglomerate — a  roitk  comi>osed  mainly  or  entirely  of  rounded  or  sab- 
angular  fragiuents.  chiefly  or  wholly  of  voUymic  rocks,  in  a  |>aste  of  the  same  materialft, 
usually  exhibiting  a  stratified  arrangement,  and  often  found  intercalated  l>etH'eeii 
successive  sheets  of  lava.  Conglomerates  of  this  kind  may  have  l^een  formed  by  the 
accumulation  of  rounded  materials  cjectetl  from  volcanic  vents  ;  or  as  the  result  of  the 
aqueous  erosion  of  previously  solidified  lavas,  or  by  a  combination  of  both  these  processes. 
Well-rounded  and  smoothed  stones  almost  certainly  indicate  long-continued  water-action, 
i-athev  than  trituration  in  a  volcanic  vent.  In  the  Western  Territories  of  the  United 
States  vast  tracts  of  countiy  are  covei-ed  with  masses  of  such  conglomerate,  some- 
times 2000  feet  thick.  Ca])tain  Dutton  has  shown  that  similar  dei)Osits  are  in  eourse 
of  fonnation  there  now,  merely  by  the  infhu^nce  of  disintegration  uiKin  exjKMed 
lavas.  * 

Volcanic  conglomerates  receive  difhirent  names  acconling  to  the  nature  of  the  com- 
]ionent  fragments :  tints  we  have  hastiIt-eoHfjIomcratc,%  where  these  fragments  are 
wholly  or  mainly  of  basalt,  fmcIiyti-confjIomeratcSf  jwrjthifritt.-couglojnerateSf  pJwtiolite- 
t'onglomcratcs,  &c. 

Volcanic  Breccia  resembles  Volcanic  Conghmierate,  except  that  the  stones  are 
angular.  Tliis  angularity  indicates  an  absimce  of  atiueous  erosion,  and,  under  the 
circumstances  in  which  it  is  found,  usually  ^Ktints  to  immediately  adjacent  volcanic 
explosions.     There  is  a  great  variety  of  breccias,  as  ftamft-birccia,  duiltasc-brrccia^  kc„ 

'   *  High  IMateaux  of  Utah,*  p.  77. 


vii  FRAGMENTAI.  ROCK>>— VOLCANIC  137 

lio  ACElomenLte — a  tumultuous  assemblage  of  blocks  of  all  siz«a  up  to  nissseH 
lids  in  dismeler,  met  with  in  the  "neoks"  or  jitjies  of  old  volcanic  oritices. 
■a  uid  paste  are  commonly  of  one  or  wore  volcanic  rocks,  such  as  felsite, 
1  or  basalt,  but  tliey  include  also  fragments  of  the  surrounding  rocks,  whatever 

ba,  through  which  the  volcanic  orifice  has  been  drilled.  As  a  rule,  agglomerate 
of  stratificatioii ;  but  Hometimes  EC  includes  |iortions  wliich  hare  a  more  or  less 
rangement  into  beds  of  coarser  and  finer  detritus,  often  placed  on  end,  or  inclined 
<t  directions  at  high  angles,  as  described  in  Book  IV.  Part  Vll.  Sect.  i.  %  4. 
llo  Tuff. — This  general  term  may  be  made  to  include  all  the  finer  kinds  of 
letritos,  ranging,  on  the  one  hand,  through  coarse  gravelly  dei-osits  into  con- 
«,  and  on  the  other,  into  exceedingly  coni|>act  fino-grained  rocks,  formed  of  the 
1  moat  impalpable  kind  of  volcanic  dust.  Some  modern  tuffs  are  full  of 
,  derived  from  the  lava  which  was  blown  into  dust.  Others  are  formed  of 
Dded  or  angular  grains  of  different  lavas,  n-itb  fragments  of  various  rocks 
rhioh  the  volcanic  funnels  have  been  drilled.  The  tuffs  of  earlier  geological 
kTB  often  been  so  much  altei'ed,  that  it  is  difficult  to  state  what  may  Imvo  I)ocn 
inal  condition.  The  alraence  of  micro- 
gUas  in  tbeni  is  no  [iroof  that  they  are 
toOii ;  for  the  preaence  of  these  liodies 
ipoD  the  iiatnre  of  the  lavas.  If  the 
re  not  vitreous  and  microlitie,  neither 

the  toffs  derived  from  theiu.  In  the 
mu  volcanic  area  of  Central  Scotland, 
m  made  u]>  of  debris  and  blocks  of  the  \ 
ata,  and,  like  these,  arc  not  microlitie. 
I  wme  places  they  abound  in  fragments 
■ic  glass  called  jialagonitc.  (Kig-  2.1. 
,  p.  138,) 

bave  consolidated  sometimes  under 
netimes  on  dry  land.  As  a  rule,  they 
etiy  stratified.     Near  the  original  vents 

in  they  commonly  present  rapid  alter-  ^*'jfc^~"^^™X»TlIffC.rBumtr.^!^!!" 
'.  Goer  and  coarser  detritus,  indicative  of        fj(( 
1   phases  of  volcanic   activity.      They 

y  shade  off  into  the  sedimentary  tonnationa  with  which  they  were  con- 
eons.  Thus,  we  have  tuffs  passing  gradnally  into  shale,  limestone,  sand- 
Tlie  iiitennediate  varieties  have  been  called  laiiy  ahalc,  tuffuctoiiH  dwir, 
(ujTj  Ac.  Kruui  the  circumstances  ot  their  formation,  tuffs  freiinently  pi'eserve 
ns  of  plants  and  animahi,  both  terrestrial  and  aquatic.  Those  of  Muute  Somma 
:agments  of  land-plants  and  sliellK.  Some  of  those  of  Carlioniferons  age  in 
Mtland  have  yieliled  crinoids,  brachiojiods,  and  other  marine  organisms.  Like 
Ingmentary  volcanic  rocks,  the  tuffs  may  be  subdivided  according  to  the  nature 
va  from  the  disintegration  of  which  they  have  lieen  formed.  Thus  we  have 
i,  irtKhyU-tiiffs,  bmnll-hif*,  piiiiiice-tiiffii,  poriiliyrite-tHfa,  4c.  A  fen-  varieties 
lal  characteristics  UMy  lie  mentioned  here.' 

— «  iiale  yellow  or  grey  rock,  rough  to  the  feel,  comi«aed  of  an  earthy  or 
ptuniceous  dust,   in  nliith  fragments  of  pumice,  trachyte,  greyuaike,  basalt, 

th«  Dccarreuce  and  structure  of  tuffa,  sea  J.  C.  Ward,  Q.  J.  Ifeol.  -Sor.  iixl.  p. 
er,  Jahri.  Oenl.  Ueich^ansl.  1881,  j^  57  ;  Geikie,  Tmnt.  /l-fi.  &"-.  Jldiii.  xxix.  ; 
,  Z.  Ufiilark.  tlent.  0\m.  x\\y.  ]•.  .'liS  ;  Peuck,  o/..  n'l.  xxxi.  p.  504.  Oo  tbe 
s  of  Scania,  K.  Eichstadt,  .•h-eri.jrs  Ueol.  CHil-rtekii,  ser.  c.  So.  E8  (1883).  On 
aoiphism  of  tuffii  into  luva-hke  rorks,  see  lliitton's  'Hi^li  llnteaui  of  Utah' 
Dgraph.  and  Geol.  Survey  of  Itocky  Mounts.),  1880,  p.  7U. 


138  GEOGNOSY  book  u 


carbonized  wood,  &c.,  are  imbedded.     It  has  tilled  up  some  of  the  valleys  of  the  Eifel, 
where  it  is  largely  <{uarried  as  a  hydraulic  mortar. 

Peperino — a  dark-brown,  earthy  or  granular  tuff,  found  in  considerable  quantity 
among  tho  Alban  Hills  near  Rome,  and  containing  abundant  cr^'stals  of  augite, 
mica,  leucite,  magnetite,  and  fragments  of  ciystalline  limestone,  basalt,  and  leucite- 
lava. 

Palagonite-TofF — a  Wddod  aggregate  of  dust  and  fragments  of  Ijasaltic  lava,  among 
wliich  ai*e  conspicuous  angular  pieces  and  minute  granules  of  the  i»ale  yellow,  green,  red, 
or  brown  basii;  glass  called  jwilagonite.  This  vitreous  substance  is  intimately  related  to 
the  basalts  (p.  1 72).  It  ap^x^ars  to  have  gathered  within  volcanic  vents  and  to  have  been 
emptied  thence,  not  in  streams,  but  by  successive  aeriform  explosions,  and  to  ha\'e  been 
subsefpiently  more  or  less  altiTKl.  The  percentage  comjKtsition  of  a  s|)ecimcn  from  the 
typical  locality,  Palagonia,  in  the  Val  di  Noto,  Sicily,  was  estimated  by  Sartorius  vou 
"NValtershausen  to  be:  silica,  41*26;  alumina,  8*60;  feme  oxide,  25*32;  lime,  5*59; 
magnesia,  4*84  ;  i^itash,  0*.'»4  ;  soda,  1*06  :  wat«T,  12*79.  This  nwk  is  largidy  developed 
among  the  ju'txlucts  of  the  Icelandic  and  Sicilian  V(»lcanoes  ;  it  occurs  also  in  the  £lfel 
and  in  Nassau.  It  has  been  found  to  be  one  of  the  characteristic  features  of  tnffs  of 
CarlK>niferous  age  in  Central  Scotland  ^  (Fig.  23). 

Schalstein. — Tnder  this  name,  Gennan  jietrographei's  have  placiMl  a  variety  of  green. 
grey,  rcil,  or  mottled  fissile  rocks,  impregnat<'«i  with  carbonate  of  lime.  They  are  inter- 
stratitied  with  the  Devonian  formations  of  Nassau,  the  Ilarz  and  Devonshire,  and  with 
the  Silurian  rocks  of  Bohemia.  They  sometimes  contain  fragments  of  clay-slate,  and 
are  <K'casionally  fossil ifenms.  They  prejicnt  amygdaloidal  and  porphyritic,  as  well  as 
IK?rfectly  laminattd  stnictures.  Probably  they  are  in  most  cas&s  true  diabase-tuffs,  bat 
sometimes  they  may  be  forms  of  diabase-lavas,  which,  like  the  stratified  formations  in 
which  they  lie,  have  undergone  alteration,  and  in  particular  have  acf^uired  a  more  or 
less  distinctly  fissile  structure,  as  the  I'esult  of  lateral  pressure  and  internal  crushing.^ 

4.  Fragmental  Rocks  of  Organic  Origin. 

This  series  includes  ileyK»sits  fonned  either  by  the  gi'owth  and  decay  of  oiganisnu 
in  sitUf  or  by  the  transjKnt  and  subse*[uent  accumulation  of  their  I'emains.  These  msy 
be  c(mveniently  gi'ouped,  according  to  their  predominant  chemical  ingredient,  into 
GalcareoiLs,  Siliceous,  r'hos])hatic,  Carbonaceous,  and  Feniiginous. 

1.  CALOAKKors. — Besiiles  the  calcareous  formations  which  occur  among  the  stratified 
fuystalline  rocks  as  results  of  the  dej^sition  of  chemical  ju-ecipitates  (p.  149),  a  more  im- 
|K)rtant  series  is  derive<l  fi*om  the  remains  of  living  organisms,  either  by  growth  on  the 
s|>ot  or  by  transport  and  accumulation  as  mwrhanical  sediment.  To  by  far  the  larger 
l»art  of  the  limestones  intercalated  in  the  rocrky  framework  of  our  continents,  an  organic 
origin  may  with  prol>ability  be  assigned.  It  is  tme,  as  has  been  above  mentioned  (p.  122), 
that  limestone,  formed  of  the  remains  of  animals  or  plants,  is  liable  to  an  internal  crysttl- 
line  rearraugt-ment,  the  effect  of  which  is  to  obliterate  the  organic  stnicture.  Hence  in 
many  of  the  older  limestones,  no  trace  of  any  fossils  can  be  detected,  and  yet  these  rocks 
were  almost  certainly  formed  of  organic  remains.  An  attentive  microscopic  study  of 
organic  calcareous  structures,  and  of  the  mode  of  their  replacement  by  crystalline  calcite, 
sometimes  detects  indications  of  former  organisms,  even  in  the  midst  of  thoroughly 
crystalline  materials.^ 

'  Trans.  Jioy.  Sik\  Kilin.  xxix.  p.  514. 

"  C.  Koch,  JahHK  Ver.  Xai.  Xassaa,  xiiL  (1858),  216,  238.  J.  A.  Phillips,  Q,  J,  QtoL 
!ktr.  xxxii.  p.  155,  xxxiv.  p.  471. 

'  Sorby,  Address  to  O'eol.  Society^  February,  1879,  and  the  paper  of  Messrs.  Comish 
and  Kendall,  cited  untey  p.  122.  Giimbel  lias  suggested  that  the  different  durability  of  the 
calcitc  and  aragonite  oigauic  forms  may  be  due  rather  to  structure  than  mineral  composition* 


PARTiigvii  FRAGMENTAL  ROCKS  139 

Limestone,  composed  of  the  remains  of  calcareous  organisms,  is  found  in  layers 
which  range  from  mere  thin  laminse  up  to  massive  beds,  several  feet  or  even  yards  in 
thickness.  In  some  instances,  such  as  that  of  the  Carboniferous  or  Mountain  limestone 
of  Britain  and  Belgium,  and  that  of  the  Coal-measures  in  Wyoming  and  Utah,  it  occurs 
in  continuous  superposed  beds  to  a  imited  thickness  of  several  thousand  feet,  and  extends 
for  hundreiis  of  square  miles,  forming  a  rock  out  of  which  picturesque  gorges,  hills,  and 
table-lands  have  been  excavated. 

Limestones  of  organic  origin  present  every  gradation  of  texture  and  structure,  from 
mere  8oft  calcareous  mud  or  earth,  evidently  comjwsed  of  entire  or  cnnnbled  organisms 
up  to  solid  com|iact  crystalline  rock,  in  which  indications  of  an  organic  source  can 
hardly  Iw  perceived.  Mr.  Sorby,  in  the  address  already  cited,  called  renewed  attention 
to  the  im^wrtance  of  the  form  in  which  carbonate  of  lime  is  built  up  into  animal 
stmctureH.  Quoting  the  opinion  of  Rose  expressed  in  1858,  that  the  diversity  in  the 
state  of  preservation  of  different  shells  might  be  due  to  the  fact  that  some  of  them  had 
their  lime  as  calcite,  others  as  aragonite,  he  showed  that  this  opinion  is  amply  supported 
by  microscopic  examination.  Even  in  the  shells  of  a  recent  raised  beach,  he  observed 
that  the  inner  aragonite  layer  of  the  common  mussel  ha<:l  been  completely  removed, 
though  the  outer  layer  of  calcite  was  well  preserved.  In  some  shelly  limestones  con- 
taining casts,  the  aragonite  shells  have  alone  disappeared,  and  where  these  still  remain 
represented  by  a  calcareous  layer,  this  has  no  longer  the  original  stnicture,  but  is  more 
or  less  coarsely  crystalline,  being  in  fact  a  pseudomorph  of  calcite  after  aragonite,  and 
qnite  unlike  contiguous  calcite  shells,  which  retain  their  original  microsco[)ical  and 
optical  characters. ' 

The  following  list  comprises  some  of  the  more  distinctive  and  im^iortant  forms  of 
organically-derived  limestones. 

Shell-Marl — a  soft,  white,  earthy,  or  crumbling  deposit,  fonne<l  in  lakes  and 
ponds  by  the  accumulation  of  the  remains  of  shells  and  Entomostraca  on  the  bottom. 
When  such  calcareous  dei)osits  become  solid  compact  stone  they  arc  known  as  fresh- 
waiUr  {lanistrine)  limestones.  These  are  generally  of  a  smooth  texture,  and  either 
dull  white,  ]iale  grey,  or  cream-coloured,  their  fracture  slightly  conchoidal,  rarely 
splintery. 

Lumachelle — a  compact,  dark  grey  or  brouTi  limestone,  charged  with  ammonites 
or  other  fossil  shells,  which  are  sometimes  iridescent,  giving  bright  green,  blue,  orange, 
and  dark  red  \mX»  (fire-marble). 

Calcareous  (Foraminiferal)  Ooze — a  white  or  grey  calcareous  mud,  of  organic 
origin,  found  covering  vast  areas  of  the  floor  of  tin;  Atlantic  and  other  oceans,  and 
formed  mostly  of  the  remains  of  Foraminifcra^  [wirticularly  of  forms  of  the  genus 
Globigerina  (Fig.  24).  Further  account  of  this  and  other  organic  deep-sea  dejwsits  is 
given  in  Book  III.  Part  II.  Section  iii. 

Shell- Sand — a  deiK>sit  comjwsed  in  great  measure  or  wholly  of  comminuted  shells, 
found  commonly  on  a  low  shelving  coast  exjwsed  to  prevalent  on-shore  winds.  When 
thrown  above  the  reach  of  the  waves  and  often  wetted  by  rain,  or  by  trickling  runnels  of 
water,  it  is  apt  to  become  consolidated  into  a  mass,  owing  to  the  solution  and  rede|)osit 
of  lime  round  the  grains  of  shell  (p.  122). 

Coral- rock — a  limestone  formed  by  the  continuous  growth  of  coral-building  ix)lyps. 
This  substance  affords  an  excellent  illustration  of  the  way  in  which  organic  stnictui-e 
may  be  eflhoed  from  a  limestone  entirely  formed  of  the  remains  of  once  living  animals. 
Though  the  skeletons  of  the  reef-building  corals  remain  distinct  on  the  upper  surface, 
those  of  their  predecessors  beneath  them  are  gradually  obliterated  by  the  passage 
through  them  of  percolating  water,  dissolving  and  redepositing  calcium  carbonate.  We 
can  thus  understand  how  a  mass  of  crystalline  limestone  may  have  been  produced  from 

*  The  student  will  find  the  address  from  which  these  citations  are  made  full  of  suggestive 
matter  in  regard  to  the  origin  and  subsequent  history  of  limestones. 


ClMlk-a  wLiV  -.ft  i.^.-k.  iiL-a-TH  i.. 
,:i].-:ir».r.,  t1<iir  <l"[h-.'l  :'r.>]ii  t<»  i^tiMiti' 
••tli-r  iiuriii-  fr'^'i]iL«iii>.  Ily  tiiakiii>;  t)i{ 
ill"  tll^.'^n■:>>|■•i,  Sorliy  Iia*  r<-uii>l  that  ¥<.< 
•I>-Mi-L>'it  •■ulU  lit i-im\aitiKivr',v  Aa\lon-\ 
■  ■{  rl.«  ^..k  l.j-  liulk  IV  ±:\'ti,-  ntiiaiii 
i.-iil'j»ni''ri-  Idnr  nf  /^'nyivmin*.  t'l^a^K-i 
-l-'iii,"-.  *i--  It  i-  not  ■)iiif  Uk"  aiiv  ki 
ii.v.r-.rEiMti-ii  •if  viMlk  frm,!  tli.-  uflJi^U^ 


.l-'l.t  itl;.':'-.li^ll 

ii.i.-  r..:l:- 
.i<Ul   Ell' 


,1.1  t-U,. 


.  ill  til-  till 

LiiuKi 


.'ur1>-iur-r< 


iiii'li.  -liliiif:  till'  liii)9'r>.  foniipd  of  a  five 
'i.vim.'v.'r.V'i.  ruliiniidi^rins,  iiiolliiski,  ud 
o  •<!  till-  iiH'k  >ttii  rxaiiiiiuiig  tliein  nndfr 
ifiTa.  i-artii'uUtly  'tl-Ai^riiui,  and  nugit 
r>-niis.  ]>r<>l<i)>ly  raiMtitutr  less  tliau  hil[ 
■u-istiii!;  cf  •U-Ui-knl  jmnna  of  the  outer 
"J,--. I.  /'.^■'.■-i.  rvhiuiKli-miH,  spii'uln  ef 
IV  kiiKivn  ii|.-Wti  c],><'[.-si-a  •lq<•>^it.  A  tuicmwoidc 
;ii>-mil..u.|  ut'  I.il1r  -linu'i^l  thut.  )«»ides  tlie  luoi] 
IIS  iiiiiiiii.>  uniiiii-  JTul  I'ly-luU  •>r<|imrtz,  tourmaliiK, 
III'"'  uiiiii'L^il.-'  l>'iii^' aniMii;;  tlii>  iii<wt  videly  diOnMil 
i-r  .-•'■liiiii'iii-'  tliiit  uro  ili'i'tvtil  from  the  denudatka 

t-.>iii--ii  rock  i,-0Tii[><>M:-t  ill  .i,Teat  jart  of  cryBtalliBt 
m.  i-ruU  ami  iiio1luAkT<.  it  varira  in  i-olmir  fron 
.  iif  liliii;.L-;,Ti-y  M-uioIliiim  ytflKni'  or  lirown,  hm 
ivii  1<Wk  iniUicr.  It  j-  aliniiilaiit  aiuoiig  Palmuotc 
•■  r<|i>i.-ially  ■.■Iiiii-au'trtUtif  <if  tlir  lower  jart  of  tUe 


'  Se«  Uitu'*  •".'•ml  ali'l  I'l^ral  Isbiid>.'  i-.  3Sl :  aUi  tlw 
(.'arlMniremu^  timc-t-ui''  in  tbe  ]'ivMn;  rultinur.  Ihii^-mt  I 
ipa-'ivt  linivtaDv*  of  Bi^l^iiini  haw  Ivtii  fonin^J  i-y  tvcf-lii 
allicil  ori;iai<nii. 

■  U  Uy*iiJi,  .1"".  .1*. ';,-.,/.  .V,. ,/.  ivii.    uw,.  ]'.  -Iss. 


►ART  n  §  vii  FRAOMENTAL  ROCKS  141 

2.  SiLK'EOUs. — Silica  is  directly  eliminated  from  both  fresh  and  salt  water  by  the 
rital  growth  of  plants  and  animals.     (Book  III.  Part  II.  Section  iii.) 

Diatom-earth,  Tripolite  (Infusorial  earth,  Kieselgiihr) — a  siliceous  de[>osit  forme<l 
;hiefly  of  the  fnistule-s  of  diatoms,  laid  down  both  in  salt  and  in  fresh  water.  "Wide 
ireas  of  it  are  now  being  deposited  on  the  bed  of  the  South  Pacitic  {Diatom-ooze^  Fig. 
181).  In  Virginia,  United  States,  an  extensive  tract  occurs  covered  with  diatom-earth 
to  a  depth  of  40  feet.  It  likewise  underlies  i)cat-mosses,  probably  as  an  original  lake- 
le|M)sit.     It  is  used  as  Tripoli  powder  for  polishing  purposes  (see  p.  481). 

Radiol ailan  ooie — a  |)ale  chalk-like  abysmal  marine  deposit  consisting  mainly  of 
the  remains  of  siliceous  radiolarians  and  diatoms.  It  is  further  referred  to  in  Book  III. 
Part  II.  Section  iil    '     U 

Flint  (Silex,  Fenerstein) — a  grey  or  black,  excessively  com)>act  rock,  with  the  hard- 
ness of  quartz  and  a  perfect  conchoidal  fracture,  its  splinters  being  translucent  on  the 
edges.  Consists  of  an  intimate  mixture  of  crystalline  insoluble  silica  and  of  amorphous 
silica  soluble  in  caustic  potass.  Its  dark  colour,  which  can  be  destroyed  by  heat,  aiises 
chiefly  from  the  presence  of  carbonaceous  matter.  Flint  occurs  principally  as  nodules, 
rli8|iersed  in  layers  through  the  Upper  Chalk  of  England  and  the  north-west  of  Euroi)e. 
It  fre«^uently  encloses  organisms  such  as  si)onges,  echini  and  brachiopods.  It  has  l>een 
deposited  from  sea-water,  at  first  through  organic  agency,  and  subsequently  by  direct 
chemical  precipitation  round  the  already  dej)osited  silica.  (Book  III.  Part  II.  Sect,  iii.) 
Chert  (phtanite)  is  a  name  a]>plied  to  impure  calcareous  varieties  of  flint,  in  layers  and 
nodules  which  are  found  among  the  Paleozoic  and  later  fonnations,  especially  but  not 
exclusively  in  limestonea^  In  some  cases,  as  in  the  spicules  of  s|K)nges,  the  silica  has 
had  a  directly  organic  origin,  having  been  secreted  from  sea-water  by  the  living 
oi^gauisms  ;  in  other  cases,  where  for  example  we  find  a  calcareous  shell,  or  echinus,  or 
coral,  converted  into  silica,  it  would  seem  that  the  sul)stitution  of  silica  for  calcium- 
carbouate  has  been  effected  by  a  process  of  chemical  pseudomorphism,  either  after  or 
daring  the  formation  of  the  limestone.  The  vertical  ramifying  masses  of  flint  in  Chalk 
show  that  the  calcareous  ooze  had  to  some  extent  accumulated  before  the  segregation 
of  these  masses.^ 

3.  Phosphatic. — A  few  invertebi-ata  contain  phosphate  of  lime.  Among  these  may 
be  mentioned  the  brachiopods  Lingula  and  Orbicula;^  also  Conulariay  SerpulitrSj  and 
some  recent  and  fossil  cnistacea.  The  shell  of  the  recent  Lingula  oralis  was  found  by 
Hunt  to  contain,  after  calcination,  61  jMjr  cent  of  fixed  residue,  which  consisted  of  85 '70 
per  cent  of  phosphate  of  lime;  11*75  carbonate  of  lime,  and  2*80  magnesia.  The 
bones  of  vertebrate  animals  likewise  contain  about  60  ])er  cent  of  phosi»hate  of  lime, 
while  their  excrement  sometimes  abounds  in  the  same  substance.  Hence  dei)osits  rich 
in  phosphate  of  lime  have  resulted  from  the  accumulation  of  animal  remains  from 
Silurian  times  up  to  the  present  day.  Associated  with  the  Bala  limestone,  in  the  Lower 
Sihirian  series  of  North  Wales  is  a  band  comjiosed  of  concretions  cemented  in  a  black, 
graphitic,  slightly  phosphatic  matrix,  and  containing  usually  64  ]»er  cent  of  phosphate 
of  lime  (phosphorite)."*  The  tests  of  the  trilobites  and  other  organisms  among  the 
Gambrian  rocks  of  "Wales  also  contain  phosphate  of  lime,  sometimes  to  the  extent  of  20 
per  cent.*    Phosphatic,  though  certainly  far  inferior  in  extent  and  importance  to  cal- 

*  Consult  Hull  and  Hardman,  Trans.  Roy.  DuUin  Soc.  i.  (1878),  p.  71.  Renard, 
BuU.  Acdd.  Roy.  Belgiqun,  2d  ser.  vol.  xlvi.  p.  471  ;  Sollas,  Ann.  Mag.  yat.  Hist,  vii. 
(1881),  p.  141  ;  Scientific  Proc.  Roy.  iJiiNin  Soc.  vi.  (1887),  part  i.  G.  J.  Hinde,  Geol. 
Mag.  1887,  p.  435.  Bands  of  radiolarian  chert  occupy  persistent  horizons  among  the 
Lower  Silurian  rocks  of  southern  Scotland. 

*  On  formation  of  chalk-flints,  see  Book  III.  Part  II.  Section  iii.  §  3. 

'  Sterry  Hunt,  Amtr.  Journ.  Soc.  xvii.  (1854),  p.  236.  Logan's  'Geology  of  Canada,' 
18^  p.  461. 

**  D.  C.  Davies,  Q.  J.  Geol,  StK.  xxxL  p.  357.  ^  Hicks,  op.  cit.  p.  368. 


142  (iEoGSn^  Y  book  n 


earrctiLs,  and  even  to  siliceous,  forniatioii.%  are  often  of  singular  geological  interest.     The 
following  examples  may  serve  as  illustrations.* 

Guano — a  (IqK)sit  consisting  niainl}'  of  th«'  droppings  of  sea-fowl,  funned  on  isUndi 
in  rainless  tracts  off  tlie  western  coasts  of  South  America  and  of  Africa.  It  is  a  brown, 
light,  }K>wdeiT  substaui-c  with  a  jieculiar  animoniacal  odour,  and  o(*cnrs  in  de|i08itt 
sometimes  more  than  100  feet  thick.  Analyses  of  American  guano  give — combustibk 
organic  matter  and  acids,  1 1  *3  ;  ammonia  (carlN^nate,  urate,  kc, ),  31  '7  :  fixed  alkaline 
salts,  sulphates,  phosphates,  clilori<ies,  &c.,  8*1  ;  phosi»liates  of  lime  and  xiiagneaiay 
22 'f> ;  oxalate  of  lime,  2*6  :  sand  and  earthy  matter,  1*6  ;  water,  22*2.  This  remarkable 
.su1)stanc«'  is  highly  valuable  as  a  source  of  artificial  manures.  (Book  III.  Part  IL 
Section  iii.; 

Bono-Brocoia — a  de^xtsit  consisting  largely  of  fragmentary'  l>ones  of  living  or 
extinct  species  of  mammalia,  found  sometimes  under  stalagmite  on  the  floors  of  lime- 
stone caverns,  more  or  less  uiixc<l  with  earth,  san«l.  or  lime.  In  some  older  ^ct^^logicd 
formations,  bone -beds  occur,  formed  lai^ly  of  the  remains  of  reptiles  or  fishes,  as  tbe 
"Lias  Ijone-lx-*!,"  and  the  **Ludluw  l>one-l»erl." 

Coprolitic  nodules  and  beds  - — ai-e  foniied  of  the  accumulateil  excrement  (coprolites)  of 
vertebrated  animals.  Among  the  Carl>oiiiferou8  shales  of  the  basin  of  the  Firth  of  Forth, 
coprolitic  nodules  are  abundant,  together  with  the  l>ones  and  scales  of  the  lai^r  ganoid 
fislics  which  voided  them  :  abundance  of  broken  scales  and  bones  of  the  smaller  ganoidi 
can  usually  l>e  observed  in  the  coprolites.  Among  the  Lower  Silurian  rocks  of  Canada, 
numerous  [ihosiiliatic  nodules,  su]i]N)sed  to  l)e  of  coprolitic  origin,  «K*cur.*  The  phos- 
phatic  Ix'ds  of  the  Cambridgeshire  Cretaceous  ro<.-ks  are  now  largely  worked  as  asouite 
of  artificial  manuiv.  In  iN>pular  and  e^Jlecially  comnicix'ial  usage,  the  word  '*  coprolitic'* 
is  applietl  to  no<liilar  deiMisits  which  can  W  woikivl  for  pho>phHte  of  lime,  thongh  tlwy 
may  contain  few  or  no  true  coprolites. 

Fhosphatic  Chalk. — In  the  ('halk  of  France  and  lii'lgium,  more  s]iariugly  in  that  of 
England,  certain  layei"s  occur  wheiv  the  original  calcareous  matter  has  been  replaced  to 
a  considerable  extent  V>y  phasphate  of  lime.  Such  1>ands  have  freijuently  a  browniili 
tint,  which  on  examination  i>  found  t<i  re^ult  from  the  abundance  of  minute  brown 
grains  comiNuu'd  mainly  of  [»h«i>pliatc.  The  foiaininifera  ami  other  minuter  or  fragment- 
ar\*  fossils  have  In-en  changeil  into  this  brown  Hibstnnce.  The  ]>ro|M)rtion  of  phosphate 
of  lime  ranges  up  to  4.'!  jht  cent  or  more."* 

4.  CAKiK>NAi*Kors. — Tlic  formations  here  inclndc<l  have  almost  always  resulted  from 
tbe  decay  and  entombment  of  vegetation  on  the  sjiot  where  it  grew,  sometimes  by  the 
drifting  of  the  plants  to  a  distance  and  their  consolidation  there.  (See  Uook  III.  Part  IL 
Section  iii.  §  3.)  In  the  latter  case,  they  may  be  mingled  with  inorganic  sediment, 
sfj  as  to  pass  into  carbonaceous  shale. 

"^esX — vegetable  matter,  more  or  less  decomiKised  and  chemically  altered,  foond 
throughout  tenqHirale  climates  in  bogg}-  places  wlieiv  mai-shy  plants  grow  and  deeiy. 
It  varies  from  a  pale  \ellow  <»r  brown  librous  sul>stance,  like  turf  or  compmMd 
hav,  in  which  the  plant -ivmains  are  abundant  and  conspicuous,  to  a  comi>act  dark 
brown  or  black  material,  res»'mbling  black  clay  when  wet,  an«l  some  varieties  of  lignitr 
when  dried.  The  nature  antl  jiroiMirtions  of  the  constituent  elements  of  peat,  afUr 
Iwing  dried  at  100*  C,  are  illustrated  by  the  analysis  of  an  Irish  example  which  gave— 
carbon,  60  48  ;  hydrogen,  6  10  :  oxygen,  32*;»5  ;  nitrogen,  0*88  ;  while  the  ash  was  3*30. 

^  For  an  exhaustive  account  of  deposits  of  pha-phate  of  lime,  see  R.  A.  F.  Penroae  Jr., 
Jli'll.  l\S.  th-ttl,  Svrr.  No.  -Itj,  1888,  also^^xWr",  Book.  III.  Part  II.  Sect.  iii.  §  8. 

-  On  the  origin  of  phosphatic  nmhiles  ami  beds,  see  Gruner,  Bull.  Soc.  Oiol.  Franetf 
xxviii.  (2nd  ser.),  p.  62.      Martin,  op.  eit.  iii.  (3rd  ser.),  \\  273. 

'  Logan's  *  Geology  of  Caiia<l:i,'  p.  461. 

*  See  A.  F.  Kenard  and  J.  Cornet,  BuU.  Acaif.  Ruy.  BcOjique,  xxi.  (1891),  p.  126. 
A.  Strahan,  Quart.  Jouni.  O'eoi.  »St«-.  xlvii.  (1891). 


PART  n  §  Tii  FRAGMENTAL  SOCKS  143 

There  is  ftlvsjs  a  Urge  proportion  of  water  wliioh  cannot  be  driven  olT  even  by 
diying  the  pe«t.  In  the  maauiocture  of  comprensed  peat  for  fuel  thin  constituent, 
which  of  couTM  leasena  the  value  of  the  ]>eat  aa  com]>arEd  with  an  eijual  weight  of 
coal,  is  driven  oiT  la  a  great  extent  by  chojiping  the  peat  into  line  pieces,  and  thereby 
«X[wang  a  large  surface  to  eva^ioration.  The  ash  varioa  in  amount  fi'om  less  than 
1-00  to  more  than  S5  per  cent,  and  consists  of  sand,  clay,  feme  oxide,  sulphuric  acid, 
and  minute  proportions  of  lime,  iK>da,  potash  and  magnesia.'  Under  a  jiresgure  of 
MOO  atmosphersB  peat  ia  converted  into  a  hard,  black,  brilliant  substance  having  the 
phjakal  aspect  of  coal,  and  allowing  no  trace  of  ovganic  structure.^ 

Ugidt*  (Biovn  Coal) — compact  or  earthy,  compressed  and  clieinjrally  altered 
rcf^etable  matter,  often  retaining  a  lamellar  or  ligneous  texture,  with  stems  sliotting 
woody  fibn  crossing  each  other  in  all  direi^tioiiB,  It  varies  from  ]>ale  brown  or  yellow 
to  deep  brown  or  black.  Some  shade  of  brown  is  the  usual  colour,  nhence  the  name 
£nnm  Ooat,  by  which  it  is  often  known.  It  contains  from  5S  to  75  per  cent  of  carbon, 
baa  a  apsciflc  gravity  of  0-5  to  I'S,  bums  easily  to  a  light  ash  with  a  sooty  flame 
and  a  strong  burnt  smell.  It  occurs  in  beds  chiefly  among  the  Tertiary  strata,  under 
conditions  similar  to  those  in  which  coal  is  found  in  older  formations.  It  may  1>e 
nguded  as  a  stage  in  the  alteration  and  mineralization  of  vegetable  matter,  inter- 
mediate between  peat  and  ti-ue  coal. 

Coal — a  compact,  usually  brittle,  velvet-black  to  pitch-black,  iron-black,  or 
dull,  sonietiiQes  brownish  rock,  with  a  greyish. black  or  brown  atreak,  and  in  some 
Torietiea  a  distinctly  cubical  cleavage,  in  others  a  conchoidal  fracture.  It  contains 
bam  75  to  90  per  cent  of  carbon,  and  a  small  [lerccntage  of  sulphur,  generally  in 
the  form  of  iron.disnlphide.  It  has  a  specific  gravity  of  1-2-1 'S.^,  and  bums  n-itb  com- 
paiatiTe  readiness,  giving  a  clear  flame,  a  strong 
anxnatic  or  bituminous  smell,  some  varieties  fus- 
icig  and  caking  into  cinder,  others  burning  away 
to  a  mere  white  or  red  ash.  Though  it  consists 
of  campresaed  vegetatiou,  no  trace  of  □rgBiLLc 
structure  is  usually  apparent.'  An  attentive 
examination,  however,  will  often  disclose  poitions 
of  stems,  leaves,  be.  or  at  least  of  carbonized 
woody  fibre.  Some  kinds  are  almost «  holly  made 
np  of  the  spore. cases  ot  lyco]iO(liaceous  plant.i 
fFig.25).  Thete  is  reason  to  believe  that  different 
Tarietiea  of  coal  may  have  arisen  from  original 
divetvties  in  the  nature  of  the  vegetation  out  of 
which  they  were  formed.  The  accompanying 
table  ahowa  the  chemical  gradation  between  uii-  tig.  ij.—'ilicTiiscvjnv  MtruitiiHollialki'ilh 
•Itarad  vegsUtion  and  the  more  highly  miner-  C.-i,.ho-nii^  LyKpodiactou-Sponin- 
Ji^d  for^  of  coal.  ^.(u,^..f,^m>V,«^Ur.X 

'  Sm  Senft's  'Humus-,  Marscli-,  Tori-  nud  Liuionlt.bililuugeu.'  Leigizig,  1862.  J.  J. 
FtUh,  '  Ueher  Torf  nod  Dopplerit,'  Ziiritb.  1883,  and  tlie  various  nieniolla  quoted  /losUa, 
p.  478. 

'  ^Ting,  Butt.  Acad.  Roy.  BmieUta.  ilix,  (1880),  p.  367. 

'  On  the  iDJInence  ot  pressure  on  tlie  formation  of  coal,  see  Frfmy,  Com/tl.  rend. 
aOth  May  1876.     Spring,  Bull.  Aead.  Hoy.  Bnixdlfs.  1880,  p.  387. 


144                                                   GEOGNOi^Y 

BOOKn 

Table  showinu  the  <;HAi)rAL  Chax<;] 

K  IN  Composition  from  Wood  to  Charcoai.' 

Substance. 

Carbon.    H 

100    ; 

100 
100 

100     ' 

100 
100 
100     1 

1 

yilrt^n 

Oxygen. 

Dinponble  Hydfo- 

gen,  i.e.,  ovn  sad 

above  what  i«  »' 

quired  to  fonu  water. 

1  1.  AVooil  (mean  of  several  analyses   . 

2.  Peat    (         „           „           „       )   . 

3.  Lignite  (mean  of  15  varieties) .     . 

4.  Ten-yarfl  coal  of  S.  Stalfonlshire  \ 
1           basin j 

1  r».  Steam  coal  from  the  Tyne  .     .     . 

1  6.   Pentrefelin  coal  of  S.  Wales     .     . 

7.  Anthracite  from  Pennsylvania,  U.S. 

12-18 
9-85 
8-37 

612 

5-91 
4.75 

2.84 

83.07 
65.67 
42.42 

21. 23 

18.32 
5.28 
174 

1-80 
2-89 
8.07 

8*47 

8-62 
4-09 
2-63 

Coal  o<'curs  in  seams  or  1)C(ls  intercalated  l>etween  strata  of  sandstone,  shale,  fireclay, 
&c.,  in  geological  formations  of  Palieozoic,  Secondaiy,  and  Tertiary  age.  It  should  In 
rcmemhei*ed  that  the  word  coal  is  rather  a  i>o])ular  than  a  scientific  term,  beiag 
indiscriminately  applied  to  any  dense,  black  mineral  substance  capable  of  beu^ 
used  as  fuel.  Strictly  cmi)loyed,  it  ought  only  to  lie  used  with  reference  to  beds  «f 
fossilized  vegetation,  the  result  either  of  the  growth  of  i)lants  on  tlie  spot  or  of  the 
drifting  of  them  thither. 

The  following  analyses  show  the  chemical  composition  of  peat,  lignite,  and  some  of 
the  i)rinci[)al  varieties  of  coal  -  : — 


real. 

Devon- 
nhiiv. 


Lignitf. 

Bovry 
Tracj'y, 
Dcvdu. 


Conl. 

Xorthuni- 
txrland. 


Non-Cak- 
ing Cotil. 

S.«taflronl. 
shire. 


Carbon .... 

Hydrogen   . 

Oxygen 

Niti"ogen 

Sulphur 

Ash       .... 

SiMJcitic  gravity 


54.02 
5-21 

28-18 
2-30 
0..'')6 
9.73 


0-850 


66-31 
5-63 

22-86 
0-57 
2-36 
2-27 


78-69 
6-00 

10-07 
2-37 
1-51 
136 


1-129        1-259 


CannH 
Coal, 

Wigan. 


78-57 
5-29 

12-88 
1-84 
0-39 
1-03 


1-278 


80-07 
5.53 
8-08 
2.12 
1-50 
270 


1276 


Anlkn- 
file, 

S.Wal6i. 


90-S9 
3-28 
2-98 
0-8S 
0*91 
]-61 


1893 


These  analyses  are  exclusive  of  water,  which  in  the  i»eat  amounted  to  25*56,  and  ii 
the  lignite  to  34-66  \^r  cent. 

Anthracite — the  most  highly  mineralized  form  of  vegetation — is  an  iron-blaok  to 
velvet -black  substance,  with  a  strong  metalloidal  to  vitreous  lustre,  hard  and  biittK 
containing  over  90  per  cent  of  carbon,  with  a  si)ecitic  gravity  of  l-SS-l*?.  It  kindkl 
with  difficulty,  and  in  a  strong  draught  burns  without  fusing,  smoking,  or  smelling;  bnt 
giving  out  a  great  heat.  It  is  a  coal  from  which  the  bituminous  parts  have  beM 
eliminated.  It  occurs  in  beds  like  ordinary  coal,  but  in  positions  where  probaUy  tt 
has  been  subjected  to  some  change  whereby  its  volatile  constituents  have  been  ezpelkdi 
It  is  found  largely  in  South  Wales,  and  sparingly  in  the  Scottish  coal-fields  where  the 
oixlinary  coal-seams  have  been  approached  by  intnisive  masses  of  igneous  rock.  It  it 
largely  dcvelo])e<l  in  the  great  coal-field  of  Pennsylvania.     Some  Lower  Silurian  ahaki 


1  Percy's  *  Metallurgy,'  vol.  i.  p.  268. 


'  From  Percy's  *  Metalluigy,'  voL  L 


PAHT  II  §  vii  FRAGMENTAL  ROCKS  145 


are  black  from  difiused  anthracite,  and  have  in  consequence  led  to  fruitless  searches  for 
coal. 

Oil-shale  {Brandxhiefer) — shale  containing  such  a  proportion  of  hydrocarbons  as 
to  be  cajMible  of  yielding  mineral  oil  on  slow  distillation.  This  substance  occurs  as 
ordinary  shales  do,  in  layers  or  beds,  intei-stratified  with  other  aqueous  deposits,  as  in 
the  Scottish  coal-fields.  It  is  in  a  geological  sense  true  shale,  and  owes  its  |)eculiarity 
to  the  quantity  of  vegetable  (or  animal)  matter  which  has  been  preserved  among  its 
inorganic  constituents.  It  consists  of  tissile  argillaceous  layers,  highly  impregnated 
with  bituminous  matter,  |)assing  on  one  side  into  common  shale,  on  the  other  into 
cannel  or  ^Mirrot  coal.  The  richer  varieties  yield  from  30  to  40  gallons  of  cnide  oil  to 
the  ton  of  shale.  They  may  be  distinguished  from  non -bituminous  or  feebly  bituminous 
shales  (throughout  the  shale  districts  of  Scotland),  by  the  j>eculiarity  that  a  thin  paring 
curbt  up  in  front  of  the  knife,  and  shows  a  brown  lustrous  streak.  Some  of  the  oil- 
shales  in  the  Lothians  are  crowded  with  the  valves  of  ostracod  crustaceans,  besides 
scales,  coprolites,  &c.,  of  ganoid  fishes.  It  is  iK>ssible  that  the  bituminous  matter  may 
in  some  cases  have  resulted  from  animal  organisms,  though  the  abundance  of  plant 
remains  indicates  that  it  is  probably  in  most  cases  of  vegetable  origin.  Under  the 
name  **pyro8chists"  Sterry  Hunt  classed  the  clays  or  shales  (of  all  geological  ages) 
which  are  hydrocarbonaceous,  and  yield  by  distillation  volatile  hydrocarbons,  in- 
flammable gas,  kc. 

Petrolenm,  a  general  term,  under  which  Ls  included  a  series  of  natural  mineral 
oils.  These  are  Huid  hydrocarbon  com[>ounds,  varying  from  a  thin,  colourless,  watery 
liquidity  to  a  black,  opaque,  tar-like  viscidity,  and  in  si)ecific  gravity  from  0*8  to  1*1. 
The  paler,  more  limpid  varieties  are  generally  called  naphtha,  the  darker,  more 
viscid  kinds  mineral  tar,  Avhile  the  name  petroleum,  or  rock-oil,  has  been  more 
generally  applied  to  the  intermediate  kinds.  Petroleum  occurs  s[>aringly  in  £uroi>c. 
A  few  localities  for  it  are  known  in  Britain.  It  is  found  in  large  (quantity  along  the 
country  stretching  from  the  Car{)athians,  through  Gallicia  and  Moldavia,  also  at 
Baku  on  the  Caspian.*  The  most  remarkable  and  abundant  dis]>lay  of  the  substance, 
however,  is  in  the  so-called  oil-regions  of  North  America,  i>articularly  in  Western  Canada 
and  Northern  Pennsylvania,  where  vast  ([uantities  of  it  have  been  obtained  in  recent 
years.  In  Pennsylvania  it  is  found  especially  in  certain  porous  beds  of  sandstone  or 
"sand-rocks,"  which  occur  as  low  down  as  the  Old  Re<l  Sandstone,  or  even  as  the  top 
of  the  Silurian  system.  In  Canada  it  is  largely  present  in  still  lower  strata.  Its 
origin  in  these  ancient  formations,  where  it  cannot  be  satisfactorily  connected  with  any 
destructive  distillation  of  coal,  is  still  an  unsolved  problem. 

Aiphalt — a  smooth,  brittle,  pitch -like,  black  or  brownish -black  mineral,  having  a 
resinous  lustre  and  conchoidal  fracture,  streak  i>aler  than  surface  of  fracture,  and 
specific  gravity  of  1  "0  to  1  '68.  It  melts  at  al)out  the  temperature  of  boiling  water, 
and  can  be  easily  kindled,  burning  with  a  bituminous  odour  and  a  bright  but  smoky 
flame.  It  is  comi>08ed  chiefly  of  hydrocarbons,  with  a  vai-iable  admixture  of  oxygen  and 
nitrogen.  It  occurs  sometimes  in  association  with  petroleum,  of  which  it  may  be 
considered  a  hardened  oxidized  form,  sometimes  as  an  impregnation  filling  the  pores  or 
chinks  of  rocks,  sometimes  in  indei>endeut  beds.  In  Britain  it  api>ears  as  a  product  of 
the  destructive  distillation  of  coals  and  carbonaceous  shales  by  intrusive  igneous  rocks, 
as  at  Binny  Quarry,  Linlithgowshire,  but  also  in  a  number  of  places  where  its  origin  is 
not  evident,  as  in  the  Cornish  and  Derbyshire  mining  districts,  and  among  the  dark 
flagstones  of  Caithness  and  Orkney,  which  are  laden  with  fossil  fishes.  At  Seyssel 
(Departement  de  I'Ain)  it  forms  a  de]>osit  2500  feet  long  and  800  feet  broad,  which 
yields  1500  tons  annually.      It  exudes  in  a  liquid  form  from  the  groimd  round  the 

*  AlHch,  Jahrb.  OeoL  Reichmmst.  xxix.  (1879),  p.  165.  Trautschold,  ZeiUch.  DeutscJi. 
GeoL  Qtt.  xzvi.  (1874),  p.  257.  See  poateay  Book  III.  Part  I.  Sect.  i.  §  2  where  other 
aathorlUes  are  cited,     y^ 


1 46  GEOGNO^  Y  book  u 

Iwrders  of  the  Dead  Sea.  In  Triuidad  it  fonns  a  lake  IJ  mile  iii  circumference,  which 
is  cool  and  solid  near  the  shore,  but  increases  in  teini)eraturc  and  softness  towards  the 
centre. 

Graphite. — This  mineral  occui*s  in  masses  of  sufficient  size  and  ini|x)rtance  to  deserre 
a  i)lac>e  in  the  enumeration  of  carbonaceous  rocks.  \Xs  mineralogical  characters  hare 
already  (p.  67)  been  given.  It  oix'ui-s  in  distinct  lenticular  lieds^  and  also  diffused  in 
minute  scales,  through  slates,  schists,  and  limestones  of  the  older  geological  formatioui^ 
as  in  Ciunberland,  Scotland,  Canada,  and  lk>hemia.  It  is  likewise  found  occasioually 
as  the  result  of  the  alteration  of  a  coal  seam  by  intiiisive  Itasalt,  as  at  New  Cumnock  iu 
Ayrshire. 

5.  Fekruoinous. — The  decomposition  of  vegetable  matter  in  marshy  places  and 
shallow  lakes  gives  rise  to  cei-tain  organic  acids,  which,  together  with  the  carbonic 
acid  so  generally  also  present,  decom|K)se  the  ferniginous  minerals  of  rocks  and  cany 
away  soluble  salts  of  iron.  Ex^tosure  to  the  air  leads  to  the  rax)id  decom|K>sitiou  and 
oxidation  of  those  solutions,  which  consequently  give  rise  to  precipitates,  consisting 
iwiitly  of  insoluble  basic  salts  and  jwrtly  of  the  hydrated  fenic  oxide.  Tliese  prci^ipitatei, 
mingled  with  clay,  sand,  or  other  mechanical  impurity,  and  also  with  dead  and  decay- 
ing organisms,  fonn  dejwsits  of  iron-ore.  OiK?rations  of  this  kind  ap|iear  to  liave 
been  in  progress  from  a  remote  geological  antiquity.  Henw  ironstones  with  traces 
of  associated  organitr  remains  l>elong  to  many  ditlerent  geological  formations,  and  an 
being  formed  still.* 

Bog  Iron-Ore  (Lake-ore,  minerai  des  marais,  Sumpferz) — a  dark-brown  to  black, 
earthy,  but  sometimes  compact  mixtui*e  of  hydmted  |»eroxide  of  iron,  phosphate  of  iron, 
and  hydrated  oxide  of  mangamise,  fre(juently  with  clay,  sand,  and  oi'ganic  matter.    An 
ordinary  specimen  yielded,  ]ieroxide  of  iron,  6'J'59  ;  oxide  of  manganese,  8*52;  saud, 
11 '37;    phosphoric  acid,    1*50;   sulphuric  acid,    traces;   water  and   organic   matter. 
16*02=100*00.     liog  iron -ore  may  either  be  fonned  in  situ  from  still  water,  or  may  be 
laid  down  by  currents  in  lakes.     Of  the  former  mode  of  foi-mation,  a  familiar  illustration 
is  furnished  by  the  "  moor-band  i>an  "  or  hard  ferniginous  crust,  which  in  boggy  placet 
and  on  some  ill-drained  land,  fonns  at  the  bottom  of  the  soil,  on  the  top  of  a  stiff  and 
tolerably  imi)ervious  subsoil.   Abun«iant  bog-iron  or  lake-ore  is  obtained  from  the  bottoms 
of  s<mie  lakes  in  Norway  and  Swedcni.      It  forms  everywhere  on  the  shallower  slopes 
near  banks  of  reeds,  where  there  is  no  strong  cunent  of  water,  occiu'ring  in  granular 
concretions  (Bohnerz)  that  vary  from   the  size  of  grains  of  coarse  gunpowder  up  to 
nodules  6  inches  in  diameter,  and  forming  layers  10  to  200  yards  long,  5  to  15  yards 
broad,  and  8  to  30  inches  thick.     These  deposits  are  worked  during  winter  by  inserting- 
l)erforated  iron  shovels  through  holes  cut  in  the  ice  ;  and  so  rapidly  do  they  acc^umulate, 
that  instances  are  known  where,  after  having  l)eeu  completely  removed,  the  ore  at  the 
end  of  twenty-six  yeai-s  was  found  to  have  gathered  again  to  a  thickness  of  several 
inehew.     A  layer  of  l(X>se  earthy  ochre  10  feet  thick  is  believed  to  have  formed  in  600 
years  on  the  floor  of  the  Lake  Ti.sken  near  the  old  coj>]»er  mine  of  Falun  in  Sweden.* 
Accoi-ding  to  Ehrenl^erg,  the  fonnation  of  bog-ore  is  due,  not  merely  to  the  chemictl 
actions  arising  from  the  decay  of  organic  matter,  but  to  a  power  ]M)ssessed  by  diatoms  of 
sei»arating  iron  from  Avater  and  depositing  it  as  hydrous  iKToxi<le  within  their  siliceous 
framework. 

AluminouB  Yellow  Iron-Ore  is  closely  ivlatcd  to  the  foregoing.  It  is  a  mixture 
of  yellow  or  pale  brown,  hydrated  peroxide  of  iron,  with  clay  and  sand,  sometimes 
with  silicate  of  iron,  hydrated  oxide  of  manganese,  and  carbonate  of  lime,  and  oeeurB 
in  dull,  usually  i)ulvenilent  grains  and  nodules.  Occasionally  these  nodules  nuiy 
\ye  observed  to  consist  of  a  shell  of  harder  material,  within  which  the  yellow  oxide 

*  See  Senft's  work  already  (p.  143)  cited,  p.  168  ;  also  jMtstea,  Book  III.  Part  II. 
Sect.  iii. 

-  A.  F.  Thoreld,  Oevi,  Fiireu.  FOrfuinif,  siiockholm,  iii.  p.  20,  pontea,  pp.  407,  488. 


FRAGMENTAL  ROCKS 


147 


become*  programTslj  softer  tawuds  the  centre,  wliich  U  goraetlmea  quite  empty.     Such 

concretiont  Me  known  as  letites  or  eagle-atones.     Tliis  ore  occurs  in  the  Coal-measurcH 

of  3«xony  aiid  Sileeia,  also  in  the  Harz,  Baden,  Bavaria,  &c.,  and  among  the  Jurassic 

rocks  in  England. 

Cakf-lrvaatoiu    (Sphi^roeiderite)    has    been    already    (p.     78)    Inferred    to.       It 

oconra  abundantly  tu  nodules  and  beds  in  the  Carboniferous  system  in  most  [arts  of 

Europe.     Tlie  nodales  are  generally  oval  and  flattened  in  foni 

Tsrying  in  size  from  a  small  bean  up  to  concretions  a  foot  c 

more  in  diameter,  and  with  an  internal  system  of  radiating 

cracks,  often  filled  with  calcite  (Fig.  26).     la  many  cases,  they    /   7^ 

contain  in  the  centre  some  organic  substance,  such  as  a  copro-    '   ~^  -^ 

lite,  fern,  oone,  shell,  or  fish,  that  has  served  as  a  surface  round 

which  the  iron  in  the  water  and  the  anrroimding  mud  could 

be  precipitated.  Seams  of  clay. ironstone  vary  in  ttiickneHs 
from  mere  paper -like  partings  up  to  beds  several  feet  deep. 
The  Cleveluid  seam  in  the  middle  Lias  of  Yorkshire  is  about 
SO  feet  thick.  In  the  Carboniferous  system  of  Scotland  certain  seams  known  as  BI/kI:- 
band  contain  from  10  to  52  per  cent  of  coaly  matter,  and  admit  of  being  calcined  with 
the  addition  of  little  or  no  fuel.  They  are  sometimes  crowded  with  organic  remains, 
eapecially  lamellibranchs  {Anthracotia.  Anlhraeomya,  kc.)  and  flsbes  {Rhizodus,  Mega- 
lidit/iyt,  *c.) 

A  microscopic  examination  of  some  block.bsnd  ironstones  reveals  a  very  perfect 
oolitic  structure,  showing  that  the  iron  has  either  replaced  an  original  calcareous 
oolit«  or  has  been  precipitated  in  water  having  such  a  gentle  movement  as  to  keep  the 
graonles  quietly  rolling  along,  while  their  successive  concentric  layers  of  carbonate  were 
being  deposited.  Hr.  Sorby  has  observed  in  the  Cleveland  ironstones  an  abnormal  form 
of  oolitic  structure,  and  remarks  that  one  specimen  bore  evidence  that  the  iron,  mostly 


in  the  form  of  small  crystals  of  the  carbonate,  had  been  introduced  snbsequeutly 
fbntuition  of  the  rock,  as  it  had  replaced  some  of  the  aragoiiito  of  the  enclosed  sliells. 
The  subjoined  analyses  show  the  comjiosition  of  some  varictieii  of  clay. 


the 


CIsy  l™....r. 

Ulack  Baii.[ 

Clev,L.i„l  „i* 

(C«l  in«.siir^ 

(Lii>), 

Sl'MllllKl. 

X-HltMTt. 

Peroxide  of  iron 

1-45     . 

.       2-72     . 

2.86 

Protoxide  of  iro 

.     40-77     . 

43-02 

Protoxide  of  manganese 

1-38     . 

0-40 

Alumina    . 

6.74      . 

687 

Lime 

2.70     . 

.       0.90     . 

G-I4  (zini') 

Magnesia  , 

217     . 

0.7-J     , 

5-21 

FoUsh       . 

0.6S    . 

Silica 

17-37     . 

'.     10.10    '. 

Carbonic  acid 

26.57     . 

.     26-41      . 

25-50 

Phosphoric  «id 
Sulpfinric  acid 

0.34     . 

1-81 

Iron  pyrites 
Water 
Organic  matter 

010    . 

'.       10      '. 

— 

2-tO     '. 

01.-. 

9S-78 

100-00 

100-til 

Penxntage  of  iron    . 

29.12    . 

.      34-80     . 

35-46 

'  Address  to  Geol.  Soc 

Februar)-  187B 

»  See  Percy's  '  Metatu 

■gy,'  vol.  ii.     B 

■chof,   'Chem 

und  Ph.v».  G« 

1.-  .npp.   (18711. 

1 48  (iKOOXOS  Y  BOOK  II 


B.    CkYSTALLINK,    INrLl'l)IN«i   Rocks   KURMKI)    KKOM   CiIF.MKAL   rilECIPITATIOX. 

This  division  consists  mainly  of  chemical  de|X)sits,  but  includes  also 
some  which,  originally  formed  of  organic  calcareous  debris,  have  acquired 
a  crystiillinc  structure.  The  rocks  included  in  it  occur  as  laminse  and 
beds,  usually  intercalated  among  clastic  formations,  such  as  sandstone 
and  shale.  Sometimes  they  attain  a  thickness  of  many  thousand  feet, 
with  hardly  any  interstratification  of  mechanically  derived  sediment 
They  are  being  formed  abundantly  at  the  present  time  by  mineral  springs 
and  on  the  floor  of  inland  seas :  while  on  the  bottom  of  lakes  and  of  the 
main  ocean,  calcareous  organic  accumulations  are  in  progress,  which  will 
doubtless  eventually  acquire  a  thoroughly  crystalline  structure  like  that 
of  many  limestones. 

Ice. — So  large  an  area  of  tlio  earth's  surface  is  covenKl  with  ice,  that  this  su1>- 
stance  deserves  notice  among  geological  formations.  Ice  is  commonly  and  conveniently 
classitied  in  tAvo  divisions,  snow-ice  and  water-ice,  accoitluig  as  it  results  from  the 
comi»ressi(ni  ami  alternate  melting  and  freezing  of  fallen  snow,  or  from  the  freezing  of 
the  surface  or  bottom  of  slutets  of  water. 

Snow-ice  (see  Book  III.  Part  II.  Sect.  ii.  §  5)  Is  of  two  kinds.  Ist,  Fallen  snow 
(m  mountain  sloiws  above  the  snow-line  gradually  assumes  a  granular  structure.  The 
little  crystalline  needles  and  stai-s  of  ice  are  melted  and  frozen  into  rounded  grannies 
which  form  a  more  or  less  com{)act  mass  known  in  Switzerland  as  Aev^  or  Fini^  2nd, 
When  the  granular  neve  slowly  slides  down  into  the  valleys,  it  iicquires  a  more  compact 
i'lystallinc  stnicture  an<l  becomes  glacier- in:  Acconling  to  the  researches  of  F.  Elocke, 
glacier-ico  is,  throughout  its  mass,  an  irregular  aggi'egatc  of  distinct  ciystalline  grains^ 
the  boundaries  of  which  form  the  minute  capillary  fissuivs  so  often  described.*  It* 
fitiiKttui'e  thus  closely  corres|M>u«ls  to  that  of  marl>lo  (p.  ir»l).  Olacier-iee  in  small 
fragments  is  white  or  colourless,  and  often  shows  innumerable  tine  bubbles  of  air, 
.sometimes  also  line  jxirtieles  of  mud.  In  larger  masses,  it  has  a  blue  or  green-blue  tint, 
and  displays  a  veined  structure,  consisting  of  parallel  v(>rtical  veinings  of  wlute  ice 
full  of  air-bubbles,  and  of  blue  clear  iee  without  air-bubbles.  Suow-ice  is  formed 
above  the  snow-line,  but  may  descend  in  glaciers  far  below  it.  It  covei-s  large  areai 
of  the  more  lofty  mountains  of  the  globe,  even  in  tropical  regions.  Towards  the 
]^oh^s  it  descends  to  the  sea,  where  large  pieces  break  olf  and  lloat  away  as  icebergs. 

Water-ice  (see  Hook  III.  Tart  II.  .Sect.  ii.  §  .'*)  is  formed,  1st,  by  the  freezing  of 
the  surface  of  fresh  water  (river-ice,  lake-ice),  or  of  the  .sea  (Jce-foot,  floe-ice,  |iaek-ice); 
this  is  a  compaet,  clear,  white  (»r  greenish  ice.  2nd,  by  the  freezing  of  the  layer  of  water 
lying  on  the  bottom  of  rivei*s,  or  the  sea  (bottom-ice,  ground-ice,  anchor-ice) ;  this 
variety  is  more  s[H)ngy,  and  often  encloses  mud,  .sand  and  stones.  * 

Rock-Salt  (Sel  gemmc.  Steinsalz.  ]>.  79)  occurs  in  layers  or  lieds  from  less  than  an  inch 
to  many  hundred  feet  in  thickness.  The  .sidt  dejwsits  at  Stassfiut,  for  example,  are  1197 
feet  thi<'.k,  of  which  the  lowest  beds  comprise  685  feet  of  pure  rock-salt,  with  thin  layers 
of  anhydrite  :J-inch  thick  dividing  the  salt  at  intervals  of  from  one  to  eight  iucLcs.  Still 
more  massive  are  the  accumulations  of  Si>erenl>erg  near  Berlin,  which  have  been  bored 
through  to  a  depth  of  4200  feet,  and  tho.se  of  Wieliczka  in  Gallicia  which  are  here  and 
there  more  than  4G00  feet  thick. 

^  Xt'HCs  Juhrh,  18S1  (i.),  p.  23.  Grad  and  Duprr  {Ann.  Cluh,  Alp.  Franc.  1874)  show 
how  the  characteristic  structure  of  glacier-ice  may  lie  revealed  by  allowing  coloured  solutions 
to  ]iernieat«  it. 

'  On  the  properties  of  ice  with  some  interesting  geological  bearings,  see  0.  PetterssoD, 
*  Vega-Expeditionens  Veteuskaiiliga  lakttagelser,'  vol.  ii.  p.  249,  Stockholm,  1883. 


PARTn§vii  STRATIFIED  CRYSTALLINE  ROCKS  149 


The  more  insoluble  salts  (notably  gypsum  or  anhydrite)  are  apt  to  appear  in  the 
lower  parts  of  a  saliferous  series.  When  purest,  rock-salt  is  clear  and  colourless,  but 
usually  is  coloured  red  (i)eroxide  of  iron),  sometimes  green,  or  blue  (chloride  or  silicate 
of  copper).  It  varies  in  structure,  being  sometimes  beautifully  crystalline  and  giving  a 
cubical  cleavage ;  laminated,  gi'anular,  or  less  frequently  fibrous.  It  usually  contains 
some  admixture  of  clay,  sand,  anhydrite,  bitumen,  &c. ,  and  is  often  mixed  with 
chlorides  of  magnesium,  calcium,  &c.  In  some  places  it  is  full  of  vesicles  (not 
infrequently  of  cubic  form)  containing  saline  water  ;  or  it  abounds  with  minute  cavities 
filled  with  hydrogen,  nitrogen,  carbon-dioxide,  or  with  some  hydrocarbon  gas. 
Occasionally  remains  of  minute  forms  of  vegetable  and  animal  life,  bituminous  wood, 
corals,  shells,  crustaceans,  and  fish  teeth  are  met  with  in  it.  Owing  to  its  ready  solu- 
bility, it  is  not  found  at  the  surface  in  moist  climates.  It  has  been  formed  by  the 
evaporation  of  very  saline  water  in  enclosed  basins — a  process  going  on  now  in  many 
salt'lakes  (Great  Salt  Lake  of  Utah,  Dead  Sea),  and  on  the  surface  of  some  deserts 
( Kirgis  Steppe).  In  different  parts  of  the  world,  deposits  of  salt  have  probably  always 
been  in  progress  from  very  early  geological  times.  Saliferous  formations  of  Tertiary  and 
Secondary  age  are  abundant  in  Europe,  while  in  America  they  occur  even  in  rocks  as  old 
as  the  Upper  Silurian  period,  and  among  the  Punjab  Hills  in  still  more  ancient  strata.* 

CSamaUite — a  chloride  of  potassium  and  magnesium  (p.  79).  It  occurs  in  a  bed  20  to  30 
metres  thick  which  overlies  the  rock-salt  in  the  saliferous  series  of  Stassfurt,  and  has  been 
found  in  other  old  salt  deposits,  as  well  as  among  the  "salterns"  or  "salines  "  along 
the  Mediterranean  coast  where  the  water  of  that  inland  sea  is  evaporated  in  the  manu- 
fiw^ture  of  salt.  It  so  closely  resembles  rock-salt  that  it  was  formerly  included  with  it, 
bnt  it  is  much  less  frequently  met  with.  It  is  a  valuable  source  for  the  manufacture  of 
potash -salts. 

Lbnestone  (Calcaire,  Kalkstein), — essentially  a  mass  of  calcium -carbonate,  some- 
times nearly  pure,  and  entirely  or  almost  entirely  soluble  in  hydrochloric  acid,  some- 
times loaded  with  sand,  clay,  or  other  intermixture.  Few  rocks  vary  more  in  texture  and 
composition.  It  may  be  a  hard,  close-gi*ained  mass,  breaking  with  a  splinterj'  or  coi^- 
choidal  fracture  ;  or  a  crystalline  rock  built  uj)  of  fine  ciystalline  grains  of  calcite,  and 
resembling  loaf-sugar  in  colour  and  texture  ;  or  a  dull  earthy  friable  chalk-like  deposit  : 
or  a  compact,  massive,  finely-granular  roc*k  resembling  a  close-grained  sandstone  or  free- 
stone. As  its  hardness  is  about  3,  it  can  easily  be  scratched  with  a  knife  and 
the  white  powder  gives  a  copious  effervescence  with  acid.  The  sjiecific  gravity  naturally 
varies  according  to  the  impurity  of  the  rock,  ranging  from  2-r)  to  2-8.  The  colours,  too, 
vary  extensively,  the  most  common  being  shades  of  blue-grey  and  cream-colour  passing 
into  white.  Some  limestones  are  highly  siliceous,  the  calcareous  matter  having  been 
accompanied  with  silica  in  the  act  of  deposition  ;  others  are  argillaceous,  sandy,  ferni- 
ginoos,  dolomitic,  or  bituminous.  By  far  the  larger  number  of  limestones  are  of  organic 
origin  ;  though  owing  to  internal  re-arrangement,  their  original  clastic  character  has 
frequently  been  changed  into  a  crystalline  one.  Under  the  present  subdivision  are 
placed  all  those  limestones  which  have  had  a  distinctly  chemical  origin,  and  also  those 
which  though  doubtless,  in  many  cases,  originally  fonned  of  organic  debris,  have  lost 
their  fragmental,  and  have  assumed  instead  a  crystalline  structure.  (For  the  organic 
limestones  see  p.  139.) 

Compact,  common  limestone, — a  fine-grained  c-r}'8talline-granular  aggregate. 
oecnrring  in  beds  or  laminie  interstratified  with  other  a«iueous  de^wsits.  When  purest  it 
is  readily  soluble  in  acid  with  effervescence,  leaving  little  or  no  residue.  Many  varieties 
oocnr,  to  some  of  which  separate  names  are  given.  HydrnuHc  limestone  contains  10  i>er 
cent  or  more  of  silica  (and  usually  alumina)  and,  when  burnt  and  subsequently  mixed 
with  water,  forms  a  cement  or  mortar,  which  has  the  i^roperty  of  "setting"  or  hard- 

^  On  salt  deposits  of  various  ages  see  A.  C.  Ramsay.  Brit.  Assin\  Rep.  1880,  p.  10  ; 
also  Index,  futb  voc.  **  Salt  Depasits. " 


1 5  0  GEOGNOH  Y  book  ii 


euiiig  under  water.  Limestones  containing  i>erhai>8  as  nnieh  as  25  \ter  cent  of  silio, 
luniina,  iron,  &c.,  that  in  themselves  would  be  unsuitable  for  many  of  the  ordiniiy 
jiurposes  for  which  limestones  are  used,  can  be  emi>loyed  for  making  hydraulic  mortar. 
The^e  limestones  occur  in  beds  like  those  in  the  Lia«  of  Lyme  Regis,  or  in  noduleg  like 
those  of  Shem^ey,  from  which  Roman  cement  is  made.  CctMntatonc  is  the  name  given  to 
many  pale  dull  feiTUginous  limestones,  which  ctmtain  an  admixture  of  clay,  and  some  of 
which  can  be  profitably  used  for  making  hydmulic  mortar  or  cement.  Fetid  limesixmc 
[stUikstein,  sxcinestonc)  gives  off  a  fetid  smell  (sulphuretted  hydrogen  gas),  when  struck 
with  a  hammer.  In  some  cases,  the  rock  seems  to  have  been  do|K>sited  by  volcuiu 
springs  containing  decomi)08able  sulphides  as  well  as  lime.  In  other  instanoes,  the 
odour  may  be  connected  with  the  decomposition  of  imbedded  organic  matter.  In  some 
i[uames  in  the  Carboniferous  Limestone  of  Ireland,  as  mentioned  by  Jukes,  the  freahly- 
broken  rock  may  be  smelt  at  a  distance  of  a  hundred  yaixls  when  the  men  are  at  v«rk, 
and  occasionally  the  sUuich  beeomtis  so  strong  that  the  workmen  are  sickened  by  it»  and 
require  to  leave  off  work  for  a  time.  Canisfone  is  an  arenaceous  or  siliceoOB  limertowi 
|»artieularly  characteristic  of  some  of  the  Pala'ozoic  red  sandstone  formations.  JfolCm- 
ittonc  is  a  decomiwsed  siliceous  limestone  from  wliieh  most  or  all  of  the  lime  has 
been  removed,  leaving  a  siliceous  skeleton  of  the  rock.  A  similar  decompoeiti<ni  takfli 
]»lace  in  some  fcnuginoiLs  limestones,  with  the  result  of  leaving  a  yellow  skeleton  of 
ochre.  Common  limestone,  having  been  deposited  in  water  usually  containing  other 
substances  in  sus])cnsion  or  solution,  is  almost  always  mixed  with  im]niTities,  and 
where  the  mixture  is  sufficiently  distinct  it  receives  a  si^'cial  name,  such  as  siliceooi 
limestone,  sandy  limestone,  argillaceous  limestone,  bituminous  limestone,  dolomitic 
limestone. 

TraviTtine  (calcareous  tufa,  rale -sinter)  is  the  ]x>rous  material  de{>osited  by  cal- 
careous springs,  usually  white  or  yellowish,  varying  in  texture  from  a  soft  chalk-like  or 
marly  substiince  to  a  conijwict  building-stone.  (See  Book  III.  Part  II.  Sect  iii.  §§  3,  6.) 
StalfU'litc  is  the  name  given  to  the  calcareous  jiendant  deposit  formed  on  the  roofs  of 
limestone  -  caverns,  vaults,  bridge^s,  &e.  ;  while  the  water,  from  which  the  hanging 
lime-icicles  are  derived,  drii)S  to  the  floor,  and  on  further  cvajjoration  there,  gives  riie 
to  the  crust-like  deix)sit  known  as  Htahufmltt'.  Mr.  Sorby  has  shown  that  in  the 
calcareous  depasits  from  fresh  water  there  is  a  constant  tendency  towards  the  produo- 
tion  of  calcite  crystals  with  the  ]»rinei}>al  axis  i)eriH*ndicular  to  the  surface  of  deposit. 
Where  that  surface  is  curved,  there  is  a  radiation  or  eonvergenc^  of  the  fibre -like 
crystals,  well  seen  in  sections  of  stalactites  and  of  some  calcareous  tufas  (Fig.  108). 

Oolite, — a  limestone  formed  wholly  or  in  ]»art  of  more  or  less  iH>rfectly  spherical 
grains,  and  having  somewhat  the  iwi)ect  of  fish -roe.  Eacrh  grain  consists  of  successiTe 
I'oncentric  shells  of  carbonate  of  lime,  frequently  with  an  internal  radiating  fibrous  struc- 
ture, which  gives  a  black  cross  between  crossed  Nicols  (Fig.  27).  The  calcareous  materud 
was  <h;pusited  round  some  minute  iwrtide  of  .sand  or  other  foreign  body  which  was  kept 
in  motion,  so  that  all  sides  could  in  turn  become  encrusted.  Oolitic  grains  of  this 
charact^T  are  now  forming  in  the  springs  of  Carlsbad  (Sprudclstein) ;  but  they  may  no 
doubt  also  be  [a-odueed  where  gentle  currents  in  lakes,  or  in  partially  enclosed  areas  of 
the  sea,  keep  grains  of  sand  or  fragments  of  shells  drifting  along  in  water,  which  is  so 
charged  with  lime  as  to  be  nyidy  to  de|X)sit  it  xxyyow  any  suitable  surfai^e.  An  oolitic  lime- 
stone may  contain  much  impurity.  Where  the  calcareous  granules  are  cemented  in  a 
somewhat  argillaceous  matrix  the  rwk  is  known  in  (lermany  as  Rogenstein.  Where  the 
individual  grains  of  an  oolitic  limestone  are  as  large  as  peas,  the  rock  is  called  a  piso- 
1  i  te  (i»ea-grit).  The  granules  sometimes  consist  of  aragtmite.  Oolitic  structure  is  found 
in  limest(mes  of  all  ages  from  Paheozoic  down  to  recent  times.*  Mr.  E.  Wethered  has 
recently  |»ointed  out  that  many  oolitic  grains  show  curious  vermifonn  twistings  in  their 

*  Oolitic  structure  is  fouinl  even  among  the  limestones  of  the  Dalradian  metamorphie 
series  of  Scotland  (Islay)  which  may  iwssibly  be  pre-Palieozoic. 


ii§vii  STRATIFIED  CBYSTiLLISE  ROCKS  l&l 

concentrio  co»ta,  which  he  regards  aa  of  organic  ongin  either  |  laiit  or  animal 
tiella).'  Id  some  instances  oolites  ha>e  had  their  calcareous  matter  replaced 
bonate  or  oxide  of  iron,  so  as  to  become  ootitic  irotistoiies 

irblo  {granular  liraestoue)  — a  crystalline  grauuUr  aggregate  composed  of 
Uiw  oalciU-grauuleii  of  remaikably  uniform  sue  «ach  of  which  las  its  own 
mdont  twin  lamella  (often  giving  interference  colours)  and  oleaiage  lines  This 
terUtic  stmeture  is  well  displajed  when  a  thin  slice  of  ordinary  statuary  marble 


•d  nnJer  the  mieroBcojie  (Fig.  28).  Tyjiical  marble  is  while,  but  tbe  rock  is  also 
.  grey,  I'luE.  green,  red,  hlack.  or  streaked  and  mottled,  as  may  be  seen  iu  the 
ana  kinds  ilsed  for  ornamental  purposes.  Its  granular  structure  gives  it  a  resem- 
to  loaf-augar,  whence  the  term  "  saccharoid  "  a]>plied  to  it.  Fine  silvery  scales  of 
r  talc  may  ofleii  be  noticed  ei-eu  in  the  purest  marble  (Cipoli«o).  Some  crystalline 
ones  aasociated  with  gneiss  and  schist  are  peculiarly  rich  in  minerals,— mica, 
,  tremolite,  actinoHte,  anthophyllite,  loisite,  vesuviauite,  pyroxenes,  and  many  other 

occurring  there  often  in  great  abundance.  These  iuvhisions  can  be  isolated  by 
iug  the  surrounding  rock  in  add  (niile,  p.  87). 

rble  ia  regarded  by  most  geologists  an  a  nietamorphic  rock,  that  is,  otic  in  which 
cium-carbonate,  whether  derived  from  an  organic  or  inorganic  sourve,  has  been 
y  pecrystalliied  i»  situ.  In  the  course  of  this  change  the  oripnal  clay,  sand  or 
mpuritics  of  the  rock  have  been  also  crystaUized,  ami  now  apjiear  as  the  crystsllins 
IS  just  referred  to.  Harble  occurs  in  beds  and  large  lenticular  masses  associated 
^Btalline  schists  on  many  different  Kcological  horizons.  In  Canada  it  occurs  of 
itian  ;  in  Scotland  of  Cambrian  ;  in  Utah  of  Up|>er  Carboniferous  ;  in  Southern 
I  of  Triassic,  Jurassic  and  Cretaceous  age. 

ondto  (Magneaian  Limestone)  consists  lypically  of  a  yellow  or  white,  crystalline, 
e  aggregate  of  the  mineral  dolomite  ;  but  the  relative  proportions  of  tbe  calcium 
ignesium.carbonates  vary  indefinitely,  so  that  every  gradation  can  t>e  found,  from 
maitone  without  magnesium -carbonate  up  to  jnire  dolomite  containing  4S-S5  ]>er 

that  carl>onate.  Ferrous  carbonate  is  also  of  eomnion  occurrence  in  this  rock.  ~ 
itore  of  dulomit«  is  usually  distinctly  crystalline,  the  individual  crystals  being 
nally  so  loosely  helil  together  that  the  rock  readily  cnimbles  into  a  ciystalline 

toL  ilng.  1889,  p.  196  ;  Qunrl.  J-mra.  <i-:.,l.  »x.  \lii.  (1890),  p.  270.  Mr.  C.  Reid 
gested  that  these  tabular  UMefi  may  )>e  due  to  the  <leposit  of  lime  rouml  organic 
t*  {Al9^)  like  the  calcareous  incrustution  formeii  rouml  fibres  of  hemp  iu  kettles  and 


1 5  2  GKOGNOH  Y  book  ii 

sand.  A  fissured  cavernous  structure?  ai)]>arcntly  due  to  a  process  of  contraction  duriiig 
the  process  of  ^^  dot  omit  izatiou  ,"  is  of  common  occurrence  :  even  in  com|)act  varieties, 
cellular  sjwices  occur,  linerl  with  crystallized  dolomite  (Rauchwacke),  the  cryaftals  of 
which  are  ofttm  hollow  and  sometimes  enclose  a  kernel  of  calcite.  Other  varieties  are 
built  up  of  spherical,  botryoidal  and  iiTegularly-shaiKjfl  concretionary  roasses.  Dolo- 
mite, in  its  more  typical  forms,  is  distinguishable  from  limestone  by  its  greater  hardneu 
(3'5-4«r))  higher  sjKJcitic  gra\ity  (2-8-2'95),  and  much  less  easy  solubility  in  acid.  It 
occurs  sometimes  in  beds  of  original  de|>osit,  associated  with  gj-psum,  rock-salt  and 
other  results  of  the  evaporation  of  satumted  saline  watei*8  ;  it  is  also  found  replacing 
what  was  once  ordinary  limestone.  The  process  by  which  carl)onate  of  lime  it 
replaced  by  carlwnate  of  magnesia,  is  referred  to  in  ]k>ok  III.  Part  I.  Sect.  iv.  §  2.* 
Dolomite  sometimes  forms  pictuivsque  mountain  masses,  as  in  the  Dolomite  Moiintaini 
of  the  Eastern  Alj»s. 

Oypsum — a  line  granular  to  com|)act.  sometimes  fibrous  or  sparry  aggregate  of  the 
mineral  gj-psum,  having  a  hardness  of  only  1  -5-2  (therefore  scratched  with  the  nail),  and  a 
specitic  gi'avity  of  about  2-32,  and  being  unaffected  by  acids ;  hence  readily  distinguishable 
from  limestone,  which  it  occasionally  resembles.  It  is  normally  white,  but  may  be 
coloured  gi'ey  or  brown  by  an  admixture  of  clay  or  bitumen,  or  yellow  and  red  by  being 
stained  with  iron-oxi«le.  It  occurs  in  beds,  lenticular  intercalations  and  strings,  usually 
associated  with  berls  of  red  clay,  ro<.'k-salt,  or  anhyrlrite,  in  formations  of  many 
various  geological  ix'rir»ds  from  Siluiian  (Xew  York)  down  to  recent  times.  The 
Triassic  gypsum  deposits  of  Thuringia,  Hanover  and  the  Harz  have  long  l)een  lamoua 
One  of  them  runs  along  the  south  flank  of  the  Harz  Mountains  as  a  great  l)and  six  niilei 
long  and  reaching  a  height  of  sometimes  430  feet. 

Gypsum  furnishes  a  good  illustration  of  the  many  different  ways  in  which  some 
mineral  substances  can  originate.  Thus  it  may  be  i>ro<iuced,  Ist,  as  a  chemical 
precipitate  from  solution  in  water,  as  when  sea-water  is  evajjorated  ;  2nd,  through  the 
decom]»osition  of  sulphides  and  the  action  of  the  resultant  sulphuric  acid  u])on  lime- 
stone ;  3nl,  through  the  mutual  deconii>ositinn  of  carbonate  of  lime  and  sulphates  of  iron, 
(ropper,  magnesia,  &c.  ;  4tli,  through  the  hydration  of  anhydrite  ;  5th,  thmugh  the 
action  of  the  suljiliuroiis  vapoui-s  and  solutions  of  volcanic  orifices  ujKjn  limestone  and 
calcareous  rocks.-  It  is  in  the  tii-st  of  these  ways  that  the  thick  beds  of  gyi^um  assod- 
ated  with  rwk-salt  in  many  geological  formations  have  been  formed.  The  first  mineral 
to  api»ear  in  the  evajjoration  of  sea-water  being  gypsum,  it  has  lieen  precipitated  on  the 
floors  of  inland  seas  and  saline  lakes  iK'fore  the  more  soluble  salts. 

Anhydrite, — the  anhydrous  variety  of  calcium -sulphate,  occurs  as  a  compact  or 
granular,  white,  grey,  bluish  or  reddish  aggiegate  in  saliferous  dejiosits.  It  is  leas 
freijuent  than  gypsum,  from  which  it  is  distinguished  by  its  nmch  greater  hardness 
(3-3-ri;i  and  into  which  it  readily  i»asses  by  taking  up  0-2025  of  its  weight  of  water.'  It 
often  occurs  in  thin  seams  or  payings  in  rock-s»ilt ;  but  it  also  forms  large  hill-like 
masses,  of  which  the  external  ]>arti  have  ]>een  converte<l  into  gypsum. 

Ironstone. — Under  this  goneml  term  are  included  various  iron-ores  in  which  the 
pt*roxide,  protoxide,  carbonate,  ^c,  are  mingled  with  cla\*  and  other  impurities.  They 
have  generally  l>een  de|»osite<l  As  chemical  pi-ecipitates  on  the  bottoms  of  lakes,  nnder 
marshy  ground,  or  within  fissures  and  cavities  of  rocks.  Some  iron-ores  are  associated 
with  schistose  and  massive  i<x*ks  ;  others  are  found  with  sandstones,  shales,  limestones 


*  On  the  mineralogioal  natun;  of  dolomite  see  0.  Meyer,  Z.  Deutsch.  O'eal.  Oes.  xxxL 
p.  445,  Loretz.  op.  cit.  xxx.  p.  387.  xxxi.  p.  756.  Reward,  BvlL  Acad,  Rt^y,  Bftg.  xhii. 
(1879),  No.  5. 

-  llotli.  <.'h*'m.  (w't?ni.  i.  p.  553. 

^  See  (jr.  Rose  on  formation  of  this  rock  in  presence  of  a  solution  of  chloride  of  sodium. 
XmesJnhrh.  Miu.  1871,  p.  932.  Also  Bischof.  *('heni.  und  Phys.  (leol.*  Suppl.  (1871), 
p.  188. 


»ART  II  §  vii  STRATIFIED  CRYSTALLINE  ROCKS  153 

nd  coals  ;  while  some  occur  in  the  form  of  mineral  veins.  Those  which  have  resulted 
rom  the  co-operation  of  organic  agencies  are  described  at  p.  146. 

Hfematite  (red  iron-ore),  a  compact,  fine-grained,  earthy,  or  fibrous  rock  of  a 
•lood-red  to  brown-red  colour,  but  where  most  crystalline,  steel-grey  and  splendent, 
rith  a  distinct  cherry-red  streak.  Consists  of  anhydrous  ferric  oxide,  but  usually  is 
lixed  i^ith  clay,  sand,  or  other  ingredient,  in  such  varying  i)ro|W)rtions  as  to  pass,  by 
isensible  gradations,  into  ferruginous  clays,  sands,  quartz,  or  jasi)er.  Occurs  as  beds, 
age  concretionary  masses,  and  veins  traversing  crystalline  rocks ;  sometimes,  as  in 
Vestmoreland,  filling  up  cavernous  spaces  in  limestone.  Is  found  occasionally  in  beds 
f  an  oolitic  structure  among  stratified  formations.  Some  at  least  of  the  oolitic  or 
•iaolitic  ironstones  have  resulted  from  the  converaion  of  original  grains  of  calcite  in 
rdinary  oolites  into  carbonate  of  iron  which  on  oxidation  has  become  magnetite, 
icmatite,  or  limonite. 

Limonite  (brown  iron-ore),  an  earthy  or  ochreous,  comjAct,  fine-gi*ained  or  fibrous 
oek,  of  an  ochre-yellow  to  a  dark  brown  colour,  di8tinguisha])le  from  hrematitc  by  being 
ijdrous  and  giving  a  yellow  streak.  Occurs  in  beds  and  veins,  sometimes  as  the  result 
f  the  oxidation  of  ferrous  carbonate  ;  abundant  on  the  floors  of  some  lakes  ;  commonly 
:>und  under  marshy  soil  where  it  forms  a  hard  brown  crust  upon  the  impervious  subsoil 
bog 'iron -ore).  Found  likewise  in  oolitic  concretions  sometimes  as  large  as  walnuts, 
onsistiug  of  concentric  layers  of  impure  limonite  with  sand  and  clay  {Bohnerz).  (See 
..  146  and  Book  III.  Part  II.  Sect.  iii.  §  3.) 

Spathic  Iron-ore,  a  coarse  or  fine  crj-stalline  or  dull  compact  aggregate  of  tlie 
aineral  siderite  or  ferrous  carbonate,  usually  with  carl)onates  of  calcium,  manganese  and 
oagnesium  ;  has  a  prevalent  yellowish  or  brownish  colour,  and  when  fresh,  its  rhombo- 
ledral  cleavage-faces  show  a  i>early  lustre,  which  soon  disappear  as  the  surface  is 
xidized  into  limonite  or  haematite.  Occurs  in  beds  and  veins,  esiwjcially  among  older 
leological  formations.  The  colossal  Erzberg  at  Eisenerz  in  Styria,  which  rises  more 
han  2700  feet  aboA'e  the  valley,  consists  almost  wholly  of  siderite.* 

Clay-ironstone  (Sphterosiderite),  a  dull  brown  or  black,  compact  form  of  siderite, 
rith  a  variable  mixture  of  olay,  and  usually  also  of  organic  matter.  Occurs  in  the 
'arboniferous  and  other  formations,  in  the  form  either  of  nodules,  wheix*  it  has  usually 
•een  deixwited  round  some  organic  centre,  or  of  beds  interstratified  with  shales  and 
oala.     It  is  more  pro[)erly  described  at  p.  147,  with  the  organically  derived  rocks. 

Magnetic  iron -ore,  a  granular  to  compact  aggi-egate  of  magnetite,  of  a  black  colour 
nd  Rtreak,  more  or  less  perfect  metallic  lustre,  and  strong  magnetism.  Commonly 
ontains  admixtures  of  other  minerals,  notably  of  hivmatite,  chrome-iron,  titanic-iron, 
yrites,  chlorite,  quartz,  hornblende,  garnet,  epidote,  felsimr.  Occurs  in  beds  and 
normous  lenticular  masses  (Stocke)  among  crystalline  schists,  likewise  in  segiegation- 
eins  of  gabbros  and  other  eruptive  rocks ;  also  occasionally  in  an  oolitic  fonn 
probably  as  a  pseudomorph  after  an  original  calcareous  oolite)  among  Paljeozoic  rocks, 
IS  in  the  so-called  **pisolitic  iron-ore"  of  North  Wales.  Among  the  Scandinavian 
;neiaaes  lies  the  iron  mountain  of  Gellivara  in  Lulea-Lappmark,  17,000  feet  long,  8500 
eet  broad,  and  52.5  feet  high.  • 

Siliewmi  Sinter  (Geyserite,  Kieselsinter),  the  siliceous  deposit  made  by  hot  springs. 
nciudiug  varieties  that  are  crumbling  and  earthy,  comi>act  and  flinty,  finely  laminated 
kod  shaly,  sometimes  dull  and  opaque,  sometimes  translucent,  with  pearly  or  waxy 
uatre,  and  with  chalcedonic  alterations  in  the  older  jiarts.  The  deposit  may  occur  as 
.n  incnistation  round  the  orifices  of  eniption,  rising  into  dome-shaiwd,  botr}'oidal, 
oralloid,  or  columnar  elevations,  or  investing  leaves  and  stems  of  plants,  shells, 
Diiects,  Ac,  or  hanging  in  pendant  stalactites  from  cavernous  sjwices  which  are  from 
ime  to  time  reached  by  the  hot  water.  When  purest,  it  is  of  snowy  whiteness,  but  is 
•(ten  tinted  yellow  or  flesh  colour.     It  consists  of  silica  84  to  91  \^v  cent,  with  small 

1  Zirkel,  Lehrb.  i.  p.  34r». 


1 5  4  GEOGNOH  Y  book  ii 


prnpoi-tions  of  aliniiiiia,  fervie  oxido,  lime,  ma^csia,  and  alkali,  and  from   5   to  S  ]«r 
rent  of  waU;r.     (See  Book  III.  Part  II.  Sect.  iii.  §  3,  i«ar.  6.) 

Flint  and  Chert  have  l>eeu  already  deseril>e<l  among  the  rocks  of  organic  origiu 
inuti^  |».  111).  Homstone,  an  excessively  comjuiet  siliceons  i*ock,  usually  of  some  dull 
dark  tint,  oecui-s  in  nodular  masses  or  in*cgular  l>ands  and  veins.  The  name  has  some- 
times Ikm'u  a])plie<l  to  fine  tliuty  fomis  of  felsitc.  Vein-Quarts  may  be  alluded  to  hereu 
a  substance  which  sometimes  oecui's  in  large  masses.  It  is  a  massiA'c  foiin  of  quarti 
found  lilling  veins  (sometimes  many  yanis  broad)  in  crystalline  and  clastic  itK^ks  ;  more 
especially  in  metamorphic  areas.     (See  Quartz  Rocks,  p.  179.) 

Some  of  the  other  varieties  of  silica  occurring  in  large  masses  may  be  classed  as  roi'ks. 
Such  are  Jasper,  and  Ferruginous  Quartz.  These,  as  well  as  common  vein-ijuartz, 
occur  as  veins  traversing  both  sti-atilied  and  unstratifiexl  rocks  ;  also  as  lieds  associated 
with  the  crystalline  schists.  AVith  them  maybe  grouiM?d  Lydian-Stone  {LydiU\  Kiftd' 
<t^hivft:i'\  a  black  or  dark -coloured,  excessively  comjiact,  hard,  infusible  rook  with  splintery 
fmcture,  (H'curriug  in  thin,  sharjOy  defined  bands,  split  by  cross  joints  into  polygoiul 
fragments,  which  arc  sometimes  cemented  by  Hue  layei's  of  ([uartz.  It  c<»iisists  of  an 
intimate  mixture  of  silica  with  alumina,  carbonaceous  materials,  and  o.xide  of  iron,  and 
under  the  microsi^ojKr  shows  minute  <|uartz-gi-anules  with  dark  amorphous  matter.  It 
oc^MU's  in  thin  laversor  liands  in  the  Silurian  and  later  Pal a'ozoic  formations  iuterstratififd 
with  ordinary  sandy  and  argillaceous  strata.  As  these  rocks  have  not  been  materially 
altered,  the  ban«ls  of  Ijy<lian -stone  may  l)e  of  original  formation,  though  the  extent  to 
which  they  are  often  veined  with  quartz  shows  that  they  have,  in  many  cases,  been 
|X'rmeated  by  siliceous  water  .since  their  dejKisit.  The  siliceous  i"ocks  due  to  the  opera- 
tions of  plant  and  animal  life  are  described  on  p.  141,  also  in  Book  III.  Part  II. 
Sfct.  iii.  §  3. 

Some  originally  clastic  siliceous  nx-ks  have  acquired  a  more  or  less  cr^^stalline 
structure  from  the  action  of  thermal  water  or  otherwise.  One  of  the  most  marked 
varieties  has  W{.'\itiiT\nvA  Cnjslallizi'd  Smidstone  (see  p.  132).  Another  variety,  knovn 
;i«  (Jiiartzife,  is  a  granular  and  comjiact  aggregate  of  tjuartz,  which  will  be  descrilied  in 
connet:tion  with  the  schistose  rocks  among  which  it  generally  jxrurs  (p.  180). 

II.  Massive — Erufi'ive — Igneous. 

.Vlmost  all  the  members  of  this  importiint  suMiAision  have  been 
|)ro(lucoil  from  within  the  crust  of  the  earth,  in  a  molteu  condition. 
Xearly  all  consist  of  two  or  more  minerals.  Considered  from  a  chemical 
p(»int  of  view,  they  may  be  (loscril>ed  as  mixtures,  in  different  propoi-tions, 
of  silicates  of  alumina,  magnesia,  lime,  potash,  and  soda,  usually  with 
magnetic  iron  and  ))hosphate  of  lime.  In  one  scries,  the  silicic  acid 
has  not  been  more  than  enough  to  combine  with  the  different  bases; 
ill  another,  it  occurs  in  excess  as  free  quartz.  Taking  this  feature  as 
a  basis  of  arrangement,  some  petrographers  have  proposed  to  divide  the 
rocks  into  an  acirl  grouj),  including  such  rocks  as  granite,  quartz-porphyry 
and  rhyolite,  where  the  percentage  of  silica  ranges  from  60  to  75  or  more, 
a  Inisic  group,  typified  by  such  rocks  as  basalt,  where  the  proportion  of 
silicii  is  only  about  50  per  cent  or  less,  and  an  intermediate  group  repre- 
sented by  the  andesites  with  a  i)roportion  of  silica  ranging  between  that 
of  the  other  two  groups.^ 

^  See  a  paper  on  the  chemical  rrlations  of  the  eruptive  rocks  by  Prof.  Kosenbnscb, 
Tst'/hi'fiifik'A  Mill.  Mitthiil.  xi.  (1SS9),  p.  144,  also  the  paper  cjuoted  in  footnote  (2)  on  p. 
156j  and  a  Memoir  on  "the  origin  of  Igneous  Rocks,"  by  .J.  V.  bUlings^  PhU,  fhc.  Wnsking- 
ton,  1S92,  p.  00. 


RT  II  §  vii  MASSIVE  ROCKS  155 

In  the  vast  majority  of  igneous  rocks,  the  chief  silicate  is  a  felspar — 
e  number  of  rocks  where  the  felspar  is  represented  by  another  silicate 
I  leucite  or  nepheline)  being  comparatively  few  and  unimportant.  As 
e  felspars  gi'oup  themselves  into  two  divisions,  the  monoclinic  or 
thoclase,  and  the  triclinic  or  plagioclase,  the  former  with,  on  the 
lole,  a  preponderance  of  silica ;  and  as  these  minerals  occur  under 
lerably  distinct  and  definite  conditions,  other  petrographers  divide  the 
Ispar-bearing  Massive  rocks  into  two  series:  (1)  the  Orthoclase  rocks, 
.ving  orthoclase  as  their  chief  silicate,  and  often  with  free  silica  in 
cess,  and  (2)  the  Plagioclase  rocks,  where  the  chief  silicate  is  some 
ecies  of  triclinic  felspar.  The  former  series  corresponds  generally  to 
e  acid  group  above  mentioned,  while  the  plagioclase  rocks  are  in- 
rmediatc  and  basic.  It  has  been  objected  to  this  arrangement  that 
e  so-called  plagioclase  felspars  are  in  reality  very  distinct  minerals, 
th  proportions  of  silica,  i-anging  from  43  to  69  per  cent;  soda  from 
to  1 2 ;  and  lime  from  0  to  20.^  In  addition  to  the  felspar-rocks, 
ere  must  be  noted  those  in  which  felspar  is  either  wholly  absent  or 
aringly  present,  and  where  the  chief  part  in  rock-making  has  been 
ken  by  nepheline,  leucite,  olivine,  or  serpentine. 

From  the  point  of  view  of  internal  structui*e,  a  classification  based 
K)n  microscopic  research  has  been  adopted  by  other  ^vriters,  who 
cognise  three  leading  types  of  micro-stnicture — Granular,  Porphjritic 
id  Glassy^  or  Holocrtjstalline,  Hemicrystalline  and  Vitreous.  MM.  Fouqu6 
id  Michel-Levy,  pointing  out  that  most  eruptive  rocks  are  the  result 

successive  stages  of  crystallization,  each  recognisable  by  its  own 
laracters,  show  that  two  phases  of  consolidation  are  specially  to  be 
«erved,  the  first  (porphyritic)  marked  by  the  formation  of  largo 
ystals  (phenocrysts)  which  were  often  broken  and  corroded  by 
echanical  and  chemical  action  within  the  still  unsolidified  magma ; 
e  second  by  the  formation  of  smaller  cry  stills,  crystallites,  Arc,  which 
e  moulded  round  the  older  series.  In  some  rocks  the  former,  in  others 
e  latter  of  these  two  phases  is  alone  present.  Two  leading  types  of 
ructure  are  recognised  by   these  authors  among   the   eruptive  rocks. 

Granitoid,  where  the  constituents  are  mainly  those  of  the  second 
K)ch  of  consolidation,  but  where  neither  amorphous  magma,  nor 
ystallites  are  to  be  seen.  This  structure  includes  three  varieties,  (a) 
le  granitoid  proper,  having  crystals  of  approximately  equal  size;  (b) 
gmaiiM^  where  there  has  been  a  simultaneous  crystallization  and 
gular  arrangement  of  two  constituents ;  {c)  ophitic^  in  which  the  felspars 
e  ranged  parallel  to  one  of  their  crystalline  faces,  forming  a  kind  of 
ansition  into  microlitic  rocks.  2.  Trachytoid,  distinguished  by  a 
ore  marked  contrast  between  the  crystals  of  the  first  and  second  con- 
•lidation,  the  usual  presence  of  an  amorphous  magma,  and  the  fluxion 
ructure.  Three  varieties  are  named :  (a)  petrosilicemis,  with  trains  and 
iherulites  of  a  finely  clouded  substance  characteristic  of  the  more  acid 
►cks ;  (b)  microlitir,  characterised  by   the   abundance    of   microlites   of 

*  Dana,  Amer,  Journ.  Sci.  1878,  p.  432.  Tlie  modern  methods  of  separating  the  fel- 
ars  remove  some  of  the  difficulty  above  referred  to. 


1 56  OKOfrXOS  ] '  BOOK  II 


felspars  iiiul  other  minci*als ;  (c)  liireou^,  derived  from  the  two  foregoing 
varieties  by  the  predominance  of  the  amorphous  paste.  ^ 

It  is  common  to  introduce  a  chronological  element  into  the  classifica- 
tion of  the  massive  rocks  and  to  divide  them  into  an  ancient  (Palaeozoic 
and  Mesozoic)  and  modern  (Tertiary  and  recent)  series.  Certain  broad 
distinctions  can  doubtless  l)e  made  between  many  anpient  and  modem 
eruptive  rocks ;  but,  for  reasons  already  stated,  it  seems  inexpedient,  in 
the  present  sttite  of  our  knowledge,  to  employ  relative  antiquity  (which 
must  be  determined  by  a  totally  distinct  branch  of  geological  inquiry, 
and  may  be  erroneously  determined)  as  a  basis  of  petrographical  arrange- 
ment.- 

In  the  following  arrangement  the  three-fold  division  first  mentioned 
al)ove  is  adopted,  according  to  the  relative  abundance  of  silica :  Ist  Acid, 
2nd  Intermediate,  3ixl  Bjisic.  In  each  of  these  series  there  is  a  range  of 
structure  from  completely  crystalline  to  completely  glassy.  The  hole- 
crystalline  rocks  are  as  a  rule  the  deep-seated  representatives  of  each 
series,  while  the  vitreous  and  semi-vitreous  are  those  which  have  either 
]»een  erupted  to  the  surface  or  have  been  connected  with  volcanic  rather 
than  plutonic  action.  No  system  of  classification  yet  proixwed  can  avoid 
incongruities,  and  it  must  be  rememl>ered  that  the  hard  and  fast  lines  of  our 
nomenclature  do  not  represent  any  reiilly  abrupt  demarcations  in  nature. 
As  one  rock  graduates  into  another,  our  terminology  should  be  elastic,  so 
as  to  include  such  transitional  forms. 

i.  Acid  Series. 

In  thij*  family  the  silicic  acid  has  been  in  such  excess  as  often  to  so|iarate  out  iu  the 
foiiM  of  flee  quartz.  Sometimes,  as  in  «(ianite,  it  lias  not  assumed  a  definitely  crystal- 
lized form,  but  is  moulded  round  the  other  crystals  as  a  later  stape  of  consulidatioii.  In 
other  iwks  (quartz-i>orphyry,  &e.}  it  occurs  as  a  ]>rcMluct  of  earlier  consolidation,  and 
oftJ'u  assumes  perfect  crystallo^a])hic  contdui-s,  occurrin;^  even  in  double  pyramids 
The  t<*xture  of  the  ro<rks  is  (1;  holwrystalline  or  crystalline-^anular  (granitoid)  SJ 
ty]>ieally  dev«'lojK*d  iu  granite  :  (2)  hemi-crystalline  (ijorphyritic,  tracliytoid),  as  in 
quartz- porphyry  or  felsite  ;  (:3)  vitreous,  as  in  obsidian. 

Granite.''— A  thoroughly  riystalline-gi-anular  admixture  of  quartz,  felspar,  and  mica, 
in  partii'les  of  tolerably  unifonn  size  (Fij^.  15  and  *29;.  Tlie  feUiiar  is  chiefly 
white  or  pink  orthoclase,  but  triclinic  felspars  (oli^joclase  and  albito)  may  often  be 
observed  in  smaller  rpiantity,  frequently  distin^iishable  by  tlieir  fine  striatlon  and 
more  waxy  lustre.  Microcline  is  not  infrequent,  as  well  as  the  interctystallization 
of  orthoclase  and    pla;j;ioelase  (Perthite).      The  niiea  may  Ik?  the  ]»ota.<ili  (muscovite) 


*  *  Minertdogie  Micrographique,'  p.  ITiO. 

"  For  a  t^diulnr  arran^renient  of  the  massive  (eruptive)  rocks  and  critical  remarks  on 
their  classification,  see  Hosenbii.^ch,  Xitus  Johrh.  18S*J,  ii.  ]».  1. 

^  On  the  structure  of  granite,  see  the  manuals  of  Zirkel  and  Rosenbusoh  and  the  niemoin 
there  cited  ;  also  Zirkel's  'Microscoj*.  Petrography,'  187H,  p.  39  ;  Philli{>s,  Q.  J.  Oeol.  Soe. 
XX xi.  p.  3.30 ;  xxxvl.  p.  1.  ,J.  (.'.  Wanl,  op.  eit.  p.  509  ;  an<l  xxxii.  p.  1.  King's 
'Systematic  Geology'  (vol.  i.  oi  Exphn',  AOfh  PonOU'l),  p.  Ill  <»/  st^q.  Michel-Levy,  BhU. 
S,^.  iit'itf.  FrancCy  3nl  ser.  iii.  p.  199.  Rosenbusch,  Xeituc/t.  Ih'iitsc/i.  f'Otf.  (resell,  xxriii. 
(1876),  p.  369.  H.  Mtihl,  yi/t.  MnJi.  Xnt.  xxiii.  p.  1  et  ser/,  J.  Lebmanu,  *  Unter- 
suchungen  ul)er  die  Kiitstehung  der  Altkrystallini.schen  Schiefergesteim*,'  1S84,  p.  8.  W.  J. 
Sollas,  TitiHs,  Ro;i.  Irish  Acad.  xxix.  Part  xiv.  (1S91). 


PART  II  §  vii  MASSrFE  ROCKS— dRAXITE  1S7 

variety,  usually  of  a  white  silvery  aspect,  but  luore  I'uliimaiily  biotite  or  ottiei'  lUrk 
brown  or  blatk  VBiiety.  The  <|iittrti  may  lie  obseived  to  foim  a  kiiiU  of  paste  or 
maf^mii  wrapping  routiil  tlie  other  iDgredients. 

Only  in  cavities  of  the  granite  do  the  coii1]hiu- 

ent  niineraU  occnr  in  iudependent  well-formeil 

crystkla,  and  there  too  the  accessory  luiuerahi 

(heryl,   topaz,    tourmaliue,    garnet,   Ac.)    are 

chiefly  found. 

From  a  microHcopic  examination  of  granite, 

it  was  formerly  inferred  that  the  rock  baa  a 

thoroughly  crystalline  structure,  with  nomega- 

BMpic  gronnd-maaa,   iior  microscopic  base   of 

any  kind  between  the  crystals  or  crystalline 

individuala.    More  recent  and  exhaustive  study 

of  the  subject,  however,   has  led  to  the  con- 

cliuiion  that  though  tiothitig  like  a  vitreous, 

or  even  j>ori>hyritic,  gronud-moss  can  be  de- 
tected, there  is  yet  sometimes  discernible  an     •■'«•  ^■'■— ""''^''J'l'*"'"^'*'™'''"''"  "''''"""'c 

analogous  kind  of  entirely  CTystatline  magma, 

in  which  the  crystals  or  crystalline  debris  of  the  rock  are  embedded,  and  in  which  they 

an  partially  dissolved.     Having  regaiil  to  the  relations  between  this  magma  and  its 

cnclwed  mineials,  M.  Michel- Liivy  has  observed  tliat  microscopic  examination  points  to 
a  distinction  between  granites  in  which  the  iiuartz  is  more  recent  than  the  other  con- 
■titufnta  and  has  consolidated  at  once,  and  those  in  which  there  are  remains  of  earlier 
bi-]iynunidal  quartz.  He  dutinguislies  these  two  si'iics  as  (A)  Ancient  granitei', 
canu{>Mied  of  black  mica,  hornblende,  oligoclase,  and  oi'thoclase,  forming  a  crystalline 
debris  embedded  in  a  more  recent  crystalline  magma  of  oi'thoclase  and  •[uart:',  'Jt) 
Porjiliyroid  granites,  generally  liner  in  grain  than  tlie  |ireccding,  and  furtlier  disCin- 
guiabed  by  the  occurrence  of  bi-|iyramidal  crystaU  of  quartz  (which  made  tbcir  apjiear* 
ance  between  the  old  felspar  and  the  recent  orthoclaxe),  and  of  ■  notal)le  ijnantity  of 
white  mica  (rare  among  the  ancient  granites)  ]«s1erior  in  advent  even  tii  the  more 
Tvcent  quartz. ' 

Amoug  the  coni|Kiucnt  minerals  of  gmnite,  the  qniLitz  jircsents  s^iecial  interest 
under  the  microscope.  It  is  often  found  to  be  (\lll  of  cavities  containing  li<)uid,  some- 
timeH  in  such  numbers  as  to  amount  to  a  thousand  millions  in  a  cubic  inch  and  to  give 
a  milky  turbid  aspect  to  the  mineral.  Tlie  tiqnld  in  tliese  cavities  ajiiiears  usually  to 
be  water  containing  sodium  and  |>otastiiuin  chloriiles.  with  sulphates  of  these  metals  and 
of  calcium  (p.  IIOJ. 

The  mean  of  eleven  analyses  of  granites  made  by  Di'.  Kangbton  gave  the  following 
average  comjiositian  ;  silica,  72-07;  alumina.  U'Hl  :  peroxide  of  iron,  2-22  ;  jiotssh, 
5-11  ;  soda,  2-79:  lime,  rS3  ;  magnesia,  O'SS  ;  loss  by  ignition,  1-09;  total,  10005, 
with  a  mean  speciRc  gisvity  of  2-flti. 

Most  large  masses  of  granite  present  differences  of  texture  in  different  ]iarts  of  their 
area.  Some  of  rhese  variations  depend  on  the  relation  of  the  mass  to  tbe  surrouniling 
rocks  (see  pottra,  p.  505).  Others  may  occur  in  any  ]iortion  of  a  granite  liosii,  and 
have  been  produced  by  the  circumstances  in  which  the  mass  consolidated.  Some 
granites  are  marked  by  the  occurrence  of  the  cavities  alH>ve  referred  to  wbere  the 
individual  minerals  hare  had  room  to  ussume  Hbarjily  delined  ciystolline  forms.  31any 
granites  are  apt  to  be  traversed  by  veins,  soinvlimes  due  to  a  segregatian  of  the 
sarroundiDg  minerals  in  rents  of  tlie  original  )>asty  magma,  sometimes  to  it  jirotiiision 
of  a  less  coarsely  ciystaUine  (micro-granitic,  felsitic)  mateiial  into  the  main  rm-k  (Fig. 
30>,     Some  of  the  more  im[iortant  of  tbesu  varieties  bit  distitigui  shell  by  speiint  names. 

'  BvU.  Soc.  (fill.  Fnince,  3rd  ser.  iti.  [Ii)75},  )>.  I'M. 


vi'  i.'iystallizml  togptLor  so  u  to 
-ii-litutioii  of  their  lolifcvr  BXH  IB 
oiii-  {{eiieral  (lirwtiou,  a»  Qttj 
oivHjiwialty  apt  to  do  iti  itegnfi' 
lioti-vciiM,  the  rock  in  tmud 
IVgTiintiU'.'  Olir  of  tin-  bum 
imiTpatiiiB  Kirucliiml  v»rieti«  b 
till'  I'onii  i)f  ]H-(nuatit«  tenntd 
r>rii)iliU-  Urauite.  in  w-Mch 
thp  uni'ntatiun  at  tliv  i|iiBrti  ud 
fi'lH]*!]'  is  HinEnlarlf  well  devrl- 
«]jv>l  iFig.  3li.  Ttie  >iiiarti  b«t 
aHHinniil  the  shajie  of  long  in- 
]KTfi'tt  cnhiniiiar  HlielU,  plmcnl 
jKirnUcI  t'l  eai'h  other  and  eiieloMil 
uithiii  llie  orthoclaae.  so  that* 
ti-Hiisi-trse  xwtioii  liean  noaif 
iiwiiililuiice  to  Hebrew  MTitiajt 
Tilt' t  nomiiiei'alii  have  ciyatallivl 
tofS'thiT,  H'ith  their  priiiei|ial  am 
jiarutlvl.  This  iiitcrgrowth  tmni 
tn  sliMH-  tliat  there  tould  haxr 
Ih'cii  tilth'  or  no  internal  mon- 
iiieiit  it(  tlie  veini.  ID  which  itn 
fi¥i|ili'iit1y  oci'urs,  when  the  eoU' 
|H>iii'iit  niiiiersht  •ntunieil  thrir 
tryntnllilie  forraB.  Where  the 
iiiter^^wtli  is  oil  a  minute  icak 
it  iskliomi  as  niieroiiegmatite, 
and  it  rtiniiH  the  li«se  of  tlie  rockbt 
u-liiuli  tlie  uiiine  of  Grauoplijre 
finiiiil  of  a  (iranite  bwooiiig 


tiup-gmiiieil.  Ihu  I'liutaiuiii);  Urjjv  Hcuttered  fctH)iar  I'l^-slais.     .Suclt  a  rock  may  liv  tcrmtd 
a  imiriJiijrUic  iiriiiiili:      Soniu  Kraidtes  b>iuiiii'1  id  dicluwil  i-rystalline  concretioui  ot 


'   i\a  an  sJiiiiralile  iind  rxliniistivi'  u 
luiuernLs  iu  Sontlwni  Nurwaj'.  see  the  { 


ot  the  Pcjiiimliti-  vtiuii,  and  their  MMKiated 
ouogniiili  liy  l-rof.  W.  C,  Briigger  in  Gioth'* 


PART  II S  vii  MASHIVE  ROCKS— GRANITE  159 


Vagmeuts.  These  are  sometimes  mere  segre^tions  of  the  materials  of  the  granite,  when 
iiey  are  usually  ovoid  in  form  and  porphyritic  in  structure  ;  in  other  cases,  they  are 
'ragmenta  of  other  rocks,  and  are  then  commonly  schistose  in  structure  and  irregular  in 
bnn.^  In  rare  exam])les  the  component  minerals  of  granite  have  crystallized  with  a 
sdial  concentric  arrangement  into  rounded  ball-like  aggregates  (spheroidal,  orbicular 
granite).  ^  In  the  centre,  as  well  as  round  the  edges  of  large  bosses  of  granite,  the 
oiiierals  occasionally  assume  a  more  or  less  perfectly  schistose  arrangement.  When  this 
akes  place,  the  rock  is  called  gneissose  or  gneiss  granite.     (See  Book  IV.  Part  VII.) 

Differences  in  the  proportions  or  nature  of  the  component  minerals  have  likewise  sug- 
;e8t«d  distinctive  names.  Of  these  the  following  are  the  more  imiK)rtant :  Granitite, 
biotite  granite) — a  mixture  of  pink  orthoclase  and  abundant  oligoclase,  with  a  little 
uartz,  some  blackish  green  magnesia-mica,  and  occasionally  with  hornblende  or  augite. 
lorn  blende -granite— a  rock  with  hornblende  added  to  the  other  normal  constituents 
f  grmnite,  and  usually  poorer  in  quartz  than  normal  granite.  A  well-known  variety 
ccors  at  Syene  in  Upi)er  Egypt,  whence  it  was  obtained  anciently  in  large  blocks  for 
belisks  and  other  architectural  works.  The  well-known  Egyptian  monoliths  are  made 
f  it.  It  was  called  by  Pliny  "Syenite," — a  name  adopted  by  Werner  as  a  general 
esignation  for  homblendic  granites  without  quartz.  The  rock  of  Syene  is  really  a  horn- 
lende-biotite-granite.  Augite-granite — a  variety  in  which  augite  occurs  with  black 
lica.  Tourmaline  granite — a  granitite  with  disseminated  tourmaline.  Greisen 
-a  rare  granitic  rock  from  which  the  felspar  has  disap^^eared,  found  in  some  granite 
jstricts,  especially  in  those  wherein  mineral-veins  occur.  A  p  1  i  t  e — a  fine-grained  mixture 
f  quartz  and  felsi»ar,  which  have  not  infrequently  intergrown  (micropegmatite) ;  found 
specially  in  veins  in  granite.  "  Elvan  "  is  a  Cornish  term  for  a  cr3rstalline-granular 
nixtnre  of  quartz  and  orthoclase,  forming  veins  which  proceed  from  granite,  or  occur 
•nly  in  its  neighbourhood,  and  are  evidently  a^ociated  with  it.^  Under  the  name 
rranulite  M.  Michel -Le>'y  includes  certain  tine -grained  gi-anites  with  white  mica, 
rbich  to  the  naked  eye  appear  to  be  composed  entirely  of  felspar  and  quartz,  or 
f  felnpar  alone,  though  both  mica  and  quartz  aj>pear  in  abundance  when  the  rocks 
jne  microscopically  examined.  He  includes  in  this  category  most  of  the  rocks  of  the 
UpB  deserib^  as  **  protogine." 

Surrounding  large  masses  of  granite  there  are  usiuilly  numerous  veins,  which  consist 
»f  granite,  quartz -porphyry,  felsite,  or  sometimes  even  sphenilitic  material  (Mull). 
rhere  can  be  no  doubt  that  these  finer-grained  protrusions  really  proceed  from  the  ciys- 
alline  granite  mass.  Lessen  has  shown  that  the  Bode  vein  in  the  Harz  has  a  granitoid 
entiVy  with  compact  por]>hyry  sides,  in  which  he  found  with  the  micrascoiH'  a  tnie  gla.s.sy 
laae.^  Sometimes  the  rocks  associated  in  this  way  with  granite  differ  in  composition  from 
he  main  granitei  Tourmaline  is  one  of  the  characteristic  mhierals  of  gianite-veins, 
hou^  lev  observable  in  the  main  body  of  the  rock  ;  with  quartz,  it  forms  Schorl-rock. 

'  Gimnite  weathers  chiefly  by  the  decay  of  its  felsjMirs.  These  are  converted  into 
laolin,  the  miea  becomes  yellow  and  soft,  while  the  ({uartz  stands  out  scarcely  affected. 
rhe  granite  of  the  south-west  of  England  has  weathered  to  a  depth  of  50  feet  and 
tpwards,  so  that  it  can  be  dug  out  with  a  si>ade,  and  is  largely  us(>d  as  a  source  of 
lonselain-elay. 

<^6ranite  ooonrs  (1)  as  an  eruptive  rock,  forming  huge  bosses,  which  rise  through  other 
ormatious  both  stratified  and  unstratified,  and  sending  out  veins  into  the  surrounding' 
nd  overlying  rocks,  which  usually  show  evidence  of  much  alteration  as  they  apjironch 

»  J.  A.  PhiUipe,  Q.  J,  0(4)1,  Soc,  xxxvi.  (1880),  p.  1. 

*  W.  C.  Brogger  and  H.  Backatrom,  Oei>i.  StockJudm  FUrhandl.  ix.  (1887),  p.  307.  Hatch. 
>Hart,  Journ.  OtU.  Soc.  xliv.  (1888),  p.  548,  and  authorities  there  citetl. 

»  J.  A.  Phillips,  Q,  J.  Gtol.  ^Soc.  xxxi.  p.  334.  Michel-Levy,  BvU.  Soc.  GitA.  Fro.na\ 
i.  Srd  ser.  p.  201. 

«  ZcUseh,  Deuiseh,  Oeol,  Ges,  zxvL  (1874),  p.  856. 


1 60  GEOGXOS  Y  book  n 

the  granite;  (2)  connected  witli  tnie  volcanic  rocks  (as  in  the  Tertiary  granophyret  of 
Mull  and  Skye),  and  forming,  ]>erha|)8,  the  lower  yiortions  of  masses  which  flowed  ou 
at  the  surface  as  lavas,  (jlranite  is  thus  a  decidedly  phUonlc  rock  ;  that  U,  it  hM 
consolidated  at  some  depth  l)eneath  the  surface,  and  in  this  resiiect  difl'erB  from  the 
suiieriicial  vulcanic  rocks,  such  as  lavas,  which  have  flowed  out  above  ground  fna 
volcanic  oiifiees. 

Quartz-Porphyry  (Microgi-anite,  Eurite).* — A  tine-grained  microgranitic  groimd- 
mass,  com]iosed  mainly  of  fels^iar  and  quartz,  through  which  are  usually  scattered  coi' 
si>icuous  i)orphyritic  crystals  of  one  or  other  or  l»oth  of  the  same  minerals. 

To  the  naked  eye  the  ground-mass  varies  from  an  exceedingly  compact  texture  toose 
where  ahundant  minute  crystals  can  be  detected.  Of  the  por]>hyritic  constituents  Xht 
({uartz  occasionally  occurs  in  bi-pyramidal  crystals :  the  felsiiar  is  usually  ortbodiM^ 
while  black  mica  occasionally  ap] Heat's.  Under  the  microscojK?  the  stnicture  of  the  rock 
is  found  to  be  microgranitic,  with  frequently  a  micropegmatitic  arrangement  of  the 
quartz  and  felsjjar  (gi-anophyre). 

The  flesh -red  (|uartz-jK)rj>hyry  of  Dobritz,  near  Meissen,  in  Saxony,  was  found  fcy 
Reutzsch  to  have  the  following  chemical  composition  :  Silica,  76«92  ;  alumina,  12*89; 
]M>tash,  4-27  ;  soda,  0-68;  lime,  O-eJS  ;  magnesia,  0-t)8  ;  oxide  of  iron,  1-15;  watff, 
1*97  ;  total,  99'54, — specific  gravity,  2«49. 

The  coloui-s  of  the  rock  dei>end  chiefly  upon  those  of  the  fels|iar, — \m\e  flesh-Ewit 
reddish-brown,  purple,  yellow,  bluish  or  slate-grey,  ]wissing  into  white,  being  in  difiv* 
ent  places  characteristic.  It  will  be  observed  in  this,  as  in  other  rocks  containing  mwh 
felsiMir,  that  the  colour,  besi<les  deiicnding  on  the  hue  of  that  mineral,  is  greatly  re- 
lated by  the  nature  and  stage  of  decomiK>sition.  A  rock,  weathering  externally  with  a 
pale  yellow  or  white  cnist,  may  be  found  to  be  dark  in  the  central  undecayed  portioiL 
AVhen  the  base  is  very  com[»act,  and  the  felspar-ciystals  well-defined  and  of  a  diflferat 
colour  from  the  base,  the  nx'k,  as  it  takes  a  good  ]K)lish,  may  be  used  with  effect  sin 
ornamental  stone.  In  popular  language,  such  a  stone  is  classed  with  the  ''marbkt,* 
under  the  name  of  "i»orphyry." 

The  Quartz-porj»h3'ries  occur  (1)  with  j>lutonic  rocks,  as  em ptive  bosses  or  veili, 
often  associated  with  gi-anite,  from  which,  indeed,  they  may  be  seen  to  proceed  directly ;  of 
freijuent  occurrence  also  as  veins  and  iiTcgularly  intrudetl  masses  among  highly  conTOl- 
uted  roi'ks,  ej*i)ecially  when  the^se  have  been  more  or  less  metamorphosed  :  (2)  in  the 
chimneys  of  old  volcanic  orifices,  forming  there  the  **  neck"  or  plug  by  which  a  ventii 
filled  up  ;  and  (3)  as  bosses  sometimes  of  large  size  which  have  been  protruded  in  oonnw* 
tion  with  volcanic  action.  Between  the  gi-anoj»hyres  which  are  charaetcriaed  by  a  micro> 
]>egmatitic  structure  and  the  felsites  or  ancient  rhyolites  there  is  a  close  relatiflB. 
4,Hiartz-iK>rphyries  are  abundant  in  Britain  among  formations  of  Lower  Silurian,  Old 
Red  Sandstone  and  Lower  Carboniferous  age.  In  the  Inner  Hebrides  they  occur  in  laigl 
bosses  or  domes  (gianophyre)  rising  through  the  older  Teitiary  basaltic  plateau. 

Many  of  the  rocks  called  '*  quartz-porphyry  "  are  not  mici'ogranitio  but  have  tfai 
*"felsitic"  structure  arising  from  the  devitrification  of  ancient  forms  of  rhyolito 
<sce  i».  161). 

Bhyolite''^  (Li|»aritc,  Quartz-trachyte)— a  rock  having  a  com^iact  liale-grey,  yellowith, 
gieenish  or  reddish  ground  -  mass,  sometimes  with  glassy  patches  and  layers  oftn 
showing  j>erfect  fl(>w-structure,  not  infrequently  also  with  spherulitio  and  peiiitk 
stnictures,  and  with  ciystals  of  orthoclase  (sanidine),  granules  of  quartz  and  minute 
ciystals  of  black  mica,  augite,  more  rarely  hornblende.  Considerable  diversity  ezisti  ii 
the  texture  of  the  rock.  Frequently  it  is  finely  cavernous,  the  cavities  being  lined 
with  chalcedony,  (juartz,  amethyst,  jasjier,  &c.     Some  varieties  are  coarse  and  gnmitoid 

^  Zirkel,  *  Microscop.  Petrog.'  p.  71.     See  particularly  Koseubusch,  '3iik.  Phy8/iLp.50. 
-  On  rliyolite  see  Richthofen,  Jahdt.  K.  K.  UcoL  Heichsanst,  zi.  156.     Zirkel,  *Mienk 
Petrog.*  p.  163.     King,  *Exi)lor.  40th  Parallel,'  vol.  i.  p.  606. 


PART  u  §  vii  MASSIVE  ROCKS—RHYOLITE  161 


in  character.  Intermediate  varieties  may  be  obtained  like  the  quartz-[)ori)liyries,  and 
these  pass  by  degrees  into  more  or  less  distinctly  vitreous  rocks.  Throughout  these 
gradations,  however,  which  doubtless  represent  different  stages  in  the  crystallization  of 
an  original  molten  glass,  a  characteristic  ground -mass  can  be  seen  under  the  micro8co]>o 
having  a  glassy,  enamel-like,  |)orcellaneous,  microlitic  character,  with  characteristic 
spherolitic  and  fluxion  structures.  In  the  ([uartz,  glass-inclusions,  having  a  dihexaliedral 
form,  may  often  be  detected  ;  but  liquid  inclusions  are  absent.  An  analysts  by  Vom 
Rath  of  a  rhyolite  from  the  Euganean  Hills  gave — silica,  76*03  ;  alumina,  13*32  ;  soda, 
5*29;  potash,  3*83;  protoxide  of  iron,  1*74;  magnesia,  0*30;  lime,  0*85  ;  loss,  0-32: 
total,  101*68,— specific  gravity,  2*553. 

The  perlitic  structure  is  so  characteristic  of  this  rock  that  the  varieties  which  specially 
exhibit  it  were  formerly  regarded  as  a  distinct  rock-species  under  the  name  of  Perlite  or 
FearfsUme.  As  the  name  indicates,  the  structure  presents  enamel -like  or  Wtreous 
globules  which,  occasionally  assuming  polygonal  forms  by  mutual  pressure,  sometimes 
constitute  the  entire  rock,  their  outer  portions  shading  off  into  each  other,  so  as  to  form 
a  compact  mass  ;  in  other  cases,  se[)arated  by  and  cemented  in  a  compact  glass  or 
enameL  They  consist  of  successive  very  thin  shells,  which,  in  a  transverse  section,  arc 
seen  as  coiled  or  spiral  rings,  usually  full  of  the  same  kind  of  hair-like  crystallites  and 
ciyataUi  as  in  the  more  glassy  i)arts  of  the  rhyolite  (Fig.  9).  As  these  bodies  both 
singly  and  in  fluxion-streams  traverse  the  globules,  the  latter  may  be  regarded  as  a 
structure  develo[)ed  by  contraction  in  the  rock,  during  its  consolidation,  analogous  to 
the  concentric  spheroidal  structure  seen  in  weathered  basalt  (Fig.  94).  Among  these 
concentrically  laminated  globules  true  spherulites  occur,  distinguished  by  their  internal 
radiating  fibrous  structure  (Figs.  7,  17). 

Rhyolite  is  an  acid  rock  of  volcanic  origin.  It  forms  enormous  masses  in  the  heart 
of  extinct  volcanic  districts  in  Eui-o^ie  (Hungary,  Euganean  Hills,  Iceland,  Li])ari), 
and  in  Xorth  America  (Wyoming,  Utah,  Idaho,  Oregon,  California). 

Nevadite — a  variety  of  rhyolite  named  by  Richthofen  from  its  development  in 
Nevada,  and  characterised  by  its  resemblance  to  granite,  owing  to  the  abundance  of  its 
porphyritic  crystals,  and  the  relatively  small  amount  of  ground-mass  in  which  they  are 
imbedded.  The  granitoid  as^iect  is  external  only,  as  the  ground-mass  is  distinct,  and 
varies  from  a  holocrystalline  character  to  one  with  abundant  glass,  and  the  texture 
ranges  from  dense  to  [lorous.^ 

Felsite  (Felstone). — Under  this  name  a  large  series  of  rocks  has  been  grouiM?d  which 
appear  for  the  most  i>art  to  have  been  originally  %itreous  lavas  like  the  rhyolites,  but 
which  have  undergone  complete  devitrification,  though  frec^uently  retaining  the  |)erlitic, 
spherulitic,  and  flow-structures.  They  varj'  in  colour  from  nearly  white  through  shades  of 
grey,  blue  and  red  or  brown  to  nearly  black,  often  weathering  with  a  white  crust. 
They  are  cloee-graine<l  in  texture,  often  breaking  with  a  sub-conchoidal  fracture  and 
showing  translucent  edges.  Por])hyritic  fels|)ars  (lx)th  orthoclase  and  plagioclase)  and 
blebs  of  quartz  are  of  freijuent  occurrence.  The  flow -structure  is  occasionally  strongly 
marked  l^  bands  of  different  colour  and  texture,  sometimes  curiously  bent  and  curled  over, 
indicating  the  direction  of  movement  of  the  still  unconsolidated  rock.  The  sphemlitic 
structure  also  may  be  found  so  strongly  marked  that  the  individual  spherules  measure  an 
inch  or  more  in  diameter,  so  that  the  rock  seems  comix)sed  of  an  aggregate  of  balls, 
and  was  formerly  mistaken  for  a  conglomerate  {Pyrom^ride).'   Under  the  niicroscoi>e  many 

*  Hague  and  hidings,  Avier.  Journ.  ScL  xxvii,  (1884),  p.  461.  These  authors  dis- 
tinguish between  Nevadite  and  Liparite,  the  latter  being  characterised  by  the  small 
number  of  porphyritic  crystals  imbedded  in  a  relatively  large  amount  of  ground-inass, 
which,  as  in  Nevadite,  may  be  holocrystalline  or  glassy.  Tliey  also  distinguish  LithoUfa! 
Rhyolite  and  Hyaline  RhydUe  as  ad<litional  varieties. 

'  On  nodular  feldtes  see  G.  Cole,  Quart.  Juvni.  O'enl.  Soc.  xli.  (1885),  p.  162  ;  xlii.  p. 
183  ;  Hiss  Raisin,  op,  dt.  xlv.  (1889),  p.  247.     Harker  ''  Bala  Volcanic  Rocks,"  1889,  p.  28. 

M 


1 62  GEOGNOS  Y  book  u 

of  the  typical  structures  of  rhyolite  can  be  detected  in  felsit^.  The  grouud-inass  of  tlie« 
rocks  has  given  rise  to  much  discussion,  but  it  is  now  generally  recognised  as  a  more  or  leai 
altered  condition  of  the  dev-itritication  of  an  original  vitreous  mass  (p.  117).  Secondiiy 
changes  have  in  large  measure  destroyed  the  original  microlitic  structure,  but  traces  of  it 
can  often  I>e  found  while  the  spherulitic  and  ])crlitic  forms  frequently  remaiu  almost  u 
fresh  as  in  a  recent  rock.  Felsites  with  a  large  proportion  of  alkalies,  especially  soda, 
have  been  called  Keratophyrcs.^ 

Felsites  have  Iteeu  found  abundantly  as  intcrbedded  lavas  with  tufis  and  a^lomei^ 
ates  associated  with  Silurian  and  older  rocks  in  Wales  and  Shropshire.'  Soda- felsites  or 
kerat-oi>hyrcs  have  l>een  found  to  play  a  considerable  |)art  among  the  materials  erupted 
by  the  Lower  Silurian  volcanoes  of  the  south-east  of  Ireland.' 

The  vitreous  acid  rocks  form  an  interesting  group  in  which  wc  may  still  detect  wlist 
was  probably  the  original  condition  of  at  least  the  rhyolites  and  felsites.     Every  grads- 
tion  can  l)e  traced  from  a  ))erfect  glass  into  a  thoroughly  devitritied  and  even  crystalline 
rock.     As  ah'cady  remarked,  the  original  vitreous  condition  of  rhyolite  can  still  be  seen 
even  with  the  naked  eye  in  the  clots  and  streaks  of  glass  that  occasionally  run  throng 
it  in  the  direction  of  its  ilow-structure.     Various  names  have  been  given  to  the  gUsiy 
rocks,  of  which  tlie  chief  are  obsidian.  ]iitchstone  and  pumice.     Tliese,  however,  are  noC 
to  be  regarded  as  distinct  nK>k-s[)ecies  but  rather  as  the  glassy  condition  of  diflereat 
lavas. 

Obsidian  (rhyolite-glass) — the  most  i»erfect  form  of  volcanic  glass,  externally  resem- 
bling  bottle  glass,  having  a  jierfect  conch oidal  fracture,  and  breaking  into  sharp  splinten^ 
transparent  at  the  edgcts.  Its  colours  are  black,  brown,  or  greyish-green,  rarely  yellow, 
blue,  or  red,  but  not  infrequently  streaked  or  banded  with  jmlcr  and  darker  hues.  A  tfain 
slice  of  obsidian  pre])arc(l  for  the  microsco|)e  is  found  to  be  very  pale  yellow,  brown, 
grey,  or  nearly  colourless,  and  on  being  magnified  shows  that  the  usual  dark  colours  an 
almost  always  produced  by  the  ] presence  of  mi nut«  oj)aque  crystallites,  which  present 
themselves  as  black  oi)aque  trichites,  sometimes  beautifully  arranged  in  eddy-like  lines 
showing  the  original  fluid  movement  of  the  rock  (Fig.  19)  ;  also  as  rod-like  transparent 
microlites.  They  occasionally  so  increase  in  abundance  as  to  make  the  rock  lose  the 
aspe<2t  of  a  glass  and  assume  that  of  a  dull  flint-like  or  enamel-like  stone.  This  devitri- 
fication can  only  l)e  properly  studieil  with  the  microscope.  Again,  dull  grey  enamel- 
like  s])lierulites  api»ear  in  some  parts  of  the  rock  in  great  abundance,  drawn  out 
into  layers  so  as  to  give  the  rock  a  tissile  structure,  while  steam-  or  gas-cavities  likewise 
occur,  sometimes  so  large  and  abundant  as  to  impart  a  C(>.llular  as])cct.  Tlie  occurrence 
of  abundant  sanidine  cr^-stals  gives  nse  to  Porphyritic  Obsidian.  Many  obsidians,  from 
the  increase  in  the  nuniljer  of  their  steam -vesicles,  pass  into  }mmice.  Now  and  then, 
the  st4;ani-porcs  are  found  in  enonnous  nimil)ers,  of  extremely  minute  size,  sa  in 
an  obsidian  from  Iceland,  a  plane  of  which,  about  one  st^uare  millimetre  in  sin, 
has  been  estimated  to  include  800,000  iK)res.  The  average  chemical  composition  of 
obsidian  is — silica,  71-0  ;  alumina,  13-S  ;  i>otash,  4*0  ;  soda,  5'2  ;  lime,  1-1  :  magnesis, 
0*6  :  oxides  of  iron  and  manganese,  8*7  :  loss,  0*6  (little  or  no  water).  Mean  speciiic 
gravity,  2*40.  Obsidian  occurs  as  a  product  of  the  volcanoes  of  late  geological  perioda 
It  is  found  in  Li)»ari,  Iceland,  and  Teneriifc  ;  in  North  America,  it  has  been  erupted 

^  Giimbel,  'Palaeolit.  Eniptivgest.  Fichtelgebirg. '  (1874),  p.  43.  Rosanbnsch,  'Mlkros- 
kop.  Physiog.'ii.  434. 

''^  Mr.  Alli)ort  descri1>ed  some  ancient  forms  of  perlitic  structure  from  Shropshire,  in 
what  were  probably  once  ordinary  rhyolites,  Q.  J,  Gcoi.  «S(t.  xxxiil.  p.  449  ;  and  Mr.  Rntky 
showed  the  presence  of  the  same  structure  among  the  Lower  Silurian  lavas  of  North  Wales. 
Op.  cU.  XXXV.  p.  508. 

'  F.  H.  Hatch,  Man.  Geol.  Surv.  Ireland,  Explanation  of  Sheet  130  ;  Oeol.  Mag,  1889, 
p.  70. 


PART  u  §  vii  MASSIVE  ROCKS— SYENITE  163 

from  many  points  among  the  Western  Territories ;  ^  it  is  met  with  also  in  New 
Zealand. 

Pitchstoneis  a  name  given  to  the  less  perfectly  glassy  acid  rocks,  which  are  distin- 
guished by  a  resinous  or  pitch-like  lustre,  and  internally  by  a  more  advanced  development 
of  microlites  than  in  obsidian.  They  thus  represent  a  further  stage  of  devitrification. 
These  rocks  are  easily  frangible,  breaking  with  a  somewhat  splintery  fracture,  translucent 
on  thin  edges,  with  usually  a  black  or  dark  green  colour,  that  ranges  through  shades  of 
green,  brown,  and  yellow  to  nearly  white.  Examined  microscopically,  they  are  found 
to  consist  of  glass  in  which  are  diffused  hair-like,  feathery  and  rod-shaped  microlites, 
or  more  definitely  formed  crystals  of  orthoclase,  plagioclase,  quartz,  hornblende,  augite, 
magnetite,  &c  The  pitchstone  of  Corriegills,  in  the  island  of  Arran,  [)resents  abundant 
green,  feathery,  and  dendritic  microlites  of  hornblende  (Fig.  14).^  Occasionally,  as  in 
Airan,  pitchstone  assumes  a  spherulitic  or  perlitic  structure.  Sometimes  it  becomes 
porphyritic,  by  the  development  of  abundant  sanidine  crystals  (Isle  of  £igg). 

Pitchstone  is  found  as  (1)  intrusive  dykes,  veins,  or  bosses,  probably  in  close  con- 
nection with  former  volcanic  activity,  as  in  the  case  of  the  dykes,  which  in  Arran 
trmyerse  Lower  Carboniferous  rocks,  but  are  probably  of  Miocene  age,  and  those  which 
in  Meissen  send  veins  through  and  overspread  the  younger  Palfpozoic  felsite-porphyries  : 
(2)  sheets  which  have  flowed  at  the  surface,  as  in  the  remarkable  mass  forming  the 
Soair  of  Eigg,  which  has  filled  up  a  river-channel  of  older  Tertiary  age.' 

Pumice  (Ponce,  Bimstein) — a  general  term  for  the  loose,  8i)ongy,  cellular,  filamentous 
or  froth-like  parts  of  lavas.  So  distinctive  is  this  structure,  that  the  term  pumiccotis 
has  oome  into  general  use  to  describe  it.  There  can  I)e  no  doubt  that  this  froth-like 
rock  owes  its  peculiarity  to  the  abundant  esca[)e  of  steam  or  gas  through  its  mass  while 
still  in  a  state  of  fusion.  The  most  perfect  forms  of  pumic«  are  fouud  among  the  acid 
lavas,  but  this  type  of  rock  may  be  met  with  in  the  other  groups.  Microscopic 
examination  of  a  rhyolitic  pumice  reveals  a  glass  crowded  with  enormous  numbers 
of  minute  gas-  or  vapour-cavities,  usually  drawn  out  in  one  direction,  also  abundant 
crystallites  like  those  of  obsidian.  Owing  to  its  porous  nature,  pumice  possesses  great 
buoyancy  and  readily  floats  on  water,  drifting  on  the  ocean  to  distances  of  many 
hundreds  of  miles  from  land,  until  the  cells  are  gradually  filled  with  water,  when 
the  floating  masses  sink  to  the  bottom.^  Abundant  rounded  blocks  of  pumice  were 
dredged  up  by  the  Challenger  from  the  floor  of  the  Atlantic  and  Pacific  Oceans. 

ii.  Intermediate  Series. 

In  this  series,  the  average  percentage  of  silica  is  considerably  less  than  in  the  acid 
series  (56-66  per  cent).  Free  quartz  is  not  found  as  a  marked  constituent,  although 
occasionally  it  occurs  in  some  quantity,  as  microscopic  examination  has  shown  in  the 
case  even  of  some  rocks  where  the  mineral  was  fonnerly  believed  to  l)e  absent.  A  range 
of  structure  is  displayed  similar  to  that  in  the  acid  series.  The  thoroughly  cr^'stalline 
▼arieties  are  typified  by  syenite  (and  diorite),  rei)resenting  the  granites  of  the  acid 
rocks,  those  which  possess  a  porphyritic  ground-mass  by  orthoclase- porphyry,  trachyte, 
and  andesite,  answering  to  quartz-porjjhyry  and  rhyolite,  while  the  vitreous  condition 
is  represented  among  ^e  trachytes  and  andcsites  by  dark  glasses  of  the  obsidian  and 
pitchstone  types. 

8j«dt«. — ^This  name,  fonnerly  given  in  England  to  agranite  with  hornblende  replacing 

'  For  an  account  of  the  obsidian  of  the  Yellowstone  Park  see  J.  P.  Iddings, 
Ith  Rept.U,  S.  OeoL  Surv.  (1885-86),  p.  255  ;  consult  also  Zirkel,  *Microscop.  Petrog.' 

-  See  F.  A.  Gooch,  Min.  MUtheU.  1876,  p.  185.     Allport,  Qeol,  Mag.  1881,  p.  438. 

>  Quart.  Joum,  GeoL  Soe,  (1871),  p.  303. 

*  On  porosity,  hydration,  and  flotation  of  pumice,  see  Bischof,  '  Chem.  und  Phys.  Geol.' 
sappL  (1871),  p.  177. 


1 64  GEOGNOS  Y  booe  U 

mica,  is  now  restricted  to  a  rock  consisting  essentially  of  a  lioloerystalliiie  mixture  of 
orthoclase  and  hornblende,  to  which  plagioclase,  biotite,  angite,  magnetite,  or  quartz  msj 
be  added.  As  already  mentioned,  the  word,  first  used  by  Pliny  in  reference  to  the  rock 
of  Syene,  was  introduced  by  Werner  as  a  scientific  designation.  It  was  applied  b^'  liin 
to  the  rock  of  the  Plauenscher-Grund,  Dresden  ;  lie  afteni'ards,  however,  made  thit 
rock  a  greenstone.  The  base  of  all  syenites,  like  that  of  granites,  is  thoroughly  cryatal- 
line,  without  an  amorphous  ground-mass.  The  typical  syenite  of  the  Plauenacher-Grond, 
fonnerly  described  as  a  coarse-grained  mixture  of  flesh-coloured  orthoclase  and  bhek 
hornblende,  containing  no  quartz,  and  with  no  indication  of  i)lagioclase,  was  regarded 
as  a  nonnal  orthoclase-hornblende  rock.  Microscopical  research  has,  however,  shovi 
that  well-striated  triclinic  fels[>ar8,  as  well  as  quartz,  occur  in  it.  Its  composition  is  ^— 
silica,  59-83  ;  alumina,  16-85  ;  protoxide  of  iron,  7-01  ;  lime.  4-43  ;  mai^esia,  2-61 ; 
l>otash,  6-57;  soda,  2-44;  water,  etc.,  1-29:  total,  101-03.  Average  siiecific  gravity, 
2-75  to  2-90. 

Syenite  is  of  much  less  fre<iuent  occurrence  than  granite.     While  always  thorouji^ilj 
granitic  in  structure,  it  varies  in  texture  from  coarse  gi-anular,  where  the  individinl 
minerals  can   readil}'  be  distinguished   by  the  naked  eye,  to  conqiact.      Among  iti 
accessory  minerals  of  common  occurrence  may  be  mentioned  titanite  (spheue),  quarts 
ajmtite,  e[>idotc,  orthite,  magnetite,  pyiite,  zii*con.     The  pit*rlominance  of  one  or  men 
of  the  ingredients  has  given  i-ise  to  the?  soimration  of  a  few  varieties  under  distinetin 
names.     In  the  typical  syenite,  the  dark  silicate  is  almost  wholly  hornblende  ;  some- 
times there  are  to  be  found  traces  of  augite  within  the  hornblende,  indicating  that  the 
former  mineral  was  the  original  constituent  and  has  been  changed  by  paramorphiaB. 
Where  the  ferro-magnesian  silicate  is  mainly  augite,  as  in  the  well-known  rock  of 
Monzoni,  the  r(K'k  is  tcnned  Augite- syenite  or  Monzonite  ;   where  brown  mioi 
preilominates  it  gives  rise  to  Mica -syenite  or  Minettc. 

Elaeolite-syenite  {XepheUac-sitf.nU^^  is  a  granitoid  rock,  characterised  by  tiie 
association  of  the  variety  of  uepheline  known  as  elaeolite  with  ortluKrlase,  and  vitk 
minor  proiwrtions  of  plagiocla-ne,  microcline,  hornblende,  augite,  biotite,  sodalite,  zirecai, 
and  spliene.  It  is  distinguished  by  the  rare  mineral.s,  upwards  of  fifty  in  number,  which 
it  contains,  and  in  which  some  of  the  rarer  elements  are  combined,  such  as  thorimn, 
yttrium,  cerium,  lanthanum,  tantalum,  niobium,  zirconium,  &c.  It  is  typically  de- 
veloi)ed  in  Southern  Xonvay  (Hrevig,  Laurvig).  Where  zircon  enters  as  an  abundant 
constituent  the  rock  is  known  as  Zircon-syenite.  Foyaite  is  the  name  given  to 
a  hornblendic  variety  found  at  Mount  Foya,  Portugal  :  Miascite  is  a  variety  nith 
abundant  mica,  found  at  Miusk  ;  Ditroite,  containing  sodalite,  spinel,  etc.,  OGcnnat 
Ditro  in  Tran.sylvania. 

Orthoclase -Porphyry  (Micro-syenite.  Quartzlcss- porphyry,  Orthophyre)  stands  to 
the  syenites  in  the  same  relation  that  quartz-ix)rphyry  or  micro-granite  does  to  the 
granites.  It  is  comiM)sed  of  a  comi>a(;t  micro-granitic  ground-mass,  with  little  or  no 
free  quailz,  but  through  which  arc  usually  scattered  numerous  cr^'stals  of  orthoclase^ 
sometimes  also  a  triclinic  fels[>ar,  black  hornblende  and  glancing  scales  of  dark  Inotite. 
It  contains  from  55  to  G5  i»er  cent  of  silica,  thus  dittering  from  quartz-porphyiy  and 
felsite  in  its  .smaller  proportion  of  this  acid.  It  is  also  rather  more  easily  scratched  with 
the  knife,  but  exce])t  by  chemical  or  microscopical  analy.sis,  it  is  often  impossible  to 
draw  a  distinction  between  this  rock  and  its  etpiivalents  in  the  acid  series. 

Orthoclase-iK)ii)hyry  occurs  in  veins,  dykes,  and  intrusive  sheets.  Probably  many 
so-called  *'  felstoncs,"  whether  occurrmg  as  lavas  or  as  intrusive  masses  among  the  oldu* 
Pala'ozoic  formations,  are  i-eally  orthoclase-iwrphyries.  Some  highly  micaceous  varieties 
have  l»een  calle<l  Mica -trap — a  vague  term  under  which  have  also  been  included 
Alinettcs,  Micaceous  Quartz -i)orphyries,  &c.  The  name  Lamprophyre,  originally 
given  by  Ci!iiml»el  to  some  mica-traps  from  the  Fichtelgebirge,  has  been  proposed  by 
Kosenbusch  as  a  general  term  for  the  Mica-traps,  divisible  into  two  groups — the  Ortho- 
clastic,  or  syenitic.  where  the  felspar  is  orthoclase  (Miuettes).  and  the  Plagioclastic  or 


PAKT  u  §  vii  MA  SSIVE  ROCKS—  DIORITE  1 6  5 

dioritic,  where  the  felspar  is  a  plagioclase  variety  (Kersantites  ).^  The  Umproph3rres 
occur  abundantly  as  dykes  or  veins  of  a  fine-grained  texture,  and  dull  reddish  to 
brownish  colour,  among  the  older  Palaeozoic  rocks  of  Britain.^ 

The  orthoclase-porphyry  of  Pieve  in  the  Vicentin  was  found  by  Von  Lasaulx  to  have 
the  following  composition  : — silica,  61-07  ;  alumina,  18'56  ;  peroxide.s  of  iron  and 
manganese,  2*60;  potash,  6*83;  soda,  3-18;  lime,  2-86;  magnesia,  1'18 ;  carbonic 
acid,  1-36  ;  loss,  2-13— specific  gravity,  2-59.* 

Diorlte.^ — Under  this  name  is  comprehended  a  group  of  rocks,  which,  possessing  a 
granitic  structure,  differ  from  the  granit'CS  in  their  much  smaller  percentage  of  silica,  and 
from  the  syenites  in  containing  plagioclase  instead  of  orthoclase  as  their  chief  con- 
stituent. They  are  sometimes  divided  into  two  sections,  the  quartz-diorites  and  the 
nonnal  diorites.  Many  of  these  rocks  were  formerly  included  in  the  general  division  of 
"Greenstones." 

Quartz-diorite — a  holocrystalline  mixture  of  plagioclase  (oligoclase,  less  fi*equently 
labrmdorite)  and  quartz  ^^-ith  some  hornblende,  augite,  or  mica.  It  outwardly  resembles 
grey  granite,  and,  indeed,  includes  many  so-called  granites.  Its  silica  ranges  up  to  67 
per  cent.  In  normal  Diorite,  quartz  is  almost  entirely  absent;  hornblende  and 
black  mica  occur  together  in  some  varieties,  while  pyroxene  characterises  others. 
Under  the  microscope  a  thoroughly  crystalline  structure  is  seen,  and  among  the  pyroxene- 
dioritea  the  felspar  and  pyroxene  are  sometimes  intergrowu  in  o])hitie  aggregates.  The 
average  chemical  composition  of  quartzless  diorite  is :  silica,  54  ;  alumina,  16-18  ; 
potash,  1  •5-2*5  ;  soda,  2-3  ;  lime,  6-7-5  ;  magnesia,  6-0  ;  oxides  of  iron  and  manganese, 
10-14  ;  mean  specific  gravity,  about  2-95. 

Among  the  varieties  of  diorite,  the  following  may  be  mentioned.  Corsite  (from 
Corsica) — a  granitoid  mixture  of  greyish-white  plagioclase,  blackish-green  hornblende, 
and  some  quartz,  which  have  grouped  themselves  into  globular  aggregations  with  an 
internal  radial  and  concentric  structure  (Orbicular  diorite,  Kugeldiorit,Napoleon- 
ite — Fig.  8).  Tonal ite'(from  Monte  Tonale,  Tyrol) — a  variety  containing  quartz,  horn- 
blende, and  biotite  in  strongly  contrasted  colours.  Ej»i(liorite — a  name  given  to  ancient 
rocks  which  have  originally  been  pyroxenic  eruptive  masses,  but,  by  mctamor})hism,  have 
acquired  a  crystalline  re-arrangement  of  their  constituents,  the  pyroxene  being  changed 
into  hornblende,  often  fibrous  or  actinolitic,  the  felsjiar  becoming  granular,  and  the  whole 
rock  having  acquired  a  more  or  less  distinct  schistose  structure.  The  dark  intrusive  sheets 
aattociated  with  the  crystalline  schists  of  the  Scottish  Highlands  and  the  north  of  Ire- 
land are  largely  epidiorites.  Some  of  these  rocks  are  rjuartziferous,  but  many  of  them 
belong  to  the  basic  series  (see  p.  627). 

As  the  granites  pass  into  fine  grained  quartz-] porphyries,  and  the  syenites  into  conii»aot 
orthoclase -porphyries,  so  the  diorites  have  their  close -textured  varieties,  which  are 


'  The  typical  locality  for  these  rocks  is  Kersanton  in  Brittany,  where  they  are  dark -green 
and  remarkably  durable.  A  singular  vein  of  kersantite,  3  to  6^  feet  broad,  has  been  traced 
for  nearly  five  miles  in  the  Harz.  Lossen,  Zeitsch.  Deutsch.  Ge^tl.  Oes.  xxxii.  (1880),  p.  445. 
Jakrh,  Preuu.  Oeol.  Landuanst.  1880.  A.  von  Groddeck,  ojk  at.  1882.  M.  Koch,  np.  cit. 
1886.     BarroiB,  Assoc.  Fmnfaise  (1880),  p.  561  ;  Ann.  Stn:.  OM.  Nortl,  xiv.  (1886),  p.  31. 

*  For  an  account  of  the  Lamprophyres  of  the  classical  district  of  the  Plauen8cher-Grun<l, 
•ee  B.  Don,  TscAennak*s  Mineral  MiUheiL  xi.  (1889). 

*  Zeitseh.  Deutsch.  Oeol.  Ues.  xxv.  p.  320. 

*  On  diorite,  its  structure  and  geological  relations,  consult  the  memoir  on  Belgian 
platonic  rocks  by  De  la  Vallee  Poussiu  and  A.  Renard,  Mim.  Acad.  Royale  Behj.  1876  ; 
Behrens,  Neues  Jahrb,  Min.  1871,  p.  460;  Zirkel,  'Microscopical  Petrog.'  p.  83.  J.  A. 
Phillips,  Q.  J,  Oeol.  Soe.  xxxii.  p.  155,  and  xxxiv.  p.  471 — two  valuable  papers  in  which 
the  constitution  of  some  of  the  ''greenstones"  of  the  older  geologists  is  clearly  worked  out. 
Many  of  these  ancient  rocks  are  there  shown  to  be  forms  of  doleritic  lava,  and  the  change 
of  their  original  sngite  into  hornblende  is  traced. 


1 66  OEOGNOS  Y  book  u 


comprised  under  the  general  tenn  Aplianite,  divisible  into  Quartz-aphanite  and  Korwal 
aphanite.  The  general  characteristic  of  these  rooks  is  that  the  constituent  minenli 
become  so  minute  as  to  disappear  from  the  naked  eye.  They  are  dark  hea^y  close- 
grained  masses.     They  merge  into  the  basic  diabases  (j).  170). 

Trachyte^  —  a  term  originally  applied  to  modem  volcanic  rocks  possessing  > 
characteristic  roughness  {jpaxvi)  under  the  finger,  is  now  restricted  to  a  compact, 
usually  i»le,  i)orphyritic,  frer^uently  cellular,  rock,  consisting  essentially  of  sanidine, 
with  more  or  less  triclinic  felspar,  augite,  hornblende,  and  biotite,  sometimes  with  apatitf, 
and  tiidymite.  It  Is  distinguished  from  rhyolite,  or  quartz- trachyte,  by  the  absence  of 
free  quartz,  and  by  the  smaller  i»roi)oi*tion  of  %itreous  or  microlitic  (micro-felsitic)  ground- 
mass.  The  sanidine  crystals  present  abundant  steam-()or6s  and  glass-inclusions,  as  wdl 
as  honiblende-microlites  and  magnetite.  In  some  varieties,  the  ground-mass  appeazi 
to  be  entirely  com|)osed  of  microlites  ;  in  others,  minor  degrees  of  devitrification  can  be 
traced,  until  the  ground-mass  passes  into  a  glass  (trachyte-glass,  obsidian).  The  trachyten 
of  Hungary  have  been  grouped  as  Augite -trachyte^  Amphihoh-traehyte^  and  Biatik' 
trachyk.  Average  comixisition  of  Trachyte:  —  silica,  CO-0-64'0  ;  alumina,  17*0;  pro- 
toxide and  (peroxide  of  iron,  6*0-8*0  ;  magnesia,  1*0  :  lime,  f3*5 :  soda,  4-0;  potiiih, 
2* 0-2* 5.     Average  specific  gravity,  2*65. 

Trachyte  is  an  abundantly  difiused  lava  of  Tertiary  and  Post-tertiar}"  date.  It 
occm's  in  most  of  the  volcanic  districts  of  Euro[)e  (Siebengebirge,  Nassau,  Transylvaniik 
Bay  of  Naples,  Euganean  Hills) ;  in  the  Western  Territories  of  tlie  United  States  * ;  iB 
Xcw  Zealand.     It  also  occurs  among  the  Carboniferous  lavas  of  Scotland. 

Do  mite  (so  named  from  the  Puy-de-I)dme)  is  a  |N)rous  loosely  aggregated  trachyte, 
liaving  a  microlitic  gi'ound-mass,  through  which  arc  dLs])ersed  tridymite,  sanidine, 
nnich  plagioclase,  hornblende,  magnetite,  biotite,  and  specular  iron.  Soda-traohyte 
(Pantellerite)  is  a  variety  rich  in  oligoclasc,  found  in  Pantelleria. 

Phonolite  (Xejiheline- trachyte,  Clinkstone)* — a  term  suggested  by  the  metallic 
ringing  sound  emitted  by  the  fresh  comimct  varieties  when  struck,  is  applied  to  • 
com^tact,  grey  or  brown,  (piartzless  mixture  of  sanidine  and  nephelinc,  with  nosean, 
hauyne,  leueite,  pyroxene,  hornblende,  or  mica.  The  rock  is  rather  subject  to  decom- 
I)osition,  hence  its  tissui'cs  and  cavities  ai'e  frequently  filled  with  zeolites.  An  avenige 
specimen  gave  on  analysis — silica,  57*7  ;  ahmiina,  20*6  ;  iK)tash,  6*0 ;  soda,  7-0 ;  lime, 
1*5  ;  magnesia,  0*5  :  oxides  of  iron  and  manganese,  3*5  ;  loss  by  ignition,  3-2  percent 
The  si)ecific  gravity  may  be  taken  as  about  2-58.  Phonolite  is  sometimes  found  splitting 
into  thin  slabs  wliich  can  be  used  for  roofing  purj^ses.  Occasionally  it  assumes  a  por- 
]ihyritic  texture  from  the  pres«nici>  of  larg(»  cr^'stals  of  sanidine  or  of  honibleude.  When 
the  rock  is  ]iurtly  decom{)Osed  and  takes  a  somewhat  jtorous  texture,  it  resembles  nofnnal 
trachyte. 

It  is  a  thoroughly  volcanic  rock,  and  generally  of  Tertiary  date.  It  occurs  some- 
times filling  the  pi]M?s  of  volcanic  orifices,  sometimes  as  sheets  which  have  been  ]ionred 

'  Ou  tracliyte,  sec  Zirkel,  *  Micro.  Petrog.'  p.  143.  King  in  vol.  i.  of  *E«xplor.  40th 
l*arallel,'  }>.  578.  Ou  the  relative  age  and  classiticuiion  of  Hungarian  trachytes,  Siab6, 
Zeitsch.  JJndsc/t.  Oeol.  Oes.  xxix.  p.  635,  and  *Compte  rend.  Cougres  intemationale  de 
Geologic'  (1878),  Paris,  1880.  For  the  Scottish  Carboniferous  trachj'tes  see  Presidential 
Address  to  the  Geological  Society  1892,  and  F.  H.  Hatch,  Trans.  Roth  Soc,  Min.  18W. 

'  It  would  apfiear  that  mivrh  of  what  has  been  regarded  as  trachyte  in  Weatem  Ameria 
is  andesite,  consisting  essentially  of  plagioclase,  and  not  of  sanidine.  The  normal  trachytes 
are  now  descril^ed  as  honibleude-mica-andesites,  and  the  augite- trachytes  are  hypenthene- 
augite-andcsitcs,  most  of  the  rest  l)eiDg  dacites.  and  some  of  them  rhyolites.  Hague  and 
Lldings,  Amcr,  Jointi.  .Srt.  xxvii.  (1884),  p.  456. 

^  Boricky, '  Petrograph.  Stud.  Phouolitgestein.  Buhmeus.' — Archiv  Landesdureh/onehung 
Bohvien,  1874.  G.  F.  Fohr,  "  Die  Phonolite  des  Hegau's,"  Vrrh.  Phys.  Med.  Oes.  Witnbury, 
xvui.  (1883).      F.  H.  Hatch,  Ttvnfi.  Hv;i.  f^in-.  Eilhi.  1892. 


PART  u  §  >-ii  MASSIVE  ROCKS— AN DESITE  167 

out  in  the  foim  of  laya-streams,  and  sometimes  in  dykes  and  veins,  as  in  Bohemia  and 
Anvergne.  Some  of  the  great  bosses  or  eruptive  vents  connected  with  the  trachyte 
lavas  of  the  Garleton  Hills,  Haddingtonshire,  have  recently  been  determined  by  Dr. 
Hatch  to  be  true  phonolites. 

With  the  phonolites  may  be  classed  Leucite-trachyte,  or  Leucite-phonolite, 
where  the  felspathoid  is  leucite  instead  of  nepheline,  and  Nosean- trachyte  (Nosean- 
phonolite),  or  Hauyne- trachyte  (Hanyne-phonolite),  with  nosean  or  hauyne  taking 
the  place  of  the  felspar  of  ordinary  phonolite. 

Andasita — a  name  originally  given  by  Yon  Buch  to  some  lavas  found  in  the  Andes, 
is  now  applied  to  a  large  series  of  rocks  distinguished  from  the  trachytes  in  that  their 
felspar  is  plagioclase,  and  i)assing  by  the  addition  of  olivine  into  dolerite  and  basalt. 
In  fresh  examples  they  are  dark  grey,  or  even  black  rocks  with  a  compact  ground-mass, 
through  which  striatcKl  felspar  prisms  may  generally  be  observed.  They  often  assume 
oellular  and  porphyritic  structures.  At  the  one  end  of  the  series  stand  rocks  containing 
free  silica  (Dadte),  while  at  the  other  are  basalt-like  masses  of  much  more  basic  com- 
position (Augite-andesite).  Under  the  microscope  the  ground-mass  presents  more  or 
leas  of  a  pale  brownish  glass  with  abundant  felspar  microlites. 

Dacite  (Quartz-andesite) — composed  mainly  of  plagioclase,  quartz,  and  mica,  with 
a  varying  amount  of  sanidine  as  an  accessory  constituent,  and,  by  addition  of  hornblende 
and  pyroxene,  graduating  into  homblende-andesite.  The  ground-mass  has  a  felsitic, 
sometimes  spherulitic,  glassy,  or  finely  granular  base.  Composition :  silica,  69*36  ; 
alumina,  16*23  ;  iron  oxides,  2*41  ;  lime,  3*17  ;  magnesia,  1*34  ;  alkalies,  7*08  ;  water, 
0*45.  Mean  specific  gravity,  2*60.  This  rock  is  extensively  developed  in  the  Great 
Basin  and  other  tracts  of  western  North  America  among  Tertiary  and  recent  volcanic 
outbursts. 

Hornblende-andesite'  consists  of  a  trielinic  felspar  (usually  oligoclase),  with  horn- 
blende, augite,  or  mica.  The  ground -mass  resembles  that  of  trachyte,  presenting 
sometimes  remains  of  a  pale  glass.  The  por])hyritic  minerals  frequently  show  evidence 
of  having  been  much  corroded  before  consolidation.  Composition :  silica,  61  '12 ;  alumina, 
11-61  ;  oxides  of  iron,  11*64  ;  lime,  4-33  ;  magnesia,  0-61  ;  potash,  3*52  :  soda,  3*85  ; 
ignition,  4*36.  Homblende-andesite  is  a  volcanic  rock  of  Tertiary  and  post-Tertiary 
date  found  in  Hungary,  Transylvania,  Siebengcbirge,  and  in  some  of  the  Western 
Territories  of  the  United  States.  According  to  researches  by  ^Lessrs.  Hague  and 
Iddings,  gradations  from  this  rock  into  basalt  and  hypersthene-andesite  can  he  traced 
in  California, 'Oregon,  and  Washington.  These  rocks,  therefore,  cannot  be  said  to  have 
sharply  defined  and  distinct  fonns.^  Under  the  name  of  Hornblende- mica -andesite 
American  petrographers  have  described  a  frequent  variety  of  rock  throughout  the  Great 
Basin,  characterised  by  the  vitreous  appearance  of  its  felspar,  its  rough  porous  trachyte- 
like ground-mass,  and  the  presence  of  mica  as  an  essential  constituent.  This  term  yn\\ 
include  a  large  proportion  of  the  rocks  hitherto  classed  as  trachyte.s,  but  in  which  the 
felspar  proves  to  be  plagioclase  and  not  sanidine.' 

Pyroxene-andesite — consisting  of  labradorite  or  oligoclase,  with  augito  (less 
frequently  a  rhombic  pyroxene)  and  abundant  magnetite,  sometimes  with  hornblende 
or  mica,  forming  a  dark  heavy  basalt-like  compound,  with  a  com])act  sometimes  more 
or  less  distinctly  vitreous  ground-mass.  Composition  :  silica,  57*15  ;  alumina,  16*10  ; 
protoxide  of  iron,  13*0  ;  lime,  5*75:  magnesia,  2*21  ;  potash,  1*81  ;  soda,  3*88.  Mean 
specific  gravity,  2*75-2*85. 

It  was  formerly  supjwsed  that  the  pyroxene  of  the  andesites  was  always  augite.  But 
rhomhic  forms  of  the  mineral  have  now  been  frequently  detected.     Under  the  name  of 


^  See  Ztrkel,  *Micro8Cop.  Petrog.'  p.  122.     King,  in  vol.  i.  of  'Explor.  40th  Parallel,' 
p.  562.     Hague  and  Iddings,  Amer.  Jovm.  Sci.  xxvi.  (1883).  p.  230. 
«  Amer.  Journ.  Sci.  Sept.  1883,  p.  233. 
'  Hague  and  Iddings,  Anier.  Journ.  Sci.  xxvii.  (1884),  p.  460. 


168  GEOGNOSY  BOOKn 


Hypersthene-andesite,  certain  Tertiary  or  recent  rocks,  stretching  oyer  Tart 
Western  America,  have  been  described  as  associated  with  other  andesites  and  faanltaL 
Tliey  are  black  to  grey,  or  reddish-grey,  in  colour,  and  vary  in  -  texture  from  denM^ 
tlioroughly  crystalline  forms,  to  others  approaching  wliite  glassy  pumioe,  the  Ian 
under  the  microscope  ranging  from  a  brown  glass  to  a  holocrystalUne  stracture.  Tht 
magnesian  silicate  is  pyroxene,  chiefly  in  the  orthorhombic  form  as  hypersthene,  bit 
partly  also  augit«.  An  analysis  of  the  pumiceous  form  of  the  rock  gave  62  per  cent  «f 
silica,  while  the  |>ercentage  of  the  same  constituent  in  the  glass  of  the  base  was  found 
to  rise  to  69-94.» 

Pyroxene -andesite  occurs  in  dykes,  lava -streams,  plateaux,  sheets,  and  neck -like 
bosses  in  regions  of  extinct  and  active  volcanoes,  as  in  the  Inner  Hebrides,  Antrim, 
Transylvania,  Hungary,  Santorin,  Iceland,  Teneriifc,  the  Western  Territories  of  North 
America,  the  Andes,  New  Zealand,  &c.  Many  of  the  rocks  of  these  regions  now  classed 
under  this  name  have  long  been  known  and  described  as  dolerites  and  basalts.  Indeed, 
there  is  the  closest  relation  between  them  and  the  true  olivine -bearing  dolerites  and 
basalts.  The  latter  occur  among  the  Tertiary  volcanic  plateaux  of  Britain,  interstratified 
with  rocks  whicli,  not  containing  oli%^e,  have  been  placed  among  the  andesitM. 
Neither  in  their  mode  of  occurrence  nor  to  the  eye  in  hand  specimens  is  there  any  good 
distinction  to  be  drawn  bet\veen  them.  Under  the  name  of  Tholeite  some  intensti^g 
augite-andesites  have  been  described,  in  which  the  felspar  prisms  form  a  network  fiUsd 
in  with  graimlar  augite  and  interstitial  matter  (intersertal  structure).  In  other  varietki 
of  andesite  the  felsi)ar-mesh  has  been  filled  with  lai^e  crystalline  patches  of  angite^ 
which  thus  encloses  the  felspar  (ophitic  structure). 

Tephrite  (Nepheline-andesite,  Leucite- andesite,  Nosean-  or  Hanyne- andesite)— a 
group  of  andesites,  in  which  the  felsiiar  is  ]>artly  replaced  by  one  of  the  felspatlioidl» 
nepheline,  leucite,  nosean,  or  hauync. 

Porphyrite — a  name  for  old  forms  of  andesite  which  have  generally  undeigaw 
considerable  alteration,  and  conse(piently  appear  as  dull,  sometimes  earthy,  genenllj 
reddish  or  brownish  rocks.  When  fresh  they  are  dark  grey  or  black.  They  an 
commonly  iK)rphyritic,  and  show  abundant  scattered  cr^'stals  of  plagioclase,  Ims 
eonmionly  of  mica.  Their  texture  varies  from  coarse  crystalline  to  exceedingly  dose- 
grained,  passing  occasicmally  into  vitreous  varieties  (Yetholm,  Cheviot  Hills).  Bo(^ 
of  this  ty]>e  have  been  abundantly  }X)ured  forth  as  lavas  during  Palteozoic  time,  and  tfasj 
occur  as  iuterstratilied  lava-lx^ds,  eruptive  sheets,  dykes,  veins,  and  irregular  bosses. 
In  Scotland  they  form  masses,  several  thousand  feet  thick,  enipted  in  the  time  of  tbe 
Lower  Old  Red  Sandstone,  and  others  of  wide  extent  and  several  hundred  feet  in 
depth  1>elonging  to  the  Lower  Carboniferous  period.  In  (lermany  porphyrites  appear 
also  At  numerous  points  among  formations  of  later  Pabeozoic  age. 

Prop y lite — a  name  given  by  Richthofen  to  certain  Tertiary  volcanio  rocks  of 
Hungar}',  Transylvania,  and  the  Western  Territories  of  the  United  States,  consisting  of 
a  triclinic  felspar  and  hornblende  in  a  fine-grained  non-vitreous  ground-mass,  and  elosdy 
relatc^d  to  the  Ilomblende-andesites.  Their  distinguishing  feature  is  the  great  alteration 
which  they  have  undergone,  whereby  their  ferro-magnesian  constituents  have  been  con- 
verted into  chlorite,  and  their  fels|>ars  into  epidote.  Some  quartziferons  propylites 
have  been  described  by  Zirkel  from  Nevada,  wherein  the  i[uartz  abounds  in  liquid  in- 
clusions containing  briskly -moving  bubbles,  and  sometimes  double  enclosures  with  an 
interior  of  liquid  carbon-dioxide.-    A  s[)ecimen  from  Storm  Canon,  Fish  Creek  Mountains, 


^  Whitman  Cross,  B»U.  U,  S.  Gtd,  Survey,  1883,  No.  1.      Hague  and  Iddings,  A 
Journ.  Sci.  xxvi.  (1883),  p.  226  ;  xxvii.  (1884),  p.  457. 

-  Zirkel's  *  Microscopical  Petrography,*  p.  110.  King,  "  Exploration  of  40th  Puallel," 
vol.  i.  p.  545.  C.  E.  Button's  ''  High  Plateaux  of  UUh ''  ( U.S.  Geographical  and  CMoffiaA 
Survci/  of  the  Rocky  Mountains),  chaps,  iii.  and  iv.  Hague  and  Iddings,  Amer,  Jaum.  ScL 
1883. 


PART  n  §  vii  MASSIVE  ROCKS— GABBRO  169 


contained  silica,  60*58;  alumina,  17*52;  ferric  oxide,  2-77;  ferrous  oxide,  2-53;  manganese, 
a  trace  ;  lime,  3-78  ;  magnesia,  2-76  ;  soda,  3-30  ;  potash,  4-46  ;  carbonic  acid,  a  trace. 
Lo68  by  ignition,  2*25 ;  specific  gravity,  2- 6-2-7.  The  geologists  of  the  Geological 
Survey  of  the  United  States  believe  that  the  rocks  included  under  the  term  "  propylite  " 
in  the  western  parts  of  America  represent  various  stages  of  the  decomposition  of  granular 
diorite,  porphyritic  diorite,  diabase,  quartz-porphyry,  homblende.-andesite,  and  augite- 
andesite.^  The  name  has  been  more  recently  applied  by  Rosenbusch  to  rocks  which 
have  ondergone  alteration  by  solfataric  action. 

iii  Basic  Seriet. 

This  third  series  of  eruptive  rocks  is  distinguished  by  its  low  silica  percentage,  and 
the  relative  abundance  of  its  basic  constituents.  A  similar  range  of  structure  can  be 
traced  in  it  as  in  the  other  two  series.  At  the  one  extreme  come  rocks  with  a  holo- 
cryvtalline  structure  like  the  gabbros.  These  pass  into  others  of  a  hemi- crystalline 
character,  where,  amid  abundant  crystals,  crystallites,  and  microlites,  there  are  still 
traces  of  the  original  glass.  At  the  other  end  lie  true  basic  volcanic  glasses,  which 
externally  might  be  mistaken  for  the  pitchstones  and  obsidians  of  the  acid  rocks. 

Gmblnro  ^  (Euphotide) — a  group  of  coarsely  crystalline  rocks  composed  of  plagioclase 
(labradorite)  or  anorthite,  magnetite  or  titaniferous  iron,  and  some  ferro-magnesian 
mineral,  which  in  the  normal  gabbros  is  augite  or  diallage,  but  may  be  a  rhombic  pyroxene, 
hornblende,  olivine,  or  mica.  These  minerals  occur  in  allotriomorphic  forms,  as  in 
granite  ;  but  they  sometimes  assume  ophitic  relations  which  lead  into  the  rock  termed 
dolerite.  The  felspar  has  often  lost  its  vitreous  lustre  and  passed  into  the  dull  opat^ue 
condition  known  as  saussurite.  Tlie  augite  is  usually  in  the  fonn  of  diallage,  distin- 
guished by  its  schiller-spar  lustre. 

Oabbro  occurs  as  an  eruptive  rock  among  the  older  fonnations,  likewise  in  large  bosses 
and  dykes  in  volcanic  cores  of  Tertiary  age  (Mull,  Skye).  Average  composition  :  silica, 
49;  alumina,  15;  lime,  9*5;  magnesia,  9*7;  oxides  of  iron  and  manganese,  11*5; 
potash,  0*3  ;  soda,  2*5.     Loss  by  ignition,  2*5  ;  specific  gravity,  2-85-3-10. 

The  following  varieties  may  be  noticed:  Olivine-gabbro — a  granitoid  or  ophiti(^ 
oom)M>und  of  plagioclase,  augite,  olivine,  and  magnetic  or  titaniferous  iron ;  good  exam])leH 
are  found  among  the  deep-seated  parts  of  some  of  the  Tertiary  volcanic  vents  of  the 
Inner  Hebrides.  Hy  persthene-gabbro  or  Norite  (Hyi)er8thenite,  Hyperite,  Schillcr- 
fels) — with  a  rhombic  pyroxene  in  addition  to  or  in  place  of  the  augite.  Troctolite 
(Forellenstein) — a  mixture  of  white  anorthite  with  dark -green  olivine,  i-eceives  its  name 
from  the  supposed  resemblance  of  its  speckled  appearance  to  that  of  the  side  of  a  trout. 
Pyroxene-granulite  (granular  diorite,  trai>-granulite) — consisting  of  plagioclase, 
pyroxene  (monoolinic  and  rhombic),  hornblende,  and  garnet,  distinguished  by  the 
granular  condition  of  these  minerals,  and  found  among  gneisses  and  other  schistose 
rocks  ;  this  is  probably  an  altered  condition  of  some  original  pyroxeuic  eruptive  rock. 

Dolsrite — an  important  group  of  basic  rocks,  which  connect  the  gabbros  with  the 
basalts  and  include  many  of  the  rocks  once  termed  *'  Greenstones."  They  are  composed 
of  labradorite  (or  anorthite),  with  some  ferro-magne.sian  mineral  (augite,  enstatite,  olivine, 
or  mica)  and  magnetic  or  titaniferous  iron.  As  a  rule,  they  are  holocrystalline,  the 
constituent  felspar  and  pyroxene  or  olivine  l>eing  characteristically  grouped  in  ophitic 


*  G.  F.  Becker  on  the  Comstock  Lode.  Reports  of  U.S.  Gedogiral  Surrci/  1880-81,  and 
his  fall  memoir  in  vol  iiL  of  the  Monographs  of  U.S.  OeoL  Sun-ei/  (1882).  Hague  and 
Iddings,  Amer.  Joum,  ScL  xxvii.  (1884),  p.  454. 

'  On  Gabbro  see  Lessen,  Z,  Deutsche  GeM.  Ges.  xix.  p.  651.  Lang,  op.  rif.  xxxi.  p. 
484.  Zirkel  on  Oabbros  of  Scotland,  op.  cii.  xxiii.  1871.  Judd,  Quart.  Joum.  GtAil.  Soc. 
xlu.  (1886),  p.  49.  G.  H.  Williams,  BuU.  U.S.  Ged.  Surv.  No.  28  (1886).  F.  D.  Cliester, 
op.  eU.  -No.  59  (1890).     M.  E.  Wadsworth,  Geol.  Surv.  Minnesota,  Bull.  2,  1887. 


1 70  GEOGNOS  Y  book  n 


structure,  but  a  little  residual  glass  may  occasionally  be  detected.  They  occur  in  boaei, 
intrusive  sheets,  and  dykes,  esi)eeially  as  the  subterranean  aecompauiments  of  tW 
volcanic  action  which  has  tln'ouii  out  augite-andosites  and  liasalts  to  the  snrlkce. 

Normal  or  oidinary  dolerite  consists  of  plagioclasc  and  augite,  with  magnetite  « 
titanic  iron  and  frequently  olivine.  Average  composition:  silica,  45-55;  alnnuai, 
12-16  :  lime,  7-13  ;  magnesia,  3-9  ;  oxides  of  iron  and  manganese.  9-18  ;  potash,  0-1 ; 
soda,  2-5.     Loss  by  ignition  (water,  &c.),  0*5-3  ;  specific  gra\'ity,  2*75-2-96. 

Diffei'ent  names  have  l>een  proposed  for  the  chief  varieties.  The  moat  imiK>rtant  «f 
these  are  Olivine-dolerite — a  dark,  heavy,  close-grained  finely -crystalline  rock,  witk 
scattered  olivine,  apt  to  weather  with  a  brown  ciiist.  OH  vine-free  dolerite— • 
similar  rock  but  containing  no  olivine.  Enstatite-dolerite  contains  enstatite  h 
addition  to  the  other  ingredients.  Nepheline-dolerite,  has  the  felspar  lai^gfly  cr 
entirely  replaced  by  nepheline  (see  Nephelinite,  p.  172). 

Ah  varieties  of  dolerite  depending  for  their  peculiarities  mainly  upon  their  antiqnitf 
and  the  consequent  alteration  they  have  undergone,  we  may  include  the  rocks  oon* 
prehended  under  the  term  Diabase.*  This  name  was  given  to  certain  dark  green  or 
black  eruptive  ro(;ks  found  in  older  geological  formations,  and  consisting  essentially  of 
triclinic  fels[>ar.  augite,  magnetite  or  titaniferous  iron,  apatite,  sometimes  oliviiK^ 
usually  with  more  or  less  of  diffused  greenish  chloritic  substances  (\iridite)  which  hiw 
resulted  from  the  alteration  of  the  augite  or  olivine.  Tlie  average  com|>08ition  of  typkil 
diabase  may  be  taken  to  be :  silica,  48-50  ;  alumina,  16*0  ;  protoxide  of  iron,  12-15; 
lime,  5-11  ;  magnesia,  4-6  ;  potash,  0.8-l'5  ;  soda,  3-4*5  ;  water,  l«5-2.  Speeifie 
gravity  about  2*9.  There  Is  generally  carbonic  acid  present,  united  with  some  of  the 
lime  as  a  decom|K)sition  ])roduct.  As  in  ordinary  dolerite,  gradations  may  he  tneed 
from  coarsely  crystalline  diabase^  into  exceedingly  fine-grained  and  compact  Tarietki 
(Diabase -aphanite),  which  sometimes  assume  a  fissile  character  (Diabase-schiefer)  what 
they  have  been  subjected  to  crushing  or  cleavage.  Some  kinds  present  a  porphyritic 
structure,  and  show  dis^iersed  ciystals  of  the  conqionent  minerals  (Diahase-porphyiji 
Labrador-iwrphyry,  Augite- i)orphyry) ;  or,  as  in  some  varieties  of  diorite,  a  concretioiiiiy 
arrangement  Ls  ])roduced  by  the  appearance  of  abundant  {)ea-like  bodies  of  a  compact  felritie 
mat(;rial,  imbedded  in  a  compact  or  finely  crystalline  ground-mass  (Variolite).  Whei 
the  green  com|)act  ground-mass  contains  small  kernels  of  carbonate  of  lime,  sometinM 
in  great  numbers,  it  is  calleil  Calcareous  aphunite  or  Calcaphanite.  Sometimes  the  roi^ 
is  abundantly  amygdaloidal.  Though,  as  a  nile,  free  silica  does  not  occur  in  it,  sone 
varieties  found  to  contain  this  mineral,  i)ossibly  a  secondary  product,  have  been 
distinguished  as  Quartz -diabase.  The  i)resence  of  olivine  has  suggested  the  name 
01ivine-dial>ase  as  distinguished  from  the  normal  kinds  in  which  this  mineral  is  absent 
A  variety  containing  hornblende  is  termed  Proterobase.  0[>hite,  a  variety  occnrring  in 
the  Pyrenees,  contains  diallagc  and  epidote  (see  p.  120). 

Diabase  occui-s  both  in  contemporaneous  beds  and  in  intnisive  dykes  and  sheets. 

Basalt  ^ — a  black,  extnmiely  conqiact,  a]>)varently  homogeneous  rock,  which  breaks  with 
a  splintery  or  conchoidal  fracture,  and  in  which  the  component  minerals  can  only  be 


*  Tlie  student  will  find  in  the  Zcitschnft.  Ihutsch.  Geol.  (fes.  1874.  p.  1,  an  importsBt 
memoir  by  Dathe  on  the  composition  and  structure  of  dial>ase.  See  also  Zix1nl*i 
*  Microscop.  Petrog."  p.  97. 

*  Michel-Levy,  BuH.  Son.  O^yf.  Francey  3rd  ser.  xi.  p.  282.  Geikie,  Trana,  Bo^,  Soc 
Edin,  xxix.  p.  487. 

^  On  basalt  rocks  see  Zirkel's  '  Basaltgesteine,'  1870.  Boricky's  '  FetrographisGhe 
Studien  en  deu  Ba.saltgesteiueu  Buhmeus,*  in  Archiv  fiir  ytUvnciss.  Lan4Uadurdi/9r- 
schuuff  von  Biihmeny  ii.  1873.  All|>ort,  Q.  J.  Geol.\  A'in:  xxx.  p.  529.  Oeikie,  2V«iu. 
Rot/.  Site,  EtUiu  xxix.  Mohl,  Xoi:  Act.  AcAui.  Lenp.  i\irol.  xxxvi.  (1873),  p.  74  ;  iVcMO 
JaJirb.  1873,  pp.  449,  824.  F.  Eiohstailt  on  Basalts  of  St^auia.  Sreriges  Geoi,  VnderwA^ 
ser.  c.  No.  51,  1882.     K.  Svedmark,  op.  cit.  No.  60,  1883. 


FART  II  §  Tii  MASSIVE  BOCKS— BASALT  171 

obaerred  with  the  microscope,  unless  where  they  are  scattered  porplijritioally  through 

the  mass  (Fig.   S2).      The  minerals  coiiaist  of  plagioclase   (tabradorite  or  anortbite), 

pjrroxene   (usnallf   augite,   but   i>ccasiaual!y  a  rhombic   form),   olivine,   mifrnetite   or 

tituiiferoDS  iroD.      Many  yea«  ago,   Andrews 

detected  native  iron  in  the  basalt  of  Antrim,  and 

more  recently  Nordenskibld  found  thia  substance 

abundantly  difliued  in  the  basalt  of  Disco  Island, 

occurring  even  in  large  blocks  like  meteorites 

(aiUV,  p.  68).     The  gronnd-mass  of  basalt  pre- 

seats  under  tbe  microscope  traces  of  glass  in 

which   are   imbedded    minute   granules,   hairs, 

needles,   and  microlites  of  felspar  and  sugile. 

The  proportion  of  this  base  varies  within  wide 

limit*,  insoinucb  that  while  iti  some  jiarts  of  a 

banlt  it  so  preponderates  that  the  individual 

ciTitala  are  scattered  widely  through  it,  or  are 

drawn  out  into  beautiful  streaks  and  eddies  of 

Bnxion  stnicturo,  in  others  it  almost  disap|>ears. 

and  the  rock  then  appears  as  a  nearly  crystalline   I 

maaa,    which  thus  graduates  into  dolerite   and  (nugnlMed^    Th*  large  sbsded  crystal- 

basic  andesite.      The  component  minerals  fre-         \^^  uI^^'ZmwUU  pti^^^n'st. 

quently  appear  porphj-ritically  dispersed,  espe-  gUMlaw.      A  few  Aiiglte  prbims  occur 

eiallj   the    olivine,    the    pale    yellow   grains   of  which,  lo  the  right  of  the  centre  of  the 

which  are  characteristic,  drawing,  are  iggn^ati'd  into  b  large  com- 

Two  types  of  basalt  have  been  recognise.!  J^nttf^""'"  '^'  '''"''  *"""'"  '" 
in   the    great    basaltic    outbursts   of    Western  '"' 

America:  (1)  the  porjihyritic,  consisting  of  a  glassy  and  microlitic  or  micro- crystal- 
line groand-maas,  bearing  relatively  large  crystals  of  olivine,  felspar,  and  occasion- 
ally augite,  a  structure  showing  close  relations  to  that  of  many  andesites  :  (2)  the 
granular  (in  tbe  sense  in  which  that  tenn  is  used  liy  Rosenbnsch,  oiite,  p.  US) — an 
aggregate  of  quite  uniform  grains,  com|iosed  of  Kell-devcloj>ed  plagioclase  and  olivine 
crystala,  with  ill^delined  patches  of  augite,  and  frequently  with  a  considerable  amount 
of^aBa-baae.  By  diminution  of  olivine  and  augmentation  of  silica,  and  the  apimarancc 
of  hypersthene,  gradations  can  lie  traced  from  true  olivinc-liasalta  into  normal  andesites. 
Basalts  with  free  qnsrtz  are  not  infreijueiit  in  Western  America.' 

Basalt  occurs  in  amorphous  and  columnar  sheets,  which  may  alternate  with  each 
other  or  withMSOciated  tuRs.  It  also  forms  abundant  dykes,  veini,  and  intrusive  bosses. 
It  frequently  assumes  a  cellular  structure,  which  becomes  aniygdaloidal  by  the  dejiosit 
of  cahnte,  leoUtes,  or  other  minerals  in  the  vesicles.  A  relation  may  be  traced 
between  the  development  of  amygdates  and  the  state  of  the  rock  ;  the  more  amygdaloidal 
the  tock,  the  more  is  it  decomposed,  showing  that  the  aniygdales  have  probably  in 
large  measure  been  derived  by  infiltrating  water  from  the  basalt  itself. 

Vitreous  Basalt  (Baaalt-gUss,  Tachylyte,  Hyalonielan).'— Basalt  jMsses  into  a 
condition  which,  even  to  the  naked  eye,  is  recognisable  o-s  that  of  a  true  glass.  This 
more  especially  takee  place  along  the  edges  of  dykes  and  intrusive  sheets.  Where  an 
eztemal  skin  of  the  original  molten  rock  has  rapidly  cooled  and  consolidated,  in  contact 
with  the  rocks  through  which  the  eniption  took  place,  a  transition  can  l>e  traced 
within  the  space  of  less  than  a  <[narter  of  an  inch  from  a  crystalline  dolerite,  anamesite, 
baaalt,  or  andesite  into  a  black  glass,  which  under  the  microscojie  assumes  a  jiule  brawn 

■  Hague  and  Iddiogs,  Amer.  Jotirn.  &i.  ixvii.  (1884),  p.  *S8.  hidings,  "/J.  til.  xxxvi. 
<ieS8),  p.  a08,  Bull.  U.  S.  aeU.  S-ff.  No».  66  and  7»  (J.  9.  Diller). 

■  See  Judd  k  Cole,  Q.  J.  Ocol.  S'k.  ixiii.  (1883),  p.  444.  Cole,  op.  cU.  >liv.  (I8SS),  p. 
300.     Cohen,  Jrnuit  Jakrb.  1876,  p.  T44  :  ISSO  (vol.  ii.),  p.  2Z  (Saudwich  Islands). 


172  GEOGNOSY  Boocn 


or  yellowish  colour,  and  is  isotropic,  but  generally  contains  abundant  microlites, 
times  with  a  globular  or  aphenilitic  concretionary  structure.  In  such  cases  it 
disputable  that  this  glass  represents  what  was  the  general  condition  of  the  whole  molten 
mass  at  the  time  of  eruption,  and  that  the  present  crystalline  structure  of  the  rook  wu 
dcyelo|)cd  during  cooling  and  consolidation.  The  glassy  forms  of  basalt  undergo  altcn* 
tion  into  a  yellowish  substance  called  Pa  1  agon  i  t e  (p.  1 38).  It  is  worthy  of  remark  tfait 
in  the  analyses  of  vitreous  basalts,  the  jtercentage  of  silica  rises  usually  above,  whik 
their  spccitic  gravity  falls  below,  that  of  ordinary  crystalline  basalt. 

Tlie  average  composition  of  basalt  is — silica,  45-55  :  alumina,  I0-]  8  ;  lime,  7-14 : 
magnesia,  3-10  ;  oxides  of  iron  and  maganese,  9-16  ;  iK)ta8h,  0'5-3  ;  soda,  2-5.  Lm 
by  ignition  (water,  &c.),  1-5  ;  specific  gravity,  2 '85-3 '10. 

The  basalt-rocks  are  thoroughly  volcanic  in  origin,  a])i>earing  in  lava-streams,  plateanXi 
sills,  necks,  dykes,  and  veins.  The  columnar  structure  is  so  common  among  the  finer- 
grained  varieties  that  the  term  '*  basaltic  "  has  been  popularly  used  to  denote  it.  M 
already  stated,  it  has  been  assumed  by  some  writers  that  luisalt  did  not  begin  to  be 
erupted  until  the  Tertiary  period.  But  true  basalt  occurs  abundantly  in  Scotland  ai  a 
product  of  Lower  Carboniferous  volcanoes,  and  exhibits  there  a  variety  of  types  rf 
minute  structure.* 

Basic  Pumice. — Though  the  acid  lavas  furnish  most  of  the  pumice  with  which  «• 
are  familiar,  some  of  the  basic  kinds  also  assume  a  similar  structure.  Thus  at  Hawui, 
the  basic,  pyroxenic  or  olivine  lavas  give  rise  to  a  pumiceous  froth. 

Melaphyre — a  name  originally  proposed  by  Brongniart  and  subsequently  applied ii 
various  senses  by  different  writers  to  include  rocks  which  range  in  structure  and  ecas- 
])osition  from  the  more  basic  andesites  to  true  olivine-basalts.  Tlie  melaphyrei  ftr 
the  moat  part  l>elong  to  pre-Tertiary  eruptions  (though  some  Tertiary  lavms  ham 
been  described  as  melaphyre)  and  have  undergone  more  or  less  alteration.  If  the  word 
is  to  be  retained  as  a  definite  rock-name  it  should  be  restricted  to  an  altered  typo,  ask 
now  generally  agreed,  and  preferentially  to  the  older  altered  basalts.  The  melaphyres  «3Di 
then  bear  somewhat  the  same  relation  to  the  basalts  that  the  diabases  do  to  the  doleiitn 
and  the  i)orphyrites  to  the  andesites.  But  it  must  necessarily  happen  that  difficulty  will 
be  experienced  in  deciding  which  of  the  three  names  would  be  best  applied  to  some  oftbi 
eruptive  rocks  of  the  older  geological  formations.  The  melaphyres,  as  thus  defined,  tit 
somewhat  dull,  dark  brown,  reddish,  or  green  rocks,  often  amygdaloidal  and  showing 
their  iK)rphyritic  minerals  in  an  altered  condition,  the  olivines  especially  being  changBd 
into  seri>entine  or  replaced  by  magnetite  or  even  by  htematite.^ 

Nepheline-basalt  (Nepheline-Basanite). — Zirkel  proved  that  certain  black  heavy 
rocks,  having  externally  the  aspect  of  ordinary  basalt,  contain  little  or  no  felsptf, 
the  i»art  of  that  mineral  being  taken  in  some  by  nepheline,  in  others  by  leucite.'  Tbtf 
are  volcanic  masses  of  late  Tertiary  age,  but  occur  much  more  sparingly  than  the  txns 
basalts.  They  are  found  in  the  Odenwald,  Thuringer  Wald,  Erzgebiige,  Baden,  ftc. 
Mean  composition — silica,  45-52;  alumina,  16'50  ;  ferric  and  ferrous  oxides,  11*30; 
lime,  10-62;  magnesia,  4-35;  }K>tash,  1-95;  soda,  5-40;  water,  2-68.  Mean  speeifie 
gravity,  2-9-3-1.     Xephelinite  is  a  fonn  of  basalt  \\'ith  no  felspar  or  olivine. 

Leucite-basalt  (Leucite-Basanite)  contains  little  or  no  felspar,  but  has  leuoite  in 
place  of  it.     Externally  it  resembles  ordinary  basalt.      This  rock  occurs  among  the 


^  See  Trans.  Royal.  Soc.  Edin.  xxix.  (1879),  p.  437,  and  Presidential  Address,  QMort 
Joitrn.  Oeol.  Soc.  (1892),  p.  129,  where  the  types  of  microscopic  structure  observed  by  Dr. 
Hatch  are  enumerated. 

'  For  some  account  of  the  use  of  the  word  melaphyre  see  Brongniart, '  Cla8sificatio&  ft 
Caracteres  mineralogiques  des  Roches  homogenes  et  heterogenes,'  1827,  p.  100.  Nannuum, 
*  Lehrbuch  der  Geognosie,'  i.  p.  587.  Zirkel,  *Petrographie,*  ii.  p.  39.  Roaenhnsdit 
'  Mikroskop.  Physiogr.'  ii.  p.  484. 

3  *  Basaltgesteine.'  1870. 


PART  n  §  N-ii  MASSIVE  ROCKS—SERPENTINE  173 


extinct  volcanoes  of  the  Eifel  and  of  Central  Italy,  and  forms  the  lavas  of  Vesuvius. 
Leucitite  contains  no  felspar  and  no  olivine. 

Melilite-Basalt. — In  continuation  of  Zirkel's  research,  A.  Stelzner  has  shown  that 
in  some  basalts  the  part  of  felspar  and  nepheline  is  played  by  melilite.^  In  outer 
appearance  the  rocks  possessing  this  com[>osition,  and  to  which  the  name  of  Melilite-basalt 
lias  been  given,  cannot  be  distinguished  from  ordinary  basalt.  Under  the  microscoi)e,  the 
ground-mass  ap|)ear8  to  be  mainly  composed  of  transparent  sections  of  melilite,  either 
disposed  without  order,  or  ranged  in  fluxion  lines  round  the  large  olivine  and  augitc 
crystals ;  but  it  also  contains  chromite  (t),  microlitic  augite,  brown  mica,  abundant 
magnetite,  with  perowskite,  a^Mitite,  and  probably  nepheline.  (Swabian  Alb,  Bohemia, 
Saxon  Switzerland,  kc.) 

Under  the  awkward  name  of  "  ultra-basic,"  the  following  group  of  rocks  is  included 
in  which  the  proportion  of  silica  sinks  to  a  still  smaller  amount  than  in  the  basalts. 

Umlmzgite  (Magma- basalt)— a  iine-graine<l  to  vitreous  rock  composed  of  augite, 
olivine,  magnetite  or  titaniferous  iron,  and  apatite.  The  base  is  generally  glassy  and  the 
proportion  of  silica  in  the  rock  is  only  about  42  per  cent.  The  ty])ical  localit}'  is  Lim- 
burg,  near  the  Kaiserstuhl  in  Baden. 

Paridotite  Group. — The  rocks  here  embraced,  stand  at  the  extreme  end  of  the  basic 
igneous  rocks  as  the  rhyolites  and  granites  stand  at  the  op]K)site  end  of  the  acid  series. 
They  contain  no  fels])ar,  or  at  least  an  insigniticaut  projiortiou  of  it,  and  consist  of 
olivine,  with  augite,  hornblende  or  mica,  magnetic  or  titaniferous  iron,  chromite  and 
other  allied  minerals  of  the  spinel  type.  Tliey  contain — silica,  39-45  ;  alumina,  0-6  : 
ferrous  oxide,  8-10 ;  lime,  0-2 ;  magnesia,  35-48  ;  and  have  a  mean  specific  gravity 
between  3*0  and  3 '8.  When  quite  fresh  these  rocks  have  a  holocr}'stalline  structure, 
but  they  are  generally  more  or  less  altered,  and  in  their  extreme  condition  of  altera- 
tion fonn  rocks  known  as  ser[)entines.  They  occur  for  the  most  ]»art  as  intmsive  masses 
belonging  to  the  deejier-seated  portions  of  volcanic  eniptions.  The  following  vaiieties 
may  be  noticed  : — 

Pikrite  *  (Palaeopikrite,  Pikrite-jKirphyry) — a  rock  rich  in  olivine,  iisuall}-  more  or 
less  serpen tinized,  with  augite,  magnetite,  or  ilmenite,  brown  biotite,  hornl)lende,  or 
apatite  ;  occurs  as  an  eniptive  rock  among  Palaeozoic  formations  ;  is  closel}'  related  to  the 
diabases  into  which  by  the  addition  of  plagioclase  it  naturally  i>asses.  When  horn- 
blende predominates  over  pyroxene  the  rock  has  been  called  hornblende- pikrite. 

Lherzolite' — so  name<l  from  L'herz  in  the  Ariege,  is  a  holocrj'stalline  rock  com- 
posed of  olivine,  diallage,  and  a  rhombic  pyroxene,  with  a  lesser  proportion  of  a  spinel- 
loid  sometimes  brown  (chromite,  picotite),  sometimes  gi*een  (pleonast),  and  iron  ores. 

Dunite,  named  by  F.  von  Hochstetter  from  the  Dun  Mountain,  New  Zealand,  con- 
sists of  a  granitoid  mixture  of  olivine  with  chromite  or  other  spinelloid.  Such  a  rock 
naturally  by  alteration  into  a  serpentine. 

Saiptntiiia.^ — Under  this  name  arc  included  rocks  which,  whatever  may  have  been 


1  Seues  Jahrb.  (BeiUgeband),  1883,  p.  369-439. 

'  So  named  from  wixp^j  bitter,  in  allu.sion  to  the  large  pro|K)rtion  of  bitter-earth  (Mag- 
character  shared  by  all  the  peridotites.     G  umbel,  '  Die  Palaeolithischen  Eruptiv- 
gtsteine  des  Fichtelgeblrges ' :  Munich,  1874. 

*  On  the  eruptive  nature  of  Lherzolite,  see  A.  Lacrois,  Compf.  rend.  cxv.  (1892),  pp.  974, 
and  976. 

*  See  Tscheniiak,  SUz,  Akad.  H'tV//,  Ivi.  July,  1867  ;  it  wa.s  this  author  who  lirsl 
showed  the  derivation  of  serpentine  from  original  olivine  rocks  ;  Bounev,  Q.  J.  GV/V.  :Sik\ 
xxxiii.  p.  884,  xxxiv.  p.  769  ;  OeoL  Jiaff,  (2)  vi.  p.  362  ;  (3)  i.  p.  406  ;  Michel-Levy,  BuN. 
Sk.  OM.  France,  vi.  3rd  ser.  p.  156  ;  Sterry  Hunt,  Tronti.  Roif,  .Sor.  f'anfufo,  i.  (1883) ; 
Datbe,  Semes  Jahrb,  1876,  pp.  236,  337,  where  Garnet-serpentine  and  Bronzite -serpentine 
ai«  described  from  the  Saxon  granulite  region.  J.  S.  Diller,  Bt'l/.  ('.  >*.  ('*•"/.  .sVr/-.  No.  38 
(1887) ;  M.  E.  Wsdsworth,  *  Uthological  Studies,'  (1884),  p.  118. 


174 


GEOGNOSY 


BOOKO 


their  □riKJiial  character  and  composition,  now  cousist  mainly  or  whoUjr  of  . 
Ab  already  stated,  olivine  readily  jiassea  into  the  condition  of  serpentiDe,  whila  tbe  otka 
miiieraU  may  rciuain  nearly  unaflwt«d,  aa  in  admirably  seen  in  aome  pikrite&  IM 
serpentine- rocks  originally  ton aiateil  principally  of  olivine  (see  Fig.  38.)    DiOTitfi,pUn^ 


Pij|.  sa.— SUhi;^ 


ttw  altentlan  talf  a 


and  oilier  i-ouks,  consisting  largely  ofmagnesianHiticatea.  likewise  pass  in 

varieties  due  to  different  jiliosex  of  alteration  were  jndged  worthy  of  separate  dt 

each  member  of  the  jieridotites  might  of  course  have  a  conceivable  or  actual  n 

tive  among  tlie  seriwn tines.     But  without  atteniptiug  this  minutciieaa  of  oloaiifintiai,     ' 

we  may  with  advantage  treat  by  itaelf,  as  deierving  special  notice,  the  maMiTa  &(■ 

of  the  mineral  ser]icntiiie  from  whatsoever  rock  it  way  have  originated. 

Massive  Herpentine  Ih  a  compact  or  finely  granular,  faintly  glimmering,  or  doll  nxl^  " 
easily  cutorscratchediliuving  a  pre  vailingdirty-greencolour.Bome  times  TariooslyitniU 
or  flecked  Hith  brown,  yellow,  or  red.  It  frefjuently  contains  other  minenli  banda 
seq)entine.  One  of  ita  commonest  accoui|iaiiiments  is  chrysotile  ur  Hbrona  Mrp«atii% 
which  in  veinings  of  a  silky  lustre  oltcn  ramiht's  through  the  rock  in  all  diractii»i 
Other  common  enclosures  are  bronzite,  enstatite,  magnetite,  and  chrome-BjqneU^  bMita 
traces  of  the  original  olivine,  pyroxene,  amphibole,  mica,  or  felsjiar  in  the  rooks  vUifc 
have  been  altered  into  serpentine. 

Serpentine  occurs  in  two  distinct  forms  ;  1st,  in  Wis  or  bauds  intercAlatod  ainilBg 
schistose  rucks,  and  associated  esjiecially  with  crystalline  limestones ;  2ndlf,  in  djfca^ 
veins,  or  liosnes  traversing  other  rocks. 

As  tu  its  mode  of  origin,  there  can  iie  no  doubt  that  in  most  cases  it  was  original^ 
an  eruptive  rock,  as  is  dearly  shown  by  ita  aMD^ 
rence  in  dykes  and  irregular  boaMa.  Tie  ft*- 
quent  occurrence  of  reeognisahle  olivine  cxTital^ 
or  of  their  still  remaining  contoura,  in  the  midrt 
•if  the  eerfientine -matrix,  affords  good  grounda  fir 
assigning  an  erujitive  origin  to  many  letpMitiBea 
which  have  no  distinctly  eruptive  estomal  tbiM 
(Fig.  34).  The  rock  cannot,  of  conna,  luv«  bMB 
ejected  as  the  hydrous  magnedaa  silioato  Wt- 
|)entine ;  we  must  regard  it  aa  hafing  bMB 
originally  an  eru]>tive  olivine  rock,  or  ■  liigh^ 
hornblendic  or  micaceous  diorite,  or  oliriM- 
gabbro.  But,  on  the  other  hand,  the  interali- 
tion  of  beds  of  serpentine  among  achiatoaa  rocki, 
and  particularly  the  frequent  occurrenoe  of  aa^ 
[lentine  in  connection  with  more  or  le^  altand 
limestones    (West   of    Ireland,    Highlands  of   Scotland)   suggests   aaothar  mods  of 


ie  (30  DiBUictcn). 


FART  II  §  vii  SCHISTO:<E  CRYSTALLINE  ROCKS  176 


origin   in  theMe  cases.     Some  writers  have  contended  that  such  ser]>entine8  are  pro- 
ductM  of  the  alteration  of  dolomite,   the  magnesia  having  been  taken  up  by  silica, 
leaving  the  carbonate  of  lime  behind  as  beds  of  limestone.     Others  have  supi)osed  the 
original  rocks,  from  which  the  seri)entines  were  derived,  to  have  1)een  a  deposit  from 
oceanic  water,  as  has  been  suggested  by  Sterry  Hunt  in  the  case  of  those  associated  with 
the  crystalline  schists.^     Beds  of  serpentine  intercalated  with  limestone  might  conceiv- 
ably have  been  due  to  the  elimination  of  magnesian  silicates  from  sea- water  by  organic 
agency,  like  the  glauconite  now  found  filling  the  chambers  oi  foraminifcra^  the  cavities 
of  corals,  the  canals  in  shells,  sea-urchin  spines  and  other  organisms  on  the  floor  of 
the    present    aea.'     Among    the    limestone     and    crystalline    schists    of    Banffshire 
(pw     183),    serpenti'ie    occurs    in    thick    lenticular    l)eds    which   possess    a    schistose 
crumpled  structure  and  agree  in  dip  with  the  surrounding  rocks.     They  may  have  lieen 
de|»0Hit8  of  contemporaneous  origin  with  the  limestones  and  schists  among  which  they 
occur,  and  in  association  with  which  they  have  undergone  the  characteristic  schistose 
puckering  and  crumpling.     Sometimes  they  suggest  a  source  from  the  alteration  of 
highly  basic  volcanic  tuffs.      In  other  cases  they  may  have  l)een  erupted  peridotites 
which  have  acquired  a  schistose  character  from  the  same  jirocess  of  mechanical  defonna- 
tion  that  has  played  so  large  a  part  in  producing  the  foliation  of  the  crystalline  schists. 

III.  Schistose  (Metamorphic). 

In  this  section  is  comprised  a  series  of  rocks  which  present  a  re- 
markable system  of  divisional  planes  that  are  not  original  but  have  been 
superinduced  upon  them.  At  the  one  end  stand  rocks  which  are  unmis- 
takably of  sedimentary  origin,  for  their  original  bedding  can  often  be  dis- 
tinctly seen,  and  they  also  contain  organic  remains  similar  to  those  found 
in  ordinary  unaltered  sedimentary  strata.  At  the  other  end  come 
coarsely  crystalline  masses,  which  in  many  respects  resemble  granite,  and  the 
original  character  of  which  is  not  obvious.  An  apjMirently  unbroken 
gradation  can  be  traced  between  these  extremes,  and  the  whole  series  has 
been  termed  "  metamorphic  "  from  the  changed  form  in  which  its  mem])er8 
are  believed  now  to  appear.  In  the  earlier  stages  the  change  has  taken 
the  form  of  cleavage  as  in  ordinary  slate.  Even  in  slate,  however,  as 
already  remarked  (p.  134),  a  beginning  may  be  detected  in  the  development 
of  crystalline  particles,  and  the  crystalline  re-arrangement  may  be  traced 
in  constantly  advancing  progression  until  the  whole  mass  has  become 
crystalline,  and  forms  what  is  known  as  a  schist. 

The  Crystalline  Schists,  properly  so  called,  constitute  a  well-defined 
series  of  rocks.  They  are  mainly  comix)sed  of  silicates.  Their  structure 
is  crystalline,  but  is  distinguished  from  that  of  the  Massive  or  Eruptive 
rocks  by  its  more  or  less  closely  imrallel  layers  or  folia,  consisting  of 
materisJs  which  have  assumed  a  crystalline  character  along  these  layers. 
The  folia  may  be  composed  of  only  one  mineral,  but  usually  consist  of 
two  or  more,  which  occur  either  in  distinct,  often  alternate  lamime,  or 
intermingled  in  the  same  layer.  This  structure  resembles  that  of  the 
stratified  rocks,  but  it  is  differentiated  (1)  by  a  prevalent  striking  want  of 
continuity  in  the  folia,  which,  as  a  rule,  are  consi)icuously  lenticular, 

1  « Chemical  Bways,'  p.  123. 

*  According  to  Beithier,  one  of  the  glanconitic  deposits  in  a  Tertiary  limestone  is  a  true 
lerptntaiic.    See  Starry  Hunt,  '  Chem.  Essays,*  p.  303. 


176  aEOGNUSY  a 

thickening  out  and  then  dying  away,  and  reappearing  after  an  inteml 
on  the  same  or  a  ditferent  plane  (Fig.  35) ;  (2)  by  a  peculiar  and  vey 
characteristic  welding  of  the  folia  into  each  other,  the  crystalline  pu-tidii 
of  one  layer  being  so  intermingled  with  those  of  the  layers  above  ud 
below  it  thiit  the  whole  coheres  as  a  tough,  not  easily  iissile  masB  ;  (S)  \ij 
a  frequent  remarkable  and  eminently  distinctive  puckering  or  crumpling 
(with  frequent  minute  faulting)  of  the  folia,  which  becomes  sometimes  ■ 


tolla,  nttunl  ikK- 


title  as  to  l>e  dieuernible  only  under  the  microscope^  (Fig.  37),  hat  ii 
often  present  conspicuouslj'  in  hand-specimena  (Fig.  36),  and  can  be  tiaced 
iti  increasing  dimensions,  till  it  connects  itself  ivith  gigantic  ctirvatnmof 
the  Htrata,  which  embrace  whole  mountains.  These  characters  are  nS- 
cient  to  indicate  a  great  difference  between  schistose  rocks  and  oidiuiT 
stratified  formations,  in  which  the  strata  lie  in  continuous  flat,  panlH 
and  more  or  less  easily  separable  layers.  ] 

In  some  instances,  the  folia  can  be  seen  to  coincide  with  original  bed-   | 
<ling,  as  where  a  band  of  ijuartzito  or  of  conglomerate  is  intercalated 
Ijetwecn  sheets  of  phyllite  or  niita-schist.     In  such  cases,  there  cannot  b* 

>  On  the  ininrosca|>ic  xtnicture  of  llie  crystnlliue  schials  nee  Zirkel,  '  Hiera«ooi<al 
Pctrograiili  J '  (vol.  vi.  of  KinK'a  Ejploralioii  of  40lli  Parallel),  1876,  p.  14.  Allpoit,  ^'. 
Gtnl.  ,**«■.  xxiii.  |i.  407.      Surliy,  n/i.  cil.   sxivi.  p.  81.  Lehniann's  '  Untcnuchnogtn  ita 


PART  II  §  v: 


SCHISTOSE  CRYtiTALLlNE  BOCKS 


any  doubt  that  the  rock,  though  now  more  or  less  re-constructed  and  crystal- 
line, was  originally  mere  accumulated  mechanical  sediment  Many  clay- 
slates,  phyllitea,  and  mica-schists  are  obviously  only  altered  marine  clays, 
and  some  of  them  still  retain  their  recognisable  fossils.  From  such  rocks, 
gradations  can  be  followed  into  chiastolitc-schist,  mica-schist,  and  fine 
gneiss.  Quartzites  and  quartz-schists  often  still  retain  the  false-bedding 
of  the  original  sandy  sediment  of  which  they  are  composed.     The  pebbly 


and  conj^lomeratic  bands  associated  with  some  schists  afford  convincing 
proof  of  their  original  clastic  nature.  Thus,  at  the  one  end  of  the  schistose 
series  we  find  rocks  in  which  an  original  sedimentary  character  remains 
nninistakable.  At  the  other  end,  after  many  intermediate  stages,  we 
encounter  thoroughly  amorphous  crystalline  masses,  that  bear  the  closest 
resemblance  to  eruptive  rocks  into  which  they  insensibly  pass.  In  such 
instances,  there  can  be  little  doubt  that  the  amorphous  structure  is  the 
original  one,  which  has  become  schistose  by  subsequent  deformation 
(Book  IV.  Part  VIII.)  The  banded  arrangement  of  many  coarse  gneisses, 
however,  may  be  an  original  segregation -structure,  like  that  obsci'vable  in 
sills  and  bosses  of  eruptive  rocks  (p.  613). 

In  the  more  thoroughly  rc-conslrncted  antl  re -crystallized  schists 
all  trace  of  the  original  structures  has  l«;en  lost.  The  foliation  is  not 
coincident  with  bedding,  nor  with  any  structure  of  eruptive  rocks,  but  has 


1 7  8  (rEOGXUS  y  iKHfE.li 


l)eeii  detcraiinod  by  planes  of  cleavage  or  of  shearing,  or  by  the  align- 
ment assumed  by  minends  crystallizing  under  the  influence  of  intense 
pressure.  Along  these  surfaces  the  constituents  have  rearranged  them- 
selves, and  new  chemical  and  mineralogical  combinations  have  1>e«n  effected 
during  the  progress  of  the  "  metamoii)hisni." 

A  rock  ])ossessing  a  crystalline  arrangement  into  separate  folia  is  in 
English  termed  a  Schist.^  This  word,  though  employed  as  a  general 
designation  to  describe  the  structure  of  all  truly  foliated  rocks,  is  also 
made  use  of  as  a  suffix  to  the  names  of  the  minerals  of  which  some  of  the 
foliated  rocks  largely  consist.  Thus  we  have  "mica-schist,"  "ehlorite- 
schist,''  "  hornblende -schist."  If  the  mass  loses  its  fissile  tendencj, 
owing  to  the  felting  together  of  the  com^wnent  mineml  into  a  tough 
coherent  whole,  the  word  rock  is  usually  substituted  for  schist,  as  in 
**  hornblende-rock,"  **actinolite-rock,"  and  so  on.  The  student  must  bear 
in  mind  that  while  the  possession  of  a  foliated  structure  is  the  distiuctive 
character  of  the  crystalline  schists,  it  is  not  always  present  in  every 
individual  ]>ed  or  mass  associated  with  these  rocks.  Yet  the  non-schistow 
portions  are  so  obviously  integi'al  parts  of  the  schistose  series  that  they 
cannot,  without  great  violation  of  natural  affinities,  be  separated  from 
them.  Hence  in  the  following  enumeration  they  are  included  as  common 
accompaniments  of  the  schists.  Quartzite  also  may  Ije  placed  in  this  sab- 
diAision,  though  in  its  typical  condition  it  shows  no  schistose  structure. 

The  origin  of  the  crystalline  schists  has  been  the  subject  of  long  dis- 
cussion among  geologists.  AVerner  held  that,  like  other  rocks  of  high 
antiquity,  they  were  chemical  precipitiites  from  a  univei'sal  ocean. 
Hutton  and  his  followers  maintiuned  that  they  were  mechanical  aqueons 
sediments  altered  by  subterranean  heat.  These  two  doctrines  in  various 
modifications  are  still  maintained  by  opposite  sch(X)ls.  In  recent  jean 
much  light  has  been  thrown  upon  the  oi-igin  of  the  schistose  structure,  which 
has  been  shown  to  be  in  many  cases  due  to  the  mechanical  crushing  and 
chemical  re-adjustment  and  re-crystallization  of  the  materials  of  both 
sedimentary  and  igneous  rocks.  This  subject  is  discussed  in  a  later  part 
of  this  volume.     (See  Book  IV.  Part  A'lll.) 

It  is  obvious  that  a  wide  series  of  rocks  embracing  variously  altered 
forms  of  both  sedimentary  and  igneous  materials  hardly  admits  of  any 
simple  syst(;m  of  classification.  Kegarding  them  from  the  j)oint  of  view 
of  the  nature  of  the  metamorphism  they  have  undergone,  geologists  have 
sometimes  grouped  these  rocks  as  resulting  either  from  contact  meta- 
morphism, that  is,  from  the  effects  of  the  protrusion  of  igneous  matter 
from  within  the  earth's  interior,  or  from  regional  metamorphism  where  the 
changes  have  been  brought  about  by  some  widespread  terrestrial  disturb- 
ance (Book  lY.  Part  YIII.)  But  this  arrangement,  though  of  value  in 
discussing  questions  of  metamorphism,  has  the  disadvantage  of  introducing 
theoretical  considerations,  and  of  placing  in  different  groups  rocks  which 

^  III  French  this  term  has  no  Micli  (lufiuilo  signitication,  l)eing  applied  both  to  scliistt 
and  to  shales.  In  Oernian  also  tho  corrv-.spondinjj  word '*  schicfer  "  designates  jichista,  bnt 
is  also  employed  for  non-rrystalline  shaly  rocks  ;  thonschiefer  =  clay-slate  :  schieferthon  = 
nhah'. 


PART  II  §  vii  SCHISTOSE  CRYSTALLINE  ROCKS  179 


undoubtedly  present  the  same  general  petrographical  characters.  Avoid- 
ing all  disputed  questions  as  to  modes  of  origin,  I  shall  group  the  schists 
according  to  their  mineral  characters,  beginning  with  those  which  are 
obviously  only  a  further  stage  of  the  alteration  of  clay-slates,  and  ending 
with  the  gneisses,  which  bear  a  close  affinity  to  granites. 

1.  Argillites,  argillaceous  schists,  Phyllites.— The  rocks  included  in  this 
groux)  may  often  be  traced  into  the  clay-slates  described  on  p.  134.  They  mark  a 
further  stage  of  metamorphism,  wherein  besides  mechanical  deformation  there  has  been 
m  more  or  less  decided  re-crystallization  of  the  materials,  which  is  demonstrated  by  the 
abundant  secondary  mica  and  by  the  api)earance  of  such  minerals  as  chiastolite, 
andalnsite,  staurolite,  garnet,  &c.  When  a  clay-slate  becomes  lustrous  by  the  develop- 
m«ut  of  mica,  it  is  known  as  Phyllite— a  term  which  may  be  regarded  as  embracing 
the  intermediate  group  of  rocks  between  normal  clay-slates  and  true  mica-schists. 

Chiastolite- slate  (schistemaole),  a  clay-slate  in  which  crystals  of  chiastolite  have 
been  developed,  even  sometimes  side  by  side  witli  still  distinctly  preserved  graptolites 
or  other  organic  remains  ^  (Skiddaw,  Aberdeenshire,  Brittany,  the  Pyrenees,  Saxony, 
Norway,  Massachusetts,  &c)  Staurolito-slate,  amicaceous  clay-slate  with  crystals  of 
staurolite  (Banffshire,  Pyrenees).  Ottrelite-slate,  a  clay -slate  marked  by  minute,  six- 
aided,  greyish  or  blackish  green  lamella;  of  ottrelitc  (Ardennes,  where  it  is  said  to  con- 
tain remains  of  trilobites,  Bavaria,  Xew  England).  Di  pyre -slate  is  full  of  small 
crystals  of  dipyre.  Sericite-phyllite  is  a  name  proposed  by  Lossen  for  those  com- 
pact, greenish,  reddish,  or  violet  sericite-schists  in  which  the  naked  eye  can  no  longer 
distinguish  the  component  minerals.  Mica-phyllite  {phyllade  gris  feuilUU  of  Du- 
mont),  a  silky,  usually  very  fissile  slate,  with  minute  scales  of  mica.  German  petro- 
graphers  have  distinguished  by  name  some  other  varieties  found  in  metamorphic  areas 
and  characterised  by  different  kinds  of  concretions,  but  to  which  no  special  designations 
have  been  given  in  English.  Knotenschicfer  (Knotted  schist)  contains  little  knots 
or  concretions  of  a  dark -green  or  brown,  fine-granular,  faintly  glimmering  substance,  of 
a  taloose  or  micaceous  nature,  imbedded  in  a  finely- laminated  matrix  of  a  talc-like  or 
mica-like  mineral.^  These  aggregations  appear  to  be  in  many  cases  incipient  stages  in 
the  formation  of  definite  crystals  of  sucli  minerals  as  andalusite.  In  Fruchtschiefer 
the  concretions  are  like  grains  of  corn  ;  in  Garbenschiefer,  like  caraway  seeds;  in 
Fleck schiefer,  like  Hecks  or  spots.  Some  of  these  rocks  might  be  included  with  the 
mica-schists,  into  varieties  of  which  they  seem  to  jxass.  Round  some  of  the  eruptive 
diabase  of  the  Harz,  the  clay-slates  have  been  altered  into  various  crystalline  masses 
to  which  names  have  been  attached.  Thus  Spilosite  is  a  greenish,  schistose  rock, 
composed  of  finely  granular  or  comi>act  felsj)athic  material,  with  small  chlorite  con- 
cretions or  scales.  Desmosite  is  a  schistose  mass  in  which  similar  materials  are  dis- 
posed in  more  distinct  alternations.' 

2.  Quartz  rocks.* — Quarti-Bchist  (schistose  (juartzite),  an  aggregate  of  gi-anular  (or 


'  A  good  illustration  of  this  association  is  figured  by  Kjerulf  in  his  '  Geologic  des  Slid* 
lichen  und  Mittleren  Norwegen,*  Plate  xiv.  fig.  246.  See  also  Brogger's  memoir  on  Upi>er 
Silurian  fossils  among  the  crystalline  rocks  of  Bergen.  Christiania,  1882.  A  similar 
association  occurs  in  the  graptolite-shales  next  the  granite  of  Galloway,  Scotland. 

*  A.  von  Lasaulx,  Ne^tes  Jahrh.  1872,  j).  840.  K.  A.  Lossen,  Z.  Deutsch.  Oeol.  Ges. 
1867,  p.  585  (where  a  detailed  description  of  the  Taunus  phyllites  will  be  found),  1872,  p.  757. 

*  Other  names  are  Bandachiefer,  Omtactschiefer^  &c.  See  K.  A.  lessen.  Zeifsch. 
iPeutscK  Oeo.   Ges,  xix.  (1867),  p.  509,  xxi.  p.  291,  xxiv.  p.  701.     Kayser,  op.  cit.  xxii. 

p.  103. 

*  J.  Macculloch,  Trans.  Oeol.  Sik.  1st  ser.  ii.  (1814),  p.  450,  iv.  (1817),  p.  264  ;  2nd 
ser.  L  (1819),  p.  63.     Loesen,  Zeitsch.  iMulsdi.  Geol.  Ges.  xix.  (1867),  pp.  615-634. 


iS> 


'iKO'iXDsy, 


^ouuliti<:)i]UBrUwitha;<uffii'ielitcl<;veloli]nviitot  tine  fuliaof  iiiicatoimiicrtamoreOTlM 
ilefiuLtelysoliiatosuHtructiire  to  tlie  iw;k.  'Jlic  (lUaii]«&raiire  uf  the mim give* quartab, 
und  the  greater  prouiineiicc  uf  tliLs  niiuovul  slfonlij  grodutious  into  mica-scbist.  Sud 
^n^atious  arc  iiuite  analogous  to  thow  auiouj;  I'ccciit  ii«diiuciitarj'  irutteriaU  from  |>an 
sand,  tlirougli  muddy  saud,  aud  sandy  luud,  into  niiid  or  rlay,  and  betn-ecn  sandttaiM 
and  Htialra.  Thv  lliglilands  nf  Scotland,  for  iii«taii<.-e,  cnibracu  large  traots  of  tjuarti- 
HciiUU— rocks  tthicli  an  not  |)rojicrly  eitlii-r  uiica-scliixt  or  ordiuary  iiuartzite.  ThFj 
uoiuist  of  Rnuiular  (grant) liti2ed)ijiiartx,  with  line  jionillel  lamiine  ormiua,  aiid  are  caiabU 
of  bctnj;  split  into  thick  or  thin  flogstoLes.     InlcrHtratJIiiHl  gieblily  vari^tJeH  ocour. 

Itacoliimitc— a  Hchistuse  iiuai'tzite,  in  whidi  the  quartz -grannies  are  separated  t^ 
line  scales  of  inica,  talc,  chlorite,  mid  xi'iieite.  Oi'i'iuiouatly  these  ]i1iable  scalca  an  id 
arranged  as  to  give  n  certain  llexibiiity  to  the  Btoiie  (flexilile  mndslone).  This  rod 
occni^  in  tliu  south  ■eastern  states  of  Korth  America  ;  also  in  Itradl,  as  the  matrix  in 
it'hicli  dianiondu  arc  found. 

Siliueuna  schist  (Lydiun  stone,  Lydiic,  Kio-cl^ehiefer)  lias  already  been  deitcribed 
(]).  154]  among  the  sti^tified  I'uvks  ;  but  it  also  occurs  among  the  crystalline  scliiib, 
suiiietinies  as  tliu  result  of  the  iinlverisatiun  of  qliartzosc  ■'ockx  (niylonite). 

Qnartilte  ((jiim-ti-ruck),  thongh  not  jirojicrly  n  schistose  rock,  may  be  most  con- 
veniently cunsidcred  here,  as  it  is  no  constant  an  nccom|>amnieutof  the  scbuta,  and,  like 
them,  can  often  be  directly  traced  to  tlie  alteration  of  former  u.-dinientary  fomiatjou. 
It  is  a  granular  to  coin[Kict  mass  of  c|iiartx,  generally  wbiti',  sometimes  yellow  or  nd 


r   t^Adin 


with  a  cliarocteristic  Instivns  fructnrc.  It  occurs  in  tbiii  an<i  thick  UhIs  in 
with  i<ehiHts,  sometimes  in  continnuns  iiias^s  several  Ihnusaud  feet  thick.  In  Scotland 
it  forms  rangra  of  mountains,  an<l  is  there  freijui-ntly  aL-conijuinied  by  buds  of  lime- 
Bloiie,  which  iu  Kntherlandbliirc  eontidii  Cambrian  fosbils.' 

Even  to  the  naked  eye,  the  finely  granuUr  or  arennc'eous  structure  of  qnartrito  is 
distinctly  visible.  Hicniseo])ic  exaoiinatiou  shows  this  stnieturc  still  more  clearly,  and 
leaves  no  doubt  that  the  rock  originally  eoiiiustrd  ofa  tolerably  pure  qnartz-sand  (Fig.  SB]. 
More  or  lesi  distinct  evidence  of  crushing  and  deformation  of  the  grains  may  oiten  be 
oinierTed.  likewise  jirouf  of  the  transfusion  ofasiliecousci'ineiitnniunK  the  [articles.  Thii 
cement  was  pmbably  ]>iiidnit.'<l  by  the  solvent  octiiin  of  heated  water  n|>on  tlic  igiiarti 
grains,  which  seem  to  shiule  olT  into  I'ai^h  other,  or  into  the  iuten-ening  silica.     It  ia 


■  8(H)  tlie  chapters  on   1 
Uiuiorphic  iiuartzose  roeki 


PART  II  §  vii  SCHISTOSE  CRYSTALLINE  ROCKS  181 


owing,  no  doubt,  to  the  purely  Riliceous  character  of  tlie  grains  that  the  blending  of 
these  with  the  surrounding  cement  is  so  intimate  as  often  to  give  the  rock  an  almost 
flinty  homogeneous  texture.  That  quartzite,  as  here  described,  is  an  original  sediment- 
ary rock,  and  not  a  chemical  deposit,  is  shown  not  only  by  its  granular  texture,  but  by 
the  exact  resemblance  of  all  its  leading  features  to  ordinary  sandstone — false-bedding, 
alternation  of  coarser  and  finer  layers,  worm-burrows,  and  fucoid-casts.  Tlie  lustrous 
fracture  that  distinguishes  this  rock  from  sandstone,  is  due  to  the  exceedingly  firm 
cohesion  of  the  component  gi'ains,  which  break  across  rather  than  separate,  and  to  the 
consequent  production  of  innumerable  minute  clear  vitreous  surfaces  of  quartz.  A  sand- 
stone, on  the  other  hand,  has  its  grains  so  loosely  coherent  that  when  the  rock  is  broken, 
the  fracture  passes  between  them,  and  the  new  surface  obtained  presents  innumerable 
doll  rounded  grains. 

Besides  occurring  in  alternation  ninth  schists,  quartzite  is  also  met  with  locally  as  an 
altered  form  of  sandstone,  which  when  traversed  by  igneous  dykes,  is  indurated  for  a 
distance  of  a  few  inches  or  feet  from  the  intrusive  mass.  These  local  productions 
of  quartzite  show  the  characteristic  lustrous  fracture,  and  have  not  yet  been  distinguished 
by  the  microscope  from  the  quartz-rock  of  wide  metamorphic  regions.  There  is  yet 
another  condition  under  which  this  rock,  or  one  of  analogous  structure,  may  be  seen. 
Highly  silicated  bands,  having  a  lustrous  aspect,  fine  grain,  and  great  hardness,  occur 
among  the  unaltered  shales  and  other  strata  of  the  Carboniferous  system.  In  such  cases 
the  supposition  of  any  general  metamorphism  being  inadmissible,  we  may  infer  either 
that  these  quartzose  bands  have  been  indurated,  for  example,  by  the  passage  through 
them  of  thermal  silicated  water,  or  that  they  are  an  original  formation. 

Bchlitoia  CkMDglomerate  Bockt. — In  some  regions  of  schists,  not  only  bands  of 
quartzite  occur,  representing  former  sandstones,  but  also  pebbly  or  conglomeratic  bands, 
in  which  pebbles  of  quartz  and  other  materials  from  less  than  an  inch  to  more  than  a 
foot  in  diameter  are  imbedded  in  a  foliated  matrix,  which  may  be  phyllite,  mica-schist, 
gneiM,  quartzite,  &c.^  Examples  of  this  kind  are  found  in  the  pass  of  the  Teto  Noire 
between  Martigny  and  Chamouni,  in  the  Saxon  granulite  region,  in  the  Bergen  region  of 
Xorway,  in  the  north-west  of  France,  in  north -west  Ireland,  in  the  islands  of  Islay  and 
Garrelloch,  and  in  Perthshire  and  other  parts  of  the  central  Highlands  of  Scotland.  The 
pebbles  are  not  to  be  distinguished  from  the  water-worn  blocks  of  ordinary  conglomerates ; 
bat  the  original  matrix  which  encloses  them  has  been  so  altered  as  to  acquire  a  micaceous 
foliated  structure,  and  to  wrap  the  pebbles  round  as  with  a  kind  of  glaze.  These  facts,  like 
those  already  referred  to  in  the  structure  of  quartzite  and  argillaceous  and  quartz-schist, 
are  of  considerable  value  in  regard  to  the  theory  of  the  origin  of  some  crystalline  schists. 

3.  Pyroxene-Rocks. — Augite-schist — a  fine  grained  schistose  aggregate  of  pale  or 
dark -green  augite,  with  sometimes  quartz,  plagioclase,  magnetite,  or  chlorite  ;  found  rarely 
among  the  crystalline  schists.  Among  the  schistose  rocks  of  the  Taunus,  Lossen  has 
described  some  interesting  varieties  under  the  name  of  Augite-schist  (Augitschiefer). 
They  are  green,  compact,  sometimes  soft  and  yielding  to  the  finger-nail,  usually  distinctly 
tchistoee,  and  interbedded  with  the  gneisses  and  schists.  They  are  composed  of  a  fine 
doll  diabase-like  ground-mass,  through  whicli  are  dispersed  crystals  of  augite,  1  to  2  mm. 
in  length,  which  in  the  typical  varieties  are  the  only  components  distinctly  recognisable 
by  the  naked  eyeu'  Augite-rook — a  granidar  aggregate  of  augite  (with  tourmaline,  sphene, 
seapolite,  etc.),  found  in  beds  in  the  Laurentian  limestone  of  Canada.  Malacolite- 
rock  is  a  pale  granular  to  compact,  or  even  fibrous  aggregate  of  malacolite  found  in  beds 

^  Prof.  Wichmann  describes  some  curious  examples  of  serpentine  conglomerates.     See 
his  paper  in  "Beitrage  zur  Geologie  Ost-Asiens  und  Australieus,"  ii.  pp.  35,  111.     On 
the   conglomerate -schists  of  Saxony,    see   A.   Sauer,  'Geol.    Si)t;cialkarte   Sachsen,'  Sect 
"  Elterlein,"  also  Lehmann's  'Altkryst.  Schiefergesteine,'  p.   124.      Reusch,    '  Silurfossiler 
og  Pressede  Konglomerater,'  Christiania,  1882.     Barrois,  Anv,  Soc.  OM,  Xord.  xi.  1884. 

*  Lossen,  Zeitich,  Deutsch,  Geoi,  Oes.  xix.  (1867),  p.  598. 


182  ir  EOGNt  )S  Y  book  ii 


in  crystalline  limestone  (Riescngeliirge).  Schistose  Gabbro — a  granular  to  seliistoae 
ag^egat*  of  plagioclase  aufl  diallage,  occurs  in  lenticular  liands  among  the  ainphibolitv 
and  gi-anulites  of  the  crystalline  schists.  The  diallage  may  occur  in  consjucuous  cryAtali, 
and  is  sometimes  associated  with  ahundant  olivine,  as  in  ordinary  gabbro  (p.  154).* 

These  j)yroxenic  intei-calations  among  the  schists,  like  the  honiblendic  and  olivine 
I>ands  mentioned  below,  seem  to  represent  bands  of  igneous  material  (lavas  or  tuft) 
either  erupted  contemi)oraneously  with  the  dcj^osition  of  the  original  material  of  the 
schists,  or  8ubse<iuently  intruded  into  it,  and  thei-eafter  exposed  to  the  metamorphifflD 
which  j)roduced  the  foliation  of  the  si^hists. 

k  HuKNDLENDK-RorK!*. — Amphibolltes— a  name  applied  to  a  grouj>  of  rocks,  con- 
jK)sed  mainly  of  hornblende,  sometimes  schistose,  sometimes  thick-bedded.  Besides  the 
hornblende,  numerous  other  minerals,  such  as  are  common  among  the  schists,  likewiie 
occur, — orthoclase,  j)lagioclase,  quartz,  augito  and  varieties,  garnet,  zoisite,  mica,  intile, 
&c.  Where  the  rock  is  schistose,  it  l)ecomes  an  amphibolite-schist  or  homblande-acfaist; 
or  if  the  hornblende  takes  the  form  of  actinolite,  Actinolite-schist.  Glaucophane- 
schist — a  bluish -grt»y  or  black  rock,  in  which  the  hornblende  occurs  in  the  form  of 
glaucophane,  forms  large  masses  in  the  Southern  Al^^,  and  occiu^  locally  in  Anglesey. 
Where  an  am[)hil)olite  Ls  not  schistose,  it  used  to  l)e  termed  hiirnhlcnde-rocJc,  Nephrite 
(Jade)  is  a  comi»act,  extremely  finely  fibrous  variety.  The  ju-esence  of  other  minerals  b 
noticeable  quantity  may  furnish  names  for  other  varieties.  Thus,  where  plagioclsae 
(and  some  orthoclase)  occurs,  the  rock  Iwcomcs  a  Felspar-amphibolite,  Dioritie 
am]>hibolite,  or  Diorite-schist.'-'  Amphiboliti's  occur  as  Itands  associated  vith 
gneiss  and  other  schistose  fonnations.  It  was  suggested  by  Jukes  that  they  msj 
[)Ossibly  rej)resent  former  l.>eds  of  hornbh»ndic  or  augitic  lava  and  tutf,  which  have  been 
metamorphosed  together  with  the  strata  among  which  they  were  intercalated.  Tim 
suggestion  has  received  confirmation  from  the  researches  of  the  Geological  Survey  in 
the  north  of  Scotland  and  in  Ireland,  where  what  were  doubtless  originally  pyroxenie 
masses  erupted  prior  to  the  metamorphism  of  the  region,  have  had  their  augite  changed  by 
])aramor]>hism  into  hornblende,  and  have  ]iartially  iissumed  a  foliated  structure,  iiasstng 
into  Epidiorite,  Epidiorite-schist,  amphibolite-schists,  and  even  serpentine.  The 
connection  of  some  schists  with  original  masses  of  diorite,  gabbro,  and  diabase  has  been 
]M)inted  out  by  Ijchmann  and  subseijuently  by  many  other  observers.' 

5.  (Jaijnkt-Roc'Ks. — Eclogite,  one  of  the  most  beautiful  memlwrs  of  the  crystalline- 
schist  series,  is  a  granular  aggregate  of  grass-green  omphacite  (jiyroxene)  and  red  garnet, 
through  which  are  freipiently  disj>ers(Hl  bluish  kyanit*  and  white  mica.  It  occurs  in 
bands  in  the  Archwan  guijiss  and  mica-schist.  To  those  varieties  where  the  kyanite 
becomes  predominant,  the  name  of  K yanite-rock  has  been  given.  Garnet-rock  lit 
crystal  line -granular  rock  composed  mainly  of  garnet,  with  hoi*nblendc  and  magnetite; 
by  the  diminution  of  the  garnet  it  i»asses  into  an  amphibolite.  K inzigite — a  crystalline 
schistose  rock,  com^wsed  of  jilagioclase,  garnet,  an<l  black  mica,  found  in  the  Black  Forest 
(Kinsig)  and  the  Odenwald. 

^  Kocks  of  this  char.icter  occur  iu  the  Saxon  **Granulitgt;birge"  and  also  in  Lower 
Austria.  F.  Becke,  Tschennak's  Mitt.  Mifih.  IV.  p.  352.  J.  I^ehniauu^s  '  Untersuchungen 
iiber  die  Kntstehuug  dor  Altkrystallinischen  Schiefergesteine,'  Bonn,  1884,  p.  190.  On  the 
diabase-schists  of  the  Taunus,  see  L.  Milch,  Zeitxch.  Dmtsch.  Otol.  Ges.  xli.  (1889),  p.  394. 

-  See  F.  Becke.  Tscherniak's  Min.  Mitth,  IV.  p.  233.  Tliis  author  likewise  distin- 
guishes diallmjc-a utphilml it€y  'jntiiKt-amphiUdite,  saUtc-amphifMiUte^  z(/isiti'-nwphibolUe, 

^  *  UntersHchungen  iiber  die  Entstehung  der  Altkrystall.  Schief.'  See  also  Glimhel, 
'Die  Paliiolitischeu  Eruptivgesteine  des  Fichtelgebirges, '  Munich,  1874,  p.  9  ;  Teall,  QMorf- 
Juuni.  arol.  .^>r.  xli.  (1883),  p.  133;  'British  Petrography,'  p.  198.  Hatch,  Mem,  GtU. 
Surifi/y  KxjAnnation  of  ShfftSf  138,  139,  Ireland j  p.  49.  Hylaud,  Mfm,  Oeol.  Swrveif, 
Kiplanations  of  North-m'st  Ihniq/ol^  oiul  nf  Smith-vrat  /}onff/alf  Petrographical  appendices* 
also  jMmtea,  Book  IV.  pt.  viii.     G.  H.  Williams,  Jhdi.  r..S.  f^'eol.  Sun:  No.  62,  1890. 


PART  II  §  vii  SCHISTOSE  CRYSTALLINE  HOCKS  183 


6.  Epidote-Rocks. — Epidofite  (Pistacite-rock) — an  aggregate  of  bright  green  epidote 
with  some  quartz,  occurs  with  chlorite-schist  (Canada),  with  granite  and  serpentine 
(Elba),  and  with  syenite.  Epidote-schist,  a  schistose  greenish  rock,  with  silvery 
lustre  on  the  foliation  surfaces,  com]K)sed  of  epidote,  sericite,  magnetite,  quartz,  calcite, 
pla^oclase,  and  specular  iron.^ 

7.  CHLORiTE-Rt)CKs. — Chloxite-scliist — a  scaly  schistose  aggregate  of  greenish  chlorite, 
usoally  with  quartz  and  often  with  felsjMir,  talc,  mica,  or  magnetite,  the  last-named 
mineral  frequently  ap])earing  in  l)eautifully  perfect  disseminated  octohedra.  Occurs  \nth 
gneiss  and  other  schists  in  evenly  bedded  masses. 

8.  Taw-Rocks. — Talc-schiit — a  schistose  aggregate  of  scaly  talc,  often  with  quartz, 
fels]Mir,  and  other  minerals  ;  having  an  unctuous  feel,  and  white  or  greenish  colour. 
Occurs  somewhat  rarely  in  beds  associated  with  mica-schist  and  clay-slate,  and  frequently 
contains  magnetite,  chlorite,  mica,  kyanite,  and  other  minerals,  including  carbonates. 
A  massive  variety,  com])Osed  of  a  finely  felted  aggregate  of  scales  of  talc,  with  chlorite 
and  serpentine,  is  called  Potstone  (Topfstein).  Many  rocks  with  a  soapy  or  unctuous 
feel  have  been  classed  as  talc-schist,  which  contain  no  talc,  but  a  variety  of  mica  (sericite- 
schiBt,  kc. )  Talc-schist,  though  not  specially  abundant,  occurs  in  considerable  mass  in 
the  Aljn  (Mont  Blanc,  Monte  Rosa,  Carinthia,  etc.),  and  is  found  also  among  the  A])en- 
nine  and  Ural  mountains. 

9.  Olivine -Rot'KS,  or  Periik>tites  pf  the  Crystalline  Schists.^  Rocks  of  which 
olivine  forms  a  main  constituent,  occur  as  subonlinate  bands  or  irregular  masses  asso- 
ciated with  gneisses  and  other  schistose  rocks.  They  were  probably  eruptive  masses, 
contemporaneous  with  or  subsequent  to  the  surrounding  gneisses  and  schists  (p.  182 ). 
The  olivine  is  commonly  associated  with  some  pyroxenic  mineral,  hornblende,  garnet,  kc. 
Some  of  the  rocks  mentioned  on  ]>.  1 73  may  also  l)e  included  here.  Dunite,  for  example, 
which  occurs  in  apparently  eruptive  form  at  Dun  Mountain,  near  Nelson,  New  Zealand, 
ia  found  in  North  Carolina  in  beds  with  laminated  structure  intercalated  in  hornblende- 
gneisa.  Many  of  these  rocks  have  undergone  much  crushing  and  deformation,  and  pass 
into  foliated  forms  of  Serpentine,  which  must  thus  be  reckoned  as  one  of  the  schistose 
as  well  as  one  of  the  eniptive  series.  Some  remarkable  schistose  serfjentines  occur  inter- 
bedded  among  phyllites,  mica-schists,  and  limestones  in  Baiitrshire. ' 

10.  Fklsitoid-Rck'Kh. — These  are  distinguished  by  an  exceedingly  comi>act  felsite- 
like  matrix.  They  occur  in  beds  or  l)ed-like  masses,  sometimes  in  districts  of  contact 
metamorphism,  sometimes  associated  with  vast  masses  of  schists. 

Httltoflinta  —  an  exceedingly  comi>act,  homstone •  like,  felsitic,  grey,  yellowish, 
greenish,  reddish,  brownish,  or  black,  rock,  comi)Osed  of  an  intimate  mixture  of  micro- 
scopic particles  of  fels[)ar  and  quartz,  ^ith  fine  scales  of  mica  and  chlorite.  It  breaks 
with  a  splintery  or  conchoidal  fracture,  presents  under  the  miero8coi»e  a  finely-crystalline 
structure,  occasionally  with  nests  of  <[uartz,  and  is  only  fusible  in  fine  splinters  l)efore 
the  blow-pipe.  Some  of  the  rocks  to  which  this  name  has  been  a]»plied  are  probably 
felaitic  lavas ;  others,  though  externally  presenting  a  resemblance  to  felsite,  occur  in 
beds  intimately  associated  with  foliate<l  rocks  (Norway),  and  may  be  metamorphic 
|)rodacts  (perhaps  altered  tine  sediments)  due  to  the  same  series  of  changes  that  gave  rise 
to  the  crystalline  schists  among  which  they  lie.^ 

Adinola  (Adinole-schist) — a  rock  externally  resembling  the  last,  but  distinguished 
from  it  by  its  greater  fusibility.  It  is  an  intimate  mixture  of  quartz  and  albite,  con- 
taining about  ten  ])er  cent  of  soda.     It  is  a  ]»roduct  of  alteration,  being  found  among  the 

*  See  Wichmann  on  Rocks  of  Timor,  "  Beitrage  zur  Geologie  Ost-Asieus  uiid  Australiens," 
II.  part  2,  p.  97,  Leyden,  1884. 

*  See  Tschermak,  ASUxb.  AkcuL  Wisscn.^  Vienna,  Ivi.  (1867).  F.  Becke,  Tschemiak's 
J/i'w.  MUth,  IV.  (1882),  p.  322.     K.  Dathe,  Xaies  Jahrh.  1876,  pp.  255-337. 

*  For  analyses  see  H.  Santesson,  ''Keniiska  Bergsartaualyser,"  8vo,  Stockholn), 
1877. 


184  aj-yxiXuSY  BOOKH 


altei-ed  Carboniferous  shnlcs  around  the  emotive  dia^iases  of  tlie  Harz,  in  tlie  altemi 
Devonian  rocks  of  the  Taunus,  and  in  the  altered  Cambrian  rocks  of  South  Wales.* 

Porphyroid — a  name  besto\ve<l  u]Km  certain  rocks  comptwied  of  a  felsite-like  gronnd- 
mass  which  has  assumed  a  more  or  less  schistose  structure  from  tlie  development  of 
micaceous  scales,  and  which  contains  ]H)r]ihyritically  scattered  or^'stals  of  fe1s|iar  and 
([uartz.  The  felsfiar  is  either  orthoclasc  or  albite,  and  may  Ite  obtained  in  tolertUy 
IKTfect  crystals.  The  quartz  occasionally  j)i*esents  doubly  torminatod  pyramids.  Tbt 
micac'eous  mineral  may  be  ]»araj]fonite  or  sencite.  Porphyroid  occurs  in  oircumstaiioei 
which  suggest  considerable  mechanical  deformation,  as  among  the  schistose  rocks  of 
Saxony,'-*  in  the  Palfeozoic  area  of  the  Ardennes,^  as  well  as  in  Westphalia  and  other 
l«irts  of  Europe.*  Some  porphyrf»i<is  ai-c  probably  sheared  fonns  of  quartz-jiorphyiy, 
felsitc,  or  some  similar  rock  ;  othei-s  mav  be  more  of  the  natui-o  of  tuffs. 

11.  <H^ARTZ-  AM)  T()ri:MALTNK-K(K'Ks. — Tourmaline-BchlBt  (Schorl-scliist,  schori- 
rock),  a  blackish,  finely  granular,  (]uart/ose  rock  i^-ith  abundant  granules  and  needles 
of  black  tourmaline  (st-horl),  which  occurs  as  one  of  the  products  of  contact-metamor* 
phism  in  the  neighbourhood  of  some  granites  (Cornwall). 

12.  QuAKTZ-  AM)  Mh;a-R<k'Ks. — Mica- schist  (Mica -slate,  Glimmerschiefer),  a 
schistose  aggregjite  f>f  fpiartz  and  mica,  the  relative  projwrtions  of  the  two  minertb 
varjing  ^Wdely  <;ven  in  the  same  mass  of  rock.  Ejich  is  an-anged  in  lenticular  waTy 
lamiuiv.  Tlie  (piartz  shows  gi'cat  inconstancy  in  the  number  and  thickness  of  its  folis. 
It  often  presents  a  granular  charact<»r,  like  that  of  (juartz-rock,  or  ]>assing  into  grannlitcL 
The  mica  lies  in  thin  ]tlates,  sometimes  so  dovetailed  into  each  other  as  to  form  long 
continuous  irn^gular  crumpled  folia,  separating  the  quaii;/  layers,  and  often  in  the  fom 
of  thin  spangles  an<l  membranes  running  in  the  (piartz.  (Figs  8t>  and  37.)  As  the  rock 
splits  oi>on  along  its  micaceous  folia,  the  i^uartz  is  not  i-eadily  seen  save  in  a  cross  fracture. 

The  mica  in  typical  mica-schist  is  generally  a  white  variety  ;  but  it  is  sometime* 
replaced  by  a  dark  s])ecies.  In  many  lustrous,  unctuous  schists  which  are  now  found 
to  have  a  wide  extent,  the  silvery  foliated  mineral  is  ascertained  to  Ix*  a  mica  (niargaio- 
dite,  damounte,  etc. ),  and  not  talc,  as  was  onct?  supiw^sed.  These  were  named  by  Dana 
hydro -mica -schists.  Among  the  accessor}'  minerals,  garnet  (specially  characteristic), 
schorl,  felsiMir,  hornblende,  kyanite.  staurolite.  chlorite,  and  talc  may  be  mentioned. 
Mica-schist  n>adily  ]»asses  into  other  membei-s  of  the  si;histose  family.  By  addition  of 
felsi)ar,  it  merges  into  gneiss.  Ry  loss  of  quartz  and  increase  of  chlorite,  it  jtasses  into 
chlorite-schist,  and  by  loss  of  mi(ra,  into  rjuartz-schist  and  (piart^-ite.  By  failure  of 
([uartz  and  diminution  of  micA,  with  an  increasing  admixture  of  calcite,  it  may  shade 
into  calc- mica -schist  (see  Iwlow),  and  even  into  marble.  Mica -.schist  varies  in  coloor 
mainly  accordin*'  to  the  hue  of  its  mira. 

^Ir.  Sorby  has  stated  that  thin  slices  of  some  mica-schists,  when  examined  under 
the  niicrosco])e,  show  traces  of  original  gi-ains  of  (piartz-siind  and  other  sedimeutaiy 
particdes  of  which  the  rock  at  lirst  consisted.  He  luus  also  found  indications  of  what  he 
su]»i>oses  to  have  been  current-bedding  «»r  ripple-drift,  like  that  seen  in  many  fine  sedi- 
mentary deposits,  and  h<>  (concludes  that  mica-schist  is  a  cr}*stalline  metamori^hooed 
sedimentary  rock."*     In  many,  if  not  in  mo>«t  cases,  however,  the  foliation  does  not 


^  Lossen,  Zvitsrh.  Detitsrh.  (iod.  Uescl.  xix.  (1867),  p.  r>73.  See  also  Quart,  Journ,  iJed. 
•Sfx\  xxxix.  (1883),  pp.  3i)2,  320.  Ro.seubusch,  'Mikroskopische  Physiographie/  ii.  p.  235. 
F.  Posepny,  Tschermak's  MineruL  Mitth,  x.  17;"). 

-  Rothj^letz,  <ii'nl.  Surraj  Snjonify  Explanation  of  Section  Rochlitz. 

■"*  De  la  Valh'*e  Poussin  and  Renard,  Mem.  (.'ouronnrr'A  Armf.  Itoy,  Belg.  1876,  p.  85. 

*  Lossen,  Sitz.  (Mst^Ifsrh.  yntvrf.  Fnmnie,  1883,  No.  9. 

•■*  Q.  J.  (rVrt/.  Nw:.  (1S63),  p.  401,  and  his  address  in  vol.  xxxvi.  (1880),  p.  85.  The 
api)arent  cmrent -bedding  of  many  granulitic  and  other  nietaniorphic  rocks  is  certainly 
deceptive,  and  must  be  due  to  planes  of  shearing  or  slip]>ing  in  the  mechanical  movementi 
which  produced  the  nietauiorphisui. 


PART  II  §  vii  SCHISTOSE  CRYSTALLINE  HOCKS  185 


corresiiond  with  original  bedding,  but  with  stnictural  planes  (cleavage,  faulting) 
sapennduced  by  pressure,  tension,  or  otherwise,  upon  rocks  which  may  not  always  have 
been  of  sedimentary  origin. 

Among  the  varieties  of  mica-schist  may  be  mentioned  Sericite- schist  (which  may 
be  also  included  among  the  phyllites),  composed  of  an  aggregate  of  line  folia  of  the  silky 
variety  of  mica  called  sericite,  in  a  compact  honestone-like  ([uartz  ;  Paragonite-schist, 
where  the  mica  is  the  hydrous  soda  variety,  paragonite  ;  Gneiss-mica-schist,  con- 
taining dispersed  kernels  of  orthoclase.  Some  of  these  rocks  contain  little  or  no  quartz, 
the  place  of  which  is  taken  by  felspar.  Calc-mica-schist,  a  schistose  calcareous  rock, 
which  in  many,  if  not  in  all  cases,  was  originally  a  limestone  with  moi-e  or  less  muddy 
impurity.  The  carbonate  of  lime  has  assumed  a  granular-crystalline  form,  while  the 
aluminous  silicates  have  re-crystallized  as  fine  scales  of  white  mica.  Tremolite,  zoisite, 
and  other  minerals  are  not  infrequent  in  this  rock. 

Normal  mica-schist,  together  with  other  schistose  rocks,  forms  extensive  regions  in  Nor- 
way, Scotland,  the  Alps,  and  other  ])arts  of  £uro]ic,  and  vast  tracts  of  the  ''Archtean" 
regions  of  North  America.  Some  of  its  varieties  are  also  found  encircling  granite 
masses  (Scotland,  Ireland,  etc.)  as  a  zone  or  aureole  of  eontact-metamorphism  from  a 
few  yards  to  a  mile  or  so  broad,  which  shades  away  into  unaltered  greywacke  or  slate 
oatside.  In  these  cases,  mica-schist  is  unquestionably  a  metamorphosed  condition  of 
ordinary  sedimentary  strata,  the  change  being  connected  with  the  extravasation  of 
granite.'    (Book  IV.  Part  VIII.) 

Though  the  possession  of  a  fissile  structure,  showing  abundant  divisional  surfaces 
covered  with  glistening  mica,  is  characteristic  of  mica-schist,  we  must  distinguish 
between  this  structure  and  that  of  many  micaceous  sandstones  which  can  be  split  into 
thin  seams,  each  splendent  with  the  sheen  of  its  mica-Hakes.  A  little  examination  will 
show  that  in  the  latter  case  the  mica  has  not  crystallized  in  situ,  but  exists  merely  in 
the  form  of  detached  worn  scales,  which,  though  lying  on  the  same  general  plane, 
are  not  welded  into  each  other  as  in  a  schist ;  also  that  the  quartz  does  not  exist  in 
folia  but  in  rounded  separate  grains. 

13.  Quartz-  and  Felspar-Rooks. — Tlie  replacement  of  the  mica  of  a  mica-schist 
by  felspar,  or  the  disap[>earance  of  the  mica  from  a  gneiss,  gives  rise  to  an  aggregate  of 
felspar  and  quartz.  Such  a  rock  may  be  observed  in  thin  liands  or  courses,  alter- 
nating with  the  surrounding  mass.  In  mineral  comi)Ositi()U,  it  may  be  com])ared  to  the 
quartz-porphyries  or  granite-porphyries  of  the  massive  rocks,  but  it  is  usually  distinguish- 
able by  a  more  or  less  foliated  stnicture,  and  by  the  absence  of  felsitic  ground-mass. 

14.  Quartz-,  Felspar-,  axd  Mica-Rocks. — GneiBs. — This  name,  formerly  restricted 
to  a  schistose  aggregate  of  orthoclase  (sometimes  microcline  or  a  plagioclastic  fels]»ar, 
either  separate  or  crystallized  together),  quartz,  and  mica,  is  now  commonly  em])loyed 
in  a  wider  sense  to  denote  the  coarser  schists  which  so  often  i)re»ent  granitoid  char- 
acters.^ Many  gneisses,  indeed,  differ  from  granite  chiefly  in  the  foliated  arrangement 
of  the  minerals.  The  quartz  sometimes  contains  abundant  liquid  inclusions,  in  which 
liquid  carbon -dioxide  has  been  detected.  The  relative  proportions  of  the  minerals,  and 
the  manner  in  which  they  are  grouped  with  each  other,  present  great  variations.  As  a 
rule,  the  folia  are  coarser,  and  the  schistose  character  less  ])erfect  than  in  mica-schist. 
Sometimes  the  quartz  lies  in  tolerably  ]>ure  bands,  a  foot  or  even  more  in  thickness,  with 
plates  of  mica  scattered  through  it.  Tliese  quartz  layers  may  be  replaced  by  a  crystal- 
line mixture  of  quartz  and  felsi)ar,  or  the  felspar  will  take  the  form  of  independent 
lenticular  folia,  while  the  laminae  of  mica  which  lie  so  abundantly  in  the  rock,  give  it 
its  fissile  structure.     The  felspar  of  many  gnei.s.ses  presents  under  the  microscoije  a 


*  See  Kalkow8ky*s  '  Gneissformation  des  Eulengebirges,'  Leipzig,  1878;  Lehniaun's 
*  Altkrystallinische  Schiefergesteine,'  1884  ;  F.  Becke,  Tschermak's  Min.  Mitth.  1882,  i>. 
194  :  E.  Weber,  op.  cit.  1884,  p.  1,  and  potttea  Book  IV.  Part  VIII.  §  ii.  and  Book  VI. 
Pre-Cambrian. 


186  GEOGXOSy^  BOOKn 


reinarkablo  fibrous  structure,  duo  to  tlic  cryst«llization  of  fine  lamellio  of  some  pUgio- 
elasK  (albite  or  oligoclasc)  in  the  main  mass  of  orthoclaije  or  microcline.^  Among  tbe 
accc'ssoiy  minerals,  garnet,  tounnaline  or  schorl,  hornblende,  apatite,  graphite,  pyritci, 
and  magnetite  may  be  enumerated. 

Tiiere  can  be  no  doubt  that  many  gneL^ses  owe  their  characteristic  sebistoee  stractnif 
to  the  cnishing  and  shearing  of  some  original  eru]>tive  rock  such  as  granite.  lustanw^ 
however,  occur  where  the  materials  ani  segi'cgated  in  bandswhich  so  closely  resemble  th<Ne 
of  tnie  How-structure  or  segregation  in  igneous  bosse^s  and  sheets  as  to  suggest  that  they 
may  i>ossibly  have  resulted  from  tlie  movement  of  a  still  unconsolidated  eruptive  hum 
(pp.  177,  615).  Analogies  to  such  structures  may  l)e  observed  among  ancient  and 
mixiern  lavas. 

Many  varieties  of  gneiss  occur.  Some  are  distinguished  by  peculiarities  of  stractnn 
or  c'om])08ition,  as  Granite-gneiss,  where  the  schistose  arrangement  is  so  coarse « 
to  be  unrccoginsable,  save  in  a  large  mass  of  the  rock  ;  Diorite -gneiss,  gabbro* 
gneiss,  comi>osed  of  the  materials  of  a  diorite  or  gabbro  but  with  a  coarsely  sebiston 
stnu^ture;  Porphyritic  gneiss  or  Augengneiss,  in  which  large  eye-like  kernels  of 
orthoclase  or  quartz  are  disixjrsed  through  a  finer  matrix  and  represent  larger  ciysttb 
or  crystalline  aggregates  whicli  have  l>een  broken  down  and  dragged  along  by  shearing 
movement*  in  the  rock.  Other  varieties  are  named  from  the  occurrence  in  them  of  mat 
or  more  distinguishing  minerals,  as  Plornblende-gneiss  (syenitic  gneiss),  in  whieh 
hornblende  occurs  instead  of  or  in  addition  to  mica;  Protogine-gneiss,  where  tbe 
ordinaiy  mica  is  altered  into  chlorite  or  a  talc-like  substance  ;  Serieitc-gueiss,  a 
schistose  aggregate  of  sericite,  albite,  quartz,  with  less  frequently  white  and  black  mioi 
and  a  chlori tic  mineral  ;'-^  Augite-gnciss,  containing  an  augitic  mineral  (not  of  the 
iliallage  group)  and  potash -fels]>ar  or  potash-soda-felsi^ar  or  scai)olite,  with  hornblende 
(which  has  often  crystallized  j>arallel  with  the  augite),  brown  mica,  more  or  le<s  quartx, 
and  also  frequently  with  garnet,  calcite,  titanite,  etc.  ;^  Plagioclase-gneiss,  with 
plagioclase  more  abundant  than  oithoclase,  sometimes  containing  hornblende,  sometimet 
augite  ;  Cordierite-gnciss,  with  the  bluish  vitreous  mineral  conlierite. 

The  most  typical  gneisses  occur  among  the  so-called  **  Archnean  rocks,**  of  which  they 
form  tlie  leading  type,  and  where  they  probably  re])resent  original  eniptive  rocks.  (Sfe 
Book  VI.  Part  I.)  They  cover  considerable  areas  in  Scandinavia,  N.-W.  Scotland, 
Bohemia,  Bavaria,  Erzge])irge,  Moravia,  Ontral  Alps,  Canada,  &c.  But  rocks  to  whieh 
the  name  of  gneiss  c^innot  be  refused  a]>[>ear  also  among  the  products  of  the  metanlo^ 
phism  of  various  stratified  formations.  Such  ai-e  the  gneisses  associated  with  many  other 
eiystalline  schists  among  the  altered  Cambrian  and  Silurian  rocks  of  Scotland,  Norway, 
and  New  England,  the  altered  Devonian  rocks  of  the  Taunus,  and  other  regions,  which 
will  l)e  descril)ed  in  Book  IV.  Part  \'IIl.  Some  of  these  may  also  l>e  eruptive  granites, 
diorites,  Ac,  which  have  undergone  shearing  and  have  acquired  a  schistose  character. 

15.  ^Ktaiitz-,  Felspar-,  ANDGAUNKT-Rfx-KS. — Ghranolite'*  (Eurite-schistoide,  Lepty- 
nite  of  French  authors,  AVeiss-stein) — a  fine-grained  granular  aggregate  of  i»ale  reddish, 

^  F.  Becke  (Tschernuik's  ^fh^.  Mitth.  1882  (iv.)  p.  198)  described  this  structure  and 
named  it  micritprrthite. 

-  K.  A.  LoHseu,  Zeitifch.  JfcHfuch.  (<lenl.  Ges.  xix.  (1S67),  p.  r>65. 

^'  The  occurrence  of  nugite  as  an  abundant  constituent  of  some  gneisses  has  been  made 
known  by  niicroscoi)ic  research.  Rocks  of  this  nature  occur  in  Sweden  (A.  Stelzner,  *V.  Jakih, 
1880  (ii.),  j>.  103},  anil  have  been  fully  described  from  liower  Austria  (F.  Becke,  Tscher- 
niak's  Min,  Mitth.  1882  (iv.),  pp.  219-3C5).  They  are  likewise  well  developed  among  thi 
oldest  gneisses  of  the  north-west  of  Sutherland  in  Scotland. 

"*  Michel-Levy  has  proi>0}*ed  to  reserve  the  names  "Leptynite''  for  achistose  and 
••Granulite"  for  erujttive  rocks.  BulL  Sac.  Ot<>l.  France,  3rd  ser.  ii.  pp.  177,  189,  iiL 
p.  287,  iv.  p.  730,  vii.  p.  7*>0  :  Lory,  oj).  cit,  viii.  ]».  14.  Scheerer,  yeifcs  Jahrb.  1878, 
p.  C73.     Dathe,  X.  Ja/irb.  1S76,  p.  225  ;  Z.  Da'tsch.  O'eol.  iifx.  1877,  p.  274.     Details  re- 


PART  II  §  vii  SCHISTOSE  CRYSTALLINE  ROCKS  187 


yellowish,  or  white  felspar  with  quartz  and  small  red  garnets,  occasionally  with  kyanite, 
biotite,  and  microscopic  rutile  and  tourmaline.  The  felspar,  which  is  the  predominant 
constituent,  presents  the  peculiar  fibrous  structure  referred  to  in  the  foregoing  descrip- 
tion of  gneiss  (microperthite,  microcline),  and  api>ears  seldom  to  be  true  orthoclase. 
The  quartz  is  conspicuous  in  thin  partings  between  thicker  more  fels()athic  bands,  giving 
a  distinctly  fissile  bedded  character  to  the  mass.  A  dark  variety,  intei-stratified  with 
the  normal  rock,  is  distinguished  by  the  presence  of  microscopic  augite  or  diallage 
(Augitgranulite  of  Saxony).  Granulite  occurs  in  bands  among  the  gneiss  and  other 
members  of  the  crystalline  schist  series  in  Saxony,  Bohemia,  Lower  Austria,  the  Vosges, 
and  Central  France.  The  term  *  *  granulite  "  is  also  employed  in  a  structural  sense  to 
denote  a  rock  which  has  been  crushed  down  by  dynamic  metamorphism,  and  has 
acquired  this  characteristic  fine  granular  structure.     (See  pp.  99,  119). 

16.  Felspar-  and  Mica-Rocks. — Rocks  composed  essentially  of  a  schistose  aggre- 
gate of  minutely  scaly  mica  with  some  felspar,  quartz,  andalusite,  or  other  mineral, 
<>ccar  in  regions  of  metamorphism.  Cornubianite  was  a  name  proposed  by  Boase 
for  a  rock  composed  of  a  felspar  base,  with  abundant  mica.*  It  is  found  around  the 
granite  of  Cornwall,  of  which  it  is  a  metamorphic  product.  By  some  ^Titers  this  rock 
has  been  associated  with  the  gneisses,  but  it  Ls  distinguished  by  the  scarcity  or  absence 
of  quartz. 


garding  the  great  development  of  the  granulite  of  Saxony  (Granulitgebirge)  will  be  found  in 
the  explanatory  pamphlets  published  with  the  sheets  of  the  Geological  Survey  of  Saxony, 
especially  those  of  sections  Rochlitz,  Geringswalde,  and  Waldheim.  The  history  of  the 
origin  of  granulite  is  discussed  by  J.  Lehmann,  ''  Uutersuchungen  iiber  die  Entstehung  der 
AltkrystalL  Schiefergesteine." 

1  *  Geologj'  of  Cornwall '  (1832),  pp.  226,  230. 


188 


GEOGNOSY 


BOOK  II  PART  n  f  TJ 


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BOOK  III. 

DYNAMICAL  GEOLOGY. 

Dynamical  GEOixxiY  investigates  the  processes  of   change  at  present 
in  progress  upon    the  earth,  whereby  modifications  are   made   on    the 
structure  and  composition  of  the  crust,  on  the  relations  between  the 
interior  and  the  surface,  as  shown  by  volcanoes,  earthquakes,  and  other 
terrestrial  distiu'bances,  on  the  distribdtion   of   land  and    sea,   on  the 
outlines  of  the  land,  on  the  form  and  depth  of  the  sea-bottom,  on  marine 
currents,   and   on    climate.     Bringing   before    us,   in    short,   the    whole 
range  of  geological  activities,  it  leads  us  to  precise  notions  regarding 
their  relations  to  each  other,  and  the  results  which  they  achieve.     A 
knowledge  of  this  branch  of  the  subject  is  thus  the  essential  groundwork 
of  a  true  and  fruitful  acquaintance  with  the  principles  of  geology.     The 
study  of  the  present  order  of  nature  j)rovides  a  key  for  the  interpre- 
tation of  the  past. 

The  operations  considered  by  Dynamical  Geology  may  be  regarded 
as  a  vast  cycle  of  change,  into  the  investigation  of  which  the  student 
may  break  at  any  point,  and  round  which  he  may  travel,  only  to  find 
himself  brought  back  to  his  starting-point.  It  is  a  matter  of  com- 
paratively small  moment  at  what  part  of  the  cycle  the  inquiry  is  begun. 
The  changes  seen  in  action  will  always  be  found  to  have  resulted  from 
some  that  preceded,  and  to  give  place  to  others  that  follow  them. 

At  an  early  time  in  the  earth's  history,  anterior  to  any  of  the  periods 
of  which  a  record  remains  in  the  visible  rocks,  the  chief  sources  of 
geological  energy  probably  lay  within  the  earth  itself.  The  planet  still 
retained  much  of  its  initial  heat,  and  in  all  likelihood  was  the  theatre 
of  great  chemical  changes.  As  it  cooled,  and  as  the  superficial  dis- 
turbances due  to  internal  heat  and  chemical  action  became  less  marked, 
the  influence  of  the  sun,  which  must  always  have  operated,  and 
which  in  early  geological  times  may  have  been  more  effective  than 
it  afterwards  became,  would  then  stand  out  more  clearly,  giving  rise  to 
that  wide  circle  of  surface  changes  wherein  variations  of  temperature 
and  the  circulation  of  air  and  water  over  the  surface  of  the  earth  come 
into  play. 


190  DYXAMICAL  GEOLOGY  book  in  pabti 


In  the  2^ui*8uit  of  his  inquiries  into  the  past  history  and  into  the 
present  economy  of  the  earth,  the  student  must  needs  keep  his  mind 
ever  oi>en  to  the  reception  of  evidence  for  kinds,  and  especially  for 
degrees,  of  action  which  he  had  not  before  encountered.  Human  experi- 
ence has  been  too  short  to  allow  him  to  assume  that  all  the  cauBM 
and  modes  of  geological  change  have  been  definitely  ascertaiiied. 
Besides  the  fact  that  both  terrestrial  and  solar  energy  were  ODoe 
probably  more  intense  than  now,  there  may  remain  for  future  discoyeiy 
evidence  of  former  operations  by  heat,  magnetism,  chemical  change,  or 
other  agency,  that  may  explain  phenomena  with  which  geology  has  to 
deal.  Of  the  influences,  so  many  and  profound,  which  the  sun  exerti 
upon  oiu*  planet,  we  can  as  yet  only  perceive  a  little.  Nor  can  we  tell 
what  other  cosmical  influences  may  have  lent  their  aid  in  the  revolutioni 
of  geology. 

In  the  present  state  of  knowledge,  all  the  geological  energy  upon  and 
within  the  earth  must  ultimately  be  traced  back  to  the  primeval  energy 
of  the  i>arent  nebula,  or  siin.  There  is,  however,  a  certain  propriety 
and  convenience  in  distinguishing  between  that  part  of  it  which  is  doe 
to  the  sui-vival  of  some  of  the  original  energy  of  the  planet,  and  thit 
part  which  arises  from  the  present  supply  of  energy  received  day  by  daj 
from  the  sun.  In  the  former  case,  the  geologist  has  to  deal  with  the 
interior  of  the  earth  and  its  reaction  upon  the  surface;  in  the  latter, 
he  is  called  ui)ou  to  study  the  surface  of  the  earth,  and  to  some  extent 
its  reaction  on  the  interior.  This  distinction  allows  of  a  broad  treatment 
of  the  subject  under  two  divisions  : — 

I.  Hypogene  or  Plutonic  Action — the  changes  within  the 
earth,  caused  by  original  internal  heat  and  by  chemical  action. 

II.  E  pi  gene  or  Surface  Action — the  changes  produced  on  the 
superficial  parts  of  the  earth,  chiefly  by  the  circulation  of  air  and  water 
set  in  motion  by  the  sun's  heat. 


Part  I.  Hypogene  Acmox, 

An  luqnin/  into  the  Geological  Changes  in  Progress  beneath  the  Suf/ace 

of  the  Earth, 

In  the  discussion  of  this  bi*anch  of  the  subject,  it  is  useful  to  carry  in 
the  mind  the  conception  of  a  globe  still  intensely  hot  within,  radiating 
heat  into  space,  and  consequently  contracting  in  bulk.  Portions  ol 
molten  rocks  from  inside  are  from  time  to  time  poured  out  at  the  fsar- 
face.  Sudden  shocks  are  generated,  by  which  earthquakes  are  propa- 
gated to  and  along  the  surface.  Wide  geographical  areas  are  upraised  or 
depressed.  In  the  midst  of  these  movements,  the  rocks  of  the  crust  are 
fi-actured,  squeezed,  sheared,  crumpled,  rendered  crystalline,  and  even 
fused. 


i  §  1  VOLCANIC  PRODUCTS  191 

Section  i.  Voleanoes  and  Volcanie  Action.^ 

§  1.  Volcanic  Products. 

rhe  term  volcanic  action  (volcanism  or  volcanicity)  embraces  all  the 
lomena  connected  with  the  expulsion  of  heated  materials  from  the 
rior  of  the  earth  to  the  surface.  Among  these  phenomena,  some 
ess  an  evanescent  character,  while  others  leave  permanent  proofs  of 
r  existence.  It  is  naturally  to  the  latter  that  the  geologist  gives  chief 
Dtion,  for  it  is  by  their  means  that  he  can  trace  former  phases  of  vol- 
c  activity  in  regions  where,  for  many  ages,  there  have  been  no  vol- 
c  eruptions.  In  the  operations  of  existing  volcanoes,  he  can  observe 
"  superficial  manifestations  of  volcanic  action.  But  examining  the 
»  of  the  earth's  crust,  he  discovers  that  amid  the  many  terrestrial 
ilutions  which  geology  reveals,  the  very  roots  of  former  volcanoes 
B  been  laid  bare,  displaying  subterranean  phases  of  volcanism  which 
,d  not  be  studied  in  any  modern  volcano.  Hence  an  acquaintance 
r  with  active  volcanoes  will  not  afford  a  complete  knowledge  of  volcanic 
an.  It  must  be  supplemented  and  enlarged  by  an  investigation  of  the 
es  of  ancient  volcanoes  preserved  in  the  crust  of  the  earth.  (Book 
Part  VII.) 

The  word  "  volcano  "  is  applied  to  a  conical  hill  or  mountain  (com- 
sd  mainly  or  wholly  of  erupted  materials),  from  the  summit  and  often 
i  from  the  sides  of  which,  hot  vapours  issue,  and  ashes  and  streams 
aolten  rock  are  intermittently  expelled.  The  term  "  volcanic  "  desig- 
ns all  the  phenomena  essentially  connected  with  one  of  these  channels 
communication  between  the  surface  and  the  heated  interior  of  the 
je.  Yet  there  is  good  reason  to  believe  that  the  active  volcanoes  of 
present  day  do  not  afford  by  any  means  a  complete  type  of  volcanic 

The  student  is  referred  to  the  following  general  works  on  the  phenomena  of  volcanoes. 
pe,  *  Considerations  on  Volcanoes,'  London,  1825;  *  Volcanoes,'  London,  2nd  edit. 
>:  'Extinct  Volcanoes  of  Central  France,'  London,  1858;  *0n  Volcanic  Cones  and 
en,'  Quart.  Juurn.  Geol.  S(K.  1859.  Daubeny,  '  A  Description  of  Active  and  Extinct 
ranoeR,'  2nd  edit.,  Loudon,  1858.  Darwin,  'Geological  Observations  on  Volcanic 
idi,*  2nd  edit.,  London,  1876.  A.  von  Humboldt,  '  Ueber  den  Bau  und  die  Wirkung 
Vulkane,'  Berlin,  1824.  L.  von  Buch,  *  Uelx;r  die  Natur  der  vulkanischen  Erscheiu- 
•n  auf  den  Cauarischen  Inseln,'  Poggeml.  Annalen  (1827),  ix.  x.  ;  *  Ueber  Erhebuugs- 
«re  nnd  Vulkane,'  Poggerul.  Annalen  (1836),  xxxvii.  K  A.  von  Hoff,  *Geschichte 
duicb  Ueberlieferuug  nachgewiesenen  uatlirlichen  Verauderungen  der  Erdobertliiche  ' 
tlL,  "Vulkane  und  Erdbeben  "),  Gotha,  1824.  C.  W.  C.  Fuchs,  *Dio  vulkanischen 
;beiiiiiiigen  der  Erde,'  Leipzig,  1865.  R.  Mallet,  "On  Volcanic  Energy,"  PhU.  Trans. 
J.  J.  Schmidt,  *  Vulkanstudieu,'  Leipzig,  1874.  Sartorius  von  Waltershauseu  and 
ron  Lasaulx,  *Der  Aetna,'  4to,  Leipzig,  1880.  E.  Reyer,  *  Beitrag  zur  Pljysik  der 
ptionen,'  Vienna,  1877  ;  *DieEuganeen  ;  Bau  und  Geschichte  eines  Vulkanes,'  Vienna. 
f,  Fouque,  'Santorin  et  ses  eruptions,'  Paris,  1879.  Judd,  'Volcanoes,'  1881.  (i. 
r^Wij  *Vulcani  e  Fenomeni  vulcanici  in  Italia,'  Milan,  1883.  Ch.  Velain,  '  Les 
:anSy'  Paris,  1884.  J.  D.  Dana,  'Characteristics  of  Volcanoes,'  1890.  'Volcanoes 
;  and  Present,*  E.  Hull,  1892.  'The  South  Italian  Volcanoes,'  H.  J.  Johnston-Ljivis, 
les  1891.     References  will  be  found  in  succeeding  pages  to  other  and  more  s|>ecial 

lOilY. 


1 92  D  YXA  MICAL  GEOLOG Y  book  m  pak  i 


action.  The  first  effort  in  the  formation  of  a  new  volcano  is  to  estaUkk 
a  fissure  in  the  earth's  crust.  A  volcano  is  only  one  vent  or  group  of 
vents  established  along  the  line  of  such  a  fissure.  But  in  many  parts  of 
the  earth,  alike  in  the  Old  World  and  the  New,  there  have  been  periodi 
in  the  earth's  history  when  the  crust  was  rent  into  innumerable  fisflum 
over  areas  thousands  of  square  miles  in  extent,  and  when  the  moltoi 
rock,  instead  of  issuing,  as  it  does  at  a  modern  volcano,  in  narrow  strcami 
from  a  central  elevated  cone,  welled  out  from  numerous  small  vents  along 
the  rents,  and  flooded  enormous  tracts  of  country  without  forming  any 
mountain  or  conspicuous  volcanic  cone  in  the  usual  sense  of  these  temoA. 
Of  these  "  fissure-eruptions,"  apart  from  central  volcanic  cones,  no  examples 
appear  to  have  occurred  ^nthin  the  times  of  human  history,  except  in 
Iceland  where  vast  lava-floods  issue<l  from  a  fissure  in  1783  (pp.  222, 
256).  They  can  best  be  studied  from  the  remains  of  former  convulsions. 
Their  importance,  however,  has  not  yet  been  generally  recognised  in 
Europe,  though  acknowledged  in  America,  where  they  have  been  lai^gelj 
developed.  Much  still  remains  to  be  done  before  their  mechanism  is  as 
well  understood  as  that  of  the  lesser  type  to  which  all  present  volcanic 
action  belongs.  In  the  succeeding  narrative  an  account  is  first  presented 
of  the  ordinary  and  familiar  volcano  and  its  products ;  and  in  §  3,  iL,  some 
details  are  given  of  the  general  asj)ect  and  character  of  fissure-eruptionsL 

The  oj^enings  by  which  heiited  materials  from  the  interior  now 
reach  the  surface  include  volcanoes  (with  their  various  associated 
orifices)  and  hot-springs. 

The  prevailing  conical  form  of  a  volcano  is  that  which  the  ejected 
materials  naturally  assume  round  the  vent  of  eruption.  The  summit  d 
the  cone  is  truncated  (Figs.  39,  45),  and  presents  a  cup-shaped  or  caldron- 
like  cavity,  termed  the  crater,  at  the  bottom  of  which  is  the  top  of  tlie 
main  funnel  or  pipe  of  communication  with  the  heated  interior.  A 
volcano,  when  of  small  size,  may  consist  merely  of  one  cone  ;  when  of 
the  largest  dimensions,  it  forms  a  huge  mountain,  with  many  subsidiary 
cones  and  many  lateral  fissures  or  pipes,  from  which  the  heated  volcanic 
])roducts  are  given  out.  Mount  Etna  (Fig.  39),  rising  from  the  sea  to* 
height  of  10,840  feet,  and  supporting,  as  it  does,  some  200  minor  conei^ 
many  of  which  are  in  themselves  considerable  hills,  is  a  magnificent 
cxani])le  of  a  colossal  volcano.^ 

The  materials  erupted  from  volcanic  vents  may  be  classed  as  (1) 
gases  and  vapoiu*s,  (2)  wat^jr,  (3)  lava,  (4)  fragmentary  substances.  A 
brief  summary  under  each  of  these  heads  may  be  given  here ;  the  share 
tuken  by  the  several  products  in  the  phenomena  of  an  active  volcano  ii 
<lescribed  in  §  2. 

^  The  structure  aud  hi:!itory  of  Etiia  are  fully  described  in  the  great  work  of  SartOTioi 
von  Waltcrshauseu  and  A.  von  Lasaulx  cited  on  p.  191 — a  treasure-honse  of  ikcti  ia 
volcanic  geology.  See  also  G.  F.  llodwell,  *Etna,  a  history  of  the  monntain  and  tti 
eniption^,'  London,  1878;  O.  Silvcatri,  '  Un  Viaggio  all'  Etna,'  1879.  Notices  of  xteanl 
erui>tiou8  of  the  mountain  will  be  found  in  J\«^?//y,  vols,  xix.,  xx.,  xxL,  xxiL,  XIT. 
(o)»servatory  on  Etna,  p.  394),  xxvii.,  xlvi. ;  Oimjjf.  rend.  Ixvi.  The  work  of  Mensalli,  dltd 
on  p.  191,  gives  descriptions  of  this  aud  the  other  Italian  volcanic  centres. 


VOLCAKIC  GAHBH  AND  VAFOVKH 


193 


1.  Gases  and  Vapours  exist  dissolved  in  the  molten  magma  within 
the  earth's  crust.  They  play  ua  important  part  in  volcanic  activity, 
showing  themselves  in  the  earliest  stages  of  a  volcano's  history,  and 
continuing  to  appear  for  centuries  after  all  other  subt«ri-anean  action 


has  ceased.  By  much  the  most  abundant  of  them  all  is  water-^'as,  which, 
ultimately  escaping  as  steam,  has  been  estimated  to  foini  y^jVa^hs  of 
the  whole  cloud  that  hangs  over  an  active  volcano  (Fig.  40).  In  great 
eruptions,  steam  rises  in  prodigious  quantities,  and  is  rapidly  condensed 


194  DVXAMICAL  GEOLOGY  book  hi  parti 

into  a  heavy  rainfall.  M.  Fouqui^  calculated  that,  during  100  days,  one  of 
the  parasitic  cones  on  Etna  had  ejected  vajmnr  enough  to  form,  if  condenaed, 
2,100,000  cubic  mfetrcs  (462,000,000  gallone)  of  water.  But  even  from 
volcanoes  which,  like  the  Solfatara  of  Naples,  have  been  dormant  for 
centuries,  steam  sometimea  still  rises  without  intennission  and  in  con- 
siderable volume.  Jets  of  vaponr  nish  out  from  clefte  in  the  sides  and 
bottom  of  a  crater  with  a  noise  like  that  made  by  the  stoam  blown  off 
by  a  locomotive.  The  number  of  these  funnels  or  '"  fumaroles  "  is  often  so 
largo,  and  the  amount  of  vapour  so  abundant,  that  only  now  and  then, 
when  the  wind  blows  the  dense  cloud  aside,  can  a  momentary  glimpte 
be  had  of  u  part  of  the  bottom  of  the  crater ;  while  at  the  same  time  tlic 


rush  and  roar  of  the  escaping  steam  remind  one  of  the  din  of  some  mt 
factory.  Aqueous  vajjour  rises  likewise  from  rents  on  the  outside  of 
the  volcanic  cone.  It  issues  so  copiously  from  some  flowing  lavas  Uut 
the  stream  of  rock  may  be  almost  concealed  from  view  by  the  cloud ;  and 
it  continues  to  escape  from  Usaurca  of  the  lava,  far  below  the  point  of 
exit,  for  a  long  time  after  the  ixick  has  solidified  and  come  to  rest.  So 
satuiHted  are  numy  molten  lavas  with  water-va|)Our  that  Mr.  Scrc^ 
thought  that  they  owed  their  mobility  to  this  cause,'  In  the  deep  vol- 
canic magma  the  water-substance  must  be  far  above  its  critical  temperatan^ 
which  is  about  773°  Kahr. 

Probably  in  no  case  is  the  st«am  mere  pure  vapour  of  water,  thou^ 

when  it  condenses  into  copious  rain,  it  is  fi-csh  and  not  salt  water.     It  ii 

associated  with  other  vapours  and  gases  disengaged  from  the  potent 

chemical  laboratory  nndei'neath.     There  seems  to  be  always  a  definite 

I  ' Cousidiinitious  on  Volcanoet'  (182G),  p.  IIO. 


5CT.  i  §  1  VOLCANIC  GASES  AND  VAPOURS  195 

tier  in  the  appearance  of  these  vapours,  though  it  may  vary  for 
ifferent  volcanoes.  The  hottest  and  most  active  "  fumaroles,"  or 
ipour- vents,  may  contain  all  the  gases  and  vapours  of  a  volcano, 
It  as  the  heat  diminishes,  the  series  of  gaseous  emanations  is  reduced. 
bus  in  the  Vesuvian  eruption  of  1855-56,  the  lava,  as  it  cooled  and 
urdened,  gave  out  successively  vapours  of  hydrochloric  acid,  chlorides, 
id  sulphurous  acid ;  then  steam ;  and,  finally,  carbon -dioxide  and 
imbustible  gases.^  More  recent  observations  tend  to  corroborate  the 
ductions  of  C.  Sainte-Claire  Deville  that  the  nature  of  the  vapours 
reived  depends  on  the  temperature  or  degree  of  activity  of  the  volcanic 
ifice,  chlorine  (and  fluorine)  emanations  indicating  the  most  energetic 
liase  of  eruptivity,  sulphurous  gases  a  diminishing  condition,  and 
xbonic  acid  (with  hydrocarbons)  the  dying  out  (of  the  activity.^  A 
solfatara,"  or  vent  emitting  only  gaseous  discharges,  is  believed  to 
IS6  through  these  successive  stages.  Wolf  observed  that  on  Cotopaxi 
hile  hydrochloric  acid,  and  even  free  chlorine  escaped  from  the  summit 

the  cone,  sulphuretted  hydrogen  and  sulphurous  acid  issued  from  the 
iddle  and  lower  slopes.  ^  Fouqu^^s  studies  at  Santorin  have  shown  also 
lat  from  submarine  vents  a  similar  order  of  appearance  obtains  among 
le  volcanic  vapours,  hydrochloric  and  sulphurous  acids  being  only  found 

points  of  emission  having  a  temperature  above  100°  C,  while  carbon- 
oxide,  sulphuretted  hydrogen,  and  nitrogen  occur  at  all  the  fumaroles, 
'en  where  the  temperature  is  not  higher  than  that  of  the  atmosphere.*^ 

Tlie  following  are  the  chief  gases  and  acids  evolved  at  volcanic  fumaroles.  Hydro- 
iloric  acid  is  abundant  at  Vesuvius,  and  probably  at  many  other  vents  whence  it 
s  not  been  recorded.  It  is  recognisable  by  its  pungent,  suffocating  fumes,  which  make 
proach  difficult  to  the  clefts  from  which  it  issues.  Sulphuretted  hydrogen  and 
Iphurous  acid  are  distinguishable  by  their  odours.  The  liability  of  the  fonner 
s  to  decomposition  leads  to  the  deposition  of  a  yellow  crust  of  sulphur  ;  occasionally, 

*  C.  Sainte-Claire  Deville  and  Leblanc,  Ann.  Vhim.  et  Phys.y  1858,  Hi.  p.  19  et  scq. 
T   accounts  of  Vesuvius  and   its   eruptions,   besides   the   general   works   already  cite<l 

p.  191,  consult  J.  PhiUips*  *  Vesuvius,'  1869  ;  'Mount  Vesuvius,'  J.  L.  Lobley,  1889  ; 

Schmidt,  *Die  Eruption  des  Vesuv.  1855,'  Vienna,  1856;  Mercalli's  *  Vulcani,  &c.* ; 
.  J.  Johnston-Lavis,  Q.  J,  Oed,  Soc,  xl.  35  ;  Geol.  Mwj,  1888,  p.  445.  A  diary  of  the 
Icano's  behaviour  for  six  months  is  given  in  Naturej  xxvi.  ;  one  for  four  years  (1882-1886) 

Dr.  Johnston- La  vis  *Spettatore  del  Vesuvio,'  Naples,  1887  ;  a  valuable  series  of  reports 

the  mountain  by  the  same  author  will  be  found  in  recent  volumes  of  the  Reports 
the  British  Association  (1885-91)  and  a  large  detailed  map  of  the  volcano,  also  by  him, 
blished  by  Philip,  London,  1891. 

^  He  distinguished  volcanic  emanations  according  to  their  order  of  appearance  as 
pkrdi  time,  nearness  to  the  vent,  and  temperature  :  viz.,  1.  Dry  fumaroles  (without 
tarn),  where  anhydrous  chlorides  are  almost  the  only  discharge,  and  where  the  temjyera- 
re  ill  very  high  (above  that  of  melted  zinc).  2.  Acid  fumaroles,  with  sulphurous  and 
drochloric  acids  and  steam.  3.  Alkaline  (ammoniacal)  fumaroles  ;  temperature  about 
0*  C.  ;  abundant  steam  with  chloride  of  ammonium.  4.  Cold  fumaroles ;  temperature 
low  100**  C,  with  nearly  pure  steam,  accompanied  by  a  little  carbon-dioxide,  and  sometimes 
Iphwetted  hydrogen.  5.  Mofettes  ;  emanations  of  carbon -dioxide  with  nitrogen  and 
ygen,  marking  the  last  phase  of  volcanic  activity. 

»  SeutsJahrh.  1878,  p.  164.  *  'Santorin  et  ses  eruptions,'  Paris,  1879. 


196  DYNAMICAL  (iEOLOCrY  book  ill  pabt  i 


also,  the  production  of  siilplniric  acid  is  observed  Ht  active  vents.  From  oWm- 
tions  made  at  Vesuvius  in  May  1878,  Mr.  Siemens  concluded  that  vast  quantitim  of 
free  hydrogen  or  of  comhustihle  compounds  of  this  gas  exist  dL»Hi)lve(1  in  thf 
magma  of  the  earth's  interior,  and  that  these,  rising  and  exploding  in  the  funnels  of 
volcanoes,  give  rise  to  the  detonations  and  clouds  of  steam.*  At  the  eniptiun  of 
Santoriu  in  1S66,  the  same  gases  were  also  distinctly  recognisetl  by  Fouque,  who  forthr 
fii-sttime  established  the  existence  of  true  volwuiii;  Hames.  These  wei-o  again  studifd 
s]>ectros(roi)ically  in  the  folli>wing  y(?ar  by  Janssen.  who  found  them  to  ariMe  essentullj 
from  the  combustion  of  free  hydrogen,  but  with  tmces  of  chlorine,  soda,  and  copjiw. 
Fou<iuc  determined  by  analysis  that,  immediately  over  the  focus  of  eruption,  tnt 
hydrogen  formed  thirty  i»er  eent  of  the  gases  emitted,  but  that  the  projM>rtion  of  tbb 
gas  m[)idly  diminisluHl  with  distance  from  the  active  vents  and  Iiotter  lavas,  while  at 
the  same  time  the  [iro[>ortion  of  marsh-gas  and  carbon -dioxide  mpidly  increased.  Tlif 
gaseous  emanations  collected  by  him  were  foiuul  to  contain  abundant  free  oxygeu  a» 
well  as  hydrogen.  One  analysis  gjive  the  following  results :  carlxin -dioxide  0*22, 
oxygen  21*11.  nitrogen  "Jl'S^O,  hydrogen  56"70,  marsh-gas  0*07,  =  100*00.  Tliift  gawoiu 
ifiixture,  on  coming  in  contact  with  a  burning  bcM.ly,  at  once  ignites  with  a  sharp 
ex])losion.  Fouquc  infci*s  that  the  water-vajMHir  of  v<»lcanic  vents  may  exist  in  a  fttste 
of  dissociation  within  th«?  molten  magma  whence  lavas  rise.-  Carbon -dioxide  risn 
chieily  (a)  after  an  eruiition  has  ceiised  and  the  volcan<>  relajwea  into  quiescence  :  or  {h) 
after  v<»lcanic  action  has  otherwise  Income  extinct.  Of  the  former  ])hase,  instances  ire 
on  reconl  at  Vesuvius  where  an  eniption  has  been  followed  by  the  emission  of  this  gw 
so  copiously  from  the  ground  as  to  suffocate  hundreds  of  hares,  pheasants,  and  i«irtridgn. 
Of  the  second  plui84\  good  examples  are  supplied  by  the  ancient  volcanic  regions  of  the 
Eifel  and  Auvergnc,  where  the  gas  still  rises  in  prodigious  quantities.  Bischof  estimated 
that  the  volume  of  carbonic  acid  evolved  in  the  Brohl  Thai  amounts  to  5,000,000  cubie 
feet,  or  300  tons  of  gas  in  one  <iay.  Nitrog(;n,  derived  perhajw*  from  the  decompoa- 
tion  of  atmospheric  air  dis.solvcd  in  the  water  which  iHMietrates  into  the  volcanie  foil, 
has  been  frequently  detecteil  among  the  gaseous  emanations.  At  Santorin  it  was  foimd 
to  fonn  from  \  to  ^8  ]»cr  cent  of  the  gas  obtained  from  ditferent  fumaroles.'  Fluorinr 
and  iodine  have  likewise  been  noticed. 

AVith  these  ga^ses  and  vaiM)ui-s  are  associated  many  substances  which,  sublimed  b)' 
the  volwinic  heat  or  resulting  from  relictions  among  the  escajung  vaix>ui'8,  apjiear  ii 
Sublimates  along  crevices  wherein  they  reach  the  air  and  are  cooled.  Uesidei 
sulphur,  there  are  several  chlorides  (imrticularly  that  <>f  sodium,  and  less  alnindaDtlr 
those  of  potassium,  iron,  copiwM",  and  lead);  als(>  free  sulphuric  acid,  sal- 
ammoniac,  specular  iron,  oxide  of  copper,  boracic  acid,  alam,  sulphate 
of  lime,  felspars,  pyroxene,  and  other  substances.  Carbonate  of  soda  occnn  in 
large  quantities  among  the  fumaroles  of  Etna.  Sodium -chloride  sometimes  ap]iean  » 
abundantly  that  wide  sjuices  of  a  volcanic  cone,  as  well  as  of  the  newly-erupted  lava, 
ai*e  crusted  with  s;ilt,  which  can  even  be  ]»r<»titably  remove<l  by  the  inhabitants  of  tBf 
district.  Ci»nsiderable  i|uantities  of  chlorMes,  &c.,  may  thus  Ik;  buried  between  mc- 
cessive  sheets  of  lava,  and  in  long  subse^iucnt  times,  may  give  rise  to  mineral  spriogi^ 
as  has  been  suggested  with  reference  to  the  saline  waters  which  issue  from  volcanic  rocks 
of  Old  Kttd  Sandstone  and  Carboniferous  age  in  Scotland."*  The  iron-chloride  forms  i 
bright  y(?lli»w  and  reddish  crust  on  the  crater  walls,  jls  well  as  on  loose  stones  on  the 
slojies  of  the  cone.  S]>eeular  inm,  from  the  decomposition  of  ii'on -chloride,  fonn» 
abundantly  as  thin  lamelhi*  in  the  tissures  of  Vesuvian  lav;us.  In  the  spring  of  1873 
the  author  observed  delic^ite  l)rown  filaments  of  tenorite  (coiqier-oxide,  CuO)  forming  in 
clefts  of  the  crater  of  Vesuvius.      They  were  u])held  by  the  ui^ti'eaming  current  of 

^  Mount sb.  K.  J^rt'uss.  A  knit.  1878.  p.  588. 
■-'  Fouque,  ' Sautoriu  et  ses  eruptions.'  p.  2*25.  •'  Fouque,  loe.  ei(. 

*  PriH'.  Rou.  Sihi.  Edxn.  ix.  p.  367. 


SECT,  i  §  1  WATER  IN  VOLCANIC  ACTION  197 

vapour  until  blown  off  by  the  wind.  Fouque  has  described  tubular  vents  in  the  lavas 
of  Sautorin  with  crystals  of  anorthite,  sphene,  and  pyroxene,  formed  by  sublimation. 
Ill  the  lava  stalactites  of  Hawaii  needle-like  fibres  of  breislakite  abound. 

2.  Water. — Abundant  discharges  of  water  accompany  some  volcanic 
explosions.  Three  sources  of  this  water  may  be  assigned: — (1)  from 
the  melting  of  snow  by  a  rapid  accession  of  temperature  previous  to  or 
during  an  eruption  ;  this  takes  place  from  time  to  time  on  Etna,  in 
Iceland,  and  among  the  snowy  ranges  of  the  Andes,  where  the  cone  of 
Cotopaxi  is  said  to  have  been  entirely  divested  of  its  snow  in  a  single 
night  by  the  heating  of  the  mountain  ;  (2)  from  the  condensation  of  the 
vast  clouds  of  steam  which  are  discharged  diuing  an  eruption ;  this 
undoubtedly  is  the  chief  source  of  the  destructive  torrents  so  frequently 
obeerv'ed  to  form  part  of  the  phenomena  of  a  great  volcanic  explosion ; 
and  (3)  from  the  disruption  of  reservoirs  of  water  filling  subterranean 
caAities,  or  of  lakes  occupying  crater-basins ;  this  has  several  times 
been  observed  among  the  South  American  volcanoes,  where  immense 
quantities  of  dead  fish,  which  inhabited  the  water,  have  been  swept 
down  with  the  escaping  torrents.  The  volcano  of  Agua  in  Guatemala, 
received  its  name  from  the  disruption  of  a  crater-lake  at  its  summit 
by  an  earthquake  in  1540,  whereby  a  vast  and  destructive  debacle  of 
water  was  discharged  down  the  slopes  of  the  mountain.^  In  the 
beginning  of  the  year  1817,  an  eruption  took  place  at  the  large  crater 
of  Idjt'n,  one  of  the  volcanoes  of  Java,  whereby  a  steaming  lake  of  hot 
acid  water  was  discharged  Avith  frightful  destruction  doA\Ti  the  slopes  of 
the  mountain.  After  the  explosion,  the  basin  filled  again  ^vith  water, 
but  its  temperature  wjis  no  longer  high.- 

In  many  cases,  the  water  rapidly  collects  volcanic  dust  as  it  rushes 
down,  and  soon  becomes  a  pasty  mud ;  or  it  issues  at  first  in  this 
condition  from  the  volcanic  reservoirs  after  violent  detonations. 
Hence  arise  what  are  termed  mud-lavas,  or  aqueous  lavas,  which  in 
many  respects  behave  like  true  lavas.  This  volcanic  mud  eventually 
consolidates  into  one  of  the  numerous  forms  of  tuff,  a  rock  which,  as 
has  been  already  stated  (p.  1.'^^)),  varies  gieatly  in  the  amount  of  its 
coherence,  in  its  com})osition,  and  in  its  internal  arrangement. 
Obviously,  unless  where  subsequently  altered,  it  cannot  possess  a 
crystalline  structure  like  that  of  true  lava.  As  a  rule,  it  betrays  its 
aqueous  origin  by  more  or  less  distinct  evidence  of  stratification,  by 
the  multifarious  pebbles,  stones,  blocks  of  rock,  tree-trunks,  branches, 
shells,  bones,  skeletons,  &c.,  which  it  has  swept  along  in  its  course  and 
preserved  within  its  mass.  Sections  of  this  comjxicted  tuff  may  be  seen 
at  Herculaneum.^     The  trass  of  the  Brohl  Thai  and  other  valleys  in  the 

*  For  an  .account  of  this  mountain  see  K.  v.  Seebacli,  Ahh.  fit'si-U.  UVs.s-.  ar,tfimji',}^ 
xxxviii.  (1892)  p.  216. 

*  See  Junghuhu's  *  Java.'  For  an  account  of  the  volcanoes  of  the  Suiula  Islaiul  and 
Moluccas,  see  F.  Scheider,  Jahrh.  Oatf.  Rckhsanttt.  Vienna,  xxxv.  (1885).  p.  1.  Consult 
alno  for  the  Javanese  volcanoes  the  works  on  Krakatoa  (pioU'd  on  ]».  212. 

'  Mallet  thought  that  the  so-called  **  mud-lavas  "  of  Herculaneum  and  Ponijieii  were  not 
aqueous  deposits  {Journ,  Jtot/.  ffeoL  Sim:  Jn'/amf,  IV.  (1876),  p.  144).  But  there  seems  no 
reason  to  doubt  that  while  an  enormous  amount  of  ashes  fell  during  the  eruption  of  a.d.  79, 


198  DYNAMICAL  GEOLOGY  book  iii  parti 


Eifel  district,  referred  to  on  p.  137,  is  another  example  of  an  ancient 
volcanic  mud. 

3.  Lava. — The  t^rm  lava  is  applied  generally  to  all  the  molten 
rocks  of  volcanoes.^  The  use  of  the  word  in  this  broad  sense  is  of 
great  convenience  in  geological  descriptions,  by  directing  attention 
to  the  leiiding  character  of  the  rocks  as  molten  products  of  volcanic 
action,  and  obviating  the  confusion  and  errors  which  are  apt  to  arise 
from  an  ill-define<l  or  incorrect  lithological  terminology.  Precise 
definitions  of  the  rocks,  such  as  those  above  given  in  Book  II.,  can 
be  added  when  required.  A  few  remarks  regarding  some  of  the 
general  lithological  characters  of  lavas  may  be  of  service  here ;  the 
behaviour  of  the  rocks  in  their  emission  from  volcanic  orifices  will  be 
described  in  5^  2. 

While  still  flowing  or  not  yet  cooled,  lavas  diflVr  from  each  otiier  in  the  extent  to 
which  they  an^  impregnated  with  gases  and  vapour?*.  Some  appear  to  be  saturated, 
others  contain  a  mnch  smaller  gaseous  im[>regnation  :  and  hence  arise  important 
distinctions  in  their  behaviour  (pp.  217-231).  After  solidification,  lavas  present  iome 
noticeable  charactei-s,  then  easily  ascertainable.  (1)  Their  average  specific  graritr 
may  ^»e  taken  as  ranging  between  2*37  and  3-22.  (2)  The  heavier  varieties  vontUD 
mueh  magnetic  or  titaniferous  iron,  with  augite  and  olivine,  their  composition  being 
l»asie,  and  their  pro]M)rtion  of  siliea  averaging  alx)Ut  4r>  to  55  i>er  cent.  In  this  groDp 
come  the  basalts,  nepheline-lavas.  and  leucite-lavas.  The  lighter  varieties  contain  com- 
monly a  minor  proiM)rtion  of  nietallie  bashes,  but  are  rich  in  silica,  their  ]>ercentage  of 
that  acid  ranging  between  70  an«l  75.  They  arc  thus  not  basic  but  acid  rocks.  Among 
their  more  ini]><)rtant  varieties  are  the  rhyolites  ami  obsidians.  Some  intermediate 
varieties  (trachytes,  jihonolites,  and  andesites)  connect  the  acid  and  Itasie  series.  (3) 
Lavas  differ  mueh  in  structure  and  texture,  (a)  Some  are  entirely  crystalline,  consisting 
of  an  interlaced  mass  of  crystals  anrl  crystalline  p^ntieles,  as  in  some  dolerites,  and 
granitoid  rhyolites.  Even  quartz,  which  used  to  be  eonsideretl  a  non-volcanic  minnal, 
cliaracteristic  of  the  older  and  ehielly  of  the  plutonie  eruptive  rocks,  has  ?>een  oUserred 
in  large  crystals  in  modern  lava  (1i]Mirite  and  ijuartz-andcsite '-').  {h)  Some  show  more  or 
less  of  a  half-glassy  or  stony  (devitritled)  matrix,  in  which  the  constituent  crystals  u« 
imbedded  ;  this  Ls  the  most  common  arrangement,  (c)  Others  arc  entirely  vitreous,  sndi 
crystals  or  crystalline  j»articles  as  occur  in  them  being  (piitc  subordinate,  and,  so  to 
s[ieak,  accidental  enclosures  in  the  main  glassy  mass.  Obsidian  or  volcanic  glass  is  the 
tyiKi  of  this  group,  {d)  They  further  differ  in  the  extent  to  which  minute  pom  or 
larger  cellular  spaces  have  been  develo[)ed  in  them.  According  to  Bischof,  the  {>oroatj 
of  lavas  dei>ends  on  their  degree  of  liquidity,  a  pnous  lava  or  slag,  when  reduced  in  lii« 
fusion-exi>erinionts  to  a  thin  Mowing  eonsistcncy.  hardening  into  a  mass  as  compact  u 
the  densest  lava  or  basit It.'*    Tlie  ]>resenee  of  interstitial  steam  in  lavas,  by  exjiandiiig 


there  were  likewise,  especially  in  the  later  phjises  of  eruption,  copious  torrents*  of  water  that 
nilngled  with  the  tine  ash  and  beoame  '*  mud-lavas."  The  shar}^)uess  of  outline  and  thr 
absence  of  any  trace  of  alMlondnal  ilistension  in  the  moulds  of  the  human  bodies  found  at 
Pompeii,  probably  show  that  these  victims  of  the  catastrophe  were  rapidly  enveloped  in  t 
firm  coherent  matrix  which  could  hanlly  have  heen  mere  loose  dust.  See  H.  J.  Johnston- 
fiavis,  ^.  ./.  Ofol,  Svc.  xl.  p.  .Sl>. 

^  '' Alles  ist  Ijava  was  ini  Vulkaue  tliesst  und  durch  seine  Fliissigkeit  neue  Lagerstltter 
einniiumt  "  is  Leopold  von  Buch's  coniprcbcusive  definition. 

-  Wolf,  yevesJohHj.  1874,  p.  377. 

^  '  Chem.  und  Fhys.  Geol.'  su])ii.  (1871),  p.  144.     On  the  production  of  the  vesicular 


SECT,  i  §  1      FRAGMENTARY  MATERIALS  FROM  VOLCANOES       199 

the  still  molten  stone,  produces  an  open  cellular  texture,  somewhat  like  that  of  sponge 
or  of  bread.  Such  a  vesicular  arrangement  very  commonly  appears  on  the  upper  surface 
of  a  lava  current,  which  assumes  a  slaggy  or  cindery  aspect.  In  some  forms  of  pumice 
the  proportion  of  air  cavities  is  8  or  9  times  that  of  the  enclosing  glass.  (4)  Lavas  vary 
greatly  in  colour  and  general  external  aspect.  The  heavy  basic  kinds  are  usually  dark 
grey,  or  almost  black,  though,  on  exposure  to  the  weather,  they  acquire  a  brown  tint 
from  the  oxidation  and  hydration  of  their  iron.  Their  surface  is  commonly  rough  and 
ragged,  until  it  has  been  sufficiently  decomposed  by  the  atmosphere  to  crumble  into  soil 
which,  under  favourable  circumstances,  supports  a  luxuriant  vegetation.  The  less  dense 
lavas,  such  as  phonolites  and  trachytes,  are  frequently  paler  in  colour,  sometimes 
yellow  or  buff,  and  decom^KMse  into  light  soils  ;  but  the  obsidians  present  rugged  black 
sheets  of  rock,  roughened  with  ridges  and  heaps  of  grey  froth -like  pumice.  Some  of  the 
most  brilliant  surfaces  of  colour  in  any  rock -scenery  on  the  globe  are  to  be  found  among 
volcanic  rocks.  The  walls  of  active  craters  glow  with  endless  hues  of  red  and  yellow. 
The  Qrand  Ca&on  of  the  Yellowstone  River  has  been  dug  out  of  the  most  mar\'ellously 
tinted  lavas  and  tuffs. 

4.  Fragmentary  Materials. — Under  this  title  may  be  included  all 
the  substances  which,  driven  up  into  the  air  by  volcanic  explosions,  fall  in 
solid  form  to  the  ground — the  dust,  ashes,  sand,  cinders,  and  blocks 
of  every  kind  which  are  projected  from  a  volcanic  orifice.  These 
materials  differ  in  composition,  texture,  and  appearance,  even  during 
a  single  eruption,  and  still  more  in  successive  explosions  of  the  same 
volcano.  For  the  sake  of  convenience,  separate  names  are  applied  to 
some  of  the  more  distinct  varieties,  of  which  the  following  may  be 
enumerated. 

(1)  Ashes  and  sand. — In  many  eruptions,  vast  (quantities  of  an  exceedingly  fine 
light  grey  powder  are  ejected.  As  this  substance  greatly  resembles  what  is  left  after  a 
piece  of  wood  or  coal  is  burnt  in  an  o\f%n  lire,  it  has  been  (K)pularly  termed  ash,  and  this 
name  has  been  adopted  by  geologists.  If,  however,  by  the  word  ash,  the  result  of  com- 
bustion is  implied,  its  employment  to  denote  any  product  of  volcanic  action  must  be 
regretted,  as  apt  to  convey  a  wrong  impression.  The  fine  ash-like  dust  ejected  by  a 
volcano  is  merely  lava  in  an  extremely  fine  state  of  comminution.  So  minute  are  the 
particles  that  they  find  their  way  readily  through  the  finest  chinks  of  a  closed  room,  and 
■rttle  down  upon  floor  and  furniture,  as  ordinary'  dust  doe«  when  a  house  is  shut  up. 
From  this  finest  form  of  material,  gradations  may  be  ti*aced,  through  what  is  termed 
Tolouiic  sand,  into  the  coarser  varieties  of  ejected  matter.  In  composition,  the  ash  and 
sand  vary  necessarily  with  the  nature  of  the  lava  from  which  they  are  derived.  Their 
microscopic  structure,  and  esjiecially  their  abundant  niicrolites,  crystals,  and  volcanic 
glass,  have  been  already  referred  to  (pp.  136,  137). 

(2)  Lapilli  or  rapilli  (p.  136)  are  ejected  fragments  ranging  from  the  size  of  a  pea 
to  that  of  a  walnut ;  round,  subangular,  or  angular  in  shape,  and  having  the  same  inde- 
finite range  of  composition  as  the  finer  dust.  As  a  rule,  the  larger  pieces  fall  neai*est 
the  focus  of  eruption.  Sometimes  they  are  solid  fragments  of  lava,  but  more  usually 
they  have  a  cellular  texture,  while  sometimes  they  arc  su  light  and  porous  as  to  float 
rcftdily  on  water,  and,  when  ejected  near  the  sea,  to  cover  its  surface.  \Vell-fonncd 
crystals  occur  in  the  lapilli  of  many  volcanoes,  and  are  also  ejected  sei^arately.  It  has 
been  observed  indeed  that  the  fragmentary  materials  not  infre<|uently  contain  finer 
crystals  than  the  accompanying  lava.' 

structure  consult  Dana,  'Characteristics  of  Volcanoes,'  p.  161.     Compare  also  Judd,  Oeol. 
Mng,  1888,  p.  7. 

'  Sartorins  von  Waltershausen,  'Sicilien  mid  Island,'  ISf'B,  p.  328. 


200 


DYXAMIfAL  OEOLOUY 


(3)  Voluonii:  Blocks  (]i.  136)  arc  larger  pipces  of  stone,  often  aiif^l&r  in  ihape.  Ii 
Koiue  vane*  they  ati[<PAr  to  lie  frnpiiriits  loowurd  fmiii  nlready  solidified  rocki  in  tkl 
rliimiwy  oftlie  voivaiio.  Uriicc  we  liiid  atiioug  them  \aecta  of  non-volcaiiic  nx^  If 
well  IB  of  older  tulfs  and  Iovsh  recogiiiiiaMy  lii'loiiginfc  to  early  eniptioiut.  In  many 
vases,  tliey  are  fjevted  in  enoriiioUB  i[uantitieH  ilurint;  the  earlier  pliaiwa  of  rioleit 
eriiiitioii.  T]ie  great  exi>loHio]i  from  the  Hide  of  Amrot  in  18J0  U'ut  Bcvompuiied  lif 
tlic  diKelmr;^  of  a  vaxt  i^uanlity  of  ftKgineiitH  over  a  tijiaco  of  many  square  niilea  amold 
the  nioiiiitaiii.  Whitney  han  ilewKlied  the  occurreura  in  California  of  beds  of  od 
fragmentary  volcailio  lirecfin,  linndredx  of  feet  thiek  and  covering  niuiy  sqiiaro  mileirf 
Hurfiiee.  Junghidin,  in  hiH  ai-coiint  of  the  eni^ition  in  Java  in  177S,  nieutiona  that  a 
valley  ten  inileH  long  was  filled  to  an  nvenige  depth  of  Hfty  feet  with  angular  Tokanie 

Among  the  earlier  eruiitioiiH  of  u  volcano,  fraginentH  of  the  rocks  through  nliicb  tk( 
vi'Tit  has  1>ct'n  drilletl  may  fii-iineiitly  he  ohiierved.  These  txv  in  many  caaes  not  Tolcanie. 
lilockH  of  ajliiat  and  granitoid  rockn  occur  in  tlic  cinder-licdg  at  the  baae  of  the  voleaaie 
Hcries  r)f  Santoriii.  In  thr  oUlei'  tulfK  iif  Soninia,  [lieces  of  altered  liinostouo  (Bonutuna 
iiieasnrilig  200  i-illnc  feet  or  more  anil  weighing  upwards  of  IS  tons)  orp  abuudant  and 
often  [contain  cavities  lineil  with  the  eliami'terintic:  "  Viwuvian  mineralH." '  Bloclci  of  a 
coarsely  (TyHtnlliuc  granitoid  (Init  really  tmchytic}  lava  liai'e  been  [larticularly  obKmd 
lioth  on  Etna'  and  VennviuB.  In  the  year  ISiO  a  niasx  of  that  kind,  weighing  Herenl 
tonn,  WOK  to  W  seen  lying  at  llie  foot  of  the  njijier  cone  iif  Vesuvius,  witliiu  the  entnaee 
to  the  Atrio  ilel  Cavallo.  Similar  lilocki  occur  among  tlie  Carboaiferoua  volcajiie  pipM 
of  iTeutral  Hcotlaiiil,  tugetliersonietinieH  with  fragments  of  santbitone,  shale,  or  limeitona, 
not  ill  frequently  full  of  Carboniferous  fowiln.''  Enonuoua  nuuses  of  variouB  schists  han 
Win  tarricl  up  liy  the  lavas  uf  the  Tertinry  volcanic  plateau  of  the  Inner  Hebrides  ' 

(4)  \  ol  a  Do  sad  slag  — Tl  ese  )  a  onginallj  formed  portiona  of  tk 
-ot  of  la  a  lUK-e    1    g  tl  e  ]  Ijir  of  a    olta  o  aud  1  a      been  dctacl  ed  •  d  hniU 

t      tie    ar     >    H      -euH         c   j  In  f   ntvn  \  (Fg     -11     is    a    roolli 


'  But  nee  the  reiuarlu  alreaily  made  on  volcanic  cougloiiiersteH,  mtlt,  p.  ISM. 
-  See  H.  J.  JolmHtan-f-ivis  y.  J.  iJoJ.  f-e.  xl.  p.  7H. 

'  For  till'    eruiiteil   blocks   (AnswilrHinge)   of  Etna  see    '  Der   Aetua,'   IL   ppL   211, 
330,  4»1. 

*  TfBU*.  Iby.  Hk.  RUh.  x\\*.  p.  -1.19.     Seejiiw'rt'.  Book  IV.  Sect.  vii.  1 1.  i. 
'  Trans,  llry.  .So,-.  Rli«.  XXXV.  (1888,i,  [^  82. 


BBCT.  i  §  1       FHAGMEXTARY  MATERIALS  FROM  VOLCAXoES       201 


elliptical,  or  i)ear-.s}ja|»ed,  often  iliscoidal  mass  of  lava,  from  a  few  inclies  to  s<>veral  feet 
ill  diameter ;  sometimes  tolerably  solid  throughout,  more  usually  coarsely  cellular  in- 
iiide.  Xc)t  iiifre<iueutly  its  interior  is  hollow,  and  the  bomb  then  consists  of  a  shell 
which  ia  most  close-grained  towards  the  outside,  or  the  centre  is  a  block  of  stone 
with  an  external  coating  of  lava.  There  can  be  no  doubt  that,  when  torn  by  eructations 
of  steam  from  the  surface  of  the  boiling  lava,  the  material  t»f  these  Iwmbs  is  in 
mK  thoroughly  molten  a  condition  as  the  rest  of  the  mass.  From  the  rotatory 
uiotion  im|)arted  by  its  ejection,  it  takes  a  circular  fonn,  an<l  in  i)ro})ortion  to  its 
rapidity  of  rotation  and  fluidity  is  the  amount  of  its  "flattening  at  the  j»ole.s."  The 
centrifugal  force  within  allows  the  expansion  of  the  interstitial  vapour,  while  the  outer 
Hurfiu'e  rapidly  cools  and  solidities  ;  hence  the  solid  crust,  and  the  ^jorous  or  cavernous 
interior.  Such  Ijomlw,  varying  from  the  size  of  an  apjile  to  that  of  a  man's  body,  were 
found  by  Darwin  abundantly  strewn  over  the  gi-ouml  in  the  Island  of  Ascension  ;  they 
were  ako  ejected  in  vast  <[uantities  during  the  eruption  of  Santorin  in  1866.^  Among 
the  tulfa  of  the  Eifel  region,  small  bombs,  consisting  mostly  of  gi-anular  olivine,  are  of 
common  occurrence,  as  also  jueces  of  wuiidine  or  other  less  fusible  minerals  which  have 
Mfp^egfttcd  out  of  the  magma  liefore  ejection.  In  like  manner,  among  the  tuffs  tilling 
volcanic  necks,  pi*ol»ably  o(  Pennian  age,  which  pierce  the  C'arlM)niferous  rocks  of  Fife, 
large  worn  cr}'stals  of  orthoclaae,  biotite,  Ac,  are  found.  AVhen  the  ejected  fragment 
of  lava  has  a  rough  irregidar  form  and  a  (lorous  stnu-tuie.  like  the  clinker  of  an  iron- 
furnace,  it  is  known  as  a  slag." 

.The  fragmentary  materials  crupteil  by  a  volcano  and  dejK>8ited  around  it  aC4iuire  by 
degrees  more  or  less  consolidation,  j>artly  from  the  mei-e  pressure  of  the  higher  \i\)on 
the  lower  strata,  jiartly  from  the  influence  of  infiltrating  water.  It  has  l)een  already 
stated  (p.  136)  that  different  names  are  applied  to  the  rocks  thus  formed.  The 
coarf«e,  tumultuous  unstratified  accumulation  of  volcanic  jlcbris  within  a  crater  or 
funnel  is  called  Agglomerate.  When  the  <lcbris,  though  still  coarse,  is  more 
roimde«1,  and  is  arranged  in  a  stratified  form  on  the  sloiies  of  the  cone  or  on  the  country 
Iteyond,  it  becomes  a  Volcanic  Conglomerate.  The  finer-giuine<l  varieties,  formed 
of  dust  and  lapilli,  are  included  in  the  general  designati(Mi  of  Tuffs.  Those  are 
iwiially  pale  yellowish,  greyish,  or  brownish,  sometimes  black  n^ks,  giJinular,  porous. 
and  often  incoherent  in  texture.  They  occur  interstratified  with  and  pass  into  ordinary' 
non-voloanic  Mdiment. 

Orgudc  remains  8<imetimes  tMvnir  in  tuff.  AVhen*  volcanic  debris  has  aicuniulate<l 
OTier  the  floor  of  a  lake,  or  of  the  sea,  the  entombing  and  ))res4'rving  of  .shells  an4l  other 
oripilic  objects  must  continually  take  place.  Kxani)»les  of  this  kind  are  lited  in  later 
|Hgea  of  this  volume  from  older  geological  formations.  IVofe.ssor  (iui.scardi  of  Na])les 
foand  about  100  species  of  marine  shells  of  living  sjkk-Ics  in  the  old  tuffs  of 
VesDTioa.  Marine  shells  have  been  picked  up  within  the  «iater  of  Monte  Nuovo,  and 
have  been  frequently  ol>serve<l  in  the  old  or  marine  tuff  of  that  district.  Showers  of 
aMh,  or  Hheets  of  volcanic  mml,  often  pivserve  land-shells,  in.sect*,  and  vegefcition  living 
on  the  area  at  the  time.  The  ohler  tuffs  of  Vesuvius  have  yielde<l  many  remains  of  the 
shrubs  and  trees  which  at  sucoes.sive  jM*ri(Kls  have  clothed  the  flanks  of  the  mountain. 
Fragments  of  coniferous  woo<l,  which  once  grew  on  the  tutl'-«-ones  of  Carlioniferous 
age  in  i-entral  Scotland,  are  abundant  in  the  *'ne(*ks"  of  that  regi<»n,  while  the  minute 
stnicture  of  some  of  the  lepi<lo4lendroid  plants  has  also  been  admirably  ]»n'served  there 
in  tutf.^ 


*  Darwin,  'Geological  Obser>'ations  on  Volcanic  Islands."    2n(l  e»lit.  p.  12.      Fou(iu«'', 
*  Santorin/  \k  79. 

*  On    the  ratio   Wtween  the  pores  and   volume  nf  the  rock   in  slagx  and  lavas,   see 
determinations  by  Bischof,  'C'heni.  und  Phys.  (Jeol.'  sup|i.  (lf<71).  p.  IAS. 

*  TrtiH*.  Rtiif,  iSfJT,  Etiin.  xxix.  p.  470;  t^usteo.  Hook  IV.  Tart  VII.  Sect.  ii.  §  2. 


202  DYNAMICAL  GEOLOGY  book  m  pami 


§  2.  Volcanic  Action. 

Volcanic  action  may  be  cither  constant  or  peiiodic.  Stromboli,  in 
the  Mediterranean,  so  far  as  we  know,  has  been  uninterruptedly  emitting 
hot  stones  and  steam,  from  a  liasin  of  molten  lava,  since  the  earlittt 
I>eriod  of  history.^  Among  the  Moluccas,  the  volcano  Sioa,  and  in  tlie 
Friendly  Islands,  that  of  Tofua,  have  never  ceased  to  be  in  eruption  since 
their  first  discovery.  The  lofty  cone  of  Sangay,  among  the  Andes  of 
Quito,  is  alwavs  giving  off  hot  vapours  ;  Cotopaxi,  too,  is  ever  constantly 
active.^  But,  though  examples  of  unceasing  action  may  thus  be  cited 
from  widely  different  quarters  of  the  globe,  they  are  nevertheless  ex- 
ceptional. The  general  rule  is  that  a  volcano  breaks  out  from  time  to 
time  with  varying  vigour,  and  after  longer  or  shorter  inter\'als  ol 
quiescence. 

Active,  Dormant,  and  Extinct  Phases. — It  is  usual  to  class  volcanoei 
as  adive,  donnant^  and  exiinci.  This  arrangement,  however,  often  presenti 
considerable  difficulty  in  its  application.  An  active  volcano  cannot  of 
course  be  mistaken,  for  even  when  not  in  eruption,  it  shows  by  its 
discharge  of  steam  and  hot  vapoure  that  it  might  break  out  into  activity 
at  any  moment.  But  in  many  cases,  it  is  im^xissible  to  decide  whetlier 
a  volcano  should  l>e  called  extinct  or  only  dormant.  The  volcanoes  of 
Silurian  age  in  Wales,  of  Car]>onifei'ous  age  in  Ireland,  of  Permian  age 
in  the  Harz,  of  Miocene  age  in  the  Hebrides,  of  younger  Tertiary  age  in  the 
Western  States  and  Territories  of  North  Ameriai,  are  certainly  all  extinct 
But  the  older  Tertiary  volcanoes  of  Iceland  are  still  represented  there 
by  Skai)tAr-r7okulK  Hecla,  and  their  neighlwurs.^  Somma,  in  the  fint 
centiu*y  of  the  Christian  era,  would  have  been  naturally  regarded  as  in 
extinct  volcano.  Us  tires  had  never  been  known  to  have  been  kindled; 
its  vast  crater  was  a  wilderness  of  ^^-ild  vines  and  brushwood,  haunted,  no 
doubt,  by  wolf  and  wild  boar.  Yet  in  a  few  days,  during  the  autumn  of 
the  year  79,  the  half  of  the  crater  walls  was  blown  out  by  a  terrific  series 
of  explosions,  the  present  Vesuvius  was  then  formed  within  the  limita  of 
the  earlier  crat^M",  and  since  that  time  volcanic  action  has  been  inter- 
mittently exhibited  up  to  the  present  day.     Some  of  the  intervak  of 

*  For  accounts  of  StroiiOwili  sec  Si>alltanzaiii's  'VoyogL's  dans  les  deux  Siciles.*  ScTOpe's 
'  Voloanops. '  Ju«M,  Oo>l.  Mmj.  lS7r».  Mercalli's  *  Vulcani,  &o.*  p.  135  ;  ami  bis  pspeniB 
Atti  i<in:.  ItiiL  Si'i.  Sat.  xxii.  xxiv.  xxvii.  xxix.  xxxi.    L.  W.  Kulclier,  ^»V*i/.  J/rrijr.  1890,p.34i* 

-  For  descriptions  of  (!otopaxi»  hee  Wolf,  iWitva  Jnhrh.  1,S7S  ;  Whyuji>er,  Sai^rt^  nin. 
p.  ri'J3  :  *  Travels  amongst  tlie  Great  Andes,'  cha]).  vi. 

^  On  the  volcanic  plK^uoniena  of  Iceland  consult  G.  Mackenzie's 'Travels  in  the  blud 
of  Iceland  during  the  Summer  of  1810.'  K.  Hemlersou's  'Iceland.'  Zirkel,  *De  gwg* 
nostica  Islandu'  constituti(me  olKservationes,'  Bonn.  1S61.  Tlioroddsen,  'Ovenigt  owde 
islandske  Vulkauers  Historie,*  translat«Ml  in  resiune  by  (4.  H.  Boehmer,  SmithsoHtaH  M. 
Jirp.  1885,  part  i.  p.  495  ;  also  ni/tnn;/  t.  Srenak.  Vft.  Akmi,  Hanff/.  14,  ii.  (1888).  17,  ti- 
(1891) :  O'eoi.  Ma,j.  1S80,  p.  -IfiS  :  Suture,  Oct.  1884.  MUlh.  K.  K.  Urogr,  Oe*,  Vienna, 
xxiv.  (1891),  p.  117.  Keilhack,  /eitsrh,  Deutwh.  ii»:oK  Citwf.  xxxviii.  (1886).  p.  376; 
Schmidt,  np.  cif.  xxxvii.  (1885),  p.  737;  A.  Helland,  'Lakis  Kratere  og  Lavm-atriime,' 
rnicrrAifefs  /*nM/rinutii''j  Christiania,  1885;  Bruon.  Mrj'ologie  do  I'lAlande,  et  des  lik« 
Foeroe,"  Paris,  1884  ;  T.  Anderson.  Joftru.  St.  Arts,  vol.  xl.  (1892).  p.  397. 


SECT,  i  §  2  VOLCANIC  ACTION  203 

quietude,  however,  have  been  so  considerable  that  the  mountain  might 
then  again  have  been  claimed  as  an  extinct  volcano.  Thus,  in  the  131 
years  between  1500  and  1631,  so  comj^tely  had  eruptions  ceased  that 
the  crater  had  once  more  become  choked  with  copsewood.  A  few  pools 
and  springs  of  very  salt  and  hot  water  remained  as  memorials  of  the 
former  condition  of  the  mountain.  But  this  period  of  (]uiescence  closed 
with  the  eruption  of  1631, — the  most  powerful  of  all  the  known  ex- 
plosions of  Vesuvius,  except  the  great  one  of  79.  In  the  island  of 
lachia,  Mont'  Epomeo  was  last  in  eruption  in  the  year  1302,  its  previous 
outburst  having  taken  place,  it  is  believed,  about  seventeen  centuiies 
before  that  date.  From  the  craters  of  the  Eifel,  Auvergne,  the  Vivarais, 
and  central  Italy,  though  many  of  them  look  as  if  they  had  only  recently 
been  formed,  no  eruption  has  been  known  to  come  during  the  times  of 
human  history  or  tradition.  In  the  west  of  North  America,  from  Arizona 
to  Oregon,  numerous  stupendous  volcanic  cones  occur,  but  even  from  the 
mo6t  perfect  and  fresh  of  them  nothing  but  steam  and  hot  vapours  has 
yet  been  known  to  proceed. ^  But  the  presence  there  of  hot  springs  and 
geysers  proves  the  continued  existence  of  one  phase  of  volcanic  action. 

In  short,  no  essential  distinction  can  be  drawn  ]>etween  dormant 
and  extinct  volcanoes.  Volcanic  action,  as  will  be  afterwards  pointed 
oat,  is  apt  to  show  itself  again  and  again,  even  at  vast  intervals,  within 
the  same  regions  and  over  the  same  sites.  The  dormant  or  waning 
condition  of  a  volcano,  when  only  steam  and  various  gases  and  sublimates 
are  given  off,  is  sometimes  called  the  Solfatara  phase,  from  the  well- 
known  dormant  crater  of  that  name  near  Naples. 

Sites  of  Volcanic  Action. — Volcanoes  may  break  through  any  geo- 
logical formation.  In  Auvergne,  in  the  Miocene  period,  they  burst 
through  the  granitic  and  gneissose  plateau  of  central  France.  In 
Lower  Old  Eed  Sandstone  times,  they  pierced  contorted  Silunan  rocks 
in  central  Scotland.  In  late  Tertiary  and  j)ost-Tertiary  ages,  they  found 
their  way  through  recent  soft  marine  strata,  and  formed  the  huge  piles 
of  Etna,  Somma,  and  Vesuvius ;  while  in  North  America,  during  the 
same  cycle  of  geological  time,  they  flooded  with  lava  and  tuff  many  of 
the  river -courses,  valleys,  and  lakes  of  Nevada,  Utah,  Wyoming,  Idaho, 
and  adjacent  territories.  On  the  banks  of  the  Rhine,  at  Bonn  and  else- 
wherCy  they  have  penetrated  some  of  the  older  alluvia  of  that  river.  In 
many  instances,  also,  newer  volcanoes  have  appeared  on  the  sites  of  older 
ones.  In  Scotland,  the  Carboniferous  volciinoes  have  risen  on  the  ruins 
of  those  of  the  Old  Red  Sandstone,  those  of  the  Permian  period  have 
broken  out  among  the  earlier  Carboniferous  eruptions,  while  the  older 
Tertiaiy  dykes  have  been  injected  into  all  these  older  volcanic  masses. 
The  newer  pays  of  Auvergne  were  sometimes  erupted  through  much  older 
and  already  greatly  denuded  basalt-streams.  Somma  and  Vesuvius  have 
risen  out  of  the  great  Neapolitan  plain  of  older  marine  tuff,  while  in 
central  Italy,  newer  cones  have  been  thrown  up  ui)on  the  wide  Roman 
plain  of  more  ancient  volcanic  debris.-     The  vast  Snake  River  lava-fields 

*  firnpikms  oocnrred perhapsjess  than  1 00  years  ago.    I  )iller,  linU.  U.  .s'.  (»<•///.  .sv rr. .  No.  79. 
'  Accofding  to  Professor  G.  Pozzi,  the  principal  volcanic  outbursts  of  Italy  are  of 


204  DYXAMICAL  HEOLOtrY  book  m  pami 


of  Idaho  overlie  deiiucled  masses  of  earlier  trachytic  lavas,  and  similir 
])roofs  of  a  long  succession  of  intermittent  and  \videly-se]>arated  volcanic 
outbursts  can  be  traced  northwanls  into  the  Yellowstone  Valley. 

When  a  volcanic  vent  is  ojKjned,  it  might  l>e  supposed  always  to 
find  its  way  to  tlie  siu'face  along  some  line  of  fissure,  valley,  or  deep 
dei)ression.  No  doubt  many,  if  not  most,  modern  as  well  as  anciail 
vents,  especially  those  of  large  size,  have  done  so.  It  is  a  curious  fad, 
however,  that  in  innumei*al»le  instances  minor  vents  have  appeared  when 
there  was  no  visible  line  of  dislocation  in  the  rocks  at  the  surface  to  aid 
tliem.  This  has  been  well  shown  bv  a  study  of  the  ancient  volcanie 
rocks  of  the  Old  Red  Sandstone,  Carboniferous,  and  Permian  formatioH 
of  Scotland.^  It  has  likewise  been  most  impressively  demonstrated  bf 
the  way  in  which  the  minor  iKisalt  cones  and  craters  of  Utah  hm 
broken  out  near  the  edges  or  even  from  the  face  of  cliffs,  rather  thai 
at  the  bottom.  Ca])Uiin  Button  remarks  that  among  the  high  plateaux 
of  Utah,  where  there  are  hundreds  of  basaltic  craters,  the  least 
l)lace  for  them  is  at  the  base  of  a  cliff,  and  that,  though  they  occur 
faults,  it  is  almost  always  on  the  lifted,  rarely  upon  the  depressed  side.' 
On  a  small  scale,  a  similar  avoidance  of  the  valley  bottom  is  shown 
on  the  Khine  and  Moselle,  where  eru})tions  have  taken  place  cloai 
to  the  edge  of  the  plateau  through  which  these  rivera  wind.  WTiy  out- 
breaks should  have  occurred  in  this  way  is  a  question  not  easily  answered. 
It  suggests  that  the  existing  depressions  and  heights  of  the  earth*! 
surface  may  sometimes  be  insignificant  features,  com|)arcd  with  the  deplk 
of  the  sources  of  volcanoes  and  the  force  employeil  in  volcanic  emptioo. 
On  the  other  hand,  it  is  remarkable  that  in  Scotland  the  Palseoioie 
erui)tions  took  place  on  the  low  gi'ound  and  valleys,  and  continued  to 
show  themselves  there  during  a  long  succession  of  volcanic  periodL 
Ksi)ecially  noteworthy  is  the  way  in  which  the  Permian  vents  were  opened 
in  lines  and  groui)s  along  the  bottom  of  long  narrow  valleys  in  the 
Silurian  uplands.' 

Ordinary  phase  of  an  active  Volcano. — Tlie  interval  between  two 
eruptions  of  an  active  volcano  shows  a  gi*jidual  augmentation  of  eues^. 
The  crater,  emptied  by  the  last  discharge,  has  its  floor  slowly  upnueed 
l>v  the  expansive  force  of  the  lava-column  underneiith.  Vajwurs  rise  in 
constant  outflow,  accompanied  sometimes  by  discharges  of  dust  or  stonefti 
Through  rents  in  the  ciat<?r-floor  red-hot  lava  may  l>e  seen  only  a  few  feet 
flown.  Where  the  lava  is  maintained  at  or  above  its  fusion-point  and 
possesses  great  lirjuidity,  it  may  form  l)oiling  lakes,-  as  in  the  great  crater 
of  Kilauea,  where  acres  of  seething  lava  may  be  watched  throwing  up 
fountains  of  molten  rock,  surging  agjiinst  the  walls  and  re-fiisiug  laige 
masses  that  fall  into  th(^  buining  flood.     The  lava-column  inside  the  ppe 

tlic  Glacial  IVrioil.  Atfi  Linrei^  .Snl  ser.  vol.  ii.  (1878\  p.  35.  Stefaiii  regards  thoM  of 
Tuscany  as  partly  Miocene,  partly  Pliocene  an»l  post-Plioi'enc.  [Pruc.  TVwc.  <S>r.  Sat.  Piitt 
1.  p.  xxi.) 

*   T I'll  lis.  Iimj.  Siu\  Etiiii.  xxix.  i».  437. 

■'  "  Ilijjli  Plateaux  of  I- tali,"  Urnl.  muJ  f^>fff.  .Si'nr.t/  nf  TfiYtforifJt,  1880,  p.  62. 

^  t^iiort,  Jiiiirn.  (r\'ni.  S»r.  vol.  xlviii.  (18P2).     l*resi«lential  Addresftf  p.  156. 


BBCT.  i  §  2  VOLCANIC  ACTION  205 


of  a  volcano  is  all  this  time  gi-adiially  rising,  until  some  weak  jwrt  of 
the  wall  allows  it  to  escape,  or  until  the  pressure  of  the  accumulated 
vapours  becomes  great  enough  to  biust  through  the  hardened  crust  of  the 
crater-floor  and  give  rise  to  the  phenomena  of  an  eruption. 

Conditions  of  Eruption. —  Leaving  for  the  present  the  general 
question  of  the  cause  of  volcanic  action,  it  may  be  here  remarked  that 
die  conditions  determining  any  i)articular  eruption  are  still  unknown. 
The  explosions  of  a  volcano  may  l)e  to  some  extent  regulated  by  the 
conditions  of  atmospheric  ])ressiu'e  over  the  area  at  the  time.  In  the 
eftBO  of  a  volcanic  fimnel  like  Stromboli,  where,  as  Scroi)e  pointed  out, 
the  expansive  subterranean  force  within,  and  the  repressive  effect  of 
atmospheric  pressure  without,  just  balance  each  other,  any  serious  dis- 
turfoance  of  that  pressure  might  be  expected  to  make  itself  evident  by 
a  change  in  the  condition  of  the  volcano.  Accordingly,  it  has  long  been 
remarked  by  the  fishermen  of  the  Lifmri  Islands  that  in  stormy  weather 
there  is  at  Stromboli  a  more  copious  discharge  of  steam  and  stones  than 
in  fine  weather.  They  make  use  of  the  cone  as  a  weather-glass,  the 
increase  of  its  activity  indicating  a  falling,  and  the  diminution  a  rising 
harometer.  In  like  manner,  Etna,  according  to  Sartorius  von  Walt^rs- 
hansen,  is  most  active  in  the  winter  months.  Mr.  Coan  has  indicated 
a  relation  between  the  eruptions  of  Kilauea  and  the  rainy  seasons  of 
Hawaii,  most  of  the  discharges  of  that  crater  taking  place  within  the  four 
months  from  March  to  June.^ 

When  we  remember  the  connection,  now  indubitably  established, 
between  a  more  copious  discharge  of  fire-damp  in  mines  and  a  lowering 
of  atmospheric  pressure,  we  may  be  prei>ared  to  find  a  similar  influence 
affecting  the  escape  of  vajwui-s  from  the  upper  surface  of  the  lavu-colunui 
of  a  volcano ;  for  it  is  not  so  much  to  the  lava  itself  as  to  the  ex|>ansive 
▼apours  impregnating  it  that  the  miuiifestations  of  volcanic  activity  arc 
due.  Among  the  Vesuvian  eruptions  since  the  middle  of  the  seventeenth 
century,  the  number  which  took  place  in  winter  and  spring  has  l)een  to 
that  of  those  which  broke  out  in  summer  and  autumn  as  7  to  4.  In 
Japan,  also,  the  greater  numl>er  of  recorded  eruptions  have  taken  place 
during  the  cold  months  of  the  year,  February  to  April. - 

There  may  be  other  causes  besides  atmospheric  pressui-e  concerned  in 

'  DaoA,  'Characteristics  of  Volcanoes/  p.  125.  For  accounts  of  the  volcanic  phenomena 
of  Hawaii,  see  W.  Ellis,  'Polynesian  Researches.'  Wilkes'  U.S.  Exjiloring  Expedition, 
1838-42,  •*Geolog}',"  by  J.  D.  Dana.  The  Rev.  T.  Coan,  a  missionary  resident  in  Hawaii, 
obeerred  the  operations  of  the  volcanoes  for  upwanls  of  forty  years,  and  publislted  from  time 
to  time  short  notices  of  them  in  the  Amerimn  Journal  n/  Sriettre^  vols.  xiii.  (1852)  xiv. 
XT.  xriiL  xzi.  zxii.  xxiiL  xxv.  xxvii.  xxxvii.  xl.  xliii.  xlvii.  xlix.  ;  3rd  ser.  ii.  (1871)  iv.  vii. 
▼iiL  xiT.  xriii.  xx.  xxi.  xxii.  (1881).  Prof.  Dana  has  recently  revisited  these  volcanoes  and 
fully  diMossed  their  phenomena  in  the  Autcr.  Jnurti.  N-«'.  vols,  xxxiii.-xxxni.  (1887-89 1, 
and  in  his  'Characteristics  of  Volcanoes.'  See  also  C.  E.  Dutton,  Amrr.  Jonr/i.  S-i. 
XXT.  (1883),  p.  219;  Rfjntrt  C.K  Oeoli^jicnl  Surcetj,  1882-83.  L.  Green,  'Vestiges  of 
the  Molten  Globe/  1887.  For  an  account  of  the  remarkable  glassy  lavas  of  Hawaii,  see 
E.  Cohen,  Sevea  Jahrb.  1880  (ii.),  p.  23  ;  and  a  general  account  of  the  petrograi)]iy  of 
the  islands,  by  E.  S.  Dana,  Amer.  Join-n.  Sci.  xxxvii.  (1889\  p.  441. 

*  J.  Milne,  Seismot.  Sue,  Japan,  IX.  Part  ii.  p.  174. 


206 


DVXAMICAL  GEOLOGY 


BOOK  UIPARl 


those  dirteronoos  ;  the  preiwnderance  of  rain  during  the  winter  and  spnn{ 
may  Ih»  one  (»f  these.  Accoi-ding  to  Mr.  Coan,  pre^ioiis  to  the  grol 
llawaian  eruption  of  18(58  there  had  lK»en  unusually  wet  weather,  aid 
to  this  fact  he  attri1»utes  the  exceptional  severity  of  the  earthquakes  ui 
voleanie  explosions.  The  p'eater  fre<iueney  of  Japanese  volcanic  eruptiooi 
and  eai-thipiakes  in  winter  ha.<  been  referred  in  explanation  to  the  fact  liyt 
the  avenii^e  Imrometrie  gradient  across  Jaixin  is  steeper  in  winter  thank 
summer,  while  the  piling  up  of  snoAv  in  the  northern  regions  gives  rise  to 
long-iH.»ntinueil  stresses,  in  conse«|uence  of  which  certain  lines  of  weaknoi 
in  the  earths  crust  are  more  prejKired  to  give  way  during  the  vrintff 
months  than  they  are  in  summer.^  The  etlects  of  vai'jnng  atmosphetie 
pressure,  however,  can  pii^lwhly  only  slightly  and  locally  modify  volcuk 
activity.  Kni]>tions.  like  the  gre-at  one  of  (.'otopaxi  in  1877,  have  ifl 
inmuneniMe  instances  taken  place  without,  so  far  as  can  be  ascertained, 
any  ivterence  to  atmospheric  cv^nilitions. 

Klui;e  has  souirht  to  trace  a  connection  between  the  vears  of  nuudnim 
and  mininnim  suii-sjk»i^  and  those  of  givatest  and  feeblest  volcanie 
.kctivitv,  and  ha^^  constnicted  lists  to  show  that  vears  ^which  hava 
Ivcn  sivcially  characteiiscvi  by  terrestrial  eruptions  have  coincided  with 
Those  marked  by  few  sun-sj>»ts  and  diminished  magnetic.  distiirbanoeL' 
Such  a  conticction  caunor  Iv  reg-ardetl  as  having  yet  been  satisfactorilf 
establisheil.  Ag-ain,  the  siune  auth«>r  hascalletl  attention  to  the  frequency 
and  \igi»ui'  of  volcanic  explosions  at  or  near  the  time  of  the  August 
meteorii-  sh«.»wci-.  l^ut  i:i  this  case,  likewise,  the  cited  examples  cu 
iianllv  vet  U'  li^^ked  u;v!i  as  more  iha!i  i  oiiu-iilences. 

PeriodieUy  of  Eruptions. — At    mai^.v  volcanic   vents   the  eruptiTe 

^■!'.c-.g\'    m;r..i!cs:s    ::sclt    wi:h   m«i:v  «•'.•    loss  rei:ularity.     At   8trombot^ 

\xhi».h  is  iov.s:a!'.!'v  ir;  a!i  .u::vi'  s:a:o.  :hr    oxi^i'.»T?it.!is  v»ccur  at   intennb 

^.ivy::'.i:   ti>^!u   th'.vo  •:■   !■•.;:■   :•■  :tv.   :i::!v.;:o<  ar.d  upwanls.      A   simikr 

•  hyrh:v.iv,il  v.i'xcmcr^:   h,is   -t-::  i::cv.    '"-irrve^l  during  the  eruptions  at 

orhcr  \cv>   \*:::vh   .*rx.    :•.•»:    o  ■!>:a!-.:'.y  ac::ve.      Volcano,   for  example^ 

.::•::•.;:  ::-  cv.i;>:>*:i  .'f  S^^vrcr.i'**:-   l>7:>.  .l:s;nayt^i  a  succession  of  ex- 

:'■  's-i^Ts  wl-.\h  :.>**.o\\i\:  t\ijh  -rhcv  a:  :r.:<rv,i>  i.f  from  twentv  to  thirtr 

\:   Kv.i  .iT-Ai  V^s*.:\"/.:'i  .i  <i:v.:\ir  rhythmical  series  of  convul- 

ry-::-.  •'•'i^vvr,:  .l::-"!:'^  :he  course  of  an  eruptioiL' 

\*.-.  ■_  :•'.,  \     ..■■  v<  v^:"  :V.c  A-.io^  .-.  ;.vr' -•■..•  ..ii-H-bArge  of  steam  has  beoi 

' ■  w  -. '. 'J- 1 .  M •    \\  :•  V :v.:\  v  ' •. . : i . -. vl  *  ::: •■  :<h l s  •  :  s:e;iir.  :•  •  proceed  at  intervak 

:"  :■■•■.  :*.^.  "tv  :o  -.rvirTy  !:::.*::ts  ::>  v.-.  :r::  s::v.r.:::  ».i  Ninpii,  while  daring 

•.  >  i  ■  > '  K '. :  :  :  ':•. -.  ^  -.wi :  -, : vi :  ^  r  ■  :"  \. *•. : ; : v* x : .  :  h is  %  •>lcaLno  iras  seen  to 

At  the  eniptioD 


....       .       .«     «    ^ 


>  \o  c "*'.::>  h.i< 


■w 


.  k- 


.1  .o 


■*  i  *       I*'      ".iTir 


Mr.    Milne    observed 


*  .    .  ■  ".  ■  .    I*" 


>?/-  y.  ri    A.  Ptaey  {ComfUi 
^1=1131  is  M2ar  $poCt.    Sec, 


j?:L  ■.^.  Ti.  IW. 


SECT,  i  §  2  VOLCANIC  ACTION  207 

that  the  explosions  occurred  nearly  every  two  seconds,  with  occasional 
pauses  of  15  or  20  seconds.^  Kilauea,  in  Hawaii,  seems  to  show 
a  regular  system  of  grand  eruptive  periods.  Dana  has  pointed 
out  that  outbreaks  of  lava  have  taken  place  from  that  volcano  at 
intervals  of  from  eight  to  nine  years,  this  being  the  time  required  to  fill 
the  crater  up  to  the  point  of  outbreak,  or  to  a  depth  of  400  or  500 
feet2 

Some  volcanoes  have  exhibited  a  remarkable  paroxysmal  phase  of 
activity,  when  after  comparative  or  complete  quiescence  a  sudden  gigantic 
explosion  has  taken  place,  followed  by  renewed  and  prolonged  repose. 
Vesuvius  supplies  the  most  familiar  illustration  of  this  character  of 
volcanic  energy.  The  great  eruption  of  A.D.  79,  which  truncated  the 
upper  part  of  the  old  cone  of  Somma,  was  a  true  paroxysmal  explosion, 
unlike  anything  that  had  preceded  it  within  historic  times,  and  far  more 
violent  than  any  subsequent  manifestation  of  the  same  volcano.  The 
crater-basin  of  Santorin,  of  which  the  islands  Thera  and  Therasia  represent 
portions  of  the  rim,  seems  to  have  been  blown  out  by  some  stupendous 
paroxysm  in  prehistoric  times.  The  vast  explosion  of  Krakatoa  in  1883 
was  another  memorable  example.  In  these  instances  there  was  an  earlier 
period  of  ordinary  volcanic  activity,  during  which  a  large  cone  was  gradu- 
ally built  up.  In  the  case  of  Somma  and  Krakatoa  the  energy  died  down 
for  a  time,  and  the  paroxysm  came  with  hardly  any  premonitory  warning. 
It  has  been  succeeded  by  a  time  of  comparatively  feeble  activity.  At 
Vesuvius  the  great  explosion  of  1631,  which  terminated  nearly  1500  years 
of  quiescence,  may  be  regarded  as  a  minor  paroxysm,  since  which  the 
mountain  has  remained  more  continuously  active. 

General  sequence  of  events  in  an  Eruption. — The  approach  of 
an  eruption  is  not  always  indicated  by  any  premonitory  symptoms,  for 
many  tremendous  explosions  are  recorded  to  have  taken  place  in  different 
parts  of  the  world  without  perceptible  warning.  Much  in  this 
respect  would  appear  to  depend  upon  the  condition  of  liquidity  of 
the  lava,  and  the  amount  of  resistance  offered  by  it  to  the  passage  of 
the  escaping  vapours  through  its  mass.  In  Hawaii,  where  the  lavas 
are  remarkably  liquid,  vast  outpourings  of  them  have  taken  place 
quietly  without  earthquakes  during  the  present  century.  But  even 
there,  the  great  eruption  of  1868  was  accompanied  by  violent  earth- 
quakes. 

The  eruptions  of  Vesuvius  are  often  preceded  by  failure  or  diminu- 
tion of  wells  and  springs.  But  more  frequent  indications  of  an  approach- 
ing outburst  are  conveyed  by  sympathetic  movements  of  the  ground. 
Subterranean  rumblings  and  groanings  are  heard ;  slight  tremors  succeed, 
increasing  in  frequency  and  violence  till  they  become  distinct  earthquake 
shocks.  The  vapours  from  the  crater  grow  more  abundant,  as  the  lava- 
column  in  the  pipe  or  funnel  of  the  volcano  ascends,  forced  upward  and 
kept  in  perpetual  agitation  by  the  passage  of  elastic  vapours  through  its 

*  Traru.  Seitm.  Soc.  Japan  y  ix.  part  ii.  p.  82. 

*  'Characteristics  of  Volcanoes.'  p.  124.     On  periodicity  of  eruptions,  see  Kluge,  Neuea 
Jahrb,  1862,  p.  582. 


208  DYXAMICAL   (GEOLOGY  book  in  part  i 


mass.  After  a  long  previous  interval  of  quiescence,  there  may  l>e  much 
solidified  lava  towards  the  toj)  of  the  funnel,  which  will  restrain  the 
ascent  of  the  still  molten  portion  underneiith.  A  vast  pressiu'e  is  thiiB 
exercised  on  the  sides  of  the  cone,  which,  if  too  weak  to  resist,  will 
open  in  one  or  more  rents,  and  the  liquid  lava  will  issue  from  the  outer 
slope  of  the  mountain  ;  or  the  energies  of  the  volcano  will  be  directed 
towards  clearing  the  obstruction  in  the  chief  throat,  until  with  tremend- 
ous explosions,  and  the  rise  of  a  vast  cloud  of  dust  and  fragments,  the 
bottom  and  sides  of  the  crater  are  finally  blown  out,  and  the  top  of  the 
cone  disa])i)ears.  The  lava  may  now  esciipe  from  the  lowest  part  of  the 
lip  of  the  crater,  while,  at  the  siime  time,  immense  numbers  of  red-hot 
lx)mbs,  scori*,  and  stones  are  shot  up  into  the  air.  The  lava  at 
first  rushes  down  like  one  or  more  rivers  of  melted  iron,  but,  as  it 
cools,  its  rate  of  motion  lessens.  Clouds  of  steam  rise  from  its  surface, 
as  well  as  from  the  central  crater.  Indeed,  every  successive  paroxysmal 
convulsion  of  the  mountain  is  marked,  even  at  a  distance,  by  the  rise  of 
huge  ball-like  wieaths  or  clouds  of  steam,  mixed  with  dust  and  stones, 
forming  a  column  which  towers  sometimes  a  couple  of  miles  or  more 
above  the  summit  of  the  cone.  By  degi'ees  these  erucUitions  diminish  in 
frequency  and  intensity.  The  lava  ceases  to  issue,  the  showers  of  stones 
and  dust  decrease,  and  after  a  time,  which  may  vary  from  hours  to  days 
or  months,  even  in  the  re(iime  of  the  same  mountfiin,  the  volcano  becomes 
once  more  trantjuil.^ 

In  the  investigation  of  the  subject,  the  student  Avill  naturally  devote 
attention  specially  to  those  aspects  of  volcanic  action  which  have 
more  particular  geological  interest  from  the  permanent  changes  with 
which  thev  are  connected,  or  from  the  way  in  M'hich  thev  enable 
us  to  detect  and  realise  conditions  of  volcanic  energy'  in  former 
periods. 

Fissures. — The  convulsions  which  culminate  in  the  formation  of 
a  volcano  usually  split  open  the  terrestrial  crust  l»y  a  more  or  less  nearly 
rectilinear  hssure,  or  by  a  system  of  fissures.  In  the  subsequent  progress 
of  the  mountain,  the  ground  at  and  around  the  focus  of  action  is  liable 
to  1)C  again  and  again  rent  open  by  other  fissures.  These  tend  to  diverge 
from  the  focus ;  but  around  the  vent  where  the  rocks  have  been  most 
exposed  to  concussion,  the  fissures  sometimes  intersect  each  other  in  aU 
directions.  In  the  great  eruption  of  Etna,  in  the  year  1669,  a  series  of 
six  parallel  Assurers  opened  on  the  side  of  the  mountain.  One  of  these, 
with  a  width  of  two  yards,  i-an  for  a  distance  of  12  miles,  in  a  somewhat 
winding  course,  to  within  a  mile  of  the  top  of  the  cone.'^  Similar  fissures, 
but  on  a  smaller  scale,  have  often  been  observed  on  Vesuvius  and  other 
volcanoes.^     A  fissure  sometimes  reoi>ens  for  a  subsecpient  eruption. 

^  See  Schmidt's  narrative  of  the  eruptiou  of  Vesuvius  in  May  1855  {anUy  p.  195),  An 
account  of  the  ijreat  eruption  of  Cotopaxi  in  June  1877,  by  Dr.  Th.  Wolf,  will  be  foand  in 
XeuefiJuhrh.  1878,  p.  UX 

^  For  tissures  on  Etna,  sm*  Silvestri,  lh»n.  R.  O'enf.  Com.  Ital.  1874. 

^  For  a  description  of  those  of  Iceland  (which  run  chiefly  N.K  to  S.W.,  aud  N.  to  S.) 
see  T.  Kjerulf.  A'.y/.  Mog.  xxi.  147. 


i§2 


vowAsif^  rissuims 


Two  obvious  causes  may  be  assigned  for  the  pushing  upward  of  a 
crater-floor  and  the  fisauring  of  a  volcanic  cone  : — (1)  the  enormous  pressure 
of  the  dissolved  vapours  or  gases  acting  upon  the  walls  and  roof 
of  the  funnel  and  convulsing  the  cone  by  successive  explosions;  and  (2) 
the  hydrostatic  pressure  of  the  lava-column  in  the  funnel,  which  may  be 
taken  to  be  about  120  lbs.  per  square  inch,  or  nearly  8  tons  on  the  square  foot, 
for  each  100  feet  of  depth.  Both  of  these  causes  may  act  simultaneously, 
and  their  united  effect  has  been  to  uplift  enormous  superincumbent  masses 
of  solid  rock  and  to  produce  a  wide-spread  series  of  long  and  continuous 
fissures  reaching  from  unknown  depths  to  various  distances  from  the  sur- 
face and  even  opening  up  sometimes  on  the  surface.  These  results  of  the 
expansive  energy  of  volcanic  action  are  of  special  interest  to  the  geologist, 
for  he  encounters  evidence  of  similar  operations  in  former  times  preserved 
in  the  crust  of  the  earth  (see  Book  I\'.  Part  '\'II.  Sect  i.) 

Into  rents  thus  formed,  the  water-substance  or  vapour  rises  with 
great  expansive  force,  followed  by  the  lava  which  solidifies  there  like  iron 
in  a  mould.  Where  fissures  are  vertical  or  highly  inclined  the  igneous  rock 
tekes  the  form  of  d^ixs  or  reins ;  where  the  intruded  material  has  forceil 
its  way  more  or  less  in  a  horizontal  direction  between  strata  of  tuff,  beds 
of  non-volcanic  sediments,  or  flows  of  lava,  it  takes  the  form  of  uluets  {siUs) 
or  bedi.     The  cliffs  of  many  an  old  crater  show  how  marvellously  they 


have  been  injected  by  such  veins,  dykes,  or  sheets  of  lava.  Those  of 
Sonuna,  and  the  A'alle  del  Bovc  on  Etna  {Fig.  -12),  which  have  long  been 
known,  project  now  from  the  softer  tuffs  like  wulla  of  masonry.'     The 

*  a.  von  WRltersliaasen,  '  Dcr  Aetna.'  li.  p.  341. 


210  inW'AMICAL  GKULOOY  hook  hi  rsxti 

cniter  cliffs  of  Stintorin  also  present  iiii  abundant  series  of  dykes.  The 
|)cnnaiieiit  separation  of  thu  walln  of  fissures  by  the  consolidation  d 
the  luv«  that  rises  in  them  iw  dykes  must  widen  the  dimensions  of  i 
lono,  for  tlie  fissures  are  not  Awn  to  shrinkage,  although  doubtless  the 
looKcly  i>ilod  fi-ajpiiciitary  materials,  in  the  course  of  their  consolidatioa, 
develop  lines  of  joint  Sometimes  the  lain  has  evidently  risen  in  a  stale 
of  oxtrcmu  fluidity,  and  has  at  once  tilled  the  rents  prepared  for  it,  cool- 
ing rapidly  on  the  outside  as  a  true  volcaniu  ginss,  but  assuming  a  da- 
tinctly  crystalline  structure  inside  (mil'-,  p.  171).  Dykes  of  this  kind, 
with  a  vitreous  crust  on  their  sides,  may  bo  seen  on  the  crater-vall  of 
Sonitna,  and  not  uncommonly  among  liasalt  dykes  in  Iceland  and  Scotland 
In  other  cases,  the  lava  had  probably  already  ac<|uircd  a  more  viscous  « 
even  litlioid  clmructer  ere  it  ruse  in  the  tissure,  and  in  this  condition  wm 


ibh  to  pn-'h  a"!  lo  and  i 
made  Its  1%  Li  (Fi„   41) 


Ln  lontort  the  itrata  of  tnlf  through  which  it 
Theie  laii  bt  littk  d  inbt  that  in  the  archibc- 
mw^t  let  tlic  part  of  huge  beams  and  gudoi 
(^l),  4  i)  liniijnig  the  loose  tufTs  and  ml* 
c.  diite  1  la\Hst  (.ethir  and  sti  cngthening  the 
tone  if^ui-'t  the  offctts  of  subsequent  coo 
\  ul*ioii« 

From  this  jioitit  of  Mon    an  explanation 
xuf^ctv  itidf  (if  the  obsencd  altemationi  in 
Kt  41    ■*«(      of  ink™  ifi-i      thechiu'-tcter  of  a\o1ianoBimption8.     TbcM 
i""^!,!"'''''""     '  "   dtewiation-*   nia\    d(|>cnd  m  groat  meaioR 

•upon  the  iclition  between  the  height  of 
the  tone,  on  thi  one  haiiil  and  the  stiength  of  ita  sides,  on  the  other 
When  the  sides  hiM  liecn  vtoll  hraied  togetbLr  b\  interlaung  d\keB,uid 
fnrthc  r  thi<.kcn<  d  b\  the  spn-ul  of  ^  olciiiic  matenals  all  o^  er  their  slopn 
the^  m  i\  resist  the  eHctts  of  explosion  and  of  the  pressure  of  the  ascend- 
ni{,  Kia-column  In  thH  e  \t^  the  voleiiiin  may  find  relief  only  from  it» 
Kumniit  "knd  if  the  1a\i  flont  forth,  it  will  do  so  from  the  top  of  the 
tone  Vs  the  (one  ineref<c-'  in  elevation,  however,  the  pressure  from 
within  uiKin  it-<  ^de-  ancient'.  Kventiially  egress  is  once  more  estab- 
li-,hed  on  the  flanks  hi  inc-iiH  of  fissures,  and  a  new  series  of  Uva- 
strenmR  is  poured  out  u^  er  the  lower  sloiies  (jimfeii,  p.  248). 


L      ftEXT.  i§2  VOLCANIC  EXPLOSIONS  211 


■  In  the  deeper  portions  of  a  volcanic  vent  the  convulsive  efforts  of  the 

E  lava-column  to  force  its  way  upward  must  often  produce  lateral  as  well 
I  as  vertical  rifts,  and  into  these  the  molten  material  will  rush,  exerting  as 
■f  it  goes  an  enormous  upward  pressure  on  the  mass  of  rock  overlying  it. 
At  a  modern  volcano  these  subterranean  manifestations  cannot  be  seen, 
but  among  the  volcanoes  of  Tertiary  and  older  time  they  have  been 
revealed  by  the  progress  of  denudation.  Some  of  these  older  examples 
teach  us  the  prodigious  upheaving  power  of  the  sheets  of  molten  rock  in- 
truded between  volcanic  or  other  strata.  An  account  of  this  structure 
(sills,  laccolites),  with  reference  to  some  examples  of  it,  will  be  found  in 
Book  IV.  Part  VII.^ 

Though  lava  very  commonly  issues  from  the  lateral  fissures  on  a 
volcanic  cone,  it  may  sometimes  approach  the  surface  in  them  with- 
out actually  flowing  out.  The  great  fissure  on  Etna  in  1669,  for 
example,  was  visible  even  from  a  distance,  by  the  long  line  of  vivid 
light  which  rose  from  the  incandescent  lava  >vithin.  Again,  it 
frequently  happens  that  minor  volcanic  cones  are  thrown  up  on  the 
line  of  a  fissure,  either  from  the  congelation  of  the  lava  round  the  point 
of  emission,  or  from  the  accumulation  of  ejected  scoriae  round  the 
*  fissure-vent.  One  of  the  most  remarkable  examples  of  this  kind  is  that 
of  the  Laki  fissure  in  Iceland,  the  whole  length  of  which  (12  miles) 
bristles  with  small  cones  and  craters  almost  touching  each  other.^ 

Explosions. — Apart  from  the  appearance  of  visible  fissures,  volcanic 
energy  may  be,  as  it  were,  concentrated  on  a  given  point,  which  will 
usually  be  the  weakest  in  the  structure  of  that  part  of  the  terrestrial  crust, 
and  from  which  the  solid  rock,  shattered  into  pieces,  is  hurled  into  the 
air  by  the  enormous  expansive  energy  of  the  volcanic  vapours.^  This  opera- 
tion has  often  been  observed  in  volcanoes  already  formed,  and  has  even  been 
witnessed  on  ground  previously  unoccupied  by  a  volcanic  vent.  The  history 
of  the  cone  of  Vesuvius  brings  before  us  a  long  series  of  such  explosions,  be- 
ginning with  that  of  a.d.  79,  and  coming  down  to  the  present  day  (Fig.  45). 
Even  now,  in  spite  of  all  the  lava  and  ashes  poured  out  during  the  last 
eighteen  centuries,  it  is  easy  to  see  how  stupendous  must  have  been  that 
earliest  explosion,  by  which  the  southern  half  of  the  ancient  crater  was  blown 
out.  At  every  successive  important  eruption,  a  similar  but  minor  operation 
takes  place  within  the  present  cone.  The  hardened  cake  of  lava  forming 
the  floor  is  burst  open,  and  with  it  there  usually  disappears  much  of  the 
apper  part  of  the  cone,  and  sometimes,  as  in  1872,  a  large  segment  of 
the  crater-wall.  The  Valle  del  Bove  on  the  eastern  flank  of  Etna  is  a 
chasm  probably  due  mainly  to  some  gigantic  prehistoric  explosion.*  The 
islands  of  Santorin  (Figs.  65  and  66)  bring  before  us  evidence  of  a  pre- 
historic catastrophe  of  a  similar  natiu*e,  by  which  a  lai'ge  volcanic  cone 

^  See  particularly  the  descriptiou  of  intnisivo  sheets  or  laccolites.    ^ 

*  A.  Helland,  'Lakis  Kratere  og  Lava-stroiue,'  cited  on  p.   Ii02.      On  this  straight 
fiMtire  some  500  craters  rise,  varying  from  5  to  450  feet  high. 

*  See  Daabr^'s  experiments  on  the  mechanical  effects  of  gas  at  liigh  i»ressures,  Comptes*    . 
Rend.  cxL  cxlL  cxiii.,  and  Bull,  Soc.  Ge^d.  France^  xix.  (1891),  p.  313. 

*  '  Der  Aetna,' p.  400. 


212  nyXAMWAL  OEOLOGY  book  ill  part  I 

was  blown  up.  The  existing  outer  islands  aie  a  chain  of  fragments  of 
the  ]>erii>hery  of  the  cone,  the  centre  of  which  ia  now  occupied  by  the  sea. 
In  the  year  1538  a  new  vnU-ano,  Monte  Xuovo,  was  formed  in  24  hours 


on  the  margin  of  the  Bay  of  Naples.  An  opening  was  drilled  by  suc- 
cessive explosions,  and  such  (juantitJes  of  stones,  scoriae,  and  ashes  were 
thrown  out  from  it  as  to  form  a  hill  that  i-osc  -140  English  feet  above  the 
sea-level,  and  was  more  than  a  mile  and  n  half  in  circumference.  Moet 
of  the  fragments  now  to  be  seen  on  the  sloiws  of  this  cone  and  inside  its 
beautifully  perfect  crater  arc  of  various  volcanic  rocks,  many  of  them 
being  black  scoriai ;  but  pieces  of  Koman  potteiy,  together  with  frag- 
ments of  the  older  underlying  tuff,  ami  some  marine  shells,  have  been 
obtained — doubtless  part  of  the  soil  and  subsoil  dislocated  and  ejected 
during  the  explosions. 

One  of  the  most  stupendous  volcanic  explosions  on  record  was  that 
of  Krakatoa  in  the  Sumla  Strait  on  the  26th  and  27th  of  August  1883,' 
After  a  series  of  eon\Tilsions,  the  greater  portion  of  the  island  was  blown 
out  with  a  succession  of  terrific  detonations  which  were  heard  more  than 
1.50  miles  away.  A  mass  of  matter,  estimated  at  about  1^  cubic  mile  in 
bulk,  was  hurled  into  the  air  in  the  foi'm  of  lapilli,  ashes,  and  the  finest 
volcanic  dust.  The  efi"ects  of  this  volcanic  outburst  were  marked  both 
upon  the  atmosphere  and  the  ocean.  A  series  of  barometrical  disturbances 
[xissed  n^innd  the  glolie  in  opposite  directions  from  the  volcano  at  the  rate 
of  about  700  miles  an  hour.  The  air-wave,  travelling  from  east  to  west 
is  supposed  t<)  have  passed  three  and  a  quarter  times  round  the  earth  (or 
82,21)0  miles)  Iwfoi'e  it  ceased  to  be  perceptible.-  The  sea  in  the  neigh- 
bourhood was  thrown  into  waves,  one  of  which  was  computed  to  have 
risen  more  than  100  feet  above  ti<le-lcvet,  destroying  towns,  villages,  and 
36,380  people.     Oscillations  of  the  water  were  perceptible  even  at  Aden, 

'  9ee'Th»Ernptic.nofKratalon,'b)-aComniitte«oflheBoya1Societ)-,  1888.  'Knkatan,' 
11.  I>.  M.  Verbtck,  Batavin,  1888. 

'  Sratt  ami  Strachey,  /'rvc.  Roy.  Soc.  ixivi.  (18831.     Eojnl  Society's  Report,  p.  67. 


SECT,  i  §  2  VOLCANIC  DUST  AND  STONES  213 

1000  miles  distant,  at  Port  Elizabeth  in  South  Africa,  5450  miles,  and 
among  the  islands  of  the  Pacific  Ocean,  and  they  are  computed  to  have 
travelled  with  a  maximum  velocity  of  467  statute  miles  in  the  hour.^ 

It  is  not  necessary,  and  it  does  not  always  happen,  that  any  actual 
solid  or  liquid  volcanic  rock  is  erupted  by  explosions  that  shatter  the 
rocks  through  which  the  funnel  passes.  Thus,  among  the  cones  of  the 
extinct  volcanic  tract  of  the  Eifel,  some  occur  which  consist  entirely,  or 
nearly  so,  of  comminuted  debris  of  the  surrounding  Devonian  greywacke 
and  slate  through  which  the  various  volcanic  vents  have  been  opened 
(see  pp.  200,  244,  585).  Evidently,  in  such  cases,  only  elastic  vapours 
forced  their  way  to  the  surface ;  and  we  see  what  probably  often  takes 
place  in  the  early  stages  of  a  volcano's  history,  though  the  fragments  of 
the  underlying  disrupted  rocks  are  in  most  instances  buried  and  lost 
under  the  far  more  abundant  subsequent  volcanic  materials.  Sections  of 
small  ancient  volcanic  "necks"  or  pipes  sometimes  afford  an  excellent 
opportimity  of  observing  that  these  orifices  were  originally  opened  by 
the  blowing  out  of  the  solid  crust  and  not  by  the  formation  of  fissures. 
Examples  will  be  cited  in  later  pages  from  Scottish  volcanic  areas  of  Old 
Red  Sandstone,  Carboniferous,  and  Permian  age.  The  orifices  are  there 
filled  with  fragmentary  materials,  wherein  portions  of  the  surrounding  and 
underlying  rocks  form  a  noticeable  proportion.  ^     (See  pp.  584-589). 

Showers  of  Dust  and  Stones. — A  communication  having  been  opened, 
either  by  fissuring  or  explosion,  between  the  heated  interior  and  the  surface, 
fragmentary  materials  are  commonly  ejected  from  it,  consisting  at  first 
mainly  of  the  rocks  through  which  the  orifice  has  been  opened,  afterwards 
of  volcanic  substances.  In  a  great  eruption,  vast  numbers  of  red-hot 
stones  are  shot  up  into  the  air,  and  fall  back  partly  into  the  crater  and 
partly  on  the  outer  slopes  of  the  cone.  According  to  Sir  W.  Hamilton, 
cinders  were  thrown  by  Vesuvius,  during  the  eruption  of  1779,  to  a 
height  of  10,000  feet.  Instances  are  known  where  large  stones,  ejected 
obliquely,  have  described  huge  parabolic  curves  in  the  air,  and  fallen  at 
a  great  distance.  Stones  8  lbs.  in  weight  occur  among  the  ashes  which 
buried  Pompeii.  The  volcano  of  Antuco  in  Chili  is  said  to  send  stones 
flying  to  a  distance  of  36  (?)  miles,  Cotopaxi  is  reported  to  have  hurled 
a  200-ton  block  9  miles,^  and  the  Japanese  volcano,  Asama,  is  said  to 
have  ejected  many  blocks  of  stone,  measuring  from  40  to  more  than  100 
feet  in  diameter.* 

But  in  many  great  eruptions,  besides  a  constant  shower  of  stones  and 
scoriae,  a  vast  column  of  exceedingly  fine  dust  rises  out  of  the  crater, 
sometimes  to  a  height  of  several  miles,  and  then  spreads  outwards  like 
a  sheet  of  cloud.  The  remarkable  fineness  of  this  dust  may  be  understood 
from  the  fact  that  during  great  volcanic  explosions  no  boxes,  watches,  or 

*  Wharton,  Royal  Society's  Report,  p.  89.     For  a  great  Japanese  explosion,  see  Nature, 
13th  Sept  1888. 

*  Trans.  Roy.   Soc.  Edin.  xxix.  p.  458  ;    Quart.  Jnurn.   Geol.  Soc.  (1892),  President's 
Address,  pp.  86,  118,  135,  143,  153. 

'  D.  Forbes,  Oeol.  Mag.  vii.  p.  320. 

*  J.  Milne,  Seism.  Soc.  Japan y  ix.  p.  179,  where  an  excellent  account  of  the  volcanoes  of 
Japan  is  given.     See  also  '  The  Volcanoes  of  Japan  *  by  J.  Milne  and  W.  K.  Burton. 


214  D\ '^A MICA  L  GEOLOGY  book  iii  part  i 


/ 


close-fitting  joints  have  been  found  to  be  able  to  exclude  it.  Mr. 
Whymper  collected  some  dust  that  fell  65  miles  away  from  Cotopaxi, 
and  which  was  so  fine  that  from  4000  to  25,000  particles  were  required 
to  weigh  a  grain.^  So  dense  is  the  dust-cloud  as  to  obsciu^e  the  sun,  and 
for  days  together  the  darkness  of  night  may  reign  for  miles  around  the 
volcano.  In  1822,  at  Vesuvius,  the  ashes  not  only  fell  thickly  on  the 
villages  round  the  base  of  the  mountain,  but  travelled  as  far  as  Ascoli, 
which  is  56  Italian  miles  distant  from  the  volcano  on  one  side,  and  as 
Casano,  105  miles  on  the  other.  The  eruption  of  Cotopaxi,  on  26th 
June  1877,  began  by  an  explosion  that  sent  up  a  column  of  fine  ashes 
to  a  prodigious  height  into  the  air,  where  it  rapidly  spread  out  and  formed 
BO  dense  a  canopy  as  to  throw  the  region  below  it  into  total  darkness.^  So 
quickly  did  it  diffuse  itself,  that  in  an  hour  and  a  half,  a  previously  bright 
morning  became  at  Quito,  33  miles  distant,  a  dim  twilight,  which  in  the 
afternoon  passed  into  such  darkness  that  the  hand  placed  before  the  eye 
could  not  be  seen.  At  Guayaquil,  on  the  coast,  150  miles  distant,  the 
shower  of  ashes  continued  till  the  1st  of  Jul  v.  Dr.  Wolf  collected  the 
ashes  daily,  and  estimated  that  at  that  place  there  fell  315  kilogrammes 
on  every  square  kilometre  during  the  first  thirty  hours,  and  on  the  30th 
of  June,  209  kilogrammes  in  twelve  hours.*'^  During  a  much  less  im- 
portant eruption  of  the  same  mountain  on  3rd  July  1880,  the  amount  of 
volcanic  dust  ejected,  according  to  Mr.  Whymper,  could  not  have  been 
less,  and  was  probably  vastly  more,  than  two  millions  of  tons,*  equivar 
lent  to  a  mass  of  lava  containing  more  than  150,000  cubic  feet. 

The  explosion  of  Kmkatoa  in  August  1883  was  accompanied  by 
the  discharge  of  enoraious  quantities  of  volcanic  dust,  some  of  which 
was  carried  to  vast  distances.  It  was  estimated  that  the  clouds  of 
fine  dust  were  hurled  from  that  volcano  to  a  height  of  17  miles,  and 
the  darkness  which  thev  caused  extended  for  150  miles  from  the 
focus  of  eruption.  The  diffusion  and  continued  suspension  of  the  finer 
particles  of  this  dust  in  the  upper  air  has  been  regarded  as  the  prob- 
able cause  of  the  remarkably  brilliant  sunsets  of  the  following  winter 
and  spring  over  a  large  part  of  the  earth's  surface.^  One  of  the  most 
stupendous  outpourings  of  volcanic  ashes  on  record  took  place,  after 
a  quiescence  of  26  years,  from  the  volcano  Coseguina,  in  Nicaragua, 
during  the  early  jjart  of  the  year  1835.  On  that  occasion,  utt^r  darkness 
prevailed  over  a  circle  of  35  miles  radius,  the  ashes  falling  so  thickly  that, 
even  8  leagues  from  the  mountain,  they  covered  the  gi^ound  to  a  depth 
of  about  10  feet.     It  was  estimated  that  the  rain  of  dust  and  sand  fell 

*  Tloyal  Society  Report  ou  Krnkatoa,  p.  183. 

-  During  the  comparatively  insignificaut  eruption  of  this  volcano  in  1880  Mr.  AVTiymper 
noticed  that  a  column  of  inky  blackness,  fonneil  doubtless  of  volcanic  dust,  went  stra^ht 
up  into  the  air  with  such  velocity  that  in  less  than  a  minute  it  had  risen  20,000  feet  above 
the  rim  of  the  crater,  or  40,000  feet  alwve  the  sea.  'Travels  amongst  the  Great  Andes,' 
I».  322. 

'*  Xeuci  Jalii'h.  1878.  p.  141.  An  account  of  this  eruption  is  given  by  Mr.  Whymper  in 
his  '  Travels  amongst  the  Great  Andes,*  chap.  vi. 

"*  'Travels  amongst  the  Great  Andes.'  p.  328. 

**  Royal  Society  Report,  pp.  ir»l-463. 


SECT,  i  §  2  VOLCANIC  D  UST  2 1 5 


over  an  area  at  least  270  geographical  miles  in  diameter.  Some  of  the  finer 
materials,  thrown  so  high  as  to  come  within  the  influence  of  an  upper 
air-current,  were  borne  away  eastward,  and  fell,  four  days  afterwards,  at 
Kingston,  in  Jamaica — a  distance  of  700  miles.  During  the  great  eruption  \ 
of  Sumbawa,  in  1815,  the  dust  and  stones  fell  over  an  area  of  nearly  one 
million  square  miles,  and  were  estimated  by  Zollinger  to  amount  to  fully 
fifty  cubic  miles  of  material,  and  by  Junghuhn  to  be  equal  to  one  hundred 
and  eighty-five  mountains  like  Vesuvius.  Towards  the  end  of  last  cen- 
tury, during  a  time  of  great  disturbance  among  the  Japanese  volcanoes, 
one  of  them,  Sakurajima,  threw  out  so  much  pumiceous  material  that 
it  was  possible  to  walk  a  distance  of  23  miles  upon  the  floating  debris  in 
the  sea. 

An  inquiry  into  the  origin  of  these  showers  of  fragmentary  materials 
brings  vividly  before  us  some  of  the  essential  features  of  volcanic  action. 
We  find  that  bombs,  slags,  and  lapilli  may  be  thrown  up  in  comparatively 
tranquil  states  of  a  volcano,  but  that  the  showers  of  fine  dust  are  dis- 
charged with  violence,  and  only  appear  when  the  volcano  becomes  more 
energetic.  Thus,  at  the  constantly,  but  quietly,  active  volcano  of  Strom- 
boli,  the  column  of  lava  in  the  pipe  may  be  watched  rising  and  falling 
with  a  slow  rhythmical  movement.  At  every  rise,  the  surface  of  the  lava 
swells  up  into  blisters  several  feet  in  diameter,  which  by  and  by  biu'st 
with  a  sharp  explosion  that  makes  the  walls  of  the  crater  vibrate.  A 
cloud  of  steam  rushes  out,  carrying  >vith  it  hundreds  of  fragments  of  the 
glowing  lava,  sometimes  to  a  height  of  1200  feet.  It  is  by  the  ascent 
of  steam  through  its  mass,  that  a  column  of  lava  is  kept  boiling  at  the 
bottom  of  the  crater,  and  by  the  explosion  of  successive  larger  bubbles  of 
steam,  that  the  various  bombs,  slags,  and  fragments  of  lava  are  torn  oflT 
and  tossed  into  the  air.  It  has  often  been  noticed  at  Vesuvius  that  each 
great  concussion  is  accompanied  by  a  huge  ball-like  cloud  of  steam  which 
rushes  up  from  the  crater.  Doubtless  it  is  the  sudden  escape  of  that 
steam  which  causes  the  explosion. 

The  varying  degi*ee  of  liquidity  or  viscosity  of  the  lava  probably 
modifies  the  force  of  explosions,  owing  to  the  different  amounts  of 
resistance  offered  to  the  upward  passage  of  the  absorbed  gases  and 
vapours.  Thus  explosions  and  accompanying  scoriae  are  abundant  at 
Vesuvius,  where  the  lavas  are  comparatively  viscid  ;  they  are  almost 
unknown  at  Kilauea,  where  the  lava  is  remarkably  liquid. 

In  tranquil  conditions  of  a  volcano,  the  steam,  whether  collecting  into 
larger  or  smaller  vesicles,  works  its  way  upward  through  the  substance 
of  the  molten  lava,  and  as  the  elasticity  of  this  compressed  vapour  over- 
comes the  pressure  of  the  overlying  lava,  it  escapes  at  the  surface,  and 
there  the  lava  is  thus  kept  in  ebullition.  But  this  comparatively  quiet 
operation,  which  may  be  watched  within  the  craters  of  many  active 
volcanoes,  does  not  produce  clouds  of  fine  dust.  The  collision  or  friction 
of  millions  of  stones  ascending  and  descending  in  the  dark  column  above 
the  crater  must  doubtless  cause  much  dust  and  sand.  But  the  explosive 
action  of  steam  is  probably  also  an  immediate  cause  of  much  trituration. 
The  aqueous  vapour  or  water-gas  which  is  so  largely  dissolved  in  many 


216  DYXAMICAL  GEOLOGY  book  hi  part  i 

lavas  must  exist  within  the  lava-cohimn,  luider  an  enormous  pressure,  at  a 
temperature  far  above  its  critical  point  (p.  194),  even  at  a  white  heat,  and 
therefore  possibly  in  a  state  of  dissociation.  The  sudden  ascent  of  lava 
so  constituted  relieves  the  pressure  rapidly  >nthout  sensibly  affecting  the 
temi)orature  of  the  mass.  Consequently,  the  white-hot  gases  or  vapours 
at  length  explode,  and  reduce  the  molten  mass  to  the  finest  powder,  like 
water  shot  out  of  a  gun.  ^ 

Evidently  no  part  of  the  operations  of  a  volcano  has  greater  geological 
significance  than  the  ejection  of  such  enormous  quantities  of  fragmentary 
matter.  In  the  first  place,  the  fall  of  these  loose  materials  round  the 
orifice  of  discharge  is  one  main  cause  of  the  growth  of  the  volcanic  cone. 
The  heavier  fragments  gather  around  the  vent,  and  there  too  the  thickest 
accumulation  of  dust  and  sand  takes  place.  Hence,  though  successive  explo- 
sions may  blow  out  the  upper  part  of  the  crater-walls  and  prevent  the 
mountain  from  gi'owing  so  rapidly  in  height,  every  eruption  must  increase 
the  diameter  of  the  cone.  In  the  second  place,  as  every  shower  of  dust 
and  sand  adds  to  the  height  of  the  ground  on  which  it  falls,  thick  volcanic 
accumulations  may  be  formed  far  beyond  the  base  of  the  moiuitain.  The 
volcano  of  Sangay,  in  Ecuador,  for  instance,  has  buried  the  country  around 
it  to  a  depth  of  4000  feet  under  its  ashes.-  In  such  loose  deposits  are 
entombed  trees  and  other  kinds  of  vegetation,  together  with  the  bodies  of 
animals,  as  well  as  the  works  of  man.  In  some  crises,  where  the  layer  of 
volcanic  dust  is  thin,  it  may  merely  add  to  the  height  of  the  soil,  without 
sensibly  interfering  with  the  vegetation.  But  it  has  been  observed  at 
Santorin  that  though  this  is  tnie  in  dry  weather,  the  fall  of  rain  with  the 
dust  at  once  acts  detrimentally.  On  the  3rd  of  June  1866,  the  vines 
were  there  withered  up,  as  if  they  had  been  burnt,  along  the  track  of  the 
smoke  cloud.^  By  the  gi-adual  accumulation  of  volcanic  ashes,  new 
geological  formations  arise  which,  in  their  component  materials,  not  only 
bear  >vitness  to  the  volcanic  eruptions  that  produced  them,  but  preserve 
a  record  of  the  land-surfaces  over  which  they  sjM'ead.  In  the  third  place, 
besides  the  distance  to  which  the  fragments  may  be  hurled  by  volcanic 
explosions,  or  to  which  they  may  be  diffused  by  the  ordinary  aerial 
movements,  we  have  to  take  into  account  the  vast  spaces  across  which  the 
liner  dust  is  sometimes  borne  by  upixjr  air-ciUTents.  In  the  instance 
already  cited,  ashes  from  Coseguina  fell  700  miles  away,  having  been 
carried  all  that  long  distance  by  a  high  counter-current  of  air,  moving 
ai)parently  at  the  rate  of  about  seven  miles  an  hour  in  an  opposite  direc- 
tion to  that  of  the  wind  which  blew  at  the  surface.  By  the  Sumbawa 
eruption,  also  referred  to  above,  the  sea  west  of  Sumatra  was  covered  with 
a  layer  of  ashes  two  feet  thick.  On  several  occasions  ashes  from  the  Ice- 
landic volcanoes  have  fallen  so  thickly  between  the  Orkney  and  Shetland 
Islands,    that   vessels   passing   there   have    had    the   unwonted   deposit 

*  Messrs.  Murray  aud  Reiiard  (/Voc.  Itoy.  S^^r.  EiUn.  xii.  (1S84),  p.  480)  concluded  that 
the  fragnieutary  condition  aud  the  fresh  fractures  of  the  dust -particles  of  the  Krakatoa 
eruption  were  due  to  a  tension  plienonienon,  which  affecU  these  vitreous  matters  in  a  manner 
analogous  to  what  is  observed  in  '*  Rupert's  drops." 

-  D.  Forbes,  Gfol.  Mag,  vii.  320.  ^  Fouqut*,  'Santorin,'  p.  81. 


SECT,  i  §  2  LA  VA'STREAMS  2 1 7 

shovelled  off  their  decks  in  the  morning.  In  the  year  1783,  during  the 
memorable  eruption  of  Skaptar-Jokull,  so  vast  an  amount  of  fine  dust 
was  ejected  that  the  atmosphere  over  Iceland  continued  loaded  with  it 
for  months  afterwards.  It  fell  in  such  quantities  over  parts  of  Caithness 
— a  distance  of  600  miles — as  to  destroy  the  crops ;  that  year  is  still 
spoken  of  by  the  inhabitants  as  the  year  of  "  the  ashie."  Traces  of  the 
same  deposit  have  been  observed  in  Norway,  and  even  as  far  as  Holland.^ 
Hence  it  is  evident  that  volcanic  accumulations  may  take  place  in  regions 
many  hundreds  of  miles  distant  from  any  active  volcano.  A  single  thin 
layer  of  volcanic  detritus  in  a  group  of  sedimentary  strata  would  not 
thus  of  itself  prove  the  existence  of  contemporaneous  volcanic  action  in 
its  neighbourhood.  Failing  other  proof  of  adjacent  volcanic  activity,  it 
might  have  been  Avind-borne  from  a  volcano  in  a  distant  region. 

Lava^streams. — At  its  exit  from  the  side  of  a  volcano,  lava 
glows  with  a  white  heat,  and  flows  with  a  motion  which  has  been 
compared  to  that  of  honey  or  of  melted  iron.  It  soon  becomes  red, 
and  like  a  coal  fallen  from  a  hot  fireplace,  rapidly  grows  dull  as  it  moves 
along,  until  it  assumes  a  black,  cindery  aspect.  At  the  same  time  the 
surface  congeals,  and  soon  becomes  solid  enough  to  support  a  heavy  block 
of  stone.  The  aspect  of  the  stream  varies  with  the  composition  and 
fluidity  of  the  lava,  form  of  the  ground,  angle  of  slope,  and  rapidity  of 
flow.  Viscous  lavas,  like  those  of  Vesuvius,  break  up  along  the  surface 
into  rough  brown  or  black  cinder-like  slags  and  irregular  ragged  cakes, 
bristling  with  jagged  points  ("a<t"2j^  which,  in  their  onward  motion, 
grind  and  grate  against  each  other  with  a  harsh  metallic  sound,  sometimes 
rising  into  rugged  mounds  or  becoming  seamed  with  rents  and  gashes,  at 
the  bottom  of  which  the  red-hot  gloAving  lava  may  be  seen  (Fig.  46).  In 
lavas  possessing  somewhat  greater  fluidity,  the  surface  presents  froth-like, 
curving  lines,  as  in  the  scum  of  a  slowly  flo^ving  river,  or  is  arranged  in 
curious  ropy  folds,  as  the  layers  have  successively  flowed  over  each  other 
and  congealed  ("/)ttAo^/«)f?"*-).  These,  and  many  other  fantastic  coiled 
shapes  were  exhibited  by  the  Vesuvian  lava  of  1858,  and  are  admirably 
displayed  by  the  peculiarly  liquid  glassy  lavas  of  Kilauea,-  Risalts 
possessing  extreme  liquidity  have  flowed  for  great  distances  vrith  singularly 
smooth  surfaces.  A  large  area  which  has  been  flooded  with  lava  is 
perhaps  the  most  hideous  and  appalling  scene  of  desolation  anywhere  to 
be  found  on  the  surface  of  the  globe. 

A  lava-stream  usually  spreads  out  as  it  descends  from  its  point  of 
escape,  and  moves  more  slowly.     Its  sides  look  like  huge  embankments, 

^  Nordenskiold,  Oeol.  Mag.  2nd  dec.  iii.  p.  292.  G.  vom  Rath,  Monatsber.  K.  Preuss. 
AkfuL  WUs.  1876,  i».  282.     Xeues  Jahrb.  1876,  p.  52,  &nd  postea,  p.  338. 

-  For  descriptions  of  Vesuvian  lava- streams,  see  the  various  memoirs  and  works  cited, 
ante^  ]>.  195.  For  those  of  Etna,  Sartorius  von  Waltershausen  and  A.  von  Lasaulx,  *  Der 
Aetna,'  ii.  p.  390.  The  rugged  scoriaceous  lava-surfaces  are  known  in  Hawaii  as  aa^  the 
smooth  coiled  and  ropy  surfaces  are  there  called  jxthoehoe.  Dana,  'Characteristics  ol 
Volcanoes,'  p.  9.  The  same  stream  of  lava  may  exhibit  both  these  aspects  in  different  parts 
of  its  course.  Ibid.  p.  209  and  Mr.  Johnston-Lavis'  papers  on  Vesuvius,  already  cited 
p.  195. 


DVXAMICAL  GEOLOGY 


BCXIK  III  PAST  I 


or  like  some  of  the  long  moiiixis  of  "  clinkers  "  in  a  great  manufacturing 
diatrict.     The  advancing  end  is  often  much  steeper,  creeping  onward  like 


Outflow  of  Lava. — This  tippcai-s  to  l>e  immediately  due   to  the 


8ECT.  i  §  2  LA  VA-STREAMS  219 

expansion  of  the  absorbed  vapours  and  gases  in  the  molten  rock.  Though 
these  vapours  may  reach  the  surface,  and  even  produce  tremendous  ex- 
plosions, without  an  actual  outcome  of  lava,  yet  so  Intimately  are  vapours 
and  lava  commingled  in  the  subterranean  reservoirs,  that  they  commonly 
rise  together,  and  the  explosions  of  the  one  lead  to  the  outflow  of  the 
other.  The  first  point  at  which  the  lava  makes  its  appearance  at  the 
surface  will  largely  depend  upon  the  structure  of  the  ground.  Two 
causes  have  been  assigned  on  a  foregoing  page  (p.  209)  for  the  Assuring 
of  a  volcanic  cone.  As  the  molten  mass  rises  within  the  chimnev  of  the 
volcano,  continued  explosions  of  vapour  take  place  from  its  upper  siu'face. 
The  violence  of  these  may  be  inferred  from  the  vast  clouds  of  steam, 
ashes,  and  stones  hurled  to  so  great  a  height  into  the  air,  and  from  the 
concussions  of  the  ground,  which  may  be  felt  at  distances  of  more  than 
1 00  miles  from  the  volcano.  It  need  not  be  a  matter  of  surprise,  there- 
fore, that  the  sides  of  a  great  vent,  exposed  to  shocks  of  such  intensity, 
should  at  last  give  way,  and  that  large  divergent  fissures  should  be  opened 
down  the  cone.  Again,  the  hydrostatic  pressure  of  the  column  of  lava 
must,  at  a  depth  of  1 000  feet  below  the  top  of  the  column,  exert  a  pres- 
sure of  between  70  and  80  tons  on  each  square  foot  of  the  surrounding  walls 
(p.  209).  We  may  well  believe  that  such  a  force,  acting  upon  the  walls  of 
a  funnel  already  shattered  by  a  succession  of  terrific  explosions,  may 
prove  too  great  for  their  resistance.  WTien  this  happens,  the  lava  pours 
forth  from  the  outside  of  the  cone.  On  a  much-fissured  cone,  lava  may 
issue  freely  from  many  points,  so  that  a  volcano  so  affected  has  been 
graphically  described  as  "  sweating  fire.'' 

In  a  lofty  volcano,  lava  occasionally  rises  to  the  lip  of  the  crater  and 
flows  out  there ;  but  more  frequently  it  escapes  from  some  fissure  or  ori- 
fice in  a  weak  part  of  the  cone.  In  minor  volcanoes,  on  the  other  hand, 
where  the  explosions  are  less  violent,  and  where  the  thickness  of  the 
cone  in  proportion  to  the  diameter  of  the  funnel  is  often  greater,  the 
lava  very  commonly  rises  into  the  crater.  Should  the  crater-walls  be 
too  weak  to  resist  the  pressure  of  the  molten  mass,  they  give  way,  and 
the  lava  rushes  out  from  the  breach.  This  is  seen  to  have  happened  in 
several  of  the  puys  of  Auvergne,  so  well  figured  and  described  by  Scrope 
(Fig.  48).^  But  if  the  crater  be  massive  enough  to  withstand  the  pressure, 
the  lava  may  at  last  flow  out  from  the  lowest  part  of  the  rim. 

In  a  tall  column  of  molten  lava,  there  mav  be  a  variation  in  the 
density  of  its  different  parts,  the  heaviest  naturally  gravitating  to  the 
bottom.  It  has  been  observed  by  Ch.  V^lain  that  at  the  Isle  of  Bourbon 
(Reunion),  the  lavas  escaping  from  the  l>ase  of  the  volcanic  cone  are 
denser  and  more  basic  than  those  which  flow  out  from  the  lip  of  the  crater. - 

*  For  descriptions  of  tliis  region,  see  Scrope's  *  Geology  and  Extinct  Volcanoes 
of  Central  France,'  2d  edit.  1858.  H.  Lecoq's  'Epoqiies  geologiqnes  de  I'Auverjrne.' 
1867.  Michel  Levy,  BuH.  S,ir.  ijdi.  France,  xviii.  (1890),  p.  688.  The  succession  of 
volcanic  rocks  in  Velay  is  described  by  M.  Boule,  Bull.  *S(h:  Oeol.  France,  xviii.  (1889), 
p.  174,  and  in  Bull.  Carte  <r^i>l.  tfe  la  France,  Xo.  28  (1892)  ;  see  also  op.  cif.  No.  13  for 
a  memoir  by  P.  Termier. 

-  *Les  Volcans,'  j».  36.     For  references  relating  to  this  island,  see  p.  243. 


220 


tiYXAMKAL  HEnLOGY 


As  soon  AB  the  molt€n  rock  reaches  the  surface,  the  superheated  water- 
vupour  or  gas,  dissolved  ^dthiIl  its  mass,  escajjcs  copioush',  and  hangs  as  a 
dense  white  cloud  over  the  moving  current.  The  lava-streams  of  Vesuving 
sometimes  appear  with  as  dense  a  steam-cloud  at  their  lower  ends  as  that 


which  escapes  at  the  same  time  from  the  main  crater.  Even  after  the 
molten  mass  has  flowed  several  miles,  steam  continues  to  rise  abundantly 
lioth  from  its  end  and  from  numerous  points  along  its  surface,  and 
continues  to  do  so  for  many  weeks,  months,  or  it  may  be  for  several 
years. 

Should  the  point  of  escape  of  a  lava-stream  lie  well  down  on  the  cone, 
far  below  the  summit  of  the  lava-column  in  the  funnel,  the  molten  rock, 
on  its  first  escape,  driven  by  hydrostatic  pressure,  will  sometimes  spout 
up  high  into  the  air — a  fountain  of  molten  rock.  This  was  observed  in 
1794  on  Vesuvius,  and  in  1^32  on  Etna.  In  the  eruption  of  1852  at 
Mauna  Loa,  an  unbroken  fountain  of  lava,  from  200  to  700  feet  in  height 
and  1000  feet  broad,  burst  out  at  the  base  of  the  cone.  Similar  "  geysers  " 
of  molten  rock  have  subsequently  been  noticed  in  the  same  region.  Thus 
in  March  and  April  186)*,  four  fiery  fountains,  throwing  lava  to  heights 
varying  from  .")00  to  1000  feet,  continued  to  play  for  several  weeks. 
According  to  Mr.  Coan,  such  outbursts  take  place  from  the  bottom  of  a 
column  of  lava  3000  feet  high.  The  volcano  of  Mauna  Loa  strikingly 
illustrates  another  feature  of  volcanic  dynamics  in  the  position  and  out- 
flow of  lava.  It  bears  u|x>n  its  Hanks  at  a  distance  of  20  miles,  but 
10,000  feet  lower,  the  huge  crater  Kilauea.  As  Dana  has  pointed  out, 
these  orifices  form  ])art  of  one  mountain,  yet  the  column  of  lava  stajids 
10,000  feet  higher  in  one  conduit  than  in  the  other.  On  a  far  smaller 
scale  the  same  independence  occurs  among  the  several  pipes  of  some  of 
the  geysers  in  the  Yellowstone  region  of  North  America. 

From  the  wide  extent  of  basalt-dykes,  such  as  those  of  Tertiary  age 
in  Britain,  which  rise  to  the  surface  at  a  distance  of  200  miles  from  the 
main  remnants  of  the  volcanic  outbursts  of  their  time,  and  are  found  over 
an  ai'ca  of  jierhaps  100,000  square  miles,  it  is  evident  that  molten  lava 


SECT,  i  5;  i  LA  VA  -ST RE  A  MS  2  2 1 

may  sometimes  occupy  a  far  greater  space  within  the  crust  than  might  be 
inferred  from  the  dimensions  and  outpourings  even  of  the  largest  volcanic 
cone.  There  can  be  no  doubt  that  vast  reservoirs  of  melted  rock,  impreg- 
nated with  superheated  vapours,  must  foimerly  have  existed,  if  they  do 
not  exist  still,  beneath  extensive  tracts  of  country  (p.  583).  Yet  even 
in  these  more  stupendous  manifestations  of  volcanism,  the  lava  should  be 
regarded  rather  as  the  sign  than  as  the  cause  of  volcanic  action.  The 
cause  of  the  ascent  of  the  lava  in  volcanic  pipes  is  still  obscure :  it  may 
possibly  be  due  to  the  compression  arising  from  the  secular  contraction 
of  the  earth.  But  it  is  doubtless  the  pressure  of  the  imprisoned  vapour, 
and  its  struggles  to  get  free,  which  produce  the  subterranean  earthquakes 
and  the  explosions  from  the  vents.  As  soon  as  the  vapoiu*  finds  relief, 
the  terrestrial  commotion  calms  down  again,  until  another  accumulation  of 
vapour  demands  a  repetition  of  the  same  phenomena. 

Rate  of  flow  of  Lava. — The  rate  of  movement  is  regulated  by  the 
fluidity  of  the  lava,  by  its  volume,  and  by  the  form  and  inclination  of 
the  ground.  Hence,  as  a  rule,  a  lava-stream  moves  faster  at  first  than 
afterwards,  because  it  has  not  had  time  to  stiffen,  and  its  slope  of  descent 
is  usually  steeper  than  farther  down  the  mountain.  One  of  the  most 
fluid  and  swiftly  flowing  lava-streams  ever  observed  on  Vesuvius  >vas 
that  erupted  on  1 2th  August  1 805.  It  is  said  to  have  rushed  down  a 
space  of  3  Italian  (3f  English)  miles  in  the  first  four  minutes,  but  to 
have  widened  out  and  moved  more  slowly  as  it  descended,  yet  finally  to 
have  reached  Torre  del  Greco  in  three  hours.  A  lava  erupted  by  Mauna 
Loa  in  1852  went  as  fast  as  an  ordinary  stage-coach,  or  fifteen  miles  in 
two  hours ;  but  some  of  the  lavas  from  that  mountain  have  in  parts  of 
their  course  moved  with  double  that  rapidity.  Long  after  a  current  has 
been  deeply  crusted  over  with  slags  and  rough  slabs  of  lava,  it  may  con- 
tinue to  creep  slowly  forward  for  weeks  or  even  months. 

It  happens  sometimes  that,  as  the  lava  moves  along,  the  still  molten 
mass  inside  bursts  through  the  outer  hardened  and  deeply  seamed  crust, 
and  rushes  out  with,  at  first,  a  motion  much  more  rapid  than  that  of  the 
main  stream.  Any  sudden  change  in  the  form  or  slope  of  the  ground 
affects  the  flow  of  the  lava.  Thus,  reaching  the  edge  of  a  steep  defile  or 
cliff,  the  molten  rock  pours  over  in  a  catiiract  of  glowing,  molten  rock, 
with  clouds  of  steam,  showers  of  fragments,  and  a  noise  utterly  indescril>- 
able.  Or,  on  the  other  hand,  encountering  a  ridge  or  hill  across  its 
path,  it  accumulates  until  it  either  finds  egress  round  the  side  or  actually 
overrides  and  entombs  the  obstacle.  The  hardened  crust  or  shell,  within 
which  the  still  fluid  lava  moves,  serves  to  keep  the  mass  from  spreading. 
Here  and  there,  inside  this  crust,  the  lava  subsides,  leaving  cavernous 
spaces  and  tunnels  into  which,  when  the  whole  is  cold,  one  may  creep, 
and  which  are  sometimes  festooned  with  stalactites  of  lava  (p.  227). 

Size  of  Lava-streams. — In  some  cases,  lava  escaping  from  craters 
or  fissures  comes  to  rest  before  reaching  the  base  of  the  slopes,  like  the 
obsidian  current  which  has  congealed  on  the  side  of  the  little  volcanic 
island  of  Volcano. ^     In  other  instances,  the  molten  rock  not  only  reaches 

*  Recent  eruptions  iu  this  island  have  consisted  entirely  of  ashes.     A.  Baltzer,  ZeUsch. 


222  JJYNAMICAL  GEOLOdY  book  iii  part  i 


the  plains  but  flows  for  many  miles  away  from  the  point  of  eruption. 
Sartorius  von  Waltershausen  computed  the  lava  emitted  by  Etna  in  1865 
at  92  millions  of  cubic  metres,  that  of  1852  at  -420  millions,  that  of  1669 
at  980  millions,  and  that  of  a  prc-historic  lava-stream  near  Randazzo  at 
more  than  1000  millions.^  The  most  stupendous  outpouring  of  lava  on 
record  was  that  which  took  place  in  Iceland  in  the  year  1 783.  Successive 
streams  issued  from  a  fissure  about  1 2  miles  long,  filling  up  river-gorges 
which  were  sometimes  600  feet  deep  and  200  feet  broad,  and  advancing 
inU)  the  alluvial  plains  in  lakes  of  molten  rock  12  to  15  miles  wide  and 
100  feet  deep.  Two  currents  of  lava  which,  filling  up  the  valley  of 
the  Skapta,  escaped  in  nearly  opposite  directions,  extended  for  \b  and 
50  miles  respectively,  their  usual  thickness  being  100  feet.  Bischof 
estimated  that  the  total  amoiuit  of  lava  poured  forth  during  this  single 
eruption  "  surjmssed  in  magnitude  the  bulk  of  Mont  Blanc."  ^ 

Varying  liquidity  of  Lava. — All  lava,  at  the  time  of  its  expulsion,  is 
in  a  molten  condition.  It  usually  consists  of  a  glassy  magma  in  which,  by 
reason  of  the  high  temjMjrature,  most  or  even  all  of  the  mineral  constituents 
exist  dissolved.  Considerable  differences,  however,  have  been  observed  in 
the  degree  of  liquidity.  Humboldt  and  Scrope  long  ago  called  attention 
to  the  thick,  short,  lumpy  forms  presented  by  masses  of  solidified  trachytic 
rocks,  which  are  lighter  and  more  siliceous,  and  to  the  thin,  widely  ex- 
tended sheets  assumed  by  l>asalts,  which  are  heavy  and  contain  much 
iron  and  Imsic  silicates.**  It  may  be  inferred  that,  as  a  rule,  the  basalts 
or  basic  lavas  have  been  more  liquid  than  the  trachytes  or  siliceous  lavas. 
The  cause  of  this  difference  has  been  variously  explained.  It  may  depend 
pirtly  uj)on  chemical  composition,  the  siliceous  being  naturally  less  fusible 
than  the  basic  rocks.  But  as  great  differences  of  fluidity  are  observable 
even  among  lavas  having  nearly  the  same  composition,  there  would  seem 
to  bo  some  further  cause  for  the  diversity.  Keyer  has  ingeniously 
maintained  that  we  must  look  to  original  differences  in  the  extent  to 
which  the  subterranean  igneous  magma  that  supplied  the  lava  has  been 
satiu*ated  with  vapours  and  gases.  Molten  rock  highly  impregnated  gives 
rise,  he  holds,  to  fragmentary  discharges,  while  when  feebly  impregnated 
it  flows  out  trampiilly.*  On  the  other  hand,  Captain  C.  E.  Dutton,  who 
has  studied  the  volcanic  phenomena  of  Western  Americji  and  Hawaii, 
suggests  that  the  different  degrees  of  liquidity  may  dej>end  not  only  on 
chemical  differences,  but  on  variations  of  temperature.  He  supposes 
that  the  basaltic  lavas  which  have  spread  so  far  in  thin  sheets,  and 
which  must  have  had  a  comparatively  great  liquidity,  flowed  at  tem- 
peratures far  above  that  of  their  melting  point,  and  were,  to  use  his 
phrase,  "  superfused.*'  ^ 

heutsch.  (real.  Gci.  xxvi.  (1875),  p.  36.     G.  M«rcalli,  *Le  Eruzioiii  dell'  Isola  Vulcano/  Rat- 
scfjna  XazionalCt  1889  ;  also  a  i>aper  by  same  author  iu  Atti.  N»c.  /tal.  Sci.  Xat,,  vol.  xxxi. 

1   'Der  Aetna,'  ii.  393. 

-  Lyell,  'Principles,' ii.  p.  49.     Ilellaud,  *  Lakis-kratere,*  cited  rtw^f,  p.  202. 

•*  Scrope,  'Considerations  on  Volcanoes*  (IS'i.*)),  p.  93. 

*  '  lieitrag  zur  Physik  der  Erui>tionen,'  p.  77. 

*  "  High  Plateaux  of  Utah,"  (wcntj,  and  O'eol.  »Sttr.  Territories.   Washington,  1880,  chap.  v. 


8BCT  L  g  2  LAVA-STREAMS  223 

The  varying  degrees  of  liquidity  are  manifested  in  a  characterietic  way 
on  the  surface  of  lava.  Thus,  in  the  great  lava-pools  of  Hawaii,  the  rock 
exhibits  a  remarkable  liquidity,  throwing  up  fountains  of  molten  rock 
to  a  height  of  300  feet  or  more.  During  its  ebullition  in  the  crater-pools, 
jets  and  dribbleU,  a  quarter  of  an  inch  in  diameter,  are  tossed  up,  and 
falling  back  on  one  another,  make  "  a  column 
of  hardened  tears  of  lava,"  one  of  which 
(Fig.  49)  was  found  to  have  attained  a  height 
of  40  feet,  while  in  other  places,  the  jets 
thrown  up  and  blown  aside  by  the  wind  give 
me  to  long  threads  of  glass  which  He  thickly 
together  like  mown  grass,  and  are  known  by 
the  natives  under  the  name  of  "  Pele's  Hair," 
after  one  of  their  divinitiea.i  Yet  although 
the  ebullition  is  caused  by  the  uprise  and 
escape  of  highly  heated  vapours,  there  is  no 
cloud  over  the  boiling  lake  itself,  heavy  white  Rg. « 

vapour    only    escaping    at    different    points        J"*  <>'  "i'"^   l*v«,  cnii«r  .>f 
along  the  edge.  ''"'""  *'^"'- 

On  the  other  hand,  the  lavas  of  Vesuvius  and  of  most  modern 
volcanoes,  which  issue  so  saturated  with  vapour  as  to  be  nearly  concealed 
from  view  in  a  cloud  of  steam,  are  accompanied  by  abundant  explosions 
of  fragmentary  materials.  Slags  and  clinkers,  torn  by  explosions  of 
steam  from  the  molten  rock,  are  strewn  abundantly  over  the  cone,  while 
the  surface  of  the  lava  ia  likewise  rugged  with  similar  clinkers,  which 
may  now  and  then  be  observed  piled  up  round  some  more  energetic 
steam- spiracle.  Sometimes  the  vapour  forces  up  the  lava  round  such  a 
spiracle  or  fumarole  and  gradually  piles  up  a  rugged  column  several  feet 
or  yards  in  height,  as  has  been  observed  on  Vesuvius*  (Figs.  46,  49, 
50).  So  vast  an  amount  of  steam  rushes  out  from  one  of  these  orifices, 
and  with  such  boiling  and  explosion,  that  the  cone  of  bombs,  slags, 
and  irregular  lumps  of  lava  forms  a  miniature  or  parasitic  volcano, 
which  will  remain  as  a  marked  cone  on  its  parent  mountain  long 
after  the  eruption  which  gave  it  birth  has  ceased.  The  laia  of  the 
eruption  at  Santorin  in  1866-67  at  first  welled  out  tranquilly,  but  after 
a  few  days  its  outflow  was  accompanied  by  explosions  and  discharges 
of  incandescent  fragments,  which  increased  until  they  had  covered  the 
lava  dome  with  ejected  scori*,  and  had  opened  a  number  of  crateriform 
mouths  on  ita  summit.' 

There  can  be  no  doubt,  as  above  remarked,  that  the  condition  of 
liquidity  of  the  tava  has  in  some  measure  determined  the  form  of  the 
eruptions.  In  one  case,  there  are  quiet  ouUwellings  of  the  more  liquid 
lavas,  as   at    Hawaii ;    in    another,  there  are   explosive   discharges    and 

'  Dana,  Otol.  i'.S.  Rrjilor.  £rj^l.,  "Geologjr,"  j).  I7S  ;  ' Chancttristici  of  Voloiiioes,' 
p.  ISO 

-  Some  8°^  einniples  were  observed  on  this  nioantain  in  the  auiunier  of  1S9I  by  Mr. 
Johnrfon-Lsvis,  BHI  J«.f.  1891.  s«[.  C. 

'  Ponqu^,  'Santorin,'  p.  xv. 


2i4 


iiYXAMI<AL  iiE<iLO»n 


BOUK  111  I'Atlll 


ciiKler-tone^  accomiMiiyiiig  the  more  viscid  lavas,  as  at  moat  modeni 
voltaiioes.  The  fonuer  has  been  the  condition  favourable  to  the  most 
colonsal  mitiwiii-ings  of  molten  i-ock,  as  we  see  in  the  basal C-plat«aux  of 
Biitain,  Faroe,  (ireoiilaiid,  Idaho,  and  Ore^ii,  the  Ghaut«,  Abysainia,  etc. 
This  subject  is  again  referred  to  at  p.  :?">5. 


glass 


ivlui  (AMdi^ 


rvstallizatioii  of  Lava. — I'uiiriiig  forth  with  a  liquidity  like  that 
olteii  iron,  lava  sjwedily  assianes  a  more  viscous  condition  and  a 
;[■  niDtiiin.  Obsidian  and  other  vitreous  iiK-ks  haie  consolidated  as 
:  yet  that  they  are  not  ahvays  extremely  fluid  is  indicated  by  the 
t  of  the  obsidian  stn<am  half-way  <lowii  the  steep  northern  Bloi>e  of 
Voleanii.  Even  in  such  jK-rfevt  iiiiturid  glass  as  obsidian,  microscojac 
crystallites  and  crystals  arc  uftnally  present,  and  in  prodigious  numbers 
(pp.  11">,  Ifi'J).  Ill  most  lavas,  deWtrification  has  proceeded  so  far  before 
the  final  stitt'enin;:,  that  the  original  glassy  magma  lias  passed  into  a  more 
or  less  completely  lithoid  or  crystalline  mass. 

That  lava  may  possess  an  appreciably  crystalline  structure  while  still 
in  motion,  has  often  been  proved  at  Vesuvius,  where  well-defined  crystals 
of  the  infusible  luucite  may  be  observecl  in  a  molten  magma  of  the  other 
minerals,  jxirtions  of  the  white-hot  rock  in  this  condition  being  ladled  out, 


.SECT,  i  §  2  LA  VA'STREAMS  225 

impressed  with  a  stamp  and  suddenly  congealed.  The  fluxion-structure 
above  described  (pp.  100,  120)  furnishes  interesting  evidence  of  this  fact 
in  many  ancient  as  well  as  modern  lavas. 

There  is  reason  to  believe  that  in  the  molten  magma  beneath  a  volcano 
considerable  progress  may  be  made  in  the  development  of  some  crystal- 
line minerals  out  of  the  surrounding  glass,  and  that  this  crystalline  portion 
may  be  to  some  extent  separated  from  the  vitreous  residue.  Hence  where 
this  has  taken  place,  subsequent  eruptions  may  give  rise  to  a  more  crystal- 
line and  probably  more  basic  lava  from  one  point  of  emission  and  a  more 
glassy  and  probably  more  acid  lava  from  another  vent  Or  we  may  con- 
ceive that  the  two  portions  of  the  magma  may  be  subsequently  mingled 
again  in  various  proportions  before  eruption.  ^  If  the  process  of  differ- 
entiation should  continue,  as  seems,  natural,  during  the  lapse  of  a  whole 
cycle  of  a  volcano's  history,  the  earlier  lavas  would  be  more  basic  than  the 
later. 

The  crystalline  structure  of  lava  has  been  supposed  to  be  in  some 
measure  determined  by  the  presence  of  the  volcanic  vapours  and  gases 
with  which  the  molten  rock  is  impregnated,  the  rapid  escape  of  these 
vapours  preventing  the  formation  of  the  crystalline  structure,  and  leaving 
the  lava  in  the  condition  of  a  more  or  less  perfect  glass.  But  the  experi- 
ments of  MM.  Fouqu6  and  Michel- L^vy  (postea^  p  302)  have  shown 
that  rocks,  having  in  every  essential  particular  the  characters  of  volcanic 
lavas,  may  be  artificially  produced  under  ordinary  atmospheric  pressiu'e  by 
simple  dry  fusion.  There  appears  to  be  no  doubt  that  the  presence  of 
water  lowers  the  fusion-point  of  silicates,  though  what  precise  influence 
the  dissolved  vapours  exert  upon  the  ultimate  consolidation  of  molten 
lava  has  yet  to  be  ascertained.  Difference  in  the  rate  of  cooling  has 
doubtless  been  an  important,  if  not  the  main,  factor  in  determining  the 
various  conditions  of  texture  of  lava-streams.  The  crystalline  stnicture 
may  be  expected  to  be  most  perfect  where,  as  within  thick  masses  of  rock, 
the  cooling  has  been  prolonged,  and  where,  consequently,  the  crystals  have 
had  ample  time  and  opportunity  for  their  formation.  On  the  other 
hand,  the  glassy  structure  will  naturally  be  most  perfectly  shown  where 
the  cooling  has  been  most  rapid,  as  in  the  vitreous  crust  on  the  walls  of 
dykes  already  referred  to  (pp.  171,  210).  Eocks  crystallizing  in  the 
deeper  parts  of  a  volcano  usually  possess  a  more  coarsely  crystalline 
structure  than  those  which  crystallize  at  or  near  to  the  surface  (p.  564). 

Temperature  of  Lava. — It  would  be  of  the  highest  interest  and 
importance  to  know  accurately  the  temperature  at  which  a  lava-stream 
first  issues.  Measurements  not  altogether  satisfactory  have  been  taken 
at  various  distances  below  the  point  of  emission,  where  the  moving  lava 
could  be  safely  approached.  Experiments  made  at  Vesuvius  by  Scacchi 
And  Sainte-Claire  Deville  in  1855,  by  thrusting  thin  wires  of  silver,  iron, 
and  copper  into  the  lava,  indicated  a  temperature  of  scarcely  700°  C. 
«(1228*  Fahr.)  Observations  of  a  similar  kind,  made  in  1819,  when  a 
diver  wire  ^(^th  inch  in  diameter  at  once  melted  in  the  Vesuvian  lava  of 

'  Compare  the  observation  of  Ch.  Velain  cited  ante,  p.  219,  and  the  remarks  jpoj^/ea,  pp. 
J262,  269,  564.     Consult  on  this  subject  a  paper  by  Prof.  Judd,  Oeol.  Mag.  1888,  p.  1. 

Q 


226  DYNAMICAL  GEOLOGY  book  m  part  i 


that  year,  gave  a  greatly  higher  temperature,  the  melting-point  of  silver 
being  about  1 800^  Fahr.  But  copper  wire  has  also  been  melted,  the  point 
of  fusion  of  this  metal  being  about  2204""  Fahr.  Evidence  of  the  hi^ 
temperatiu'e  of  lava  has  likewise  been  adduced  from  the  alteration  it  has 
effected  upon  refractory  substances  in  its  progress,  as  where,  at  Torre  del 
Greco,  it  overflowed  the  houses,  and  was  after^vards  found  to  have  fused 
the  fine  edges  of  flints,  to  have  decomposed  brass  into  its  component 
metals,  the  copper  actually  crystallizing,  and  to  have  melted  silver,  and 
even  sublimed  it  into  small  octahedral  crystals  (p.  230).  The  lava  of 
Santorin  has  caught  up  pieces  of  limestone,  and  has  formed  out  of  them 
nodules  containing  crystallized  anorthite,  augite,  sphene,  black  garnet^ 
and  particularly  wollastonite.^  The  initial  temj^erature  of  lava,  as  it  fint 
issues  from  the  Vesuvian  fimnel,  is  probably  considerably  more  than 
2000''  Fahr.  Obviously  the  dissolved  water  (or  water-substance,  for,  as 
already  remarked,  the  temperatui'e  is  far  above  the  critical  point  of  wator, 
and  its  comiK)nent  gases  may  exist  dissociated)  must  possess  as  high  a 
temperatiu'e  as  that  of  the  white-hot  lava  in  which  it  is  contained.  The 
existence  of  the  elements  of  water  at  a  white  heat,  even  in  rocks  which 
have  reached  the  surface,  is  a  fact  of  no  little  significance  in  the  theoretical 
consideration  of  hypogene  action. 

Inclination  and  thickness  of  lava-flows. — It  was  at  one  time 
supposed  that  lava  coidd  not  consolidate  in  beds  on  such  steep  slopes  as 
those  of  most  volcanoes.  Hence  arose  the  "  elevation-crater  theory " 
(described  at  p.  241),  in  which  the  inclined  position  of  lavas  round  a  vol- 
canic vent  was  explained  by  upheaval  after  their  emission.  Observations 
all  over  the  world,  however,  have  now  demonstrated  that  lava,  with  all 
its  characteristic  features,  can  consolidate  on  slopes  of  even  35°  and  40*.* 
The  lava  in  the  Hawaii  Islands  has  cooled  rapidly  on  slopes  of  25**,  that 
from  Vesuvius,  in  1855,  is  here  and  there  as  steep  as  30**,  while  the  older 
lavas  in  Monte  Somma  are  sometimes  inclined  at  45"".  On  the  east  side 
of  Etna,  a  cascade  of  lava,  which  in  1689  poured  into  the  vast  hollow  of 
the  Cava  Grande,  has  an  inclination  varying  from  18^  to  48"*,  with  an 
average  thickness  of  16  feet.  On  Mauna  Loa  some  lava-flows  are  said  to 
have  congealed  on  slopes  of  49",  60°,  and  even  90",^  though  in  these 
cases  it  coidd  only  be  a  layer  of  rock,  stiflening  and  adhering  to  the  surface 
of  the  dediWtv.  On  the  other  hand,  lava-streams  have  travelled  consider- 
able  distances  over  ground  that  to  the  eye  looks  quite  level.  Among  the 
Hawaiian  islands  a  declivity  of  1^  or  less  has  been  quite  sufficient  for  the 
flow  of  the  extremely  liquid  and  mobile  lavas  of  that  region.  In  the 
great  lava-fields  of  the  Snake  River  region  of  the  Western  Territories  <rf 
the  United  States  the  basalts,  which  must  also  have  been  extremely  liquid, 
have  flowed  over  slopes  of  much  less  than  l®."*  The  breadth  and  length 
of  a  lava-stream,  as  well  as  the  form  of  its  surface,  depend  mainly  upaa 
the  liquidity  of  the  molten  material  at  the  time  of  outflow.     Even  when 

^  Fouque,  *  Santoriu,'  p.  206. 
■"'  Lyell  on  the  cousolidation  of  lava  on  steep  slopes,  Phif.  Trans.  1858. 
^  J.  D.  Dana.  Amer.  Juio-.  iki.  xxxv.  (1888),  p.  32. 
*  J.  D.  Dana,  '  Characteristics  of  Volcanoes,"  p.  12. 


8BCT.  i  §  2  LA  VA-STREAMS  227 


it  consolidates  on  a  steep  slope,  a  stream  of  lava  forms  a  sheet  with  parallel 
upper  and  under  surfaces,  a  general  uniformity  of  thickness,  and  often 
greater  evenness  of  surface,  than  where  the  angle  of  descent  is  low.  The 
thickness  varies  indefinitely ;  many  basalts  which  have  been  poured  out  in 
a  remarkably  liquid  condition  have  solidified  in  beds  not  more  than  1 0 
or  1 2  feet  thick.  On  the  other  hand,  more  pasty  lavas,  and  lavas  which 
have  flowed  into  narrow  valleys,  may  be  piled  up  in  solid  masses  to  a 
thickness  of  several  hundred  feet  (pp.  222,  229). 

Structure  of  a  lava-stream. — Lava-streams  are  sometimes  nearly 
homogeneous  throughout.  In  general,  however,  they  each  show  three 
component  layers.  At  the  bottom  lies  a  rough,  slaggy  mass,  produced  by 
the  rapid  cooling  of  the  lava,  and  the  breaking  up  and  continued  onward 
motion  of  the  scorifoim  layer.  The  central  and  main  portion  of  the 
stream  consists  of  solid  lava,  often,  however,  with  a  more  or  less  carious 
and  vesicular  texture.  The  upper  part,  as  we  have  seen,  may  be  a%nass 
of  rough  broken-up  slabs,  scoriae,  or  clinkers.  The  proportions  borne  by 
these  respective  layers  to  each  other  vary  continually.  Some  of  the  more 
fluid  ropy  lavas  of  Vesuvius  have  an  inconstant  and  thin  slaggy  crust ; 
others  may  be  said  to  consist  of  little  else  than  scorise  from  top  to 
bottom.  Throughout  the  whole  mass  of  a  lava -current,  but  more 
especially  along  its  upper  surface,  the  absorbed  or  dissolved  water-vapour 
expands  with  diminution  of  pressure,  and  pushing  the  molten  rock  aside, 
segregates  into  small  bubbles  or  irregular  cavities.  Hence,  when  the  lava 
solidifies,   these    steam -holes   are 

seen  to  be  sometimes  so  abundant   "     —  ^c^r-^  <c2c:^  tcx: 
that  a  detached  portion  of  the  rock 
containing  them  will  float  in  water 
(pumice).      They   are  often  elon- 
gated   in    the    direction    of    the  "  "^"^ 

motion    of    the    lava -stream     (Fig.     FIr.  51.-Elongation  of  vesicles  in  direction  of  flow 

51).      Sometimes,   indeed,   where 

the  cells  are  numerous,  their  elongation  in  one  direction  gives  a  fissile 

structure  to  the  rock. 

A  singular  feature  in  many  lava-streams  are  the  tunnels  and  caverns 
already  referred  to  (p.  221)  as  obsei-vable  in  them.  These  cavities  have 
doubtless  arisen  during  the  flow  of  the  mass  when  the  upper  and  under 
portions  had  solidified  and  were  creeping  sluggishly  onward,  while  the  still 
molten  interior  was  able  to  move  faster  and  thus  to  leave  empty  spaces 
behind  it.  Such  tunnels  may  frequently  be  seen  among  the  Vesuvian  lava- 
streams.  Some  remarkable  examples  are  described  from  the  highly  glassy 
lavas  of  Hawaii,  where  they  are  sometimes  from  2  to  10  feet  in  height 
and  30  feet  broad,  but  with  large  lateral  expansions.  The  walls  of  these 
Hawaiian  lava-chambers  are  smooth  and  even  glassy,  and  from  their  roofs 
hang  slender  stalactites  of  lava  20  to  30  inches  long,  while  on  the  floor 
below  little  mounds  of  lava-stalagmite  have  formed.  The  precise  mode 
of  origin  of  these  curious  appendages  is  not  yet  understood.^ 

In  passing  from  a  fluid  to  a  solid  condition,  and  thus  contracting, 

^  See  Dana's  *  Characteristics  of  Volcanoes,*  pp.  209,  33'2. 


228  DYNAMICAL  GEOLOGY  book  ra  parti 

lava  acquires  different  sti-uctures.  Lines  of  divisional  planes  or 
joints  traverse  it,  es]>ecially  ])eqx*ndicular  to  the  upi)er  and  under 
surfaces  of  the  sheet  These  sometimes  assume  prismatic  forms, 
dividing  the  rock  into  columns,  as  is  so  frequently  to  be  observed  in 
basalt.  They  are  described  in  Book  IV.  Part  IL,  together  with  other 
forms  of  joints. 

Vapours  and  sublimations  of  a  lava-stream. — Besides  steam, 
many  other  vaix)urs,  absorbed  in  the  original  subterranean  molten  magma, 
escape  from  the  fissures  of  a  lava-stream.  Such  vapours  are  copiously 
disengaged  at  fumarftlfs  (pp.  194,  195).  Among  the  exhalations^ 
chlorides  abound,  particularly  chloride  of  sodium,  which  appears,  not 
only  in  fissures,  but  even  over  the  cooled  crust  of  the  lava,  in 
small  crystals,  in  tufts,  or  as  a  granidar  and  even  glassy  incrustation. 
Chloride  of  iron  is  de}K)sited  as  a  yellow  coating  at  fumaroles,  where  also 
briglft  emerald-gi*een  films  and  scales  of  chloride  of  copper  may  be  more 
rarely  observied.  Many  chemical  changes  take  place  in  the  escape  of 
these  vapours.  Thus  sj^ecular-iron,  either  the  result  of  the  mutual  decom- 
}X)sition  of  steam  and  iron-chloride,  or  of  the  oxidation  of  magnetite, 
forms  abundant  scales,  plates,  and  small  crystals  in  the  fumaroles  and 
vesicles  of  some  lavas.  Sal-ammoniac  also  appears  in  large  quantity  on 
many  lavas,  not  merely  in  the  fissures,  but  also  on  the  upper  surface.  In 
these  cases,  it  is  not  directly  a  volcanic  product,  but  results  from  acme 
decomposition,  possibly  from  the  gases  evolved  by  the  sudden  destruction 
of  vegetation.  It  has,  however,  been  observed  also  in  the  crater  of  Etna, 
where  the  co-operation  of  organic  substance  is  hanlly  conceivable,  and 
where  perhaps  it  may  arise  from  the  decomposition  of  aqueous  vapour, 
whereby  a  combination  is  formed  with  atmosphenc  nitrogen.  Sulphur, 
breislakite,  szaboite,  tenorite,  alum,  sulphates  of  iron,  soda  and  potash, 
and  other  minerals  are  also  found. 

Slow  cooling  of  lava. — The  hardened  crust  of  a  lava-stream 
is  a  bad  conductor  of  heat.  Consequently,  the  surface  of  the  stream 
may  have  become  cool  enough  to  be  walked  upon,  though  the  red- 
hot  mass  may  be  observed  through  the  rents  to  lie  only  a  few  inches 
IkjIow.  Many  years,  therefore,  may  elapse  before  the  temperature  of 
the  whole  mass  has  fallen  to  that  of  the  siu*rounding  soil  Eleven 
months  after  an  eruption  of  Etna,  Spallanzani  could  see  that  the 
lava  was  still  red-hot  at  the  bottom  of  the  fissures,  and  a  stick  thrust 
into  one  of  them  instantly  took  fire.  The  Vesiivian  lava  of  1785  was 
found  by  Breislak,  seven  years  afterwards,  to  be  still  hot  and  steaming 
internally,  though  lichens  had  already  taken  root  on  its  surface.  The 
ropy  lava  enipted  by  Vesuvius  in  1858  was  observed  by  the  author  in 
1870  to  be  still  so  hot,  even  near  its  termination,  that  steam  issued 
abundantly  from  its  rents,  many  of  which  were  too  warm  to  allow  the 
hand  to  be  held  in  them,  and  three  years  later  it  was  still  steaming 
abundantly.  Hoffmann  records  that  from  the  lava  which  flowed  from 
Etna  in  1787,  steam  was  still  issuing  in  1830.  Yet  more  remarkable 
is  the  case  of  Jorullo,  in  Mexico,  which  sent  out  lava  in  1759.  Twenty- 
one  years  later  a  cigar  could  be  lighted  at  its  fissures ;  after  44  years  it 


SECT,  i  §  2  LAVA-STREAMS  229 


was  still  visibly  steaming;  and  even  in  1846,  that  is,  after  87  years  of 
cooling,  two  vapour-columns  were  still  rising  from  it.^ 

This  extremely  slow  rate  of  cooling  has  justly  been  regarded  as  a 
point  of  high  geological  significance,  in  regard  to  the  secular  cooling  and 
probable  internal  temperature  of  our  globe.  Some  geologists  have 
argued,  indeed,  that  if  so  comparatively  small  a  portion  of  molten  matter 
as  a  lava-stream  can  maintain  a  high  temperature  under  a  thin,  cold  crust 
for  so  many  years,  we  may,  from  analogy,  feel  little  hesitation  in  believ- 
ing that  the  enormously  vaster  mass  of  the  globe  may,  beneath  a  relatively 
thin  crust,  still  continue  in  a  molten  condition  within.  More  legitimate 
deductions,  however,  might  be  drawn  from  mor^  accurate  and  precise 
measurements  of  the  rate  of  loss  of  heat,  and  of  its  Variations  in  different 
lava -streams.  Lord  Kelvin,  for  instance,  has  suggested  that,  by 
measuring  the  temperature  of  intrusive  masses  of  igneous  rock  in 
coal-workings  and  elsewhere,  and  comparing  it  with  that  of  other  non- 
volcanic  rocks  in  the  same  regions,  we  might  obtain  data  for  calculating 
the  time  which  has  elapsed  since  these  igneous  sheets  were  enipted 
(aniej  p.  50). 

Effects  of  lava-streams  on  superficial  waters  and  topo- 
graphy.— In  its  descent,  a  stream  of  lata  may  reach  a  water-course,  and, 
by  throwing  itself  as  an  embankment  across  the  stream,  may  pond  back 
the  water  and  form  a  lake.  Such  is  the  origin  of  the  picturesque  Lake 
Aidat  in  Auvergne.  Or  the  molten  current  may  usurp  the  channel  V)f 
the  stream,  and  completely  bury  the  whole  valley,  as  has  happened  again 
and  again  among  the  vast  lava-fields  of  Iceland.  Few  changes  in  physio- 
graphy are  so  rapid  and  so  enduring  as  this.  The  channel  which  has 
required,  doubtless,  many  thousands  of  years  for  the  water  laboriously 
to  excavate,  is  sealed  up  in  a  few  houi-s  under  100  feet  or  more  of  stone, 
and  another  vastly  protracted  interval  may  elapse  before  this  newer  pile 
is  similarly  eroded.*-^ 

By  suddenly  overflowing  a  brook  or  pool  of  water,  molten  lava  some- 
times has  its  outer  crust  shattered  to  fragments  by  a  sharp  explosion  of 
the  generated  steam,  while  the  fluid  mass  within  rushes  out  on  all  sides.** 
The  lava  emitted  by  Mauna  Loa,  Hawaii,  in  the  spring  of  1868  flowed 
out  to  sea,  and  added  half  a  mile  to  the  extent  of  the  island  at  that  point. 
At  the  end  of  the  stream  three  cinder-cones  formed  from  the  contact  of 
the  lava  with  the  water,  and  Captain  Dutton  calls  special  attention  to 
the  fact  that  not  only  in  this  instance,  but  in  other  examples  among  the 
Hawaiian  lavas  which  have  reached  the  sea,  there  is  clear  evidence  of  the 
formation  of  ordinary  volcanic  craters  by  the  accidental  contact  of  lava 
with  water."*     The  lavas  of  Etna  and  Vesuvius  have  also  protruded  into 

'  E.  Schleiden,  quoted  by  Naumanu,  *  Geognosie,'  i.  p.  1 60. 

*  For  an  example  of  tlie  conversion  of  a  lava-buried  river-bed  into  a  hill- top  by  long- 
continued  denudation,  see  Quart.  Journ.  Oeol.  Six;.  1871,  p.  803. 

'  Explosions  of  tliis  nature  have  been  observed  on  Etna,  where  tlie  lava  has  suddenly 
come  in  contact  with  water  or  snow,  considerable  loss  of  life  being  sometimes  the  result. 
Sartorins  von  Waltershausen  and  A.  von  Lasaulx,  *  I)er  Aetna,*  i.  pp.  295,  300. 

<  U.S.  Qeol,  Report  for  1882-83,  p.  181. 


230  DYNAMICAL  GEOLOGY  book  m  part  i 

the  sea,  but,  owing  probably  to  their  more  viscous  and  lithoid  oonditicm 
and  lower  temperature,  they  do  not  seem  to  have  given  rise  to  exploeiye 
action  at  their  seaward  ends.  Thus  a  current  from  the  latter  mountain 
entered  the  Mediterranean  at  Torre  del  Greco  in  1794,  and  pushed  its 
way  for  360  feet  outwards,  with  a  breadth  of  1100  and  a  height  of  16 
feet.  So  quietly  did  it  advance,  that  Breislak  could  sail  round  it  in  a 
boat  and  observe  its  progi'ess. 

By  the  outpoiuing  of  lava,  two  important  kinds  of  geological  change 
are  produced.  (1)  Stream-courses,  lakes,  ravines,  valleys,  in  shorty  aO 
the  minor  features  of  a  landscape,  may  be  completely  overwhelmed  under 
a  thick  sheet  of  lava.  The  drainage  of  the  district  being  thus  effectually 
altered,  the  numerous  changes  which  flow  from  the  operations  of  running 
water  over  the  land  are  arrested  and  made  to  begin  again  in  new  channek 
(2)  Considerable  alterations  may  likewise  be  caused  by  the  effects  of  the 
heat  and  vapoiurs  of  the  lava  upon  the  subjacent  or  contiguous  ground. 
Instances  have  Ijeen  observed  in  which  the  lava  has  actually  melted 
down  opposing  rocks,  or  masses  of  slags  on  its  own  surface.  Interesting 
observations,  already  referred  to  (p.  226),  have  been  made  at  Torre  del 
Greco  under  the  lava-stream  which  overflowed  part  of  that  town  in  179i. 
It  was  foimd  that  the  window-panes  of  the  houses  had  been  devitrified 
into  a  white,  translucent,  stony  substance ;  that  pieces  of  limestone  had 
acquired  an  open,  sandy,  granular  texture,  without  loss  of  carbon-dioxide, 
and  that  iron,  brass,  lead,  copj)er,  and  silver  objects  had  been  greatly 
altered,  some  of  the  metals  being  actually  su])limed.  We  can  understand, 
therefore,  that,  retaining  its  heat  for  so  long  a  time,  a  mass  of  lava  may 
induce  many  crystalline  structures,  rearrangements,  or  decompositions  in 
the  rocks  over  which  it  comes  to  rest,  and  proceeds  slowly  to  cool.  This  is  a 
(fuestion  of  considerable  importance  in  relation  to  the  behaviour  of  ancient 
lavas  which  after  having  been  intnided  among  rocks  beneath  the  surface, 
have  subsefjuently  been  exposed  by  denudation  (Book  IV.  Part  VII.) 

But  on  the  other  hand,  the  exceedingly  trifling  change  produced,  even 
by  a  massive  sheet  of  lava,  has  often  been  remarked  with  astonishment 
On  the  flank  of  Vesuvius,  vines  and  trees  may  be  seen  still  flourishing 
on  little  islets  of  the  older  land-surface,  completely  surrounded  by  a  flood 
of  lava.  Dana  has  given  an  instructive  account  of  the  descent  of  a  laTa- 
stream  from  Kilauea  in  June  1840.  Islet-like  spaces  of  forest  were  left 
in  the  midst  of  the  lava,  many  of  the  trees  being  still  alive.  Where  the 
lava  flowed  round  the  trees,  the  stumps  were  usually  consumed,  and 
cylindrical  holes  or  casts  remained  in  the  lava,  either  empty  or  filled 
with  charcoal  In  many  cases,  the  fallen  crown  of  the  tree  lay  near,  and 
so  little  damaged  that  the  epiphytic  plants  on  it  began  to  grow  again. 
Yet  so  fluid  was  the  lava  that  it  himg  in  pendent  stalactites  from  the 
branches,  which  nevertheless,  though  clasped  round  by  the  molten  rock, 
had  barely  their  bark  scorched.  Again,  for  nearly  100  years  there  has 
lain  on  the  flank  of  Etna  a  large  sheet  of  ice,  which,  originally  in  the 
form  of  a  thick  mass  of  snow%  was  ovci'flowed  by  lava,  and  has  thereby 
been  protected  from  the  evaporation  and  thaw  which  would  certainly 
have  dissipated  it  long  ago,  had  it  been  exposed  to  the  air.     The  heat  oi 


8ECT.  i  §  2     ELEVATION  AND  SUBSIDENCE  AT  VOLCANOES        231 

the  lava  has  not  sufficed  to  melt  it.  Extensive  tracts  of  snow  were  likewise 
overspread  by  lava  from  the  same  mountain  in  1879.  In  other  cases, 
snow  and  ice  have  been  melted  in  large  quantities  by  overflowing  lava. 
The  great  floods  of  water  which  rushed  down  the  flank  of  Etna,  after  an 
eruption  of  the  mouuDain  in  the  spring  of  1755,  and  similar  deluges  at 
Cotopaxi,  are  thus  explained. 

One  further  aspect  of  a  lava-stream  may  be  noticed  here — the  effect 
of  time  upon  its  surface.  While  all  kinds  of  lava  must,  in  the  end, 
crumble  down  under  the  influence  of  atmospheric  waste  and,  where  other 
conditions  permit,  become  coated  with  soil,  and  support  some  kind  of 
vegetation,  yet  extraordinary  differences  may  be  observed  in  the  facility 
with  which  different  lavarstreams  yield  to  this  change,  even  on  the  flank 
of  the  same  mountain.  Every  one  who  ascends  the  slopes  of  Vesuvius 
remarks  this  fact.  After  a  little  practice,  it  is  not  difficult  there  to  trace 
the  limits  of  certain  lavas  even  from  a  distance,  in  some  cases  by  their 
verdure,  in  others  by  their  barrenness.  Five  hundred  years  have  not 
sufficed  to  clothe  with  green  the  still  naked  surface  of  the  Catanian  lava 
of  1381  ;  while  some  of  the  lavas  of  the  present  century  have  long  given 
footing  to  bushes  of  furze. ^  Some  of  the  younger  lavas  of  Auvergne, 
which  certainly  flowed  in  times  anterior  to  those  of  history,  are  still 
singularly  bare  and  rugged.  Yet,  on  the  whole,  where  lava  is  directly 
exposed  to  the  atmosphere,  without  receiving  protection  from  occasional 
showers  of  volcanic  ash,  or  where  liable  to  be  washed  bare  by  heavy 
torrents  of  rain,  its  surface  decays  in  a  few  years  sufficiently  to  afford 
soil  for  stray  plants  in  the  crevices.  When  these  have  taken  root  they 
help  to  increase  the  disintegration  ;  at  last,  as  the  rock  is  overspread, 
the  traces  of  its  volcanic  origin  fade  away  from  its  surface.  Some  of  the 
Vesuvian  lavas  of  the  present  century  already  support  vineyards. 

Elevation  and  Subsidence. — Proofs  of  elevation  are  frequent  among 
volcanic  vents  which,  lying  near  the  sea  and  containing  marine  sediments 
among  their  older  erupted  materials,  supply,  in  the  enclosed  marine 
organisms,  evidence  of  the  movement.  In  this  way,  it  is  known  that 
Etna,  Vesuvius,  and  other  Mediterranean  volcanoes,  began  their  history 
as  submarine  vents,  and  that  they  owe  their  present  dimensions  not  only 
to  the  accumulation  of  ejected  materials,  but  also,  to  some  extent,  to  an 
elevation  of  the  sea-bottom. 

Proof  of  subsidence  is  less  easily  traced,  but  indications  have 
been  observed  of  a  sinking  of  the  ground  beneath  a  volcanic  vent. 
During  the  eruption  of  Santorin  in  1866-67,  very  decided  but  extremely 
local  subsidence  took  place  near  the  vent  in  the  centre  of  the  old 
crater.  The  discharge  of  such  prodigious  quantities  of  material 
may  tend  to  produce  cavernous  spaces  in  the  terrestrial  crust,  and  the 
weight  of  the  ejected  lavas  and  tuffs  may  still  further  contribute  to  a 
general  settlement  of  the  gi'ound  around  a  volcanic  focus. 

If  we  consider  the  records  of  volcanic  action  in  past  geological  time  we 
meet  with  many  proofs  that  it  took  place  in  areas  where  the  predominant 
terrestrial  movement  was  one  of  subsidence.     Thus  among  the  Palaeozoic 

'  On  the  weathering  of  the  Etna  lavas,  see  *  Der  Aetna,*  ii.  p.  397. 


232  DYNAMICAL  GEOLOGY  book  m  part  i 


systems  of  Britain  the  Cambrian,  Siliuian,  Devonian,  Carboniferous,  and 
Permian  volcanoes  successively  appeared,  and  their  lavas  and  tuflfs  were 
carried  down  and  buried  under  thousands  of  feet  of  sedimentary 
deposits.^ 

Torrents  of  Water  and  Mud. — We  have  seen  that  large  quantities 
of  water  accompany  many  volcanic  eruptions.  In  some  cases,  where 
ancient  crater-lakes  or  internal  reservoirs,  shaken  by  repeated  detonations, 
have  been  finally  dismpted,  the  mud  which  has  thereby  been  liberated 
has  issued  from  the  mountain.  Such  "mud-lava''  (lava  d'acqua),  on 
account  of  its  li(|uidity  and  swiftness  of  motion,  is  more  dreaded  for 
destructiveness  than  even  the  true  melted  lavas.  On  the  other  hand, 
rain  or  melted  snow  or  ice,  rushing  down  the  cone  and  taking  up  loose 
volcanic  dust,  is  converted  into  a  kind  of  mud  that  grows  more  and  more 
l)asty  as  it  descends.  The  mere  sudden  rush  of  such  large  bodies  of 
water  down  the  steej)  declivity  of  a  volcanic  cone  cannot  fail  to  effect 
much  geological  change.  Deep  trenches  are  cut  out  of  the  loose  volcanic 
slopes,  and  sometimes  largo  areas  of  woodland  are  swept  away,  the  debris 
being  strewn  over  the  plains  below. 

One  of  these  mud- lavas  invaded  Herculaneum  during  the  great 
eniption  of  79,  and  by  quickly  enveloping  the  houses  and  their  contents, 
has  preserved  for  us  so  many  precious  and  i>eri8hable  monuments  of 
antiquity.  In  the  same  district,  during  the  eruption  of  1622,  a  torrent 
of  this  kind  j)oured  down  ui)on  the  villages  of  Ottajano  and  Massa,  over- 
throwing walls,  filling  up  streets,  and  even  burying  houses  with  their 
inhabitants.  Diuing  the  gi*eat  eruj)tion  of  Cotoj>axi,  in  June  1877, 
enonnoiis  torrents  of  water  and  nuid,  produced  by  the  melting  of  the 
snow  and  ice  of  the  cone,  rushed  down  from  the  mountiiin.  Huge  portions 
of  the  glaciers  of  the  mountain  were  detached  by  the  heat  of  the  rocks 
below  them  and  rushed  down  Inxlily  breaking  up  inU)  blocks.  The  villages 
all  roiuid  the  mountain  to  a  distiince  of  sometimes  more  than  ten  geo- 
graphical miles  were  left  deeply  buried  under  a  deposit  of  mud  mixed 
with  blocks  of  lava,  ashes,  pieces  of  wood,  lumps  of  ice,  &c.-  Many  of 
the  volcanoes  of  Central  and  South  America  discharge  large  quantities  of 
mud  directly  from  their  craters.  Thus,  in  the  year  1691,  Imliaburu,  one 
of  the  Andes  of  Quito,  emitted  floods  of  mud  so  largely  charged  with 
dead  fish  that  ])estilential  fevers  arose  from  the  subsequent  effluvia. 
Seven  years  later  (1698),  during  an  explosion  of  another  of  the  same 
range  of  lofty  mountains,  Carguairazo  (14,706  feet),  the  summit  of  the 
cone  is  said  to  have  fallen  in,  while  torrents  of  mud  containing  immense 
numbers  of  the  flsh  PipnehHlus  (/f/clojmm,  jx)ured  forth  and  covered  the 
ground  over  a  sjmce  of  four  square  leagues.  The  carbonaceous  mud 
(locally  called  mot/a)  emitted  by  the  Quito  volcanoes  sometimes  escapes 
from  lateral  fissures,  sometimes  from  the  cratere.  Its  organic  contents, 
and  notably  its  siluroid  fish,  which  are  the  same  as  those  found  living 
in  the  streams  above  ground,  prove  that  the  water  is  derived  from  the 
surface,  and   accumulates  in  craters  or  undergi-ound  cavities  until  dis- 

*  Presidential  Addresses,  Quart.  Jovrn.  iieol.  ^s>c.  xlvii.  (1891),  xlviii.  (1892'r. 

-  Wolf.  XfMsJohrf,.  1878,  p.  133. 


8ECT  L  §  2  SILLS  AND  DYKES  233 


charged  by  volcanic  action.  Similar  but  even  more  stupendous  and 
destructive  outpourings  have  taken  place  from  the  volcanoes  of  Java, 
where  wide  tracts  of  luxuriant  vegetation  have  at  different  times  been 
buried  under  masses  of  dark  grey  mud,  sometimes  100  feet  thick,  with 
a  rough  hillocky  surface  from  which  the  top  of  a  submerged  palm-tree 
would  here  and  there  protrude. 

Between  the  destructive  effects  of  mere  water- torrents  and  that  of 
these  mud-floods  there  is,  of  course,  the  notable  difference  that,  whereas 
in  the  former  case  a  portion  of  the  surface  is  swept  away,  in  the  latter, 
while  sometimes  considerable  demolition  of  the  surface  takes  place  at  first, 
the  main  result  is  the  burying  of  the  ground  under  a  new  tumultuous 
deposit  by  which  the  typography  is  greatly  changed,  not  only  as  regards 
its  temporary  aspect,  but  in  its  more  permanent  features,  such  as  the 
position  and  form  of  its  water-courses. 

Effects  of  the  Closingr  of  a  Volcanic  Chimney — Sills  and  Dykes. — 
A  study  of  the  volcanic  phenomena  of  former  geological  periods,  where 
the  structure  of  the  interior  of  volcanoes  and  their  funnels  has  been  laid 
bare  by  denudation,  shows  that  in  many  cases  a  vent  becomes  plugged  up 
by  the  ascent  and  consolidation  of  solid  material  in  it,  while  yet  the 
eruptive  energy  of  the  volcano,  though  lessened,  has  not  ceased.  A  time 
is  reached  when  the  ascending  magma,  impelled  by  pressure  from  below, 
can  no  longer  overcome  the  resistance  of  the  coliunn  of  solid  lava  or  com- 
pacted agglomerate  which  has  sealed  up  the  orifice  of  discharge,  or  at  least 
when  it  can  more  easily  force  a  passage  for  itself  between  the  sedimentary 
strata  on  which  the  whole  volcanic  pile  may  rest,  or  between  the  lava 
sheets  at  the  base  of  the  pile,  or  into  fissui'es  in  either  or  both  of  these 
groups.  Hence  arise  intrusive  sheets  or  sills  and  dykes  or  veins  (see  pp. 
573,  577).  That  these  later  manifestations  of  volcanic  energy  have  some- 
times taken  place  on  a  great  scale  is  shown  by  the  number  and  size  of 
the  sills  which  are  found  at  the  base  of  the  Palaeozoic  volcanic  groups  of 
Britain.  This  feature  is  a  remarkably  striking  feature  of  the  rocks  that 
underlie  the  great  Lower  Silurian  volcanic  outflows  of  Arenig  and 
Cader  Idris  in  North  Wales.  It  recurs  so  frequently,  not  only  among 
Palaeozoic  volcanic  phenomena  but  quite  as  markedly  among  those  of 
Tertiary  age  in  the  British  Isles,  that  it  must  be  regarded  as  marking  an 
ordinary  phase  of  volcanic  action.  But  it  remains  of  course  invisible  until 
in  the  progress  of  denudation  a  volcanic  cone  is  cut  down  to  the  roots. 

Exhalations  of  Vapours  and  Gases. — A  volcano,  as  its  activity 
wanes,  may  pass  into  the  Solfatara  stage,  when  only  volatile  emana- 
tions are  discharged.  The  well-known  Solfatara  near  Naples,  since 
its.  last  eruption  in  1198,  has  constantly  discharged  steam  and 
sulphurous  vapours.  The  island  of  Volcano  has  now  passed  also 
into  this  phase,  though  giving  vent  to  occasional  explosions.  Numerous 
other  examples  occur  among  the  old  volcanic  tracts  of  Italy,  where 
they  have  been  termed  soffioni.  Steam,  escaping  in  conspicuous  jets, 
sulphiuretted  hydrogen,  hydrochloric  acid,  and  carbonic  acid  are 
particularly  noticeable  at  these  orifices.  The  vapours  in  rising  condense. 
The  sulphuretted  hydrogen  partially  oxidises  into  sulphuric  acid,  which 


234  DYNAMICAL  GEOLOGY  book  hi  part  i 

powerfully  corrodes  the  surrounding  rocks.  The  lava  or  tuff  through 
which  the  hot  vapours  rise  is  bleached  into  a  white  or  yellowish  crumbling 
clay,  in  which,  however,  the  less  easily  corroded  crystals  may  still  be 
recognised  in  situ.  At  the  same  time,  sublimates  of  sulphur  or  of 
chlorides  may  be  foimed,  or  the  sulphuric  acid  attacking  the  lime  of  the 
silicates  gives  rise  to  gypsum,  which  spreads  in  a  network  of  threads 
and  veins  through  the  hot,  steaming,  and  decomposed  mass.  In  this 
way,  at  the  island  of  Volcano,  obsidian  is  converted  into  a  snow-white, 
dull,  clay-stone-like  substance,  with  crystals  of  sulphur  and  gypsum  in  its 
crevices.  Silica  is  likewise  deposited  from  solution  at  many  orifices,  and 
coats  the  altered  rock  Avith  a  crust  of  chalcedony,  hyalite,  or  some  form 
of  siliceous  sinter.  As  the  result  of  this  action,  masses  of  rock  are  decom- 
posed beloAv  the  surface,  and  neAv  deposits  of  alum,  sulphur,  sulphides  of 
iron  and  copper,  &c.,  are  formed  alxjve  them.  Examples  have  been 
described  from  Iceland,  Li})ari,  Hungary,  Terceira,  Teneriffe,  St.  Helem^ 
and  many  other  localities.^  The  lagoons  of  Tuscany  are  basins  into  which 
the  waters  from  suftioni  are  discharged,  and  where  a  precipitation  of  their 
dissolved  salts  takes  place.  Among  the  substances  thus  deposited  are 
gypsum,  sulphur,  silica,  and  various  alkaline  salts ;  but  the  most  important 
is  boracic  acid,  the  extraction  of  which  constitutes  a  thriving  industry. 
In  Chili  many  solfataras  occur  among  extinct  volcanoes.- 

Aiiothcr  class  of  gaseous  emanations  betokens  a  condition  of  volcanic 
activity  further  advanced  towards  final  extinction.  In  these,  the  gas 
is  carbou-<lioxide,  either  issuing  directly  from  the  rock  or  bubbling  up 
with  water  which  is  often  quite  cold.  The  old  volcanic  districts  of 
Kurope  furnish  many  examples.  Thus  on  the  shores  of  the  Laacher 
See — ^an  ancient  crater-lake  of  the  Eifel — the  gas  issues  from  numerous 
openings  called  nutffette,  round  Avhich  dead  insects,  and  occasionally 
mice  and  birds,  may  be  found.  In  the  same  region  occiu*  hundreds  of 
springs  more  or  less  charged  with  this  gas.  The  famous  Valley  of 
Death  in  Java  contains  one  of  the  most  remarkable  gas -springs  in 
the  Avorld.  It  is  a  deep,  bosky  hollow,  from  one  small  space  on  the 
bottom  of  which  carbon-<lioxide  issues  so  copiously  as  to  form  the 
lower  stratum  of  the  atmosphere.  Tigers,  deer,  and  wild-boar,  enticed 
by  the  shelter  of  the  spot,  descend  and  are  si>eedily  suffocated. 
Many  skeletons,  including  those  of  man  himself,  have  been  observed. 

As  a  distinct  class  of  gas-springs,  Ave  may  group  and  describe  here 
the  emanations  of  volatile  hydrocarlx)ns,  which,  when  they  take  fire, 
are  known  as  Fire-wells.  These  are  not  of  volcanic  origin,  but  arise 
from  changes  within  the  solid  rocks  underneath.  They  occur  in  many 
of  the  districts  Avhere  mud- volcanoes  apj^ear,  as  in  northern  Italy,  on 
the  Caspian,  in  Mesopjtamia,  in  southern  Kurdistan,  and  in  many  parts 
of  the  United  States.     It  has  been  observed  that  they  frequently  rise 

>  Von  Buch,  *Caiiar.  Inseln/ p.  232.  Hoffmann,  PfHjg.  Ann,  1832,  pp.  38,  40,  60. 
Hanson,  Ann.  Ch^m.  Pharm.  1847  (Ixii.),  p.  10.  Darwin,  *  Volcanic  Islands,'  p.  29.  Hie 
name  Propyl ite^  as  already  mentioned  {antr^  p.  169)  has  been  proposed  by  Rosenbnsch  to  be 
restricted  to  certain  andesiten  and  allied  rocks  alt«re<l  by  solfataric  action. 

-  Doineyko,  Ann.  Mines^  ix,  (7®  s«'t.)     Large  numbers  of  solfataras  occur  also  in  loelaad. 


SECT,  i  §  2  GEYSERS  236 

in  regions  where  beds  of  rock-salt  lie  underneath,  and  as  that  rock 
has  been  ascertained  often  to  contain  compressed  gaseous  hydrocarbons, 
the  solution  of  the  rock  by  subterranean  water,  and  the  consequent 
liberation  of  the  gas,  has  been  offered  as  an  explanation  of  these  fire- 
wells. 

In  the  oil  regions  of  Pennsylvania,  certain  sandy  strata  occur  at 
various  geological  horizons  whence  large  quantities  of  petroleum  and 
gas  are  obtained  (p.  1 45).  In  making  the  borings  for  oil-wells,  reservoirs 
of  gas  as  well  as  subterranean  courses  or  springs  of  water  are  met  with. 
When  the  supply  of  oil  is  limited  but  that  of  gas  is  large,  a  contest  for 
possession  of  the  bore-hole  sometimes  takes  place  between  the  gas  and 
water.  When  the  machinery  is  removed  and  the  boring  is  abandoned, 
the  contest  is  allowed  to  proceed  unimpeded  and  results  in  the  intermittent 
discharge  of  columns  of  water  and  gas  to  heights  of  1 30  feet  or  more. 
At  night,  when  the  gas  has  been  lighted,  the  spectacle  of  one  of  these 
"  fire-geysers  "  is  inconceivably  grand.^ 

Geysers. — Eruptive  fountains  of  hot  Avater  and  steam,  to  which  the 
general  name  of  Geysers  {i.e.  gushers)  is  given,  from  the  examples  in  Ice- 
land, which  were  the  first  to  be  seen  and  described,  mark  a  declining 
phase  of  volcanic  activity.  The  Great  and  Little  Geysers,  the  Strokkr 
and  other  minor  springs  of  hot  water  in  Iceland,  have  long  been  celebrated 
examples.  More  recently  another  series  has  been  discovered  in  New 
Zealand.  But  probably  the  most  remarkable  and  numerous  assemblage 
is  that  which  has  been  brought  to  light  in  the  north-west  part  of  the 
territory  of  Wyoming,  and  Avhich  has  been  included  within  the  "  Yellow- 
stone National  Park  " — a  region  set  apart  by  the  Congress  of  the  United 
States  to  be  for  ever  exempt  from  settlement,  and  to  be  retained  for  the 
instruction  of  the  people.  In  this  singular  region  the  ground  in  certain 
tracts  is  honeycombed  with  passages  which  communicate  with  the  surface 
by  hundreds  of  openings,  whence  boiling  water  and  steam  are  emitted. 
In  most  cases,  the  water  remains  clear,  tranquil,  and  of  a  deep  green- 
blue  tint,  though  many  of  the  otherwise  quiet  pools  are  marked 
by  patches  of  rapid  ebullition.  These  pools  lie  on  mounds  or  sheets  of 
sinter,  and  are  usually  edged  round  with  a  raised  rim  of  the  same 
substance,  often  beautifully  fretted  and  streaked  with  brilliant 
colours.  The  eruptive  openings  usually  appear  on  small,  low, 
conical  elevations  of  sinter,  from  each  of  which  one  or  more  tubular  pro- 
jections rise.  It  is  from  these  irregular  tube-like  excrescences  that  the 
eruptions  take  place. 

The  term  geyser  is  restricted  to  active  openings  whence  coliunns  of 
hot  water  and  steam  are  from  time  to  time  ejected ;  the  non-eruptive 

*  Asbburner,  Proc.  Artier.  Phil.  *Soc.  xvii.  (1877),  p.  127.  StmcdVs  PetroUuvi  Reporter, 
15th  Sept.  1879.  Second  Oeol.  Survey  of  Pennsylvania,  containing  Reports  by  J.  Carll, 
1877,  1880.  J.  S.  Newberry,  *The  First  Oil  Well.'  Harper's  Magazine,  Oct.  1890.  On 
the  naphtha  districts  of  the  Caspian  Sea,  Abich,  Jdhrb.  Oeol.  ReicJis.  xxix.  (1879),  p.  165. 
H.  Sjogren,  op.  cit.  xxxvii.  (1887),  p.  47.  C.  Marvin,  *  Region  of  Eternal  Fire,'  London, 
1884.  See  also  for  phenomena  in  Gallicia,  Jahrb.  Oeol,  Rtichs.  xv.  pp.  199,  351  ;  xviL  p. 
291  ;  xviii.  p.  311  ;  xxxi.  (1881),  p.  131.     Proc.  Inst.  Civ.  Engineers,  xlii.  (1875),  p.  843. 


236  DYXAMIOAL  GEIiLOGY  book  in  pam  i 

pools  are  only  hot  springs.  A  true  geyser  should  thus  possess  an  under- 
ground pipe  or  passage,  terminating  at  the  surface  in  an  opening  built 
round  with  deposits  of  sinter.  At  more  or  less  regular  intervals,  rumblingi 
and  sharp  <leUiiiations  in  the  pipe  are  fullon-ed  by  an  agitation  of  t^e 
water  in  the  basin,  and  then  by  the  violent  e.xpulsion  of  a,  column  of  water 
and  steam  to  a  considerable  height  in  the  air.  In  the  Upper  Fire  Hole 
basin  of  the  Yellowstone  Park,  one  of  the  geysers,  named  "  Old  Faithful " 
(Fig.  52),  has  ever  since  the  discovery  of  the  region,  sent  out  a  column  of 
mingled  water  and  steam  every  sixty-three  minutes  or  thereabouts.  The 
column  rushes  up  vtith  a  loud  roar  to  a  height  of  more  than  100  feet,  the 
whole  eruption  not  occupying  more  than  about  five  or  six  minutes.  The 
other  geysers  of  the  same  district  are  more  capricious  in  their  movements, 


ttBPCc.  Fin  Hoi*  River, 


and  some  of  them  more  stupendous  in  the  volume  of  their  discharge. 
The  eruptions  of  the  Castle,  Giant,  and  Beehive  vents  are  marvellously 
impressive.' 

lu  examining  the  Yellowstone  Geyser  region  in  1679,  the  author  was 
s{>ecially  struck  by  the  evident  independence  of  the  vents.  This  was 
shown  by  thoii-  very  different  levels,  as  well  as  by  their  capricious  and 
unsymitathetic  ei-uptions.  On  the  same  hill-slope,  dozens  of  quiet  pools, 
as  well  as  some  true  geysers,  were  noticed  at  dilfcrent  levels,  from  the 
edge  of  the  Fire  Hole  River  up  to  a  height  of  at  least  80  feet  above  it. 
Yet  the  lower  pools,  from  which,  of  course,  had  there  been  underground 
1  Sc«  Hayd«Q's  ReporU  for  1S70  and  for  ISTS.  in  the  latter  at  which  will  be  (Oand  • 
volniiiinous  nionognph  on  the  Hot  SpringH  by  A,  C.  Peale :  Cornatock'i  Report  in  Jona'a 
KecuDDaisxance  of  N'.W.  Wyomiug,  kr..  I87t.  Tlie  depmlta  of  liot  Bpringi  «i«  tartber 
referred  to  ai.  |<|<.  153,  4S3. 


SECT,  i  §  2  GEYSERS  237 

connection  between  the  different  vents,  the  drainage  should  have  princi- 
pally discharged  itself,  were  often  found  to  be  quiet  steaming  pools 
without  outlet,  while  those  at  higher  points  were  occasionally  in  active 
eruption.  It  seemed  also  to  make  no  difference  in  the  height  or  tranquil- 
lity of  one  of  the  quietly  boiling  cauldrons,  when  an  active  projection 
of  steam  and  water  was  going  on  from  a  neighbouring  vent  on  the  same 
gentle  slope. 

Bunsen  and  Descloiseaux  spent  some  days  experimenting  at  the  Ice- 
landic geysers,  and  ascertained  that  in  the  Great  Geyser,  while  the  sur- 
face temperature  is  about  212*  Fahr.,  that  of  lower  portions  of  the  tube 
is  much  higher — a  thermometer  giving  as  high  a  reading  as  266"  Fahr. 
The  water  at  a  little  depth  must  consequently  be  64°  above  the  normal 
boiling-point,  but  it  is  kept  in  the  fluid  state  by  the  pressure  of  the  over- 
lying column.  At  the  basin,  however,  the  water  cools  quickly.  After  an 
explosion  it  accumulates  there,  and  eventually  begins  to  boil.  The 
pressure  on  the  column  below  being  thus  relieved,  a  portion  of  the  super- 
heated water  flashes  into  steam,  and  as  the  change  passes  down  the  pipe, 
the  whole  column  of  water  and  steam  rushes  out  with  great  violence.  The 
water  thereafter  gradually  collects  again  in  the  pipe,  and  after  an 
interval  of  some  hours  the  operation  is  renewed.  The  experiments  made 
by  Bunsen  proved  the  source  of  the  eruptive  action  to  lie  in  the  hot  part 
of  the  pipe.  He  hung  stones  by  strings  to  different  depths  in  the  funnel 
of  the  geyser,  and  found  that  only  those  in  the  higher  part  were  cast 
out  by  the  rush  of  water,  sometimes  to  a  height  of  100  feet,  Avhile,  at 
the  same  time,  the  water  at  the  bottom  was  hardly  disturbed  at  all. 
These  observations  give  much  interest  and  importance  to  the  phenomena 
of  geysers  in  relation  to  volcanic  action.  They  show  that  the  eruptive 
force  in  geysers  is  steam  ;  that  the  water  column,  even  at  a  comparatively 
small  depth,  may  have  a  temperature  considerably  above  212° ;  that  this 
high  temperatiu*e  is  local ;  and  that  the  eruptions  of  steam  and  water  take 
place  periodically,  and  with  such  vigour  as  to  eject  large  stones  to  a 
height  of  100  fect.^ 

The  hot  water  comes  up  with  a  considerable  percentage  of  mineral 
matter  in  solution.  According  to  the  analysis  of  Sandberger,  water 
from  the  Great  Geyser  of  Iceland  contains  in  10,000  parts  the  following 
proportions  of  ingredients  :  silica,  5097;  sodium-carbonate,  1*939;  ammo- 
nium-carbonate, 0083;  sodium-sulphate,  107;  potassium-sulphate,  0*475  ; 
magnesium-sulphate,  0  042  ;  sodium-chloride,  2  5 21  ;  sodium-sulphide, 
0-088  ;  carbonic  acid,  0557  =  11-872.2 

When  the  water  has  reached  the  surface,  it  deposits  the  silica  as  a 

»  Comptes  Rtndus,  xxiii.  (1846),  p.  934  ;  Pogg.  AnncU.  Ixxu.  (1847),  p.  159  ;  Ixxxiil 
(1851),  p.  197.  Ann.  ChimiCy  xxxviii.  (1853),  pp.  215,  385.  The  explanation  proposed 
for  the  phenomena  observed  at  the  Great  Geyser  is  probably  not  applicable  in 
those  cases  where  the  mere  local  accumulation  of  steam  in  suitable  reservoirs  may  be 
snfficient. 

'  Annal.  Chem,  und  PharvL  1847,  p.  49.  A  series  of  detailed  analyses  of  the  hot 
spriogs  of  the  Yellowstone  National  Park  will  be  found  in  No.  47  of  the  BuU,  U.S.  Geol. 
Surv.  1888. 


238  DYXAMICAL  GEOLOGY  book  ill  parti 

sinter  on  the  surfaces  over  which  it  floAvs  or  on  which  it  rests.^  The 
deposit,  which  is  not  due  to  mere  cooling  and  evaporation,  is  curioady 
aided  by  the  presence  of  liA-ing  algie  (jxistea,  p.  483).  It  naturally  takes 
place  fastest  along  the  margins  of  the  pools.  Hence  the  curiously  fretted 
rims  by  which  these  sheets  of  water  are  surrounded,  and  the  tubular  or 
cylindrical  protulxyrances  which  rise  from  the  growing  domes.  Where 
numerous  hot  springs  have  issued  along  a  slope,  a  succession  of  badm 
gives  a  curiously  picturesque  terraced  aspect  to  the  ground,  as  at  the 
Manunoth  Springs  of  the  Yellowstone  Park  and  at  the  now  destroyed 
terraces  of  Kotamahana  in  Ncav  Zealand. 

In  coiu*se  of  time,  the  network  of  imderground  passages  undergoei 
alteration.  Oiifices  that  were  once  active  cease  to  erupt,  and  even  the 
water  fails  to  overflow  them.  Sinter  is  no  longer  formed  round  them, 
and  their  siuiaces,  exposetl  to  the  weather,  crack  into  fine  shaly  rubbish 
like  commiiuited  oyster-shells.  Or  the  cylinder  of  sinter  grows  upward 
luitil,  by  the  continued  deposit  of  sinter  and  the  failing  force  of  the 
geyser,  the  tul>e  is  finally  filled  up,  and  then  a  dry  and  crumbling  white 
])illar  is  Ml  to  mark  the  site  of  the  extinct  geyser. 

Mud- Volcanoes. — ^These  are  of  two  kinds :  1st,  where  the  chief  source 
of  movement  is  the  escajH?  of  gsiseous  discharges ;  2nd,  where  the  actiye 
agent  is  steam. 

(1)  Although  not  volcanic  in  the  proper  sense  of  the  term,  certain 
remarkable  orifices  of  eruption  may  be  noticed  here,  to  which  the  names 
of  fnu(il-r*fitvnf'i'.<,  i^ilsifs,  air-ritlcano€.<y  and  maccalnbas  have  been  applied 
(Sicily,  the  Apennines,  Caucasus,  Kertch,  Taman).  These  are  corneal 
hills  formeil  by  the  accumidation  of  fine  and  usually  saline  mud,  which, 
\nth  various  gases,  is  contiiuiously  or  intermittently  given  out  from  the 
onfice  or  crater  in  the  centre.  They  occur  in  groui)s,  each  hillock  being 
sometimes  less  than  a  yaiil  in  height,  biu  ranging  up  to  elevations  of  100 
feet  or  moi-e.  Like  true  volcanoes,  they  have  their  periods  of  repose, 
when  either  no  dischai'ge  takes  phice  at  all,  or  mud  oozes  out  tranquilly 
from  the  crater,  and  their  eix>chs  of  activity,  Avhen  Islrge  volumes  of  gis, 
and  sometimes  columns  of  fiame,  rush  out  with  considerable  violence  and 
explosion,  and  throw  up  mud  and  stones  to  a  height  of  several  hundred 
feet.  The  gjises  play  much  the  s;ime  jiart,  therefore,  in  these  phenomena 
that  steam  di^s  in  those  of  true  volcanoes.  They  consist  of  marsh-gss 
and  other  hydriH*arl>ons,  carbon-dioxide,  sulphiu-etted  hydrogen,  and  nitro- 
gen, with  }x*ti*oleuni  A-ajxnu^  The  mud  is  usually  cold.  In  the  water 
occur  various  s;iliiie  ingredients,  among  which  common  salt  generally 
appears  *,  hence  the  name,  <S({/,'!^vs.  Naphtha  is  likewise  frequently 
present.  Large  piei-es  of  stone,  diifering  from  those  in  the  neighbonr- 
hood,  have  been  oliserveil  among  the  ejections,  indicative  doubtless  of  a 
somewhat  deeper  soiu*ce  than  in  ortlinary  nises.  Heav}*  rains  may  wash 
down  the  minor  mud-cones  and  spread  out  the  material  over  the  ground ; 
but  gas-bubbles  again  appear  through  the  sheet  of  mud.  and  by  degrees 
a  new  series  of  moimds  is  once  more  thrown  up. 

^  For  an  accoant  of  Xht  sreyserite  of  the  Tellow>to?itf  district,  see  I^peis  by  W.    tL 
Weed,  Am^r.  Jo»rn.  Sci.  xxxrii.  (1SS9\  and  9fA  .Un.  /?./>.  U.S.  Gf>^.  Surr,  1890. 


8BCT.  i  §  3  STRUCTURE  OF  VOLCANOES  239 

There  can  be  little  doubt  that  this  type  of  mud-volcano  is  to  be  traced 
to  chemical  changes  in  progress  underneath.  Dr.  Daubeny  explained 
them  in  Sicily  by  the  slow  combustion  of  beds  of  sulphur.  The 
frequent  occurrence  of  naphtha  and  of  inflammable  gas  points,  in  other 
cases,  to  the  disengagement  of  hydrocarbons  from  subterranean  strata.^ 

(2)  The  second  class  of  mud-volcano  presents  itself  in  true  volcanic 
regions,  and  is  due  to  the  escape  of  hot  water  and  steam  through  beds  of 
tuff  or  some  other  friable  kind  of  rock.  The  mud  is  kept  in  ebullition 
by  the  rise  of  steam  through  it.  As  it  become  more  pasty  and  the  steam 
meets  with  greater  resistance,  large  bubbles  are  formed  which  burst,  and 
the  more  liquid  mud  from  beloAv  oozes  out  from  the  vent.  In  this  way, 
small  cones  are  built  up,  many  of  which  have  perfect  craters  atop.  In 
the  Geyser  tracts  of  the  YelloAvstone  region,  there  are  instructive  examples 
of  such  active  and  extinct  mud-vents.  Some  of  the  extinct  cones  there 
are  not  more  than  a  foot  high,  and  might  be  carefully  remoA^ed  as  museum 
specimens. 

Mud-volcanoes  occur  in  Iceland,  Sicily  (Maccaluba),  in  many  districts 
of  northern  Italy,  at  Tamar  and  Kertch,  at  Baku  on  the  Caspian,  near 
the  mouth  of  the  Indus,  and  in  other  parts  of  the  globe.^ 

§  3.  Structure  of  Volcanoes. 

We  have  now  to  consider  the  manner  in  which  the  various  solid 
materials  ejected  by  volcanic  action  are  built  up  at  the  surface.  This 
inquiry  ivill  be  restricted  here  to  the  phenomena  of  modern  Aolcanoes, 
including  the  active  and  dormant,  or  recently  extinct,  phases.  Obviously, 
however,  in  a  modern  volciino  we  can  study  only  the  upper  and  external 
portions,  the  deeper  and  fundamental  parts  l>cing  still  concealed  from 
view.  But  the  interior  structure  has  been,  in  many  cases,  laid  open 
among  the  volcanic  products  of  ancient  vents.  As  these  belong  to  the 
architecture  of  the  terrestnal  crust,  they  are  described  in  Book  IV.  The 
student  is  therefore  requested  to  take  the  descriptions  there  given,  in 
connection  with  the  foregoing  and  present  sections,  as  related  chaptei*s  of 
the  study  of  volcanism. 

Confining  attention  at  present  to  modern  volcanic  action,  we  find  that 
the  solid  materials  emitted  from  the  earth's  interior  are  arranged  in  two 
distinct  types  of  structure,  according  as  the  eruptions  proceed  from  large 
central  cones  or  from  less  prominent  vents  connected  with  fissiu'es.  In 
the  former  case,  volcanic   cones  are  produced ;    in  the   latter,  volcanic 

*  The  "burning  hills  "  of  Turkestan  are  referred  to  the  subterranean  combustion  of  beds 
of  Juraasic  Coal.     J.  Muschketoff,  Neues  Jahrb.  1876,  p.  516. 

'  On  mud  -  volcanoes,  see  Bunsen,  Liebujs  Annuti/^  Ixiii.  (1847),  p.  1  ;  Abich,  Mew. 
Acad.  St,  Petertburgy  7®  ser.  t.  vi.  No.  r»,  ix.  No.  4  ;  Daubeny's  V(tfca.m)c,Sj  pp.  264,  539  ; 
Bnist,  TV-ans.  Bombay  Geograph.  Sftc.  x.  p.  154  ;  Roberts,  Joum.  Rot/.  Asiatic  Snc.  1850  ; 
De  Vemeuil,  Mem,  Soc.  Qeol.  France,  iii.  (1838),  p.  4  ;  Stiflfe,  Q.  J,  Oeol.  Sue.  xxx.  p.  60  ; 
Von  Lasaulx,  Z.  Deutsch.  Oeol.  Ges.  xxx\.  p.  457  ;  Giimbel,  Sitzb.  Akad.  Miinclt.  1879  ; 
F.  R.  Mallet,  Rec.  Oeoi.  Surr.  Indi<i,  xi.  ji.  188.  H.  Sjogren,  JaJirb.  Geo/.  Reichaanst. 
xxxviL  (1887),  p.  233. 


240  nVXAMICAL  GEOLOGY  book  lii  paw i 


plateaux  or  plains.     The  tyjje  of  the  volcanic  cone,  or  ordinary  volcano, 
is  noAv  the  most  abundant  and  Inist  known. 

i.    Vcicanic  Cones. 

From  some  weaker  jwint  of  a  fissure,  or  from  a  vent  opened  directly 
by  explosion,  A'olcanic  discharges  of  gas  and  vapours  with  their  liquid 
and  solid  accom^xiniments  make  their  wkj  to  the  siu^ace  and  gradually 
build  up  a  volcanic  hill  or  mountain.     Occasionally,  eruptions  have  pro- 
ceeded no  further  than  the  first  stage  of  gaseous  explosion.     A  cauldron- 
like cavity  has  l>een  torn  open  in  the  ground,  and  ejected  fragments  of 
the  solid  rocks,  through  which  the  explosion  has  emerged,  have  fall«i 
Ivick  into  and  ix»und  the  vent     Subsequently,  after  possible  subsidence 
of  the  fragmentary  materials  in  the  vent,  and  even  of  the  sides  of  the 
orifice,  Avater  supplieil  by  rain  and  filtering  from  the  neighbouring  ground 
may  pju-tially,  or  wholly,  fill  up  the  cavity,  so  as  to  produce  a  lake  either 
with  or  without  a  superficial  outlet.     Under  favoiu^ble  circumstances, 
vegetation  creeping  over  bare  earth  and  stone  may  so  conceal  all  evidence 
i\i  the  origiiuil  volcanic  action  as  to  make  the  quiet  sheet  of  water  look 
as  if  it  had  always  1>een  an  essential  part  of  the  landscape.     £3q)lo6ion- 
lakes  (Crater-lakes)  of  this  kind  occur  in  districts  of  extinct  volcanoes,  as 
in  the  Eifel  (maare\  central  Italy,  and  Auvergne.    The  craterifonn  hollow 
called  the  Gour  de  Tazenat,  in  Vebiv,  has  a  diameter  of  half  a  mile  and 
lies  in  the  giimite,  while  another  cavity  near  Confolens,  on  the  left  bank 
of  the  Loire,  has  also  been  blown  out  of  the  granite  and  has  given  passag? 
to  no  volcanic  materials,  but  only  to  broken-up  granite.^     Other  illustra- 
tions in  central  France  are  to  be  found  in  the  I^kes  of  Pa^in,  Mont 
Sinoire,  Chauvet,  Beurilouse,  ChamjHHlaze  and  Lji  Ciodival.-    A  remarkable 
example  is  supplioii  by  the  Lonar  l^ike  in  the  Indian  peninsula,  half-way 
between  Rnnlxiy  and  Nag]nir.     It  lies  in  the  midst  of  the  volcanic  plateau 
of  the  IVccan  trai>s,  which  extend  around  it   for  hundreds  of  miles  in 
nearly  flat  IhhIs  that  slightly  dip  aA^iiy  from  the  lake.     An  almost  circular 
dqnvssion,  rather  more  than  a  mile  in  diameter,  and  from  300  to  400 
tVet  divp,  contains  at  the  Ixittom  a  shallow  lake  of  Intter  saline  water, 
de|Kv<iting  crystals  of  tixma  (native  carlxMiate  of  soila,  the  nitnun  of  the 
ancients).      Except    to  the  north  and   north-east,  it   is  encircled  with  a 
raisi^l  rim  of  irregularly  pileil  bUx'ks  of  liasiilt,  identical  with  that  of  the 
IkiIs  thnnigh  which  the  cavity  has  Kvn  opened.     The  rim  never  exceeds 
100  foot,  and  is  often  not  more  th.in  40  or  oO  feet  in  height,  and  cannot 
ixMitain  a  thousandth  (Xirt  of  the  material  which  once  tilled  the  crater. 
No  other  evidence  of  volcanic  discharge  from  this  vent  is  to  be  seen. 
Some  of    the  contents  of    the  cavity   may  have  been  ejected    in   fine 
p;4rtioles,  which  have  sulisetpiently  been  removed  by  denudation ;   bat 
it  seems  more  pn.^bable  that  the  existence  of  the  cavity  is  mainly  dae  to 
subsidence  after  the  original  explosion.' 

-  To*.iTT.»iiv,  DW  Si<.  twV'.t*.   Fr^.xncf,  xivi.     1S^9  .  p.   1166;  Dunbrvc,  Compiet  rend, 

* 

-  Son>:»e.  'Voloanoe*  of  iVntral  Frano*/  pp.  SI.  14:>.  144. 
i^Thu  cavi:T  miT  posdablv  mark  oue  of  the  v^nts  fn>m  whicli  the  htaalt  Hoods  u 


BsctigS  ELEVATION-CRATER  THEOBY  241 

In  moflt  cases,  explosions  are  accompanied  by  the  expulsion  of  so 
much  solid  material  that  a  cone  gathers  round  the  point  of  emission.  As 
the  cone  increases  in  height,  by  successive  additions  of  ashes  or  lava  to 
its  surface,  these  volcanic  sheets  are  laid  down  upon  progressively  steeper 
slopes.  The  inclination  of  beds  of  lava,  which  must  have  originally 
issued  in  a  more  or  less  liquid  condition,  offered  formerly  a  difficulty 
to  observers,  and  suggested  the  famous  theory  of  Elevation-craters  {Er- 
helMnffskratere)  of  L.  von  Buch,'  £lie  de  Beaumont,^  and  other  geologists. 
According  to  this  theory,  the  conical  shape  of  a  volcanic  cone  arises 
mainly  from  an  upheaval  or  swelling  of  the  ground,  round  the  vent  from 
which  the  mat«rials  are  finally  expelled.  A  portion  of  the  earth's  crust 
(represented  in  Fig.  53  as  composed  of  stratified  deposits,  ab  g  h)  was 
believed  to  have  been  pushed  up  like  a  huge  blister,  by  forces  acting 
from  below  (at  c)  until  the  summit  of  the  dome  gave  way  and  volcanic 
materials  were  emitted.      At  first  these  might  only  partially  fill  the 


Fig.  SS.— SecOon  illurtniUve  of  Uke  Eleralion-crater  Theory. 

.  cavity  (as  at  f),  but  subsequent  eruptions,  if  sufficiently  copious,  would 
cover  over  the  truncated  edges  of  the  pre-volcanic  rocks  (as  at  g  ft),  and 
would  be  liable  to  further  upheaval  by  a  renewal  of  the  original  upward 
swelling  of  the  site. 

It  was  a  matter  of  prime  importance  in  the  interpretation  of  volcanic 
action  to  have  this  question  settled.  To  Poulett  Scrope,  Constant 
Prevost,  and  Lyell,  belongs  the  merit  of  disproving  the  Elevation-crater 
theory,  Scrope  showed  conclusively  that  the  steep  slope  of  the  lava-beds 
of  a  volcanic  cone  was  original.^  Constant  Provost  pointed  out  that 
there  was  no  more  reason  why  lava  should  not  consolidate  on  steep  slopes 
than  that  tears  or  drops  of  wax  should  not  do  so,'  Lyell,  in  successive 
Oq  eipltMion ' enters  ui<t  lakes,  9»s  Scrope'a  'Volcaaoes.'  Lecoq,  'Epoqnea  geologiques 
d«  TAnvtrgne,' tome  it.  ;  eompara  also  VogeUsng,  'Vnleane  dec  Eifel, '  Bod  in  Smt»  Jahrb, 
1870,  pp.  199,  32S,  460.  Od  Lanar  Lake,  see  Malcolmaon,  Tram.  Oeol.  Soc.  2nd  aet.  t. 
p.  S82.      Hedlicott  and  Blanford'a  '  Geo]ogj  of  India,'  p.  379. 

'  Pops-  '*«•>■  !«,  I.  "vii,  p.  168, 

'  Jiifl.  Sac  atot.  FrajKi,  iv.  p.  357,      Ann.  da  Ml«a,  ii.  and  i. 

'  '  Considerationa  on  Volcanoea,'  1825.      Quart.  Jonm.  Geai.  Soc.  lil.  p.  328. 

*  CompUt  Rendas,  i.  (1835),  480;  ill.  (1855),  p.  919.  GM.  Soc  Franct :  Mtmiim,^. 
p.  105,  and  Bail.  liv.  217.      Social  PhiUm.  Paris,  PnK.   Verb.  1843,  p.  13. 


BYNAMICAL  GEOLOGY 


BOOK  III  PABT  I 


editionB  of  his  works,  and  subsequently  by  an  examination  of  the  Canarr 
Islands  with  Hartung,  brought  forward  cogent  ai^uments  against  tlie 
Elevation-crater  theory.'     A  comparison  of  Fig.  53  with  Fig,  54  will  show 


iS^-Q.'- 


u  bj  tlie  <lotU<l  llu> 


at  a  glance  the  difference  Ivetwecn  thi^  theorj-  and  the  views  of  volcanic 
atnicture  now  universally  accej)ted.  The  steep  decli^-ities  on  which  lara 
can  actually  consolidate  have  licoi)  referred  to  on  p.  226. 

The  conical  form  of  a  volcano  in  that  naturally  assumed  by  a  self- 
supporting  Tnas^4  of  coherent  material.  It  varies  slightly  accot^ng  to 
the  nature  of  the  matenals  of  the  cune,  the  progress  of  atmoapheric 
denudation,  the  gmsitioii  of  the  crater,  the  direction  in  which  materials 
are  ejected,  the  foii;e  and  dii-ection  of  the  wind  diuing  an  eruption, 
the  growth  of  |Nira»itic  cones,  and  the  collaiise  due  to  the  dying  out  of 
volcanic  cnerg_v.- 

The  cone  gi-ows  by  additions  made  to  its  surface  during  successive 
eruptions,  and  though  liable  to  great  local  variation  of  contour  and  topo- 
graphy, preserves  its  general  form  with  singular  i>ersistence.  Many 
exaggerated  pictui-es  have  l>een  drawn  of  the  stccimess  of  slope  in  volcanic 
cones,  but  it  is  obvious  that  the  angle  cannot  as  a  whole  exceed  the 
maximum  inclination  of  repose  of  the  detrital  niatt«r  ejected  from  the 
central  chimney.^  A  scries  of  profiles  of  volcanic  cones  taken  from 
photographs  shows  how  nearly  they  approach  to  a  common  average  type.* 
One  of  the  most  potent  and  constant  agencies  in  modifying  the  out«r 
forms  of  these  eoues  is  undoubtedly  to  Ije  found  in  rain  and  torrents, 
which   sweep   down   the   loose  detiitus   and  excavate   ravines   on   the 

'  Pkil.  Tram.  18.'>S,  |i.  703.     See  the  ceraarks  of  Fouqu.',  '  Santorin, '  pp.  400-422. 

=  J.  Milne,  Oe-U.  ilaj.  1878.  p.  336  ;  1879,  p.  BOB.  Aamnliy.  Sue  Japan,  fr.  p.  178. 
G,  F.  Becker,  Amfr.  Joiim.  S^i.  xxx.  1885,  p.  283. 

»  Cotopaxi  a  a  uotablc  example  of  such  <r™)at«rateii  represenUtion.  Mr.  Wbfniper 
totuid  that  the  general  anglis  o(  the  nortbeni  and  souCheni  nlope*  at  ths  nma  mra  ntber 
lewi  than  30°  ('Travels  Auiongat  the  Great  Andei,'  |j.  133].  Uuniboldt  depicted  tht  an^e  u 
ona  of  SO*  ! 

*  See  MUne,  Sehm.  Soc.  JapaK,  ii.,  and  Otol.  Mag.  1878,  plale  ii. 


SECT,  i  §  3  ELEVATIOK-CSATER  THEORY  243 

declivities  till  a  cone  may  be  so  deeply  trenched  as  to  resemble  a  half- 
opened  umbrella.^ 

The  crater  doubtless  owes  its  generally  circular  form  to  the  equal 
expansion  in  all  directions  of  the  explosive  vapours  from  below.  In 
some  of  the  mud-cones  already  noticed,  the  crater  is  not  more  than  a  few 
inches  in  diameter  and  depth.  From  this  minimum,  every  gradation  of 
size  may  be  met  with,  up  to  huge  precipitous  depressions,  a  mile  or  more 
in  diameter,  and  several  thousand  feet  in  depth.  In  the  crater  of  an 
active  volcano,  emitting  lava  and  scorite,  like  Vesuvius,  the  walls  are 
steep,  rugged  cliffs  of  scorched  and  blasted  rock — red,  yellow,  and  black. 
Where  the  material  erupted  is  only  loose  dust  and  lapilli,  the  sides  of  the 
crater  are  slopes,  somewhat  steeper  than  those  of  the  outside  of  the  cone. 

The  crater-bottom  of  an  active  volcano  of  the  first  class,  when 
quiescent,  forms  a  rough  plain  dotted  over  with  hillocks  or  cones,  from 
many  of  which  steam  and  hot  vapours  are  ever  rising.  At  night,  the 
glowing  lava  may  be  seen  lying  in  these  vents,  or  in  fissures,  at  a  depth 
of  only  a  few  feet  from  the  surface.  Occasional  intermittent  eruptions 
take  place  and  miniature  cones  of  slag  and  scoriie  are  thrown  up.  In 
some  instances,  as  in  the  vast  crater  of  Gurung  Tengger,  in  Java,  the 
crater'bottom  stretches  out  into  a  wide  level  waste  of  volcanic  sand, 
driven  by  the  wind  into  dunes  like  those  of  the  African  deserts. 

A  volcano  commonly  possesses  one  chief  crater,  often  also  many  minor 
ones,  of  varying  or  of  nearly  equal  size.  The  volcano  of  the  Isle  of 
Bourbon  (or  Reunion)  has  three  craters.^  Kot  infrequently  craters  api>ear 
successively,  owing  to  the  blocking  up  of  the 
pipe  below.  Thus  in  the  accompanying  plan  of 
the  volcanic  cone  of  the  island  of  Volcanello 
(Fig.  55),  one  of  the  Lipari  group,  the  volcanic 
funnel  has  shifted  its  position  twice,  ho  that 
three  craters  have  successively  appeared  ii|>om 
the  cone,  and  partially  overlap  each  other.  It 
may  be  from  this  cause  that  some  volcanic 
mountains  are  now  destitute  of  craters,  or  in 
other  cases,  because  the  lava  has  welled  up  in 

dome  form  covered  perhaps  with  masses  of  ,how1pH"three~iuc™i»7cnii»r'- 
acorite,  but  without  the  production  of  a  definite 

crater.  Mount  Ararat,  for  example,  is  said  to  have  no  crater ;  but  so 
late  as  the  year  1840  a  fissure  opened  on  its  side  whence  a  considerable 
eruption  took  place.  The  trachytic  pnys  of  Auvergne  are  dome-shaped 
hilb  without  craters. 

Though  the  interior  of  modem  volcanic  cones  can  be  at  the  best  but 

•  On  the  deandation  of  volcanic  i-oue»,  «e  H.  J.  Jolinaton-Lavi*,  Q.  J.  Oeol.  Sue.  il. 

p.  loa 

'  For  recent  informalion  regarding  this  volcanic  island,  tm  R.  vou  Drasche,  In  Verhaadl. 
Oeol.  JUichtantL  1875,  ]>.  266,  and  in  TBChemak'a  Min.  Mittheil.  1376  (3),  p.  217  (4). 
p.  39,  and  hii  work  'Die  lusel  Wunion  (Bourbon),'  4to,  Vienna,  1878.  C.  Velain. 
*  Description  giologique  de  Is  Preaqn'ile  d'Aden,  de  I'lla  de  U  ftinnion,  &c.,'  Paris,  4to. 
1878  ;  and  his  work,  '  Le~  Volcmis,'  ISS4. 


244  DYNAMICAL  GEOLOGY  book  m  pabii 

very  partially  examined,  the  study  of  the  sites  of  long-extinct  cones,  laid 
bare  after  denudation,  shows  that  subsidence  of  the  ground  has  commoDly 
taken  place  at  and  round  a  vent.     Evidence  of  subsidence  has  also  been 
observed  at  some  modern  volcanoes  (ante,  p.  231).     Theoretically  two 
causes  may  be  assigned  for  this  structure.     In  the  first  place,  the  mere 
piling  up  of  a  huge  mass  of  material  round  a  given  centre  tends  to  prm 
down   the   rock   underneath,    as   some   railway  embankments  may  be 
observed  to  have  done.     This  pressiu*e  must  often  amount  to  eeTflnl  .^ 
hundred  tons  on  the  square  foot.     In  the  second  place,  the  expolsioQ 
volcanic  material  to  the  surface  may  leave  cavities  underneath,  into 
the  overlying  crust  will  naturally  gravitate.     These  two  causes 
as  suggested  by  Mr.  Mallet,  afford  a  probable  explanation  of  the 
sha])ed  depressions  in  which  many  ancient  and  some  modem  venta 
to  lie.i  •"'•'^^ 

The  following  are  the  more  important  types  of  volcanic  cones :  ^ 


1.  Cones  of  Non-Yolcanic  Materials. — These  are  due  to  the  discharge  of 
other  aeriform  product  through  the  solid  crust  without  the  emission  of  any  troe 
lava.  The  materials  ejected  from  the  cavity  are  wholly,  or  almost  wholly,  partB  of  tke 
Kurrouuding  rocks  through  which  the  volcanic  pipe  has  heen  drilled.  Some  of  the  eoifes 
surrounding  the  crater  lakes  {niaarc)  of  the  Eifel  consist  chiefly  of  fragmentB  of  the 
underlying  Devonian  slates  (pp.  200,  213). 

2.  Toff-Cones,  Cinder-Cones. — Successive  eruptions  of  fine  dust  and  stones^  often 
rendered  i)a8ty  by  mixture  vdth  the  water  so  copiously  condensed  during  an  eruption, 
form  a  cone  in  which  the  materials  are  solidified  by  pressure  into  tuff.  Cones  made  np 
only  of  loose  cinders,  like  Monte  Nuovo  in  the  Bay  of  Baiae,  often  arise  on  the  flanks  or 
round  the  roots  of  a  great  volcano,  as  happens  to  a  small  extent  on  Vesuvine,  and  on  a 
larger  scale  ujwn  Etna.  They  likewise  occur  by  themselves  apart  from  any  lava-prodnoing 
volcano,  though  usually  they  atford  indications  that  columns  of  lava  have  risen  in  their 
funnels,  and  even  now  and  then  that  this  lava  has  reached  the  surface. 

The  cones  of  the  Eifel  district  have  long  been  celebrated  for  their  wonderjful  perfec- 
tion. Though  small  in  size,  they  exhibit  A^ith  singular  clearness  many  of  the  leading 
features  of  volcanic  structure.  Those  of  Auvergne  are  likewise  exceedingly  instmctive.' 
The  high  plateaux  of  Utah  are  dotted  with  hundreds  of  small  volcanic  cinder-cones,  the 
singular  [>o8itions  of  which,  close  to  the  edge  of  i)rofound  river-gorges  and  on  the  upthrow 
side  of  faults,  have  already  (p.  204)  been  noticed.     Among  the  Carboniferous  volcanic 


*  Mallet,  Q.  J.  Oed,  Soc.  xxxiii.  p.  740.  See  also  the  account  of  *  *  Volcanic  Necks,'*  in 
Book  IV.  Part  VII. 

2  Von  Seebach  {Z.  JJeuisch.  Geol.  Ges.  xviil  644)  distinguished  two  volcanic  types,  let, 
Fkddfd  Vofcam>€s  (Strato-Vulkane),  comix>sed  of  successive  sheets  of  lava  and  tnfis,  and 
embracing  the  great  mjyority  of  volcanoes.  2nd,  Dome  Volcanoes^  forming  hills  com- 
posed of  homogeneous  protrusions  of  lava,  with  little  or  no  accompanying  fragmentary 
discharges,  without  craters  or  chimneys,  or  at  least  with  only  minor  examples  of  these 
volcanic  features.  He  believed  that  the  same  volcano  might  at  different  periods  in  its  history 
belong  to  one  or  other  of  these  types — the  determining  cause  being  the  nature  of  the  erupted 
lava,  which,  in  the  case  of  the  dome  volcanoes,  is  less  fusible  and  more  viscid  than  in  that  of 
the  bedded  volcanoes.     (See  below,  under  "  Lava-cones.") 

^  For  Auvergne,  see  works  cited  on  p.  219.  For  the  Eifel,  consult  Hibbert,  'History 
of  the  Extinct  Volcanoes  of  the  Basin  of  Neuwied  on  the  Lower  Rhine,'  Edin.  1832. 
Von  Dechen,  ' Geoguostischer  Fiihrer  zu  dem  Laacher  See,*  Bonn,  1864.  '  Geognoatitelier 
Flihrer  in  das  Siebengebirge  am  Rhein,'  Bonn,  1861. 


VOLCANIC  CONES 


roclcB  of  oentral  Scotland  tlie  atiimpa  of  ancient  tiiff-coneB,  frequenti;  with  a  central  core 
of  liuatt,  or  with  dykes  sod  veine  of  that  rock,  are  of  common  occurrence.' 

The  naterials  of  a  toff-cone  are  arranged  in  more  or  leu  regularly  stratified  lieds. 


-^:5SI**t 


nfrSrt.— ViewoftheTt 


On  the  out«r  side,  tliey  dip  down  tlie  slopes  of  the  cone  at  the  average  angle  of  repose, 
which  may  range  between  30°  and  40°.  From  the  summit  of  the  crater-lip  they  likewise 
dip  inward  toward  the  erater-bottom  at  similar  angles  of  inclination  (Fig.  57). 

3.  Hitd-Conu  resemble  tulT-cones  in  form,  but  are  usually  smaller  iu  size  and  less 
steep.  They  are  produced  hy  the  hardening  of  auccessire  outpourings  of  mud  from  tlic 
oriGcee  already  described  (p.  238).  In  the  region  of  the  Lower  Indus,  where  they  are 
abundantly  distributed  over  an  area  of  1000  square  miies,  some  of  them  attain  a  height 
of  100  feet,  with  cratera  30  yards  across.' 


4.  LftTA-^MinM. — Volcanic  cones  composed  entirely  of  lava  are  comparatively  rare, 
bnt  occur  in  aome  younger  Tertiary  and  modem  voloanoea.  Fouijue  descrilws  tlie  lava 
of  1806  at  Santorin  as  having  formed  a  dome-shaped  elevation,  Hewing  out  quietly  and 

I   Tram.  Roy.  Sk.  Hdin.  iiii.  p.  455.      Ste posUa.  Book  IV.  Part  VIl. 
'  Lyell,  ■  Principles,'  ii.  p.  77. 


246  DYNAMICAL  aEOLOGY  book  hi  pami 


ragiidty  without  fxpltHiaiiM.     Af(«r  iieveral  days,  hawever,  its  etiiistaon  vai  Mcompaokd 

with  copious  discharges  of  fraijmeutary  malerialn  and  the  formation  of  several  uratOTidaa 

,  tiioutha  on  the  toji  of  the  dome.     Wliere  lava  ]>08aefiaea  extreme  liquidi^, 

I  and  given  rine  to  little  or  no  fragmciitarj'  matter,  it  may  build  up  a  flat 

I'Oiie  as  ill  the  remarkable  examples  described  by  Dana  from  the  Hamli 

iHlaiidnJ    On  the  mmmlt  of  llauua  Loa  (Fig.  68),  a  flat  Un-cone  13,7N 

feet  above  the  aea,  lies  a  crater,  which  in  its  deejiest  part  is  about  8000 

feet  broad,  nith  vertical  walls  of  strati&ed  lava  riding  on  one  nde  to  a 

height  of  7S4  feet  above  the  black  lava-plain  of  the  rrater-bottom.     From 

the  edges  of  this  elevated  cauldron,  the  mountain  slopes  outward  at  u 

angle  of  not  more  than  fl',  until,  at  a  level  of  about  10,000  feet  lower,  it« 

auifscc  is  indented  hj  tlie  vast  pit-crater,  Kilauea,  abont  two  tnilea  long^ 


^'S^> 


(Dana,  1S11.>) 


Biiil  nearly  a  mile  broad.  So  Ion-  are  the  siiriiinudiiig  slo]ies  that  thesr 
vast  eiaters  have  been  compared  lo  o]ien  iiuarricii  on  a  hill  or  moor.  The 
biittom  of  Kiianea  is  a  Iava-]>lain,  clntted  with  lake^  of  extremely  Huii) 
lava  in  constant  eliuUition.  The  level  of  the  lava  box  varied,  for  the  walli 
surrounding  the  tier)'  Hood  consist  of  l)nls  of  similar  lava,  and  are  marked 
by  leilKcs  or  platforms  (Fig.  5B)  indicative  of  former  sucooasive  heighla  of 
lava,  as  lake  terraces  show  former  levels  of  water.  In  the  accompanying 
wrtion  {Via.  80)  the  walls  rising  almve  the  Inivcr  jut  ipp')  were  found  to 


Kiii.  ciO,-Swti(inDfLav«-l«™cc«in  Kil.u,^  (D«n»). 

be  'ii-i  feet  hiRh,  those  bounding  the  higher  terrace  (o  n  «  o')  were  960 
feet  high,  all  being  composed  of  innamcrable  beds  of  lava,  as  in  cliffs  of 
stratified  rocks.  Much  of  the  l«ttom  of  the  lower  lara-plain  has  been 
i:ni»teii  over  liy  the  solidification  of  the  molten  rock.  But  large  areas, 
which  shift  their  {losition  froni  time  to  time,  remain  in  perpetual  rapid 
ebullition.  The  glowing  flood,  as  it  boils  up  with  a  fluidity  more  like  that 
of  water  than  what  is  commonly  shown  by  molten  rock,  surges  against 


{■■pod  of  i:  -^  Exploring  Kritr-lilioa, 

See  the  works  cited  on  p.  205. 
it  niajis  showing  the  variations  of  this  a 


8-42,  t 


id  Dana's  '  Character- 


i  'Choractattitic*.' 


VOLCANIC  CONES 


247 


the  suiToanding  terrace  walls.  Large  segments  of  the  cliSa  undermined  by  the  fUaion  of 
their  b«se,  fall  at  intervals  into  the  fiery  wavea  and  are  soon  inelled.  Recent  obeervatioDS 
by  Captain  DattoD  point  to  a  diminution  of  the  actiritjof  tliia  lava-ctater.  In  Iceland, 
and  in  the  Western  Territories  of  North  America,  low  domcB  of  laTa  appear  to  mark  the 
T«nta  from  which  exteunve  basalt-floods  have  issued. 

Where  the  lava  assumes  a  more  viscid  character,  as  in  trachyte  and  liparite,  dome- 
shaped  eminences  may  be  protruded.  Ah  the  moHs  increases  in  size  by  the  advent  of 
Ireah  material  injected  from  below,  the  outer  layer  will  be  pushed  outward,  and  sQcceuJve 
shells  will  in  like  manner  be  enlarged  as  tlie  eruption  advances.  On  the  cessation  of 
discharges,  we  may  conceive  that  a  volcuiic  hill  formed' in  this  way  will  present  an 
onion-like  arrangement  of  its  component  sheets  of  rock.  Mors  or  less  perfect  examples 
of  this  structure  have  been  observed  In  Bohemia,  Auvergne,  and  the  Eifel.'  The 
trachytic  domee  of  Aavergne  form  a  conspicuous  feature  among  the  cinder  cones  of  that 
region.  Hnge  conical  protuberances  of  granophyre  occur  among  the  Tertiary  volcanic 
rocks  of  the  Inner  Hebrides,  and  similar  hills  of  liparite  rise  through  the  basalts  of  Iceland. 

6.  CanM  of  Tuff  and  Lmva.— This  is  by  far  the  most  abundant  type  of  volcanic 


Fig  0 


rian  or  til 


and  includes  the  great  volcanoes  a!  the  globe  Beginning  perhaps  as  mere 
luff-cones,  these  eiTiinenoes  have  grailually  been  built  np  by  successive  outpourings  of 
lava  from  dtfTerent  side.'t,  anil  by  showers  of  dust  and  scoriae.  At  first,  the  lava,  if  the 
sides  of  the  cone  are  strong  enough  to  resist  its  pressure,  may  rise  until  it  overflows 
from  the  crater.  Subsequently,  as  the  funnel  becomes  choked  up,  and  the  cone  is  shat- 
tered by  repeated  explosions,  the  lava  finds  egress  from  different  fissures  and  openings 
an  the  cone.  As  the  mountain  increases  in  height,  the  number  of  lava-currents  from 
its  summit  will  usually  decivase.  Indeed,  the  taller  a  volcanic  cone  grows,  the  less 
frequently  as  a  rule  does  it  enipt.     The  lofty  volcanoes  of  the  Andes  have  each  seldom 

'  E.  Reyer  {Jahiii.  Oeol,  Reichx.  1879.  p.  ^63)  has  experimentally  imitated  the  process 
of  extrusion  by  forcing  np  plaster  of  Paris  through  a  hole  in  a  board.  For  drawingH  of  the 
Pay  de  Sarcouy  and  other  ilome-shajwd  hills  which  presumably  have  had  this  mode  of 
origin,  see  Scrope's  '  Geology  nuU  Extinct  Volcanoes  of  Central  France.'  Refer  al»o  to  the 
lemarki  already  made  nn  the  liqnidity  of  lava  (nn(f,  pp.  322-S),  aud  the  accoont  of 
"  Valkanische  Kuppen."  poslta,  p.  25S. 


S48 


DTKAMICAL  GEOLOGY 


BOOK  in  PABTI 


1«eu  more  thau  once  in  eruption  duriug  >  century.  The  pe&k  of  Tenerifle  (Fig.  61)  m 
three  times  active  daring  S70  jesrs  prior  to  178S.'  The  earlier  efforts  of  a  Tolcano  tnd 
to  increase  its  height,  ae  well  as  its  breadth  ;  the  later  eruptiona  chieflj  angmgnt  ttt 
breadth,  and  are  often  apt  to  diminish  the  height  by  blowing  away  the  nppetpartoftlit 
cone.  The  formation  of  liasurca  and  tlie  consequent  intmaion  of  a  network  of  IsTk-dfkM, 
tend  to  bind  the  frameworlc  of  the  volcano  and  strengthen  it  against  subsequent  ezplOMU. 
In  thisway,  a  kind  of  oscillation  is  established  in  the  form  of  the  cone,  periodaofaattr 
eruptions  being  succeeded  by  others  when  the  emissions  take  place  only  latnsUy  (Mir, 
p.  210). 

One  consequence  or  lateral  eruption  is  the  formation  of  minor  parasitic  cones  on  the 


I.  Uvi 


flanks  of  the  parent  volcano  (p.  1S2).  Those  on  Etna,  more  than  200  in  number,  an 
really  miniature  volcanoes,  some  of  them  reaching  a  height  of  700  feet  (Fig.  62),  As  the 
lateral  vents  successively  become  extinct,  the  cones  are  buried  under  sheets  of  lava  and 
showers  of  debris  thronii  out  from  younger  oi>enings  or  from  the  parent  cone.  It  some- 
times happens  tliat  the  original  funnel  is  disused,  and  that  the  eruptions  of  the  volcaito 


'  For  a  recent  ai 


it  of  Teoeriffe,  lee  A.  Rothpleti,  I'ettrmanWi  iliUhtU.  x\ 


'■  (18W, 


S3 


SUBMARINE  VOLCANOES 


MM  fivm  a  newer  main  vent,  Veauriua,  for  exanipU  (48  bIiowii 
m  the  uto  of  a  portion  of  the  rim  of  the  more  ancient 
«h  Urgw  vent  of  Monte  Sommo.  The  present  crater 
I  liei  to  the  north-vest  of  the  former  vaster  crater. 
»t^  little  example  of  this  shifting  furnished  by  Vol- 
haa  been  tlready  noticed  (p.  243]. 
ile,  therefore,  a  volcano,  and  more  particularly  one  of 
le,  throwing  out  both  lava  and  fragmentary  materials, 
e  to  continual  modification  of  its  external  form,  as  the 
e  eruptJona,  its  contonr  is  likewise  usually 
I  alteration  by  the  effects  of  ordinary 
bene  erosion,  aa  well  aa  from  the  condensation  of  the 
:  vapours.  Heavy  and  sudden  floods,  produced  by  the 
aiufall  consequent  upon  a  copious  discharge  of  steam, 
iwn  the  slopes  with  such  volume  and  force  as  to  cut 
lilies  in  the  loose  or  only  partially  consolidated  tuffs 
irice.  Ordinary  rain  continues  the  erosion  until  the 
lopes,  unless  occasionally  renewed  by  fresh  showers  of 
1,  aasame  a  curiously  furrowed  aspect,  like  a  half-ojiencd 
A,  the  ridges  being  separated  by  furrows  that  narrow 
B  towards  the  summit  of  the  cone.  The  outer  declivi- 
Uonte  Somma  afford  an  eicellent  ilhiatrstion  of  this 
surface,  the  numerous  raviuea  on  that  side  of  the 
in  presenting  instructive  sections  of  the  pre-hiatoric 
id  tu^  of  the  earlier  and  more  important  iteriod  in  the 
of  this  volcano. '  Similar  trenches  have  been  eroded 
Muthem  or  Vesuvian  side  of  the  original  cone,  bat 
ave  in  great  measure  been  filled  up  by  the  lavas  of  the 
:  mountain.  The  ravines,  in  fact,  form  natural  chan- 
■  the  lava,  as  may  unfortunately  be  seen  round  the 
0)  observatory.  This  building  is  ]>laced  on  one  of  the 
between  two  deep  ravines  ;  but  the  lava-streams  of 
'ewa  have  poured  into  these  ravines  on  either  side,  and  \ 

dly  filling  them  up. 

bmarltie  Volcanoes. — It  is  not  only  on  the 
:  of  the  land  that  volcanic  action  ehows  itself, 
es  place  likewise  under  the  sea,  and  as  the 
ical  recprds  of  the  earth's  \>a»t    history  arc 

marine  fonnations,  the  characteristics  of  sub- 
)  volcanic  action  have  no  small  interest  for  the 
ist.  In  a  few  instances,  the  actual  outbreak 
ibmarine  erujition  has  been  witnessed.  Thus, 
i  early  summer  of  1783,  a  volcanic  eruption 
lace  about  thirty  miles  from  Cape  Keykjaiiaes 
!  west  coast  of  Iceland.  An  island  was  built 
im  which  fire  and  smoke  continued  to  issue, 

less  than  a  year  the  waves  had  washed  the 
Dumice  away,  leaving  a  submerged  reef  from 
)  thirty  fathoms  below  sea-level.     About  a 

'  See  H.  J,  Johnston- Lav i«,  Q.  J.  Gtol.  Sm.  il.  p.  103. 


y 


il 

■If 
hi 

jsl 

I  2-a 

an 


Hit 

s  £,^ 

E   I  ■<    . 

ill 
5|I 

s-sa 
I  If- 


280  DYNAMICAL  GEOLOGY  bookotpaoi 

month  after  this  eruption,  the  frightful  outbreak  of  Sk&ptar  JokaD, 
already  refened  to  (p.  233),  began,  the  diBtance  of  this  mountain  from 
the  submarine  vent  being  nearly  200  mileB.'  A  century  afterwards,  m 
in  July  11384,  another  volcanic  island  is  said  to  have  been  thrown  iq) 
near  the  same  spot,  having  at  first  the  form  of  a  flattened  cone,  but  soon 
yielding  to  the  power  of  the  bi'eakcrs.  Many  submarine  eruptions  have 
taken  place  within  historic  times  in  the  Mediterranean.  The  niost 
noted  of  these  occurred  in  the  year  1831,  when  a  new  volcanic  idand 
(Graham's  Island,  lie  Julia)  was  thrown  up,  with  abundant  dlschujB 
of  steam  and  showei-R  of  scoriee,  between  Sicily  and  the  coast  of  Afaio. 
It  reached  an  extreme  height  of  200  feet  or  more  above  the  sea-lerel 
(800  feet  above  sea-bottom)  with  a  circumference  of  3    miles,  but  on 


'Sktlcliof  submuiiie  volunlc  cruptLun(ttabriiu  l>lind)a)rst  Hichul'i,  Jnne  1S1L 


the  cessation  of  the  eruptions,  was  attacked  by  the  waves  and  soon 
demolished,  leaving  only  a  shoal  to  mark  its  site.-  In  the  year  1811, 
another  island  was  formed  by  submarine  eruption  of  the  coast  off 
St.  Michaela  in  the  Azores  (Fig.  04).  Consisting,  like  the  Mediterranean 
example,  of  loose  cinders,  it  rose  to  a  height  of  about  300  feet,  with 
a  circumference  of  about  a  mile,  biit  subsequently  disappeared.'  In 
the  year  1796  the  island  of  Johaima  Bogoslawa,  in  Alaska,  appeared 
above  the  water,  and  in  four  years  had  grown  into  a  large  volcanic 
cone,  the  summit  of  which  was  .^000  feet  above  sea-level.* 

'  Ljell,  '  Priiiciiiks,'  ii.  p.  49. 

'  Pkil.  Tram.  IS32.  CoDBtunt  Piijvojit.  Ann.  ilia  Set.  Xal.  ixir.  JTAn.  Sac  OM. 
France,  ii.  ji.  91.  JIiTcalli's  '  Vulcani,  fcc.,'  p.  117.  For  a  recent  labmirine  arnptkni  in 
Ihe  Mediterranean,  we  Kioco,  Coiiiiif.  rriiil.  Xov.  23ril,  1891, 

'  De  In  Beclie,  '  Geological  Oliserter,'  p,  "0,  *  D.  Forbes,  G*ot.  Mag,  vU,  p.  923. 


i«3 


SUBMARINE  VOLCANOES 


[JufortunAtely,  the  phenomena  of 
it  volcanic  eruptions  under  the 
ire  for  the  most  part  inacceBeibltj. 
1  and  there,  as  in  the  Bay  of 
les,  at  Etna,  among  the  islands  of 


ca,  cr  Buntorin ;  h,  Th^nulii ; 
toKmlmcni.  Tlie  llgiirHi  Ui 
>mi,thf  I 

xreek  Archipelago,  and  at  Tahiti, 
ktion   of  the  sea-Wl  has  taltcn 
;,  and  brought  Xo  the  siirface  beds 
iff  or  of  lava  which  have  consoli- 
1  under  water.      Both  Vesuvius 
Etna  began  their  career  as  sub- 
ne  volcanoes.'      It  will  be   seen 
the   accompanying  chart  (Fig. 
that  the  Islands  of  Santorin  and 
aaia  form  the  unsubmergcd  por- 
of  a  great  crater-rim  rising  round 
ater  which   descends    1278   feet 
w    sea- level.       The    matci'ials  of 
)  islands  consist  of  a  nucleus 
narhles    and    schists,   nearly 
>d  under  a  pile  of  tufTs  (trass),        a 
m,  and  sheets  of   lava,  the 
ed  character  of  which  is  well 
n  in  the  accompanying  sketch 
aee,  M  regan 
327. 


252  DYNAAflCAL  GEOLOGY  book  iif  paw  i 

by  Admiral  Spratt  (Fig.  66),  who,  with  the  late  Professor  Edward  Forbes, 
examined  the  geology  of  this  interesting  district  in  1841.     They  found 
some  of  the  tuffs  to  contain  marine  shells,  and  thus  to  bear  witness  to  an 
elevation  of  the  sea-floor  since  volcanic  action  began.     More  recently  the 
islands  have  been  carefully  studied  by  various  observers.     K.  von  Fritsch 
has  found  recent  marine  shells  in  many  places  up  to  heights  of  nearly 
600   feet   above   the   sax.      The    strata    containing   these   remains  he 
estimates   to   be  at  least   100  to   120  metres  thick,  and   he  remarks 
that  in  every  case  ho  found  them  to  consist  essentially  of  volcanic  debris 
and  to  rest  upon  volcanic  rocks.     It  is  eWdent,  therefore,  that  these  shell- 
Ijearing  tuff's  were  originally  deposited  on  the  sea-floor  after  volcanic 
action  had  begun  here,  and  that  during  later  times  they  were  upraised, 
together  with  the  submarine  lavas  associated  with  them.^     Fouqu^  con- 
cludes that  the  volcano  formed  at  one  time  a  large  island  with  wooded 
slopes   and   a    somewhat    civilised    human    population,    cultivating    a 
fertile  valley  in  the  south-western  district,  and  that  in  prehistoric  times 
the  tremendous  explosion  occurred  whereby  the  centre  of  the  island  was 
blown  out. 

The  similaritv  of  the  structure  of  Santorin  to  that  of  Somma  and 
Etna  is  obvious.  Volcanic  action  still  continues  there,  though  on  a 
diminished  scale.  In  1866-67  an  eruption  took  place  on  Neo  E^imeni, 
one  of  the  later-formed  islets  in  the  centre  of  the  old  crater,  and  greatly 
added  to  its  area  and  height.  The  recent  eniptions  of  Santorin,  which  have 
])een  studied  in  great  detail,  are  specially  interesting  from  the  additional 
infoimation  they  have  supplied  as  to  the  nature  of  volcanic  vapours  and 
gases.  Among  these,  as  already  stated  (p.  196),  free  hydrogen  plays  an 
important  part,  constituting,  at  the  focus  of  discharge,  thirty  per  cent  of 
the  whole.  By  their  ei-uption  under  water,  the  mingling  of  these  gases 
with  atmospheric  air  and  the  combustion  of  the  inflammable  compounds 
is  there  prevented,  so  that  the  gaseous  discharges  can  be  collected  and 
analysed.  Probably  were  operations  of  this  kind  more  practicable  at 
terrestrial  volcanoes,  free  hydrogen  and  its  compounds  would  be  more 
abundantly  detected  than  has  hitherto  been  possible. 

The  numerous  volcanoes  which  dot  the  Pacific  Ocean,  probably  in 
most  cases  began  their  career  as  submarine  vents,  their  eventual  appear- 
ance as  subaerial  cones  being  mainly  due  to  the  accumulation  of  erupted 
material,  but  also  jiartially,  as  in  the  case  of  Santorin,  to  actual  upheaval 
of  the  sea-bottom.  The  lonely  island  of  St.  Paul  (Figs.  67  and  69), 
lying  in  the  Indian  Ocean  more  than  2000  miles  from  the  nearest  land, 
is  a  notable  example  of  the  summit  of  a  volcanic  mountain  rising 
to  the  sea-level  in  mid -ocean.      Its   circular   crater,  broken  down  on 

1  See  Fritsch,  Z.  Deuisch.  Gfol.  Ges.  xxiii.  (1871),  pp.  125-213.  The  moat  complete 
and  elaborate  work  is  Fouque's  monograph  (already  cited),  '  Santorin  et  ses  Smptiona,' 
Paris.  4to,  1880,  where  copious  analyses  of  rocks,  minerals,  and  gaseous  emanatioiis. 
with  maps  and  numerous  admirable  views  and  sections,  are  given.  In  this  Tolame  a 
bibliography  of  the  locality  will  be  found.  Compare  C.  Doelter  on  the  Ponza  Islands. 
IknlcAch.  Ahul.  Wisgrnsdi.  Vienna,  xxxvi.  p.  HI.  Sitz.  Akad.  iriMfftMA.  Yienot. 
Ixxi.  (1875),  p.  49. 


tigs  SUBMARIXE   VOLCANOES  853 

Qortb-eaat    side,    is   filled   with   water,    having    a    depth   of    30 

Observations  by  R.  von  Drasche 
^e  shown  that  at  Bourbon  {Reunion), 
ring  the  early  submarine  eruptions  of 

t  volcano,   coarsely  crystalline    rocks     f:\''\''i^^t^^2k*  ")" 

bbro)  were  emitted,  that  these  were  '' 

«eeded  by  andoaitic  and  trachytic  lavas : 
*.  that  when  the  vent  rose  above  the 
,  basalts  were  poured  out.*  Fouque 
<erves  that  at  Santorin  some  of  the 
•\j  submarine  lavas  are  identical  with 
ne  of  later  subaerial  origin,  but  that 
I  greater  part  of  them  belong  to  an 
irely  different  series,  being  acid  rocks,  fi«.  bt-— VuiouiccimterofBi,  p»uii«i«nj, 
onging  to   the  group  of    hornblende-  indisn  ocan. 

lesites,  while  the  subaerial  rocks  are  angite-andesites.  The  acidity  of 
jse  lavaa  has  been  largely  increased  by  the  infusion  into  them  of  much 
ca,  chiefly  in  the  form  of  opal  They  differ  much  in  aspect,  being 
aetimes  compact,  scoriaceous,  hard,  like  millstone,  with  perlitic  and 
lerulitic  structures,  while  they  frequently  present  the  characters  of 
B8  impregnated  with  opal  and  zeolites.  Among  the  fragmental 
ctions  there  occur  blocks  of  schist  and  granitoid  rocks,  probably 
vesenting  the  materials  below  the  sea-floor  through  which  the  first 
iloeion  took  place  (pp.  200,  213,  244).  During  the  eruption  of  1866 
ne  islets  of  lava  rose  above  the  sea  in  the  middle  of  the  bay,  near  the 
ivo  vent.  The  rock  in  these  cases  was  compact,  vitreous,  and  much 
eked,* 

Among  submarine  volcanic  formations,  the  tulTs  differ  from  those  laid 
(TO  on  land  chiefly  in  their  organic  contents ;  but  partly  also  in  their 
re  distinct  and  originally  less  inclined  bedding,  and  in  their  tendency 
the  admixture  of  non-volcanic  or  ordinary  mechanical  sediment  with 
!  volcanic  dust  and  stones.  No  appreciable  difference  either  in 
:emal  aspect  or  in  internal  structure  seems  yet  to  have  been  established 
;ween  subaerial  and  submarine  lavas.  Some  undoubtedly  submarine 
-aa  are  highly  scoriaceous.  There  is  no  reason,  indeed,  why  slaggy 
•a.  and  loose,  non-buoyant  scoriie  should  not  accumulate  under  the 

'  For  a  g«nenl  accoiiut  of  the  volcanic  isUmls  of  the  ocean,  see  Darwin's  '  Vokanic 
iBdi,'  Snd  edit.  1S7S.  For  the  Philippine  volcanoea,  see  R.  von  Draiche,  TsrAemali'» 
utmlogiiiAe  MiWieii.  1 876  ;  Semper's  '  Die  Philippinen  und  ihre  Bevrohner, '  WUrz- 
g,  1869.  For  the  Kurile  Islands,  J.  Milne,  Geol.  Mag.  1879,  1860,  18S1  ;  Volcanoea 
J«y  of  BeDgal  (Barren  Islan.l.  !;c.),  V.  Ball,  Qtol.  Mag.  1879,  p.  18  ;  1888,  p.  ^04  ;  F. 
SI«llet,  Man.  Orel.  Sun:  India,  iii.  part  iv.  St.  Paul  (Indian  Ocean),  C.  va»ln,  Aaoc. 
in.  1875,  p.  581  ;  '  Mission  i.  Tile  St.  Paul,"  1879  ;  'Description  geologiqne  de  la 
■qa'ilfl  d'Aden,'  &c..  4to,  Paris,  1S7S  ;  and  '  Les  Volcans,'  1684.  For  Iile  of  Bourbon, 
■athorities  citeil  on  p.  243,  and  for  Hawaii,  the  references  on  p.  205. 

*  Tschermok's  MinenilogUche  MUtkeil.  1876,  pp.  42,  1S7.  A  similiir  stmetnre  occuis 
lUmA  (Coheo,  .Vewj  JiArb.  1879,  p.  482)  and  in  SL  Paul  (Velain  as  above  cited). 

*  Fonqne,  '  Sontorid. ' 


254  IiYKAMlCAL  GEOLOGY  BOOsmPiBii 

pressure  of  a  deep  coliunn  of  the  ocean.  At  the  Hawaii  IslandB,  on  25di 
February  1877,  masses  of  pumice,  during  a  submaiine  volcanic  explodon, 
were  ejected  to  the  surface,  one  of  which  struck  the  bottom  of  a  boat 
with  considerable  violence  and  then  floated.  When  we  reflect,  indeed, 
to  what  a  considerable  extent  the  bottom  of  the  great  ocean-buini  ii 
(lotted  over  with  volcanic  cones,  rising  often  solitary  from  profoond 
depths,  we  can  believe  that  a  lai-ge  proiwiiion  of  the  actual  enipdon 
in  oceanic  areas  may  take  place  under  the  sea.  The  immense  abunduce 
and  wide  diffusion  of  volcanic  detritus  (including  blocks  of  pumice)  over 


the  bottom  of  the  Pacific  and  Atkntic  oceans,  even  at  distances  remote 
from  land,  as  made  known  by  the  voyage  of  the  CkalUoffer^  doubt 
less  indicate  the  prevalence  and  pei-sistenco  of  submarine  volcamc 
action,  even  though,  at  the  same  time,  an  extensive  difluaion  of  volcanic 
debris  from  the  islands  is  admitted  to  Iw  effected  liy  winds  and  ocewi- 
currents. 

Volcanic-  islands,  unless  continually  augmented  by  renewed  eruptions. 


iiK-pIn  Rock.  *  itwli  of  Ivrdrr  nick  left  by  thf  bf>  : 
tththiGlicrriuKc)  liy  w»vf..  aud  »ul«rtal  wtnU-.j 


:iM  (Ca]i1.  ItlukiTDal  Id  Admlnl^  Chut). 
liguon  (HC  Pig.  «n; 


are  attacked  by  the  waves  and  cut   down.     Graham's  Island  and  the 
other  examples  above   cited  show  how   rapid  this   disappearance  may 


BBCT.  i  §  3  FISSURE^ERUPTIONS  265 

be.  The  island  of  Volcano  has  the  base  of  its  slopes  truncated  by  a  line 
of  cliff  due  to  marine  erosion.  The  island  of  Teneriffe  shows,  in  the 
same  way,  that  the  sea  is  cutting  back  the  land  towards  the  great  cone 
(Pig.  68).  The  island  of  St.  Paid  (Figs.  67,  69)  brings  before  us  in  a 
more  impressive  way  the  tendency  of  volcanic  islands  to  be  destroyed 
unless  replenished  by  continual  additions  to  their  surface.  At  St. 
Helena  lofty  cliffs  of  volcanic  rocks  1000  to  2000  feet  high  bear  witness 
to  the  enormous  denudation  whereby  masses  of  basalt  two  or  three  miles 
long,  one  or  two  miles  broad,  and  1000  to  2000  feet  thick,  have  been 
entirely  removed.^ 

ii.  Fissure  (Massive)  Eruptions. 

Under  the  head  of  massive  or  homogeneous  volcanoes  some  geologists 
have  included  a  great  number  of  bosses  or  dome-like  projections  of  once- 
melted  rock  which,  in  regions  of  extinct  volcanoes,  rise  conspicuously 
above    the  surface   without   any   visible   trace   of   cones   or  craters   of 
fragmentary  material       They  are  usually   regarded   as   protrusions  of 
lava,  which,  like  the  Puy  de  Ddme  in  Auvergne,  assumed  a  dome-form 
at  tiie  surface  without  spreading  out  in  sheets  over  the  surrounding 
coantry,  and  with  no  accompanying  fragmentary  discharges.     But  the 
mere  absence  of  ashes  and  scoriae  is  no  proof  that  these  did  not  once 
exist,  or  that  the  present  knob  or  boss  of  lava  may  not  originally  have 
solidified  within  a  cone  of  tuff  which  has  been  subsequently  removed  in 
denudation.     The  extent  to  which  the  surface  of  the  ground  has  been 
changed  by  ordinary  atmospheric  waste,  and  the  comparative  ease  with 
which  loose  volcanic  dust  and  cinders  might  have  l:>een  entirely  removed, 
require  to  be  considered.     Hence,  though  the  ordinary  explanation  is  no 
doubt  in  some  cases  correct,  it  may  be  doubted  whether  a  large  propor- 
tion of  the  examples  cited   from  the  Rhine,  Bohemia,   Hungar}^  and 
other  regions,   ought  not  rather  to  be    regarded  like  the  "  necks  "  so 
abundant  in  the  ancient  volcanic  districts  of  Britain  (Book  IV.  Part  VII.) 
as  the  remaining  roots  of  ordinary  volcanic  cones.      If  the  tuff  of  a 
cone,  up  the  funnel  of  which  lava  rose  and  solidified,  were  swept  away, 
we  should  find  a  central  lava   plug  or   core    resembling   the   volcanic 
"heads"  (vidkanisclie  Knppen)  of  Germany.      Unquestionably,  lava  has 
in   innumerable   instances  risen  in  this    way    within    cones   of  tuff  or 
cinders,  partially  filling  them  without  flo^ring  out  into  the  surrounding 
country.2 

But  while,  on  either  explanation  of  their  origin,  these  volcanic  "  heads  " 
find  their  analogues  in  the  emissions  of  lava  in  modern  volcanoes,  there 
are  numerous  cases  in  old  volcanic  areas  where  the  eruptions,  so  far  as 
can  now  be  judged,  were  not  attended  with  the  production  of  any  central 
cone  or  crater.     Such  emissions  of  lava  may  have  resembled  those  which 

^  Darwin,  *  Volcanic  Islands,'  p.  104.  For  a  more  detailed  account  of  this  island,  see 
J.  C.  Melli.Hs'  'St.  Helena,'  London,  1875. 

*  Von  Seebach,  Z.  Deutsch.  OeoL  Ges.  xviii.  p.  643.  F.  von  Hochstetter,  Neues 
JaKrh.  1871,  p.  469.  Reyer,  Jahrb.  K,  K,  OeoL  ReichsansUdU  1878,  p.  81  ;  1879,  p. 
463. 


256  TiYSAMICAL  GEOUJGY  book  ni  pabt  I 


in  recent  times  have  occurred  at  the  Hawaiian  volcanoes,  where  enormoiu 
accumulations  of  lava  have  gradually  been  built  up  into  flat  domefl,  of 
which  Mauna  Loa  rises  to  a  height  of  1 3,675  feet.  Vast  floods  of  remark- 
ably liquid  basic  lava  have  from  time  to  time  flowed  out  tranquilly  without 
explosion  or  eaithquake,  and  with  no  accom|)animcnt  of  fragmental  dis- 
charges. These  currents  of  molten  rock  have  spread  out  into  wide  sheets, 
sloping  at  so  low  an  angle  that  they  look  horizontal.  The  lower  and  older 
portions  of  them  have  l>een  eroded  by  streams  so  as  to  present  escarp- 
ments and  outliers  not  unlike  those  of  western  North  America  or  the 
older  basaltic  plateaux  of  Britain  and  India.^ 

The  most  stupendous  modern  basaltic-floods  of  Iceland  issued  from 
vents  along  a  fissure.  Acconling  to  Thoroddsen  the  post-glacial  lavsr 
fields  of  Odadahraun,  covering  an  area  of  about  4390  square  kilometres, 
have  issued  from  about  20  distinct  vents,  while  in  the  east  of  Iceland  the 
lava  has  flowed  from  the  lips  of  fissures.-  It  would  seem  that  for  the 
discbarge  of  such  wide  and  flat  sheets  of  lava,  great  mobility  and 
tolerably  complete  fusion  of  the  molten  mass  is  necessary.  The 
phenomenon  occurs  among  the  more  basic  lavas  (basalts,  &c)  rather  than 
among  the  more  lithoid  acid  lavas  (trachytes,  rhyolites.  Sec.) 

In  former  geological  ages,  extensive  eruptions  of  lava,  without  the 
accompaniment  of  scoriae,  with  hardly  any  fragmentary  materials,  and 
with,  at  the  most,  only  flat  dome-shaped  cones  at  the  points  of  emission, 
have  taken  place  over  wide  areas  from  scattered  vents,  along  lines  or 
systems  of  fissures.  Vast  sheets  of  lava  have  in  this  manner  been  poured 
out  to  a  depth  of  many  hundred  feet,  completely  burying  the  previous 
sui*face  of  the  land  and  forming  wide  plains  or  plateaux.  These  truly 
"  massive  eruptions "  have  been  held  by  Kichthofen  ^  and  others  to 
represent  the  grand  fundamental  character  of  volcanism,  ordinary  volcanic 
cones  being  regarded  merely  as  i)arasitic  excrescences  on  the  subterranean 
lava-reservoirs,  \qy\  much  in  the  relation  of  minor  cinder  cones  to  their 
parent  volcano.* 

Though  a  description  of  these  old  fissure  or  massive  eruptions  ought 
pn>|x»rly  to  Ihj  included  in  Book  IV.,  the  subject  is  so  closely  connected 
with  the  dynamics  of  existing  active  volcanoes  that  an  account  of  the 
subject  may  be  given  here.  Perhai)s  the  most  stupendous  example  of  this 
type  of  volcanic  structiue  occims  in  Western  North  America.  The  extent 
of  country  which  has  been  flooded  with  hisalt  in  Oregon,  Washington, 
California,  Idaho,  and  Montana  has  not  yet  been  acciurately  surveyed,  but 
has  been  estimated  to  cover  a  larger  area  than  France  and  Great  Britain 
combined,  with  a  thickness  averaging  2000  but  reaching  in  some  places 
to  3700  feet.^     The  Snake  River  plain  in  Idaho  (Fig.  70)  forms  part  of 

^  For  a  gi-aj>hic  account  of  the  Hawaiian  lava-fieUls,  see  Captain  Dutton,  Foarth  Animal 
Report,  U.  S.  Geol.  Survey  for  1882-83.     See  also  Dana's  *  Characteristics  of  Volcanoes.' 

»  See  W.  L.  Watts'  ''  Across  the  Vatna  Jokull,"  Proc.  Roy.  Qtiig.  Soc  1876.  W.  G. 
Lock,  Oeol.  Mag.  1881,  p.  212  ;  and  papers  by  Thoro<ldsen  and  Helland,  quoted  antej  pw  202. 

'   Trans.  Akatl.  Sci.  California,  1868. 

"*  Proc.  Ruy.  Phys.  Sac.  JS<lin.  v.  236  ;  Sature,  xxiii.  p.  3. 

*  J.  LeConte.     Amer.  Journ.  Sci.  3rd  ser.  vii.  (1874),  167,  259. 


nor.  i  §  3 


FISSURE-ER  UPTIONS 


257 


this  lava-flood,  Surrounded  on  the  north  and  east  by  lofty  mountains,  it 
BtretcheB  westward  as  an  apparently  boundless  deeort  of  sand  and  bare 
sheets  of  black  basalt.  A  few  streams  descending  into  the  plain  from  the 
hills  are  soon  swallowed  up  and  tost.  The  Snake  River,  however,  flows 
across  it,  and  has  cut  out  of  its  lava-beds  a  series  of  picturesque  gorges 
and  rapids.  Looked  at  from  any  point  on  it«  surface,  it  appears  as  a  vast 
level  plain  like  that  of  a  lake-bottom,  though  more  detailed  examination 
may  detect  a  slope  in  one  or  more  directions,  and  may  thereby  obtain 
evidence  as  to  the  sites  of  the  chief  openings  from  which  the  basalt  was 
poured  forth.  The  uniformity  of  suriace  has  been  produced  either  by  the 
lava  flowing  over  a  plain  or  lake-bottom,  or  by  the  complete  effacomont  of 
an  original  and  undulating  contour  of  the  ground  under  hundreds  of  feet 
of  volcanic  rock  in  successive  sheets.     The  lava  rolling  up  to  the  base  of 


the  mountains  has  followed  the  sinuosities  of  their  margin,  as  the  waters 
of  a  lake  follow  its  promontories  and  bays.  The  author  crossed  the 
Snake  River  plain  in  1879,  and  likewise  rode  for  many  miles  along  its 
northern  edge.  He  found  the  surface  to  be  everywhere  marked  with 
low  hummocks  or  ridges  of  bare  black  liasalt,  the  surfaces  of  which 
exhibited  a  reticulated  pavement  of  the  ends  of  columns.  In  some  places, 
there  was  a  perceptible  tendency  in  these  ridges  to  range  themselves  in 
one  general  north-easterly  direction,  when  they  might  be  likened  to  a 
series  of  long,  low  waves,  or  ground-swells.  In  many  instances  the  crest 
of  each  ridge  had  cracked  open  into  a  fissure  which  presented  along  its 
walls  a  series  of  tolerably  symmetrical  columns  (Fig.  70).  That  these 
ridges  were  original  undulations  of  the  lava,  and  had  not  been  produced 
by  erosion,  was  indicated  by  the  fact  that  the  columns  were  perpendicular 
to  their  surface,  and  changed  iu  direction  according  to  the  form  of  the 
ground  which  was  the  original  cooling  surface  of  the  lava.     Though  the 


258  nVXAMIGAL  GEOLOGY  book  hi  part  x 


basalt  was  sometimes  vesicular,  no  layers  of  slag  or  scorisB  were  anywhere 
observed,  nor  did  the  surfaces  of  the  ridges  exhibit  any  specially  scorifomi 
character. 

There  are  no  great  cones  whence  this  enormous  flood  of  basalt  could 
have  flowed.  It  probably  escaped  from  orifices  or  fissui^es  still  concealed 
under  the  sheets  which  issued  from  them,  the  points  of  escape  being 
marked  only  by  such  low  domes  as  could  readily  be  buried  under  the 
succeeding  eruptions  from  other  vents.^  That  it  was  not  the  result  of 
one  sudden  outpouring  of  rock  is  shown  by  the  distinct  bedding  of  the 
basalt,  which  is  well  marked  along  the  river  ravines.  It  arose  from  what 
may  have  been,  on  the  whole,  a  continuous  though  locally  intermittent 
welling-out  of  lava,  probably  from  vents  on  many  fissures  extending  over 
a  wide  tract  of  Western  America  duiing  a  late  Tertiary  period,  if,  indeed, 
the  eruptions  did  not  partly  come  within  the  time  of  the  human  occupation 
of  the  continent.  The  discharge  of  lava  continued  until  the  previous  topo- 
graphy was  buried  under  some  2000  feet  of  lava,  only  the  higher  summits 
still  projecting  above  the  volcanic  flood.-  At  a  few  points  on  the  plain 
and  on  its  northern  margin,  the  author  observed  some  small  cinder  cones 
(Fig.  70).  These  were  evidently  formed  during  the  closing  stages  of 
volcanic  action,  and  may  be  compared  to  the  minor  cones  on  a  modem 
volcano,  or  better,  to  those  on  the  surface  of  a  recent  lava-stream. 

In  Europe,  during  older  Tertiary  time,  similar  enormous  outpourings 
of  basalt  covered  many  hundreds  of  square  miles.  The  most  important  of 
these  is  that  which  occupies  a  large  part  of  the  north-east  of  Ireland, 
and  in  disconnected  areas  extends  through  the  Inner  Hebrides  and  the 
Faroe  Islands  into  Iceland.  Throughout  that  region,  the  paucity  of 
evidence  of  volcanic  vents  is  truly  remarkable.  So  extensive  has  been 
the  denudation,  that  the  inner  structure  of  the  volcanic  plateaux  has  been 
admirably  revealed.  The  ground  beneath  and  around  the  basalt-sheets 
has  been  rent  into  innumerable  fissures  which  have  been  filled  by  the 
rise  of  basalt  into  them.  A  vast  number  of  basalt-dykes  ranges  from 
the  volcanic  area  eastwards  across  Scotland  and  the  north  of  England  and 
the  north  of  Ireland.  Towards  the  west  the  molten  rock  reached  the 
surface  and  was  poured  out  there,  while  to  the  eastward  it  does  not 
appear  to  have  overflowed,  or,  at  least,  all  evidence  of  the  outflow  has 
been  removed  in  denudation.  When  we  reflect  that  this  system  of  dykes 
can  be  traced  from  the  Orkney  Islands  southwards  into  Yorkshire  and 
across  Bntain  from  sea  to  sea,  over  a  total  area  of  probably  not  less  than 
100,000  square  miles,  we  can  in  some  measure  appreciate  the  volume 
of  molten  basalt  which  in  older  Tertiary  times  underlay  large  tracts  of 
the  site  of  the  British  Islands,  rose  up  in  so  many  thousands  of  fissures, 
and  poui'ed  forth  at  the  surface  over  so  wide  an  area  in  the  north-west.' 

In  Africa,  basaltic  plateaux  cover  large  tracts  of  Abyssinia,  where  by 

^  Captain  Dutton  has  remarked  the  absence  of  any  conspicuous  feature  at  the  •onroes 
from  which  some  of  the  largest  lava-streams  of  Hawaii  have  issued. 

'■^  Professor  J.  LeConte  believes  that  the  chief  fis.sures  opened  in  the  Cascade  and  Blue 
Mouutaiji  Ranges.     Amer.  Jnnm.  Sci.  3rd  series,  \'ii.  (1874),  p.  168. 

'  Trans,  Roy.  Soc.  Edin.  xxxv.  (1888),  p.  21. 


SECT,  i  S  4  DISTRIBUTION  OF  VOLCANOES  259 

the  denuding  effect  of  heavy  rains  they  have  been  carved  into  picturesque 
hills,  valleys,  and  ravines.^  In  India,  an  area  of  at  least  200,000  square 
miles  is  covered  by  the  singidarly  horizontal  volcanic  plateaux  of  the 
"  Deccan  Traps  "  (lavas  and  tuffs),  which  belong  to  the  Cretaceous  period 
and  attain  a  thickness  of  6000  feet  or  more.^  The  underlying  platform 
of  older  rock,  where  it  emerges  from  beneath  the  edges  of  the  basalt  table- 
land, is  found  to  be  in  many  places  traversed  by  dykes ;  but  no  cones 
and  craters  are  anywhere  visible.  In  these,  and  probably  in  many  other 
examples  still  undescribed,  the  formation  of  great  plains  or  plateaux  of 
level  sheets  of  lava  is  to  be  explained  by  "  fissure-eruptions  "  rather  than 
by  the  operations  of  volcanoes  of  the  familiar  "  cone  and  crater  "  type. 

§    4.     Geographical    and   geological   distribution   of 

volcanoes. 

Adequately  to  trace  the  distribution  of  volcanic  action  over  the 
globe,  account  ought  to  be  taken  of  dormant  and  extinct  volcanoes,  like- 
wise of  the  proofs  of  volcanic  outbreaks  during  earlier  geological  periods. 
When  this  is  done,  we  learn,  on  the  one  hand,  that  innumerable  districts 
have  been  the  scene  of  prolonged  volcanic  activity,  where  there  is 
now  no  imderground  commotion,  and  on  the  other,  that  volcanic  out- 
bursts have  been  apt  to  take  place  again  and  again  after  wide  intervals 
on  the  same  ground,  some  modern  active  volcanoes  being  thus  the 
descendants  and  representatives  of  older  ones.  Some  of  the  facts 
regarding  former  volcanic  action  have  been  already  stated.  Others  will 
be  given  in  Book  W.  Part  VII. 

Confining  attention  to  vents  now  active,  of  which  the  total  number 
may  be  about  300,^  the  chief  facts  regarding  their  distribution  over  the 
globe  may  be  thus  summarised.  (1)  Volcanoes  occur  along  the  margins 
of  the  ocean-basins,  particularly  along  lines  of  dominant  mountain- 
ranges,  which  either  form  part  of  the  mainland  of  the  continents  or  extend 
as  adjacent  lines  of  islands.  The  vast  hollow  of  the  Pacific  is  girdled 
with  a  wide  ring  of  volcanic  foci.  (2)  Volcanoes  rise,  as  a  striking 
feature,  from  the  submarine  ridges  that  traverse  the  ocean  basins.  All 
the  oceanic  islands  are  either  volcanic  or  formed  of  coral,  and  the 
scattered  coral-islands  have  in  all  likelihood  been  built  upon  the  tops  of 
submarine  volcanic  cones.  (3)  Volcanoes  are  situated  not  far  from  the  sea. 
The  only  exceptions  to  this  rule  are  certain  vents  in  Mantchouria  and  in 
the  tract  lying  between  Thibet  and  Siberia ;  but  of  the  actual  nature  of 
these  vents  very  little  is  yet  knoAvn.  (4)  The  dominant  arrange- 
ment of  volcanoes  is  in  series  along  subterranean  lines  of  weakness, 

»  Blanford's  'Abyssinia,'  1870,  p.  181. 

'  Medlicott  and  Blanford,  *  Geology  of  India,'  p.  299. 

'  Thia  number  is  probably  below  the  truth.  Prof.  J.  Milne  has  enumerated  in  Japan 
alone  no  fewer  than  fifty-three  volcanoes  which  are  either  active  or  have  been  active  within 
a  recent  period.  He  remarks  that,  "if  we  were  in  a  position  to  indicate  the  volcanoes 
which  had  been  in  eruption  during  the  last  4000  years,  the  probability  is  that  they  would 
number  seyeral  thousands  rather  than  four  or  five  hundred."  '  Earthquakes  and  other  Earth - 
movements,'  1886,  p.  227.     Compare  Fisher,  '  Physics  of  Earth's  Crust,'  2nd  ed.  chap.  xxiv. 


260  DYNAMICAL  GEOLOGY  book  m  part  i 

as  in  the  chain  of  the  Andes,  the  Aleutian  Islands,  and  the  Malay 
Archipelago.  A  remarkable  zone  of  volcanic  vents  girdles  the  globe 
from  Central  America  eastward  by  the  Azores  and  Canary  Islands  to 
the  Mediterranean,  thence  to  the  Red  Sea,  and  through  the  chains  of 
islands  from  the  south  of  Asia  to  New  Zealand  and  the  heart  of  the 
Pacific.  (5)  On  a  smaller  scale  the  linear  arrangement  gives  place  to  one 
in  groups,  as  in  Italy,  Iceland,  and  the  volcanic  islands  of  the  great  oceans. 

In  the  European  area  there  are  six  active  volcanoes — Vesuvius,  Etna, 
Stromboli,  Volcano,  Santorin,  and  Nisyros.  Asia  contains  twenty-four, 
Africii  ten.  North  America  twenty,^  Central  America  twenty-five,  and 
South  America  thirty-seven.*'  By  much  the  largest  number,  however, 
occur  on  islands  in  the  ocean.  In  the  Arctic  Ocean  rises  the  solitary 
Jan  Mayen.  On  the  ridge  se^mrating  the  Arctic  and  Atlantic  basins,  the 
group  of  Icelandic  volcanoes  is  found.  Along  the  great  central  ridge  of 
the  Atlantic  bottom,  numerous  volcanic  vents  have  risen  above  the  sur- 
face of  the  sea — the  Azores,  Canary  Islands,  and  the  extinct  degraded 
volcanoes  of  St.  Helena,  Ascension  and  Tristan  d'Acunha.  On  the 
eastern  border  lie  the  volcanic  vents  of  the  islands  off  the  African  coast, 
and  to  the  west  those  of  the  West  India  Islands.  Still  more  remarkable 
is  the  development  of  volcanic  energy  in  the  Pacific  area.  From  the 
Aleutian  Islands  southwards,  a  long  line  of  volcanoes,  numbering 
upwards  of  a  hundred  active  vents,  extends  through  Kamtschatka  and 
the  Kurile  Islands  to  Japan,^  whence  another  numerous  series  carries 
the  volcanic  band  far  south  towards  the  Malay  Archipelago,  which  must 
be  regarded  as  the  chief  centre  of  the  present  volcanic  activity  of  our 
planet.  In  Sumatra,  Java,  and  adjoining  islands,  no  fewer  than  fifty 
active  vents  occur.  The  chain  is  continued  through  New  Guinea  and  the 
groups  of  islands  to  New  Zealand.*  Even  in  the  Antarctic  regions, 
Mounts  Ei'ebus  and  Ten*or  are  cited  as  active  vents ;  while  in  the  centre 
of  the  Pacific  Ocean  rise  the  gi-eat  lava  cones  of  the  Sandwich  Islands. 
In  the  Indian  Ocean,  the  Red  Sea,  and  ofl*  the  east  coast  of  Africa  a 
few  scattered  vents  appear. 

Besides  the  existence  of  extinct  volcanoes  which  have  obviously 
been  active   in   comparatively  recent  times,  the  geologist   can    adduce 

^  For  au  account  of  tlie  remarkable  extinct  volcanoes  of  Northern  California,  Oregon,  and 
Washington  Territory,  see  A.  Hague  and  J.  P.  hidings.  Ainer.  Journ.  Sci.  xxvi.  (1883), 
j>.  222.  On  Volcanoes  of  Mexico  see  H.  Lenk,  *  Beitriige  zur  Geologic  und  Pabeontologie 
(ler  Repbulik  Mexico,'  Leipzig,  1890  :  of  Central  America,  A.  Dolfuiis  and  E.  de  Monsemt, 
'Voyage  Gtologique. '  Paris,  folio,  1868  ;  K.  von  Seebach,  Ahh.  Kiin.  Oes.  Wiss,  GUttiHgen^ 
xxxviii.  (1892). 

•^  For  a  recent  account  of  the  volcanoes  of  the  Andes  of  the  Equator  see  Whympers 
'Travels  Amongst  the  Great  Andes.' 

•*  For  the  volcanoes  of  Jajjau,  besides  papers  (juote*!  on  p.  213,  see  W.  J.  Holland, 
Appalachid,  vi.  (1890),  109.  E.  Xanmann,  Zeiisch,  Deutsch.  Geol.  Gen.  1877,  p.  864.  Mr. 
Milne  enumerates  100  active  vents  from  the  Kuriles  to  Kinshu  (2000  miles). 

*  The  great  eruption  of  Tarawera,  New  Zealand,  in  1886,  is  describcil  by  Prof.  A.  P.  W. 
Thomas,  *  Report  on  the  Eruption  of  Tarawera,'  published  by  the  Government  in  1888  :  also 
Prof,  llutton's  *  Rei>ort  on  the  Tarawera  Volcanic  District,  Wellington,  1887,'  Qu€arL  Jaum. 
Geol.  Soc.  xliii.  (1887),  p.  178. 


8ECT.i§4  DISTRIBUTION  OF  VOLCANOES  261 

proofs  of  the  former  presence  of  active  volcanoes  in  many  countries 
where  cones,  craters,  and  all  the  ordinary  aspects  of  volcanic  mountains, 
have  long  disappeared,  but  where  sheets  of  lava,  beds  of  tuff,  dykes,  and 
necks  representing  the  sites  of  volcanic  vents  have  been  recognised 
abundantly  (Book  IV.  Part  VII.)  These  manifestations  of  volcanic 
action,  moreover,  have  as  wide  a  range  in  geological  time  as  they  have  in 
geographical  area.  Every  great  geological  period,  back  into  pre-Cambrian 
time,  seems  to  have  had  its  volcanoes.  In  Britain,  for  instance,  there 
were  probably  active  volcanic  vents  in  pre-Cambrian  ages.  The  Archaean 
gneiss  of  N.-W.  Scotland  includes  a  remarkable  series  of  dykes  presenting 
some  points  of  resemblance  to  the  great  Tertiary  system.  The  Torridon 
sandstone  of  the  same  region,  which  is  now  known  to  be  pre-Cambrian, 
contains  pebbles  of  various  finely  vesicular  porphyrites,  and  in  one  place 
includes  a  band  of  true  tuff.  In  the  lower  Cambrian  period  the 
tuffs  and  diabases  of  Pembrokeshire  were  erupted.  Still  more  vigorous 
were  the  volcanoes  in  the  Lower  Silurian  period,  when  the  lavas  and  tuffs 
of  Snowdon,  Aran  Mowddwy,  and  Cader  Idris  were  ejected.  During 
the  deposition  of  the  Upper  Silurian  rocks  a  few  volcanoes  were  active  in 
the  west  of  Ireland.  The  Lower  Old  Red  Sandstone  epoch  was  one  of 
prolonged  activity  in  central  Scotland.  The  earlier  half  of  the  Carbon- 
iferous period  likewise  witnessed  two  distinct  epochs  of  volcanic  activity 
over  the  same  region.  In  the  earlier  of  those,  lavas  (andesites  and 
trachytes)  were  poured  out  in  wide  level  plateaux  from  many  vents, 
while  in  the  later,  groups  of  minor  cones  like  the  puys  of  Auvergne  were 
dispersed  among  the  lagoons.  During  Permian  time,  more  than  a  hundred 
small  vents  rose  in  scattered  groups  across  the  centre  and  south-west  of 
Scotland,  while  a  few  similar  points  of  eruption  appeared  in  the  south- 
west of  England.  No  trace  of  any  British  Mesozoic  volcanoes  has  been 
met  with.  The  vast  interval  between  Permian  and  older  Tertiary  time 
appears  to  have  been  a  period  of  total  quiescence  of  volcanic  activity. 
The  older  Tertiary  ages  were  distinguished  by  the  outpouring  of  the 
enormous  basaltic  plateaux  of  Antrim  and  the  Inner  Hebrides.^ 

In  France  and  Germany,  likewise,  Palaeozoic  time  was  marked  by  the 
eruption  of  many  diabase,  porphyrite,  and  quartz-porphyry  lavas.  In 
Brittany,  for  example,  Dr.  Barrois  has  found  a  remarkable  series  of 
older  Palaeozoic  diabases  and  porphyrites  with  tuffs  and  agglomerates. 
He  distinguishes  four  principal  periods  of  eruption — 1.  Cambrian  and 
Lower  Silurian ;  2.  Middle  and  Upper  Silurian ;  3.  Upper  Devonian  ; 
4.  Carboniferous.*  The  Permian  period  was  marked  in  Germany  and 
also  in  the  south  of  France  by  the  discharge  of  great  masses  of  various 
quartz-porphyries.  The  Triassic  period  likewise  witnessed  numerous 
eruptions.  But  from  that  period  onward  the  same  remarkable  quiescence 
appears  to  have  reigned  all  over  Europe,  which  characterised  the 
geological  history  of  Britain  during  Mesozoic  time.^     In  Tertiary  time  a 

*  For  a  detailed  summary  of  the  volcanic  history  of  Britain,  see  Presidential  addreases 
to  the  Geological  Society,  Quart.  Journ.  Geal.  Soc.  xlvii.  xlviii.  (1891-92). 

*  CarU  Giol,  dStaill.  France,  No.  7,  1889. 

'  Some  trifling  exceptions  to  this  general  statement  are  said  to  occur.     C.  E.  M.  Rohrbach 


262  DYXAMICAL  GEOLOGY  book  in  part  i 

prodigious  outpouring  of  lavas,  both  acid  and  basic,  continued  from  the 
Miocene  epoch  down  even  perhaps  to  the  historic  period.  Examples  of 
this  great  series  are  met  with  in  Central  France,  the  Eifel,  Italy,^  Bohemia, 
and  Hungary,  almost  to  the  existing  period.^  Recent  research  has 
brought  to  light  evidence  of  a  long  succession  of  Tertiary  and  post- 
Tertiary  volcanic  outbursts  in  Western  America  (Nevada,  Oregon,  Idaho, 
Utah,  &c.)  Contemporaneous  volcanic  rocks  are  associated  with  Palaeo- 
zoic, Secondary,  and  Tertiary  formations  in  New  Zealand,  and  volcanic 
action  there  is  not  yet  extinct. 

Thus  it  can  be  shown  that,  within  the  same  comparatively  limited 
geogi*aphical  space,  volcanic  action  has  been  rife  at  intervals  during  a 
long  succession  of  geological  ages.  Even  round  the  sites  of  still  active 
vents,  traces  of  far  older  eruptions  may  be  detected,  as  in  the  case  of  the 
existing  active  volcanoes  of  Iceland,  which  rise  from  amid  Tertiary  lavas 
and  tuffs.  Volcanic  action,  which  now  manifests  itself  so  conspicuously 
along  certain  lines,  seems  to  have  continued  in  that  linear  development 
for  protracted  periods  of  time.  The  actual  vents  have  changed,  dying 
in  one  place  and  breaking  out  in  another,  yet  keeping  on  the  whole 
along  the  same  tracts.  Taking  all  the  manifestations  of  volcanic  action 
together,  both  modern  and  ancient,  we  see  that  the  subterranean  forces 
have  operated  along  great  lines  in  the  earth's  crust,  and  that  the  existing 
volcanoes  form  but  a  small  proportion  of  the  total  number  of  once  active 
vent*?. 

Looking  broadly  at  the  geological  history  of  volcanic  action  we 
observe  that,  while  there  is  eWdence  of  the  protinision  of  both  acid  and 
basic  materials  from  the  remotest  periods,  the  earlier  discharges  were 
preponderantly  acid.  In  Britain,  for  example,  the  vast  piles  of  lavas 
ejected  during  the  Silurian  period  were  mainly  of  a  felsitic  character, 
though  considerable  accumulations  of  andesites  were  not  wanting.  On 
the  other  hand,  the  wide  sheets  of  lava  poured  out  in  this  country 
during  Tertiary  time  were  chiefly  basalts,  the  acid  protrusions  occurring 
for  the  most  ]>art  as  dykes  and  bosses.  A  similar  broad  sequence  has 
been  observed  in  other  countries. 

When,  however,  we  proceed  to  consider  more  closely  the  nature  of 
the  successive  eruptions  during  the  continuance  of  one  of  the  volcanic 
periods  of  which  records  are  preserved  among  the  geological  formations^ 
we  discover  proofs  of  a  remarkable  variation  in  the  character  of  the  lavas.^ 

describirs  Cretaceous  tescheuites  and  diabases  in  Silesia  {Tsrfiermak's  Min.  Miitheii.  vii. 
(1885),  p.  1.5).  P.  Choffat  refers  to  Cenomanian  emptions  in  Portugal  {Joum.  Seiencias 
Math.  Phys.  Xatui\  Lisbon,  1884).  A.  £.  T.«agorio  )ias  found  in  the  Crimea  a  series  of 
sheets,  dykes,  and  bosses,  ranging  from  nevadites  to  basalts. 

^  For  early  and  classical  accounts  of  the  Italian  volcanic  districts,  see  SpaUaniani's 
'  Voyages  dans  les  deux  Siciles,'  ai)d  Breislak's  '  Voyages  Physiques  et  Lythologiques  dams 
la  Cani]ianie.'  Consult  also  Mercalli's  '  Vulcani,  &c.,'  and  Johnston-La\is'  'South  Italian 
Volcanoes,*  already  cited. 

'^  For  a  recent  attempt  to  give  a  stratigraphical  and  geographical  view  of  the  distribntion 
of  igneous  rocks  in  Europe,  see  M.  Bertrand,  Bull.  Str.  GM.  France^  xvi.  (1888),  p.  678. 

^  Tn  some  volcanoes  {e.g.  Teneriffe)  the  lower  lavas  arc  heavier  and  more  basic  than  the 
upi>er. 


SECT,  i  §  5  CAUSES  OF  VOLCANIC  ACTION  263 

Various  observers  have  noticed  that  volcanic  rocks  have  succeeded 
each  other  in  a  certain  order  in  different  regions.  Baron  von  Eichthofen 
deduced  from  observations  in  Europe  and  America  a  general  sequence  of 
volcanic  succession,  which  he  arranged  in  the  following  order : — 1.  Pro- 
pylite;  2.  Andesite ;  3.  Trachyte;  4.  Rhyolite;  5.  Basalt^  This 
sequence  he  believed  to  be  seldom  or  never  complete  in  any  one  locality ; 
sometimes  only  one  member  of  the  series  may  be  found ;  but  when  two  or 
more  occur  they  follow,  in  his  opinion,  this  sequence,  basalt  being  every- 
where the  latest  of  the  series.  The  subject  has  been  more  recently 
discussed  by  M.  Bertrand,  who  remarks  that  in  Europe  each  of  the  great 
areas  of  plication  has  given  rise  to  the  formation  of  eruptive  rocks  of 
every  composition  and  structure.  He  recognises  a  recurrence  of  the 
phenomena  in  successive  geological  periods,  and  speaks  of  a  definite  order 
of  eruptions  in  the  same  series.^ 

The  great  volcanic  series  of  Auvergne  presents  a  marvellous  succession 
of  varied  eruptions  within  a  limited  region  during  what  was  probably 
a  single  volcanic  period.  The  first  eruptions  app>ear  to  have  been 
basalts,  and  rocks  of  similar  character  reappeared  again  and  again  in  later 
stages  of  the  history,  the  intervening  eruptions  consisting  of  phonolites, 
trachytes,  rhyolites,  or  andesites.  The  latest  lavas  were  scoriaceous 
basalts.^  Among  the  later  Palaeozoic  volcanic  eruptions  of  Britain  a  more 
definite  and  regular  recurrence  of  rocks  appears  to  be  traceable.  The 
earlier  lavas  of  the  Old  Red  Sandstone  and  Carboniferous  series  were 
generally  either  intermediate  or  basic,  sometimes  remarkably  basic,  while 
the  late  protrusions  were  decidedly  acid.  At  the  one  end  we  find  basalts 
or  diabases  and  picrites,  followed  sometimes  by  copious  outpourings  of 
andesites,  while  at  the  other  end  come  intrusions  of  felsites  and  quartz- 
porph3rries.  Again,  among  the  Tertiary  lavas,  the  basalts  of  the  great 
plateaux  are  pierced  by  bosses  and  dykes  of  granophyre  and  allied  acid 
rocks.  In  these  various  examples  the  facts  point  to  some  gradual  change 
in  the  composition  of  the  subterranean  magma  during  the  lapse  of  a  single 
volcanic  period — a  change  in  which  there  was  a  separation  of  basic 
constituents  and  the  discharge  of  more  basic  lavas,  leaving  a  more  acid 
residuum  to  be  erupted  towards  the  end  of  the  activity."* 

§  5.  Causes  of  Volcanic  Action. 

The  modus  operandi  whereby  the  internal  heat  of  the  globe  manifests 
itself  in  volcanic  action  is  a  problem  to  which  as  yet  no  satisfactory 
solution  has  been  found.  Were  this  action  merely  an  expression  of  the 
intensity  of  the  heat,  we  might  expect  it  to  have  manifested  itself  in  a 
far  more  powerful  manner  in  former  periods,  and  to  exhibit  a  regularity 
and  continuity  commensurate  with  the  exceedingly  slow  diminution  of 
the  earth's  temperature.     But  there  is  no  geological  evidence  in  favour 

'  'The  Natural  System  of  Volcanic  Rocks,'  Calif om,  Acad,  Set,  1868. 
»  Bull.  Soc.  Giol.  France^  xvl  (1888),  p.  611. 

*  Carte  Oiol.  dHaili.  France^  Feuille  166  (Clermont  Ferrand). 

*  Quart.  Journ.  OeoL  Soc.  vol.  xhiii.  (1892),  p.  177. 


264  DYNAMICAL  GEOLOGY  book  in  parti 

of  greater  volcanic  intensity  in  ancient  than  in  more  recent  periods ;  on 
the  contrary,  it  may  be  doubted  whether  any  of  the  Palaeozoic  yolcanoes 
equalled  in  magnitude  those  of  Tertiary  and  perhaps  even  post-Tertiary 
times.  On  the  other  hand,  no  feature  of  volcanic  action  is  more  con- 
spicuous than  its  spasmodic  fitfulness.^ 

As  physical  considerations  negative  the  idea  of  a  comparatively  thin 
crust,  surmounting  a  molten  interior  whence  volcanic  energy  might  be 
derived  {ante,  p.  53),  geologists  have  found  themselves  involved  in  great 
perplexity  to  explain  volcanic  phenomena,  for  the  production  of  which  a 
source  of  no  great  depth  would  seem  to  be  necessary.  Some  have  sup- 
posed the  existence  of  pools  or  lakes  of  liquid  lava  lying  beneath  the 
crust,  and  at  an  inconsiderable  depth  from  the  surface.  Others  have 
appealed  to  the  influence  of  the  contraction  of  the  earth's  mass,  assuming 
the  contraction  to  be  now  greater  in  the  outer  than  in  the  inner 
portions,  and  that  the  effect  of  this  external  contraction  must  be  to 
squeeze  out  some  of  the  internal  molten  matter  through  weak  parts  of 
the  crust  2 

That  volcanic  action  is  one  of  the  results  of  terrestrial  contraction 
can  hardly  be  doubted,  though  we  are  still  without  satisfactory  data  as 
to  the  connection  between  the  cause  and  the  effect.  It  will  be  observed 
that  volcanoes  occur  chiefly  in  lines  along  the  crests  of  terrestrial  ridges. 
^^There  is  probably,  therefore,  a  connection  between  the  elevation  of  these 
ridges  and  the  extravasation  of  molten  rock  at  the  surface.  The  forma- 
tion of  continents  and  mountain-chains  has  already  been  referred  to  as 
prolmbly  consequent  on  the  subsidence  and  readjustment  of  the  cool 
outer  shell  of  the  planet  u])on  the  hotter  and  more  rapidly  contracting 
nucleus.  Every  such  movement,  by  relieving  pressure  on  regions  below 
the  axis  of  elevation,  will  tend  to  bring  up  molten  rock  nearer  the 
surface,  and  thus  to  promote  the  formation  and  continued  activity  of 
volcanoes. 

The  fissure-eniptions,  wherein  lava  has  risen  in  innumerable  rents 
in  the  ground  across  the  whole  breadth  of  a  country,  and  has  been 
poured  out  at  the  surface  over  areas  of  many  thousand  square  miles, 
flooding  them  sometimes  to  a  depth  of  several  thousand  feet,  undoubtedly 
prove  that  molten  rock  existed  at  some  depth  over  a  large  extent 
of  territory,  and  that  by  some  means  still  unknown,  it  was  forced  out  to 
the  surface  (aiifCy  p.  256).  In  investigating  this  subject,  it  would  be 
important  to  discover  whether  any  evidence  of  great  terrestrial  crumpling 
or  other  movement  of  the  crust  can  be  ascertained  to  have  taken  place 
about  the  same  geological  period  as  a  stupendous  outpouring  of  lava — 

*  Consult  Dana,  'Characteristics  of  Volcanoes/  p.  15  et  seq.  Dutton,  U.  &  Oeoi,  JUp. 
1882-83,  p.  183  et  ««?.  Prestwich.  Proc.  Roy.  Soc.  xlL  (1886),  p.  117.  Uiwl,  Jahrh,  Qtol, 
Reichsanst.  1886,  p.  315. 

^  Conlier,  for  example,  calculated  that  a  contraction  of  only  a  single  millimetre  (about 
^s^th  of  an  inch)  would  suffice  to  force  out  to  the  surface  lava  enough  for  500  eropUoiu. 
allowing  1  cubic  kilometre  (about  1300  million  cubic  yards)  for  each  eruption.  Prof.  Ptest- 
wich  invokes  a  slight  contraction  of  the  crust  as  the  initial  cause  of  volcanic  action.  Brit, 
Assoc,  1881,  Sects,  p.  610. 


SECT,  i  §  5  CAUSES  OF  VOLCANIC  ACTION  265 

whether,  for  example,  the  great  lava-fields  of  Idaho  may  have  had  any 
connection  with  contemporaneous  flexure  of  the  North  American 
mountain-system,  or  whether  the  basalt-plateaux  of  Antrim,  Scotland, 
Faroe,  and  Iceland  may  possibly  have  been  in  their  origin  sympathetic 
with  the  postrEocene  upheaval  of  the  Alps  or  other  Tertiary  movements 
in  £urope.  The  most  striking  instance  of  an  apparent  connection  between 
such  terrestrial  disturbances  and  volcanic  phenomena  is  that  supplied  by 
the  great  semicircle  of  eruptions  that  sweeps  from  Central  France  by  the 
£ifel,  Hochgau,  and  Bohemia  into  Hungary,  and  which  has  been  referred 
to  the  dislocations  consequent  on  the  upheaval  of  the  Alps.^ 

In  the  ordinary  phase  of  volcanic  action,  marked  by  the  copious 
evolution  of  steam  and  the  abundant  production  of  dust,  slags,  and  cinders, 
from  one  or  more  local  vents,  the  main  proximate  cause  of  volcanic 
excitement  is  obviously  the  expansive  force  exerted  by  vapours  dissolved 
in  the  molten  magma  from  which  lavas  proceed.  Whether  and  to  what 
extent  these  vapours  are  parts  of  the  aboriginal  constitution  of  the 
earth's  interior,  or  are  derived  by  descent  from  the  surface,  is  still  an 
unsolved  problem.  The  abundant  occlusion  of  hydrogen  in  meteorites, 
and  the  capacity  of  many  terrestrial  substances,  notably  melted  metals, 
to  absorb  large  quantities  of  gases  and  vapours  without  chemical  com- 
bination, and  to  emit  them  on  cooling  with  eruptive  phenomena,  not 
unlike  those  of  volcanoes,  have  led  some  observers  to  conclude  that  the 
gaseous  ejections  at  volcanic  vents  are  portions  of  the  original  con- 
stitution of  the  magma  of  the  globe,  and  that  to  their  escape  the  activity 
of  volcanic  vents  is  due.  Prof.  Tschermak  in  particular  has  advocated 
this  opinion,  and  it  is  meeting  with  increasing  acceptance.^ 

On  the  other  hand,  since  so  large  a  proportion  of  the  vapour  of  active 
volcanoes  consists  of  steam,  many  geologists  have  urged  that  this  steam 
has  in  great  measure  been  supplied  by  the  descent  of  water  from  above 
ground.  The  floor  of  the  sea  and  the  beds  of  rivers  and  lakes  are  all 
leaky.  Moreover,  during  volcanic  eruptions  and  earthquakes,  fissures  no 
doubt  open  under  the  sea,  as  they  do  on  land,  and  allow  the  oceanic  water 
to  find  access  to  the  interior.^  Again,  rain  sinking  beneath  the  surface  of 
the  land,  percolates  down  cracks  and  joints,  and  infiltrates  through  the 
very  pores  of  the  rocks.  The  presence  of  nitrogen  among  the  gaseous 
discharges  of  volcanoes  may  indicate  the  decomposition  of  water  containing 
atmospheric  gases.  The  abundant  sublimations  of  chlorides  are  such  as 
might  probably  result  from  the  decomposition  of  sea-water.  To  some 
extent  surface-waters  doubtless  do  reach  the  volcanic  magma. 

*  Sucss,  'AutlitzderErde,'  i.  p.  358,  pi.  iii.  ;  Jiilien,  Anmuiiredu  Club  Alpin,  1879-80, 
p.  446  ;  Michel-Levy,  BuU,  Soc.  mol,  France,  xviii.  (1890),  pp.  690,  841. 

^  He  has  suggested  that  if  190  cubic  kilometres,  of  the  constitution  of  cast  iron,  be 
supposed  to  solidify  annually,  and  to  give  off  50  times  its  volume  of  gases,  it  would 
suffice  to  maintain  20,000  active  volcanoes.  Sitz.  Akad.  Wissen,  fftCTi,  Ixxv.  (1877), 
p.  151.    Reyer  (•  Beitrag  zur  Physik  der  Eruptionen,'  Vienna,  1877)  advocates  the  same  view. 

•  Professor  Moseley  mentions  that  during  a  submarine  eruption  off  Hawaii  in  1877  "a 
fissure  opened  on  the  coast  of  that  island,  from  a  few  inches  to  three  feet  broad,  and  in 
some  places  the  water  was  seen  pouring  down  the  opening  into  the  abyss  below."  '  Notes 
by  a  Naturalist  on  the  "  Challenger,  '"p.  508. 


266  DYNAMICAL  GEOLOGY  book  iii  part  i 


Whatever  may  be  its  source,  we  cannot  doubt  that  to  the  enonnooB 
expansive  force  of  superheated  wat^r  (or  of  its  component  gases,  disso- 
ciated by  the  high  temperatiu'e),  in  the  molten  magma  at  the  roots  of 
volcanoes,  the  explosions  of  a  crater  and  the  subsequent  rise  of  a  lava- 
column  are  mainly  due.  The  water  or  gas  dissolved  in  the  lava  is 
retained  there  by  the  enormous  overlying  pressure  of  the  lava-colomu, 
but  when  the  molten  material  is  brought  up  to  the  surface  the  pressure 
is  relieved  and  the  water  vaporizes  and  esca}3es.  Where  the  relief  is  rapid 
the  effect  may  be  to  froth  up  the  lava  into  a  pasty  mass  of  pumice,  while 
where  it  is  sudden  and  extreme  the  escape  of  the  water-vapour  may  be  by 
an  explosive  discharge. 

It  has  been  supposed  that,  somewhat  like  the  reservoirs  in  which 
hot  water  and  steam  accumulate  under  geysers,  re8er\'oirs  of  molten  rock 
receive  a  constant  influx  of  water  from  the  surface,  which  cannot  escape 
by  other  channels,  but  is  absorbed  by  the  internal  magma  at  an 
enormously  high  temperature  and  under  vast  pressure.  In  the  course  of 
time,  the  materials  filling  up  the  chimney  are  unable  to  withstand 
the  upward  expansion  of  this  imprisoned  vapour  or  water-substance,  so 
that,  after  some  premonitory  rumblings,  the  whole  opposing  mass  is  blown 
out,  and  the  vapour  escapes  in  the  well-known  masses  of  cloud.  Mean- 
while, the  removal  of  the  overlying  column  relieves  the  pressure  on  the 
lava  underneath,  saturated  with  vapours  or  superheated  water.  Hus 
lava  therefore  begins  to  rise  in  the  furmel  until  it  forces  its  way  through 
some  weak  i>art  of  the  cone,  or  pours  over  the  top  of  the  crater. 
After  a  time,  the  vapour  being  expended,  the  energy  of  the  volcano 
ceases,  and  there  comes  a  variable  period  of  repose,  until  a  renewal  of  the 
same  phenomena  brings  on  another  einiption.  By  such  successive 
paroxysms,  the  forms  of  the  internal  reservoirs  and  tunnels  may  be 
changed  ;  new  spaces  for  the  accumulati<jn  of  superheated  water  being 
opened,  whence  in  time  fresh  volcanic  vents  issue,  while  the  old  ones 
gradually  die  out. 

An  obvious  objection  to  this  explanation  is  the  difficulty  of  con- 
ceiving that  water  should  descend  at  all  against  the  expansive  forc^ 
within.  But  Daubr<^e's  experiments  have  shown  that,  owing  to  capil- 
larity, water  may  permeate  rocks  against  a  high  counter-pressure  of 
steam  on  the  further  side,  and  that  so  long  as  the  water  is  supplied, 
whether  by  minute  fissures  or  through  pores  of  the  rocks,  it  may,  under 
pressiu*e  of  its  own  superincumbent  column,  make  its  way  into  highly 
heated  regions.^  Experience  in  deep  mines,  however,  rather  goes  to  show 
that  the  permeation  of  water  through  the  pores  of  rocks  gets  feebler  as 
we  descend. 

Keference  may  be  made  here  to  a  theory  of  volcanic  action  in  which 
the  influence  of  terrestrial  contraction  as  the  grand  source  of  volcanic 
energy  was  insisted  upon  by  the  late  Mr.  Mallet.^     He  maintained  that 


1 


Daubrt^e,  *  Geologie  Ex  penmen  tale,'  p.  274  (criticised  adversely  by  Fisher,  *  Physics  of 
Earth's  Crust,'  2iid  ed.  p.  144).  Tscherniuk,' cit^d  on  previous  page.  Reyer,  *  Beitrag  nir 
Physik  der  Eruptionen,'  §  L 

'  Phii.     Trans.     1873.       See    also    Daubree's    experimental    deterrolDation    of    the 


SECT,  i  §  5  CAUSES  OF  VOLCANIC  ACTION  267 

all  the  present  manifestations  of  hypogene  action  are  due  directly  to  the 
more  rapid  contraction  of  the  hotter  internal  mass  of  the  earth  and  the 
consequent  crushing  in  of  the  outer  cooler  shell.  He  pointed  to  the 
admitted  difficulties  in  the  way  of  connecting  volcanic  phenomena  with 
the  existence  of  internal  lakes  of  liquid  matter,  or  of  a  central  ocean  of 
molten  rock.  Observations  made  by  him,  on  the  effects  of  the  earth- 
quake shocks  accompanying  the  volcanic  eruptions  of  Vesuvius  and  of 
Etna,  showed  that  the  focus  of  disturbance  could  not  be  more  than  a 
few  miles  deep;  that,  in  relation  to  the  general  mass  of  the  globe,  it 
was  quite  superficial,  and  could  not  possibly  have  lain  under  a  crust  of 
800  miles  or  upwards  in  thickness.  The  occurrence  of  volcanoes  in  lines, 
and  especially  along  some  of  the  great  mountain-chains  of  the  planet,  was 
likewise  dwelt  upon  by  him  as  a  fact  not  satisfactorily  explicable  on  any 
previous  hypothesis  of  volcanic  energy.  But  he  contended  that  all  these 
difficulties  disappear  when  once  the  simple  idea  of  cooling  and  contraction 
is  adequately  realised.  "  The  secular  cooling  of  the  globe,"  he  remarks, 
"  is  always  going  on,  though  in  a  very  slowly  descending  ratio.  Contrac- 
tion is  therefore  constantly  providing  a  store  of  energy  to  be  expended 
in  crushing  parts  of  the  crust,  and  through  that  providing  for  the  volcanic 
heat.  But  the  crushing  itself  does  not  take  place  with  uniformity ;  it 
necessarily  acts  per  solium  after  accumulated  pressure  has  reached  the 
necessary  amount  at  a  given  point,  where  some  of  the  pressed  mass,  un- 
equally pressed  as  we  must  assume  it,  gives  way,  and  is  succeeded 
perhaps  by  a  time  of  repose,  or  by  the  transfer  of  the  crushing  action 
elsewhere  to  some  weaker  point.  Hence,  though  the  magazine  of  volcanic 
energy  is  being  constantly  and  steadily  replenished  by  secular  cooling, 
the  effects  are  intermittent."  He  offered  an  experimental  proof  of  the 
sufficiency  of  the  store  of  heat  produced  by  this  internal  crushing  to  cause 
all  the  phenomena  of  existing  volcanoes.^  The  slight  comparative  depth 
of  the  volcanic  foci,  their  linear  arrangement,  and  their  occurrence  along 
lines  of  dominant  elevation  become,  he  contended,  intelligible  under  this 
hypothesis.  For  since  the  crushing  in  of  the  crust  may  occur  at  any 
depth,  the  volcanic  sources  may  vary  in  (lepth  indefinitely ;  and  as  the 
crushing  will  take  place  chiefly  along  lines  of  weakness  in  the  crust,  it  is 
precisely  in  such  lines  that  crumpled  mountain-ridges  and  volcanic 
funnels  should  appear.  Moreover,  by  this  explanation  its  author  sought  to 
harmonise  the  discordant  observations  regarding  variations  in  the  rate  of 
increase  of  temperature  downward  within  the  earth,  which  have  already 
been  cited  and  referred  to  unequal  conductivity  in  the  crust  (p.  51).  He 
pointed  out  that  in  some  parts  of  the  crust  the  crushing  must  be  much 
greater  than  in  other  parts  ;  and  since  the  heat  "  is  directly  proportionate 

quantity  of  heat  evolved  by  the  iuteraal  crushing  of  rocks.  'Geologie  Experimentale,' 
p.  448.     For  an  adverse  criticism  of  Mallet's  view,  see  Fisher,  op.  cit.  chap.  xxii. 

*  The  elaborate  and  careful  exj)erimeutal  researches  of  this  observer  will  reward  attentive 
perusal.  Mallet  estimates  from  experiment  the  amount  of  heat  given  out  by  the  crushing  of 
different  rocks  (syenite,  granite,  sandstone,  slate,  limestone),  and  concludes  that  a  cubic  mile 
of  the  crust  taken  at  the  mean  density  would,  if  crushed  into  powder,  give  out  heat  enough 
to  melt  nearly  3}  cubic  miles  of  similar  rock,  assuming  the  melting-point  to  be  2000**  Fahr. 


268  DYNAMICAL  GEOLOGY  book  m  paw  i 

to  the  local  tangential  pressure  which  produces  the  crushing  and  the 
resistance  thereto,"  it  niay  vary  indefinitely  up  to  actual  fusioiL  So  long 
as  the  crushed  rock  remains  out  of  reach  of  a  sufficient  access  of 
subterranean  water,  there  would,  of  course,  be  no  disturbance. 
But  if,  through  the  weaker  parts,  water  enough  should  descend 
and  \ye  absorbed  by  the  intensely  hot  crashed  mass,  it  would  be  raised  to 
a  very  high  temperature,  and,  on  sufficient  diminution  of  pressure, 
would  flash  into  steam  and  produce  the  commotion  of  a  volcanic 
eruption. 

This  ingenious  theory  recjuires  the  operation  of  sudden  and  violent 
movements,  or  at  least  that  the  heat  generated  by  the  crushing  should 
be  more  than  can  be  immediately  conducted  away  through  the  crust 
Were  the  crushing  slow  and  equable,  the  heat  developed  by  it  might  be 
so  tranquilly  dissipated  that  the  temi^eraturo  of  the  ciiist  would  not  be 
sensibly  affected  in  the  process,  or  not  to  such  an  extent  as  to  cause  any 
appreciable  molecular  rearrangement  of  the  particles  of  the  rocks.  But 
an  amount  of  internal  crushing  insufficient  to  generate  volcanic  action 
may  have  been  accompanied  by  such  an  elevation  of  temperature  as  to 
induce  important  changes  in  the  structure  of  rocks,  such  as  are  embraced 
under  the  t^rm  "  metamorphic." 

There  is,  indeed,  strong  e\'idence  that,  among  the  consequences  arising 
from  the  secular  contraction  of  the  globe,  masses  of  sedimentary  strata, 
many  thousands  of  feet  in  thickness,  have  been  crumpled  and  crushed, 
and  that  the  crumpling  has  often  l)een  accompanied  by  such  an  amount 
of  heat  and  evolution  of  chemical  acti>dty  as  to  produce  an  interchange 
and  rearrangement  of  the  elements  of  the  rocks, — this  change  sometimes 
advancing  perhaps  to  the  point  of  actual  fusion.  (See  posiea,  p.  298,  and 
Book  IV.  Part  VIII.)  There  is  reason  to  l^elieve  that  some  at  least  of 
these  periods  of  intense  terrestrial  disturbance  have  been  followed  by 
periods  of  prolonged  volcanic  action  in  the  disturl>cd  areas.  Mr.  Mallet's 
theory  is  thus,  to  some  extent,  supported  by  independent  geological  testi- 
mony. The  existence,  however,  of  large  reservoirs  of  fused  rock,  at  a 
comi>aratively  small  depth  beneath  the  surface,  may  be  conceived  as  prob- 
able, a|>art  from  the  effects  of  crushing.  The  connection  of  volcanoes 
with  lines  of  elevation,  and  consequent  weakness  in  the  earth's  crust,  i« 
what  might  have  been  anticipated  on  the  view  that  the  nucleus,  though 
practiciilly  solid,  is  at  such  a  temperatui*e  and  pressure  that  any  diminu- 
tion of  the  pressure,  by  cormgation  of  the  cinist  or  otherwise,  will  cause 
the  subjacent  portion  of  the  nucleus  to  melt.  Along  lines  of  elevation 
the  pressure  is  relieved,  and  consequent  melting  may  take  place.  On 
these  lines  of  weakness  and  fracture,  therefore,  the  conditions  for  volcanic 
excitement  may  be  conceived  to  be  best  developed,  whether  arising  from 
the  explosive  energy  of  water  dissolved  in  the  magma  or  from  water 
descending  to  the  intensely  heated  materials  underneath  the  crust  The 
periodicity  of  eruptions  may  thus  depend  upon  the  length  of  time 
required  for  the  storing  up  of  sufficient  steam,  and  on  the  amount  of 
resistance  in  the  crust  to  be  overcome.  In  some  volcanoes,  the  intervals 
of  activity,  like  those  of  many  geysers,  return  with  considerable  regularity. 


8BCT.  i  §  6  CAUSES  OF  VOLCANIC  ACTION  269 


In  other  cases,  the  shattering  of  the  crust,  or  the  upwelling  of  vast  masses 
of  lava,  or  the  closing  of  subterranean  passages  for  the  descending  water, 
or  other  causes  may  vary  the  conditions  so  much,  from  time  to  time,  that 
the  eruptions  follow  each  other  at  very  unequal  periods,  and  with  very 
discrepant  energy.  Each  great  outburst  exhausts  for  a  while  the  vigour 
of  the  volcano,  and  an  interval  is  needed  for  the  renewed  accumulation 
of  vapour. 

But  beside  the  mechanism  by  which  volcanic  eruptions  are  produced, 
further  problems  are  presented  by  the  varieties  of  materials  ejected,  by 
the  differences  which  these  exhibit  at  neighbouring  vents,  even  some- 
times in  successive  eruptions  from  the  same  vent,  by  the  alternation  or 
recurrence  of  lavas  from  basic  to  acid  in  the  continuance  of  a  single 
volcanic  period,  and  by  the  repetition  of  a  similar  cycle  in  successive 
periods.  Observations  are  yet  needed  from  a  larger  number  of  ancient 
volcanic  districts  and  in  greater  detail,  before  these  problems  can  be 
satisfactorily  discussed  and  solved.  It  is  obvious  that  in  such  a  great 
series  of  eruptions  as  that  of  Central  France,  where  over  a  comparatively 
limited  area  an  alternation  of  basic  and  acid  lavas  has  been  many  times 
repeated,  the  subterranean  magma  must  have  undergone  a  succession  of 
changes  in  composition.  Perhaps  a  definite  cycle  of  such  alternations  may 
be  made  out.  The  sequence  from  basic  to  acid  protrusions,  observable 
among  the  British  Palaeozoic  volcanic  rocks,  is  suggestive  of  a  separation 
of  the  more  basic  constituents  of  the  magma  with  consequent  increasing 
acidity  of  the  residue.  The  earliest  lavas  mark  the  more  basic  condition 
of  the  magma,  while  the  latest  felsite  and  quartz-porphyry  intrusions 
show  its  impoverishment  in  bases  at  the  close  of  a  volcanic  period. 
During  the  interval  before  the  next  period  the  magma  had  in  some  way 
been  renewed,  for  when  eruptions  began  anew  they  were  once  more 
basic.  But  by  the  close  of  the  volcanic  activity  the  magma  had  again 
lost  a  large  proportion  of  its  basic  constituents  and  had  become 
decidedly  acid. 

Reference  has  already  (p.  61)  been  made  to  the  speculation  of 
Durocher  as  to  the  existence  within  the  crust  of  an  upper  siliceous  layer 
with  a  mean  of  71  per  cent  of  silica,  and  a  lower  basic  layer  with  about  51 
per  cent  of  silica.  Bunsen  also  came  to  the  conclusion  that  volcanic  rocks 
are  mixtures  of  two  original  normal  magmas — the  normal  trachytic  (with 
a  mean  of  76*67  silica),  and  the  normal  pyroxenic  (with  a  mean  of  47*48 
silica).  The  varying  proportions  in  which  these  two  original  magmas 
have  been  combined  are,  in  Bunsen's  view,  the  cause  of  the  differences 
of  volcanic  rocks.  We  may  conceive  these  two  layers  to  be  superposed 
upon  each  other,  according  to  relative  densities,  and  the  composition 
of  the  last  material  erupted  at  the  surface  to  depend  upon  the  depth 
from  which  it  has  been  derived.^  The  earliest  explosions  may  be 
supposed  to  have  taken  place  usually  from  the  upper  lighter  and  more 

*  See  R.  Bunsen,  Pogff.  Anti.  Ixiii.  (1851),  p.  204;  Sartorius  von  Waltershauseu, 
'Sicilien  und  Island,'  p.  416  ;  Reyer.  '  Beitragzur  Physik  der  Eruptionen,'  iiL  Scrope  had 
long  before  snggesteil  a  classification  of  volcanic  rocks  into  Trachyte,  Greystone,  and 
Basalt,  Joum.  Sdence^  xxi. 


270  DYNAMICAL  GEOLOGY  book  iii 


siliceous  layer,  and  the  lavas  ejected  would  consequently  be  in  general 
acid,  while  later  eruptions,  reaching  down  to  deeper  and  heavier  zonet 
of  the  magma,  brought  up  such  basic  lavas  as  basalt.  Certainly  the 
general  similarity  of  the  volcanic  rocks  all  over  the  globe  would  appear 
to  prove  that  there  must  be  considerable  uniformity  of  composition  in 
the  zones  of  intensely  hot  material  from  which  volcanic  rocks  are 
derived.^ 

Many  difRculties,  however,  remain  yet  to  be  explained  before  our 
knowledge  of  volcanic  action  can  be  regarded  as  more  than  rudimentary. 
In  Book  IV.  Part  VII.  a  description  is  given  of  the  part  volcanic 
rocks  have  played  in  building  up  what  we  see  of  the  earth's  crust,  and 
the  student  will  there  find  other  illustrations  of  facts  and  deductions 
which  have  been  given  in  the  previous  pages. 

Section  ii.     Earthquakes.^ 

By  the  more  delicate  methods  of  observation  which  have  been 
invented  in  recent  years,  it  has  been  ascertained  that  the  ground  beneath 
our  feet  is  apparently  everywhere  subject  to  continual  slight  tremors  and 
to  minute  pulsations  of  longer  duration.  The  old  expression  "  terra 
firma  "  is  not  only  not  strictly  true,  but  in  the  light  of  modem  research 
seems  singularly  inappropriate.  liapid  changes  of  temperature  and 
atmospheric  pressure,  the  fall  of  a  shower  of  rain,  the  patter  of  birds'  feet, 
and  still  more  the  tread  of  larger  animals,  produce  tremors  of  the  ground 
which,  though  exceedingly  minute,  are  capable  of  being  made  clearly 
audible  by  means  of  the  microphone  and  visible  by  means  of  the  galvano- 
meter. Some  tremors  of  varying  intensity  and  apparently  of  irregular 
occurrence,  may  be  due  to  minute  movements  or  displacements  in  the 
crust  of  the  earth.  Less  easily  traceable  are  the  slow  pulsations  of  the 
crust,  which  in  many  cases  are  periodic,  and  may  depend  on  such  causes 
as  the  diurnal  oscillation  of  the  thermal  or  barometric  conditions  of  the 
atmosphere,  the  rise  and  fall  of  the  tides,  <fec.  So  numerous  and  well 
marked  are  these  tremors  and  pulsjitions,  that  the  delicate  observations 

^  In  the  memoir  by  Captain  Dutton,  cited  iu  a  ])revioiis  note,  the  hypothesis  is  main' 
tained  that  the  order  of  ai>pearance  of  the  lavas  is  determined  by  their  relative  density 
and  fusibility,  the  most  basic  and  heaviest,  though  most  easily  fused,  requiring  fhe 
highest  temperature  to  diminish  their  density  to  such  an  extent  as  to  permit  them  to  be 
enipted. 

-  On  the  phenomena  of  earthquakes  consult  Mallet,  Brit,  Asuoc.  1847,  part  ii.  p.  80; 
1850,  p.  1 ;  1851,  p.  272 ;  1852,  p.  1  ;  1858,  p.  1  ;  1861,  p.  201  ;  'The  Great  Neapolitan 
Earthquake  of  1857,'  2  vols.,  1862  ;  D.  Milne,  Iklin.  New  Phil.  Journ,  zxzi.-xxzvi.  ;  A. 
Perrey,  MSm.  Cvuronn.  Bruxelles,  xviii.  (1844),  Comptes  renduSf  Iii.  p.  146  ;  Otto  Volger, 
*  Untersuchungen  iiber  die  Phanomeue  der  Erdbeben  in  der  Schwelz,'  Gk>tha,  1857-8; 
Z.  JJeiitsch.  Oed.  Ges,  xiii.  p.  667  ;  R.  Falb,  *  Grundziige  einer  Theorie  der  Erdbeben 
und  Vulkanensausbriiche,'  Graz,  1871  ;  *Gedankeu  und  Studien  ttber  den  Vnlkanismns, 
&c.,'  1874:  Pfaflf,  *Allgemeine  Geologie  als  exactc  Wissenschaft,*  I^ipzig,  1873,  p^  224. 
Rcconls  of  observed  earthquakes  will  be  found  in  the  memoirs  of  Mallet  and  Perrey  ;  also 
i>i  pauers  by  Fuchs  iu  Seues  Jahrb.  1865-1871,  and  in  Tschermak's  Mineradog,  MiUheSi- 
1873  and  subsequent  years.     See  also  Schmidts  '  Studien  iiber  Erdbeben,'  2nd  edit 


'ipauers 
'^fc8 


PART  I  SECT,  ii  EARTHQUAKES  271 


which  were  set  on  foot  to  determine  the  lunar  disturbance  of  gravity  had 
to  be  abandoned,  for  it  was  found  that  the  minute  movements  sought  for 
were  wholly  eclipsed  by  these  earth  tremors.^  "* 

The  term  Earthquake  denotes  any  natural  subterranean  concussion, 
varying  from  such  slight  tremors  as  to  be  hardly  perceptible  up  to 
severe  shocks,  by  which  houses  are  levelled,  rocks  dislocated,  landslips 
precipitated,  and  many  human  lives  destroyed.  The  phenomena  are 
analogous  to  the  shock  communicated  to  the  ground  by  explosions  of 
mines  or  powder-works.  They  may  be  most  intelligibly  considered  as 
wave-like  undulations  propagated  through  the  solid  crust  of  the  earth. 
In  Mr.  Mallet's  language,  an  earthquake  may  be  defined  as  "  the  transit 
of  a  wave  of  elastic  compression,  or  of  a  succession  of  these,  in  parallel 
or  intersecting  lines  through  the  solid  substance  and  surface  of  the 
disturbed  country."  Mr.  Milne  has  since  remarked  that  the  disturbance 
may  also  be  due  to  the  transit  of  waves  of  elastic  distortion.  The 
passage  of  the  wave  of  shock  constitutes  the  real  earthquake. 

Besides  the  wave  of  shock  transmitted  through  the  solid  crust,  waves 
su'e  also  propagated  through  the  air,  and,  where  the  site  of  the  impulse 
is  not  too  remote,  through  the  ocean.  Earthquakes  originating  under  the 
sea  are  numerous  and  specially  destructive  in  their  effects.  They  illustrate 
well  the  three  kinds  of  waves  associated  with  the  progress  of  an  earth- 
quake. These  are,  1st,  The  true  earth- wave  through  the  earth's  crust  ; 
2nd,  a  wave  propagated  through  the  air,  to  which  the  characteristic  sounds 
of  rolling  waggons,  distant  thunder,  bellowing  oxen,  &c.,  are  due ;  3rd, 
Two  sea-waves,  one  of  which  travels  on  the  back  of  the  earth-wave  and 
reaches  the  land  with  it,  producing  no  sensible  effect  on  shore ;  the  other 
EUi  enormous  low  swell,  caused  by  the  first  sudden  blow  of  the  earth- wave, 
but  travelling  at  a  much  slower  rate,  and  reaching  land  often  several 
hours  after  the  earthquake  has  arrived. 

Amplitude  of  earth -movements. — The  popular  conception  of  the 

1879;  *Stu(lien  iiber  Vulkane  und  Erdbeben,'  1881  ;  Dieffenbach,  NeuesJahrb.  1872,  p.  155  ; 
M.  S.  di  Rossi,  '  La  Meteorologia  Eudogena,'  2  vols.  1879  and  1882  ;  M.  Gatta,  •  L'ltalia, 
in  Tolcanl  e  terremoti,'  1882;  J.  Milne,  *  Earthquakes  and  other  Earth -movements,' 
1886,  and  his  beautifuUy  illustrated  volume  on  the  Japan  Karthquake  of  October  1891. 
6.  Mercalli,  in  bis  '  Yulcaui  e  Fenomeni  Vulcanici  in  Italia  '  (1883),  gives  an  account  of  the 
Italian  earthquakes  from  1450  b.c.  to  1881  a.d.  ;  he  separately  describes  the  great  Ischian 
earthquake  of  1883  :  *  L'Isola  d'Ischia,'  Milan,  1884.  Much  interesting  information  will  be 
found  in  the  BuUeiino  del  Vulcanismo  ItalianOt  which  began  to  be  published  in  1874  ;  also 
in  the  Transactions  of  the  Setsmological  Society  of  Japan — a  society  instituted  in  the  year 
1880  for  the  investigation  of  earthquake  phenomena,  especially  in  Japan,  where  they  are  of 
freqnent  occurrence.     Others  papers  are  quoted  in  the  following  pages. 

*  A.  d'Abbadie,  *  Etudes  sur  la  verticale,*  1872.  Plantamour,  Comptes  rend.  June  1878, 
February  1881  ;  Archives  Sciences  Phys.  Nat.  Geneva,  iu  p.  641 ;  v.  p.  97  ;  vii.  p.  601  ; 
viiL  p.  551  ;  x.  p.  616  ;  xii.  (1884),  p.  388.  G.  H.  Darwin,  BrU.  Assoc.  1882,  p.  95.  In 
this  paper  Prof.  Darwin  discusses  the  amount  of  disturbance  of  the  vertical  near  the  coasts 
of  continents,  caused  by  the  rise  and  fall  of  the  tide.  J.  Milne,  Trans.  SeisnidtHfical  Sac. 
Japan,  vi.  (1888),  p.  1  ;  Oecl.  Mag.  1882,  p.  482  ;  Naiurey  xxvi.  p.  125.  The  numerous 
Dbaervations  made  by  Rossi  in  Italy  are  summarised  by  G.  Mercalli  in  his  work  cited  above, 
p.  332. 


272  DYNAMICAL  GEOLOGY  book  m 

extent  to  which  the  ground  moves  to  and  fro  or  up  and  down  during  an 
earthquake  is  a  great  exaggeration  of  the  truth.  As  the  result  of  very 
careful  me^urement  ^ith  delicate  instnunents,  there  appears  to  be  reason 
to  believe  that  the  horizontal  motion  at  the  time  of  a  small  earthquake 
is  usually  only  the  fraction  of  a  millimetre,  and  seldom  exceeds  three  or 
four  millimetres.  A\Tien  the  motion  rises  to  five  or  six  millimetres  brick 
and  stone  chimneys  are  shattered.  Yet  even  with  such  an  intensity  of 
shock  a  person  walking  in  an  open  place  might  be  quite  unconscious  of 
any  perceptible  movement  of  the  ground.  The  vertical  motion  also 
appears  to  be  exceedingly  small.  ^ 

Velocity. — Experiments  have  been  made  to  determine  the  velocity 
of  the  earth -wave,  and  its  variation  with  the  natm*e  of  the  material 
through  which  it  is  propagated.  Mi*.  Mallet  found  that  the  shock 
produced  by  the  explosion  of  gunpowder  travelled  at  the  rate  per  second 
of  825  feet  in  sand;  1088  feet  in  schists,  slates,  and  quartzites ;  1306 
feet  in  friable  g!-anite ;  and  1664  feet  in  solid  granite.  General  Abbot, 
by  observing  the  effects  of  the  explosion  of  dynamite  and  gunpowder, 
found  the  velocity  of  transmission  of  the  shock  to  vary  from  1240  to 
8800  feet  per  second,  and  to  be  gi'catest  where  the  shock  is  most  violent.* 
Observations  of  the  time  at  which  an  earthquake  has  successively  visited 
the  different  places  on  its  track  have  shown  similar  variations  in  the  rate 
of  movement.  Thus  in  the  Calabrian  earthquake  of  1857,  the  wave  of 
shock  varied  from  658  to  989  feet  per  second,  the  mean  rate  being  789 
feet.  The  earthquake  at  Vi<Vge  in  1855  was  estimated  to  have  travelled 
northwards  towards  Sti-asburg  at  the  rate  of  2861  feet  per  second,  and 
southwards  towards  Turin  at  a  mtc  of  1398  feet,  or  loss  than  half  the 
northern  speed.  The  earthquake  of  7th  October  1874,  in  northern  Italy, 
travelled  at  rates  varying  from  273  to  874  feet  per  second.  That  of 
12th  March  1873  showed  a  velocity  per  second  of  2734  feet  between 
Ragusa  and  Venice;  4101  feet  from  Spoleto  to  Venice ;  601  feet  from 
Perugia  to  Orvicto ;  1640  feet  from  Perugia  to  Ancona;  and  1640  (or 
2188)  feet  from  Perugia  to  Rome.  The  rate  of  the  central  £ui*opean 
earthquake  of  1872  was  estimated  to  have  been  2433  feet,  that  of 
Herzogenrath,  June  24,  1877,  1555  feet,  that  of  an  earthquake  at 
Travancore,  in  Southern  Hindustan,  656  feet  in  a  second.*  The  most 
accurate  metvsuremcnts  and  computiitions  of  the  velocity  of  earthquake 

^  Milne. '  Rirth quakes,'  pp.  75,  76.  An  ingenious  nicxlel  in  wire  has  been  made  by  Prof. 
Sekiya  to  illustrate  the  highly  complex  i>nth  pursued  by  a  particle  on  the  surface  of  the 
jjrouujl  duriug  an  earthquake  at  Tokio,  Japan,  on  15th  January  1887. 

'^  Avfi'.  Jmirn,  Sci.  xv.  (1878).  l*rof.  J.  Milne,  exj)erimenting  in  Jaium,  has  likewise 
ascertaineil  that  a  close  relation  exists  between  the  initial  violence  of  the  shock  and  the 
velocity  of  propagation,  and  that  there  is  a  progressive  diminution  in  speed  as  the  ware  of 
sho<;k  travels  outward  from  tlic  centre  of  disturbance.     'Earthquakes,'  p.  65. 

»  K.  von  Seebach,  *Das  M  itteldeutsche  Erdbel)en  von  6  Marz,  1872,'  Leipzig,  1873. 
Hiifer,  SU^Jb.  Akad.  Wien,  Dec.  1876  ;  A.  von  Lasaulx,  'Das  Erdbeben  von  Herzogenrath, 
22ud  Oct.  1873,'  Bonn,  1874.  *Das  Erdbeben  von  Herzogenrath,  24  Juni,  1877/ Bonn, 
1878.  G.  C.  Laube,  on  Earth<iuake  of  31st  January  1883,  at  Trautenau,  Jahrb.  Otol,  lUidu. 
1883,  p.  331.  11.  Credner  on  the  Earthquakes  of  the  Erzgebirge  and  Vogtland  from  1878 
to  1884,  ZeitHch.  fiir  Xattirtnss.  vol.  Ivii.  (1884).     F.  Wahner,  on  Agram  earthquake  of  9 


PART  I  SECT,  ii  EARTHQUAKES  273 

movements  are  probably  those  made  by  Prof.  J.  Milne  and  his  associates 
in  Japan.  The  rates  of  movement  diu-ing  the  Tokio  earthquake  of 
25th  October  1881  are  estimated  to  have  ranged  between  4000  and 
9000  feet  per  second.  As  the  result  of  prolonged  observation,  Prof. 
Milne  concludes  that  "different  earthquakes,  although  they  may  travel 
across  the  same  country,  have  very  variable  velocities,  varying  between 
several  hundreds  and  several  thousands  of  feet  per  second ;  that  the 
same  earthquake  travels  more  quickly  across  districts  near  to  its  origin 
than  it  does  across  districts  which  are  far  removed ;  and  that  the  greater 
the  intensity  of  the  shock,  the  greater  is  the  velocity."  ^ 

Duration.  —  The  niunber  of  shocks  in  an  earthquake  varies  in- 
definitely, as  well  as  the  length  of  the  intervals  between  them.  Some- 
times the  whole  earthquake  only  lasts  a  few  seconds :  thus  the  city  of 
Caracas,  with  its  fine  churches  and  10,000  of  its  inhabitants,  was 
destroyed  in  about  half-a- minute ;  Lisbon  was  overthrown  in  five 
minutes.  But  a  succession  of  shocks  of  varying  intensity  may  continue 
for  days,  weeks,  or  months.  The  Calabrian  earthquake,  which  began  in 
February  1783,  was  continued  by  repeated  shocks  for  nearly  four  years 
until  the  end  of  1786. 

Modifying  influence  of  geologrical  structure.  —  In  its  passage 
through  the  solid  terrestrial  crust  from  the  focus  of  origin,  the  earth- 
wave  must  be  liable  to  continual  deflections  and  delays,  from  the  varying 
geological  structure  of  the  rocks.  To  this  cause,  no  doubt,  must  be  in 
large  measure  ascribed  the  marked  differences  in  the  rate  of  propagation 
of  the  same  earthquake  in  different  directions.  The  wave  of  disturbance, 
as  it  passes  from  one  kind  of  rock  to  another,  and  encounters  materials 
of  very  different  elasticity,  or  as  it  meets  with  joints,  dislocations,  and 
curvatures  in  the  same  rock,  must  be  liable  to  manifold  changes  alike  in 
rate  and  in  direction  of  movement.  Even  at  the  surface,  one  effect  of 
differences  of  material  may  be  seen  in  the  apparently  capricious  demo- 
lition of  certain  quarters  of  a  city,  while  others  are  left  comi>aratively 
scatheless.  In  such  cases,  it  has  often  been  found  that  buildings  erected 
on  loose  inelastic  foundations,  such  as  sand  and  clay,  are  more  liable  to 
destruction  than  those  placed  upon  solid  rock.  In  illustration  of  this 
statement  the  accompanying  plan  (Fig.  71)  of  Port  Royal,  Jamaica,  was 
given  by  De  la  Beche  -  to  show  that  the  portions  of  the  town  which  did 
not  disappear  during  the  earthquake  of  1692  were  built  upon  solid  white 
limestone,  while  the  parts  built  on  sand  were  shaken  to  pieces.^ 

It  has  been  observed  that  an  earthquake  shock  ^vill  pass  under  a 
limited  area  without  disturbing  it,  while  the  region  all  around  has  been 
affected,  as  if  there  were  some  superficial  stratum  protected  from  the 

Nov.  1880,  SUz,  Akad.  Wiefif  Ixxxviii.  (1883),  p.  15.     Di  Rossi,  '  Meteorologia  Endogena,* 
L  p.  306  ;  P.  Serpieri,  InstitiUo  LombardOj  1873. 

*  *  Earthquakes,'  p.  94. 

*  'Geological  Observer,'  p.  426. 

'  The  opposite  effect  has  been  observed  on  tlie  island  of  Ischia,  the  houses  built  on  loose 
subsoil  generaUy  havjng  suffered  much  less  than  the  others.  There  appears,  indeed,  to  be  a 
considerable  conflict  of  testimony  on  this  subject.     See  Milne,  *  Earthquakes,'  p.  130. 

T 


374  nVNAMICAL  GEOLUGY  book  ui 

earth-wave.  Humboldt  cited  a  caae  wheie  miners  were  driven  up  from 
below-gi-ound  by  earthquake  shocks  not  i>erce])tible  at  the  surface,  and  on 
the  other  hand,  an  instance  where  they  expei-ieiiced  no  Bensation  (rf  ui 
efiithquake  which  shook  the  surface  with  considerable  \-iolence.'    Sudi 


Fii:.  ri.— I-laD  i>r  Fort  RoyBl,  Juiiuk's.  nlxin-iMx  tlic  effrctH  ut  Uii'  Eiutbcia*kE  uT  lnoi  (a> 

facts  bring  impressively  l^efure  the  mind  the  extent  to  which  the  coiine 
of  the  earth-wave  nuist  be  modified  by  geological  structure.  In  some 
instances,  the  shuck  extends  outwards  from  a  common  centre,  so  that  « 
series  of  concentric  circles  may  be  drawn  round  the  foais,  each  of  which 
will  denote  a  cei-tain  a]>pi-oximately  unifonu  intensity  of  shock  ("coaeinnic 
lines  "  of  Mallet),  this  intensity,  of  course,  diminishing  with  distance  from 
the  focus.  The  C'alabriau  eai-thquake  of  1857  and  that  of  Central  Europe 
in  1872,  may  ))o  taken  in  itiuiitration  of  thii>  central  type.  In  other 
cases,  however,  the  earth<|iiake  travels  chiefly  along  a  certain  Iwind  or 
zone  (particularly  along  the  flanks  of  a  niouiUain-chain)  without  advanc- 
ing far  from  it  latenilly.  This  tyi)e  of  linear  earthtpuike  is  exemplified 
by  the  freijuent  shocks  which  ti-averse  Chili,  Pern,  and  Ecuador,  between 
the  line  of  the  Andes  an<l  the  Pacific  coaat.- 

Extent  of  country  afTected.— The  area  shaken  by  an  earthquake 
ViH-icH  with  the  intensity  of  the  shoik,  from  a  mere  local  ti-act  where  a 
slight  tremor  has  lieen  experienced,  up  to  such  catastrophes  as  that  of 
Lisbon  in  17."iij,  which,  besides  convulsing  the  Portuguese  coasts,  extended 
into  the  north  of  Africa  on  the  one  hand  and  to  Scandinavia  on  the  other, 
and  was  even  felt  as  far  as  the  east  of  Xoith  America.  Humboldt  com- 
puted that  the  urea  shaken  by  this  great  earthquake  was  foiu*  times 
greater  than  that  of  the  whole  of  Eurojie.  The  South  Amencan  earth- 
({uakes  are  remarkable  for  the  gi-eat  distances  to  which  their  effects 
extend  in  a  linear  directioiu  Thus  the  strip  of  country  in  Peru  and 
Ecuador  severely  shaken  hy  the  earthquake  of  I8C8,  had  a  length  of 
aOOO  miles. 

'  'CoiCHOs,'  Art.  Earlhquakti. 

■  For  a  lint  oF  Peruvian  eartliqiiakcs  from  a.D.  1570  ta  1875,  Ke  Oeograph.  Mag.  i<r. 
(1877),  ji.  208.  The  enrtliquake  of  »  Slny,  1877,  at  Iqiiique,  aud  ita  oc^n-wsve  ue  dsKriM 
by  E.  GeiniU,  JVoki  Ad.  Ac.  Cat.  LeepoIJ.  Car.  xl.  <187S),  pp.  383-444. 


PABTisBCT.  ii  EARTHQUAKES  276 

Depth  of  source, — Accordii^  to  Mallet's  observations,  over  the 
centre  of  origin  the  shock  is  felt  as  a  vertical  up-and-down  movement 
{SeismK  vetiicat) ;  while,  receding  from  this  centre  in  any  direction, 
it  is  felt  as  an  undulatory  movement,  and  comes  up  more  and  more 
obliquely.  The  angle  of  emergence,  as  he  termed  it,  was  obtained  by  him 
by  taking  the  mean  of  observations  of  the  rente  and  displacements  of 


walls  and  buildings.  In  Fig.  72,  for  example,  the  wall  there  represented 
has  been  rent  by  an  earthquake  which  emerged  to  the  surface  in  the  path 
marked  by  the  arrow. 

By  observations  of  this  nature.  Mallet  estimated  the  approximate 
depth  of  origin  of  an  earthquake.  -Let  Fig.  73,  for  example,  represent 
a  portion  of  the  eai-th's  crust  in  which  at  a  an  caithquake  arises.     The 


wave  of  shock  will  travel  outwards  in  successive  spherical  shells.  At 
the  point  e  it  yn\l  be  felt  as  a  vertical  movement,  and  loose  objects, 
such  as  paving-stones,  may  be  jerked  up  into  the  air,  and  descend  bottom 
uppermost  on  their  previous  sites.  At  rf,  however,  the  wave  will  emerge 
at  a  lower  angle,  and  will  give  rise  to  an  undulation  of  the  ground,  and 
the  oscillation  of  objects  projecting  above  the  surface.  In  rent  buildings, 
the  fissures  will  be  on  the  whole  perpendicular  to  the  path  of  emergence. 
By  a  series  of  observations  made  at  different  points,  as  at  ?  and  /,  a 
number  of  angles  are  obtained,  and  the  point  where  the  various  lines 
cut  the  vertical  (a)  will  mark  the  area  of  origin  of  the  shock.     By  this 


276  PYX  A  MICA  L  GEOLOGY  bookih 


means,  Mallet  computed  that  the  depth  at  which  the  impulse  of  the 
Calabrian  earthquake  of  1857  was  given  was  about  five  miles.  As  the 
general  result  of  his  inquiries,  he  concluded  that,  on  the  whole,  the  origin 
of  carthfjuakes  must  be  sought  in  comparatively  superficial  parts  of  the 
crust,  probably  never  exceeding  a  depth  of  30  geographical  mileB. 
Following  another  method  of  calculation,  Von  Seebach  computed  tliat 
the  earthquake  which  affected  Central  Euroi)e  in  1872  originated  at  a 
depth  of  9 '6  geographical  miles;  that  of  Belluno  in  the  same  year  wu 
estimated  by  Hofer  to  have  had  its  source  rather  more  than  4  miles  deep; 
while  that  of  Hcrzogenrath  in  1873  was  placed  by  Von  Lasaulx  at  i 
depth  of  a1x)ut  14^  miles,  and  that  of  1877  in  the  same  region  at  about 
14  miles. ^ 

Geologrieal  EflTects. — These  are  dependent  not  only  on  the  strsngli 
of  the  concussion  but  on  the  structure  of  the  ground,  and  on  the  site  of 
the  disturbance,  whether  underneath  land  or  sea.  They  include  ^^IwmgM 
superinduced  on  the  surface  of  the  land,  on  terrestrial  and  oceanic  wiUl% 
and  on  the  relative  levels  of  land  and  sea. 

1.  Effects  upon  the  soil  and  general  surface  of  a  country. — 
The  earth- wave  or  wave  of  shock  underneath  a  country  may  timveiie  a 
wide  region  and  atfect  it  violently  at  the  time,  without  leaving  permaiient 
traces  of  its  i>assage.  Blocks  of  rock,  however,  already  disengaged  from 
their  jxirent  masses  on  declivities,  may  be  rolled  down  into  the  vmUeyn 
liiindslips  are  ])roduced,  which  may  give  rise  to  considerable  sufasequent 
changes  of  drainage.  In  some  instances,  the  siuiaces  of  solid  rocks  are 
shattered  as  if  by  ginipowder,  as  was  i>articularly  noticed  to  have  taken 
place  among  the  Primary*  rocks  in  the  Concepcion  earthquake  of  1835.^ 
It  has  nfteu  been  observed  also  that  the  soil  is  rent  by  fissures  vhidi 
vary  in  size  from  mere  cracks,  like  those  due  to  desicciition,  up  to  chasms 
a  mile  or  more  in  length  and  200  feet  or  more  in  depth.  Permanent 
ni<Hliticatii»ns  of  the  landscai>e  may  thus  be  produced.  Trees  are  thrown 
down,  and  buried,  wholly  or  in  jwrt,  in  the  rentes.  These  superficial 
ctlects  may,  indeoil,  be  soon  ettaced  by  the  levelling  jwwer  of  the  atmo- 
spheric. \\'here,  however,  the  chasms  are  wide  and  deep  enough  to 
intercept  rivulets,  or  to  serve  as  channels  for  heavy  rain-torrents,  they 
are  sometime**  further  excavated,  so  as  to  become  graduidly  enlarged  into 
ravines  and  valleys  i^s  has  hapiKMied  in  the  case  of  renU  caused  by  the 
earth«iuakos  of  ISll-l-,  in  the  Mississippi  valley.  In  the  earthquake 
whii'h  sluH^k  the  Si^uth  Island  i*f  New  Zealand  in  1848,  a  fissure  was 
formed,  averairiuir  IS  inches  in  width  and  traceable  for  a  distance  of  60 
miles  jKirallel  to  the  axis  of  the  adjacent  mountain-chain.  The  subsequent 
eartln|uake  of  L^T*.""*.  in  the  same  region,  gjive  rise  to  a  fnicture  which 
could  l»e  tractxl  along  the  Ivise  of  a  line  of  cliff  for  a  distance  of  about 
00  miles.  l>r.  i.>klham  has  desi-rilnxi  a  remarfcible  series  of  fissurings 
which    ran    jKU-allel    with    the    river  oi  Calhar,   Eastern    British  India, 

■  Siv  i..'.iH'r'»  ^^y  H  -hr  .uul  A,  von  Uln^iuIx.  i:;u»»l  i  xi  y.  1*72.  For  an  acconnt  of  the 
\.iri«v;<  n;r:]uM>  i*::'.vli\\otl  i;i  «>:iii;:i:iiii:  iho  iUj»:h  of  orijiin  of  earthquakes,  see  Milne'f 
•  H.ir:::,|U.iki>/  iliaj'trVN  x.  aiiil  \i.     iVnsul:  «l>o  ::se  T:\:r.s.  Sdsmolog,  Soc  Japan. 

-  D.iiwra.  "Journal  of  Keseaiv-ho,"  154.\  ]»,  30?. 


CI.  ii 


EARTHQUAKES 


277 


raryjng  with  it  to  every  point  of  the  compass  and  traceable  for  100 
niles.'  The  great  Japanese  earthquake  of  26tb  October  1891  gave  rise 
jO  Bome  remarkable  fractures  of  the  ground,  in  one  of  which  one  eide 
iras  placed  pei-raanently  at  a  different  level  from  the  other  (Fig.  74). 


fig.  7<.-Ft*iur« 


It«m&rkable  circulai*  cavities  have  1>een  noticed  in  Ciilabria  mid 
ilsewhere,  formed  in  the  ground  during  the  passjige  of  the  earth-wave. 
!n  many  cases,  these  holes  serve  as  funnels  of  escape  for  an  abundant 
luch&rgo  of  water,  so  that  when  the  disturbance  ceases  they  ap[)ear  an 
x>ole.  They  are  beliei-ed  to  be  ttiused  by  the  sudden  collapse  of  aul)- 
crranean  water-channels  and  the  conse<]uent  forcible  ejection  of  the  water 
o  the  surface.  Besiiles  water,  discharges  of  various  gases  and  vapours, 
lometimes  combustible,  have  Iwen  noted  .it  the  fissures  formed  during 
tarthquakes. 

2.  Effects  upon  terrestrial  waters.^ — Springs  are  temporarily 
ifTected  by  earthquake  movements,  becoming  greater  or  smaller  in 
volume,  sometimes  muddy  or  discoloured,  and  sometimes  increasing 
n  temperature.  Brooks  and  rivers  have  been  observed  to  flow  with  an 
ntemipted  course,  incre;tsing  or  diminishing  in  size,  stopping  in  their 
low  so  as  to  leave  their  chaimels  dry,  and  then  rolling  forward  with 
ncreased  rapidity.  Lakes  are  still  more  sensitive.  Their  waters 
wcasionally  rise  and  fall  for  several  hours,  even  at  a  distance  of  many 
lundred  miles  from  the  centre  of  disturlmncc.     Thus,  on  the  day  of  the 

■  Q.  J.  Otol.  ,4^.  xxviii.  ]:.  -.'57.      For  a  catalogue  of  Inilian  Eiuthitiiiikes  ilown  to  tlm 
ltd  Of  ISeS,  Ke  T.  OUllinm,    Mem.  f;r.il.  .•inn:  India,  lii.  imrt  -2. 
'  Klnge,  StHttJukrb.  18B1,  ]..  777. 


278  nYXAMICAL  GEOLOGY  BOOKm 

great  Lisbon  earthquake,  many  of  the  lakes  of  central  and  north-western 
Europe  were  so  aflected  as  to  maintain  a  succession  of  waves  rising  to  a 
height  of  2  or  3  feet  above  their  usual  level.  Cases,  however,  have  been 
observed  where,  owing  to  excessive  subterranean  movement,  lakes  have 
been  emptied  of  their  contents  and  their  beds  have  been  left  permanentlj 
dry.  On  the  other  hand,  areas  of  dry  ground  have  been  depressed,  and 
have  become  the  sites  of  new  lakes. 

Some  of  the  most  important  changes  in  the  fresh  water  of  a  region, 
however,  are  produced  by  the  fall  of  masses  of  rock  and  earth,  which, 
by  damming  up  a  stream,  may  so  arrest  its  water  as  to  form  a  lake.    If 
the  barrier  be  of  sufRcient  strength,  the  lake  will  be  permanent ;  thou^ 
from  the  usually  loose,  incoherent  character  of  its  materials,  the  dam 
thrown  iicross   the  pathway  of  a  stream   runs  a   great  risk  of  being 
undermined  by  the  percolating  water.      A  sudden  giving  way  of  tfce 
barrier  allows   the  confined   water  to  nish  >\'ith  great  violence  down 
the  valley,  and  to  produce  perhaps  tenfold  more  havoc  there  than  may 
have  been  caused  by  the  original  earthquake.      When  a  landslip  is  d 
sutficient  dimensions  to  divert  a  stream  from  its  previous  course,  the 
new  channel  thus  taken  may  become  permanent,  and  a  valley  may  be 
cut  out  or  ^Wdcned. 

3.  Effects  upon  the  sea. — The  gi*eat  sea- wave  propagated  outward 
from  the  centre  of  a  sub-oceanic  earthquake  and  reaching  the  land  after 
the  earth-wave  has  arrived  there,  gives  rise  to  much  destruction  along  the 
maritime  parts  of  the  disturbed  region.  AVhen  it  approaches  a  low  shcve, 
the  littoral  waters  retreat  seawards,  sucked  up,  as  it  were,  by  the  advano- 
iug  wall  of  water,  which,  reaching  a  height  of  sometimes  60  feet  or  more, 
rushes  over  the  bare  beach  and  sweeps  inland,  carrying  with  it  everything 
which  it  can  dislodge  and  bear  away.  Loose  blocks  of  rock  are  thus 
lifted  to  a  considerable  distance  from  their  former  position,  and  left  at  a 
higher  level.  Deposits  of  sand,  gravel,  an<l  other  superficial  accumula- 
tions are  torn  up  and  swept  away,  while  the  sm*face  of  the  country,  as  far 
as  the  limit  reached  by  the  wave,  is  strewn  with  debris.  If  the 
district  has  been  already  shattered  by  the  passage  of  the  earth- 
wave,  the  advent  of  the  great  sea-wave  augments  and  completes  the 
devastation.  The  havoc  caused  by  the  Lisbon  earthquake  of  1755,  and 
by  that  of  Peni  and  Ecuador  in  1868,  was  much  aggi^vated  by  the  co- 
oi)eration  of  the  oceanic  wave.  Where  the  wave  breaks  on  land  rising 
out  of  deep  water  little  damage  may  be  done. 

4.  Permanent  changes  of  level. — It  has  been  observed,  after  the 
pasirage  of  an  earthquake,  that  the  level  of  the  disturbed  country  has 
sometimes  been  changed.  Thus  after  the  terrible  earthquake  of  19th 
November  1822,  the  coast  of  Chili,  for  a  long  distance,  was  found  to 
have  risen  from  3  to  4  feet,  so  that  along  shore,  littoral  shells  were 
exposed  still  adhering  to  the  rocks,  amid  multitudes  of  dead  fish.  The 
same  coast-line  has  been  further  upraised  by  subsequent  earthquake 
shocks.  •  On  the  other  hand,  many  instances  have  been  observed  where 
the  effect  of  an  earthquake  has  been  to  depress  permanently  the  disturbed 
gi-ound.     For  example,  by  the  Bengal  earthquake  of  1762,  an  area  of  60 


PART  I  SECT,  ii  EARTHQUAKES  279 

square  miles  on  the  coast  near  Chittagong,  suddenly  went  down  beneath 
^he  sea,  leaving  only  the  tops  of  the  higher  eminences  above  water. 
rhe  succession  of  earthquakes  which  in  the  years  1811  and  1812 
levastated  the  basin  of  the  Mississippi,  gave  rise  to  widespread  depressions 
)f  the  ground,  over  some  of  which,  above  alluded  to,  the  river  spread  so 
IS  to  form  new  lakes,  with  the  tops  of  the  trees  still  standing  above  the 
nirface  of  the  water. 

Distribution  of  Earthquakes.^ — While  no  large  space  of  the  earth's 
(orface  seems  to  be  free  from  at  least  some  degree  of  earthquake-move- 
nent,  there  are  regions  more  especially  liable  to  the  visitation.  As  a 
•ule,  earthquakes  are  most  frequent  in  volcanic  districts,  the  explosions 
)f  a  volcano  being  generally  preceded  or  accompanied  by  tremors  of 
preater  or  less  intensity.  In  the  Old  World,  a  great  belt  of  earthquake 
listurbance  stretches  in  an  east  and  west  direction,  along  that  tract  of 
'emarkable  depressions  and  elevations  lying  between  the  Alps  and  the 
nountains  of  northern  Africa,  and  spreading  eastward  so  as  to  enclose 
.he  basins  of  the  Mediterranean,  Black  Sea,  Caspian,  and  Sea  of  Aral, 
kod  to  rise  into  the  great  mountain-ridges  of  Central  Asia.  In  this  zone 
ie  numerous  volcanic  vents,  both  active  and  extinct  or  dormant,  from  the 
V^ores  on  the  west  to  the  basaltic  plateaux  of  India  on  the  east.  The 
Pacific  Ocean,  surrounded  with  a  vast  ring  of  volcanic  vents,  has  its 
)orders  likewise  subject  to  frequent  earthquake  shocks.  Some  of  the 
nost  terrible  earthquakes  within  human  experience  have  been  those 
rhich  have  affected  the  western  seaboard  of  South  America.  2  It  is  worthy 
rf  notice  that  the  coasts  of  the  Pacific  Ocean  more  specially  liable  to  con- 
nilsions  of  this  nature  plunge  steeply  down  into  deep  water  with  slopes 
rf  one  in  twenty  to  one  in  thirty,  while  shore -lines  such  as  those  of 
.^.ostralia,  Scandinavia,  and  the  east  of  South  America,  where  the  slope 
s  no  more  than  from  one  in  fifty  to  one  in  two  hundred  and  fifty,  are 
aardly  ever  affected  by  earthquakes.  It  should  also  be  remarked  that 
^hile  earthquakes  are  apt  to  occur  along  the  flanks  of  mountain-chains  and 

*  For  European  earthquakes  an  alphabetical  catalogue  has  been  compiled  bj'  Professor 
D'Reilly,  Trans.  Roy.  Inah  Aatdemy,  xxviii.  (1886),  p.  489.  Catalogue  of  British  earth- 
laakes,  op.  cit.  xxviii.  (1884),  p.  285.  C.  Davidson,  Geol.  May.  1891,  p.  450.  Qxiart.  Jour. 
Geci.  Snc.  xlvii.  (1891),  p.  618.  Detailed  observations  of  the  effects  of  some  recent  European 
uirthqaakes  will  be  found  in  the  following  Memoirs.  The  Andalusian  earthquake  of  25th 
Dec  1884,  T.  Taramelli  and  G.  Mercalli,  Real.  Accad.  Lincei,  1885-86,  p.  116,  Hebert, 
Campt.  rend.  1885,  Fouque,  ihid.  20th  April  1885,  and  the  large  quarto  volume  of  reports 
by  the  mission  specially  sent  to  study  the  phenomena  of  this  earthquake,  MSmoircs  Ac/ui. 
SeL  1889  ;  the  Ligurian  earthquake  of  23rd  Feb.  1887,  T.  Taramelli  and  G.  Mercalli,  Ann. 
Uficw  Vent  rale  Meteordoy.^ih'OiUnam.  part  iv.  vol.  viii.  (1888),  Real.  Accail.  Lincei,  iv. 
[1888)  ;  the  Agram  earthquake  of  9th  Nov.  1880,  *Grundzuge  der  Abyssodjiiamik,  &c.,' 
by  8.  Pilar,  Agrara,  1881  ;  the  middle  German  earthquake  of  6th  March  1872,  *Das 
Mitteldeutsche  Erdbeben  von  6  Marz  1872.'  by  K.  von  Seebach,  Leipzig,  1873.  See  also  the 
papers  cited  on  pp.  270-273. 

'  The  Charleston  Earthquake  of  31st  August  1886,  has  been  fully  discussed  by  Captain 
Dutton,  yinth  Ann.  Rejtort  V.  S.  Oeol.  Survey,  1887-88,  p.  209.  The  earthquakes  of 
Central  America  are  discussed  by  F.  de  Montessus  de  Ballore  in  a  Memoir  rewarded  by  the 
Acad.  Sci.  Nat.  Saone  et  Loire,  and  published  at  Dijon,  1888. 


PART  I  SECT,  iii  UPHEAVAL  AND  DEPRESSION  281 

conceivably  affect  mountainous  areas ;  but  we  do  not  know  how  it 
would  affect  the  sea-floor.  In  mountainous  districts,  many  different 
degrees  of  shock,  from  mere  tremors  up  to  imporUmt  earthquakes,  have 
been  observed,  and  these  are  not  improbably  due  to  sudden  more  or  less 
extensive  fractures  of  rocks  still  under  great  strain.^  Hoernes,  from  a 
study  of  European  earthquake  phenomena,  concludes  that  though  some 
minor  earth-tremors  may  be  due  to  the  collapse  of  underground  caverns, 
and  others  of  local  character  to  volcanic  action,  the  greatest  and  most 
important  earthquakes  are  the  immediate  consequences  of  the  formation 
of  mountains,  and  he  connects  the  lines  followed  by  earthquakes  >nth  the 
structural  lines  of  mountain-axes.'-^ 

From  what  was  stated  at  the  beginning  of  the  present  section,  it  is 
evident  that  where  the  earth's  crust  in  any  region  is  in  a  critical  condition 
of  equilibrium,  some  connection  may  be  exi)ected  to  l^e  traceable  between 
the  frequency  of  earthquakes  and  the  earth's  p>osition  with  regard  to  the 
moon  and  sun,  on  the  one  hand,  and  changes  of  atmospheric  conditions, 
on  the  other.  ^  A  comparison  of  the  dates  of  recorded  earthquakes  seems 
to  bear  out  the  following  conclusions;  1st.  An  earthijuake  maximum 
occurs  about  the  time  of  new  moon  ;  2nd.  Another  maximum  appears 
two  days  after  the  first  quarter  .•  3rd.  A  diminution  of  activity  occurs 
about  the  time  of  full  moon  ;  4th.  The  lowest  earthquake  minimum  is  on 
the  day  of  the  last  quarter.^  There  is  likewise  observable  a  seasonal 
maximum  and  minimum,  earthquakes  over  most  of  the  northern  hemi- 
sphere occurring  most  frequently  in  winter,  and  least  frequently  in 
summer.'*  Out  of  656  earthquakes  chronicled  in  France  up  to  the  year 
1845,  three-fifths  took  place  in  the  winter,  and  two-fifths  in  the  summer 
months.  In  Switzerland  they  have  been  observed  to  be  alx)ut  three  times 
more  numerous  in  winter  than  in  summer.  The  same  fact  is  remarked 
in  the  history  even  of  the  slight  earthquakes  in  Britain.  A  daily  maxi- 
mum appears  to  occur  about  2.30  A.M.,  and  a  minimum  a>K)Ut  three- 
quarters  of  an  hour  after  noon.  No  connection  has  yet  been  satisfactorily 
established  between  the  oecuiTence  of  earthquakes  and  sun-spots.  The 
greater  frequency  of  earth<iuake8  in  winter  might  be  exi)ected  to  indicate 
a  relation  between  their  occurrence  and  atmospheric  pressure,  and 
possibly  earthquakes  are  more  fre<iuent  with  a  low  than  with  a  high 
barometer.^ 


Section  iii.     Secular  Upheaval  and  Depression. 

Besides  scarcely  perceptible  tremors  and  more  or  less  violent  move- 
ments due   to   earthquake -shocks,  the   crust  of  the   earth   is  generally 

*  See  poateut  p.  311.     Suess,  'Entstehung  der  Alpen,*  Vienna,  1875. 
'  "Erdbeben  Studien,"  Jahrb.  Geof.  RficJi.s.  xxviii.  (1878),  p.  448. 

'  J.  F.  J.  Schmidt,  "Studien  iiber  ErdbeWn,"  2ud  ed.  (1879),  p.  18. 

*  Jln<i.  p.  20.     See  the  work.s  of  Perrey  cite<l  on  p.  270. 

*  Schmidt,  op.  cit.  j),  23.  F.  Griiger,  Seues  Jahrh.  1878,  p.  928.  There  does  not 
appear  to  be  any  marked  connection  between  the  state  of  the  barometer  and  the  occurrence 
of  earthquakes  in  Japan — J.  Milne,  *  Earthquake^,'  p.  268. 


282  DYXAMICAL  GEOLOGY  BOOKin 


believed  to  undergo  in  many  places  oscillations  of  an  extremely  quiet  and 
uniform  character,  sometimes  in  an  upm'ard,  sometimes  in  a  downward 
direction.  So  tranquil  may  these  changes  be,  as  to  produce  from  day  to 
day  no  appreciable  alteration  in  the  aspect  of  the  ground  affected,  so  that 
only  after  the  lapse  of  several  generations,  and  by  means  of  careful 
measurements,  can  they  reiilly  be  proved.  Indeed,  in  the  interior  of  a 
country  nothing  but  a  series  of  accurate  levellings  from  some  unmoved 
datum-line  might  detect  the  change  of  level,  unless  the  effects  of  the 
terrestrial  disturbance  showed  themselves  in  altering  the  drainaga  Only 
along  the  sea-coast  is  a  ready  measure  afforded  of  any  such  movement 

It  is  customary  in  p>opular  language  to  speak  of  the  sea  rising  or 
falling  relatively  to  the  land.  We  cannot  conceive  of  any  possible 
augmentation  of  the  oceanic  waters,  nor  of  any  diminution,  save  what 
may  be  due  to  the  extremely  slow  processes  of  abstraction  by  the  hydrar 
tion  of  minerals  and  absorption  into  the  earth  s  interior.  Any  changes, 
therefore,  in  the  relative  levels  of  sea  and  land  must  be  due  to  some  re- 
adjustment in  the  form  either  of  the  solid  globe  or  of  its  watery  envelope 
or  of  l)oth.  Playfair  argued  at  the  Iwginning  of  this  century  that  no 
sulisidence  of  the  sea-level  could  l)e  local,  but  must  extend  over  the  globc.^ 
But  it  is  now  recognised  that  what  is  called  the  sea-level  cannot  possess  the 
uniformity  formerly  attributed  to  it ;  that  on  the  contrary  it  must  be  liable 
to  lociil  distortion  from  the  attractive  influence  of  the  land.  Not  only  so, 
but  the  level  of  the  surface  of  large  inland  sheets  of  water  must  be 
affected  by  the  suiTounding  high  lands. 

Mr.  R.  S.  AVoodward,  whose  lecent  memoir  on  this  subject  has  been 
cited  (p.  35)  has  calculated  that  in  a  lake  HO  miles  broad  and  1000 
feet  deep  in  the  middle,  the  diflerence  of  level  of  the  water-surface  at 
the  centre  and  at  the  margin  may  amount  to  between  three  and  four 
feet.-  As  already  stated  he  has  further  computed  that  the  effect  of  the 
continents  of  Europe  and  Asia  at  the  centre  in  disturbing  the  sea-level 
must  amount  to  about  2900  feet,  if  we  suppose  that  there  is  no  deficiency 
of  density  underneath  the  continent,  and  to  only  about  10  feet  if  we 
suppose  that  the  very  existence  of  the  continent  implies  such  a  deficiency.* 

Various  suggestions  have  been  made  regiirding  possible  causes  of 
alteration  of  the  sea-level.  (1)  A  shifting  of  the  present  distribution 
of  density  within  the  nucleus  of  the  planet  would  aflect  the  position  and 
level  of  the  oceans  (anfey  p.  47).  (2)  As  permanent  snow  and  ice 
represent  so  much  removed  from  the  general  Ixxly  of  water  on  the  globe^ 
any  large  increase  or  diminution  in  the  extent  and  thickness  of  the  polar 
ice-caps  must  cause  a  corresponding  variation  in  the  sea-level  {anky 
p.  20).  (3)  A  change  in  the  earth's  centre  of  gravity,  such  as  might 
result  from  the  accumulation  of  large  masses  of  snow  and  ice  as  an  ice- 
cap at  one  of  the  poles,  has  been  already  referred  to  (p.  20)  as  tending  to 

*  '  Illustrations  of  the  Huttoniau  Theory/  1802.  The  same  conclusion  was  announced 
liv  L.  von  Buch,  *  Reise  durch  Norwegen  un<l  Lapland,*  1810. 

-  Bull.  U.  S.  Geol.  Surv.  No.  48  (1888),  p.  59. 

2  Op.  i'U.  p.  85.  See  Stokes,  Traus.  Camh.  Phil.  Srn:.  \iii.  (1849),  p.  672 ;  Sei,  Proc. 
Roy.  JtuUin  S>c.  v.  (1887),  p.  652. 


PART  I  SECT,  iii  UPHEA  VAL  AND  DEPRESSION  283 

raise  the  level  of  the  ocean  in  the  hemisphere  so  affected,  and  to  diminish 
it  in  a  corresponding  measure  elsewhere.  The  return  of  the  ice  into  the 
state  of  water  would  produce  an  opposite  eftect.  The  atti-active  influence 
of  the  ice-sheets  of  the  Glacial  Period  upon  the  sea-level  over  the  northern 
hemisphere  has  been  discussed  by  various  mathematicians,  especially  by 
Croll,  Pratt,  Heath,  and  Lord  Kelvin.  Considerable  diff'erences  appear  in 
their  results,  according  to  the  conditions  which  they  postulate,  but  they 
agree  that  a  decided  elevation  of  the  sea-level  must  be  attributed  to  the 
accumulation  of  thick  masses  of  snow  and  ice.  The  rise  of  the  sea-level 
^ong  the  border  of  an  ice-cap  of  SS''  angular  radius  and  10,000  feet 
thick  in  the  centre  is  estimated  at  from  139  to  573  feet.^  (4)  A  still 
further  conceivable  source  of  geographical  disturbance  is  to  be  found  in 
the  fact  that,  as  a  consequence  of  the  diminution  of  centrifugal  force 
owing  to  the  retardation  of  the  earth's  rotation  caused  by  the  tidal  wave, 
the  sea-level  must  have  a  tendency  to  subside  at  the  equator  and  rise 
at  the  poles. ^  A  larger  amount  of  land,  however,  need  not  ulti- 
mately be  laid  bare  at  the  equator,  for  the  change  of  level  resulting 
from  this  cause  would  be  so  slow  that,  as  Dr.  Croll  has  pointed  out,  the 
general  degradation  of  the  surface  of  the  land  might  keep  pace  with  it 
and  diminish  the  terrestrial  area  as  much  as  the  retreat  of  the  ocean 
tended  to  increase  it.  The  same  writer  has  further  suggested  that  the 
waste  of  the  equatorial  land,  and  the  deposition  of  the  detritus  in  higher 
latitudes,  may  still  further  counteract  the  effects  of  retardation  and  the 
consequent  change  of  ocean-level.  (5)  Some  geologists  have  supposed 
that  where  the  earth's  crust  is  loaded  with  thick  deposits  of  sediment  or 
massive  ice-sheets  it  will  tend  to  sink,  while  on  the  other  hand  denudation 
by  unloading  it  promotes  upheaval. 

The  balance  of  evidence  at  present  available  seems  adverse  to  any 
theory  which  would  account  for  ancient  and  modern  changes  in  the 
relative  level  of  sea  and  land  by  variations  in  the  figure  of  the  oceanic 
envelope,  save  to  a  limited  extent  by  the  attraction  caused  by  extensive 
masses  of  upraised  land,  and  possibly  in  northern  and  southern  latitudes 
by  the  attractive  influence  of  large  accumulations  of  snow  and  ice.  Such 
changes  are  rather  to  be  regarded  as  due  to  movements  of  the  solid  crust. 
The  proofs  of  upheaval  and  subsidence,  though  sometimes  obtainable  from 

*  See  CroU,  *  Climate  and  Time,'  chap,  xxiii.  xxiv.  (MJ.  Mwj.  1874.  Pratt,  '  Figure  of  the 
Earth.*  D.  D.  Heath,  PhU.  Mag.  xxxi.  (1866),  pp.  201,  323,  xxxii.  (1866),  p.  34.  Thomson 
(Lord  Kelvin),  op.  cit.  xxxi.  p.  305.  A.  Penck,  Jahrb.  O'eofjrajjh.  Gesel.  Munich,  vii.  De 
Lapparent,  Bull.  Sot-,  (t^tl.  France^  xiv.  (1886),  p.  368,  Revvc  Ofn^rale  des  Sciences^  May 
1890.  R.  S.  Woodward,  Bull.  U.  S.  Gf<^.  Survey,  No  48.  Von  Drygalski,  '  Bewegungen 
der  Kontinente  zur  Eiszeit,'  Berlin,  1889.  Prof.  Suess  believes  that  the  limits  of  the  dry  land 
depend  upon  certain  large  indeterminate  oscillations  of  the  statical  figure  of  the  oceanic 
envelope  ;  that  not  only  are  "raised  beaches"  to  be  thus  explained,  but  that  there  are 
absolutely  no  vertical  movements  of  the  crust  save  such  as  may  form  part  of  the  plication 
arising  from  secular  contraction  ;  and  that  the  doctrine  of  secular  fluctuations  in  the  level 
of  the  continents  is  merely  a  remnant  of  the  old  "Erhebungstheorie,"  destined  to  speedy 
extinction.  *  Antlitz  der  Erde,'  Leipzig,  1883.  Pfaff  defends  the  general  opinion  against 
these  views  in  Zeitsch.  Jteutsch.  Ci*'d.  Oes.  1884. 

'  Croll,  Phil.  Mtnj.  1868,  p.  382.     Thomson,  Trans.  Gcol.  Soc.  Glasgow,  iii.  p.  223. 


284  DYNAMICAL  GEOLOGY  BOOKniPABTi 

^vide  areas,  are  marked  bv  a  want  of  uniformitv  and  a  local  and  variable 
cbanicter,  indicative  of  an  action  local  and  variable  in  its  operations,  such 
as  the  folding  of  the  terrestrial  crust,  and  not  regular  and  iiddespread, 
such  as  might  be  predicated  of  any  alteration  of  sea-level.  While  admit- 
ting therefore  that,  to  a  certjiin  extent,  oscillations  of  the  relative  level 
of  sea  and  land  may  have  arisen  from  some  of  the  causes  above  enumer- 
ated, we  may  hold  that,  on  the  whole,  it  is  the  land  which  rises  and  sinks 
rather  than  the  sea.^ 

§  1.  Upheaval. — Various  maritime  tracts  of  land  have  Ijeen  asceitained 
to  have  undergone  in  recent  times,  or  to  be  still  undergoing,  what  appears 
to  be  a  gradual  elevation  above  the  seji.  On  the  coast  of  Siberia,  for  600 
miles  to  the  east  of  the  river  Lena,  round  the  islands  of  Spitzbergen  and 
Xovaja  Zemlja,  along  the  shores  of  the  Scandinavian  peninsula  with  the 
exception  of  a  small  area  at  its  southern  apex,  and  along  a  maritime  strip 
of  western  South  America,  it  has  been  proved  that  the  sea  stands  now 
at  a  lower  level  with  regard  to  the  land  than  it  formerly  did.  In 
searching  for  proofs  of  such  movements  tlie  student  must  be  on  his  guard 
against  being  deceived  by  any  apparent  retreat  of  the  sea,  which  may  be 
due  merely  to  the  dei)08it  of  gi'avel,  sand,  or  mud  along  the  shore,  and 
the  consequent  gain  of  land.  Local  accumulations  of  gravel  or  "  storm 
beaches,"  are  often  thrown  up  by  storms,  even  above  the  level  of  ordinary 
high-tide  mark.  Li  estuaries,  also,  considerable  tracts  of  low  ground  are 
gradually  raised  above  the  tide-level  by  the  slow  deposit  of  mud.  The 
following  j)roofs  of  actual  rise  of  the  land  are  chiefly  relied  on  by 
geologists. - 

Evidence  from  dead  organisms. — Rocks  covered  with  l>arnacles  or  other  littoral 
adlierunt  animals,  or  jtierced  by  lithodomous  shells,  aft'ord  i»resimi[)tivo  proof  of  the 
presence  of  the  sea.  A  single  stone  with  these  creatures  on  its  surface  would  not  be 
satisfactory  evidence,  for  it  might  have  been  cast  up  by  a  stonn  ;  but  a  line  of  large 
boulders,  which  had  evidently  not  been  moved  since  tlie  cirripedes  and  mollusks  lived  upon 
tliem,  and  still  more  a  solid  clilf  with  these  marks  of  littoral  or  sub-littoral  life  U]»on  its 
base,  now  raistjd  above  high-water  mark,  would  be  sufficient  to  demonstrate  a  change  of 
level.  The  amount  of  this  change  might  be  jjretty  accuratel}'  determined  by  measuring 
the  vertical  distance  between  the  up|>er  edge  of  the  barnacle  zone  upon  the  upraised  rock, 
and  the  limit  of  the  same  zone  on  the  present  shore.  By  this  kind  of  eridence,  the 
recent  uprise  of  the  coast  of  Scandinavia  has  been  proved.  The  shell-borings  on  the 
pillai-s  of  the  temple  of  Jupiter  Serapis  in  the  Bay  of  Naples  prove  first  a  depression  and 
then  an  elevation  of  the  ground  to  the  extent  of  more  than  twenty  feet.'  Raised  coral- 
reefs,  forme<l  by  living  sjiccies  of  corals,  are  a  conspicuous  feature  of  the  geolog)* 
of  the  "West  Indian  Region.     The  teiraces  of  Barbadoes  are  jvirticularly  striking.     In 


'  For  the  arguments  against  the  view  alx)ve  adopted  and  in  favour  of  the  doctrine  that 
the  increase  of  the  land  above  sea-level  is  due  to  the  retirement  of  the  sea,  see  H.  Trast- 
KchoM,  IhiJIetin  Soci^tf  Imp.  ife^i  yatvralistrs  de  Moscou,  xlii.  (1869),  part  i.  p.  1  ;  188S, 
No.  2,  p.  341  ;  Bifll,  .S'>r.  Cteo/.  France  (3),  viii.  (1S7P),  p.  134  ;  but  more  especially  Saess, 
in  his  great  work  the  *  Antlitz  der  Erde.' 

-  See  "  Earthquakes  and  Volcanoes  *'  (A.  G.),  Cliambers's  A[iscelhny  of  TracU. 

3  Babbage,  E(Hii.  Phil.  Jovrn.  xi.  (1824),  91.  J.  D.  Forbes.  Edin.  Joum.  Sd,  i. 
(1829),  p.  260.     Lyell,  *  Principles,'  ii.  p.  164. 


CT.  iii  §  1  EVIDENCE  OF  UPHEAVAL  285 


Ua,  a  raised  coral-reef  occurs  at  a  height  of  1000  or  1100  feet  above  the  sea.*  In  Peni, 
dem  coral- limestone  has  been  found  2900  or  3000  feet  above  sea-level.'''  Again,  in 
J  Solomon  Islands,  evidence  of  recent  uprise  is  funiislied  by  coral  reefs  lying  at  a 
ght  of  1100  feet,'  and  similar  evidence  occurs  among  the  New  Hebrides  at  1500  feet. 
The  elevation  of  the  sea-lx)ttom  can  in  like  manner  be  proved  by  dead  organisms 
ed  in  their  (>osition  of  growth  beneath  high-water  mark.  Thus  dead  si)e«imens  of 
ta  trunaUa  occur  on  some  parts  of  the  coast  of  the  Firth  of  Forth  in  considerable 
mbers,  still  placed  with  their  siphuncular  end  up])ermost  in  the  stiff  clay  in  which 
>y  burrowed.  The  position  of  these  shells  is  about  high -water  mark,  but  as  their  exist- 
;  descendants  do  not  live  above  low- water  mark,  we  may  infer  that  the  coast  has  been 
9ed  by  at  least  the  difference  between  high  and  low- water  mark,  or  eighteen  feet."* 
ad  shells  of  the  large  Pholas  dadyhis  occur  in  a  similar  position  near  high-water  mark 
the  Ayrshire  coast.  Even  below  low-water,  examples  have  been  noted,  as  in  the 
eresting  case  observed  by  Sars  on  the  Drobaksbank  in  the  Christiania  Fjord,  where 
d  stems  of  OciUina  proUfera  (L. )  occur  at  depths  of  only  ten  or  fifteen  fathoms, 
is  coral  is  really  a  deep-sea  form,  living  on  the  western  and  northern  coasts  of  Nor- 
jr,  at  depths  of  one  hundred  and  fifty  to  three  hundred  fathoms  in  cold  water.  It 
st  have  been  killed  as  the  elevation  of  the  area  brought  it  up  into  upper  and  warmer 
ers  of  water.  ^  It  has  even  been  said  that  the  pines  on  the  edges  of  the  Norwegian 
•w-fields  are  dying  in  consetjuente  of  the  secular  elevation  of  the  land  bringing  them 
into  colder  zones  of  the  atmosphere. 

Any  stratum  of  rock  containing  marine  organisms  which  have  manifestly  lived  and 
d  where  their  remains  now  lie,  may  be  held  to  prove  a  change  of  level  between  sea 
I  land  In  this  way  it  can  be  shown  that  most  of  the  solid  land  now  visible  to  us 
(  once  l>een  under  the  sea.  High  on  the  flanks  of  mountain -chains  (as  in  the  Alps 
I  Himalayas),  undoubted  marine  shells  occur  in  the  solid  rocks. 
Sea- worn  Caves. — A  line  of  sea- worn  caves,  now  standing  at  a  distance  above 
:h- water  mark  beyond  the  reach  of  the  sea,  affords  evidence  of  recent  change  of  level. 
the  accom])anying  diagram  (Fig.  75)  examples  of  such  caves  are  seen  at  the  l^ase 
the  cliff,  once  the  sea-margin,  now  sei>arated  from  tlie  tide  by  a  platform  of  meadow- 
d. 

Raised  Beaches  furnish  one  of  the  most  striking  proofs  of  change  of  level.  A 
-ch  or  space  between  tide-marks,  where  the  sea  is  constantly  grinding  down  sand  and 
vel,  mingling  with  them  the  remains  of  shells  and  other  organisms,  sometimes  piling 
^  dei>osits  up,  sometimes  sweeping  them  away  out  into  oi)ener  water,  forms  a  familiar 
race  or  j»latform  on  coast-lines  skirting  tidal  seas.  When  this  margin  of  littoral 
>osits  has  Ikjcu  i)laced  above  the  reach  of  the  waves,  the  flat  terrace  thus  elevated  is 
)wn  as  a  **  raised  beach  "  (Figs.  75,  76,  77,  78).  The  former  high-water  mark  then  lies 
lud,  and  while  its  sea-worn  eaves  are  in  time  hung  with  ferns  and  mosses,  the  beach 
OSS  which  the  tides  once  flowed  furnishes  a  platfonn  on  which  meadows,  fields, 
dens,  roads,  houses,  villages,  and  towns  spring  up,  while  a  new  beach  ia  made  below  the 
rgin  of  the  uplifted  one.  A  series  of  raised  beaches  may  occur  at  various  heights 
»ve  the  sea.  Each  terrace  marks  a  fonner  lower  level  of  the  land  with  regard  to  the 
,  and  probably  a  lengthened  stay  of  the  land  at  that  level,  while  the  intervals 
ween  them  represent  the  vertical  amount  of  each  variation  in  the  relative  levels  of  sea 
I  land,  and  show  that  the  interval  lietween  the  changes  was  too  brief  for  the  forma- 
1  of  terraces.     A  succession  of  raised  beaches,  rising  above  the  j»resent  sea -level,  may 

^  A.  Agassiz,  Amer.  Aciid.  xi.  (1882),  p.  119. 
'  A.  Agassiz,  Bull.  Mus.  Comp.  Zool.  vol.  iii. 
'  H.  B.  Guppy,  Nature,  3rd  January  1884. 

*  Hugh  Miller's  '  Edinburgh  and  its  Neighbourhood,'  p.  110. 

*  Quoted  by  Vom  Rath  in  a  paper  entitled  "Aus  Norwegen,"  Neues  Jahrb,  1869,  p. 
i.     For  another  exanii)le,  see  Owyn  Jeffreys,  Bn(,  Assoc.  1867,  p.  431. 


lyYNAAflCAL  dEOLOGY 


BOOK  UI  PABt  I 


therefore  be  lukeli  as  ]ioiiiting  tn 
iiitprrupte<l  liy  lung  [lauHcii,  iliirhig 
unlesa  in  tegimis  where  there  in  renai 


11  riimier  iiitvnnittcut  nplipnvBl  of  the  vountrj, 
L'hicli  the  funeral  level  did  not  niateriallj  chuige, 
II  to  Wlieve  that  tlie  Burfac«  of  the  sea  bu  nndtr' 


accumulation  or  melting  of  large  n 


.'»  ud  bvntail  liy  ( 


gone  a  cliaiige  of  level  ivi: 
ice  {ante,  p.  20). 

Raised  beaelied  obonnd  ill  the  higher  latitiideit  of  the  northern  mid  touthem  heni- 
Hpheres,  niid  tliis  di^tribntion  huh  bevti  cluimed  as  a  strong  argument  in  favour  of  the 
view  tlial  tliey  are  due  to  ii  fall  of  the  touil  level  of  the  sva-HUrface  from  the  djsa|>pear- 
ance  or  diminution  of  former  iee-ca])^.  Unit  Hume  at  least  of  tlie  rained  beachea  in  thew 
regions  may  Iw  due  to  this  eiiiise  niay  l<e  granteil.  The  grailual  rise  of  level  of  the 
lieaches  when  traced  ili>  the  Ijords.  whieh  has  been 
i-ejienteilly  asserted  for  sonic  districts,  would  be  the 
natural  elfei't  of  the  gi-pater  uioaa  of  ii-e  in  the  in' 
terior.  In  the  exploration  of  the  lake  regions  of 
Xorth  America  numerous  instances  iiave  bren 
dewri1)ed  of  a  slo]>e  u|iv'ard  of  the  former  water- 
levels  towards  the  main  ice-liclds.  A  ramariuble 
example  is  fumiiihol  by  the  terraces  of  the  vanished 
glacial  sheet  ot  Water  called  T^ke  Agaraii  which 
OUL1-  tilled  tint  basin  of  the  Ked  River  of  the  Korth. 
Mr.  1Vam-ii  Upliam  has  found  that  these  ancient 
lines  of  watci'-levpl  gradually  rise  from  south  to 
,  north  and  from  west  to  en.st,  in  the  direction  of  tlie 
I  former  ice-fields,  the  amount  of  olojie  ranging  irom 
1.1  blown  «nJ  (.()cffln|Hei«l bj^Uw  i»ro  to  1-3  feet  |K!r  mile,'  Mr.  O.  K.  Gilbert  has 
noticed  a  rise  of  as  iiinch  as  S  feet  in  a  mile  among 
the  old  teiTocuH  of  I^ake  Ontario.' 
r  ivund  many  parts  of  the  coast-line  of  Britain.  De  la  Becbe 
!W  (Fig.  77)  of  tt  Corniah  locality  whcm  the  existing  beach  ii 
flanked  by  a  ctitf  of  slate,  b,  continually  cut  away  by  the  sea  so  that  the  overlying  rwisvd 
iH'Och,  a,  r,  nil!  ere  long  disajijH'sr.     The  coast  line  on  both  sides  of  ScotUnd  is  likowiae 


ml  Hull,  Cornwall  (D.) 
Rained  beaches  oct 
i  the  subjoined  v 


'  B«U.  r.  S.  Oeol.  Sun:  Xo.  i 


],  pp.  IS.  20. 


'  .leiena,  i.  p.  221. 


EVIDENCE  (IF  UPHEAVAL 


friliged  with  raised  beachea,  sometimes  four  or  live  occun-itig  above  each  other  at  heights 
of  25,  40,  50,  eO,  75,  and  100  feot  aboTe  the  present  high-water  mark.'  Others  are  found 
on  both  Bides  of  the  Enghsh  CI  annrl       The  s  lea    f  tt  e     ou  ta  nous  tjonls  of  Is'ortlieni 


}Te  thau  000  feet  above  sea-level,  are  marked  with  coiiapieuous  litiee  of 
These  teiraees  are  partlj-  ordinary  l>each  de[Hiait3,  partly  notches  cut 


out  of  rock,  probably  with  tlie  aid  of  driftiug  coast-ioe.'    Pi™>f3  of  recent  elevation  of 
the  ahores  of  the  Mediterranean  are  furnished  by  raised  beaches  at  various  heights  alrave 


Die  British  raised  benches,  see  De  la  Beche,  *  Report  on  Geology  of 
lap.  liii.  ;  C.  Maclareu,  '  Geology  of  Fife  sud  the  Lothians,'  1838  ; 
Sea  Maitfins ; '  Prestwieh,  Q.  J.  Oed.  Sue.  xiviil.  p.  38  ;  mi.  p. 
V.    Holmea,  Brit.   Ataor.    1876,  Seels,  p.    95;  Uasher,  (iti-l.  ilwj. 


'  For  icconnta  of 
Devon  and  Cornwall,' 
R.  Chambers,  '  Anciet 
29  ;  B.  Russell  and  1 

1878,  p.  16fl. 

'  On  the  raiaed  beach  of  Sangntte.  near  Calais,  see  Preatwich,  RvU.  .Sor.  GM.  France 
(3),  viii.  (1880),  p.  547  ;  on  those  of  Finisterre,  C,  Barrois,  Ann.  .foe.  GM.  Nord.  ix.  (188^). 

1  See  R.  Chambers,  '  Trociugs  of  the  North  of  Europe'  (1850),  p.  172  tl  atq.  Bravais, 
'Voyages  de  !a  CommiasioQ  Scientlfique  du  Nord,'  tc,  translated  in  C-  ■!■  Qeol.  Soc.  L  p. 
SS4.  Kjemlf,  Z.  Dcutudi.  Oeul.  Ore.  :txii.  p.  1  ;  'Die  Geologie  des  aud.  und  mittl.  Nor- 
wagen,' 1880,  p.  7  ;  Otol.  Mag.  vjii.  p.  74.     S.  A.  Sexe,  "  On  Rise  of  Land  in  Scandltuivia," 


288  />  YXAMR  'AL  GEOLOG  Y  book  in  part  i 


tin*  present  water-level.     In  Corsiea  such  terracesi  occur  at  heights  of  from  15  to  20 
metres.* 

On  the  we.st  eoast  r»f  South  America,  lines  of  raised  terrace  containing  recent  shells 
have  heen  traee<l  by  Daiwin  as  [irool's  of  a  f^CAt  upheaval  of  that  i)art  of  the  globe  in 
moilern  freolofjieal  time.  The  terraces  are  not  quite  horizontal  but  rise  towards  tlw 
south.  On  the  frontier  of  Bolivia,  they  weur  at  from  65  to  80  feet  above  the  existing  set- 
level,  hut  nearer  the  higher  mass  of  the  Chilian  Andes  they  are  found  at  1000,  and  n«r 
Valj^iraiso  at  130<)  feet.  That  >ome  of  these  ancient  sea-margins  l)elong  to  the  humsn 
I«eri(Kl  was  shown  by  Mr.  Darwin's  distH»very  of  shells  with  bones  of  birds,  ears  of 
maize,  plaited  ivimIs  and  i-otton  thread,  in  one  of  the  terraces  opposite  Callao  at  a  height 
<»f  bo  fe»*t.-  Riiisetl  Wai-hes  (K-eur  in  Xew  Zealand,  and  indicate  a  greater  change  of 
level  in  the  southt'rn  than  in  the  northern  [^ai-t  of  the  country.'  It  should  be  obserred 
that  this  inci-easeil  rise  of  the  terrat.*es  indewanls  occurs  both  in  the  northern  and 
southern  hemisj>heivs.  and  is  one  of  the  chief  facts  insisted  ujx)n  by  those  who  would 
exi'lain  the  ten•aee^  by  dis[>lacements  of  the  sea  rather  than  of  the  land. 

Human  Reconls  and  Traditions. — In  countries  which  have  been  long 
si'ttlfd  hy  a  human  [lopulatiun.  it  is  sometimes  i^^ssihle  to  ]»rove,  or  at  least  to  render 
prolwhle.  the  fact  of  i-ei-ent  change  of  It'Vel  hy  reference  to  tradition,  to  local  names,  and 
to  works  of  human  construction.  IMei-s  and  harlK>urs,  if  now  found  to  stand  above  the 
u[»i>er  limit  of  high-water,  furnish  indecMl  indis]>utahle  endencoof  a  rise  of  land  or  fidlof 
sca-levcl  since  their  erection.  Numerous  pr«>ofs  of  a  recent  change  of  level  in  the  coast  of 
the  Arctic  Ocean  from  .S[»itzWrgen  eastwanl  have  l»een  observed.  The  Finnish  coast  is 
rcjtortetl  to  have  risen  tJ  feet  4  inches  in  127  yeai-s.*  At  SpitzWrgen  itself,  besides  its 
rai^-d  beacho.  )»earing  witne.ss  to  j»revious  elevations,  small  islands  which  existed  two 
hundrtnl  yeai'>  ago  are  now  joine«l  to  larger  i»ortions  of  land.  At  Novaja  Zem\ja,  where 
>ix  rai>ed  InMebes  were  foun<l  by  Xonlenskjold,  the  highest  Wing  600  feet  above  set- 
level,''  theiv  M'cms  to  have  U'cn  a  rising  of  the  sea-lx)ttom  to  the  extent  of  100  feet  (W 
more  since  the  Dutch  e.\[K;dition  of  ir»iM.  On  the  north  coast  of  Siberia  the  island  of 
Diomida.  observe<l  in  1760  byChalaourof  to  the  east  of  Cape  Sviatoj,  was  found  by  Wraugel 
-sixty  years  aft«rwan.ls  to  have  l»een  unitt^il  to  the  mainland.^  From  marks  made  on  the 
c»K\>t  in  the  middle  of  la>t  century  it  ap[»ears  that  the  north  of  Sweden  has  risen  about 
7  feet  in  the  last  l.'»l  yeai>.  but  that  the  movement  has  lessened  southwards  until  in 
Srauia  it  ha^  l»een  re]ilaced  by  ""ne  in  a  downwanl  tlirection  .see  p.  291). 

jj  2.  Subsidence. — It  is  more  ditticult  to  trace  a  downward  movement 
of  land.  t»>r  tht*  evidence  of  each  successive  sea-margin  is  carried  down 
and  washe<l  away  or  covered  up.  Tlie  student  >W11  take  care  to  guanl 
himself  apunst  heing  niishnl  V>y  mere  proofs  uf  the  advance  of  the  sea  on 

/.■./.  .:  .<;.  .*/',■.'.„  -f'  I'..:,'  r<if'/,  Ohri-ti-inia.  1S72.  H.  M.»hn.  yt/t.  Aftitj,  Xaf.  xxH,  p.  1. 
D.ikyii^.  fJ'  '.  M"t.  1^77.  p.  7ii.  K.  IVtter-en,  .I-./'.  M".fh.  Xot.  rhriMinnin,  1878,  p.  182, 
V.  il^"-.'.'  :  *i-  \  M  '•!.  1*«7'J.  I',  ii*^  :  Tr-ms-.i  .\f.\-'"»s  A,triihrthr,  Til.  1880.  Sitz,  Akad. 
\V'  ,  ::■  viii.  1-*^'.*.  I.rlniiann.  '  reV»er-eliemali;re  Strandlinier,  &c.,"  Halle,  1879  ;  ^iisek. 
y. ..  .V"*  /-•<-.  l-*^*:',  p.  2>«».  A.  «J.  H.i*:l»o!!i.  ».'..-•.  F'ii:  FurhaihU,  ikt>ckhiilm,  ix.  (1887), 
p.  IV.     r.  Saii-llrr.  /•■'.■-,/."«/?.<  .\[:tth.i:.  xxwi.  tlS9u».  pj..  200.  2:35. 

'  /;"7.  .<■;.  '.Vv.  Fr.t,..-,'   :V.  iv.   j..  <»;. 

-     Geo^vi^^al  i.>b^crvatioiis*  chap.  ix.     r>ee  ftV.7.  J/,/./.  Is77,  p.  28. 

•  Haa>t'.>  M;t:«jloir>-  «'lH"aiiicrbury.*  1>79.  p.  ot>»j. 

*  y^t"...  xxvi.  i».  201.  »  //././.  XV.  jK  123. 

'^  i;ra.l.  /;.'.■/.  Sr,  r;.',^.  Fr.'n,.\  3nl  ser.  ii.  p.  04 S.  Traces  of  oscillations  of  level 
>*itinn  hi>roric  time«i  have  JK-eu  ob-iervevl  in  tlie  Netherlands  Flanders,  and  Upper  Italy. 
/;•..".  .ji'-.  '.''/.  F,''"h'.  2n.l  ser.  xix.  p.  .Xitf  :  :lr.l  ser.  ii.  pp.  46,  222;  -4«fi.  Sk*e.  GioL 
y  /■■'.  V.  p.  'J IS.  For  aileu'ed  cliauje>  of  level  in  the  estuary  of  the  Garonne,  see 
\xfifmp.  Ait.  S..\    Linn.  lk»nleaux.  xxxi.  (1^76  .  p.  2S7,  and  Delfortrie,  ib,  xzxii.  p.  79. 


BBCT.  iii  §  2  EVIDENCE  OF  SUBSIDENCE  289 

the  land.  In  the  great  majority  of  cases,  where  such  an  advance  is  taking 
place,  it  is  due  not  to  subsidence  of  the  land,  but  to  erosion  of  the  shores. 
It  is,  indeed,  the  converse  of  the  deposition  above  mentioned  (p.  284)  as 
liable  to  be  mistaken  for  proof  of  upheaval.  The  results  of  mere  erosion 
by  the  sea,  however,  and  those  of  actual  depression  of  the  level  of  the 
land,  cannot  always  be  distinguished  without  some  care.  The  encroach- 
ment of  the  sea  upon  the  land  may  involve  the  disappearance  of  suc- 
oeasive  fields,  roads,  houses,  villages,  and  even  whole  parishes,  without 
any  actual  change  of  level  of  the  land.  Certain  causes,  however,  referred 
to  below,  may  come  into  operation,  producing  an  actual  submergence  of  land 
without  any  real  subsidence  of  the  land  itself.  The  following  kinds  of 
evidence  are  usually  cited  to  prove  subsidence. 

Sabmerged  Formats. ^As  the  land  is  brouftbt  within  reach  of  the  iravea,  and  its 
elianctcnstic  aurTace'featuree  are  effaced,  the  sabmerged  area  maj  retain  little  or  no 
erideTice  of  its  having  been  a  land-surface.  It  will  be  covered,  as  a  rule,  with  sea-worn 
Mud  or  silt.  Hence,  no  doubt,  the  reosoD  v--\tf,  among  the  marine  strata  which  form 
■o  much  of  the  stratified  portion  of  the  earth's  crast,  and  contain  so  many  proofs  of 
depreaaion,  actual  traces  of  land-BUiftcea  are  comparatively  rare.  It  is  only  under  very 
(avtmrable  circumstances,  as,  for  instance,  where  the  ares  is  sheltered  from  prevalent 
winds  and  waves,  and  where,  therefore,  the  sarface  of  the  land  can  sink  tranquilly  under 
tlie  sea,  that  Fragments  of  that  surface  ma;  be  preserved  under  overlying  marine 
Mcnmulationa.    It  is  in  such  places  that  "submerged  forests  "  occur  (Fig.  79).    These  are 


j|ifti>,  ^Kd',\i^^  -.  ■„  >S!iji^y-^.  I  h  jt ,  Ma 


■tamps  of  trees  still  in  their  positions  of  growth  Jn  their  native  soil,  often  associated 
with  beds  of  peat,  full  of  tree-roots,  hazel-nuts,  branches,  leaves,  and  other  indications 
of  a  terrestrial  surface.  Tliere  is  sometimes,  however,  considerable  risk  of  deception  in 
regard  to  the  nature  and  value  of  such  evidence  of  depression.  Where,  for  instance, 
■bingia  or  sand  is  banked  up  against  a  nhore'or  river-month,  considerable  spaces  may  be 
enclosed  and  tilled  with  frexb-water.  the  bottom  of  which  may  be  some  way  below  higb- 
water  mark.  In  such  lagoons  terrestrial  vegetation  slid  debris  from  the  land  may 
be  deposited.  Eventually,  if  the  protecting  barriers  should  be  cut  away  the  tides  may 
Itoir  over  tbe  layers  of  terrestrial  |ieat,  giving  a  false  appearance  of  subsidence.  Agalli, 
vwing  to  removal  of  subterranean  aandy  dejiosits  by  springs,  overlying  ^cat-beds  may 
■ink  below  sea-level. 

De  la  Beche  has  described,   round   the  shores  of  Devon,  Cornwall,  and  westein 


290  DYNAMICAL  GEOLOGY  book  m  parti 

Somerset,  a  vegetable  accumulation,  consisting  of  plants  of  the  same  species  as  thon 
which  now  grow  freely  on  the  ac^oining  land,  and  occurring  as  a  bed  at  the  mouths  of 
valleys,  at  the  bottoms  of  sheltered  bays,  and  in  front  of  and  under  low  tracts  of  land, 
of  which  the  seaward  side  dips  beneath  the  present  level  of  the  sea.^  Over  this  sab* 
merged  land -surface,  sand  and  silt  containing  estuarine  shells  have  generally  been 
deposited,  whence  we  may  infer  that,  in  the  submergence,  the  valleys  first  became 
estuaries,  and  then  sea-bays.  If  now,  in  the  course  of  ages,  a  series  of  such  submciyd 
forests  should  be  formed  one  over  the  other,  and  if,  finally,  they  should,  by  upheaval  of 
the  sea-bottom,  be  once  more  laid  dry,  so  as  to  be  capable  of  examination  by  boring, 
well-sinking,  or  otherwise,  they  would  prove  a  former  long-oontinued  depression,  witii 
intervals  of  rest.  These  intervals  would  be  marked  by  the  buried  forests,  and  the  progren 
of  depression  by  the  strata  of  sand  and  mud  lying  between  them.  In  short,  the  evidenoe 
would  be  strictly  on  a  i)arallel  with  tliat  famished  by  a  succession  of  raised  beaches  ai 
to  a  former  protracted  intermittent  elevation. 

Along  the  coasts  of  Holland  and  the  north  of  France^-  submerged  beds  of  peat  have 
been  regarded  as  proofs  of  submergence  during  historic  times.  The  amount  of  change 
varies  considerably  in  different  places,  and  here  and  there  can  hardly  be  appreciated. 
The  sinking  during  the  350  years  preceding  1 850  is  estimated  to  have  amounted  in  the 
l)oldcrs  of  Groningen  to  a  mean  annual  rate  of  8  millimetres.^  In  the  north  of  Franee 
numerous  examples  of  submerged  forests  have  been  observed.  In  1846,  in  digging  the 
harbour  of  St.  Servan,  near  St.  Malo,  a  Gaulish  cemetery  containing  ornaments  and  ooiiii, 
and  resting  on  a  still  more  ancient  prehistoric  cemetery,  was  met  with  at  a  level  of  6 
metres  below  the  level  of  high  tide,  so  that  the  submergence  must  have  been  at  least  to 
that  extent.' 

Coral-islands. — Evidence  of  widespread  depression,  over  the  area  of  the  Faoifie 
and  Indian  Oceans,  has  been  adduced  from  the  structure  and  growth  of  coral-reefr  and 
islands.  Mr.  Darwin,  many  years  ago,  stated  his  belief  that,  as  the  reef-building  corals 
do  not  live  at  depths  of  more  than  20  to  30  fathoms,  and  yet  their  reefs  rise  out  of  deep 
wat«r,  the  sites  on  which  they  have  formed  these  structures  must  have  subsided,  the 
rate  of  subsidence  being  so  slow  that  the  upward  growth  of  the  reefs  has  on  the  whole 
kept  i>ace  with  it.^  More  recent  researches,  however,  show  that  the  phenomena  of  coral- 
reefs  are  in  some  cases,  at  least,  capable  of  satisfactory  explanation  without  subsidence, 
and  hence  that  their  existence  can  no  longer  be  adduced  by  itself  as  a  demonstratioii 
of  the  subsidence  of  large  areas  of  the  ocean. ^  The  formation  of  coral-reefs  is  described  in 
Book  III.  Part  II.  Section  iii.,  and  Mr.  Darwin's  theory  is  there  more  fully  explained. 

Distribution  of  plants  and  animals. — Since  the  appearance  of  Edward  Forbes*! 
essay  upon  the  connection  between  the  distribution  of  the  existing  fauna  and  flora  of  the 
British  Isles,  and  the  geological  changes  which  have  affected  that  area,^  much  attention 
has  been  given  to  the  evidence  furnished  by  the  geographical  distribution  of  plants  and 

1  "Geology  of  Devon  and  Cornwall,"  Mem.  Geol.  Survey.  For  further  accounts  of 
British  submerged  forests,  see  Q.  J.  Geol,  Soc.  xxii.  p.  1.  ;  xxxiv.  p.  447.  OeaL  Mag,  vL 
p.  76  ;  vii.  p.  64  ;  iii.  2nd  ser.  p.  491  :  vi.  pp.  80,  251.  Mr.  D.  Pidgeon  has  aiguedin 
favour  of  the  submerged  forest  of  Torbay  having  been  formed  without  subsidence  of  the  land. 
Quart.  Journ.  Gtd.  Soc  xli.  (1885),  p.  9.    See  also  W.  Shone,  op,  cit  xlviii.  (1892),  p.  96. 

'  Lorii",  Archives  da  Musie  Teyler^  ser.  ii.  vol.  iii.  Part  5  (1890),  p.  421. 

^  Lorie,  ibid,  p.  438,  and  papers  cited  postea,  ]>.  292.  But  see  Suess,  '  Antlitz  der  Erde,* 
ii.  p.  547. 

*  See  Darwin's  'Coral  Islands,'  Dana's  'Corals  and  Coral  Islands,'  and  the  works  dted 
jtostea.  Book  III.  Part  II.  Section  iii.  §  3,  under  "Coral-reefs  "  (p.  488).  The  various  theories 
on  the  subject  are  discussed  by  K.  Langenbeck  iu  his  '  Theorien  uber  die  Entstehung  der 
Koralleninseln  und  Korollenriffe,*  1890. 

^  See  Proc.  Roy.  Phys.  Soc.  Edinhurghy  viii.  p.  1. 

«  Mem.  Geol.  Survey,  vol  i.  1846,  p.  336. 


SECT,  ui  §  2  EVIDENCE  OF  SUBSIDENCE  291 

anim&ls  as  to  geological  revolutions.  In  some  cases,  the  former  existence  of  land  now 
submerged  lias  been  inferred  with  considerable  confidence  from  the  distribution  of  living 
organisms,  although,  as  Mr.  Wallace  has  shown  in  the  case  of  the  supposed  *'  Lemuria," 
some  of  the  inferences  have  been  unfounded  and  unnecessary.'  The  present  distribution 
of  plants  and  animals  is  only  intelligible  in  the  light  of  former  geological  changes.  As 
a  single  illustration  of  the  kind  of  reasoning  from  present  zoological  groupings  as  to 
former  geological  subsidence,  reference  may  be  made  to  the  fact,  that  while  the  fishes 
and  mollusks  living  in  the  seas  on  the  two  sides  of  the  Isthmus  of  Panama  are  on  the 
whole  very  distinct,  a  few  shells  and  a  large  number  of  fishes  are  identical ;  whence  the 
inference  has  been  drawn  that  though  a  broad  water-channel  originally  separated  North 
and  South  America  in  Miocene  times,  a  series  of  elevations  and  subsidences  has  since 
oocurred,  the  most  recent  submersion  having  lasted  but  a  short  time,  allowing  the 
jiassage  of  locomotive  fishes,  yet  not  admitting  of  much  change  in  the  comparatively 
stationary  mollusks.^ 

Fjords. — An  interesting  proof  of  an  extensive  depression  of  the  north-west  of  Europe 
is  famished  by  the  fjords  or  sea-lochs  by  which  that  region  is  indented.  A  Qord  is  a 
long,  narrow,  and  often  singularly  deep  inlet  of  the  sea,  which  terminates  inland  at  the 
mouth  of  a  glen  or  valley.  The  word  is  Norwegian,  and  in  Norway  Qords  are  character- 
istically developed.  The  English  word  "  firth,"  however,  is  the  same,  and  the  western 
coasts  of  the  British  Isles  furnish  many  excellent  examples  of  Qords,  such  as  the  Scottish 
Loch  Houm,  Loch  Nevis,  Loch  Fyne,  Gareloch  ;  and  the  Irish  Lough  Foyle,  Lough  Swilly, 
Bantry  Bay,  Dunmanus  Bay.  Similar  indentations  abound  on  the  west  coast  of  British 
North  America  and  of  the  South  Island  of  New  Zealand.  Some  of  the  Alpine  lakes 
(Lucerne,  Garda,  Maggiore,  and  others),  as  well  as  many  in  Britain,  are  inland  examples 
of  Qords. 

There  can  be  little  doubt  that,  though  now  filled  with  salt  water,  Qords  have  been 
originally  land-valleys.  The  long  inlet  was  first  excavated  as  a  valley  or  glen.  The 
adjacent  valley  exactly  corresi^nds  in  form  and  character  with  the  hollow  of  the  Qord, 
and  must  be  regarded  as  merely  its  inland  prolongation.  That  the  glens  have  been 
excavated  by  subaerial  agents  is  a  conclusion  borne  out  by  a  great  weight  of  evidence, 
which  will  be  detailed  in  later  parts  of  this  volume.  If,  therefore,  we  admit  the  sub- 
aerial  origin  of  the  glen,  we  must  also  grant  a  similar  origin  to  its  seaward  prolongation. 
Every  Qord  will  thus  mark  the  site  of  a  submerged  valley.  This  inference  is  confirmed 
by  the  fact  that  Qords  do  not,  as  a  rule,  occur  singly,  but,  like  glens  on  land,  lie  in  grou^ts  ; 
so  that,  when  found  intersecting  a  long  line  of  coast,  such  as  that  of  the  west  of  Norway  or 
the  west  of  Scotland,  they  show  that  the  sea  now  runs  far  up  and  fills  submerged  glens. 

Human  constructions  and  historical  records. — Should  the  sea  be  observed 
to  rise  to  the  level  of  roads  and  buildings  which  it  never  used  to  touch,  should  former 
half-tide  rocks  cease  to  be  visible  even  at  low  water,  and  should  rocks,  previously  above 
the  reach  of  the  highest  tide,  be  turned  first  into  shore- reefs,  then  into  skerries  and  islets, 
we  infer  that  the  coast-line  is  sinking.  Such  kind  of  evidence  is  found  in  Scania,  the 
most  southerly  part  of  Sweden.  Streets,  built  of  course  above  high-water  mark,  now 
lie  below  it,  with  older  streets  lying  beneath  them,  so  that  the  subsidence  is  of  some 
antiquity.  A  stone,  the  position  of  which  had  been  exactly  determined  by  Linnaeus 
in  1749,  was  found  after  87  years  to  be  100  feet  nearer  the  water's  edge.'  The  west 
coast  of  Greenland,  for  a  space  of  more  than  600  miles,  is  perceptibly  sinking.     It  has 

'  *  Island  Life,'  1880,  p.  394.  In  this  work  the  question  of  distribution  in  its  geological 
relations  is  treated  with  admirable  lucidity  and  fulness. 

•  A.  R.  Wallace,  '  Geographical  Distribution  of  Animals,*  i.  pp.  40,  76. 

'  According  to  Erdmann,  the  subsidence  has  now  ceased,  or  has  even  been  exchanged  for 
an  upward  movement  {Oeol.  FOr.  Stockholm  Forhandl.  i.  p.  93).  Nathorst  also  thinks  that 
Scania  is  now  sharing  in  the  general  elevation  of  Scandinavia  {^id.  p.  281).  It  appears  that 
the  zero  of  movement  now  passes  through  Bornholm  and  Laalaiid. 


292  DYXAMICAL  GEOLOGY  book  in  part  I 

there  been  noticed  that,  over  ancient  buildings  on  low  shores,  as  well  as  oyer  entire 
islets,  the  sea  has  risen.  The  Mora\'ian  settlers  have  been  more  than  once  driven  to 
shift  their  boat -poles  inland,  some  of  the  old  poles  remaining  visible  under  water.^ 
Historical  evidence  likewise  exists  of  the  salwidence  of  ground  in  Holland  ind 
Belgium.^  On  the  coast  of  Dalmatia,  Roman  roads  and  viUaa  are  said  to  be  visible 
below  the  seA.^ 

§  3.  Causes  of  Upheaval  and  Depression  of  Land. — These  move- 
ments must  again  be  traced  l)ack  mainly  to  consequences  of  the  internal 
heat  of  the  earth.  There  are  various  ways  in  which  this  cause  may  have 
acted.  As  rocks  expand  when  heated,  and  contract  on  cooling,  we  may 
suppose  that,  if  the  crust  underneath  a  tract  of  land  has  its  temperature 
slowly  raised,  as  no  doubt  takes  place  round  areas  of  nascent  volcanoes, 
a  gradual  uprise  of  the  gi'ound  above  will  be  the  result.  The  gradual 
transference  of  the  heat  to  another  quarter  may  produce  a  steady  subsidence. 
Basing  on  the  calculations  of  Colonel  Totten,  cited  on  p.  299,  Lyell 
estimated  that  a  mass  of  red  sandstone  one  mile  thick,  having  its  temperar 
tui'e  augmented  200°  Fahr.,  would  raise  the  overlying  rocks  10  feet,  and 
that  a  portion  of  the  earth's  crust  of  similar  character  50  miles  thick,  with 
an  increase  of  600'  or  800'',  might  produce  an  elevation  of  1000  or  1600 
feet^  But  this  computation,  as  Mr.  Mellard  Keade  has  pointed  out^ 
takes  account  only  of  linear  expansion.  If  from  any  cause  the  mass  of 
rock  whose  temperature  was  augmented  could  not  expand  horizontally  it 
would  rise  vertically,  and  unless  some  of  the  surplus  volume  could  be 
disposed  of  by  condensation  of  the  rock,  the  uprise  would  be  three  times 
as  much  as  the  linear  extension.  Taking  this  view  of  the  case,  we  find 
that  a  mass  of  the  earth's  crust  twenty  miles  thick,  heated  lOOO'^  Fahr., 
and  prevented  from  extending  laterally,  would  rise  1650  feet.* 

Again,  rocks  expand  by  fusion  and  contract  on  solidification.  Hence, 
by  the  alternate  melting  and  solidifying  of  subterranean  masses,  upheaval 
and  depression  of  the  surface  may  possiblv  be  produced  (see  pp.  299, 
304). 

But  evidently  processes  of  this  nature  can  only  eflfect  changes  of  level 
limited  in  amount  and  local  in  area.  When  we  consider  the  wide  tracts 
over  which  terrestrial  movements  are  now  taking  place,  or  have  occurred 

^  Tliese  obsen'atious,  which  have  been  accM'.pted  for  at  least  a  generation  past  (Prvc  Oeol. 
S()C.  ii.  1835,  }>.  208),  have  recently  been  called  in  queatiou,  but  the  alleged  disproof  is  not 
convincing,  and  they  are  here  retained  as  worthy  of  credence.  See  Suess,  Verliand.  Oeti. 
Ht'icJimnstaltf  1880,  No.  11,  and  'Antlitz  der  Enle,'  ii.  p.  415  ef  seq, 

^  Besides  the  ]>a].>er  of  Lorie,  quoted  on  p.  290,  consult  Lavaleye,  '  Affaissement  da  sol  et 
envasement  des  fleuves,  sur\'enus  dans  les  temps  hi.stori(pies,'  Brussels,  1859.  Grad,  BmR, 
Soc.  Gktl,  France^  ii.  3rd  ser.  p.  46.  Arends,  'Physische  Geschichte  der  Nordseektute,' 
1833.  Com{)are  also  R.  A.  Peacock  on  '  Physical  and  Historical  Evidences  of  vast  Sinkings 
of  land  on  the  North  and  West  Coasts  of  France/  &c.,  London,  1868.  For  submerged 
peat-beds  on  French  coast,  see  A.  Gas])ard,  Ann,  Sue,  G6i)l,  Xonl^  1870-74,  p.  40.  On 
o.scillations  of  Frencli  coast,  T.  Girard,  Bull.  Sue,  G^njrnph,  Paris,  st-r.  6,  vol.  x.  p.  225 : 
E.  Delfortrie,  Act.  Soc.  Linn.  Bortleaux^  ser.  4,  vol.  i.  p.  79. 

3  BoU,  Cam,  U*'ol,  Italiano,  1874.  p.  57. 

*  *  Principles,*  ii.  p.  235. 

*  Melhird  Reade,  *  Origin  of  Mountain  Ranges'  (1886),  pp.  112,  114. 


SECT.  iii§3    CAUSES  OF  UPHEAVAL  AND  SUBSIDENCE  293 

in  past  time,  the  explanation  of  them  must  manifestly  be  sought  in  some 
far  more  widespread  and  generally  effective  force  in  geological  dynamics. 
It  must  be  confessed,  however,  that  no  altogether  satisfactory  solution  of 
the  problem  has  yet  been  given,  and  that  the  subject  still  remains  beset 
with  many  difficulties. 

Professor  Darwin,  in  one  of  his  memoirs  already  cited  (ante,  p.  21), 
has  suggested  a  possible  determining  cause  of  the  larger  features  of  the 
Barth's  surface.  Assuming  for  his  theory  a  certain  degree  of  viscosity  in 
the  earth,  he  points  out  that,  under  the  combined  influence  of  rotation 
ind  the  moon's  attraction,  the  polar  regions  tend  to  outstrip  the  equator, 
md  to  acquire  a  consequent  slow  motion  from  west  to  east  relatively  to 
t^he  equator.  The  amount  of  distortion  produced  by  this  screwing  motion 
tie  finds  to  have  been  so  slow,  that  45,000,000  years  ago,  a  point  in  lat. 
30**  would  have  been  4 J',  and  a  point  in  lat.  60"  14 J'  further  west,  with 
reference  to  the  equator,  than  they  are  at  present.  This  slight  transfer- 
3nce  shows  us,  he  remarks,  that  the  amount  of  distortion  of  the  surface 
strata  from  this  cause  must  be  exceedingly  minute.  But  it  is  conceivable 
.hat,  in  earlier  conditions  of  the  planet,  this  screwing  action  of  the  earth 
may  have  had  some  influence  in  determining  the  surface  features  of  the 
planet.  In  a  body  not  perfectly  homogeneous  it  might  originate  wrinkles 
it  the  surface  running  perpendicular  to  the  direction  of  greatest  pressure. 
^In  the  case  of  the  earth,  the  wrinkles  would  run  north  and  south  at  the 
Mjuator,  and  would  bear  away  to  the  eastward  in  northerly  and  southerly 
atitudes,  so  that  at  the  north  pole  the  trend  would  be  north-east,  and  at 
ihe  south  pole  north-west.  Also  the  intensity  of  the  wrinkling  force 
varies  as  the  square  of  the  cosine  of  the  latitude,  and  is  thus  greatest  at 
.he  equator  and  zero  at  the  poles.  Any  wrinkle,  when  once  formed, 
rould  have  a  tendency  to  turn  slightly,  so  as  to  become  more  nearly  east 
md  west  than  it  was  when  first  made." 

According  to  the  theory,  the  highest  elevations  of  the  earth's  surface 
.hould  be  equatorial,  and  should  have  a  general  north  and  south  trend, 
vhile  in  the  northern  hemisphere  the  main  direction  of  the  masses  of 
and  should  bend  round  towards  north-east,  and  in  the  opposite  hemi- 
phere  towards  south-east.  Prof.  Darwin  thinks  that  the  general  facts  of 
errestrial  geography  tend  to  corroborate  his  theoreticjil  views,  though  he 
dmits  that  some  are  very  unfavourable  to  them.  In  the  discussion  of 
uch  a  theory,  however,  we  must  remember  that  the  present  mountain 
hains  on  the  earth's  surface  are  not  aboriginal,  but  arose  at  many 
uccessive  and  widely-separated  epochs.  Now  it  is  quite  certain  that  the 
ounger  mountain-chains  (and  these  include  the  loftiest  on  the  surface  of 
he  globe)  arose,  or  at  least  received  their  chief  upheaval,  during  the 
'ertiary  periods — a  comparatively  late  date  in  geological  history.  Unless 
re  are  to  enlarge  enormously  the  limits  of  time  which  physicists  are 
filling  to  concede  for  the  evolution  of  the  whole  of  that  history,  we  can 
ardly  suppose  that  the  elevation  of  the  great  mountain-chains  took  place 
t  an  epoch  at  all  approaching  an  antiquity  of  45,000,000  years.  Yet, 
ccording  to  Prof.  Darwin's  showing,  the  supei-ficial  effects  of  internal 
istortion  must  have  been  exceedingly  minute  during  the  past  45,000,000 


294  DYNAMICAL  GEOLOGY  BOOKmPAKri 

years.  We  must  either  therefore  multiply  enormously  the  periods  re- 
quired for  geological  changes,  or  find  some  cause  which  could  have 
elevated  great  mountain-chains  at  more  recent  intervals. 

But  it  is  well  Worth  consideration  whether  the  cause  suggested  hy 
Prof.  Darwin  may  not  have  given  their  initial  trend  to  the  masses  of  land, 
so  that  any  subsequent  wrinkling  of  the  terrestrial  surface,  due  to  any 
other  cause,  would  bo  apt  to  take  place  along  the  original  lines.  To  be 
able  to  answer  this  question,  it  is  necessary  to  ascertain  the  dominant  line 
of  strike  of  the  older  geological  formations.  But  information  on  this 
subject  is  still  scanty.  In  Western  Eiu'ope,  the  prevalent  line  along 
which  terrestrial  plications  took  place  during  Palaeozoic  time  was  certainly 
from  S.W.  or  S.S.W.  to  N.E.  or  N.N.E.,  and  the  same  direction  is  recog- 
nisable in  the  eastern  Stat<;s  of  North  America.  But  the  trend  of  later 
formations  is  more  varied.  The  striking  contradictions  between  the 
actual  direction  of  so  many  mountain-chains  and  masses  of  land,  and 
what  ought  to  be  their  line  according  to  the  theory,  seem  to  indicate  that 
while  the  effects  of  internal  distortion  may  have  given  the  first  outlines 
to  the  land-areas  of  the  globe,  some  other  cause  has  been  at  work  in  later 
times,  acting  sometimes  along  the  original  lines,  sometimes  across  them. 

The  main  cause  to  which  geologists  ;ire  now  disposed  to  refer  the 
corrugations  of  the  earth's  suiiace  is  secular  cooling  and  consequent  con- 
traction.^ If  our  planet  has  been  steadily  losing  heat  by  radiation  into 
si)ace,  it  must  have  progressively  diminished  in  volume.  The  cooling 
implies  contraction.  According  to  Mallet,  the  diameter  of  the  earth  is 
less  by  at  least  189  miles  since  the  time  when  the  planet  was  a  mass  of 
liquid.^  But  the  conti-action  has  not  manifested  itself  uniformly  over  the 
whole  surface  of  the  planet.  The  crust  varies  much  in  structure,  in 
thermal  resistance,  and  in  the  position  of  its  isogeothcrmal  lines.  As  the 
hotter  nucleus  contracts  more  rapidly  by  cooling  than  the  cooled  and 
hardened  crust,  the  latter  must  sink  down  by  its  own  weight,  and  in  so 
doing  requires  to  accommodate  itself  to  a  continually  diminishing  diameter. 
The  descent  of  the  crust  gives  rise  to  enormous  tangential  pressures.  The 
rocks  are  crushed,  cnimpled,  and  broken  in  many  places.  Subsidence  must 
have  been  the  general  rule,  but  every  subsidence  would  doubtless  be 
accompanied  with  upheavals  of  a  more  limited  kind.  The  direction  of 
these  upheaved  tracts,  whether  determined,  as  Prof.  Darwin  suggests,  by 
the  effects  of  internal  distortion,  or  by  some  original  features  in  the 
structiu'e  of  the  crust,  would  l>e  apt  to  be  linear.  The  lines,  once  taken 
as  lines  of  weakness  or  relief  from  the  intense  strain,  would  probably  be 
made  use  of  again  and  again  at  successive  paroxysms  or  more  tranquil 
}>eriods  of  contraction.  Mallet  ingeniously  connected  these  movements 
with  the  linear  direction  of  mountain-chains,  volcanic  vents,  and  earth- 
quake shocks.  If  the  initial  trend  to  the  land-masses  were  given  as 
hypothetically  stated  by  Prof.  Darwin,  we  may  conceive  that  after  the 
outer  parts  of  the  globe  had  attained  a  considerable  rigidity  and  coukl 

*  For  an  able  criticism  of  this  view  see  Fisher's  *  Physics  of  Earth's  Crust,*  2nd  Edit 
Consult  also  Mr.  Reade's  'Origin  of  Mountain  Ranges.' 
-  Phil.  Trans.  1873,  \\  205. 


SECT.  iii§3    CAUSES  OF  UPHEAVAL  AND  SUBSIDENCE  296 


then  be  only  slightly  influenced  by  internal  distortion,  the  effects  of 
continued  secular  contraction  would  be  seen  in  the  intermittent  subsidence 
of  the  oceanic  basins  already  existing,  and  in  the  successive  crumpling  and 
elevation  of  the  intervening  stiffened  terrestrial  ridges. 

This  view,  variously  modified,  has  been  Avidely  accepted  by  geologists 
as  furnishing  an  explanation  of  the  origin  of  the  upheavals  and  subsid- 
ences of  which  the  earth's  crust  contains  such  a  long  record.  But  it  is  not 
unattended  with  objections.  The  difficulty  of  conceiving  that  a  globe 
poesessing  on  the  whole  a  rigidity  equal  to  that  of  glass  or  steel  could  be 
corrugated  as  the  crust  of  the  earth  has  been,  has  led  some  writers  to 
adopt  the  hypothesis  already  described  {ante,  p.  56),  of  an  intermediate 
viscous  layer  between  the  solid  crust  and  the  solid  nucleus,  while  others 
have  suggested  that  the  observed  subsidence  may  have  been  caused,  or  at 
least  aggravated,  by  the  escape  of  vapours  from  volcanic  orifices.  But 
with  modifications,  the  main  cause  of  terrestrial  movements  is  still  sought 
in  secular  contraction. 

Some  observers,  following  an  original  suggestion  of  Babbage,^  have 
supposed  that  upheaval  and  subsidence,  together  with  the  solidification, 
crystallization,  and  metamorphism  of  the  layers  of  the  earth's  crust,  may 
have  been  in  large  measure  due  to  the  deposition  and  removal  of  mineral 
matter  on  the  surface.  There  can  be  no  doubt  that  the  lines  of  equal 
internal  temperature  (isogeothermal  lines)  for  a  considerable  depth  down- 
ward, follow  approximately  the  contours  of  the  surface,  curving  up  and 
down  as  the  surface  rises  into  mountains  or  sinks  into  plains.  The  de- 
position of  a  thousand  feet  of  rock  will,  of  course,  cause  a  corresponding 
rise  in  the  isogeotherms,  and  if  we  assume  the  average  rise  of  temperature 
to  be  1°  Fahr.  for  every  50  feet,  then  the  temperature  of  the  crust 
immediately  below  this  deposited  mass  of  rock  will  be  raised  20°.  But 
masses  of  sediment  of  much  greater  thickness  have  been  laid  down,  and 
we  may  admit  that  a  much  greater  increase  of  temperatiu'e  than  20°  has 
been  effected  by  this  means.  On  the  other  hand,  the  denudation  of  the 
land  must  lead  to  a  depression  of  the  isogeotherms,  and  a  consequent 
cooling  of  the  upper  layers  of  the  crust. 

It  may  be  conceded  that  in  so  far  as  the  internal  structure  of  rocks 
may  be  modified  by  such  progressive  increase  of  temperature  as  would 
arise  from  superficial  deposit,  this  cause  of  change  must  have  a  place  in 
geological  dynamics.  But  it  has  been  urged  that  besides  this  effect,  the 
removal  of  rock  by  denudation  from  one  area  and  its  accumulation  upon 
another  affects  the  equilibrium  of  the  crust ;  that  the  portions  where  de- 
nudation is  active,  being  relieved  of  weight,  rise,  while  those  where 
deposition  is  prolonged,  being  on  the  contrary  loaded,  sink.^  This  hypo- 
thesis has  recently  been  strongly  advocated  by  some  of  the  geologists  who 
have  been  exploring  the  Western  Territories  of  America,  and  who  point 

'  Journ.  Oeol.  Soc.  iii.  (1834),  p.  206. 

^  Similarly  it  has  been  contended  that  the  accumulation  of  a  massive  ice-sheet  on  the 
land  would  cause  a  depression  of  the  terrestrial  surface.  N.  S.  Shaler,  Proc.  Boston  Nat. 
Hist.  Soc.  xvii.  p.  288.  T.  F.  Jamieson,  Quart.  Journ.  Oeol.  fihc.  1882,  and  Oeol.  Mag. 
1882,  pp.  400,  526.     Fisher,  *  Physics  of  Earth's  Cnist,'  p.  223. 


296  JJYXAMICAL  HEOLOGY  book  lU  past  I 

in  proof  of  its  truth  to  evidence  of  continuous  subsidence  in  tracts  where 
there  was  prolonged  deposition,  and  of  the  uprise  and  curvature  of 
originally  horizontal  strata  over  mountain  ranges  like  the  Uinta  Mountaiiu 
in  Wyoming  and  Utah,  which  have  been  for  a  long  time  out  of  water. 
To  suppose,  however,  that  the  removal  and  deposit  of  a  few  thousand  feet 
of  rock  should  so  seriously  affect  the  equilibrium  of  the  crust  as  to  cause 
it  to  sink  and  rise  in  proportion,  would  e\ince  such  a  mobility  in  the  earth 
as  could  not  fail  to  manifest  itself  in  a  far  more  powerful  way  under  the 
influence  of  lunar  and  solar  attraction.  That  there  has  always  been  the 
closest  relation  between  upheaval  and  denudation  on  the  one  hand,  and 
subsidence  and  deposition  on  the  other,  is  undoubtedly  true.  But  denuda- 
tion has  been  one  of  the  consequences  of  upheaval,  and  deposition  has 
been  kept  up  only  by  continual  subsidence. 

We  are  concerned  in  the  present  part  of  this  volume  only  with  the 
surface  features  of  the  land  in  so  far  as  they  bear  on  questions  of  geo- 
logical dynamics.  The  history  of  these  features  will  be  more  conveniently 
treated  in  Book  VIL  after  the  structure  and  history  of  the  crust  have 
been  described.  Before  quitting  the  subject,  however,  we  may  observe 
that  the  larger  terrestrial  features,  such  as  the  great  ocean  basins,  the 
lines  of  submarine  ridge  surmounted  here  and  there  by  islands  chiefly  of 
volcanic  materials,  the  continental  masses  of  land,  and  at  least  the  cores 
of  most  great  mountain  chains,  are  in  the  main  of  high  antiquity,  stamped 
as  it  were  from  the  earliest  geological  ages  on  the  physiognomy  of  the 
globe,  and  that  their  present  aspect  has  been  the  result  not  merely  of 
original  hypogene  operations,  but  of  long-continued  superficial  action  by 
the  epigene  forces  described  in  Book  III.  Part  II. 


Section  iv.  Hypogene  Causes  of  Changes  in  the  Texture* 
Structure,  and  Composition  of  Rocks. 

The  phenomena  of  hy|X)gene  action  considered  in  the  foregoing  pages 
relate  almost  wholly  to  the  effects  produced  at  the  surface.  It  is  evident, 
however,  that  these  phenomena  chiefly  arise  from  movements  within  or 
beneath  the  earth's  crust,  and  must  be  accom^mnied  by  very  considerable 
internal  changes  in  the  rocks  which  form  that  crust  These  rocks^ 
subjected  to  enormous  pressure,  have  been  contorted,  crumpled,  and 
folded  back  u])on  themselves,  as  if  thousands  of  feet  of  solid  limestones, 
sandstones,  and  shales  had  been  merely  a  few  layers  of  carpet ;  they  have 
been  shattered  and  fractured  ;  they  have  in  some  places  been  pushed  far 
above  their  original  position,  in  others  depressed  far  beneath  it :  so  great 
has  l>een  the  compression  which  they  have  imdergone  that  their  com- 
ponent pai-ticles  have  in  many  places  been  rearranged,  and  even  crystal- 
lized. They  have  here  and  there  probably  been  reduced  to  actual  fusion, 
and  have  been  abundantly  invaded  by  masses  of  molten  rock  from  below. 

In  the  present  section,  the  student  is  asked  to  consider  chiefly  the 
nature  of  the  agencies  by  which  such  changes  can  be  effected ;  the  results 
achieved,  in  so  far  as  they  constitute  part  of  the  architecture  or  structure 


8BCT.  iv  §  1  EFFECTS  OF  HEAT  ON  ROCKS  297 

of  the  earth's  crusty  will  be  discussed  in  Book  IV.  At  the  outset,  it  is 
evident  that  we  can  hardly  hope  to  detect  many  of  these  processes  of 
subterranean  change  actually  in  progress  and  watch  their  effects.  The 
very  vastness  of  some  of  them  places  them  beyond  our  direct  reach,  and 
we  can  only  reason  regarding  them  from  the  changes  which  we  see  them 
to  have  produced.  But  a  good  number  are  of  a  kind  which  can  in  some 
measure  be  imitated  in  laboratories  and  furnaces.  It  is  not  requisite, 
therefore,  to  speculate  wholly  in  the  dark  on  this  subject.  Since  the  early 
and  classic  researches  of  Sir  James  Hall,  great  progress  has  been  made  in 
the  investigation  of  hypogene  processes  by  experiment.  The  conditions 
of  nature  have  been  imitated  as  closely  as  possible,  and  varied  in  different 
ways,  with  the  result  of  giving  us  an  increasingly  clear  insight  into  the 
I^ysics  and  chemistry  of  subterranean  geological  changes.  The  following 
pages  are  chiefly  devoted  to  an  illustration  of  the  nature  of  hypogene 
action,  in  so  far  as  that  can  be  inferred  from  the  results  of  actual  experi- 
ment. The  subject  may  be  conveniently  treated  under  three  heads — 
1.  The  effects  of  mere  heat ;  2.  the  influence  of  the  co-operation  of  heated 
water  ;  3.  the  effects  of  compression,  tension,  and  fracture. 

§  1.  Effects  of   Heat. 

The  importance  of  heat  among  the  transformations  of  rocks  has 
been  fully  admitted  by  geologists,  since  it  used  to  be  the  watchword  of 
the  Huttonian  or  Vulcanist  school  at  the  end  of  last  century.  Three 
sources  of  subterranean  heat  may  have  at  different  times  and  in  different 
degrees  co-operated  in  the  production  of  hypogene  changes — the  original 
Internal  heat  of  the  globe,  the  heat  arising  from  chemical  changes  within 
the  crust  or  beneath  it,  and  the  heat  due  to  the  transformation  of  mechanical 
energy  in  the  crumpling,  fracturing,  and  crushing  of  the  rocks  of  the 
i^rust 

Rise  of  temperature  by  depression. — As  stated  above  (p.  295),  the 
mere  recession  of  rocks  from  the  surface  owing  to  superposition  of  newer 
lepoeits  upon  them  will  cause  the  isogeotherms,  or  lines  of  equal  sub- 
terranean temperature,  to  rise — in  other  words,  >vill  raise  the  temperature 
>f  the  masses  so  withdrawn.  This  can  take  place,  however,  to  but  a 
limited  extent,  unless  combined  with  such  depression  of  the  crust  as  to 
idmit  of  thick  sedimentary  formations.  From  the  rate  of  increment 
)f  temperature  downwards  it  is  obvious  that,  at  no  great  depth,  the  rocks 
must  be  at  the  temperature  of  boiling  water,  and  that  further  down,  but 
jtill  at  a  distance  which,  relatively  to  the  earth's  radius,  is  small,  they 
nay  reach  and  exceed  the  temperatures  at  which  they  would  fuse  at  the 
nirface.  Mere  descent  to  a  great  depth,  however,  will  not  necessarily 
"esult  in  any  marked  lithological  change,  as  has  been  shown  in  the  cases 
)f  the  Nova  Scotian  and  South  Welsh  coal-fields,  where  sandstones,  shales, 
;lays,  and  coal-seams  can  be  proved  to  have  been  once  depressed  14,000 
;o  17,000  feet  below  the  sea-level,  under  an  overlying  mass  of  rock,  and 
ret  to  have  sustained  no  more  serious  alteration  than  the  partial  conversion 
)f  the  coal  into  anthracite.      They  have  been   kept  for  a  long  period 


298  DYNAMICAL  GEOLOGY  book  m  parti 

exposed  to  a  temperature  of  at  least  212''  Fahr.  Such  a  temperature  would 
have  been  sufficient  to  set  some  degree  of  internal  change  in  progress, 
had  any  appreciable  quantity  of  water  been  present,  whence  the  absence 
of  alteration  may  perhaps  be  exi)licablc  on  the  supposition  that  these  rocks 
were  comparatively  dry  (p.  305). 

Rise  of  temperature  by  chemical  transformation. — To  what  extent 
this  cause  of  internal  heat  may  be  operative,  forms  part  of  an  obscure 
problem.  But  that  the  access  of  water  from  the  surface,  and  the  con- 
sequent hydration  of  previously  anhydrous  minerals  must  produce  local 
augmentation  of  temperature,  cannot  be  doubted.  The  conversion  of 
anhydrite  into  gypsum,  which  takes  place  rapidly  in  some  mines,  gives  rise 
to  an  increase  of  volume  of  the  substance  (p.  345).  Besides  the  remark- 
able manner  in  which  the  rock  is  torn  asunder  by  minute  clefts,  crystals 
of  bitter-spar  and  quartz  are  reduced  to  fragments.^  The  amount  of  heat 
evolved  during  this  process  is  capable  of  measurement  The  conversion 
of  limestone  into  dolomite,  on  the  other  hand,  which  involves  a  diminution 
of  volume,  might  likewise  be  made  the  subject  of  similar  experimental 
inquiry.  Experiments  with  various  kinds  of  rocks,  such  as  clay-elate, 
clay,  and  coal,  show  that  when  these  substances  are  reduced  to  powder 
and  mixed  with  water,  they  evolve  hoat.- 

Rise  of  temperature  by  rock-crushing. — A  fiu-ther  store  of  heat 
is  pro\4ded  by  the  internal  crushing  of  rocks  during  the  collapse  and 
re-adjustment  of  the  cnist.  The  amount  of  heat  so  produced  has  been 
made  the  subject  of  direct  experiment.  Daubr^e  has  shown  that,  by  the 
mutual  friction  of  its  parts,  firm  brick-clay  can  be  heated  in  three-quarters 
of  an  hour  from  a  temj)erature  of  18^  to  one  of  40'  C.  (65°  to  104°  Fahr.)* 
The  most  elaborate  and  carefully  conducted  series  of  experiments  yet 
made  in  this  subject  are  those  conducted  by  Mallet.  He  subjected  16 
varieties  of  stone  (limestone,  marble,  porphyry,  granite,  and  slate)  in  cubes 
averaging  rather  less  than  1^  inches  in  height  to  pressures  sufficient  to 
crush  them  to  fragments,  and  estimated  the  amount  of  pressure  required, 
and  of  heat  produced.  The  following  examples  may  be  selected  from  his 
table :  * — 

*  The  microscopic  structure  of  the  stages  in  the  couversion  of  anhydrite  into  g^'psom  is 
described  by  F.  Hamnierschuudt,  Tschermak's  Mineral.  MitthirU,  v.  (1883),  p.  272. 

-  W.  Skey,  Ch^m.  Xcws^  xxx.  p.  290. 

*'  'Gi'ol.  ExpiTinientale, '  p.  448  et  seq.  This  distinguished  chemist  and  geologist  has 
during  the  last  forty  years  devoted  much  time  to  researches  designed  to  illostnte  ex- 
j>erimentany  the  processes  of  geology.  His  numerous  imix)rtant  memoirs  are  scattered 
through  the  Annnles  des  Mines,  Ctnnptes  Rendus  de  VAauUmie,  Bulletin  de  la  SociUi 
Otologiqiie  de  Franre,  and  other  ]>ublicationR.  But  he  has  collected  and  republished 
them  as  'Etudes  Synthrtiques  de  Geologic  Expt'rimentale,'  8vo,  1879 — a  storehouse  of 
information.  The  admirable  memoirs  of  Delesse  in  the  same  journals  should  also  be  studied. 
The  transformation  of  aragonite  into  calcite  has  been  shown  by  Favre  and  Silbermaim  to 
give  rise  to  a  relatively  large  disengagement  of  heat.  H.  Le  Chatelier,  Compf,  rend.  (1893), 
p.  390. 

*  Phil.  Tram.  1873,  p.  187. 


ECT.  iv  §  1 


SOURCES  OF  HEAT 


299 


Rock. 

Temperature 

(Fahr.)  in 

1  cubic  foot  of 

rock  due  to  work 

of  crushing. 

Number  of  cubic 
feet  of  water  at 
32  deg.  evapo- 
rated Into  steam 
at  212  deg. 

Volume  of  ice  at 
82  deg.  melted  to 

water  at  32  deg. 

by  one  volume  of 

rock. 

Caen  Stone,  Oolite  .... 
Sandstone,  Ayre  Hill,  Yorkshire 
Slate,  Conway         .... 
Granite,  Aberdeen  .... 
Scotch  furnace-clay  porphyry  . 
Rowley  Rag  (basalt) 

8^004 
47".79 
132^85 
155'-94 
198*.97 
213**.23 

0-0046 

0-0234 

0-07 

0-072 

0-083 

0-109 

0-04008 

0-2026 

0-596 

0-617 

0-724 

0-925 

Within  the  crust  of  the  earth,  there  are  abundant  proofs  of  enormous 
tresses  under  which  the  rocks  have  been  crushed.  The  weight  of  rock 
nvolved  in  these  movements  has  often  been  that  of  masses  several  miles 
hick.  We  can  conceive  that  the  heat  thus  generated  may  have  been 
ufBcient  to  promote  many  chemical  and  mineralogical  rearrangements 
hrough  the  operation  of  water  (postea,  p.  305),  and  may  even  have  been 
ere  and  there  enough  for  the  actual  fusion  of  the  rocks  by  the  crushing 
f  which  it  was  produced. 

Rise  of  temperature  by  intrusion  of  erupted  rock. — The  great 
leat  of  lava,  even  when  it  has  flowed  out  over  the  surface  of  the  earth, 
as  been  already  referred  to,  and  some  examples  have  been  given  of  its 
fleets  (pp.  226,  230).  Where  it  does  not  reach  the  surface,  but  is  injected 
nto  subterranean  rents  and  passages,  it  must  effect  considerable  changes 
ipon  the  rocks  with  which  it  comes  in  contact.  That  such  intruded 
^eous  rocks  have  sometimes  melted  down  portions  of  the  crust  in  their 
lassage,  can  hardly  be  doubted.  But  probably  still  more  extensive 
hanges  may  take  place  from  the  exceedingly  slow  rate  of  cooling  of 
rupted  masses,  and  the  consequently  vast  period  during  which  their 
leat  is  being  conveyed  through  the  adjacent  rocks.  Allusion  vnH  be 
aade  in  later  pages  to  the  observed  amount  of  such  "  contact-meta- 
Qorphism  "  (p.  597  et  seq). 

Expansion. — Rocks  are  dilated  by  heat.  The  extent  to  which  this 
akes  place  has  been  measured  with  some  precision  for  various  kinds  of 
ock,  as  shown  in  the  subjoined  table  : — 


Rock. 


Linear  expansion  for 
every  1°  Fahr. 


Black   marble,  Galway,   Ire-  \  , 
land  .         .         .         .  /  I 

Grey  granite,  Aberdeen 

Slate,  Penrhvu,  Wales . 

White  marble,  Sicily    . 

Red      sandstone,     Poitland, 
Connecticut 


Authority. 


':} 


-00000247  =  ;joiVin» 
-00000438  =  5yT,»5r^ 

-00000576  =  rnWT 
•00000613  =  iT,^»n7 

-00000953  =  TTnVuir 


{Adie,  Trans.  Hoy.  Soc.  Edin. 
xiii.  p.  366. 
Ihid. 
Ibid. 
Ibid. 
/Totten,    Amer.    Journ.    Sci. 
\      xxii.  (1832).  136.1 


*  For  additional  results,    see  Mellard  Reade's    *  Origin  of   Mountain  Ranges '  (1886), 
.  109. 


300  DYXAMICAL  GEOLOGY  book  in  paw  i 

According  to  these  data,  the  expansion  of  ordinary  rocks  ranges  from 
jilK)ut  2  4  7  to  9 '6  3  million ths  for  V  Fahr.      Even  ordinary  daily  and 
seasonal   changes  of  temperature  suffice  to  produce  considerable  super- 
ficial changes  in  rocks  (see  p.  328).     The  much  higher  temperatures  to 
which  rocks  are  exposed  by  subsidence  within  the  earth's  crust  must 
have  far  greater  effects.     Some  experiments  by  Pfaff  in  heating  from  an 
ordinary  tem|>erature  up  to  a  red  heat,  or  about  1180'  C,  small  columns 
of  granite  from  the  Fichtelgobirge,  red  porphyry  from  the  Tyrol,  and 
basalt  from  Auvergne,  gave  the  expansion  of  the  granite  as  0*016808,  of 
the  porph}Ty  0*012718,  of  the  Ijasalt  0  01199.^      The  expansion  and 
contraction  of  rocks  by  heating  and  cooling  have  been  already  referred 
to  as  possible  sources  of  upheaval  and  depression  (p.  292).     Mr.  Mellaid 
Heade  concludes   from   his  exi)eriments   that   the   mean  co-efficient  of 
eximnsion  for  various  classes  of  rocks  may  be  taken  as  -nnri"5Y  ^or  each 
degree  Fahr.,  which  would  be  equivalent  to  an  expansion  of  2*77  feet 
per  mile  for  every  lOO""  Fahr.^ 

Crystallization. — In  the  experiments  of  Sir  James  Hall,  pounded 
chalk,  hemietically  enclosed  in  gun-barrels  and  exposed  to  the  temperature 
oi  melting  silver,  was  melted  and  partially  crystallized,  but  still  retained 
its  carbonic  acid.  Chalk,  similarly  exposed,  viith  the  addition  of  a  little 
water,  was  transformed  to  the  state  of  mar})le.'  These  experiments  have 
been  repeated  by  G.  Rose,  who  produced  l)y  dry  heat  from  lithographic 
limestone  and  chalk,  fine-gi'ained  marble  without  melting.  The  dis- 
tinction of  marble  is  the  independent  crystalline  condition  of  its  component 
granules  of  calcit^.  This  stnicture,  therefore,  can  be  superinduced  by 
heat  under  pressure.  In  natiu'e,  portions  of  limestone  which  have  been 
invaded  by  intrusive  masses  of  igneous  rock,  have  been  converted  into 
marble,  the  gradations,  fi*om  the  unaltered  into  the  altered  rock  being 
distinctly  traceable,  as  will  be  shown  in  subsequent  pages  (p.  602). 

Production  of  prismatic  structure. — The  long-continued  high 
temperature  of  iron-fm*naces  has  been  observed  to  have  superinduced 
a  prismatic  or  columnar  structure  upon  the  hearth-stones,  and  on  the 
sand  in  which  these  are  bedded.*  This  fact  is  of  interest  in  geology, 
seeing  that  sandstones  and  other  rocks  in  contact  Anth  eruptive  masses 
of  igneous  matter  have  at  various  depths  below  the  siu-face  assumed  a 
similar  internal  arrangement  (p.  599). 

Dry  fusion. — In  an  interesting  series  of  experiments,  the  illustrioiis 
Dc  Saussure  (1779)  fused  some  of  the  rocks  of  S>vitzerland  and  France, 
and  inferred  from  them,  contrary  to  the  opinion  j^reviously  expressed 
by  Desmarest,^  that  basalt  and  lava  have  not  been  produced  from  granite, 
but  from  hornstone  (pierre  de  come),  varieties  of  "  schorl,"  calcareous 
clays,  marls,  and  miciiceous  earths,  and  the  cellular  varieties  from  different 
kinds  of  slate.®    He  observed,  however,  that  the  artificial  products  obtained 

'  /f.  Ik'titsch.  iieitl,  Ges.  xxiv.  ji.  403.  -  *  Orijfin  of  Monntain  RaiigeK/  !»•  HO. 

•'  Trans.  Hoy,  Site.  Eitin.  vi.  (1805),  pp.  101,  121.     See  note  on  next  iwge. 

*  C  Coclirane,  Proc.  Jindley  Oetjl.  Stc,  iii.  ]>.  54. 

•^  Mt'm,  Acail.  $Scien.  1771,  p.  273. 

*'  I>e  Saussure,  'Voyages  dans  les  Alpes,*  edit.  1803,  tome  i.  p.  178. 


SCT.  iv  §  1  EXPERIMENTS  IN  FUSION  301 


y  fusion  were  glassy  and  enamel-like,  and  did  not  always  recall  volcanic 
xks,  though  some  exactly  resembled  porous  lavas.  Dolomieu  (1788)  also 
mtended  that  as  an  artificially-fused  lava  becomes  a  glass,  and  not  a  cry- 
balline  mass  with  crystals  of  easily  fusible  minerals,  there  must  be  some  flux 
resent  in  the  original  lava,  and  he  supposed  that  this  might  be  sulphur.^ 

Sir  James  Hall,  about  the  year  1790,  began  an  important  investiga- 
iOn,  in  which  he  succeeded  in  reducing  various  ancient  and  modem 
olcanic  rocks  to  the  condition  of  glass,  and  in  restoring  them,  by  slow 
3oling,  to  a  stony  condition  in  which  distinct  crystals  (probably  pyroxene, 
livine,  and  perhaps  enstatite)  were  recognisable.^  Gregory  Watt  after- 
wards obtained  similar  results  by  fusing  much  larger  quantities  of  the 
3cks.  In  more  recent  years,  this  method  of  research  has  been  resumed 
ad  pursued  with  the  much  more  effective  appliances  of  modern  science, 
otably  by  Mitscherlich,  G.  Kose,  C.  Sainte-Claire  Deville,  Delesse,  Daubr^e, 
*(mqu^,  Michel-L^vy,  Friedel,  and  Sarasin.  It  has  been  experimentally 
roved  that  all  rocks  undergo  molecular  changes  when  exposed  to  high 
9mperature,  that  when  the  heat  is  sufficiently  raised,  they  become  fluid, 
liat  if  the  glass  thus  obtained  is  rapidly  cooled  it  remains  vitreous,  and 
iiat^  if  allowed  to  cool  slowly,  a  more  or  less  distinct  crystallization  sets 
1,  the  glass  is  devitrified,  and  a  lithoid  product  is  the  result. 

A  glass  is  an  amorphous  substance  resulting  from  fusion,  perfectly 
lOtropic  in  its  action  on  transmitted  polarized  light  (ante,  pp.  114,  120). 
bB  specific  gravity  is  rather  lower  than  that  of  the  same  substance  in  the 
rystallized  condition.  By  being  allowed  to  cool  slowly,  or  being  kept  for 
ome  hours  at  a  heat  which  softens  it,  glass  assumes  a  dull  porcelain-like 
Bpect  This  devitrification  possesses  much  interest  to  the  geologist, 
9eing  that  most  volcanic  rocks,  as  has  been  already  described  (p.  120), 
resent  the  characters  of  devitrified  glasses.  It  consists  in  the  appearance 
f  minute  crystallites,  and  other  imperfect  or  nidimentary  crystalline 
>rms,  accompanied  with  an  increase  of  density  and  diminution  of  volume, 
b  must  be  regarded  as  an  intermediate  stage  between  the  perfectly  glassy 
nd  the  crystalline  conditions.  Rocks  exposed  to  temperatures  as  high  as 
heir  melting-points  fuse  into  glass  which,  in  the  great  majority  of  cases, 
I  of  a  bottle-green  or  black  colour,  the  depth  of  the  tint  depending 
lainly  on  the  proportion  of  iron.  In  this  respect  they  resemble  the 
atural  glasses — ^pitchstones  and  obsidians.  Microscopic  investigation  of 
uch  artificially-fused  rocks  shows  that,  even  in  what  seems  to  be  a  tolerably 
omogeneous  glass,  there  are  abundant  minute  hair-like,  feathered,  needle- 
haped,  or  irregularly-aggregated  bodies  diffused  through  the  glassy  paste, 
liese  crystallites,  in  some  cases  colourless,  in  others  opaque,  metallic 
xides,  particularly  oxides  of  iron,  resemble  the  crystallites  observed  in 
lany  volcanic  rocks  (p.  115).  They  may  be  obtained  even  from  the 
iision  of  a  granitic  or  granitoid  rock,  as  in  the  well-known  case  of  the 

*  *Ile«  Ponces/  p.  8  <^  seq.  At  temperatures  between  2000°  and  SOOO**  C,  various 
letallic  oxides  are  fused  and  crystallize.     H.  Moissan,  Compt.  rend,  cxv.  (1892),  p.  1034. 

•  Trans,  Roy,  Soc.  Edin.  v.  p.  48.  Hall's  actual  products  have  been  microscopically 
camined  by  Fouque  and  Michel-L^vy.  CoinpUs  rend.  May  1881.  For  repetitions  of  his 
isioD  of  limestone,  op.  cit,  cxv.  (1892),  pp.  817,  934,  1009,  1296. 


302  DYNAMICAL  GEOLOGY  book  ra  paw  i 

Mount  Sorrel  syenito  near  Leicester,  which,  being  fused  and  slowly  cooled, 
yielded  to  Mr.  Sorby  abundant  crystallites,  including  exquisitely-grouped 
octohedra  of  magnetite.^ 

According  to  the  observations  of  Delesse,  volcanic  rocks,  when  reduced 
to  a  molten  condition,  attack  briskly  the  sides  of  the  Hessian  crucibles  in 
which  they  are  contained,  and  even  eat  them  through.  This  is  an 
interesting  fact,  for  it  helps  to  explain  how  some  intrusive  igneous  rocks 
have  come  to  occupy  positions  previously  filled  by  sedimentai^  strata,  and 
why,  under  such  circumstances,  the  composition  of  the  same  noass  of  rock 
should  be  found  to  vary  considerably  from  place  to  place.* 

The  most  elaborate  and  successful  experiments  yet  made  regarding 
the  fusion  of  igneous  rocks,  are  those  of  MM.  Fouqu^  and  Michel-Levy. 
These  observers,  by  mixing  the  chemical  elements  and,  in  other  cases,  the 
minemlogical  constituents,  of  certain  minerals  and  rocks,  and  fusing  these 
in  platinum  crucibles  in  a  gas-furnace,  have  been  able  to  produce  both  rock- 
forming  minerals,  such  as  several  felspars,  augite,  leucite,  nepheline,  and 
garnet,  and  also  rocks  possessing  the  composition  and  microscopic  structore 
of  augite-andesites,  leucite-tephrites,  and  true  basalts.  By  rapid  cooling, 
they  obtained  an  isotropic  glass,  often  full  of  bubbles,  and  varying  in 
colour  with  the  nature  of  the  mixture  from  which  it  was  formed.  Where 
the  mixture  contains  the  elements  of  pyroxene,  enstatite,  or  melilite,  it 
must  bo  cooled  very  rapidly  to  prevent  these  minerals  from  partially 
crystallizing  out  of  the  glass.  Nepheline  also  crystallizes  easily.  The 
felspars,  on  the  other  hand,  pass  much  more  slowly  from  the  viscous  to 
the  crystalline  condition.  In  these  experiments,  use  was  made  of  the  law 
that  the  fusion-temperature  of  a  crystallized  silicate  is  usually  higher  than 
that  of  the  same  substance  in  the  glassy  state.  Hence  if  such  a  glass  be 
kept  sufliciently  long  at  a  temperature  slightly  higher  than  that  at  which 
it  softens,  the  most  favourable  conditions  are  obtained  for  the  production 
of  molecular  arrangements  and  the  formation  of  those  crystalline  bodies 
which  can  solidify  in  the  midst  of  a  viscous  magma.  The  limits  of 
tem]>erature  for  the  production  of  a  given  mineral  must  thus  be  comprised 
>vithin  the  narrow  range  between  the  fusion-point  of  the  mineral  and  that 
of  its  glass.  By  varying  the  temj)erature  in  the  experiments,  distinct 
minerals  can  be  obtained  from  the  same  magma.  Minerals  such  as  olivine, 
leucite,  and  felspar,  which  solidify  at  higher  temperatures  than  the  others, 
appear  first,  and  the  later  forms  are  moulded  round  them.  Thus  an 
artificial  basalt,  like  a  natural  one,  always  shows  that  its  olivine  has 
crystallized  first.     By  providing  facilities  for  the  crystallization  of  the 

1  Zirkcl,  Mik.  Brsch.  p.  92  ;  Sorby,  Address  Geol,  Sect,  BriL  Assoc  1880.  On  the 
microscopic  structure  of  slags,  &c.,  see  Vogelsang's  *  Krystalliten. ' 

-  BuU,  Soc.  OSU.  France^  2nd  ser.  iv.  1882  ;  see  also  Trans,  Edin,  Roy,  Soc,  xzix.  pw 
492.  In  the  more  recent  experiments  by  Doelter  and  Hussak  no  change  was  obaenred  in  the 
porcelain  crucibles  in  which  basalt,  audesite  and  phonolite  were  melted.  Ne^tea  Jahrb,  1884, 
p.  19.  Bischof  has  described  a  series  of  experiments  on  the  fusion  of  lavas  with  diilerent 
proportions  of  clay-slate.  He  found  that  the  lava  of  Niedermendig,  kept  an  hoar  in  a 
bellows-furnace,  was  reduced  to  a  black  glassy  substance  without  pores,  and  that  a  similar 
product  was  obtained  even  after  30  ])er  cent  of  clay-slate  had  been  added  and!the  whole  had 
been  kept  for  two  hours  in  the  furnace.     *Chem.  und  Phys.  (Jeol.'  supp.  (1871),  p.  98. 


SECT,  iv  §  1  EXPERIMENTS  IN  FUSION  303 

minerals  in  the  inverse  order  of  their  fusibilities,  the  characters  of 
naturally  formed  crystalline  rocks  can  thus  be  artificially  produced  by 
simple  igneous  fusion. 

Certain  well-known  facts  which  appear  to  militate  against  the  principle 
of  these  experiments  have  been  successfully  explained  by  MM.  Fouqu^ 
and  Michel-L6vy.  Some  minerals,  very  difficult  to  fuse,  contain  crystals  of 
others  which  are  easily  fusible,  as  if  the  latter  had  crystallized  first,  as  in 
the  case  of  pyroxene  enclosed  within  leucite.  But  in  reality  the  pyroxene 
has  slowly  crystallized  out  of  inclusions  of  the  surrounding  glass  which 
were  caught  up  in  the  leucite.  Where  the  same  silicates  are  found  to 
have  crystalliz^  firat  in  large  and  subsequently  in  smaller  forms,  they  may 
reveal  stages  in  the  gradual  cooling  and  consolidation  of  the  mass,  one 
set  of  crystals,  for  example,  being  formed  in  a  lava  while  still  within  the 
vent  of  a  volcano,  and  another  during  the  more  rapid  cooling  after  expul- 
sion from  the  vent. 

The  rocks  obtained  artificially  by  these  observers  are  thus  classed  by 
them  : — 1.  Andesites  and  andesitic  porphy rites — from  the  fusion  of  a 
mixture  of  four  parts  of  oligoclase  and  one  of  augite.  2.  Labradorites 
and  labradoric  porphyrites — from  the  fusion  of  three  parts  of  labrador 
and  one  of  augite.  3.  A  microlitic  rock  formed  of  pyroxene  and 
anorthite.  4.  Basalts  and  labradoric  melaphyres — from  the  fusion  of  a 
mixture  of  six  parts  of  olivine,  two  of  augite  and  six  of  labrador. 
5.  Nephelinites — from  the  fusion  of  a  mixture  of  three  parts  of  nepheline 
and  1  '3  of  augite.  6.  Leucitites — from  the  fusion  of  nine  parts  of  leucite 
and  one  of  augite.  7.  Leucite-tephrite — from  the  fusion  of  a  mixture  of 
silica,  alumina,  potash,  soda,  magnesia,  lime,  and  oxide  of  iron,  represent- 
ing one  part  of  augite,  four  of  labrador,  and  eight  of  leucite.  8.  Lherzolite. 
9.  Meteorites  without  felspar.  10.  Meteorites  with  felspar.  11.  Dia- 
bases and  dolerites  with  ophitic  structiu'e.  In  these  artificially  produced 
compounds  the  most  complete  resemblance  to  natural  rocks  was  observed, 
down  even  to  the  minutiae  of  microscopic  structure.  The  crystals  and 
microlites  ranged  themselves  exactly  as  in  natural  rocks,  with  the  same 
distribution  of  vitreous  base  and  vitreous  inclusions.  It  is  thus 
demonstrated  that  a  rock  like  basalt  may  be  produced  in  nature  in  the 
dry  way,  by  a  process  entirely  igneous.^ 

More  recently,  another  series  of  experiments  has  been  carried  on  by 
Messrs.  Doelter  and  Hussak  of  Gratz,  to  determine  the  effect  of  immersing 
various  minerals  in  molten  basalt,  andesite,  or  phonolite.  Among  the 
results  obtained  by  them  are  the  production  of  a  granular  structure  in 

*  See  the  work  of  Messrs  Fouque  and  Michel  -  LiW-j-,  *S3nithese  dee  Min^raux  et  des 
Roches,'  1882,  from  which  the  above  digest  of  their  researches  is  taken.  Since  this  was 
written  I  have  had  the  advantage  of  being  shown  by  M.  Michel-L^vy  the  original  slides 
prepared  from  the  products  obtained  by  him  and  M.  Fouqu^,  and  I  can  entirely  corroborate 
the  results  at  which  these  observers  have  arrived.  They  have  succeeded  in  imitating  all  the 
essential  features  of  such  rocks  as  basalt,  down  even  into  minute  microscopic  details.  They 
have  produced  rocks,  not  only  showing  microlitic  forms,  but  with  crystals  of  the  con- 
stituent minerals  as  definitely  formed  as  in  any  natural  lava.  Indeed  it  would  be  hardly 
possible  to  distinguish  between  one  of  their  artificial  products  and  many  true  lavas. 


304  DYNAMICAL  GEOLOGY  book  m  parti 


I 


pyroxene  and  hornblende,  e8()ecially  along  the  lK>rder8,  as  may  be  observed 
in  the  hornblende  of  recent  eruptive  rocks ;  the  conversion  of  a  hornblende 
crystal,  which  still  retains  its  form,  into  an  aggregate  of  augite  prisms  and 
magnetite,  as  observed  also  in  some  basalts ;  the  conversion  of  garnet  into 
various  other  minerals,  such  as  meionite,  melilite,  anorthite,  Iime>oIivine, 
lime-nepheline,  specular  iron,  and  spinel,  the  garnet  itself  never  reappear- 
ing in  the  molten  magma.  ^ 

While  experiment  has  thus  shown  that  certain  eruptive  rocks  of  the 
l>asic  order,  such  as  basalts  and  augite-andesites,  may  be  produced  by 
mere  dry  fusion,  the  acid  rocks  present  difficulties  which  have  as  yet  proved 
insuperable  in  the  laboratory.  MM.  Fouque  and  Michel-L^vy  have  vainly 
endeavoured  to  reproduce  by  igneous  fusion  rocks  with  quartz,  orthoclaae, 
white  mica,  black  mica,  and  amphibole.  We  may  therefore  infer  that 
these  rocks  have  been  produced  in  some  other  way  than  by  dry  igneoos 
fusion.  The  acid  rocks,  terminating  in  granite,  fonn  a  remarkable  series, 
regarding  the  origin  of  which  we  are  still  completely  ignorant.  Some 
data  relating  to  their  production  >\ill  be  given  in  JJ  2  (p.  308)  in  connection 
\nth  the  co-ojjeration  of  undergi'ound  water. 

Contraction  of  rocks  in  passing  ft>om  a  glassy  to  a  stony  state. — 
Reference  has  been  made  (pp.  56,  292,  299)  to  the  expansion  of  rocks 
by  heat  and  their  contraction  on  cooling  ;  likewise  to  the  difference  between 
their  volume  in  the  molten  and  in  the  solid  state.  It  would  appear  that 
the  diminution  in  density,  as  rocks  pass  from  a  crystalline  into  a  vitreous 
condition,  is,  on  the  whole,  greater  the  more  silica  and  alkali  are  present^ 
and  is  less  as  the  proportion  of  iron,  lime,  and  alumina  increases. 
According  to  Delesse,  granites,  quartziferous  porphyries,  and  such  highly 
silicated  rocks  lose  from  8  to  1 1  per  cent  of  their  density  when  they  are 
reduced  to  the  condition  of  glass,  basalts  lose  from  3  to  5  per  cent,  and 
lavas,  including  the  vitreous  varieties,  from  0  to  4  per  cent.*  More 
recently.  Mallet  observed  that  plate-glass  (taken  as  representative  of  add 
or  siliceous  rocks)  in  passing  from  the  liquid  condition  into  solid  glass 
contracts  l'r)9  per  cent,  100  parts  of  the  molten  liquid  measuring  98*41 
when  solidified  ;  while  iron-slag  (ha>ing  a  composition  not  unlike  that  of 
many  basic  igneous  rocks)  contracts  6*7  per  cent,  100  parts  of  the 
molten  mass  measuring  93*3  when  cold.^  By  the  contraction  due  to  such 
changes  in  the  internal  condition  of  subterranean  masses  of  rock,  minor 
oscillations  of  level  of  the  surface  may  be  accounted  for,  as  already  stated 
(p.  292).     Thus,  the  vitreous  solidification  of  a  molten  mass  of  siliceous 

*  XtHes  Jnhrh.  1884,  \i\\.  18,  158.  Compare  also  A.  Becker's  experiments  in  melting 
pjToxenes  and  ampliiboles,  Zcitsch.  Deutsch.  fiettl.  (f'eself.  xxxvii.  (1885),  p.  10. 

'  Bull.  *Soc.  OkJ.  France,  1847,  p.  1390.  Bischof  had  determined  the  contraction 
of  granite  to  Ije  as  much  as  25  per  cent  (Leonhard  nnd  Bronn,  Jahrb,  1841).  The 
correctness  of  this  detennination  was  disputed  by  D.  Forbes  {Oed,  Mttg,  1870, 
p.  1),  who  found  from  his  own  experiments  that  the  amount  of  contraction  mwt 
lie  much  less.  Tlie  values  given  by  him  were  still  much  in  excess  of  thoee  afterwaidi 
obtained  vnih.  much  care  by  Mallet.  Compare  0.  Fisher,  *  Physics  of  the  Earth's  Cnut,' 
2nd  Edit.,  p.  45,  and  Bams  quoted  anU^  p.  56. 

"'  Phil.  Trans,  clxiii.  pp.  201,  204  ;  clxv.  ;  Proc.  Roy.  Soc.  xxii.  p.  828. 


fiECE.  iv  §  2  INFLUENCE  OF  HEATED  WATER  305 

rock  1000  feet  thick  might  cause  a  subsidence  of  about  16  feet,  while,  if 
the  rock  were  basic,  the  amount  of  subsidence  might  be  67  feet 

Sublimation. — It  has  long  been  known  that  many  mineral  substances 
can  be  obtained  in  a  crystalline  form  from  the  condensation  of  vapours 
(pp.  196,  228).  This  process,  called  Sublimation,  may  be  the  result  of  the 
mere  cooling  and  reappearance  of  bodies  which  have  been  vaporised  by 
heat  and  solidify  on  cooling,  or  of  the  solution  of  these  bodies  in  other 
vapours  or  gases,  or  of  the  reaction  of  different  vapours  upon  each  other. 
These  operations,  of  such  common  occurrence  at  volcanic  vents,  and  in 
the  crevices  of  recently  erupted  and  still  hot  lava-streams,  have  been  suc- 
cessfully imitated  by  experiment.  In  the  early  researches  of  Sir  James 
Hall  on  the  effects  of  heat  modified  by  compression,  he  obtained  by  sub- 
limation "  transparent  and  well-defined  crystals,"  lining  the  unoccupied 
portion  of  ia  hermetically-sealed  iron  tube,  in  which  he  had  placed  and 
exposed  to  a  high  temperature  some  fragments  of  limestone.^  Numerous 
experiments  have  been  made  by  Delesse,  Daubr6e,  and  others  in  the  pro- 
duction of  minerals  by  sublimation.  Thus,  many  of  the  metallic  sulphides 
found  in  mineral  veins  have  been  produced  by  exposing  to  a  comparatively 
low  temperature  (between  that  of  boiling  water  and  a  dull-red  heat)  tubes 
containing  metallic  chlorides  and  sulphide  of  hydrogen.  By  varying  the 
materials  employed,  corundum,  quartz,  apatite,  and  other  minerals  have 
been  obtained.  It  is  not  difi^cult,  therefore,  to  understand  how,  in  the 
crevices  of  lava-streams  and  volcanic  cones,  as  well  as  in  mineral  veins, 
sulphides  and  oxides  of  iron  and  other  minerals  may  have  been  formed  by  the 
ascent  of  heated  vapours.  Superheated  steam  is  endowed  with  a  remark- 
able power  of  dissolving  that  intractable  substance,  silica  ;  artificially 
heated  to  the  temperature  of  the  melting-point  of  cast-iron,  it  rapidly 
attacks  silica,  and  deposits  the  mineral  in  snow-white  crystals  as 
it  cools.  Sublimation,  however,  can  hardly  be  conceived  as  having 
operated  in  the  formation  of  rocks,  save  here  and  there  in  the  infilling  of 
open  fissures. 

§  2.     Influence   of   Heated    Water. 

In  the  geological  contest  fought  at  the  beginning  of  the  century 
between  the  Neptunists  and  the  Plutonists,  the  two  great  battle-cries 
were,  on  the  one  side,  Water,  on  the  other,  Fire.  The  progress  of  science 
since  that  time  has  shown  that  each  of  the  parties  had  some  truth  on  its 
side,  and  had  seized  one  aspect  of  the  problems  touching  the  origin  of 
rocks.  If  subterranean  heat  has  played  a  large  part  in  the  construction 
of  the  materials  of  the  earth's  crust,  water,  on  the  other  hand,  has  per- 
formed a  hardly  less  important  share  of  the  task.  They  have  often  co- 
operated together,  and  in  such  a  way  that  the  results  must  be  regarded  as 
their  joint  achievement,  wherein  the  respective  share  of  each  can  hardly 
be  exactly  apportioned.  In  Part  II.  of  this  Book  the  chemical  operation 
of  infiltrating  water,  at  ordinary  temperatures  at  the  surface,  and  among 
rocks  at  limited  depths,  is  described.       We  are  here  concerned  mainly 

^   Trans.  Roy,  Soc.  Edin.  vi.  p.  110. 

X 


306  DYNAMICAL  GEOLOGY  book  ra  parti 

with  the  work  done  by  water  when  within  the  influence  of  subterranean 
heat,  and  the  manner  in  which  this  work  can  be  experimentally  imitated. 

Presence  of  water  in  all  rocks. — Besides  its  combinations  in  hydrous 
minerals,  water  may  exist  in  rocks  either  (1)  retained  interstitially  among 
minute  crevices  and  i)ores,  or  (2)  imprisoned  within  the  microscopic  cells 
of  crystals. 

(1.)  By  numerous  observations  it  has  been  proved  that  all  rocks  within 
the  accessible  portion  of  the  earth's  crust  contain  interstitial  water,  or, 
as  it  is  sometimes  called,  quarry-water  {euu  de  carrikre).      This  is  not 
chemically   combined   with   their   mineral  constituents,  but   is   merely 
retained  in  their  pores.     Most  of  it  evaporates  when  the  stone  is  taken 
out  of  the  parent  rock,  and  freely  exposed  to  the  atmosphere.     The 
absorbent  powers  of  rocks  vary  gi*eatly,  and  chiefly  in  proportion  to  their 
degi'ee  of  porosity.     Gyi>sum  absorbs  from  about  0*50  to  1'50  per  cent 
of  water  by  weight;  granite,  about  0*37  \yer  cent;  quartz  from  a  vein  in 
granite,   0*08 ;    chalk,  about  20*0 ;    plastic   clay,    from    19*5    to    24*5. 
These  amounts  may  be  increased  by  exhausting  the  air  from  the  speci- 
mens and  then  immersing  them  in  water.^      No  mineral   substance  is 
strictly  impervious  to  the  passage  of  water.     The  well-known  artificial 
colouring  of  agates  proves  that  even  mineral  substances,  apparently  the 
most  homogeneous  and  impervious,  can  be  traversed  by  liquids.     In  the 
series  of  exi^eriments  above  referred  to  (p.  266),  Daubree  has  illustrated 
the  power  possessed  by  water  of  penetrating  rocks,  in  virtue  of  their 
porosity  and  capillarity,  even  against  a  considerable  counter-pressure  of 
vapour ;  and,  without  denying  the  presence  of  original  water,  he  concludes 
that  the  interstitial  water  of  igneous  rocks  may  all  have  been  derived  by 
descent  from  the  surface.     The  masterly  researches  of  Poiseuille  have 
shown  that  the  rate  of  flow  of  liquids  through  capillaries  is  augmented  by 
heat.     He  proved  that  water  at  a  temj)erature  of  45^  C.  in  such  situations 
moves  nearly  three  times  faster  than  at  a  temperature  of  0**  C.'*     At  the 
high  temperatures  under  which  the  water  must  exist  at  some    depth 
within  the  crust,  it^  ix)wer  of  penetrating  the  capillary  interstices  of  rocks 
must  be  increased  to  such  a  degree  as  to  enable  it  to  l>ecome  a  powerful 
geological  agent. 

(2.)  Reference  has  already  (p.  110)  l)een  made  to  the  presence  of 
minute  cavities,  containing  water  and  various  solutions,  in  the  crystals  of 
many  rocks.  The  water  thus  imprisoned  was  obviously  enclosed  with 
it<3  gases  and  saline  solutions,  at  the  time  when  these  minerals  crystallized 
out  of  their  jmrent  magmiu  The  quartz  of  granite  is  usually  full  of  such 
water- vesicles.  "  A  thousand  millions,"  says  Mr.  J.  Clifton  Ward, 
'^  might  easily  be  contained  within  a  cubic  inch  of  quartz,  and  sometimes 
the  contained  water  must  make  up  at  least  5  per  cent  of  the  whole 
volume  of  the  containing  quartz." 

^  See  au  interestiug  paper  by  Delcsse,  Bvfl.  *^»c.  <»Vf)/.  France,  2me  ser.  xix.  (1861-2), 
p.  65. 

-  Cowptes  Hendus  (1840),  xi.  p.  1048.  Pfaff  (*  AUgemeine  Geologic,'  p.  141)  con- 
chules  from  calculations  as  to  the  relations  between  pressure  and  tension  that  water 
may  descend  to  any  depth  in  fissures  and  remain  in  a  tluid  state  even  at  high  temperatni^a. 


3BCT.  iv  §  2         EFFECTS  OF  WATER  AND  PRESSURE  307 

Solvent  power  of  water  among  rocks. — The  presence  of  interstitial 
water  must  affect  the  chemical  constitution  of  rocks.  It  is  now  well 
understood  that  there  is  probably  no  terrestrial  substance  which,  under 
proper  conditions,  is  not  to  some  extent  soluble  in  water.  By  an  interest- 
ing series  of  experiments,  made  many  years  ago  by  W.  B.  and  H.  D.  Rogers, 
it  was  ascertained  that  the  ordinary  mineral  constituents  of  rocks  could 
be  dissolved  to  an  appreciable  extent  even  by  distilled  water,  and  that 
the  change  was  accelerated  and  augmented  by  the  presence  of  carbonic 
acid.^  Water,  as  pure  as  it  ever  occurs  in  a  natural  state,  can  hold  in 
solution  appreciable  proportions  of  silica,  alkaliferous  silicates,  and  iron 
oxide,  even  at  ordinary  temperatures.  The  mere  presence,  therefore,  of 
water  within  the  pores  of  subterranean  rocks  cannot  but  give  rise  to 
changes  in  the  composition  of  these  rocks.  Some  of  the  soluble  materials 
must  be  dissolved,  and,  as  the  water  evaporates,  will  be  redeposited  in  a 
new  form.* 

This  power  increased  by  heat. — The  chemical  action  of  water  is 
increased  by  heat,  which  may  be  either  the  earth's  original  heat  or  that 
which  arises  from  internal  crushing  of  the  crust  Mere  descent  from  the 
surface  into  successive  isogeotherms  raises  the  temperature  of  permeating 
water  until  it  may  greatly  exceed  the  boiling-point  But  a  high  tempera- 
ture is  not  necessary  for  many  important  mineral  rearrangements. 
Daubr^e  has  proved  that  very  moderate  heat,  not  more  than  50°  C. 
(122°  Fahr.)  has  sufficed  for  the  production  of  zeolites  in  Roman  bricks 
by  the  mineral  waters  of  Plombi^res.*  He  has  experimentally  demon- 
strated the  vast  increase  of  chemical  activity  of  water  with  augmentation 
of  its  temperature,  by  exposing  a  glass  tube  containing  about  half  its 
weight  of  water  to  a  temperature  of  about  400''  C.  At  the  end  of  a  week 
he  found  the  tube  so  entirely  changed  into  a  white,  opaque,  powdery  mass, 
as  to  present  not  the  least  resemblance  to  glass.  The  remaining  water 
was  highly  charged  with  an  alkaline  silicate  containing  63  per  cent 
of  soda  and  37  per  cent  of  silica,  with  traces  of  potash  and  lime.  The 
white  solid  substance  was  ascertained  to  be  composed  almost  entirely  of 
crystalline  materials,  partly  in  the  form  of  minute  perfectly  limpid  bi- 
pyramidal  crystals  of  quartz,  but  chiefly  of  very  small  acicular  prisms  of 
wollastonite.  It  was  found,  moreover,  that  the  portion  of  the  tube  which 
had  not  been  directly  in  contact  with  the  water  was  as  much  altered  as 
the  rest^  whence  it  was  inferred  that,  at  these  high  temperatures  and 
pre8sm*es,  the  vapour  of  water  acts  chemically  like  the  water  itself. 

Co-operation  of  pressure. — The  effect  of  pressure  must  be  recognised 
as  most  important  in  enabling  water,  especially  when  heated,  to  dissolve 
and  retain  in  solution  a  larger  quantity  of  mineral  matter  than  it  could 
otherwise  do,*  and  also  in  preventing  chemical  changes  which  take  place 
at  once  when  the  pressiu*e  is  removed.^     In  Daubr^e's  experiments  above 

^  American  Journ.  *Scie)ice  (2),  v.  p.  401. 

*  See  further  on  this  subject,  jKtstea^  pp.  343,  364.      '  *  Geologic  Experimentale,'  p.  462. 

*  Sorby  has  shown  that  the  solubility  of  all  salts  which  exhibit  contraction  in  solution 
is  remarkably  increased  by  pressure.     Proc.  Roy.  Soc.  (1862-3),  p.  340. 

»  See  Cailletet,  Xatur/orscher,  v.  ;  Pfaff,  Xeues  Jahrb.  1871  ;  W.  Spring,  Bull,  Acad. 


308  1)  YNA  MICA  L  GEOL  OG  Y  book  ui  pari  i 


cited,  the  tubes  were  heiTaetically  sealed  and  secured  against  fracture,  ao 
that  the  pressure  of  the  greatly  superheated  vapoiu*  had  full  effect    By 
this  means,  with  alkaline  water,  he  not  only  produced  the  two  minenls 
above  mentioned,  but  also  felspar  and  diopside.      The  high   pressures 
under  which  many  crystalline  rocks  have  solidified  is  indicated  by  the 
liquid  carbon-dioxide  in  the  vesicles  of  their  crystals.    Besides  the  pressure 
due  to  their  varying  depth  from  the  surface,  they  must  have  been  subject 
to  the  enormous  expansion  of  the  superheated  water  or  vapour  which 
filled  all  their  ca\dties,  and  sometimes,  also,  to  the  compression  resulting 
from  the  secular  contraction  of  the  globe  and  consequent  corrugation  of 
the  crust.     Mr.  Sorby  inferred  that  in  many  cases  the  pressure  under 
which  granite  consolidated  must  have  been  equal  to  that  of  an  overlying 
mass  of  rock  50,000  feet,  or  more  than  9  miles  in  thickness,  while  De  h 
Vallee  Poussin  and  Kenard  from  other  data  deduced  a  pressure  equal  to 
87  atmospheres  (p.  112). 

Aquo-igneous  ftision. — As  far  back  as  the  year  1846,  Scheerer 
observed  that  there  exist  in  granite  various  minerals  which  could  not 
have  consolidated  save  at  a  comparatively  low  temperature.^  He 
instanced  especially  gadolinites,  orthites,  and  allanites,  which  cannot 
endure  a  higher  temperature  than  a  dull-red  heat  without  altering  their 
physical  characters ;  and  he  concluded  that  granite,  though  it  may  have 
possessed  a  high  tempei-ature,  cannot  have  solidified  from  simple  igneous 
fusion,  but  must  have  been  a  kind  of  i)asty  mass  containing  a  considerable 
proportion  of  water.  It  is  common  now  to  speak  of  the  "  aquo-igneoua " 
origin  of  some  eniptive  rocks,  and  to  treat  their  production  as  a  part  of 
what  are  termed  the  "  hydro-thermal "  operations  of  geology. 

Scheerer,  £lio  de  Beaumont,  and  Daubr^e  have  sho\ni  how  the  presence 
of  a  comparatively  small  quantity  of  water  in  eniptive  igneous  rocks  may 
have  contributed  to  suspend  their  solidification,  and  to  promote  the 
crystallization  of  their  silicates  at  temperatiu*es  considerably  below  the 
point  of  fusion  and  in  a  succession  difterent  from  their  relative  order  of 
fusibility.  In  this  way,  the  solidification  of  (juartz  in  granite  after  the 
crystallization  of  the  silicates,  which  would  l)e  unintelligible  on  the 
snpj)osition  of  mere  dry  fusion,  becomes  explicable.  The  water  may  be 
regarded  as  a  kind  of  mother-liquor  out  of  which  the  silicates  crystallize 
without  reference  to  relative  fusibility. 

The  researches  of  the  late  Professor  Guthrie  on  the  influence  of  water 
in  lowering  the  fusing  points  of  various  substances  have  an  important 
geological  bearing.  He  showed  that  while  the  melting-point  of  nitre  by 
itself  is  320  C,  an  admixture  of  only  1*14  per  cent  of  water  reduced 
the  temperature  of  fusion  by  20^,  while  by  increasing  the  proportion  of 
water  to  29  07  per  cent  he  lowered  the  melting-point  to  97*6°,  and  he 
concluded  that  "the  phenomenon  of  fusion  is  nothing  more  than  an 
extreme  case  of  liquefaction  by  solution."  He  could  see  no  reason  why 
water  should  not  exist  even  at  the  earth's  centre,  for  even  granting  that 

/^V/.   IMifiqnt;  2ikI  Ser.  xlix.  (1880),  p.  369.     Pfaff  found  that  plaster  does  not  absorb 
water  uuder  a  pressure  of  40  atnio«plifres. 
1  Hull.  Si,c.  (r'^»/.  Fmnct',  iv.  i>.  468. 


Krr.  iv  §  2  EXPERIMENTS  IN  METAMORPHISM  309 

has  a  "  critical  temperature,"  still,  "  at  high  pressures  it  will  be  com- 
'essible  as  a  vapour  to  a  density  at  least  as  great  as  that  of  liquid  water." 
!e  concluded  that  "  water  at  a  high  temperature  may  not  only  play  the 
urt  of  a  solvent  in  the  ordinary  restricted  sense,  but  that  there  is  in 
any  cases  no  limit  to  its  solvent  faculty ;  in  other  words,  that  it  may  be 
ixable  with  certain  rocks  in  all  proportions ;  that  solution  and  mixture 
•e  continuous  with  one  another,  in  some  cases  at  temperatures  not  above 
le  temperature  of  fusion  of  those  bodies  per  se.'^  ^ 

Professor  Guthrie  was  disposed  to  doubt  whether  the  replenishment 
•  water  by  capillary  descent  from  the  surface  was  necessary  for  the 
reduction  of  these  phenomena  of  fusion  and  volcanic  eruption.  Prof, 
aubree's  experiments,  however,  enable  us  to  see  how  the  supply  of 
ater  may  be  kept  up  from  superficial  sources,  while  from  those  of  Prof, 
uthrie  we  learn  that  when  the  descending  water  reaches  masses  of 
ighly-heated  but  still  solid  rock,  it  may  allow  them  to  pass  into  a  fused 
mdition  and  to  exert  a  powerful  expansive  force  on  the  overlying  crust. 

Artificial  production  of  minerals. — As  the  result  of  experiments,  both 
I  the  dry  and  moist  way,  various  minerals  have  been  produced  in  the 
ystalline  form.  Among  the  minerals  successfully  reproduced  are  quartz, 
idymite,  olivine,  pyroxene,  enstatite,  wollastonite,  zircon,  emerald,  ruby, 
lelanite,  melilite,  several  felspars,  leucite,  nepheline,  meionite,  petalite, 
iveral  zeolites,  dioptase,  rutile,  brookite,  anatase,  perowskite,  sphene, 
klcite,  aragonite,  dolomite,  witherite,  siderite,  cerusite,  malachite,  corun- 
iim,  diaspore,  spinel,  haematite,  vivianite,  apatite,  anhydrite,  diamond 
ith  many  metallic  ores.'^ 

Artificial  alteration  of  internal  structures. — Besides  showing  the 
Jvent  power  of  superheated  water  and  vapour  upon  glass  in  illustration 
:  what  happens  within  the  crust  of  the  earth,  Daubr^e's  experiments 
Msess  a  high  interest  and  suggestiveness  in  regard  to  the  internal  re- 
Tangements  and  new  structures  which  water  may  superinduce  upon 
jcks.  Hermetically  sealed  glass  tubes  containing  scarcely  one-third  of 
leir  weight  of  water,  and  exix)sed  for  several  days  to  a  temperature 
5I0W  an  incipient  red  heat,  showed  not  only  a  thorough  transformation 
:  structure  into  a  white,  porous,  kaolin-like  substance,  encrusted  with 
inumerable  bipyramidal  crystals  of  quartz,  like  those  of  the  drusy 
kvities  of  rocks,  but  had  acquired  a  very  distinct  fibrous  and  even  an 
ninently  schistose  structure.  The  glass  was  found  to  split  readily  into 
acentric  laminae  arranged  in  a  general  way  parallel  to  the  original 
irfaces  of  the  tube,  and  so  thin  that  ten  of  them  could  be  counted  in  a 
*eadth  of  a  single  millimetre.  Even  where  the  glass,  though  attacked, 
tained  its  vitreous  character,  these  fine  zones  appeared  like  the  lines  of 
I  agate.  The  whole  structure  recalled  that  of  some  schistose  and 
ystalline  rocks.  Treated  with  acid,  the  altered  glass  crumbled  and 
jrmitted  the  isolation  of  certain  nearly  opaque  globules  and  of  some 
inute  transparent  infusible  acicular  crystids  or  microlites,  sometimes 
ouped  in  bundles  and  reacting  on  jwlarized  light.     Reduced  to  thin 

•  Phil.  May.  xviii.  (1884),  p.  117. 
^  Fouque  and  Michel-Li'vy,  '  Synthese  des  Mineraux  et  des  Roches.* 


310  DYXAMICAL  GEOLOGY  bookiupabti 

slices  and  examined  under  the  microscope  with  a  magnifying  power  of 
300  diameters,  the  altered  glass  presented :  1st,  Sphendites,  yV  of  a 
millimetre  in  radius,  nearly  opaque,  yellowish,  bristling  with  points  which 
perhaps  belong  to  a  kind  of  crystallization,  and  with  an  internal  radiating 
fibrous  structure  (these  resist  the  action  of  concentrated  hydrochloric  acid, 
whence  they  cannot  be  a  zeolite,  but  may  be  a  substance  Uke  chalcedony); 
2nd,  innumerable  colourless  acicular  microlites,  with  a  frequently  stellate, 
more  rarely  solitary  distribution,  resisting  the  action  of  acid  like  quartz 
or  an  anhydrous  silicate  ;  3rd,  dark  green  crystals  of  pyroxene  (diopside). 
Daubr^e  satisfied  himself  that  these  enclosures  did  not  pre-exist  in  the 
glass,  but  were  developed  in  it  during  the  process  of  alteration.^ 

But  beside  the  effects  from  increase  of  temperature  and  pressure,  we 
have  to  take  into  account  the  fact  that  water  in  a  natural  state  is  never 
chemically  pure.  Eain,  falling  through  the  air,  absorbs  in  particular 
oxygen  and  carbon-dioxide,  and  filtering  through  the  soil,  abstracts  more 
of  this  oxide  as  well  as  other  results  of  decomposing  organic  matter.  It 
is  thus  enabled  to  effect  numerous  decompositions  of  subterranean  rocks, 
even  at  ordinary  temperatures  and  pressures.  But  as  it  continues  its 
underground  journey,  and  obtains  increased  solvent  iK)wer,  the  very 
solutions  it  takes  up  augment  its  capacity  for  effecting  mineral  tran8form&> 
tions.  The  influence  of  dissolved  alkaline  carbonates  in  promoting  the 
decomposition  of  many  minerals  was  long  ago  pointed  out  by  Bischot 
In  1857  Sterry  Hunt  showed  by  experiments  that  water  impregnated 
with  these  carbonates  would,  at  a  temperature  of  not  more  than  212*^^ 
Fahr.,  produce  chemical  reactions  among  the  elements  of  many  sedi- 
mentary rocks,  dissolving  silica  and  generating  various  silicates.^  Daubr^ 
likewise  proved  that  in  presence  of  dissolved  alkaline  silicates,  at  tempera- 
tures above  700'  Fahr.  various  siliceous  minerals,  as  quartz,  felspar,  and 
pyroxene,  could  be  crystallized,  and  that  at  this  temperature  the  silicates 
would  combine  with  kaolin  to  form  felspar.  •"* 

The  presence  of  fluorine  has  been  proved  experimentally  to  have  a 
remarkable  action  in  facilitating  some  precipitates,  especially  tin  oxides, 
as  well  as  in  other  parts  of  the  mechanism  of  mineral  veins.*  Further 
illustrations  of  the  important  part  probably  played  by  this  element  in 
the  crystallization  of  some  minerals  and  rocks  have  been  published  by 
Ste.  Claire  Deville  and  Hautefeuille,  who  by  the  use  of  compounds  of 
fluorine  have  obtained  such  minerals  as  rutile,  brookite,  anatase  and 
corundum  in  crystalline  form.^       £lio  de   Beaumont  inferred  that  the 

^  'Gi'ol.  ExpcTiiii.  *  p.  158  et  acq.  The  production  of  crystals  and  microlites  in  the 
devitrification  of  glass  at  eomi)aratively  low  temperatures  by  the  action  of  water  is  of  great 
interest.  Tlie  first  observer  who  described  the  phenomenon  api^ears  to  have  been  Brewster. 
who,  in  the  second  decade  of  this  century,  studied  the  effect  upon  polarized  light  of  glass 
decomposed  by  ordinary  meteoric  action.  {Phil.  Trana.  1814,  Tnms.  Roy.  Soe.  £iiN. 
xxii.  (1860),  p.  607.     See  on  the  weathering  of  rocks,  p.  345.) 

'^  Phil.  May.  xv.  p.  68. 

2  Bull.  .Soc.  Ofol.  France,  xv.  (1885),  p.  103. 

■*  First  suggested  by  Daubree,  .-l?j//.  ihs  Mines  (1841),  3me  ser.  xx.  p.  65. 

'  ('ompfes  Itendusy  xlvi.  p.  764  (1858)  ;  xlvii.  p.  89  ;  Ivii.  p.  648  (1865).  Foaqa^  and 
Michel-Levy,  *Synthese  des  Mineruux  et  des  Roches.' 


SECT,  iv  §  3     COMPRESSION,  TENSION,  AND  FRACTURE  311 

mineralizing  influence  of  fluorine  had  been  effective  even  in  the  crystalliza- 
tion of  granite.  He  believed  that  "the  volatile  compound  enclosed 
in  granite,  before  its  consolidation  contained  not  only  water,  chlorine, 
and  sulphur,  like  the  substance  disengaged  from  cooling  lavas,  but 
also  fluorine,  phosphorus  and  boron,  whence  it  acquired  much  greater 
activity  and  a  capacity  for  acting  on  many  bodies  on  which  the  volatile 
matter  contained  in  the  lavas  of  Etna  has  but  a  comparatively  insignificant 
action."  ^ 

• 

§  3.  Effects  of  compression,  tension,  and  fracture. 

Among  the  geological  revolutions  to  which  the  crust  of  the  earth 
has  been  subjected,  its  rocks  have  been  in  some  places  powerfully  com- 
pressed ;  elsewhere  they  have  undergone  enormous  tension,  and  almost 
everywhere  they  have  been  more  or  less  ruptured.  Hence  internal 
structures  have  been  developed  which  were  not  originally  present  in 
the  rocks.  These  structures  will  be  more  properly  considered  in  Book 
IV.  We  are  here  concerned  mainly  with  the  nature  and  operation  of 
the  agencies  by  which  they  have  been  produced. 

The  most  obvious  result  of  pressure  upon  rocks  is  consolidation,  as 
where  a  mass  of  loose  sand  is  gradually  compacted  into  a  more  or  less 
coherent  stone,  or  where,  with  accompanying  chemical  changes,  a  layer 
of  vegetation  is  compressed  into  peat,  lignite,  or  coal.  The  cohesion  of 
a  sedimentary  rock  may  be  due  merely  to  the  pressure  of  the  superin- 
cumbent strata,  but  some  cementing  material  has  usually  contributed  to 
bind  the  component  particles  together.  Of  these  natural  cements  the 
most  frequent  are  peroxide  of  iron,  silica,  and  carbonate  of  lime.  Moderate 
pressure  equally  distributed  over  a  rock  presenting  everywhere  nearly 
the  same  amount  of  resistance  will  promote  consolidation,  but  may  pro- 
duce no  further  internal  change.  Where  the  component  particles  are 
chiefly  crystalline,  pressure  may  induce  a  crystalline  structure  upon  the 
whole  mass,  as  recent  experiments  have  shown. ^  If,  however,  the  pres- 
sure becomes  extremely  unequal,  or  if  the  rock  subjected  to  it  can  find 
escape  from  the  strain  in  one  or  more  directions,  it  may  undergo  shear  in 
certain  planes,  or  may  be  crumpled,  or  the  limit  of  its  rigidity  may  be 
passed,  and  rupture  may  take  place.  Some  consequences  of  these  move- 
ments may  be  briefly  alluded  to  here  in  illustration  of  hypogene  action 
in  dynamical  geology. 

(1.)  Minor  Ruptures  and  Noises. — Among  mountain- valleys,  in 
railway-tunnels  through  hilly  regions,  or  elsewhere  among  rocks  subjected 
to  much  lateral  pressure,  or  where  owing  to  the  removal  of  material  by 
running  water,  and  the  consequent  formation  of  cavities,  subsidence  is  in 

*  **Sur  les  Emanations  Volcaniques  et  Metalliferes,"  BuU,  Soc.  OSol.  France^  iv.  (1846), 
p.  1249.  This  admirable  and  exhaustive  memoir,  one  of  the  greatest  monuments  of  ^lie  de 
Beaumont's  genius,  should  be  consulted  by  the  student.  See  also  De  Lapparent  {Bull.  Soc. 
GSd.  France,  xvii.  (1889),  p.  282)  on  the  part  played  by  mineralizing  agents  in  the  forma- 
tion of  eruptive  rocks. 

'  W.  Spring,  Bull.  Acad.  Ray.  Bdg.  1880,  p.  875. 


312  DYXAMICAL  GEOLOGY  book  m  paw  i 

1 

progress,    sounds   as   of   explosions   are   occasionally  heard.     In  many 
instances,  these  noises  are  the  result  of  relief  from  great  lateral  compreB- 
sion,  the  rocks  having  for  ages  been  in  a  state  of  strain,  from  which  u 
denudation  advances,  or  as   artificial  excavations   are   made,   they  are 
relieved.     This  relief  takes  place,  not  always  uniformly,  but  sometiiiiei 
cumulatively  by  successive  shocks  or  snaps.     Mr.  W.  H.  Nilea  of  Boston 
has  described  a  number  of  interesting  cases  where  the  effects  of  ndi 
ex|)ansion  could  be  seen  in  quarries ;  large  blocks  of  rock  being  rent  and 
crushed  into  fragments,  and  smaller  pieces  being  even  discharged  vitk 
explosion  into  the  air.^     More  recently  Mr.  A.  Strahan  has  called  attsn- 
tion  to  the  occurrence  of  slickensided  surfaces  in  the  lead-mines  of  Derhj- 
shire  which  on  being  stiiick  or  even  scratched  with  a  miner's  pick  braak 
ofi'  with  explosive  violence,  and  he  suggests  that  the  spars  and  ores  along 
those  surfaces  are  in  "  a  state  of  molecular  strain,  resembling  that  of  the 
liu}>ert's  Drop  or  of  toughened  glass,  and  that  this  condition  of  strain  is 
the  result  of  the  earth  movements  which  produced  the  slickensides."  * 

If  such  is  the  state  of  strain  in  which  some  rocks  exist  even  at  the 
surface  or  at  no  great  distance  beneath  it,  we  can  realise  that  at  great 
depths,  where  escape  from  strain  is  for  long  i)eriods  impossible,  and  the 
compression  of  the  masses  must  be  enormous,  any  sudden  relief  from  this 
strain  may  well  give  rise  to  an  earthquake-shock  (p.  280).  A  continued 
condition  of  strain  must  also  influence  the  solvent  power  of  water  per- 
meating the  rocks  (p.  307). 

(2.)  Consolidation  and  Welding. — That  pressure  consolidates  rocks 
is  familiar  knowledge.  Loose  sedimentary  msiterials  may  by  mere  pres- 
sure be  converted  into  more  or  less  firm  and  hard  masses.  Experiments 
by  W.  Spring  uix)n  many  substances  in  the  state  of  ix)wder  have  shown 
that  under  high  pressure  they  become  welded  into  solid  substances. 
Under  a  pressure  of  6000  atmospheres,  coal-dust  becomes  a  brilliant  solid 
block,  taking  the  mould  of  the  cavity  in  which  it  is  placed,  and  thereby 
giving  evidence  of  plasticity.  Peiit,  in  like  manner,  becomes  a  brilliant 
black  substance  in  which  all  tmce  of  the  original  stnicture  is  gone.^ 

(3.)  Cleavage. — Over  extensive  tracts  of  country  a  jjeculiar  structure 
has  been  superinduced  by  powerful  lateral  pressure,  especially  upon  fine- 
grained argillaceous  rocks,  which  are  then  termed  slates.  They  split  along 
a  set  of  planes  which,  as  a  rule,  are  highly  inclined  or  vertical,  and  inde- 
pendent of  the  original  bedding.  Examined  more  minutely,  it  is  found  that 
their  component  particles,  which  in  most  cases  have  a  longer  and  shorter 
axis,  have  grouped  themselves  with  their  long  axes  generally  in  one  com- 
mon direction,  and  parallel  with  the  planes  of  fissility.  An  ordinary  shale 
may  present  under  the  microscope  such  a  structure  as  is  shown  in  Fig. 
81.  But  where  it  has  undergone  the  change  here  referred  to,  it  has 
acquired  the  structure  represented  in  Fig.  80.  Rocks  which,  having  been 
thus  acted  on,  have  acquired  this  superinduced  fissility,  are  said  to  be 
cleaved,  and  the  fissile  structui*e  is  termed  cleavage.     In  Fig.  82,  for 

^  Proc.  Boston  &>€.  yat.  Hist  xviii.  p.  272  (1876). 

'^  tied.  Mag.  1887,  p.  400.     See  also  the  same  volume,  pi».  511,  522. 

»  hull.  Acad.  Roy.  Bdg.  1880,  p.  325,  and  ante,  p.  142. 


■ECT.  ivg  3     COMPRESSION,  TENSION,  AND  FRACTURE  313 

)xample,  where  the  strata,  at  first  in  even  parallel  beds,  have  been  sub- 
lOcted  to  great  compression  from  the  directions  (a)  and  (b),  the  original 
ilanes  of  stratification  are  represented  by  wavy  lines,  and  the  new  system 


of  cleavage-planes  by  tine  upright  lines.     The  fineness  of  the  cleavage 
depends  in  large  measure  upon  the  texture  of  the  original  rock.     Sand- 


isisting  as  they  do  of  rounded  olxlurate  quartz-grains,   take 
rude  cleavage  (or  jointing)  or  none  at  all.     Finc-giained 


argillaceous  rocks,  consisting  of  minut*  particles  or  flakes,  that  can  adjust 
their  long  axes  in  a  new  direction,  are  those  in  which  the  structure  is  best 
developed.  In  a  scries  of  cleaved  rocks,  therefore,  cleavage  may  be 
perfect  in  argillaceous  beds  (b  b.  Figs.  83  and  84),  and  imperfect  or  absent 


314  DYNAMICAL  GEOLOGY  book  m  paw  i 

in  intcrstratified  beds  of  sandstone  (a  a.  Fig.  83)  or  of  limestone  (as  at 
Clonea  Castle,  Waterford,  a  a,  Fig.  84). 

That  cleavage  may  be  produced  in  a  mechanical  way  by  lateral  pres- 
sure has  been  proved  experimentally   by  Sorby,  who  effected  perfect 
cleavage  in  pipe-clay  through  which  scales  of  oxide  of  iron  had  previously 
been  mixed.^     Tyndall  superinduced  cleavage  on  bees-wax  and  other  sub- 
stances by  subjecting  them  to  severe  pressure.      More  recently,  Fisher 
has  proposed  the  view  that  in  nature  it  is  not  to  the  pressure  which 
plicated  the  rocks  that  cleavage  is  to  be  attributed,  but  to  the  shearing 
movements  generated  in  large  masses  of  rock  left  in  a  position  too  lofty 
for  equilibrium.^     If  such,  however,  had  been  the  origin  of  the  structure, 
it  is  difficult  to  understand  why  there  should  be  such  a  prevalent  relation 
between  the  strike  and  the  cleavage  ;  for  if  descent  by  gravitation  were  the 
main  cause,  we  should  ex]>ect  to  find  the  rocks  sheared  far  more  irregu- 
larly than  even  the  most  irregular   disposition  of   cleavage.     That  in 
cleavage  there  has  been  a  true  distortion  of  the  rocks  is  indubitable ;  and 
the  ahiount  of  distortion  may  be  ascertained  by  the  extent  of  the  altera- 
tion of  shape  of  fossils  (Figs.  85-88).       Microscopic  study  of  cleaved 
rocks  shows  that  their  fissility  is  not  always  due  merely  to  a  rearrange- 
ment of  original  clastic  particles,  but  to  the  development  of  new  minends, 
particularly  varieties  of  mica,  along  the  planes  of  cleavage.     This  relation 
is  well    seen  in  the  folded   and    cleaved  Devonian  and   Carboniferous 
rocks  of  S.W.  Ireland  and  Cornwall,  in  the  Carboniferous  shales  of  Laval, 
Mayenne,  and  in  the  Jurassic  and  Eocene  shales  of  the  Alps.^      Just 
as  shales  graduate  into  true  cleaved  slates,  so  slates  by  augmentation 
of   their  superinduced   mica  pass  into  phyllites,  and  these  into  mica- 
schists.      The  structure  of  districts  with  cleaved  rocks  is  described  in 
Book  IV.  Part  V. 

(4.)  Deformation. — Further  evidence  of  the  powerful  pressures  to 
which  rocks  have  been  exposed  is  furnished  by  the  way  in  which 
contiguous  pebbles  in  a  conglomerate  have  been  squeezed  into  each  other, 
and  even  sometimes  have  been  elongated  in  a  certain  general  direction. 
The  coarseness  of  the  grain  of  such  rocks  permits  the  effects  of  compres- 
sion or  tension  to  be  readily  seen.  Similar  effects  may  take  place  in 
fine-grained  rocks  and  escape  observation.  l)aubr(^e  has  imitated  exi)eri- 
mentally  indentations  produced  by  the  contiguous  ix)rtions  of  conglomerate 
pebbles.^ 

^  Hopkius,  Camhridifc  Phil.  Trans,  viii.  (1847),  p.  456.  D.  Sharpe,  Quart.  Joum,  Oeei, 
Soc.  iii.  (1846),  p.  74  ;  v.  (1848),  p.  111.  Sorby,  Edin.  New  Phil.  Joum.  Iv.  (1853),  p. 
137.  W.  King,  Rny.  Irish  Acad.  xxv.  (1875),  p.  605.  The  student  will  find  recent  interesting 
additions  to  our  knowleilge  of  the  microscopic  structure  and  the  history  of  cleaved  rocks  in 
Mr.  Sor)>y'8  address,  Q.  ./.  CJeol.  »Sf»c.  xxxvi.  p.  72,  and  in  Mr.  Harker's  very  able  essay, 
Brit.  Assoc.  1885,  Reports,  pp.  813-852.  See  also  E.  Jannettaz,  Bull.  Soc.  OfoL  France, 
ix.  (1881),  p.  196  ;  xi.  (1884),  p.  211.  G.  F.  Becker,  Bull.  GeU.  Soc.  Amer,  iv.  (1898), 
p.  13.  ^  Oeol.  Mag.  1884,  p.  396. 

^  JiinnettAZ,  Renevier  and  Lory,  Bull.  Soc.  Oiol.  Franc^^  ix.  p.  649. 

*  Omptes  RenduSy  xliv.  p.  823  ;  also  his  *  Geologic  Exp^rimentale,'  part  i.  sect.  ii.  chap. 
iii.,  where  a  series  of  important  experiments  on  deformation  is  given.  For  various  examples 
and  opinions,  see  Rothpletz,  Z.  Deutsch.  (Jfttl.  Oes.  xxxi.  p.  355.    Heim,  *  Mechanismns  der 


SECT.  IT  §3     COMPRESSION,  TENSION,  AND  FRACTURE  315 

In  discussing  the  cause  of  these  indentations  it  must  be  remembered 
that  imprints  of  pebbles  upon  each  other,  particularly  when  the  material 
is  limestone  or  other  tolerably  soluble  rock,  may  have  been  to  some 
extent  produced  by  solution  taking  place  most  actively  where  pressure 
was  greatest  (p.  307).     But  there  are  indubitable  evidences  of  crushing 


and  deformation,  even  in  what  would  be  termed  solid  and  brittle  rocks. 
Of  these  evidences,  perhaps  the  most  instnictive  and  valuable  arc 
furnished  by  the- remains  of  plants  and  animals  occurring  as  fossils,  and 
of  which  the  unaltered  shapes  are  well  known.  Where  fossiliferous  rocks 
have  undergone  a  shear,  the  extent  of  this  movement,  as  above  rem.irked. 


in  be  measured  ii 

7  drawings  are  g 


the  resultant  distortion  of  the  fossils.    In  Figs.  85  and 
veil  of  two  Lower  Silurian  fossils  in  their  natural  forms. 


Gebirgsbildung,'  1878,  vol.  ii.  p.  31.  Hitchcock,  'Geology  of  Vermont,'  i.  p.  28.  I'mr. 
B-ul.  Sue.  Nat.  lliit.  vii,  pp.  209,  358  ;  xviii.  p.  97  :  xv.  p.  1  ;  xx.  j>.  313.  Amtr,  Asiof. 
1866,  |..  83.  A<>.f.:  Jour.  Sci.  (2)  «ii.  ii.  372.  Sorby.  fifp.  ranlif  ,Va(.  Sot.  187S,  p. 
21.     H.  H.  Reiisch,  ■  Fosailieii-fiih render  Krj'st,  Sfhiefer,'  p.  25. 


316  J)yXAMICAL  GEOLOGY  book  iii  parti 

In  Fig.  86  a  specimen  of  the  same  species  of  trilobite  as  in  Fig.  85  is 
represented  where  it  has  been  distorted  during  the  shearing  of  the 
enclosing  rock.  In  Fig.  88  four  examples  of  the  same  shell  as  in  Fig.  87 
are  shown  greatly  distorted  by  a  strain  which  has  elongated  the  rock  in 
the  direction  a  h.^  Amorphous  crystalline  rocks  (pegmatite,  granite, 
diorite)  have  been  so  crushed  as  to  acquire  a  schistose  structure  (pp. 
544,  545,  597,  615,  625,  630). 

Another  illustration  of  the  effects  of  pressure  in  producing  deforma- 
tion in  rocks,  is  supplied  by  the  so-called  "  lignilites,"  "epsomites,"  or 
"  stylolites."     These  are  cylindrical  or  columnar  bodies  varying  in  length 
up  to  more  than  four  inches,  and  in  diameter  up  to  two  or  more  inches. 
The  sides  are  longitudinally  striatod  or  grooved.     Each  column,  usually 
>vith  a  conical  or  rounded  cap  of  clay,  beneath  which  a  shell  or  other 
organism  may  frequently  be  det^ct^d,  is  placed  at  right  angles  to  the 
bedding  of  the  limestones,  or  calcareous  shales  through  which  it  passes, 
and  consists  of  the  same  material.     This  structure  has  been  referred  by 
Professor  Marsh  to  the  difference  between  the  resistance  offei'ed  by  the 
column  under  the  shell,  and  by  the  surrounding  matrix  to  superincumbent 
pressure.     The  striated  surface  in  this  view  is  a  case  of  *' slickensidee." 
The  same  observer  has  suggested  that  the  more  complex  structure  known 
as  "  cone-in-cone ''  may  be  due  to  the  action  of  pressure  upon  concretions 
in  the  course  of  fortnation.^ 

The  ingenious  experiments  of  M.  Tresca  ^  on  the  flow  of  solids  have 
thrown  considerable  light  upon  the  internal  defoi-mations  of  rock-masses. 
He  has  j)roved  that,  even  at  ordinary  atmospheric  temperatures,  solid 
resisting  bodies  like  lead,  cast-iron,  and  ice,  may  be  so  comj^ressed  as  to 
undergo  an  internal  motion  of  their  parts,  closely  analogous  to  that  of 
fluids.  Thus,  a  solid  jet  of  lead  has  been  produced,  by  placing  a  piece 
of  the  met*il  in  a  cavity  between  the  jaws  of  a  ^KDwerful  compressing 
machine.  Iron,  in  like  maimer,  has  been  forced  to  flow  in  the  solid  state 
into  cavities  and  take  their  shape.  On  cutting  sections  of  the  metals 
so  compressed,  their  particles  or  crystals  are  found  to  have  ranged  them- 
selves in  lines  of  flow  which  follow  the  contour  of  the  space  into  which 
they  have  been  squeezed.  Such  experiments  are  of  considei-able  geological 
interest.  They  illustrate  how  in  certain  circumstances,  under  great  strain, 
rocks  may  not  only  be  made  to  undergo  internal  deformation  along 
certain  shearing  planes,  as  in  cleavage,  but  may  even  be  subjected  to  such 
stresses  as  to  acquire  a  "  shear-structure  '  resembling  the  fluxion-structure 
seen  in  rocks  which  have  been  truly  liquid  (p.  544)."* 

»  See  I).  Sharpe,  Q.  J.  UeoL  !S»k.  iii.  (18-16),  p.  75.  W.  Hopkins,  Camlnidfje  Phil. 
Trans,  viii.  (1847),  p.  466.  S.  Haiighton,  PhU.  Mag.  (1856),  xii.  p.  409.  0.  Fisher, 
(icol.  Mwj.  1884,  p.  399.     Harker,  Brit.  Assoc,  1885,  Reports,  p.  824. 

-  Proc.  American  Assoc.  Science^  1867.  Giimbel,  Zeitsck.  Ueutsch.  Gcol.  Oea,  xzxiy.  p. 
642. 

•♦  Comptes  Rcmbis,  1864,  p.  754  ;  1867,  y.  809.  M6m.  .Sar.  £t ran (/ers,  xyiil  p.  788; 
XX.  p.  75.  Inst.  Mech.  Enyineers^  June  1867  ;  June  1878.  See  also  W.  C.  Roberts- 
Austen,  Pruc.  Roy.  Institvtioni  xi.  (1886),  p.  395. 

^  This  remarkable  kind  of  structure  has  Ijeeu  developed  to  an  enormous  extent  among 
the  crystalline  rocks  of  the  north-west  Highlands  of  Scotland  (p.  624). 


ECT.iv§3     COMPRESSION,  TENSION,  AND  FRACTURE  317 

Numerous  examples  have  been  found  during  the  last  few  years  in  the 
lorth-west  Highlands  of  Scotland  where  rocks  have  been  subjected  to 
uch  mechanical  movements  as  to  have  been  crushed  down  and  made  to 
ow  in  certain  directions.  Massive  crystalline  pegmatites  may  there  be 
raced  through  successive  stages  until  the  material  becomes  a  fine  compact 
dlsitic  substance  with  thin  lines  of  flow  so  like  the  "  flow-structure  "  of 
lava  that  it  would  deceive  even  a  practised  geologist,  and  sometimes 
plitting  into  thin  lamiiisB  like  those  of  shale.  Further  reference  to  this 
Abject  will  be  made  in  Book  IV.  Part  VIII.  §  2. 

(5.)  Plication. — On  the  assumption  of  a  more  rapid  contraction  of 
he  inner  hot  nucleus  of  the  globe,  and  the  consequent  descent  of  the 
ool  upper  shell,  a  subsiding  area  of  the  curved  surface  of  the  earth 
equires  to  occupy  less  horizontal  space,  and  must  therefore  sufier 
powerful  lateral  compression.  De  la  Beche  long  ago  pointed  out  that  if 
ontorted  and  tilted  beds  were  levelled  out,  they  would  require  more 
pace  than  can  now  be  obtained  for  them  without  encroaching  on  other 
feas.^  The  magnificent  example  of  the  Alps  brings  before  the  mind 
he  enormous  extent  to  which  the  crust  of  the  earth  has  in  some  places 
»een  compressed.  According  to  the  measurements  and  estimates  of 
Professor  Heim  of  Zurich,  the  diameter  of  the  northern  zone  of  the 
entral  Alps  is  only  about  one  half  of  the  original  horizontal  extent  of 
he  component  strata,  which  have  been  corrugated  and  thrown  back  upon 
tach  other  in  huge  folds  reaching  from  base  to  summit  of  lofty  mountains, 
tnd  spreading  over  many  square  miles  of  surface.  He  computes  the 
lonzontal  compression  of  the  whole  chain  at  120,000  metres,  that  is  to 
ay,  that  two  points  on  the  opposite  sides  of  the  chain  have,  by  the 
olding  of  the  crust  that  produced  the  Alps,  been  brought  120,000  metres, 
)r  74  miles,  nearer  each  other  than  they  were  before  the  movement.^ 
[hough  the  sight  of  such  colossal  foldings  of  solid  sheets  of  rock  impresses 
18  with  the  magnitude  of  the  compression  to  which  the  crust  of  the  earth 
las  been  subjected,  it  perhaps  does  not  convey  a  more  vivid  picture  of 
he  extent  of  this  compression  than  is  afforded  by  the  fact  that  even  in 
he  minuter  and  microscopic  structure  of  the  rocks  intricate  puckerings 
ire  visible  (Fig.  37).  So  intense  has  been  the  pressure,  that  even  the 
iny  flakes  of  mica  and  other  minerals  have  been  forced  to  arrange  them- 
elves  in  complex,  frilled,  crimped,  and  goffered  foldings.  On  an  inferior 
cale,  local  compression  and  contortion  may  be  caused  by  the  protrusion 
)f  eruptive  rocks.  The  characters  of  plicated  rocks  as  part  of  the  frame- 
ivork  of  the  terrestrial  crust  are  given  in  Book  IV.  Part  IV. 

As  may  be  supposed,  it  is  difficult  to  illustrate  experimentally  the 
processes  by  which  vast  miisses  of  rock  have  been  plicated  and  crumpled. 
rhe  early  devices  of  Sir  James  Hall,  however,  may  be  cited  from  their 
nterest  as  the  first  attempts  to  demonstrate  the  origin  of  the  contortion 
)f  rocks.  He  placed  layers  of  cloth  under  a  weight,  and  by  compressing 
.hem  from  two  sides  produced  corrugations  closely  resembling  those  of 
he  Silurian  strata  of  the  Berwickshire  coast  .(Fig.  89).     Professor  Favre 

*  'Report,  Devon  and  Cornwall,'  ]>.  187. 
^  *  Mechauismus  der  Gebirgsbildung,*  1878,  vol.  ii.  p.  213. 


318  DYXA.mCAL  GEOLOGY  bookiiipabti 


of  Geneva  devised  an  exi>eriment  which  more  closely  imitates  the  con- 
ditions in  nature.     U}X)n  a  tightly  stretched  band  of  india-rubber  he 

places  various  layers  of  clay,  making 
them  adhere  to  it  as  firmly  as  possible. 
By  then  allowing  the  band  to  contract 
he  produces  in  the  overlying  strata  of 
clay  a  series  of  contortions,  inversions, 
and  dislocations  which  at  once  recall 
those  of  a  great  mountain-chain.^  More 
recently  this  subject  has  been  illustrated 
Fig.  bj>.— Hall's  Experiment  iUustrating  exi)erimentally  by  Mr.  H.  M.  Cadell,  who 
contortion.  j^^  obtained  results  curiously  like  those 

exhibited  by  the  crumpled  and  dislocated  rocks  of  the  N.W.  Highlands 
of  Scotland.  '^ 

(6.)  Jointing  and  Dislocation. — Almost  all  rocks  are  traversed  by 
vertical  or  highly  inclined  divisional  planes  termed  joinis  (Book  IV.  Part 
II.)  These  have  been  regardfsd  as  due  in  some  way  to  contraction 
during  consolidation  (fissures  of  retreat) ;  and  this  is  no  doubt  their 
origin  in  innumerable  cases.  But,  on  the  other  hand,  their  frequent 
regularity  and  persistence  across  materials  of  very  varying  texture  suggest 
rather  the  effects  of  internal  pressure  and  movement  within  the  crust 
In  an  ingenious  series  of  ex2)enments,  Daubree  has  imitated  joints  and 
fiuctures  by  subjecting  different  substances  to  undulatory  movement  by 
torsion  and  by  simple  pressure,  and  he  infers  that  they  have  been 
produced  by  analogous  movements  in  the  t^rrestiial  cnist.^ 

But  in  many  cases  the  rupture  of  continuity  has  been  attended  with 
relative  displacement  of  the  sides,  producing  what  is  termed  a  fauU, 
Daubiee  also  shows  experimentally  how  faults  may  arise  from  the  same 
movements  as  have  caused  joints,  and  from  bending  of  the  rocks.  As  the 
solid  crust  settles  down,  the  subsidence,  where  unequal  in  rate,  may 
cause  a  rupture  between  the  less  stable  and  more  stable  areas.  When  a 
tract  of  ground  has  been  elevated,  the  rocks  underlying  it  get  more  room 
by  being  pushed  up,  and  are  placed  in  a  |>osition  of  more  or  leas 
insta1)ility.  As  they  cannot  occupy  the  additional  space  by  any  elastic 
expansion  of  their  mass,  they  accommodate  themselves  to  the  new  position 
by  a  series  of  dislocations.*  Those  segments  having  a  broad  base  rise 
more  than  those  with  narrow  bottoms,  or  the  latter  sink  relatively  to  the 
former.  Each  broad-bottomed  segment  is  thus  bounded  by  two  sides 
sloping  towards  the  upper  iwrt  of  the  block.  The  plane  of  dislocation  is 
nearly  always  inclined  from  the  vertical,  and  the  side  to  which  the 
inclination  rises,  and  from  which  it  "  hades,"  is  the  upthrow  side.  Faults 
of  this  kind  are  termed  wnmal,  and  are  by  far  the  most  common  in  natura 
In  mountainous  regions,  however,  insUinces  freijuently  occur  where  one 

*  Xntun:,  xiv.  (1878),  p.  103. 
-  Trans.  Hoy.  S»c.  EtUn.  xxxvi.  (1888).  i».  337. 

^  *(frol.   KxixTim.'  Part  I.   sect.  ii.  cliaj).  ii.      See  W.   King,  Roy,  Irish  AcaiL   xxf. 
(187.')).  i».  60r»,  ami  the  theorieii  of  jointing  given  postea,  j).  526. 

"*  See  J.  M.  Wilson,  Ocitl.  M(uj.  v.  p.  206  ;  O.  Fisher,  op.  ciU  1884. 


**KCT.  iv  §  4  METAMORPHISM  3 1 9 

side  has  been  pushed  over  the  other,  so  that  lower  are  placed  above  higher 
beds.  Such  a  fault  is  said  to  be  reversed.  It  indicates  an  upward  tlurust 
^^^ithin  the  crust,  and  is  often  to  be  found  associated  with  lines  of  plica- 
Won.  Where  a  sharp  fold,  of  which  one  limb  is  pushed  forward  over  the 
other,  gives  way  along  a  line  of  rupture,  the  result  is  a  reversed  fault. 
The  details  of  these  features  of  geological  structure  are  reserved  for  Book 
n^  Part  VI. 

§  4.    The  Metamorphism  of  Rocks. 

Metamorphism  is  a  crystalline  (usually  also  a  chemical)  rearrange- 
ment of  the  constituent  materials  of  a  rock.^  In  its  production  the  fol- 
lowing conditions  have  been  mainly  operative.  (1)  Temperature,  from 
the  lowest  at  which  any  change  is  possible  up  to  that  of  complete  fusion ; 
(2)  pressure,  the  potency  of  the  action  of  heat  being,  within  certain 
limits,  increased  with  increase  of  pressure ;  (3)  mechanical  movements, 
which  so  often  have  induced  molecular  rearrangements  in  rocks ;  (4)  pre- 
sence of  water,  usually  containing  various  mineral  solutions,  whereby 
chemical  changes  would  be  effected  which  would  not  be  possible  in  dry 
heat ;  (5)  nature  of  the  materials  operated  upon,  some  being  much  more 
susceptible  of  change  than  others. 

A  metamorphosed  rock  is  one  which  has  suffered  such  a  mineralogical 
rearrangement  of  its  substance.  It  may  or  may  not  have  been  a 
crystalline  rock  originally.  Any  rock  capable  of  alteration  (and  all  rocks 
must  be  so  in  some  degree)  will,  when  subjected  to  the  required  conditions, 
be  metamorphosed.  The  resulting  structure,  however,  will,  save  in  extreme 
cases,  bear  witness  to  the  original  character  of  the  mass.  In  some 
instances,  the  change  has  consisted  merely  in  the  reamingement  or 
crystallization  of  one  mineral  originally  present,  as  in  limestone  converted 
into  marble ;  in  others,  there  has  been  a  process  of  paramorphisni,  as 
where  augite  has  been  changed  into  hornblende  in  the  alteration  of 
dolerites  into  epidiorites ;  in  others,  the  constituents  have  been 
forced  by  mechanical  movements  to  rdiige  themselves  in  parallel 
lamime,  as  where  a  diorite  or  pyroxenic  rock  l)econies  a  hornblende-schist ; 
in  others,  partial  or  complete  transformation  of  the  original  constituents, 
whether  crystalline  or  clastic,  into  new  crystalline  minerals  has  been 
accompanied  by  a  complete  recrystallization  and  change  of  structure 
in  the  rock.  Quartzite  is  evidently  a  compacted  sandstone,  either 
hardened  by  mere  pressure,  or  most  frequently  by  the  deposit  of  silica 
between  its  granules,  or  a  slight  solution  of  these  granules  by  permeating 
water,  so  that  they  have  l>ecome  mutually  adherent.  A  clay-slate  is  a 
hardened,  cleaved,  and  partially  metamorphosed  form  of  muddy  sediment, 
which  on  the  one  hand  may  be  found  full  of  organic  remains,  like  any 
common  shale,  while  on  the  other,  by  the  appearance  and  gradual  increase 
of  some  form  of  mi«i  and  other  minerals,  it  may  be  traced  becoming 
more  and  more  crystalline,  until  it  passes  into  phyllite,  chiastolite-slate,  or 

*  See  A.  Barker  ou  the  Physics  of  Metamorphism,  Oeol.  Mng,  vi.  (1889),  p.  15.     J.  W. 
Judd,  ib,  p.  243,  and  Book  IV.  Part  VIII.  of  this  Text-lxwk. 


320  DVXAMICAL  GEOLOGY  book  in  parti 


sonio  other  schistose  rock.  Yet  remains  of  fossils  niav  l>e  obtained  even 
in  the  same  hand-specimens  with  crystals  of  andalusite,  garnet,  or  other 
minerals.  The  calcareous  matter  of  corals  is  sometimes  replaced  by  horn- 
blende, garnet,  and  axinite,  without  deformation  of  the  fossils.^ 

Since  exj)eriment  has  proved  that  in  presence  of  water  under  pressure, 
even  at  comiximtively  low  temperatures,  mineral  sul^tancos  are  >igorou8lT 
attacked  (p.  307),  we  may  expect  to  find  that  as  these  conditions  abund- 
antly exist  within  the  earth^s  crust,  the  rocks  exposed  to  them  have  been 
more  or  less  altered.  A  large  proportion  of  the  accessible  crust  consists 
of  sedimentiiry  materials  which  were  laid  down  on  the  ocean -bottom, 
and  which  were  still  abundantlv  soaked  ^nth  sea- water  even  after  thev 
had  l>een  covered  over  with  more  recent  fonnations.  The  gradual  growth 
of  submai-ine  accumidations  would  of  couree  depnve  the  lower  strata  of 
most  of  their  original  water,  but  some  proportion  of  it  would  probably 
remain.  If,  according  to  Dana,  the  average  amount  of  interstitial  water  iu 
strdtiiied  rocks,  at  the  earth's  surface,  such  as  limestones,  sandstones  and 
shales,  be  assumed  to  Ije  2*67  per  c^jnt,  which  is  prolwibly  less  than  the 
truth,  "  the  amount  will  correspond  to  two  quarts  of  water  for  every  cubic 
foot  of  rock."-  There  is  certaiidy  a  considerable  store  of  water  ready  for 
chemical  action  when  the  reijuired  conditions  of  heat  and  pressure  are 
obtained.  We  must  also  remember  that  the  water  in  which  the  sedimentarv 
formations  of  the  cnist  were  formed,  being  mostly  that  of  the  ocean,  already 
possessed  chlorides,  sulphates,  and  other  Sidts  with  which  to  begin  its  re- 
actions. The  inference  may  therefore  l)e  dmwn,  that  rocks  possessing 
not  more  than  3  per  cent  of  intei^stitiid  water  cannot  be  depressed  to 
depths  of  8evt»i*jil  thousjind  feet  beneath  the  level  of  the  earth's  surhce, 
and  undergo  great  pressure  and  cnishing,  without  suffering  more  or  less 
marked  internal  change  or  metamorphism. 

A  few  illustrative  examples  of  metamorphism  may  be  given  here  ;  the 
structure  of  metamori)hic  rocks,  ^\^th  the  phenomena  of  **  contact "  and 
*'  regional "  metamorphism,  will  be  discussed  in  B(X)k  IV.  Part  VIII. 

Pridndinn  of  marble  from  Iwwshme, — One  of  the  most  obvious  cases  of 
alteration — the  artificial  conversion  of  limestone  into  crvstalline  saccharoid 
marble — has  been  alre;i(ly  referred  to  (p.  300).'*  The  calcite  ha>'ing 
un<lergf)no  comj)lete  ti-jinsformation,  its  original  stnicture,  whether  organic 
or  not,  has  l)een  effaced,  and  a  new  stnicture  has  been  developed,  consist- 
ing of  an  aggregjite  of  miinite  rounded  gi-jiins,  each  with  an  independent 
ciystalline  arrangement.  The  production  of  a  crystalline  stnicture  in 
annjrphous  calcite  may  be  effected  by  the  action  of  mere  meteoric  water 
at  or  near  the  surface  {ante,  p.  151,  an<l  yw.s7m,  p.  365).  But  the  genera- 
tion of  the  peculiar  gnmular  stnicture  of  marble  always  demands  heat 
and  pressure,  and  prol^ably  usually  the  presence  of  water ;  the  details  ol 
the  process  are,  however,  still  involved  in  ol>scurity.  We  know  that 
where  a  dyke  of  }>asidt  or  other  intrusive  rock  has  involved  limestone,  it 

'  Ann.  dfs  Minfs,  5me  s/t.  xii.  p.   318.      II.  H.  Ileuscli,  'Die  Fossilien  fuhrendea 
krystallinisi.lien  Schiefer  voii  Beixen  *  (traiislate«l  l)y  H.  Baldauf),  Leipzig,  1883. 
-  •  Manual.'  3r.l  ed.  (1880).  p.  758. 
'^  Sec  also  ''  Mnrmarosis"  in  Book  IV.  Part  VIII. 


SECT,  iv  §  4  METAMORPHISM  32 1 

has  sometimes  been  able  tx)  convert  it  for  a  short  distance  into  marble. 
The  heat  (and  perhaps  the  moisture)  of  the  invading  lava  have  sufficed  to 
l)roduce  a  granular  structure,  which  even  under  the  microscope  is  identical 
with  that  of  marble.  The  conversion  of  wide  areas  of  limestone  into 
marble  is  a  regional  metamorphism,  associated  usually  with  the  alteration 
of  other  sedimentary  masses  into  schists,  &c. 

DolomUfzaiion. — Another  alteration  which,  from  the  labours  of  Von 
Buch,  received  in  the  early  decades  of  this  century  much  attention  from 
geologists,  is  the  conversion  of  ordinary  limestone  into  dolomite.  Some 
dolomite  appears  to  be  an  original  chemical  precipitate  from  the  saline 
water  of  inland  lakes  and  seas  (p.  412).  But  calcareous  formations  due 
to  organic  secretions  are  often  weakly  dolomitic  at  the  time  of  their 
formation,  and  may  have  their  proportion  of  magnesium  carbonate 
increased  by  the  action  of  permeating  water,  as  is  proved  by  the 
conversion  into  dolomite  of  shells  and  other  organisms,  consisting 
originally  of  calcite  or  aragonite,  and  forming  portions  of  what  was  no 
doubt  originally  a  limestone,  though  now  a  continuous  mass  of  dolomite. 
This  change  may  have  sometimes  consisted  in  the  mere  abstraction  of 
carbonate  of  lime  from  a  limestone  already  containing  carbonate  of 
magnesia,  so  as  to  leave  the  rock  in  the  form  of  dolomite ;  or  probably 
more  usually  in  the  action  of  the  magnesium  salts  of  sea-water,  especially 
the  chloride,  upon  organically-formed  limestone ;  or  sometimes  locally  in 
the  action  of  a  solution  of  carbonate  of  magnesia  in  carbonated  water 
npon  limestone,  either  magnesian  or  non-magnesian.  £lie  de  Beaumont 
calculated  that  on  the  assumption  that  one  out  of  every  two  equivalents 
of  carbonate  of  lime  was  replaced  by  carbonate  of  magnesia,  the  conver- 
sion of  limestone  into  dolomite  would  be  attended  with  a  reduction  of  the 
volume  of  the  mass  to  the  extent  of  12*1  per  cent.  It  is  certainly 
remarkable  in  this  connection  that  large  masses  of  dolomite,  which  may 
be  conceived  to  have  once  been  limestone,  have  the  cavernous,  fissured 
structiire  which,  on  this  theory  of  their  origin,  might  have  been  looked 
for. 

Dolomite  has  been  produced  both  on  a  small  and  on  a  great  scale. 
In  the  north  of  England  and  elsewhere,  the  Carboniferous  Limestone,  has 
been  altered  for  a  few  feet  or  yards  on  either  side  of  its  joints  into  a  dull 
yellow  dolomite,  locally  termed  "dunstone."  Similar  vertical  zones  of 
dolomite  occur  also  in  the  Carboniferous  Limestone  of  Ireland.  Harkness 
pointed  out  that  the  dolomite  appears  in  vertical  ribs  where  the  rocks  are 
much  jointed,  and  in  beds  where  they  have  few  or  no  joints.^  No  doubt 
percolating  water  has  been  the  agent  of  change  in  the  vertical  zones. 
The  beds,  however,  which  in  Ireland  and  elsewhere  constitute  important 
masses  in  the  Carboniferous  Limestone,  were  more  probably  formed 
contemporaneously  with  the  rocks  among  which  they  lie.  They  may 
have  been  deposited  as  limestone  in  shallow  lagoons  where  the  magnesian 
salts  of  concentrated  sea-water  would  act  upon  them.  Dolomite  some- 
times forms  great  ranges  of  mountains,  as  in  the  Eastern  Alps,  where  it 
has  by  some  writers  been  regarded  as  altered  ordinary  limestone.     But 

*  Q.  J.  Oed.  So€.  XV.  p.  100. 
Y 


322  DYNAMICAL  GEOLOGY  book  ra  part  i 

these  masses,  may  have  partly,  at  least,  become  dolomite  at  the  beginning 
by  the  action  of  the  magnesian  salts  of  the  concentrated  waters  of  inland 
seas  upon  organic  or  inorganic  calcareous  deix>sits  accumulated  previous  to 
the  concentration,  their  metamorphism  ha\ang  consisted  mainly  in  the 
subsequent  generation  of  a  crystalline  stnictiure  analogous  to  that  of  the 
conversion  of  limestone  into  marble.^ 

Conversion  of  vegetable  substance  into  coal. — Exposed  to  the  atmosphere, 
dead  vegetation  is  decomposed  into  humus,  which  goes  to  increase  the 
soil.  But  sheltered  from  the  atmosphere,  exposed  to  the  action  of  water, 
especially  with  an  increase  of  temperature,  and  under  some  pressure,  it 
is  converted  into  lignite  and  coal.  An  example  of  this  alteration  has 
been  observed  in  the  Dorothea  mine,  Clausthal.  Some  of  the 
timber  in  a  long-disused  level,  filled  with  slate  rubbish,  and  saturated 
with  the  mine-water  from  decomposing  pyrites,  was  found  to  have  a 
leathery  consistence  when  wet,  but,  on  exix)siu*e  to  the  air,  hardened  to 
a  firm  and  ordinary  brown -coal,  with  the  typical  brown  colour  and 
external  fibrous  structure,  and  having  the  intei*nal  fracture  of  a  black 
glossy  pitch-coal.^  This  change  must  have  l)een  produced  within  less 
than  four  centuries — the  time  since  the  levels  were  opened.  According 
to  Bischof's  determinations  the  conversion  of  wood  into  coal  may  take 
place,  1st,  by  the  separation  of  carbonic  acid  and  carburetted  hydrogen; 
2nd,  by  the  separation  of  carbonic  acid,  and  the  formation  of  water 
either  from  oxidation  of  hydrogen  by  meteoric  oxygen,  or  from  the 
hydrogen  and  oxygen  of  the  wood ;  3rd,  by  the  separation  of  carbonic 
acid,  carbiu'etted  hydrogen,  and  water.^  The  circumstances  under  which 
the  vegetable  matter  now  forming  coal  has  been  accumulated  were 
favourable  for  this  slow  transmutation.  The  carbon-dioxide  (choke- 
damp)  of  old  cofd-mines  and  the  carburetted  hydrogen  (fire-damp,  CHJ 
given  off  in  such  large  quantities  by  coal  seams,  are  products  of  ihe 
alteration  which  would  appear  to  be  accelerated  by  terrestrial  movementSi 
such  as  those  that  compress  and  plicate  rocks.  During  the  process  these 
gases  escape,  and  the  proportion  of  carbon  progressively  increases  in  the 
residue,  till  it  reaches  the  most  highly  mineralised  anthracite  (p.  144),  or 
may  even  pass  into  nearly  pure  carbon  or  graphite.  In  the  coal-basins 
of  Mons  and  Valenciennes,  the  same  seams  which  are  in  the  state  of 
bituminous  coal  (gras)  at  the  surface,  gradually  lose  their  volatile  con- 
stituents as  they  are  traced  downward  till^  they  pass  into  anthracite.  In 
the  Pennsylvanian  coal-field  the  coals  become  more  anthracitic  as  they 
are  followed  into  the  eastern  region,  where  the  rocks  have  undergone 

^  Ou  dolomitization,  see  L.  von  Bach,  in  Leonhard's  Minercdog,  TaMhenbu/tk^  1824; 
Naumann*s  *  Geognosie,'  i.  p.  763 ;  Bischof  s  '  Chemical  Geology,'  iii.  ;  &ie  de  Beranumt, 
null.  Soc.  Otd.  France,  viii.  (1836),  p.  174  ;  Sorby,  BriL  Assoc,  Rep.  1856,  part  iL  p.  77, 
an*l  Adiiressj  Q.  J.  (Jeof.  Sttc.  1879.  A  full  statement  of  the  literature  of  this  subject  will 
1)e  found  in  a  suggestive  memoir  by  C.  Doelter  and  R.  Hoernes,  Juhrb.  Oeoi.  ReieksatutdUf 
XXV.  The  dolomite  mountains  of  the  Eastern  Alps  have  been  well  described  by  MojaiMmct. 
See  account  of  Triassic  system,  posted.  Book  VI. 

-  Hirscliwald,  Z.  Devlsch.  Ocol.  Oes.  xxv.  p.  364. 

3  Bischof,  'Chem.  Geol.'  i.  p.  274. 


SECT,  iv  §  4  METAMORPHISM  323 

great  plication,  and  where,  possibly  during  the  subterranean  movements, 
they  were  exposed  to  an  elevation  of  temperature.^  Daubr^e  has 
produced  from  wood,  exposed  to  the  action  of  superheated  water,  drop- 
like globules  of  anthracite  which  had  evidently  been  melted  in  the 
transformation,  and  which  presented  a  close  resemblance  to  the  anthracite 
of  some  mineral  veins.^ 

Production  of  New  Minerah, — ^Where  metamorphism  is  well  developed 
the  chemical  reactions  which  have  been  set  up  have  given  rise  to  more  or 
less  complete  re-combination  of  the  chemical  constituents  of  the  rock. 
New  minerals  have  thus  been  formed  either  entirely  out  of  the  materials 
already  comprising  the  rock,  or  with  some  addition  or  replacement  of 
substance  introduced  from  without,  by  aqueous  solution  or  otherwise. 
Some  of  the  commonest  secondary  minerals  are  micas ;  andalusite,  chiasto- 
lite,  and  garnet  are  also  of  frequent  occurrence.    (See  Book  IV.  Part  VIII.) 

Production  of  the  sdiistose  structure, — All  rocks  are  not  equally  per- 
meable by  water,  nor  is  the  same  rock  equally  permeable  in  all  directions. 
Among  the  stratified  rocks  especially,  which  form  so  large  a  proportion 
of  the  visible  terrestrial  crust,  there  are  great  differences  in  the  facility 
with  which  water  can  travel,  the  planes  of  sedimentation,  or  those  of 
deavage  or  shearing  where  these  have  been  developed,  being  naturally 
those  along  which  water  passes  most  easily.  It  is  along  these  planes  that 
differences  of  mineral  structure  and  composition  are  ranged.  Alternate 
layers  of  siliceous,  argillaceous,  and  calcareous  material  vary  in  porosity 
and  capability  of  being  changed  by  permeating  water.  We  may,  there- 
fore, expect  that  unless  the  original  stratified  stnicture  has  been  effaced 
or  rendered  inoperative  by  any  other  superinduced  structure,  it  will 
guide  the  metamorphic  action  of  underground  water,  and  will  remain 
more  or  less  distinctly  traceable  even  after  very  considerable  mineralogical 
transformations  have  taken  place.  Even  without  this  guiding  influence, 
superheated  water  can,  to  a  certain  extent,  produce  a  schistose  structure, 
parallel  to  its  bounding  surfaces,  as  Daubr^e's  experiments  upon  glass, 
above  cited,  have  proved. 

The  stratified  formations  consist  largely  of  silica,  silicates  of  alumina, 
lime,  magnesia,  soda  and  potash,  and  iron  oxides.  These  mineral  sub- 
stances exist  there  as  original  ingredients,  partly  in  recognisable  worn 
crystals,  partly  in  a  granular  or  amorphous  condition,  ready  to  be  acted 
on  by  permeating  water  under  the  requisite  conditions  of  temperature 
and  pressure.  We  can  understand  that  any  re-combination  and  re- 
crystallization  of  the  silicates  will  probably  follow  the  laminae  of  deposit 
or  of  cleavage,  and  that  in  this  way  a  crystalline  foliated  structure  may 
be  developed.  Round  masses  of  granite  erupted  among  Palaeozoic  rocks, 
instructive  sections  may  be  observed  where  a  transition  can  be  traced 
from  ordinary  unaltered  sedimentary  strata,  such  as  sandstones,  grey- 
wackes  and  shales  containing  fossils,  into  foliated  crystalline  rocks,  to 
which  the  names  of  mica-schist  and  even  gneiss  may  be  applied.     (Book 

^  Daabree,  '  Geologic  Experimentale/  p.  463.  Part  of  the  framework  below  a  steam- 
hammer  has  been  found  after  twenty  years  to  be  converted  into  lignite.  F.  Seeland,  Verh, 
OeoL  Rekha,  1883,  p.  192.  '^  Op,  cU.  p.  177. 


324  m'XAMICAL  GEOLOGY  book  iii 


IV.  Part  VIII.)  Not  only  can  the  gradual  change  into  a  crystalline 
foliated  structure  1x5  readily  followed  with  the  naked  eye,  but  with  the 
aid  of  the  microscope  the  finer  details  of  the  alteration  can  be  traced. 
Minute  plates  of  some  micaceous  mineral  and  small  concretions  of  anda* 
lusite,  garnet,  quartz,  &c.,  may  l>e  observed  to  have  crystallized  out  of 
the  siu'rounding  amorphous  sediment.  These,  especially  the  mica,  can  be 
seen  gradually  to  increase  in  size  and  number  towards  the  granite,  until 
the  rock  assumes  a  thoroughly  foliated  stnicture  and  passes  into  a  true 
schist.  Yet  even  in  such  a  schist,  traces  of  the  original  and  durable 
water- worn  quartz-granules  may  be  detected.^  Foliation  is  a  crystalline 
segregation  of  the  mineral  matter  of  a  rock  in  certain  dominant  planes 
which  may  l^e  those  of  original  stratification,  of  joints,  of  cleavage,  of 
shearing,  or  of  fractiu^e.^  Mr.  Sorby  has  recognised  foliation  in  three  sets 
of  planes  even  among  the  same  rocks.^ 

Scrope  many  years  ago  called  attention  to  the  analogy  between  the 
foliation  of  schists  and  the  ribbanded  or  streaked  stnicture  of  trachyte, 
obsidian,  and  other  lavas.*  This  analogy  has  even  l)een  regarded  as  an 
identity  of  structure,  and  the  idea  has  found  supporters  that  the 
schistose  rocks  have  l^een  in  a  condition  similar  to  or  identical  with  that 
of  many  volcanic  masses,  and  have  acquired  their  peculiar  fissility  by 
differential  movements  within  the  viscous  or  pasty  magma,  the  solidified 
minerals  being  drawn  out  into  layers  in  the  direction  of  shearing. 
Daubr6e,  availing  himself  of  the  researches  of  Tresca  on  the  flow  of 
solids  (p.  316),  has  endeavoured  to  imitate  artificially  some  of  the 
phenomena  of  foliation  by  exposing  clay  and  other  substances  to  great 
but  unequal  pressure.^  That  some  of  the  lenticular  wavy  laminie  of 
different  minerals  in  gneiss  and  other  foliated  rocks  may  be  due  to 
original  segregation  or  fiow  in  still  imconsolidated  igneous  rock  seems  to 
l)e  rendered  highly  prol>able  by  the  curious  analogies  to  this  structure  to 
be  observed  in  the  deeper  pjvrts  of  large  intnisive  lx)sse8  of  rock,  such  as 
granite,  dial>ase,  and  gabbro.  These  layers  may  thus  l)e  the  remains  of 
the  oldest  stnicture  now  retiiined  by  the  gneiss.  But  subsequent 
pressure  and  deformation  have  frequently  produced  a  foliation  cutting 
obliquely  across  this  original  lamination  and  even  entirely  effacing  it. 

That  the  schistose  stnicture  has  been  largely  induced  by  mechanical 
movements  cannot  be  doubted.  The  evidence  in  the  field  and  under  the 
microscope  has  now  rendered  it  certain  that  many  rocks  have  been 
subjected  to  enormous  mechanical  stresses  within  the  earth's  crust ;  that 
they  have  yielded  to  the  pressure  both  by  disniption  and  by  molecular 
shearing,  that  in  some  cases  they  have  l>een  crushed  into  minute 
fragments  or  dust,  and  have  then  been  made  to  flow  and  to  simulate  the 
flow-structure  of  lavii,  while,  in  other  cases,  the  crushed  particles  have 
crystallized  into  a  granulitic  structure,  or  the  recrystallization  has  taken 
place  along  the  flow-planes  and  hiis  given  rise  to  a  perfect  foliation.     The 

^  Sorby,  Q.  J.  Oeol.  &>c.  xxxvi.  j).  82. 

-  Darwin,  *  Geological  Observations,' p.  16*2.     Ramsay,  "Geology  of  North  Wales,"  iu 
Memoirs  of  Oeol.  Survey y  vol.  iii.  p.  182.  '  Op,  eit.  p.  84. 

*  'Volcanoes,'  pp.  140,  300.  «  'Geologic  Exptirimentale,'  p.  410. 


PART  II  EPIGENE  OR  SURFACE  ACTION  325 


action  that  produced  cleavage,  if  further  developed,  might  be  accompanied 
with  sufficient  augmentation  of  temperature  to  permit  of  extensive  mineral- 
ogical  transformation  along  the  cleavage-planes.  But  probably  a  rise  of 
temperature  was  not  essential.  The  conversion  of  pyroxene  into  hornblende, 
which  has  been  observed  in  regions  of  crystalline  schists,  points  indeed  to 
a  lower  temperature  than  that  required  for  the  crystallization  of  the 
original  mineral.^  A  schistose  structure  of  almost  any  degree  of  coarse- 
ness might  conceivably  be  produced.  A  mixed  rock,  such  as  granite,  has 
l)een  converted  into  a  foliated  gneiss.  Diorite,  diabase,  or  gabbro  has 
likewise  by  mechanical  movement,  with  accompanying  chemical  and 
crystallographic  transformation,  been  made  to  assume  a  schistose 
stnicture  and  pass  into  amphibolite-schist. 

The  study  of  metamorphism  and  metamorphic  rocks  leads  us  from 
unaltered  mechanical  sediments  at  the  one  end,  into  thoroughly  crystal- 
line masses  at  the  other.  We  are  presented  with  a  cycle  of  change 
wherein  the  same  particles  of  mineral  matter  pass  from  crystalline  rocks 
into  sedimentary  deposits,  then  by  increasing  stages  of  alteration  back 
into  crystalline  masses,  whence,  after  being  reduced  to  detritus  and 
redeposited  in  sedimentary  formations,  they  may  be  once  more  launched 
on  a  similar  series  of  transformations.  The  phenomena  of  metamor- 
phism appear  to  be  linked  together  with  those  of  igneous  action  as 
connected  manifestations  of  hypogene  change. 

Part  II.  Epigene  or  Surface  Action  : 
An  Inquiry  into  the  Geological  Changes  in  progress  upon  the  Earth's  Surface, 

On  the  surface  of  the  globe  and  by  the  operation  of  agents  working 
there,  the  chief  amount  of  ^^sible  geological  change  is  now  effected. 
This  branch  of  inquiry  is  not  involved  in  the  preliminary  difficulty, 
regarding  the  very  nature  of  the  agents,  which  attends  the  investigation 
of  plutonic  action.  On  the  contrary,  the  surface  agents  are  carrying 
on  their  work  under  our  eyes.  We  can  watch  it  in  all  its  stages,  measure 
its  progress,  and  mark  in  many  ways  how  well  it  represents  similar 
changes  which  for  long  ages  previously  must  have  been  effected  by 
similar  means.  But  in  the  systematic  treatment  of  this  subject,  a 
difficulty  of  another  kind  presents  itself.  While  the  operations  to  be 
discussed  are  numerous  and  often  complex,  they  are  so  interwoven  into 
one  great  network  that  any  separation  of  them  under  different  sub- 
divisions is  sure  to  be  more  or  less  artificial,  and  is  apt  to  convey  an 
erroneous  impression.  While,  therefore,  under  the  unavoidable  necessity 
of  making  use  of  such  a  classification  of  subjects,  we  must  bear  always 
in  mind  that  it  is  employed  merely  for  convenience,  and  that  in  nature, 
superficial  geological  action  must  be  viewed  as  a  whole,  since  the  work 
of  each  agent  has  close  relations  with  that  of  the  others  and  is  not 
properly  intelligible  unless  this  connection  be  kept  in  view. 

The  movements  of  the  air ;  the  evaporation  from  land  and  sea ;  the 

*  See  G.  H.  Williams,  Ajtier.  Journ,  Set.  3rd  ser.  xxviii.  (1884),  p.  259. 


326  DYNAMICAL  GEOLOGY  book  hi  part  ii 

fall  of  rain,  hail,  and  snow ;  the  flow  of  rivers  and  glaciers ;  the  tides, 
currents,  and  waves  of  the  ocean ;  the  growth  and  decay  of  organised 
existence,  alike  on  land  and  in  the  depths  of  the  sea : — in  short,  the 
whole  circle  of  movement,  which  is  continually  in  progress  upon  the 
surface  of  our  planet,  are  the  subjects  now  to  be  examined.  It  would  be 
desirable  to  adopt  some  general  term  to  embrace  the  whole  of  this  range 
of  inquiry.  For  this  end  the  word  epigene  may  be  suggested  as  a  con- 
venient term,  and  antithetical  to  hypogene,  or  subterranean  action. 

The  simplest  arrangement  of  this  part  of  Geological  Dynamics  will 
be  into  three  sections  : — 

I.  Air. — The  influence  of  the  atmosphere  in  destroying  and  forming 
rocks. 

II.  Water. — The  geological  functions  of  the  circulation  of  water 
through  the  air  and  between  sea  and  land,  and  the  action  of  the  sea. 

III.  Life. — The  part  taken  by  plants  and  animals  in  preserving 
destroying,  or  originating  geological  formations. 

The  words  destructive,  reproductive,  and  conservative,  employed  in 
describing  the  operations  of  the  epigene  agents,  do  not  necessarily  imply 
that  anything  useful  to  man  is  destroyed,  reproduced,  or  pre8er\'ed.  On 
the  contrar}',  the  destructive  action  of  the  atmosphere  may  cover  hare 
rock  with  rich  soil,  while  its  reproductive  effects  may  bury  fertile  soQ 
under  sterile  desert.  Again,  the  conservative  influence  of  vegetation  has 
sometimes  for  centuries  retained  as  barren  morass  what  might  otherwise 
have  become  rich  meadow  or  luxiuiant  woodland.  The  terms,  therefore, 
are  used  in  a  strictly  geological  sense,  to  denote  the  removal  and  re- 
deposition  of  material,  and  its  agency  in  preserving  what  lies  beneath  it 


Section  i.  Air. 

The  geologiail  action  of  the  atmosphere  arises  partly  from  its  chemical 
composition  and  j)artly  from  its  movements.  The  composition  of  the 
atmospheric  envelope  has  Ijeen  already  discussed  (p.  32),  and  further 
information  \vill  l)e  found  under  the  head  of  Rain.  The  movements  of 
the  atmosphere  are  due  to  variations  in  the  distribution  of  pressure  or 
density,  the  law  lieing  that  air  always  moves  spirally  from  where  the 
pressure  is  high  to  where  it  is  low.  Atmospheric  pressiu^e  is  understood 
to  l>e  determined  by  two  causes,  temperatiu-e  and  aqueous  vapour.  Since 
warm  air,  l>eing  less  dense  than  cold  air,  ascends,  while  the  latter  flows  in  to 
take  its  place,  the  unequal  heating  of  the  earth's  surface,  by  causing 
upward  currents  from  the  warmed  portions,  produces  horizontal  currents 
from  the  surrounding  cooler  regions  inwaixis  to  the  central  ascending  mass 
of  heated  air.  The  familiar  land  and  sea  breezes  offer  a  good  example 
of  this  action.  Again,  the  density  of  the  air  lessens  with  increase  of 
water- vai)our.  Hence  moist  air  tends  to  rise  as  warmed  air  does,  with  a 
corresponding  inflow  of  the  drier  and  consequently  heavier  air  from  the 
surrounding  tracts.  Moist  air,  ascending  and  diminishing  atmospheric 
pressiu-e,  as  indictited  by  the  fall  of  the  barometer,  rises  into  higher 


SECT,  i  §  1  GEOLOGICAL   WORK  OF  THE  AIR  327 

regions  of  the  atmosphere,  where  it  expands,  cools,  condenses  into  visible 
cloud  and  into  showers  that  descend  again  to  the  earth. 

Unequal  and  rapid  heating  of  the  air,  or  accumulation  of  aqueous 
vapour  in  the  air,  and  possibly  some  other  influences  not  yet  properly 
understood,  give  rise  to  extreme  disturbances  of  pressure,  and  con- 
sequently to  storms  and  hurricanes.  For  instance,  the  barometer  some- 
times indicates  in  tropical  storms  a  fall  of  an  inch  and  a  half  in  an  hour, 
showing  that  somewhere  about  a  twentieth  part  of  the  whole  mass  of 
atmosphere  has,  in  that  short  space  of  time,  been  displaced  over  a  certain 
area  of  the  earth's  surface.  No  such  sudden  change  can  occur  without 
the  most  destructive  tempest  or  tornado.  In  Britain  the  tenth  of  an  inch 
of  barometric  fall  in  an  hour  is  regarded  as  a  large  amount,  such  as  only 
accompanies  great  storms.^  The  rate  of  movement  of  the  air  depends  on 
the  difference  of  barometric  pressure  between  the  regions  from  and  to 
which  the  wind  blows.  Since  much  of  the  potency  of  the  air  as  a 
geological  agent  depends  on  its  rate  of  motion,  it  is  of  interest  to  note 
the  ascertained  velocity  and  pressiu-e  of  wind  as  expressed  in  the  sub- 
joined table :  ^ — 

Velocity  in  Miles      PreRsare  in  Pounds 
j>er  hour.  per  square  foot. 

Calm 0  0 

Light  breeze 

Strong  breeze 

Strong  gale  . 

Hurricane    . 

While  the  paramount  importance  of  the  atmosphere  as  the  vehicle  for 
the  circulation  of  moisture  over  the  globe,  and  consequently  as  powerfully 
influencing  the  distribution  of  climate  and  the  growth  of  plants  and 
animals,  must  be  fully  recognised  by  the  geologist,  he  is  specially  called 
upon  to  consider  the  influence  of  the  air  in  directly  producing  geological 
changes  upon  the  surface  of  the  land,  and  in  augmenting  the  geological 
work  done  by  water. 

§  1.  Geological  work  of  the  atmosphere  on  land. 

Viewed  in  a  broad  way,  the  air  is  engaged  in  the  twofold  task  of 
promoting  the  disintegration  of  superficial  rocks  and  in  removing  and  re- 
distributing the  finer  detritus.  These  two  operations,  however,  are  so 
intimately  bound  up  with  each  other  that  they  cannot  be  adequately 
understood  unless  considered  in  their  mutual  relations. 

1.  Destructive  action. — Still  dry  air,  not  subject  to  much  range  of 
temperature,  has  probably  little  or  no  effect  on  minerals  and  rocks.  The 
chemical  action  of  the  atmosphere  takes  place  almost  entirely  through 
dissolved  moisture.  This  subject  is  discussed  in  the  section  devoted  to 
Kain.  But  sunlight  produces  remarkable  changes  on  a  few  minerals. 
Some  lose  their  colours  (celestine,  rose -quartz),  others  change  it,  as 
cerargyrite  does  from  colourless  to  black,  and  realgar  from  red  to  orange- 

^  Buchan's  *  Meteorology,'  p.  266. 
*  For  another  atatenient  see  Czerny,  PeUnnan,  MUL  1876,  Erganzungsheft. 


14 

1 

42 

9 

70 

25 

84 

36 

328  DYNAMICAL  GEOLOGY  book  ni  part  ii 


yellow.  Some  of  these  alterations  may  be  explained  by  chemical 
moclificjitions  induced  by  such  causes  as  the  loss  of  organic  matter  and 
oxidation. 

Effects  of  lightning. — Hibbert  has  given  an  account  of  the 
disniption  by  lightning  of  a  solid  mass  .of  rock  105  feet  long,  10  feet 
broad,  and  in  some  places  more  than  4  feet  high,  in  Fetlar,  one  of  the 
Shetland  Islands,  about  the  middle  of  last  century.  The  dislodged  mass 
was  in  an  insUint  torn  from  its  bed  and  broken  into  three  large  and 
several  lesser  fragments.  ^'One  of  these,  28  feet  long,  17  feet  broad,  and 
5  feet  in  thickness,  was  hurled  across  a  high  point  of  rock  to  a  distance 
of  50  yards.  Another  broken  mass,  about  40  feet  long,  was  thrown 
still  farther,  but  in  the  same  direction  and  quite  into  the  sea.  There 
were  also  many  lesser  fragments  scattered  up  and  down."  ^ 

The  more  usual  effect  of  lightning,  however,  is  to  produce  in  loose 
sand  or  more  compact  rock  patches  of  vitreous  drops  or  bubbles  coating 
the  surface,  also  tul)es  termed  fill  (fit  rites,  which  range  up  to  2  J  inches  in 
diameter.  These  tulx5s  descend  vertically,  but  sometimes  obliquely,  from 
the  siu'facc,  occasionally  branch,  and  rapidly  lessen  in  dimensions  till  they 
disiippeiir.  They  are  formed  by  the  actual  fusion  of  the  particles  of  the 
soil  or  rock  surrounding  the  pathway  of  the  electric  spark.  They  have 
been  most  frequently  found  in  loose  sand.  Abich  has  observed  examples 
of  such  tubular  perforations  Avith  vitreous  walls  in  the  porous  reddish- 
white  andesite  at  the  summit  of  Little  Ararat.-  A  piece  of  the  rock  about  a 
foot  long  nuiy  be  obtained  perforated  all  over  with  irregular  tubes  having 
an  average  diameter  of  3  centimetres.  Esich  of  these  is  lined  with  a 
blackish-green  glass.  As  the  whole  summit  of  the  moimtain,  owing  to  its 
frequent  stonns,  is  drilled  in  this  manner,  it  is  e\'ident  that  the  action  of 
lightning  may  considerably  motlify  the  structiu-e  of  the  su];>erficial  portions 
of  any  mass  of  rock  ex|)osed  on  lofty  eminences  to  frequent  thunderstorms. 
Humlx)ldt  collected  fulgurites  from  a  trachyte  peak  in  Mexico,  and  in  two 
of  his  specimens  the  fusetl  mass  of  the  walls  has  actually  overflowed  from 
the  tul)es  on  the  surrounding  siu^ace.^ 

Effects  of  changes  of  temperature. — Of  far  wider  geological 
importance  are  the  effects  that  arise  among  rocks  and  soils  from  the 
alternate  ex^mnsion  and  contraction  caused  by  daily  or  seasonal  changes 
of  temperature.  In  countries  with  a  great  annual  range  of  temperature, 
considerable  difficulty  is  sometimes  experienced  in  selecting  building- 
materials  liable  to  be  little  affectetl  by  rapid  or  extreme  variations  in 
tcmpemture,  which  induce  an  alternate  expansion  and  contraction  that 

*  Hibbert's  'Shetlaud  Islands,'  p.  3S9,  quoting  from  the  MS.  of  Rev.  George  Low. 

-  Sitzb.  AkmL  M'iss.   Wien,  Ix.  (1870),  p.  155. 

'  G.  Rose,  ZfUsch.  Ik'uUrh,  Genl.  Ofs.  xxv.  p.  112;  Giiml^el,  op.  ciL  zzziT..(1882), 
p.  G47  ;  A.  Wichniann,  op,  cit.  xxxv.  (1883),  p.  849.  Fusion  by  lightning  was  obserred 
by  De  Saussure  in  hornblende-schiat  on  the  suiuniit  of  Mont  Blanc  (see  also  F.  Riitley, 
Quart.  Journ,  Geal.  Soc.  1885,  p.  152) ;  by  Raniond  in  mica-schist  and  limestone  on  a  peak 
of  the  IVrenees  ;  by  J.  S.  Diller  on  the  baxalt  of  Mount  Thielson,  Oregon,  and  on  the  top  of 
Mount  Shasta,  California,  Amfr.  Jovni.  Sci,  Oct.  1884  :  by  J.  Eccles  in  glanoophane  schist 
on  Monte  Viso,  F.  Rutley,  Quaii.  Journ,  Ued.  Soc,  xlv.  (1889),  i>.  60. 


SECT,  i  §  1  EFFECTS  OF  WIND  329 

prevents  the  joints  of  masonry  from  remaining  close  and  tight.  ^  If  the 
daily  thermometric  variations  are  large,  the  effects  are  frequently  striking. 
In  Western  America,  where  the  climate  is  remarkably  dry  and  clear,  the 
thermometer  often  gives  a  range  of  more  than  80°  in  the  twenty-four 
hours.  Thus  in  the  Yellowstone  district,  at  a  height  of  9000  feet  above 
the  sea,  the  author  found  the  temperature  of  rocks  exposed  to  the  sun  at 
noon  to  be  more  than  90°  Fahr.,  and  the  thermometer  at  night  to  sink 
below  20°.  In  the  Sahara  and  other  African  regions,  as  well  as  in 
Central  Asia,  the  daily  range  is  considerably  greater.  This  rapid  nocturnal 
contraction  produces  such  a  superficial  strain  as  to  disintegrate  rocks  into 
sand,  or  cause  them  to  crack  or  peel  off  in  skins  or  irregular  pieces.  Dr. 
Livingstone  found  in  Africa  (12°  S.  lat,  34°  E.  long.)  that  surfaces  of 
rock  which  during  the  day  were  heated  up  to  137°  Fahr.,  cooled  so  rapidly 
by  radiation  at  night  that,  unable  to  sustain  the  strain  of  contraction,  they 
split  and  threw  off  sharp  angular  fragments  from  a  few  ounces  to  100  or  200 
lb.  in  weight.2  In  the  plateau  region  of  North  America,  though  the  climate 
is  too  dry  to  afford  much  scope  for  the  operation  of  frost,  this  daily  vicissi- 
tude of  temperature  produces  results  that  quite  rival  those  usually 
associated  with  the  work  of  frost.  Cliffs  are  slowly  disintegrated,  the 
surface  of  arid  plains  is  loosened,  and  the  fine  debris  is  blown  away  by 
the  wind. 

Effects  of  wind. — The  geological  work  directly  due  to  the  air  itself 
is  mainly  performed  by  wind.^  A  dried  surface  of  rock  or  soil,  when 
exposed  to  wind,  has  the  finer  disintegrated  particles  blown  away  as  dust 
or  sand.  This  process,  which  takes  place  familiarly  before  our  eyes  on 
every  street  and  roadway,  over  cultivated  ground,  as  well  as  on  siu^aces 
with  which  man  has  not  interfered,  is  most  marked  in  dry  climates. 
Aridity  indeed  is  its  main  cause.  Mr.  Flinders  Petrie,  the  able  Egyptian 
archaeologist  and  explorer,  has  brought  forward  evidence  of  the  abrading 
influence  of  the  wind  upon  mud-brick  walls  and  other  buildings,  and  he 
estimates  that  in  some  parts  of  the  Nile  delta  about  eight  feet  of  soil  has 
been  swept  away  by  the  wind  during  the  last  2600  years,  or  nearly  four 
inches  in  a  centiury.'*  Many  old  fortifications  in  Northern  China  have 
been  laid  bare  to  the  very  foundations  by  the  removal  of  the  surrounding 
soil   through  long-continued  action  of  wind.^      In  the  dry  plateaux  of 

^  In  the  United  States,  with  an  annual  thermometric  range  of  more  than  90^  Fahr.,  this 
difficulty  led  to  some  experiments  on  the  amount  of  expansion  and  contraction  in  different 
kinds  of  building-stones,  caused  by  variations  of  temperature.  It  was  found  that  in  fine- 
grained granite  the  rate  of  expansion  was  '000004825  for  every  degree  Fahr.  of  increment  of 
heat ;  in  white  crystalline  marble  it  was  '000005668  ;  and  in  red  sandstone  '000009532,  or 
about  twice  as  much  as  in  granite.  Totten,  in  Sillimana  Amer.  Joum.  xxii.  p.  136.  See 
ante,  pp.  292,  299. 

^  Livingstone's  '  Zambesi,'  pp.  492,  516.'  According  to  Stanley,  cold  rain  falling  on  these 
sun-heated  African  rocks  causes  them  to  split  ojieu  and  peel  off.  Proc.  Roy.  Qeog,  Soc.  xx. 
(1876),  p.  142. 

*  The  general  geological  effects  of  wind  are  discussed  by  F.  Czemy,  Petermanna  MiUheil, 
Ergdnzungah^ty  No.  48.     Saiure,  xv.  p.  2311. 

*  Proc  Roy,  Geograph,  Soc.  1889,  p.  648. 

«  Richthofen's  'Chintv'  Berlin,  1877,  I  p.  97. 


330  DYXAMICAL  GEOLOGY  book  ra  pabt  n 

Xoi-th  America,  too,  though  no  human  memorials  serve  there  as  measures, 
extensive  denudation  from  the  same  cause  is  in  progress. 

It  is  not  merely  that  the  wind  blows  away  what  has  already  been 
loosened  and  pulverised.  The  grains  of  dust  and  sand  are  themselves 
employed  to  rub  down  the  surfaces  over  which  they  are  driven.  The 
nature  and  potency  of  the  erosion  done  by  sand-grains  in  rapid  motion  jb 
well  illustrated  by  the  artificial  sand-blast,  in  which  a  spiay  of  fine 
siliceous  sand,  driven  with  great  velocity,  is  made  to  etch  or  engrave 
glass.  ^  The  abrading  and  polishing  effects  of  wind-blown  sand  have  long 
been  noticed  on  Egyptian  monuments  exposed  to  sand-drift  from  the 
Libyan  dese^t.^  Similar  effects  have  been  observed  on  dry  volcanic  plains 
of  l>arren  sand  and  ashes,  as  on  the  island  of  Volcano.^  On  the  sandy 
plains  of  Wyoming,  Utah,  and  the  adjacent  territories,  surfaces  even  of 
such  hard  materials  as  chalcedony  are  etched  into  furrows  and  wrinkles, 
acquiring  at  the  same  time  a  peculiar  and  characteristic  polish.  There, 
also,  large  blocks  of  sandstone  or  limestone  which  have  fallen  from  an 
adjacent  cliff  are  attacked,  chiefly  at  their  base,  by  the  stratum  of 
drifting  sand,  until  hy  degrees  they  seem  to  stand  on  narrow  pedestals. 
As  these  supports  are  reduced  in  diameter  the  blocks  eventually  tumble 
over,  and  a  new  basal  erosion  leads  to  a  renewal  of  the  same  stages  of 
waste.*  Hollows  on  rock-siu-faces  may  also  be  noticed  where  grains  of 
sand,  or  small  pel>bles  kept  in  gyration  by  the  wind,  gradually  erode  the 
shallow  cavities  in  which  thev  lie. 

As  the  result  of  the  protracted  action  of  wind  upon  an  area  exposed 
at  once  to  great  drought  and  to  rapid  vicissitudes  of  temperature,  a 
continuous  lowering  of  the  general  level  takes  place.  The  great  sandy 
deserts  thus  produced  represent,  however,  only  a  portion  of  the  disintegn- 
tion.  Vast  quantities  of  the  finer  dust  are  borne  away  by  the  wind  into 
other  regions,  where,  as  will  be  immediately  |)ointed  out,  they  tend  to 
raise  the  general  level.  Again,  a  considerable  amount  of  fine  dust  and 
sand,  blown  into  the  neighl)ouring  rivers,  is  carried  down  in  their  waters. 
In  inland  areas  of  drainage,  indeed,  like  that  of  Central  Asia,  this  transport 
does  not  finally  remove  the  river-borne  sediment  from  the  basin  of 
evaporation,  but  tends  to  fill  up  the  lakes.  "NMiere,  however,  as  in  North 
America,  rivers  cross  from  the  desert  areas  to  the  sea,  there  must  be  a 

^  The  student  will  flud  much  valuable  information  on  this  subject  in  the  experimental 
results  obtaineil  by  Thou  let,  Compfes  Rend.  civ.  p.  381.  Amu  des  Mints,  1887  ;  and  in  the 
essay  by  Walther  cited  below. 

^  An  excellent  account  of  the  denudation  phenomena  of  the  Egyptian  deserts  wiU  be 
found  in  an  essay  by  J.  Walther  in  vol.  xvi.  (1891)  of  the  Ahhand,  KSnigL  SBchnsek, 
(resellsch.  d.  Wisscnscfi,  The  |)olLshing  of  rocks  by  the  sand  of  the  Sahara  is  described  by 
M.  Choisy  in  his  report  '  Documents  relatifs  a  la  mission  dirig^e  an  Sud  de  I'Alg^rie,'  1890, 
p.  327. 

*  Kayser,  Z.  Devtsch.  Geol.  Ges.  xxvii.  p.  966. 

*  See  Gilbert  in  A\Tieeler*s  Ueport  of  V.  S.  Geograph,  Surr.  If.  o/lOOih  Meridian,  iii. 
p.  82.  W.  P.  Blake,  Union  Pdcijic  Jtailnxui  Report. ^  v.  pj>.  92,  230.  Arner,  Joum.  ScL 
XX.  (1885),  i».  17S.  Naumaun,  Seiies  Jahrh.  1874,  p.  337.  Cazalis  de  Fondonce,  A$90c, 
Fran^aisej  1S79,  p.  646.  Many  goo<l  illustrations  are  given  by  Walther  in  the  essay  above 
cited. 


SECT,  i  §  1  GROWTH  OF  DUST  331 

permanent  removal  of  wind-swept  detritus  by  these  streams.  In  the  arid 
plateaux  drained  by  the  Colorado  and  its  tributaries,  so  great  has  been 
the  subaerial  denudation  that  a  thickness  of  thousands  of  feet  of  horizontal 
strata  has  been  removed  from  the  surface  of  level  plains  thousands  of 
square  miles  in  extent.  This  denudation,  the  extent  of  which  is  attested 
by  the  remaining  cliffs  and  "  buttes,"  or  outliers,  of  the  strata,  appears  to 
be  in  great  measure  due  to  the  causes  here  discussed,  augmented  in  some 
districts  by  the  effects  of  occasional  heavy  storms  of  rain. 

One  further  effect  produced  by  air  in  violent  motion  may  be  seen  in 
the  destruction  caused  by  cyclones.  Not  only  are  houses  demolished, 
with  much  damage  to  other  property  and  loss  of  life,  but  permanent 
changes  of  more  or  less  importance  are  produced  upon  the  surface  of  a 
countrv.  Loose  rocks  on  the  face  of  cliffs  are  hurled  down,  and  blocks 
of  stone  and  loose  gravel  are  swept  away.  .  But  the  most  obvious  effects 
are  those  in  wooded  districts,  where  the  trees  are  prostrated  far  and  near 
in  the  path  of  the  storm.  On  the  18th  and  19th  of  May  1883,  a 
succession  of  hurricanes  passed  over  the  States  of  Illinois  and  Wisconsin, 
with  such  fury  that  the  brick  chimney  of  a  factory  was  carried  to  a 
distance  of  three-quarters  of  a  mile,  an  entire  house  was  lifted  into  the 
air  and  blown  to  pieces,  and  an  oak  two  feet  in  diameter  was  dashed 
through  a  house.  When  such  a  storm  passes  over  forest -ground  in 
temperate  latitudes,  the  surface-drainage  may  be  so  obstructed  by  the 
fallen  stems,  that  marsh-plants  spring  up,  and  eventually  the  site  of  a 
forest  may  be  occupied  by  a  peat-moss  (p.  478). 

2.  Reproductive  action. — Growth  of  Dust.  The  fine  dust  and 
sand  resulting  from  the  general  superficial  disintegration  of  rocks  would, 
if  left  undisturbed,  accumulate  in  situ  as  a  layer  that  would  serve  to  protect 
the  still  undecayed  portions  underneath.  Such  a  layer,  indeed,  partially 
remains,  but,  being  liable  to  continual  attack  and  removal,  may  be  taken 
to  represent,  where  it  occurs,  the  excess  of  disintegration  over  removal. 
In  the  vast  majority  of  cases,  however,  the  superficial  coating  of  loose 
material  is  not  due  merely  to  the  direct  action  of  the  sun's  rays  and  of 
the  air,  but  in  far  greater  degree  to  the  work  of  rain,  aided  by  the 
co-operation  of  plants  and  animals.  To  the  layer  thus  variously  produced, 
the  name  of  Soil  is  given.     Its  formation  is  described  at  p.  351. 

That  wind  plays  an  effective  part  in  the  re-distribution  of  supei'ficial 
detritus  is  demonstrated  by  every  cloud  of  dust  blown  from  dessicated 
groimd.  We  only  need  to  take  into  account  the  multiplying  power  of 
time,  to  realise  how  extensively  the  soil  of  a  district  may  be  lowered,  or, 
in  other  cases,  may  be  replenished  and  heightened  by  the  dust-storms  of 
centuries.  Dust  and  sand,  intercepted  by  the  leaves  of  plants,  gradually 
descend  into  the  soil,  whither  they  are  washed  down  by  rain,  so  that 
even  a  permanently  grassy  surface  may  be  slowly  and  imperceptibly 
heightened  in  this  way,  and  a  soil  may  be  formed  differing  considerably 
in  chemical  composition  from  what  would  result  merely  from  the  decay  of 
the  subsoil.^ 

On  the  sites  of  ancient  monuments  and  cities,  this  reproductive  action 

*  C.  Reid,  OeoL  Mag.  1884,  p.  165. 


332  DYNAMICAL  GEOLOGY  ikx)k  in  partu 


of  the  atmosphere  can  l)e  most  impressively  seen  and  most  easily 
measured.  In  Europe,  on  sites  still  inhabited  by  an  abundant  population, 
the  deep  accumulations  beneath  which  ancient  ruins  often  lie  are  doubtless 
mainly  to  be  assigned  to  the  successive  destnictions  and  rebuildings  of 
generation  after  generation  of  occupants.  But  at  Nineveh,  Babylon,  and 
many  other  eastern  sites,  mounds  which  have  been  practically  untouched 
by  man  for  many  centuries  consist  of  fine  dust  and  sand  gradually  drifted 
l>y  the  wind  round  and  over  abandoned  cities,  and  protected  and 
augmented  by  the  growth  of  vegetation.^  In  those  arid  lands,  the  air  is 
often  laden  with  fine  detritus,  which  drifts  like  snow  round  conspicuouB 
objects  and  tends  to  bury  them  up  in  a  dust-drift.  In  Central  Asia,  even 
when  there  is  no  wind,  the  air  is  often  thick  with  fine  dust,  and  a  yellow 
sediment  settles  from  it  over  everything.  In  Khotan  an  exceedingly  fine 
dust  sometimes  so  obscures  the  sun  that  even  at  midday  one  cannot  read 
large  print  without  a  lamp.  This  dust,  deposited  on  the  soil,  heightens 
and  fertilises  it,  and  is  regarded  by  the  inhabitants  as  a  kind  of  manure, 
without  which  the  ground  would  be  barren.^ 

Loess. — This  name  has  been  given  to  a  remarkable  deposit,  first 
descril>ed  in  the  valley  of  the  Khine,  but  which  has  l>een  found  to  cover 
vast  areiis  both  in  the  Old  World  and  in  the  New.^  It  is  usually  a 
yellowish  homogeneous  clay  or  loam,  unstratified,  and  presenting  a 
singular  uniformity  of  composition  and .  stnicture.  When  carefully 
examined,  its  quartz-grains  are  found  to  l>e  remarkably  angular,  and  its 
mica-flakes,  instead  of  being  deposited  horizontally,  as  they  are  by  water, 
occur  dispersedly  in  every  possible  position  and  with  no  definite  order.* 
The  chief  constituent  of  loess  is  always  hydrated  silicate  of  alumina,  in 
which  the  scattered  grains  of  quartz  and  flakes  of  mica  are  distributed. 
It  is  in  some  measure  calcareous,  the  lime  being  here  and  there  segregated 
into  curious  concretionary  forms  (Lossmanchen,  Lr»sspup{)en,  p.  512)  by 
the  action  of  infiltrating  water.  Though  a  firm  unstratified  mass,  it  is 
traversed  by  innumerable  tubes,  formed  by  the  descent  of  roots  and  mostly 
crusted  with  carbonate  of  lime.  These  have  generally  a  vertical  position, 
and  ramify  downwards.  Where  the  surface  is  covered  with  vegetation, 
they  may  l)e  seen  occupied  by  rootlets  to  a  depth  of  a  foot  or  a  few  feet 
from  the  surface.  By  means  of  these  pipes  a  tendency  is  given  to  a 
vertical  jointing  of  the  mass.  With  these  characters,  the  loess  unites  a 
remarkable  peculiarity  in  respect  of  its  organic  remains,  which  consist 
chiefly  of   land-shells,  sometimes  in  immense   numbers,  likewise  of   the 

^  The  nibbish  which,  in  the  course  of  many  centuries,  has  accumulated  above  the 
foundations  of  the  Assyrian  buildings  at  Kouyunjik  was  found  by  Layard  to  be  in  some 
])laces  twenty  feet  deep.  It  consisted  partly  of  ruins,  but  mostly  of  fine  sand  and  dnst 
l>lown  from  off  the  plains  and  mixed  with  decayed  vegetable  matter.  Layanl,  'Nineveh 
and  its  Remains,'  3nl  edit.  ii.  p.  120.     See  also  Richthofen's  'China,'  i.  p.  97. 

-  Johnson's  'Journey  to  Hohi,  the  capital  of  Khotan,'  Journ.  Geog.  Soc,  xzxvii.  1867, 
p.  1.     H.  B.  Guppy,  Xatnre,  xxiv.  (1881),  p.  126. 

'*  The  calcareous  clays  of  the  arid  regions  of  North  America  have  been  largely  used  for  the 
manufacture  of  sun-drie<l  bricks  called  in  Spanish  *'adol)e," — a  term  which  has  been  pro- 
])osed  as  a  geological  designation  for  these  deitosits.     I.  C.  Russell,  Geol,  Mag,  1889,  p.  291. 

V^'^^b,  Russell's  pai>er  cited  in  the  previous  note,  p.  294. 


SECT,  i  §  1  LOESS  333 

bones  of  various  herbivorous  and  carnivorous  mammals,  which  are  either 
identical  with  or  closely  allied  to  living  species  that  abound  on  steppes 
and  grassy  plains.  Freshwater  shells  are  usually  rare,  and  marine  forms 
do  not  occur.  Loess  is  found  at  all  elevations,  up  to  5000  feet  among 
the  Carpathians,  8000  feet  in  Shansi,  China,  and  probably  to  still  higher 
altitudes  further  west.  In  hilly  regions  it  fills  up  the  valleys,  shading  off 
on  either  side  up  the  slopes  into  the  angular  debris  of  the  adjoining  rock. 
Elsewhere,  it  spreads  over  the  surface  so  as  completely  to  conceal  the 
original  inequalities  of  the  ground.  In  Northern  China,  Richthofen  found 
it  to  have  a  thickness  of  1500  or  possibly  over  2000  feet,  and  to  be  cut 
into  deep  valleys  and  precipitous  ravines,  with  cliffs  500  feet  high,  which 
are  excavated  into  tiers  of  chambers  and  passages  by  a  teeming  popula- 
tion.^ In  the  arid  tracts  of  North  America  the  loess  or  "adobe"  is 
estimated  to  be  sometimes  2000  or  3000  feet  thick.* 

Various  theories  have  been  proposed  in  explanation  of  this  singular 
deposit.  By  some  it  has  been  referred  to  the  operation  of  the  sea ;  by 
others  to  the  work  of  lakes  or  of  rivers.  But  its  wide  extent,  its 
independence  of  the  altitude  or  contoiu^  of  the  ground,  its  uniform  and 
unstratified  character,  the  unworn  condition  of  its  component  particles, 
and  the  natiu^e  of  its  organic  remains,  show  that  it  cannot  be  assigned 
to  the  action  of  large  Ixxiies  of  water.  Richthofen  propounded  in  1870 
the  opinion  that  the  loess  is  mainly  due  to  the  long-continued  drifting 
and  deposit  of  fine  dust  by  wind  over  areas  more  or  less  covered 
with  grassy  vegetation,  aided  by  the  washing  influence  of  rain,  and  this 
view  has  been  widely  accepted.  Where  rain  is  distributed  somewhat 
equally  throughout  the  year  little  dust  is  formed ;  but  where  dry  and 
wet  seasons  alternate,  as  in  Central  Asia,  vast  quantities  of  dust  may  be 
moved  during  the  months  of  dry  weather.  When  the  dust  falls  on  Imre 
ground,  it  is  eventually  swept  away  by  the  wind ;  but  where  it  settles 
down  on  ground  covered  with  vegetation  it  is  in  great  measure  protected 
from  further  transport,  and  thus  heightens  the  soil.^ 

For  atmospheric  accumulations  of  this  natiure,  Trautschold  has  pro- 
posed the  name  eluviuvi.  They  originate  in  situ,  or  at  least  only  by 
wind-drift,  whereas  alhmum  requires  the  operation  of  water,  and  consists 
of  materials  brought  from  a  greater  or  less  distance.*  For  wind-formed 
deposits  the  term  "  seolian  "  is  sometimes  used. 

*  See  Richthofen's  description,  Geol.  Mag.  1882,  p.  293,  and  bis  *  China,'  above  cited. 
=  Russell,  Oeol.  Mag.  1889,  p.  292. 

%  *  Richthofen,  Oeol,  Mag.  1882,  p.  297.  For  some  of  the  more  important  contributions 
to  this  subject,  see  Richthofen's  *  China,'  vols.  i.  and  ii.  ;  also  Verh,  Oeol.  Reichs.  1878, 
p.  289;  E.  Tietre,  Verh.  Oeol.  Reichs.  1878,  p.  113;  1881,  p.  87;  Jahrb.  Oed.  Reichs. 
1881,  p.  80;  1882,  p.  11  ;  1883,  p.  279;  R  Pumpelly,  Aimt.  Journ.  Sci.  xvii.  (1879)  ; 
E.  W.  Hilgard,  op.  cit.  xviii.  (1879),  p.  106  (p.  427)  ;  I.  C.  Russell,  Oeol.  Mag.  1889,  pp. 
288,  342  ;  F.  Wahnschaffe,  Z.  Deutsch.  Oeol.  Oes.  1886.  Jahrb.  Preuss.  Laiidesanst.  1889, 
p.  328.  A.  Sauer,  Zeitsch.  f'dr  Natuncissensch.  Ixii.  (1889) ;  and j[>o«/ea,  Book  VI.  Pail  V. 
Sect.  i.  On  the  loess  of  Alsace,  see  E.  Schumacher,  Commiss.  LandesunUrsuch.  Elsass- 
Lothringen,  vol.  ii.  Part  I.  (1889),  p.  79  ;  on  the  loess  of  the  Pampas,  S.  Roth,  Zeitsch. 
Dtutseh.  Oed.  OeseU.  xl.  (188S),  p.  422. 

*  Z.  Deutsch.  Oeol.  Oca.  xxxi.  p.  578. 


334  1> YSA  MICA  L  (iEOLOG  Y  book  hi  pam  ii 


SHiulhillK    or   Duiics. — Winds   blowing   continuously   upon   sand 
dnvi*   it    oiiwunl,   uiid   pile   it   into  irregular  heaps   and    ridges,   called 
"dunoH/'      This  takes  place  more  especially  on  windward  coasts  either 
t»f  tho  Hoji  or  of  largo  inland  lakes,  where  sandy  shores  are  exposed  to 
tho  drying  iiiHuonoe  of  solar  heat  and  wind  ;  but  similar  effects  may  be  seen 
\}\v\\  in   tho  hrart  of  a  continent,  as  in  the  sandy  deserts  of  the  Sahara,^ 
An^bia.  jiihI   iu  tho  arid  lands  of  Utah,  Arizona,  &c.     The  dunes  travel 
in  iviniUol,  irn»gular,  and  often  confluent  ridges,  their  general  direction 
Umuj;  imn5»vorso  to  the  prevalent  coiu^e  of  the  wind.     Local  wind-eddies 
oatiHO   uiHuy    irrt^giilantios  of  form.      In  humid  climates,  rain-water  or 
tho  dnuuj*j:\*  «*t*  small  brooks  is  sometimes  arrested  between  the  ridges 
to  fonu    iHH^ls  {ttafifjs  of  the  French  coasts),   where  formations  of  peat 
^HVi^siionaUy    tako   place.      On  the  coast  of  Gascony,   the  sea  for  100 
nulos  is   *o  Ku-riHl  by  sand-dunes  that  in  all  that  distance  only  two  out- 
lot*  o\^st    for  tho  discharge  of  the  drainage  of  the  interior.     As  fast  as 
ouo  nd>:o  is  driven  away  from  a  beach  another  forms  in  its  place,  so  that 
.^  aoMOs  of    hugi^  8juidy  billows,  as  it  were,  is  continiially  on  the  move 
\\y^\\\    tho    soa- margin   towards  the  interior.      A  stream   or  river   may 
loinpovaiily  anv8t  their  progress,  but  eventually  they  push  the  obstacle 
,\^\\W  or  in  front  of  them.     In  this  way  the  river  Adour,  on  the  west  coast 
^  of  France,  has  had  its  mouth  shifted 

*'  ^^*'MSl^^  ^^^'®  ^^  ^^^®®  ™^®®-     OccasionaUy, 

JTteS              ^*w^  ^   ^^    ^^®   mouths   of    estuaries, 

*^%ll  r\     (r)     J   ^l^  ^^®  ^"^  ^^   blown   across,   so  as 

.4        ^'W^Jr^Jf  J  gradually  to  exclude  the  sea,  and 

^        *  •■9r**?      '  ^^C^      /^^^^^        ^^^^  ^^  ^^^  ^^®  fluviatile  deposits  in 
vVov^-;.  «^  adding  to  the  breadth  of  the  land 

*  J4fl^i;|^  a  ^**  ^^^'  ^^  »  stream  (t  e)  is  repre- 

sented as  crossing  a  plain  (a)  at  the 
margin  of  the  sea  or  of  a  large 
inland  sheet  of  water,  bounded  bv 
a  range  of  sand-dunes  {h  h)  extend- 
ing between  the  two  lines  of  cliff 
(r  </).  The  stream  has  been  turned 
ri„  iH».  Haii.i  aunrn  uffei'tinK' inna-iiraiiiaKo  (/i.)     to  its  right  l>ank  by  the  advance 

of  the  dunes  driven  by  a  prevalent 
^uiiil  Mowing  in  tho  direction  of  the  arrows,  A  brook  (J)  has  been 
ain-hii'il  uiiiong  the  sjindy  wastes,  whence,  after  forming  a  few  pools,  it 
innl"  I'f^reHrt  by  soaking  through  the  sandy  barrier. 

Tho  imturo  of  the  gi-ains  of  sand  depends  on  the  character  of  the 
iiitkh  frtitn  tho  tlostniction  by  which  they  are  derived,  and  their  form 
.Mill  hi/o  are  largely  ivg\datoil  by  the  force  of  the  wind  and  the  relative 
fthiiic'  taken  by  suKierial  and  subaqueous  action  in  their  production. 
ijiiaii/  i«  the  most  fnninent  constituent,  but  the  other  minends  of  rocks 
,(Im»  iiiiur,  eHpeeially  thi>se  which  are  most  capable  of  resisting  mechanical 
1 1 1 1 1 1 1  a  I  ioi  I.    In  Honie  ea^os,  organic  i*emains,  such  as  particles  of  shells,  nuUi- 

^Mkf  «u  ui«'ti(int  of  tho  Minil-dmios  of  the  Sahara  see  *  Documents  relatib  k  Im  MiatUiB 
^'    ^  Villi  lie  rAlgrrii'.'  .\.  I'hoiay,  1890,  p.  323. 


SECT,  i  §  1  SAND-DUNES  335 

pores,  &c.,  form  the  main  mass  of  the  sand  (see  p.  336).^  The  sand-grains 
liberated  by  inland  subaerial  disintegration  are  apt  to  be  more  angular 
than  those  brought  within  the  influence  of  the  wind  along  a  shore-line.  ^ 

Perfect  "  ripple-marks  "  (p.  507)  may  often  be  observed  on  blown  sand. 
The  sand-grains,  pushed  along  by  the  wind,  travel  up  the  long  slopes 
and  fall  over  the  steep  slopes.  Not  only  do  the  particles  travel,  but  the 
ridges  also  more  slowly  follow  each  other,  as  in  Fig.  91.^ 


S^^iiLiA^:"^^ 


mM^m^^^^^&^^r^S::^. 


Fig.  tfl.— Diagram  of  Ripples  in  blown  Sand.    The  ridges  61,  62,  6S,  impelled  in  the  direction  of  W  W, 

succesKively  come  to  occupy  the  hollows  oi,  a*,  a'  (J5.) 

The  western  sea-board  of  Europe,  exposed  to  prevalent  westerly  and  south-westerly 
winds,  atfords  many  instructive  examples  of  these  aeolian  or  wind- formed  deposits.  The 
coast  of  Norfolk  is  occasionally  fringed  with  sand-hills  50  to  60  feet  high.  On  parts  of 
the  coast  of  Cornwall,*  the  sand  consists  mainly  of  fragments  of  shells  and  corallines,  and 
through  the  action  of  rain  ui>on  these  calcareous  particles,  becomes  sometimes  cemented 
by  carbonate  of  lime  (or  oxide  of  iron)  into  a  stone  so  compact  as  to  be  fit  for  building 
purposes.  Long  tracts  of  blown  sand  are  likewise  found  on  the  Scottish  and  Irish* 
coast-lines.  Sand-dunes  extend  for  many  leagues  along  the  French  coast,  and  thence, 
by  Flanders  and  Holland,  round  to  the  shores  of  Courland  and  Pomerania.  On  the 
coast  of  Holland  they  are  sometimes,  though  rarely,  260  feet  high — a  common  average 
height  being  50  to  60  feet.<* 

The  breadth  of  this  maritime  belt  of  sand  varies  considerably.  On  the  east  coast  of 
Scotland  it  ranges  from  a  few  yards  to  3  miles  ;  on  the  opjwsite  side  of  the  North 
Sea  it  attains  on  the  Dutch  coast  sometimes  to  as  much  as  5  miles.  The  rate  of 
progress  of  the  dunes  towards  the  interior  depends  upon  the  wind,  the  direction  of  the 
coast,  and  the  nature  of  the  ground  over  which  they  have  to  move.  On  the  low 
and  exposed  shores  of  the  Bay  of  Biscay,  when  not  fixed  by  vegetation,  they  travel 
inland  at  a  rate  of  about  16J  feet  per  annum,  in  Denmark  at  from  3  to  24  feet.  In  the 
course  of  their  march  they  envelop  houses  and  fields  ;  even  whole  jtarishes  and  districts 
once  populous  have  been  overwhelmed  by  them.^ 

^  Mr.  Russell  (Geol,  Mag.  1889)  refers  to  some  parts  of  the  sands  of  the  arid  lands 
of  North  America  as  being  composed  mainly  of  the  cases  of  cyprids,  blown  away  from  the 
beds  of  dried-up  lakes. 

'  Engravings  of  some  of  the  sand-grains  from  the  Egyptian  deserts  are  given  by  Walther 
in  the  essay  already  cited. 

'  On  the  origin  of  ripple-mark,  see  Book  IV.  Part  I.  p.  509. 

*  Ussher,  Oeol.  Mag.  (2),  vi.  p.  30/,  and  authorities  there  cited.  The  upper  parts 
of  the  blown  sand  are  sometimes  crowded  with  land-shells,  the  decay  of  which  furnishes 
the  cementing  material  (see  Fig.  76). 

^  See  Kinahan,  Oeol.  Mag.  viii.  p.  155. 

^  On  the  growth  of  Holland  through  the  operation  of  the  wind  and  the  sea,  see  l^Iie  de 
Beaumont,  'Lemons  de  Geologie  pratique,'  i.  A  detailed  description  of  the  dunes  of 
Holland  is  given  by  J.  Lorie,  Arch.  MusSe  Teyler,  ser.  ii.  vol.  iii.  Part  V.  (1890),  p.  375. 
For  an  account  of  the  sand-dunes  of  Western  Euroi>e,  see  W.  Topley,  Pop.  Science  Rev.  xiv. 
(1875),  p.  133. 

^  This  destruction  has  more  recently  been  averted  to  a  great  extent  by  the  planting  of 
pine  forests,  the  turpentine  of  which  has  become  the  source  of  a  large  revenue. 


336  DYXAMICAL  GEOLOGY  book  hi  part  n 


Along  the  margins  of  large  lakes  and  inland  seas  many  of  the  phenomena  of  in 
i!X])08ed  sea -coast  are  rejteated  on  a  scarcely  inferior  scale.  Among  these  must  be 
included  sand-dunes,  such  as  those  which,  reaching  heights  of  100  to  200  feet  ou  the 
south-e^istern  shores  of  Lake  Michigan,  have  entombed  forests,  the  tofis  of  the  trees 
being  still  visible  above  the  drifting  sand.  Large  dunes  occiur  also  on  the  eastern 
Iwrders  of  the  Ca.si»ian  Sea,  where  the  sand  spreads  over  the  desert  region  between  that 
sea  and  the  Sea  of  Aral,  into  which  latter  sheet  of  water  the  spread  of  the  sand  hat 
driven  the  course  of  the  Oxus,  once  a  tributary'  of  the  Cas])ian. 

In  the  interior  of  continents,  the  existence  of  vast  arid  wastes  of  loose  sand,  situated 
far  inland  and  remote  from  any  sheet  of  fresh  water,  suggest  curious  problems  iu 
physical  geography.  In  some  instances,  these  tracts  have  been  at  a  comparatiTely 
recent  geological  i>erio<l  covered  by  the  sea.  Yet  the  disintegration  of  rock  in  tonid 
and  rainless  i-egions  is  so  great  (rot^,  ]).  328),  that  the  existing  sand  is  doubtless  mainly, 
if  nut  entirely,  of  subaerial  origin.  The  sandy  deserts  of  the  high  plateaux  of  Westeni 
North  America,  which  have  never  been  under  the  sea  for  a  long  series  of  geological  agei^ 
show,  as  we  have  aheady  found  (p.  329),  the  mode  and  progress  of  their  formation  from 
atmospheiic  disintegration  alone.  In  Asia  lie  the  vast  deserts  of  Gobi,  where  iu  some 
places  ancient  cities  have  been  buried  under  the  sand.*  In  Rajputana,  ^ide  tracts  of 
sandy  <lesert  present  a  succession  of  nearly  parallel  ridges  or  waves  of  sand,  varying  up 
to  180  feet  from  trough  to  crest,  and  presenting  long  gentle  slopes  towards  south-west, 
whence  the  ]>revalent  winds  blow,  but  with  noith -eastern  fronts  as  steep  as  the  sand 
will  lie.-  To  the  east  of  the  Red  Sea  stretch  the  great  sand-wastes  of  Arabia  ;  and  to 
the  west  those  of  Libya.  The  sandy  wastes  of  the  Sahara  have  in  recent  years  been 
partially  ex])lored,  especially  by  French  ol>servers  fix)m  the  Algerian  frontier.  Accord- 
ing to  M.  Rolland,  the  sand  is  entirely  due  to  the  action  of  the  wind,  and  though  there 
is  a  trans]>ort  of  sand  and  fine  dust,  the  }K)sition  of  the  large  dunes,  sometimes  70 
metres  in  height,  remains  on  the  whole  unchanged. '"*  In  the  south-east  of  Europe,  orer 
the  st<^})]>es  of  southern  Russia  and  the  adjacent  territories,  wide  areas  of  sandy  desert 
occur.  Captain  Sturt  found  vast  deserts  of  sand  in  the  interior  of  Australia,  with  long 
bands  of  dunes  200  feet  high,  united  at  the  ba.se  and  stretching  in  straight  lines  as  &t 
as  the  eye  could  reach."* 

Some  of  the  most  remarkable  leolian  formations  arc  in  course  of  accumulation  at 
Bermuda  and   other   coral-islands.       The   finer   coral -sand,   with   remains  of  shells, 


'  For  important  information  regarding  the  Central  Asiatic  wastes,  see  Richthofen's 
*  China,'  i.  Also  Tchihatchef,  Brif.  Assoc.  1882,  ]k  356.  T.  D.  Forsyth,  Joum.  Ji<»y. 
(feof/.  Soc.  xlvii.  (1878),  p.  1. 

'  Major  C.  Strahan  in  *  Report  of  Survey  of  India,'  1882-83. 

•'  (t.  Rolland,  BuiL  *S>c".  (f^ol.  France^  3rd  si'r.  x.  i».  30.  See  also  A.  Parran,  op,  cit, 
xviii.  (1890),  p.  245. 

*  For  accounts  of  sand -dunes,  their  extent,  progress,  structure,  and  the  means 
employed  to  arrest  their  progress,  the  student  may  consult  Andersen's  '  Klitfonnationen,'  1 
vol.  8vo,  Copenhagen,  1861  ;  Laval  in  Annales  jdcs  Ponts-et-ChaMssicSf  1847,  2me  sem. 
Marsh's  '  Man  and  Nature,'  1864,  and  the  works  cited  by  him.  Forchhanimer,  Jldin,  JVinp 
P/iiI.  Journ.  xxxi.  (1841),  p.  61.  £llie  de  Beaumont,  *  Lemons  de  Geologic  pratique,*  voL 
i.  p.  183.  Winkler,  Cong,  Internai.  (weol.  1878,  p.  181.  Information  regarding  the  sands 
of  the  interior  of  continents  will  be  found  in  Palgrave's  *  Travels  in  Arabia '  ;  Blake,  in 
Union  P((cijic  Railroad  Itejwrty  v.  ;  Tristram,  *The  Great  Sahara,'  1860;  Deaor,  **Le 
Sahara,  ses  ditferents  types  de  deserts,"  Buil.  Soc,  Sci,  Sat.  SeufchAttf^  1864 ;  -E.  Fachs, 
Peten}wnns  Mitthcil.  1879  ;  A.  Poniel,  Assw.,  Frnn^nise,  1877,  p.  428 ;  G.  RoUand, 
Ball.  Soc.  Hid.  France,  3me  ser.  x.,  La  Xatvre,  1882,  Soc.  de  Ukuj.  1890  ;  Richthofens 
'China,'  i. ;  I.  C.  Russell  on  the  subaerial  deposits  of  North  America,  Qtol,  Mag»  1889, 
p.  289. 


SECT,  i  §  1  DUST'SHOWERS,  BLOOD-RAIN  337 

echinoderins,  calcareous  algte,  and  other  organisms,  is  driven  by  the  wind  into  dunes, 
the  surface  of  which  by  the  action  of  rain-water  soon  becomes  cemented  into  coherence, 
while  by  degrees  the  whole  mass  of  calcareous  debris  is  converted  into  a  hard  compact 
rook  which  nngs  under  the  hammer.  The  highest  point  of  Bermuda  is  245  feet  above 
the  sea,  and  the  whole  land  up  to  that  height  is  composed  of  these  hardened  calcareous 
t«olian  deposits.* 

Dust-showers,  Blood -rain. — Besides  the  universal  transport  and 
deposit  of  dust  and  sand  already  described,  a  phenomenon  of  a  more 
aggravated  nature  is  observed  in  tropical  countries,  where  great  droughts 
are  succeeded  by  violent  hiu-ricanes.  The  dust  or  sand  of  deserts  and  of 
dried  lakes  or  river-beds  is  then  sometimes  borne  away  into  the  upper 
regions  of  the  atmosphere,  where,  meeting  with  strong  aerial  currents 
which  may  transport  it  for  many  hundreds  of  miles,  it  descends 
again  to  the  surface,  in  the  form  of  "  red  fog,"  "  sea-dust,"  or  "  sirocco- 
dust."  This  transported  material,  usually  of  a  brick-dust  or  cinnamon 
colour,  is  occasionally  so  abundant  as  to  darken  the  air  and  obscure  the 
sun,  and  to  cover  the  decks,  sails,  and  rigging  of  vessels  which  may  even 
be  hundreds  of  miles  from  land.  Rain  falling  through  such  a  dust-cloud 
mixes  with  it,  and  descends,  either  on  sea  or  land,  as  what  is  popularly 
called  "  blood-rain."  Occasionally  the  dust  is  brought  down  to  the  surface 
of  the  ground  by  snow. 

This  phenomenon  is  frefjueiit  on  the  north-west  of  Africa,  about  the  Cape  Verd  Islands, 
in  the  Mediterranean,  and  over  the  bordering  countries.  A  microscopic  examination  of 
this  dust  by  Ehrenberg  led  him  to  the  l)elief  that  it  contains  numerous  diatoms  of  South 
American  si)ccies  ;  and  he  inferred  that  a  dust-cloud  must  be  swimming  in  the  atmo- 
sphere, earned  forward  by  continuous  currents  of  air  in  the  region  of  the  trade-winds  and 
anti-trades,  but  suffering  |>artial  and  i)eriodical  deviations.  But  much  of  the  dust  seems 
to  come  from  the  sandy  plains  and  desiccated  pools  of  the  north  of  Africa.  Daubr^e 
recognised  in  1865  some  of  the  Sahara  sand  which  fell  in  the  Canary  Islands.  On  the 
coast  of  Italy,  a  film  of  sandy  clay,  identical  with  that  from  parts  of  the  Libyan  desert, 
is  occasionally  found  on  windows  after  rain.  In  the  middle  of  last  centui-y  an  area  of 
Xorthern  Italy,  estimated  at  about  200  square  leagues,  was  covered  with  a  layer  of  dust 
wliieh  in  some  places  reached  a  depth  of  one  inch.  In  1846  the  Sahara  dust  reached 
Lyons,  and  it  is  said  to  have  been  since  detected  as  far  as  Boulogiie-sur-Mer.  Should  the 
travelling  dust  encounter  a  cooler  temperature,  it  may  be  brought  to  the  ground  by  snow, 
as  has  happened  in  the  North  of  Italy,  and  more  notably  in  the  east  and  south-east  of 
Russia,  where  the  snows  are  sometimes  rendered  dirty  by  the  dust  raised  by  winds  on  the 
Caspian  steppes.-*  It  is  easy  Xjo  see  how  widespread  deposits  of  dust  may  arise,  mingled 
with  the  soil  of  the  land,  and  with  the  silt  and  sand  of  lakes,  rivers,  or  the  sea ;  and 
how  the  minuter  organisms  of  tropical  regions  may  thus  come  to  be  preserved  in  the  same 
formations  with  the  terrestrial  or  marine  organisms  of  temi^erate  latitudes.' 

The  transport  of  volcanic  dust  by  wind,  already  referred  to  (p.  216),  maybe  again 
cited  here,  as  another  example  of  the  geological  work  of  the  atmosphere.     Thus,  from 

*  Nelson,  Q.  J.  Geol,  Sue.  ix.  p.  200.    Wyville  Thomson's  'Atlantic, '  vol.  i.,  and  ante,  p.  128. 

^  Consult  an  interesting  paper  by  C.  von  Camerlander  on  snow  with  dust  which  fell  in 
SUesia,  >k>ravia,  and  Hungary  iu  February  1888,  JaJirb.  Geol.  Reichsanst.  xxxviii.  (1888), 
p.  281. 

'  See  Humboldt  on  dust  whirlwinds  of  Orinoco,  *  Aspects  of  Nature  '  ;  also  Maury,  *  Phys. 
6eog.  of  Sea,'  chap.  vi.  ;  Ehrenberg's  *  Passat-Staub  und  Blut-Regen,'  Berlin  Akad.  1847. 
A.  von  Lasanlx  on  so-called  "cosmic  dust,"  Tschermak's  Mineral,  MittheU.  1880,  p.  617. 

Z 


338  DYXAMICAL   GEOLOGY  book  ill  part  u 

the  Icelandic  eruptions  of  1874-75,  vast  showers  of  tine  ashes  not  only  fell  on  Iceland  to 
a  depth  of  six  inches,  destroying  the  i^asturcs,  but  were  borne  over  the  sea  and  aotM 
Scandinavia  to  the  east  coast  of  Sweden.^  The  remarkable  sunsets  of  Europe  during  the 
winter  and  spring  of  1883-84  ai-e  ascribed  to  the  diti'usion  of  the  line  dust  from  the  great 
Krakatoa  eruption  of  August  1883  (p.  214).  Considerable  deposits  of  volcanic  material 
may  thus  be  formed  in  the  course  of  time  even  far  remote  from  any  active  volcano. 

Transportation  of  Plants  and  Animals. — Besides  the  trausportof 
dust  for  distances  of  perhaps  thousands  of  miles,  wind  may  also  transport 
living  seeds  or  spores,  which,  finally  reaching  a  congenial  climate  and 
soil,  may  survive  and  spread.  "We  are  yet,  however,  very  ignorant  as 
to  the  extent  to  which  this  cause  has  actually  o{)erated  in  the  establish- 
ment of  any  given  local  flora.  With  regard  to  the  minute  forms  of 
vegetable  life,  indeed,  there  can  be  no  doubt  as  to  the  efKcacy  of  the 
wind  to  transi)ort  them  aci'oss  vast  distances  on  the  surface  of  the  globe. 
Upwards  of  300  species  of  diatoms  have  been  found  in  the  deposits  left 
by  dust -showers.  Among  the  millions  of  organisms  thus  transported 
it  is  hai-dly  conceivable  that  some  should  not  fall  still  alive  into  a  fitting 
locality  for  their  continued  existence  and  the  i)erpetuation  of  their  species 
Animal  fomis  of  life  are  likewise  diffused  through  the  agency  of  winds. 
Insects  and  birds  are  often  met  with  at  sea,  many  miles  distant  from 
the  land  from  which  they  have  been  l)lown.  Such  organisms  are  in 
this  way  introduced  into  oceanic  islands,  as  is  well  shown  in  the  case  of 
Bermuda.  Hurricanes,  by  which  large  quantities  of  water  are  sucked 
up  from  lakes  and  rivers  over  which  they  pass,  may  also  transport  part 
of  the  fauna  of  these  waters  to  other  localities. 

Efflorescence  products. — Among  the  formations  due  in  lai^ 
measure  to  atmospheric  action  must  be  included  the  saline  efflorescences 
which  form  upon  the  ground  in  the  dry  interior  l)asins  of  continents. 
The  steppes  of  Southern  Russia,  and  the  plains  round  the  Great  Salt 
Lake  of  Utah,  may  be  taken  as  ilhistnitive  examples.  Water,  rising  by 
capillary  attraction  through  the  soil  to  the  surface,  is  there  evaporated, 
leaving  behind  a  white  crust,  by  which  the  upi>er  portion  of  the  soil  is 
covered  and  permeiited.  The  incrustations  consist  of  sodium-chloride, 
sodium-  and  calcium-carbonates,  calcium-  sodium-  and  poUissium-sulphates 
in  various  proportions,  these  being  the  Siilts  present  also  in  the  salt  lakes 
of  the  same  regions  (p.  408).*- 

§  2.   Influence  of  the  Air  on  Water. 

The  results  of  the  action  of  the  air  uiwii  water  will  be  more  fitly 
noticed  in  the  section  devoted  to  Water.  It  will  be  enough  to  notice 
here — 

1.  Ocean  currents. — These  are  mainly  dependent  for  their  existence 

1  Nordenskiold.  Oeol.  Mag.  (2),  iii.   i>.  292.     F.  Zirkel,  Xeuen  J^thrh.  1879,  p.  8W 
(i.  voiii  Rati),  ibid.  p.  506,  and  antf,  }>.  216. 

-  Oil  efflorescence  of  Great  Salt  Lake  region,  see  Exploration  of  Adth  ParaUd,  i.  sect.  t. 
Consult  also  E.  Tietze,  '*Eutstebung  der  Salzsteppeu,"  JaJtrb.  Otvl.  Reichsanst.  1877,  and 
H.  Ic  Chatelier  on  the  salt-crusts  of  Algeria,  Comptes  Mend,  Ixxxiv.  p.  396. 


BBCT.  i  §  2  INFLUENCE  OF  AIR  ON  WATER  339 

and  direction  on  the  circulation  of  the  atmosphere.  The  in-streaming  of 
air  from  cooler  latitudes  towards  the  equator  causes  a  drift  of  the  sea- 
water  in  the  same  direction.  As,  owing  to  the  rotation  of  the  earth 
these  aerial  currents  tend  to  take  a  more  and  more  westerly  trend  in 
approaching  the  equator,  they  communicate  this  trend  to  the  marine 
currents,  which,  likewise  moving  into  regions  with  a  greater  velocity  of 
rotation  than  their  own,  are  all  the  more  impelled  in  the  same  westerly 
direction.  Hence  the  dominant  equatorial  current  which  flows  westward 
across  the  great  ocean.  Owing,  however,  to  the  position  of  the  continents 
across  its  path,  this  great  current  cannot  move  uninterruptedly  round  the 
earth.  It  is  split  into  branches  which  tiu*n  to  right  and  left,  and,  bathing 
the  shores  of  the  land,  carry  some  of  the  warmth  of  the  tropics  into  more 
temperate  latitudes.  Return  currents  are  thus  generated  from  cooler 
latitudes  towards  the  equator  (p.  434). 

2.  Waves. — The  impulse  of  the  ^vind  upon  a  surface  of  water  throws 
that  surface  into  pulsations  which  range  in  size  from  mere  ripples  to  huge 
billows.  Long-continued  gales  from  the  seaward  upon  an  exposed  coast 
indirectly  effect  much  destniction,  by  the  foimidable  battery  of  billows 
which  they  bring  to  bear  upon  the  land  (p.  444).  Wave-action  is  like- 
wise seen  in  a  marked  manner  when  wind  blows  strongly  across  a  broad 
inland  sheet  of  water,  such  as  Lake  Superior  (p.  406). 

3.  Alteration  of  the  Water-level. — Wind  bloM'ing  freshl}'  across 
a  lake  or  narrow  sea  drives  the  water  before  it,  and  keeps  it  temporarily 
at  a  higher  level  on  the  farther  or  "wandward  side.  Li  a  tidal  sea,  such  as 
that  which  surrounds  Great  Britain,  and  which  sends  alnmdant  long  arms 
into  the  land,  a  high  tide  and  a  gale  are  sometimes  synchronous.  This 
conjunction  makes  the  high  tide  rise  to  a  gi*eater  height  than  elsewhere 
in  those  bays  or  firths  which  look  windward,  occasionally  causing  consider- 
able damage  to  property  by  the  flooding  of  warehouses  and  stores,  with 
even  a  sensible  destniction  of  cliffs  and  sweeping  away  of  loose  materials. 
On  the  other  hand,  a  ^vnnd  from  the  opposite  quarter  coincident  with  an 
ebb  tide,  by  driving  the  water  out  of  the  inlet,  makes  the  water-level  lower 
than  it  would  otherwise  be.  In  inland  seas  where  tides  are  small  or  im- 
perceptible, considerable  oscillations  of  water-level  may  arise  from  the 
action  of  the  wind.  At  Naples,  for  example,  a  long-continued  south-west 
wind  raises  the  level  of  the  water  several  inches.  Similar  results  attend 
prolonged  gales  on  large  fresh-water  lakes  (p.  405). 

Eapid  and  great  diminution  of  atmospheric  pressiu^e  may  also  cause  a 
rise  in  the  level  of  the  sea  and  produce  great  destruction  (p.  437). 

Section  11.     Water. 

Of  all  the  terrestrial  agents  by  which  the  surface  of  the  earth  is 
geologically  modified,  by  far  the  most  important  is  water.  We  have 
already  seen,  when  following  hypogene  changes,  how  large  a  share  is 
taken  by  water  in  the  phenomena  of  volcanoes  and  in  other  subterranean 
processes.  Returning  to  the  surface  of  the  earth  and  watching  the 
operations  of   the  atmosphere,  we  soon  learn  how  imjjortant  a  part  of 


340  nVXAMICAL  GEoLOdY  book  III  part  ii 

these  is  sustained  by  the  aqueous  vapour  by  which  the  atmosphere  is 
pervaded. 

The  8ul)stance  which  we  term  water  exists  on  the  earth  in  three  well- 
known  forms — (1)  gaseous,  as  invisible  vapour;  (2)  liquid,  as  water;  and 
(3)  solid,  Jis  ice.  The  gaseous  form  has  already  l)een  noticed  as  one  of 
the  characteristic  ingredients  of  the  atmosphere  (p.  32).  Vast  quantities 
of  vapour  are  continually  rising  from  the  surface  of  the  seas,  rivers,  lakes, 
snow-fields,  and  glaciers  of  the  world.  This  vapour  remains  invisible 
until  the  air  containing  it  is  cooled  down  below  its  dew-point,  or  point  of 
saturation, — a  result  which  follows  upon  the  luiion  or  collision  of  two 
aerial  currents  of  dift'erent  t<?mi)cratures,  or  the  rise  of  the  air  into  the 
upper  cold  regions  of  the  atmosphere,  where  it  is  chilled  by  expansion,  by 
radiation,  or  by  contact  with  cold  mountains.  According  to  recent 
researches,  condensation  appears  only  to  take  place  on  free  surfaces,  and 
the  formation  of  cloud  and  mist  is  explained  by  condensation  upon  the 
fine  microscopic  dust  of  which  the  atmosphere  is  full.^  At  first  minute 
jwiticles  of  water-vapour  apjK?ar,  which  either  remain  in  the  liquid 
condition,  or,  if  the  temperature  is  sufficiently  low,  are  frozen  into  ice. 
As  these  changes  take  place  over  considerable  spaces  of  the  sky,  they  give 
rise  to  the  phenomena  of  clouds.  Further  condensation  augments  the 
size  of  the  cloud-ptirticles,  and  at  last  they  fall  to  the  surface  of  the  earth, 
if  still  liquid,  as  rain ;  if  solid,  as  snow  or  hail ;  and  if  partly  solid  and 
partly  liquid,  as  sleet.  As  the  vai)our  is  largely  raised  from  the  ocean- 
sui-face,  so  in  great  measure  it  falls  l>ack  again  directly  into  the  ocean. 
A  considerable  proportion,  hcjwever,  descends  upon  the  land,  and  it  is  this 
pirt  of  the  condensed  vapour  which  we  have  now  to  follow.  Upon  the 
higher  elevations  it  falls  as  snow,  and  gathers  there  into  snow-fields,  which, 
by  means  of  glaciers,  send  their  drainage  towanls  the  valleys  and  plains. 
Elsewhere  it  falls  chiefly  as  rain,  some  of  M'hich  sinks  underground  to 
gush  forth  again  in  springs,  while  the  rest  poiu^s  down  the  slopes  of  the 
land,  feeding  brooks  and  ton'ents,  which,  swollen .  fiu^her  by  springs, 
gather  into  broader  and  yet  broader  rivers  that  bear  the  acciunulated 
drainage  of  the  land  out  to  sea.  Thence  once  more  the  vapour  rises,  con- 
densing into  clouds  and  rain  to  feed  the  iimumerable  water-channels  by 
which  the  land  is  furrowed  from  mountain-top  to  seashore. - 

In  this  vast  system  of  circulation,  ceaselessly  renewed,  there  is  not  a 
drop  of  water  that  is  not  busy  with  its  allotted  task  of  changing  the  face 
of  the  earth.  When  the  vapour  ascends  into  the  air,  it  is  comparatively 
speaking  chemically  pure.  But  when,  after  being  condensed  into  visible 
form,  and  working  its  way  over  or  under  the  suiiace  of  the  land,  it  once 
more  entera  the  sea,  it  is  no  longer  pure,  but  more  or  less  loaded  with 
material  taken  by  it  out  of  the  air,  rocks,  or  soils  through  which  it  has 
travelled.  Day  by  day  the  process  is  advancing.  So  far  as  we  can  tell, 
it   has   never  ceased  since  the  first  shower  of  rain  fell  upon  the  earth. 

'  Coulier  auJ  Mascart,  Satucforschcr,  1875,  \\  400.     Aitkeu,  Proc,  Roy.  Sue.  JSdin^ 
Deo.  1880. 

-  For  estimates  of  the  distribution  of  rain  over  the  globe,  see  Murray,  Scottish  Oeol.  Mag. 

1887. 


SECT,  ii  §  1  CHEMICAL  ACTION  OF  RAIN  341 


We  may  well  believe,  therefore,  that  it  must  have  worked  marvels 
upon  the  surface  of  our  planet  in  past  time,  and  that  it  may  effect 
vast  transformations  in  the  future.  As  a  foundation  for  such  a  belief 
let  us  now  inquire  what  it  can  be  proved  to  l>e  doing  at  the  present 
time. 

§   1.  Eain. 

Rain  effects  two  kinds  of  changes  upon  the  surface  of  the  land. 
(1)  It  acts  cJiemically  upon  soils  and  stones,  and,  sinking  under  ground, 
continues,  as  we  shall  find,  a  great  series  of  similar  reactions  there.  (2) 
It  acts  mechanically,  by  washing  away  loose  materials,  and  thus  power- 
fidly  affecting  the  contours  of  the  land. 

1.  Chemical  Action. —  This  depends  mainly  upon  the  nature  and 
proportion  of  the  substances  abstracted  l>y  rain  from  the  air  in  its 
descent  to  the  earth.  Rain  absorbs  a  little  air,  which  always  contains 
carbonic  acid  as  well  as  other  ingredients,  in  addition  to  its  nitrogen 
and  oxygen  (p.  32).  Rain  thus  washes  the  air  and  takes  impurities  out 
of  it,  by  means  of  which  it  is  enabled  to  work  many  chemical  changes 
that  it  could  not  accomplish  were  it  to  reach  the  ground  as  pure  water. 

Composition  of  Rain-water. — Numerous  analyses  of  rain-water 
show  that  it  contains  in  solution  al)out  25  cubic  centimetres  of  gases 
|)er  litre.  ^  An  average  proportional  percentage  is  by  measure — nitrogen, 
64*47:  oxygen,  33*76;  carbonic  acid,  1*77.  Carbonic  acid  being  more 
soluble  than  the  other  gases,  is  contained  in  rain-water  in  proportions 
between  30  and  40  times  greater  than  in  the  atmosphere.  Oxygen  too 
is  more  soluble  than  nitrogen.  These  differences  acquire  a  considerable 
importance  in  the  chemical  operations  of  rain.  Other  sul>stances  are 
present  in  smaller  quantities.  In  England  there  is  an  average  of  3*95 
parts  of  solid  impurity  in  100,000  parts  of  niin.*-  Nitric  acid  sometimes 
occiu^  in  marked  proportions  :  at  Bale  it  was  found  to  retich  a  maximum 
of  13*6  parts  in  a  niiAion,  '  with  20*1  parts  of  nitrate  of  ammonia. 
Sidphuric  acid  likewise  occurs,  especially  in  the  rain  of  towns  and 
manufacturing  districts.^  Sulphates  of  the  alkalies  and  alkaline  earths 
have  been  detected  in  rain.  But  the  most  almndant  siilt  is  chloride  of 
sodium,  which  appears  in  marked  proportions  on  coasts,  as  well  as  in  the 
rain  of  towns  and  industrial  districts.  R;iin  taken  at  the  Land's  End  in 
Cornwall  during  a  strong  south-west  wind  was  found  to  contain  2*180 
of  chlorine,  or  3*591   parts  of  common  salt,  in   every    10,000   of  rain. 

*  Baumert,  Ann.  Chein.  Pharm.  Ixxxviii.  ji.  17.  The  proportion  of  carbonic  acid  found 
by  Peligot  was  2*4.  See  also  Bunsen,  op.  cit.  xciii.  p.  20.  Roth,  *Cheni.  Geol.'  i.  p.  44. 
Angus  Smith,  *  Air  and  Rain,'  1872,  p.  225. 

'  Rivers  PoUutioti  Cammission,  6th  Rej).  p.  29. 

*  The  occurrence  of  sulphuric  and  nitric  acids  in  the  air,  e8]>ecially  noticeable  in 
large  towns,  leads  to  considerable  corrosion  of  metallic  surfaces,  as  well  as  of  stones  and 
lime.  The  mortar  of  walls  may  often  be  observed  to  be  slowly  swelling  out  and  dropping 
off,  owing  to  the  conversion  of  the  lime  into  sulphate.  Great  injury  is  likewise  done, 
from  a  similar  cause,  to  marble  monuments  in  exposed  graveyards.  See  Angus  Smith, 
*  Air  and  Rain,'  p.  444.     Geikie,  Proc.  Roy.  St^-.  Edin.  1879-80,  p.  518.      • 


342  DYNAMICAL  GEOLOGY  book  in  part  ii 

The  mean  proportion  of  chlorine  ovei*  England  is  about  0*022  in  every 
10,000  parts  of  rain ;  at  Ootacamund  0003  to  0-004.^ 

In  washing  the  air,  rain  cairies  down  also  inorganic  particles  or 
motes  floating  there ;  likewise  organic  dust  and  li"\'ing  germs. ^  As  the 
result  of  this  process  the  soil  comes  to  he  not  merely  watered  but 
fertilised  by  the  min.  Angus  Smith  cites  the  expenence  of  J.  J. 
Pierre,  who  found  by  analysis  that  in  the  neighbourhood  of  Caen,  in 
Fi-ance,  a  hectare  of  land  receives  armually  from  the  atmosphere  by 
means  of  rain  ^  : — 

Cliloridc  of  sodiiini .37  '5  kilogrannnes. 

IM)ta88ium 8 '2 

magnesium 2*5 

r^Icium 1*8 


ft 


Sulphate  of  8<H la 8*4  kilogrammes 

,,  {totasli    ......  8*0  ., 

lime        ......  6*2  ,, 

,,  magnesia 5 '9 


•)i 


Not  only  i*ain,  but  also  dew  and  hoar-frost  abstmct  impuiities  from 
the  atmos})here.  The  analyses  perforaied  by  the  Rivers  Pollution 
Commission  show  that  dew  and  hoar-frost,  condensing  from  the  lower 
and  more  impure  layei*s  of  the  air,  are  oven  more  contaminated  than 
rain,  iis  they  contain  on  an  average  in  England  4*87  parts  of  solid 
impurity  in  100,000  parts,  with  0*198  of  ammonia."* 

It  is  manifest  that  min  reaches  the  surface  by  no  means  chemically 
pure  water,  but  ha\dng  absorbed  from  the  air  various  ingreilients  which 
enable  it  to  accomplish  a  series  of  chemical  changes  in  rocks  and  soils. 
So  far  as  we  know  at  })resent,  the  three  ingi*edients  which  are  chiefly 
cftective  in  these  operations  are  oxygon,  carbonic  acid,  and  organic 
matter.  As  soon  as  it  touches  the  earth,  however,  rain-water  liegins  to 
absorb  additional  impuiities,  notably  increasing  its  proportion  of  carbonic 
acid  and  of  organic  matter,  from  decomposing  animals  and  plants. 
Among  the  organic  products  most  efficacious  in  promoting  the  corrosion 

-  Angus  Smith,  *  Air  and  Rain.'  Rivfis  PolliftioH  Commi^atuoif  6tli  Rep.  1874,  p.  425. 
During  a  westerly  ||:ale  on  tlie  Atlantic  coasts  of  Britain,  when  the  sea  is  white  with  foam, 
the  air,  elsewhere  clear,  may  \ye  seen  to  be  quite  misty  alongshore  from  the  clouds  of  fine 
spray  swept  by  the  wiml  from  the  crests  of  the  brwikers.  This  suit-water  dust  is  borne 
far  inland.  From  the  investigations  carried  on  at  the  Agricultunil  Laboratory,  Rothamsted, 
it  ai)i»ears  that  the  average  proportion  of  chlorine  is  2*01  per  million  parts  of  rain,  which  in 
a  rainfall  of  31  '65  inches  is  equal  to  a  discharge  of  21  lbs,  of  pure  sodium  chloride  per  acre. 
At  Cirencester,  where  the  rainfall  is  33 '31  inches,  the  proportion  of  chlorine  is  3*25  per 
million,  which  is  etpiivalent  to  40*3  lbs.  of  sodium  chloritle  per  acre.  R.  Warington,  Joum. 
rlifin.  SiK\  1887,  p.  502. 

'"  Among  the  inorganic  contents  of  rain  and  snow.  Hue  terrestrial  dust  ami  spherules  of 
iron,  proliubly  in  ]>art  of  cosmic  origin,  have  been  N]>ecially  noted.  See  authorities  cited 
aiUr,  p.  08  ;  A.  von  Lasaulx,  as  cited  on  j).  337.  The  orgauit;  matter  of  rain  is  revealed  by 
the  putrid  smell  which  lung-kept  rain-water  gives  out. 

'^  Angus  Smith,  '  Air  and  Rain,'  p.  233. 

*  Rii'ers  Poll  lit  ion  Commission  ^  6th  Rep.  p.  32.  • 


SECT,  ii  §  1  CHEMICAL  ACTION  OF  RAIN  343 

of  minerals  and  rocks  are  the  so-called  ulmic  or  humous  substances  that 
form  with  alkalies  and  alkaline  earths  soluble  compounds,  which  are 
eventually  converted  into  carbonates.^  Hence  as  rain-water,  already 
armed  with  gases  absorbed  from  the  atmosphere,  proceeds  to  take  up 
these  organic  acids  from  the  soil,  it  is  endowed  with  considerable  chemical 
activity  even  at  the  very  beginning  of  its  geological  career. 

Chemical  and  mineralogical  changes  due  to  rain-water. — 
In  previous  pages,  it  was  pointed  out  that  all  rocks  and  minerals  are, 
in  varying  degrees,  porous  and  permeable  by  water,  that  probably  no 
known  substance  can,  imder  all  conditions,  resist  solution  in  water,  and 
that  the  subsequent  solvent  power  of  water  is  greatly  increased  by  the 
solutions  which  it  effects  and  carries  with  it  in  its  progress  through  rocks 
(pp.  306,  307).  The  chemical  work  done  by  rain  may  be  conveniently 
considered  under  the  five  heads  of  Oxidation,  Deoxidation,  Solution, 
Formation  of  Carbonates,  and  Hydration. 

1.  Oxidation. — The  prominence  of  oxygen  in  rain-water,  and  its 
readiness  to  unite  with  any  substance  that  can  contain  mpre  of  it, 
render  oxidation  a  marked  feature  of  the  passage  of  rain  over  rocks. 
A  thin  oxidized  pellicle  is  formed  on  the  surface,  and  this,  if  not  at  once 
washed  off,  is  thickened  from  inside  until  a  crust  is  formed  over  the 
stone,  while  at  the  same  time  the  common  dark  green  or  black  colour  of 
the  original  rock  changes  into  a  yellowish,  brownish,  or  reddish  hue. 
This  process  is  simply  a  rusting  of  those  ingredients  which,  like  metallic 
iron,  have  no  oxygen,  or  have  not  their  full  complement  of  it.  The 
ferrous  and  manganous  oxides  so  frequently  found  as  constituents  of 
minerals  are  specially  liable  to  this  change.  In  hornblende  and  augite, 
for  example,  one  cause  of  weathering  is  the  absorption  of  oxygen  by  the 
iron  and  the  hydration  of  the  resultant  peroxide.  Hence  the  yellow 
and  brown  sand  into  which  rocks  abounding  in  these  minerals  are  apt 
to  weather.  Sulphides  of  the  metals  give  rise  to  sulphates,  iind  some- 
times to  the  liberation  of  free  sulphuric  acid.  Iron  disulphide,  for 
example,  becomes  copperas,  which  on  oxidation  of  the  iron,  gives  a 
precipitate  of  limonitc,  with  the  escape  of  free  sulphuric  acid. 

2.  Deoxidation. — Kain  becomes  a  reducing  agent  by  absorbing  from 
the  atmosphere  and  soil  organic  matter  which,  having  an  affinity  for 
oxygen,  decomposes  peroxides  and  reduces  them  to  protoxides.  This 
change  is  especially  noticeable  among  iron-oxides,  as  in  the  familiar  white 
spots  and  veinings  so  common  among  red  sandstones.  These  rocks  are 
stained  red  by  ferric  oxide  (haematite),  which,  reduced  by  decaying  organic 
matter  to  ferrous  oxide,  is  usually  removed  in  solution  as  an  organic  salt  or 
a  carbonate.  When  the  deoxidation  takes  place  round  a  fragment  of  plant 
or  animal,  it  usually  extends  as  a  circular  spot ;  where  water  containing 
the  organic  matter  permeates  along  a  joint  or  other  divisional  plane,  the 

^  Senft,  Z.  Deatsch.  Grnl.  O'es.  xxiii.  p.  665,  xxvi.  p.  954.  Tliis  subject  has  been 
well  treated  in  a  paper  by  A.  A.  .Tulien  "On  the  Geological  Action  of  the  Humous  Acids'* 
{Proc.  Amer.  Ahsoc.  xxviii.  1879,  p.  311),  to  which  further  reference  is  made  in  later 
page».  See  also  his  excellent  paper  on  the  decomposition  of  pyrites,  Ann.  New  York  Acad, 
Sci.  vol.  iv.  (1888). 


344  DYXAMWAL  GKOLOUY  book  hi  part  ii 


decoloration  follows  that  line.  Another  common  effect  of  the  presence  of 
organic  matter  is  the  reduction  of  sulphates  to  the  state  of  sulphides. 
Gypsum  is  thus  decomposed  into  sulphide  of  calcium,  which  in  water 
readily  gives  c^lciiun  carbonate  and  sulphuretted  hydrogen,  and  the  latter 
by  oxidation  leaves  a  deposit  of  sulphur.  Hence  from  original  l)ed8  of 
gypsum,  layers  of  limestone  and  sulphur  have  been  formed,  as  in  Sicily 
and  elsewhere  (p.  67).^ 

3.  Soluiioh. — A  few  minerals  (halite,  for  example)  are  readily  soluble 
in  water  without  chemical  change,  and  without  the  aid  of  any  intermediate 
element ;  hence  the  copious  Iwine-springs  of  salt  regions.  In  the  great 
majority  of  cases,  however,  solution  is  effected  through  the  medium  of 
carl)onic  acid  or  other  re-agent.  Limestone  is  soluble  to  the  extent  of 
a])out  1  |)art  in  1000  of  water  saturated  with  carlx)nic  acid.  The  solu- 
tion and  removal  of  lime  from  the  mortar  of  a  bridge  or  vault,  and  the 
deposit  of  the  material  so  removed  in  sti\lactites  and  stalagmites  (p.  365), 
likewise  the  rapid  effacement  of  marble  epitaphs  in  our  churchyards,  are 
instances  of  this  solution.  It  has  1>een  shown  that  in  the  atmosphere  of 
a  large  town,  with  abundant  coal-smoke  and  rain,  exposed  inscriptions  on 
marble  become  illegible  in  half  a  century.  Pfaff  determined  that  a  slab 
of  Solenhofen  limestone,  2520  square  millimetres  in  superficies,  lost  in 
two  years,  by  the  solvent  action  of  rain,  0*180  gramme  in  weight,  in 
three  years  0*548,  the  original  jx^lish  being  replaced  l)y  a  dull  earthy 
surface  on  which  fine  cmcks  and  incipient  exfoliation  liegan  to  appear. 
Taking  the  specific  gra\ity  of  the  stone  at  2*6,  the  yearly  loss  of  surface 
amounts  to  -f-.i-s  millimetre,  so  that  a  crag  of  such  limestone  would  be 
lowered  1  metre  in  72,800  vciirs  bv  the  solvent  action  of  i*ain.*  J.  G. 
Goodchild,  from  observations  of  dressed  surfaces  of  Carboniferous  limestone 
in  the  north  of  England,  has  inferred  that  these  surfaces  have  l)een  lowered 
at  rates  varying  from  one  inch  in  240  yt»ars  to  the  same  amount  in  500 
veai-s.^  Dolomite  is  much  moi*c  feel>lv  soluble  than  limestone.  As  rain- 
water  attacks  the  carbonate  of  lime  more  readilv  than  the  airbonate  of 
magnesia,  the  rock  is  a})t  to  acquire  a  somewhat  porous  or  carious  textiu^ 
with  a  corresponding  increjise  in  the  propoition  of  its  magnesian  carbonate. 
Eventiudly  the  latter  carbonate  is  dissolved  and  redeposited  in  the  pore-s 
of  the  rock,  which  then  assumes  a  chanicteristic  crystiillino  as|)ect. 
Among  the  sulphates,  g\'psum  is  the  most  important  example  of  solu- 
tion. It  is  dissolved  in  the  proportion  of  about  1  |>ivrt  in  400  parts  of 
water. 

4.  Forniotinn  of  Carbjiuite.^. — Silicates  of  lime,  jMDtash,  and  sodti,  with 
the  ferrous  and  manganous  silicates  which  exist  so  abundantly  in  rocks, 
are  atUicked  by  rain-water  containing  carbonic  acid,  with  the  formation 
of  carlK)nates  of  these  Imses  and  the  lil)enition  of  silica.  The  felspars  are 
thus  decomposed.     Theii*  crystals  lose  their  lustre  and  colour,  becoming 

'  The  reducing  action  of  organic  acids  is  further  described  in  Section  iii. 

-  Pfaff,  Z.  jMiftsch.  Uettl.  Hes.  xxiv.  p.  405:  and  *Allgemeine  Geologic  ahf  ezacte 
Wissenscliaft,'  j».  317.  Roth,  *  Allgenieiue  und  Cheni.  Geol.'  i.  p.  70.  Geikie,  Proc.  Roy, 
Soc.  Edin.  x.  1879-80,  p.  518. 

'  aeoL  Man.  1890,  p.  466. 


SECTT.  ii  §  1  WEATHERING  345 

dull  and  earthy  on  the  outside,  and  the  change  advances  inwards  until 
the  whole  substance  is  converted  into  a  soft  pulverulent  clay.  In  this 
decomposition  the  whole  of  the  alkali,  together  with  about  two-thirds  of 
the  silica,  is  removed,  leaving  a  hydrous  aluminous  silicate  or  kaolin 
behind.  But  the  rapidity  and  completeness  of  the  process  vary  greatly, 
especially  in  proportion  to  the  abundance  of  carbonic  acid.  Where  it 
advances  with  sufficient  slowness,  most  of  the  silica,  after  the  abstraction 
of  the  alkali,  may  be  left  behind.  In  the  case  of  magnesian  minerals 
(augite,  hornblende,  olivine,  &c.)  the  silicates  of  magnesia  and  alumina, 
being  less  soluble,  may  remain  as  a  dark  brown  or  yellow  clay,  coloured  by 
the  oxidation  of  the  iron,  while  the  lime  and  alkalies  are  removed.^ 
Evidence  of  the  progress  of  these  changes  may  be  obtained  even  for  some 
distance  from  the  surface  in  many  massive  rocks.  Diabase,  basalt,  diorite, 
and  other  crystalline  rocks,  which  may  appear  to  be  quite  fresh,  will 
often  reveal,  by  the  effervescence  produced  when  acid  is  dropped  on  their 
newly  broken  and  seemingly  undecomposed  surfaces,  that  their  silicates 
have  been  attacked  by  meteoric  water  and  have  l)een  partially  converted 
into  carbonates. 

5.  Hydration. — Some  anhydrous  minerals,  when  exposed  to  the  action 
of  the  atmosphere,  al)sorb  water  (l>ecome  hydrous),  and  may  then  l)e  more 
prone  to  further  change.  Anhydrite  becomes,  by  addition  of  water, 
gypsum,  the  change  being  accompanied  by  an  increase  of  bidk  to  the 
extent  of  alx)ut  33  per  cent.  Local  uplifts  of  the  ground  and  crumpling 
or  fracture  of  rocks  may  sometimes  be  caused  by  the  hydration  of 
subterranean  beds  of  anhydrite  (p.  298).  Many  substances  on  oxidizing 
likewise  become  hydrous.  The  oxidation  of  ferrous  oxide  in  damp  air 
gives  rise  to  hydrous  ferric  oxide,  with  its  chai-aeteristic  yellow  and  brown 
colours  on  weathered  siu*faces. 

Weathering. — This  term  expresses  the  general  result  of  all  kinds  of 
meteoric  action  upon  the  superficial  parts  of  rocks.  As  these  changes 
almost  invariably  lead  to  disintegration  of  the  surface,  the  word  weather- 
ing has  come  to  be  naturally  associated  in  the  mind  with  a  loosened 
crumbling  condition  of  stone.  But  the  influence  of  the  atmospheric 
agents  is  not  invariably  to  destroy  the  coherence  of  the  integi'al  |3articles 
of  rocks.  In  some  cases,  stones  harden  on  exposure.  Certtiin  sandy 
rocks,  for  example,  like  the  "grey  weathei-s"  and  scattered  Tertiary 
blocks  in  the  Ardeinies,  l>ecome  under  meteoric  influence  a  kind  of 
lustrous  quartzite.  In  other  cases,  there  may  Ije  more  complex  molecular 
rearrangements,  such  as  those  remarkable  transformations  to  which 
Brewster  first  called  attention  in  the  case  of  artificial  glass.^  He  showed 
that  in  thin  films  of  decomposed  glass,  obtained  from  Nineveh  and  other 
ancient  sites,  concentric  agate -like  rings  of  devitrification  are  formed 
roiuid  isolated  points,  closely  analogous  to  those  alx)ve  descrilied  as 
artificially  produced  by  the  action  of  heated  alkaline  watei-s  (p.  309),  and 
that  groups  of  crystals  or  crystallites,  "  i)rol>ably  of  silex,"  are  developed 
from  many  independent  points  in  the  decomposing  layer.     Coloured  films 

*  Roth,  op.  rif.  i.  ]».  112. 
*  Trans.  Ro}/.  S)i\  Kdin.  xxii.  607  ;  xxiii.  193.     See  ante,  p.  310. 


346  DYXAMICAL  GEOLOGY  book  m  pabt  n 


indicative  of  incipient  decomposition  have  l)een  observed  on  surfaces  of 
glass  exposed  only  to  the  air  of  the  atmosphere  for  twenty  or  thirty 
veal's.  Bnlliantly  iridescent  films  have  l>een  produced  on  the  glass  of 
windows  exposed  for  not  more  than  twenty  years  to  the  air  and 
ammoniacal  vapours  of  a  stable.^  That  similar  transformations  take 
place  in  the  natural  silicates  of  rocks  seems  in  the  highest  degree 
probable.  They  may  form  the  earliest  stages  of  the  change  to  the  usual 
opaque  eiiithy  decomposing  crust,  in  which,  of  course,  all  trace  of  any 
stnicture  develoj)ed  in  the  preliminary  weathering  is  lost.^ 

In  humid  and  temperate  climates,  weathering  is  mainly  due  to  the 
combined  influence  of  rain  and  sinishine.  Saturated  with  rain-water, 
which  dissolves  more  or  less  of  any  soluble  constituents  that  may  be 
present,  and  thereafter  exjx)sed  to  the  desiccating  and  expanding  influenee 
of  the  warm  rays  of  the  sun,  rock-surfaces  are  disintegrated,  breaking 
up  into  angular  fragments  or  cnimbling  into  dust.-*  In  high  mountainous 
situations,  as  well  as  in  lower  regions  where  the  temperature  falls  below 
the  freezing-point  in  winter,  weathering  is  in  large  measure  caused  by  the 
action  of  frost  (p.  413) ;  in  arid  lands  suliject  to  great  and  rapid  alterna- 
tions of  temperature,  it  may  be  mainly  due  to  the  strain  of  alternate 
expansion  and  contraction  (p.  328)  and  the  mex^hanical  action  of  the 
wind  (p.  329  et  seq.)  As  the  name  denotes,  weathering  is  dependent  on 
meteorological  conditions,  and  varies,  even  in  the  same  rock,  as  these 
conditions  change,  but  is  likewise  almost  infinitely  diversified  according 
to  the  stnicture,  texture,  and  composition  of  rocks. 

Mere  hardness  or  softness  forms  no  sure  index  to  the  comparative 
l>ower  of  a  rock  to  resist  weathering.  Many  granites,  for  instance, 
we;ither  to  clay,  deep  into  their  mass,  while  much  softer  limestones 
retjiin  smooth,  hard  surfaces.  Xor  is  the  depth  of  the  weathered 
surface  any  better  guide  t^>  the  rehitive  rapidity  of  waste.  A  tolerably 
jniie  limestone  may  weather  ^nth  little  or  no  crust,  and  yet  may  be 
contiiuially  losing  an  appreciable  portion  of  its  surface  by  solution, 
while  an  igneous  rock,  like  a  diorite  or  basalt,  may  l>e  encased  in  a  thick 
decomposed  crust  antl  weather  with  extreme  slowness.  In  the  former 
ease,  the  sul>stance  of  the  rock  being  removed  in  solution,  few  or  no 
insoluble  portions  are  left  to  mark  the  progress  of  decay,  while  in  the 
igneous  rock,  the  removal  of  but  a  comparatively  small  proportion  causes 
disintegi-ation,  and  the  remaining  insoluble  parts  are  f ound  as  a  cnunbling 
crust.  Impure  limestone,  however,  yields  a  weathered  cnist  of  more  or 
less  insoluble  i)aiticle8.  Hence,  as  we  have  already  seen  (p.  81)  the  relative 
purity  of  limestones  may  be  roughly  determined  from  their  weathered 

*  Tliis  fact  lias  Iweu  obstTved  In*  uiy  friend  Mr.  P.  Dudgeon,  of  Caiigen,  in  an  ill- 
veiitilate<l  oow-house,  and  1  have  seen  the  plates  of  glass  removed  from  the  windows.  The 
process  of  decay  in  glass  lias  lH*en  treated  of  in  ^eat  detail  by  Mr.  James  Fowler,  Trann, 
S>ir.  Ant  iff  I'd  firs,  xlvi.  (1?<79),  i»p.  65-102. 

-  Reference  may  be  made  liere  to  the  liquid  inclusions  alreatly  allude«i  to  as  deTeloi^ed 
in  feUpar  iluring  the  decomposition  of  gneiss,  a/iff',  ]k  112. 

•  Tliis  action  can  be  instructively  imitated  by  boiling  and  drying  shales  in  the  manner 
described  in  Book  V.  Sect.  vii. 


n  g  1  WEATHERING  347 

G6B,  where,  if  they  contain  much  sand,  the  grains  wi]l  be  seen 
cting  from  the  calcareous  matrix :  should  they  he  very  femiginouB, 
■ellow  hydrous  peroxide,  or  ochre,  will  be  found  as  a  powdery  crust ; 
they  be  fosailiferous,  they  will  commonly  present  the  fossils  stand- 
)Ut  in  reUef.  An  experienced  fossil -collector  will  always  carefully 
h  weathered  surfaces  of  Hmestone,  for  he  often  fiuds  there,  delicately 
id  out  by  the  weather,  minute  and  frail  fossils,  which  are  wholly 
ible  on  the  freshly  broken  atone.  This  difference  arises  from  the 
alline  calcite  of  the  organic  remains  being  less  soluble  than  the 
granular  calcite  in  which  these  are  imbedded  Limestones  frequently 
ae  a  remarkable  channelled  rugose  surface,  with  projecting  knobs, 
8,  and  pinnacles  especially  developed  in  high  bare  tracts  of  ground 
renf elder).' 
locks    liable    to    little    chemical    change    are    best    fitted    to    resist 


hering,  provided  their  particles  have  sufficient  cohesion  to  with- 
1  the  mechanical  processes  of  disiutegmtion.^     Siliceous  sandstones 

excellent  examples  of  this  i>ennanence.  Consisting  mainly  of  the 
^le  minemble  <|uartz,  they  are  sometimes  able  bo  to  withstand  decay 
buildings  made  of  them  still  retain,  after  the  lapse  of  centuries,  the 
1-marks  of  the  builders,  Jlariy  sandstones,  however,  contain  argilla- 
i,  calcareous,  or  ferruginous  concretions  which  weather  more  rapidly 

the  siui'oundiRg  rock,  and  canse  it  to  assume  a  honeycombed 
ce  ;  others  are  full  of  a  diffused  cement  {clay,  lime,  iron)  the  decay 
hich  makes  the  rock  cnimble  down  into  sand.  In  sandstones,  as 
d  in  most  utratifieil  i-ocks,  there  is  a  tendency  towards  more  rapid 
bering  alonj;  the  planes  of  stiratification,  so  that  the  stratified 
Jtim,  Jail-*.  .S.'J,ir^h.  A/jf-.tdulii,  liii,  (1978). 

)a  wentheriiig  of  biiiliiing- stones,  we  I'rw.  Roy,  li^.  fklin.  1878-80,  p,  S18,  Jalten, 
.  ,Vnp  I'wi  ,l,<(,/.  Sri.  Jan.  IKSS.    W.  Wallnce,  I'm.  I'liil.  Snr.  Hint.  iit.  (1882-3),  p.  22. 


DYXAmCAL  GEOLOGY 


BOOK  III  PART  II 


atnicture  is  brought  owt  veiy  clearly  on  imtuml  cliffs  (Fig.  92).  In 
many  ferniginous  sandstones  and  clay  iroiiBtones,  eucceseive  yellow  or 
brown  /ones  or  shells  may  l)e  traced  inwai-d  from  the  surface,  freqnentlf 
(luc  to  changes  of  the  ferrous  carbonate 
into  liraonite,  the  interior  remaining  still 
fresh.  Ill  many  prismatic  massive  roclu 
(basalt,  diorJte,  &c.),  segments  of  the  prisms 
weather  into  spheroids,  in  which  successive 
weathered  rings  form  cnists  like  the  con- 
centric coats  of  an  onion  (Figs.  93,  94). 
Where  one  of  these  rocks  has  been  intruded 
FL».  e;i.— RUiRK  »r  wndiFdnit.  '^^  >*  dyke,  it  sometimes  decomposes  to  t 
considerable  depth  into  a  mass  of  brown 
femiginoTia  balls  in  a  aun-oimding  sandy  matrix — the  whole  hanng 
lit  fii'st  a  reseinltlanue  to  a  ouiiglomei'ate  made  of  i^olled  and  transported 
fragments  (Fig.  95). 

No  rock  presents  greater  vaiiety  of  weathering  than  gianite.     Some 
remarkably  dui'able  kinds  only  yield  slowly  at  the  edges  of  the  joints, 


the  sc[)<intted  masses  giiuliukl])  assuming  the  form  of  rounded  blocks 
like  uatoi  worn  lH>uldcrs  Othei  kind-i  doooinpose  to  a  depth  of  50 
feet  or  nuirf.  aiul  i^ii  lie   dug  ont  uith  .i  s]Hido.      In  Coniwall  and 


SECT,  ii  §  1 


WEATHERING 


349 


Devon,  the  kaolin  from  the  rotted  granite,  largely  extracted  for  pottery 
purposes,  is  found  down  to  a  depth  of  occasionally  600  feet.     That  what 

appears  to  be  mere  loose  sand  and 
clay  is  really  rock  decomposed  in 
situ,  is  proved  by  the  quartz -veins 
and  bands  of  schorl -rock  which 
ascend  from  the  solid  rock  (a.  Fig. 
96)  into  the  friable  part  (6),  and 
by  the  entire  agreement  in  structure 
between  the  two  portions.  Here  and 
there,  kernels  of  still  undecomposed 
granite  may  be  seen  (as  at  c  c  in 
Fig.  97),  surrounded  by  thoroughly 
decayed  material,  and,  like  the  solid  cores  of  basalt  above-mentioned, 
presenting  a  deceptive  resemblance  to  accumulations  of  transported 
materials.  There  can  be  no  doubt  that  the  granite  boulders,  so  abund- 
antly transported  by  the  ice-sheets  and  glaciers  of  the  Ice  Age,  originated 
in  great  measure  in  this  way.     Owing  to  its  numerous  joints,  granite 


Fig.  95. — Felsite  Dyke  weathering  into  spheroids, 

Cornwall  (B.) 


■.••'.'. 


Pip^  96. — Decomposition  of  Granite,  a.  Solid 
{granite  ;  6,  decomposed  granite  ;  c,  vege- 
table soil. 


Fig.  1)7.— Decomposition  of  Granite,  a.  Solid  gran- 
ite ;  h,  decoini)08ed  granite ;  c,  c,  kernels  of  still 
nudecomiHised  granite. 


occasionally  weathers  into  forms  that  resemble  ruined  walls.  Large  slabs, 
each  defined  by  joint  planes,  weather  out  one  above  another  like  tiers  of 
masonry  (Fig.  98),  until,  loosened  by  disintegration,  they  slip  off  and 
expose  lower  parts  of  the  rock  to  the  same  influences.     Here  and  there. 


<;::;;y 


Fig.  98.— Weathering  of  Granite  along  its  joints  (i?.) 

a  separate  block  }>ecomes  so  poised  that  it  may  be  readily  moved  to 
and  fro  by  the  hand,  as  in  the  so-called  "  rocking-stones "  of  granite 
districts.  As  the  disintegration  varies  with  local  differences  in  durability, 
some  portions  weather  into  cavities,  others  into  prominences,  often  with  a 
singularly  artificial  appearance,  as  in  the  "rock  basins"  (Fig.  99)  and 
"tors"  (Fig.  98)  of  the  south-west  of  England.  The  ruin-like  weathering  of 
dolomite  gives  rise  in  the  Cevennes  to  some  singularly  picturesque  scenery. 
To  the  influence  of  weathering,  many  of  the  most  familiar  minor 
contours  of  the  land  may  be  traced.  So  characteristic  are  these  forms 
for  particrular  kinds  of  rock,  that  they  serve  as  a  means  of  recognising 
them  even  from  a  distance.     (Book  VII.) 


350  liYXAMICAI.  OEOLOGV  bookuipabtii 

In  coiintiiea  which  hnve  not  l»een  under  water  for  a  \-a8t  lapse  of 
tiniu,  and  where  coiuicqucnth'  the  auperticial  rot^ks  hxve  >>een  continuoiulf 
expoeed  to  subaerial  disintegration,  thick  at-cumulations  of  "rotted  rock" 
are   found  on    the   sni-faci'.     The   extent    of   this    change    is   sometimei 


impressively  marked  m  areas  of  Lulcitreoiis  iixks.  Limestone  bemg 
mostly  sohilile,  its  surface  is  t.'ontiniiaDy  dissolved  by  r^iu,  while  the 
insolubk  portiunK  i-enmin  1)ehind  us  n  slowly  increasing  deposit.  Id 
regions  which,  ixisscssing  the  necessiiiy  conditions  of  climate,  have  been 
for  It  long  period  iinsubmerged,  traotit  of  limestone,  unprotected  by  ^^acial 
01'  other  ncetumilations,  are  found  to  lie  covered  with  a  ■■od  loam  or 
eai^h.  This  (.hiiracteristic  layer  occurs  on  a  limited  scale  over  the 
chalk  of  the  south-east  of  England,  where,  with  its  abundant  fiinU,  it 
lieii  iis  the  undissolved  ferniginoiis  residue  <jf  the  chalk  that  has  been 
removed  to  a  depth  of  manj'  yards.  It  occurs  likewise  in  ewallow-hcdes 
and  other  [Kissages  dissolved  out  of  calcareous  masses,  and  forms  the 
well-kuown  iwl  earth  of  Imne  kuvk*.  In  south-eastern  Europe  it  plays  an 
important  jNirt  among  superficiid  deposits,  being  extensively  developed 
ovei'  the  limestone  districts,  especially  in  Istiii  and  Dalmatia,  where  it  is 
known  iia  the  feiTuginous  red  earth  or  tr^nv  rojwr.' 

Other  remai'kable  examples  of  similar  sul>aerial  waste  have  l>een 
specially  notiwwl  among  crystalline  schists  and  eruptive  rocks.  Li  Brazil, 
it  has  lieen  remarkeil  with  astonishment  that  the  crystalline  rocks  are 
sometimes  decayed  to  a  depth  of  moi-e  than  300  feet.-     In  Massachusetts, 

'  Ou  the  origiu  of  "Titmi  Itossn."  ere  M.  Kfiiiimyr.  Verliamd.  Ueol.  RtiduMHitt.  187,'., 
p.  .10  ;  Til.  Fnclis,  i-p.  dl.  p.  104  :  E.  voii  .Mojsisovifs,  JnhH;  Oeoi.  Reiclmantt.  m. 
(1880),  [I.  -JIO  ;  E.  Tietip,  oji.  cil.  xxx.  (1&80).  n  720  :  Loreiw.  Verh.  OfJ.  Reidu.  1881, 
[I.  81 ;  C.  de  Otorp,  IMI.  Com.  lirnl.  IM.  rii.  p.  204.  It  is  includsd  mmong  tbr 
fBrruginoiia  deposits  by  Stoppimi  (■Como  i]i  Geologia,'  iii.  p.  534).  Nenm.iyr  ahows  that  h 
i.1  ot  various  ii|{i.-)i ;  in  th«  Karxt  it  encloneg  MinccDf  iuniiiiiial». 

'  Liais,  'G.'ologii!  dii  »ri-»il,' ji.  2.  .!mm.  des  .W/ncj,  Tmenrr.  viiL  p.  688.  T,  Belt, 
'  >'aturali!it  in  Nirarngiia'  (1874),  p.  8S.  T.  iSturry  Hunt,  Amrr.  JourH.  Sci.  Sid  aer.  Tii. 
ii.  00  ;  iiTi.  (1883),  p.  198  ;  (/,-ol.  Mag.  1883.  p.  310  ;  Amtrimn  NalmvUtt,  ix.  (187S), 
p.  471.  Tbia  vrril«r  ilvrells  i-spiK^iiilly  Ou  tliK  gnaX.  geological  autiquity  o[  the  wcstherad 
cnut.  On  the  Hsculsr  rork-wvatlieriDg  ol  tlie  Sniilish  monutains  see  Nathorst,  Oai.  f^mt. 
StockhiJBi.  rorhanj.  1S79,  iv.  Ko.  13. 


BEtT.  iigl  FORM ATIOK  VF  SOIL  351 

Pennsylvania,  and  generally  in  the  middle  nnd  southern  Atlantic  States 
of  North  America,  the  depth  of  disintegration  appears  gradually  to 
increase  southward  from  the  limits  where  the  country'  has  been 
"glaciated"  by  ice-sheets  during  theGlacial  Periwl.'  In  central  Asia,  a 
simitar  superficial  decay  has  been  observed.^  l)r.  Sterry  Hunt  has 
specially  drawn  attention  to  the  geological  imjKirtance  of  this  prolonged 
disintegration  in  situ.  Miv  Pumpelly  points  out  that,  as  masses  of 
decomposed  rock  may  be  oljserved  to  a  depth  of  over  100  feet,  the  surface 
of  the  still  solid  rock  underneath  presents  ridges  and  hollows,  succeeding 
each  other  according  to  larying  durability  under  the  influence  of  i>er- 
colatiug  carbonate<i  water.  In  this  kind  of  weathering,  where  erosion 
does  not  come  into  play,  it  is  e^'ident  that  the  resulting  topography  must, 
in  some  important  res]>ects,  differ  from  that  of  an  onlinary  surface  of 
superficial  denudation.  In  particular,  rock-basins  may  be  gradually  eaten 
out  of  the  solid  rock.  These  will  remain  full  of  the  decomposed  material, 
but  any  sulraequeiit  action,  snch  as  that  of  glacier-ice,  which  could  scoop 
o»»t  the  detritTis,  would  loaie  the  Iwsins  and  their  intervening  ridges 
exposed.^ 

Formation  of  Soil. — On  level  surfaces  of  rock  the  weathered  crust 
may  remain  with  comparatively  little  rearrangement  until  plants  take 
root  on  it,  and  by  their  decay  supply  organic  matter  to  the  decomposed 
layer,  which  eventually  becomes  what  we  term  "vegetable  soil." 
Ajiimals  also  furnish  a  smaller  proportion  of  organic  ingredients.  Though 
the  character  of  soil  depends  primarily  on  the  imtims  of  the  rock  out  of 
which  it  has  been  formed,  its  fertility  largely  depends  on  the  commingling 
of  decayed  animal  and  \egetablti  matter  with  decomposed  rock. 

A  gradation  may  Iw  traced  from  the  soil  dowtiwaitls  into  what  is 
termed  the  "  sultsoil,"  and  thence  into  the 
solid  rock  underneath  (Fig.  1 00).  Between 
soil  and  subsoil  a  marked  difTerence  in 
colour  is  often  ol>servable,  the  former  being 
yellow  or  l»rown,  when  the  latter  is  blue, 
grey,  red,    or    other   colour   of    the  rock 

beneath.*     This  contrast,  evidently  due  to  |^  ^ 

oxidation  and  hydration,  especially  of  the  lassmge  of  Buck  (n)  into  Sui»iii  {(.). 
iron,  extends    downwards  as    far   as    the        *"  tienci'  uto   encta  ii^si.iifr). 

"  I.  C.  RusstU,  Bull.  I:K  Otfl.  Si-rrfi/,  No,  Eli  (185»),  ]>.  l:i  et  »■./.  Tliere  a  >  u>e- 
fal  biblic^rapbf  of  pipers  ou  Ihe  Hubaerial  ilecay  oC  rocks  apiwuiled  to  tliia  essny.  See  a]mi 
W.  O.  Crcwby,  Pr-f.  X-il.  Ilinl.  Soc.  BestuH,  isiil.  ]>.  219. 

'  On  B  eiualler  scale  it  is  also  to  Ik  nateil  iii  the  grsuite  oud  killaii  (pliylliU)  of  Coniuall 
and  Devon,  wbicli,  not  liaving  .suffi-Terl  froni  tlie  abrading  action  of  thu  ice  of  tht  Glacial 
Period,  show  a  d<iep  cover  of  ratted  rock,  aud  afford  aonie  judicatiou  ot  wliat  may  have 
been  elsewhere  the  condition  of  Britain  brfore  tlie  period  of  glaciation.  The  aea-cliffa  along 
the  north  coaat  of  Cornwall  eipoue  instructive  siwtioua  of  tlie  deep  iipju-r  Uecoiiii-oseil,  and 
of  the  lower  blue  .wild  killas.  with  the  reiiiarkably  uneven  boimilary  along  which  tliey  pans 
into  each  other. 

'  Panjpetly,  .-imfi:  Jouni.  .'kl.  3rd  «r.  xviii.  13C  :  U  8.  Burbonk,  /'n'-.  Btil.  Sat. 
Bitl.  Sue  i<ri.  (ISii),  part  2,  p.  ISO  ;  nlso poilea.  ]>.  431. 

■  Deceptive  appearances  of  a  break  hetneeu  the  soil  or  suImolI  sud  what  liea  beneath  are 


352  nyXAMICAL  GEOLOGY  book  hi  part  il 


sul)S<)il  is  opened  up  by  rootlets  and  fibres  to  the  ready  descent  of  rain- 
water. The  yellowing  of  the  sul)soil  may  even  occasionally  be  noticed 
around  sonic  stray  rootlet  which  has  struck  down  further  than  the  rest, 
Ik'Iow  the  general  lower  limit  of  the  soil  {iumtea^  p.  473). 

Mr.  Darwin  observed  many  years  ago  that  a  layer  of  soil,  three 
inches  in  depth,  had  grown  above  a  layer  of  burnt  marl  spread  over  the 
land  fifteen  yeai's  previously  ;  also  that  in  another  example,  a  similar 
layer  had,  as  it  were,  sunk  bcnciith  the  soil,  to  a  depth  of  twelve  or 
thirteen  inches  in  eighty  years.  He  connected  these  facts  with  the 
work  of  the  conmion  eaith-worni,  and  concluded  that  the  fine  loam  which 
had  gi'own  alK)ve  these  original  supei*ficial  layei*s  had  Ijeen  carried  up  to 
the  surface,  and  had  been  voided  there  in  the  familiar  form  of  worm- 
castings.^  This  action  of  the  earth-worm  is  doubtless  highly  important, 
but,  as  Kichthofen  has  pointed  out,  we  have  to  take  also  into  account 
the  gradual  augmentiition  of  level  due  to  the  daily  deposit  of  dust  (ank^ 
p.  3.*H,  and  poatm,  p.  473). 

Soil  being  composed  mainly  of  inorganic,  and  to  a  slight  extent  of 
organic  materials,  the  proixjrtion  l)etween  these  two  elements  is  a 
<[uestion  of  high  economic  importonce.  With  regaitl  to  the  organic 
matter,  it  is  the  exjx'nencc  of  practical  agi-iculturists  in  Britain  that 
oiits  and  rye  will  gi*ow  upon  a  soil  with  1  i  per  cent  of  organic  matter, 
but  that  wheat  requires  from  4  to  8  per  cent.-  To  a  geologist,  this 
orgjinic  matter  has  nnich  interest,  as  the  source  of  most  of  the  carbonic 
acid,  with  which  so  wide  a  series  of  changes  is  M-orked  by  subterranean 
watei".  The  inorgjinic  portion  of  soil,  or  still  undissolve<l  residue  of  the 
original  surface-rock,  vanes  from  a  loose  open  sul)sUince  with  90  per 
cent  or  more  of  sand,  to  a  stift',  cold,  retentive  material  wHth  more  than 
00  per  cent  of  clay.  AVhen  this  sand  and  clay  are  more  equally  mixed 
thev  form  a  'Mofim."^ 

« 

Keferenco  has  just  l>eeu  made  to  the  thick  accumulation  of  rock 
decomposed  in  situ  observable  in  certain  regions  which,  having  been 
above  the  sea  for  a  lengthened  period,  have  been  long  exi)osed  to  the 
a(;tion  of  weathering.  Where  this  action  has  been  supplemented  by  that 
of  rain,  ^\'idespread  formations  of  loam  and  earth  have  been  gathered 
together.  These  are  well  illustrated  by  the  *'  })rick-oarth,"  "  head,"  and 
"rain-wash"  of  the  south  of  England — ea,rthy  deposits,  with  angular  stones, 
derived  from  the  subaerial  waste  of  the  rocks  of  the  neighbourhood.* 

soiiietinies  i»ro<Uu!e(l  liy  this  means.  See  W.  Wliitoker,  Q.  J.  O'eol.  Soc.  xxxiii.  ju  122. 
E.  Van  den  Broeck,  Mtni.  i'ouronn.  Acad.  Brnssels,  1881. 

^  ff'cff/.  Trtnt.i.  V.  1840,  p.  ,')05  ;  and  liis  more  recent  researches  in  his  volame  on 
•  Ve^'etahle  Mould.'     See  also  C.  Reid.  Gfol.  Mag.  1884,  p.  165. 

-  Jolmston's  'Elements  of  Agricultural  Chemistry,'  p.  80. 

•*  For  measurements  of  tlie  i>ermeability  of  soils,  see  Houdaille  and  Semirhow,  Campt. 
,rnJ,  cxv.  (1892),  p.  lOlf). 

*  (todwin- Austen,  (^.  J.  f'tfl.  Sx:,  vi.  p.  94,  vii.  p.  121  ;  Foster  and  Topley,  ()p.  cit,  xxi. 
p.  446.  The  vast  extent  of  some  superficial  formations,  like  the  "loes-s''  above  referred 
to  (p.  032),  has  often  su^^sted  submergence  l>elow  the  sea.  But  when,  instead  of  marine 
organisms,  only  terrestrial,  Huviatile,  or  lacustrine  remains  occur  in  them,  as  in  the  brick- 
earths  an<l  loess,  the  idea  of  marine  submergence  cannot  be  entertained.     The  remarkable 


SECT,  ii  §  1  MECHANICAL  ACTION  OF  RAIN  363 

• 

2.  Mechanical  Action. — Besides  chemically  corroding  rocks  and 
thereby  loosening  the  cohesion  of  their  particles,  rain  acts  mechanically 
by  washing  off  these  particles,  which  are  held  in  suspension  in  the  little 
rain-runnels  or  are  pushed  by  them  along  the  surface.  The  amount  and 
rapidity  of  this  action  do  not  depend  merely  on  the  annual  quantity  of  rain. 
A  comparatively  large  rainfall  may  be  so  equably  distributed  through  a 
year  or  season  as  to  produce  less  change  than  may  be  caused  by  a  few 
heavy  rain-storms  which,  though  inferior  in  total  amount  of  precipitated 
moistiu*e,  descend  rapidly  in  great  volume.  Such  copious  rains,  by  deluging 
the  surface  of  a  country  and  rapidly  flooding  its  water-courses,  may  trans- 
port in  a  few  hours  an  enormous  amount  of  sand  and  mud  to  lower  levels. 
Another  feature  to  be  kept  in  view  is  the  angle  of  declivity :  the  same 
amount  of  rain  will  perform  vastly  more  mechanical  work  if  it  can  swiftly 
descend  a  steep  slope,  than  if  it  has  to  move  tardily  over  a  gentle  one. 

Removal  and  Renewal  of  Soil. — Elie  de  Beaumont  drew  attention 
to  what  appeared  to  be  proofs  of  the  permanence  or  long  duration  of  the 
layer  of  vegetable  soil.^  But  the  cases  cited  by  him  are  not  inconsistent 
with  a  belief  that  the  doctrine  of  the  persistence  of  the  soil  is  true  rather 
of  the  layer  as  a  whole,  than  of  its  individual  particles. ^  Were  there  no 
provision  for  its  renewal,  soil  woidd  comparatively  soon  be  exhausted,  and 
would  cease  to  support  the  same  vegetation.  This  result,  indeed,  occurs 
jxirtially,  especially  on  flat  lands,  but  would  be  far  more  widespread  were 
it  not  that  rain,  gradually  washing  off"  the  upper  part  of  the  soil,  exposes 
what  lies  beneath  to  further  disintegration.  This  removal  takes  place 
even  on  grass -covered  surfaces,  through  the  agency  of  earth-worms,  by 
which  fine  particles  of  loam  are  brought  up  and  exposed  to  the  air,  to  be 
dried  and  blown  away  by  wind,  or  washed  down  by  rain.  The  lower 
limit  of  the  layer  of  soil  is  thus  made  to  travel  downward  into  the  subsoil, 
which  in  turn  advances  into  the  underlying  rock.  As  Hutton  long  ago 
insisted,  the  superficial  covering  of  soil  is  constantly,  though  slowly, 
travelling  to  the  sea.'  In  this  ceaseless  transport,  rain  acts  as  the  great 
carrying  agent.  The  particles  of  rock  and  of  soil  are,  step  by  step,  moved 
downwani  over  the  face  of  the  land,  till  they  reach  the  nearest  brook  or 
river,  whence  their  seaward  progress  may  be  rapid.  A  heavy  rain  dis- 
colours the  water-courses  of  a  country,  because  it  loads  them  with  the  fine 
debris  which  it  removes  from  the  general  surface  of  the  land.  In  this  way, 
rain  serves  as  the  means  where])y  the  work  of  other  disintegrating  forces 
is  made  conducive  to  the  general  degradation  of  the  land.  The  decomposed 
crust  produced  by  weathering,  which  would  otherwise  accumulate  over 
the  solid  rock,  and  in  some  measure  protect  it  from  decay,  is  removed 
by  rain,  and  a  fresh  surface  is  thereby  laid  bare  to  further  decomposition. 

*•  tundnw"  or  steppes  of  Siberia,  and  the  "black  earth  "  of  Russia,  are  examples  of  such 
exteusive  formations,  -which  are  certainly  not  of  marine  origin,  but  point  to  long-continued 
emergence  above  the  sea.  See  Murchison,  Keyserling,  and  De  Venieuil's  'Geology  of 
Russia,'     Belt,  Q,  J.  (^eol.  »Sr«r.  xxx.  p.  490  ;  also poslea,  p.  478. 

*  'Le<;ons  de  Geologic  pratique,'  i.  p.  140. 

^  Geikie,  Trans.  Oeol.  Sfn:  (Hasijnir^  iii.  p.  170. 

*  *  Theory  of  the  Earth,'  Part  II.  chaps,  v.  vi. 

2  A 


3&4 


DYXAMICAL  GEOLOGY 


BOOK  lU  PART  II 


Movement  of  Soil-cap. — In  some  coimtries,  vrhere  the  ground  it 
covered  with  a  thick  spongy  mase  of  vegetation  exposed  to  coneddenUe 
variation  of  tempeniture  and  moisture,  appearances  have  been  obflerred  of 
an  oxtensire  slipping  of  the  layer  of  soil  to  lower  levels,  bearing  with  it  wha^ 
ever  may  be  growing  or  lying  upon  it.  Such  are  the  ao-called  "stone-riven" 
of  the  Falkland  Islands,  and  the  superficial  debris  of  certain  parts  of  the 
west  coast  of  Patagonia.*  In  Western  Europe,  slight  indications  of  a 
similar  movement  may  often  be  noticed  on  the  sides  of  hillii  or  valleys. 

Unequal  Erosive  Action  of  Rain. — While  the  result  of  rain  actioD 
is  the  general  lowering  of  the  level  of  the  land,  this  process  necessarily 
advances  very  unequally  in  different  places.     On  flat  ground,  the  waste 


may  I>e  quite  inappreciable  except  after  long  lnter^als  and  by  the  most 
accurate  measurements,  or  it  may  even  gi\e  place  to  deposiboo,  the  fine 
detritus  n-ashed  off  the  slopes  being  spre.id  out,  so  as  actually  to  heighten 
the  alluvial  surface  In  numerous  localities,  great  variations  m  the  rate 
of  erosion  by  iiiin  may  be  observed.  Thus,  from  the  pitted,  channelled 
ground  lying  immediately  under  the  drip  of  the  eaves  of  a  houM^ 
fragments  of  stone  and  gravel  stand  up  prominently,  because  the  earth 
around  and  above  them  has  been  washed  away  by  the  falling  drops,  and 
))ecause,  Iteiiig  hard  they  resist  the  erosive  action  and  screen  the  earth 
Iielow  thom.  On  a  larger  scale  the  same  kind  of  operation  may  be  noticed 
in  districts  of  conglomerate  vvhere  the  Urger  blocks,  serving  as  a  protec- 
tion to  the  rock  underneath,  come  to  form,  ua  it  were,  the  capitals  of  slowly- 

'  Wyvillu  ITionisod'n  'A11«iitic,'  vol.  ii.  p.  21.'i.     K.  W.  Coppingsr,  ^.  J.  Oeol.   Sue. 
1SS1,  p.  as.     Seefflwfai,  uudcr  >' Landslipa,"  p.  U70. 


8ECT.  ii  g  1  MECHASICAL  ACTION  OF  RAIN  355 

deepening  columns  of  rock  (Fig.  101).  In  certain  valleys  of  the  Alps  a 
stony  clay  is  cut  \>y  the  rain  into  pillars,  each  of  which  is  protected  by, 
and  indeed  owes  its  existence  to,  a  large  block  of  stone  which  lay 
originally  in  the  heart  of  the  mass  (Fig.  103).  These  columns,  or  "earth- 
pillars,"  are  of  all  heights,  according  to  the  original  positions  of  the  stones. 
More  colossal  examples  have  been  described  by  Hayden  from  the  con- 
glomerates of  Colorado. 

There  are  instances,  however,  where  the  disintegration  has  l>eon  so 


complete  that  only  a  few  scattered  fragments  remain  of  a  once  extensive 
stratum,  and  where  it  may  not  be  easy  to  realise  that  these  fragments  are 
not  transported  boulders.  In  Dorsetshire  and  Wiltehire,  for  example,  the 
surface  of  the  country  is  in  some  parts  so  thickly  strewn  with  fragments 
of  sandstone  and  conglomerate  "  that  a  person  may  almost  leap  from  one 
stone  to  another  without  touching  the  ground.  The  stones  are  frequently 
of  considerable  size,  many  being  four  or  five  yards  across,  and  about  four 
feet  thick."  •     They  are  found  lying  abundantly  on  the  Chalk,  suggestive 

'  Thty  h«TO  been  used  for  the  hugs  blocka  of  which  Slonaheuge  and  other  of  tbe  so-called 
Ihuidical  circles  hive  been  con^lructeil,  heiice  they  have  been  termed  Druid  StoDes.  Other 
names  are  Sotmh  StoDCS  (8U|iposvd  to  inditate  tbiit  their  Bccumulatton  haa  been  popaUrly 
ucribed  to  the  Sinceni),  and  Grey  Wetbera,  from  their  reseiiiblauce  iD  the  distance  lo  flocka 


356  nVXAMICAL  GEOLOGY  book  hi  part  n 


at  first  of  some  former  agent  of  tniiisport  by  which  they  were  brought  from 
a  distance.  They  are  now,  however,  generally  admitted  to  be  simply 
fragments  of  some  of  the  sandy  Tertiary  strata  which  once  covered  the 
disti-icta  where  they  occur.  While  the  softer  ix)rtion8  of  these  strata  have 
been  earned  away,  the  harder  parts  (their  hardness  perhaps  increasing  by 
exposure)  have  remained  l>ehind  as  "Grey  Wethers,"  and  have  sub- 
sequently suffered  from  the  inevitable  splitting  and  crumbling  action  of 
the  weather.  Similar  blocks  of  quartzite  and  conglomerate,  referable  to 
the  disintegration  of  Lower  Tertiary  beds  in  sihiy  are  traceable  in  the 
north-east  of  France  up  into  the  Ardennes,  showing  that  the  Tertiar}' 
deposits  of  the  Paris  l^asin  once  had  a  much  wider  extension  than  they 
now  possess.  1  On  a  far  grander  scale;  the  apparent  caprice  of  general 
subaerial  disintegration  is  exhibited  among  the  "  buttes  "  and  "  bad  lands  " 
of  Wyoming  and  the  neighbouring  territories  of  North  America.  Colossal 
pyramids,  ban-ed  horizontally  by  level  lines  of  stratification,  rise  up 
one  after  another  far  out  into  the  plains,  which  were  once  covered  by  a 
continuous  sheet  of  the  formations  whereof  these  detached  outliers  are 
only  fnigments. 

As  a  consequence  of  this  inequality  in  the  rate  of  waste,  depending 
on  so  many  conditions,  notably  upon  declivity,  amount  and  heaviness  of 
rain,  lithological  texture  and  composition,  and  geological  structure,  great 
N'ariotie«  of  contour  arc  worked  out  uix)n  the  land.  A  sur\'ey  of  this 
deiwrtment  of  geological  actinty  shows,  indeed,  that  the  unequal  wasting 
by  rain  has  in  large  measure  pi*oduced  the  details  of  relief  on  the 
present  surface  of  the  continents,  those  tracts  where  the  destruction  has 
been  greiitest  forming  hollows  and  valleys,  others,  where  it  has  been  less, 
rising  into  ridges  and  hills.  Even  the  minuter  f cultures  of  crag  and 
pinnacle  may  1x3  refeiTcd  to  a  similar  origin.     (Book  VII.) 

§  2.  Underground  Water. 

A  gi'eat  i)art  of  the  rain  that  falls  on  land,  sinks  into  the  ground  and 
apiwirently  disfippears ;  the  rest,  flowing  off  into  nmnels,  brooks,  and 
rivers,  moves  doMiiward  to  the  sea.  It  is  most  convenient  to  follow 
firat  the  course  of  the  subterranean  water. 

All  rocks  being  more  or  less  porous,  and  traversed  by  abundant  joints 
and  cracks  (p.  306),  it  results  that  from  the  bed  of  the  ocean,  from  the 
bottoms  of  lakes  and  nvers,  as  well  as  from  the  general  surface  of  the 
land,  water  is  continually  filtering  downward  into  the  rocks  beneath. 
To  what  depth  this  descent  of  surface-water  may  go,  is  not  known.  As 
stated  in  a  former  section,  it  may  reach  as  far  as  the  intensely  hejited 
interior  of  the  planet,  for,  as  the  already  quoted  researches  of  Daubree 
have  shown,  capillary  water  am  penetrate  rocks  even  against  a  high 
counter-pressure  of  vapour  {tmfe,  p.  306).     Proliably  the  depth  to  which 

of  (wetlier)  sheep.  See  /hscn'jttirr  Catahnjiie  nf  Tinck  Sjtecimens  in  Jermyn  Street  Mvteum^ 
'ird  eil.  ;  rreslwiob,  (^.  ./.  d'enf.  *S('C.  x.  j).  123  ;  Whitaker,  (Jeuloffiatf  Survey  Afemoir  on 
parts  of  MHhUnfn!.i\  &(.•..  p.  71. 

^  Barrois.  Ann.  Sh\  0(ol.  du  Sonl,  vi.  ]>.  306. 


8ECT.  ii  §  2 


UNDERGROUND  WATER 


357 


the  water  descends  varies  indefinitely  according  to  the  varying  nature 
of  the  rocky  crust.  Some  shallow  mines  are  practically  quite  dry,  others 
of  great  depth  require  large  pumping  engines  to  keep  them  from  being 
flooded  by  the  water  that  pours  into  them  from  the  surrounding  rocks. 
Yet,  as  a  rule,  the  upper  layers  of  rock  in  the  earth's  crust  are  fuller  of 
moisture  than  those  deeper  down. 

Underground  Circulation  and  Ascent  of  Springs. — The  water 
which  sinks  below  ground  is  not  permanently  removed  from  the  surface, 
though  there  must  be  a  slight  loss  due  to  absorption  and  chemical  altera- 
tion of  rocks.  Finding  its  way  through  joints,  fissures,  or  other  divisional 
planes,  it  issues  once  more  at  the  surface  in  springs.  This  may  happen 
either  by  continuous  descent  to  the  point  of  outflow,  or  by  hydrostatic 
pressure.  In  the  former  case,  rain-water,  sinking  underneath,  flows  along 
a  subterranean  channel  until,  when  that  chaimel  is  cut  by  a  valley  or 
other  depression  of  the  ground,  the  water  emerges  again  to  daylight. 
Thus,  in  a  district  having  a  simple  geological  structure  (as  in  Fig.  103),  a 


' -"iffiTr^  b^^^MfaaS^ 


Fig.  103.— Simple  or  Surface  SpriugM. 

sandy  porous  stratum  (^/),  through  which  water  readily  finds  its  way, 
may  rest  on  a  less  easily  permeable  clay  («),  followed  underneath  by  a 
second  sandy  pervious  bed  (c),  resting  as  before  upon  comparatively 
impervious  ^  strata  {a),  Eain  falling  upon  the  upper  sandy  stratum  ((l\ 
will  sink  through  it  to  the  surface  of  the  clay  (/^),  along  which  it  will 
flow  until  it  issues  either  as  springs,  or  in  a  general  line  of  wetness  along 
the  side  of  the  valley  {h).  The  second  sandy  bed  (c)  will  serve  as  a 
reservoir  of  subterranean  water  so  long  as  it  remains  below  the  surface, 
but  any  valley  cutting  down  below  its  base  will  drain  it. 

Except,  however,  in  districts  of  gently  inclined  and  unbroken  strata, 
springs  are  more  usually  of  the  second  class,  where  the  water  has 
descended  to  a  greater  or  less  distance,  and  has  risen  again  to  the  sur- 
face in  fissures,  as  in  so  many  syphons.  Tjines  of  joint  and  fault  afford 
ready  channels  for  subterranean   drainage   (Fig.   104).     Powerful  faults 


Fig.  104.— Deep-seated  Springs  {*,  s)  rising  through  Jointa  and  a  fault  (J). 

which  bring  different  kinds  of  rock  against  each  other  (as  a  and  g  are  by 
the  fault /in  Fig.  104)  are  frequently  marked  at  the  surface  by  copious 

^  This  tenn  impervious  must  evi<lently  be  used  in  a  relative  and  not  in  an  absolute 
Heuse.  A  stiff  clay  is  practically  impervious  to  the  trickle  of  underground  water ;  hence 
its  employment  as  a  material  for  puddling  (that  is,  making  water-tight)  canals  and  reservoirs. 
But  it  contains  abundant  interstitial  water,  on  which,  indeed,  its  characteristic  plasticity 
depends. 


DVXAMIOAL  GEOLOGY 


BOOK  III  PART  U 


springs.  So  complex  is  the  network  of  di^-iaioiia)  planes  by  which  rocks 
are  trnverserf,  that  water  may  often  follow  a  most  labyrinthine  coune 
before  it  cumplctoR  its  undergi'ound  circulation  (Fig.  105).  In  countries 
with  a  sufficient  rainfall,  nicka  arc  Baturatetl  with  water  below  a  certain 


II.  lUJ — InttiMtc  Biilitmiii 


limit  teimed  the  iraln-letvl.  Owing  to  varying  Btnictme,  and  relative 
cajjacity  for  water  among  rocks,  this  line  is  not  strictly  horizontal,  h'ke 
that  of  the  Burfnce  of  a  lake.  Moi-eover,  it  is  liable  to  rise  and  fall 
aixording  as  the  seasons  are  wet  or  dry.  In  some  places  it  lies  qiiite 
near,  in  others  far  l>elow,  the  surface.  A  well  is  an  artificial  hole  dug 
down  I>elow  the  water-level,  so  that  the  water  may  percolate  into  it. 
Hence,  when  the  water-level  hapi>ens  to  be  at  a  small  depth,  wells  are 
shallow  ;  when  at  a  greater  depth,  they  reipn're  to  l>e  deeper. 

Since  rocks  vary  greatly  in  [Kirosity,  some  contain  far  more  water 
than  others.  It  often  happens  that,  percolating  along  some  porous  bed, 
subterranean  water  finds  its  way  downward  until  it  passes  under  some 
more  impervious  rock.  Hindei-od  in  its  progress,  it  accumulates  in  the 
porous  l>ed,  from  which  it  may  l>e  able  to  find  its  way  up  to  the  surface 
again  only  by  a  tedious  cii-cuitous  passage.  If,  however,  a  Iwre-hole  be 
STink  through  the  upper  imperious  \>ed  down  to  the  water-charged 
stratum  1>eluw,  the  water  will  a^'ail  itself  of  this  artificial  chaimel  of 
escai>c,  and  will  rise  in  the  bole,  or  even  gush  out  as  a  jet  d'ean  above 
ground.     Wells  of  this  kin<l  ai-e  now  largely  employed.     They  bear  the 


-iXJ:.-^ 


vend  by  « 


rii(0,thraunliwblch,i' 


name  of  Artestimi,  from  the  old  pnivince  of  Artois  in  France,  whore  they 
have  long  l>ecn  in  use^  (Fig.  lOli). 

'  Sev  I'restwich  <j.  J.  (l/iil.  ^K.  xiviii.  |>.  Ivii.,  anil  the  referencM  there  girai.  Oiw 
of  tlie  lisit  recent  esmyB  on  tlic  nnhjept  of  Arlesim  Wells  it  thnt  by  Pnrfi»*or  T.  C. 
Chaiiiberliti  in  tlis  Slli  AniiusI  Re|wrt  of  tlie  U.ij.  Geo).  Survey  (1883-81),  p.  131. 


SECT.  ii§2  UNDERGROUND  WATER  359 

That  the  water  really  circulates  underground,  and  passes  not  merely 
through  the  pores  of  the  rocks,  but  in  crevices  and  tunnels,  which  it  has 
no  doubt  to  a  large  extent  opened  for  itself  along  natural  joints  and 
fissures,  is  proved  by  the  occasional  rise  of  leaves,  twigs,  and  even  live 
fish,  in  the  shaft  of  an  Artesian  well.  Such  testimony  is  particularly 
striking  when  found  in  districts  without  surface-waters,  and  even  perhaps 
with  little  or  no  rain.  It  has  been  met  with,  for  instance,  in  sinking 
wells  in  some  of  the  sandy  deserts  on  the  southern  borders  of  Algeria.^ 
In  these  and  similar  cases,  it  is  clear  that  the  water  may,  and  sometimes 
does,  travel  for  many  leagues  undergroimd,  away  from  the  district 
where  it  fell  as  rain  or  snow,  or  where  it  leaked  from  the  bed  of  a  river 
or  lake. 

The  temperature  of  springs  affords  a  convenient,  but  not  always 
quite  reliable  indication  of  the  relative  depth  from  which  they  have 
risen.  Some  springs  are  just  one  degree  or  less  above  the  temperature 
of  ice  (C.  0**,  Fahr.  32**).  Others,  in  volcanic  districts,  issue  with  the 
temperature  of  boiling  water  (C.  100**,  Fahr.  212°).  Between  these  two 
extremes  every  degree  may  be  registered.  Very  cold  springs  may  be 
regarded  as  probably  deriving  their  supply  from  cold  or  snow-covered 
mountains.  Certain  exceptional  cases,  however,  occur,  where,  owing  to 
the  subsidence  of  the  cold  winter  air  into  caverns  (glacih'es),  ice  is  formed 
which  is  not  wholly  melted  even  though  the  summer  temperature  of  the 
caves  may  be  above  freezing-point.  Water  issuing  from  these  ice-caves 
is  of  course  cold.^  On  the  other  hand,  springs  whose  temperature  is 
higher  than  the  mean  temperatiu'e  of  the  places  at  which  they  emerge 
must  have  been  warmed  by  the  internal  heat  of  the  earth.  These  are 
termed  Thermal  Springs.^  The  hottest  springs  are  found  in  volcanic 
districts  (see  p.  235).  But  even  at  a  great  distance  from  any  active 
volcano,  springs  rise  with  a  temperature  of  120°  Fahr.  (which  is  that  of 
the  Bath  springs)  or  even  more.  These  have  probably  ascended  from  a 
great  depth.  If  we  could  assume  a  progressive  increase  of  1*  Fahr.  of 
subterranean  heat  for  every  60  feet  of  descent,  the  water  at  1 20°,  issuing 
at  a  locality  whose  ordinary  temperature  is  50*,  should  have  been  down 

1  Desor,  Bull.  Soc.  Sci.  Nat.  Neu/chdUl,  1864.  On  the  hydrology  of  the  Sahara 
consult  G.  RoUand,  Assoc.  Fran^ise,  1880,  p.  547.  Tchihatchef,  Brit.  Assoc.  1882,  p. 
356.     Choisy,  'Documents  relatifs  a  la  Mission  dirigee  au  Sud  de  I'Alg^rie.*     Paris,  1890. 

^  A  remarkable  example  of  a  glaci^re  is  that  of  Dobschau,  in  Hungary,  of  which  an 
account,  with  a  series  of  interesting  drawings,  was  published  in  1874  by  Dr.  J.  A. 
Krenner,  keeper  of  the  national  museum  in  Buda-Pesth.  See  also  Murchison,  Keyserling, 
and  De  Vemeuil,  'Geology  of  Russia.'  Thury,  Biblioth.  Univ.  Greneva,  1861.  Browne, 
'  Ice-Caves  in  France  and  Switzerland,'  1865.  Fifty-six  of  these  caves  are  known  in  the 
Alps,  some  in  the  Jura,  and  many  elsewhere. 

'  Studer  points  out  that  some  springs  which  are  thermal  in  high  latitudes  or  at 
great  elevations,  would  be  termed  cold  springs  near  the  equator,  and,  consequently,  that 
springs  having  a  lower  temperature  than  that  of  the  inter-tropical  zone,  that  is  from 
C.  0°  to  80'  (Fahr.  82" -84"),  should  be  called  "relative,"  those  which  surpass  that 
limit  (C.  80*-100')  "absolute,"  and  he  gives  a  series  illustrative  of  each  group:  *Phy- 
sikalische  Geographie,'  ii.  (1847),  p.  49.  For  volcanic  thermal  springs  see  ante,  p.  235, 
and  poMtea^  p.  368. 


360  DYNAyriCAL  GEOLOGY  book  hi  part  ii 


at  least  4200  feet  below  the  surface.  But  from  what  has  been  already 
stated  (p.  51)  regarding  the  irregular  stratification  of  temperature  within 
the  earth's  crust,  such  estimates  of  the  probable  depth  of  the  sources  of 
springs  are  not  quite  reliable.  The  source  of  heat  in  these  cases  may  be 
some  crushing  of  the  crust  or  ascent  of  heated  matter  from  underneath, 
which  does  not,  however^  produce  volcanic  phenomena. 

1.  Chemical  Action. — Every  spring,  even  the  clearest  and  most 
sparkling,  contains  dissolved  gases,  also  solid  matter  abstracted  from  the 
soils  and  rocks  which  it  has  traversed.  The  gases  include  those  absorbed 
by  rain  from  the  atmosphere  (p.  341),  also  carbon-dioxide  supplied  by 
decomposing  organic  matter  in  the  soil,  sulphuretted  hydrogen,  and 
marsh-gas  or  other  hydrocarbon  derived  from  decompositions  within  the 
crust.  The  solid  constituents  consist  partly  of  organic,  but  chiefly  of 
mineral  matter.  Where  spring- water  has  been  derived  from  an  area 
covered  A\dth  ordinary  humus,  organic  matter  is  always  present  in  it 
Organic  acids  are  abstracted  from  the  soil  by  descending  water,  and  these, 
before  they  are  oxidized  into  carbonic  acid,  are  effective  in  decomposing 
minerals  and  forming  sohible  salts  (p.  343).  The  mineral  matter  (rf 
spring-water  consists  principally  of  carbonates  of  calciimi,  magnesium,  and 
sodium,  sulphates  of  calcium  and  sodium,  and  chloride  of  sodium,  with 
minute  traces  of  silica,  phosphates,  nitrates,  &c.  The  nature  and  amount 
of  mineral  impregnation  depend,  on  the  one  hand,  upon  the  chemical 
energ\'  of  the  water,  and  on  the  other,  upon  the  composition  of  the  rocks. 

Various  sources  of  augmentation  of  its  chemical  energy  are  available 
for  subterranean  water.  (1)  The  abundant  organic  matter  in  the  soil 
partially  abstracts  oxygen  from  the  water,  but  supplies  organic  acids, 
especially  carbonic  acid.  In  so  far  as  the  water  carries  down  from  the 
soil  any  oxidizable  organic  substance,  its  action  must  be  to  reduce  oxides 
(p.  343).  Ordinary  vegetable  soil  possesses  the  power  of  removing  from 
permeating  water  potash,  silica,  phosphoric  acid,  ammonia,  and  organic 
matter,  elements  which  had  been  already  in  great  measure  abstracted 
from  it  by  li\ing  vegetation,  and  which  are  again  ready  to  be  taken  up 
by  the  same  organic  agents.  (2)  Carlx)n-dioxide  is  here  and  there  largely 
evolved  within  the  earth's  crust,  especially  in  regions  of  extinct  or 
dormant  volcanoes.  Subtemincan  water  coming  in  the  way  of  this  gas 
dissolves  it,  and  thereby  obtains  increased  solvent  power.  (3)  The 
capacity  of  water  for  dissohing  mineral  substances  is  augmented  by 
increase  of  temperature  {anky  p.  307).  It  is  conceivable  that  cold  springs, 
containing  a  large  percentage  of  mineral  sohitions,  may  have  acquired 
this  impregnation  at  a  great  depth  and  at  a  higher  temperature.  As  a 
rule,  however,  thermal  water,  as  it  cools,  deposits  its  dissolved  minerals 
on  the  walls  of  the  fissures  up  which  it  ascends.  Hence,  no  doubt,  the 
successive  layers  in  mineral  veins.  (4)  Pressure  likewise  raises  the  solvent 
power  of  water  (p.  307).  (5)  Some  of  the  solutions,  duo  to  decomposi- 
tions effected  by  the  water,  increase  its  ability  to  accomplish  further 
decompositions  (p.  310).  Thus  the  alkaline  carbonates,  which  are  among 
the  earliest  products,  enable  it  to  dissolve  silica  and  decompose  silicates. 
These  carbonates  likewise  promote  the  decomposition  of  some  sulphates 


SECT,  ii  §  2 


CHEMICAL  ACTION  OF  SPRINGS 


361 


and  chlorides.  Calcium-carbonate,  which  is  found  in  the  water  of  most 
springs,  is  the  result  of  decomposition,  and  by  its  presence  leads  to  the 
further  disintegration  of  various  minerals.  "  Carbonic  acid,  bicarbonate 
of  lime,  and  the  alkaline  carbonates  bring  about  most  of  the  decompositions 
and  changes  in  the  mineral  kingdom.  It  is  a  matter  of  great  importance 
to  find  that  the  same  substances  which  give  rise  to  so  many  decomposi- 
tions in  the  mineral  kingdom  are  the  chief  ingredients  in  the  waters."  ^ 

The  nature  of  the  changes  effected  by  the  percolation  of  water  through 
subterranean  rocks  will  be  best  understood  from  an  examination  of  the 
composition  of  spring-water.  Springs  may  be  conveniently,  though  not 
very  scientifically,  grouped  into  two  classes :  1st,  Common  springs,  such 
as  are  fit  for  ordinary  domestic  purposes,  and  2nd,  mineral  springs,  in 
which  the  proportions  of  dissolved  mineral  matter  are  so  much  higher  as 
to  remove  the  water  from  the  usual  potable  kinds. 

1.  Common  Springs  possess  a  temperature  not  higher  but  frequently  lower  than 
that  of  the  localities  at  which  they  rise,  and  ordinarily  contain,  besides  atmospheric  air 
and  its  gases,  calcium-carbonate  and  sulphate,  common  salt,  with  chlorides  of  calciimi  and 
magnesium,  and  sometimes  organic  matter.  The  amount  of  dissolved  mineral  contents 
in  ordinary  drinking  water  does  not  exceed  0  "5,  or  at  most  1  '0  gramme  per  litre  ;  the  best 
waters  contain  less.  The  amount  of  organic  matter  should  not  exceed  from  0*000 
to  0*01  gramme  per  litre  in  wholesome  drinking  water.'  Spring- water  containing  a  very 
minute  ^lercentage  of  mineral  matter,  or  in  which  this  matter,  even  if  in  more  consider- 
able quantity,  consists  chiefly  of  alkaline  salts,  dissolves  common  soap  readily,  and  is 
known  in  domestic  economy  as  *'soft"  water.  Where,  on  the  other  hand,  the  salts  in 
solution  are  calcic  or  magnesic  carbonates,  sulphates,  or  chlorides,  they  decompose  soap, 
forming  with  its  fatty  acids  insoluble  compounds  which  appear  in  the  familiar  white 
curdy  precipitate.  Such  water  is  termed  "hard."  Where  the  hardness  is  due  to  the 
l>re8ence  of  bicarbonates  it  disappears  on  boiling,  owing  to  the  loss  of  carbonic  acid  and 
the  consequent  precipitation  of  the  insoluble  carbonate,  while  in  the  case  of  sulphates 
and  chlorides  no  such  change  takes  place. 

The  extensive  investigations  carried  on  by  the  Rivers  Pollution  Commission  in 
Britain  have  thrown  much  light  on  the  relation  bet>veen  the  amount  of  mineral  matter 
in  solution  in  springs  and  wells,  and  the  character  of  the  underlying  rock.  The  follow- 
ing table  of  analyses  of  waters  from  different  kinds  of  rocks  gives  a  summary  of  results 
obtained — 


1.  Fluviomarine,  Drift  and  Gravel 

2.  Chalk         .... 
Hastings  Sand  and  Greensands 

4.  Oolites       .... 

5.  Lias  ..... 
New  Red  Sandstone  . 
Magnesian  Limestone 

8.  Coal-Measures    . 

9.  Yoredale  beds  and  Millstone-Grit 

10.  Mountain  Limestone  . 

11.  Devonian  and  Old  Red  Sandstoi 

12.  Silurian     .... 

13.  Granite  and  Gneiss     . 


3 


6. 


No.  of 
Analyses. 

.  10 
.  30 
.  19 
.     35 

7 
.     15 

1 
.     14 

8 

.     13 

.     32 

15 

8 


Mean  amount  of  Solid 

Contents  in  10,000 

Parts  of  Watfir. 

6-132 
2-984 
3-005 
3-033 
3-641 
2-869 
6-652 
2-430 
1-773 
3  "206 
2-506 
1-233 
0-594 


>  Bischof,  'Chem.  Geol.'  i.  p.  17.         '^  Dr.  B.  H.  Paul 


n  Watts'  *  Diet.  Chem.'  v.  p.  1022. 


362  DYXA^MICAL  GEOLOGY  book  hi  part  ii 


From  this  table  it  is  evident  how  greatly  the  i>roportion  of  dissolved  mineral 
sub»tanee  augments  in  those  waters  which  rise  in  calcareous  tracts,  and  how  it  corre- 
8iN)ndingly  sinks  in  those  where  the  rocks  are  mainly  siliceous.  The  maximum 
jx'rcentage  in  group  No.  13  was  less  than  1  part  in  every  10,000  of  water,  the  minimum 
being  0*140  from  granite.  In  No.  1,  on  the  contrary,  the  maximum  was  22*524,  in  No.  6 
it  was  7-426,  and  in  No.  10  it  was  9*850.1 

2.  Mineral  Springs  are  in  some  instances  cold,  in  others  warm,  or  even  boiling. 
Thermal  springs  are  more  usually  mineral  waters  than  cold  springs,  but  there  does  not 
ajipcar  to  be  any  necessary  relation  between  temperature  and  chemical  composition. 
Mineral  springs  may  be  roughly  classified  for  geological  purposes  according  to  the  pre- 
vailing mineral  sul)stance  contained  in  them,  which  may  range  in  amount  from  1  to  800 
grammes  per  litn^- 

Cttlcareaus  Springs  contain  calcium -carbonate  in  such  quantity  as  to  be  deposited  in 
the  fonn  of  a  white  crust  round  objects  over  which  the  water  flows.  Calcium -carbonate, 
according  to  Fresenius,  is  dissolved  by  10,600  of  cold  and  by  8834  parts  of  warm  water.' 
But  in  nature,  the  proportion  of  this  carbonate  present  in  springs  depends  mainly  on  the 
proiK)rtion  of  free  carbonic  acid,  which  retains  the  lime  in  solution.  On  the  loss  of 
carbonic  acid  by  exjK>sure  and  evaporation,  the  carl>onate  is  thrown  do^'n  as  a  white 
precipitate.  This  deposition  is  frec^uently  brought  about  by  the  action  of  living  plants 
(Book  HI.  Part  II.  Sect.  iii.  §  3.)  Water  saturated  with  carbonic  acid  will  at  the 
freezing-point  dissolve  0*70  gramme  and  at  10**  C.  0'88  gramme  of  calcium-carbonate 
])er  litre.  Calcareous  springs  occur  abundantly  in  limestone  districts,  and  indeed  may 
bo  looked  for  wherever  the  nx'ks  are  of  a  markedly  calcareous  character.  In  some 
regions,  they  have  brought  up  such  enormous  quantities  of  lime  as  to  form  cousideFable 
hills  {postra,  p.  366). 

Ferrwjiiwics  or  Chalybeate  Springs  contain  a  large  ])roiK)rtion  of  ferrous  sulphate  (iron- 
vitiiol,  copperas)  in  the  total  mineral  ingredients,  and  are  known  by  their  inky  taite, 
and  the  yellow,  brown,  or  red  ochry  deposit  along  their  channel.  They  may  be  freqnentiy 
oVwerved  in  districts  where  beds  or  veins  of  pyritous  ironstone  occur,  or  where  the 
rocks  contain  nmeh  iron-disulphide  in  combination,  j»articularly  in  the  waters  of  old 
mines.  By  the  weathering  of  this  sulphide  (marcasite),  so  abundantly  contained  among 
stratified  ro<-ks,  ferrous  sulphate  is  produced  and  brought  to  the  surface,  but  in  presence 
of  carbonates,  ])articularly  of  the  ubiquitous  carbonate  of  lime,  is  decomposed,  the  add 
being  taken  up  by  the  alkaline  earth  or  alkali,  and  the  ii-on  l>ecominga  ferrous  carbonate, 
which  ra])idly  oxidizes  and  falls  as  the  familiar  yellow  or  brown  crust  of  hydrous  peroxide. 
The  mjadity  with  which  ferrous-carbonate  is  thus  oxidized  and  precipitated  was 
well  shown  by  Fresenius  in  the  case  of  the  Langenschwalbach  chalyl:teate  spring.  In  its 
fresh  state  the  water  contains  in  1000  parts  0-37696  of  protoxide  of  iron.  After  standing 
twenty-four  hours  it  was  found  to  contain  only  87*7  per  cent  of  the  original  amount 
of  iron  ;  after  sixty  hours  62*9  i»er  cent,  and  after  eighty-four  hours  53'2  per  cent.* 

Brine- Springs  (Soolquellen)  bring  to  the  surface  a  solution  in  which  sodium  chloride 
greatly  predominates.  Springs  of  this  kind  apj)ear  where  beds  of  solid  rock-salt  exist 
underneath,  or  where  the  rocks  are  im])regnated  with  that  mineral.     Most  of  the  brines 

'  liiirrs  Pollution  Commission,  6th  ReiK)rt,  1874,  pp.  107-131.  See  also  Reports  of 
Brit.  Assoc,  (.-oiumittee  on  underground  circulation  of  water,  l>eginning  in  1876,  and  B. 
Warington's  Report  on  experiments  at  the  Rothamsteil  Laboratory,  Jouni.  CJiem,  Soe,  1887. 

-  Paul,  Watts'  *Dict.  Chem.'  v.  p.  1016. 

^  Roth,  *Chem.  Geol.'  i.  p.  48.  **One  litre  of  water,  either  cold  or  boiling,  dissolves 
alwut  18  niilligraninies."     Roscoe  and  Schorl  em  nier,  'Cheniistrj*,*  iL  p.  208. 

*  Juumol  far  Prakt.  Chtm.  Ixiv.  368,  (juoted  by  Roth,  op.  cit.  i.  p.  565.  The  river  in 
the  Vale  of  Avoca,  Ireland,  formerly  contained  so  nmch  ferrous  sulphate,  carried  into  it  by 
mine-waters,  that  its  bed  and  Kinks  for  several  miles  down  to  the  sea  were  covered  with 

ochreous  de]K)sit. 


a&oc 


SECT,  ii  §  2  CHEMICAL  ACTION  OF  SPRINGS  363 

worked  as  sources  of  salt  are  derived  from  artificial  boriugs  into  saliferous  rocks.  Those 
of  Cheshire  in  England,  the  Salzkammergnt  in  Austria,  Bex  in  Switzerland,  &c.,  have 
long  been  well  known.  That  of  Clemenshall,  Wiirtemberg,  yields  upwards  of  26  per 
cent  of  salts,  of  which  almost  the  whole  is  chloride  of  sodium.  The  other  substances 
contained  in  solution  in  the  water  of  brine-springs  are  chlorides  of  potassium,  magnesium, 
and  calcium  ;  sulphates  of  calcium,  and  less  frequently  of  sodium,  potassium,  magnesium, 
barium,  strontium,  or  aluminium  ;  silica ;  compounds  of  iodine  and  fluorine  ;  with 
phosphates,  arseniates,  borates,  nitrates,  organic  matter,  carbon-dioxide,  sulphuretted 
hydrogen,  marsh-gas,  and  nitrogen.^ 

Medicinal  Springs,  a  vague  term  applied  to  mineral  springs  which  have  or  are  believed 
to  have  curative  effects  in  different  diseases.  Medical  men  recognise  various  qualities, 
distinguished  by  the  particular  substance  most  conspicuous  in  each  variety  of  water — 
AVcaliiUi  Waters,  containing  lime  or  soda  and  carbonic  acid — Vichy,  Saratoga  ;  Bitter 
Waiers,  with  sulphate  of  magnesia  and  soda — Sedlitz,  Kissingen  ;  Salt  or  Muriaied 
Waters,  with  common  salt  as  the  leading  mineral  constituent — Wiesbaden,  Cheltenham  ; 
Earthy  Waters,  lime,  either  a  sulphate  or  carbonate  being  the  most  marked  ingredient 
— Bath,  Lucca ;  Sulphurous  Waters,  with  sulphur  as  sulphuretted  hydrogen  and  in 
sulphides — Aix-la-Chapelle,  Harrogate.  Some  of  these  medicinal  springs  are  thermal 
waters.  Even  where  no  longer  warm,  the  water  may  have  acquired  its  peculiar  medicinal 
characters  at  a  great  depth,  and  therefore  under  the  influence  of  increased  temperature 
and  pressure.  Sulphur  springs  are  sometimes  warm,  but  also  occur  abundantly  cold, 
where  the  water  rises  through  rocks  containing  decomposing  sidphides  and  organic 
matter.  Sulphates  are  there  first  formed,  which  by  the  reducing  effect  of  the  organic 
matter  are  decomposed,  with  the  resultant  formation  of  sulphuretted  hydrogen  (p.  67). 
Sulphuretted  hydrogen  and  sulphurous  acid  are  sometimes  oxidized  into  sulphuric  acid, 
which  remains  free  in  the  water  .^ 

Hot  Springs,  Oeysers. — The  thermal  waters  of  volcanic  districts  usually  contain  a 
marked  percentage  of  dissolved  mineral  matter,  notably  silica,  with  sulphates,  carbonates, 
chlorides,  bromides,  and  other  combinations.  Perhaps  the  most  detailed  examination 
yet  made  of  any  such  group  of  springs  is  the  series  of  analyses  performed  by  the  Geological 
Survey  of  the  United  States  on  the  waters  of  forty-three  hot  springs  in  the  Yellowstone 
National  Park.  The  temperatures  of  these  waters  ranged  up  to  93**  C,  and  the  total 
amount  of  dissolved  mineral  matter  up  to  2*8733  grammes  in  every  kilogramme.  The 
silica  sometimes  amounted  to  0*6070  gramme,  the  sulphuric  acid  to  1*9330,  the  carbonic 
acid  to  1-2490,  the  chlorine  to  1*0442,  the  calcium  to  0*3076,  the  magnesium  to  00797, 
the  |x>tassium  to  0*1603,  the  sodium  to  0*4407,  and  there  were  minute  quantities  of 
numerous  other  constituents.* 

Oil  Springs. — Petroleum  is  sometimes  brought  up  in  drops  floating  in  spring- water 
(St.  Catherine's,  near  Edinburgh).  In  many  countries  it  comes  up  by  itself  or  mingled 
with  inflammable  gases.  Reference  has  already  been  made  (pp.  145,  235)  to  the  abund- 
ance of  this  product  in  North  America.  In  western  Pennsylvania,  some  oil-wells  have 
yielded  as  much  as  2000  to  3000  barrels  of  oil  per  day.  That  the  oil,  which  is  specially 
confined  to  particular  layers  of  rock  in  the  Carboniferous  and  Devonian  systems,  arises 
from  the  alteration  of  organic  substances  embedded  in  the  rocks  of  the  crust,  appears  to 
be  probable,  but  no  satisfactory  explanation  has  been  given  of  the  nature  and  distribu- 
tion of  the  organisms  which  yielded  the  oil.* 

^  Roth,  *Chem.  Geol.'  L  p.  442.  Bischof,  '  Chem.  Geol. '  ii.  Many  subterranean  waters, 
though  not  deserving  the  name  of  brines,  contain  considerable  proportions  of  chlorides.  On 
the  alkaline  chlorides  of  the  Coal-measures  see  R.  Malherbe,  Bull.  Acad.  Boy.  Bdgiq^i^, 
1875,  p.  16  ;  also  R.  Laloy,  Ann.  Soc.  UM.  Xard,  1875,  p.  195. 

2  Roth,  op.  cit.  i.  pp.  444,  452. 

*  F.  A.  Gooch  and  J.  E.  Whitfield,  Bull.  C.S.  Oeol.  Surrey,  No.  47,  1888. 

*  See  the  authorities  cited  ante,  p.  235. 


364  DYXAMICAL  OEOLffGY  book  m  pahi  li 


Kesults  of  the  Chemical  Action  of  Underground  Water. — 
Three  remtirkable  results  of  the  chemical  operations  of  undergroiind  water 
are  :  Ist,  The  internal  comiK>sition  and  minute  stnicture  of  rocks  are 
altered.  2nd,  Enormous  quantities  of  mineral  matter  are  carried  up  to 
the  surface,  whore  they  arc  jwrtly  deposited  in  visible  form,  and  partlj 
conveyed  by  brooks  and  rivers  to  the  sea.  3rd,  As  a  consequence  of  this 
transport,  subterranean  tunnels,  passiiges,  caverns,  grottos,  and  other 
cavities  of  many  varied  shapes  and  dimensions  are  formed. 

(1)  Alteration  of  llocks. — The  processes  of  oxidation,  deoxidation,  solu- 
tion, hydration,  and  the  formation  of  carlx>nates,  described  (pp.  343,  344) 
as  c<iiTied  on  al)ove  f^ound  by  rain,  are  likewise  in  progress  on  a  great 
scide  underneiith.  Since  the  pemieiibility  of  subterranean  rocks  permits 
water  to  find  it^j  way  through  their  pores  as  well  as  .along  their  divisional 
planes,  chemical  changes,  of  a  kind  like  those  in  ordinary  weathering, 
take  place  in  them,  and  at  some  depth  may  lie  intensified  by  internal 
terrestrial  heat  and  pressure.  This  8ubt43rranean  alteration  of  rocks  may 
consist  in  the  mere  addition  of  substances  intro<luced  in  chemical  solution ; 
in  the  simple  solution  and  removal  of  some  one  or  more  constituents :  or 
in  a  complex  process  of  removal  and  replacement,  wherein  the  origiiial 
subsUmce  of  a  rock  is  molecule  by  molecule  removed,  while  new  in- 
^•edients  are  simultaneously  or  afterwards  substituted.  In  tracing  these 
alterations  of  rocks,  the  study  of  pseudomorphs  l)ecomes  important^  for  we 
thereby  learn  what  was  the  original  composition  of  the  mineral  or  ro^ 
The  mere  existence  of  a  pseudomorph  points  to  the  removal  and  substita- 
tion  of  mineral  matter  by  permeating  water. ^ 

The  extent  to  which  such  mineral  replacement  has  been  carried 
among  rocks  of  the  most  varied  stnicture  and  composition  is  probably 
best  shown  by  the  abundant  i)etrified  organic  forms  in  formations  of  idl 
geological  ages.  The  minutest  stnictures  of  plants  and  animals  have  been, 
particle  by  jxirticle,  removed  and  replaced  by  mineral  matter  introduced 
in  solution,  and  this  so  imi)erceptibly,  and  yet  thoroughly,  that  even 
minutiae  of  organisation,  requiring  a  high  power  of  the  microscope  for  their 
invostigjition,  have  been  preserved  without  distortion  or  disarrangement 
From  this  perfect  condition  of  i)reservation,  gradations  may  be  traced 
until  the  organic  structure  is  gradually  lost  amid  the  crystalline  or 
amorphous  infiltrated  substance  (Fig.  107).  The  most  impoilant  petrifying 
media  in  nature  are  c^dcium-carbonate,  silica,  and  iron-disiUphide  (marcasite 
more  usually  than  i)yrite).     (See  Book  V.) 

Another  ])roof  of  the  alteration  which  rocks  have  suffered  from 
permeating  water  is  supplied  by  the  abundance  of  veins  of  calcite  and 
cjuartz  by  which  they  are  traversed,  these  minerals  having  been  introduced 
in  solution  and  often  from  the  decomposition  of  the  enclosing  rock.     As 

'  It  is  not  needful  to  take  account  liere  of  sucli  exceptional  cases  as  tlie  artificial  convonioii 
of  arnjjTonite  into  calcite  by  exposure  to  a  high  temperature.  In  such  paramorpht  the  change 
is  a  molecular  or  crystalline  rather  than  a  chemical  one,  though  how  it  takes  place  is  still 
unknown.  Pseuiloniorphs  may  be  artificially  formed.  Crystals  of  atacamite  (Cn4QjClj  + 
4H./>)  i>lacfd  in  a  solution  of  bicarbonate  of  soila  are  completely  changed  into  malachite  in 
four  years.     Tschermak's  Min.  MUth.  1S77,  ]».  07. 


BECT.  ii  §  2  CHEMICAL  ACTION  OF  SPRINGS  395 

Bischof  pointed  out,  a  drop  of  acid  seldom  fails  to  give  efFeireacence  on 
pieces  of  rock,  composed  of  Bilicat«8,  which  have  been  taken  even  at  some 
little  depth  from  the  surface,  thus  indicating  the  decomposition  and 
deposit  caused  by  permeating  water.  As  already  stated,  one  of  the  most 
remarkable  results  of  the  application  of  the  microscope  to  geological 
inquiry  is  the  extent  to  which  it  has  revealed  these  all-pervading  altera- 
tions, even  in  what  might  be  supposed  to  be  perfectly  freah  rocks. 
Among  the  silicates,  the  most  varied  and  complex  interchanges  have  1>een 
effected.  Besides  the  production  of  calcium-carbonate  by  the  decomposi- 
tion of  guch  minerals  as  the  lime -felspars,  the  series  of  hydrous  green 
femiginous  silicates  (delessite,  saponite,  chlorite,  serpentine,  fee),  so 
roininonly  met  with  in  crystalline  rocks,  are  usually  witnesses  of  the 
influence  of  infiltrating  water.  The  changes  visible  in  olivine  (p.  173) 
offer  instructive  lessons  on  the  progress  of  transformation.  One  further 
mple  may  be  cited  as  supplied  by  the  zeolites,  so  common  in  cai'ities  and 


veins  among  many  ancient  volcanic  and  other  ciystalline  rocks.  These  hH^'e 
commonly  resulted  from  the  decomposition  of  felspars  or  allied  miiiemls. 
Their  mode  of  formation  is  indicated  by  the  oliservation  already  cited 
(p.  307),  that  Roman  masonry  at  the  baths  of  Plombi^res  has  in  the 
course  of  ccutiiries  been  so  decomposed  by  the  slow  percolation  of  alkaline 
water  at  a  temperatiire  not  exceeding  50'  C.  (122"  Fahr.)  under  ordinary 
atmospheric  pressure,  that  various  zcolitic  silicates  have  been  developed 
in  the  brick.' 

(2)  Chemical  Deposils. — Of  these  by  far  the  most  abundant  is  calciimi- 
carbonate.  The  way  in  which  this  substance  is  removed  and  rc-deposited 
by  permeating  water  can  I*  instnictively  studied  in  the  formation  of  the 
familiar  slalneltUi'  and  dalai/miks  )>encath  damp  arches  and  in  limestone 
caves  (p.  150).  As  each  drop  gathers  on  the  roof  and  logins  to  e^aiwrate 
and  lose  carlmnic  acid,  the  excess  of  cjirlHinate  which  it  can  no  longer 
retain  is  depoeit«d  round  its  edges  as  a  ring  (Fig.  lOS).  iJrop  succeeding 
'  Danbri'*,  "(iiHilogie  PIiiiiti'imeLlale.' p.  179  ri  *v/. 


366  DYXAMICAL  UEOLOGY  book  hi  part  ii 


drop,  the  original  ring  gi-ows  into  a  long  pendant  tul)e,  which,  by  sub- 
sequent deposit  inside,  l>ecomes  a  solid  stalk,  and  on  reaching  the  floor 
may  thicken  into  a  massive  pillar.  At  first  the  calcareous  su1)6tance  is  soft 
and,  when  dry,  pidvenilent,  but  by  prolonged  saturation  and  the  internal 
deposit  of  calcite  it  becomes  by  degrees  crystalline.  Each  stalactite  is 
found  to  2)ossess  an  internal  radiating  fibrous  stnicture,  the  fibres  (prisms) 
pissing  across  the  concentiic  zones  of  growth.  The  stalactite  remains 
saturated  with  calciireous  water,  and  the  divergent  prisms  are  developed 
and  continued  as  i-adii  frc»m  the  centre  of  the  stalk.  This  process  may 
be  complet^jd  within  a  short  period.  At  the  North  Bridge,  Edinburgh, 
for  exami)le,  which  was  erected  in  1772,  stalactites  were  obtained  in 
1874,  some  of  which  measure  an  inch  and  a  half  in  diameter  and  possess 
the  characteristic  radiating  stnicture.^  It  is  doubtless  by  an  analogous 
process  that  limestones,  originally  composed  of  the  debris  of  calcareous 
orgjinisms  and  interstratified  among  perfectly  unaltered  shales  and  sand- 
stones, have  acquired  a  crystalline  structiu'e  (p.  122).- 

Some  cidcareous  springs  deposit  abundiintly  a  precipitate  of  carbonate 
of  lime  uix)n  mosses,  twigs,  leaves,  stones,  and  other  objects.  The  preci- 
pitjite  tjikes  2)lace  when  from  any  cause  the  water  jMrts  with  carbonic 
acid.  This  may  arise  from  mere  evaiX)ration,  but  is  frequently  due  to 
the  action  of  bog-mosses  and  water-plants,  which,  decomposing  the  car- 
lx)nic  acid,  cause  a  crust  of  carlK)nate  of  lime  to  be  deposited  round  their 
stt»ms  and  branches  {posteu,  p.  482).  Hence  calcareous  springe  «* 
l)0i>ularly  called  "]>etrifving,"  though  they  merely  encrust  organic  bodies, 
and  do  not  conveil  them  into  stone.  Calc-sinter  or  travertine,  as  this 
precipitate  is  wdled,  may  Ije  found  in  couree  of  formation  in  most  lime- 
stone districts,  sometimes  in  masses  large  enough  to  form  hills,  and 
compict  enough  to  furnish  excellent  building-stone.  The  travertine  of 
Tusciiny  is  (kq)osited  at  the  Baths  of  San  Yignone  at  the  rate  of  six  inches 
a  year,  at  San  FilipjK)  one  foot  in  four  months.  At  the  latter  locality  it 
has  been  i)iled  up  to  a  depth  of  at  legist  250  feet,  forming  a  hill  a  mile 
and  a  (juiirter  long  and  a  thinl  of  a  mile  broad.^ 

Chaly])eate  springs  give  rise  to  a  deposit  of  hydrous  peroxide  of  iron. 
This  has  already  been  referred  to  as  a  yellow  and  reddish-brown  deposit 
along  the  channels  of  the  water.  Some  acidulous  springs,  like  those  of 
the  Liuicher  See,  deposit  large  quantities  of  ochre.     In  undrained  districts 

^  The  rate  of  deposit  in  the  Ingleborough  Cave  is  stated  to  be  •2946  inch  per  annnm, 
or  about  2^  feet  in  a  century  (Boyd  Da'^^kins,  Brit.  Assoc.  1880,  sects,  p.  578).  This  is 
probably  an  exceptionally  i*apid  growth. 

-  Sorby,  Address  to  Geological  Society,  Q.  J.  Oeol.  Soc.  1879,  p.  42  et  aeq.  The  finely 
tibrous  structure  seen  in  chalce<lony  under  the  microscoi)e  with  polarized  light  passes  in  a 
similar  way  through  the  bands  of  growth  of  pebbles. 

*  Lyell,  *  Principles,'  i.  j>.  402.  At  Narni,  the  greater  the  velocity  of  flow,  the  greater 
the  de]>o>it  of  lime,  very  little  Iteing  deposited  in  stagnant  water.  The  amount  thrown  down 
increases  with  tem])erature  and  dist^mce  from  source,  exposure  to  the  air  being  necessary  for 
deposition.  B.  Fabri,  iVr*t\  Inst.  Cir.  Entjinfers,  xli.  (1876),  p.  246.  The  student  will  find 
much  detail  regarding  the  abstraction  and  deposit  of  carlK>nate  of  lime  by  subteirauean 
water  in  a  pai>er  by  Senft,  "  Die  Wanderungen  und  Wandelungen  des  kohlensiinren  Kalkes,*' 
Z.  D^iUtich.  <»>»»/.  <ies.  xiii.  p.  263. 


SECT,  ii  §  2  CHEMICAL  ACTION  OF  SPRINGS  367 

of  temperate  latitudes  in  Northern  Europe  and  America,  much  iron  is  also 
deposited  beneath  soil  which  rests  on  a  retentive  subsoil.  When  the 
descending  water  is  arrested  on  this  subsoil,  the  iron,  in  solution  as 
organic  salts  that  oxidize  into  ferrous  carbonate,  is  gradually  converted 
into  the  insoluble  hydrous  ferric  oxide,  which  is  precipitated  and  forms  a 
dark  ferruginous  layer,  known  to  Scottish  farmers  as  "  moorband  pan." 
So  effectually  does  this  layer  interrupt  the  drainage  that  the  soil  remains 
permanently  damp  and  unfertile.  But  when  the  "  pan  "  is  broken  up  and 
spread  over  the  surface  it  quickly  disintegrates,  and  improves  the  soil, 
which  can  then  be  properly  drained  (postea^  p.  483). 

Siliceous  springs  form  important  masses  of  sinter  round  the  point  of 
outflow.  The  basins  and  funnels  of  geysers  have  already  been  described 
(p.  235).  One  of  the  sinter-beds  in  the  Iceland  geyser  region  is  said 
to  be  two  leagues  long,  a  quarter  of  a  league  wide,  and  a  hundred  feet 
thick.  Enormous  beds  of  similar  material  have  been  formed  in  the 
Yellowstone  geyser  region.  Such  accumulations  usually  point  to  proximity 
to  former  volcanic  centres,  and  are  formed  during  one  of  the  latest  phases 
of  volcanic  action. 

(3)  Formation  of  subterranean  channels  and  caverns. — Measurement  of 
the  yearly  amount  of  mineral  matter  brought  up  to  the  surface  by  a 
spring,  furnishes  an  approximate  idea  of  the  extent  to  which  underground 
rocks  undergo  continual  loss  of  substance.  The  warm  springs  of  Bath, 
for  example,  with  a  mean  temperature  of  120°  Fahr.,  are  impregnated 
with  sulphates  of  lime  and  soda,  and  chlorides  of  sodium  and  magnesium. 
Sir  A.  C.  Ramsay  estimated  their  annual  discharge  of  mineral  matter 
to  be  equal  to  a  square  column  9  feet  in  diameter  and  1 40  feet  in  height. 
Again,  the  St.  Lawrence  spring  at  Lou^che  (Leak)  discharges  every 
year  1620  cubic  metres  (2127  cubic  yards)  of  dissolved  sulphate  of 
lime,  equivalent  to  the  lowering  of  a  bed  of  gypsum  one  square  kilometre 
(0*3861  square  mile)  in  extent,  more  than  16  decimetres  (upwards  of  five 
feet)  in  a  century.^ 

By  prolonged  abstraction  of  this  nature,  subterranean  tunnels,  channels, 
and  caverns  have  been  formed.  In  regions  abounding  in  rock-salt  deposits, 
the  result  of  the  solution  and  removal  of  these  by  underground  water  is 
visible  in  local  sinkings  of  the  ground  and  the  consequent  formation  of 
pools  and  lakes.  The  landslips  and  meres  of  Cheshire  are  illustrations  of 
this  process.  In  that  county,  owing  to  the  pumping  out  of  the  brine  in 
the  manufacture  of  salt,  tracts  of  ground  sometimes  more  than  100  acres 
in  extent  have  sunk  down  and  become  the  sites  of  lakes  of  varying  depth, 
some  being  45  feet  deep.^  In  calcareous  districts,  still  more  striking  effects 
are  observable.  The  ground  may  there  be  found  drilled  with  vertical 
cavities  {sioaMow-holes,  sinks,  dolinas),  by  the  solution  of  the  rock  along  lines 
of  joint  or  of  faults  that  serve  as  channels  for  descending  rain-water.  The 
line  of  outcrop  of  a  limestone-band,  among  non-calcareous  stnita,  may 
often  be  traced,  even  under  a  covering  of  superficial  deposits,  by  its  row 

^  E.  Recliw,  •  La  Terre,*  i.  p.  340. 

*  T.    Ward,    **  History  and  Cause  of  the  subsidences  at  Northwich,  &c."  1887,  Geol. 
Mag,  1887,  p.  617. 


368 


JfYXAMICAL  GEOLOGY 


BOOK  III  PART  II 


of  swallow-holes.  Surface-clminage,  thus  intercepted,  passes  at  onoe  under- 
ground, where,  in  course  of  time,  an  elaborate  system  of  spacious  tunnelB 
and  chambera  may  l>e  dissolved  out  of  the  solid  rock  (Fig.  111).     Sueh  has 


Fig.  10t>.— Section  of  a  Limestone  Cavern  (If.) 

1 1,  A  lini«.>Htone  hill,  )ierforat«<l  by  a  cavern  (b  6)  which  conininniGat«8  with  tho  ^'alley  (r)  by  an  opening 
(u).  The  botttnn  of  the  cavern  is  covered  with  ossiferons  loam,  above  which  lies  a  layer  of  stabg- 
mite  ((/  d\  while  stalactites  han^  from  the  roof,  and  by  Joining  the  floor  sepante  the  cavern  into 
two  chamWrrt. 

been  the  origin  of  the  Peak  caverns  of  Derbyshire,  the  intricate  grottos 
of  Antiparos  and  Adelsberg,  and  the  vast  lab3rrinths  of  the  Alammoth 
Cave  of  Kentucky.^  In  the  coiu^e  of  time,  the  imderground  rivers  open 
out  new  courses,  and  leave  their  old  ones  dry,  as  the  Poik  has  done  at 
Adelsberg.  By  the  falling  in  of  the  roofs  of  caverns,  a  communication  is 
established  with  the  surface,  and  land-shells  and  land-animals  fall  into  the 
holes,  or  the  caverns  are  used  as  dens  by  beasts  of  prey,  so  that  the 
remains  of  terrestrial  animals  are  preserved  under  the  stalagmite.  Not 
unfre(iuently  caverns,  once  open  and  freely  used  as  haunts  of  camivora, 
have  had  their  entmnces  closed  by  the  fall  of  debris,  as  at  d  in  Fig.  110, 


j^*^ 


Fi^.  110.— Sectiun  of  a  Llm-'stone  Cavcni  with  fullcn-in  roof  and  conctialed  entranci*  (i^) 

where  also  th(?  partial  filling-up  of  a  cavern  (a  a)  from  the  same  cause  is 
setin.  Where  the  collapse  of  a  cavern  roof  tiikes  place  l>elow  a  water- 
course, the  stream  is  engidfed.  In  this  way,  brooks  and  rivers  suddenly 
(lisapi)ear  from  the  surface,  and  after  a  long  subterranean  course,  issue 
again  in  a  totally  different  surface-area  of  river-drainage  from  that  in 
which  they  took  their  rise,  and  sometimes  vriih  volume  enough  to  1)6 

^  For  accounts  of  the  remarkable  houeyconibed  region  of  Camiola,  &c.,  see  Mojsisoyic5, 
'  Ot'ologie  von  Bosnien-IIercegovina,*  pp.  44-60  ;  Zeitsch.  Daitsch.  Al}>enfereins,  1880.  E 
Tiftze,  Jiihrh.  <ift,l.  Reichsunsi.  xxx.  (1880),  p.  729,  and  paperti  citetl  by  him.  Dr.  J.  H- 
Kloo.>  and  Dr.  Max  Miiller,  ilesoriptiou  and  photographs  of  the  Hermann's  Care  of  RiibeUnd 
jwick  (Weimar,  1889). 


y^J^uUil 


MECHANICAL  ACTION  OF  SPRINGS 


zed 


navig&ble  almost  up  to  their  outflow.  In  such  circumBtanceB,  lakes,  either 
temporary,  like  the  Lake  ZJrknJtz  in  Camiola,  or  perennial,  may  be 
formed  over  the  sites  of  the  broken-in  caverns  ;  and  valleys  may  thus  be 
deepened,  or  gorges  may  be  formed,'  Mud,  sand,  and  gravel,  with  the 
remains  of  plants  and  animals,  are  swept  below  ground,  and  eometimes 
accumulate  in  deposits  of  loam  and  breccia,  such  as  are  so  often  found  in 
ossiferous  caverns  (Figs,  109,  110). 

As  from  time  to  time  the  roofs  of  underground  chambers,  weakened 
)>y  the  constant  abstraction  of  mineral  matter,  collapse,  or  large  portions 
are  detached  from  them  and  fall  on  the  floors  below,  sudden  shocks  are 


generated  which  are  felt  above  ground  as  earthquakes.  In  subsiding 
to  fill  up  hollows  from  which  the  rock  has  been  removed  in  solution,  the 
overlying  strata  may  be  greatly  contorted  and  fractured,  those  underneath 
remaining  undisturbed. 

2.  Mechanical  Action. — In  its  passage  along  fissures  and  channels, 
underground  water  not  merely  dissolves  and  removes  materials  in 
solution,  it  likewise  loosens  finer  particles  and  carries  them  along  in 
mechanical  suspension.  This  removal  of  material  sometimes  produces 
remarkable  surface-changes  along  the  sides  of  steep  slopes  or  cliffs.  A 
'  3«e  iatemtiug  Hccounts  by  M.  Martcl  of  the  subUrTBnesn  chumela  of  the  Cauues  or 
Juiauic  limeatoDS  plateaux  ot  Gard  and  Loiere  In  the  South  of  France,  and  of  the  fomutiDa 
of  caHoaa  there.  Cmipt  rem/.  1888.  BuU.  Soc.  Olol.  France,  nil  (1889),  p.  910. 
2  B 


370  DYNAMICAL  GEOLOGY  book  m  paw ii 

thin  porous  layer,  such  as  loose  sand  or  ill-compacted  sandstone,  lying 
between  more  impervdous  rocks,  such  as  masses  of  clay  or  limestone,  and 
sloping  down  from  higher  ground,  so  as  to  come  out  to  the  surface  near 
the  base  of  a  line  of  abrupt  cliff,  serves  as  a  channel  for  undeirground 
water  which  issues  in  springs  or  in  a  more  general  oozing  at  the  foot  of 
the  decli\4ty.  Under  these  circumstances  the  support  of  the  overlying 
mass  of  rock  is  apt  to  l>e  loosened ;  for  the  water  not  only  removes  piece- 
meal the  sandy  layer  on  which  that  overlying  mass  rests,  but,  as  it  were, 
lubricates  the  rock  underneath.  Ck>nsequcntly,  at  intervals,  portions  ol 
the  upper  rock  break  off  and  slide  dovm  into  the  valley  or  plain  below. 
Such  dislocations  are  known  as  landslips,^  The  movement  may  be  gradual, 
as  in  the  case  of  the  Bee  Rouge  in  the  Tarentaise,  where  the  side  of  the 
mountain  is  slowly  ovenvhelming  the  village  of  Miroir,*  or  it  may  be 
sudden  and  disastrous. 

Along  sea-coasts  and  river  valleys,  at  the  base  of  cliffs  subject  to  continual  or  frequent 
removal  of  mateiial  by  running  water,  the  phenomena  of  landslips  are  best  seen.     The 

coast-line  of  tlie  British  IsUnds  abounds  with  in- 

^C^^     structivc  examples.   On  the  shores  of  Dorsetshini 

r^  h::: --.•.rr:  d    for  instance  (Fig.  112),  impervious  Liassic  cUjb 

p         f'^  {a)  are  overlain  by  porous  greensand  (6),  aboTe 

/^V^j-jy  which  lies  clialk  (c)  capped  with  irrayel  (d).    In 

nJ^i^^S^frQ^^^X^^  :y;:%y^  h    consequence  of  the  percolation  of  water  throug)! 

j~"  ^.^33      the  sandy  zone  (ft),  the  support  of  the  overlying 

''C'^  n   mass  is  destroyed,  and  hence,  from  time  to  time, 

Fig.  ii2.-8eotiou  of  Landslip  forming  segments  are  launched  down  towards  the  sea. 

undercliff,  Pinhay,  Lyme-Regis  (B.)  In  this  way,  a  confused  medley  of  mounds  and 

hollows  (/)  forms  a  characteristic  strip  of  ground 
termed  the  ''  Undercliff"  on  this  and  otlier  parts  of  the  English  coasts.  This  recession 
of  the  upper  or  inland  cliff  through  the  operation  of  springs  is  here  more  rapid 
than  that  of  the  lower  cliff  {g)  washed  by  the  sca.^  In  the  year  1839,  after  a  season 
of  wet  weather,  a  mass  of  chalk  on  the  same  coast  slipped  over  a  bed  of  clay  into 
the  sea,  leaving  a  rent  three-quarters  of  a  mile  long,  150  feet  deep,  and  240  feet  wide. 
The  shifted  mass,  bearing  with  it  liouses,  roads,  and  fields,  was  cracked,  broken,  and 
tilted  in  various  directions,  and  was  thus  prei>ared  for  further  attack  and  removal  by  the 
waves.'*  In  February  1891  a  mass  of  chalk-cliff  calculated  to  contain  some  10,000  tons 
of  material  gave  way  on  the  cliffs  to  the  east  of  Brighton,  and  fell  to  the  beach,  breaking 


^  Boltzer,  in  his  work  '*  Ueber  Bergstiirze  in  den  Alpen "  (Ziirich,  1875),  classifies 
Swiss  landslips  into  four  categories,  viz.,  1st,  Rock-falls  (Felssttlrze)  ;  2nd,  Earth -alipe 
(Erdschliffe) ;  3rd,  Mud-streams  (Schlaniinstrome),  where  soft  strata  saturated  with  water 
are  crushe<l  by  the  weight  of  overlying  rock  and  move  down  in  mass,  like  lava  ;  4th,  Mixed 
falls  (gemischte  Stilrze),  where,  as  in  most  instances,  rock,  earth,  and  mud  are  launched 
down  the  declivities.  More  recently  he  has  offered  another  classification  of  landslips, 
according  to  the  dimensions  of  the  mass  moved  and  the  solid  or  muddy  condition  of 
the  material,  Nevej<  Jahrb.  1880  (ii.),  p.  198.  See  A.  Rothpletz,  ZeUseh.  Deutsch.  OeoL 
(ies,  1881,  p.  540;  also  op,  cU.  1882,  pp.  430,  435.  E.  Buss  and  A.  Helm,  'Der 
Bergsturz  von  Elms,'  Ziuich,  1881. 

-  L.  Borrell,  BulL  Soc.  GH.  France,  ser.  3,  vi.  (1877),  p.  47. 

3  De  la  Beche,  '  Geol.  Observer,'  p.  22. 

**  Conybeare  and  Buckland's  '  Axmouth  LandsUp,'  London,  1840.     Lyell,  '  Principles,' L 

536. 


SECT,  ii  §  3  BROOKS  AND  RIVERS  371 


away  part  of  the  main  road  above.  In  March  1893  by  an  extensive  slipping  of  the 
Lower  Greensand  towards  the  beach  a  large  part  of  the  town  of  Sandgate  on  the  coast  of 
Kent  was  destroyed.  The  antiquity  of  many  landslips  is  shown  by  the  ancient  build- 
ings occasionally  to  be  seen  upon  the  fallen  masses.  The  undercliff  of  the  Isle  of 
Wight,  the  cliffs  west  of  Brandon  Head,  county  Kerry,  the  basalt  escarpments  of 
Antrim,  and  the  edges  of  the  great  volcanic  plateaux  of  Mull,  Skye,  and  Raasay, 
furnish  illustrations  of  such  old  and  prehistoric  landslips. 

On  a  more  imposing  scale,  and  interesting  from  its  melancholy  circumstances  being 
so  well  known,  was  the  celebrated  fall  of  the  Rossberg,  a  mountain  (a,  Fig.  113)  situated 
behind  the  Rigi  in  Switzerland,  rising  to  a  height  of 
more  than  5000  feet  above  the  sea.  After  the  rainy 
summer  of  1806,  a  large  part  of  one  side  of  the 
mountain,  consisting  of  steeply  sloping  beds  of  hard 
red  sandstone  and  conglomerate  (6),  resting  upon  soft 
sandy  layers  {e  c),  gave  way.     The  lubrication  of  the 

lower  surface  by  the  water  having  loosened  the  cohesion      ^    ,,„     „    ,.     ..i    *    *i     ^v 
r  XV  1    •  ^1  J      ^x  *      ,.1        1  Fig.  113.— Section  lUuatrating  the 

Of  the  overlying  mass,  thousands  of  tons  of  solid  rock,  p^n  ^f  the  Rossberg. 

set  loose  by  mere  gravitation,  suddenly  swept  across 

the  valley  of  Goldau  (rf),  burying  about  a  square  German  mile  of  fertile  land,  four  villages 

containing  330  cottages  and  outhouses,  with  457  inhabitants.^     In  1855  a  mass  of  debris, 

8500  feet  long,  1000  feet  wide,  and  600  feet  high,  slid  into  the  valley  of  the  Tiber, 

which,  dammed  back  by  the  obstruction,  overflowed  the  village  of  San  Stefano  to  a 

depth  of  50  feet,  until  drained  off  by  a  tunneh 


§  3.  Brooks  and  Rivers. 

These  will  be  considered  under  four  aspects : — (1)  sources  of  supply, 
(2)  discharge,  (3)  flow,  and  (4)  geological  action.'- 

1.  Sources  of  Supply. — Rivers,  as  the  natural  drains  of  a  land- 
surface,  carry  out  to  sea  the  surplus  water  after  evaporation,  together 
with  a  vast  amoimt  of  material  worn  off  the  land.  Their  liquid  contents 
are  derived  partly  from  rain  (including  mist  and  dew)  and  melted  snow, 
partly  from  springs.  In  a  vast  river-system,  like  that  of  the  Mississippi, 
where  the  area  of  drainage  is  so  extensive  as  to  embrace  different 
climates  and  varieties  of  rainfall,  the  amount  of  discharge,  being  in  a 
great  measure  independent  of  local  influences  of  weather,  remains 
tolerably  uniform,  or  is  subject  to  regular  periodically-recurrent  varia- 
tions. In  smaller  rivers,  such  as  those  of  Britain,  whose  basins  lie  in  a 
region  having  the  same  general  features  of  climate,  the  quantity  of  water 
is  regulated  by  the  local  rainfall.  A  wet  season  swells  the  streams,  a 
dry  one  diminishes  them.  Hence,  in  estimating  and  comparing  the 
geological  work  done  by  different  rivers,  we  must  take  into  account 
whether  or  not  the  sources  of  supply  are  liable  to  occasional  great 
augmentation  or  diminution.  In  some  rivers,  there  is  a  more  or  less 
regularly  recurring  season  of  flood  followed  by  one  of  drought.  The 
Nile,  fed  by  the  spring  rains  of  Abyssinia,  floods  the  plains  of  Egypt 

*  Zay,  *  Goldau  und  seine  Gegend. '  B&ltzer  Seu^  Jahri).  1875,  p.  15.  Upwards  of  150 
destructive  landslips  have  been  chronicled  in  Switzerland.     Riedl,  Nenes  Jahrb.  1877,  p.  916. 

^  An  excellent  monograph  on  a  river  is  C.  Lenth(''ric's  *■  Le  RhOne,  histoire  d'un  fleuve,' 
2  vols.  Paris,  1892. 


372  m'XAMICAL  GEOLOGY  book  m  part  ii 


every  summer,  rising  in  Upper  Eg\'pt  from  30  to  35  feet,  at  Cairo  23  to 
24  feet,  and  in  the  seaward  part  of  the  delta  al)out  4  feet.  The  Granges 
and  its  adjuncts  l)egin  to  rise  every  April,  and  continue  doing  so  until 
the  plains  ai*e  converted  into  a  vast  lake  32  feet  deep.  In  other  rivers, 
sudden  and  heavy  niins,  occurring  at  irregular  inten'als,  swell  the  usual 
volume  of  water  and  give  rise  to  floods,  freshets,  or  **  spates."  This  is 
markedly  the  case  with  the  rivers  of  Western  Europe.  Thus  the  Rhone 
sometimes  rises  Hi  feet  at  Lyons  and  23  feet  at  Avignon;  the  Sadne 
from  20  to  24 i  feet.  In  the  middle  of  March  1876,  the  Seine  rose  20 
feet  at  Paris,  the  Oise  17  feet  near  Compi^gne,  the  Mame  14  feet  at 
Damery.  The  Ardeche  at  Gournier  exceeded  a  rise  of  69  feet  during  the 
inundations  of  1827.^  The  causes  of  floods,  not  only  as  regards  meteoro- 
logical conditions,  but  in  respect  to  the  geological  structure  of  the 
ground,  merit  the  careful  attention  of  the  geological  student.  He 
may  occasionally  observe  that,  other  things  being  equal,  the  volume  of 
a  fltxxi  is  less  in  proportion  to  the  permeability  of  a  hydrographic  basin, 
and  the  consequent  case  with  which  rain  can  sink  beneath  the  surface. 

Were  rivers  entirely  dependent  upon  direct  supplies  of  rain,  they 
would  only  flow  in  rainy  seasons  and  disappear  in  drought.  This  does 
not  happen,  however,  l)ecause  they  derive  much  of  their  water  not 
directly  from  rain,  Init  indirectly  through  the  intermediate  agency  of 
springs.  Hence  they  contiinie  to  flow  even  in  very  dry  weather,  because, 
though  the  superficial  supplies  have  l)een  exhausted,  the  underground 
sources  still  continue  available.  In  a  long  drought,  the  latter  begin  at 
length  to  fail,  the  surface  springs  ceasing  fii'st,  and  gradually  drying  up 
in  their  order  of  depth,  until  at  last  only  deep-seated  springs  furnish  a 
perhaps  daily  diminishing  quantity  of  >vater.  Though  it  is  a  matter  of 
great  economic  as  well  as  scientific  interest  to  know  how  long  any  river 
would  continue  to  yield  a  certain  amount  of  water  during  a  prolonged 
drought,  no  nile  seems  iK)S8ible  for  a  generally  applicable  calculation, 
ever}'  area  having  its  own  peculiarities  of  underground  drainage,  and 
varying  gi'ciitly  from  year  to  year  in  the  amount  of  rain  which  is 
al>sorbed.  The  river  Wandle,  for  instance,  drains  an  area  of  51  square 
miles  of  the  chalk  downs  in  the  south-east  of  England.  For  eighteen 
months,  from  May  1858  to  October  1859,  as  tested  by  gauging,  there 
was  very  little  a]>soq)tion  of  rainfall  over  the  drainage  Wsin,  and  yet  the 
minimum  recorded  flow  of  the  Wandle  was  10,000,000  gallons  a  day, 
which  represents  not  more  than  '4090  inch  of  rain  absorbed  on  the  51 
square  miles  of  chalk.  The  rock  is  so  saturated  that  it  can  continue  to 
supply  a  large  Weld  of  water  for  eighteen  months  after  it  has  ceased  to 
receive  supplies  from  the  siuiace,  or  at  least  has  received  only  very  much 
diminished  supplies.- 

^  For  a  graphic  account  of  rivers  swollen  by  heavy  rainfall,  see  Sir  T.  D.  Laudei^s 
'  Morayshire  Floo«ls. '  On  torrents  consult  Surell  and  Cezanne,  *  j^tudes  sur  les  Torrents  des 
Hautes  Alpes.* 

-  Lucas,  *  Horizontal  Welk,'  London,  1874,  pi>.  40,  4L  See  also  Braithwaite,  Min,  Proc. 
Inst.  Or.  Kiujin.  xx.  It  is  much  to  be  desired  that  such  observations  as  those  of  Sir  J.  B. 
I^wes.  I)r.  Gilbert,  and  Sir  John  Evans  on  the  i)ercolation  of  rain  through  soils  and  chalk 


SECT,  ii  §  3  DISCHARGE  OF  RIVERS  373 


2.  Discharsre. — What  proportion  of  the  total  rainfall  is  discharged  by 
rivers  is  another  question  of  great  geological  and  industrial  interest. 
From  the  very  moment  that  water  takes  visible  form,  as  mist,  cloud,  dew, 
rain,  snow,  or  hail,  it  is  subject  to  evaporation.  When  it  reaches  the 
ground,  or  flows  off  into  brooks,  rivers,  lakes,  or  the  sea,  it  imdergoes 
continual  diminution  from  the  same  cause.  Hence  in  regions  where  rivers 
receive  no  tributaries,  they  grow  smaller  in  volume  as  they  move  onward, 
till  in  dry  hot  climates  they  even  disappear.  Apart  from  temperature, 
the  amount  of  evaporation  is  largely  regulated  by  the  nature  of  the 
surface  from  which  it  takes  place,  one  soil  or  rock  differing  from  another, 
and  all  of  them  probably  from  a  surface  of  water.  Full  and  detailed 
observations  are  still  wanting  for  determining  the  relation  of  evaporation 
to  rainfall  and  river  discharge.^  During  severe  storms  of  rain,  the  water 
discharged  over  the  land  finds  its  way,  to  a  very  large  extent,  at  once 
into  brooks  and  rivers,  by  which  it  reaches  the  sea.  Mr.  David  Stevenson 
remarks  that,  according  to  diff^erent  observations,  the  amount  carried  off" 
in  floods  varies  from  1  to  100  cubic  feet  per  minute  per  acre.^  In 
estimating  and  comparing,  therefore,  the  ratios  between  rainfall  and  river 
discharge  in  different  regions,  regard  must  be  had  to  the  nature  of  the 
rainfall,  whether  it  is  crowded  into  a  rainy  season  or  diffused  over  the 
year.  Thus,  though  floods  cannot  be  deemed  exceptional  phenomena, 
forming  as  they  do  a  part  of  the  regular  system  of  water-circulation  over 
the  land,  they  do  not  represent  the  ordinary  proportions  between  rainfall 
and  river  discharge  in  such  a  climate  as  that  of  Britain,  where  the  rainfall 
is  spread  more  or  less  equally  throughout  the  year.  According  to 
Beardmore's  table,*  the  Thames  at  Staines  has  a  mean  annual  discharge 
of  32*40  cubic  inches  per  minute  per  square  mile,  equal  to  a  depth  of  7*31 
inches  of  rainfall  run  off",  or  less  than  a  third  of  the  total  rainfall.  The 
most  carefully  collected  data  at  present  available  are  probably  those  given 
by  Humphreys  and  Abbot  for  the  basin  of  the  Mississippi  and  its  tribut- 
aries, as  shown  in  the  subjoined  table  ^  : — 

{Min.  Proc.  Inst.  Civ.  Engin,  xlv.  p.  208  ;  see  also  Greave.s,  op.  cit.  j).  19)  should  be  tried 
in  maiiy  different  areas. 

^  Id  the  present  state  of  our  information  it  seems  almost  useless  to  state  any  of  the 
results  already  obtained,  so  widely  discrepant  and  irreconcilable  are  they.  In  some  cases, 
the  evaporation  is  given  as  usually  three  times  the  rainfall :  and  that  evaporation  always 
excee<led  rainfall  was  for  many  years  the  belief  among  the  French  hydraulic  engineers.  (See 
AnnalM  d€8  Fonts -et-Chduas^,  1850,  p.  383.)  Observations  on  a  larger  scale,  and  with 
greater  precautions  against  the  undue  heating  of  the  evaporator,  have  since  shown,  as  might 
have  been  anticipated,  that  as  a  rule,  save  in  exceptionally  dry  years,  evaporation  is  lower 
than  rainfall.  As  the  average  of  ten  years  from  1860  to  1869,  Mr.  Greaves  found  that  at 
Lea  Bridge  the  evaporation  from  a  surface  of  water  was  20*946  inches,  while  the  rainfall  was 
25*534  (Symons's  British  Rain/all  for  1869,  p.  162).  But  we  need  an  accumulation  of 
observations,  taken  in  many  different  situations  and  exposures,  in  different  rocks  and  soils, 
and  at  various  heights  above  the  sea.  (For  a  notice  of  a  method  of  trying  the  evaporation 
from  soil,  see  British  Rain/all^  1872,  p.  206.) 

'  *  Reclamation  and  Protection  of  Agricultural  Laud,*  Edin.  1874,  p.  15. 

^  *  Hydrology,  p.  201.    Comp.  Report  of  Royal  Commission  on  Water  Supply,  1869,  p.  Uii* 

*  *  Physics  and  Hydraulics  of  the  Mississippi  River,'  Washington,  1861,  p,  186. 


374  DYXAMICAL  GEOLOGY  book  in  part  n 


Ratio  of  Dfachaise 
toRainfUL 
Ohio  River 0*24 

Missouri  River 0*15 

Upper  Mississippi  River 0*24 

Small  Tributaries 0*90 

Arkansas  and  White  River 0*15 

Red  River 0*20 

Yazoo  River 0*90 

St.  Francis  River 0*90 

Entire  Mississippi,  exclusive  of  Red  River  .        .        .0*25 

In  the  Mississippi  basin,  one-fourth  of  the  rainfall  is  thus  di8chai*ged 
into  the  sea.  The  Elbe,  from  the  beginning  of  July  1871  to  the  end  of 
June  1872,  was  estimated  to  carry  off  at  most  a  quarter  of  the  rainfall 
from  Bohemia.^  The  Seine  at  Paris  appears  to  carry  off  about  a  third  of 
the  rainfall.  In  Great  Britain  from  a  fourth  to  a  third  part  of  the  rain- 
fall is  perhaps  carried  out  to  sea  by  streams.^ 

In  comparing  also  the  discharges  of  different  rivers,  regard  should  be 
paid  to  the  influence  of  geological  structure,  and  particularly  of  the 
permeability  or  impermeability  of  the  rocks,  as  regulating  the  supply 
of  water  to  rivers.  Thus  the  Thames,  from  a  catchment  basin  of  3670 
square  miles  and  wnth  a  rainfall  of  27  inches,  has  a  mean  annual 
discharge  at  Kingston  of  1 250  millions  of  gallons  a  day,  and  rather  more 
than  688  millions  of  gallons  in  summer.  The  Severn,  on  the  other  hand, 
which  gathers  its  supplies  mainly  from  the  hard,  impervious  slate  hills  of 
Wales,  has  a  drainage  area  alwve  Gloucester  of  3890  square  miles,  with 
an  average  rainfall  of  probably  not  less  than  40  inches.  Yet  its  daily 
summer  discharge  does  not  amount  to  298  millions  of  gallons,  and  its 
minimum  sinks  as  low  as  100  millions  of  gallons,  while  that  of  the  Thames 
in  the  driest  season  never  falls  below  350  millions.  In  the  one  case,  the 
water  is  stored  up  within  the  rocks  and  is  dispensed  gradually ;  in  the 
other,  it  in  great  measure  runs  off  at  once.^  It  is  likewise  deserving  of 
note  that  the  operations  of  man,  particularly  in  draining  land  and 
deforesting,  may  materially  alter  the  mean  level  of  a  river  and  increase 
the  volume  of  floods.  The  mean  level  of  the  Elbe  at  Dresden  is  said  to 
have  been  perceptibly  diminished  by  human  interference,  while  in  the 
Rhine  the  low- water  level  has  been  lowered,  and  the  floods  have  been 
augmented.* 

*  Verliaiidl.  (ted.  lie.khmnsialt^  Vienna,  1876,  p.  173. 

-  In  mountainoiLs  tracts  having  a  large  rainfall  and  a  short  descent  to  the  sea,  the  pro- 
portion of  water  returned  to  the  sea  must  be  very  much  greater  than  this.  Mr.  Bateman's 
observations  for  seven  years  in  the  Loch  Katrine  district  gave  a  mean  annual  rainftdl  of  87^ 
inches  at  the  head  of  the  lake,  with  an  outflow  equivalent  to  a  depth  of  81  *70  inches  of  rain 
removed  from  the  drainage  basin  of  71}  square  miles.  See  a  paper  by  Graeve  on  the 
quantity  of  water  in  German  rivers,  and  on  the  relation  between  rainfall  and  dischaiige,  Der 
Civil- Ingenievr,  1879,  p.  591  ;  Satvre^  xxiii.  p.  94.     J.  Murray,  Scott,  Oeog,  Mag.  1887. 

'  Prestwich,  Q.  ./.  (rfol,  Soc.  xxviii.  p.  Ixv.  Compare  the  conditions  of  the  catchment 
basin  of  the  Seine  as  given  by  A.  Delaire,  Ann.  Conserv.  Arts  et  M&iers,  No.  138,  p.  385. 

*  Report  of  (Austrian)  Committee  on  Diminution  of  Water  in  Springs  and  Riyera,  Proc 
Inst.  Civ.  Entjineers,  xlii.  (1875),  p.  271. 


SECT,  ii  §  3  FLOJV  OF  RIVERS  376 

3.  Flow. — ^While,  in  obedience  to  the  law  of  gravitation,  a  river 
always  flows  from  higher  to  lower  levels,  great  variations  in  the  rate  and 
character  of  its  motion  are  caused  by  inequalities  in  the  angle  of  slope  of 
its  channel.  A  vertical  or  steeply  inclined  face  of  rock  originates  a  water- 
fall ;  a  rocky  declivity  in  the  channel  gives  rise  to  rapids ;  a  flat  plain 
allows  the  stream  to  linger  with  a  scarcely  visible  current ;  while  a  lake 
renders  the  flow  nearly  or  altogether  imperceptible.  Thus  the  rate  of 
flow  is  regulated  in  the  main  by  the  angle  of  inclination  and  form  of  the 
channel,  but  partly  also  by  the  volume  of  water,  an  increase  of  volume  in 
a  narrow  channel  increasing  the  rate  of  motion  even  without  an  increase 
of  slope.  ^ 

The  coxu^e  of  a  great  river  may  be  divided  into  three  parts : — 1.  The 
MourUain  Track, — ^where,  amidst  clouds  or  snows,  it  takes  its  rise  as  a 
mere  brook,  and,  fed  by  innumerable  similar  torrents,  dashes  rapidly 
down  the  steep  sides  of  the  mountains,  leaping  from  crag  to  crag  in 
endless  cascades,  and  growing  every  moment  in  volume,  until  it  enters 
lower  groimd.  2.  The  Valley  Track, — where,  now  flowing  through  lower 
hills  or  undulations,  the  stream  is  found  at  one  time  in  a  wide  fertile 
valley,  then  in  a  dark  gorge,  now  falling  headlong  into  a  cataract,  now 
expanding  into  a  broad  lake.  This  is  the  part  of  its  career  where  it 
assumes  the  most  varied  aspects,  and  receives  the  largest  tributaries. 
3.  The  Plain  Track, — where,  having  quitted  the  undulating  region,  the 
river  finally  emerges  upon  broad  plains,  probably  wholly  or  in  great  part 
composed  of  alluvial  formations  deposited  by  its  own  waters.  Here 
winding  sluggishly  in  wide  curves,  it  may  eventually  bifurcate,  as  it 
approaches  the  sea  and  spreads  through  its  delta,  enclosing  tracts  of  flat 
meadow  or  marsh,  and  finally,  amid  banks  of  mud  and  sand,  passing  out 
into  the  great  ocean.  In  Europe,  the  Rhine,  Rhone,  and  Danube ;  in 
Asia,  the  Ganges  and  Indus ;  in  America,  the  Mississippi  and  Amazon ; 
in  Africa,  the  Nile  and  Niger — illustrate  this  typical  course  of  a  great 
river. 

If  we  draw  a  longitudinal  section  of  the  course  of  any  such  river  or 
of  any  of  its  tributaries  from  its  source,  or  from  the  highest  peaks  around 
that  source,  to  its  mouth,  we  find  that  the  line  at  first  curves  steeply  from 
the  moimtain  crests  down  into  the  valleys,  but  grows  less  and  less  inclined 
through  the  middle  portion,  until  it  finally  can  hardly  be  distinguished 
from  a  horizontal  line.  This  feature,  however,  is  not  confined  to  stream 
courses  but  belongs  to  the  architecture  of  the  continents. 

It  is  evident  that  a  river  must  flow,  on  the  whole,  fastest  in  the  first 
portion  of  its  course,  and  slowest  in  the  last.  The  common  method  of 
comparing  the  fall  or  slope  of  rivers  is  to  divide  the  difference  of  height 
between  their  source  and  the  sea-level  by  their  length,  so  as  to  give  the 
declivity  per  mile.  This  mode,  however,  often  fails  to  bring  out  the  real 
resemblances  and  differences  of  rivers,  even  in  regard  to  their  angle  of 
slope.  For  example,  two  streams  rising  at  a  height  of  1000  feet,  and 
flowing  100  miles  to  the  sea,  would  each  have  an  average  slope  of  10  feet 
per  mile ;  yet  they  might  be  wholly  unlike  each  other,  one  making  its 

*  See  A.  Tylor  on  the  Laws  of  River-action,  Oeol.  Mag.  1875,  p.  443. 


376 


DYNAMICAL  GEOLOGY 


BOOK  in  PABT  U 


descent  almost  entirely  in  the  first  or  mountain  part  of  its  course,  and 
lazily  winding  for  most  of  its  way  through  a  vast  low  plain ;  the  other 
toiling  through  the  mountains,  then  keeping  among  hills  and  table-lands, 
so  as  to  form  on  the  whole  a  tolerably  equable  and  rapid  flow.  The  great 
rivers  of  the  globe  bive  prolmbly  a  less  average  slope  than  2  feet  per  mile, 
or  1  in  2640.  The  Missouri,  which  has  a  descent  of  28  inches  per  mile, 
is  a  tumultuous  rapid  ciurent  even  down  as  far  as  Kansas  City.  The 
average  slope  of  the  chaimel  of  the  Thames  is  2 1  inches  per  mile ;  of  the 
Shannon  about  11  inches  per  mile,  but  between  KiUaloe  and  Limerick 
about  6  J  feet  per  mile  ;  of  the  Nile,  below  Cairo,  3*25  to  5 '5  inches  per 
mile ;  of  the  DouIds  and  Rhone,  from  Besan9on  to  the  Mediterranean, 
24*18  inches  per  mile;  of  the  Volga  from  its  soiu'ce  to  the  sea,  a  little 
more  than  3  inches  per  mile.  Higher  angles  of  descent  are  those  <rf 
torrents,  as  the  Ai*ve,  with  a  slope  of  1  in  616  at  Chamounix,  and  the 
Durance,  whose  angle  varies  from  1  in  467  to  1  in  208.  The  Colorado 
river  rushes  through  its  caiions  with  an  average  declivity  of  7*72  feet  per 
mile,  or  1  in  683.  The  slope  of  a  navigable  river  ought  hardly  to  exceed 
10  inches  per  mile,  or  1  in  6336.^ 

But  not  only  does  the  rate  of  flow  of  a  river  vary  at  different  parts  erf 
its  course,  it  is  not  the  same  in  every  ^xirt  of  the  cross-section  of  the  river 


a 


d 


a 


V/Vy'/A/ 


taken  at  any  given  point.  A  rivOT 
channel  (Fig.  114)  supports  a  succes- 
sion of  layers  of  water  (a,  ft,  c^  rf), 
moWng  with  different  velocities,  die 
greatest  movement  l)eing  at  the  centre 
(//),  and  the  least  in  the  layer  which 
lies  directly  on  the  channel.     At  the 


7WAi 
'/  ^'//,  //f 

Fig.  114. — CrosR-soction  of  a  Rivpr. 

same  vertical  depth,  therefore,  the  velocity  is  greater  in  proportion  as  the 
point  approaches  the  centre  of  the  stream.  The  water  next  the  sides  and 
bottom  (a  a),  being  retarded  by  finction  against  the  channel,  moves  less 
rapidly  than  the  layers  (ft  ft,  c  c)  towards  the  centre  (il).  The  central  piers 
of  a  bridge  have  consequently  a  greater  velocity  of  river-current  to  bear 
than  those  at  the  banks.  The  motion  of  the  siuiace-water,  however,  is 
retarded,  on  the  other  hand,  by  upward  currents,  generated  chiefly  by 
irregularities  of  the  bottom.^  It  follows  that  whatever  tends  to  diminish 
the  friction  of  the  mo\dng  current  will  increase  its  rate  of  flow.  The  same 
l)ody  of  water,  other  conditions  being  equal,  ^vill  move  faster  through  a 
narrow  gorge  with  steep  smooth  walls  than  over  a  broad  rough  rocky  bed. 
For  the  same  reason,  when  two  streams  join,  their  united  ciurent,  having 
in  many  cases  a  channel  not  much  larger  than  that  of  one  of  the  single 
streams,  flows  faster,  because  the  water  encounters  now  the  friction  of 
only  one  channel.  The  average  rate  of  flow  is  much  less  than  might  be 
supposed,  even  in  what  are  termed  swift  rivers.  A  moderate  current  is 
about  \\  mile  in  the  hour;  even  that  of  a  torrent  does  not  exceed  18  or 
20  miles  in  the  hour.     Mr.  D.  Stevenson  states  that  the  velocity  of  such 


*  D.  Stevenson,  'Canal  and  River- Engineering,'  j),  224. 

-  J.   Tliomson,   Proc,    Roy,   Soc.  xxviii.   (1878),  p.  114.      Comp.  Collignon, 
d' Hytiraulique,'  p.  301. 


Ck>iirt 


SECT,  ii  §  3  GEOLOGICAL  ACTION  OF  RIVERS  377 

rivers  as  the  Thames,  the  Tay,  or  the  Clyde  may  be  found  to  vary  from 
about  one  mile  per  hour  as  a  minimum  to  about  three  miles  per  hour  as 
a  maximum  velocity.^ 

It  may  be  remarked,  in  concluding  this  part  of  the  subject,  that 
elevations  and  depressions  of  land  must  have  a  powerful  influence  upon 
the  slope  of  rivers.  The  upraising  of  the  axis  of  a  country,  by  increasing 
the  slope,  augments  the  rate  of  flow,  which,  on  the  contrary,  is  diminished 
by  a  depression  of  the  axis  or  by  an  elevation  of  the  maritime  regions. 

4.  Geological  Action. — Like  all  other  forms  of  moving  water,  streams 
have  both  a  chemical  and  iiiechanical  action.  The  latter  receives  most  atten- 
tion, as  it  undoubtedly  is  the  more  important ;  but  the  former  ought  not 
to  be  omitted  in  any  survey  of  the  general  waste  of  the  earth's  surface. 

i.  Chemical. — The  water  of  rivers  must  possess  the  powers  of  a 
chemical  solvent,  like  rain  and  springs,  though  its  actual  work  in  this 
respect  can  be  less  easily  measured,  seeing  that  river -water  is  directly 
derived  from  rain  and  springs,  and  necessarily  contains  in  solution 
minenil  substances  supplied  to  it  by  them.  Nevertheless,  that  streams 
dissolve  chemically  the  rocks  of  their  channels  can  be  strikingly  seen 
in  limestone  districts,  where  the  lower  portions  of  the  ravines  may  be 
found  enlarged  into  wide  cavities  or  pierced  with  tunnels  and  arches, 
presenting  in  their  smooth  surfaces  a  great  contrast  to  the  angular  jointed 
faces  of  the  same  rock  where  exposed  to  the  influence  only  of  the  weather. ^ 

Daubree  endeavoured  to  illustrate  the  chemical  action  of  rivers  uiwn  their  transported 
pebbles  by  exposing  angular  fragments  of  felspar  to  prolonged  friction  in  revolving 
cylinders  of  sandstone  containing  distilled  water.  He  found  that  they  unjierwent  con- 
siderable decomiX)sition,  as  was  shown  by  the  presence  of  silicate  of  potash,  rendering  the 
water  alkaline.  Three  kilogrammes  of  felspar  fragments  made  to  revolve  in  an  iron 
cylinder  for  a  period  of  192  hours,  which  was  equal  to  a  journey  of  460  kilometres  (287 
miles),  yielded  2.720  kilogrammes  of  mud,  while  the  five  litres  of  water  in  which  they 
were  kept  moving  contained  12-60  grammes  of  jwtash,  or  2-52  grammes  i>er  liti*e.^ 

The  mineral  matter  held  in  solution  in  river-water  is,  doubtless,  partly 
derived  from  the  mechanical  trituration  of  rocks  and  detritus ;  for 
Daubr^e's  experiments  show  that  minerals  which  resist  the  action  of  acid 
may  be  slowly  decomposed  by  mere  mechanical  trituration,  such  as  takes 
place  along  the  bed  of  a  river.  But  in  sluggish  streams  the  main  supply 
of  mineral  solution  is  doubtless  furnished  by  springs. 

^  The  proportion  of  mineral  matter  in  river -water  varies  with  the 
season,  even  for  the  same  stream.  It  reaches  its  maximum  when  the 
water  is  mainly  derived  from  springs,  as  in  very  dry  weather  and  during 
frost ;  it  attains  its  minimum  in  rainy  seasons  and  after  rain.*  Its 
amount  and  composition  depend  upon  the  nature  of  the  rocks  forming 
the    drainage-basin.      Where  these  are    on  the  whole   impervious,  the 

^  *  Reclamation  of  Land,'  p.  18. 

^  For  an  illustration  of  this  action  by  the  Rhone  in  the  marine  molasse,  see  F.  Cuvier, 
Bull.  Sue,  GM,  France f  3me  s^r.  viii.  p.  164. 

'  *  Geologic  Experimentale,  p.  271 ;  Fayol,  Bull.  Soc.  Giol.  Franc^j  8me  s^r.  xvi.  p.  996, 
posfea,  p.  385. 

*  Roth,  *Chem.  Geol.'  i.  p.  454. 


378  DYNAMICAL  GEOLOGY  book  hi  part  n 

water  runs  off  with  comparatively  slight  abstraction  of  minend 
ingredients ;  but  where  they  are  permeable,  the  water,  in  sinking  through 
them  and  rising  again  in  springs,  dissolves  their  substance  and  carries  it 
into  the  rivers. 

The  composition  of  the  river-waters  of  Western  Europe  is  well  shown  by  nnmeiroiu 
analyses.  The  substances  held  in  solution  include  variable  proportions  of  the  atmo* 
spheric  gases,  carbonates  of  lime,  magnesia,  soda,  iron,  and  ammonia  ;  silica  ;  peroxides 
of  iron  and  manganese  ;  alumina  ;  sulphates  of  lime,  magnesia,  potash,  and  soda ; 
chlorides  of  sodium,  potassium,  calcium,  and  magnesium  ;  silicate  of  potash  ;  nitrates ; 
phosphoric  acid  ;  and  organic  matter.  The  minimum  proportion  of  mineral  matter 
among  the  analyses  collected  by  Bischof  was  2*61  in  100,000  ])arts  of  water  in  the  Moll, 
near  Heiligenblut — a  mountain  stream  3800  feet  above  the  sea,  flowing  from  the 
Pasterzen  glacier  over  crystalline  schists.  On  the  other  hand,  as  much  as  54*5  parts  in 
the  100,000  were  obtained  in  the  waters  of  the  Beuvronne,  a  tributary  of  the  Loire 
above  Tours.  The  average  of  the  whole  of  these  analyses  is  about  21  parts  of  mineral 
matter  in  100,000  of  water,  whereof  carbonate  of  lime  usually  forms  tlie  half,  its  mean 
quantity  being  11*34.^  Bischof  calculated  that,  assuming  the  mean  qmuitity  of  carbonate 
of  lime  in  the  Rhine  to  be  9*46  in  100,000  of  water,  which  is  the  proportion  ascertained 
at  Bonn,  enough  of  this  substance  is  carried  into  the  sea  by  this  river  for  the  annual 
formation  of  three  hundred  and  thirty-two  thousand  millions  of  oyster-shells  of  the 
usual  size.  The  mineral  next  in  abundance  is  sulphate  of  lime,  which  in  some  liven 
constitutes  nearly  half  of  the  dissolved  mineral  matter.  Less  in  amoimt  are  sodium 
chloride,^  magnesium  carbonate  and  sulphate,  and  silica.  Of  the  last-named,  a  per- 
centage amounting  to  4*88  parts  in  100,000  of  water  has  been  found,  in  the  Rhine,  near 
Strasburg.'  Tlie  largest  amount  of  alumina  was  0-71  in  the  Loire,  near  Orleans.  The 
proportion  of  mineral  matter  in  the  Thames,  near  London,  amounts  to  about  33  parts  in 
100,000  of  water.* 

It  requires  some  reflection  proi>erly  to  appreciate  the  amount  of  solid  mineral  matter 
which  is  every  year  carried  in  solution  from  the  rocks  of  the  land  and  difiused  by  rivers 
into  the  sea.  Accurate  measurements  of  the  amount  of  material  so  transported  are  still 
much  required.  The  Thames  carries  past  Kingston  19  grains  of  mineral  salts  in  eveiy 
gallon,  or  1502  tons  every  twenty-four  hours,  or  548,230  tons  every  year.  Of  thii 
quantity  about  two-thirds  consist  of  carbonate  of  lime,  the  rest  being  chiefly  sulphate  of 
lime,  with  minor  proportions  of  the  other  ordinaiy  salts  of  river- water.  Mr.  Prestwich 
estimates  that  the  quantity  of  carbonate  of  lime  removed  from  the  limestone  areas  of  the 
Thames  basin  amounts  to  140  tons  annually  from  every  S([uare  mile.  This  quantity, 
assuming  a  ton  of  chalk  to  measure  15  cubic  feet,  is  equal  to  a  loss  of  ^^  of  an  inch  from 
each  square  mile  in  a  century,  or  one  foot  in  13,200  years.'  According  to  monthly 
observations  and  estimates  made  in  the  year  1866  at  Lobositz,  near  the  exit  of  the  Elbe 
from  its  Bohemian  basin,  this  river  may  be  regarded  as  carrying  every  year  oat  of 
Bohemia  from  an  area  of  880  German  square  miles,  or,  in  round  numbers,  20,000  English 

^  BLschof,  'Cheiii.  Geol.'  1.  chap.  v.  Of  the  analyses,  chiefly  of  European  rivers, 
published  by  Roth,  the  mean  of  thirty-eight  gives  a  proportion  of  19*988  in  100,000  parts 
of  water.     Op,  cit.  p.  456.     Compare  I.  C.  Russell,  Bull,  U,S.  Geol,  Surv,  1889. 

^  On  the  variations  of  the  chlorine  in  the  Nile  and  Thames,  see  J.  A.  Wanklyn,  Chem. 
News,  xxxii.  (1875),  pp.  207,  219. 

'  Of  the  total  solid  matter  dissolved  in  the  water  of  the  River  Uruguay  as  much  as  abont 
46  per  cent  consists  of  soluble  si  Ilea,  "chiefly  as  hydrated  silicic  acid.  Hence  the  "  petrifying  ** 
property  of  the  water.     J.  Kyle,  Chern,  News^  xxxviii.  (1878),  p.  28, 

"*  Bischof,  op,  et  loc,  cit.  ;  Roth,  op.  cit.  i.  p.  454.  For  composition  of  British  river- 
water,  see  'Rivers  Pollution  Commission  Report.' 

*  Prestwich,  Q,  J,  Geol,  Soc.  xxviii.  p.  Ixvii. 


SECT,  ii  §  3  GEOLOGICAL  ACTION  OF  RIVERS  379 

square  miles,  6,000,000,000  cubic  metres  of  water,  containing  622,680,000  kilogrammes 
of  dissolved  and  547,140,000  of  suspended  matter,  or  a  total  of  1169  millions  of  kilo- 
grammes. Of  this  total,  978  millions  of  kilogrammes  consist  of  fixed  and  192  millions 
of  volatile  (chiefly  organic)  matter.  The  proportions  of  some  of  the  ingredients  most 
important  in  agriculture  were  estimated  as  follows :  lime,  140,380,000  kilogrammes ; 
magnesia,  28,130,000  ;  potash,  64,520,000  ;  soda,  39,600,000  ;  chloride  of  sodium, 
25,320,000  ;  sulphuric  acid,  45,690,000  ;  phosphoric  acid,  1,500, 000.  ^ 

Mr.  T.  Mellard  Reade  has  estimated  that  a  total  of  8,370,630  tons  of  solids  in 
solution  is  every  year  removed  by  running  water  from  the  rocks  of  England  and  Wales, 
which  is  equivalent  to  a  general  lowering  of  the  surface  of  the  country,  from  that  cause 
alone,  at  a  rate  of  '0077  of  a  foot  in  a  century,  or  one  foot  in  12,978  years.  The  same 
writer  computes  the  annual  discharge  of  solids  in  solution  by  the  Rhine  to  be  equal  to 
92-3  tons  per  square  mile,  that  of  the  Rhone  at  Avignon  282  tons,  that  of  the  Danube 
72*7  tons,  and  that  of  the  Mississippi  120  tons.  He  supposes  that  on  an  average  over 
the  whole  world  there  may  be  every  year  dissolved  by  rain  about  100  tons  of  rocky 
matter  jier  English  square  mile  of  surface.' 

If  the  average  proportion  of  mineral  matter  in  solution  in  river-water 
be  taken  as  only  2  parts  in  every  10,000  by  weight,  then  it  is  obvious 
that  in  every  5000  years  the  rivers  of  the  globe  must  carry  to  the  sea 
their  own  weight  of  dissolved  rock. 

ii.  Mechanical. — The  mechanical  work  of  rivers  is  threefold: — (1) 
to  transport  mud,  sand,  gravel,  or  blocks  of  stone  from  higher  to  lower 
levels ;  (2)  to  use  these  loose  materials  in  eroding  their  channels ;  and  (3) 
to  deposit  these  materials  where  possible,  and  thus  to  make  new  geological 
formations.3 

1.  Transporting  Power .^ — One  of  the  distinctions  of  river-water,  as 
compared  with  that  of  springs,  is  that  as  a  rule,  it  is  less  transparent,  in 
other  words,  contains  more  or  less  mineral  matter  in  suspension.^  A 
sudden  heavy  shower,  or  a  season  of  wet  weather,  suffices  to  render  turbid 
a  river  which  was  previously  clear.  The  mud  is  washed  into  the  main 
streams  by  rain  and  brooks,  but  is  partly  produced  by  the  abrasion  of  the 
water-channels  through  the  operations  of  the  streams  themselves.  The 
channels  of  the  mountain-tributaries  of  a  river  are  choked  with  large  frag- 
ments of  rock  disengaged  from  cliffs  and  crags  on  either  side.  Traced 
downwards,  the  blocks  become  gradually  smaller  and  more  rounded. 
They  are  ground  against  each  other  and  upon  the  rocky  sides  and  bottom 
of  the  channel,  becoming  more  and  more  reduced  as  they  descend,  and  at 
the  same  time  abrading  the  rocks  over  or  against  which  they  are  driven. 
Of  the  detritus  thus  produced,  the  finer  portions  are  carried  in  suspension, 

*  Breitenlohner,  Verhand.  Oeol.  ReichsansL  Vienna,  1876,  p.  172.  Taking  the 
978,000,000  kilogrammes  to  be  mineral  matter  in  solution  and  suspension,  this  is  equal  to 
an  aonual  loss  of  about  48  tons  per  English  square  mile.  But  it  includes  all  the  materials 
discharged  by  the  drainage  of  an  abundant  population. 

^  Addresses,  Liverpool  Oeol.  Soc.  1876  and  1884. 

'  On  the  behaviour  of  rivers,  consult  Dausse,  *  Etudes  relatives  aux  inondations,*  Paris, 
1872. 

•*  See  Login,  Naturey  i.  pp.  629,  654  ;  ii.  p.  72. 

*  The  brown  colour  of  river  or  estuary  water  is  not  always  due  to  mud.  In  the  Southamp- 
ton Water  it  is  caused  in  summer  by  the  presence  of  protozoa  {Peredinium  fuscum).  A. 
Angell,  Bri(.  Assoc,  1882,  sects,  p.  589. 


380  DYXAMWAL  GEOLOGY  book  hi  part  ii 

and  impart  the  characteristic  turbidity  to  rivers ;  the  coarser  sand  and 
gi-avel  are  driven  along  the  river-bottom.^ 

The  presence  of  a  moving  stratum  of  coarse  detritus  on  the  bed  of  a 
brook  or  river  may  be  detected  in  transit,  for  though  invisible  beneath 
the  overlying  discoloured  wat^r,  the  stones  of  which  it  is  composed  may 
be  hetird  knocking  against  each  other  as  the  current  sweeps  them  onward. 
Al>ove  Bonn,  and  again  a  little  below  the  Lurelei  Rock,  while  drifting 
down  the  Khine,  the  observer,  by  laying  his  ear  close  to  the  bottom  of  the 
0])en  boiit,  may  hear  the  harsh  gmting  of  the  gravel-stones  over  each 
other,  as  the  current  pushes  them  onwards  along  the  bottom.  On  the 
Moselle  also,  between  Cochem  and  Coblentz,  the  same  fact  may  be 
noticed. 

The  transporting  capacity  of  a  stream  depends  («)  on  the  volume  and 
velocity  of  the  current,  (h)  on  the  size,  shape,  and  specific  gra\'ity  of  the 
sediment,  and  (c)  pjirtly  on  the  chemical  composition  of  the  water,  (a) 
According  to  the  cAlcidations  of  Hopkins,^  the  capacity  of  transport 
increases  as  the  sixth  power  of  the  velocity  of  the  current ;  thus  the 
motive  power  of  the  current  is  increased  64  times  by  the  doubling  of  the 
velocity,  729  times  by  trebling,  and  4096  times  by  quadnipliug  it.  If  a 
stream  which,  in  its  ordinary  state,  can  just  move  pebbles  weighing  an 
ounce,  has  its  velocity  doubled  by  a  flood,  it  can  then  sweep  forward 
stones  weighing  4  lb.  Mr.  David  Stevenson*  gives  the  subjoined  table 
of  the  power  of  transport  of  different  velocities  of  river  currents : — 

Tn.  i>er         Mile  per 
Second.  Hour. 

3  =  0'170    will  just  begin  to  work  on  fine  clay. 

6  =  0-340    will  lift  fine  sand. 

8  =  0*4545  will  lift  sand  as  coarse  as  linseed. 

12  =  0*6819  will  sweep  along  fine  gravel. 

24  =  1*3638  will  roll  along  ronnded  pebbles  1  inch  in  diameter. 

36  =  2*045    will  sweep  along  slippery  angular  stones  of  the  size  of  an  egg. 

It  is  not  the  surface  velocity,  nor  even  the  mean  velocity,  of  a  river  which 
can  be  taken  as  the  measure  of  its  power  of  transport,  but  the  bottom 

*  These  operations  of  rumiiiig  water  may  be  studied  with  great  advantage  on  a  small 
scale,  wliere  brooks  descend  from  high  grounds  into  valleys,  rivers,  or  lakes.  A  single  flood 
sufiices  for  the  transport  of  thousands  of  tons  of  stones,  gravel,  sand,  and  mud,  even  by  a 
small  streamlet.  At  Lybster,  for  exam]>le,  on  the  coast  of  Caithness,  as  the  author  "wms 
informed  by  Mr.  Thomas  Stevenson,  C.E.,  a  small  streamlet  carries  down  annually  into  a 
harbour  which  has  there  been  made,  between  400  and  500  cubic  yards  of  gravel  and  sand. 
A  weir  or  dam  has  been  constructed  to  protect  the  harbour  from  the  inroad  of  the  coarser 
sediment,  and  this  is  cleaned  out  regularly  every  summer.  But  by  far  the  greater  portion  of 
the  tine  silt  is  no  doubt  swept  out  into  the  North  Sea.  The  erection  of  the  artificial  barrier, 
by  arresting  the  seaward  course  of  the  gravel,  reveals  to  us  what  must  be  the  normal  state  of 
this  stream  and  of  similar  streams  descending  from  maritime  hills.  The  area  drained  by  the 
stream  is  about  four  square  miles  ;  consequently  the  amount  of  loss  of  surface,  which  is  re- 
presented by  the  coarse  gravel  and  saud  alone,  is  ^g^o^  of  a  foot  per  annum, 

^  Q.  J.  OeoL  Si)c.  viii.  i>.  xxvii. 

^  *  Canal  and  River  Engineering,'  p.  315.  See  also  Thoulet,  Ann.  des  Afines^  1884, 
p.  507. 


SECT,  ii  §  3  GEOLOGICAL  ACTION  OF  FIVERS  381 

velocity — that  is,  the  rate  at  which  the  stream  overcomes  the  friction  of 
its  chaimel.  (b)  The  average  specific  gravity  of  the  stones  in  a  river 
ranges  between  two  and  three  times  that  of  pure  fresh  water ;  hence  these 
stones  when  borne  along  by  the  river  lose  from  a  half  to  a  third  of 
their  weight  in  air.  Huge  blocks  which  could  not  be  moved  by  the  same 
amount  of  energy  applied  to  them  on  dry  ground,  are  swept  along  when 
they  have  found  their  way  into  a  strong  river-current.  The  shape  of  the 
fragments  greatly  affects  their  portability,  when  they  are  too  large  and 
heavy  to  be  carried  in  mechanical  suspension.  Rounded  stones  are  of 
course  most  easily  transported :  flat  and  angular  ones  are  moved  with 
comparative  difficulty  (see  p.  385).  (c)  Piu-e  water  will  retain  fine  mud 
in  suspension  for  a  long  time ;  but  the  introduction  of  mineral  matter  in 
solution  diminishes  its  capacity  to  do  so,  probably  by  lessening  the  mole- 
cular cohesion  of  the  liquid.  Thus  the  mingling  of  salt  with  fresh  water 
causes  a  rapid  precipitation  of  the  suspended  mud  (p.  398).  Probably  each 
variety  of  river-water  has  its  own  capacity  for  retaining  mineral  matter  in 
suspension,  so  that  the  mere  mingling  of  these  varieties  may  be  one  cause 
of  the  precipitation  of  sediment.^ 

Besides  inorganic  sediment,  rivers  sweep  seaward  the  remains  of 
land-animals  and  vegetation.  The  great  rafts  of  the  Mississippi  and  its 
tributaries  are  signal  examples  of  this  part  of  river-action.  The  Atcha- 
falaya  has  been  so  obstructed  by  drift-wood  as  to  be  fordable  like  dry  land, 
and  the  Red  River  for  more  than  a  hundred  miles  flows  under  a  matted 
cover  of  dead  and  living  vegetation.  The  Amazon,  Ganges,  and  other 
tropical  rivers  furnish  abundant  examples  of  the  transport  of  a  terrestrial 
fauna  and  flora  to  the  sea.  Minute  forms  of  life  sometimes  constitute  a 
considemble  proportion  of  the  so-Ciilled  "  solid  impurity  "  of  river-water. 
The  mud  of  the  Ganges,  for  instance,  is  estimated  to  contain  from  12  to 
25  per  cent  of  infusoria,  and  that  of  the  Nile  4*6  to  10  per  cent. 

Beyond  their  ordinary  powers  of  transport,  rivers  gain  at  times  con- 
siderable additional  force  from  several  causes.  Those  liable  to  sudden  and 
heavy  falls  of  rain,  or  to  a  rapid  augmentation  of  their  volume  l)y  the 
quick  melting  of  snow,  acquire  by  floo<ling  an  enormous  increase  of 
transporting  and  exaivating  power.  More  work  may  thus  l)e  done  by  a 
stream  in  a  day  than  could  be  accomplished  by  it  during  years  of  its 
ordinary  condition.'^     Another  cause  of  sudden  increase  in  the  efficacy  of 

*  T.  Sterry  Hunt,  Proc.  Boston  Xat,  Hist.  Soc.  1874  ;  W.  Durham,  Chem.  Xews,  xxx. 
(1874),  p.  57  ;  xxxvii.  (1878),  p.  47  ;  W.  Ramsay,  Quart.  Journ.  Geol.  JSoc.  xxxii.  (1876), 
p.  129  ;  C.  Banis.  BuU.  C.S.  Geol.  Surv.  No.  36  (1886)  ;  Tlioulet,  Ann. Mines,  xix.  (1891). 
p.  5.  lu  this  last  memoir  M.  Tboulet  concludes  as  the  result  of  his  experiments  that  tlie 
precipitation  of  clays  takes  place  in  fresh  water  which  has  had  an  addition  of  ten  per  cent 
of  sea- water  (and  consequently  of  density  equal  to  1  '002)  exactly  as  in  pure  sea-water,  and 
that  this  observation  furnishes  a  measure  for  determining  the  true  limits  of  the  ocean  and 
the  continents. 

^  The  extent  to  whicli  heavy  rains  can  alter  the  usual  characters  of  rivers  is  forcibly 
exemplified  in  Sir  T.  Dick  Lauder's  'The  Morayshire  Floods.'  In  the  year  1829  the  rivers 
of  that  region  rose  10,  18,  and  in  one  case  even  50  feet  above  their  common  summer  level, 
producing  almost  incredible  havoc.  See  also  G.  A.  Koch,  "  Ueber  Murbriiche  in  Tyrol," 
Jahrb.  Geol.  Reichsanst.  xxv.  (1875),  p.  97. 


382  DYNAMICAL  GEOLOGY  book  m  paw  n 

river-action  is  provided  when,  from  landslips  formed  by  earthquakes,  by  the 
undermining  influence  of  springs,  or  otherwise,  a  stream  is  temporarily 
dammed  back,  and  the  barrier  subsequently  gives  way.  The  bursting  out 
of  the  arrested  waters  produces  great  destruction  in  the  valley.  Blocks  as 
big  as  houses  may  be  set  in  motion,  and  carried  down  for  consideraUe 
distances.  Again,  the  transporting  power  of  rivers  may  be  greatly 
augmented  by  frost  {see  postea,  p.  415).  Ice  forming  along  the  banks  or 
on  the  bottom,  encloses  gravel,  sand,  and  even  blocks  of  rock,  which,  when 
thaw  comes,  are  lifted  up  and  carried  down  the  stream.  In  the  rivers  d 
Northern  Russia  and  Siberia,  which,  flowing  from  south  to  north,  have  the 
ice  thawed  in  their  higher  courses  before  it  breaks  up  farther  down,  much 
disaster  is  sometimes  caused  by  the  piling  up  of  the  ice,  and  then  by  the 
bursting  of  the  impeded  river  through  the  temporary  ice -barrier.  In 
another  way,  ice  sometimes  vastly  increases  the  destructive  power  of  small 
streams,  where  avalanches  or  an  advancing  glacier  cross  a  valley  and  pond 
back  its  drainage.  The  valley  of  the  Dranse,  in  Switzerland,  has  several 
times  suffered  from  this  cause.  In  1818,  the  glacier-barrier  extended  acrosB 
the  valley  for  more  than  half  a  mile,  with  a  breadth  of  600  and  a  height 
of  400  feet.  The  waters  above  the  ice -dam  accumulated  into  a  lake 
containing  800,000,000  cubic  feet.  By  a  tunnel  driven  through  the  ice^ 
the  water  was  drawn  off  without  desolating  the  plains  below. 

The  amount  of  sediment  borne  downwards  by  a  river  is  not  necessarily 
determined  by  the  carrying  power  of  the  ciu'rent.  The  swiftest  streams 
are  not  always  the  muddiest.  The  proportion  of  sediment  is  partly 
dependent  upon  the  hardness  or  softness  of  the  rocks  of  the  channel, 
the  number  of  tributaries,  the  nature  and  slope  of  the  ground  forming 
the  di-ainage-basin,  the  amoimt  and  distribution  of  the  rainfall,  the  size 
of  the  glaciers  (where  such  exist)  at  the  sources  of  the  river,  the  chemical 
composition  of  the  water,  and  probably  other  causes.  A  rainfall  spread 
with  some  uniformity  throughout  the  year  may  not  sensibly  darken  the 
rivers  with  mud,  but  the  same  amount  of  fall  crowded  into  a  few  days 
or  weeks  may  be  the  means  of  sweeping  a  vast  amoimt  of  earth  into  the 
rivers,  and  sending  them  down  in  a  greatly  discoloured  state  to  the  sea. 
Thus  the  rivers  of  India,  swollen  diuing  the  rainy  season  (sometimes  by 
a  niinfall  of  25  inches  in  40  hours,  as  at  the  time  of  the  destructive 
landslip  at  Naini  Tal  in  September  1880),  become  rolling  currents  of 
mud.^ 

The  amoimt  of  mineral  matter  transported  by  rivers  can  bo  cstinLated  by  examining 


*  la  his  journeys  through  equatorial  Africa,  Liviugstoue  came  upon  rivers  which  appemr 
usually  to  consist  more  of  sand  than  of  water.  lie  describes  the  Zingesi  as  "a  sand-rivnlet 
in  flood,  60  or  70  yards  wide,  and  waist  dee]).  Like  all  these  sand-rivers,  it  is  for  the  most 
part  dry  ;  but,  by  digging  down  a  few  feet,  water  is  to  be  found  which  is  percolating  along 
the  bed  on  a  stratum  of  clay.  In  trying  to  ford  it,"  he  remarks,  "  I  felt  thousands  of 
jmrticles  of  coarse  sand  striking  my  legs,  which  gave  me  the  idea  that  the  amount  of  matter 
removed  by  every  freshet  must  be  very  great.  .  .  .  These  sand-rivers  remove  vast  masses 
of  disintegrated  rock  before  it  is  fine  enough  to  form  soil.  In  most  rivers  where  mnch  wear- 
ing is  going  on,  a  person  diving  to  the  bottom  may  hear  literally  thousands  of  stones 
knocking  against  each  other. " 


SECT,  ii  §  3  GEOLOGICAL  ACTION  OF  RIVERS  383 

their  waters  at  different  periods  and  places,  and  determining  their  solid  contents.  A 
complete  analysis  should  take  into  account  what  is  chemically  dissolved,  what  is 
mechanically  suspended,  and  what  is  driven  or  pushed  along  the  bottom.  We  have 
already  dealt  with  the  chemically  dissolved  ingredients.  In  determinations  of  the 
mechanically  mixed  constituents  of  river-water,  it  is  most  advantageous  to  obtain  the 
proportion  first  by  weight,  and  then  from  its  average  specific  gravity  to  estimate  its 
bulk  as  an  ingredient  in  the  water.  According  to  experiments  made  upon  the  water  of 
the  Rhone  at  Lyons,  in  1844,  the  proportion  of  earthy  matter  held  in  suspension  was 
by  weight  Trhnr*  Earlier  in  the  century  the  results  of  similar  experiments  at  Aries 
gave  j^Tf  as  the  proportion  when  the  river  was  low,  yfy  during  floods,  and  ttjW  i^  the 
mean  state  of  the  river.  The  greatest  recorded  quantity  is  tV  ^y  weight,  which  was 
found  **when  the  river  was  two- thirds  up,  with  a  mean  velocity  of  probably  about  8 
feet  per  second."  *  A.  Gu^rard,  who  has  more  recently  made  observations  at  the  mouth  of 
this  river,  estimates  the  total  annual  discharge  of  sediment  to  amount  to  23,540,000  cubic 
yards,  or  t^Vt  o^  the  volume  of  the  water. ^  Lombardini  gives  s^  as  the  proportion  by 
volume  of  the  sediment  in  the  water  of  the  Po.  In  the  Vistula,  according  to  Spittell, 
the  proportion  by  volume  reaches  a  maximum  of  ■^.'  The  Rhine,  according  to 
Hartsoeker,  contains  j-^  by  volume  as  it  passes  through  Holland,  while  at  Bonn  the 
experiments  of  L.  Horner  gave  a  proportion  of  only  rvhnf  by  volume.*  Stiefensand 
found  that,  after  a  sudden  flooding,  the  water  of  the  Rhine  at  Uei-dingen  contained 
tAt  by  weight.  Bischof  measured  the  quantity  of  sediment  in  the  same  river  at  Bonn 
during  a  turbid  state  of  the  water,  and  found  the  proportion  to  be  y^  by  weight, 
while  at  another  time,  after  several  weeks  of  continuous  dry  weather,  and  when  the 
water  had  become  clear  and  blue,  he  detected  only  Tyirnr-*  ^^  the  Meuse,  according  to 
the  experiments  of  Chandellon,  the  maximum  of  sediment  in  sus^jension  in  the  month 
of  December  1849  was  ^^,  the  minimum  jrhifi  a^^d  the  mean  TTk^Tnr**  In  the  Elbe, 
at  Hamburg,  the  proportion  of  mineral  matter  in  suspension  and  solution  has  been 
found  by  experiment  to  average  'about  y^v-  The  Danube,  at  Vienna,  yielded  to 
Bischof  about  ^^W  o^  suspended  and  dissolved  matter.^  Tlie  Durance  has  ordinarily  a 
maximum  of  30  grammes  of  sediment  to  one  litre  of  water,  or  ^  by  weight.  In 
exceptional  floods  it  rises  to  100  grammes  per  litre  of  water,  or  ^  by  weight.  In 
extreme  low  water  the  proportion  may  sink  to  about  y^its  5  the  average  for  nine  years 
from  1867  to  1875  was  about  yIv®  The  Garonne  is  estimated  to  contain  perhaps  y^.' 
In  the  Avon,  which  falls  into  the  Severn,  the  mean  amount  of  suspended  mud  is 
estimated  at  s^^.^^  The  observations  of  Mr.  Everest  upon  the  water  of  the  Ganges 
show  that,  during  the  four  months  of  flood  in  that  river,  the  proportion  of  earthy 
matter  is  ^fr  l>y  weight,  or  -g^  by  volume  ;  and  that  the  mean  average  for  the  year 
is  yi-ff  by  weight,  or  toVt  by  volume."  According  to  Mr.  Login,  the  waters  of  the 
Irrawaddy  contain  y^Vff  by  weight  of  sediment  during  floods,  and  ^^Vt  during  a  low 

*  Surell,  "  Memoire  snr  I'am^lioration  des  embouchures  du  Rhdne."  Humphreys  and 
Abbot,  'Report  upon  the  Physics  and  HydrauHcs  of  the  Mississippi,  1861,  p.  147. 

«  Min,  Proc  Inst.  Civ,  Engirt,  Ixxxii.  (1884-85),  p.  309. 

'  Ibid.  p.  148.  •*  Edin.  New  Phil.  Joum.  xviii.  p.  102. 

»  *  Chemical  Geology,'  L  p.  122. 

*  Annales  d^  Travaux publics  de  Beigique,  ix.  204. 

7  Op.  cit.  i.  p.  130.  More  recent  observations  by  Sir  Charles  Hartley  show  that  the 
mean  proportion  of  sediment  by  weight  in  the  Danube  water  for  ten  years  from  1862  to 
1871  was  xcVvi  or  (at  specific  gravity  1*9)  y^Vr  by  volume. 

*  G.  Wilson,  Min  Proc  Inst.  Civ.  Engin.  li.  (1877-8),  p.  216. 

*  Baumgarten,  cited  by  R<^clus,  *  La  Terre.* 
w  T.  Howard,  BrU.  Assoc,  1875,  p.  179. 

"  Joum,  Asiatic  Society  of  Calcutta^  March  1832. 


384  JfYyAMICAL  'GEOLOGY  book  in  pabt  ii 


state  of  the  river.*  In  the  Yaiigtso  the  proijortion  of  sediment  by  weight  is  estiuiatcil 
hy  ^Ir.  H.  I>.  Ouinty  at  tiVs*"  The  amount  in  the  water  of  tlie  River  Pl&te  is  com- 
puted to  be  --Vy  by  weight.^  The  Nile  has  been  estimated  to  contain  159  jjarts  of 
solid  niateiial  in  every  100,000  i»arts  of  water. 

With  reganl  to  the  amount  of  coarser  and  heavier  sediment  pushed  along  the  bottom 
of  a  river  by  the  downward  current,  it  is  more  difficult  to  obtain  accurate  measurements. 
Ihit  it  must  sometimes  constitut^^  a  large  i>roiM)rtion  of  tlie  total  bulk  of  solid  material 
discharged  into  the  sea.  In  the  case  of  the  Rhone,  for  example,  it  is  concluded  by  M. 
(Jueraixl,  that  the  (quantity  of  sand  rolled  along  the  bwl  of  this  river  into  the 
Mediterranean  in  the  course  of  a  year  is  nmch  greater  than  the  lighter  matter  held  in 
susiMHision  in  the  water,  and  that  ''when  the  river,  on  approac^hing  the  sea,  is  no  longpr 
confined  by  embankments,  the  greater  jwirt  of  its  alluvium  is  rolled  along  its  bed."  In 
tloiMl-time  it  is  not  unconnnon  for  whole  lianks  of  .sand  to  travel  bodily  down  the 
river.  ^ 

The  mast  extensive  and  m:curate  determinations  yet  made  u^ion  tlie  pliysics  and 
hydraulics  of  a  river  are  those  of  the  United  States  Goveniment  u^ion  the  Mississipiu. 
As  tlie  mean  of  many  observations  carried  on  continuously  at  different  parts  of  the 
river  for  months  together,  Humphreys  and  Abbot,  the  engineers  charged  with  the 
investigation,  found  that  the  aveiiige  proportion  of  sediment  contained  in  the  water  of 
the  Mississippi  is  ^^'^tv  by  weight  or  ^^\jf  by  volume.'  J5ut  1>esidcs  tlie  matter  held  in 
susi»ension,  they  observed  that  a  large  amount  of  eoarsi^  detritus  is  constantly  being 
jmshed  abmg  tlie  bottom  of  the  river.  They  estimated  that  this  moving  stratum  carrie* 
every  year  into  the  dulf  of  Mexico  about  Tf^OjOOOjOOO  cubic  feet  of  sand,  earth,  and 
gravel.  Their  observations  led  them  to  conclude  that  the  annual  discharge  of  water  by 
the  Mississippi  is  19,r>00,000,000,000  cubic  feet,  and  consequently  that  the  weight  of 
mud  annually  carric»l  into  the  sea  by  this  river  must  reach  the  sum  of  812,500,000,000 
jtounds.  Taking  the  total  annual  contributions  of  earthy  matter,  whether  iu  suspension 
or  moving  along  the  bottom,  they  found  them  to  e<[ual  a  prism  268  feet  in  height  with 
a  base  of  one  sipiarc  mile. 

The  value  »>f  these  data  to  the  geologist  consists  mainly  in  the  fact  that  they  furnish 
him  with  materials  for  an  approximate  measurement  of  the  rate  at  which  the  sui*face  of 
thf  land  is  h)wered  by  subaerial  waste.     This  subject  is  discussed  at  p.  -160. 

2.  E^ffirofi/ifj  Povyr. — It  was  a  prominent  pvrt  of  the  teaching  of 
Hutton  anil  Plavfair,  tliat  rivers  have  exaivated  the  channels  in  which 
they  flow.  Exi)erience  in  all  jwrts  of  the  world  has  confirmed  this 
doctrine.  The  mechanical  erosive  work  of  ninning  water  dei>eiids  for  its 
rate  and  chanicter  uixm  {a)  the  friction  of  the  detritus  driven  by  the 
current  a<i;ainst  the  sides  and  l)ottom  of  a  watercourse,  modified  by  (i) 
the  varying  declivity  and  the  geological  stnicture  of  the  groimd. 

{o)  Driven  downward  by  the  descending  water  of  a  river,  the  loose 
grains  and  stones  are  nibbed  against  each  other,  as  well  as  upon  the 
wwky  bed,  until  they  are  reduced  to  fine  sand  and  mud,  and  the  sides 
and  l>()ttoni  of  the  channel  are  smoothed,  widened,  and  deepened.  The 
familiar  efl'ect  of  ruiniing  water  upon  fragments  of  rock,  in  reducing 
them  to  rounded  pebbles,  is  expressed  by  the  common  epithet  "water- 

*  Proc,  liny.  i<iic.  Ellin.  1857. 

-  ScUure,  xxii.  p.   4S6.     According  to  Dr.  A.  Woeikolf,  this  estimate  is  much  under 
the  truth  ;  xxiii.  p.  9.     See  also  op.  cit.  p.  684. 
'^  G.  Higgiu,  Sature^  xix.  p.  55o. 

*  Mem,  Pnn:.  Inst.  Civ.  Engin.  Ixxxii.  (1884-85),  p.  309. 

*  '  Keport,'  p.  148.     The  specific  gravity  of  the  silt  of  the  Mississippi  is  given  as  1*9. 


BECT.  ii  §  3  GEOLOGICAL  ACTION  OF  RIVERS  385 


worn.'  A  stream  which  descends  from  high  rocky  ground  may  l)e 
compared  to  a  grinding  mill ;  large  boulders  and  angular  blocks  of  rock, 
disengaged  by  frosts,  springs,  and  general  atmospheric  waste,  fall  into  its 
upper  end  ;  fine  sand  and  silt  are  discharged  into  the  sea. 

Ill  the  series  of  experimeuts  akeady  referred  to  (p.  377),  Prof.  Daubree  made  frag- 
ments of  granite  and  quartz  to  slide  over  each  other  in  a  hollow  cylinder  jyartially  filled 
Mrith  water,  and  rotating  on  its  axis  with  a  mean  velocity  of  0*80  to  1  metre  in  a  second. 
He  found  that  after  the  first  25  kilometres  (about  15^  English  miles)  the  angular 
fragments  of  granite  had  lost  -^  of  their  weight,  while  in  the  same  distance  fi^agments 
already  well  rounded  had  not  lost  more  than  ^  to  -f^j^.  The  fragments  rounded  by 
tliis  journey  of  25  kilometres  in  a  cylinder  could  not  be  distinguished  either  in  form  or 
in  general  aspect  from  the  natural  detritus  of  a  river-bed.  A  second  product  of  these 
ex[)eriments  was  an  extremely  fine  imi>alpable  nmd,  which  remained  susi>ended  in  the 
water  several  days  after  the  cessation  of  the  movement.  During  the  production  of  this 
fine  sediment,  the  water,  even  though  cold,  was  found  in  a  day  or  two  to  have  acted 
chemically  upon  the  granite  fragments.  After  a  journey  of  160  kilometres,  3  kilo- 
grammes (about  6^  lb.  avoirdupois)  yielded  3*3  grammes  (about  50  grains)  of  soluble 
salts,  consisting  chiefly  of  silicate  of  potash.  A  third  product  was  an  extremely  fine 
angular  sand  consisting  almost  wholly  of  quartz,  with  scarcely  any  felspar,  nearly  the 
whole  of  the  latter  mineral  having  {Missed  into  the  state  of  clay.  Tlie  sand  grains,  as 
they  are  continually  pushed  onward  over  each  other  upon  the  bottom  of  a  river,  become 
rounded  as  the  larger  pebbles  do.  But  a  limit  is  placed  to  this  attrition  by  the  size  and 
specific  gravity  of  the  grains.^  Asa  rule,  the  smaller  [)articles  suffer  proportionately 
less  loss  than  the  larger,  since  the  friction  on  the  bottom  varies  directly  as  the  weight 
and  therefore  as  the  cube  of  the  diameter,  while  the  surface  exposed  to  attrition  varies 
as  the  square  of  the  diameter.  Mr.  Sorby,  in  calling  attention  to  this  relation, 
remarks  that  a  grain  ^  of  an  inch  in  diameter  would  l>e  worn  ten  times  as  nmch  as 
o"e  rJv  of  an  inch  in  diameter,  and  a  pebble  1  inch  in  diameter  would  be  worn  relatively 
more  by  being  drifted  a  few  hundred  yards  than  a  sand  grain  yj^^  of  an  inch  in  diameter 
would  be  by  being  drifted  for  a  hundred  miles.'  So  long  as  the  jwtrticles  are  borne 
along  in  suspension,  they  will  not  abrade  each  other,  but  remain  angular.  Prof.  Daubree 
found  that  the  milky  tint  of  the  Rhine  at  Strasburg  in  the  months  of  July  and  August 
was  due,  not  to  mud,  but  to  a  fine  angular  sand  (with  grains  about  ^  millimetre  in 
diameter)  which  constitutes  TTrAinj  of  the  total  weight  of  water.  Yet  this  sand  had 
travelled  in  a  rapidly  flowing  tumultuous  river  from  the  Swiss  mountains,  and  had  been 
tossed  over  waterfalls  and  rajdds  in  its  jouniey.  He  ascertained  also  that  sand-grains 
with  a  mean  diameter  of  ^  mm.  will  float  in  feebly  agitated  water  ;  so  that  all  sand  of 
finer  grain  must  remain  angular.  Tlie  same  observer  has  noticed  that  sand  composed  of 
grains  with  a  mean  diameter  of  4  mm.,  and  carried  along  by  water  moving  at  a  rate  of 
1  metre  i)er  second,  is  rounded,  and  loses  about  rxr^THF  of  its  weight  in  every  kilometre 
travelled.^ 

The  effects  of  abrasion  upon  the  loose  materials  on  a  river-l^ed  are  but 
a  minor  part  of  the  erosive  work  performed  by  the  stream.  A  layer  of 
debris,  only  the  upper  portion  of  which  is  pushed  onward  by  the  normal 
current,  will  protect  the  solid  rock  of  the  river-channel  which  it  covers, 
but  it  is  apt  to  l)e  swept  away  from  time  to  time  by  \iolent  floods. 
Sand,  gravel,  and  boulders,  in  those  parts  of  a  river-channel  where  the 
ciurent  is  strong  enough  to  keep  them  moving  along,  rub  down  the  rocky 

*  •  Geologic  Experimentale,'  p.  250  et  seq. 
«  Q.  J.  Geol.  Soc.  xxxvi.  p.  59.  ^  'Giiologie  Exp^rimenUle,"  pp.  256,  258. 

2  C 


386  DYNAMICAL  GEOLOGY  boo«  in  pabt  n 

bottom  over  which  they  are  driven.  As  the  shape  and  declivity  of  the 
channel  vary  constantly  from  point  to  point,  with,  at  the  aame  time, 
freqnent  changes  in  the  nature  of  the  rocks,  this  erosive  action  is  liable  to 
continual  modifications.  It  advances  most  briskly  in  the  numerous  hoUowi 
and  grooves  along  which  chiefly  these  loose  materials  travel.  Wher- 
ever un  eddy  occurs  in  which  gravel  is  kept  in  gyration,  erosion  is  much 
increased.  The  stones,  in  their  movement,  excavate  a  hole  in  the 
channel,  while,  as  they  themselves  are  reduced  to  sand  and  mud,  or  are 
swept  out  by  the  force  of  the  current,  their  places  are  taken  by  fresh 
stones  brought  down  by  the  stream  (Fig.  113).     Such  pot-hoUs,  as  they 


— ii.-pi^/ 


arc  termed,  vary  in  size  from  mere  cup-like  depressions  to  huge 
Kiuldroiis  or  pools.  As  they  often  coalesce,  by  the  giving  way  of  the 
intervening  walls  Ixitwcen  two  or  more  of  them,  they  materially  increase 
the  deepening  of  the  rivcr-I)cd. 

That  a  river  enxics  its  channel  by  means  of  its  transported  sediment 
anil  not  by  the  mere  friction  of  the  water,  is  sometimes  admiraUy 
illustrated  in  the  course  of  streiims  filtered  by  one  or  more  lakes.  As 
the  Khone  escaix's  from  the  Lake  of  Geneva,  it  sweeps  with  a  swift 
clear  ciirreut  over  ledges  of  i-ock  that  have  not  yet  been  very  deeply 
enxled.  The  Niagara  siipplies  a  still  more  impressive  example.  Issuing 
from  Lake  Erie,  and  flowing  through  a  level  country  for  a  few  miies,  it 
approaches  its  falls  by  a  scries  of  rapids.     The  water  leaves  the  lake 


SECT,  ii  §  3  GEOLOGICAL  ACTION  OF  RIVERS  387 

with  hardly  any  appreciable  sediment,  and  has  too  brief  a  journey  in 
which  to  gather  it,  before  beginning  to  rush  down  the  rocky  channel 
towards  the  cataract.  The  sight  of  the  vast  body  of  clear  water,  leaping 
and  shooting  over  the  sheets  of  limestone  in  the  rapids,  is  in  some 
respects  quite  as  striking  a  scene  as  the  great  falls.  To  a  geologist  it  is 
specially  instructive ;  for  he  can  observe  that,  notwithstanding  the 
tremendous  rush  of  water  which  has  been  rolling  over  them  for  so  many 
centuries,  these  rocks  have  been  comparatively  little  abraded.  The 
smoothed  and  striated  surface  left  by  the  ice-sheet  of  the  Glacial  Period 
can  be  traced  upon  them  almost  to  the  water's  edge,  and  the  flat  ledges 
at  the  rapids  are  merely  a  prolongation  of  the  ice-worn  siuiace  which 
passes  under  the  banks  of  drift  on  either  side.  The  river  has  hardly 
eroded  more  than  a  mere  suj>erficial  skin  of  rock  here  since  it  began  to 
flow  over  the  glaciated  limestone. 

Similar  evidence  is  offered  by  the  St.  Lawrence.  This  majestic  river 
leaves  Lake  Ontario  as  pure  as  the  waters  of  the  lake  itself.  The  ice- 
worn  hummocks  of  gneiss  at  the  Thousand  Islands  still  retain  their 
characteristic  smoothed  and  polished  surface  down  to  and  beneath  the 
surface  of  the  current.  In  descending  the  river,  I  was  astonished  to 
observe  that  the  famous  rapids  of  the  St.  Lawrence  are  actually  hemmed 
in  by  islets  and  steep  banks  of  boulder-clay,  and  not  of  solid  rock.  So 
little  obvious  erosion  does  the  ciurent  perform,  even  in  its  tumultuous 
billowy  descent,  that  a  raw  scar  of  clay  betokening  a  recent  slip  is  hardly 
to  be  seen.  The  banks  are  so  grassed  over,  or  even  covered  with  trees, 
as  to  prove  how  long  they  have  remained  imdistiu'bed  in  their  present 
condition.  That  very  considerable  local  destruction  of  these  clay-ishmds, 
however,  has  been  caused  by  floating  ice  will  be  alluded  to  further  on. 

Mere  volume  and  rapidity  of  current,  therefore,  will  not  cause  much 
erosion  of  the  channel  of  a  stream  unless  sediment  be  present  in  the 
water.  A  succession  of  lakes,  by  detaining  the  sediment,  must 
necessarily  enfeeble  the  direct  excavating  power  of  a  river.  On  the 
other  hand,  by  the  disintegrating  action  of  the  atmosphere,  and  by  the 
operations  of  springs  and  frosts,  loose  detritus  as  well  as  portions  of  the 
river-banks  are  continually  being  launched  into  the  currents,  which,  as 
they  roll  along  are  thus  supplied  with  fresh  materials  for  erosion. 

(b)  Besides  the  obvious  relation  between  the  angle  of  slope  of  a 
river-bed  and  the  scouring  force  of  the  river,  a  dominant  influence,  in  the 
gradual  excavation  of  a  river-channel,  is  exercised  by  the  lithological 
nature  and  geological  structure  of  the  rocks  tlirough  which  the  stream 
flows.  This  influence  is  manifested  in  the  form  of  the  channel,  the 
angle  of  declivity  of  its  banks,  and  in  the  details  of  its  erosion.  On  a 
small  but  instructive  scale  these  phenomena  are  revealed  in  the  opera- 
tions of  brooks.  Thus,  one  of  the  most  characteristic  features  of  streams, 
whether  large  or  small,  is  the  tendency  to  wind  in  serpentine  curves 
when  the  angle  of  declivity  is  low,  and  the  general  surface  of  the  country 
tolerably  level.  This  peculiarity  may  be  observed  in  every  stream  which 
traverses  a  flat  alluvial  plain.  Some  slight  weakness  in  one  of  its 
banks  enables  the  current  to  cut  away  a  portion  of  the  bank  at  that 


388 


DYXAMFCAL  GEOLOGY 


BOOK  III  PART  n 


Fig.  110.— Meanleriiip  course  of  a  brook. 


point.  By  degrees  a  concavity  is  formed  round  which  the  upper  water 
sweeps  with  increased  velocity,  while  under -currents  tend  to  carr}' 
sediment  across  to  the  opposite  side.  The  outer  bank  is  accordingly 
worn  away,  while  the  inner  or  concave  side  of  the  bend  is  not  attacked, 
but  is  even  protected  by  a  deposit  of  sand  or  gravel.^  Thus,  bending 
alternately  from  one  side  to  the  other,  the  stream  is  led  to  describe  a 
most  sinuous  course  across  the  plain.  By  this  process,  howeyer,  while 
the  course  is  greatly  lengthened,  the  velocity  proportionately  diminishes, 
until,  before  quitting  the  plain,  the  stream  may  become  a  lazy,  creeping 
current,  in  England  commonly  bordered  with  sedges  and  willows.     A 

stream  may  eventually  cut  through 
the  neck  of  land  between  two  loops, 
as  at  a,  h,  and  c,  in  Fig.  116,  and 
thus  for  a  while  shorten  its  channel. 
Instances  of  this  nature  may  fre- 
quently be  observed  in  streams  flow- 
ing through  alluvial  land.  The  old 
deserted  loops '^^  are  converted,  first  into  lakes,  and  by  degrees  into 
stagnant  pools  or  bogs,  until  finally,  by 
growth  of  vegetation  and  infilling  of 
sediment  by  rain  and  wind,  they  become 
dry  ground. 

Although  most  frequent  in  soft  allu- 
vial plains,  serpentine  water-courses  may 
also  be  eroded  in  solid  rock  if  the  ori- 
ginal form  of  the  surface  was  tolerably 
flat.  The  windings  of  the  gorges  of  the 
Moselle  (Fig.  117)  and  Rhine  through 
the  table -land  between  Treves,  Mainz, 
and  the  Siebengebirge  form  a  notable 
illustration. 

Abrupt  changes  in  the  geological 
structure  or  lithological  character  of  the 
rocks  of  a  river- channel  may  give  rise 
to  waterfalls.  In  many  cases,  this  feature 
of  river-scenery  has  originated  in  lines  of 
escarpment  over  which  the  water  at  first 
found  its  way,  or  in  the  same  geological 
an-angement  of  hard  and  soft  rocks  by 
which  the  escarpments  themselves  have 
been  produced.  The  occurrence  of  hori- 
zontal, tolerably  compact  strata,  traversed 
by  marked  lines  of  joint,  and  resting 
upon  softer  beds,  presents  a  structure 
well  adapted  for  showing  the  part  played 
by  waterfalls  in  river-erosion.    The  water- 


Fig.  117.— Windings  of  the  gorge  of  Uie- 
Moselle  above  Cochem. 


^  J.  Thomson,  /Vtxr.  Roy.  tSoc.  xxv.  (1876),  p.  5. 
-  '*  Aigues-inortes, "  or  dead  waters.     See  p.  408,  noU. 


BKi-T.  ii  S  3 


GEOLOGICAL  Ai^ION  OF  RIVERS 


fall  acts  with  special  potency  [gainst  the  softer  underlying  materials  at 

its  base.  These  are  hollowed  out,  and  as  the  foundations  of  the  super- 
incumbent more  solid  rocks  are  destroyed,  slices  of  tbe  latter  from  time 
to  time  fall  off  into  the  boiling  whirlpool,  where  they  are  reduced  to 
fragments,  and  carried  down  the  stream.  Thus  the  waterfall  cuts  its 
way  backward  up  the  stream,  and  as  it  advances  it  prolongs  the  excava- 
tion of  the  ravine  into  which  it  descends.  The  student  will  frequently 
observe,  in  the  recession  of  waterfalls  and  consequent  erosion  of  ravines, 
the  important  part  taken  by  lines  of  joint  in  the  rocks.  These  lines 
have  often  determined  the  direction  of  the  ravine,  and  the  vertical  walls 
on  either  side  depend  for  their  precipitousness  mainly  upon  these 
divisional  planes  in  the  rock. 

The  gorge  o!  the  KiHgHra  alTords  b  niagiiilicent  and  remarkably  ■omple  illuatration 
of  these  features  of  river-action.  At  its  lower  end,  wli^re  it  enters  tlie  «ide  plain  that 
pxtiMula  to  I^ke  Ontario,  there  stretches  away,  ou  eitlier  side  of  the  nver,  ■  Une  of  chff 
and  ateep  wooded  bank  formed  ly  the  encarj'nieiit  of  the  massive  Niagara  limestone. 
Back  from  tliii  line  ofehtr  tlirough  which  it  i<i3iie<i  nito  the  lacnaCnne  plain,  the  gorge 
of  the  ri*er  extends  for  about  /  miles,  »ith  a  »iUh  of  from  200  to  400  yards,  and  a 
<lepth  of  from  200  to  300  feet  At  tlie  U[  [ler  end  lie  the  ivorld  renovned  falU.  The 
whole  of  tins  great  ravine  lias  un  |uestioQal>U  been  cut  out  b\  the  recession  of  the 
fnllf.  Wlien  the  nver  firat  liegaii  to  flow  n  ma\  liHie  found  the  escarpment  running 
across.its  course  and  maj  then  have  begun  the  eKca\ation  of  its  gorge  ilore  prob 
al>lv,  however  the  escarpment  and  waterfall  liegan  to  anw  simultaneously,  and  from 
the  lame  )i;eological  stnicture  As  the  former  ;^ew  in  height  it  receded  from  ita 
Mtarting  point  The  nver  ravine  likevMse  crej  t 
liackuard  but  at  a  moi e  ra|  id  rate  anil  the  result 
h«»  been  that  while  at  present  the  cliff,  worn  down 
1>)  atmospheric  disintegi'ation,  standij  at  Qoeeii^- 
town  the  ravine  dug  by  the  river  extends  7  miles 
furtbi  r  inland      Tlie  waterfall  will  coutintie  to  cut 

ts     aj  liaik.  as  long  aa  the  structure  of  the  gorge 

■out      es  as    t  19  now  — thick  l)edB  of  limestone         f 
resti  g  loi^ntally  upon  soft   shales   (Fig.   118). 

n  e  softer  strata  at  the  base  are  undermined,  and 
•.1    e  alter  al  ee  is  cat  off  from  the  cliff  over  wliicli 
ll  e     atara  t  i>o  rs.      The  [larallcl   walls  of  thin 
l!V  at  j,orj,   o   e  their  dii'eclion  and  mural  character    kIk. 
t     |«rall  I  J    uts  of  the  strata.      Tlie  lesser  or 

i    enca    fall  (A    i>  Fig.  119),  enters  lij' the  side   «.M 

r  ll      'a      e  a  d  falls  over  iU  lateral  wall.     Tlie        '- 
[a  (.        r  Canadan  (Horse-slioe)  fall  (Cj  occupies'        '1^^ 
Ihc  head  iif  tlie  ravine,  and  owes  its  form  to  the         visible  »l  the  fcil. 
intersection  of  two  sets  of  joints.     Tlie  atructnre  (if 

the  gorge  being  the  same  at  l>otli  falls,  it  seems  reasonable  to  infer  that  at 
fall,  which  appears  to  be  iliminisliing  in  volume,  has  cut  back  only  somewhei-e  about 
140  feet  from  the  original  face  of  tlie  ravine,  this  braln-ll  of  the  river  lias,  eomiai-atively 
speaking,  only  recently  begun  to  work,  (ioat  Island,  which  now  s«|«irates  the  two  falls, 
is  ail  ontlier  of  drift  resting  on  the  limestone.  It  has  been  cut  off  from  the  rest  of  the 
]^und  on  the  right  bank  of  the  river  by  the  branch  which  rejoins  (lie  main  stream  by 
the  American  fall.  From  tbe  jiOHition  of  the  glacial  striie  it  may  be  eoiictniled  that  a 
grrat  ]<art,  if  not  the  whole,  of  the  ravine  has  been  excavated  since  tbe  IJlaeial  Period. 
There  are  indications,  indeed,  of  a  [ire-glacial  valley  by  whicli  the  waters  of  Lake  Erie 


<ne.  MOfMt;  A,  Clinton 
Ethalc,  30  fret ;  (,  Xia. 
feet;  J,  Niagara  Lime- 


DYNAMICAL  GEOLOGY 


BOOK  HI  PAST  n 


joined  those  of  Ontario,  before  the  erosion  of  the  preunt  goi^.  Bakewell.  (ran 
hintonctil  notices  and  the  testimony  of  old  residents,  inferred  that  the  nte  of  Tecmian 
of  the  falls  is  three  feet  in  a  year.  L;ell,  on  no  better  kind  of  evidence,  conclnded 
that  "the  average  of  one  foot  a  year  would  be  a  much  more  probable  coi^ectnre,"  and 
estimated  the  length  of  time  required  for  the  e;[c«vation  of  the  whole  Niagan  nvine  4t 


35,000  years.'  A  conimission  recently  appointed  ta  survey  the  falls  and  b 
ratf  of  recession  has  rejwrted  (18S0)  that  since  1742,  when  the  first  snrvey  waa  mad«,  the 
total  mean  recession  of  the  Home-shoe  falls  has  been  104  feet  6  inches.  The  mudlDDm 
recession  at  one  point  is  270  feet.  The  mean  recession  of  the  American  falls  is  SO  feet 
e  inches.  The  length  of  the  crest  has  increased  frotn  22S0  to  3010  feet  bj  the  mahing 
away  of  the  embankment.  Tlie  total  area  of  recession  of  the  American  falla  ia  83,900 
square  feet,  and  that  of  the  Horse-shoe  falls  275,400  feet. 

A  feature  of  interest  in  the  future  history  of  the  Niagara  river  deserves  to  be  noticed 
here.  It  is  evident  that  if  the  structure  o(  the  gorge  continued  the  same  from  the  bUt 
to  Lake  Erie,  the  I'ccessioii  of  the  falls  would  eventually  tap  the  lake,  and  reduce  its 
surface  to  tlie  level  of  the  bottom  of  the  ravine.  Successive  stages  in  this  retreat  of  tha 
falls  are  shown  in  Kip.  120,  by  the  letters/  to  ti,  and  in  the  consequent  lowering  of  the 


to  lUiutiule  the  luwetlng  gf  Lake  Br 

lake  by  the  letters  a,  bta  e.  It  is  believed,  however,  that  a  slight  inclination  of  tha 
strata  carries  the  soft  underlying  shale  out  of  (lossible  reach  of  the  fall,  whicb  will 
retard  indefinitely  the  lowering  of  the  lake. 

A  waterfall  may  occasionally  be  observed  to  have  been  produced  by 
the  existence  of  a  haitier  and  more  Fesisting  band  or  barrier  of  rock 
crossing  the  course  of  tlie  stream,  as,  for  instance,  where  the  rocks  have 
been  exit  by  an  intrusive  dyke  or  mass  of  basalt,  or  where,  as  in  the  cue 
'  LjelL  'Travels  in  Nortii  America,'  i.  p.  32  ;  ii.  p.  03.  '  Principles,'  i.  p.  358.  Cota- 
Iiare  Lesley's  'Coal  aud  its  Topograjihy '  (1866),  p.  169.  On  recent  changes  at  the  fills, 
tee  Marcou.  BuU.  Soc.  Geol.  France  {2).  xxii.  p.  260.  The  Falls  of  St  Anthony  on  tli« 
Misaissippi  show,  according  to  Vr'incbell,  a  rale  of  receision  varying  from  3-4B  to  6-71  fMl 
per  annum,  the  whole  recession  since  the  discovery  of  the  falls  in  lfl80  to  the  present  tlu 
being  906  feel.     Q.  J.  Otol.  Soc.  wxiv.  p.  899. 


SECT,  ii  §  3  OEOLOaiCAL  ACTION  OF  RIVEBS  391 

of  the  Khine  at  Schaffhausen,  and  possibly  in  that  of  the  Niagara,  the 
stream  has  been  diverted  out  of  its  ancient  course  by  glacial  or  other 
deposits,  BO  aa  to  be  forced  to  carve  out  a  new  channel,  and  rejoin  its 
older  one  by  a  fait'  In  these  and  all  other  cases,  the  removal  of  the 
harder  mass  destrgya  the  waterfall,  which,  after  passing  into  a  series  of 
rapids,  is  finally  lost  in  the  general  abrasion  of  the  river-channel 

The  reeemblance  of  a  deep  narrow  river-gorge  to  a  rent  opened  in  the 
ground  by  subterranean  agency,  has  often  led  to  a  mistaken  belief  that 
such  marked  superficial  features  could  only  have  arisen  from  actual 
violent  dislocation.  Even  where  something  is  conceded  to  the  river, 
there  is  a  natiural  tendency  to  assume  that  there  most  have  been  a  line 
of  fault  and  displacement  as  in  f^g.  121,  or  at  least  a  line  of  crack,  and 
consequent  weakness  (Fig.  122).     But  the  existence  of  an  actual  fracture 


Fig.  III.— ny<sr.goisc  in  line  of  Fault.  Fig. : 

is  not  necessary  for  the  formation  of  a  ravine  of  the  first  magnitude. 
The  gorge  of  the  Niagara,  for  example,  has  not  been  determined  by  any 
dislocation.  Still  more  impressive  proof  of  the  same  fact  is  furnished  by 
the  most  marvellous  river-gorges  in  the  world — those  of  the  Colorado 
region  in  North  America.  The  rivers  there  flow  in  ravines  thousands 
of  feet  deep  and  hundreds  of  miles  long,  through  vast  table-lands  of  nearly 
horizontal  strata.  The  Grand  Canon  (ravine)  of  the  Colorado  river  is 
300  miles  long,  and  in  some  places  more  than  6000  feet  in  depth.  In 
many  instances  there  are  two  canons,  the  upper  being  several  miles  wide, 
with  vast  lines  of  cliff-walls  and  a  broad  plain  between  them,  in  which 
runs  the  second  cafion  as  another  deep  gorge  with  the  river  winding  over 
its  bottom.  The  country  is  hardly  to  be  crossed  except  by  birds,  so 
profoundly  has  it  been  trenched  by  these  numerous  gorges.  Yet  the 
whole  of  this  excavation  has  been  effected  by  the  erosive  action  of  the 
streams  themselves.^  Some  idea  of  the  vastness  of  the  erosion  of  these 
plateaux  may  be  formed  from  Fig.  123,  the  Frontispiece  to  this  volume, 
and  the  illustrations  in  Book  VIL 

In  the  excavation  of  a  ravine,  whether  by  the  recession  of  a  waterfall 

1  WUrUnbergCT,  Neva  Jahrb.  1871,  p.  682. 

*  For  descriptioDS  aod  Ggurea  of  this  remarkable  region,  see  Ives  and  Hewberrf, 
'  Biplorations  of  the  Colorado  River  of  the  West, '  1861  ;  J.  W.  Powell,  '  Exploration  of  the 
Colorado  River  of  the  West  and  its  Tribataries,'  1876  ;  Captain  Dutlon,  '  Tertiary  Hiitory 
of  the  Qruid  Cailon  of  the  Colorado ' ;  Moncgraph  II.  U.S.  QtUogical  Survey,  4to,  1882  ; 
anipoHea,  Book  VII. 


nrXAMICl  I.  (IKOLIXIY 


IKX>K  UI  PAST  a 


or  of  a  series  of  nipids,  the  action  of  the  river  is  more  eifective  tlian  that 
of  tho  atmospheric  agonts.     The  sides  of  the  ravine  consequently  retain 


thi'ii  VLTLicul  charictor  whuh  where  thej  coincide  with  lines  of  joint, 
is  further  preserved  b\  tlie  way  in  which  atmospheric  weathenng  acts 
along  the  joints.     But  wheic   from  the  iiatnrc  of  the  ground  or  of  the 


SECT,  ii  S  3 


GEOLOGICAL  ACTIOX  OF  RIVERH 


climate,  the  denuding  action  of  rein,  frost,  and  general  weathering  is 
more  rapid  than  that  of  the  river,  a  wider  and  opener  valley  is  hollowed 
out,  through  which  the  river  flows,  carrying  away  the  materials  washed 
into  it  from  the  surrounding  slopes  by  rain  and  brooks. 

3.  Beprodti/iite  Poteer. — Every  body  of  water  which,  when  in  motion, 
carries  along  sedinient,  drops  it  when  at  rest.  The  moment  a  current 
has  its  rapidity  checked,  it  is  deprived  of  some  of  its  carrying  power, 
and  begins  to  lose  hold  upon  its  sediment,  which  tends  more  and  more 
to  sink  and  halt  on  the  bottom  the  slower  the  motion  of  the  water.  In 
Fig.  1 24  the  river  in  flowing  from  »  to  b  has  a  less  angle  of  declivity 


and  a  smaller  transporting  power,  and  will  therefore  have  a  greater 
tendency  to  throw  down  sediment,  than  in  descending  the  steeper 
gradient  from  h  to  c. 

In  the  course  of  every  brook  and  river,  there  are  frefiuent  checks  to 
the  current  If  these  are  examined,  they  will  usually  be  found  to  be 
each  marked  by  a  more  or  less  conspicuous  deposit  of  sediment  We 
may  notice  seven  difl'erent  situations  in  which  stream- deposits  or  allunum 
may  be  accumulated. 

(a)  At  the  foot  of  Mountain  Slopes. — When  a  runnel  or  torrent 
descends  a  steep  declivity  it  tears  down  the  soil  and  rocks,  cutting  a 
gash  out  of  the  side  of  the  mountain  <Fig.  \'2^)-     On  reaching  the  more 


level  ground  at  the  base  of  the  slope,  the  water,  abruptly  checked  in  its 
velocity,  at  once  drops  its  coarser  sediment,  which  gathers  in  a  fan-shaped 
pile  or  cone  ("  cone  tie  lUjolum  "  :  "  Mail-rii'-ke  "  '),  with  the  apex  pointing 

'  G.  A.  Koch,  Jahrb.  ifeol.  ReMiMHil.  uv.  (1875),  p.  97.  liescfiliea  the  debacles  of  the 
Tyrol.     Conxult  ilao  the  work  of  Surell  and  t'eianne  citeil  on  p.  372. 


394 


DYNAMICAL  GEOLOGY 


BOOK  m  PABT  n 


up  the  water- course.  Huge  accumulations  of  boulders  and  shin^e 
may  thus  be  seen  at  the  foot  of  such  torrents, — the  water  flowiDg 
through  them,  often  in  several  channels  which  re-unite  in  the  plain 
beyond.  From  the  deposits  of  small  streams,  every  gradation  of  size 
may  be  traced  up  to  huge  fans  many  miles  in  diameter  and  sevend 
hundred  feet  thick,  such  as  occur  in  the  upper  basin  of  the  Indus  ^  and 
on  the  flanks  of  the  Rocky  Mountains,^  as  well  as  other  ranges  in  Nortii 
America  (Fig.  126).*     The  level  of  the  valleys  in  the  Tyrol  has  been 


Fig.  126.— Fans  of  Alluvium.    Madison  River,  Montana. 


sensibly  raised  within  historic  times  by  the  detritus  swept  into  ^them 
from  the  surrounding  mountains.  Old  churches  and  other  buildings  are 
half-buried  in  the  accumulated  sediment.* 

(b)  In  River-beds. — The  deposition  of  alluvium  on  river-beds  is 
characteristically  shown  by  the  accumulation  of  sand  or  shingle  at  the 
concave  side  of  each  sharj)  bend  of  a  river-course.  While  the  main  upper 
current  is  making  a  more  rapid  sweep  round  the  opposite  bank,  undei^ 
currents  pass  across  to  the  inner  side  of  the  curve  and  drop  their  freight 
of  loose  detritus,  which,  when  laid  bare  in  dry  weather,  forms  the  familiar 
sand-bank  or  shingle-beach.  Again,  when  a  river,  well  supplied  with 
sediment,  leaves  mountainous  ground  where  its  course  has  been  rapid, 
and  enters  a  region  of  level  plain,  it  l)egins  to  drop  its  burden  on  the 


Fig.  127.— Section  of  a  River-plain,  showing  heightening  of  channel  by  dep<Mit8  of  sediment  (&)' 

channel,  which  is  thereby  heightened,  till  it  may  actually  rise  above  the 
level  of  the  surrounding  plains  (Fig.  127). 

^  On  the  alluvial  dei)osit8  of  this  region,  see  Drew,  Q.  J.  Oeol.  Soc.  zxix.  p.  441 ;  alao 
his  *  Juramoo  and  Kashinere  Territories,*  1875. 

3  See  Dutton's  'High  Plateaux  of  Utah.'  Hayden's  '  Reports  of  the  U.S.  Geological 
and  Geographical  Surveys  of  the  Territories.' 

^  In  the  great  inland  ba^in  of  Nortli  America,  which  includes  the  arid  tracts  of  Great  Salt 
Lake  and  other  saline  waters,  the  depth  of  accumulated  detritus  must  amount  in  many  placet 
to  several  thousand  feet.  See  on  this  subject  I.  C.  Russell,  Oeof.  Mag.  1889|  and  Gilbert'i 
Essay  on  Lake-Shores  in  the  5(h  Annual  Report  of  the  U.S.  Oeol.  Surv. 

*  G.  A.  Koch,  Jahrb.  Oeol.  Reichsansi.  xxv.  (1876),  p.  123. 


SECT,  ii  §  3  GEOLOGICAL  ACTION  OF  RIVERS  396 

This  tendency  is  displayed  by  the  Adige,  Reno,  and  Brenta,  which,  descending  from 
the  Alps  well  supplied  with  detritus,  debouch  on  the  plains  of  the  Po.^  The  Po  itself  has 
been  quoted  as  an  instance  of  a  river  continuing  to  heighten  its  bed,  while  man  in  self- 
defence  heightens  its  embankments,  until  the  surface  of  the  river  becomes  higher  than 
the  plains  on  either  side.  It  has  been  shown  by  Lombardini,  however,  that  the  bed  of 
this  river  has  undergone  very  little  change  for  centuries  ;  that  only  here  and  there  does 
the  mean  height  of  the  water  rise  above  the  level  of  the  plains,  being  generally  con- 
siderably below  it,  and  that  even  in  a  high  flood  the  surface  of  the  river  is  scarcely  ten 
feet  above  the  pavement  in  front  of  the  Palace  at  Ferrara.'  The  Po  and  its  tributaries 
have  been  carefully  embanked,  so  that  much  of  the  sediment  of  the  rivers,  instead  of 
accumulating  on  the  plains  of  Lombardy,  as  it  naturally  would  do,  is  carried  out  into 
the  Adriatic.  Hence,  partly,  no  doubt,  the  remarkably  rapid  rate  of  growth  of  the 
delta  of  the  Po.  But  in  such  cases,  man  needs  all  his  skill  and  labour  to  keep  the  banks 
secure.  Even  with  his  utmost  efforts,  a  river  will  now  and  then  break  through, 
sweeping  down  the  barrier  which  it  has  itself  made,  as  well  as  any  additional  embank- 
ments constructed  by  him,  and  carrying  its  flood  far  and  wide  over  the  plain.  Left  to 
itself,  the  river  would  incessantly  shift  its  course,  until  in  turn  every  part  of  the  plain  had 
been  again  and  again  traversed.  It  is  indeed  in  this  way  that  a  great  alluvial  plain  is 
gradually  levelled  and  heightened.  The  most  stupendous  example  of  the  gradual 
heightening  of  a  plain  by  river  deposits,  and  of  the  devastation  caused  by  the  bursting 
of  the  artificial  barriers  raised  to  control  the  stream,  is  that  of  the  Hoang  Ho  or  Yellow 
River.  So  frequently  has  this  river  changed  its  course  across  the  great  eastern  plain, 
and  so  appalling  has  been  the  consequent  devastation,  that  it  has  received  the  name  of 
"China's  sorrow."  The  last  great  inundation  took  i)lace  in  the  autumn  of  1887,  when 
hundreds  of  villages  were  submerged  and  more  than  a  million  human  beings  were 
drowned.  Breaking  down  its  frail  embankment,  the  stream  poured  through  the  breach, 
which  was  some  1200  yards  wide,  and  spread  out  over  a  width  of  30  miles  in  a  current 
ten  to  twenty  feet  deep  in  the  middle. 

(c)  On  River-banks  and  Flood-plains. — As  is  partly  implied  in  the 
action  described  in  the  foregoing  paragraph,  alluvium  is  laid  down  on  the 
level  tracts  or  flood-plain  over  which  a  river  spreads  in  flood.  It  consists 
usually  of  fine  silt,  mud,  earth,  or  sand ;  though  close  to  the  channel,  it 
may  be  partly  made  up  of  coarser  materials,  ^^^len  a  flooded  river  over- 
flows, the  portions  of  water  which  spread  out  on  the  plains,  by  losing 
velocity,  and  consequently  power  of  transport,  are  compelled  to  let  fall 
more  or  less  of  their  mud  and  sand.  If  the  plains  happen  to  be  covered 
'with  wood,  bushes,  scrub,  or  tall  grass,  the  vegetation  acts  the  part  of  a 
sieve,  and  filters  the  muddy  water,  which  may  rejoin  the  main  stream 
comparatively  clear.  The  height  of  the  plain  is  thus  increased  by  every 
flood,  until,  partly  from  this  cause  and  partly,  in  the  case  of  a  rapid 
stream,  from  the  erosion  of  the  channel,  the  plain  can  no  longer  be  over- 
spread by  the  river.  As  the  channel  is  more  and  more  deepened,  the 
river  continues,  as  before,  to  be  liable,  from  inequalities  in  the  material 
of  it«  banks,  sometimes  of  the  most  trifling  kind,  and  from  the  behaviour 

^  It  is  in  the  north  of  Italy  that  the  struggle  between  man  and  nature  has  been  roost 
persistently  waged.  See  Lombardini,  iu  Ann.  des  PorUs-et-ChaussSeSf  1847.  Beardmore's 
*  Tables,*  p.  172.  The  bed  of  the  Yang-tse-Kiang  has  been  raised  in  places  far  above  the 
level  of  the  surrounding  country  by  embanking.  E.  L.  Oxenham,  Joum,  Qeog.  Soc.  xlv. 
(1875),  p.  182. 

^  Between  Mantua  and  Modena  the  Po  is  said  to  have  raised  its  bed  more  than  5^  metres 
since  the  15th  century.     Dausse,  Bui.  Soc.  GioL  France^  iii.  (3me  ser.),  p.  137. 


JtYXAiriCAl,  CEOLOdY 


BOOK  m  PABC  n 


uf  water  flowing  in  irregular  channels,  to  wind  from  side  to  side  in  wide 
curves  and  loops,  itnd  cuts  into  its  old  alluvium,  making  eventuallf  a 
newer  plain  at  a  lower  level.  Prolonged  erosion  carries  the  ch&nnel  to  a 
still  lower  level,  where  the  stream  can  attack  the  later  alluvial  deposit,  and 
form  a  still  lower  and  newer  one.  The  river  comes  by  this  means  to  be 
fringed  with  a  series  of  terraces  (Fig.  128),  the  surface  of  each  of  whidi 
represents  a  former  flood-level  of  the  stream. 


Fig.  12S.-S«tlan  of  lUrfi 

III  Britain,  it  \%  coiiraion  ti>  tiiiil  tlirce  such  tcrracra,  liiit  soiuGlimes  as  many  m  ni  or 
soven  or  even  more  may  oimir.  Oil  the  Seine  and  other  rivers  of  tlie  North  of  Fiance, 
there  is  a  ninrkeil  ten-oce  at  ahcightnf  12  to  17  metrea  above  the  present  water-lcTcL 
III  Xortli  America,  the  liver-teiTsce*  exist  ou  so  grsiid  a  scale  tliat  the  geologists  of  that 
countrj  Iiavc  named  one  of  the  Inter  Jieriods  of  geological  litstor;,  during  which  thiM 
lieposits  were  fonned,  the  Termee  Epoth  (Fig.  129}.  Tlie  modem  alluvium  of  tha 
lliiisiiuiipin,  from  the  month  of  the  Ohio  to  the  Gulf  of  Mexico,  covers  an  area  of  J9,4G0 
miles,  and  lias  a  breadth  of  from  25  to  75  miles  and  a  depth  of  from  25  to  40  feet 
The  old  alluvium  of  the  AmaitDn  likewise  forms  extensive  lines  of  cliff  for  handreda 
of  miles,  Wiipsth  whieli  a  newer  ]i!atfonn  of  detritus  is  lieing  formed.^ 


■>■>..-  - 


Hit.  1311.— OUTormcm 


CUon.    Munbna. 


In  the  attcmjjt  to  recoiistnict  the  history  of  the  old  river- terraces  of  a 
couTitry,  we  have  to  consider  whether  they  huve  been  entirely  cut  out  of 
older  alluvium  (iii  which  ease,  of  course,  the  \'alley8  must  have  been 
deeper  aiul  broader  than  now,  l)efoi'e  the  formation  of  the  terraces.  Fig. 

'  I'be  stagei  of  terrace -nisking  in  the  ri'giiiic  of  n  ^reat  river  are  well  brought  oat  in  tbi 
case  of  the  Aniaion.  C.  B.  Brown,  ly.  J.  Hv^.  .•)«■.  Jixxv.  p.  763.  The  sal^teX  «f  the 
origin  ofriver-l«rroces  la  diSfUHSed  by  Mr.  H.  Miller  of  the  Oeologieal  Survey  tn /Vof,  Jtof. 
fh^a.  H'C.  Kiliii.  1883,  p.  263. 


SECT,  ii  S5  3 


GEOLOGICAL  ACTION  OF  RIVERS 


397 


1 28)  ;  whether  they  afford  any  indications  of  ha\'iiig  l)een  formed  during  a 
period  of  greater  rainfall,  when  the  rivers  were  larger  than  at  present ; 
whether  they  point  to  upheaval  of  the  interior  of  the  country,  which 
would  accelerate  the  erosive  action  of  the  streams,  or  to  depression 
of  the  interior  or  rise  of  the  seaward  tracts,  which  would  diminish  that 
action  and  increase  the  deposition  of  alluvium.  Professor  Dana  has 
connected  the  terraces  of  America  with  the  elevation  of  the  axis  of  thiit 
continent.  There  can  be  no  doubt  that  lx)th  in  Europe  and  North 
America  the  rivers  at  a  comparatively  recent  geological  period  had  a 
much  greater  volume  than  they  now  possess. 

(d)  In  Lakes. — When  a  river  enters  a  lake  or  inland  sea  its  current  is 
checked,  and  its  sediment  begins  to 
spread  in  fan-shape  over  the  bottom  (c 
in  Fig.  130).  Every  tributary  stream 
brings  in  its  contribution  of  detritus. 
In  this  way,  a  series^  of  shoals  is  pushed 
out  into  the  lake  (Fig.  131  and  p.  406). 
This  phenomenon  may  frequently  l)e 
instructively  obser\'ed  from  a  height 
overlooking  a  small  lake  among  mountains.  At  the  mouth  of  each 
torrent  or  brook  lies  a  little  tongue  of  its  alluvium  (a  true  delta),  through 

d 


Fig.  130.— Streamlet  (6)  entering  a  nuiall  lake 
(a),  and  depositing  a  fan  of  sediment  (r). 


Fig.  131.— Plan  of  a  lake  entered  by  three  streams        Fig.  132.— Lake  (as  in  Fig.  131)  fllleil  »i>  and  con- 
(f,  d,  0,  each  of  which  dejiosits  a  cone  of  «wii-  vertod  into  an  alluvial  jilain  by  tin*  three 

ment  (a,  b)  at  its  moittli.  streams,  c,  d,  r. 

which  the  streamlet  winds  in  one  or  more  branches,  before  mingling  its 
waters  with  those  of  the  lake.  Two  streams  entering  from  opposite  sides 
(as  at  c,  d  Fig.  131)  may  join  their  allu\da  and  divide  a  lake  into  two,  like  the 
once  united  lakes  of  Thun  and  Brienz  at  Interlaken.  Or,  by  the  advance 
of  the  alluvial  deposits,  the  lake  may  l>e  finally  filled  up  altogether,  as  has 
happened  in  innumerable  cases  in  all  mountainous  coimtries  (Fig.  132). 

The  rapidity  of  tlie  infilling  is  sometimes  not  a  little  remarkable.  Since  the  year 
1714,  the  Kander  is  said  to  have  thrown  into  the  Lake  of  Thun  a  delta  measuring  230 
acres,  now  partly  woodland,  j>artly  meadow  and  marsh.  The  Aar,  at  its  entrance  into 
the  Lake  of  Brienz,  has  deposited  a  delta  3500  to  4000  feet  broad,  formed  of  detritus  which 
at  the  mouth  of  the  river  has  an  outward  slope  of  30°,  that  gradually  falls  to  the  nearly 
level  lake  floor.  In  twenty -seven  years  after  its  rectification  the  Reuss  had  laid  down 
in  the  Lake  of  Lucerne  a  delta  estimated  to  contain  ui>wards  of  141  million  of  cubic 
feet  of  sediment,  which  is  equivalent  to  a  discharge  of  19,350  cubic  feet  in  a  day,  or 
nearly  7,000,000  cubic  feet  in  a  year.* 


^  A.  Ueiin,  Jahrb.  Schweizer  Alpenldubsy  1879. 


398  DYXAMICAL  GEOLOGY  book  m  pabt  n 

In  tlie  case  of  a  large  lake  who8e  length  is  great  in  proportion  to  the  volume  of  the 
tributary  river,  the  whole  of  the  detritus  may  be  deposited,  so  that,  at  the  outflow,  the 
river  becomes  as  clear  as  when  its  infant  waters  began  their  course  from  the  8priQgl^ 
snows,  and  mists  of  the  far  mountains.  Thus,  the  Rhone  enters  the  Lake  of  Genevi 
turbid  and  impetuous,  but  escapes  at  Geneva  as  blue  translucent  water.  Its  sediment  is 
laid  down  on  the  floor  of  the  lake,  and  chiefly  at  the  upper  end,  as  an  important  delta 
which  quite  rivals  that  of  a  great  river  in  the  sea.  Hence,  lakes  act  as  filters  or  sieves 
to  intercept  the  sediment  which  is  travelling  in  the  rivers  from  the  high  grounds  to  the 
sea  (p.  405).^ 

(e)  Estuarine  Deposits ;  Bars  and  Lagoon -barriers. — If  we  take  a 
broad  view  of  terrestrial  degradation,  we  must  admit  that  the  deposit  of 
any  sediment  on  the  land  is  only  temporary ;  the  inevitable  destination 
of  all  detrital  material  is  the  floor  of  the  sea.  Where  a  gently  flowing 
river  comes  within  the  influence  of  the  alternate  rise  and  fall  of  the 
tides,  a  new  set  of  conditions  is  established  in  regard  to  the  disposal  of 
the  sediment.  During  the  flow  of  the  tide  in  the  Severn,  for  example,  the 
suspended  mud  is  carried  up  the  estuary,  and  sometimes  far  up  its  tribu- 
taries. For  two-thirds  of  the  el)b,  though  the  surface-water  is  running  out 
rapidly,  the  bottom-water  is  practically  at  rest :  only  during  the  remaining 
third  of  the  ebb  does  the  bottom-water  flow  outwards  and  with  sufficient 
velocity  to  scour  the  channel.  But  this  lasts  for  so  short  a  time  that  it 
hardly  removes  as  much  mud  or  sand  as  has  been  laid  down  during  flood 
and  the  earlier  part  of  ebl>-tide.  Hence  the  sediment  is  in  a  state  of 
contiiuial  oscillation  upward  and  downward  in  the  estuary.  At  the 
lower  end,  some  portion  of  it  is  continually  being  swept  out  to  sea.  At 
the  upper  end,  fresh  material  of  similar  kind  is  being  supplied  by  the 
river.  But,  between  these  two  limits,  the  same  sediment  may  be  kept 
in  suspension  or  may  be  alternately  deposited  and  removed  for  many 
weeks  or  months  l)efore  it  finally  escapes  to  sea  and  is  spread  out  on  the 
bottom.  To  this  cause,  doubtless,  the  remarkable  turbidity  of  many 
estuaries  is  to  be  attri])uted.- 

AVhere  a  river,  with  a  considerable  velocity  of  current,  enters  the  sea, 
its  mouth  is  commonly  obstructed  by  a  bar  of  gravel,  sand,  or  mud.  The 
formation  of  this  barrier  results  from  the  conflict  between  the  river  and 
the  ocean.  The  muddy  fresh  water  floats  on  the  heavier  salt  water,  its 
current  is  lessened,  and  it  can  no  longer  push  along  the  mass  of  detritus 
at  the  bottom,  which  therefore  accumulates  and  tends  to  form  a  bar. 
Moreover,  tis  already  mentioned  (p.  381),  though  fresh  water  can  for  a 
long  time  retain  fine  mud  in  suspension,  this  sediment  is  rapidly 
thrown  down  when  the  fresh  is  mixed  with  saline  water.  Hence,  apart 
from  the  necessary  loss  of  transporting  power  by  the  checking  of  the 
current  at  the  river- mouth,  the  mere  mingling  of  a  river  with  the 
sea  must  of  itself  be  a  cause  of  the  deposit  of  sediment.  Moreover, 
in  many  cases  the  sea  itself  piles  up  great  part  of  the  sand  and  gravel  of 

^  Cousult  a  suggestive  essay,  G.  K.  Gilbert  ou  the  topographic  features  of  lake-shorai, 
5tk  Ann.  Rf'p.  U.S.  Oeoi.  Snrv.  1885,  p.  7r>. 

^  See  au  interesting  paper  by  Prof.  Sollas,  Q.  J.  Oeoi.  Soc.  xxxix.  (1883),  p.  611,  and 
authorities  there  cited. 


8BCT.  ii  §  3 


GEOLOGICAL  ACTION  OF  RIVERS 


399 


the  bar.  Heavy  river- floods  push  the  bar  further  to  sea,  or  even 
temporarily  destroy  it ;  storms  from  the  sea,  on  the  other  hand,  drive  it 
further  up  the  stream. 

Some  of  these  facts  in  the  economy  of  rivers  have  been  well  studied  at  the  mouths  of 
the  Mississippi.  At  the  south-west  pass,  the  bar  is  equal  in  bulk  to  a  solid  mass  one 
mile  square  and  490  feet  thick,  and  advances  at  the  rate  of  338  feet  each  year.  It  is 
formed  where  the  river  water  begins  to  ascend  over  the  heavier  salt  water  of  the  gulf, 
and  consists  mainly  of  the  sediment  that  is  pushed  along  the  bed  of  the  river.  A 
singular  feature  of  the  Mississippi  bars  is  the  formation  upon  them  of  "mud  lumps." 
These  are  masses  of  tough  clay,  varying  in  size  from  mere  protuberances  like  tree-trunks, 
up  to  islands  several  acres  in  extent.  They  rise  suddenly,  and  attain  heights  of  from  3 
to  10,  sometimes  even  18  feet  above  the  sea-level.  Salt  springs  emitting  inflammable 
gas  rise  upon  them.  After  the  lapse  of  a  considerable  time,  the  springs  oease  to  give  ofif 
gas,  and  the  lumps  are  worn  away  by  the  currents  of  the  river  and  the  gulf.  The 
origin  of  these  excrescences  has  been  attributed  to  the  generation  of  carburetted  hydrogen 
by  the  decomposing  vegetable  matter  in  the  sediment  underlying  the  tenacious  clay  of 
the  bars.^ 

Conspicuous  examjiles  of  the  formation  of  detrital  bars  may  occasionally  be  observed 
at  the  mouths  of  narrow  estuaries,  as  at  r 
in  Fig.  133.  A  constant  struggle  takes 
place  in  such  situations  between  the  tidal 
currents  and  waves  which  tend  to  heap  up 
the  bar  and  block  the  entrance  to  the 
estuary,  and  the  scour  of  the  river  and 
ebb-tide  which  endeavours  to  keep  the 
passage  open. 

Another  remarkable  illustration  of  the 
contest  between  alluvium-carrying  streams 
and  the  land -eroding  ocean  is  shown  by 
the  vast  lines  of  bar  or  bank  which  stretch 
along  the  coasts  both  of  the  Old  and  the 
New  World.  The  streams  do  not  flow 
straight  into  the  sea,  but  nin  sometimes 
for  many  miles  parallel  to  the  shore-line,  accumulating  behind  the  barriers  into  broad 
and  long  lagoons,  but  eventually  breaking  through  the  barriei-s  of  alluvium  and  entering 
the  sea.  On  a  small  scale,  examples  occur  on  the  coasts  of  the  British  Islands,  as  at 
Stert  Bay,  Devon  (Fig.  134),  where  the  slaters  (<•)  with  their  weathered  surface  {d)  are 


Fig.  133.— Shingle  and  sand-spit  (e)  at  the  mouth  of 
an  estuary  (c).  entered  by  a  river,  and  opening 
upon  an  exposwl  rocky  coast-line  {li.) 


Fig.  134.— Section  of  bar  and  lagc»on.    81apton  Pool,  Start  Bay,  Devon  (B.) 

flanked  by  a  fresh -water  lake  (c),  iK)nded  back  by  a  bar  (h)  from  the  sea  (a).  The 
lagoons  of  the  shores  of  the  Mediterranean,^  and  the  Kurische  and  Frische  Haf  in 
the  Baltic,  near  Dantzic,  are  familiar  examples.  A  conspicuous  series  of  these  alluvial 
bars  fronts  the  American  mainland  for  many  hundred  miles  round  the  Gulf  of  Mexico 
and  the  shores  of  Florida,  Georgia,  and  North  Carolina  (Fig.  135).*    A  space  of  several 


*  Humphreys  and  Abbot,  '  Report  on  Mississippi  River,'  1861,  p.  452. 

*  For  an  account  of  these  see  Ansted,  Min.  Proc.  Inst,  Civ.  Engin.  xxviii.  (1869),  p.  287. 
'  See  Report  by  H.  D.  Rogers,  Brit.  Assoc,  iii.  p.  13. 


Iir.\'A.VI'-AL  HHOLOUy 


nooK  III  PAST  II 


hundred  miles  nil  Ihe  rast  coa.it  al  India  U  HiniiUrty  bordered.     Elie  de   Bmimont. 
indeed,  estimated  that  aluut  a  third  iif  tlic  wliole  of  the  coast-Uiieii  of  the  contiDcnti  u 

■  with  such  alhivisl  lars.' 


Oil  a  ciiaat-liiif  siK'h  as  tliat  of  Western  EuTO|>e,  subjofl  boUi  to  ix)»-erful  tidal  actiim 
■lid  tu  strong  (jail's  of  wind,  many  iuti-restiuf;  illuHtrations  may  be  atiidied  of  the  Htttigf[le 
lietween  the  riven  and  the  sea,  an  to  the  disposal  of  the  sediment  borne  from  tlie  land. 
De  la  Bei'he  ilmerilird  an  exani)>le  from  the  coa«t  of  Soutli  Waleii  vhire  tvo  strMmi, 
the  Tovey  and  NVhl  (u  ami  b.  Tig.  13Si.  inttr  Swansea  Bay.  bearing  with  them  a 


B»)r(ft) 


euniiiderablu  .1  mount  of  Handy  and  muddy  >e(liniclit.  Tlie  line  mud  is  carried  hy  tht 
el>li-tide  ,7  /  /;.  into  tim  shetteml  lay  Iwtwwn  Swaniea  (f  1  and  the  Mumble  Bocki  [f] 
Ixit  in  partly  suept  round  tliia  headlalul  into  tlie  Hri^itut  Channel.  Tlie  «wiMr  landf 
KcliiiiGiit,  more  raj-idly  tlirunii  tliwiu  is  stirr«<l  up  and  driven  sliorewards  by  the  breaken 
caustiil  by  the  iireval.-iit  *ve*t  uud  .south-w«it  wiiidK  or).  The  sandy  flats  thereby  formed 
are  [lartly  ununvenil  at  Uin-  water,  and  liring  then  dried  by  the  wind,  supply  it  with  the 
kuiiil  Kliich  it  bhiws  inUnd  to  form  the  liuen  of  uand-duiK-*  (//).' 

(/)  Dcltiis  in  the  Sf;i. — The  tendfiicy  of  swimieiit  to  acciimtilate  in  a 
toiigtic  of  flat  IhikI  u'liL-ii  n  river  loses  itself  in  h  lake,  la  exhibited  on  a 
vustLT  iti-itle  whei-e  tht-  jpvut  rivera  of  the  i-ontiiients  enter  the  Bca.     It 

'  '  I.*raiiis  de  Gi'-ologir  i>r.itii(iii-,'  i.  p.  2i9,  In  tins  volonje  some  interestiog  examjilcf 
of  Ihi»  kind  of  deposit  are  .le.'wril>e<l. 

'  'Geological  ObserviT,"  p.  SS. 


SECT,  ii  §  3  GEOLOGICAL  ACTION  OF  RIVERS  401 

was  to  one  of  these  maritime  accumulations,  that  of  the  Nile,  that  the 
Greeks  gave  the  name  delta,  from  its  resemblance  to  their  letter  A,  with 
the  apex  pointing  up  the  river,  and  the  base  fronting  the  sea.  This  shape 
being  the  common  one  in  all  such  alluvial  deposits  at  river  mouths,  the 
term  delta  has  become  their  general  designation.  A  delta  consists  of 
successive  layers  of  detritus,  brought  down  from  the  land  and  spread  out 
at  the  mouth  of  a  river,  until  they  reach  the  surface,  and  then,  partly  by 
growth  of  vegetation  and  partly  by  flooding  of  the  river,  form  a  plain,  of 
which  the  inner  and  higher  portion  comes  eventually  to  be  above  the 
reach  of  floods.  Large  quantities  of  drift-wood  are  often  carried  down, 
and  bodies  of  animals  are  swept  off"  to  be  buried  in  the  delta,  or  even  to 
be  floated  out  to  sea.  Hence,  in  deposits  formed  at  the  mouths  of  rivers, 
we  may  always  expect  to  find  terrestrial  organic  remains. 

A  delta  does  not  necessarily  form  at  every  river-mouth,  even  where 
there  is  plenty  of  sediment.  In  particular,  where  the  coast-line  on  either 
side  is  lofty,  and  the  water  deep,  or  where  the  coast  is  swept  by  powerful 
tidal  ciurents,  there  is  no  delta.  ^  In  some  cases,  too,  the  sediment  spreads 
out  over  the  sea-bottom  without  being  allowed  by  the  sea  to  build  itself 
up  into  land,  as  happens  at  the  mouths  of  some  of  the  rivers  in  the  north- 
west of  France.  Considerable  influence  may  be  exerted  by  tides  and 
ciurents  in  arresting  or  facilitating  the  spread  of  sediment  over  the  sea- 
floor.  The  deltas  of  the  Ehone,  Nile,  Tiber,  and  Danube  ^re  formed  in 
tideless  or  nearly  tideless  seas.^ 

When  a  river  enters  upon  the  delta  portion  of  its  course,  it  assumes  a 
new  character.  In  the  previous  parts  of  its  journey  it  is  augmented  by 
tributaries ;  but  now  it  begins  to  split  up  into  branches,  which  wind  to 
and  fro  through  the  flat  alluvial  land,  often  coalescing  and  thus  enclosing 
insular  spaces  of  all  dimensions.  The  feeble  current,  no  longer  able  to 
l)eiir  along  all  its  weight  of  sediment,  allows  much  of  it  to  sink  to  the 
lx)ttom  and  to  gather  over  the  tracts  which  are  from  time  to  time 
submerged.  Hence  many  of  the  channels  are  choked  up,  while  others  are 
opened  out  in  the  plain,  to  be  in  tiu*n  abandoned ;  and  thus  the  river 
restlessly  shifts  its  channels.  The  seaward  ends  of  at  least  the  main 
channels  grow  outwards  by  the  constant  accumulation  of  detritus  pushed 
into  the  sea,  unless  this  growth  chances  to  be  checked  by  any  marine 
current  sweeping  past  the  delta. 

These  features  are  nowhere  more  strikingly  displayied  than  by  the  great  delta  of  the 
Mississippi  (Fig.  137).  The  area  of  this  vast  expanse  of  alluvium  is  given  at  12,300 
square  miles,  advancing  at  tlie  rate  of  262  feet  yearly  into  the  Gulf  of  Mexico  at  a  point 
which  is  now  220  miles  from  the  head  of  the  delta. ''  On  a  smaller  scale  the  rivers  of 
Euro|ie  furnish  many  excellent  illustrations  of  delta-growth.     Thus  the  Rhine,  Meuse, 

^  Consult  Admiral  Spratt's  memoir,  *  An  investigation  of  the  effect  of  the  prevailing 
wave  influence  on  the  Nile's  deposit,'  folio,  LondoB,  1859. 

-  For  a  discnssiou  on  non-tidal  rivers,  see  Min.  Proc,  Inst.  Civ.  Kngin,  Ixxxii.  (1885), 
])p.  2-68,  where  information  is  given  about  the  Tiber  and  some  other  rivers. 

'  Humphreys  and  Abbot,  op.  cit.  ;  see  also  C.  Hartley,  Min.  Proc.  Inst,  Civ.  Engin, 
xl.  p.  185.  The  tide  has  a  mean  rise  of  15  inches  every  24  hours  at  the  Mississippi 
mouths. 

2  D 


Wi 


DISAMICAL  GEOLOGY 


BOOK  m  TAXI  II 


Suiilin'.  ScLddt,  and  other  riven  liave  fiirnied  the  wide  m&ritiine  ]ilain  of  Holland  ud 
tll«  X^tlmlandi.  Tlir  Kliune,  wLkh  bw  dei>ositeU  an  iiapoibuit  delta  in  the  Hedila- 
nuieMi  Sea.  U  cotnpiited  to  furuUli  evtry  year  (by  tbe  Petit  RhQne)  about  four  millioiu 
of  cuHv  nietreit  of  seiliuient  to  tbe  sliores.'  The  upper  reaches  of  tbe  Adriatic  Sea  m 
Iieiug  so  npidly  ahallowed  and  filled  up  by  the  Fo,  Adi)^,  and  other  ■treama,  that 
Kavcnua.  ori^Hiuilly  l>iiitt  in  a  latioon  like  Venice,  ia  lion  4  miles  ^m  th«  sea ;  and  the 
jiiirt  of  Ailria,  ao  welt  known  in  andent  limes  as  to  have  given  its  name  to  the  Adrialk, 
is  now  14  utiles  inland,  while  <m\  other  jiarisof  that  const* line  the  breadth  of  land  gained 
within  tlw  last  ISOO  years  has  been  as  nioi-h  as  20  miles.  Borings  for  water  nearTeiuM 
to  n  depth  of  .ITJ  feet  have  diwlosed  a  su<>«e^)un  of  nearly  horizontal  clajs,  sanda,  and 
lipiiliferuuii  tviU.  Marine  shelta  ^Oirrfi'Hin.  &c.'i  occur  in  the  sandy  lajers  ;  the  lignites 
and  lifniitileruns  claya  niutaiu  laud- vegrlat  ion  and  terrestrial  shells  (Suneiiua,  Pwpa, 
Htlij;\  the  whole  fut-cessioti  ofdejioiitts  iudintiii(;an  alternation  of  marine  and  terrestrisl 


■:f  :h-.  Miftnitt.     T-.*  ' 


'  On  ihe  opiy^iM  tii*  ii  zL*  IcalLin  peninsula,  gnmx  additioni 
**:-'.::;<  «;;!;:•.:  ;>.e  ^tstori-.-i  ^«^i^d,  It  U  compatcd  that  tlw 
.  ::;'ii;>.  i»  li  r.;;''i-,u  -."siVii'  yird*  of  sediment  enerr  year  viiiun 
T:'r<r.  a  it  was  aptly  leraed  by  Hr 
'■i*  -.-J  .<.-!  ''^T  :.;  :he  VL'US'iat:!.-*  vf  xht  ««dixea:  which  it  arries  to  •>«.  It 
i:i  ii.liL^  :.>  th*  AM»;-li:ie  a?.';i::d  i;s  :::->uth  s:  the  race  of  from  It  to  13  itet 
V-r  is.y.e~:  '^.arSsiTi'fOftijt  it  r.-v  ,v~<e^i^:e3:Iy9Mln  tkanSmOeaiBkDd. 
,1V;  Sn-  i^rtially  e.i.-avateii.  b~i:  it^:t  £^•>i  .-f  :h«  rivo'  l<«n*  a  thick 
i-;i  ■::  :i.-  «r«>  o/i  oa  tie  tvw  ■;:  :!■;  -^^..--jverwi  toiun.  Heoec  it  woqjd 
:1;  Ti'.cr  '^u  ~::  .nly  ainrii.ini  t»  .Mu:-'.i::<  ':&:  hat  raised  in  bed  sn  Ac 
z.t  ii;r^:-,:i,'.'.i-L--i-n,ifii.i:i:-^-:v  .-.T.-d:*!  [-Lk-es  which.  :^>» yean  a^ 


.  JL  tix-nid,  Jfia.  Pthl 
•>«"-  Jf-i^ix.  115721, 


GEOLOGICAL  ACTION  OF  RIVERS 


403 


couid  not  have  been  bo  frequently  uuder  water.'  In  the  Black  Sea,  a  great  delt»  is 
rapidly  growing  at  the  moutha  of  the  I>anul>e.  At  the  Kilia  outlets  the  water  ia 
shallowiDg  so  fast  that  the  lines  of  soundings  of  6  feet  and  30  feet  are  advancing  into  the 
sea  at  the  rate  of  between  300  and  400  feet  per  annum.'  The  typical  delta  of  the  Kile 
has  a  seaward  border  180  miles  in  length,  the  distance  from  which  to  the  apex  of  the 
plain  where  the  river  bifurcates  is  90  mileH.*  The  united  delta  of  the  Ganges  and  Brahma- 
putra (Fig.  138)  covers  a  space  of  between  60,000  and  flO,000  square  miles,  and  has  been 
bored  through  to  a  depth  of  481  feet,  the  whole  luass  of  deposits  consiating  of  fine  sands 
and  clays,  with  occasional  pebble-beds,  a  bed  of  ]ieat  and  remains  of  trees,  but  with  no 
trace  of  any  marine  organism.* 


(?)  Sea-borne  Sediment- — Although  more  properly  to  be  noticed 
under  the  section  on  the  Sea,  the  final  course  of  the  materials  worn  by 
rains  and  rivers  from  the  surface  of  the  land  may  be  referred  to  here.  By 
far  the  larger  part  of  these  materials  sinks  to  the  bottom  close  to  the 
land.     It  is  only  the  fine  mud  carried  in  suspension  in  the  water  which  is 

'  See  an  interesting  article  by  Profes.sor  Charles  Martins  on  the  Aigues- Stories  (i.e. 
dead  waters  or  disused  river- channels),  in  Reevt  da  iMiu:  ilonda.  1874,  p,  780.  I 
accompanied  the  diatinguiahed  French  geologist  on  the  occasion  of  his  visit  to  Ostia  Id  the 
spring  o(  1873,  and  was  much  struck  with  the  proofs  of  the  rapidity  of  deposit  in  favourable 
situations.  In  the  article  just  cited,  and  in  another  in  Complea  rtnd.  Ixxviii.  p.  1748,  some 
valuable  icrormation  is  given  regarding  the  progre^  of  the  delta  of  the  Rhone  in  the 
Mediterranean.  Interesting  historii:al  data  as  to  geological  changes  at  the  mouths 
of  the  Rhine,  Heuse,  EilK^,  Po,  Rhone,  and  other  European  rivers,  as  well  us  of  the 
Nile,  will  be  found  in  £lie  de  Beaumont's  '  Lei,'otis  de  Gi'ologie  pratique,'  vol.  i.  p.  '15i. 

'  Hartley,  Mia.  Pnx.  Inst.  Civ.  Enffia.  ixivii.  p.  216. 

'  For  a  detailed  study  of  the  Nile  delta  in  its  geological  aspects,  see  an  essay  by  Dr.  J. 
Jank6,  HiUhaL  Jakrb.  ffftn.   Vngariacktn  Gevl.  AtuI.  viii.  (1890),  p.  236. 

'  For  a  full  account  of  the  alluvium  of  the  Indo-Gangetic  plain,  see  Medlicott  and 
Blanford's  'Qeology  of  India,'  chap,  xvii.,  and  authorities  there  cited  ;  also  a  more  recent 
paper  by  Mr.  Medlicott,  Raordi  Gtnl.  Surr.  India   1881,  p.  220, 


404  DYXAMirAL  GEOLOGY  book  m  partii 

carried  out  to  sea.  In  none  of  the  niimeroiis  soundings  and  dredgings  in 
the  Gulf  of  Mexico  has  Mississippi  mud  l)een  obtained  from  the  bottom 
more  than  100  miles  eastwanl  from  the  mouth  of  the  river.^  The  sound- 
ings taken  by  the  Clwllenijer,  however,  ])rought  up  land-derived  detritus 
from  depths  of  1500  fathoms — 200  miles  or  more  from  the  nearest  shores 
(p.  465).  The  sea  fronting  the  Amazon  is  sometimes  discoloured  for  300 
miles  by  the  mud  of  that  river. 

§  4.  Lakes. 

Depressions  filled  with  water  on  the  surface  of  the  land,  and  known 
as  Lakes,  occur  abundantly  in  the  northern  })arts  of  both  hemispheres, 
and  more  sjjaringly,  but  often  of  large  size,  in  warmer  latitudes.  For 
the  most  part,  they  do  not  belong  to  the  normal  system  of  erosion  in 
which  ruruiing  water  is  the  prime  agent,  and  to  which  the  excavation  of 
valleys  and  ravines  must  be  attributed.  On  the  contrary,  thev  are 
exceptional  to  that  system,  for  the  constant  tendency  of  running  water 
is  to  fill  them  up.  Their  origin,  therefore,  nuist  l>e  sought  among  some 
of  the  other  geological  processes.     (See  Book  VII.) 

Lakes  are  conveniently  classed  as  fresh  or  salt.  Those  which  possess 
an  outlet  contain  in  almost  all  cases  fresh  water;  those  which  have  none 
are  usually  salt. 

1.  Fresh -WAtar  Lakes. — In  the  northern  paits  of  Europe  and 
America,  as  first  emphasised  by  Sir  Andrew  C.  Ramsay,  lakes  are 
prodigiously  abuiidftnt  on  use-worn  rock-surfaces,  irrespective  of  dominant 
lines  of  drainAgOf  Tbey  seem  to  be  distributed  as  it  were  at  ran- 
dom, being  found  now  on  the  summits  of  ridges,  now  on  the  sides  of 
hills,  and  now  over  broad  plains.  They  lie  for  the  most  part  in  rock- 
basins,  but  many  of  them  have  Ixirriers  of  detritus.  Their  coimection 
with  the  operations  of  the  glacial  period  will  l)c  afterwards  alluded  to. 
In  the  mountainous  regions  of  tem])erate  and  polar  latitudes,  lakes 
abound  in  valleys,  and  are  coiuiected  -vrith  main  drainage -lines.  In 
North  America  and  in  Equatorial  Africa,  vast  sheets  of  fresh  water  occiu* 
in  depressions  of  the  land,  and  are  rather  inland  seas  than  lakes. 

The  water  of  many  lakes  has  l>een  observed  to  rise  alx)ve  its  normal 
level  for  a  few  minutes  or  for  more  than  an  hoiu*,  then  to  descend 
beneath  that  level,  and  to  continue  this  vibration  for  some  time.  In  the 
Ijake  of  Geneva,  where  these  movementvS,  locally  known  there  as  Seicheiiy 
have  long  been  net  iced,  the  amplitude  of  the  oscillation  ranges  up  to  a 
metre  or  even  sometimes  to  two  metres.  These  disturbances  may  some- 
times l>e  due  to  subterranean  movements  ;  Init  probably  they  are  mainly 
the  effect  of  atmospheric  perturbations,  and,  in  particular,  of  local  stonna 
with  a  vertical  descending  movement.*- 

*  A.  Agassiz,  Amer.  Acad.  xii.  (1882),  j).  108. 

-  F.  A.  Forel,  Coiaptes  raid.  Ixxx.  (1875),  p.  107,  Ixxxiii.  (1876),  p.  712,  IxxxvL  (1878),. 
p.  1500,  Ixxxix.  (1879),  p.  859  ;  Assoc.  Fran(:aist\  1879,  p.  493.  P.  du  Bois,  CompitM 
r*'iid.  «;xii.  (1891),  p.  122.  For  a  valuable  mouograph  on  the  regime  of  a  typical  lake, 
Forel'M  *  Le  Ijuinau,'  Ijausanne,  1892. 


SECT,  ii  15  4  LAKES  405 

The  distribution  of  temperature  in  lakes  is  a  question  of  considerable 
geological  interest,  in  regard  to  which  careful  measurements  are  much 
needed. 

The  observations  of  Sir  Robert  Christison,  at  Loch  Lomond  in  Scotland,  show  that 
ill  this  sheet  of  water,  which  lies  25  feet  above  sea-level,  with  a  depth  of  about  600 
feet,  and  is  in  great  measure  surrounded  with  high  hills,  a  tolerably  constant  tempera- 
ture of  about  42**  Fahr.  is  found  to  pervade  the  lowest  100  feet  of  water.  ^  More 
extended  observations  have  since  been  made  by  Dr.  John  Murray  and  the  staff  of  the 
Scottish  Marine  Station  in  Lochs  Ness,  Oich,  Morar,  and  Shiel,  as  well  as  in  some  of  the 
Qords  and  sounds  of  the  west  of  Scotland,  and  the  earlier  observations  have  been 
con6rmed.  The  surface  of  Loch  Morar  in  September  1887  was  found  to  have  a 
temi)erature  of  57 "S**  Fahr.,  but  at  a  depth  of  160  fathoms  the  thermometer  had  fallen 
to  42*1°.  The  surface  temperature  of  Loch  Ness  in  the  same  month  was  54**,  but  at  120 
fathoms  42*1".-  Again,  in  the  Lake  of  Geneva  the  surface  temjjerature  in  autumn  is 
78*  Fahr.,  while  the  bottom  water  at  a  depth  of  950  feet  was  found  to  mark  41**  7'. 
The  Lago  Sabatino  near  Rome  has  a  temperature  of  77"  at  the  surface,  but  one  of  44** 
at  a  depth  of  490  feet.  Similar  observations  on  other  deep  lakes  in  Switzerland  and 
Northern  Italy  indicate  the  existence  in  all  of  them  of  a  permanent  mass  of  cold  water 
at  the  bottom.  The  cold  heavy  wa%er  of  the  surface  in  winter  must  sink  down,  and  as 
the  upper  layers  cannot  be  heated  by  the  direct  rays  of  the  sun,  save  to  a  trifling  and 
superficial  extent,  the  temperature  of  the  deep  parts  of  these  basins  is  kept  i)ermanently 
low. 

Geological  functions. — Among  the  geological  functions  discharged 
by  lakes  the  following  may  be  noticed  : 

Ist  Lakes  equalise  the  temperature  of  the  localities  in  which  they 
lie,  preventing  it  from  falling  as  much  in  \vinter  and  rising  as  much  in 
summer  as  it  would  otherwise  do.^  The  mean  annual  temperature  of 
the  surface  water  at  the  outflow  of  the  Lake  of  Geneva  is  nearly  4° 
warmer  than  that  of  the  air. 

2nd.  Lakes  regulate  the  drainage  of  the  area  below  their  outfall, 
thereby  preventing  or  lessening  the  destnictive  effects  of  floods.* 

3rcl.  Lakes  filter  river-water  and  permit  the  undisturbed  accumulation 
of  new  deposits,  which  in  some  modern  cases  may  cover  thousands  of 
square  miles  of  siu^face,  and  may  attain  a  thickness  of  nearly  3000  feet 
(Lake  Superior  has  an  area  of  32,000  s(|uare  miles ;  Lago  Maggiore 
is    2800    feet   deep).       How    thoroughly    lakes    can    filter   river -water 

*  For  observations  on  the  freezing  of  this  and  other  lakes,  see  J.  Y.  Buchanan,  Nature, 
xix.  p.  412.  On  the  deep-water  temperature  of  lakes,  A.  Buchan,  BriL  Assoc,  1872, 
Sects,  p.  207. 

^  Pwc,  Roy,  Sac.  Edin.  xviii.  (1890-91),  p.  139. 

^  The  lakes  of  Sweden,  which  cover  one-twelfth  of  the  surface  of  the  country,  exercise 
an  important  influence  on  climate  according  as  they  are  frozen  or  open.  See  Prof. 
Hildebrandsson  on  the  freezing  and  breaking- up  of  the  ice  on  the  Swedish  lakes.  Ann.  Bur. 
Central  Mitiorol,  France,  1878. 

*  Winds,  by  blowing  strongly  down  the  length  of  a  lake,  sometimes  considerably 
increase,  for  the  time  being,  the  volume  of  the  outflow.  If  this  takes  place  coincidently 
with  a  heavy  rainfall,  the  flood  of  the  escaping  river  is  greatly  augmented.  These 
features  are  noticed  in  Loch  Tay  (D.  Stevenson,  'Reclamation  of  Land,'  p.  14).  Hence, 
though  on  the  whole  lakes  tend  to  moderate  floods  in  the  outflowing  rivers,  they  may,  by  a 
combination  of  circumstances,  sometimes  increase  them. 


406 


DYNAMICAL  GEOLOGY 


BOOR  ni  PART  II 


is  typically  displayed  by  the  contrast  between  the  muddy  river  which 
flows  in  at  the  head  of  the  Lake  of  Geneva,  and  the  "  blue  rushing  of 
the  arrowy  Rhone,"  which  escapes  at  the  foot.  The  mouths  of  small 
brooks  entering  lakes  aiford  excellent  materials  for  studjnng  the  behaviour 
of  silt-bearing  streams  when  they  reach  still  water.  £ach  rivulet  may 
be  observed  pushing  forward  its  delta  composed  of  successive  sloping 
layers  of  sediment  (ante,  p.  397).  On  a  shelving  bank,  the  coarser 
detritus  may  repose  directly  upon  the  solid  rock  of  the  district  (Fig.  139). 


Fig.  13J».— Section  of  a  delta-cone  imslied  by  a  brook  into  a  lake. 

But  as  it  advances  into  the  lake,  it  may  come  to  rest  upon  some  older 
lacustrine  deposit  (Fig.  140).  The  river  Linth  since  1860  has  annually 
discharged  into  Lake  Wallenstadt  some  62,000  cubic  metres  of  detritus. 


Fig.  140.— Streaui-tletritus  piisheil  forwanl  over  a  previous  lacustrine  silt  (B.) 

A  river  which  flows  through  a  succession  of  lakes  cannot  cany  much 
sediment  to  the  sea,  unless  it  has  a  long  course  to  nin  after  it  has  passed 
the  lowest  lake,  and  receives  one  or  more  muddy  tributaries  (see  p.  397). 
Let  us  suppose,  for  example,  that,  in  a  hilly  region,  a  stream  passes 
through  a  series  of  lakes  (as  a,  h,  c,  in  Fig.  141).     As  the  highest  lake 


Fig.  141.— Filling  up  of  a  Huccession  of  lakes  (6.) 

will  intercept  much,  perhaps  all,  of  this  sediment,  the  next  in  succession 
will  receive  little  or  none  until  the  first  is  either  filled  up  or  has  been 
drained  by  the  cutting  of  a  gorge  through  the  intervening  rock  at  /. 
The  same  process  will  be  repeated  at  e  and  d  until  the  lakes  are  effaced, 
and  their  places  are  taken  by  allurial  meadows.  Examples  of  this 
sequence  of  events  are  of  frequent  occurrence  in  Britain. 

Besides  the  detrital  accumulations  due  to  the  influx  of  streams,  there 
are  some  which  may  properly  l)e  regarded  as  the  work  of  lakes  them- 
selves. Even  on  small  sheets  of  water,  the  eroding  influence  of  wind- 
waves  may  be  observed ;  but  on  large  lakes  the  wind  throws  the  water 
into  waves  which  almost  rival  those  of  the  ocean  in  size  and  destructive 
power.  Beaches,  sand-dunes,  shore-cliifs,  and  other  familiar  features  of 
the  meeting-line  between  land  and  sea,  reappear  along  the  margins  of 
such  great  fresh-water  seas  as  Lake  Superior.     Beneath  the  level  of  the 


SECT,  ii  §  4    *        GEOLOGICAL  FUNCTIONS  OF  LAKES  407 

water  a  terrace  or  platform  is  formed,  of  which  the  distance  from 
shore  and  depth  vary  with  the  energy  of  the  waves  by  which  it  is 
produced.     This  platform  is  well  developed  in  the  Lake  of  Geneva.^ 

Some  of  the  distinctive  features  of  the  erosion  and  deposition  that 
take  place  in  lake-basins  have  been  admirably  laid  open  for  study  in 
those  basins  of  vanished  lakes  which  have  been  so  well  described  by 
Gilbert,  Dutton,  Russell,  and  Upham  in  the  Western  Territories  of  the 
United  States.  They  have  been  treated  of  in  a  masterly  way  by  Gilbert 
in  his  essay  on  "  The  Topographic  featiu*es  of  Lake-shores."  ^ 

4th.  Lakes  serve  as  badns  in  which  chemical  deposits  may  take 
place.  Of  these  the  most  interesting  and  extensive  are  those  of  iron-ore, 
which  chiefly  occur  in  northern  latitudes  (pp.  146,  483).^ 

5th.  Lakes  fiu*nish  an  abode  for  a  lacustrine  fauna  and  flora,  receive 
the  remains  of  the  plants  and  animals  washed  down  from  the  surround- 
ing country,  and  entomb  these  organisms  in  the  growing  deposits,  so  as 
to  preserve  a  record  of  the  lacustrine  and  terrestrial  life  of  the  period 
diudng  which  they  continue.     Besides  the  more  familiar  pond-snails  and 
fishes,  lakes  possess  a  peculiar  pelagic  fauna,  consisting  in  large  measure 
of  entomostracous  crustaceans,   distinguished   more  especially  by  their 
transparency.*      These,  as  well  as   the   organisms  of  shallower  water, 
doubtless  furnish  calcareous  materials  for  the  mud  or  marl  of  the  lake 
bottoms.     But  it  is  as  receptacles  of  sediment  from  the  land,  and  as 
localities  for  the  preservation  of  a  portion  of  the  terrestrial  fauna  and 
flora,   that   lakes   present   their  chief   interest   to   a   geologist.      Their 
deposits  consist  of  alternations  of  sand,  silt,  mud,  gravel,  and  occasional 
irregular  seams  of  vegetable  matter,  together  with  layers  of  calcareous 
marl    formed    of    lacustrine    shells,    Entomostraca,    &c.    (p.    484).       In 
lakes    receiving   much    sediment,   little    or    no   marl    can    accumulate 
during  the  time  when  sediment  is  being  deposited.     In  small,  clear,  and 
not  very  deep  lakes,  on  the  other  hand,  where  there  is  little  sediment, 
or  where  it  only  comes  occasionally  at  intervals  of  flood,  thick  beds  of 
white  marl,   formed   entirely  of   organic  remains,   may  gather  on   the 
bottom,  as  has  happened  in  numerous  districts  of  Scotland  and  Ireland. 
The  fresh-water  limestones  and  clays  of  some  old  lake-basins  (those  of 
Miocene    time    in    Auvergne   and    Switzerland,   and    of   Eocene   age  in 
Wyoming,   for  example)    cover  areas    occasionally   hundreds  of  square 

^  D.  CoUadon,  BuU.  Soc.  OSoL  Frarice  (3),  iu.  p.  661. 

'  hth  Ann.  Report  U.S.  Oeol.  Survet/y  1885.  See  also  Dutton,  in  2nd  Report  of  same 
Survey,  1880-81,  p.  169  ;  I.  C.  Russell,  3nrf  Report  U.S.  Survey,  1881-82,  p.  195  ;  ith 
Report,  1882-83,  p.  435  ;  Sth  Report,  1886-87,  p.  201  ;  and  Monograph  XI.  (1885)  of 
Hame  Survey.  W.  Upham  on  the  beaches  and  terraces  of  a  former  glacial  lake  (Lake 
Agassiz)  BiUl.  U.S.  Geol.  Survey,  No.  39  (1887) ;  also  ^th  Ann,  Report  Oeol.  and  Nat. 
Hist.  Survey  Minnesota  (1879),  i)p.  84-87  ;  H.  W.  Turner  on  a  vanished  lake  in  Mohawk 
Valley,  Plumas  County,  California,  Bull.  Phil.  Soc.  Washington,  xi.  (1891),  p.  385. 

'  For  an  elaborate  paper  on  these  lake-ores  (See-erze),  see  StapflF,  Z,  Deutach.  Oeol.  Oes. 
xviiu  pp.  86-173  ;  aUso  A.  F.  Thoreld,  Oeol.  FUren.  Stockholm.  Forh.  iii.  p.  20  ;  a,nd  postea, 
Section  iii.  p.  483. 

*  F.  A.  Forel,  Archives  d.  Sciences,  Sept.  1882.  0.  E.  Imhof,  Ann.  Mag,  Nat.  Hitt. 
1884,  p.  69. 


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HECT.  ii  S  J  SALIXE  LAKES  409 

IB,  hoHevei',  merely  tlie  shrunk  rcmoant  of  a  once  far  more  exteosirc  sheet  of  water,  to 
uliich  the  name  of  Lake  Bonneville  haa  been  given  by  Qilbert  It  is  |«rt]y  surrounded 
with  mountains,  along  the  sides  of  which  well-Jeliued  lines  of  terrace  mark  former  levels 
of  the  water  (Fig.  142).  The  high^tof  these  terraces  lies  about  940  feet  above  the  iiresent 
surface  of  the  lake,  so  that  when  at  its  greatest  dimensions,  this  vast  sheet  of  water  must 
have  stood  at  a  level  of  about  5200  feet  above  the  sea,  aud  covered  an  area  of  300  miles 
from  north  to  sonth,  and  180  miles  in  extreme  width  from  east  to  west.  It  was  then  cer- 
tainly fresh,  for,  having  an  outlet  to  the  north,  it  drained  into  the  Pacilic  Ocean,  and  in 
its  stratified  deposits  ail  abundant  lacustrine  molliiscanfaunahos  been  found.^  According 
to  Gilbert  there  are  proofs  that,  previous  to  the  great  extension  of  Lake  Bonneville,  there 
was  a  dry  period,  during  which  considerable  accumulations  of  subaerial  detritus  were 
formed  along  the  slopes  of  the  mountains.  A  great  meteorological  change  tlien  took 
placp,  and  the  whole  vast  basin,  not  only  that  termed  Lake  Bonneville,  hut  a  second 
large  basin.  Lake  Lahontaii  of  Kiug,  lying  to  the  west  and  hardly  inferior  in  area,  was 
gradually  filled  with  fresh  water.  Again,  another  meteorological  revolution  supervened 
and  the  climate  once  mora  became  dry.  The  waters  shrank  back,  and  in  so  doing,  when 
they  had  sunk  below  the  level  of  their  outlet,  began  to  grow  increasingly  saline.  The 
decreara  of  the  water  and  the  increase  of  salinity  were  in  direct  relation  to  each  otiier 


UDtil  the  present  dtgrce  of  concentration  has  been  reached,  as  shown  in  the  table  (p.  41 1 ). 
The  Creat  Salt  Iiake,  at  present  having  an  extreme  depth  of  less  than  50  feet,  is  still 
subject  to  oscillations  of  level.  When  surveyed  by  the  Stanshuiy  Expedition  in  1349, 
its  level  was  11  feet  lower  than  in  1877,  when  tlie  Survey  of  the  40tli  I'arallel  examined 
the  ground.  From  1866,  however,  a  slow  sulMidenee  of  the  lake  has  been  in  progress, 
consequent  upon  a  dindnntion  of  the  rainfall.  Large  tracts  of  flat  land,  formerly  under 
water,  are  being  laid  bare.  As  the  water  recedes  from  them  and  they  are  exposed  to  the 
remarkably  dry  atmosphere  of  these  regions,  they  soon  become  crusted  with  a  white  sali- 
ferous  and  alkaline  deposition,  which  likewise  jiermeatea  the  dried  mud  undemeatli.  So 
strongly  saline  are  the  waters  of  the  lake,  and  so  rapid  the  evaporation,  as  I  found  on 
trial,  that  one  Hoats  in  spite  of  oneself,  and  tbc  under  surfaces  of  the  wooden  steps 
leading  into  the  water  at  the  I >athing' places  are  hung  with  short  stalactites  of  salt  from 
the  evs)ioratiou  of  the  drip  of  the  emergent  bathers.^ 

1  For  an  account  of  this  fauna  sea  K.  E.  Call,  Jtull.  U.S.  Ucd.  Shtt.  No  11  (1884]. 

'  Much  information  regarding  the  Great  Basin  an<l  its  takes  is  to  be  found  in  vol.  iii.  of 
Wheeler's  SMmtff  H'eK  o/ 1  OOf/i  .l/enrfinn,  vols.  i.  and  iv.  ot  the  Survey  of  Iht  iOth  Paralld, 
and  Report  oj  C.K  Vei^.  Siirreg,  1880-81,  1.  C.  Kussell,  'Geological  Hislorj-  o(  Lake 
Lahontan,'  F'.X  Geul.  Surerg  Monognip/u,  No.  XI.,  and  in  the  papers  cited  ante.  p.  407. 


410  DYXAMICAL  GEOLOGY  book  iii  part  ii 

Some  of  the  smaller  lakes  in  tlie  great  arid  basin  of  Xorth  America  are  intenwlj 
bitter,  and  contain  large  quantities  of  carbonate  and  sulphate  as  well  as  chloride  of 
so<Iium.  The  Big  Soda  Lake  near  Ragtown  in  Nevada  contains  129*013  grammeB  of  aalti 
in  the  litre  of  water.  These  salts  consist  largely  of  chloride  of  sodium  (55*42  per  cent  of 
the  whole),  sulphate  of  soda  (14*86  i)er  cent),  carbonate  of  soda  (12*96  per  cent),  and  chloride 
of  iH>tassium  (3*73  per  cent).     Soda  is  obtained  from  this  lake  for  commercial  purpoees.' 

(b)  Salt  lakes  of  oceanic  origin  are  comparatively  few  in  number. 
In  their  case,  portions  of  the  sea  have  been  isolated  by  movements  of  the 
earth's  cnist ;  and  these  detached  areas,  exposed  to  evaporation,  which  is 
only  partially  compensated  by  inflowing  rivers,  have  shrunk  in  level,  and 
at  the  same  time  have  sometimes  grown  much  Salter  than  the  parent  ocean. 

The  Caspian  Sea,  180,000  square  miles  in  extent,  and  with  a  maximum  depth  of  from 
2000  to  3000  feet,  is  a  magnificent  example.  The  sliells  living  in  its  waters  are  chiefly 
the  same  as  those  of  the  Black  Sea.  Banks  of  them  may  be  traced  between  the  two 
seas,  with  salt  lakes,  marshes,  and  other  evidences  to  prove  that  the  Caspian  was  once 
joined  to  the  Black  Sea,  and  had  thus  communication  vrith.  the  main  ocean.  In  this  cHe 
also  there  are  proofs  of  considerable  changes  of  water-level.  At  present  the  suriaoe  of 
the  Caspian  is  85^  feet  l)elow  that  of  the  Black  Sea.  The  Sea  of  Aral,  also  sensibly  atit 
to  the  taste,  was  once  probably  united  with  the  Caspian,  but  now  rests  at  a  level  of  242*7 
feet  above  that  sheet  of  water.  The  stepjies  of  south-eastern  Russia  are  a  vast  depression 
with  numerous  salt  lakes  and  abundant  saline  and  alkaline  deposits.  It  has  been 
supposetl  that  this  depression  continued  far  to  the  north,  and  that  a  great  firth,  running 
up  between  Eurojte  and  Asia,  stretched  completely  across  what  are  now  the  steppes  and 
plains  of  the  Tundras,  till  it  merged  into  the  Arctic  Sea.  Seals  of  a  species  (/%oas 
caspica)  which  may  be  only  a  variety  of  the  common  northern  form  {Ph.faetida),  abound 
in  the  Caspian,  which  is  the  scene  of  one  of  the  chief  seal-fisheries  of  the  world.*  On 
the  west  side  of  the  Ural  chain,  even  at  present,  by  means  of  canals  connecting  the  riven 
Volga  and  Dwina,  vessels  can  pass  from  the  Ca.spian  into  the  White  Sea.' 

The  cause  of  the  isolation  of  the  Caspian  and  the  other  saline  basins  of  that  region 
is  to  be  sought  in  uiulerground  movements  which,  according  to  Helmersen,  are  still  in 
]»rogress,  but  jiartly,  and,  in  tlie  case  of  the  smaller  basins,  probably  chiefly  in  a  generd 
diminution  of  the  water-supply  all  over  Central  Asia  and  the  neighbouring  regions.  The 
rivers  that  flow  from  the  north  towards  Lake  Balkash,  and  that  once  doubtless  emptied 
into  it,  now  lose  themselves  in  the  wastes  and  are  cvaix>rated  before  reaching  that  sheet 
of  water,  which  is  fed  only  from  the  mountains  to  the  south.  The  channels  of  the  Amor 
Darya,  Syr  Darya,  and  other  streams  bear  witness  also  to  the  same  general  desiccation.^ 
At  present,  the  amount  of  water  supplie<l  by  rivers  to  the  Caspian  Sea  appears  on  the 


^  Bi'li.  U.S,  fJfiol.  Surv.  No.  9  (1884),  p.  25.  T.  M.  Chatard,  Amer,  Jaum,  Set. 
xxxvi.  (1888),  p.  148,  and  xxxviii.  (1889),  p.  59. 

-  Another  variety  or  species  of  seal  inhabits  Lake  Baikal.  For  an  account  of  the  stmctnre 
and  distribution  of  seals  see  an  interesting  monograph  by  J.  A.  Allen  in  Afiscelliinetnis 
Pubficalions  of  U.S.  (leolngical  and  (veographical  Survey  of  Oie  Territwies,  Washington, 
1880. 

^  Count  von  Helmersen,  however,  has  stated  his  belief  that  for  this  extreme  northern 
]irolougation  of  the  Aralo-Caspian  Sea  there  is  no  evidence.  The  shells,  on  the  presence  <A 
which  over  the  Tundras  the  opinion  was  chiefly  based,  are,  according  to  him,  all  fresh-water 
s]>ecies,  and  there  are  no  niariue  shells  of  living  species  to  be  met  with  in  the  plains  at  the 
foot  of  the  Ural  Mountains. 

**  Ilnll.  Aaui.  Imp.  St.  Ptfrrsbourtjf,  xxv.  p.  535  (1879).  For  an  account  of  these  rivers 
antl  Lake  Aral,  see  H.  Wood,  Joum.  Roy.  iieog.  S(k.  xlv.  (1875),  p.  867,  where  an  estimate 
is  given  of  the  annual  amount  of  evaporation. 


SECT,  ii  §  4 


SALINE  LAKES 


411 


whole  to  balance  that  removed  by  evaporation,  though  there  are  slight  yearly  or  seasonal 
fluctuations.  In  the  Aral  basin,  liowever,  there  can  be  no  doubt  that  tlie  waters  are 
progressively  diminishing,  the  rate  in  the  ten  years  between  1848  and  1858  having  been 
18  inches,  or  1  '8  inch  per  annum. 

Owing  to  the  enormous  volume  of  fresh  water  poured  into  it  by  its  rivers,  the  Caspian 
Sea  is  not  as  a  whole  so  salt  as  the  main  ocean,  and  still  less  so  than  the  Mediterranean 
Sea.  Nevertheless  the  inevitable  result  of  evaporation  is  there  manifested.  Along  the 
shallow  pools  which  border  this  sea,  a  constant  deposition  of  salt  is  taking  place,  forming 
sometimes  a  pan  or  layer  of  rose-coloured  crystals  on  the  bottom,  or  gradually  getting 
dry  and  covered  with  drift-sand-  This.concentration  of  the  water  is  particularly  marked 
in  the  great  offshoot  called  the  Karaboghaz,  which  is  connected  with  the  middle  basin 
of  the  Caspian  Sea  by  a  channel  1 50  yards  wide  and  5  feet  deep.  Through  this  narrow 
mouth  there  flows  from  the  main  sea  a  constant  current,  which  Von  Baer  estimated  to 
carry  daily  into  the  Karaboghaz  350,000  tons  of  salt.  An  appreciable  increase  of  the 
saltness  of  that  gulf  has  been  noticed  ;  seals,  which  once  frequented  it,  have  forsaken  its 
barren  shores.  Layers  of  salt  are  gathering  on  the  mud  at  the  bottom,  where  tliey  have 
formed  a  salt  bed  of  unknown  extent,  and  the  sounding  line,  when  scarcely  out  of  the 
water,  is  covered  with  saline  crystals.^ 

The  following  table  shows  the  proportions  of  saline  ingredients  in  1000  parts  of  the 
water  of  some  salt  lakes  : — 


Caspian  Sea. 

In. 

dertsch 

Lake 

(Giibel). 

239-28 
17-86 

101 

0*05 
0-42 

8-46 

Great  Salt  Lake, 

Utah  (0.  D. 

Allen). 

Elton 

Uke, 

Kirghis 

Rose). 

Dead 
Sea,from 
a  depth 

of  185 

fath- 

oniH. 

Conatituents  (except 
where  otherwise  stated^ 

Near  mouth  of 
R.  Ural 
(Gobel). 

At  Baku 
(Abich). 

Chloride  of  Sodinm  .    . 
,,          Magneninra 
,,         Calcium     . 

,,          Potassium 

Bromide  of  Magnesium 
Sulphate  of  Calcium 

„          Potassium . 

„          Magnesium 

3-673 
0-632 
0*013  (MgCOa) 

0*070 

trace 

0*490 

0*171  (CaCOj) 

1*289 

8-5267 
0*3039 

•  ■ 

trace 

1-0742 

0  0554  (CaCOs) 

8-2493 

118-628 
14*908 

•  • 

/  0862  (excess  ^ 
t      Chlorine)    i 

0*858* 

5*363 

9-321  (XaS04) 

38-3 
197-5 

2-3 
53*2 

78-554 

145-897 

31-075 

6-586 

1-374 
0-701 

Deposits  in  Salt  and  Bitter  Lakes. — The  study  of  the  precipitations 
which  take  place  on  the  floors  of  modern  salt  lakes  is  important  in 
throwing  light  upon  the  history  of  a  number  of  chemically-formed  rocks. 
The  salts  in  these  waters  accumulate  until  their  point  of  saturation  is 
reached,  or  until  by  chemical  reactions  they  are  thrown  down.  The  least 
soluble  are  naturally  the  first  to  appear,  the  water  becoming  progressively 
more  and  more  saline  till  it  reaches  a  condition  like  that  of  the  mother- 
liquor  of  a  salt  work.  Gypsum  begins  to  be  thrown  down  from  sea-water, 
when  37  per  cent  of  water  has  been  evaporated,  but  93  per  cent  of  water 
must  be  driven  off  before  chloride  of  sodium  can  begin  to  be  deposited. 
Hence  the  concentration  and  evaporation  of  the  water  of  a  salt  lake 
ha^'ing  a  composition  like  that  of  the  sea  would  give  rise  first  to  a  layer  or 
sole  of  gypsum,  followed  by  one  of  rock-salt.  This  has  been  found  to  be 
the  normal  order  among  the  various  saliferous  formations  in  the  earth's 

*  Von  Baer,  B*fU.  Acnd.  St.  PHershourg  (1855-56).  See  also  Carpenter,  Proc,  Roy. 
Oeog.  Soc.  xviii.  No.  4.  For  the  conipo.sition  of  the  water  of  salt  and  bitter  lakes,  see  the 
analyses  collected  by  Roth  in  his  '  CHiemische  Geologic, '  i.  p.  463  et  seq. 


412  DYNAMICAL  GEOLOGY  book  m  fart  ii 

crust.  But  gypsum  may  be  precipitated  without  rock-salt,  either  because 
the  water  was  diluted  before  the  point  of  saturation  for  rock-salt  was 
reached,  or  because  the  salt,  if  deposited,  has  been  subsequently  dissolved 
and  removed.  In  every  case  where  an  alternation  of  layers  of  gypsum 
and  rock-salt  occurs,  there  must  have  been  repeated  renewals  of  the  water- 
supply,  each  gypsum  zone  marking  the  commencement  of  a  new  series  of 
precipitates. 

But  from  what  has  now  been  adduced  it  is  obvious  that  the  composi- 
tion of  many  existing  saline  lakes  is  strikingly  unlike  that  of  the  sea  in 
the  proportions  of  the  different  constituents.  Some  of  them  contain 
carbonate  of  sodium ;  in  others  the  chloride  of  magnesium  is  enormously 
in  excess  of  the  less  soluble  chloride  of  sodium.  These  variations  modify 
the  effects  of  the  evaporation  of  additional  supplies  of  water  now  poured 
into  the  lakes.  The  presence  of  the  sodium -carbonate  causes  the 
decomposition  of  lime  salts,  with  the  consequent  precipitation  of  calcium- 
carbonate  accompanied  with  a  slight  admixture  of  magnesium-carbonate, 
while  by  further  addition  of  the  sodium-carbonate  a  hydrated  magnesium- 
carbonate  may  be  eventually  precipitated.  Hunt  has  shown  that  solutions 
of  bicarbonate  of  lime  decompose  sulphate  of  magnesia  with  the  consequent 
precipitation  of  gypsum,  and  eventually  also  of  hydrated  carbonate  of 
magnesia,  which,  mingling  with  carbonate  of  lime,  may  give  rise  to 
dolomite.^  By  such  processes  the  marls  or  clays  deposited  on  the  floors 
of  inland  seas  and  salt  lakes  may  conceivably  be  impregnated  and  inter- 
stratified  with  gypseous  and  dolomitic  matter,  though  in  the  Trias  and 
other  ancient  formations  which  have  been  formed  in  enclosed  saline  waters, 
the  magnesium-chloride  has  probably  been  the  chief  agent  in  the  produc- 
tion of  dolomite  (anie,  p.  321). 

The  Dead  Sea,  Elton  Lake,  and  other  very  salt  waters  of  the  Aralo-Cas|iian 
depression,  are  interesting  examples  of  salt  lakes  far  advanced  in  the  process  of  con-. 
centration.'-^  The  great  excess  of  the  magnesium -chloride  shows,  as  Bischof  pointed  ont, 
that  the  waters  of  these  basins  are  a  kind  of  mother-liquor,  from  which  most  of  the 
sodium -chloride  has  already  been  de[K)sit«d.  The  greater  the  proportion  of  the 
magnesium-chloride,  tlie  less  sodium-chlonde  can  be  held  in  solution.  Hence,  as  soon 
as  the  waters  of  the  Jordan  and  other  streams  enter  the  Dead  Sea,  their  proportioii  of 
sodium-chloride  (which  in  the  Jordan  water  amounts  to  from  '0525  to  '0608  jier  cent) 
is  at  once  ]>reci  pita  ted.  With  it  gypsum  in  crystals  goes  down,  also  the  carbonate 
of  lime  which,  though  present  in  the  tributary  streams,  is  not  found  in  the  waters  of  the 
Dea<l  Sea.  In  spring,  the  rains  bring  large  quantities  of  muddy  water  into  this  sea. 
Owing  to  dilution  and  diminished  evai)oration,  a  check  must  be  given  to  the  de|X)6ition 
of  common  salt,  and  a  layer  of  mud  is  formed  over  the  bottom.  As  the  summer 
advances  and  the  supply  of  water  and  mud  decreases,  while  evaporation  increases, 
the  deposition  of  salt  and  g}'i)sum  begins  anew.^  As  the  level  of  the  Dead  Sea  is  liable 
to  variations,  i)arts  of  the  bottom  are  from  time  to  time  exposed,  and  show  a  surface  of 
bluish -grey  clay  or  marl  full  of  crystals  of  common  salt  and  gypsum.     Beds  of  similar 

^  Sterry  Hunt,  in  'Geologj-  of  Canada'  (1863),  p.  575. 

-  The  Dead  Sea,  like  the  Great  Salt  Lake,  was  originally  fresh,  as  proved  by  shells 
of  Mdania^  &c.,  found  in  lacustrine  terraces  1300  feet  above  its  present  level.  Hull, 
'  Mount  Seir,'  1885,  pp.  100,  180. 

»  Bischof,  'Chera.  Geol.*  i.  p.  397.     Roth,  '  aiem.  Geol.'  i.  p.  476. 


SECT.  ii§5  TERRESTRIAL  ICE  413 

saliferous  and  gypsiferous  clays,  with  bands  of  gyjisum,  rise  along  the  slopes  for  some 
height  above  the  present  snrface  of  the  water,  and  mark  the  deposits  left  when  tlH> 
Dead  Sea  covered  a  larger  area  than  it  now  does.  Save  occasional  impressions  of 
diifted  teiTestrial  plants,  these  strata  contain  no  organic  remains.^  Interesting  details 
regarding  saliferous  deposits  of  recent  origin,  on  the  site  of  the  Bitter  Lakes,  were 
obtained  during  the  construction  of  the  Suez  Canal.  Beds  of  salt,  interleave<l  with 
laminae  of  clay  and  gypsum -crystals,  were  found  to  fonn  a  deposit  upwards  of  30  feet 
thick  extending  along  21  miles  in  length  by  about  8  miles  in  breadth.  No  fewer  than 
42  layers  of  salt,  from  8  to  18  centimetres  thick,  could  be  counted  in  a  depth  of  2*46 
metres.  A  deposit  of  earthy  gyi)sum  and  clay  was  ascertained  to  liavc  a  thickness 
of  367  feet  (112  metres),  and  another  bed  of  nearly  pure  crumbling  gyitsum  to  be  about 
230  feet  (70  metres)  deep.'' 

The  desiccated  floors  of  the  great  saline  lakes  of  Utah  and  Nevada  have  revealed 
some  interesting  facts  in  the  history  of  saliferous  deposits.  The  ancient  terraces 
marking  former  levels  of  these  lakes  are  cemented  by  tufa,  wliich  appears  to  have  l>een 
abundantly  formed  along  the  shores  where  the  brooks,  on  mingling  with  tlie  lake, 
immediately  parted  with  their  lime.  Even  at  ])resent,  oolitic  grains  of  carbonate  of 
lime  are  to  be  found  in  course  of  formation  along  the  margin  of  Great  Salt  Lake, 
though  carbonate  of  lime  has  not  been  detected  in  the  water  of  the  lake,  being  at  once 
precipitated  in  the  saline  solution.  The  site  of  the  ancient  salt  lake  which  has  been 
termed  Lake  Lahontan  displays  areas  several  sc^uare  miles  in  extent  covered  with 
deiKJsitji  of  calcareous  tufa,  20  to  60  and  even  150  feet  thick.  This  tufa,  however, 
presents  a  remarkable  |)eculiarity.  It  is  sometimes  almost  wholly  composed  of  what 
have  been  determined  to  be  calcareous  pseudomorphs  after  gaylussite  (a  mineral 
composed  of  carbonates  of  calcium  and  sodium  with  water) — the  sodium  of  the 
mineral  having  l)een  replaced  by  calcium.  When  this  variety  of  tufa,  distinguished 
by  the  name  of  thlnolite,  was  originally  formed,  the  waters  of  the  vast  lake  must  liavo 
been  bitter,  like  those  of  the  little  soda -lakes  which  now  lie  on  its  site — a  dense 
solution  in  which  carlK)nate  of  soda  predominated.  On  the  margin  of  one  of  the  present 
Swla  Lakes,  ciystals  of  gaylussite  now  form  in  the  drier  season  of  the  year.  Yet  no 
trace  of  carbonate  of  lime  has  been  detected  in  the  water.  The  carbonate  of  lime  in  the 
crystals  must  l)e  derived  from  water  which  on  entering  the  saline  lakes  is  at  once 
-deprived  of  its  lime.' 

S  5.  Terrestrial   Ice. 

Fresh  water,  under  ordinary  circumstances,  when  it  reaches  a  tempera- 
ture of  32°  Fahr.  passes  into  the  solid  state  by  crystallizing  into  ice. 
In  this  condition,  it  performs  a  series  of  important  geological  operations 
before  being  again  melted  and  relegated  to  the  general  mass  of  liquid 
terrestrial  waters.  Five  conditions  under  which  ice  occurs  on  the  land 
deserve  notice,  viz.,  frost,  frozen  rivers  and  lakes,  hail,  snow,  and 
glaciers. 

Frost — Water,  if  perfectly  still,  may  fall  below  the  freezing-point 

*  Lartet,  Bull,  Soc.  (Jiol.  France  (2),  xxii.  p.  450  et  seq.  Below  the  high  terraces,  con- 
taiuing  lacustrine  shells,  evidence  of  shrinkage  and  concentration  is  supplied  by  gyiJseous 
marls  and  a  bed  of  salt  (30  to  50  feet),  600  feet  above  the  present  water-level. 

2  Lesseps,  CompUs  rend.  Ixxviii.  p.  1740,  Ann.  Chim.  et  Phi/s.  (5),  iii.  p.  139.  Bader, 
Verb.  Geol.  Reichsanst.  1869,  p.  288. 

^  King,  Exploration  of  the  4Qth  Parallel,  i.  p.  510.  See  also  on  the  crystallographic 
form  and  chemical  composition  of  the  thinolite  and  its  original  mineral,  E.  S.  Dana,  Bull. 
r.S,  Oeol.  Sure.  No.  12  (1884). 


414  DYNAMICAL  GEOLOGY  book  hi  part  ii 


without  freezing,  but  when  it  is  then  moved,  it  at  once  freezes  over.  In 
freezing,  water  expands.  If  it  be  confined  in  such  a  way  that  expansion 
is  impossible,  it  remains  liquid  even  at  temperatures  below  the  freezing- 
point  ;  but  the  instant  that  the  pressure  is  removed  this  chilled  water 
becomes  ice.  There  is  a  constant  effort  on  the  part  of  the  water  to 
expand  and  become  solid,  very  considerable  pressure  being  needed  to 
counterbalance  this  expansive  power,  which  increases  as  the  temperature 
sinks.  At  30°  Fahr.  the  pressure  must  amount  to  146  atmospheres^ 
or  the  weight  of  a  column  of  ice  a  mile  high,  or  138  tons  on  the  square 
foot.  Consequently  when  the  water  freezes  at  a  lower  temperature,  its 
pressure  on  the  walls  of  its  enclosing  cavity  must  exceed  138  tons  on  the 
square  foot.  Bombshells  and  cannon  filled  with  water  and  hermetically 
sealed  have  been  burst  in  strong  frosts  by  the  expansion  of  the  freezing 
water  within  them.  In  nature,  the  enormous  pressures  which  can  be 
obtained  artificially  occur  rarely  or  not  at  all,  because  the  spaces  into 
which  water  penetrates  can  hardly  ever  be  so  securely  closed  as  to  permit 
the  water  to  be  cooled  down  considerably  below  32°  Fahr.  before  freezing. 
But  ice  forming  in  cavities  at  even  two  or  three  degrees  below  the 
freezing-point  exerts  an  enormous  disruptive  force. 

Soils  and  rocks,  being  all  porous,  and  usually  containing  a  good  deal 
of  moisture,  have  their  particles  pushed  asunder  by  the  freezing  of  this 
interstitial  water.  Stones,  stumps  of  trees,  or  other  objects  imbedded  in 
the  ground,  are  scjueezed  out  of  it  When  a  thaw  comes,  the  soil  seems 
as  if  it  had  been  ground  down  in  a  mortar.  Water,  freezing  in  the 
innumerable  joints  and  fissures  of  rocks,  exerts  great  pressure  upon  the 
walls  between  which  it  lies,  pushing  them  asunder  as  if  a  wedge  were 
driven  between  them.  When  this  ice  melts,  the  separated  masses  do  not 
return  to  their  original  position.  Their  centre  of  gravity  in  successive 
winters  becomes  more  and  more  displaced,  until  the  sundered  masses 
fall  apart.  In  mountainous  districts,  where  the  winters  are  severe,  and 
in  high  latitudes,  much  waste  is  thus  produced  on  exposed  cliffs  and 
loose  blocks  of  rock.  Some  measure  of  its  magnitude  may  be  seen  in  the 
heaps  of  angular  rubbish  which  in  these  regions  so  frequently  lie  at  the 
foot  of  crags  and  steep  slopes.  At  Spitzbergen  and  on  the  coast  of  Green- 
land, the  observed  amount  of  destruction  caused  by  frost  is  enonnous. 
The  short  warm  summer,  melting  the  snow,  fills  the  pores  and  joints  of 
the  rocks  with  water,  which  when  it  freezes  splits  off  large  blocks, 
launching  them  to  the  base  of  the  declivities,  where  they  are  farther 
broken  up  by  the  same  cause.  In  some  countries,  where  the  winters  are 
severe,  the  soil-cap  has  been  observed  to  be  pushed  or  to  creep  down- 
hill from  the  action  of  frost.  ^ 

Frozen  Rivers  and  Lakes. — In  countries  such  as  Canada,  the  lakes 
and  rivers  are  frozen  over  in  winter  with  a  cake  of  ice  IJ  to  2 J 
feet  thick.  This  cake  as  it  forms  expands  and  presses  against  the 
shores.  A  continuance  of  frost  leads  to  a  contraction  of  the  ice  already 
formed  and  to  the  consequent  opening  of  vertical  fissures,  into  which  the 
water  from    below  ascends   and  freezes.     When  a  subsequent  rise  in 

^  Kerr,  Amer,  Journ.  ScL  xxi.  (1881),  p.  345  ;  C.  Davidson,  Ged,  Mag,  1889,  p.  255. 


SECT,  ii  §  6  GEOLOGICAL  ACTION  OF  ICE  415 

temperature  causes  an  expansion  of  the  superficial  crust,  the  ice  once 
more  presses  against  the  shores.  When  these  are  steep  the  ice  yields 
and  either  breaks  up  along  its  margin  or  assumes  an  undulating  surface 
over  the  lake ;  but  where  they  are  sloping  it  is  pushed  up  the  slope, 
carrying  with  it  earth  and  boulders.  Similar  results  are  repeated  during 
subsequent  rises  and  falls  of  temperature,  the  debris  being  driven  further 
up  the  shore,  until  it  sometimes  accumulates  in  a  mound  or  wall  along 
the  outer  edge  of  the  broken  ice.  When  the  ice  melts  this  embankment 
of  displaced  material  is  left  as  a  memorial  of  the  severity  of  the  climate. 
Such  "  shore-walls  "  are  of  common  occurrence  on  the  margins  of  many 
lakes  in  Canada  and  the  United  States.^  Under  certain  conditions,  also, 
what  is  called  "  anchor-ice  "  forms  on  the  bottoms  of  the  rivers  and  rises 
to  the  surface.^  In  several  ways,  geological  changes  are  thus  effected. 
Mud,  gravel,  and  boulders  encased  in  the  anchor-ice  or  pushed  along  by 
it  on  the  bottom,  are  moved  from  their  position.  This  ice,  formed  in 
considerable  quantity  in  the  rapids  of  the  Canadian  rivers,  is  carried  down 
stream  and  accumulates  against  the  bars  and  banks,  or  is  pushed  over 
upon  the  surface  of  the  upper  ice.  By  its  accumulation  a  temporary 
barrier  is  formed,  the  bursting  of  which  causes  destructive  floods.  When 
the  ice  breaks  up  in  early  summer,  cakes  of  it  which  have  been  formed 
along  shore,  and  have  enclosed  beach-pebbles  and  boulders,  float  off  so  as 
either  to  drop  these  in  deeper  water  or  to  strand  them  on  some  other 
part  of  the  shore. 

This  kind  of  transport  takes  place  on  a  great  scale  on  the  St.  Lawrence.  The 
islets  of  boulder-clay  and  solid  rock  are  fringed  with  blocks  which  have  been  stranded 
by  ice  and  which  are  ready  to  be  again  enclosed,  and  floated  off  further  down  stream. 
Should  a  gale  arise  during  the  breaking  up  of  the  frost,  vast  piles  of  ice,  with  mingled 
gravel  and  boulders,  may  be  driven  ashore  and  pushed  up  the  beach  ;  even  blocks  of 
stone  of  considerable  size  are  sometimes  forced  to  a  height  of  several  yards,  tearing  up 
the  soil  on  their  way,  and  helping  to  form  a  bank  above  the  water-level.  In  the  same 
river,  great  destruction  of  banks  has  been  caused  by  rafts  of  ice,  and  jwrticularly  of 
anchor-ice.  Crab  Island,  for  example,  which  was  about  an  acre  and  a  half  in  extent  at 
the  beginning  of  this  century,  has  entirely  disappeared,  its  place  being  indicated  merely 
by  a  strong  ripple  of  the  water,  which  is  every  year  getting  deeper  over  the  site.^ 
Other  islands  have  also  been  destroyed.  Great  damage  is  frequently  done  to  quays  and 
bridges  in  the  same  region,  by  masses  of  river-ice  driven  against  them  on  the  arrival  of 
spring.  Reference  has  already  been  made  to  the  increased  power  of  transport  and 
erosion  acquired  by  frozen  rivers,  and  esj>ecially  when,  as  in  Siberia,  their  ice 
breaks  up  in  the  higher  parts  of  their  courses,  before  it  gives  way  in  the  lower  (p.  382). 

Hail,  the  formation  of  which  is  not  yet  well  understood,*  falls  chiefly 

'  C.  A.  White,  Amer.  Naturaliat,  ii.  (1868),  p.  148  ;  G.  K.  Gilbert,  bth  Ann.  Rep. 
r.  S.  Geol.  Survey,  1885,  p.  109. 

*  These  conditions,  according  to  Dr.  Rae  {Nature,  xxi.  p.  538),  are:  1st,  u  rocky  or 
stony  bottom  ;  2Dd,  shallow  water  as  compared  with  that  higher  up  the  stream ;  3rd,  a 
swifter  current  and  rougher  water,  in  comparison  with  a  smooth  and  slower  motion 
immediately  above.  It  is  a  loose,  slushy,  adhesive  kind  of  ice.  See  also  Nature,  xxi. 
p.  612  ;   xxii.  31,  64. 

*  BleasdeU,  Q.  J.  Oed.  Soc.  xxvi.  p.  669 ;  xxviii.  p.  292. 

*  For  an  account  of  the  different  theories  proposed  to  account  for  hail,  see  Prof. 
Viguier,  Assoc.  Frangaise,  1879,  p,  543  ;  1880,  p.  436. 


416  DYNAMICAL  GEOLOGY  book  ni  part n 


in  summer  and  during  thunderstx)rm8.  When  the  pellets  of  ice  are  froasen 
together  so  as  to  reach  tlie  ground  in  lumps  as  large  as  a  pigeon's  ^g,  or 
larger,  great  damage  is  often  done  to  cattle,  flying  birds,  and  vegetation. 
Trees  have  their  leaves  and  fruit  torn  off*,  and  farm  crops  are  beaten 
down. 

Snow. — In  those  parts  of  the  earth's  surface  where,  either  from 
geographical  position  or  from  elevation  into  the  upper  cold  regions  of  the 
atmosphere,  the  mean  annual  temperature  is  below  the  freezing-pointy 
the  condensed  moisture  falls  chiefly  as  snow,  and  remains  in  great 
measure  unmelted  throughout  the  year.  A  line,  termed  the  snow-line^  can 
be  traced,  below  which  the  snow  disappears  in  summer,  but  above  which 
it  continues  to  cover  the  whole  or  great  part  of  the  surface.  The  snow- 
line comes  down  to  the  sea  around  the  poles.  Between  these  limita 
it  rises  gradually  in  level  till  it  reaches  its  highest  elevation  in  tropical 
latitudes.  South  of  lat.  TS""  N.  it  begins  to  retire  from  the  searlevel,  so 
that  on  the  coast  of  northern  Scandinavia  it  is  already  nearly  3000  feet 
above  the  sea.  None  of  the  British  mountains  quite  reach  it.  In  the 
Alps  it  stands  at  8500  feet,  on  the  Andes  at  18,000  feet,  and  on  the 
northern  slopes  of  the  Himalayas  at  19,000  feet. 

Snow  exhibits  two  difl'erent  kinds  of  geological  behaviour:  (1)  con 
servative,  and  (2)  destructive.  (1)  Lying  stationary  and  unmelted,  it 
exercises  a  protective  influence  on  the  face  of  the  land,  shielding  rocks, 
soils,  and  vegetation  from  the  effects  of  frost.  On  IcAv  grounds  this  is 
doubtless  its  chief  function.  (2)  «.  When  snow  falls  in  a  partially 
melted  state  it  is  apt  to  accumulate  on  branches  and  leaves,  until  by  its 
weight  it  breaks  them  off",  or  even  bears  down  entire  trees.  Great 
destruction  is  thus  ciuised  in  dense  forests,  b.  Snow  accumulating  on 
gentle  slopes  and  slowly  sliding  downwards,  pushes  soil  or  loose  stones 
down-hill.  Considerable  transport  of  rotted  rock  and  boulders  may  thus 
arise.^  c.  Snow  on  steep  mountain  slopes  is  frequently  during  spring 
and  summer  detached  in  sheets  from  10  to  more  than  50  feet  thick  and 
several  hundred  yards  broad  and  long,  which  rush  down  as  avalanchfii 
(Lawinen),  sweep  away  trees,  soil,  or  rocks,  and  heap  them  up  in  the  valleys.* 
Besides  the  destruction  caused  by  the  avalanche  itself,  sometimes  much 
damage  arises  from  the  sudden  violent  wind  to  which  it  gives  rise.' 
(L  Another  indirect  efl'ect  of  snow  is  seen  in  the  sudden  rise  of  rivers 
when  warm  weather  rapidly  melts  the  mountain  snows.  Many  summer 
freshets  are  thus  caused  in  Switzerland.  It  is  to  the  melting  of  the 
snows,  rather  than  to  rain,  that  rivers  descending  from  snowy  mountains 
owe  their  periodical  floods.  Hence  such  rivers  attain  their  greatest  volume 
in  summer,     e.  A  curious  destructive  action  of  snow  has  been  observed 

1  H.  Y.  HiiKl,  Canadian  Xaturalist,  viii.  (1878),  pp.  967,  976. 

'--'  All  avalauclie  near  Ormons  Dessus,  Canton  Vaud  (Dec.  1882),  piled  up  a  insn 
<if  ice  and  snow  200  feet  thick  (some  of  the  ice-blocks  being  18  feet  long),  and  covered 
3  s(|uare  km.  of  ground.  Xaturey  xxvii.  p.  181.  Streams  may  be  thus  blocked  up,  at 
tlie  Inn  was  at  Siis  in  1827.  For  accounts  of  avalanches,  see  J.  Coaz,  *Die  Lawinen  in 
den  Sehweizeralpen,*  Berne,  1881. 

»  aeol.  Ma{f.  1888   p.  155. 


SECT,  ii  §  5  GLACIERS  AND  ICE-SHEETS 


Ml 


on  the  sides  of  the  Rocky  Mountains,  where  the  drifting  of  snow-crystals 
by  the  wind  in  some  of  the  passes  has  damaged  and  even  killed  the 
pine-trees,  wearing  away  the  foliage,  cutting  off  the  bark,  and  even 
sawing  into  the  wood  for  several  inches.^ 

Glaciers^  and  Ice-sheets. — Glaciers  are  rivers  of  ice  formed  by 
the  slow  movement  and  compression  of  the  snow,  which,  by  gravitfttio^, 
creeps  downward  into  valleys  descending  from  snow-fields.  The  sn8w 
in  the  higher  regions  is  loose  and  granular.  As  it  moves  downward  it 
becomes  firmer,  passing  into  the  condition  of  ndv^  or  Jim  (p.  148). 
Gradually,  as  the  separate  granules  are  pressed  together  and  the  air  is 
squeezed  out,  the  mass  assumes  the  character  of  blue  compact  crystalline 
ice.  From  a  geological  point  of  view,  a  glacier  may  be  regarded  as  the 
drainage  of  the  snowfall  above  the  snow-line,  as  a  river  is  the  drainage 
of  the  rainfall  A  glacier,  like  a  river,  is  always  in  motion,  though  so 
slowly  that  it  seems  to  be  solid  and  stationary.  It  descends  as  a 
brittle,  thick -flowing  substance,  like  pitch  or  resin.  The  motifln  is 
unequal  in  the  different  parts,  the  centre  moving  faster  than  the  sides 
and  bottom,  as  was  first  ascertained  through  accurate  measurement  by 
J.  D.  Forbes,  who  found  that  in  the  Mer  de  Glace  of  Chamouni,  the  mean 
daily  rate  of  motion  in  the  summer  and  autumn  was  from  20  to  27  inches 
in  the  centre,  and  from  13  to  19i  near  the  side.  Helland  has  observed 
that  on  the  west  coast  of  Greenland  the  glacier  of  Jacobshavn  has  a 
remarkably  rapid  motion,  its  rate  for  twenty-four  hours  ranging  from 
48*2  feet  to  64*8  feet.  The  ice  of  the  fjord  of  Torsukatak,  nearly  five 
miles  wide,  moves  with  a  mean  rate  of  24  feet  in  a  day ;  that  of  Karajak, 
four  and  a  half  miles  broad,  moves  30  feet  daily.  G.  F.  Wright,  from 
observations  made  by  him  in  Alaska,  inferred  that  the  Muir  glacier  there, 
enters  a  sea-inlet  at  an  average  rate  of  forty  feet  per  day  (70  feet  in  the 
centre  and  10  feet  near  the  margin)  in  the  month  of  August;^  but 
a  more  recent  measurement  by  Dr.  Reid  in  the  summer  of  1890  gives  a 
maximum  rate  of  only  seven  feet  in  a  day. 

The  consequence  of  this  differential  motion  is  seen  in  the  internal 
banded  structure  of  a  glacier,  in  the  downward  curvature  of  the  transverse 
fissures  (crevasses),  and  in  the  arrangement  of  the  lines  of  rubbish  thrown 
down  at  the  termination,  which  often  present  a  horse-shoe  shape,  corre- 
sponding to  that  of  the  end  of  the  ice  by  which  they  were  discharged.* 

^  Clarence  King,  ExplcmUion  of  4Qth  Parallel,  i.  p.  527. 

'■*  On  glaciers  and  their  geological  work,  see  De  Saussure,  *  Voyages  dans  les  Alpes,* 
§  535;  Agassiz,  *]Studes  sur  les  Glaciers,*  1840;  Rendu,  'Theorie  des  Glaciers  de  la 
Savoie,'  Mem.  Acad.  Savoie,  x.,  translated  into  English,  1875  ;  J.  D.  Forbes,  'Travels  in 
the  Alps,*  1843  ;  'Norway  and  its  Glaciers,*  1853  ;  'Occasional  Papers  on  Glaciers,'  1859  ; 
TyndaU,  'Glaciers  of  the  Alps,'  1857  ;  Mousson,  'Gletsclier  der  Jetztzeit,'  1854  ;  A.  Heim, 
'Handbuch  der  Gletscherkunde,'  Stuttgart,  1885;  E.  Richter,  'Gletscher  der  Ostalpen,' 
Stuttgart,  1888. 

'  Amer.  Jaum.  Sci.  xxxiii.  (1887),  p.  10.  For  the  glaciers  of  the  United  States  see 
Wright's  'Ice-Age  in  America'  ;  H.  P.  Gushing,  American  (Jeologist^  1891,  p.  207  ;  Hayes, 
Naiional  Geographic  Magazine,  iv.  (1892),  p.  150  ;  Russell,  Am^r,  Journ.  Sci.  xliii.  (1892), 
p.  169.     Uk.  Ann.  Rep.  U.  S.  Geol.  Sure.  (1885). 

^  The  cause  of  glacier  motion  has  been  a  much-Texed  question  in  physics.     See,  besides 

2  E 


418  DYNAMICAL  GEOLOGY  book  ni  part u 

Under  the  term  Ice-sheet  is  included  the  deep  mantle  of  snow 
and  ice  which,  in  the  Polar  regions,  covers  the  land  and  creeps  oat  to 
sea.  In  high  Arctic,  and  still  more  in  Antarctic  latitudes,  land -ice, 
formed  from  the  drainage  of  a  great  snow -field,  attains  its  greatest 
dimensions.  The  land  in  these  regions  is  buried  under  an  ice-cap  which 
ranges  up  to  a  thickness  (in  the  South  Polar  circle)  of  10,000  feet 
(2  miles)  or  even  more.  Greenland  lies  under  such  a  pall  of  snow  that 
all  its  inequalities,  save  only  the  steep  mountain- crests  and  peaks 
near  the  coast,  are  concealed.  The  snow,  creeping  down  the  slopes,  and 
mounting  over  the  minor  hills,  [msses  beneath  by  pressure  into  compact 
ice.  From  the  main  valleys  great  glaciers,  like  vast  tongues  of  ice,  2000 
or  3000  feet  thick,  and  sometimes  50  miles  or  more  in  breadth,  push  oat 
to  sea,  where  they  break  off  in  huge  fragments  that  float  away  as 
icebergs.^  As  far  back  as  1777,  Captain  Cook  gave  interesting  descrip- 
tions of  the  glaciers  of  South  Georgia  (Lat.  54^  S.),  which  reach  the  sea 
in  a  line  of  cliffs  (Fig.  149). 

Glaciers,  though  naturally  most  abundantly  developed  in  arctic  and 
antarctic  regions,  may  be  met  witli  in  any  latitude  wherever  a  sufficiently 
extensive  area  of  snow  accumulates  and  remains  permanent  throughout 
the  year.  They  occur  even  in  equatorial  regions  where  the  ground  rises 
sufficiently  high  above  the  snow-line.  They  are  found  in  great  force 
among  the  Himalaya  mountains,  while  among  the  Andes  of  Quito,  close 
to  the  equator,  many  glaciers  have  been  noted ;  the  great  mountain  of 
Chimborazo  (20,498  feet),  for  example,  being  capped  with  ice  and 
sending  glaciers  out  in  all  directions.'^  Hence  the  peculiar  geological 
results  effected  by  glacier-ice  are  not  restricted  to  definite  latitudes,  but 
may  be  encountered,  under  the  necessary  limitations,  from  the  equator  to 
the  poles. 

Some  featiu'es  of  geological  importance  in  the  behaviour  of  a  glacier 
as  it  descends  its  valley  deserve  mention  here.  When  the  ice  has  to 
travel  over  a  very  uneven  floor,  some  portions  may  get  embayed,  while 
overlying  parts  slide  over  them.     A  massive   ice -sheet  may  thus  have 

the  works  cited  in  tlie  foregoing  note,  J.  Thomson,  Proc.  Roy.  Sue.  1856-7  ;  Mosely,  fj». 
cit.  1869;  CVoll,  'Climate  and  Time,'  1875;  Hopkins,  Phil.  Mag.  1845;  PhiL  Tran*, 
1862  ;  Hehnlioltz,  Ueidelheiy  Verhandl.  Nat.  Metl.  1865,  p.  194  ;  PkU.  May.  1866,  p.  22  ; 
Pfaff,  AkaiL  Bn^yfr.  1876.  A  valuable  hUtorj-  of  the  controversy  regarding  glacier  motion 
has  been  prepared  by  Sir  H.  H.  Howorth,  Mem.  Pi-vc.  Manchester  Lit.  PhU.  Soc,  iv.  (1891). 
The  conclusion  to  which  the  most  recent  researches  ^voint  coincides  essentially  with  that 
enunciated  upwards  of  40  years  ago  by  J.  D.  Forbes,  that  the  motion  of  a  glacier  **  is  that 
of  a  slightly  viscous  mass,  partly  sliding  upon  its  bed,  partly  shearing  upon  itself  under 
the  influence  of  gravity."  Trotter,  Proc.  Roy.  Soc.  xxxviii.  p.  107.  The  banded  stmctore 
of  glacier-ice  may  be  compared  with  shear-structure  (pp.  316,  544). 

^  The  Greenland  snow -fields  and  glaciers  are  well  described  in  the  '^  Meddelelser  om 
Griinland  " — the  detailed  report  of  a  Danish  commission  ap]>ointed  to  investigate  that  conntiy. 
The  first  volume  was  published  in  1879,  and  ten  have  subsequently  appeared.  See  alto 
Nordenskiiild,  UeoL  Mag.  1872,  Marr,  Geo!.  Mag.  1887,  p.  151.  H.  Rink,  Edin.  Geoi, 
Sac.  V.  (1887),  p.  286.  E.  von  Drygalski,  Zeitsc^.  Gesell.  /.  Erdkunde,  Berlin  (1892). 
See  also[,Nansen,  Petern,  MUUieU.  Erganzungsheft,  No.  105  (1892). 

^  On  glaciers  of  Ecuador  see  Whymper,  *  Travels  Amongst  the  Great  Andes,'  p.  348. 


SECT,  ii  §  5  GLACIERS  AND  ICE-SHEETS  419 

many  local  eddies  in  its  lower  portions,  the  ice  there  even  travelling  for 
various  distances,  according  to  the  nature  of  the  ground,  obliquely  to  the 
general  flow  of  the  main  mass,  as  is  remarkably  displayed  in  the  Green- 
land ice  where  it  flows  round  the  isolated  rocks  or  "  Nimatakker  "  which 
rise  out  of  it.  It  there  acquires  in  some  places  a  remarkably  beautiful 
banded  structure,  which  in  lenticular  banding  and  folding  presents  a 
close  resemblance  to  the  characteristic  banded  and  plicated  structure  of 
many  ancient  gneisses.^  In  descending  by  a  steep  slope  to  a  more  level 
part  of  its  course,  a  glacier  becomes  a  mass  of  fissured  ice  in  great  con- 
fusion. It  descends  by  a  slowly  creeping  ice-fall,  where  a  river  would 
shoot  over  in  a  rushing  waterfall.  A  little  below  the  fall  the  fractured 
ice,  with  all  its  chaos  of  pinnacles,  bastions,  and  chasms,  is  pressed  together 
again,  and  by  regelation  becomes  once  more  a  solid  mass  (Fig.  143). 

ShumfiM 


~....-^ 


Fig.  143.— Sectiou  of  Glacier  with  Ice-fallR,  Fondalen,  Holands  Fjord,  Arctic  Norway. 

The  body  of  the  glacier  throughout  its  length  is  traversed  by  a  set  of 
fissures  called  crevassesy  which,  though  at  first  as  close-fitting  as  cracks 
in  a  sheet  of  glass,  widen  by  degrees  as  the  glacier  moves  on,  till  they 
form  wide  yawning  chasms,  reaching,  it  may  be,  to  the  bottom  of  the 
ice,  and  travelling  down  with  the  glacier,  but  apt  to  be  effaced  by  the 
pressing  of  their  walls  together  again  as  the  glacier  winds  down  its 
valley.  The  glacier  continues  to  descend  until  it  reaches  that  point 
where  its  rate  of  advance  is  just  equalled  by  its  liquefaction.  There  it 
ends,  its  place  down  the  rest  of  the  valley  being  taken  by  the  tumultuous 
river  of  muddy  water  which  escapes  from  under  the  melting  extremity  of 
the  ice.  A  prolonged  augmentation  of  the  snowfall  will  send  the  foot  of 
the  glacier  further  down  the  valley  ;  a  diminution  of  the  snowfall  or  a 
general  rise  of  temperature  will  cause  it  to  retreat  further  up. 

Considerable  variations  in  the  thickness  and  length  of  glaciers  have  been  observed 
within  the  last  two  or  three  generations,  due  to  oscillations  of  temperature  and  wetness. 
Thus  the  glacier  of  La  Brenva,  on  the  Italian  side  of  Mont  Blanc,  shrank  to  such  an 
extent  in  the  twenty-four  yeai*s  succeeding  1818,  that  its  surface  at  one  place  was  found 
to  have  subsided  no  less  than  300  feet.-  The  glaciers  of  Mont  Blanc  had  ceased  to 
advance  about  1854,  and  in  twelve  years,  from  1854  to  1865,  the  Glacier  des  Bossons 

*  See  by  way  of  illustration  plates  ix.-xii.  of  a  paper  on  the  glaciers  and  inland  ice  of 
Greenland  by  E.  von  Drygalski,  Zeitsch.  Oesdl.  f.  Erdkunde,  Berlin  (1892). 

*  J.  D.  Forbes,  'Travels  in  the  Alps,'  p.  205. 


420 


DYNAMICAL  GEOLOGY 


BOOK  in  PART  II 


lioil  receded  332  tiictr»i,  that  of  Boia  188  nietreo,  that  of  Argciitiere  181  metni,  and 
that  of  Tour  5'20  metres.  Similar  facts  liave  been  obHervrd  in  the  Bemeae  Oberlud  ind 
the  Tyrol,  but  with  some  local  exceptions,  in  (isrticiitar  the  Gomer  and  Aar  gUcam' 
At  the  Pastencu  glacier,  which  tilirank  back  about  6  or  8  metres  annually,  ths  retnat 
was  changed  in  1888  into  a  forward  movement,  [mneibly  indicating  tliat  ths  miiii- 
ninni  hail  been  reached  and  that  a  new  advance  of  the  ice  had  begun.'  Since  IS6S  tbt 
glaciers  of  the  Pyrenees  and  CaucaHUH  have  also  shrunli.'  Tlie  glaciers  of  GTeenland  and 
Alaska  were  fomierty  much  lar^r  than  tlicy  are  now.  The  Uuir  glaoter  in  Alaaka  it 
said  to  have  retreated  lialf  a  7iiile  in  four  yeani  jircceiling  1890.* 

In  a  mauntainous  region  such  aa  tlie  Alps,  or  a  t&ble-luKl  lilu 
Scandinavia,  where  a  considenible  mass  of  ground  lies  above  the  nmr^ 
line,  tliree  varieties  of  glaciers  may  be  observed. 


(1)  (ilaciers  of  the  first  order  (valley-glaciers)  come  down  well  below 
the  snow,  and  extend  into  the  valleys.  In  high  latitudes  they  reach  the 
sea.  The  Hunibolilt  Glacier  in  Korth  Greenland  presents  a  w^l  of  ice  60 
miles  long  and  rising  300  feet  ubove  the  sen,  which  washes  the  base  of 
the  cliff.  The  spiry  jwaks  and  sharp  crests  of  the  Alps  rise  through  the 
snow,  which  they  thus  isolate  into  distinct  basins  (Fimmuldeu),  averagii^ 
perhaps  two  square  miles  in  area,  whence  glaciers  proceed.     The  number 

■  L.  Gniiier,  Couiplcn  rfml.  lx»iii.  p.  833.     Bull.  .*c.  iflol.  Fran.  it.  (3«  B#r.)    On 
(leriodic  variations  of  Alpiue  Glaciers,  see  Forel,  Arcli.  .lei.  Bib.  I'liiv.  Geneva,  July  1881. 
'  F.  Seehind,  Zcitxii.  Ufiili--h-tfji<err.  Alixmrneint,  1884,  p.  Bl. 

=  l-li.Uufour.^Mw./'mHpiur,  1880, 11.449.    Tlie  Norwegian  glaciers  are  now  wtnatti* 
*.H.  P.  t'usbing,  Ameriam  Oeo/ogM.  1891,  j-.  315. 


BKCT.  ii  §  5  GLACIEJtS  AND  ICE-SHEETS  421 

of  glaciers  among  the  Alps  has  been  estimated  at  2000,  covering  a  total 
area  of  from  3000  to  4000  square  kilometres  (Figs.  144,  145).  They 
average  perhaps  from  3  to  5  miles  in  length.     The  Groat  Aletach  Glacier 


Ftg.  lii.--aiiu:iv  dn  Lcclui 


is  nearly  10  (or,  including  the  anow-fielJ,  nearly  15)  miles  long,  with  a 
mean  breadth  of  5900  feet,  and  descending  to  4439  feet  above  the  sea. 
The  thickness  of  the  ice  in  the  Alpine  glaciers  must  often  be  as  much  as 
800  to  1 300  feet.  .  It  has  been  computed  that  the  Gorner  Glacier  is  large 


enough  to  make  three  cities  as  big  as  London.  The  great  snow-ttelds 
of  Arctic  Norway  accumulate  on  broad  table-lands,  from  which  they  send 
glaciers  down  into  the  valleys  (Figs.  143,  146). 


J22  DYNAMICAL  ISEOI.OGY  BoOKiUPARtn 

(2)  Glaciers  of  the  second  order  (Gorrie- glaciers,  Hangegletscher) 
hivrdty  creep  beyond  tlie  high  recesses  wherein  they  are  formed,  and 
do  not  therefore  reach  as  far  as  the  Dearest  valley.  Many  beaulaful 
examples  of  this  type  may  be  seen  along  the  steep  declivities  which 
intervene  between  the  snow-covered  plateau  of  Arctic  Norway  and 
the  sea. 

(3)  Ee-cemented  Glaciers  (GlacUrs  renumiis).  —  These  consist  of 
fragments  which,  falling  from  an  ice-cliff  crowning  precipices  of   rock, 


V\g.  147.— View  utre^JcluenteU  Glacier.  Jipkuls  Fjurd,  Anlic  Nonrmy. 

are  re-frozen  at  the  bottom  into  a  solid  mass  that  creeps  downward 
as  a  glacier  usually  of  the  second  order.  Probably  the  best  illustrationB 
in  Eiiro|>e  are  furnished  by  the  Niia  Fjord,  and  other  parts  of  the 
north  of  Norway.  In  some  cases  a  cliff  of  "  firn "  resting  on  blue  ice 
appears  at  the  top  of  the  precipice — the  edge  of  the  great  "  sneefond,"  or 
^now-held — while  several  hundred  feet  below,  in  the  corrie  or  cwm  at  the 


ila  FJorrl  Glwdcr. 


bottom,  lies  the  re-cemented  glacier,  white  at  iu  upper  edge,  but 
acf|uiring  somewhat  of  the  characteristic  blue  gleam  of  compact  ice  as  it 
moves  towards  its  lower  margin.  A  beautiful  example  of  this  kind  was 
visited  by  me  at  the  head  of  the  Jdkuls  Fjord  in  Arctic  Norway  in 
180.">.  When  making  the  sketch  from  wliich  Fig.  147  is  taken,  I 
observed  that  the  ice  from  the  edge  of  the  anow-field  above  slipped  off 
in  occasional  avalanches,  which  sent  a  roar  as  of  thunder  down  the 
valley,  while  from  the  shattered  ice,  as  it  nished  down  the  prectpicef, 
clouds  of  white  snow-dust  rose  into  the  air.     The  debris  thus  launched 


SECT,  ii  §  5  GLACIERS  AND  ICE-SHEETS  423 

into  the  defile  beneath  accumulates  there  by  mutual  pressure  into  a 
tolerably  solid  mass,  which  moves  downward  as  a  glacier,  and  actually 
reaches  the  sea-level — the  only  example,  so  far  as  I  am  aware,  of  a 
glacier  on  the  continent  of  Europe  which  attains  so  low  an  altitude. 
As  it  descends  it  is  crevassed,  and  when  it  comes  to  the  edge  of  the 
fjord,  slices  from  time  to  time  slip  off  into  the  water,  where  they  form 
fleets  of  miniature  icebergs,  with  which  the  surface  of  the  fjord  (/  in  Fig. 
148)  is  covered. 

Great  destruction  is  sometimes  caused  by  the  breaking  ofl*  of  the  end 
of  glaciers  which  terminate  on  steep  ground.  The  sudden  dislocation  of 
the  ice  and  its  reduction  to  fragments,  and  even  to  powder,  causes  a 
considerable  proportion  of  it  to  melt.  A  mingled  mass  of  ice  and  water 
is  thus  discharged,  which,  meeting  with  loose  moraine  stuff,  may  speedily 
become  a  moving  debacle  of  mud.  Such,  according  to  M.  Forel,  was  the 
origin  of  the  destructive  avalanche  which  on  12th  July  1892  swept 
away  some  thirty  houses  and  killed  about  150  people,  in  the  valley  of 
Montjoie,  which  joins  that  of  the  Arve,  not  far  below  Chamouni.^ 

Another  incidental  effect  of  the  movement  of  glaciers  is  to  be  seen 
when  the  ice,  barring  the  mouth  of  a  tributary  valley,  dams  back  the 
streams  flowing  therein,  and  causes  a  lake  to  form.  This  result  may  be 
observed  at  the  Marjelen  See,  on  the  great  Aletsch  Glacier,  and  else- 
where on  the  Alpine  chain.  If  this  arrest  of  the  water  is  temporary, 
great  damage  may  be  done  by  the  bursting  of  the  ice-dam  and  the  conse- 
quent sudden  rush  of  the  liberated  water.  If,  on  the  other  hand,  the 
glacier  is  massive  enough  to  form  a  permanent  barrier,  the  water  may 
rise  behind  it  so  as  to  fill  the  tributary  valley,  and  even  escape  by  a  pass 
at  its  head.  Successive  diminutions  of  the  mass  of  ice  will  lead  to 
corresponding  lowerings  of  the  level  of  the  lake,  each  prolonged  rest  of 
the  water  at  one  level  being  marked  by  a  shelf  or  terrace  formed  as  a 
beach-line  along  the  shore.  The  famous  "  parallel  roads  "  of  Glen  Roy 
are  a  striking  illustration  of  this  kind  of  geological  history.  (Book  VI. 
Part  V.  Sect.  i.  §  1.) 

Work  done  by  Glaciers. — Glaciers  have  two  important  geological 
tasks  to  perform — (1)  to  carry  the  debris  of  the  mountains  down  to 
lower  levels ;  and  (2)  to  erode  their  beds. 

{a)  Transport. — This  takes  place  chiefly  on  the  surface  of  the  ice. 
Descending  its  valley,  the  glacier  receives  and  bears  along  on  its  margin 
the  earth,  stones,  and  rubbish  which,  loosened  by  frost,  or  washed  down 
by  rain  and  rills,  slip  from  the  cliffs  and  slopes.  In  this  part  of  its  work, 
the  glacier  resembles  a  river  which  carries  down  branches  and  leaves 
from  the  woods  on  its  banks.  Most  of  the  detritus  rests  on  the  surface 
of  the  ice.  It  includes  huge  masses  of  rock,  sometimes  as  big  as  a  large 
cottage,  all  which,  though  seemingly  at  rest,  are  slowly  travelling  down 
the  valley  with  the  ice,  liable  at  any  moment  to  slip  into  the  crevasses 
which  may  open  below  them.  When  they  thus  disappear,  they  may 
descend  to  the  bottom  of  the  ice,  and  move  with  it  along  the  rocky  floor, 

^  Comptes  rend.  cxv.  (1892),  p.  193.     Other  writers  assign  the  bursting  of  a  glacier-lake 
as  the  cause. 


424  I'YXAMICAL  GEOLOGY  book  iu  part  u 

which  is  no  doubt  the  fat«  of  a  large  projKtrtion  of  the  smaller  atones 
anil  sand.  But  the  large  stones  seem,  sometimes  at  least,  to  be  cast  up 
again  by  the  ice  to  the  surface  o£  the  glacier  at  a  lower  part  of  its  course. 
Whether  therefore  on  the  ice,  in  the  ice,  or  under  the  ice,  a  vast  quantity 
of  detritus  is  contitiuallj  travelling  with  the  glacier  down  towards  the 


|)lains.  Tlie  ruhliish  lying  on  the  surface  is  called  mtmiinf  stuff. 
Naturally  it  accumulates  on  either  side  of  the  glacier,  where  it  forms 
the  so-called  hti-rui  wnrainr.'.  When  two  glaciers  unite,  their  two 
adjacent  lateral  moraines  ai-e  brought  together,  and  travel  thereafter 
down  the  centre  of  the  glacier  as  a  medial  Moraiiir.  In  Fig.  150  the  left 
lateral  moraine  (3)  of  glacier  B  imites  with  the  right  lateral  moraine 
(:;i  of  A  to  form  the  medial  moraine  h,  while  the  other  moraines  (1, 


4)  continue  their  coin-se  and  become  resi>ectively  the  right  and  left  lateral 
moraines  (r,  a)  of  the  united  glacier.  A  glacier  formed  by  the  union 
of  many  tributaries  in  its  U]>j>er  parts,  may  have  numerous  medial  lines 
of  moraine,  so  many  indee<l  as  sometimes  to  be  covered  with  d^Inis, 


SECT,  ii  S  5  GLACIERS  AND  ICE-SHEETS  425 

to  the  complete  concealment  of  the  ice.  At  such  parts  the  glacier 
appears  to  be  a  bare  field  or  earthy  plain,  rather  than  a  solid  mass  of 
clear  ice  of  wliich  only  the  surface  is  dirty  irith  rubbish.  At  the  end 
of  the  glacier,  the  pile  of  loose  materials  is  tumbled  upon  the  valley  in 
what  is  called  the  lerminal  moraine. 

Beneath  the  ice  of  the  Swiss  glaciers  lies  a  thin  inconstant  layer 
of  fine  wot  mud,  aaiid,  and  stones,  derived  partly  from  the  descent  of 
materials  from  the  surface  down  the  crevasses,  partly  from  the  rocks 
of  the  sides  and  bottom  of  the  glacier-bed.  These  materials  may 
be  seen  fixed  sometimes  in  the  ice  itself.  Though  it  may  locally 
accumulate,  this  layer  is  apt  to  be  removed  by  the  ice  or  by  the 
water  that  flori's  under  the  glacier.  It  is  known  to  Swiss  geologists 
as  the  moraine  prnfmide  or  Grandmorarte  ( =  boulder  clay,  till  or  bottom- 
moraine).  The  sheet  of  ice  that  once  filled  the  broad  central  plain  of 
Switzerland,  between  the  Alps  and  the  Jura,  certainly  pushed  a  vast 
deal  of  mud,  sand,  and  stones  over  the  floor  of  the  valley,  and  this 
material  has  been  left  as  a  covering,  like  the  till  of  Northern  Europe.' 

When  from  any  cause  a  glacier  diminishes  in  size,  it  may  drop 
it^  blocks  upon  the  sides  of  its  valley,  and  leave  them  there,  sometimes 
in  tliB  moat  threatening  positions.  Such  stranded  stones  are  known  as 
pnrhfd  blocks.  Those  of  each  valley  belong  to  the  rocks  of  that  valley  ; 
and  if  there  he  any  difference  between  the  rocks  on  the  two  sides,  the 
j>erchcd  blocks,  carried  far  down  from  their  sources,  still  point  to  that 
difference,  for  they  remain  on  their  own  original  side.  But  during  a 
former  great  extension  of  the  glaciers  of  the  northern  hemisphere,  blocks 
of  rock  have  been  carried  out  of  their  native  valleys,  across  plains,  valleys, 
and  even  considerable  ranges  of  hills. 


Such  "erratics"  (Fiiidlidge)  not  ouly  Bboiimi  in  llic  Swiss  valleys,  Imt  Ci-osa  llic 
jfreat  plain  or  Sn-itzerkiiil,  and  fk|ij>ear  in  iiuiiiIhts  liigli  uiwli  the  Hanks  of  tlie  Juni, 
Siiice  the  latter  mountains  conniHC  chiefly  of  limettonc,  and  the  blocks  are  of  variini!! 
crystalline  rocks  belonging  to  the  higher  iwrts  of  tbc  Aliw,  the  jiroof  of  tniiis]iort  in 
irrefragable.  Thousands  of  thein  Tonn  a  great  belt  of  Wulilers  extending  for  miles  ut  an 
■veragc  height  of  800  feet  above  tlie  Ulte  of  XeufchiUel  (Fig.  151).    These  consist  of 

>  In  1869  I  eiamined  n  charncterixltc  Hc-tloii  of  au  ancient  morniitr-  ]>r-f-H.I'  i.eiir 
Solothuni,  fall  of  wratclied  stones,  au<l  lying  on  the  striated  pavenient  of  rock  to  he 
immedimteljr  described  as  fiirllier  character i slit  of  ice-ai'tioii.  It  clowly  rescniblcl  the 
boulder-clay  of  Northern  Euroi>c 


426  DYXAmCAL  CEOLOGY  BOOKinPAStn 

the  protogine  granite  of  the  Mont  Blanc  group  of  mountains,  and  must  haTe  trareUal 
at  least  60  or  70  uiiUs.  One  ot  tlie  most  noted  of  them,  the  Pierre  i  Bot  (to«d-»tone), 
whicli  lies  about  two  miles  went  o[  NearchSt«),  ineasarea  50  {French)  feet  in  length  bj 
20  in  widtli,  and  40  in  heij^ht.  It  is  estimated  to  contain  40,000  cubic  feet,  and  to 
weigh  about  3000  tona.i  The  celeliroted  "blocks  of  Monthey  "  consist  of  buge  nuuv* 
of  granite,  dis]io«cd  in  a  belt,  which  extends  for  iiiilea  along  the  moimtain  slopes  on  tfa« 
left  bank  of  the  Rhone,  iiuar  its  union  with  the  Lake  of  Geneva.  On  the  southern  side 
of  the  Alps,  similar  evidence  oC  tlie  transport  of  blocks  from  the  central  mountains  is  to 
be  found.  On  the  flanks  of  the  limestone  heights  on  the  farther  aide  of  the  Lake  at 
Como,  blocks  of  granite,  gneiss,  and  other  i-rystalline  rocks  lie  scattered  about  in 
hundreds  (Fig.  152). 


»,  lake  ot  Como  (B.): 

Before  the  numerous  facts  had  heen  collected  and  underatood 
which  prove  a  former  gi-eat  iingmentation  in  the  size  of  the  Alpine 
glaciers,  it  was  believed  by  many  geologists  that  the  erratics  stranded 
along  the  flanks  of  the  Jura  mountains  had  been  transported  on 
Heating  ice,  and  that  Central  Europe  iits  tlun  in  great  part  sub- 
merged beneath  an  icy  sea.  It  is  now  unncrsally  adnutted,  however, 
that  the  transport  has  been  entirely  the  \«ork  of  glaciers  Instead  of 
being  confined,  as  at  present,  to  the  highei  parts  of  their  valleys,  the 
glaciers  extended  down  into  tlie  plains  \s  already  stated,  they  filled 
the  great  depression  between  the  01>erlanil  and  the  Tun,  and,  rising  high 


upon  tlie  flanks  of  the  latter  chain,  actually  overrode  some  of  its  ridges. 
Similar  evidence  in  the  hilly  parts  of  Britain,  as  well  as  in  other  parts  of 
Eurdpo  and  America,  no  longer  the  abode  of  glaciers,  shows  that  a  great 
extension  of  snow  and  ice  at  a  recent  geological  period  prevailed  in  tiie 
northern  hemisphere,  as  will  be  described  in  the  account  of  the  Glacul 
Period  in  Book  VI.  Extensive  as  arc  the  present  ice-sheeta  and  glacien 
of  ftreenland,  they  are  undoubtedly  much  reduced  from  their  former  rite, 
for  bare  ice-worn  rocks  are  found  beyond  their  limits,  as  in  Scandinavia.' 

'  Forbes.  'Travels  in  tlie  Alp^,'  p.  J9. 

'  Mecliielelser  om  Gninland.  H.  Rink.  Pelrnnniw'!  Milthtilangm,  188*.  p.  188,  giwi 
some  recent  results  of  Graeuland  exploration.  Muuli  u.ietal  iDfonnatiou  regarding  the  Arctic 
regions  is  given  in  the  '  Manual  and  Instructions  for  the  Arctic  Expedition,'  1875. 


BErr.  ii  §  S  GLACIERS  AND  ICE-SHEETS  427 

There  ie  proof  also  that  the  glaciers  of  New  Zealand  were  formerly  much 
lai^er.' 

As  De  la  Beche  has  well  pointed  out,  the  student  must  be  on  his  guard 
lest  he  be  led  to  mistake  for  true  erratics  mere  weathered  blocks  belonging 
to  a  rock  that  has  disintegrated  in  siiii. 
If,  for  example,  he  should  encounter  a 
block  like  that  represented  in  Fig.  154, 
he  would  properly  conclude  that  it  Iiad 
travelled,  because  it  did  not  belong  to  the 
rock  on  which  it  lay.  But  he  would 
re<|uire  to  prove  further  that  there  was  no 
rock  in  the  immediat«  neighbourliood  from      ..,  „,    ,    , 

which  it  could  have  fallen  as  the  result  of  iDciinai  ■tnu<i(.> 

mere  weathering.      The  granite  (c)  shown 

ia  Fig.  155  disintegrates  at  the  summit,  and  the  blocks  into  which  it 
splits  find  their  way  by  gravitation  down  the  slope." 


Fig.  1^.— artuil1e(c><]ecoaipoBlnK  Into  blnclii  (n),  wlikli  tirodualLy  roU  duwu  uiH)n  the  iiiTTODndlng 


(b)  Erosion. — The  manner  and  results  of  erosion  in  the  channel  of  a 
glacier  differ  from  those  associated  with  other  geological  agents,  and  form 
therefore  distinguishing  features  of  ice-action.  This  erosion  is  effected 
not  by  the  mere  contact  and  pressure  of  the  ice  upon  the  rocks  (though  un- 
doubtedly blocks  of  rock  may  thereby  be  detached),  but  by  means  of  the 
fine  sand,  stones,  and  blocks  of  rock  that  fall  between  the  ice  and  the 
rocks  on  which  it  moves.  The  detritus  thus  introduced  is,  for  the  moat 
part,  fresh  and  angular.  Its  trituration  by  the  glacier  reduces  the  size 
of  the  particles,  but  retains  their  angular  character,  so  that,  as  Daubr^e 
has  pointed  out,  the  sand  that  escapes  from  the  end  of  a  glacier  appears 
in  sharp  freshly -broken  grains,  and  not  as  roimded  water-worn  particles.^ 

The  surface  of  a  glacier  being  often  strewn  with  earth  and  stones, 
these  materials  are  frequently  precipitated  into  the  crevasses,  and  may  thus 
reach  the  rocky  floor  over  which  the  ice  is  moving.     They  likewise  fall 

'  For  New  Zealaml  gin.iers  see  A.  P.  Harper,  Gfiymph.  Joum.  i,  (1893),  ]>.  32. 

'  De  la  Beche,  '  Geological  Observer,'  p.  257.  The  surface  of  some  parts  of  the  granite 
djatricts  at  Cornwall  are  atrewn  with  large  bonlilers  of  granite,  schorl-rock,  vein  .quartz,  >cc., 
but  theae,  though  resembling  erratics  in  fonii,  are  all  ilui^  to  ilecoui]><i9ition  of  the  parent-rocki 
in  ritu. 

'  'Oeologie  Ei]>erim.'  p.  254. 


DY.\A.\riCAl  GEOLOGY 


BOOK  III  1-ARI  II 


ito  the  narrow  sptce  which  soroetimes  intervene)!  between  the  margin  of 

glacier  and  the  side  of  the  valley  (d  in  Fig.  156).     Held  by  the  ice  as  it 

creeps  along,  they  are  pressed  against  the 

rocky  sides  and  bottom  of  the  valley  so 

fii-mly  and  persistently  as  to  descend  into 

each  little   hollow  and   mount  over  each 

ridge,    yet    all    the   while    moving  along 

steadily  in  one  dominant  dii-ection  with 

the   genciral    movement    of    the    glacier. 

;   Here  ami  there  the  ice,  with    grains  of 

,  sand  and  pieces  of  stone  imbedded  in  it« 

■  surface,  can  be  caught  in  the  very  act  of 

'  polishing  and  scoring  the  rocks.      In  Fig. 

KnriiM,)  '"       •"■•-'"'      >"■  ■      J--    ^    ^.jg^j,    jg    given    of    the    "angle" 

on  the  Mcr  de  Glace,  Chamouni,  where 

blocks  of  granite  are  jammed  between  the  mural  edge  of  the  ice  and 

the  precipice  of  rock  along  which  it  moves,  and  which  is  scored  and 


..  pmiirilouH  r. 


jjolisheil  in  the  direction  of  motion  of  the  blocks.  Under  the  slow, 
continuous,  and  enormoiuily  erosive  power  of  the  creeping  ice,  the  most 
coiniMict'    resisting    rocks    are    ground    down,    smoothed,    polished,    and 


SECT,  ti  j^  & 


GLACIERS  AND  ICE-SHEETS 


striated  (Fig.  158).  The  strise  vary  from  such  fine  lines  as  may  be  made 
by  the  smallest  grains  of  quartz  up  to  deep  ruts  and  grooves.  They  some- 
times cross  each  other,  one  set  partially  effacing  an  older  one,  and  thus 
{winting  to  shiftings  in  the  movement  of  the  ice.  On  the  retirement  of 
the  glacier,  hummocky  bosses  of  rock,  having  smooth  undulating  forms 
like  dolphins'  backs,  are  conspicuous.  These  have  received  the  name  of 
roches  numUmaies.  The  stones  by  which  this  scratching  and  polishing  are 
effected  suffer  in  exactly  the  same  way.  They  are  ground  down  and 
striat«d,  and  since  they  must  move  in  the  line  of  least  resistance,  or  "  end 
on,"  their  striae  run  in  a  general  sense  lengthwise  (Fig.  160).     It  will  be 


seen,  when  we  come  to  notice  the  traces  of  former  glaciers,  how  important 
is  the  evidence  given  by  these  striated  atones. 

Besides  its  proper  and  characteristic  rock-erosion,  a  glacier  is  aided  in 
a  singidar  way  by  the  co-operation  of  running  water.  Among  the  Alps, 
during  day  in  summer,  much  ice  is  melted,  and  the  water  courses  over 
the  glaciers  in  brooks  which,  as  they  reach  the  crevasses,  tumble  down  in 
rushing  waterfalls,  and  are  lost  in  the  depths  of  the  ice.  Directed,  how- 
ever, by  the  form  of  the  ice-passf^e  against  the  rocky  floor  of  the  valley, 
the  water  descends  at  a  particular  spot,  carrying  with  it  the  sand,  mud, 
and  stones  which  it  may  have  swept  away  from  the  surface  of  the  glacier. 
By  means  of  these  materials  it  erodes  deep  pot-holes  (raoulins)  in  the  solid 
rock,  in  which  the  rounded  detritus  is  left  us  the  crevasse  closes  up  or 
moves  down  the  valley.  On  the  ice-worn  surface  of  Xorway,  singular 
cavities   of    this    kind,    known    as    "giants'   kettles"    or   "cauldrons" 


430  T>¥NAMICAL  GEOLOGY  book  lu  paw  n 

(Kieseiitupfe,  Kiesenkesael,  Fig.  159),  exist  in  great  numbers.*  There  cm 
be  little  doubt  that  they  have  had  an  origin  under  the  massive  ice-cover 
H  hich  once  spread  over  that  peninsula.  Similar 
ca\itie8  filled  with  transported  boulders  occur 
in  the  molasse  sandstone  near  Bem,^  and  a  large 
^loup  of  them  is  now  one  of  the  sights  of 
Lucerne  They  have  been  recognised  in  North 
(  ermanj  *  and  generally  over  the  glaciated 
areas  of  llurope.  As  the  Greenland  ice-sheet 
18  tra^  ersed  in  ummer  by  powerful  rivefra  which 
^rc  s«allo\ved  up  in  the  crevasses,  excavations 
of  the  same  nature  are  no  doubt  also  in  progress 
there 

Since  locks  present  great  diversities  of  struc- 
ture and  hardness,  and  consequently  vary  much 
in  the  resistance  they  offer  to  denudation,  they 
are  necea.  arilj  worn  down  unequally.  The 
softer  moie  easily  eroded  portions  are  scooped 
out  by  the  grinding  action  of  the  ice,  and 
l>asin  shaped  or  I'arious  irregular  canties  are 
dug  out  below  the  level  of  the  general  surface. 
h  ui       »  cb    t  SimiUr  effects  may  bo  produced    by    a    local 

augmentation  of  the  excavating  power  of  ■ 
glacier  as  where  the  ico  is  strangled  in  some  narrow  part  of  a  valley,  or 
where  from  change  m  declivit}  it  is  allowed  to  accumulate  in  greater 
mass  as  it  mo\es  more  sJowlj  onward  Such  hollows,  on  the  retirement 
of  the  ice  become  receptacles  for  water  and  form   }X)ols,  tarns,  or  lakes, 


unless,  indeed,  they  chance  to  have  been  already  filled  up  with  glacial 
rubbish. 

Among  the  proofs  of  great  erosion  by  ice  on  hard  rocky  surfaces  the 

existence  of  basins  scooped  out  of  the  solid  rock  are  perhaps  the  most 

striking.      The  striro  and  scorings  may  in  such  cases  be  traced  down 

below  the  water  at  the  end  of  a  tarn  or  lake,  and  maybe  found  emerging 

'  S.  A.  Sen,  Cniaertii.  Program.  ChrUtiania,  1874.    Briigger  Bnd  Benlch,  Q.  J.  OoL 


SECT,  ii  §  5  GLACIERS  AND  ICE-SHEETS  431 

at  the  other  end  with  the  same  steady  direction  as  on  the  surrounding 
ground  or  enclosing  valley.  In  the  year  1862  the  late  Sir  A.  C.  Eamsay 
drew  attention  to  this  peculiar  power  of  land-ice,  and  affirmed  that  the  abund- 
ance of  excavated  rock-basins  in  Northern  Europe  and  America,  was  due  to 
the  fact  that  these  regions  had  been  extensively  eroded  by  sheets  of  land-ice, 
when  the  more  northern  parts  of  the  two  continents  were  in  a  condition  like 
that  of  North  Greenland  at  the  present  day.^  It  is  among  the  ice-fields 
of  Greenland,  rather  than  among  the  valley-glaciers  of  isolated  mountain- 
groups,  that  the  operations  which  produced  the  widespread  general  glacia- 
tion  of  the  period  of  the  rock-basins  find  their  nearest  modern  analogies. 
A  single  valley-glacier  retires  towards  its  parent  snow-field  as  the  climate 
ameliorates,  leaving  its  roches  moutonnSes,  moraine -mounds,  and  rock- 
basins,  yet  at  times  discharging  its  water-drainage  in  such  a  way  as  to 
sweep  down  the  moraine-mounds,  fill  up  the  basins,  bury  the  ice-worn 
hummocks  of  rock,  and  strew  the  valley  with  gravel,  earth,  sand  and  big 
blocks  of  rock.  Hence  the  actual  floor  of  the  glacier  is  apt  to  be  obscured. 
But  in  the  case  of  a  vast  sheet  of  land-ice  covering  continuously  a  wide 
region,  there  can  be  but  little  superficial  debris.  When  such  a  mass  of 
ice  retires,  it  must  leave  behind  it  an  ice-worn  surface  of  country,  more  or 
less  strewn  with  the  detritus  which  accumulated  under  the  ice  and  was 
pushed  along  by  it.  This  infra-glacial  debris  forms  the  Grundmcn'ane 
{moraine  profonde),  or  bottom-moraine  above  referred  to  (p.  425).  We 
know  as  yet  very  little  regarding  its  formation  in  Greenland.  Most  of 
our  knowledge  regarding  it  is  derived  from  a  study  of  the  till  or  boulder- 
clay  in  more  southern  latitudes,  which  is  believed  to  represent  the 
bottom-moraine  of  an  ancient  ice-sheet.  In  countries  where  true  boulder- 
clay  occurs,  numerous  rock-basins  are  commonly  to  l)e  met  with  among 
the  uncovered  portions  of  the  rocks.  These  and  other  features  of 
glaciated  Europe  and  America  will  be  more  fully  described  in  the  account 
of  the  Glacial  Period  (Book  VI.)2 

But  while  the  proofs  of  great  erosion  by  land-ice  are  .indisputable, 
many  instances  have  now  been  collected  where  glaciers  have  over-ridden 
moraines,  gravel-beds,  or  other  soft  material,  and  have  moved  across  them 
for  perhaps  long  periods  without  removing  them.  It  is  obvious  that  in 
these  places  the  ice  can  have  no  marked  or  at  least  rapid  erosive  power. 
The  preservation  of  detritus  below  the  ice  seems  generally  to  arise  from 
flatness  of  the  ground,  thinning  away  of  the  ice,  or  some  other  local 

^  Q.  ./.  Oeoi.  Sh)c.  xviii.  (1862),  p.  185.  See  also  a  paper  by  A.  Helland  (oj).  cit.  xxxiii. 
p.  142),  on  the  Ice-fjords  of  North  Greenland,  and  the  fonijation  of  Fjords,  Lakes,  and 
Cirques.  That  glaciers  nib  down  rocks  is  demonstrated  by  tlie  roches  Tnoutonnies  which 
they  leave  behind  them.  That  they  can  dig  out  hollows  has  been  denied  by  some  able 
observers,  but  that  they  can  do  so  to  some  extent  at  least,  seems  to  be  proved  by  the  way  in 
which  the  ice-striae  descend  into  and  rise  out  of  rock-basins.  For  arguments  against  this 
view  see  especially  W.  D.  Freshfield,  Proc.  Roy.  Oeog.  Soc.  1888,  p.  779,  and  authorities 
there  cited. 

^  See  the  remarks  already  made  (p.  851)  on  the  possibility  of  the  rotting  out  of  basin- 
sha|)ed  receptacles  in  solid  rock  through  the  operations  of  superficial  weathering — a  process 
which  may  account  for  many  rock-basins  that  have  subsequently  had  their  decomposed  rock 
swept  out  of  them  by  ice. 


432  DYNAMICAL  GEOLOGY  book  in  PAW  n 

cause  sufficient  to  indicate  that  the  glacier  cannot  there  act  with  eioeiTe 
effect.^ 

Hardly  anything  has  yet  been  done  in  the  way  of  actual  measure- 
ment of  the  rate  of  erosion  by  different  glaciers.  An  approximation  to 
the  truth  might  be  obtained  from  the  abundant  fine  sediment  which, 
giving  the  chai'acteristic  milky  turbidity  to  all  streams  that  escape  from 
the  melting  ends  of  glaciers,  is  an  index  of  the  amount  of  this  erosion. 
The  average  quantity  of  sediment  discharged  from  the  melting  end  of 
a  glacier  during  a  year  having  been  estimated,  it  would  be  easy  to 
determine  its  eciuivalent  in  the  precise  fraction  of  a  foot  of  rock  annually 
removed  from  the  area  drained  by  the  glacier. 

From  the  end  of  tlie  Aar  glacier  (which  with  its  aiHueuts  is  computed  to  have  an 
area  of  60  square  kilometres,  and  is  therefore  by  no  means  one  of  the  largest  in  Switzer- 
land) it  has  been  estimated  that  there  e8cai>e  every  day  in  the  month  of  August  two 
milli(m  cubic  metres  (440  million  gallons)  of  water,  containing  284,374  kilogFamiiMs 
(280  tons)  of  sand.  The  amount  of  tine  sand  discharged  from  the  meltiug  glacier  into 
the  Ijord  of  Isortok,  Greenland,  is  estimated  at  4062  million  kilogrammes  per  day.*  Mr. 
A.  llclland  has  computed  that  from  the  Justcdal  glacier,  Noru'ay,  one  million  kilo* 
grammes  of  sediment  are  discharge<l  in  a  July  day,  and  that  the  total  annual  dischar;ge 
from  the  ice-tield,  830  si[uare  miles  in  area,  amount^j  to  180  millions  of  kilogranunes, 
besides  13  million  kilogrammes  of  mineral  matter  in  solution.  Taking  the  s|)eeiiic  gravity 
of  the  suspended  matter  at  2*6,  he  linds  that  the  basin  of  the  glacier  loses  69,000  cuW 
nu'trt's  of  solirl  rock  every  year,  or  a  cu])ic  mass  measuring  41  metres  on  the  side.* 
Thei-i'  is  some  dittioulty,  however,  in  ilet«rmining  what  ]>roi>ortion  of  the  sediment  may 
have  been  washe<l  in  U'low  the  ice  by  streams  issuing  from  springs  and  melted  snow^ 
Estimates  of  the  work  done  by  glaciei's,  so  far  as  ba.sed  uiM)n  the  amount  of  sediment 
disrhargcd  )>y  them,  may  oonseipiently  be  rather  over  the  truth. 


J^  ().  OcHanic  Waters. 

The  area,  clei)th,  temperature,  density,  and  composition  of  the  sea 
having  been  already  treated  of  (Book  II.),  wc  have  now  to  consider  its 
place  among  the  dynamical  agents  in  geology.  In  this  relation  it  may  be 
studied  under  two  aspects:  1st,  its  movements,  and  2nd,  its  geological 
work. 

I.  Movements. — (1)  Tides. — These  oscillations  of  the  mass  of  the 
oceanic  waters,  caused  by  the  attraction  of  the  sun  and  moon,  require 
notice  here  only  as  regards  their  geological  bearings.  They  are  scarcely  per- 
ceptible in  enclosed  seas,  such  as  the  Mediterranean  and  Black  Seas,  which 
are  commonly  si)oken  of  as  tideless.  In  strictness,  however,  a  feeble  but 
quite  recognisal)le  tide  may  be  observed  in  the  Mediterranean.  On  the 
coast  of  the  Alpes  Maritimes  it  has  a  mean  rise  of  6  to  8  inches,  the  least 
rise  being  4  and  the  highest  not  exceeding  10  inches.  The  Mediterranean 
tides  are  most  strongly  developed  in  the  Bay  of  Gibraltar  (where  thej  rise 

^  For  a  striking  example  of  the  way  in  which  a  glacier  may  spread  over  depositB  of 
gravel,  see  the  plate  accompanying  Mr.  H.  P.  Cushing's  paper  on  the  Muir  Glacier  of  Alaakft, 
A/nerioin  (ivohujist^  1801. 

-  Meddelelser  oui  Griinlaml,  vol.  ii. 

=  r.W.  FOren.  Stockholm  FiirJiandl.  1874,  No.  21.  Band  ii.  No.  7. 


MCT.  ii  g  6  OCEANIC  TIDES  433 

from  5  feet  to  6  feet  6  inches),  the  upper  Adriatic,  and  the  Gulf  of  Gab«s. 
At  Brindisi  the  rise  is  8  inches,  at  Ancona  1  foot  4  inches,  at  Venice  1 
foot  8  inches,  and  at  Trieste  2  feet  4  inches.  With  a  rise  of  the 
)>arometer  the  level  of  the  water  falls  sometimes  a  fourth  lower  than  the 
limit  of  the  normal  ebb.  Observations  at  Nice,  Monaco,  Cannes,  and 
other  places  show  that  from  atmospheric  disturbances  the  level  of  the 
sea  may  be  lowered  as  much  as  1  foot  8  inches.' 

In  a  wide  deep  ocean,  tidal  elevation  probably  produces  no  per- 
ceptible geological  change.  It  passes  at  a  great  speed  ;  in  the  Atlantic, 
its  rate  is  500  geographical  miles  an  hour.  But  as  this  is  merely  the 
passing  of  an  oscillation  whereby  the  particles  of  water  are  gently  raised 
up  and  let  down  again,  there  can  hardly  be  any  appreciable  effect  upon 
the  deep  ocean-bottom.  When,  however,  the  tidal  wave  enters  a  narrow 
and  shallow  sea,  it  has  to  accommodate  itself  to  a  smaller  channel,  and 
encounters  more  and  more  the  friction  of  the  bottom.  Hence,  while  its 
rate  of  motion  is  diminished,  its  height  and  force  are  increased.  It  is  in 
shallow  water,  and  along  the  shores  of  the  land,  that  the  tides  acquire 
their  main  geological  importance.  They  there  show  themselves  in  an 
alternate  advance  uiwn  and  retreat  from  the  coast.     Their  upper  limit 


baa  received  the  name  of  kiffh-waler  mark,  their  lower  that  of  lov-imfer 
mark,  the  littoral  space  between  being  termed  the  beach  (Fig.  161).  If 
the  coast  is  precipitous,  a  beach  can  only  occur  in  shelving  bays  and 
creeks,  since  elsewhere  the  tides  will  rise  and  fall  against  a  face  of  rock, 
as  they  do  on  the  piers  of  a  port  On  such  rocky  coasts,  the  line  of 
high-water  is  sometimes  admirably  defined  by  the  grey  crust  of  barnacles 
aiihering  to  the  rocks.  Where  the  beach  is  flat,  and  the  rise  and  fall  of 
the  tide  great,  many  square  miles  of  sand  or  mud  may  be  laid  bare  in  one 
bay  at  low- water. 

The  height  of  the  tide  varies  from  zero  up  to  60  or  70  feet  It 
is  greatest  where,  from  the  form  of  the  land,  the  tidal  wave  is  cooped 
up  within  a  narrow  inlet  or  estuary.  Under  such  circumstances  the 
advancing  tide  sometimes  gathers  itself  into  one  or  more  large  waves, 
and  rushes  furiously  up  between  the  converging  shores.  This  ts  the 
origin  of  the  "bore"  of  the  Severn,  which  rises  to  a  height  of  9  feet, 
while  the  rise  and  fall  of  the  tide  at  Chepstow  amounts  to  a.  maximum 
of  50  feet.  In  like  manner,  the  tides  which  enter  the  Hay  of  Fundy, 
between  Nova  Scotia  and  New  Brunswick,  are  more  and  more  cooped  up 

•  Heeeliert,  Iie«i»che  Jtujidtcliau  /iir  Gnxjraphit,  July  18S7.  BvU.  Amtr:  OeojrapH. 
Soc.  lit.  (1887).  p.  3H.     J.  de  Pulligny,  Jwnc.  Fmnf.  1881,  ii.  p.  28", 


434  DYNAMICAL  GEOLOGY  book  in  part  n 

and  rise  higher  as  they  ascend  that  strait,  till  they  reach  a  height  of  70 
feet.  The  bore  on  the  Tsien-Tang  Kiang,  70  miles  from  Shanghai,  rushes 
up  the  estuary  as  a  huge  breaker  20  feet  or  more  in  height,  with  a  loud 
roar  and  a  speed  of  sometimes  eight  knots  an  hour.^ 


Fig.  102.— Effect  of  converging  shores  upon  the  Tidal  Wave. 

The  tidal  wave,  running  up  in  the  direction  of  the  arrows,  rises  successively  higher  at  a,  6,  and  c  to  tf, 
after  which  it  slackens  and  dies  away  at  the  upper  limit  of  tides,  /. 

While  the  tidal  swelling  is  increased  in  height  by  the  shallowness 
and  convergence  of  the  shores  between  which  it  moves,  it  gains  at  the 
same  time  force  and  rapidity.  No  longer  a  mere  oscillation  or  pulsation 
of  the  great  ocean,  the  tide  acquires  a  true  movement  of  translation,  and 
gives  rise  to  ciurents  which  rush  past  headlands  and  through  narrows  in 
powerful  streams  and  eddies. 

The  rocky  and  intricate  navigation  of  the  west  of  Scotland  and  Scandinavia  furnishes 
many  admirable  illustrations  of  the  rapidity  of  these  tidal  currents.  The  famous  whirl- 
IK)ol  of  Corryvreckan,  the  lurking  eddies  in  the  Kyles  of  Skye,  the  breakers  at  the  Bore 
of  Duncansby,  and  the  tumultuous  tideway,  giiinly  named  by  the  northern  fishermen 
**  the  Merry  Men  of  Mey,"  in  the  Pentland  Firth,  bear  witness  to  the  strength  of  these 
sea  rivers.  At  the  last  -  mentioned  strait,  the  cun'cnt  or  "  race  "  at  its  strongest  runs  at 
the  rate  of  10  miles  an  hour,  which  is  fully  three  times  the  s^ieed  of  most  of  our  luge 
rivers. 

(2)  Cwrreiiis. — Recent  researches  in  ocean-temperature  have  disclosed 
the  remarkable  fact  that,  beneath  the  surface-layer  of  water  affected  by 
the  temperature  of  the  latitude,  there  lies  a  vast  mass  of  cold  water,  the 
l)ottora-temperature  of  every  ocean  in  free  communication  with  the  poles 
being  little  above,  and  sometimes  actually  below,  the  freezing-point  of 
fresli  water. *^  In  the  North  Atlantic,  a  temperature  of  40°  Fahr.  is 
reached  at  an  average  depth  of  about  800  fathoms,  all  beneath  that  depth 
being  progressively  colder.  In  the  equatorial  parts  of  that  ocean,  the 
same  temperature  comes  to  within  300  fathoms  of  the  surface.  In  the 
South  Atlantic,  off  Cape  of  Good  Hope,  the  mass  of  cold  water  (below 
40")  rises  likewise  to  about  300  fathoms  from  the  surface.  This 
distribution  of  temperature  proves  that  there  must  be  a  transference  of 

*  Report  to  the  Admiralty  by  Commander  Moore,  R.N.,  1888. 

-  See,  in  particular,  memoirs  by  Carpenter  and  Wyville  Thomson,  Proc.  Roy.  Soc  xrii 
(1868) ;  Brit.  Assoc,  xli.  el  seq.  ;  Proc.  Roy.  Oeograph.  Soc.  xv.  Reports  to  the  Admiralty 
of  the  Challenger  Exploring  Expedition.  Wyville  Thomson's  *  Depths  of  the  Sea,*  1878, 
and  '  Atlantic,'  1877.  Narrative  volume  of  *  ChalUnger  *  Report.  Prince  of  Monaco,  BriL 
Assoc.  1892. 


8KCT.  ii  §  6  OCEANIC  TIDES  AND  CURRENTS  436 

« 

cold  polar  water  towards  the  equator,  for  in  the  first  place,  the  temper- 
ature of  the  great  mass  of  the  ocean  is  much  lower  than  that  which  is 
normal  to  each  latitude,  and  in  the  second  place,  it  is  much  lower  than 
that  of  the  superficial  parts  of  the  earth's  crust  underneath.  On  the 
other  hand,  the  movement  of  water  from  the  poles  to  the  equator  requires 
a  return  movement  of  compensation  from  the  equator  to  the  poles,  and 
this  must  take  place  in  the  superficial  strata  of  the  ocean.  Apart  there- 
fore from  those  rapid  river- like  streams  which  traverse  the  ocean,  and 
to  which  the  name  of  Currents  is  given,  there  must  be  a  general  drift  of 
warm  surface-water  towards  the  poles.  This  is  doubtless  most  markedly 
the  case  in  the  North  Atlantic,  where,  besides  the  current  of  the  Gulf 
Stream,  there  is  a  prevalent  set  of  the  surface-waters  towards  the  north- 
east As  the  distribution  of  life  over  the  globe  is  everywhere  so  depend- 
ent upon  temperature,  it  becomes  of  the  highest  interest  to  know 
that  a  truly  arctic  submarine  climate  exists  everywhere  in  the  deeper 
parts  of  the  sea.  With  such  uniformity  of  temperature,  we  may  antici- 
pate that  the  abysmal  fauna  will  be  found  to  possess  a  corresponding 
sameness  of  character,  and  that  arctic  types  may  be  met  with  even  on 
the  ocean-bed  at  the  equator. 

But  besides  this  general  drift  or  set,  a  leading  part  in  oceanic 
circulation  is  taken  by  the  more  defined  currents.  The  tidal  wave 
only  becomes  one  of  translation  as  it  passes  into  shallow  water,  and 
is  thus  of  merely  local  consequence.  But  a  vast  body  of  water,  known 
as  the  Equatorial  Current,  moves  in  a  general  westerly  direction  round 
the  globe.  Owing  to  the  way  in  which  the  continents  cross  its  path, 
this  current  is  subject  to  considerable  deflections.  Thus,  that  portion 
which  crosses  the  Atlantic  from  the  African  side  strikes  against  the 
mass  of  South  America,  and  divides,  one  portion  turning  towards  the 
south  and  skirting  the  shores  of  Brazil ;  the  other  bending  north-west- 
ward into  the  Gulf  of  Mexico,  and  issuing  thence  as  the  well-known 
Gulf  Stream.  This  equatorial  water  is  comparatively  warm  and  light. 
At  the  same  time,  the  heavier  and  colder  polar  water  moves  towards 
the  equator,  sometimes  in  surface -currents  like  those  which  skirt  the 
eastern  and  western  shores  of  Greenland,  but  more  generally  as  a  cold 
under-current  which  creeps  over  the  floor  of  the  ocean  even  as  far  as  the 
equator. 

A  large  body  of  information  has  now  been  gatliered  as  to  the  great 
marine  currents  which  traverse  the  upper  parts  of  the  ocean,  but 
comparatively  little  is  yet  known  of  the  velocity  of  the  movement  of 
the  water  at  great  depths.  Where  the  bottom  is  covered  with  a  deep 
fine  ooze  we  may  infer  that  the  rate  of  movement  must  be  so  feeble  as 
not  to  disturb  the  deposition  of  the  finest  sediment.  Where,  on  the 
other  hand,  "hard-bottom"  is  found,  we  may  probably  conclude  that 
a  sufficiently  strong  current  flows  there  to  prevent  the  accumulation 
of  sediment,  for  all  over  the  ocean  there  is  enough  of  organic  and 
inorganic  particles  diff*used  through  the  water  to  form  a  deposit  on  the 
floor  if  the  conditions  are  favourable  A  few  observations  have  been 
made  showing  that  at  considerable  depths  among  submarine  ridges  or 


436  DYXAMICAL  GEOLOGY  book  in  part  n 

islands  strong  currents  exist.  At  a  depth  of  3000  feet  near  Gibraltar  the 
telegraph  cable  from  Falmouth  was  ground  like  the  edge  of  a  razor,  and 
the  scouring  effects  of  strong  currents  have  been  noted  at  depths  of  6000 
feet  between  the  Canary  Islands.^ 

Much  discussion  has  arisen  in  recent  years  as  to  the  cause  of  oceanic 
circulation.  Two  rival  theories  have  been  given.  According  to  one  of 
these,  the  circulation  entirely  arises  from  that  of  the  air.  The  trade- 
winds,  blowing  from  either  side  of  the  equator,  drive  the  water  before 
them  until  the  north-east  and  south-east  currents  unite  in  equatorial 
latitudes  into  one  broad  westerly-flowing  current.  Owing  to  the  form  of 
the  land,  ])ortions  of  this  main  current  are  deflected  into  temperate 
latitudes,  and,  as  a  consequence,  an  equivalent  bulk  of  polar  water 
requires  to  move  towards  the  equator  to  restore  the  equilibrium. 
According  to  the  other  view,  the  currents  arise  from  differences  of 
temperature  (and  according  to  some,  of  salhiity  also) ;  the  warm  and 
light  equatorial  water  stands  at  a  higher  level  than  the  colder  and 
heavier  polar  water ;  the  former,  therefore,  flows  down  as  it  were  pole- 
wards, while  the  latter  moves  as  a  bottom-inflow  towards  the  equator; 
the  cold  bottom-water  under  the  tropics  slowly  ascends  to  the  warmer 
upper  layers,  and  rises  in  temj)erature  towards  the  surface,  whence  it 
drifts  away  as  warm  water  towards  the  pole,  and,  on  being  cooled  down 
there,  descends  and  begins  another  journey  to  the  equator.  There  can 
be  no  doubt  that  the  Avinds  are  directly  the  cause  of  such  ciurents  as  the 
Gulf  Stream,  and  therefore,  indirectly,  of  return  cold  currents  from  the 
polar  regions.  It  seems  hardly  less  certain  that,  to  some  extent  at  least, 
diff*erences  of  temperature,  and  therefore  of  density,  must  occasion 
movements  in  the  mass  of  the  oceanic  waters.- 

Apart  from  disputed  questions  in  physics,  the  main  facts  for  the 
geological  reader  to  grasp  are — that  a  system  of  circulation  exists  in  the 
ocean ;  that  warm  currents  move  round  the  equatorial  regions,  and  are 
turned  now  to  the  one  side,  now  to  the  other,  by  the  form  of  the 
continents  along  and  around  which  they  sweep  ;  that  cold  currents  set 
in  from  poles  to  equator ;  and  that,  apart  from  actual  currents,  there  is 
an  extremely  slow  "  creep  "  of  the  polar  water,  under  the  wanner  upper 
layers,  to  the  equator. 

(3)  J  raves  and  Grou  nil-Swell. — A  gentle  breeze  curls  into  ripples  the 
surface  of  water  over  which  it  blows.  A  strong  gale  or  furious  storm 
raises  the  surface  into  waves.  The  agitation  of  the  water  in  a  storm  is 
prolonged  to  a  great  distance  beyond  the  area  of  the  original  disturb- 
ance, and  then  takes  the  form  of  the  long  heaving  undulation  termed 
G-roiind-siceU.  Waves  which  break  upon  the  land  or  sunken  rocks  are 
called  Breakers,  and  the  same  name  is  applied  to  the  ground-swell  as  it 

1  T.  M.  Reade,  Phil.  Morj.  xxv.  (1888\  p.  342. 

-  The  student  may  consult  Maury's  *  Physical  Geography  of  the  Sea,*  but  more  par- 
ticularly Dr.  Carpenter's  pai>ers  in  the  Proceedhigs  of  the  Royal  Society  for  1S69-78,  and 
Journal  of  the  Royal  Geographical  Society  for  1871-77,  on  the  side  of  tempenture  ;  and 
Herschel's  'Physical  Geography,'  and  CroU's  'Climate  and  Time,*  on  the  side  of  tbe 
winds. 


SECT,  ii  §  6        OCEANIC  WAVES  AND  GROUND-SWELL  437 

bursts  into  foam  and  spray  upon  submarine  reefs  and  shoals.  The 
concussion  of  earthquakes  sometimes  gives  rise  to  very  disastrous  ocean- 
waves  (pp.  271,  278). 

The  height  and  force  of  waves  depend  upon  the  strength  and  con- 
tinuance of  the  wind,  the  breadth  and  depth  of  sea,  and  the  form  and 
direction  of  the  coast-line.  The  longer  the  "  fetch,"  and  the  deeper  the 
water,  the  higher  the  waves.  A  coast  directly  facing  the  prevalent 
wind  will  have  larger  waves  than  a  neighbouring  shore  which  presents 
itself  at  an  angle  to  the  wind  or  bends  round  so  as  to  form  a  lee- 
shore.  The  highest  waves  in  the  narrow  British  seas  probably  never 
exceed  15  or  20  feet,  and  usually  fall  short  of  that  amount.  The 
greatest  height  observed  by  Scoresby  among  the  Atlantic  waves  was 
43  feet.i 

Ground-swell  propagated  across  a  broad  and  deep  ocean  produces  by 
far  the  most  imposing  breakers.  So  long  as  the  water  remains  deep  and 
no  wind  blows,  the  only  trace  of  the  passing  ground-swell  on  the  open 
sea  is  the  huge  broad  heaving  of  the  surface.  But  where  the  water 
shallows,  the  superficial  part  of  the  swell,  travelling  faster  than  the 
lower,  which  encounters  the  friction  of  the  bottom,  begins  to  curl  and 
crest  as  a  huge  billow  or  wall  of  water,  that  finally  bursts  against  the 
shore.  Such  billows,  even  when  no  wind  is  blowing,  often  cover  the 
cliffs  of  the  north  of  Scotland  with  sheets  of  water  and  foam  up  to 
heights  of  100  or  even  nearly  200  feet.  During  north-westerly  gales, 
the  windows  of  the  Dunnet  Head  lighthouse,  at  a  height  of  upwards  of 
300  feet  above  high-water  mark,  are  said  to  be  sometimes  broken  by 
stones  swept  up  the  cliffs  by  the  sheets  of  sea-water  which  then  deluge 
the  building. 

A  single  roller  of  the  ground-swell  20  feet  high  falls,  according  to 
Mr.  Scott  Russell,  with  a  pressure  of  about  a  ton  on  every  square  foot. 
Mr.  Thomas  Stevenson  conducted  some  years  ago  a  series  of  experiments 
on  the  force  of  the  breakers  on  the  Atlantic  and  North  Sea  coasts  of 
Britain.  The  average  force  in  summer  was  found  in  the  Atlantic  to 
be  611  lb.  per  square  foot,  while  in  the  winter  it  was  2086  lb.,  or 
more  than  three  times  as  great.  On  several  occasions,  both  in  the 
Atlantic  and  North  Sea,  the  winter  breakers  were  found  to  exert  a 
pressure  of  three  tons  per  square  foot,  and  at  Dunbar  as  much  as  three 
tons  and  a  half.^  Besides  the  waves  produced  by  ordinary  wind  action,, 
others  of  an  extraordinary  size  and  destructive  power  are  occasionally 
caused  by  local  atmospheric  disturbances.  Such  are  probably  the  raz  de 
marie  of  the  French  coast,  which  occasionally  rise  to  a  height  of  several 
feet,  and,  where  the  shores  converge  inland,  do  considerable  damage. 
Still  more  serious  are  the  effects  of  a  violent  cyclone-storm.  The  mere 
diminution  of  atmospheric  pressure  in  a  cyclone  must  tend  to  raise  the 
level  of  the  ocean  within  the  cyclone  limits.  But  the  further  furious 
spiral  in-rushing  of  the  air  towards  the  centre  of  the  low-pressure  area 

^  Brit.  Assoc  Hep.    1850,  p.   26.     A  table  of  the  observed  heig^its  of  waves  round 
Great  Britain  is  given  in  Mr.  T.  Stevenson's  treatise  on  *  Harbours/  p.  20. 

'  T.  Stevenson,  Trans.  Roy.  Soc.  Edin,  zvi.  p.  25  ;  treatise  on  '  Harbours,'  p.  42. 


438  DYNAMICAL  GEOLOGY  book  ra  pabt  n 

drives  the  sea  onward,  and  gives  rise  to  a  wave  or  succession  of  waves 
having  great  destructive  power.  Thus,  on  5th  October  1864,  during  a 
great  cyclone  which  passed  over  Calcutta,  the  sea  rose  in  some  places 
24  feet,  and  swept  everything  before  it  with  irresistible  force,  drowning 
upwards  of  48,000  people. 

Besides  the  height  and  force  of  waves  it  is  important  to  know 
the  depth  to  which  the  sea  is  affected  by  such  superficial  moyements. 
Sir  G.  Airy  states  that  ground-swell  may  break  in  100  fathoms  water.^ 
It  is  common  to  find  boulders  and  shingle  disturbed  at  a  depth  of  10 
fathoms,  and  even  driven  from  that  depth  to  the  shore,  and  waves  may 
be  noticed  to  become  muddy  from  the  working-up  of  the  silt  at  the 
bottom,  when  they  have  reached  water  of  7  or  8  fathoms  in  deptL' 
In  the  English  Channel  coarse  sediment  is  disturbed  at  depths  of  30 
or  more  fathoms.^  It  is  stated  by  Delesse  that  engineering  operationB 
have  shown  submarine  constructions  to  be  scarcely  disturbed  at  a 
greater  depth  than  5  metres  (16*4  feet)  in  the  Mediterranean  and  8 
metres  (26*24  feet)  in  the  Atlantic.^  In  the  Bay  of  Gascony,  the  depth 
at  which  the  sea  breaks  and  is  effective  in  the  transport  of  sand  along 
the  bottom,  is  said  to  vary  from  scarcely  3  metres  in  ordinary  weather 
to  5  metres  in  stormy  weather,  and  only  exceeds  10  metres  (32*8  feet) 
in  great  hurricanes.  According  to  Commander  Cialdi,  the  movement  <^ 
waves  may  disturb  fine  sand  on  the  bottom  at  a  depth  of  40  metree  (131 
feet)  in  the  English  Channel,  50  metres  (164  feet)  in  the  Mediterranean, 
and  200  metres  (656  feet)  in  the  ocean.^  Off  the  Florida  coast  the  dis- 
turbing action  of  the  waves  is  believed  to  cease  below  100  fathoms.'  As 
above  remarked,  the  infiuence  of  currents  has  been  detected  at  much 
greater  depths. 

(4)  Ice  on  the  Sea. — In  this  place  may  be  most  conveniently  noticed 
the  origin  and  movements  of  the  ice  which  in  circumpolar  latitudes 
covers  the  sea.  This  ice  is  derived  from  two  sources — a,  the  freezing  of 
the  sea  itself,  and  ^,  the  seaward  prolongation  of  land-ice.^ 

a.  Three  chief  types  of  sea-ice  have  been  observed,  (a)  In  the 
Arctic  sounds  and  bays,  the  littoral  waters  freeze  along  the  shores,  and 
form  a  cake  of  ice  which,  upborne  by  the  tide  and  adhering  to  the  land, 
is   thickened  by  successive  additions  below,  as  well  as  by  snow  above, 

*  Kncyclopedia  Afetrqpoliiana,  art.  "  Waves."     Gentle  movement  of  the  bottom  water 
'is  said  to  be  sometimes  indicated  by  ripple-marks  on  the  fine  sand  of  the  sea-floor  at  a 

depth  of  600  feet.  2  t   Stevenson's  '  Harbours,'  p.  15. 

^  A.  R.  Hunt,  Proc.  Roy.  Dublin  Soc.  iv.  (1884),  p.  285.  For  further  information  on 
this  subject  aee  jfostea,  pp.  451,  455. 

*  "Lithologie  des  Mers  de  France'  (1872),  p.  110. 
^  Quoted  by  Delesse,   op.  cit.  p.  111. 

*  A.  Agassiz,  Amtr.  Acad.  xii.  (1882),  p.  108. 

'  Consult  on  the  whole  of  this  subject  K.  Weyprecht's  *Die  Metamorphceen  dea 
Polareises,'  Vienna,  1879  ;  Payer's  '  New  Lands  within  the  Arctic  Circle,*  1876,  chap.  i.  The 
physics  of  sea-ice  ai^  discussed  by  0.  Pettersson  ( '  Vega-Expeditionens  Vetenskapliga  lakt- 
tagelser,'  ii.  p.  299,  Stockholm,  1883),  who  concludes  that  instead  of  being  contracted  by 
cold,  tlie  volume  of  the  frozen  sea  increases  to  an  extraordinary  degree,  and  that  the  rapture  of 
the  ice  is  thus  due  to  expansion  instead  of  contraction. 


SKCr.  ii  §  6  OCEANIC  WA  VES  AND  ICE  439 

until  it  forms  a  shelf  of  ice  120  to  130  feet  broad,  and  20  to  30  feet  high. 
This  shelf,  known  sa  the  Ice-foot,  eerrcB  as  a  platform  on  which  the 
abundant  debris,  looseued  by  the  severe  frosts  of  an  Arctic  winter,  gathers 
at  the  foot  of  the  cliffs.  It  is  more  or  less  completely  broken  up  in 
summer,  nut  forms  again  with  the  early  frosts  of  the  ensuing  autumn. 
(6)  The  surface  of  the  open  sea  likewise  freezes  over  into  a  continuous 
solid  sheet,  which,  when  undisturbed,  becomes  in  the  Arctic  regions 
about  eight  feet  thick,  but  which  in  summer  breaks  up  into  separate 
masses,  sometimes  of  large  extent,  and  is  apt  to  be  piled  up  into  huge, 
irregular  heaps  (Fig.  163).  This  is  what  navigators  term  Floe-ice,  and 
the  separate  floating  cakes  are  known  as  fioes.     Ships  fixed  among  these 


floes  have  been  drifted  with  the  ice  for  hundreds  of  miles,  until  at  last 
liberated  by  its  disruption,  {c)  In  the  Baltic  Sea,  off  the  coast  of 
Labrador  and  elsewhere,  ice  has  been  observed  to  form  on  the  aea-bottom. 
It  is  known  as  Ground-ice  or  Anchor-ice.  In  the  Labrador  fishing- 
grounds,  it  forms  even  at  considerable  depths.  Seals  caught  in  the  lines 
at  those  depths  are  said  to  be  brought  up  sometimes  solidly  frozen.' 

ji.  In  the  Arctic  regions,  vast  glaciers  drain  the  snow-fields,  and, 
descending  to  the  sea,  extend  for  some  distance  from  shore  uutil  large 
fragments  break  off  and  float  away  seawards  (Fig.  164).  These  detached 
masses  are  Icebergs.  Their  shape  and  size  greatly  vary,  but  lofty  peaked 
formsarecommon(Fig.  165),  and  they  sometimes  rise  from  200  to  300  feet 
'  S*e  H.  Y.  Hinil,  Canadian  Xaliiralial.  viii.  (ISrS),  pp.  227,  282. 


DYNAMICAL  QEOLOGY 


BOOK  nt  PAH  n 


above  the  level  of  the  sea.  As  the  part  that  appears  above  water  is  onljr 
about  one-ninth  of  the  whole  mass  of  ice,  these  largerbergsmay  sometjioet 
be  from  1800  to  2700  feet  thick  from  base  to  top,  though  the  submuine 


o™i.tlono(lMbergii(B.) 

jrDUdd  (b)  to  tbs  m-levcl  (t),  beuing  mondne  stuff  on  Uif 
iw  (d).  indHiidingnfriceberfiB  (n),  which  nujr  cury  datijttu  uxt 
ciroii  i[  aver  tne  sa-Dottom  :  [,  [',  a,  Unu  othlgh  and  loir  water. 

|)art  of  the  ice  may  be  as  irregular  in  form  and  thickness  as  the  portion 
above  water.^  Icobergs  of  tlie  largest  size  consequently  require  water  of 
some  depth  to  float  them,  but  are  sometimes  seen  aground  la  the 
Antarctic  regions,  where  one  vast  sheet  of  ice  envelops  the  land  and 
protrudes  into  the  sea  as  a  long,  lofty  rampart  of  ice,  the  detached  ke- 


ic  leelierg 


bergs  often  reach  a  great  size,  and  are  characterised  by  the  frequency  of  a 
flat  tabular  form  (Fig.  1C6). 

II.  Geological  Work. — (1)  Influence  on  Climate. — Were  there 
no  agencies  in    nature   for  distributing  temperature,  there  would  1m  a 
regular  and  uniform  diminution  in  the  mean  annual  temperature  from 
■  Od  natation  of  icebergs,  see  Gtiii.  Hag.  {lai  aec ),  iii.  p.  303,  379  ;  It.  65,  p.  136. 


BBCT.  ii  S  6  GEOLOGICAL  tVilRK  OF  THE  SEA  441 

equator  to  poles,  and  the  xaoOurmal  lines,  or  lines  of  equal  heat,  would 
coincide  with  lines  of  latitude.  But  no  such  general  correspondence 
actually  exists.     A  chart  of  the  globe,  with  the  isothermal  lines  drawn 


across  It,  shows  that  their  divergences  from  the  parallels  are  striking 
and  must  so  where  they  approach  and  cross  the  ocean.  Currents  from 
warm  regions  raise  the  temperature  of  the  tracts  into  which  they  flow ; 
those  from  cold  regions  lower  it.  The  ocean,  in  short,  is  the  great 
distributor  of  temperature  over  the  globe. 

As  an  illnstratioD,  the  tno  opposite  sides  of  tbe  North  Atlautie  may  be  taken.  The 
cold  Arctic  current,  flowing  southward  along  the  north-east  coast  of  America,  reduces  the 
mean  annnsl  temperature  of  that  region.  On  the  other  hand,  the  Gidf  Stream  brings 
to  the  nhorea  of  the  north-west  of  Europe  a  temjieratnre  much  above  (chat  they  would 
otherwise  enjoy.  Dnliiin  and  the  south-eastern  headlands  of  Labrador  lie  on  the  same 
parallel  of  latitude,  yet  differ  as  much  as  18°  in  their  mean  annual  temperature,  that  of 
Dublin  heing  50°,  and  that  of  Labrador  33°  Fahr.  Dr.  CroU  has  calculated  tliat  the 
Gutf  Stream  conveys  nearly  half  as  much  heat  from  the  tropics  as  is  receivcil  from  the 
sun  by  the  entire  Arctic  Regions.' 

(2)  Erosion.  A,  Chemical. — The  chemical  action  of  the  sea  upon 
the  rocks  of  its  bed  and  shores  has  not  yet  been  properly  studied.^  It 
is  evident,  however,  that  changes  analogous  to  those  efl^ected  by  fresh 
water  on  the  land  must  be  in  progress.  Oxidation,  solution,  and  the 
formation  of  carbonates,  no  doubt  continually  take  place.  The  solvent 
action  of  sea-water  on  calcareous  organisms,  already  referred  to  (p.  38), 
has  in  recent  years  been  made  the  subject  of  discussion  and  experiment. 
Dr.  Murray,  in  calling  attention  to  the  gradual  disappearance  of  such 
organisms,  as  the  deposits  of  the  sea-bottom  are  traced  down  into  the 
abysses,  explained  it  by  the  solvent  influence  of  the  water  containing 

'  See  a  series  of  papeis  by  him  on  the  "  Gulf  Stream  and  Ocean  Corrents,"  in  Otol.  Mag. 
and  na.  Jloff.  (or  1866,  1870-71,  and  bis  work  'aimnteand  Time';  likewise  a  series  of 
controversial  papers  on  tbla  subject  by  bim  aad  Prof,  Newcombe,  PliU.  Hay.  1883-4. 
Prof,  Haughton  has  offered  some  cnlculations  of  the  actual  amount  of  influence  exercised  by 
ocean -currents  upon  climate,  and  of  tlie  effect  of  a  current  between  tbe  Indian  and  Arctic 
Oceans  across  Meanpotamia  anil  tbe  Aralo-Cospiao  deprcHiion.  Brit.  As>oc,  I88I,  Reports, 
pp.  451-463. 

*  See  Bischori  'Chemical  Geolo^,'  vol.  i.  chap,  vii. 


442  DYNAMICAL  GEOLOGY  book  in  pari  n 

carbonic  acid  in  solution,  and  he  has  more  recently  conducted  a  series  of 
experiments  to  demonstrate  the  truth  of  this  view.  Ten  specimens  of 
coral  of  different  species  were  immersed  in  sea-water  and  allowed  to  remain 
for  periods  varying  from  20  to  60  days.  In  each  case  a  perceptible  loss 
of  material  took  place,  varying  from  0*0725  to  0*1707  of  their  weight, 
which  he  estimated  to  be  equal  to  a  rate  of  loss  amounting  to  from  0*453 
to  01 860  from  one  square  inch  of  surface  in  a  year.  The  more  areolar 
or  amorphous  corals  were  attacked  more  rapidly  than  the  hardo' 
crystalline  varieties.^  The  complex  chemical  changes  that  take  plaee 
in  the  sea  through  the  operation  of  living  and  dead  organisms  are  referred 
to  on  pp.  482,  484,  492,  493. 

We  may  judge,  indeed,  of  the  nature  and  rapidity  of  some  of  these  changes  by 
watching  the  decay  of  stones  and  material  employed  in  the  oonstniction  of  piers.  Mr. 
Mallet — as  the  result  of  experiments  with  specimens  sunk  in  the  sea — concluded  that 
from  ^\  to  yV  of  ^^  ^ch  in  depth  in  iron  castings  1  inch  thick,  and  about  iV  of  *n 
inch  of  wrought  iron,  will  bo  destroyed  in  a  century  in  clear  saltwater.  Mr.  Stevenson, 
in  referring  to  these  experiments,  remarks  that  at  the  Bell  Rock  lighthouse,  twenty-five 
different  kinds  and  combinations  of  iron  were  exposed  to  the  action  of  the  sea,  and  all 
yielded  to  corrosion.  In  some  of  these  eastings,  the  loss  has  been  at  the  rate  of  an  inch 
in  a  century.  "  One  of  the  bars  which  was  free  from  air-holes  had  its  specific  graTity 
reduced  to  5*63,  and  its  transverse  strength  from  7409  lb.  to  4797  lb.,  and  yet  presented 
no  external  appearance  of  decay.  Another  apparently  sound  specimen  was  reduced  in 
strength  from  4068  lb.  to  2352  lb.,  having  lost  nearly  half  its  strength  in  fifty  year&**' 
Similar  results  were  observed  by  Mr.  Grothe,  resident  engineer  at  the  construction  of 
the  ill-fated  railway  bridge  across  the  Firth  of  Tay.  A  cast-iron  cylinder  (such  m  wai 
employed  in  constructing  the  concrete  basements  for  the  piers),  which  had  been  below 
water  for  only  sixteen  months,  was  found  to  be  so  corroded  that  a  penknife  could  be 
stuck  through  it  in  many  places.  An  examination  of  the  shore  will  sometimes  reveal  a 
good  deal  of  (^uiet  chemical  change  on  the  outer  crust  of  wave-washed  rocks.  Basalt, 
for  instance,  has  its  felsj^r  decomi>08ed,  and  shows  the  presence  of  carbonates  by 
effervescing  briskly  with  acid.  The  augite  is  occasionally  replaced  by  ferrous  carbonate. 
The  solvent  action  of  sea-water  on  calcareous  organisms  is  referred  to  on  pp.  38,  491. 

B.  MechanicaL — It  is  mainly  by  its  mechanical  action  that  the  sea 
accomplishes  its  erosive  work.  This  can  only  take  place  where  the 
water  is  in  motion,  and,  other  things  being  equal,  is  greatest  where 
the  motion  is  strongest.  Hence  we  cannot  suppose  that  erosion  to  any 
appreciable  extent  can  be  effected  in  the  abysses  of  the  sea,  where  the 
only  motion  is  probably  the  slow  creeping  of  the  polar  water.  But  where 
the  currents  are  powerful  enough  to  move  grains  of  sand  and  gravel,  a 
slow  erosion  may  take  place  even  at  considerable  depths.  It  is  in  the 
upper  portions  of  the  sea,  however, — ^the  region  of  currents,  tides,  and 
waves, — that  mechanical  erosion  is  chiefly  performed.  The  depth  to 
which  the  influence  of  waves  and  ground-swell  may  extend  seems  to  vary 
greatly  according  to  the  situation  {ante,  p.  438).  A  good  test  for  the 
absence  of  serious  abrasion  is  furnished  by  the  presence  of  fine  mud  on 

*  Pruc.  Roy,  Soc.  Edin.  xvii.  (1889),  p.  109.  See  also  R.  Irvine,  Xaiurt,  1888,  p.  461 ; 
J.  G.  Ross,  ihid.  p.  462.  Compare  A.  Agassiz,  Bull,  Mus.  Comp,  Zooi.  Harvard,  zvii 
No.  3  (1889),  p.  125. 

'  T.  Stevenson  on  •Harbours,*  p.  47. 


SECT,  ii  §  6 


OEOLOQICAL   WORK  OF  THE  SEA 


443 


the  bottom.  Wherever  that  is  found,  we  may  be  tolerably  sure  that 
the  bottom  at  that  place  lies  beyond  the  reach  of  ordinary  breaker-action.^ 
From  the  superior  limit  of  the  accumulation  of  mud  up  to  high-water 
mark,  and  in  exposed  places  up  to  100  feet  or  more  above  high-water 
mark,  lies  the  zone  within  which  the  sea  does  its  work  of  abrasion.  To 
this  zone,  even  where  the  breakers  are  heaviest,  a  greater  extreme  vertical 
range  can  hardly  be  assigned  than  300  feet,  and  in  most  cases  it  probably 
falls  far  short  of  that  extent. 

The  mechanical  work  of  erosion  by  the  sea  is  done  in  six  ways. 

(i.)  The  enormous  force  of  the  breakers  suffices  to  tear  off  frag- 
ments of  the  solid  rocks. 

Abundant  examples  are  furnished  by  the  precipitous  shores  of  Caithness,  and  of  the 
Orkney  and  Shetland  Islands.  It  sometimes  happens  that  demonstration  of  the  height 
to  which  the  effective  force  of  breakers  may  reach  is  furnished  at  lighthouses  built  on 
exposed  parts  of  the  coast.  Thus,  at  Unst,  the  most  northerly  point  of  Shetland,  walls 
were  overthrown  and  a  door  was  broken  open  at  a  height  of  196  feet  above  the  sea.  At 
the  Bishop  Rock  lighthouse,  on  the  w^est  of  England,  a  bell  weighing  3  cwt.  was 
wrenched  off  at  a  level  of  100  feet  above  high-water  mark.'  Some  of  the  most  remark- 
able instances  of  the  power  of  breakers  have  been  observed  by  Mr.  Stevenson  among  the 
islands  of  the  Shetland  group.  On  the  Bound  Skerry  he  found  that  blocks  of  rock,  up 
to  9i  tons  in  weight,  had  been  washed  together  at  a  height  of  nearly  60  feet  above  the 
sea  ;  that  blocks  weighing  from  6  to  13^  tons  had  been  actually  quarried  out  of  their 
original  bed,  at  a  height  of  from  70  to  75  feet ;  and  that  a  block  of  nearly  8  tons  had 
been  driven  before  the  waves,  at  the  level  of  20  feet  above  the  sea,  over  very  rough 
ground,  to  a  distance  of  73  feet.  He  likewise  records  the  moving  of  a  50-ton  block  by 
the  waves  at  Barrahead,  in  the  Hebrides.*  At  Plymouth,  also,  blocks  of  several  tons  in 
weight  have  been  known  to  be  washed  about  the  breakwater  like  pebbles.* 

(ii.)  The  alternate  compression  and  expansion  of  air  in  crevices 
of  rocks  exposed  to  heavy  breakers  dislocates  large  masses  of  stone,  even 
above  the  direct  reach  of  the  waves.  It  is  a  fact  familiar  to  engineers 
that,  even  from  a  vertical  and  apparently  perfectly  solid  wall  of  well-built 
masonry  exposed  to  heavy  seas,  stones  will  sometimes  be  started  out  of 
their  places,  and  that  when   this  happens,  a  rapid  enlargement  of  the 

*  T.  Stevenson  on  *  Harbours,'  p.  15. 

*  T.  Stevenson,  op,  cU.  p.  31.  D.  A.  Stevenson,  Min.  Proc.  Inst.  Civ.  Engin.  xlvi. 
(1876),  p.  7. 

'  T.  Stevenson,  op.  cit  pp.  21-37. 

*  The  student  will  bear  in  mind  that  the  relative  weight  of  bodies  is  greatly  reduced 
when  in  water,  and  still  more  in  sea-water.  The  following  examples  will  illustrate  this  fact 
(T.  Stevenson's  *  Harbours,'  p.  107)— 




Specific 
Gravity. 

No.  of  cubic  feet  to  a 
ton  in  air. 

No.  of  feet  to  a  ton  in 

sea- water  of  upecific 

gravity  1-028. 

Basalt 

Red  granite 
San£tone .... 
Cannel  Coal 

2-99 
2-71 
2-41 
1-54 

11-9 
13-2 
14-8 
233 

18-26 
21-30 
26-00 
70-00 

444  DYXAMICAL  GEOLOGY  book  ni  part  n 

cavity  may  be  effected,  as  if  the  walls  were  breached  by  a  severe 
bombardment.  At  the  Eddystone  lighthouse,  during  a  storm  in  1840,  a 
door  which  had  been  securely  fastened  against  the  force  of  the  surf  irom 
without,  was  actually  driven  outward  by  a  pressure  acting  from  within 
the  tower,  in  spite  of  the  strong  bolts  and  hinges,  which  were  broken. 
We  may  infer  that,  by  the  sudden  sinking  of  a  mass  of  water  hurled 
against  the  building,  a  partial  vacuum  was  formed,  and  that  the  air  inside 
forced  out  the  door  in  its  efforts  to  restore  the  equilibrium.^  This  e2q)laDar 
tion  may  partly  account  for  the  way  in  which  the  stones  are  started  from 
their  places  in  a  solidly  built  sea-wall.  But  besides  this  cause,  we  must 
also  consider  a  perhaps  still  more  effective  one  in  the  condensation  of  the 
air  driven  before  the  wave  between  the  joints  and  crevices  of  the  stones, 
and  its  subsequent  instantaneous  expansion  when  the  wave  drops. 
During  gales,  when  large  waves  are  driven  to  shore,  many  tons  of  water 
are  poured  suddenly  into  a  cleft  or  cavern.  These  volumes  of  water,  as 
they  rush  in,  compress  the  air  into  every  joint  and  pore  of  the  rock  at 
the  further  end,  and  then,  quickly  retiring,  exert  such  a  suction  as  from 
time  to  time  to  bring  down  i)art  of  the  walls  or  roof.  The  sea  may  thus 
gradually  form  an  inland  passage  for  itself  to  the  surface  above,  in  a 
** blow-hole,'*  or  "puffing-hole,"  through  which  spouts  of  foam  and  spray 
are  in  storms  shot  high  into  the  air. 

On  the  more  exiwseil  ix)rtioiis  of  th«  west  coast  of  Ireland,  and  on  the  north  coast  of 
Cornwall,  numerous  examples  of  such  blow-holea  occur.  In  Scotland,  likewise,  they  may 
often  be  observed,  as  in  the  Bullers  (boilers)  of  Buchan  on  the  coast  of  Aberdeenshire, 
and  the  (icary  Pot  near  Arbroath.  Magnificent  instances  occur  among  the  Orkney  and 
Shetland  Islands,  some  of  the  more  shattered  rocks  of  these  northern  coasts  being,  as  it 
were,  hcmeycombed  by  sea-tunnels,  many  of  which  ojwn  up  into  the  middle  of  fields  or 
mooi-s. 

(iii.)  The  hydraulic  pressure  of  those  portions  of  large  waves  that 
enter  fissures  and  passages  tends  to  force  asunder  masses  of  rock.  The 
sea-water  which,  as  part  of  an  inrushing  wave,  fills  the  gullies  and  chinks 
of  the  shore-rocks,  exerts  the  same  pressure  upon  the  walls  between  which 
it  is  confined  as  the  rest  of  the  wave  is  doing  upon  the  face  of  the  cliff. 
Each  cleft  so  circumstanced  becomes  a  kind  of  hydraulic  press,  the  potency 
of  which  is  to  be  measured  by  the  force  with  which  the  waves  fall  upon 
the  rocks  outside — a  force  which  often  amounts  to  three  tons  on  the 
square  foot.  There  can  be  little  doubt  that  by  this  means  considerable 
pieces  of  a  clif!'  are  from  time  to  time  dislodged. 

(iv.)  The  waves  make  use  of  the  loose  detritus  within  their  reach  to 
break  down  cliffs  exposed  to  their  fury.  Pro])ably  by  far  the  largest 
amount  of  erosion  is  thus  accomplished.  The  blows  dealt  against  shore- 
cliffs  by  boulders,  gravel,  and  sand  swung  forward  by  breakers,  were 
aptly  compared  by  Playfair  to  a  kind  of  artillery.^  During  a  storm  upon 
a  shingly  coast  we  may  hear,  at  a  distance  of  several  miles,  the  grind  of 
the  stones  upon  each  other,  as  they  are  dragged  back  by  the  recoil  of  the 

'  Walker,  Proc.  Inst.  Civ.  Engin,  i.  p.  15  ;  Stevenson's  'Harboun,*  p.  10. 
*  'Illustrations  of  the  Huttonian  Theory,'  sec.  97. 


8ECT.  u  §  6  GEOLOGICAL  WORK  OF  THE  SEA  445 

waves  which  had  launched  them  forward.*  In  this  tear  and  wear,  the 
loose  stones  are  ground  smaller,  and  acquire  the  smooth  round  form  bo 
characteristic  of  a  surf-beaten  beach.  At  the  same  time,  they  bruise  and 
wear  down  cliffs  a^inst  which  they  are  driven.  A  rock,  much  jointed, 
or  from  any  cause  presenting  leas  resistance  to  attack,  is  excavated  into 
gullies,  creeks,  and  caves ;  its  harder  parts  standing  out  as  promontories 
are  pierced  ;  gradually  a  series  of  detached  buttresses  and  sea-stacks 
appears  as  the  cliff  recedes,  and  these  in  turn  are  wasted  until  they  become 
mere  skerries  and  sunken  aurf- beaten  reefs  (Fig,  167).  The  surface  of  the 
beach  is  likewise  ground  down.  The  reality  of  this  erosion  and  consequent 
lowering  of  level  is  sometimes  instructively  displayed  where  a  block 
of  harder  rock  serves  for  a  time  to  protect  the  portion  of  rocky  beach 
lying  beneath  it.  The  block  by  degrees  comes  to  rest  on  a  growing  pedestal, 
which  is  eventually  cut  round  by  the  waves,  until  the  overlying  mass, 


losing  its  support,  rolls  down  upon  the  beach.     Thereafter  the  same  process 
is  renewed,  and  the  boulder  continual!}'  diminishes  in  size  (Fig.  168).- 

Of  tlie  progress  of  msritie  eronion,  the  rame  exposed  |>ai'ts  of  the  British  eoaat-liiie 
furnish  m&iiy  admireble  eiBmples.  Tlie  sea-Iioanl  of  Coliiwall  prcaeuti)  a  tiioat  impresnive 
range  of  cliffs,  Be&-atacka,  oavea,  gullies,  tutiiiels,  reefa,  and  skerries,  iiliowiug  eveij  stage 
iu  the  proceas  of  demolition  {Kig.  1B7).  The  «est  coast  of  Ireland,  eijiosed  to  the  full 
Hwell  of  tlie  Atlantie,  is  in  innumerable  localities  completely  uiiilemiined  by  caverns,  into 
wliich  the  sea  enters  from  both  aides.  Tlie  pretipitous  coasts  of  Skye,  Sutlierlaml, 
Caithoess,  Aberdeen,  Kincardiue,  and  Forfar  abound  in  the  most  ini[>regaive  lessons  of 
the  waste  of  a  rocky  sea-liiargin  ;  while  the  same  juLturesque  features  are  |irolonged 
into  the  Orkney  and  Shetland  Islands,  the  niagnihccut  chOit  of  Hoy  towering  as  a  vast 
wall  some  lliOO  feet  above  the  Athtntie  breakers   uhicli  are  tunnelling  and  fi-etting  their 


'  For  n  graphic  account  of  the  heavy  roll  of  the  boulders  and  thundering  of  the  hillows 
ma  heard  in  a  mine  under  the  sea  during  a  storm,  see  J.  ^V .  Uenwood,  Ttant.  Rog.  Gtol.  Sac. 
CamteaU,  v.  p.  11. 

•  See  on  the  action  of  waves  on  sea-beaches  and  sca-bottouis,  A.  K  Hunt,  froc.  Roy. 
DuUinSoc.  1S84,  p.  241. 


DYNAMICAL  GEOLOGY 


BOOK  tn  PART  n 


If  such  is  the  pragresa  of  vaste  where  the  nuiterUls  consist  of  tbe  most  aolid  ioAm, 
we  may  e.tiiert  to  meet  with  still  more  iiiipresaiv^  proofs  of  decay  where  the  o 
can  opjiose  only  soft  sand  or  clay  to  the  march  of  the  breakers.  Again,  the  gi 
student  in  Britain  can  examine  for  himself  many  illustrations  of  this  kind  of  destmctian 
around  the  shores  of  these  islands.  Within  the  last  few  hundred  yeara  entire  ptriihea, 
with  their  farms  and  village^  have  been  washed  away,  and  the  tide  now  ebbs  and  flows 
over  districts  which  in  old  times  were  cultivated  Kelds  and  cheerful  hatnleta.  ITn 
ooast  of  Yorkshire  between  Flamborough  Head  and  the  mouth  of  the  Humber,  and  abe 
that  between  the  Wash  and  the  mouth  of  the  TliamBs,  sulTeT  at  a  specially  rapid  rats, 
for  the  clilfB  in  these  parts  consist  in  great  measure  of  soft  clay.  In  some  |d»OM 
between  3pura  Point  and  Flamborough  Head  tliis  loss  is  said  to  amount  to  &Te  yards 
l«r  anniuji.' 

Other  [larts  of  the  European  sea-hoard  likewise  furnish  instmctive  lessoUB  as  to  tbe 
progress  of  marine  erosion.     The  destruction  of  Heligoland,  in  tbe  North  Sea,  has  been 


^caudinai 


for  oeuturii's,  the  stnf^es  in  tliu  disappearance  of  this  island  being  eaiUy 
I  the  cohorts  of  successive  jierio'la.''  Kven  the  hard  crystalline  rocks  of 
,  are  unable  wholly  to  withstand  the  assaults  of  the  Atlantic  breakers.' 


While  investigating  the  progress  of  waste  along  a  coast-line,  the 
geologist  has  to  consider  the  vaiyiiig  powers  of  resistance  possessed  by 
rocks,  and  the  e.^tont  to  which  the  action  of  the  waves  is  assisted  by  that 
of  tlie  subaeiial  agents.  Bocks  of  little  tenacity,  and  readily  susceptible 
of  disintegration,  obviously  present  least  resistance  to  the  advance  of 

'  R  Fickwell.  J'n^.  latl.  Ct'v.  Jingin.  ii.  p.  191.  On  the  waste  of  the  coast  between 
the  Thames  and  Waah.Beo  J.  B.  Redman,  op.  rit.  iiiii.  (1861),  P-  186;  C.  Reid,  Oal.  Mag. 
3Dd  dec.  iv.  p.  136.  '  Ueology  of  Halderness,' .Urni.  Urol.  Surv.  1885.  The  Reports  g( 
the  British  AssoclBtioD  Committee  an  the  erosion  of  the  sea-coaata  of  England,  18S5-8S,  give 
much  interesting  information  on  this  subject. 

-'  K.  W.  M.  Wiebels  'Die  Insel  Helgoland,'  4to,  Hamburg,  1848, 

*  H.  ReuBch,  .Vami  Jahrb.  18;9.  p.  214. 


p.  ii  §  6  GEOLOGICAL  WORK  OF  THE  SEA  447 

waves.     A  clay,  for  example,  is  readily  eaten  away.     If,  however,  it 

old  contain  numerous  hard  nodules  or  imbedded  boulders,  these,  as 

y  drop  out,  may  accumulate  in  front  beneath  the  cliff,  and  serve  as  a 

tial  breakwater  against  the  waves  (Fig.  169).     On  the  other  hand,  a 

d  band  or  boss  of  rock  may  withstand  the  destruction  which  overtakes 

softer  or  more  jointed  surrounding  portions,  and  may  consequently  be 

projecting  into  the  sea,  as  a  line  of  headland  or  promontory,  or  rising 

m  isolated  stack  (Fig.  167).     But,  besides  mere  hardness  or  softness, 

geological  structure  of  the  rocks  powerfully  influences  the  nature  and 

3  of  the  encroachment  of  the  sea      Where,  owing  to  the  inclination  of 

iding,  joints,  or  other  divisional  planes,  sheets  of  rock  slope  down  into 

water,  they  serve  as  a  kind  of  natural  breakwater,  up  and  down  which 

surges  rise  and  fall  during  caJms,  or  rush  in  crested  billows  during 

3S,  the  abrasion  being  here  reduced  to  the  smallest  proportions.     In  no 

t  of  the  degradation  of  the  land  can  the  dominant  influence  of  rock- 

icture  be  more  conspicuously  observed  and  instructively  studied  than 

ig  marine   cliffs.      Where  the  lines  of   precipice  are  abrupt,  with 

nerous  projecting  and  retiring  vertical  walls,  it  will  almost  invariably 


ig.  100.— Cliffs  of  clay  full  of  septarian  nodules,  the  accumulation  of  which  serves  to  arrest  the 

progress  of  the  waves.  • 

found  that  these  perpendicular  faces  have  been  cut  open  along  lines  of 
jrsecting  joint.  The  existence  of  such  lines  of  division  permits  a 
>p  or  vertical  front  to  be  presented  by  the  land  to  the  sea,  because,  as 
B  after  slice  is  removed,  each  freshly  bared  surface  is  still  defined  by 
)int-plane  (see  p.  524). 

During  the  study  of  any  rocky  coast  where  these  features  are 
ibited,  the  observer  will  soon  perceive  that  the  encroachment  of  the 

upon  the  land  is  not  due  merely  to  the  action  of  the  waves,  but 
t,  even  on  shores  where  the  gales  are  fiercest  and  the  breakers  most 
Drous,  the  demolition  of  the  cliffs  depends  largely  upon  the  sapping 
uence  of  rain,  springs,  frosts,  and  general  atmospheric  disintegration. 
Fig.  170,  for  example,  which  gives  a  view  of  a  portion  of  the  northern 
thness  coast,  exposed  to  the  full  fury  of  the  gales  and  rapid  tidal 
rents  which  rush  from  the  Atlantic  through  the  Pentland  Firth,  we 
at  once  that  though  the  base  of  the  cliff  is  scooped  out  by  the  restless 
ye  into  long  twilight  caves,  nevertheless  the  recession  of  the  precipice 
^used  by  the  wedging  off  of  slice  after  slice,  along  lines  of  vertical 
it,  and  that  this  process  begins  at  the  top,  where  the  subaerial  forces 

not  the  waves  are  the  sculptors.     Undoubtedly  the  sea  plays  its  part 


DYXAMII'AL  (iKOLiiC.Y 


BOOK  III  PABI  n 


by  removing  the  materials  dislodged,  iind  j  ire  venting  them  from  accntnnlat- 
ing  against  and  protecting  the  face  of  the  pretipice.  But  were  it  not  for 
the  potent  influence  of  Bubtcrial  decay,  the  progresH  of  the  sea  would  br 
coraparativcly  fooble.     The  very  blocks  of  stone  which  give  the  waves  bo 


lI«Hl.  Cnlthiin-i. 


much  of  their  etticiicj-  us  abrading  agents,  arc  in  great  measure  funiishnl 
to  them  liy  the  action  of  tlie  meteoric  agents.  If  sru-cIifTs  were  mainly 
due  to  the  destructive  effects  of  the  waves,  they  ought  to  overhang  their 
base,  for  only  at  or  noar  their  Iwse  does  the  sea  act  (Fig.  171).      But 


the  fact  that,  in  the  vast  majority  of  cases,  sca-clifis,  instead  of  ovcrhan^'- 
ing,  slope  backward,  at  a  greater  or  less  angle,  from  the  sea  (Fig.  167), 
shows  tliat  the  waste  from  subaerial  action  is  really  greater  than  tfaftt  from 


BBCT.  ii  § 


GEOLOGICAL   WORK  OF  THE  SEA 


449 


the  action  of  the  breakers '  Even  when  a  cliff  actually  overlianga,  how- 
ever It  may  often  be  shown  that  the  apparent  greater  recession  of  its  base, 
and  inferentially  the  more  powerful  denuding  action  of  the  sea,  are  decep- 
tive In  Fig  172  one  of  innumerable  examples  from  the  Old  Red 
Sandstone  cliffs  of  Caithness  and  the  Orkney  and  Shetland  Islands,  we  at 
once  perceive  that  the  process  of  demolition  is  precisely  similar  to  that 
alreadj  cited  m  Fig  1 70  The  cliff  recedes  by  the  loss  of  successive 
slices  from  its  sea-front,  which  are  wedged  off  not  by  the  waves  below,  but 
by  the  subaenal  agents  above  along  lines  of  parallel  joint.  To  the  inclin- 
ation of  these  divisional  planes  at  a  high  angle  from  the  sea,  the  precipice 
owea  Its  slope  towards  the  land 

{^  )  Tidal  Eroswn  —Reference  has  already  been  made  (pp.  436,  438)  to 


existence  of  currents  at  considerable  depths  in  the  ocean,  though  not  in 
the  profoiinder  abjsses  These  movements  have  been  observed  in  straits 
between  islands  oi  submanne  ridges,  and  they  are  doubtless  con- 
nected with  the  tidji  itaie  They  seem  to  possess  sufficient  scour  to 
prevent  tl  e  ace  imulatioi  ot  aed  ment,  but  whether  they  are  effective  in 
eroding  hollows  on  the  sea  floor,  as  Ima  been  claimed  for  them,  may  be 
doibted  Their  power  to  di^  out  hollows  or  to  deepen  and  widen 
ch  Lnnela  must  depend  not  merely  on  their  velocity  but  upon  the  presence 
of  detntus  which  they  can  use  in  abrasion,  for  without  this  detritus  they 
could  not  remove  the  surface  of  hard  rocks.- 

(vi )  /'■''  h    itnn  — \mon^  the  erosive  ojjerations  of  the  sea,  must  be 
included  what  is  performed  b\  floating  ice.     Along  the  margin  of  Arctic 

Wli  taker  (lail    V        i   \    H 

The  polcDcy  of  t  dnl  act  on  hns  long  beea  mniiitained  by  Mr.  T.  Mellard  lieaile.  Proc 
Geoi   SiK   Ltverpooi  18  3    PA      V  g   iiv.  (1868),  p.  338. 


450 


DYXAMICAL  GEOLOGY 


BOOK  lU  PABT  II 


lands,  a  good  deal  of  work  is  done  by  the  broken-up  floe-ice  and  ice-foot, 
both  in  abrasion  and  in  deposit.  Cakes  of  ice,  driven  ashore  by  storms, 
tear  up  and  redistribute  the  soft  shallow-water  or  littoral  deposits,  rub 
and  scratch  the  rocks,  and  push  gravel  and  blocks  of  rock  before  them 
as  they  strand  on  the  beach.  Icebergs  also,  when  they  get  aground  in 
deep  water,  must  greatly  disturb  the  sediment  accumulating  there,  and 
may  grind  down  any  submarine  rock  on  which  they  grate  as  they  are 
driven  along.  The  geological  operations  of  floating  ice  were  formerly 
invoked  by  geologists  to  explain  much  that  is  now  believed  to  have 
been  entirely  the  work  of  ice  on  land.^ 

(3)  Transport. — By  means  of  its  currents,  the  sea  transports 
mechanically-suspended  sediment  to  varying  distances  from  the  land. 
The  distance  will  depend  on  the  size,  form,  and  specific  gravity  of  the 
sediment  on  the  one  hand,  and  on  the  velocity  and  transporting  power 
of  the  marine  current  on  the-  other.  Babbage  estimated  that  if,  from 
the  mouth  of  a  river  100  feet  deep,  suspended  limestone  mud,  of  diflferent 
degrees  of  fineness,  were  discharged  into  a  sea  having  a  uniform  depth 
of  1000  feet  over  a  great  extent,  four  varieties  of  silt,  falling  respectively 
through  10,  8,  5,  and  4  feet  of  water  per  hour,  would  be  distributed  as 
in  the  following  table  :  ^ — 


No. 

! 

Velocity  of  fall 
|H;r  hour. 

Nearest  distance  of 
deiKMit  to  river. 

miles. 

Length  of  deposit. 

Greatest  dlstanoe 

of  deposit  fhnn 

riyer. 

feet. 

miles. 

miles. 

1. 

10 

180 

20 

200 

2. 

8 

225 

25 

250 

3. 

5 

360 

40 

400 

4. 

4 

450 

60 

500 

It  must  be  }yome  in  mind,  however,  that  mechanical  sediment  sinks 
faster  in  salt  than  in  fresh  water.^  The  chief  part  of  the  fine  mud  in  the 
layer  of  river  water,  which  floats  for  a  time  on  the  Salter  and  heavier  sea- 
water,  sinks  to  the  bottom  as  soon  as  the  two  waters  commingle.  It  has 
been  ascertained,  nevertheless,  by  direct  observation  that  an  appreciable 
amount  of  extremely  fine  clay  is  present  in  ocean-water  even  far  away 
from  land,  the  proportion  so  transported  depending  not  only  on  the  size 
and  weight  of  the  particles,  but  on  the  temperature  and  to  a  less  extent 
on  the  salinity,  being  greater  the  lower  the  temperature  and  salinity. 
In  specimens  of  surface-water  taken  from  various  oceans  the  amount  of 
mechanically  suspended  silicates  (clay)  was  found  to  be  as  follows  * : — 

^  For  an  account  of  the  work  of  floating  ice  ("pan-ice")  see  H.  Y.  Hind,  Caneuiian 
Naturalist,  viii.  (1878),  p.  229. 

2  Q.  J.  Ueol.  Sac,  xii.  368. 

^  See  ante  J  \)\\  381,  398  and  authorities  there  cited. 

■*  Murray  and  Irvine,  Proc.  Roy.  Soc.  JCdin.  xviii.  (1891),  p.  243.  These  authors  ngud 
the  silica  thus  mechanically  suspended  in  sea- water  aa  the  probable  source  of  most  of  tbii 
substance  secreted  by  marine  plants  and  animals. 


SECT.  ii§  6  GEOLOGICAL  WORK  OF  THE  SEA  451 


In  14  litres 

Per  cubic  mile 

of  water. 

of  water. 

0-0062  grm. 

=r 

1604  tons 

0-0063    „ 

^ 

1946    „ 

0-0065    ,, 

= 

2031    „ 

0-0105    „ 

^ 

3200    „ 

0-0006    „ 

:= 

264    „ 

,  0-0006    .. 

^^ 

264    „ 

Atlantic  Ocean,  lat  51"  20',  long.  31"  W. 

Grerman  Ocean,  30  miles  E.  of  May  Island 

Mediterranean,  centre  of  Eastern  basin 

Baltic  Sea,  salinity  1005-5 

Red  Sea,  off  Brothers  Island 

Indian  Ocean,  lat.  15"  46'  N.,  long.  58"  51'  E.  0-0006   ,, 

Near  the  land,  where  the  movements  of  the  water  are  active,  much 
coarse  detritus  is  transported  along  shore  or  swept  further  out  to  sea.  A 
prevalent  wind,  by  creating  a  current  in  a  given  direction,  or  a  strong 
tidal  current  setting  along  a  coast-line,  will  cause  the  shingle  to  travel 
coastwise,  the  stones  getting  more  and  more  rounded  and  reduced  in  size 
as  they  recede  from  their  source.  The  Chesil  Bank,  which  runs  as  a 
natural  breakwater  16  miles  long,  connecting  the  Isle  of  Portland  with 
the  mainland  of  Dorsetshire,  consists  of  drifted  rounded  shingle.^  On  the 
Moray  Firth,  the  reefs  of  quartz-rock  about  Cullen  furnish  abundance  of 
shingle,  which,  urged  by  successive  easterly  gales,  moves  westwards  along 
the  coast  for  more  than  1 5  miles.  The  coarser  sediment  probably  seldom 
goes  much  beyond  the  littoral  zone.  Returning  to  the  subject  of  the 
depth  to  which  wave-action  extends  (ante,  p.  438)  we  may  take  note  that 
it  has  been  observed  by  the  fishermen  at  Land's  End  that  their  lobster- 
pots  are  often  filled  with  coarse  sand  and  shingle  in  depths  up  to  30 
fathoms  during  heavy  ground-swells,  and  that  some  of  the  stones  weigh  as 
much  as  one  pound.-  From  a  depth  of  even  600  fathoms  in  the  North 
Atlantic,  between  the  Faroe  Islands  and  Scotland,  small  pebbles  of 
volcanic  and  other  rocks  are  dredged  up  which  may  have  been  carried  by 
an  Arctic  under -current  from  the  north.  Mr.  Murray  and  Captain 
Tizzard,  however,  have  brought  up  large  blocks  of  rounded  shingle  from 
that  bank  at  a  depth  of  300  fathoms.  Such  detritus  can  hardly 
be  due  to  any  present  action  of  the  sea,  for  at  these  depths  the  force  of 
currents  at  the  bottom  is  probably  too  feeble  to  push  along  coarse  shingle. 
It  may  be  moraine-stuflf  dating  back  to  the  ice-sheets  of  the  Glacial  Period, 
its  finer  particles  having  been  swept  away  while  it  is  prevented  from  being 
buried  under  submarine  mud  by  the  scour  of  the  currents  over  the  bank. 
Blocks  of  stone  brought  up  from  depths  of  more  than  2000  fathoms  in 
the  Atlantic  (Lat.  49"N.,  Long.  43°-44°W.)  have  probably  been  dropped 
by  icebergs  from  the  north.* 

*  On  the  Chesil  Bank,  see  J.  Coode,  Min.  Proc.  Inst.  Civ.  Engin.  xiL  p.  520.  J.  B 
Redman,  op.  cit.  xi.  p.  201  ;  xxiii.  p.  226  ;  Nature^  xxvi.  pp.  30,  104,  150  ;  J.  Prestwich, 
Min.  Proc.  Inst,  Civ.  Engin.  xL  p.  115  ;  H.  W.  Bristow  and  W.  Whitaker,  Oeol.  Mag. 
vi.  (1869),  p.  433 ;  0.  Fisher,  op.  cit.  1874,  p.  285  ;  G.  H.  Kinahan,  op.  cU.  1874.  A.  R. 
Hunt,  Proc.  Roy.  Dublin  Soc.  iv.  (1884),  p.  241.  The  general  transport  of  littoral  detritus 
in  the  English  Channel  is  from  west  to  east ;  Prof.  I*restwich,  however,  thinks  that  at  the 
Chesil  Bank  this  direction  is  locally  reversed. 

*^  J.  N.  Douglas,  Min.  Proc.  Inst.  Civ.  Engin.  xl.  (1875),  p.  103. 

'  See  charts  of  part  of  North  Atlantic  by  Messrs.  Siemens  Brothers  &  Co.,  London, 
1882.  Some  specimens  shown  to  me  by  Messrs.  Siemens  are  pieces  of  basalt  which  may  have 
come  from  Greenland. 


452  DYNAMICAL  GEOLOGY  book  m  part  ii 


Much  fine  sediment  is  visibly  carried  in  suspension  by  the  sea  for  long 
distances  from  land.  The  Amazon  poiu*s  so  much  silt  into  the  sea  as 
to  discolour  it  for  several  hundred  miles.  After  wet  weather,  the  sea 
around  the  shores  of  the  British  Islands  is  sometimes  made  turbid  by 
the  quantity  of  mud  washed  by  rain  and  streams  from  the  land.  Dr.  Car- 
penter found  the  bottom-waters  of  the  Meditermnean  to  be  everywhere 
permeated  by  an  extremely  fine  mud,  derived  no  doubt  from  the  rivers 
and  shores  of  that  sea.  He  remarks  that  the  characteristic  blueness  of  the 
Mediterranean,  like  that  of  the  Lake  of  Geneva,  may  be  due  to  the 
diffusion  of  exceedingly  minute  sedimentary  particles  through  the  water. 

The  great  oceanic  currents  are  i)robably  powerful  agents  in  the 
transport  of  fine  detritus  and  of  living  and  dead  organisms.  Coral-reefs 
appear  to  flourish  best  where  these  currents  bring  a  continuous  and 
abundant  supply  of  food  to  the  reef -builders.  The  reefs,  in  turn,  famish 
an  enormous  quantity  of  fine  silt,  produced  by  the  pounding  action  of 
breakers  upon  them.  Before  the  silt  can  sink  to  the  bottom,  it  may  be 
transported  to  vast  distances.  The  lower  portion  of  the  Gulf  Stream, 
from  its  exit  in  the  Florida  Channel  northward  to  Cape  Hatteras,  a 
distance  of  700  miles,  has  been  compared  to  a  huge  muddy  river,  carrying 
its  silt  to  the  steep  slope  south  of  that  cape,  and  depositing  here  and  there 
patches  of  green  sand  along  the  sides  of  its  course,  while  the  upper  waters 
remain  perfectly  clear  and  of  the  deepest  blue.  The  silt  is  partly  derived 
from  the  abrasion  of  coral-reefs,  partly  from  the  decay  of  the  abundant 
pelagic  fauna  swe^)t  onward  by  the  current.  Professor  A.  Agassiz  has 
recently  called  attention  to  the  important  part  which  the  great  oceanic 
currents,  in  ancient  as  in  modern  times,  may  have  played  in  the  accumula- 
tion of  limestones,  not  only  hy  transporting  calcareous  organisms,  but  by 
bringing  an  abundant  food-supply  and  thereby  nourishing  a  prolific  fauna 
along  their  track. ^ 

During  the  voyage  of  the  CJuilleiujer,  from  the  abysses  of  the  Pacific 
Ocean,  at  remote  distances  from  land,  the  dredge  brought  up  bushels  of 
rounded  pieces  of  pumice  of  all  sizes  up  to  blocks  a  foot  in  diameter. 
These  fragments  were  all  evidently  waterworn,  as  if  derived  from  land, 
though  we  are  still  ignorant  of  the  extent  to  which  they  may  have  been 
suj)])lied  by  submarine  volcanic  eruptions.  Some  small  pieces  were  taken 
on  the  surface  of  the  ocean  in  the  tow-net.  Round  volcanic  islands,  and 
oft'  the  coasts  of  volcanic  tracts  of  the  mainland,  the  sea  is  sometimes 
covered  with  floating  pieces  of  water-worn  ])uniice  swept  out  by  flooded 
rivers.  These  fragments  may  drift  away  for  hundreds  or  even  thousands 
of  miles  until,  becoming  water-logged,  they  sink  to  the  bottom.  The 
universal  distribution  of  pumice  was  one  of  the  most  noticeable  features 
in  the  dredgings  of  the  Challenger.  The  clay  which  is  found  on  the 
l)ottom  of  the  ocean,  at  the  greatest  distances  from  any  shore,  contains  only 
volcanic  minerals,  and  appears  to  be  due  to  the  trituration  of  volcanic 
detritus.  In  apjiroaching  the  continents,  at  a  distance  of  several  hundred 
miles  from  shore,  traces  of  the  minerals  of  the  crystalline  rocks  of  the 
land  begin  to  make  their  appearance.- 

»  Ajner.  Acruf.  xi.  (1882),  p.  126.  -  Murray,  Proc.  Hoy.  Soc.  Edin,  1876-7,  p.  247. 


SECT,  ii  i^  6  GEOLOGICAL   WORK  OF  THE  SEA  453 

Another  not  unimportant  process  of  marine  transport  is  that  performed 
by  floating  ice.  Among  the  Arctic  glaciers,  moraine  stuff  is  not  abundant ; 
but  occasional  blocks  of  rock  and  heaps  of  earth  and  stones  fall  from  the 
cliffs  which  rise  above  the  general  waste  of  snow.  Hence,  on  the  ice- 
bergs that  float  off*  from  these  glaciers,  rock  debris  may  sometimes  be 
observed.  It  is  transported  southwards  for  hundreds  of  miles  until,  by 
the  shifting  or  melting  of  the  bergs,  it  is  dropped  into  deep  water.  The 
floor  of  certain  pcfrtions  of  the  North  Atlantic  in  the  pathway  of  the  bergs 
may  be  plentifully  strewn  with  this  kind  of  detritus.  By  means  of  the 
ice-foot  also,  an  enormous  quantity  of  earth  and  stones  is  every  year  borne 
away  from  the  shore  on  the  disrupted  ice,  and  is  strewn  over  the  floor  of 
the  sounds,  bays,  and  channels. 

(4)  Reproduction. — ^The  sea,  being  the  receptacle  for  the  material 
worn  away  from  the  land,  must  receive  and  store  up  in  its  depths  all  that 
vast  amount  of  detritus  by  the  removal  of  which  the  level  and  contours 
of  the  land  are  in  the  course  of  time  so  greatly  changed.  The  deposits 
which  take  place  within  the  area  covered  by  the  sea  may  be  divided  into 
two  groups — the  inorganic  and  organic.  It  is  the  former  with  which 
we  have  at  present  to  deal ;  the  latter  will  be  discussed  with  the 
other  geological  functions  of  plants  and  animals  (see  pp.  477,  481, 
seq.)  The  inorganic  deposits  of  the  sea-floor  are  (i.)  chemical  and  (ii.) 
mechanical. 

(i.)  Of  Chemical  deposits  now  forming  on  the  sea-floor  we  know  as 
yet  very  little.  At  the  mouth  of  the  Rhone  a  crystalline  calcareous 
deposit  accumulates,  in  which  the  debris  of  the  sea-floor  is  enveloped. 
Bischof  estimated  that  no  precipitation  of  carbonate  of  lime  could  take 
place  from  sea- water  until  after  ^  of  the  water  had  evaporated.^  No 
deposit  of  lime  in  the  open  sea  is  possible  from  concentration  of  sea-water. 
But  the  calcareous  formation  on  the  sea-bottom  opposite  rivers  like  the 
Rhone,  if  not  the  result  of  the  precipitation  of  lime  by  plants  or  animals, 
may  i>erhaps  be  explained  by  supposing  that  as  the  layer  of  river-water 
floats  and  thins  out  over  the  surface  of  the  sea  in  warm  weather  with 
rapid  evaporation,  its  comparatively  large  proportion  of  carbonate  of  lime 
may  be  partially  precipitated.  It  has  been  observed  near  Nice,  as  well 
as  on  the  African  coasts  and  other  parts  of  the  Mediterranean  shores,  that 
on  shore-rocks  within  reach  of  the  water  a  hard  varnish-like  crust  is 
deposited.  This  substance  consists  essentially  of  carbonate  of  lime.  As 
it  extends  over  rocks  of  the  most  various  composition,  it  has  been  regarded 
as  a  deposit  of  lime  held  in  soliition  in  the  shore  sea-water,  and  rapidly 
evaporated  in  pools  or  while  bathing  the  surface  of  rocks  exposed  to 
strong  sun-heat.*  But  it  may  possibly  be  due  to  organic  agency  like  the 
amorphous  crust  of  limestone  formed  by  nullipores  (see  postea,  p.  477). 
During  the  researches  of  the  Challenger  expedition,  important  facts  in  the 
history  of  marine  chemistry  have  been  obtained  from  the  abysses  of  the 
Atlantic  and  Pacific  oceans  (see  pp.  455,  457,  405). 

^   'Chera.  Geol.'i.  p.  178. 

'^  Bull.  Soe,  OSol,  France  (3),  ii.  p.  219,  iii.  p.  46,  vi.  p.  84.     Seeposteay  p.  492,  where 
the  evaporation  in  the  coral-seas  is  referred  to. 


454  DYXAMIOAL  GEOLOGY  book  m  part  n 


(ii.)  The  Mechanical  deposits  of  the  sea  may  be  grouped  into  sub- 
divisions according  as  they  are  directly  connected  with  the  waste  of  the 
land,  or  have  originated  at  great  depths  and  remote  from  land,  when 
their  source  is  not  so  obvious,^ 

A.  Land-derivfd  or  Terrlgenmis, — These  may  be  conveniently  grouped 
according  to  their  relative  places  on  the  sea-bed. ^  , 

a.  Shore  Deposits. — The  most  conspicuous  and  familiar  are  the  layers 
of  gravel  and  sand  which  accumulate  between  tid^-marks.  As  a  rule, 
the  coarse  materials  are  thrown  up  about  the  upper  limit  of  the  beach. 
They  seem  to  remain  stationary  there ;  but  if  watched  and  examined 
from  time  to  time,  they  will  be  found  to  be  continually  shifted  by  high 
tides  and  storms,  so  that  though  the  bank  or  bar  of  shingle  retains  its 
place,  its  component  pebbles  are  being  constantly  moved.  During  gales 
coincident  with  high  tides,  coarse  gravel  may  be  piled  up  considerably 
above  the  ordinary  limit  of  the  waves  in  the  form  of  what  are  termed 
storin-beavhes,^  Below  the  limit  of  coarse  shingle  upon  the  beach  lies  the 
zone  of  fine  gravel,  d,nd  then  that  of  sand,  the  sediment,  though  liable 
to  irregular  distribution,  yet  tending  to  arrange  itself  according  to 
coarseness  and  specific  gravity,  the  rougher  and  heavier  detritus  lying  at 
the  upper,  and  the  finer  and  lighter  towards  the  lower  edge  of  the  shore. 
The  nature  of  the  littoral  accumulations  on  any  given  part  of  a  coast-line 
must  depend  either  upon  the  character  of  the  shore-rocks  which  at  that 
locality  are  broken  up  by  the  waves,  or  upon  the  set  of  the  shore- 
currents,  and  the  kind  of  detritus  they  bear  with  them.  Coasts  exposed 
to  heavy  surf,  especially  where  of  a  rocky  character,  are  apt  to  present 
beaches  of  coarse  shingle  between  their  projecting  promontories.  Shel- 
tered bays,  on  the  other  hand,  where  wave-action  is  comparatively  feeble, 
afford  a  gathering  ground  for  finer  sediment,  such  as  sand  and  mud 
Estuaries  and  inlets,  into  which  rivers  enter,  frequently  show  wide  muddy 
flats  at  low  water  (p.  398).  Deposits  of  comminuted  shells,  coral-sand,  or 
calcareous  organic  remains  thrown  up  on  shore,  may  be  cemented  into 
compact  rock  by  the  solution  and  redeposit  of  carbonate  of  lime  (p.  492). 
Where  tidal  currents  sweep  along  a  coast  yielding  much  detritus,  long 
bars  or  shoals  may  form  parallel  with  the  shore.  On  these  the  shingle 
and  sand  are  driven  coastwise  in  the  direction  of  the  prevalent  current* 
They  not  infrequently  accumulate  as  long  barriers  completely  protecting 
the  shores  from  which  they  are  separated  by  a  channel  or  lagoon  of  fresh 
or  brackish  water  (p.  399).  Into  this  lagoon  sediment  is  washed  from  the 
land  and  aquatic  vegetation  takes  root  there,  until  not  infrequently  a  salt 

^  See  on  this  subject  an  important  memoir  by  Messrs.  Murray  and  Renard,  Proc.  Rojf. 
S(.K.  Edin.  1884,  and  Nature,  xxx.  (1884) ;  also  Murray,  Proc.  Roy,  Soc,  1876  ;  Proc  Roy, 
Soc.  Edin.  ix.  ;  Murray  and  Renard,  Brit,  Assoc,  1879,  sects,  p.  340 ;  aJso  for  the  North 
Atlantic,  *Deu  Norske  Nordhavs-Expedition,*  jmrt  ix.  (on  Oceanic  Deposits),  1882^  J.  Y. 
Buchanan,  Proc.  Roy,  Soc.  Edin.  xviii.  (1891),  p.  131.  But  the  chief  source  of  informatioii 
is  now  the  great  Memoir  on  *  Deep  Sea  Deposits '  by  Messrs.  Murray  and  Renard  in  the 
Report^s  of  the  ChaUenger  Expedition,  1891. 

-  On  this  subject  consult  the  *  Deep-Sea  Deposits  *  of  the  Challenfier  Report,  chap.  v. 

^  See  Kiuahan  on  Sea-beaches,  Pmc.  Roy.  In'sh  Aatd,  (2nd  ser.),  iiL  p.  101. 

^  See  the  authorities  cited  on  p.  451,  regarding  the  Chesil  Bank. 


SECT,  ii  §  6  GEOLOGICAL   WORK  OF  THE  SEA  465 

marsh  or  swamp  is  formed.  Extensive  accumulations  of  this  kind  are  to 
be  found  along  the  eastern  coast  of  the  United  States.^ 

Among  the  deposits  cast  ashore  by  the  sea,  not  the  least  interesting 
are  the  masses  of  driftwood  which,  carried  down  by  rivers  are  borne  by 
marine  currents,  sometimes  for  hundreds  of  miles,  and  thrown  down 
in' huge  accumulations  in  protected  bays.  It  is  in  the  Arctic  seas  that 
this  phenomenon  obtains  its  greatest  development  Prodigious  quantities 
of  terrestrial  vegetation  are  swept  by  the  Siberian  rivers  into  these  waters 
and  are  carried  westwards  imtil  stranded  in  sheltered  bays  of  the  coast 
and  of  the  islands.  Every  shoal-coast  of  Spitzbergen  presents  examples 
of  these  heaps  of  driftwood.* 

p.  Infra- Littoral  and  Deeper- Water  Deposits. — These  extend  from 
below  low-water  mark  to  a  depth  of  sometimes  as  much  as  2000  fathoms, 
and  reach  a  distance  from  land  varying  up  to  200  miles  or  even  more. 
Near  land,  and  in  comparatively  shallow  water,  they  consist  of  banks 
or  sheets  of  sand,  more  rarely  mixed  with  gravel.  The  bottom  of  the 
North  Sea,  for  example,  which  between  Britain  and  the  continent  of 
Europe  lies  at  a  depth  never  reaching  100  fathoms,  is  irregularly  marked 
by  long  ridges  of  sand,  enclosing  here  and  there  hollows  where  mud  has 
been  deposited.  In  the  English  Channel,  large  banks  of  gravel  extend 
through  the  Straits  of  Dover  as  far  as  the  entrance  to  the  North  Sea.* 
These  features  seem  to  indicate  the  line  of  the  chief  mud-bearing  streams 
from  the  land,  and  the  general  disposition  of  currents  and  eddies  in  the 
sea  which  covers  that  region,  the  gravel  ridges  marking  the  tracts  or 
junctions  of  the  more  rapidly  moving  currents,  while  the  muddy  hollows 
point  to  the  eddies  where  the  fine  sediment  is  permitted  to  settle  on  the 
bottom.  The  more  prominent  features  on  the  floor  of  the  North  Sea, 
however,  are  probably  of  much  older  date  than  the  deposits  now 
accumulating  there.  Some  of  them  are  doubtless  relics  of  the  time  when 
the  floor  of  that  sea  was  a  broad  terrestrial  plain.  The  Dogger  Bank, 
for  instance,  is  probably  a  prolongation  of  the  Jurassic  escarpment  of  the 
Yorkshire  coast.  Other  minor  submarine  featiu^s  may  be  partly  due  to 
irregular  deposition  of  glacial  drift. 

During  the  course  of  the  voyage  of  the  Challenger,  the  approach  to 
land  could  always  be  foretold  from  the  character  of  the  bottom,  even  at 
distances  of  150  and  200  miles.  The  deposits  were  found  to  consist  of 
blue  and  green  muds  derived  from  the  degradation  of  older  crystalline 
rocks.     The  blue  or  dark  slate-coloured  mud  takes  its  colour  from  de- 

^  N.  S.  Shaler  on  sea-coast  swamps,  6th  Ann.  Rep.  U.  S.  Oecl,  Surv.  1884-85,  p.  353. 
F.  J.  H.  Merill  on  barrier  beaches  of  Atlantic  coast,  Popular  Science  Monthly^  Oct.  1890. 

*  Nordenskiold's  *  Vega  Expedition.'  Petermann,  Geograph.  MUtheil.  Erganzungaheft^ 
No.  16,  where  a  map  of  these  accumulations  on  the  Arctic  coasts  is  given. 

'  For  information  as  to  the  English  Channel  and  other  parts  of  the  British  seas,  see  J. 
T.  Harrison,  Min.  Proc.  Inst.  Civ.  Engin.  vii.  (1848),  p.  327  (where  a  map  of  the  submarine 
deposits  will  be  found) ;  R.  A.  C.  Godwin -Austen,  Quart.  Joum.  Oeol.  Soc,  vi.  (1849),  p. 
69 — a  paper  of  singular  interest  and  importance  ;  Lebour,  Proc.  Oeol.  Assoc,  iv.  p.  158  ; 
John  Murray,  Min.  Proc.  Inst.  Civ.  Engin.  xx.  (1860-1),  where  a  map  of  the  North  Sea 
floor  is  given. 


456  nVXAMICAL  GEOLOGY  book  m  part  ii 

composing  organic  matter  and  sulphide  of  iron,  frequently  giving  off  the 
odour  of  sulphuretted  hydrogen,  and  assuming  a  brown  or  red  hue  at  the 
surface,  owing  to  oxidation.  Besides  occurring  in  deposits  of  deep  water, 
iron  disulphide  is  met  with  on  some  coasts,  cementing  sand,  gravely  and 
shells  into  a  coherent  mass.^  The  chemical  changes  that  result  in  the 
elimination  of  sulphides  from  sea-water  may  be  explained  by  supposing 
that  the  decomposing  animal  and  vegetable  matter  of  the  sea-floor  reduces 
the  sulphates  to  sulphides,  which  in  turn  react  on  the  iron  and  manganese 
minerals  (principally  silicates)  in  the  mud,  forming  sulphides  of  those 
metals.  Subsequently  the  oxygen  of  the  water  converts  the  sulphides  to 
oxides,  which  gather  into  concretionary  forms.^  The  green  muds  found 
at  depths  of  100  to  700  fathoms  are  characterised  by  the  presence  of  a 
considerable  quantity  of  glauconite  grains,  either  isolated  or  united  into 
concretions,  and  frequently  filling  the  chambers  of  Foraminifera  or  other 
organisms.  Round  volcanic  islands,  the  bottom  is  covered  with  grey 
volcanic  mud  and  sand  derived  from  the  degradation  of  volcanic  rocks. 
These  deposits  can  be  traced  to  great  distances ;  from  Hawaii  they  extend 
for  200  miles  or  more.  Pieces  of  pumice,  scoriae,  &c.,  occur  in  them, 
mingled  with  marine  organisms,  and  more  particularly  with  abundant 
gi'ains,  incnistations,  and  nodules  of  an  earthy  peroxide  of  manganese 
(Fig.  175).  Near  coral-reefs  the  sea-floor  is  covered  with  a  white 
calcareous  mud  derived  from  the  abrasion  of  coral,  and  frequently 
containing  95  per  cent  of  carbonate  of  lime.  Beyond  a  depth  of  1000 
fathoms,  coral  mud  gives  place  to  a  Globigerina  ooze  or  red  clay.  The 
east  coast  of  South  America  supplies  a  peculiar  red  mud  which  is 
spread  over  the  Atlantic  slope  down  to  depths  of  more  than  2000 
fathoms. 

Throughout  these  land-derived  sediments  are  found  minute  particles 
of  recognisable  minerals.  Of  these,  quartz,  often  in  rounded  gr^ns,  plays 
the  chief  part.  Next  come  mica,  felspar,  augite,  hornblende,  and  other 
less  abundant  constituents  of  terrestrial  rocks,  the  materials  becoming 
coarser  towards  land.  Occasional  pieces  of  wood,  portions  of  fruits,  and 
leaves  of  trees  in  the  same  deposits  further  indicate  the  reality  of  the 
transport  of  material  from  the  land.  Shells  of  pteropods,  larval  gastero- 
pods,  and  lamellibranchs  are  tolerably  abundant  in  these  muds,  with 
many  infra-littoral  species  of  Fitramin'ifera^  and  diatoms.     Below  1500  or 

^  H.  \\^mc\\,Xeues  Jahrh.  1879,  p.  255. 

'  J.  Y.  Buchanan,  Brit.  Assoc.  1881,  p.  584.  Mr.  Buchanan,  in  renewing  this  inves* 
tigation  and  obtaining  many  illustrations  from  the  seas  around  Scotland,  has  shown  that  the 
mud  on  many  parts  of  the  sea-bottom  is  being  continually  passeil  and  repassed  throngh  the 
bodies  of  animals  which  live  upon  it.  The  mineral  matter  is  thus  brought  in  contact  with 
the  oi*ganic  secretions  of  the  animals  and  is  ground  up  with  these  in  their  milling  oigmnL 
The  reducing  action  of  the  secretions  produces,  Mr.  Buchanan  believes,  sulphides  firom  the 
sulphates  of  sea- water,  and  these  sulphides,  acting  on  the  ochreous  matter  of  the  bottom,  give 
rise  to  sulphides  of  iron  and  manganese,  which  being  very  unstable  in  presence  of  wmter  and 
oxygen  are,  where  they  lie  on  the  surface,  soon  transformed  into  oxides.  Proc  Rof.  Soc> 
Kdin,  xviii.  (1890),  p.  17,  'On  the  occurrence  of  sulphur  in  marine  mnds.*  Another  Tiew 
of  the  decomposition  of  the  sulphates  of  sea- water  is  proposed  by  Dr.  Murray  and  Mr.  IrHoe. 
See  papers  quoted  on  p.  484. 


SECT,  ii  §  6  GEOLOGICAL   WORK  OF  THE  SEA  467 

1700  fathoms,  pteropod  shells  seldom  appear,  while  at  3000  fathoms 
hardly  a  foraminifer  or  any  calcareous  organism  remains.^ 

In  some  regions  vast  quantities  of  terrestrial  vegetation  are  strewn 
over  the  sea-bottom,  even  at  depths  of  2000  fathoms,  and  at  distances 
of  several  hundred  miles  from  land.  This  fact  has  been  observed  by 
Professor  Agassiz  off  Central  America,  both  in  the  Atlantic  and  Pacific 
Oceans,  hardly  a  single  haul  of  the  dredge  failing  to  bring  up  much 
vegetable  matter,  and  frequently  logs,  branches,  twigs,  seeds,  leaves,  and 
fruits.^ 

R  Abysmal  or  Pelagic,^ — Passing  over  at  present  the  organic  deposits 
which  form  so  characteristic  a  feature  on  the  floor  of  the  deeper  and  more 
open  parts  of  the  ocean,  we  come  to  certain  red  and  grey  clays  found  at 
depths  of  more  than  2000  fathoms,  down  to  the  bottoms  of  the  deepest 
abysses.  These,  by  far  the  most  widespread  of  oceanic  deposits,*  consist 
of  exceedingly  fine  clay,  coloured  sometimes  red  by  iron-oxide,  sometimes 
of  a  chocolate  tint  from  manganese  oxide,  with  grains  of  augite,  felspar, 
and  other  volcanic  minerals,  pieces  of  palagonite  and  pumice,  nodules 
of  peroxide  of  manganese,  and  other  mineral  substances,  together 
with  Foraminifera,  and  in  some  regions  a  large  proportion  of  siliceous 
Radiclaria,  These  clays  result  from  the  decomposition  of  pumice  and 
fine  volcanic  dust,  transported  from  volcanic  islands  into  mid-ocean,  or 
from  the  accumulation  of  the  detritus  of  submarine  eruptions.  The 
extreme  slowness  of  deposit  is  strikingly  brought  out  in  the  tracts  of  sea- 
floor  furthest  removed  from  land.  From  these  localities  great  numbers 
of  sharks'  teeth,  with  ear-bones  and  other  bones  of  whales,  were  dredged 
up  in  the  Challenger  expedition, — some  of  them  quite  fresh,  others 
partially  crusted  with  peroxide  of  Dianganese,  and  some  wholly  and 
thickly  surrounded  with  that  substance.  We  cannot  suppose  that 
sharks  and  whales  so  abounded  in  the  sea  at  one  time  as  to  cover  the 
floor  of  the  ocean  with  a  continuous  stratum  of  their  remains.  No 
doubt  each  haul  of  the  dredge,  which  brought  up  so  many  bones, 
represented  the  droppings  of  many  generations.  The  successive  stages 
of  manganese  incrustation  point  to  a  long,  slow,  undisturbed  period, 
when  so  little  sediment  accumulated  that  the  bones  dropped  at  the 
beginning  remained  at  the  end  still  uncovered,  or  only  so  slightly 
covered  as  to  be  easily  scraped  up  by  the  dredge.  In  these  deposits, 
moreover,  occur  numerous  minute  spherular  particles  of  metallic  iron 
and  '*chondres,"  or  spherical  internally  radiated  particles  referred  to 
bronzite,  which  are  in  all  probability  of  cosmic  origin — portions  of  the 
dust  of  meteorites  which  in  the  course  of  ages  have  fallen  upon  the 

*  See  papers  by  Messrs.  Murray  and  Renard,  quoted  on  p.  454,  and  vol.  of  Challenger 
Report  on  *  Deep-Sea  Deposits,'  p.  190. 

*  'Three  Cruises  of  the  Blake,'  and  Bull.  Mus.  Comp,  Zool,  xxiiL  No.  1  (1892),  p.  11. 

'  For  information  regarding  the  fauna  and  deposits  of  the  ocean -abysses,  besides  the 
works  quoted  on  page  454,  note  1,  consult  the  various  writings  of  Prof.  A.  Agassiz,  especially 
his  *  Three  Cruises  of  the  Blake,'  and  papers  in  Bull.  Mus.  Cotnp.  Zool,  xxi.  No.  4,  and 
xxxiii.  No.  1  ;  also  Haeckel's  '  Plankton-Studien,'  1890. 

*  They  are  estimated  to  cover  upwards  of  50,000,000  square  miles  of  the  sea-floor. 
Murray  and  Irvine,  Proc.  Roy,  Soc.  Kdin.  xvii.  (1889),  p.  82. 


liYXAMlCAL  GEOLOGY 


BOOK  lit  FART  U 


Bea-lxtttom  (Figs.  1 73,  1 74).      Such  particles,  no  doubt,  fftU  all  over  the 
ocean  .;  l)ut  it  is  only  on  those  parts  of  the  bottom  which,  by  reasoa  of 


lie  n-an.bgllaru.    (Mum]- Hid  Bnud^ 
ini^n)  frcno  ■  if  fib  ot  t/fli  Btthomi  In  Bgntk 
*  inrticl^  aod  ibowB  the  anal  dcprpsiloD  <ja 


their  distance  from  any  land,  receive  accessions  of  deposit  wiUi  a 
slowness — and  nhcrc  therefore  the  present  surface  may  coctftin  ths dut 
of  a  long  succession  of  yeus — thit  it 
may  be  expected  to  be  poauble  to 
detect  them.' 

The  abundant  deposit  of  piWT^ip^ 
of  manganese  over  the  floor  of  dM 
deep  sea  is  one  of  the  most  ""g"*«^ 
features  of  recent  discovery.  It  occnn 
as  an  earthy  incrustation  round  bits 
of  pumice,  bones,  and  other  objects 
(Fig.  1(5).  The  nodules  possess  a 
concentric  arrangement  of  lines  not 
unlike  those  of  urinarj'  calculi  That 
they  are  formed  on  the  spot,  and  not 
drifted  from  a  distance,  was  made 
abundantly  clear  from  their  containing 
,  abysmal  oi^anisms,  and  enclosiI^; 
'  more  or  less  of  the  surrounding  bot- 
*  torn,  whatever  its  uature  might  happen 
to  }>e.  More  recently  Mr,  J.  Y. 
Buchanan  dredf^ed  similar  small  manganese  concretions  from  some  of 
the  deeper  ]iart«  of  Loch  Fyne,*  and  subsequently  Dr.  John  Murray 
found  them  abundantl)-  at  10  fathoms  in  the  Firth  of  Clyde.  The 
formation  of  such  concretions  may  be  analogous  to  the  aolution 
and    deiKwition    of  oxides    of    iron   and    manganese    by    organic   acids, 

'  Mnrniy  aiid  Rvnard  on  Coamic  Diiat,  Ptoc.  Ri<y.  fiK.   Edin.  1834  ;    Xabm,  nU. 
rhnU'Hgrr  Es|K.slition  Reiwrt,  vol.  oii  'Deep-Sea  De|)ositB,'  p.  327  rt  leq. 

''  Xoli-r'.  iviii.  (1878),  p.  828.     Brit.  Ai-K.  1881,  p.  583.     Prot.  Rofi.  Sue.  Riiit.  ti. 
■^  a87.      Tri.«>.  II.  .?.  /ilOi.  iriivi.  (18S1),  459.      Dieulafait,  Compla  Tend.  1SS4,  p.  68». 


8EIT.  ii  %  e  GEOLOaiCAL   WORK  OF  THE  SEA  459 

as  on  lake-floors,  bogs,  &c.  (p.  483).'  In  connection  with  the  chemical 
reactions  indicated  by  these  nodules  as  taking  place  on  the  sea-bottom, 
reference  may  be  made  to  a  still  more  remarkable  discovery  made  by 
Messrs.  Murray  and  Benard  in  the  course  of  their  ezaminatians  of  the 
materials  brought  up  from  the  same  abysmal  deposits.  Minute  crystals, 
simple,  twinned,  or  in  spheroidal  groups,  which  occur  abundantly  in  the 
tj-pical  red  clay  of  the  central  Pacific,  have  been  identified  with  the 
zeolite  koown  as  cbristianite.  These  crystals  have  certainly  been  formed 
directly  on  the  sea-bottom,  for  they  are  found  gathered  round  abysmal 
organisms,  and  their  production  has  been  elfected  at  about  the  tempera- 
ture of  32°  Fahr.  The  importance  of  this  fact  in  reference  to  the 
chemistry  of  marine  deposits  is  at  once  obvious. 

From  a  comparison  of  the  results  of  the  dredgings  made  in  recent 
years  in  all  parts  of  the  oceans,  it  is  impossible  to  resist  the  conclusion 


IS  Bonh  PBeillc.    Two-ihirila  imlural  "iie.J 
A,  Sodnle  (romaoOO  (kthom*  ihowlng  eitemal  form.    B,  Sccttnn  of  nodule  from  !T40  fetboms,  BhowLng 
iDtenwl  ctmCBiitfic  deposit  round  ft  rrft^cmeDt  of  purnlc«. 

that  there  is  little  in  the  character  of  the  deep-sea  deposits  which  finds 
a  parallel  among  the  marine  geological  formatioits  visible  to  us  on  land. 
It  is  only  among  the  comparatively  shallow-water  accumulations  of  the 
existing  sea  that  we  encounter  obvious  analogies  to  the  older  formations. 
And  thus  we  reach,  by  another  and  a  new  approach,  the  conclusion 
which  on  other  and  very  different  grounds  has  been  arrived  at,  viz., 
that  the  present  continental  axes  have  existed  from  the  remotest  times, 
and  that  the  marine  strata  which  constitute  so  large  a  portion  of  their 

'  Different  viewn  huve  been  expressed  by  Dr.  John  Mum)'  Bud  Hr.  J.  Y.  Bu<3bBiiaa  aa 
to  the  mode  of  origio  of  the  mBrine  manganese  deposits.  See  R.  Irvine  and  J.  Gibson, 
Ptoc.  Roy.  Soc.  Edia.  xviii.  (1891),  p.  E4. 

»  These  and  Fig.  174  are  taken  from  plate  x.'iiiii.  of  the  vol.  on  '  Deep-Sea  Depoiita '  in 
tbe  Reports  of  the  Challenger  Ei|iedi(ion.  The  detailed  investigation  by  Messrs.  Murray 
and  Benard  of  the  deep-sea  deiwsits  obtained  by  this  expedition  fornis  the  most  important 
contribution  yet  made  to  our  knowledge  of  the  oceanic  abysses. 


460  DYNAMICAL  GEOLOGY  book  m  part  ii 

present  mass  have  been  accumulated  not  as  deep-water  deposits,  but  in 
comparatively  shallow  water  along  their  flanks  or  over  their  submerged 
ridges.^ 


j5  7.  Denudation  and  Deposition. — The  results  of  the  action  of 

Air  and  Water  upon  Land.- 

It  may  be  of  advantage,  before  passing  from  the  subject  of  the 
geological  work  of  water,  to  consider  the  broad  results  achieved  by  the 
co-operation  of  all  the  forces  by  which  the  surface  of  the  land  is  worn 
down.  These  results  naturally  group  themselves  under  the  two  heads 
of  Denudation  and  Deposition. 

1 .  Subaerial  Denudation — tht  general  lowering  of  land. 

The  true  measure  of  denudation  is  to  be  sought  in  the  amount  of 
mineral  matter  removed  from  the  surface  of  the  land  and  carried  into 
the  sea.  This  is  an  appreciable  and  measurable  quantity.  There  may 
be  room  for  discussion  as  to  the  way  in  which  the  waste  is  to  be 
apportioned  to  the  different  forces  that  have  produced  it,  but  the  total 
amount  of  sea-borne  detritus  must  Ije  accepted  as  a  fact  about  which, 
when  properly  verified,  no  further  question  can  possibly  arise.  In  this 
manner  the  subject  is  at  once  disencumbered  of  difficulty  in  fixing  the 
relative  importance  of  rain,  rivers,  frost,  glaciers,  &c.,  considered  as 
denuding  agents.  We  have  simply  to  deal  with  the  sum-total  of  results 
achieved  by  all  these  forces  acting  severally  and  conjointly.  Thus 
considered,  this  subject  casts  a  new  light  on  the  origin  of  existing  land- 
surfaces,  and  affords  some  fresh  data  for  approximating  to  a  measure  of 
past  geological  time. 

Of  the  mineral  substances  received  by  the  sea  from  the  land,  by  much 
the  larger  portion  is  brought  down  by  streams ;  a  relatively  small 
amount  is  washed  off  by  the  waves  of  the  sea  itself.  It  is  the  former, 
or  stream -borne  part,  which  is  at  present  to  be  considered.  The 
quantity  of  mineral  matter  carried  every  year  into  the  ocean  by  the 
rivers  of  a  continent  represents  the  amount  by  which  the  general  surface 
of  that  continent  is  annually  lowered.  Much  has  been  written  of  the 
vastness  of  the  yearly  tribute  of  silt  borne  to  the  ocean  by  such  streams 
as  the  Ganges  and  Mississippi;  but  "the  mere  consideration  of  the 
number  of  cubic  feet  of  detritus  annually  removed  from  any  tract  of 
land  by  its  rivers  does  not  produce  so  striking  an  impression  upon  the 
mind  as  the  statement  of  how  much    the  mean  surface-level   of   the 

*  Pn»c.  liny,  (iettgraph.  Soc.  July  1879. 

-  This  section  is  mainly  taken  from  an  essay  by  the  author,  Trans.  Oeol.  Soe,  GUugovc, 
iii.  p.  153.  Tlie  subject  has  been  discussed  anew  on  the  Imsis  of  more  exact  knowledge  of 
the  interior  of  tlie  continents  and  the  depths  of  the  sea  by  Dr.  John  Murray,  ScoiiiA 
frftifft'oph.  Motj.  1887.  See  also  a  note  by  Mr.  C.  Davison,  Oeol,  Mag.  1889,  p.  409.  A. 
De  Lappareut,  Bull.  Soc.  G4ol.  France^  xviii.  (1890),  p.  351. 


SECT.  ii§7  SUBAERIAL  DENUDATION  461 

district  in  question  would  be  reduced  by  such  a  removal."^  This 
method  of  inquiry  is  so  obvious  and  instructive  that  it  probably  received 
attention  from  early  geologists,  though  data  were  still  wanting  for  its 
proper  application.  Playfair,  for  instance,  in  speaking  of  the  trans- 
ference of  material  from  the  surface  of  the  land  to  the  bottom  of  the  sea, 
remarks  that  "  the  time  requisite  for  taking  away  by  waste  and  erosion 
2  feet  from  the  surface  of  all  our  continents  and  depositing  it  at  the 
bottom  of  the  sea  cannot  be  reckoned  less  than  two  hundred  years. "  ^ 
This  estimate  does  not  appear  to  have  been  based  on  any  actual  measure- 
ments, and  must  greatly  exceed  the  truth ;  but  it  serves  to  indicate  how 
broad  was  the  view  that  Playfair  held  of  the  theory  which  he  undertook 
to  illustrate.  The  first  geologist  who  appears  to  have  attempted  to 
form  any  estimate  on  this  subject  from  actually  ascertained  data,  was  Mr. 
Alfred  Tylor,  who  in  the  year  1850  published  a  paper  in  which  he 
estimated  the  probable  amount  of  solid  matter  annually  brought  into  the 
ocean  by  rivers  and  other  agents.  He  inferred  that  the  quantity  of 
detritus  now  distributed  over  the  sea-bottom  every  year  would,  at  the 
end  of  10,000  years,  cause  an  elevation  of  the  ocean-level  to  the  extent 
of  at  least  3  inches.^  The  subject  was  afterwards  taken  up  by  Dr.  Croll, 
who  specially  drew  attention  to  the  Mississippi  as  a  measure  of  denuda- 
tion and  thereby  of  geological  time.'* 

When  the  annual  discharge  of  mineral  matter  carried  seaward  by  a 
river,  and  the  area  of  country  drained  by  that  river,  are  both  known,  the 
one  sum  divided  by  the  other  gives  the  amount  by  which  the  drainage- 
area  has  its  mean  general  level  reduced  in  one  year.  For  it  is  clear  that 
if  a  river  carries  so  many  millions  of  cubic  feet  of  sediment  every  year 
into  the  sea,  the  area  drained  by  it  must  have  lost  that  quantity  of  solid 
material,  and  if  we  could  restore  the  sediment  so  as  to  spread  it  over  the 
basin,  the  layer  so  laid  down  would  represent  the  fraction  of  a  foot  by 
which  the  surface  of  the  basin  had  been  lowered  during  a  year. 

It  has  been  already  shown  that  the  material  removed  from  the  land 
by  streams  is  twofold — one  portion  is  chemically  dissolved,  the  other  is 
mechanically  suspended  in  the  water  or  pushed  along  the  bottom. 
Properly  to  estimate  the  loss  sustained  by  the  surface  of  a  drainage- 
basin,  we  ought  to  know  the  amount  of  mineral  matter  removed  in  each 
of  these  conditions,  and  also  the  volume  of  water  discharged,  from 
measurements  and  estimates  made  at  different  seasons  and  extending 
over  a  succession  of  years.  These  data  have  not  yet  been  fully  collected 
from   any    river,    though    some   of    them    have   been    ascertained   with 

^  Tylor,  PhU.  Mag.  4th  series,  v.  p.  268,  1850. 

-  'Illustrations,'  p.  424.  Manfredi  had  previously  made  a  calculation  of  the  amount 
of  rain  that  falls  over  the  glolie,  and  of  the  quantity  of  earthy  matter  carried  into  the 
»ea  by  rivers.  He  estimated  that  this  earthy  matter  distributed  over  the  sea-bed  must 
raiite  the  level  of  the  latter  five  inches  in  348  years.  Von  Hoff,  '  Verandeningen  der 
Erdoberflache,'  Band  i.  p.  232.     See  the  other  authorities  there  cited. 

^  Phil.  Miuf.  loc.  cit. 

*  Phil.  Mag.  for  February  1867  and  May  1868;  and  his  'Climate  and  Time.'  See 
also  Geol.  Mag.  June  1868  ;  Trans.  Ueol.  Stx.  <ilasgoWy  iii.  p.  153. 


462 


DYXAMICAL  GEOLOGY 


BOOK  lU  PART  n 


approximate  accuracy,  as  in  the  Mississippi  Survey  of  Messrs.  Hum- 
phreys and  Abbot,  and  the  Danube  Survey  of  the  International  Com- 
mission. As  a  rule,  more  attention  has  been  shown  to  the  amount  of 
mechanically  suspended  matter  than  to  that  of  the  other  ingredients. 
It  will  be  borne  in  mind,  therefore,  that  the  following  estimates,  in  so 
far  as  they  arc  based  upon  only  one  portion  of  the  waste  of  the  land 
— that  carried  in  mechanical  suspension, — are  understatements  of  the 
truth.  ^ 

The  proportion  of  mineral  substances  held  in  suspension  in  the  water 
of  rivers  has  been  already  (p.  379)  discussed.  It  is  most  advantageoos 
to  determine  the  amount  of  mineral  matter  by  weighty  and  then  from  its 
average  specific  gravity  to  estimate  its  bulk  as  an  ingredient  in  river- 
water.  The  proportion  by  weight  is  probably,  on  an  average,  about  half 
that  by  bulk. 

It  may  seem  superlluous  to  insist  that  the  earthy  matter  borne  into 
the  sea  from  any  given  area  represents  so  much  actual  loss  from  the 
surface  of  that  area.  Yet  this  self-evident  statement  is  probably  not 
realised  by  many  geologists  to  the  extent  which  it  deserves.  If  a 
stream  removes  in  one  year  one  million  of  cubic  yards  of  earth  from  its 
drainage-basin,  that  basin  must  have  lost  one  million  of  cubic  yards 
from  its  surface.  From  the  data  and  authorities  which  have  already 
been  adduced  (p.  383),  the  subjoined  table  has  been  constructed,  in 
which  arc  given  the  results  of  the  measurement  of  the  proportion  of 
sediment  in  a  few  rivers.  The  last  column  shows  the  fraction  of  a  foot 
of  rock  (reckoning  the  specific  gravity  of  the  silt  at  1  '9  and  that  of  rock 
at  2*5)  which  each  river  must  remove  from  the  general  surface  of  its 
drainage-basin  in  one  year. 


Name  of  River. 


Mis.si.ssi pin  . 
fiHiiges  (UpiKir) 
Huang  Ho    . 
Rliono . 
Danube 
Po 


Area  of  basin  in 
squaru  miles. 


1,147,000 

143,000 

700,000 

25,000 

234,000 

30,000 


Fraction  of  foot  of 

Annual  dincliarge  of 

rock  by  which  th« 

sediment  in  cubic  feet 

area  of  drainage  is 

lowered  in  on«  ymr. 

7,468,694,400 

ir^Arr 

6,368,077,440 

nil 

17,520,000,000(?) 

tAi 

600,381,800 

tA¥ 

1,253,738,600 

«i4t 

1,510,137,000 

Th 

At  the  present  rate  of  erosion,  the  rivers  named  in  this  table  remove 
one  foot  of  rock  from  the  general  surface  of  their  basins  in  the  following 
ratio  : — The  Mississippi  removes  one  foot  in  6000  years ;    the  Ganges 

^  Geologi.st.s  are  largely  indebted  to  Mr.  Mellaril  Reade  for  the  attention  which  he  hai 
given  to  the  important  part  played  by  chemical  solution  in  the  general  denudation  of  the 
land.  From  the  data  collected  by  him  he  infers,  as  the  jtroportion  of  solids  in  solutimi  in 
the  water  of  the  Mi8ai8si]»pi  is  ^^j^^  by  weight,  about  150  millions  of  tons  of  dissolTed 
mincnil  must  be  carried  by  this  river  annually  into  the  sea.  In  the  River  Plate  the  propor- 
tion is  TiiVn*  ill  ^^^  3^  Lawrence  ^^jst  in  the  Amazon  iviirv*  Presidential  AddrBii» 
JAverjjotA  (Jeoi.  Soc.  1884. 


ECT.  ii  §  7  SUBAERIAL  DENUDATION  463 

above  Ghazipilr  does  the  same  in  823  years  ;^  the  Hoang  Ho  in  1464 
years;  the  Rhone  in  1528  years;  the  Danube  in  6846  years;  the  Po  in 
729  years.  If  these  rates  should  continue,  the  Mississippi  basin  will  be 
lowered  10  feet  in  60,000  years,  100  feet  in  600,000  years,  1000  feet  in 
6,000,000.  Assuming  Humboldt's  estimate  of  the  mean  height  of  the 
North  American  continent,  748  feet,^  we  find  that  at  the  Mississippi's 
rate  of  denudation,  this  continent  would  be  worn  away  in  about  four 
and  a  half  million  years.  The  Ganges  works  still  more  rapidly.  It 
removes  one  foot  of  rock  in  823  years,  and  if  Humboldt's  estimate  of  the 
average  height  of  the  Asiatic  continent  be  accepted,  viz.,  1132  English 
feet,  that  mass  of  land,  worn  down  at  the  rate  at  which  the  Ganges 
destroys  it,  would  be  reduced  to  the  sea-level  in  little  more  than  930,000 
years.  Still  more  remarkable  is  the  extent  to  which  the  River  Po 
denudes  its  area  of  drainage.  £ven  though  measurements  had  not  been 
made  of  the  ratio  of  sediment  contained  in  its  water,  we  should  be 
prepared  to  find  that  proportion  a  remarkably  large  one,  if  we  look  at 
the  enormous  changes  which,  within  historic  times,  have  been  made 
by  the  alluvial  accumulations  of  this  river  (p.  395).  If  the  Po  removes 
one  foot  of  rock  from  its  drainage  basin  in  729  years,  it  will  lower  that 
basin  10  feet  in  7290  years,  100  feet  in  72,900  years.  If  the  whole 
of  £urope  (taken  at  a  mean  height  of  671  feet)  were  denuded  at  the 
same  rate,  it  would  be  levelled  in  rather  less  than  half  a  million  of 
years. 

It  is  not  pretended  that  these  results  are  strictly  accurate.  On  the 
other  hand,  they  are  not  mere  guesses.  The  amount  of  water  flowing 
into  the  sea,  and  the  annual  discharge  of  sediment,  have  been  in  each 
case  measured  with  greater  or  less  precision.  The  areas  of  drainage 
may  perhaps  require  to  be  increased  or  lessened.  But  though  some 
change  may  be  made  upon  the  ultimate  results  just  given,  it  is  hardly 
possible  to  consider  them  attentively  without  being  forced  to  ask 
whether  those  enormous  periods  which  geologists  have  been  in  the  habit 
of  demanding  for  the  accomplishment  of  geological  phenomena,  and 
more  especially  for  the  very  phenomena  of  denudation,  are  not  in  reality 
far  too  vast.  If  the  Mississippi  is  carrying  on  the  process  of  denudation 
so  rapidly  that  at  the  same  rate  the  whole  of  North  America  might  be 
levelled  in  four  and  a  half  millions  of  years,  surely  it  is  most  un- 
philosophical  to  demand  unlimited  ages  for  similar  but  often  much  less 
extensive  denudations  in  the  geological  past.  Moreover,  that  rate  of 
erosion  appears,  on  the  whole,  to  be  rather  below  the  average  in  point  of 

'  In  my  original  paper  the  area  of  drainage  of  the  Gauges  was  given  as  432,480  square 
miles.  Bat  the  area  from  which  the  annual  discharge  of  silt  was  there  given  was  only  that 
part  of  the  Grangetic  basin  above  Ghazipur,  which  Dr.  Haughton  estimates  at  148,000  square 
miles  {Proc  Roy,  Dublin  Soc  1879,  No.  xxxix.)  Hence,  as  he  has  pointed  out,  the  rate 
of  erosion  is  reaUy  much  greater  than  I  had  made  it.  I  have  recalculated  the  rate  from  the 
altered  data,  and  the  result  is  as  given  above. 

'  AnUf  pp.  89,  40,  where  other  and  more  probable  estimates  of  the  height  of  the  land  are 
given.     But  as  the  numbers  do  not  aflfect  the  argument,  those  originally  assumed  are  here 


464  DYXAMICAL  GEOLOGY  book  hi  part  ii 

rapidity.  The  Po,  for  instance,  works  more  than  eight  times  as  fast 
But  as  the  physics  of  the  Mississippi  have  been  more  carefully  studied 
than  those  of  perhaps  any  other  river,  and  as  that  river  drains  so 
extensive  a  region,  embracing  so  many  varieties  of  climate,  rock,  and 
soil,  we  shall  probably  not  exaggerate  the  result  if  we  assume  the 
Mississippi  ratio  as  an  average.  It  is,  of  course,  obvious  that  as  the 
level  of  the  land  is  lowered,  the  rate  of  subaerial  denudation  decreases, 
so  that  on  the  supposition  that  no  subterranean  movements  took  place  to 
aid  or  retard  the  denudation,  the  last  stages  in  the  demolition  of  a 
continent  must  be  enormously  slower  than  during  earlier  periods. 

It  must  not  be  forgotten,  however,  that  as  already  remarked,  the 
estimates  here  given,  inasmuch  as  they  are  based  only  on  the  material 
removed  in  mechanical  suspension,  arc  probably  understatements  of  the 
truth.  If  wo  take  into  account  also  the  material  carried  away  in  chemical 
solution,  the  rate  of  subaerial  denudation  will  be  considerably  heightened. 
It  is  difficult,  however,  to  apportion  the  loss  of  dissolved  substance  from 
the  surface  of  the  land.  The  salts  contained  in  solution  in  river-water 
are  derived  not  only  from  the  superficial  rocks,  but  probably  to  a  much 
greater  extent  from  springs  which  sometimes  carry  up  dissolved  substances 
from  considerable  depths.  In  the  end,  no  doubt,  as  the  level  of  the  land 
is  reduced  by  subaerial  waste,  this  subterranean  solution  will  tell,  but  it 
can  hardly  be  said  sensibly  to  affect  the  lowering  of  the  level  from  cen- 
tury to  century.  Mr.  Mellard  Rcadc,  from  his  researches  into  this  sub- 
ject, believes  that  the  amount  of  solids  in  solution  is  on  the  whole  about 
one-third  of  that  of  those  in  susi>ension.  He  finds  this  to  be  the  ratio  in 
tlie  Nile,  the  Danube,  and  the  Mississippi,  the  last-named  being  in  many 
respects  a  typical  river.  If,  as  he  proposes,  we  add  this  additional  loss 
by  cliomical  solution  to  the  amount  of  material  removed  in  mechanical 
suspension  from  the  Mississippi  basin,  the  annual  lowering  of  the  level  of 
the  basin  will  be  raised  from  witttit  ^^  tAtf  of  a  foot.^  It  is  quite  true 
that  the  loss  of  mineral  matter  from  the  whole  basin  would  be  equivalent 
to  that  sum,  but  there  would  obviously  not  be  strictly  a  lowering  of  the 
level  of  the  basin  to  that  amount.  It  is  difficult  to  see  how  we  are  to 
discriminate  between  superficial  and  subterranean  solution ;  and  until 
some  separation  of  this  kind  is  made,  it  seems  hardly  legitimate  to  class 
the  whole  of  the  dissolved  matter  with  that  carried  in  mechanical  suspen- 
sion as  a  measure  of  the  annual  loss  from  the  surface  of  the  land. 

There  is  another  point  of  view  from  which  a  geologist  may 
advantageously  contemplate  the  active  denudation  of  a  country.  He 
may  estimate  the  annual  rainfall  and  the  proportion  of  water  which 
returns  to  the  sea.  If  he  can  obtain  a  probable  average  ratio  for  the 
earthy  su])staiices  contained  in  the  river-water  which  enters  the  sea,  he 
will  be  able  to  estimate  the  mean  amount  of  loss  sustained  by  the  whole 
country.  Thus,  taking  the  average  rainfall  of  the  British  Islands  at 
3G  inches  annually,  and  the  superficial  area  over  which  this  rain  is 
discharged  at  1 20,000  sqiuire  miles,  then  it  will  be  found  that  the  total 
(luantity  of  rain  received  in   one  year  by  the   British   Isles  is   equal  to 

*  T.  Mellaril  Reade,  Presideutial  Address,  Licerjntttl  Oeof,  Sac.  1884-85. 


BBcr.  ii  §  7  SUBAERIAL  DENUDATION  465 

about  68  cubic  miles  of  water.  If  the  proportion  of  rainfall  returned  to 
the  sea  by  streams  be  taken  at  a  third,  there  are  23  cubic  miles ;  if  at  a 
fourth,  there  are  1 7  cubic  miles  of  fresh  water  sent  off  the  surface  of  the 
British  Islands  into  the  sea  in  one  year.  Assuming,  in  the  next  place, 
that  the  average  ratio  of  mechanical  impurities  is  only  -^j^jy  by  volume 
of  the  water,  the  proportion  of  the  rainfall  returned  to  ^e  sea  being  J, 
then  it  will  follow  that  ^^q  of  a  foot  of  rock  is  removed  from  the 
general  surface  of  Britain  every  year.  One  foot  will  be  planed  away 
in  8800  years.  If  the  mean  height  of  the  British  Islands  be  taken  at 
650  feet^  then,  if  the  ratio  now  assumed  were  to  continue,  these  islands 
might  be  levelled  in  about  five  and  a  half  millions  of  years.  Much  more 
detailed  observation  is  needed  before  any  estimate  of  this  kind  can  be 
based  upon  accurate  and  reliable  data.  But  it  illustrates  a  method  of 
vividly  bringing  before  the  mind  the  reality  and  extent  of  the  denudation 
now  in  progress. 

2.  Sttbaerial  Denudation — the  unequal  erosion  of  land. 

It  is  obvious  that  the  earthy  matter  annually  removed  from  the 
surface  of  the  land  does  not  come  equally  from  the  whole  surface.  The 
determination  of  its  total  quantity  furnishes  no  aid  in  apportioning  the 
loss,  or  in  ascertaining  how  much  each  part  of  the  surface  has  contributed 
to  the  total  amount  of  sediment.  On  plains,  watersheds,  and  more  or 
less  level  ground,  the  proportion  of  loss  may  be  small,  while  on  slopes 
and  in  valleys  it  may  be  great,  and  it  may  not  be  easy  to  fix  the  true 
ratios  in  these  cases.  But  it  must  be  borne  in  mind  that  estimates  and 
measurements  of  the  sum-total  of  denudation  are  not  thereby  affected. 
If  we  allow  too  little  for  the  loss  from  the  surface  of  the  table-lands,  we 
increase  the  proportion  of  the  loss  sustained  by  the  sides  and  bottoms  of 
the  valleys,  and  vice  versd. 

While  these  proportions  vary  indefinitely  with  the  form  of  the  surface, 
rainfall,  &c.,  the  balance  of  loss  must  always  be,  on  the  whole,  on  the 
side  of  the  sloping  surfaces.  In  order  to  show  the  full  import  of  this  part 
of  the  subject,  certain  ratios  may  here  be  assumed  which  are  probably 
understatements  rather  than  exaggerations.  Let  us  take  the  proportion 
between  the  extent  of  the  plains  and  table-lands  of  a  country,  and  the  area 
of  its  valleys,  to  be  as  nine  to  one ;  in  other  words,  that,  of  the  whole 
surface  of  the  country,  nine-tenths  consists  of  broad  undulating  plains,  or 
other  comparatively  level  ground,  and  one-tenth  of  steeper  slopes.  Let  it 
be  further  assumed  that  the  erosion  of  the  surface  is  nine  times  greater 
over  the  latter  than  over  the  former  area,  so  that  while  the  more  level 
parts  of  the  country  have  been  lowered  one  foot,  the  valleys  have  lost  nine 
feet.  If,  following  the  measurements  and  calculations  idready  given,  we 
admit  that  the  mean  annual  quantity  of  detritus  carried  to  the  sea 
may,  with  some  probability,  be  regarded  as  equal  to  the  yearly  loss  of 

Thhru  ^^  *  ^^^^  ^^  ^^^^  ^^^^  ^^®  general  surface  of  the  country, 
then,  apportioning  this  loss  over  the  surface  in  the  ratio  just  given, 
we  find  that  it  amounts  to  {-  of  a  foot  from  the  more  level  grounds 

2  H 


466  DYNAMICAL  GEOLOGY  book  m  part  n 

in  6000  years,  and  5  feet  from  the  valleys  in  the  same  space  of  time. 
Now,  if  ^  oi  a.  foot  be  removed  from  the  level  grounds  in  6000  years,  1 
foot  will  be  removed  in  10,800  years;  and  if  5  feet  be  worn  out  of  the 
valleys  in  6000  years,  1  foot  will  be  worn  out  in  1200  years.  This  is 
equal  to  a  loss  of  only  ^,j  of  an  inch  from  the  table-lands  in  75  years,  while 
the  same  amount  is  excavated  from  the  valleys  in  8i  years. 

It  may  seem  at  first  sight  that  such  a  loss  as  only  a  single  line  from 
the  surface  of  the  open  country  during  more  than  the  lapse  of  a  long 
human  life  is  almost  too  trifling  to  be  taken  into  account^  as  it  is  certainly 
too  small  to  be  generally  appreciable.  In  the  same  way,  if  we  are  told 
that  the  constant  wear  and  tear  which  is  going  on  before  our  eyes  in 
valleys  and  water-courses,  does  not  effect  more  than  the  removal  of  one 
line  of  rock  in  eight  and  a  half  yeai's,  we  may  naturally  enough  regard 
such  a  statement  as  probably  an  under-estimate.  But  if  we  only  permit 
the  multiplying  power  of  time  to  come  into  play,  the  full  force  of  those 
seemingly  insignificant  quantities  is  soon  made  apparent  For  we  find 
by  a  simple  piece  of  arithmetic  that,  at  the  rate  of  denudation  which  has 
been  just  postulated  as  probably  a  fair  average,  a  valley  of  1000  feet  deep 
may  be  excavated  in  1,200,000  years,  a  period  which,  in  the  eyes  of  mo^ 
geologists,  will  seem  short  indeed. 

Objection  may  be  taken  to  the  ratios  from  which  this  average  rate  of 
denudation  is  computed.  Without  attempting  to  decide  what  this  average 
rate  actually  is — a  question  which  must  be  determined  for  each  region 
upon  much  fuller  data  than  are  at  present  available — the  geologist  will 
find  advantage  in  considering,  from  the  point  of  view  now  indicated,  what^ 
according  to  the  most  probable  estimates,  is  actually  in  progress  around 
him.  Let  him  assume  any  other  apportioning  of  the  total  amount  of 
denudation,  he  does  not  thereby  lessen  the  measurement  of  that  amount^ 
which  can  bo  and  has  been  ascertained  in  the  annual  discharge  of  rivers. 
A  certain  determined  quantity  of  rock  is  annually  worn  off  the  surface  of 
the  land.  If,  as  already  remarked,  we  represent  too  large  a  proportion 
to  be  derived  from  the  valleys  and  water-courses,  we  diminish  the  loss 
from  the  open  country ;  or,  if  we  make  the  contingent  derived  from  the 
latter  too  great  we  lessen  that  from  the  former.  Under  any  ascertained 
or  assumed  proportion,  the  facts  remain,  that  the  land  loses  a  certain 
ascertainable  fraction  of  a  foot  from  its  general  siu'face  per  annum,  and 
that  the  loss  from  the  valleys  and  water-courses  is  larger  than  that  fraction, 
while  the  loss  from  the  level  ground  is  less. 

3.  Marine  Denudation — its  comparative  rate. 

From  the  destructive  effects  of  occasional  storms  an  exaggerated 
estimate  has  been  formed  of  the  relative  potency  of  marine  erosion. 
That  the  amount  of  waste  by  the  sea  must  be  inconceivably  less  than  ihtt 
effected  by  the  subaerial  agents,  will  be  evident  if  we  consider  how  small 
is  the  extent  of  surface  exposed  to  the  power  of  the  waves,  when  con- 
trasted with  that  which  is  under  the  influence  of  atmospheric  waste.  Id 
the  general  degradation  of  the  land,  this  is  an  advantage  in  favour  of  the 


SECT,  ii  §  7  MARINE  DENUDATION  467 


subaerial  agents  which  would  not  be  counterbalanced  unless  the  rate  of 
waste  by  the  sea  were  many  thousands  or  millions  of  times  greater  than 
that  of  rains,  frosts,  and  streams.  But  in  reality  no  such  compensation 
exists.  In  order  to  see  this,  it  is  only  necessary  to  place  side  by  side 
measurements  of  the  amount  of  work  actually  performed  by  the  two 
classes  of  agents.  Let  us  suppose,  for  instance,  that  the  sea  eats  away  a 
continent  at  the  rate  of  ten  feet  in  a  century — an  estimate  which  probably 
attributes  to  the  waves  a  much  higher  rate  of  erosion  than  can,  as  the 
average,  be  claimed  for  them.^  Then  a  slice  of  about  a  mile  in  breadth 
will  require  about  52,800  years  for  its  demolition,  ten  miles  will  be  eaten 
away  in  628,000  years,  one  hundred  miles  in  5,280,000  years.  Now  we 
have  already  seen  that,  on  a  moderate  computation,  the  land  loses  about 
a  foot  from  its  general  surface  in  6000  years,  and  that,  by  the  continuance 
of  this  rate  of  subae^al  denudation,  the  continent  of  Europe  might  be 
worn  away  in  about  4,000,000  years.  Hence,  before  the  sea,  advancing 
at  the  rate  of  ten  feet  in  a  century,  could  pare  off  more  than  a  mere 
marginal  strip  of  land,  between  70  and  80  miles  in  breadth,  the  whole 
land  might  be  washed  into  the  ocean  by  atmospheric  denudation. 

Some  such  results  as  these  would  necessarily  be  produced  if  no  dis- 
turbance took  place  in  the  relative  levels  of  sea  and  land.  But  in 
estimating  the  amount  of  influence  to  be  attributed  to  each  of  the 
denuding  agents  in  past  times,  we  require  to  take  into  account  the  com- 
plicated effects  that  would  arise  from  the  upheaval  or  depression  of  the 
earth's  crust  If  frequent  risings  of  the  land,  or  elevations  of  the  sea-floor 
into  land,  had  not  taken  place  in  the  geological  past,  there  could  have 
been  no  great  thickness  of  stratified  rocks  formed,  for  the  first  continents 
must  soon  have  been  washed  away.  But  the  great  depth  of  the  stratified 
part  of  the  earth's  crust,  and  the  abundant  breaks  and  unconformabilities 
among  the  sedimentary  masses,  show  how  constantly,  on  the  one  hand, 
the  waste  of  the  land  was  compensated  by  elevatory  movements,  while, 
on  the  other,  the  continued  upward  growth  of  vast  masses  of  sedimentary 
deposits  was  rendered  possible  by  prolonged  depression  of  the  sea-bed. 

When  a  mass  of  land  is  raised  to  a  higher  level  above  the  sea,  a 
larger  surface  is  exposed  to  denudation.  As  a  rule,  a  greater  rainfall  is 
the  result,  and  consequently,  also,  a  more  active  waste  of  the  surface  by 
subaerial  agents.  It  is  true  that  a  greater  extent  of  coast-line  is  exposed 
to  the  action  of  the  waves,  but  a  little  reflection  will  show  that  this 
increase  will  not,  on  the  whole,  bring  with  it  a  proportionate  increase  in 
the  amount  of  marine  denudation.  For,  as  the  land  rises,  the  cliffs  are 
removed  from  the  reach  of  the  breakers,  and  a  more  sloping  beach  is 
produced,  on  which  the  sea  cannot  act  with  the  same  potency  as  when  it 
beats  against  a  cliff- line.  Moreover,  as  the  sea-floor  approaches  nearer 
the  surface  of  the  water,  it  is  the  former  detritus  washed  off  the  land,  and 
deposited  under  the  sea,  which  first  comes  within  the  reach  of  the  currents 
and  waves.     This  serves,  in  some  measure,  as  a  protection  to  the  solid 

^  It  may  be  objected  that  this  rate  is  far  below  that  of  parts  of  the  east  coast  of  England 
{omUj  p.  446).  But  along  the  rocky  western  coast  of  Britain  the  loss  is  perhaps  not  so  much 
M  one  foot  in  a  century. 


468  DVXAMICAL  GEOLOOY  book  hi  part  ii 


rock  below,  and  must  be  cut  away  by  the  ocean  before  that  rock  can  be 
exposed  anew.  While,  therefore,  elevatory  movements  tend -on  the  whole 
to  accelerate  the  action  of  subaerial  denudation,  they  in  some  degree  check 
the  natural  and  ordinary  influence  of  the  sea  in  wasting  the  land.  Again, 
the  influence  of  movements  of  depression  will  probably  be  found  to  tend 
in  an  opposite  direction.  The  lowering  of  the  general  level  of  the  land 
will,  as  a  rule,  help  to  lessen  the  rainfall,  and  consequently  the  rate  of 
subaerial  denudation.  At  the  same  time,  it  will  aid  the  action  of  the 
waves,  by  removing  under  their  level  the  detritus  produced  by  them  and 
heaped  up  on  the  beach,  and  by  thus  bringing  constantly  within  reach 
of  the  sea  fresh  portions  of  the  land -surface.  But  even  with  these 
advantages  in  favour  of  marine  denudation,  the  balance  of  power  will,  on 
the  whole,  remain  always  on  the  side  of  the  subaerial  agents. 


4.  Marine  Denudation — its  final  remit. 

The  general  result  of  the  erosive  action  of  the  sea  on  the  land  is  the 
production  of  a  submarine  plain.  As  the  sea  advances,  the  sites  of  success- 
ive lines  of  beach  pass  under  low-water  mark.     Where  erosion  is  in  full 


Fig.  17C. — .Section  of  rocks  ground  down  to  a  jdain  on  the  bt^eh  by  wave4u:Uon. 

operation,  the  littoral  belt,  as  far  down  as  wave-action  has  influence,  is 
ground  down  by  moving  detritus.  This  result  may  often  be  instruct- 
ively observed,  on  a  small  scale,  upon  rocky  shores  where  sections  like 
that  in  Fig.  176  occur.  We  can  conceive  that,  should  no  change  of  le'vel 
between  sea  and  land  take  place,  the  sea  might  slowly  eat  its  way  far 
into  the  land,  and  produce  a  gently  sloping,  yet  apparently  almost  hori- 
zontal selvage  of  plain,  covered  permanently  by  the  waves.  In  such  a 
submarine  plain,  the  influence  of  geological  structure,  and  notably  of  the 
relative  powers  of  resistance  of  different  rocks,  would  make  itself  conspic- 
uous, as  may  be  seen  even  on  a  small  scale  on  any  rocky  beach  (Fig.  167). 
The  present  promontories  caused  by  the  superior  hardness  of  their 
component  rocks  would  no  doubt  be  represented  by  ridges  on  the  sub- 
aqueous plateau,  while  the  existing  bays  and  creeks,  worn  out  of  softer 
rocks,  would  be  marked  by  lines  of  valley  or  hollow.^ 

This  tendency  to  the  formation  of  a  submarine  plain  along  the  margin 
of  the  land  deserves  special  attention  by  the  student  of  denudation. 
The  angle  at  which  a  mass  of  land  descends  to  the  sea -level  serves 
roughly  to  indicate  the  depth  of  water  near  shore.     A  precipitous  coast 

'  >fr.  Whitaker,  iu  the  excellent  paper  on  subaerial  denudation  cited  on  p.  449  hu 
I)ointe<l  out  the  different  results  which  are  obtained  by  tlie  subaerial  forces  from  those  of  set- 
action  in  the  proiluction  of  lines  of  cliff. 


SECT.  >i  §  7 


MARINE  UENUDATinS 


commonly  rises  out  of  deep  water ;  a.  low  coast  is  usually  skirted  with 
shallow  water,  the  line  of  alope  ahove  sea-level  being  in  a  general  way 
prolonged  below  it  The  belt  of  beach  forms  a  kind  of  terrace  or  notch 
along  the  maritimB  slope.  Sometimes,  where  the  coast-line  is  preci- 
pitous, this  terrace  is  nearly  or  wholly  wanting.  In  other  places,  it  runs 
out  a  good  way  beyond  low-water  mark.     On  a  great  scale,  the  floor  of 


FJtr.  177.— Map  of  Briti»h  lubnui: 
Hm  darksr  tlnl  lepreMntg  Bea-botunn  more  than  100  bthoms  deep,  wbile  the  pi1«r  shading  sham  the 
UM  of  Ism  de[itha.    The  Hgnns  mark  th«  depth  in  ftithniin.    The  narrow  chHnnel  bttween  Norway 
*nd  DtDimrk  li  !&W  feet  deep. 

tbe  North  Sea  and  that  of  the  Atlantic  Ocean,  for  some  distance  to  the 
west  of  Ireland,  may  be  regarded  as  a  marine  platform  that  once  formed 
part  of  the  European  continent  (Fig.  177),  and  has  been  reduced  by 
denudation  and  subsidence  to  its  present  position. 

So  far  as  the  present  regime  of  nature  has  been  explored,  it  would 
aeem  to  be  inevitable  that,  unless  where  subterranean  movements  interfere, 


470  DYXAMICAL  GEOLOGY  book  ni  part  n 

or  where  volcanic  rocks  are  poured  forth  at  the  surface,  a  submarine  plain 
should  be  formed  along  the  margin  of  the  land.     This  final   result  of 
denudation  has  been  achieved  again  and  again  in  the  geological  past,  as 
is  shown  by  the  existence  of  table-lands  of  erosion  (aiiie,  p.  43).     To  these 
table-lands  the  name  of  '^  plains  of  marine  denudation  "  has  been  applied 
by  Sir  A.  C.  Ramsay.     From  what  has  now  been  said,  however,  it  will  be 
seen  that  in  their  actual  production  the  sea  has  really  had  less  to  do  than 
the  meteoric  agents.     A  "  plain  of  marine  denudation  "  is  that  base-level  of 
erosion  to  which  a  mass  of  land  had  been  reduced  mainly  by  the  subaerial 
forces — the  line  below  which   further  degradation   became  impossible, 
because  the  land  was  thereafter  protected  by  being  covered  by  the  sea. 
Undoubtedly  the  last  touches  in  the  long  process  of  sculpturing  were 
given  by  marine  waves  and  currents,  and  the  surface  of  the  plain,  save 
where  it  has  subsided,  may  correspond  generally  with  the  lower  limit  of 
wave-action.     Nevertheless,  in  the  past  history  of  our  planet,  the  influence 
of  the  ocean  has  probably  been  far  more  conservative  than  destructive. 
Beneath  the  reach  of  the  waves,  the  surface  of  the  abraded  land  has 
escaped  the  demolition  which  sooner  or  later  overtakes  all  that  rises  above 
them. 

5.  Deposition — tlie.  franuicm'k  of  new  land. 

If  a  survey  of  the  geological  changes  in  daily  progress  upon  the 
surface  of  the  earth  leads  us  to  realise  how  momentously  the  land  is 
being  worn  down  by  the  various  epigene  agents,  it  ought  also  to  impress 
us  with  the  vast  scale  on  which  new  formations — the  foundation  of  futare 
land — are  being  continually  accumulated.  Every  foot  of  rock  removed 
from  the  surface  of  a  country  is  represented  by  a  corresponding  amount 
of  sedimentary  material  arranged  somewhere  beneath  the  sea.  Denuda- 
tion and  deposition  are  synchronous  and  co-equal. 

On  land,  va.st  accumulations  of  detrital  origin  are  now  in  progress. 
Alluvial  plains  of  every  size,  from  those  of  mere  brooks  up  to  those  of 
the  largest  rivers,  are  built  up  of  gravel,  sand,  and  mud  derived  from  the 
disintegration  of  higher  ground.  From  the  level  of  the  present  streams, 
successive  terraces  of  these  materials  can  be  followed  up  to  heights  of 
several  hundred  feet.  Over  wide  regions,  the  daily  changes  of  tempera- 
ture, moisture  and  wind  supply  a  continual  dust,  which,  in  the  course  of 
centuries,  has  accumulated  to  a  depth  of  sometimes  1500  feet,  and  covers 
thousands  of  square  miles  of  the  surface  of  the  continents.  The  numerous 
lakes  that  dot  the  surface  of  the  land  serve  as  receptacles  in  which  a 
ceaseless  deposition  of  sediment  takes  place.  Already  an  unknown 
number  of  once  existent  lakes  has  been  entirely  filled  up  with  detrital 
accumulations,  and  every  stage  towards  extinction  may  be  traced  in  those 
that  remain. 

But  extensive  though  the  teiTestrial  sedimentary  deposits  may  be, 
they  can  be  regarded  merely  as  temporary  acciunulations  of  the  detritus. 
Save  where  protected  and  concealed  under  the  water  of  lakes,  they  are 
everywhere  exposed  to  a  renewal  of  the  denudation  to  which  they  owe 
their  origin.     Only  where    the   sediment  is  strewn  over  the  sea-floor 


SECT,  iii  §  1  GEOLOGICAL  ACTION  OF  PLANTS  471 

« 

beneath  the  limit  of  breaker-action,  is  it  permitted  to  accumulate  undis- 
turbed. In  these  quiet  depths,  are  now  growing  the  shales,  sandstones, 
and  limestones,  which  by  future  terrestrial  revolutions  will  be  raised  into 
land,  as  those  of  older  times  have  been.  Between  the  modern  deposits 
and  those  of  former  sea-bottoms  which  have  been  upheaved,  there  is  the 
closest  parallel.  Deposition  will  obviously  continue  as  long  as  denudation 
lasts.  The  secular  movements  of  the  crust  seem  to  have  been  always 
sufficiently  frequent  and  extensive  to  prevent  cessation  of  these  operations. 
And  so  we  may  anticipate  that  it  will  be  for  many  geological  ages  yet  to 
come.  Elevation  of  land  will  repair  what  has  been  lost  by  superficial 
waste,  and  subsidence  of  sea-bottom  will  provide  space  for  continued 
growth  of  sedimentary  deposits. 

Section  ill.  Life. 

Among  the  agents  by  which  geological  changes  are  now,  and  have 
in  past  time  been  effected  upon  the  earth's  surface,  living  organisms 
take  by  no  means  an  unimportant  place.  They  serve  as  a  vehicle  for 
continual  transferences  from  the  atmosphere  into  the  mineral  world,  and 
from  the  mineral  world  back  into  the  atmosphere.  Thus,  they  decompose 
atmospheric  carbon-dioxide,  and  in  this  process  have  gradually  removed 
from  the  atmosphere  the  vast  volumes  of  carbon  now  locked  up  within 
the  earth's  crust  in  beds  of  solid  coal.  By  their  decomposition,  organic 
acids  are  produced  which  partly  enter  into  mineral  combinations,  and 
partly  return  to  the  atmosphere  as  carbon -dioxide.  Plants  abstract 
from  the  soils  silica,  alkalies,  calcium -phosphate,  and  other  mineral 
substances,  which  enter  largely  into  the  composition  of  the  hard  parts 
of  animals.  On  the  death  and  decomposition  of  animals,  these  substances 
are  once  more  relegated  to  the  inorganic  world,  thence  to  enter  upon 
a  new  circulation  through  the  tissues  of  living  organisms. 

From  a  geological  point  of  view,  the  operations  of  organic  life 
may  be  considered  under  three  aspects — destructive,  conservative,  and 
reproductive. 

§  1.  Destructive  Action. 

Plants  in  several  ways  promote  the  disintegration  of  rocks. 

1.  By  keeping  the  surfaces  of  rocks  moist,  plants  provide  means  for 
the  continuous  solvent  action  of  water.  This  influence  is  particularly 
observable  among  liverworts,  mosses,  and  similar  moisture-loving  plants. 

2.  By  their  decay,  plants  supply  an  important  series  of  organic  acids, 
which  exert  a  powerful  influence  upon  soils,  minerals,  and  rocks.  The 
humus,  or  organic  portion  of  vegetable  soil,  consists  of  the  remains  of 
plants  and  animals  in  all  stages  of  decay,  and  contains  a  complex  series 
of  organic  compounds  still  imperfectly  understood.  Among  these  are 
humic,  ulmic,  crenic  and  apocrenic  acids. ^  The  action  of  these  organic  acids 
is  twofold.  (1)  From  their  tendency  to  oxidation,  they  exert  a  markedly 
reducing  influence  {ante,  pp.  343,  360,  456).     Thus  they  convert  metallic 

^  See  J.  Both,  '  Allgemeine  und  Chemische  Geologie/  1883,  p.  596. 


472  DYNAMICAL  GEOLOGY  book  iii  part  ii 

■ 

sulphates  into  sulphides,  as  in  the  blue  marine  muds,  and  the  abundant 
2)yritous  incrustations  of  coal-seams,  shell-l>earing  clays,  and  even  some- 
times of  mine-timbers.  Metallic  salts  are  still  further  reduced  to  the  state 
of  native  metals.  Native  silver  occiu^  among  silver  ores  in  fossil  wood 
among  the  Permian  rocks  of  Hesse.  Native  copper  has  been  frequently 
noticed  in  the  timber-props  of  mines ;  it  was  found  hanging  in  stalactites 
from  the  timbers  of  the  Ducktown  copper  mines,  Tennessee,  when  the 
mines  were  re-opened  after  being  shut  up  during  the  civil  war.  Fossil 
fishes  from  the  Kupferschiefer  have  been  incrusted  with  native  copper, 
and  fish-teeth  have  been  obtained  from  Liguria  completely  replaced  by 
this  metal.  (2)  They  exert  a  remarkable  power  of  dissolving  mineral 
substances.^  This  phase  of  their  activity  has  probably  been  under- 
valued by  geologists.^  Experiments  have  shown  that  many  of  the 
common  minerals  of  rocks  are  attacked  by  organic  acids.  There  is 
reason  to  })elieve  that  in  the  decomposition  effected  by  meteoric  waters^ 
and  usually  attributed  mainly  to  the  operation  of  carbonic  acid,  the 
initial  stages  of  attack  are  due  to  the  powerful  solvent  capacities  of 
the  humus  acids.  Owing,  however,  to  the  facility  with  which  these 
acids  ]^)ass  into  higher  states  of  oxidation,  it  is  chiefly  as  carbonates 
that  the  results  of  their  action  are  carried  down  into  deeper  parts  of  the 
crust  or  brought  up  to  the  siu*face.  Although  carbonic  acid  is  no  doubt 
the  final  condition  into  which  these  unstable  organic  acids  pass,  yet 
during  their  existence,  they  attack  not  merely  alkalies  and  alkaline 
earths,  but  even  dissolve  silica.  The  relative  proportion  of  silica  in 
river-waters  has  been  referred  to  the  greater  or  less  abundance  of  humus 
in  their  hydrographical  basins,^  the  presence  of  a  large  percentage  of 
silica  being  a  concomitant  of  a  large  proportion  of  organic  matter. 
Further  evidence  of  the  important  influence  of  organic  acids  upon  the 
solution  of  silica  is  supplied  by  many  siliceous  deposits  (p.  483). 

AXHierever  a  layer  of  humus  has  spread  over  the  surface  of  the  land, 
traces  of  its  characteristic  decompositions  may  be  found  in  the  soils,  sub- 
soils and  underlying  rocks.  Next  the  surface,  the  normal  colour  of  the 
subsoils  is  usually  changed  by  oxidation  and  hydration  into  tints  of 
brown  and  yellow,  the  lower  limit  of  the  weathered  zone  being  often 
sharply  defined.  Where  the  humus  acids  can  freely  attack  the 
hydrated  peroxide  of  iron,  tltey  remove  it  in  solution,  and  the  decomposed 
rock  or  soil  is  thereby  bleached.  This  may  be  observed  where  pine-trees 
grow  on  ferruginous  sand,  a  rootlet  one-sixth  of  an  inch  in  diameter  being 
by  its  decay  capable  of  whitening  the  sand  to  a  distance  of  from  one  to 

^  Professor  Sollas  has  noticed  the  formation  of  minute  hemispherical  pits  on  limestmie 
by  the  solvent  action  of  a  lichen,  Verrucaria  rupestria  {Bn'L  Assoc,  1880,  sects,  p.  586). 
See  also  J.  G.  Goodchild,  Oeol.  Mag.  1890,  p.  464. 

^  This  has  been  strongly  insisted  upon  by  A.  A.  Julien  in  a  memoir  on  the  Geological 
Action  of  the  Humus  Acids.  Aiwr,  Assoc.  1879,  p.  311.  Professor  H.  C.  Bolton  has  ex- 
perimented on  the  action  of  citric  acid  on  200  different  mineral  species,  and  he  finds  that 
this  organic  acid  possesses  a  power  of  dissolving  minerals  only  slightly  less  than  that  of 
hydrochloric  acid  :  Brit.  Assoc.  1880,  sects,  p.  505. 

^  Sterry  Hunt's  *  Chemical  and  Geological  Essays,'  pp.  126-150. 


SECT,  iii  §  1  ACTION  OF  PLAXTS  AND  ANIMALS  473 

two  inches  around  it.^  It  has  recently  been  proposed  to  ascribe  mainly 
to  the  operation  of  the  humus  acids  the  thick  layer  of  decomposed  rock 
above  noticed  (p.  350)  as  observable  so  frequently  south  of  the  limits  of 
the  ice  of  the  Glacial  Period,  and  the  inference  has  been  drawn  that,  even 
where  the  surface  is  now  comparatively  barren,  the  mere  existence  of 
this  thick  decomposed  layer  affords  a  presumption  that  it  once  underlay 
an  abundant  vegetation,  such  as  a  heavy  primeval  forest-growth.^  Nor  is 
the  chemical  action  confined  to  the  superficial  layers.  The  organic  acids 
are  carried  down  beneath  the  surface,  and  initiate  that  series  of  altera- 
tions which  carbonic  acid  and  the  alkaline  carbonates  effect  among  sub- 
terranean rock  masses  {ante,  p.  360). 

3.  Plants  insert  their  roots  or  branches  between  the  joints  of  rock, 
or  penetrate  beneath  the  soil.  Two  marked  effects  are  traceable  to 
this  action.  In  the  first  place,  large  slices  of  rock  may  be  wedged  off 
from  the  sides  of  wooded  hills  or  cliffs.  Even  among  old  ruins,  an  occa- 
sional sapling  ash  or  elm  may  be  found  to  have  cast  its  roots  round  a 
portion  of  the  masonry,  and  to  be  slowly  detaching  it  from  the  rest  of 
the  wall.  In  the  second  place,  the  soil  and  subsoil  are  opened  up  to  the 
decomposing  influences  of  the  air  and  descending  water.  The  distance 
to  which,  under  favourable  circumstances,  roots  may  penetrate  downward 
are  much  greater  than  might  be  supposed.  Thus  in  the  loess  of  Nebraska 
the  buffalo-berry  {Shepherdia  argophylla)  has  been  observed  to  send  a  root 
65  feet  down  from  the  surface,  and  in  that  of  Iowa  the  roots  of  grasses 
penetrate  from  5  to  25  feet.* 

4.  By  attracting  rain,  as  thick  forests,  woods,  and  mosses,  more 
particularly  on  elevated  ground,  are  believed  to  do,  plants  accelerate 
the  general  scouring  of  a  country  by  running  water.  The  indiscri- 
minate destruction  of  the  woods  in  the  Levant  has  been  assigned, 
with  much  plausibility,  as  the  main  cause  of  the  present  desiccation 
of  that  region.* 

5.  Plants  promote  the  decay  of  diseased  and  dead  plants  and  animals,  as 
when  fungi  overspread  a  damp  rotting  tree  or  the  carcase  of  a  dead  animal. 

Animals. — The  destructive  influences  of  the  animal  kingdom  like- 
wise show  themselves  in  several  distinct  ways. 

1.  The  surface-soil  is  moved,  and  exposed  tliereby  to  attack  by 
rain,  wind,  &c.  As  Darwin  showed,  the  common  earth-worm  is  con- 
tinually engaged  in  bringing  up  the  fine  particles  of  soil  to  the  surface. 
He  found  that  in  fifteen  years  a  layer  of  burnt  marl  had  been  buried 
under  3  inches  of  loam,  which  he  attributed  to  this  operation.^  It  has 
been  already  pointed  out  that  part  of  the  growth  of  soil  may  be  due  to 
wind-action  {(mky  p.  331).  There  can  be  no  doubt,  however,  that  the 
materials  of  vegetable  soil  are  largely  commingled  and  fertilised  by  the 

*  Kindler,  Pogrflrem/.  Annal,  xxxvii.  (1836),  p.  203.  J.  A.  Phillips,  'Ore  Deposits,' 
1884,  p.  14.  2  Jiilien,  Amer.  Assoc.  1879,  p.  378. 

'  Aughey*8  'Physical  Geography  and  Geology  of  Nebraska,'  1880,  p.  275. 

*  See  on  this  disputed  question  the  works  cited  by  RoUeston,  Jounu  Roy,  Oeog.  Soe. 
xlix.  (1879).  The  destruction  of  forests  is  also  alleged  to  increase  the  number  and  severity 
of  hail-storms.  *  Trans.  Geol,  Sac.  v.  p.  605.     'Vegetable  Mould,'  1881. 


474  DYNAMICAL  GEOLOGY  book  ni  pakt  n 

earth-worm,  and  in  particular  that^  by  being  brought  up  to  the  surface, 
the  fine  particles  are  exposed  to  meteoric  iaflueaces,  notably  to  wind 
and  rain.  Even  a  grass-covered  surface  may  thns  snffer  slow  deaudft- 
tion.  Lob-worms  on  sandy  shores  possibly  aid  transport  by  wavee  and 
tides,  inasmuch  as  they  bring  up  large  quantities  of  fresh  sand.' 

Burrowing  animals,  by  throwing  up  the  soil  and  subsoil,  expose 
these  to  be  dried  and  blown  away  by  the  wind.  At  the  same  time, 
their  subterranean  passages  serve  to  drain  off  the  superficial  water, 
and  to  injure  the  stability  of  the  surface  of  the  ground  above  tbem.  In 
Britain,  the  mole  and  rabbit  are  familiar  examples.  In  North  America, 
the  prairie  dog  and  gopher  have  undermined  extensive  tracts  of  pastnre- 
land  in  the  west.  In  Cape  Colony,  wide  areas  of  open  country  eeem 
to  be  in  a  constant  state  of  eruption  from  the  burrowing  operations  of 
multitudes  of  Batliyergi  and  C'krysochioris — small  mole-like  animals  which 
bring  up  the  soil  and  bury  the  grassy  vegetation  under  it.  The 
decomposition  of  animal  remains  produces  chemical  changes  similar  to 
those  resulting  from  the  decay  of  plants. 

2.  The  flow  of  streams  is  sometimes  interfered  with,  or  even 
diverted,  by  the  operations  of  animals.  Thns  the  beaver,  by  cutting 
down  trees  (sometimes  1  foot  or  more  in  diameter)  and  constnicdng 
dams  with  the  stems  and  branches,  checks  the  flow  of  water-course^ 
intercepts  floating  materials,  and  sometimes  even  diverts  the  wat«r  into 
new  channels.  This  action  is  typically  displayed  in  Canada  and  in  the 
Rocky  Mountain  regions  of  the  United  States.  Thousand  of  acres 
in  many  valleys  have  been  converted  into  lakes,  which,  intercepting  Uie 
sediment  carried  down  by  the  streams,  and  being  likewise  invaded  by 
marshy  vegetation,  have  subsequently  become  morass  and  finally 
meadow-land.  The  extent  to  which,  in  these  regions,  the  alluvial 
formations  of  valleys  have  been  modified  and  extended  by  the 
operations  of  the  beaver,  is  almost  incredible.  The  embankments  of  the 
Mississippi  are  sometimes  weakened  to  such  an  extent  by  the  burrowings 
of  the  cray-fish  as  to  give  way,  and  allow  the  river  to  inundate  the 
surrounding  country.  Similar  results  have  happened  in  Europe  from 
the  subterranean  ojierations  of  rats, 

3.  Some   molhisks   (Pho!as,   Sariaiea,    Teredo,  &c..   Fig.    178)   bore 
into  stone  or  wood,  and  by  the  number 

J  *y^,t'-       of  contiguous  perforations  greatly  weaken 
"/■■'\  1     the  materials.     Pieces  of  drift-wood  are 
-  "  '    -      soon  riddled  with  long  boles  by  the  tere- 
do ;  while  wooden  piere,  and  the  bottoms 
of  wooden  ships,  are  often  rapidly  per- 
l^pr       't^         forated.     Saxicavous  shells,  by  piercing 
'  ^1'        ; '       '       stone  and  leaving  open  cavities  for  rain 
Fig.  178— aheU-boiiniMiniirpejtone       ^"^  sea-water  to  fill,  promote  its  decay. 
A    potent    cause  of    the    destruction  of 
coral-reefs  is  to  be  found  in  the  borings  of  moUusks,  annelids,  and  echino- 

'  Mr.  Daviaon  rstiinutes  the  nmouDt  to  be  somelimei  nrarlT  2000  toni  uunudlvotw 
aoBcre.    Gro/.  Mny.  ISBl. 


BECT.  iii  §  2  ACTION  OF  PLANTS  AND  ANIMALS  475 

derms,  whereby  masses  of  coral  are  weakened  so  as  to  be  more  easily 
removed  by  breakers. 

4.  Many  animals  exercise  a  ruinously  destructive  influence  upon 
vegetation.  Of  the  various  insect-plagues  of  this  kind  it  will  be  enough 
to  enumerate  the  locust^  phylloxera,  and  Colorado  beetle.  The  pasture 
in  some  parts  of  the  south  of  Scotland  has  in  recent  years  been  much 
damaged  by  mice,  which  have  increased  in  numbers  owing  to  the 
indiscriminate  shooting  and  trapping  of  owls,  hawks,  and  other 
predaceous  creatures.  Grasshoppers  cause  the  destruction  of  vegeta- 
tion in  some  parts  of  Wyoming  and  other  Western  Territories  of  the 
United  States.  The  way  in  which  animals  destroy  each  other,  often 
on  a  great  scale,  may  likewise  be  included  among  the  geological  opera- 
tions now  under  description.  As  an  illustration  of  this  action,  reference 
may  be  made  to  the  occasionally  enormous  development  of  the  protozoon 
genera  Peridinium  and  Ghnodinium,  and  the  consequent  killing  off  of  the 
oysters  and  other  mollusks  in  the  waters  of  Port  Jackson.^ 


§  2.  Conservative  Action. 

Plants. — Tlie  protective  influence  of  vegetation  is  well  known. 

1.  The  formation  of  a  stratum  of  turf  protects  soil  and  rocks  from 
being  rapidly  removed  by  rain  or  wind.  Hence  the  surface  of  a  district 
so  protected  is  denuded  with  extreme  slowness,  except  along  the  lines  of 
its  water-courses.  A  crust  of  lichens  doubtless  on  the  whole  protects 
the  rock  underneath  it  from  atmospheric  agents.^ 

2.  Many  plants,  even  without  forming  a  layer  of  turf,  serve  by 
their  roots  or  branches  to  protect  the  loose  sand  or  soil  on  which 
they  grow  from  being  removed  by  wind.  The  common  sand-carex 
and  other  arenaceous  plants  bind  littoral  sand-dunes,  and  give  them 
a  permanence  which  would  at  once  be  destroyed  were  the  sand  laid 
bare  again  to  the  storms.  In  North  America,  the  sandy  tracts  of  the 
Western  Territories  are  in  many  places  protected  by  the  sage-brush 
and  grease -wood.  The  growth  of  shrubs  and  brushwood  along  the 
course  of  a  stream  not  only  keeps  the  alluvial  banks  from  being  so 
easily  undermined  and  removed  as  would  otherwise  be  the  case,  but 
serves  to  arrest  the  sediment  in  floods,  filtering  the  water,  and  thereby 
adding  to  the  height  of  the  flood-plain.  On  some  parts  of  the  west 
coast  of  France,  extensive  ranges  of  sand-hills  have  been  planted  with 
pine  woods,  which,  while  preventing  the  destructive  inland  march  of  the 
sand,  also  jrield  a  large  revenue  in  timber,  and  have  so  influenced  the 

^  An  occurrence  of  this  kind  in  March  1891  led  to  an  almost  complete  destruction  of  the 
oysters,  mussels,  and  other  bivalves  ;  the  rest  of  the  littoral  fauna — limpets  and  other  uni- 
valves, starfisbf  worms,  ascidians,  and  other  lower  forms  of  life — were  so  seriously  affected 
that  dead  and  dying  were  strewn  about  in  great  numbers,  while  the  higher  forms,  able  to 
move  rapidly,  had  retired  to  deep  water.  T.  Whittelegge,  Records  of  Australian  Museum^ 
L  No.  9  (1891),  p.  179. 

*  But  see  the  remark  already  made,  antCj  p.  472,  note  1. 


/ 

>■ 


476  It  YXA  MICA  L  UEO  L  OG  Y  book  iii  part  ii 


climate  as  to  make  these  districts  a  resort  for  pulmonary  invalids,^  In 
tropical  countries,  the  mangrove  grows  along  the  sea-margin,  and  not 
only  protects  the  land,  but  adds  to  its  breadth,  by  forming  and  increasing 
a  maritime  alluvial  belt. 

3.  Some  marine  })lants  likewise  aftbrd  protection  to  shore  rocka. 
This  is  done  by  the  hard  incrustration  of  calcareous  nullipores;  like- 
wise by  the  tangles  and  smaller  fuci  which,  growing  abundantly  on  the 
littoral  zone,  break  the  force  of  waves,  or  diminish  the  effects  of  ground- 
swell. 

4.  Forests  and  brushwood  protect  soil,  especially  on  slopes,  from 
being  washed  away  by  rain.  This  is  shown  by  the  disastrous  results 
of  the  thoughtless  destruction  of  woods.  According  to  Reclus,-  in  the 
three  centuries  from  1471  to  1776,  the  "  vigueries,"  or  provostry-districts 
of  the  French  Alps,  lost  a  third,  a  half,  and  even  three-fourths  of  their 
cultivated  ground,  and  the  population  has  diminished  in  somewhat 
similar  proportions.  From  1836  to  1866  the  departments  of  Hautes 
and  Basses  Alpes  lost  25,000  inhabitants,  or  nearly  one-tenth  of  their 
population — a  diminution  which  has  with  plausibility  been  assigned  to 
the  reckless  removal  of  the  pine  forests,  whereby  the  steep  mountain 
sides  have  been  washed  bare  of  their  soil.  The  desiccation  of  the 
countries  bordering  the  eastern  Mediterranean  has  been  ascribed  to  a 
similar  cause.*^ 

o.  In  mountain  districts,  pine-forests  exercise  also  an  important  con- 
servative function  in  preventing  the  formation  or  arresting  the  progress 
of  avalanches.  In  Switzerland,  some  of  the  forests  which  cross  the  lines 
of  frequent  snow-falls  are  carefully  preserved. 

Animals  do  not  on  the  whole  exert  an  important  conservative  action 
upon  the  earth's  surface,  save  in  so  far  as  they  form  new  de|)Osits,  as  will  be 
immediately  referred  to.  On  many  shores,  however,  by  thickly  encrusting 
rocks,  they  act  like  the  marine  vegetation  above  alluded  to,  and  protect 
these  to  a  considerable  extent  from  abrasion  by  the  waves.  The  most 
familiar  example  in  Europe  of  this  action  is  that  of  the  common  acorn- 
shell  or  barnacle  {Bahinu^  balanoides),  Seqiulae  often  encrust  considerable 
masses  of  a  coral-reef,  and  act  like  nullipores,  in  protecting  decaying  and 
dead  corals  from  being  so  rapidly  broken  up  by  the  waves  as  they  would 
otherwise  be.  But  even  soft -bodied  animals,  such  as  sponges  and 
ascidians,  when  they  spread  over  rocks  near  low-water,  afford  protection 

'  De  Lavergiie,  '  Economic  rurale  de  la  France  depnis  1789,'  p.  297.  Edin.  Review, 
Oct.  18()4,  article  on  Coniferous  Trees. 

-  '  I^  Terre,'  p.  410.  J.  C.  Brown,  '  Reboisement  en  France,*  London  1876.  Accord* 
ing  to  Dr.  J.  Garret,  however,  the  deterioration  of  the  climate  of  Savoy  and  the  diminatioo 
of  the  population  there  cannot  be  attributed  to  drboiseinent  Tlie  cutting-down  of  tha 
forests  dates  from  the  First  Empire,  but  replanting  has  been  going  on  for  some  time,  and  the 
forest  area  is  now  a  little  larger  than  it  was  last  century.  Nevertheless  the  depopnlatko 
of  the  higlier  tracts,  which  had  begun  before  last  century,  continues,  notwithttaadiag  tlie 
replanting  of  the  slopes  :  Assoc.  Fran^-aisCf  1879,  p.  538. 

^'  Recent  attempts  to  reclothe  the  desiccated  stone-wastes  of  Dalmatia  with  trees  have 
been  attended  with  success.     See  Mojsisovics,  Jtihrb.  Geol.  Reichsarul,  1880,  p.  210. 


SECT,  iii  §  3  ACTION  OF  PLANTS  AND  ANIMALS  477 

from  at  least  the  less  violent  attacks  of  the  breakers.  Professor  Herdman, 
who  has  called  attention  to  this  subject,  enumerates  as  the  more  important 
animals  in  protecting  shore  rocks :  Foraminifera  (such  as  Flanorbulina 
vulgaiis),  calcareous  and  fibrous  sponges,  hydroid  zoophytes,  sea  anemones, 
corals,  annelides  (serpula),  polyzoa,  cirripedes,  mollusks  (such  as  gregarious 
forms  like  the  mussel  and  oyster,  and  gasteropods  like  the  limpet),  and 
simple  and  compound  ascidians.^ 

In  the  prairie  regions  of  Wyoming  and  other  tracts  of  North  America, 
some  interesting  minor  effects  are  referable  to  the  herds  of  roving  animals 
which  migrate  over  these  territories.  The  trails  made  by  the  bison,  the 
elk,  and  the  big-horn  or  mountain  sheep,  are  firmly  trodden  tracks  on 
which  vegetation  will  not  grow  for  many  years.  All  over  the  region 
traversed  by  the  bison,  numerous  circular  patches  of  grass  are  to  be  seen 
which  have  been  formed  on  the  hollows  where  this  animal  has  wallowed. 
Originally  they  are  shallow  depressions,  formed  in  great  numbers  where 
a  herd  of  bisons  has  rested  for  a  time.  On  the  advent  of  the  rains  they 
become  pools  of  water ;  thereafter  grasses  spring  up  luxuriantly,  and  so 
bind  the  soil  together  that  these  grassy  patches  or  "  bison- wallows,"  may 
actually  become  slightly  raised  above  the  general  level,  if  the  surrounding 
ground  becomes  parched  and  degraded  by  winds.^ 

§  3.  Reproductive  Action. 

Plants. — Both  plants  and  animals  contribute  materials  towards  new 
geological  formations,  chiefly  by  the  aggregation  of  their  remains,  partly 
from  their  chemical  action.  Their  remains  are  likewise  enclosed  in 
deposits  of  sand  and  mud,  the  bulk  of  which  they  thus  help  to  increase. 
Of  plant-formations  the  following  illustrative  examples  may  be  given  : — 

1.  Sea-weeds. — It  was  long  ago  shown  by  Forchhammer  that  fucoids 
abstract  an  appreciable  amount  of  lime,  magnesia,  soda,  and  other  com- 
ponents of  sea-water,  and  he  believed  that  these  plants  probably  played  an 
important  part  in  the  accumulation  of  the  older  Palaeozoic  sediments.*  The 
calcareous  nuUipores  which  encrust  shore  rocks  provide  solid  material 
which,  either  growing  in  situ  or  broken  off  and  distributed  by  the  waves, 
gives  rise  to  a  distinct  geological  deposit.  Considerable  masses  of  a 
structureless  limestone  are  formed  in  the  Bay  of  Naples  mainly  by 
calcareous  algae.  By  the  infiltration  of  water  into  the  dead  parts  of  the 
material  the  organic  structure  is  destroyed.* 

2.  Humus,  Black  Soils,  &c. — Long-continued  growth  and  decay  of 
vegetation  upon  a  land-surface  not  only  promotes  disintegration  of  the 
superficial  rock,  but  produces  an  organic  residue,  the  intermingling  of 
which  with  mineral  debris  constitutes  vegetable  soil.  Undisturbed 
through  long  ages,  this  process  has,  under  favourable  conditions,  given  rise 
to  thick  accumulations  of  a  rich  dark  loam.     Such  are  the  "  regur,"  or 

^  Proc.  Liverpool  Oeol.  »Soc.  1884-85. 

2  Conistock,  in  Captain  Jones's  'Reconnaissance  of  N.W.  Wyoming,*  1875,  p.  175. 

»  Brit.  Assoc,  1844,  p.  155. 

*  J.  Walther,  Zeitsch.  Deiitscii.  Oeol.  Oesdl.  xxxvii.  (1885),  p.  329. 


478  DYXAMICAL  GEOLOGY  book  m  part  u 


rich  black  cotton  soil  of  India,  the  "  tchernayzem,"  or  black  earth  of 
Eussia,  containing  from  6  to  10  per  cent  of  organic  matter,  and  the  deep 
fertile  soil  of  the  American  prairies  and  savannahs.  These  formations 
cover  plains  many  thousands  of  square  miles  in  extent.  The  "  tundras  " 
of  northern  latitudes  are  frozen  plains  of  which  the  surface  is  covered  with 
arctic  mosses  and  other  plants.^ 

3.  Peatmosses  and  Bogs.*- — In  temperate  and  arctic  latitudes, 
marshy  vegetation  accumulates  in  sit^  to  a  depth  of  sometimes  40  or  50 
feet,  in  what  are  termed  bogs  or  peat-mosses.  In  northern  Europe  and 
America  these  vegetable  deposits  have  been  largely  formed  by  mosses, 
especially  species  of  SpJuiffnum,  which,  growing  on  hill-tops,  slopes,  and 
valley-bottoms  as  a  wet  spongy  fibrous  mass,  die  in  their  lower  parts  and 
send  out  new  fibres  above.  Some  peaty  deposits  have  been  formed  in 
lakes,  either  by  the  growth  of  aquatic  plants  on  the  bottom,  or  by  the 
precipitation  of  decaying  vegetation  from  the  layer  of  matted  plant- 
growth  which  creeps  from  shore  along  the  surface  of  the  water.^  In  some 
cases,  peat  may  possibly  have  arisen  in  brackish-water  conditions.  There 
are  even  instances  cited  of  marine  peat  formed  of  sea-weeds  {ZosUra^ 
Fucus,  &c.)*  Among  the  Alps,  as  also  in  the  northern  parts  of  South 
America,  and  among  the  Chatham  Islands,  east  of  New  Zealand,  various 
phanerogamous  plants  form  on  the  surface  a  thick  stratum  of  peat 

A  succession  cmi  sometimes  be  detected  in  the  vegetation  out  of  which  the  peat  has 
l)een  formed.  Tlius  in  Euro]:ie,  among  the  1)ottom-layers  traces  of  rush  {Juneus),  sedge 
(/rw),  and  fescue-grass  {F^'siuca)  may  be  observed,  while  not  infrequently  an  underlying 
layer  of  fresh-water  marl,  full  of  mouldering  shells  of  Limnea,  Planorbis,  and  other 
lacustrine  moUusks,  shows  that  the  area  was  originally  a  lake  which  has  been  filled  up 
with  vegetation.  The  next  and  chief  layer  of  the  peat  will  usually  be  found  to  consist 
mainly  of  matted  fibres  of  diirerent  mosses,  i)artieularly  Sphw/nntn,  Pohflrichum,  and 
Bryum,  mingled  with  roots  of  coarse  grasses  and  aquatic  plantjj.  The  higher  layers 
frequently  abound  in  the  remains  of  heaths.     Ev«?ry  stage  in  the  formation  of  jjeat  may 

^  Sec  a  pamphlet,  '  Uber  deu  Humus,'  by  Dr.  vou  Ollech,  Berlin,  Bodo  Gnmdmanii, 
1890.  It  may  be  well  to  take  note  here  again  of  the  extensive  accumulation  of  red  loam  in 
limestone  regions  which  have  long  been  exposed  to  atmospheric  influences.  To  what  extent 
vegetation  may  co-operate  in  the  productiou  of  this  loam,  has  not  been  determiDcd.  Faclu 
believes  that  the  *'  terra  rossa"  is  only  pre^nt  in  dry  climates  where  the  amount  of  hmnnt 
is  small  {ayiiCy  p.  SfiO,  and  authorities  there  cited). 

2  For  a  general  account  see  T.  R.  Jones,  Proc.  (icol.  Assoc,  vi.  (1880),  p.  207.  On  tiie 
composition,  structure,  and  history  of  peat-mosses,  consult  Kennie's  *  Essays  on  Peat-mots,' 
Ediuburgh,  1810;  Steele's  'Natural  and  Agricultural  History  of  Peat-moss,'  £dinlnii|^, 
1826;  Templeton,  Trans,  G&jl,  Soc.  v.  p.  608;  H.  Schinz-Gessner,  'Der  Torf^  &c,' 
Zurich,  1857;  Pokoruy,  Verhand,  Geol.  Reidisanst.  Vienna^  1860;  Senft,  'Humus-, 
Marsch-,  Torf-,  und  Limonit-bildungeu,'  Leipzig,  1862;  G.  Thenius,  *Die  Torftnoore 
Oesterrcichs,'  Vienna,  1874  ;  J.  Geikie,  Trans,  Roy.  Soc.  Rdin.  xxiv.  p.  363.  For  a  list  of 
plants  that  supply  material  for  the  formation  of  peat,  see  J.  Macculloch's  '  Western  Islands,' 
vol.  i.  ;  T.  R.  Jones,  above  quoted  ;  J.  Friih,  ' '  Kritische  BeitrUge  zur  Kenntniss  des  Torfes," 
Jahrb.  O'eul.  Rcichsaiist.  xxxv.  (1885),  p.  677  ;  and  BnU.  Soc.  Botan.  Suisse,  i.  (1891). 

^  For  accounts  of  matted  vegetation  covering  lakes,  see  Land  and  Water,  1876,  pp.  180, 
282. 

■*  J.  MaccuUoch,  *  System  of  Geolog}','  1831,  vol.  ii.  p.  341.  Sirodot,  CompL  rtnd, 
Ixxxvii.  (1878),  p.  267.     Bobierre,  A7in,  Mines,  7me  ser.  x.  (1876),  p.  469. 


ACTION  OF  PLANTS  AND  ANIMALS 


479 


be  observed  where  moeies  are  cot  for  tiiel ;  the  portions  at  the  bottom  are  more  or  lesa 
compact,  dark  brown  or  black,  with  comparatively  little  external  appearance  of  vegetable 
atnichire,  while  those  at  the  top  are  loose,  spoogy,  and  BbrouB,  where  the  living  and 
dead  parts  of  the  mossea  commingle  (Fig.  179). 


.,%4- 


It  frequently  happens  that  remains  of  treen  occur  in  pcat-mossca.  Sometinies 
roots  are  imbedded  in  soil  underlying  the  moHs,  ahoHing  that  the  moss  has  formed  si 
the  growth  of  the  trees  (sre  p.  331).     In  other  cases,  the  roots  and  tniuks  occur  in 


heart  of  the  peat,  proving  that  the  trees  grew  ui">ii  the  mossy  surface,  and  were  finally,  on 
their  decay,  enclosed  in  growing  peat  (Fig.  180).  A  succession  of  trees  has  been  observed 
among  the  Danish  peat-mosses,  the  Scotch  fir  (Pinna  lylvtttris)  and  nhite  birch  {Btljtla 
alba)  being  characteristic  of  the  lower  layers  ;  higher  jiortions  of  the  peat  being  marked 


480  DYNAMICAL  GEOLOGY  book  ni  part  ii 


hy  remains  of  the  oak,  wliile  at  the  top  comes  the  common  1>eech.     Remains  of  the  suae 
kinds  of  trees  are  abundant  in  the  bogs  of  Scotland  and  Ireland. 

The  rate  of  ;^owth  of  jwat  varies  T^ithin  wide  limits.  An  interesting  example  of  the 
formation  and  growth  of  i)eat-mos8  in  the  latter  half  of  the  seventeenth  century  b  on 
record.^  In  the  year  1651  an  ancient  pine-forest  occupied  a  level  tract  of  land  among 
the  hills  in  the  west  of  Ross-shire.  The  trees  were  all  dead,  and  in  a  condition  to  be 
blown  down  by  the  wind.  About  fifteen  ye^rs  later  every  vestige  of  a  tree  had  di»- 
a])pearcd,  the  site  being  occupied  by  a  spongy  green  bog  into  which  a  man  would  rink 
up  to  the  arm-pits,  liefore  the  year  1699  the  tratjt  had  become  firm  enough  to  yield  good 
|>eat  for  fuel.  In  the  valley  of  the  Somme,  three  feet  of  peat  will  grow  in  from  80  to 
40  yeai's.*'^  On  a  moor  in  Hanover,  a  layer  of  jieat  from  4  to  6  feet  thick  formed  in  alKXit 
30  years.  Near  the  I^ke  of  Constance,  a  layer  of  3  to  4  feet  grew  in  24  yeank 
Among  the  Danish  mosses,  a  period  of  250  to  300  years  has  been  required  to  form  a  layer 
10  feet  thick.  Much  must  dejiend  uiK)n  the  climate,  sloi>e,  drainage,  and  soiL  Some 
£uroi>ean  peat-mosses  are  probably  of  extreme  antiquity,  having  begun  to  form  soon  after 
the  surface  was  freed  from  the  snow  and  ice  of  the  glacial  period.  In  the  lower  parts  of 
these  mosses,  traces  of  the  arctic  flora  which  then  oversj^read  so  much  of  the  continent 
are  to  be  met  with.  In  other  instances,  the  mosses  are  at  least  as  late  as  Roman  timc&' 
Change  of  climate  and  likcAi^se  of  drainage  may  stop  the  formation  of  peat,  so  thit 
shnibs  and  trees  spring  up  on  the  firm  surface.  Along  the  Flemish  coast  a  layer  of  pest 
containing  mosses,  rushes,  and  other  fresh-water  ])lantH  underlies  four  or  five  feet  of  clap 
and  sands  with  marine  shells,  indicating  a  subsidence  and  re-elevation  of  the  country.^ 

Peat  -  mosse^s  cover  many  thousand  scpiare  miles  of  Europe  and  North  Americi.' 
About  one-seventh  of  Ireland  is  covered  with  bogs,  that  of  Allen  alone  comprisiiig 
238,5<)0  acres,  with  an  average  depth  of  25  feet  "Where  lakes  are  gradually  converted  into 
bogs,  the  marshy  vegetation  advances  from  the  shores,  and  sometimes  forms  a  matted 
treacherous  grevn  surface,  l)eneath  which  the  waters  of  the  lake  still  lie.  The  decayed 
vegetable  matter  from  the  under  part  of  this  crust  sinks  to  the  bottom  of  the  water,  fonn- 
ing  there  a  fine  i^aty  nnid,  which  slowly  grows  upward.  Eventually,  as  the  spongf 
covering  spreads  over  the  lake,  a  layer  of  brown  muddy  water  may  be  left  between  the 
still  growing  vegetation  above  and  the  muddy  dejiosit  at  the  bottom.  Heavy  rains,  by 
augmenting  this  intermediate  watery  layer,  sometimes  make  the  centre  swell  up  until  tlie 
matted  skin  of  moss  bursts,  and  a  deluge  of  black  mud  iK)urs  into  the  surrounding  country. 
The  inuHilated  ground  is  covered  i)ennanently  with  a  layer  of  black  (leaty  earth. 

From  the  treacherous  nature  of  their  surfac<\  ]>eat-mosses  have  frequently  been  the 
receptacles  for  bodies  of  men  and  animals  that  ventured  u]>on  them.  As  peat  possesws 
great  antiseptic  power,  these  remains  are  usually  in  a  stat«  of  excellent  preservation. 
In  Irelantl,  the  remains  of  the  extinct  large  Irish  elk  {Megactros  hibemieus)  have  been 
dug  up  from  many  of  the  bogs.     Human  weapons,   tools,  and  ornaments  have  been 


'  Rarl  of  Cromarty,  PhiL  Tmns,  xxvii. 

"  J.  Kolb,  Pirn:.  Inat.  Civ.  Eagin.  xl.  (1875),  p.  35. 

^  On  mosses  of  Flanders  and  north  of  France  see  H.  Debroy,  BulL  Soc.  OSoL  iVniKr, 
3me  Sir.  ii.  p.  46.  Ann.  .Soc.  WM.  Sord,  1870-74,  p.  19.  Lorie,  Arch.  Mus.  Teyler^  2me irf. 
iii.  part  5  (1890),  i»p.  423,  439.  Below  the  moors  of  Oldenburg,  Roman  coins,  weapons,  and 
plank-roads  are  found  at  a  depth  of  13  feet  and  upwards  {Pettmianns  MitdUU.  1883,  t.) 
On  the  Bolieniian  peat-bogs,  F.  Sitensky,  Archiv  Landesclurch-forscJi.  Bohmen^  vi.  (1891); 
on  those  lying  east  of  the  Chrlstiania  Fjord,  G.  E.  Stangeland,  *  TorvmjTer,'  Xorges  Otob»g. 
rniU'i'sHg.  1892  ;  on  those  of  Schleswig-Holstein,  R.  v.  Fischer- Benzon,  Ahk,  XaturKm. 
\\-r.  Hamburg,  xi.  (1891). 

■*  Ann.  Mines  J  7  me  s^'t.  x.  p.  468. 

''  For  an  account  of  the  fi-esh-water  morasses  and  swamps  of  the  United  States 
Shaler,  lOth  Ann.  licjt.  C'.S.  (/col  Surv.  1890,  p.  255. 


r.  iii  §  3 


ACTION  OF  PLANTS  AND  ANIMALS 


481 


omed  from  pest-moeMe  ;  likewise  cnnnoges,  or  pile -dwellings  (construGtwl  in  the 
ins]  lakes  that  preceded  tha  mosaeB),  and  canoea  hollowed  out  of  single  treea. 

4,  Mangrove-SwampB.  —  On  the  low  moist  shores  and  river- 
aths  of  tropical  countries,  the  mangrove-tree  plays  an  important 
logical  part  It  grows  in  such  situations  in  a  dense  jungle,  some- 
ea  twenty  miles  broad,  which  fringes  the  coast  as  a  green  selvage, 
1  nins  up,  if  it  does  not  quite  occupy,  creeks  and  inlets.  The  maU' 
ve  flourishes  in  sea-water,  even  down  to  low-water  mark,  forming 
re  a  dense  thicket,  which,  as  the  trees  drop  their  radicles  and  take 
t,  grows  outward  into  the  sea.  It  is  singular  to  find  terrestrial  birds 
tling  in  the  branches  above,  and  crabs  and  barnacles  living  among  the 
tfl  below.  By  this  network  of  subaqueous  radicles  and  roots,  the 
XT  that  flows  off  the  land  is  filtered  of  its  sediment,  which,  retained 
ing  the  vegetation,  helps  to  turn  the  spongy  jungle  into  a  firm  soil.'    On 

coast  of  Florida,  the  mangrove  swamps  stretch  for  long  distances,  as  a 
t  from  five  t«  twenty  miles  broad,  which  winds  round  the  creeks  and 
its.  At  Bermuda,  the  mangroves  co-operate  with  grasses  and  other 
nta  to  choke  up  the  creeks  and  brackish  lakes.  In  these  waters  cal- 
eous  alf,Ee  abound  and  as  their  remains  are  thrown  up  amidst  the  sand 
I  vegetation   they  form  a  remarkablpralcareous  soil  (pp  138    337).^ 

5.  Diatom  Larth  or  Ooze  — As  the  mmute  siliceous  plants 
ed  diatoms  occur  both  in  fresh  and  salt  water  the  deposit  formed 
n  their  congregated  remains  is  found  both  on  the   sites  ol   lakes  and 


the  sea-floor.  The  most  extensive  terrestrial  accumulations  of  this 
ure  now  in  course  of  formation  are  probably  those  of  the  warm  water 
rshes  supplied  by  the  hot  springs  of  the  Yellowstone  Park,  where  the 
y  deposits  and  drier  meadows  cover  many  square  miles,  sometimes  to 
epth  of  six  feet.^  "Infusorial"  earth  and  "tripoli  powder"  consist 
inly  of  the  frustules  and  fragmentary  debris  of  diatoms,  which  have 
umulated   on    the  bottoms  of  lacustrine  areas,  the   purer   varieties 


'  For  an  accoont  of  the  growth  o 
1.  Oeoi.  SUTT.  1890,  p.  291. 
'  See  Nelson.  Q.  J.  Oeol.  Soc,  ix. 
1872-3,  p.  139  ;   Wyville  ThomB 
<  W.  H.  WeeJ.  iiotaitiaU  UmetU 


mangrove  swamps,  see  K.  S.  Shaler.  lOIA  A 


I.  Rep. 


.  200  el  ttq. ;  J.  J.  Kcin,  Btriehl  Senekaib.  Xaturf. 
[■a  '  Atlaatic,'  L.  ]'.  2B0.      (See  anie,  pp.  128,  337.) 
[iv.  (1889),  p.  117. 


482  DYNAMICAL  GEOLOGY  book  hi  part  ii 

containing  90  to  97  per  cent  of  silica.  They  form  beds  sometimes  upwards 
of  30  feet  thick.  (Richmond,  Virginia ;  Bilin,  Bohemia ;  Aberdeen- 
shire.) Diatamacece  occur  in  abundance,  both  in  the  surface-waters  of  the 
ocean  and  on  the  bottom.  In  the  Ai'ctic  Ocean  and  in  the  seas  around  the 
Shetland  Islands  living  diatoms  sometimes  form  vast  floating  banks  of  a 
yellowish  slimy  mass,  which  impedes  the  prosecution  of  the  herring 
fishery.^  The  frustules  of  these  plants  accumulate  at  depths  of  from 
1260  to  1975  fathoms,  as  a  pale  straw-coloured  deposit^  which  when 
dried  is  white  and  very  light  (Fig.  181).^ 

6.  Chemical  Deposits. — But,  besides  giving  rise  to  new  formatioDs 
by  the  mere  accumulation  of  their  remains,  plants  do  so  also  both 
directly  and  indirectly  by  originating  or  precipitating  chemical  solutions. 
The  most  conspicuous  example  of  this  action  is  the  production  of  cak- 
sinter.  Some  plants  (several  species  of  67wira,  for  instance)  have  the 
power  of  decomposing  the  carbonic  acid  dissolved  in  water,  and  pre- 
cipitating calcium-carbonate  within  their  own  cell  walls.  Others  (such 
as  the  mosses  Hypnum,  Bri/um,  &c.^)  precipitate  the  carbonate  as  an  in- 
organic incrustation  outside  their  own  substance.  Some  observers  have 
even  maintained  that  this  is  the  normal  mode  of  production  of  calc-sinter 
in  large  masses  like  those  of  ^ivg^  It  is  certainly  remarkable  that 
this  substance  may  be  observed  encnisting  fibrous  bunches  of  moss 
{Hypnum,  &c.),  when  it  can  be  found  in  no  other  part  of  the  water-course, 
and  this,  too,  at  a  spring  containing  only  0*034  of  carbonate.  It  is 
evident  that  if  the  deposit  of  calc-sinter  were  due  to  mere  evaporati<Hi, 
it  would  be  more  or  less  equally  spread  along  the  edges  and  shallow  parts 
of  the  channel.  It  appear  to  arise  first  from  the  decomposition  of  dissolved 
carbonic  acid  by  the  living  plants,  and  it  proceeds  along  their  growing 
stems  and  fibres.  Subsequently,  evaporation  and  loss  of  carbon-dioxide 
cause  the  carbonate  to  be  precipitated  over  and  through  the  fibroos 
sinter,  till  the  substance  may  become  a  solid  crystalline  stone.  Varieties 
of  sinter  are  traceable  to  original  differences  in  the  plants  precipitating  it 
Thus  at  Weissenbrunnen,  near  Schalkau,  in  Central  Germany,  a  cavernous 
but  compact  sinter  is  made  by  Hypnum  violltiscum,  while  a  loose  porous 
kind  gathers  upon  Didymodon  capillaceus} 

Some  marine  alga;,  as  above  noticed,  abstract  calcium-carbonate  from 
sea -water  and  build  it  up  into  their  own  substance.  A  nuUipore 
{Lithothamniuvi  nodosum)  has  been  found  to  contain  about  84  per  cent  of 
calcium-carbonate,  5|  of  magnesium-carbonate,  with  a  little  phosphoric 
acid,  alumina,  and  oxides  of  iron  and  manganese.^     Vegetable  life  has 

^  Murray  and  Irvine,  Proc.  Roy.  Soc.  Edin.  xviii.  (1891),  p.  231.  On  the  source  whence 
marine  plants  and  animals  obtain  their  silica,  see  anle^  p.  450,  and  poUea,  p.  494. 

-  Messrs.  Murray  and  Irvine  estimate  the  area  of  sea-bottom  covered  with  diatom  con 
at  10,420,600  sijuare  miles,  and  the  mean  depth  of  the  surface  of  the  deposit  at  1477 
fathoms  below  Kca-level,  Proc.  Roy.  Soc.  Edin.  xvii.  (1889),  p.  82. 

*  Also  i>hauerogams,  as  Ranunculus  and  Potaviogeion. 

^  See  V.  Schauroth,  Z.  Dcutsch.  Ocd.  Oes.  ili.  (1S51),  p.  137.  Cohn,  jVetce*  /oAi*. 
1864,  p.  580,  gives  some  interesting  information  as  to  the  plants  by  which  the  sinter  ii 
fornieil,  and  their  work.     In  Scotland  Hypnum  covimutatum  is  a  leading  sinter-former. 

*  Giimbel,  Abhandi.  Bayerisch.  Akcui.  Wissensch.  xi.  1871. 


SECT,  iii  §  3  ACTION  OF  PLANTS  AND  ANIMALS  483 

likewise  the  power  of  precipitating  silica  from  solution  in  hot  springs 
and  forming  siliceous  sinter.  In  the  geyser  district  of  the  Yellowstone 
Park  it  has  been  ascertained  that  the  extensive  sinter  deposits  are  largely 
formed  by  vegetation,  which  causes  the  siliceous  material  to  be  thrown 
down  as  a  stifi*  gelatinous  substance,  in  many  varied  forms.  Algae  are 
chiefly  concerned  in  this  process.  On  the  death  of  the  plant  the  jelly-like 
mass,  which  consists  of  the  siliceous  filaments  of  the  algse  and  their  slimy 
envelope,  loses  part  of  its  water,  becomes  cheese-like  in  consistency,  and 
finally  hardens  into  stone.  ^ 

In  the  formation  of  extensive  beds  of  bog-iron-ore,  the  agency  of 
vegetable  life  is  of  prime  importance.  In  marshy  flats  and  shallow  lakes, 
where  the  organic  acids  are  abundantly  supplied  by  decomposing  plants, 
the  salts  of  iron  are  attacked  and  dissolved.  Exposure  to  the  air  leads 
to  the  oxidation  of  these  solutions,  and  the  consequent  precipitation  of 
the  iron  in  the  form  of  hydrated  ferric  oxide,  which,  mixed  with  similar 
combinations  of  manganese,  and  also  with  silica,  phosphoric  acid,  lime, 
alumina,  and  magnesia,  constitutes  the  bog -ore  so  abundant  on  the 
lowlands  of  North  Germany  and  other  marshy  tracts  of  northern 
Europe. 2  On  the  eastern  sea-board  of  the  United  States,  large  tracts  of 
salt  marsh,  lying  behind  sand-dunes  and  bars,  form  receptacles  for 
much  active  chemical  solution  and  deposit.  There,  as  in  the  European 
bog- iron  districts,  ferruginous  sands  and  rocks  containing  iron  are 
bleached  by  the  solvent  action  of  humus  acids,  and  the  iron  removed 
in  solution  is  chiefly  oxidized  and  thrown  down  on  the  bottom.  In 
presence  of  the  sulphates  of  sea-water  and  of  organic  matter,  the  iron 
of  ferruginous  minerals  is  partially  changed  into  sulphide,  which  on 
oxidation  gives  rise  to  the  precipitation  of  bog-iron.^  The  existence  of 
beds  of  iron-ore  among  sedimentary  formations  aff'ords  strong  presumption 
of  the  existence  of  contemporaneous  organic  life  by  which  the  iron  was 
dissolved  and  precipitated. 

The  humus  acids,  which  possess  the  power  of  dissolving  silica, 
precipitate  it  in  incrustations  and  concretions.  Julien  describes  hyalite 
crusts  at  the  Palisades  of  the  Hudson,  due,  as  he  thinks,  to  the  action 
of  the  rich  humus  upon  the  fallen  debris  of  diabase.  The  frequent 
occurrence  of  nodules  of  flint  and  chert  in  association  with  organic 
remains,  the  common  silicification  of  fossil  wood,  and  similar  close 
relations  between  silica  and  organic  remains,  point  to  the  action  of 
organic  acids  in  the  precipitation  of  this  mineral.  This  action  may 
consist  sometimes  in  the  neutralisation,  by  organic  acids,  of  alkaline 
solutions  charged  with  silica ;  *  sometimes  in  the  solution  and  re- 
deposit  of  colloid  silica  by  albuminoid  compounds,  developed  during 
the  decomposition  of  organic  matter  in  deposits  through  which 
silica    has   been  disseminated,   the    deposit   taking   place  preferentially 

*  W.  H.  Weed,  Ninth  Ann.  Rep.  U.  S.  Geol.  Survey,  1889.     Amer.  Joum.  Set.  xxxvii. 
(1889),  p.  351. 

*  Forchhammer,  Neues  Jahrb.  1841,  p.  17,  ante,  p.  146. 
'  Julien,  Amer.  Assoc.  1879,  p.  347,  and  ante,  p.  455. 

*  Leconte,  Amer.  Joum.  Set.  1880,  p.  181. 


484  IJ  YXA  MICA  L  GEOL  OG  Y  book  hi  part  ii 


round  some  decaying  organism,  or  in  the  hollow  left  by  its  re- 
moval.^ 

Animals. — Animal  formations  are  chiefly  composed  of  the  remum 
of  tlio  lower  grades  of  the  animal  kingdom,  especially  of  Molluscn, 
Adiiwzoa,  and  Fomminifera. 

1.  Calcareous. — Lime,  chiefly  in  the  form  of  carbonate,  is  the 
mineral  substance  of  which  the  solid  parts  of  invertebrate  animals  are 
mainly  built  up.  The  proportion  of  carbonate  of  lime  in  sea-water  is  so 
small  as  to  have  presented  a  difficulty  in  the  endeavour  to  account  for 
the  vast  (quantities  of  this  substance  eliminated  by  marine  organisms.  Mr. 
J.  Y.  Buchanan,  however,  has  suggested  that  the  testaceous  denizens  of 
the  sea  assimilate  their  lime  from  the  gypsum  dissolved  in  sea-water, 
forming  sulphide  in  the  interior  of  the  animal,  which  is  transformed  into 
carbonate  on  the  outside.'^  Messrs.  Murray  and  Irvine  have  experi- 
mentally proved  that  sea-animals  can  secrete  carbonate  of  lime  from  sei- 
water  from  which  carbonate  of  lime  is  rigidly  excluded,  and  thus  that 
the  other  lime  salts,  notably  the  sulphate,  are  made  use  of  in  the  process. 
They  infer  that  the  living  tissues  of  the  lower  animals  and  the  effete 
secretions  of  higher  forms,  produce  carbonate  of  ammonia,  which  in 
presence  of  the  sulphate  of  lime  of  soa-wat€r  becomes  carbonate  of  lime 
and  sulphate  of  ammonia.^  The  great  majority  of  the  accumulations 
formed  of  animal  remains  are  calcareous.  Those  organisms  which  secrete 
their  lime  as  calcite  produce  much  more  durable  skeletons  or  tests  than 
those  which  accumulate  it  in  the  form  of  aragonite.  Hence  among 
geological  formations  aragonite  shells  have  in  large  measure  disappeared.^ 

In  fresh  water,  accumulations  of  animal  remains  are  represented  by 
the  marl  of  lakes — a  white,  chalky  deposit  consisting  of  the  mouldering 
remains  of  MollnsiVy  Entomostrara,  and  partly  of  fresh-water  algce.  On 
the  sea-bottom,  in  shallow  water,  they  consist  of  beds  of  shells,  as  in 
oyster-banks.  Under  favourable  conditions,  extensive  deposits  of  lime- 
stone are  now  being  formed  on  the  sea-floor  in  tropical  latitudes.  Mr. 
Muniiy,  from  observations  made  during  the  Challenger  voyage,  estimates 
that  in  a  square  mile  of  the  tropical  ocean  down  to  a  depth  of  100 
fathoms  there  are  more  than  1 6  tons  of  calcareous  matter  in  the  form  of 
animal  and  vegetable  organisms.^  These  surface  organisms,  when  dead, 
are  continually  falling  to  the  bottom,  where  their  remains  accumulate  as 
a  soft  ooze.  On  the  floor  of  the  West  Indiari  seas,  where  an  extra- 
ordinarily abundant  fauna  is  supjKjrted  by  the  plentiful  supply  of  food 
brought  by  the  great  ocean  currents  which  enter  that  region  from  the 
South  Atlantic,  a  calcareous  deposit  is  being  formed  out  of  the  hard  parts 
of  the  animals  that  live  on  the  l)ottom  (mollusks,  echinoderms,  corals, 

>  Julien,  ojK  cit.  .S96.  Sollas,  Ann.  Mufj.  Xat.  Hist.  Nov.  Dec.  1880.  J.  Roth, 
'  Allgein.  C'heiu.  Geologic,'  i).  576,  and  Dr.  von  Ollech's  pami)hlet  cited  ante,  p.  478. 

-  Jirit.  Assoc.  1881,  sects,  p.  584. 

='  Pruc.  Hoy.  Soc.  Edin.  xvii.  (1889),  p.  89. 

*  Sorby,  Presidential  Address  Geol.  Soc.  18/9  ;  P.  F.  Kendall,  Geol,  Mag,  1883,  p.  497; 
V.  Cornish  and  P.  F.  Kendall.  Ueol.  Mag,  1888,  p.  60.     i^e  pastea,  Book  V.  §  ii.  2. 

^  Prtn;.  Roy.  Stn:.  Edin.  x.  (1880).  p.  608. 


§  3  CORAL-REEFS  486 

ids,  annelids,  Crustacea,  &c.),  mingled  with  what  may  fall  from 
er  water.  This  deposit  accumulates  as  a  vast  submarine  plateau 
3  of  broad  banks,  and  is  comparable  in  extent  to  some  of  the  more 
nt  limestones  of  older  geological  time.  Some  portions  of  it  have 
id  there  (Barbados,  Guadeloupe,  Cuba,  &c.)  been  elevated  above 
,  so  that  its  composition  and  structure  can  be  studied.  The 
ns  in  these  upraised  limestones  are  the  same  as  those  which  still 
1  form  a  similar  limestone  in  the  surrounding  seas.  In  Yucatan 
c  is  perforated  with  caverns,  one  of  which  is  70  fathoms  deep.^ 
e  and  there  considerable  deposits  of  broken  shells  have  been  pro- 
y  the  accumulation  of  the  excrement  of  fishes,  as  Verrill  has  pointed 
.he  north-eastern  coasts  of  the  United  States.  Deposits  of  broken 
aised  above  sea-level  either  by  breakers  and  winds  or  by  sub- 
in  movements,  are  solidified  into  more  or  less  compact  shelly 
la  Extensive  beds  of  this  nature,  composed  mainly  of  species  of 
Uraria,  Mactra,  i^c,  form  islands  fronting  the  shores  of  Florida,  and 

underlie  the  soil  of  that  State.  Some  of  the  shells  still  retain 
lours.  The  whole  mass  is  in  layers  1  to  18  inches  thick,  quite 
3re  exposure  to  the  air,  but  hardening  thereafter,  and  much  of  it 
ng  a  confused  crystallization.^  It  is  known  locally  as  Coquina. 
:areous  dunes  of  Bermuda  have  been  already  referred  to  (p.  336). 
Jrreefs.^ — But  the  most  striking  calcareous  formations  now  in 
;  are  the  reefs  and  islands  of  coral.  These  vast  masses  of  rock 
Qed  by  the  continuous  growth  of  various  genera  and  species  of 
n  tracts  where  the  mean  temperature  is  not  lower  than  68°  Fahr. 
owth  is  prevented  by  colder  water,  and  by  the  fresh  and  muddy 
lischarged   into   the  sea  by  large  rivers.     One   of  the   essential  _. 

ns  for  the  formation  of  coral-reefs  is  abundance  of  food  for  the  11 

ders,  and  this  seems  to  be  best  supplied  by  the  great  equatorial 
^  It  is  observed  that  on  the  eastern  coasts  of  Africa,  Central 
k,  and  Australia,  bathed  by  ocean  currents,  extensive  coral-reefs 

while  on  the  western  coasts,  in  corresponding  latitudes,  where 
powerful  currents  flow,  only  isolated  patches  of  coral  exist.* 


.gassiz,  Amer,  Acad.  xi.  (1882),  p.  Ill  :  and  his  "Three  Cruises  of  the  Blake,** 
).  Rogers,  Brit.  Assoc.  RejK  1834,  p.  11. 

Darwin,  *The  Structure  and  Distribution  of  Coral  Islands,'  1842  ;  2nd  edit.  1874  ; 
)ral8  and  Coral  Islands,'  1872  ;  2nd  edit.  1890  ;  Jukes's  'Narrative  of  Voyage  of 
7y,'  1847  ;  C.  Semper,  Zeitsch.  Wissen,  Zool.  xiil.  (1863),  p.  558  ;  Verhandl.  Phys. 
dlscA.  Wiirzburg,  Feb.  1868  ;  'Die  Philippinen  und  ihre  Bewohner,'  1869,  p.  100  ; 
I,  Senckenh.  Xaturf.  Oes.  Wiirzburg,  1869-70,  p.  157.  Murray,  Proc,  Roy.  Soc. 
K  605,  xvii.  (1889),  p.  79  ;  A.  Agassiz,  Mem.  Amer,  Acad.  xi.  (1882),  p.  107  ;  Bxdl. 
vpar.  Zool.  Jfannrd,  1889,  No.  3.  C.  P.  Sluiter,  on  the  coral-reefs  of  the  Java  Sea, 
nd.  Tijd.  Nederlatuhch.  Indie,  xlix.  (1890) ;  J.  Walther,  on  the  coral-reefs  of  the 
Qsula,  Ahharul.  Math.- Phys.  KOn.  Sachs.  Oesell.  xiv.  (1888)  ;  H.  B.  Guppy,  Trans, 
Edin.  xxxii.  (1885),  'The  Solomon  Islands,'  1887  ;  J.  C.  Bourne,  Nature,  1888,  pp. 
i  J.  C.  Wharton,  ibid.  p.  393  ;  A.  Heilprin,  '*The  Bermuda  Islands,"  1889,  Proe, 
i,  Scu  Philadelphia,  1890,  j).  303  ;  Jukes  Brown  and  Harrison,  Barbados,  Quart, 
cl.  Soc.  xlvii.  (1891),  p.  197  ;  Walther,  Peterm.  Mitth.  Krgdnz,  No.  102  (1891). 
Lgassiz,  Amer.  Acad.  xi.  (1882),  p.  120. 


I 


486  DYNAMICAL  GEOLOGY  book  m  part  ii 

Darwin  and  Dana  have  shown  that  reef-building  corals  cannot  live  at 
depths  of  more  than  about  fifteen  or  twenty  fathoms  ;  they  appear,  indeed, 
not  to  thrive  below  a  depth  of  six  or  seven  fathoms.  They  cannot  survive 
exposure  to  sun  and  air,  and  consequently  are  unable  to  grow  above  the 
level  of  the  lowest  tides.  They  are  likewise  prevented  from  growing  by 
the  presence  of  much  mud  in  the  water.  Various  observations  and 
estimates  have  been  made  of  the  rate  of  growth  of  coraL  Individual 
specimens  of  Ma^andrina  have  been  found  to  increase  from  half  an  inch 
to  an  inch  in  a  year,  and  others  of  Madrepora  have  grown  three  inches 
in  the  same  time.^  Specimens  of  Orbicella,  Manicina,  and  Isophyllia, 
taken  from  the  submarine  telegraph -cable  between  Havana  and  Key 
West^  showed  a  growth  of  from  one  to  two  and  a  half  inches  in  about 
seven  years.  A.  Agassiz  estimates  that  in  the  Florida  reef  the  corak 
could  build  up  a  reef  from  a  depth  of  seven  fathoms  to  the  surface  in 
1000  or  1200  years.2  When  coral-reefs  begin  to  grow,  either  fronting 
a  coast-line  or  on  a  submarine  bank,  they  continue  to  advance  outward, 
the  living  portion  being  on  the  outside,  while  on  the  inside  the  mass 
consists  of  dying  or  dead  coral,  which  becomes  a  solid  white  compact 
limestone.  In  the  coral  area  of  the  Pacific  there  are,  according  to  Dana, 
290  coral-islands,  besides  extensive  reefs  round  other  islands.  The 
Indian  Ocean  contains  some  groups  of  large  coral-islands  ;  others  occor 
in  the  Hed  Sea.  Reefs  of  coral  occur  less  abundantly  in  the  tropical 
paits  of  the  Atlantic,  among  the  West  Indian  Islands  and  on  the  Florida 
coast,  but  they  are  absent  from  the  Pacific  side  of  Central  America — a 
fact  attributed  by  Prof  Agassiz  not  to  a  cold  marine  current^  as  suggested 
by  Prof.  Dana,  but  to  the  enormous  amount  of  mud  poured  into  the  sea 
on  this  side  during  the  rainy  season.^  The  great  reef  of  Australia  is  1250 
miles  long  and  from  10  to  90  miles  broad. 

Coral-rock,  though  formed  by  the  continuous  growth  of  the  polyps, 
gradually  loses  any  distinct  organic  structure,  and  acquires  an  internal 
crystalline  character  like  an  ancient  limestone,  owing  to  the  infiltration  of 
water  through  its  mass,  whereby  calcium-carbonate  is  carried  down  and 
deposited  in  the  pores  and  crevices,  as  in  a  growing  stalactita  Great 
quantities  of  calcareous  sand  and  mud  are  produced  by  the  breakers  which 
beat  upon  the  outer  edge  of  the  reefs.  This  detritus  is  partly  washed  up 
upon  the  reefs,  where,  being  cemented  by  solution  and  redeposit^  it  aids 
in  their  consolidation,  sometimes  acquiring  an  oolitic  structure ;  *  but 
much  of  it  is  swept  away  by  the  ocean  currents  and  distributed  over  the 
sea-floor,  the  water  becoming  milky  ^ritli  it  after  a  stonn.*     Around  vol- 

1  Dana,  'Corals  and  Coral  Islamla,'  2nd  edit.  1890,  p.  123. 

-  Amer,  Acaif.  xi.  (1882),  p.  129.  See  also  Bidl.  Alus.  Comp.  Zocl,  Harvard^  xx.  (18«0), 
p.  Gl. 

3  Bull.  Mhh.  Comp.  Zoof.  xxiii.  (1892),  p.  70. 

*  See  Dana's  'Corals  and  Coral  Islands,'  pp.  152,  194  ;  A.  Agassiz,  Mem.  Amer,  Atad, 
xi.  (1882),  p.  128. 

^  A.  Agassiz  mentions  that  after  a  stomi,  the  sea  is  sometimes  discoloured  by  this  silt  to 
a  distance  of  six  to  ten  niile^  from  the  outer  reef,  and  he  adds  that  he  has  seen  between  two 
and  three  inches  of  tine  silt  deposited  in  the  inter\'al  between  two  tides  after  a  pndongcd 


CORAL-REEFS 


canic  islands  much  lava-detritiM  may  be  mixed  with  the  coral-sand  and 
mud.     Thus  at  Hawaii,  where  great  abrasion  by  the  waves  takes  place 


on  the  ends  of  the  lava-streama  which  have  run  out  to  sea,  large  quanti- 
ties of  olivine  sand  are  formed,  the  grains  of  this  mineral  vatying  from 


irt  of  Keeling  Atoll,  Inilian  OcBm  fytttt  Duwln). 
Th[  wbIM  portion  repra«iiU  the  reef  Bbova  iM-levcl,  the  Inner  «h»ded  ipue  theUgoon.  of  nhieli 
the  iteepeat  portion  la  mirkiit  by  the  darker  tint. 

the  size  of  a  bean  or  pea  downwards  to  the  finest  particles.     This  sand 

Itorm  :  Amer.  Acad.  xi.  )>.  126.  The  total  area  of  )!ea-f1oor  corerctl  irith  coral  Mnd  aod 
mud  is  eitimated  b^  Messrs.  Murray  nnil  Irvine  at  3.219.S00  square  miles.  Froc.  Roy. 
Soc  Edin.  xvii.  (1889).  p.  82. 


nrXAMICAL  OKOLOGY  book  hi  part  ii 

becomes  mixed  with  the  coral  detritiu,  and 
is  alao  iiiterstratified  with  it  in  layers.* 

As  already  mentioned  (p.  290^  thefor- 

raation  of  coral-islands  has  bean  explained 

;:       by  Darwin  on  the  hypothesis  of  a  suhsi- 

7  dence  of  the  sea-floor.  The  circular  islands, 
^      or  atolls,  rising   in  mid-ocean,  have  the 

8  general  aspect  shown  in  Fig.  182.  Their 
°  external  funu  may  he  understood  from  the 
I  chart  (Fig.  183),  and  their  structure  and 
s  the  character  of  their  surface  from  the 
I  section  (Fig.  184).  They  rise  with  some- 
.=  times  tolerably  steep  slopes  from  a  depth 
I  of  '2000  feet  and  upwards,  until  they 
£  reach  the  surface  of  the  sea.  But  as  the 
I  coral  polyps  do  not  live  at  a  greater  depth 
^  .<;  than  about  15  or  20  fathoms,  and  could  not 
&  ^    have    grown   upward    therefore  from   the 

^  ^.1  bottom  of  a  deep  soa,  Darwin  inferred  thtt 
^  S  I  the  sites  of  these  coral-reefs  had  undergone 
J  V  ^  a  progressive  subsidence,  the  rate  of  their 
upward  growth  keeping  pace,  on  the 
whole,  with  that  of  their  depreaaion.  On 
this  view,  what  is  termed  a  /Wn^njr  B^ 
(a  b,  Fig.  185)  would  first  be  formed  front 
ing  the  land  (l)  between  the  limit  of  the 
20-fathom  line  and  the  sea-level  (s  t\ 
'"It  (iFowing  upwanl  until  it  reached  the  snr- 
%  f  face  of  the  water,  it  would  be  exposed  to 
I  -  the  dash  of  the  waves,  which  wotild  Imst 
I  olT  pieces  of  the  coral  and  heap  them  upon 
~L  the  reef.  In  this  way  islets  vonld  bt 
£  formed  upon  it,  which,  by  succeasiTe  sc- 
\  cumulations  of  materials  thrown  ap  by 
f  the  breakers  or  brought  by  winds,  would 
^  remain  permanently  above  water.  On 
^  these  islets,  irnlms  and  other  plants,  whose 
*  Rccds  might  be  drifted  from  distant  or 
^  adjoining  land,  would  take  root  and 
£  flourish.  Inside  the  reef,  there  would 
-  be  a  shallow  channel  of  water,,  com- 
municating, tlirough  gaps  in  the  reef,  with 

'  W.  L.  Green,  Jtnira.    Hoy.    Utd.   Soe.  Iniead, 

iv.  (I8H7),  i>,  140.     Thi9  author  niggMtiTelT  point) 

out  t1i«  rewmliUnce  of  such  a  mingling  of  calcannu 

material  nnd  iiiBgiieaian  liHcato  to  thfe  ntingled  limt- 

m.l  oiihicttlcitwof  thecrj'stalline  echists.    SoUm,  Proe.  Rogal  DutliK  *t 


13  = 
I  =^ 

5  II 


HECT.  iii  g  3  CORAL-REEFS  489 

the  main  ocean  outside  Fringing  reefs  of  this  character  are  of  common 
occurrence  at  the  present  time  In  the  case  of  a  continent^  they  front  its 
coast  for  a  long  distance,  but  they  mar  entirely  surround  an  island 


If,  according  to  the  Darwinian  explanation,  the  site  of  a  fnnging  reef 
undei^oes  depression  at  a  rate  sufficiently  slow  to  allow  the  corals  to  keep 


ifter  Beech  y). 


pace  with  it,  the  reef  may  be  conceived  to  grow  upward  as  fast  as  the 
bottom  sinks  downward.  As  the  reef  grows  mainly  on  its  upper  seaward 
edge,  the  lagoon  channel  inside  will  become  deeper  and  wider,  while,  at 
the  same  time,  the  depth  of  water  outside  will  increase  until  a  Barrier 


490  DYNAMICAL  GEOLOGY  book  in  pabtii 

Reef  {a!  b',  Fig.  185)  is  formed.  In  Fig.  186,  for  example,  the  Gambler 
Islands  (1248  feet  high)  are  shown  to  be  entirely  surrounded  by  an 
interrupted  barrier  reef,  inside  of  which  lies  the  lagoon.  Prolonged  slow 
depression  would  continually  diminish  the  area  of  the  land  thus  encircled, 
while  the  reef  might  retain  much  the  same  size  and  position.  At  last  the 
final  peak  of  the  original  island  might  disappear  under  the  lagoon  (c,  Fig. 
185),  and  an  Atoll,  or  true  coral-island,  would  be  formed  (a"  a",  Fig.  185, 
and  Figs.  182  and  183).  Should  any  more  rapid  or  sudden  downward 
movement  take  place,  it  might  carry  the  atoll  down  beneath  the  surface, 
like  the  Great  Chagos  bank  in  the  Indian  Ocean,  which  is  a  submarine  atoll. 

This  simple  and  luminous  explanation  of  the  histor}'  of  coral-reefs 
accorded  w^ell  with  all  the  known  facts,  and  led  up  to  the  impressive  con- 
clusion that  a  vast  area  of  the  Pacific  Ocean,  fully  6000  geographical 
miles  from  east  to  west,  has  undergone  a  recent  subsidence,  and  may  be 
slowly  sinking  still. 

Mr.  Darwin's  views  having  been  universally  accepted  by  geologists, 
coral-islands  have  b^en  regarded  with  special  interest  as  furnishing 
proof  of  vast  oceanic  subsidence.  In  the  year  1868,  C.  Semper  pointed 
to  some  cases  of  atolls  which,  he  said,  could  not  be  explained  by 
Darwin's  theory.  The  Pelew  Islands,  at  the  western  end  of  the  Caroline 
archipelago,  show  tnie  atolls  at  their  northern  extremity,  whOe  at 
their  southern  end,  only  60  miles  away,  there  are  raised  coral-reefs, 
and  an  island  entirely  destitute  of  reefs.  Semper  considered  that  the 
atolls  had  grown  up  under  the  infiuence  of  peculiar  conditions  of  marine 
currents  and  erosion,  simultaneously  with  elevation  rather  than  sub- 
sidence.^ In  1870  J.  J.  Rein  cited  the  case  of  Bermuda  as  one  capable 
of  explanation  by  upgrowth  of  calcareous  accumulations  from  the  bottom 
without  subsidence.*^  More  recently,  Mr.  Murray,  whose  researches 
in  the  Chnlknijer  Expedition  led  him  to  make  detailed  examination 
of  many  coral  reefs,  has  suggested  that  barrier-reefs  do  not  necessarily 
prove  subsidence,  seeing  that  they  may  grow  outward  from  the  land 
upon  the  top  of  a  talus  of  their  o>\ti  debris  broken  down  by  the  waves, 
and  may  thus  appear  to  consist  of  solid  coral  which  had  grown  upward 
from  the  bottom  during  depression,  although  only  the  upper  layer,  20 
fathoms  or  thereabouts  in  thickness,  is  composed  of  solid,  unbroken  coral 
growth.  He  points  out  that  in  the  coral-seas  the  islands  appear  to  have 
always  started  on  volcanic  ejections,  at  least  that  all  the  non-calcareous 
rock  now  visible  is  of  volcanic  origin.  Where  the  submarine  peak  lay 
below  the  inferior  limit  of  coral  growth,  it  may  have  been  brought  up 
to  the  requisite  level  by  the  gradual  accumulation  of  the  remains  of 
organisms.^     Where  the  original  eminence  rose  above  the  sea,  the  pro- 

^  See  Semper's  papers  quoted  in  footuoto  on  p.  485.  In  the  Appendix  to  the  second 
edition  of  his  '  Coral  Reefs '  (p.  223)  Mr.  Darwin  replies  to  Semper's  criticism,  maintain- 
ing  tliat  his  objections  present  no  insuperable  difficulty  in  the  theory  of  subsidence. 

-  See  pajMjr  cited  in  footnote  on  p.  485. 

'  "  A  submarine  peak,"  says  Professor  A.  Agassiz,  "is  built  tip  by  the  carcases  of  the 
invertebrates  that  live  upon  it,  and  for  which  the  pelagic  fauna  serves  in  part  as  food," 
Bull.  Mus,  Comji.  Zool.  Harvard,  xvii.  No.  3  (1889),  p.  127. 


SECT,  iii  §  3 


CORAL-REEFS 


491 


jecting  portion  (Fig.  187)  may  be  supposed  to  have  been  cut  down  to 
the  lower  limit  of  breaker  action  (a  a),  so  as  to  offer  a  platform  on 
which  corals  might  build  reefs  (i  k)  up  to  the  level  of  high  water 
{b  b).  Or  with  less  denudation,  or  a  loftier  cone,  a  nucleus  of  the 
original  volcano  might  remain  as  an  island  (Fig.  188),  from  the  sides 


Fig.  1S7. — Section  of  a  volcanic  cone  of  loose  ashes  supposed  to  have  been  thrown  up  on  the  sea-floor 

and  to  have  reached  the  sea-level  (B.) 

of  which  a  barrier  reef  might  grow  outward,  on  a  talus  of  its  own 
debris  (r  r),  and  maintain  a  steep  outer  slope.  According  to  this  view 
the  breadth  of  a  reef  ought,  in  some  degree,  to  be  a  measure  of  its 
antiquity. 

To  the  obvious  objection  that  this  explanation  requires  the  existence  of 
so  many  volcanic  peaks  just  at  the  proper  depth  for  coral-growth,  and  that 
the  number  of  true  atolls  is  so  great,  Mr.  Murray  replies  that  in  several 


w.--    ' '-/J^i 


Fig.  188.— Section  of  denuded  volcanic  island  with  lava  nucleus  and  surrounding  coral-reef  (7i.) 

ways  the  limit  for  the  commencement  of  coral-growth  may  be  reached. 
Volcanic  islands  may  be  reduced  by  the  waves  to  mere  shoals  (Fig. 
187)  like  Graham's  Island,  in  the  Mediterranean.  On  the  other  hand, 
submarine  volcanic  peaks,  if  originally  too  low,  may  conceivably  be 
brought  up  to  the  coral-zone  by  the  constant  deposit  of  the  detritus  of 
marine  life  (foraminifera,  radiolaria,  pteropods,  &c.),  which  as  above  stated, 
is  found  to  be  very  abundant  in  the  upper  waters,  whence  it  descends 
as  a  kind  of  organic  rain  into  the  depths.  Mr.  Murray  holds  also  that 
the  dead  coral,  attacked  by  the  solvent  action  of  the  sea -water,  is 
removed  in  solution  both  from  the  lagoon  (which  may  thus  be  deepened) 
and  from  the  dead  part  of  the  outer  face  of  the  reef,  which  may  in  this 
way  acquire  greater  steepness.^ 

Professor  A.  Agassiz  has  arrived  at  similar  conclusions  from  detailed 
explorations  among  the  coral-reefs  and  submarine  banks  of  the  West 
Indian  seas  and  the  Hawaiian  Islands.  He  believes  that  barrier-reefs  and 
atolls  have  arisen  without  the  aid  of  subsidence,  upon  a  platform  prepared 
for  them  by  the  upward  growth  of  submarine  calcareous  banks,  under 

1  Proc,  Roy.  Soc.  Edin.  1880,  p.  505,  ante^  pp.  38,  441,  442. 


492  DVXAMICAL  GEOLOGY  book  m  pari  ii 

the    most   favourable   condition    of   ocean -currents,    temperature,   and 
food.i 

Tliat  the  wide -spread  oceanic  subsidence  demanded  by  Darwin's 
theory  cannot  be  demonstrated  by  coral-reefs  must  now,  I  think,  be 
conceded.  The  co-existence  of  fringing  and  barrier-reefs,  and  of  atolls, 
in  the  same  neighbourhood  with  proofs  of  protracted  stability  of  level  or 
even  with  evidence  of  upheaval,  likewise  the  successive  stages  whereby  a 
true  atoll  may  be  formed  without  subsidence,  have  been  demonstrated  so 
clearly  in  the  West  Indian  region,  that  we  must  admit  the  possibility  that 
the  same  mode  of  formation  may  extend  all  over  the  coral-seas.  At  the 
same  time,  it  must  be  granted  that  the  necessary  conditions  for  the  forma- 
tion of  barrier -reefs  and  atolls  might  sometimes  be  brought  about  by 
subsidence.  So  long  as  a  suitable  bottom  is  provided  for  coral-growth  it 
is  prol)ably  immaterial  whether  this  is  done  by  the  submergence  of  land 
or  by  the  ascent  of  the  sea-iioor.  That  subsidence  has  in  some  cases 
taken  place  seems  to  be  proved  by  the  depth  of  some  atoU-Iagoons — 40 
fathoms — unless  this  depth  can  be  supposed  to  be  due  to  solution  by 
sea- water,  and  not  to  the  progressive  deepening  during  a  subsidence  with 
which  the  upward  growth  of  the  reef  could  keep  pace. 

Oor-e. — The  bed  of  the  Atlantic  and  other  oceans  is  covered  with  a 
calcareous  ooze  formed  of  the  remains  of  Farandnifera,  chiefly  species  of  the 
genus  GloUtjcniia,  It  has  been  observ^ed  that  in  these  deep-sea  deposits, 
the  larger  and  relatively  thinner  |)elagic  shells  are  rare  or  absent  at  greater 
depths  than  2000  fathoms,  while  the  thicker -shelled  varieties  abound. 
This  lias  been  referred  to  the  solvent  action  of  sea-water,  whereby  the 
more  fragile  forms  are  attacked  and  removed  in  solution  (ait^  pp.  38,  441). 
Among  abysmal  deposits,  foraminiferal  ooze  ranks  next  in  abundance  to 
the  red  and  grey  clays  of  the  deep  sea  (p.  457).  It  is  a  pale-grey  marl, 
sometimes  red  from  peroxide  of  iron,  or  brown  from  peroxide  of 
manganese ;  and  it  usually  contains  more  or  less  clay,  even  with 
occasional  fragments  of  pumice.  It  covers  an  area  of  the  North  Atlantic 
probably  not  less  than  1300  miles  from  east  to  west,  by  several  hundred 
miles  from  north  to  south.  The  total  area  of  ocean-bottom  occupied  by 
globigerina  -  ooze  is  estimated  at  47,752,500  square  miles,  the  mean 
<lepth  of  the  surface  of  the  deposit  below  sea-level  is  computed  to  be 
1996  fathoms,  and  the  mean  proportion  of  carbonate  of  lime  in  the  ooze 
64*53  per  cent.^ 

The  consolidation  of  a  soft  calcareous  ooze  or  a  mass  of  broken  shells, 
corals,  and  other  calcareous  organisms,  effected  by  the  percolation  of  water 
containing  ciirbonic  acid  {antCj  pp.  365,  454,  486),  is  most  rapid  witii 
copious  evaporation,  as,  for  instance,  on  coral-reefs  where  exposure  to  the 
air  in  the  interval  between  two  tides  suffices  for  the  deposit  of  a  thin 
crust  of  hard  limestone  over  a  surface  of  broken  coral  or  coral-fiand.' 
Recently-upraised  limestone  and  coral-rock  have  in  some  places  assumed 

'  Amer.  Acad.  xi.  (1882),  p.  107  :  BuU.  Mus.  Comp.  Xool.  Harvard,  xviL  (1889),  Ko.  5. 
See  also  the  papers  of  Messrs.  Ouppy,  Wharton,  Bourne,  and  Sluiter,  cited,  anU,  p.  485. 
^  Murray  and  Irvine,  Proc.  Roy.  fihc.  Edin.  xvii.  (1889),  p.  82. 
'  A.  Agassiz,  Amer.  Acad.  xi.  (1882),  p.  128. 


SECT,  iii  g  3  GROWTH  OF  UMKHTOiiE  AXD  OOZE  493 

ft  cryBtalline  Bti-ucture  by  this  process,  and  the  more  delicate  organisms 
have  disappeared  from  them.  But  the  calcareous  deposits  may  acquire, 
even  under  the  sea,  sufficient  cobesion  to  be  capable  of  being  broken  up 
into  blocks.  On  the  submarine  plateau  off  Florida,  the  trawl  or  dredge 
frequently  brings  up  lai^  fragments  of  the  limestone  now  in  course  of 
formation  on  the  bottom,  consisting  of  the  dead  carcases  of  the  very 
species  that  hve  upon  the  surface  of  the  growing  deposit* 

2.  Siliceous  deposits  formed  from  anima)  exuvice  are  illustrated 
by  another  of  the  deep-sea  formations  brought  to  light  by  the  Challenge- 
researches.  In  certain  regions  of  the  western  and  middle  Pacific  Ocean, 
the  bottom  was  found  to   be  covered  with   an   ooze   consisting  almost 


FiK.  IBB.. 

entirely  of  Radiohiria.  These  minute  organisms  occur,  indeed,  more  or 
less  abundantly  in  almost  all  deep  oceanic  deposits.  From  the  deepest 
sounding  taken  by  the  Challenger  (4475  fathoms,  or  more  than  5  miles) 
a  radiolarian  ooze  was  obtained  (Fig.  189).  The  spicules  of  sponges 
likewise  furnish  materials  towards  these  siliceous  accumulations.  The 
number  of  marine  plants  and  animals  which  secrete  silica  is  so  great, 
and  the  proportion  of  that  constituent  in  sea-water  so  minute  that  some 
difficulty  has  been  felt  to  account  satisfactorily  for  the  vast  quantities  of 

'  A.  Agusiz,  0^1.  cil.  p.  112.  An  sccoiint  of  Ih*  upraised  oeeanic  ciBjiaiit*  of  Bsrbsdoa 
is  glveo  by  Mmsts.  Juke-"  Brown  suil  Harriaon,  Quiirl.  Jmitk.  Oeol.  .t^.  xlviii.  (1892),  p. 
170.  Some  of  these  deposits  (iresent  a  close  Ksemblance  to  those  uicertaiaeil  by  ilredgiDf; 
to  be  lecn  in  progruB  of  accuruulH 


494  DYNAMICAL  GEOLOGY  book  m  part  ii 

silica  continually  being  abstracted  from  the  ocean  by  organic  agencies. 
Messrs.  Murray  and  Irvine,  however,  as  already  stated,  have  shown  that 
an  appreciable  amount  of  fine  clay  is  present  even  in  the  water  of  mid- 
ocean,  and  they  have  ascertained  by  actual  experiments  with  living 
diatoms  that  these  plants  can  obtain  their  silica  from  diffused  clay  in 
suspension.^ 

3.  Phosphatic  deposits,-  in  the  great  majority  of  cases,  betoken 
some  of  the  vei-tebrat^  animals,  seeing  that  phosphate  of  lime  enters 
largely  into  the  composition  of  their  bones  and  occurs  in  their  excrement 
(p.  141).  The  most  typical  modern  accumulations  of  this  nature  are  the 
guano  beds  of  rainless  islands  off  the  western  coasts  of  South  America  and 
Southern  Africa.  In  these  regions,  immense  flocks  of  sea-fowl  have,  in 
the  course  of  centuries,  covered  the  ground  with  an  accumulation  of  their 
droppings  to  a  depth  of  sometimes  30  to  80  feet,  or  even  more.  This 
deposit,  consisting  chiefly  of  organic  matter  and  ammoniacal  salts,  with 
about  20  per  cent  of  phosphate  of  lime,  has  acquired  a  high  value  as  a 
manure,  and  is  being  rapidly  cleared  off.  It  could  only  have  been 
preserved  in  a  rainless  or  almost  rainless  climate.  In  the  west  of  Europe, 
isolated  stacks  and  rocky  islands  in  the  sea  are  often  seen  to  be  white 
from  the  droppings  of  clouds  of  sea-birds ;  but  it  is  merely  a  thin  crusty 
which  is  not  allowed  to  grow  thicker  in  a  climate  where  rains  are 
frequent  and  heavy.  From  observations  made  on  phosphatic  deposits 
such  as  the  phosphatic  chalk  of  France,  Belgium,  and  England,  it  is  evident 
that  phosphate  of  lime  derived  from  the  decomposition  of  animals  (fish, 
&c.)  may  be  held  in  solution  and  gather  round  any  organic  body,  or  fill  its 
cavities  and  replace  original  carbonate  of  lime.  Grains  and  concretions  of 
phosphate  are  thus  formed,  especially  in  the  interior  of  shells  and 
forarainifera.^ 

Wherever  terrestrial  mammalia  congregate,  and  especially  where  they 
die  and  leave  their  carcases,  phosphatic  deposits  may  be  formed  if  the 
conditions  are  favourable  for  the  preservation  of  the  remains.  Caves 
haunted  by  hyauias  serve  as  receptacles  not  only  for  the  bones  and 
excrement  of  these  animals  but  also  for  bones  of  the  various  animals 
which  they  have  dragged  there  as  food.  Hence  in  limestone  countries 
"  osseous  breccias  "  are  often  found  below  the  layer  of  stalagmite  on  the 
floor.  Again,  along  the  swampy  margins  of  lakes  and  salt-marshes  the 
bodies  of  wild  animals  are  oftenr  mired  in  the  boggy  ground  and  perish 
there,  and  their  bodies  gradually  sink  below  the  surface.  Hence 
j)hosphatic  accumulations  arise  sometimes  on  an  extensive  scale,  as  has 
happened  in  different  parts  of  the  United  States.* 

In   connection  with    the  organic    deposits  of   the   sea -floor,  further 

^  Murray  Jiud  Irvine  on  siliceous  deposits  of  modern  seas,  Proc.  Roy,  Soc.  Edin.  xviii. 
(1891),  p.  229,  and  ante,  p.  450. 

-  A  useful  compendium  of  information  on  these  deposits  is  given  by  R.  A.  F.  Penrose  isn 
Bull.  U.  .v.  Ged.  Surv.  No.  46  (1888)  already  cited  (p.  142). 

3  A.  Y.  Reuard  and  T.  Cornet,  Ihdl.  Acad,  Roy.  Rdij.  xxi.  (1891),  p.  126.  A.  Strahao, 
(^uart.  Joiini.  Oed.  Sue.  xlvii.  (1891),  p.  356. 

*  Penrose,  Bull.  U.  S,  Ued.  Siirv.  No.  46  (1888),  p.  127. 


SECT,  iii  §  4  MAN  AS  A  GEOLOGICAL  AGENT  496 

reference  may  be  made  here  to  the  chemical  processes  in  progress  there, 
and  to  the  probable  part  taken  in  these  processes  by  living  organisms 
and  decaying  animal  matter.  The  transformation  of  sulphate  of  lime  into 
carbonate,  which  may  now  be  regarded  as  the  chief  source  of  the 
calcareous  constituents  of  marine  plants  and  animals,  takes  place  on  a 
gigantic  scale  in  the  ocean.  The  precipitation  of  manganic  oxide  and  its 
segregation  in  concretions,  often  round  organic  centres  (p.  458),  presents 
a  close  analogy  to  the  formation  of  concretionary  bog-iron  ore  through 
the  operation  of  the  humus  acids  in  stagnant  water.  The  crystallization 
of  silicates  observed  during  the  Challenger  expedition  is  possibly  also  to 
be  connected  with  the  action  of  organic  compounds  (p.  459).  The 
formation  of  Hint  concretions  has  been  for  many  years  a  vexed  question 
in  geology.  The  constant  association  of  flints  with  traces,  more  or  less 
marked,  of  former  abundant  siliceous  organisms  seems  to  make  the 
inference  irresistible,  that  the  substance  of  the  flint  has  been  precipitated 
through  the  agency  of  these  creatures.  The  silica  has  first  been  abstracted 
from  sea-water  by  living  organisms.  It  has  then  been  re-dissolved  and  re- 
deposited  (probably  through  the  agency  of  decomposing  organic  matter), 
sometimes  in  amorphous  concretions,  sometimes  replacing  the  calcareous 
parts  of  echini,  mollusks,  &c.,  while  the  surrounding  matrix  was,  doubt- 
less, still  a  soft  watery  ooze  under  the  sea.^ 


§  4.  Man  as  a  Geological  Agent. 

No  survey  of  the  geological  workings  of  plant  and  animal  life  upon 
the  surface  of  the  globe  can  be  complete  which  does  not  take  account  of 
the  influence  of  man — an  influence  of  enormous  and  increasing  consequence 
in  physical  geogi'aphy ;  for  man  has  introduced,  as  it  were,  an  element  of 
antagonism  to  nature.  Not  content  with  gathering  the  fruits  and 
capturing  the  animals  which  she  has  offered  for  his  sustenance,  he  has, 
with  advancing  civilisation,  engaged  in  a  contest  to  subdue  the  earth  and 
possess  it.  His  warfare,  indeed,  has  often  been  a  blind  one,  successful  for 
the  moment,  but  leading  to  sure  and  sad  disaster.  He  has,  for  instance, 
stripped  off"  the  woodland  from  many  a  region  of  hill  and  mountain, 
gaining  his  immediate  object  in  the  possession  of  their  stores  of  timber, 
but  thereby  laying  bare  the  slopes  to  parching  droughts  or  fierce  rains. 
Countries  once  rich  in  beauty,  and  plenteous  in  all  that  was  needful  for 
his  support,  are  now  burnt  and  barren,  or  washed  bare  of  their  soil.  It 
is  only  in  comparatively  recent  years  that  he  has  learnt  the  truth  of  the 
aphorism — **  Honio  Xaturce  minisier  et  interpresy 

1  See  Wallich,  Q.  J.  (Jeol.  Soc.  xxxvi.  p.  68;  Sollas,  Ann.  and  Mag.  Nat.  Hist.  5th 
series,  vi.  p.  437  ;  and  ante,  pp.  141,  483;  Brit.  Assoc.  1882,  sects,  p.  549 ;  Hull  and  Hardraan, 
Tran^.  Roy.  Dublin.  Soc.  new  series  (1878),  vol.  i.  p.  71.  Julien  observes  that  a  substance 
corresponding  to  humus  appears  to  enter  universally  into  the  constitution  of  the  oceanic  oozes, 
resulting  from  the  decomposition  of  organisms  and  containing  a  high  percentage  of  silica 
(Proc.  Amer.  Assoc,  xxviii.  p.  359).  Consult  also  the  paper  of  Messrs.  Murray  and  Irvine 
already  cited  {Proc.  Rf>y.  Soc.  Edin.  xviii.  (1891),  p.  229)  and  the  suggestive  experiments  there 
described  as  to  the  solution  of  silica  in  sea- water  containing  living  and  dead  organisms. 


496  DYXAMJCAL  GEoLWY  book  m  paet  ii 

But  now,  when  that  truth  is  coming  more  and  more  to  be  recognised 
and  acted  on,  man's  influence  is  none  the  less  marked.  His  object  still  ii 
to  subdue  the  earth,  and  he  attains  it,  not  by  setting  nature  and  her 
laws  at  defiance,  but  by  enlisting  her  in  his  service.  Within  the  com- 
pass of  this  volume  it  is  impossible  to  give  more  than  merely  a  brief  oat- 
line  of  so  vast  a  subject.^  The  action  of  man  is  necessarily  confined 
mainly  to  the  land,  though  it  has  also  to  some  extent  influenced  the 
marine  fauna.  It  may  be  witnessed  on  climate,  on  the  flow  of  water,  on 
the  character  of  the  terrestrial  surface,  and  on  the  distribution  of  life. 

1.  On  Climate. — Human  interference  afiects  meteorological  con- 
ditions— ( 1 )  by  removing  forests  and  laying  bare  to  the  sun  and  winds 
areas  which  were  previously  kept  cool  and  damp  under  trees,  or  which, 
lying  on  the  lee  side,  were  protected  from  temi)ests ;  as  alread}'  stated, 
it  is  supposed  that  the  wholesale  destruction  of  the  woodlands  formerly 
existing  in  countries  bordering  the  Mediterranean  has  been  in  part  the 
cause  of  the  present  desiccation  of  these  districts,  while  in  the  Tyrol  the 
great  increase  and  destructiveness  of  the  debacles  has  been  attributed  to 
the  wholesale  deforesting  of  that  region,  and  the  consequent  exposure  of 
the  soil  to  rain  and  melted  snow  ;  (2)  by  drainage,  the  effect  of  diis 
operation  being  to  remove  rapidly  the  discharged  rainfall,  to  raise  the 
temperature  of  the  soil,  to  lessen  the  evai>oration,  and  thereby  to  diminish 
the  rainfall  and  somewhat  increase  the  general  temperature  of  a  countiy ; 
(3)  by  the  other  processes  of  agriculture,  such  as  the  transformation  of 
moor  and  bog  into  cultivated  land,  and  the  clothing  of  bare  hillsides  with 
green  crops  or  plantations  of  coniferous  and  hard-wood  trees. 

2 .  O n  t h e  F 1  o w  of  Water. — ( 1 )  By  increasi ng  or  diminish ing  the 
rainfall  man  diroctlv  affects  the  circulation  of  water  over  the  bind. 
(2)  By  the  drainage-operations,  which  cause  the  rain  to  run  off  more 
rapidly  than  before,  he  increases  floods  in  rivers.  (3)  By  wells,  bores, 
mines,  or  other  subterranean  works,  he  interferes  with  underground 
waters  and  conse(iucntly  with  the  discharge  of  springs.  (4)  By  embank- 
ing rivers  he  confines  them  to  narrow  channels,  sometimes  increasing 
their  scour,  and  enabling  them  to  carry  their  sediment  further  seaward, 
sometimes  causing  them  to  deposit  it  over  the  plains  and  raise  their 
level. 

3.  On  the  Surface  of  the  Land. — Man's  operations  alter  the  aspect 
of  a  country  in  many  ways  : — (1)  by  changing  forest  into  bare  mountain, 
or  clothing  bare  mountain  with  forest ;  (2)  by  promoting  the  growth  or 
causing  the  removal  of  peat-mosses ;  (3)  by  heedlessly  uncovering  sand- 
dunes,  and  thereby  setting  in  motion  a  process  of  destruction  which  may 

'  See  Marsh's  *  Man  and  Nature,'  a  work  which,  as  its  title  denotes,  Bpecially  treats  of 
this  subject,  and  of  which  a  new  and  enlarged  edition  was  published  in  1874  under  the  title 
of  *  The  Earth  as  modified  by  Human  Action.'  It  contains  a  copious  bibliography.  See  abo 
Holleston,  .fi'ur.  Itoij.  Ueorj.  Soc.  xlix.  p.  320,  and  works  cited  by  him,  particokrly  De 
Candnlle.  *  (J«'o^raphie  botanique  raisonnt'^e,*  1855:  Unger's  *  Botanische  Streifziige/  is 
SitzUr.  Vienna  Acad.  1857-1859  ;  J.  G.  St  Hilaire,  *Histoire  naturelle  gen^rale  desBtfg&efl 
Organiques,'  torn.  iii.  1862;  Oscar  Peschel,  *  Physische  Erdkunde;'  Link,  *Urwelt  mid 
Alterthum'  (1822)  :  O.  A.  Koch,  JaJirb.  OcoL  Rcichtan^t.  xxv.  (1875),  p.  114. 


SECT,  iii  §  4  MAN  AS  A  GEOLOGICAL  AGENT  497 

convert  hundreds  of  acres  of  fertile  land  into  waste  sand,  or  by  prudently 
planting  the  dunes  with  sand-loving  herbage  or  pines,  and  thus  arresting 
their  landward  progress;  (4)  by  so  guiding  the  course  of  rivers  as  to 
make  them  aid  him  in  reclaiming  waste  land  and  biinging  it  under  culti- 
vation ;  (5)  by  piers  and  bulwarks,  whereby  the  ravages  of  the  sea  are 
stayed,  or  by  the  thoughtless  removal  from  the  beach  of  stones  which  the 
waves  had  themselves  thrown  up,  and  which  would  have  served  for  a 
time  to  protect  the  land  ;  (6)  by  forming  new  deposits  either  designedly 
or  incidentally.  The  roads,  bridges,  canals,  railways,  tunnels,  villages, 
and  towns  with  which  man  has  covered  the  surface  of  the  land  will  in 
many  cases  form  a  permanent  record  of  his  presence.  Under  his  hand, 
the  whole  surface  of  civilised  countries  is  very  slowly  covered  by  a 
stratum,  either  formed  wholly  by  him,  or  due  in  great  measure  to  his 
operations,  and  containing  many  relics  of  his  presence.  The  soil  of  old 
cities  has  been  increased  to  a  depth  of  man}-  feet  by  the  rubbish  of  his 
buildings  :  the  level  of  the  streets  of  modern  Rome  stands  high  above 
that  of  the  pavement  of  the  Caesars,  and  this  again  above  the  roadways 
of  the  early  Republic.  Over  cultivated  fields  potsherds  are  turned  up  in 
abundance  by  the  plough.  The  loam  has  risen  within  the  walls  of  our 
graveyards,  as  generation  after  generation  has  mouldered  there  into  dust. 

4.  On  the  Distribution  of  Life. — It  is  under  this  head,  perhaps, 
that  the  most  subtle  of  human  influences  come.  Some  of  man's  doings 
in  this  dominion  are  indeed  plain  enough,  such  as  the  extirpation  of  wild 
animals,  the  diminution  or  destruction  of  some  forms  of  vegetation,  the 
introduction  of  plants  and  animals  useful  to  himself,  and  especially  the 
enormous  predominance  given  by  him  to  the  cereals  and  to  the  spread  of 
sheep  and  cattle.  But  no  such  extensive  distur])ance  of  the  normal  con- 
ditions of  the  distribution  of  life  can  take  place  without  carrying  with  it 
many  secondary  effects,  and  setting  in  motion  a  wide  cycle  of  change  and 
of  reaction  in  the  animal  and  vegetable  kingdoms.  For  example,  the 
incessant  warfare  waged  by  man  against  birds  and  beasts  of  prey,  in  dis- 
tricts given  up  to  the  chase,  leads  sometimes  to  unforeseen  results.  The 
weak  game  is  allowed  to  live,  which  would  otherwise  be  killed  oft'  and 
give  more  room  for  the  healthy  remainder.  Other  animals,  which  feed 
perhaps  on  the  same  materials  as  the  game,  are  from  the  same  cause  per- 
mitted to  live  unchecked,  and  thereby  to  act  as  a  further  hindrance  to  the 
spread  of  the  protected  species.  But  the  indirect  results  of  man's  inter- 
ference with  the  regime  of  plants  and  animals  still  require  much  pro- 
longed observation.^ 

This  outline  may  suffice  to  indicate  how  important  is  the  place  filled 
by  man  as  a  geological  agent,  and  how  in  future  ages  the  traces  of  his 
interference  may  introduce  an  element  of  difficulty  or  uncertainty  into 
the  study  of  geological  phenomena. 

^  See  on  the  subject  of  mau's  iuHuence  ou  organic  nature,  tlie  paper  by  Professor 
RoUestOD,  quoted  on  the  previous  j»age,  and  the  numerous  authorities  cited  by  liiin. 


2  K 


BOOK   IV. 

GEOTECTONIC  (STRUCTURAL)  GEOLOGY, 

OR   THE   ARCHITECTURE   OF  THE   EARTH'S   CRUST. 

The  nature  of  minerals  and  rocks  and  the  operations  of  the  different 
agencies  by  which  they  are  produced  and  modified  having  been  discussed 
in  tlie  two  foregoing  books,  there  remains  for  consideration  the  manner  in 
which  these  materials  have  been  arranged  so  as  to  build  up  the  crust  of 
the  earth.  Since  by  far  the  largest  visible  portion  of  this  crust  consists 
of  sedimentary  or  aqueous  rocks,  it  will  be  of  advantage  to  treat  of  them 
first,  noting  both  their  original  charactera,  as  resulting  from  the  circum- 
stances' under  which  they  were  formed,  and  the  modifications  subse- 
quently effected  upon  them.  Many  superinduced  structures,  not  peculiar 
to  sedimentary,  but  occurring  more  or  less  markedly  in  all  rocks,  may  be 
conveniently  described  together.  The  distinctive  characters  of  the  igneous 
or  eniptive  rocks,  as  portions  of  the  architecture  of  the  crust,  will  then 
be  described ;  and  lastly,  those  of  the  crystalline  schists  and  other 
associated  rocks  to  which  the  name  of  metamorphic  is  usually  applied. 

Part  I.     Stratification  and  its  Accompaniments. 

The  term  "  stratified,'*  so  often  applied  as  a  general  designation  to  the 
aqueous  or  sedimentary  rocks,  expresses  their  leading  structural  feature. 
Their  mateiials,  laid  down  for  the  most  pai-t  on  the  bed  of  the  sea  and 
the  floors  of  lakes  and  rivers,  under  conditions  which  have  been  already 
discussed  in  Book  IIL,  are  disposed  in  layers  or  strata,  an  arrangement 
characteristic  of  them  alike  in  hand-specimens  and  in  cliffs  and  mountains 
(Figs.  190,  191,  252  and  253).  Not  that  every  morsel  of  aqueous  rock 
exhibits  evidence  of  stratification.  But  it  is  this  feature  which  in  a 
sutticiently  large  mass  of  material  is  least  frequently  absent  The  general 
characters  of  stratification  will  be  best  understood  from  an  explanation  of 
the  terms  by  which  they  are  expressed. 

Forms  of  Bedding. — Lamina}  are  the  thinnest  paper-like  layers  in 


\ 


BOOK  IV  PART  I 


STRATIFICATION 


the  planes  of  deposit  of  a  stratified  rock.  Such  fine  layers  only  occur 
where  the  material  is  fine-grained,  as  in  mud  or  shale,  or  where  fine  scales 
of  BOmo  mineral  have  been  plentifully  deposit«d,  as  in  micaceous  sandstone. 
In  some  laminated  rocks,  the  laminie  cohere  so  firmly  that  they  can  hardly 


be  split  open,  and  the  rock  will  break  more  readily  across  them  than  in 
their  direction.  More  usually,  however,  the  planes  of  lamination  serve  as 
convenient  divisional  surfaces  by  means  of  which  the  rock  can  be  split 
open.^  The  cause  of  this  structure  has  been  generally  assigned  to  inter- 
initt«nt  deposit,  each  lamina  being  assumed  to  have  partially  consolidated 
before  its  successor  was  laid  down  upon  it.  Mr.  Sorby,  however,  has  re- 
cently suggested  that  in  fine  argillaceous  rocks  it  may  be  a  kind  of  cleavage- 
structure  (see  pp.  314,  645),  due  to  thepressureof  the  overlying  rocks,  with 
the  consequent  squeezing  out  of  interstitial  water  and  the  rearrangement 
of  the  argillaceous  particles  in  lines  per)>endicular  to  the  pressure." 

Much  may  be  learnt  as  to  former  geographical  and  geological  changes 
by    attending    to    the    characters    of    strata. 
In  Fig.  101,  for  example,  there  is  evidence 
of  a  gradual  diminution  of  movement  In  the 
waters  in  which  the  layers  of  sediment  were 
deposited.     The    conglomerate  (a)  points  to 
currents  of  some  force  ;  the  sandstones  (b  c  d) 
mark  a  progressive  quiescence  and  the  advent 
of   finer    sediment ;     the    shales   (if)  show  a 
deposition  of  fine  mud  and  accretion  of  fer- 
rous  carbonate  into    nodules   round  oi^anic 
remains;  while  the  shell-limestone  (/)  proves 
that  the  water  no  longer  carried  much  sedi-  ] 
ment,  but  had  become  clear  enough  for  an  , 
abundant  growth  of  marine  organisms.     The       hiysiiniiiitone;i:.thin-i*id"iii»iia- 
existence,  therefore,  of   alternations  of   fine      wiui'ironitone'noduies; /i""*- 
lamime  of  deposit  may  be  conceived  as  pointr       aione  vnh  murine  org«ui«ni». 
ing  to  tranquil  conditions  of  slow  intermittent 

'  H.  Daubr^  has  projiosed  the  term  diaitromt  to  eipnsi  the  nplitting  of  rocks  aloDg 
tb«ir  bedding  planes.      BhU.  f!oc.  Gtei.  France  (3|,  x.  p.  137. 
•  (iuart.  Joum.  Qeol.  Soe.  iiivi.  p,  6"  (1880). 


500  GEOTECTONW  {HTRUCTURAL)  GEOLOGY  book  iv 

sedimentation,  where  silt  has  been  borne  at  intervals  and  has  fallen  over 
the  same  area  of  undisturbed  water.  Eegularity  of  thickness  and  pe^ 
sistence  of  lithological  characters  among  the  laminae  may  be  taken  to 
indicate  periodic  currents,  of  approximately  equal  force,  from  the  same 
quarter.  In  some  cases,  successive  tides  in  a  sheltered  estuary  may  have 
been  the  agents  of  deposition.  In  others,  the  sediment  was  doubtless 
brought  by  recurring  river-floods.  A  great  thickness  of  laminated  rock, 
like  the  massive  shales  of  Palaeozoic  formations,  suggests  a  prolonged 
period  of  quiescence,  and  probably,  in  most  cases,  slow,  tranquil  subsid- 
ence of  the  sea-floor.  On  the  other  hand,  the  alternation  of'  thin  bands 
of  laminated  rock  with  others  coarser  in  texture  and  non-laminated. 
indicates  considerable  oscillation  of  currents  from  different  quarters 
bearing  various  qualities  and  amounts  of  sediment.^ 

Strata  or  Beds  are  layers  of  rock  varying  from  an  inch  or  less  up 
to  many  feet  in  thickness.  A  stratum  may  be  made  up  of  numerous 
laminae,  if  the  nature  of  the  sediment  and  mode  of  deix)sit  have  favoured 
the  production  of  this  structure,  as  has  commonly  been  the  case  with  the 
flncr  kinds  of  sediment.  In  materials  of  coarser  grain,  the  strata,  as  a 
rule,  are  not  laminated,  but  form  the  thiimest  parallel  divisions.  Strata, 
like  laminae,  sometimes  cohere  firmly,  but  are  commonly  se^mrable  with 
more  or  less  ease  from  each  other.  In  the  fonner  case,  we  may  suppose 
that  the  lower  bed  before  its  consolidation  was  followed  by  the  deposit  of 
the  upper.  The  common  merging  of  a  stratum  into  that  which  overlies 
it  must  no  doubt  be  regarded  as  evidence  of  more  or  less  gradual  change 
in  the  conditions  of  deposit.  AVhere  the  overlying  bed  is  abruptly 
separable  from  that  below  it,  the  interval  was  probably  of  some  duration, 
though  occasionally  the  want  of  cohesion  may  arise  from  the  nature  of  the 
sediment,  as  for  instance,  where  an  intervening  layer  of  mica-flakes  has 
been  laid  down.  A  stratum  may  be  one  of  a  series  of  similar  beds  in  the 
same  mass  of  rock,  jis  where  a  thick  sandstone  includes  many  individual 
strata,  varying  considerably  in  their  respective  thicknesses  ;  or  it  may  be 
complete  and  distinct  in  itself,  as  where  a  band  of  limestone  or  ironstone 
runs  through  the  heart  of  a  series  of  shales.  As  a  general  rule,  the  con- 
clusion appears  to  be  legitimate  that  stratification,  when  exceedingly  well- 
murked,  indicates  slow  intermittent  deposition,  and  that  when  weak  or 
absent  it  points  to  more  rapid  deposition,  intervals  and  changes  in  the 
nature  of  the  sediment  and  in  the  direction  of  force  of  the  transporting 
currents  being  necessary  for  the  production  of  a  distinctly  stratified 
structure. 

Lines  due  to  original  stratification  must  be  carefully  distinguished 
from  other  divisional  ])lanes  which,  though  somewhat  like  them,  are  of 
entirely  different  origin.  Five  kinds  of  fissility  may  be  recognised  among 
rocks: — 1st,  lammttion  of  original  deposit;  2nd,  cleavage^  as  in  slate; 
3rd,  sheariwjy  as  near  faidts  and  thrust-planes  (pp.  316,  544)  ;  4th,  foliaim, 
iis  in  schists  ;  5th,  Jiow-stnidurr,  when  extremely  developed  in  some  lavas, 

^  For  a  series  of  experimeut<<  to  illustrate  the  origiu  of  the  sedimentation  of  the  cod* 
measures,  see  H.  Fayol,  Bifll.  Nw.  Iiuiustric  Mintrale,  St.  ElUnne,'  2nie  ser.  xv.  (1886!- 
'  Etudes  sur  le  terrain  houiller  de  Commentry.*  with  atlas. 


PART  I 


FALSE-BEDDING 


601 


wherein,  by  the  development  of  steam -holes  or  spherulitic  concretions 
and  the  drawing-out-  of  these  into  planes  during  the  movement 
of  the  molten  mass,  a  kind  of  fissility  is  [^oduced  which  at  first  might 
be  mistaken  for  the  lamination  of  deposit.  Close-set  joints  likewise  give 
rise  to  divisional  planes,  which,  like  cleavage,  may  now  and  then  deceive 
an  observer  by  their  resemblance  to  stratification. 

Originally  the  planes  of  stratification,  in  the  great  majority  of  cases, 
were  nearly  horizontal.  As  most  sedimentary  rocks  are  of  marine  origin, 
and  have  accumulated  on  the  shallower  slopes  of  the  sea-floor,  they  have 
generally  had  from  the  first  a  slight  inclination  seawards ;  but,  save  on 
rapidly  shelving  shores,  the  angle  of  declivity  has  been  usually  so  slight 
as  to  be  hardly  appreciable  by  the  eye.  Slight  departures  from  this 
predominant  horizontality  would  be  caused  where  sediment  accumulated 
unequally,  or  where  the  floor  on  which  deposition  took  place  was  of  an 
undulating  or  more  markedly  uneven  character. 

False-bedding,  Current-bedding. — Some  strata,  particularly  sand- 
stones, are  marked  by  an  irregular  lamination,  wherein  the  laminae, 
though  for  short  distances  parallel 


to  each  other,  are  oblique  to  the 
general  stratification  of  the  mass,  at 
constantly  varying  angles  and  in 
different  directions  (a  b  c  d  in  Fig. 
192).  This  structure,  known  as 
false -bedding  or  current- bedding, 
points  to  frequent  changes  in  the 
direction  of  the  currents  by  which 
the  sediment  was  carried  along  and  deposited.  Sand  pushed  over  the 
bottom  of  a  sheet  of  water  by  varying  currents  tends  to  accumulate 
irregularly  in  banks  and  ridges,  which  often  advance  with  a  steep 
slope  in  front.  The  upper  and  lower  surfaces  of  the  bank  or  bed  of 
sand  (*  *  in  Fig.  192)  may  remain  parallel  with  each  other  as  well 
as  with  the  underlying  bottom  (a),  yet  the  successive  laminae  com- 
posing it  may  lie  at  an  angle  of  30°  or  even  more.  We  may 
illustrate  this  structure  by  the  familiar  formation  of  a  railway  em- 
bankment The  top  of  the  embankment,  on  which  the  permanent  way 
is  to  be  laid,  is  kept  level ;  but  the  advancing  end  of  the  earthwork 
shows  a  steep  slope  over  which  the  workmen  are  constantly  discharging 
waggon-loads  of  rubbish.  Hence  the  embankment,  if  cut  open  longi- 
tudinally, would  present  a  "false-bedded"  structure,  for  it  would  be 
found  to  consist  of  many  irregular  layers  inclined  at  a  high  angle  in  the 
direction  in  which  the  formation  of  the  mound  had  advanced.  Among 
geological  formations  of  all  ages,  occasional  sections  of  the  upper  surfaces 
of  such  false-bedded  strata  show  the  singular  irregularity  of  the  structure, 
and  bring  vividly  before  the  imagination  the  feeble  shifting  currents  by 
which  the  sediment  was  drifted  about  in  the  shallow  water  where  it 
accumulated  (Fig.  193).  A  noticeable  feature  is  the  markedly  lenticular 
character  of  false-bedded  strata.  Even  where  the  usual  diagonal  lamina- 
tion is  feeble  or  absent  this  lenticular  structure  may  remain  distinct 


Fig.  192.— Section  of  False-bedded  Strata. 


GEOTEVTOKIC  {STRUCTURAL)  GEOLOGY 


(Fig.  194).  Examples  may  also  be  observed,  in  whieli,  while  all  the  bedi 
are  well  lamintited,  in  some  the  lamtnEe  run  parallel  with  the  geneni 
bedding,  and  in  olhere  obliquely  (Fig.  195).  Iliough  current^bedding  is 
most  frequent  among  sandstones,  or  markedly  arenaceous  strata,  it  mif 
be  observed  occasionally  in  detrital  formations  of  ot^nic  origin,  as  in  x 


section  (Fig.  196)  by  De  la  Beche,  where  a  portion  of  one  of  the 
calcareous  members  of  the  'Jurassic  series  of  England,  consists  of  bedi 
composed  mostlyof  orgaqpc  fra^ents  with  a  strongly  marked  cuneDt- 
bedding  (a  a),  while  others,  formed  of  nmddy  layers  and  not  obliquely 
laminated  (b  b),  point  to  intervals  when,  with  the  cessation  of  the  silt- 
bearing  currents,  the  water  became  still  enough  to  allow  the  mud 
suspended  in  it  to  settle  on  tlie  bottom.' 


Intercalated  (.'oiitortion.  —  Diagonal  lamination  is  somediBN 
contorted  as  well  as  steejily  inclined,  and  highly  contorted  beds  are  inte^ 
ixjsed  between  othci-s  which  are  undisturbed  and  horizontal.  Currcd 
'  'Geologipnl  Obwner,'  p.  536.  The  memoEr  by  H.  Payol  citeil  od  i>.  MH},  It  k- 
comjianied  «itli  an  ntlaa  uhieL  contains  many  encellent  illuitratioui  oT  tb«  e 
irregular  atratlficatiaD  of  the  CoaUmeaauien. 


PART  I  FALSE-BEDDING  503 

and  contorted  Umination  is  of  frequent  occurrence  among  Palieozoic 
sandstonea.  In  Fig.  196,  an  example  ia  given  from  one  of  the  oldesl 
formations  in  Britain,  and  in  Fig.  197  another  from  one  of  the  yonngeat. 


MttoD  and  coirent-luniiuclon,  Upper  Old  Bed  SudilODe 

WtMrtOtd  <B.) 
lilt  dppoft1t«d  borlioiitAUjr  tnd  HppAmilly  tt<im  in«)unEcJ 
la  af  und  which  hive  been  piuhed  mioug  the  botloDi. 


The  cause  of  this  structure  is  not  well  understood  Among  glacial 
deposits  local  examples  of  contortion  occur  which  maj  be  accounted 
for  by  the  intercalation  and  subsequent  meltmg  of  sheets  of  fiozen  mud, 


or  by  the  stranding  of  heavy  masses  of  drift-ice  upon  still  unconsolidated 
sand  and  mud.  The  removal  of  mineral  matter  in  solution  (as  an  ong 
saliferous  and  gypseous  deposits)  leads  to  the  subsidence  and  crumpi  ng 


of  overlying  beds.  The  hydration  of  anhydrite  (pp.  298,  34.5),  by  augment- 
ing the  volume  of  the  mass,  subjects  the  adjacent  strata  to  crushing  and 
contortion.     It  is  possible  that  some  of  the  extraordinary  labyrinthine  and 


GEOTECTOXIC  {STRUCTURAL)  GEOLOOY 


complex  contortions  of  certain  scluetoBe  rocks  way  be  due  to  the  snbee- 
quent  crumpling  of  sti'ata  already  full  of  diagonal  or  contorted  laminatiOD. 
Irregularities  of  Bedding  due  to  Inequalities  of  Deposition 
or  of  Erosion. — A  sharp  ridge  of  sand  or  gravel  may  be  laid  down 
under  water  by  current-action  of  some  strength.  Should  the  motion  of 
the  water  diminish,  finer  sediment  may  l>e  brought  to  the  place  and  be 
deposited  around  and  above  the  ridge.  In  such  a  case,  the  stratification 
of  the  later  accumulation  may  end  off  abruptly  against  the  flanks  of  the 
older  ridge,  which  will  appear  to  rise  up  through  the  overlying  bed. 
Appearances  of  this  kind  are  not  uncommon  in  coal-fields,  where  they 
are  known  to  the  miners  as  "rolls,"  ''swells,''  or  "horses'  backs."  A 
structure  exactly  the  reverse  of  the  preceding,  where  a  stratum  has  been 
scooped  out  before  the  deposition  of  the  layers  which  cover  it,  has  also 
often   been  observed  in  mining  for  coal,  when  it  is  termed  a  "  want" 


(.'hnnuels  have  been  cut  out  of  a  coal-seam,  or  rather  out  of  the  bed  of 
vegetation  which  ultimately  became  coal,  and  these  winding  and  branching 
channels  have  been  filled  up  with  sandy  or  muddy  sediment.  The 
accompanying  plan  (Fig.  199)  represents  a  jKirtion  of  a  remarkable  seri« 
of  such  channels  traversing  the  Coleford  High  Delf  coal-seam  in  the 
Forest  of  Dean.  The  chief  one,  locally  known  as  the  "  Horse  "  (a  h),  ha 
lieen  traced  for  iibout  two  miles,  anil  varies  in  width  from  170  to  340 
yards.  It  is  joined  by  smaller  tributaries  (e  (■),  which  run  for  some  way 
a[)proximately  parallel  to  it.  The  coal  has  either  been  prevented  from 
accumulating  in  contemporaneous  water-channels,  or,  while  still  in  the 
condition  of  soft  bog-like  vegetation,  has  been  eroded  by  streamleU 
flowing  through  it.'  A  section  drawn  across  such  a  buried  channel 
exhibits  the  structure  represented  in  Fig.  300,  where  a  bed  of  fire-clay 
(c),  full  of  roots  and  eviilently  an  old  soil,  sup[)orts  a  bed  of  coal  (d)  and 
of  shale  (c),  which,  during  the  deposition  of  this  series  of  strata,  have 
been  cut  out  into  a  channel  at  /.     A  deposition  of  sand  (b)  has  then 

'   Hud.i1*.  i;coi.   Tnt,,>.  vi.  (1842).  p,  215. 


IRREGVtAKITIES  OF  BEDDING 


the  excavation,  and  a  layer  of  mud  (a)  haa  covered  up  the 

its  of  very  unequal  force  and  transporting  power  may  alternate 
way  that  after  fine  silt  has  for  some  time  been  accumulated, 


ingle  may  next  be  swept  along,  and  may  be  so  irregularly 
ith  the  softer  strata  as  to  simulate  the  behaviour  of  an  intrusive 
201).^  The  section  (Fig.  202)  taken  by  De  la  Beche  from  a 
lal-measures  on  the  coast  of  Pembrokeshire,  shows  a  deposit  of 


ihat  during  the  course  of  its  formation  was  eroded  by  a  channel 
which  sand  was  carried  ;  after  which,  the  deposit  of  fine  mud 
ced,  and  similar  shale  was  again  laid  down  upon  the  top  of  the 
er,  iintil,  >>y  a  more  potent  current,  the  shale  deposit  was  cut 


Fig,  iOi ConUmponneous  Eroi[on  ind  DapoiLt  iJt.) 

.he  left  side  of  the  section,  and  a  series  of  sandbeds  (c)  was  laid 
n  its  eroded  edges.  An  intemiption  of  this  kind,  however,  may 
sly  disturb  the  earlier  conditions  of  a  deposit,  which,  as  shown 
oe  section,  may  be  again  resumed,  and  new  layers  {d)  may  be 
conformably  over  the  whole.  Among  the  lessons  to  be  learnt 
'  De  la  Beche,  •  Geol.  Observer,'  p.  533. 


GEOTECTONIV  {i>TBUCTURAL)  GEOLOGY 


from  such  sections  of  local  irregularity,  one  of  the  most  uaefial  is  t^ 
reminder  that  the  inclination  of  strata  maj'  not  always  be  due  to  subter- 
ranean movement.  In  Fig.  203,  for  exumple,  the  lower  strata  of  shale  lod 
sandstone  are  nearly  horizontal.  The  upper  thick  sandstone  (&')  has  been 
cut  away  towards  the  left,  and  a  series  of  shales  (a')  and  a  coal-seam  (d) 
have  l>een  deposited  against  and  over  it.  If  the  sandstone  waa  then  leiiel, 
the  shales  must  have  been  laid  down  at  a  considerable  angle,  or,  if  thnt 
were  deposited  in  horizontal  sheets,  the  earlier  sandstone  miist  bsie 
accumulated  on  a  marked  slope.  As  deposition  continued,  the  iuclioed 
plane  of  sedimentation  would  gradually  become  horizontal  until  tl» 
strata  were  once  more  parallel  with  the  series  ab  c  below.  A  stmctare 
of  this  kind,  not  unfrequent  in  the  Coal-measures,  must  be  looked  upon 
as  a  larger  kind  of  false-bedding,  where,  however,  terrestrial  i 
may  sometimes  have  intervened. 


V  V  r^ 


In  the  instances  here  cited  it  is  ei  idcnt  that  the  erosion  took  plw^ 
in  a  general  sense  during  the  same  period  with  the  accumulation  (rf  tli* 
strata,  i'or,  after  the  interruption  was  coieied  up  sedimentation  veil 
on  as  before,  and  there  is  usuiUj  in  obiious  close  sequence  betweea  tM 
continuous  strata.  Though  it  may  lie  impossible  to  decide  as  W  the 
relative  length  of  the  interval  that  elapsed  between  the  formation  of  * 
given  stratum  aiul  that  of  the  next  stratum  which  lies  upon  its  eroded 
surface,  or  to  ascertain  bow  much  depth  of  rock  lias  been  removed  in  thi 
erosion,  yet,  when  the  structure  occurs  among  conformable  stnU, 
evidently  united  as  one  litliologically  continuous  series  of  deposits,  « 
may  reasonably  infer  that  the  missing  (lortions  are  of  small  moment,  ai» 
that  the  erosion  was  merely  due  to  the  in'egular  and  more  violent  actloB 
of  the  very  currents  by  which  the  sediment  of  the  Euccessive  atratawW 
supplied. 


PAM  I  SURFACE-MARKINGS  607 

The  case  is  very  different  when  the  eroded  Btrata,  besides  being 
inclined  at  a  different  angle  from  those  above  them,  are  strongly  marked 
off  by  lithological  distinctiona,  particularly  when  fragments  of  them  occur 
in  the  oyerlying  deposits.  In  some  of  the  coal-mines  in  central  Scotland, 
for  instance,  deep  channels  have  been  met  with  entirely  filled  with  sand, 
gravel,  or  clay  belonging  to  the  general  superficial  drift  of  the  country. 
These  channels  have  evidently  been  water-courses  worn  out  of  the  Coal- 
measure  strata  at  a  comparatively  recent  geological  period,  and  subse- 
qaently  buried  under  glacial  accumulations.  There  is  a  complete  dis- 
cordance between  them  and  the  Paleozoic  strata  below,  pointing  to  the 
existence  of  a  vast  interval  of  time. 

Surface-markings. — The  surface  of  many  beds  of  sandstone  is  marked 
with  lines  of  wavy  ridge  and  hollow,  such  -as  may  be  seen  on  a  sandy 
shore  from  which  the  tide  has  retired,  on  the  floors  of  shallow  lakes  and 
of  river-pools,  and  on  surfaces  of  dry  wind-blown  sand.  To  these 
markings  the  general  name  of  Ripple-mark  has  been  given.  They  have 
been  produced  by  an  oscillation  of  the  medium  (water  or  air)  in  which 


n«.a 


n  of  RLppled  Suitece. 


the  loose  sand  has  lain.  In  water,  an  oscillatory  movement,  sometimes 
also  with  a  more  or  less  marked  current,  is  generated  by  wind  blowing  on 
its  surface.  The  sand-graina  are  carried  backwards  and  forwards.  By 
degrees,  inequalities  of  surface  are  produced,  which  give  rise  to  vortices 
in  the  water.  In  in-egular  ripple-mark,  the  direct  current  carries  the 
sand  up  the  weather-alope,  while  the  vortex  pushes  it  up  the  lee-slope, 
until  the  surface  of  the  sand  becomes  mottled  over  with  little  prominences 
or  dunes.  In  regular  ripple-mark,  the  forms  are  produced  by  water 
oscillating  relatively  to  the  bottom  and  the  consequent  establishment  of 
a  series  of  vortices.'  The  long  gentle  slope  towards  the  wind,  and  the 
short  steep  slope  away  from  it,  are  well  marked  (Fig.  20i,  compare  also 
Fig.  91).  Considerable  diversity  in  the  form  of  the  ripple,  however,  may  be 
observed  (as  at  a  i  c  in  Fig.  205),  depending  on  conditions  of  wind,  water, 
and  sediment  which  have  not  been  thoroughly  studied.  No  satisfactory 
inference  can  be  drawn  from  the  existence  of  ripple-marks  as  to  the  precise 

>  Prof.  Datwin,  Prac.  Roy.  Soc.  iiivi.  (1883),  p,  18.  See  al^o  H.  C.  Sorby,  Edia. 
IffB  PhU.  Journ.  new  ser.  iii.  ir.  v.  vii.  ;  OeolBgal.  ii.  (1858),  p.  137  ;  A.  R.  Hunt.  Proc. 
Roy.  Soc  uxiv.  p.  1  ;  C.  de  Candolle,  Arch.  Sa.  Phg:  Ifat.  Omem,  ix.  (1883) :  M.  Forel, 
in  uhm  rolnme. 


508  GEOTECTONIG  {STRUCTURAL)  GEOLOGY  bookiv 

depth  of  water  in  which  the  sediment  was  accumulated.  As  a  rule,  it  is 
in  wat«r  of  only  a  few  feet  or  yards  in  depth  that  this  characteristic 
surface  is  formed.  But  it  may  be  produced  at  any  depth  to  which  the 
agitation  caused  by  wind  on  the  upper  waters  may  extend  (p.  438). 
Examples  of  it  may  l)e  observed  among  arenaceous  deposits  of  all  ages 
from  pre-Cambrian  upwards.  In  like  manner,  we  may  frequently  detect, 
among  these  formations,  small  isolated  or  connected  linear  ridges  (rill- 
marks)  directed  from  some  common  quarter,  like  the  current-marks 
frequently  to  be  found  behind  projecting  fragments  of  shell,  stones,  or 
bits  of  seaweed  on  a  beach  from  which  the  tide  has  just  retired. 

On  an  ordinary  beach,  each  tide  usually  effaces  the  ripple-marks 
made  by  its  predecessor,  and  leaves  a  new  series  to  be  obliterated  by  the 
next  tide.  In  the  process  of  obliteration,  the  tops  of  the  ridges  are 
levelled  off  (see  h  in  Fig.  205),  while  sometimes  the  hollows,  where  they 
serve  as  receptacles  for  surface  drainage,  are  deepened.  Where  the 
markings  are  formed  in  water  which  is  always  receiving  fresh  accumu- 
lations of  sediment,  a  rippled  surface  may  be  gently  overspread  by  the 
descent  of  a  layer  of  sediment  upon  it.  and  may  thus  be  preserved.  By 
a  renewal  of  the  oscillation  of  the  water  another  series  of  ripples  may 
then  be  made  in  the  overlying  layers,  which  in  turn  may  be  buried  and 
preserved  under  a  renewed  deposit  of  sand.  In  this  way,  a  considerable 
thickness  of  such  ripple-marked  strata  may  be  accumulated,  as  has  fre- 
quently taken  place  among  geological  formations  of  all  ages. 

Sun-cracks,  Rain-prints,  Vestiges  of  former  Shores.— 
One  of  the  most  fascinating  parts  of  the  work  of  a  field-geologist  con- 
sists in  tracing  the  shores  of  former  seas  and  lakes,  and  in  endeavouring 
thereby  to  reconstruct  the  geography  of  successive  geological  periods. 
There  are  not    a    few  pieces  of  evidence,  which,  though  in  themselves 


Fig.  i>()«;.  -Sun-cracked  surface  of  mud  or  muddy  sand. 

individually  of  apparently  small  moment,  combine  to  supply  him  with 
reliable  data.  Among  these  he  lays  special  emphasis  upon  the  proofs 
that,  during  their  deposition,  strata  have  at  intervals  been  laid  hare  to 
sun  and  air. 

The  nature  and  validity  of  the  arguments  founded  on  this  evidence 
will  be  best  realised  In'  the  student  if  he  can  make  observations  at  the 
margin  of  the  sea,  or  of  any  inland  sheet  of  water,  which  from  time  to 
time  leaves  tracts  of  mud  or  fine  ?and  exposed  to  sun  and  rain.  The 
way  in  which  the  muddy  bottom  of  a  dried-up  i)ool  cracks  into  polygonal 


PART  I 


SUN-CRACKS,  RAIN-PRINTS 


509 


cakes  when  exposed  to  the  sun  may  be  illustrated  abundantly  among  sedi- 
mentary rocks.  These  desiccation -cracks,  or  sun-cracks  (Fig.  206), 
could  not  have  been  produced  so  long  as  the  sediment  lay  under  water. 
Their  existence  therefore  among  any  strata  proves  that  the  surface  of 


Fig.  207.— Footprints  from  the  Triassic  Sandstone  of  Connecticut  (Hitchcock). 

rock  on  which  they  lie  was  exposed  to  the  air  and  dried,  before  the  next 
layer  of  water-borne  sediment  was  deposited  upon  it. 

With  these  markings  are  occasionally  associated  prints  of  rain-drops. 
The  familiar  effects  of  a  heavy  shower  upon  a  surface  of  moist  sand  or 
mud  may  be  witnessed  among  rocks  even  as  old  as  the  Cambrian  period. 
In  some  cases,  the  rain-prints  are  found  to  be  ridged  up  on  one  side,  in 
such  a  manner  as  to  indicate  that  the  rain-drops  as  they  fell  were  driven 


Fig.  -208.— Footprints  and  Sun-cracks.  Hildlmrphauscu,  Saxony  (SJcklor.. 

**l«int  by  the  wind.     The  prominent  side  of  the  markings,  therefore,  indi- 
cates the  side  towards  which  the  wind  blew. 

Numerous  proofs  of  shallow  shore-water,  and  likewise  of  exposure 
^  the  air,  are  supplied  by  markings  left  by  animals.  Castings,  tubular 
^burrows  and  trails  of  worms,  tracks  of   mollusks  and   crustaceans,  fin- 


510  GEOTEGTOXW  {STRUCTURAL)  GEOLOGY  book  iv 

marks  of  fishes,  footprints  of  reptiles  (Fig.  207),  birds,  &nd  maininalH,  may 
all  be  preserved  and  give  their  evidence  regarding  the  physical  conditioiu 
under  which  sedimentary  formations  were  accumulated.  It  may  fre- 
quently be  noticed  that  such  impressions  are  associated  with  ripple- 
marks,  rain-prints,  or  sun-cracks  (Fig.  208) ;  ao  that  more  than  one  kind 
of  evidence  may  be  gleaned  from  a  locality  to  show  that  it  was  sometimes 
laid  bare  of  wat«r. 

The  more  striking  indications  of  littoral  conditions  being  comparatively 
infrequent,  the  geologist  must  usually  content  himself  with  tracing  the 
gravelly  detritus,  which  suggests,  if  it  does  not  always  prove,  proximity 
to  some  former  line  of  shore.  Such  a  section,  for  instance,  aa  that  de- 
picted in  Fig.  209  may  often  be  found,  where  lower  strata  (a)  having 
been  tilted,  raised  into  land,  and  worn  away,  have  yielded  materials  for 
a  coarse  littoral  boulder  bed  (b),  over  which,  as  it  was  carried  down  into 
deeper  and  clearer  water,  limestone  eventually  accumulated.  Beds  of 
conglomerate,  especially  where,  as  in  this  example,  they  accompany  an 
uncon  form  ability  in  the  stratification,  are  of  much  service  in  tracing  the 
limits  of  ancient  seas  and  lakes  (see  Part  X.) 


Gas-spurts. — The  surfaces  of  some  strata,  usually  of  a  dark  colour 
and  containing  organic  matter,  may  be  observed  to  be  raised  into  little 
heaps  of  various  indefinite  shapes,  not  like  the  heaps  associated  ffill> 
worm-burrowa,  connected  with  pipes  descending  into  the  rock,  nor  com- 
posed of  different  material  from  the  surrounding  sandstone  or  »b»lt 
These  may  be  conjectured  to  be  due  to  the  intermittent  escape  of  gu 
from  decomposing  organic  matter  in  the  original  sand  or  mud,  as  we  nuj 
sometimes  witness  in  operation  among  the  mud-flats  of  rivers  and  estnuieii 
whore  much  organic  matter  is  decomposing  among  the  sediments  On  » 
small  scale,  these  protmsions  of  the  upper  surface  of  a  deposit  may  be 
compared  with  the  mud-lumps  at  the  mouths  of  the  Mississippi,  already 
described  (p.  399). 

Concretions. — Many  sedimentary  rocks,  more  particularly  days* 
ironstones,  and  limestones,  exhibit  a  concretionary  structure.  Thi^ 
arrangement  may  be  part  of  the  original  sedimentation,  or  may  be  du.^ 
to  subsequent  segregation  from  decomposition  round  a  centre.  Con>-- 
cretionary  structures  of  contemporaneous  origin,  particularly  in  calcareoO-'^ 
materials,  may  lie  so  closely  adjacent  as  to  form  continuous  or  nearly  ooi^ 
tinuoiis  beds  (Fig.  210).  The  Magnesi an  Limestone  of  Durham  iabontu^M 
of  variously  shaped  concretionary  masses,  sometimes  like  cannon-ball^' 


CONCRETIONS 


grape-shot,  or  buncbeB  of  com).     Connected  with  concretionary  beds  are 
the  seams  of  g}'psuni,  which  may  occasionally  be  observed  to  send  out 


iWWg«ki*iMar 


^^'^^H^^. 


veins  into  other  gypsum  beds  above  and  below  them.  De  la  Beche 
describes  a  section  at  Watchet,  Somersetshire,  where,  amid  the  Triassic 
marls  (b  b  in  Fig.  211),  beds  of  gypsum  (a  a)  connect  themselves  by  means 
of  fibrous  veins  with  the  overlying  and 
underlining  beds,  ^ 

The  most  frequent  form  of  concretions  „ 

is  that  of  isolated  spherical,  elliptical,  or  '■  "^■^^T^/^^T'OK^ 
variously  shaped  nodules,  disposed  in  ~^^^f'^j  "  '"»^j»i'  '^S'^ 
certain  layers  of  a  stratum  or  dispersed  ''  'fTf^*^''^^^^f^y^r^^x^  " 
irregularly  through  it  (Fig.  212).  They  i,^  Jii -s«tuimof  i»i>  .ud  «.m«cUns 
most  commonly  consist  of  ferrous  or  «tringi  of  npsum  id  the  Trt>^  WBichct, 
calcic  carbonates,  or  of  silica.  Many  clay-  soinerwtaiiire  (b.). 
ironstone  beds  assume  a  nodular  form,  and  this  mineral  occurs  abundantly 
in  the  shape  of  separate  nodules  in  shales  and  clay-rocks.  The  nodules 
have  frequently  formed  round  some  oi^nic  body,  such  as  a  fragment  of 
plant,  a  shell,  bone,  or  coproHte.     That  the  carbonate  was  slowly  precipi- 


tated during  the  fomiatiou  of  the  bed  of  shale  in  which  its  nodules  lie, 
may  often  be  satisfactorily  proved  by  the  lines  of  deposit  passing  contin- 
uously through  the  nodules  (Fig.  213).  In  many  cases,  the  internal  first- 
formed  parts  of  a  nodule  have  contracted  more  than  the  outer  and  more 
^'Ompact  crust ;  and  have  cracked  into  open  polygonal  spaces,  which  are 
commonly  filled  with  calcite  (Fig.  26).     Such  spptarian  wdules,  whether 


geotectonk;  ^structural)  geology 


comjioijed  of  clay-iron  stone  or  limestone,  iire  abundant  in  many  shalea,  u 
it)  the  Carlwiiiferoxis  and  Liassic  aeries  of  England. 

Alluvial  clays  somctinios  contain  fantastically  shaped  concretions  due  to 
the  consolidation  of  the  clay  liy  a  calcarooiia  or  feiruginouB  cement  round 
a  centre.  These  are  known  in  Scotland  as  fairy-atones,  in  the  ^'alley 
of  the  Khine  as  Losspupi)en,  Liissmanchcn,  and  in  Finland  as  Imatra-stonef 
(Fig.  214  and  p.  332).  They  not  uncommonly  show  the  bedding  of  the 
clay  in  which  they  may  have  l«on  fomied.  Their  qnaint  imitative  forms 
have  natni-.dly  given  rise  to  a  jwpnlar  belief  that  they  are  petrifications  of 
various  kinds  of  organic  Ixxlies  and  eien  of  articles  of  human  manufacture. 


In  Norway  thev  occur  in  <!lacial  and  jiost-glacial  dejwsits  up  to  heights  el 
3r>U  feet  above  Kt;a-level,  and  enclosi-  ri'mains  of  fishes  (of  which  16  apedK 
ha^e  been  noticed),  as  well  as  other  orjxanisms.' 

(.'oncrctions  of  silica  occur  in  limestJinc  of  many  geological  ages  (p.  4951- 
The  flint.4  of  tlie  English  Chalk  are  a  familiar  example,  but  simibi 
siliceous  concretions  occur  In  CarliOTuferous  and  Cambrian  limestooH- 
Tlie  silica,  in  these  case^,  has  not  infrc<|uently  been  deposited  round 
urgiiiiic  Inxlies,  such  as  s]longe^  sea-urchins,  and  mollneks,  which  are  com- 
pli'tely  enveloped  in  it,  and  have  even  themselves  Iwen  silicified.  Iron- 
disnlphide  often  assumes  the  foi-m  of  conditions,  more  particularly  amon^ 
'  Kj^riilf,   ■(;M.l0Bie  .lis  MiJl,  UU.I  iiiiKl.   XorweKtiw"  (ISSOt.  !■.  6;   B.  Cdlrt.  J¥ 


ALTERNATIONS  OF  STRATA 


&I3 


ctay-rocks,  and  these,  though  presenting  many  eccentricities  of  shape — 
round,  like  pistol-shot  or  cannon-balls,  kidney-shaped,  botryoidal,  &c. — 
agree  in  usually  poe&efising  an  internal  fibrous  mdiated  structure. 
Phosphate  of  lime  ia  found  as  concretions  in  formationB  where  the 
coprolites  and  bones  of  reptiles  and  other  animals  have  been  collected 
together  (see  p.  494). 

Concretions  produced  subsequently  to  the  formation  of  the  rock  occur 
in  some  sandstones,  which,  when  exposed  to  the  weather,  decomjxise  into 
large  round  balls.  In  other  instances,  a  ferruginous  cement  is  gradually 
aggregated  by  percolating  water  in  lines  which  curve  round  so  as  to 
enclose  portions  of  the  rock.  These  lines,  owing  to  abstraction  of  iron 
from  within  the  spheroid  and  partly  from  without,  harden  into  dark 
crusts,  inside  of  which  the  sandstone  becomes  quite  bleached  and  soft.* 
Some  shales  exhibit  a  concretionary  structure  in  a  still  more  striking 
manner,  inasmuch  as  the  concretions  consist  of  the  general  mass  of  the 
laminated  shale,  and  the  lines  of  stratification  pass  through  them  and 
mark  them  out  distinctly  as  superinduced  upon  the  rock.  Examples  of 
this  structure  are  not  infrequent  among  the  argillaceous  strata  of  the 
Carboniferous  system.  The  concretionary  olive-green  shales  and  mud- 
stones  of  the  Ludlow  group,  in  the 
Upper  Silurian  system,  exhibit  on 
weathered  surfaces,  all  the  way 
from  South  Wales  into  central  Scot- 
land, a  peculiar  structure  which 
consists  in  the  development  of  con- 
centric spheroids  varying  from  less 
than  an  inch  up  to  several  feet  in 
diameter,  the  successive  shells  being 
separated  from  each  other  by  a  fine 
dark  ferruginous  film  (Fig.  '2\5). 
The  lines  of  stratification  are  some-  shire{B.) 
times    well   marked    by   layers  of 

fossils,  but  the  rock  splits  up  mainly  along  the  curved  surfaces  separating 
the  concentric  shells.  Concretionary  structures  are  found  also  in  rocks 
formed  from  chemical  precipitation,  as  for  instance  in  beds  of  rock-salt. 
The  pseudo-concretions  probably  due  to  pressure  (stylolites)  have  been 
already  described  (p.  316). 

Alt«Fttatlons  and  Associations  of  Strata. — Though  great  variations 
occur  in  the  nature  of  the  strata  com{Kising  a  mass  of  sedimentary  rocks, 
it  may  often  be  observed  that  certain  rejietitions  occur.  Sandstones,  for 
example,  are  found  to  be  interleaved  with  shale  above,  and  then  to  pass 
into  shale ;  the  latter  may  in  turn  become  sandy  at  the  top  and  be  finally 
covered  by  sandstone,  or  may  assume  a  calcareous  character  and  pass  up  into 
limestone.  Such  alternations  bring  before  us  the  conditions  under  which 
the  sedimentation  took  plitce.  A  siindstone  group  indicates  water  of 
comparatively  little  depth,  moved  by  changing  currents,  bringing  the  sand, 
now  from  one  side,  now  fri>m  another.      The  passage  of  sudi  a  group 

■  Set  Penning,  f/wi.  Mag.  Det.  2,  iii.  May  1S76. 


514 


GEOTECTONIC  {STRUCTURAL)  GEOLOGY 


BOOK  IV 


into  one  of  shale  points  to  a  diminution  in  the  motion  and  transporting 
power  of  the  water,  perhaps  to  a  sinking  of  the  tract,  so  that  only  fine  mud 
was  intermittently  brought  into  it.  The  advent  of  limestone  above  the 
shale  serves  to  show  that  the  water  cleared,  owing  to  a  deflection  of  the 
sediment-carrying  currents,  or  to  continued  and  perhaps  more  rapid  sub- 
sidence, and  that  foraminifera,  corals,  crinoids,  moUusks,  or  other  lime- 
secreting  organisms,  established  themselves  upon  the  spot.  Shale  over- 
lying the  limestone  would  tell  of  fresh  inroads  of  mud,  which  destroyed 
the  animal  life  that  had  been  flourishing  on  the  bottom ;  while  a  return 
of  sandstone  beds  would  mark  how,  in  the  course  of  time,  the  original 
conditions  of  troubled  currents  and  shifting  sandbanks  returned. 
Such  alternating  groups  of  sandy,  calcai'eous,  and  argillaceous  strata 
are  well  illustrated  among  the  Jurassic  formations  of  England  (Fig.  216). 
Certain  kinds  of  strata  commonly  occur  together,  because  the  con- 
ditions under  which  they  were  formed  were  apt  to  arise  in  succession. 
One  of  the  most  familiar  examples  is  the  association  of  coal  and  fire-clay. 
In  Britain  a  seam  of  coal  is  generally  found  to  lie  on  a  bed  of  fire-clay,  or 


a 


Fig.  21(5. —Section  of  strata  from  the  base  of  the  Liaa  doD^-n  to  the  top  of  the  TriM,  Sliepton  Mallet  (&) 

a,  Grey  Lias  liutestone  and  marls  ;  h,  (>arthy  whitish  limestone  and  marls ;  c,  earthy  white  limMtooe; 
(f,  arenaceous  limestone ;  /,  grey  marls ;  g,  red  marls ;  A,  sandstone  with  calcarGOOs  cement ;  i,  blw 
marl ;  k,  red  marl :  /,  blue  marl ;  m,  red  marls. 

on  some  argillaceous  stratum.  The  reason  of  this  union  becomes  at  once 
apparent  when  we  learn  that  the  fire-clay  was  the  soil  on  which  the 
plants  grew  that  went  to  form  the  coal.  Where  the  clay  was  laid  down 
under  suitable  circumstances,  vegetation  sprang  up  upon  it  This 
appears  to  have  taken  place  in  wide  shallow  lagoon-like  expansions  of 
the  sea,  bordering  land  clothed  with  dense  vegetation,  and  to  have  been 
accompanied  by  slow,  intermittent,  but  prolonged  subsidence  of  the  sesr 
l>ottom.  Hence,  during  pauses  of  the  downward  movement^  when  the 
water  shoaled,  an  abundant  growth  of  water-loving  or  marshy  plants 
sprang  up  on  the  muddy  l)ottom,  somewhat  like  the  mangrove-swamps  <rf 
the  j)resent  day,  and  continued  to  flourish  until  the  muddy  soil  was 
exhausted,^  or  until  subsidence  recommenced  and  the  matted  jungles, 
carried  under  the  water,  were  buried  under  fresh  inroads  of  sand  or  muA 

'  Sterry  Hunt  has  called  attention  to  the  fact  that  the  underclays  of  the  Coal-meftfom 
have  generally  been  deprived  of  their  alkalies  by  the  vegetable  growth  which  tlwy 
supported.  In  the  little  coal-basins  of  France  evidence  has  been  obtained  that  much  of  the 
coal  was  formed  ont  of  vegetation  that  had  been  swept  down  and  buried  by  rapid  caiwnts.. 
See  the  Memoir  of  M.  Fayol  cited  on.  p.  500. 


PART  I  RELATIVE  PERSISTENCE  OF  STRATA  616 

Each  coal-field  thus  conCaina  a  succession  of  buried  forests  with  a  coDatant 
repetition  of  the  same  kind  of  interrening  strata  (Fig.  217). 

For  obvious  reasons,  conglomerate  and  sandstone  occur  together, 
rather  than  conglomerate  and  shale.  The 
agitation  of  the  water  which  could  form  and 
deposit  coarse  detritus,  like  that  composing  . 
conglomerate,  was  too  great  to  admit  of  the 
accumulation  of  fine  silt.  On  the  other  hand, 
we  may  look  for  shale  or  clay  rather  than 
sandstone,  as  an  accompaniment  of  limestone, 
inasmuch  as  when  the  gentle  currents  by 
which  fine  argillaceous  silt  was  carried  in  sus- 
pension ceased,  they  would  be  succeeded  by 
intervals  of  quiet  clearing  of  the  water,  during 
which  calcareous  material  might  be  elaborated 
either  chemically  or  by  the  action  of  living 
organisms. 

Relative  persistence  of  Strata. — A  little 
reflection  will  convince  the  student  that  all 
sedimentary  rocks  must  tiiin  out  and  disappear, 
and  that  even  the  most  persistent^  when 
regarded  on  the  great  scale,  are  local  and  lentic- 
ular accumulations.  Derived  from  the  degrada- 
tion of  land,  they  hare  accumulated  near  land. 
They  are  necessarily  thickest  in  mass,  aa  wel! 
as  coarsest  in  texture,  nearest  to  tbe  i 
of  supply,  and  become  more  attenuated  and 
fine-grained  as  tiiey  recede  from  it.  We  have 
only  to  observe  what  takes  place  at  the  present 
time  on  lake-bottoms,  estuaries,  or  sea-margins, 
to  be  assured  that  this  ie  now,  and  must  always 
have  been,  the  law  of  sedimentation.  C*^  urown).' 

But  while    all   sedimentary  deposits  must  ">  "I'l'tonM  ^  i..  nhsiei ;  c.  war 
be  regarded    as  essentially  local,  some    kinds       utd  gcain|ia  <»  hib. 
possess  a  far  greater  persistence  than  others. 

As  a  general  rule,  it  may  be  aaid  that  the  coarser  the  grain,  the  more  local 
the  extent  of  a  rock.  Conglomerates  are  thus  by  much  the  most  variable 
and  inconstant  of  all  sedimentary  formations.  They  suddenly  sink  down 
from  a  thickness  of  several  hundred  feet  to  a  few  yards  or  die  out  alto- 
gether, to  reappear,  perhaps  further  on,  in  the  same  wedge-like  fashion. 
Sandstones  are  less  liable  to  such  extremes  of  inconstancy,  but  they  too 
are  apt  to  thin  away  and  to  swell  out  again.  Shales  are  much  more  per- 
sistent, the  same  zone  being  often  traceable  for  many  miles.  Limestones 
sometimes  occur  in  thick  local  masses,  as  among  the  Silurian  formations,  but 
they  often  also  display  remarkable  coatinuity.  Three  thin  limestone  bands, 
eacb  of  them  only  two  or  three  feet  in  thickness,  and  separated  by  a  con- 
'  See  R.  Brown,  Quart.  Joum.  Otot.  Soc.  vi.  p.  115  ;  uid  De  la  Beche.  'Geol.  Observer,' 
p.  SOS. 


ty  Cnnl-Fielcl,  C«pe  Bnl 


616  GEOTECTOXia  (STRUCTUSAL)  GEOLOGY  book  it 

Biderabla  thickness  of  intervening  sandstones  and  shales,  can  be  tmced 
through  the  coal-fields  of  central  Scotland  over  an  area  of  at  least  1000 
square  miles.  Coal-seams  also  posBess  great  i>ersi8tence.  The  same 
seams,  varying  slightly  in  thickness  and  quality,  may  often  be  traced 
throughout  the  whole  of  an  extensive  coal-field. 

What  is  thus  tnie  of  individual  strata  may  be  affirmed  also  of  group* 
of  such  strata.  A  thick  mass  of  sandstone  will  be  found  as  a  rule  to  be 
more  continuous  than  one  of  conglomerate,  but  less  so  than  one  of  shale. 
A  series  of  limestone  beds  usually  stretches  further  than  either  arenaceooi 
or  argillaceous  sediments.  But  even  to  the  most  extensive  stratum  or 
group  of  strata  there  must  be  a  limit  It  must  end  off,  and  give  place 
to  others,  either  suddenly,  as  a  bank  of  shingle  is  succeeded  by  the  sheet 
of  sand  heaped  against  its  base,  or,  as  is  more  usual,  very  gradually,  by 
insensibly  passing  into  otlier  strata  on  all  sides. 

Great  variations  in  the  character  of  stratified  rocks  may  frequently  be 
observed  in  passing  from  one  part  of  a  country  to  another  along  the  out- 
crop of  the  same  rocks.     Thus,  at  one  end,  we  may  meet  with  a  thick 


scries  of  sandstones  which,  traced  in  a  certain  direction,  may  be  found 
iwasiug  into  shales  (Fig.  218).  A  group  of  strata  may  consist  of  massive 
conglomerates  at  one  locality,  and  may  graduate  into  fine  fissile  flagstones 
in  another.  A  thick  mass  of  clay  may  Ije  found  to  alternate  more  and 
more  with  shelly  sands  as  it  is  traced  outward,  until  it  loses  its  ai^li- 
ceous  nature  altogether. 

IiitciTiiting  illuslratioiis  of  such  arisDgem(Mita  occur  in  Che  Bouth-w»st  of  Endami, 
whpre  what  are  now  groa\is  of  hiUa,  like  the  Sleiidip,  Malvern,  anrt  other  erointviaa 
formerly  existeii  as  islands  iu  the  Mcsoioic  sea.  IX'  la  Beclie  pointed  out  that  tbe 
uiitunieil  Cariwiiiftniuji  linieatoiio  (a  a  in  Vig,  219)  haa  formed  the  short  agaiott  whidi 


tlie  voaiM  ^liiiigU'  of  tlie  dulomitic  congloiiit'raU 
trnci:d  awny  from  its  shore-line,  iiasiteit  on  the  sa 
duriiif^  II  gradual  .nubsidcucc,  the  clays  und  tinu-! 
ilepre»!icd  shore-line.     He  likewiao  ealleil  attctitit 


{!>  b)  accumulated  ;  that  the  latter, 
le  i>laue  into  red  marl  (c),  and  that 
(ones  of  the  Lias  (d)  crept  anr  tht 
1  to  tliu  important  fact  that,  in  nch 


PART  I  INFLUENCE  OF  STRATA    UPON  DIP  517 

cases,  a  continuous  zone  of  conglomerate  may  belong  to  many  successive  horizons.  In 
Fig.  220  a  section  is  given  from  one  of  the  islands  in  the  south-west  of  England,  round 
which  the  Trias  and  Lias  were  deposited.  Denudation  has  stripped  off  a  jwrtion  of  the 
overlying  red  marls.  If  the  rest  of  the  section  to  the  left  of  the  dotted  line  (d  d)  were 
removed,  there  would  remain  a  continuous  mass  of  conglomerate,  which,  in  default  of 
other  evidence  to  the  contrary,  would  be  regarded  as  one  bed  laid  down  upon  the  sloping 
surface  of  limestone,  instead  of,  what  it  really  is,  a  series  of  shore  gravels  piled  upon 
each  other,  and  belonging  to  a  consecutive  series  of  deposits. 

ft 


Fig.  220.  -Sectirm  of  part  of  the  flank  of  the  Mendip  Hills  (B.), 

showing  the  Carboniferous  Limestone  (a  a)  overlaid  by  dolomitic  conglomerate  (b  b), 

and  that  by  red  marls  (c). 

Mere  difference  of  lithological  character,  even  within  a  limited 
geographical  space,  does  not  necessarily  mean  diversity  of  age.  At  the 
present  day,  coarse  shingle  may  be  formed  along  the  beach,  at  the  same 
time  that  the  finest  mud  is  being  laid  down  on  the  same  sea-bottom 
further  from  land.  The  existing  differences  of  character  between  the 
deposits  of  the  shore  and  of  the  opener  sea  would  no  doubt  continue  to 
be  maintained,  with  slight  geographical  displacements,  even  if  the  whole 
area  were  undergoing  subsidence,  so  that  a  thick  group  of  littoral  deposits 
might  gather  in  one  tract,  and  of  deeper-water  accumulations  in  another. 

Among  the  formations  of  former  geological  periods,  the  same  conditions  of  deposition 
appear  sometimes  to  have  continued  for  enormous  periods.  The  thick  Carboniferous 
Limestone  of  western  Europe  evidently  accumulated  during  a  slow  subsidence,  when 
the  same  conditions  of  clear  water  with  abundant  growth  of  crinoids,  corals,  &c. ,  con- 
tinued for  a  period  vast  enough  to  admit  of  the  gradual  gro^'th  of  thousands  of  feet  of 
calcareous  matter.  Traced  northwards  into  Scotland,  this  massive  limestone  is  gradually 
replaced  by  sandstones,  shales,  ironstones,  and  coal-seams.  These  strata  prove  that  the 
deeper  and  clearer  water  of  Belgium,  central  England,  and  Ireland  passed  northwards  into 
muddy  flats  and  sandy  shoals,  which  at  one  time  were  overspread  with  coal-growths, 
and  at  another,  owing  to  more  rapid  subsidence,  were  depressed  beneath  the  clearer  sea 
which  brought  with  it  the  corals,  crinoids,  moUusks,  &c.,  whose  remains  are  now  to  be 
seen  in  intercalations  of  crinoidal  limestone. 

Influence   of   the   Attenuation    of    Strata  upon    apparent  Dip. 

— Where  a  thick  mass  of  sedimentary  materials  rapidly  thins  away  in  a 
given  direction,  a  deceptive  resemblance  to  the  effects  of  underground 
movement  may  be  observed.  If,  for  example,  we  suppose  that  on  a 
perfectly  level  bottom,  a  series  of  sedimentary  beds  is  accumulated  at  one 
place  to  a  depth  of  5000  feet,  and  that  this  series  dies  out  in  a  distance 
of  80  miles,  the  inclination  due  to  this  attenuation  will  amount  to  a  slope 
of  about  62  feet  in  a  mile.  That  this  structure  has  not  been  without 
considerable  influence  on  the  apparent  dip  of  stratified  rocks  has  been 
well  shown  by  Mr.  W.  Topley  with  reference  to  the  Mesozoic  rocks  of  the 
south-east  of  England.^ 

*  Quart.  Journ.  Gtd.  Soc.  xxx.  (1874),  p.  186. 


518  GEOTECTONIG  (STRUCTURAL)  GEOLOGY  book  iv 

Overlap. — Sediment  laid  down  in  a  subsiding  region,  wherein  the 
area  of  deposit  is  gradually  increased,  spreads  over  a  progressively  aug- 
menting surface.  Under  such  circumstances,  the  later  portions  of  a  for- 
mation, or  series  of  sedimentary  accumulations,  will  extend  beyond  the 
limits  of  the  older  parts,  and  will  repose  directly  upon  the  shelving 
bottom.  This  relation,  called  Overlap  (Fig.  221),  in  which  the  higher 
or  newer  members  are  said  to  **  overlap "  the  older,  may  often  be 
detected  among  formations  of  all  geological  ages.  It  brings  before  ns 
the  shore-lines  of  ancient  land-surfaces,  and  shows  how,  as  these  sank 
under  water,  the  gravels,  sands,  and  silts  gradually  advanced  and  covered 
them. 

This  structure  must  be  carefully  distinguished  from  Unconformability 
(posteu,  p.  64 1 ).  In  Overlap  there  is  no  break  in  the  sequence  of  formations ; 
the  strata  that  overlap  follow  on  continuously  upon  these  which  are  over- 
lapped. But  in  unconformability  there  is  a  break  in  the  succession,  the 
overlying  rocks  have  been  laid  down  on  the  previously  uptilted  and 


Fig.  221.— Section  of  Overla])  in  the  Lower  Jurassic  series  of  the  South-west  of  BngUnd  (£.) 
Thie  Old  Red  Sandstone  (r).  Lower  Limestone  Shale  (b\  and  Carboniferous  Limestone  (a)  having  been 
previously  npraised  and  denuded,  the  older  beaches  (d  m),  laid  down  nnconfonnably  upon  them, 
were  successively  covered  by  conformable  Jurassic  beds.  The  Lias  (e),  with  its  uppersands  (/X  is  otw- 
lapped  by  the  extension  of  the  Inferior  Oolite  (g)  completely  across  their  edges,  untal  this  formaticio 
comes  to  rest  directly  on  the  Paleozoic  strata  at  tu  Tlie  corresponding  extension  of  the  overljing 
Fuller's  Earth  (h  I)  and  limestone  (i)  has  been  removed  by  deuudation.i 

denuded  edges  of  those  below  them.  In  Fig.  221,  for  example,  the  upper 
or  Mesozoic  formations  (d  to  i)  form  an  unbroken  series,  so  do  the  lower 
or  Palaeozoic  strata  (a  b  c),  but  the  latter  have  been  disturbed  and  worn 
down  before  the  deposition  of  the  strata  above  them.  The  two  series  are 
said  therefore  to  be  unconformable. 

Relative  Lapse  of  Time  represented  by  Strata  and  by  the  Intervals 
between  them. — Of  the  absolute  length  of  time  represented  by  any  strata 
or  groups  of  strata,  no  satisfactory  estimates  have  yet  been  possible. 
Certain  general  conclusions  may  indeed  be  drawn,  and  comparisons  may 
l)e  made  between  different  series  of  rocks.  Sandstones  full  of  false- 
bedding  were  probably  accumulated  more  rapidly  than  finely-laminated 
shales  or  clays.  It  is  not  uncommon  in  certain  Carboniferous  sandstones 
to  find  huge  sigillarioid  and  coniferous  trunks  imbedded  in  upright  or 
inclined  positions.  Where,  as  in  Fig.  222,  the  trees  actually  grew  on  the 
spot  where  their  stems  remain,  it  is  evident  that  the  rate  of  deposit  of  the 
sediment  which  entombed  them  must  have  been  sufficiently  rapid  to  have 
allowed  a  mass  of  twenty  or  thiity  feet  to  accumulate  before  the  decay  of 
the  wood.  Of  the  durability  of  these  ancient  trees  we  of  course  know 
nothing  ;  though  modern  instances  are  on  record  where,  under  certain 
circumstances,  submerged  trees  may  last  for  some  centuries.     We  may 

^  De  la  Beche,  •  Gaol.  Observer,'  p.  485. 


RELATIVE  LAPSE  OF  TIME 


619 


conjecture  that  wbere  upright  or  inclined  stems  are  enveloped  in  one  con- 
tinuous stratum,  the  rate  of  accumulation  was  probably,  on  the  whole, 
somewhat  rapid.  The  general  character  of  the  strata  among  which  such 
erect  tree-tranks  occur,  obviously  indicates  extremely  shallow  water  con- 
ditions with  continuous  or  intermittent  subsidence.  Unless  soon  submerged, 
dead  trees  would  be  subject  to  speedy  subaerial  decomposition.  It 
occasionally  happens  that  an  erect  trunk  has  kept  its  position  even  during 
the  accumulation  of  a  series  of  strata  aroun<l  it  (Fig.  223).  We  can  hardly 
believe  that  in  such  cases  any  considerable  number  of  years  could  have 
elapsed  between  the  death  of  the  tree  and  its  final  entombment    From  the 


Fig.  242.— Erect  troii 


i«  Museum  or  th'  Rejvl  I] 


decayed  condition  of  the  interior  of  some  imbedded  trees,  we  may  likewise 
infer  that  accumulation  of  sediment  is  not  always  an  extremely  slow 
process.  Instances  occur  where,  as  Fig.  224,  while  sand  and  mud  have 
been  accumulating  round  the  submerged  stem,  its  interior  has  been  rotting, 
BO  that  eventually  a  mere  hollow  cylinder  has  been  left,  into  which  sediment 
and  different  plants  (sometimes  with  the  bodies  of  land  animals)  were 
introduced  from  above.^  Large  coniferous  trunks  (as  in  the  neighbour- 
hood of  Edinburgh)  have  been  imbedded  in  sandstone,  and  have  had  their 

'  De1aB«cbe,  'Geol.  Observer,*  p.  501. 

'  The  hollow  tree-trunks  of  the  Nova  Scotian  coal. fields  have  yivldeil  n  most  interestiug 
Mries  of  terraBtrial  orgatiiiimn — land-SLnils  and  reptiles.  For  illustmtioni  of  trees  in  Coal- 
iQ«uare  strata  and  the  dejiositien  of  sediineat  round  them  see  the  Atlas  to  M.  Payora 
Memoir  cil«d  on  p.  500. 


620 


GEOTECTONIC  {STRUCTURAL)  GEOLOGY 


BOOK  IV 


a 


internal  microscopic  structure  well  preserved.      In  siich  examples,  the 
drifted  trees  seem  to  have  sunk  with  their  heavier  or  root-end  touching 

the  bottom,  and  their  upper  end  pointing 
upward  in  the  direction  of  the  current, 
like  the  snags  of  the  Mississippi,  and 
to  have  been  completely  buried  in  sedi- 
ment before  decay. 

Continuous  layers  of  the  same  kind 
of  deposit  suggest  a  persistence  of  geo- 
logical conditions ;  numerous  alternations 
of  different  kinds  of  sedimentary  matter 
point  to  vicissitudes  or  alternations  of 
conditions.  As  a  rule,  we  should  infer 
that  the  time  represented  by  a  given 
thickness  of  similar  strata  was  less  than 
that  shown  by  the  same  thickness  of  dis- 
''    -^^  ^— mfflnfflffllBBftTO     similar  strata,  because  the  changes  needed 

to  bring  new  varieties  of  sediment  into 
the  area  of  deposit  would  usually  require 
Fig.  'JJ3.— Erect  tree-tnink  rising  through    the  lapse  of   somo  time  for  their  com- 

a  snecfission  r>f  >stmta,  Killingworth  C«l-     pletiou.       But  this  COnclusioU  might  oftCU 

»  V  J^^t^*^,  ^       .X.  be  erroneous.     It  would  be  best  supported 

a,  High  Mam  Cuil-seam ;    />,   bitnininouK         i  r  i  i-     i_  -i 

shale ;  c,  blue  shale ;  c/,  compact  sand-  wheu,  f  rom  the  ver}'  uaturo  of  the  rocks, 
stone;  f,  siiaiea  and  sandstones;  /,  wide  Variations  in  the  character  of  the 
l^Z'TlZ^!''  1/.  micaceous  sand-    ^ater-bottom  could  be  established.    Thus 

a  group  of  shales  followed  by  a  fossiliferous 
limestone,  would  mark  a  j)eriod  of  slow  deposit  and  quiescence,  almost 
always  of  longer  duration  than  would  be  indicated  by  an  equal  depth 
of  sandy  strata,  pointing  to  more  active  sedimentation.  Thick  limestones, 
made  up  of  remains  of  organisms  which  lived  and 
died  upon  the  spot,  and  whose  remains  are  crowded 
together  generation  above  generation,  must  haA'e 
demanded  prolonged  periods  for  their  formation. 

But  in  all  speculations  of  this  kind,  we  must  bear 
in  mind  that  the  relative  length  of  time  represented 
by  a  given  depth  of  strata  is  not  to  be  estimated 
merely  from  thickness  or  litholoj'ical  characters.  It 
lias  already  been  pointed  out  that  the  interval  be- 
tween the  deposit  of  two  successive  laminje  of  shale  Fig.  224. — Erect  tfve-tnmic 
may  have  been  as  long  as,  or  even  longer  than,  that  («  «)  imbedded  in  mnd- 
required  for  the  formation  of  one  of  the  lamina;. 
In  like  manner,  the  interval  needed  for  the  transi- 
tion from  one  stratum  or  kind  of  strata  to  another 
may  often  have  been  more  than  equal  to  the  time 
required  for  the  formation  of  the  strata  of  either  kind. 
But  the  relative  chronological  importance  of  the  bars  or  lines  in  the 
geological  record  can  seldom  be  satisfactorily  discussed  merely  on  litho- 
logical  grounds.     This  must  mainly  be  decided  on  the  evidence  of  oi^ganic 


stones  (c  c)  and  thak» 
{d  d),  its  interior  filkd 
with  dilTerent  sandy  and 
clayey  stimta  (c  t\  and 
the  whole  coTered  by  a 
sandstone  bed  (b)  (A) 


A.RT  I  TERNARY  SUCCESSION  OF  STRATA  521 

^mains,  as  will  be  shown  in  Book  V.  By  this  kind  of  evidence,  it  can 
e  made  nearly  certain  that  the  intervals  represented  by  strata  were 
I  many  cases  much  shorter  than  those  not  so  represented, — in  other 
ords,  that  the  time  during  which  no  deposit  of  sediment  went  on  at  any 
articular  locality  was  longer  than  that  wherein  deposit  did  take  place. 

Ternary  Succession  of  Strata. — In  following  the  order  of  sedimenta- 
on  among  the  stratified  rocks  of  the  earth's  crust,  the  observer  will  be 
id  to  remark  a  more  or  less  distinct  threefold  ai'rangement  or  succession 
I  which  the  sandy,  muddy,  and  calcareous  sediments  have  followed  each 
ther.  Professor  John  Phillips  and  Mr.  Hull  have  called  attention  to  this 
-ructure,  illustrating  it  by  reference  to  the  geological  formations  of  Great 
ritain,  while  Professor  Newberry,  Dr.  Sterry  Hunt,  and  Principal 
^wson  have  discussed  it  in  relation  to  the  stratigraphical  series  of  North 
merica.  According  to  Mr.  Hull  a  natural  cycle  of  sedimentation  consists 
:  three  phases  ;  1st,  a  lower  stage  of  sandstones,  shales,  and  other  sedi- 
lentary  deposits,  representing  prevalence  of  land  with  downward  move- 
lent ;  2nd,  a  middle  stage,  chiefly  of  limestone,  representing  prevalence 
:  sea  with  general  quiescence  and  elaboration  of  calcareous  organic 
irmations;  3rd,  an  upper  stage,  once  more  of  mechanical  sediments 
idicative  of  proximity  to  land.^  Where  the  strata  are  interrupted  by 
isturbance  and  unconformability,  we  may  suppose  the  cycle  of  sedimenta- 
on  to  have  been  completed  by  upheaval  after  prolonged  subsidence, 
at  where  the  continuity  of  the  formations  is  unbroken,  as  it  is  over  such 
ist  tracts  in  North  America,  upheaval  is  not  required,  and  the  facts  seem 
cplicable,  as  Phillips  long  ago  showed,  on  the  idea  of  prolonged  but 
itermittent  subsidence.  Let  us  suppose  a  downward  movement  to 
)mmence,  and  to  depress  successive  sheets  of  gravel,  shingle,  sand,  and 
;her  shallow  water  accumulations,  derived  from  the  erosion  of  neigh bour- 
ig  land.  If  the  depression  be  comparatively  rapid,  the  bottom  may  soon 
5  carried  beyond  the  reach  of  at  least  the  coarser  kinds  of  sediment,  and 
arine  lime-secreting  organisms  may  afterwards  begin  to  form  a  calcareous 
yoT  beneath  the  sea.  Let  us  imagine  further,  that  the  subsidence  ceases 
ir  a  time,  and  that  by  the  accumulation  of  organic  remains,  and  partly 
so  by  the  deposit  of  fine  muddy  sediment,  the  water  is  shallowed. 
Tith  this  gradual  change  of  depth,  the  coarser  detritus  begins  once  more 
>  be  able  to  stretch  seawards,  and  to  overspread  the  limestones,  which, 
ider  the  altered  circumstances,  cease  to  be  formed.  A  gradual  silting 
y  of  the  area  takes  place,  marked  by  beds  of  sand  and  mud,  until  a 
tnewal  of  the  subsidence,  either  suddenly  or  slowly,  restores  the  previous 
5pth  and  clearness  of  water,  and  allows  either  the  old  marine  organisms, 
hich  had  been  driven  off,  or  their  modified  descendants  to  reoccupy  the 
■ea  and  build  new  limestone. 

1  Phillips,  Mem.  Oeol.  Surv.  ii.  ;  'Geol.  Yorkshire,'  ii.  ;  'Geol.  Oxford,'  p.  293;  Hull, 
tart.  Jour.  Sci.  July,  1869  ;  Newberry,  Proc.  Amer.  Assoc.  1873,  p.  185  ;  Proc.  Lyceum 
al.  Hist.  New  Vorkf  2nd  ser.  No.  4,  p.  122;  Hunt,  in  Logan's  *  Geology  of  Canada,' 
<6S,  p.  627  ;  Amer.  Journ.  Sci.  (2nd  series),  xxxv.  p.  167  ;  Dawson,  Q,  J.  Oeol.  Soc  xxii. 
102  ;  *  Acadian  Geology*,'  p.  135.  Compare  on  this  subject  E.  van  den  Broeck,  BhU,  Mvs. 
oy,  BruxdUsy  ii.  (1883),  p.  341  ;  A.  Rutot,  op,  cit,  p.  41. 


522  GEOTECTONIC  (STRUCTURAL)  GEOLOGY  book  iv 

Groups  of  Strata. — Passing  from  individual  strata  to  large  masses  of 
stratified  rock,  the  geologist  finds  it  needful  for  convenience  of  reference 
to  subdivide  these  into  groups.  He  avails  himself  of  two  bases  of  classifi- 
cation— (1)  lithological  character,  and  (2)  organic  remains. 

1.  The  subdivision  of  stratified  rocks  into  groups  according  to  their 
minenil  aspect  is  an  obvious  and  easily  applied  classification.  Moreover, 
it  often  serves  to  connect  together  rocks  formed  continuously  in  certain 
circumstances  which  differed  from  those  under  which  the  strata  above  and 
below  were  laid  down — so  that  it  expresses  natural  and  original  subdivi- 
sions of  strata.  In  the  middle  of  the  English  Carboniferous  system  of 
rocks,  for  example,  a  zone  of  sandy  and  pebbly  beds  occurs,  known  as  the 
Millstone  Grit.  No  abrupt  and  sharp  line  can  be  drawn  between  these 
strata  and  those  above  and  below  them.  They  shade  upwanl  and  down- 
ward into  the  beds  between  which  they  lie.  Yet  they  form  a  conspicuous 
belt,  traceable  for  many  miles  by  the  scenery  to  which  it  gives  rise. 
Again,  the  red  rocks  of  central  England,  with  their  red  sandstones,  marls, 
rock-salt,  and  gypsum,  form  a  well-marked  group,  or  rather  series 
of  gi'oups.  It  is  obvious,  however,  that  characters  .of  this  kind,  though 
sometimes  wonderfully  persistent  over  wide  tracts  of  country,  must  be  at 
best  but  local.  The  physical  conditions  of  deposit  must  always  have 
been  limited  in  extent.  A  group  of  strata,  showing  great  thickness  in  one 
region,  will  be  found  to  die  away  as  it  is  traced  into  another.  Or  its 
I)lace  is  gradually  taken  by  another  group  which,  even  if  geologically 
contemporaneous,  possesses  totally  different  lithological  characters.  Just 
as  at  the  present  time  a  group  of  sandy  deposits  gradually  gives  place 
along  the  sea-floor  to  others  of  mud,  and  these  to  others  of  shells  or  of 
gravel,  so  in  former  geological  periods,  contemporaneous  deposits  were  not 
always  lithologically  similar.  Hence  mere  resemblance  in  mineral  aspect 
cannot  usually  be  regarded  as  satisfactory  evidence  of  contemporaneity, 
except  within  comparatively  contracted  areas.  The  Carboniferous  Lime- 
stone has  already  (p.  517)  been  cited  as  a  notable  example.  Typically  in 
Belgium,  central  England,  and  Ireland,  it  is  a  thick  calcareous  group  of 
rocks,  full  of  corals,  crinoids,  and  other  organisms,  which  bear  witness  to 
the  formation  of  these  rocks  in  the  open  sea,  But  traced  into  the  north 
of  England  and  Scotland,  it  passes  into  sandstones  and  shales,  Avith  numerous 
coal-seams,  and  only  a  few  thin  beds  of  limestone.  The  soft  clay  beneath 
the  city  of  London  is  represented  in  the  Alps  by  liard  schists  and  contorted 
limestones.  We  conclude,  therefore,  that  lithological  agreement,  when 
pushed  too  far,  is  apt  to  mislead  us,  partly  because  contemporaneous 
strata  often  vary  greatly  in  lithological  character,  and  partly  because  the 
samti  lithological  characters  may  appear  again  and  again  in  different  ages. 
By  trusting  too  implicitly  to  this  kind  of  evidence,  we  may  be  led  to  class 
together  rocks  belonging  to  very  different  geological  periods,  and,  on  the 
other  hand,  to  separate  groups  which  really,  in  spite  of  their  seeming 
distinction,  were  formed  contemporaneously. 

2.  It  is  by  the  remains  of  plants  and  animals  imbedded  among  the 
stratified  rocks  that  the  most  sjitisfactory  subdivisions  of  the  geological 
record  can  be  made,  as  will  be  more  fully  stated  in  Books  V.  and  VI. 


iT  II  JOINTS  623 

chronological  succession  of  organic  forms  can  be  made  out  among  the 
ks  of  the  earth's  crust.  A  certain  common  facies  or  type  of  fossils  is 
md  to  characterise  particular  groups  of  rocks,  and  to  hold  true  even 
mgh  the  lithological  constitution  of  the  strata  should  greatly  vary. 
>reover,  though  comparatively  few  species  are  universally  diffused,  they 
«ess  remarkable  persistence  over  wide  areas,  and  even  when  they  are 
laced  by  others,  the  same  general  facies  of  fossils  remains.  Hence 
I  stratified  formations  of  two  countries  geographically  distant,  and 
ring  little  or  no  lithological  resemblance  to  each  other,  may  be  compared 
I  paralleled  simply  by  means  of  their  enclosed  organic  remains. 

Order  of  Superposition — ^the  Foundation  of  Geological  Chrono- 
y. — As  sedimentary  strata  were  laid  down  upon  one  another  in  a 
re  or  less  nearly  horizontal  position,  the  underlying  beds  must  be  older 
n  those  which  cover  them.     This  simple  and  obvious  truth  is  termed 

Law  of  Superposition.  It  furnishes  the  means  of  determining  the 
onology  of  rocks ;  and  though  other  methods  of  ascertaining  this 
nt  are  employed,  they  must  all  be  based  originally  upon  the  observed 
er  of  superposition.  The  only  case  where  the  apparent  superposition 
y  be  deceptive  is  when  the  strata  have  been  inverted,  as  in  the  Alps 
.  540,  541),  where  the  rocks  composing  huge  mountain  masses  have 
n  so  completely  overturned  that  the  highest  beds  appear  as  if  regularly 
ered  by  others  which  ought  properly  to  underlie  them.     But  these 

exceptional  occurrences,  wherein  the  true  order  can  usually  be  made 

from  other  sources  of  evidence. 


Part  II.  Joints. 

All  rocks  are  traversed  more  or  less  distinctly  by  vertical  or  highly 
ined  divisional  planes  termed  Joints.^  Soft  rocks,  indeed,  such  as 
je  sand  and  uncomimcted  clay,  do  not  show  these  lines ;  but  where 
jdimentary  mass  has  acquired  some  degree  of  consolidation,  it  usually 
ws  them  more  or  less  distinctly.  It  is  by  means  of  the  intersection  of 
its  that  rocks  can  be  removed  in  blocks ;  the  art  of  quarrying  consists 
aking  advantage  of  these  natural  planes  of  division.  Joints  differ  in 
racter  according  to  the  nature  of  the  material  which  they  traverse ; 
se  in  sedimentary  rocks  are  usually  distinct  from  those  in  crystalline 
»es. 

M.  Daubr^e  has  proposed  a  classification  of  the  various  divisional  planes  of  rocks 
to  rupture  of  original  continuitj',  which  he  groups  together  as  Lithodaaes.  1.  Under 
:enn  Leptocinse  he  classes  minor  fractures,  which  may  be  either  (a)  syndases,  produced 
>me  internal  mechanical  or  molecular  action,  and  generally  by  contraction,  as  in  cooling 
drying  ;  or  {b)  piesodases,  produced  by  some  external  mechanical  movement,  particularly 
ressure,  as  in  the  structures  called  cone-in-cone,  stylolites,  and  ruiniform  marble.  2. 
loses  correspond  to  what  in  English  are  called  joints.  3.  Paradases  are  faults.  BuU. 
Oiol.  France  (3),  x.  p.  136.     On  jointing,  faulting,  and  cleavage  in  rocks  see  O.  Fisher, 

Mag,  1884,  204.  A.  Harker,  Ged.  Mag.  1885,  Brit.  Assoc.  1885,  p.  813.  G.  K. 
Jrt,  Amcr.  Joum.  iki.  xxiii.  (1882),  p.  25,  xxiv.  (1882),  p.  50,  xxvii.  (1884),  p.  47  ;  W. 
roeby,  Proc.  Boston  Soc.  Xat.  Hist.  xxii.  (1882),  p.  72,  xxiii.  p.  243. 


524  liKOTK'-TOSir    STRl'iTrUM..  GEUl.OGY  book  u 

1.  In  Stratified  Rocks. — To  the  presence  of  joints  some  of  the  rocwt 
familiar  features  of  rock-scenery  are  due  (Fig,  225).  Joints  rarr  in  the 
angles  at  which  they  cut  the  planes  of  bedjing,  in  the  sharpness  of  their 
definition,  in  the  regiiiikrity  of  their  ]icrpendiculBr  and  horizontal  conne, 
in  their  lateral  persistence,  in  number,  and  in  the  directions  of  their  inter 
section.  As  a  rule,  they  are  most  shaq^ly  defined  in  proiiortion  to  the 
fineness  of  grain  of  the  rock.  In  limestones  and  close-grained  slulea,  fw 
exanipte,  they  often  occur  so  clean-cut  as  to  be  invisible  until  revealed  br 
fracture  or  by  the  slow  disintegrating  effects  of  the  weather.  The  rock 
splits  up  along  these  conceale<l  lines  of  division,  whether  the  agent  of 
demolition  lie  the  hammer  or  frost.  In  coarse -textured  rocks,  on  the 
other  hand,  joints  are  apt  to  show  themselves  as  more  irregular  sinuon* 
rents. 

As  ii  rule,  they  run  [lerpendicular  or  approximateU  so  to  the  pUnee 
of  bedding,  and  descend  ^erticali\  at  not  ^er}  unequal  distances,  so  tbit 


tiie  puilions  of  rock  between  them,  when  seen  in  profile,  ap[>ear  marked 
off  into  so  many  wall-like  masses.  But  this  symmetry  often  gives  place 
to  a  more  or  less  tortuous  course  with  lateral  joints  in  various  random 
directions,  more  especially  where  the  ditferent  strata  vary  considerably  in 
lithological  characters.  A  single  joint  may  be  traced  for  many  yards, 
sometimes,  it  is  sai<l,  for  several  miles,  more  |Nirticularly  when  the  rock 
is  finegrained,  as  in  limestone.  But  where  the  texture  is  coarse  and 
unei[uul,  the  joints,  though  abundant,  run  into  each  other  in  such  a  way 
that  no  one  in  particular  can  be  identiUed  for  more  than  a  limited 
distance.  The  number  of  Joints  in  a  mass  of  stratified  rock  varies  witiiin 
wide  limitx.  Among  strata  which  have  undergone  little  disturbance  the 
joints  may  be  separated  from  each  other  by  intervals  of  several  yardt. 
But  in  other  cases  where  terrestrial  movement  has  been  considerable,  the 
rocks  are  so  jointed  as  to  have  acquired  therefrom  &  fissile  character  thit 
has  nearly  or  wholly  obliterated  their  tendency  to  split  along  the  lin« 
of  bediling. 

An  impoitant  featui-e  in  the  joints  of  stratified  rocks  is  the  directioa 
in  which  they  intersect  each  other.     In  general  they  have  two  domiiuut 


PiRTii  JOINTS  IN  STRATIFIED  ROCKS  52S 

trends,  one  coincident,  on  the  whole,  with  the  direction  in  which  the 
strata  are  inclined  from  the  horizon,  and  tlie  other  running  transversely 
at  a  right  angle  or  nearly  bo.  The  former  set  ia  known  aa  dip-joivts, 
because  they  run  with  the  dip  or  inclination  of  the  rocks ;  the  latter  ie 
termed  strike-joinls,  inaamuch  as  they  conform  to  the  atriie  or  general 
outcrop.  It  ia  owing  to  the  existence  of  this  double  aeries  of  joints  that 
ordinary  quarrying  operations  can  be  carried  on.  Large  quadrangular 
blocks  can  be  wedged  off,  which  would  be  shattered  if  exposed  to  the 
risk  of  bloating.  A  quarry  is  usually  worked  to  the  dip  of  a  rock  ;  hence 
the  strike-joints  form  clean-cut  faces  in  front  of  the  workmen  as  they 
lidvance.  These  are  known  as  "  backs,"  and  the  dip-joiuts,  which  traverse 
them,  us  "cutters."  The  way  in  which  this  double  set  of  joints  occurs 
in    a  quarry  may  be  seen  in  Fig.   326,  where  the  close  parallel  lines 


traversing  the  shaded  and  unsimded  faces  mark  the  planes  of  stratification, 
which  here  are  inclined  from  the  spectator.  The  steep  faces  in  light  are 
defined  by  the  strike-joints  or  "backs."  Tlie  faces  in  shadow  have  been 
quarried  out  along  dip-joints  or  "cutters."  It  will  be  observed  that  the 
long  face  in  sunlight  is  cut  by  parallel  lines  of  dip-joints  not  yet  opened 
in  quarrying,  while  in  like  manner,  the  shaded  face  to  the  right,  is  that 
of  a  dip-joint  which  is  traversed  by  parallel  lines  of  strike-joint 

Ordinary  household  coal  presents  a  remarkably  well-developed  system 
of  joints.  A  block  of  such  coal  may  be  observed  to  be  traversed  by  fine 
laminse,  the  surfaces  of  many  of  which  arc  soft  and  soil  the  fingers. 
These  are  the  planes  of  stratification.  Perpendicular  to  them  run 
divisional  planes,  which  cut  each  other  at  right  angles  or  nearly  so,  and 
thus  divide  the  mineral  into  cubical  fragments.  One  of  these  sets  of  joints 
makes  clean  sliarply  defined  surfaces,  and  is  known  as  the  face,  slyne,  cleat, 
or  bord ;  the  other  has  rougher,  less  regular  surfaces,  and  ia  known  as  the 


520  GEUTECTONIC  {STRUCTURAL)  GEOLOGY  book  if 


end.     The  face  remains  persistent  over  wide  areas;  it  serres  to  define 
the  direction  of  the  roadways  in  coal-mines,  which  must  run  with  it 

According  to  observations  made  by  Jukes,  both  strike -joints  and 
dip-joints  occur  in  beds  of  recently-formed  coral-rock  in  the  Australian 
and  other  reefs.  ^  In  like  manner,  a  remarkably  definite  system  of 
jointing  has  been  noticed  by  Mr.  Gilbert  in  the  recent  clays  and  mads 
of  the  dried-up  bed  of  the  Sevier  lake  in  Utah.  Such  modem  sediments 
have  certainly  never  been  subject  to  the  pressure  of  any  superincumbent 
rock,  nor  to  the  torsion  or  other  disturbance  incident  to  subterranean 
movement.  That  great  force  has  sometimes  been  concerned  in  the 
production  of  the  structure  is  instructively  shown  in  some  conglomerates, 
Avhere  the  joints  traverse  the  enclosed  pebbles,  as  well  as  the  surroond- 
ing  matrix,  in  such  a  way  that  large  blocks  of  hard  quartz  are  cut 
through  by  them  as  sharply  as  if  they  had  been  sliced  in  a  lapidary's 
machine,  and  the  same  joints  can  be  traced  continuously  through  many 


ONE  FOOT 


Fig.  2i7.— Plan  of  coare«i  cungluinerati'  of  blocks  of  Cambrian  rocks  in  CarboniferoiiB  Limastone, 
traversed  by  a  line  of  Joint  cutting  the  individual  boulders  in  the  line  a  h.  Coast  near  Skoiieft, 
Dublin  County  (/J.) 

yards  of  the  rock  (Fig.  227).-  Indication  of  relative  movement  of  the 
sides  of  a  joint  is  often  supplied  by  their  rubbed  and  striated  surfaces, 
termed  slickejisu/es,  which  have  evidently  been  ground  against  each  other. 
They  are  often  coated  with  haematite,  calcite,  chlorite,  or  other  mineral, 
which  has  taken  a  cast  of  the  strije  and  then  seems  itself  to  be  striated. 

The  ciiiiso  of  jointing  has  not  been  satisfactorily  explained.  Various 
theories  have  been  proposed  to  account  for  the  structure  ;  but  as  no  one 
will  explain  every  case,  it  is  probable  that  what  we  call  joints  may  have 
originated  in  several  different  ways,  or,  in  other  words,  that  the  results 
of  several  distinct  natural  processes  are  all  indiscriminately  comprised 
under  the  term  joint.     The  following  theories  may  be  enumerated. 

(1)  Contraction. — The  contraction  of  rocks  gives  rise  to  fissures  of 
retreat  in  their  mass,  whether  it  results  from  the  drying  and  consolidation 
of  aqueous  sediments  or  from  the  cooling  of  masses  that  have  been 

^  '  Manual  of  Geology.'  3rd  edition,  p.  184. 
2  De  la  Beche,  'Geol.  Observer,'  p.  628. 


PART  II  JOINTS  IN  MASSIVE  ROCKS  627 

molten  or  have  been  highly  heated.  The  prismatic  or  columnar  system 
of  joints  observable  in  the  gypsum  of  the  Paris  Basin,  of  which  the  beds 
are  divided  from  top  to  bottom  into  vertical  hexagonal  prisms,  may  be 
an  instance  of  this  causa  ^  A  columnar  structure  has  often  been  super- 
induced upon  stratified  rocks  (sandstone,  shale,  coal)  by  contact  with 
intrusive  igneous  masses  (p.  599). 

(2)  Crystalline  or  Magnetic  Forces. — Jointing  has  been  regarded 
as  referable  to  forces  analogous  to  those  that  have  produced  the  cleavage 
of  minerals,  the  difference  between  the  two  arising  perhaps  from  the 
forces  in  the  case  of  jointing  being  subordinated  to  terrestrial  magnetism, 
while  those  concerned  in  mineral  cleavage  are  obedient  to  crystalline 
polarity. 2     But  this  theory  has  met  with  little  support 

(3)  Compression. — Jointing  has  been  associated  by  some  authors 
with  cleavage  as  a  result  of  the  lateral  compression  of  rocks  (p.  312). 

(4)  Torsion. — From  experiments  on  the  behaviour  of  various  sub- 
stances under  the  strain  of  torsion,  M.  Daubr^e  concludes  that  a  system 
of  joints  may  be  explained  as  the  results  of  the  torsion  of  strata  arising 
during  the  movements  to  which  the  crust  of  the  earth  has  been 
subjected.^ 

(5)  Earthquakes. — The  existence  of  joints  has  been  referred  to  the 
results  of  the  earth -waves  generated  during  earthquakes,  the  rocks 
through  which  the  waves  pass  being  exposed  to  such  powerful  alternate 
compression  and  tension  as  to  rupture  them.^ 

Joints  form  natural  lines  for  the  passage  downward  and  upward 
of  subterranean  water.  They  likewise  furnish  an  effective  lodgment 
for  the  action  of  frost,  which  wedges  off  blocks  of  rock  in  the  manner 
already  described  (p.  414).  As  they  serve,  in  conjunction  with  bedding, 
to  divide  stratified  rocks  into  large  quadrangular  blocks,  their  influence 
in  the  weathering  of  these  rocks  is  seen  in  the  symmetrical  and  archi- 
tectmral  as  well  as  splintered,  dislocated  aspects  so  familiar  in  the  scenery 
of  sandstone  and  limestone  districts. 

2.  In  Massive  (Igrneous)  Rocks. — While  in  stratified  rocks,  the 
divisional  planes  consist  of  lines  of  bedding  and  of  joint,  cutting  each 
other  usually  at  a  high,  if  not  a  right  angle  ;  in  massive  (igneous)  rocks, 
they  include  joints  only ;  and  as  these  do  not,  as  a  rule,  present  the  same 
parallelism  as  lines  of  bedding,  unstratified  rocks,  even  though  as  full  of 
joints,  have  not  the  regularity  of  arrangement  of  stratified  formations. 
Some  massive  rocks  indeed  may  have  one  system  of  divisional  planes 
so  largely  developed  as  to  acquire  a  bedded  or  fissile  character.  This 
structure,  characteristically  shown  by  phonolites,  may  also  be  detected 
among  ancient  porphyries  (Fig.  228).  Most  massive  rocks  are  traversed 
by  two  intersecting  sets  of  chief  or  "  master  "  joints,  whereby  the  rock  is 
divided  into  long  quadrangular,  rhomboidal,  or  even  polygonal  columns. 
A  third  set  may  usually   be  noticed  cutting  across  the  columns  and 

*  Jukes's  'Manual,'  3rd  edition,  p.  180. 
'  Prof.  W.  King,  Trans.  Roy.  Irish  Acad.  xxv.  (1875),  p.  641. 
'  'Etudes  de  Geologie  Experimentale, '  p.  300,  and  nntCf  p.  318. 
♦  W.  0.  Crosby,  Proc,  Boston  Soc.  Nat.  Hist.  xxii.  (1882),  p.  72. 


528 


tiEOTECTiiSir  ISTKUCTUHAL)  GEOLOGY 


iirticutating  them  into  se^^ents,  though  generally  less  oontinnoiu  « 
(lominunt  than  the  others  (Fig.  229).  When  these  last-named  cm 
joints  lire  absent  or  feebly  develo|>ed,  columns  many  feet  in  length  e 
Ik  quarried  out  entire.  Such  monoliths  have  been  from  early  tin 
employed  in  the  con&tmction  of  obelisks  and  pillars. 


Vie.  236.-  Poniliytj-.  "8"  Clyni«  V«irr,  C«« 


In  large  masses  of  granite,  un  outward  inclination  of  the  nabml 
divisional  jdaiies  of  the  rock  may  sometimes  be  observed,  as  if  the 
Ignite  were  really  a  rudely  bedded  mass,  having  a  dip  towards  ud 
under  the  strata  which  rest  u;ion  its  flanks.  It  is  not  a  foliated  aml]g^ 
nient  of  the  constituent  minerals  analogous  to  the  foliation  of  gneii^ 
for  it  can  be  traced  in  perfectly  amorphous  and  thoroughly  crj'stalliiM 
griinite,  but  is  undoubtedly  a  form  of  jointing  by  reason  of  vhid  tbe 


rock  weathers  into  large  blocks  piled  one  upon  another  like  a  kind  "^ 
mde  Cyclopean  masonry.'  In  the  quarrying  of  granite,  the  workmen 
recognise  that  the  rock  splits  into  blocks  much  more  easily  in  <x» 
direction,  though  externally  there  is  do  tnice  of  any  structure  wbid 
coidd  give  rise  to  this  tendency. 

Kocks  of  finer  grain  than  granite,  such  as  many  diorites  and  doleiitMi 
acquire  a  prismatic  structure  from  the  number  and  intersection  li 
[K!t|>cndicHlar  joints.  The  prisms,  however,  are  unequal  in  dimeDBi(Mi 
as  well  as  in   the   number  and  proportions   of  their  sides,   a  freqiittt 

>  111  tlie  iimiiitc  uf  the  axes  nf  Hit  Rocky  Mauataius  and  parallel  isngu  to  tbe  WHtnA 
n  Viail  n(  liedilcil  litruuluie  hiu  been  clescribvil  as  pauiuE  under  the  crjrMallina  H^iuti. 


PART  II  PRISMATIC  JOINTING  OF  BASALT  529 


diameter  being  2  or  3  feet,  though  they  may  sometimes  be  observed 
three  times  thicker,  and  extending  up  the  face  of  a  cliff  for  300  or  400 
feet.  It  is  by  means  of  joints  that  precipitous  faces  of  crystalline,  no 
less  than  of  sedimentary  rock  are  produced  and  maintained,  for  they 
serve  as  openings  into  which  frost  drives  every  year  its  wedges  of  ice. 
They  likewise  give  rise  to  the  formation  of  the  fantastic  pinnacles  and 
fretted  buttresses  characteristic  of  massive  rocks. 

As  lava,  erupted  to  the  surface,  cools  and  passes  into  the  solid 
condition,  a  contraction  of  its  mass  takes  place.  This  diminution  of  bulk 
is  accompanied  by  the  development  of  divisional  planes  or  joints,  more 
especially  diverging  from  the  upper  and  under  surfaces,  and  intersecting 
at  irregular  distances,  so  as  to  divide  the  rock  into  rude  prisms. 
Occasionally  another  series  of  joints,  at  a  right  angle  to  these,  traverses 
the  mass,  parallel  with  its  upper  and  under  surfaces,  and  thus  the  rock 
acquires  a  kind  of  fissile  or  bedded  appearance.  The  most  characteristic 
structure,  however,  among  volcanic  rocks  is  the  prismatic,  or,  as  it  is 
incorrectly  termed  "basaltic."  Where  this  arrangement  occurs,  as  it 
so  commonly  does  in  basalt,  the  mass  is  divided  into  tolerably  regular 
pentagonal,  hexagonal,  or  irregularly  polygonal  prisms  or  columns,  set 
close  together  at  a  right  angle  to  the  main  cooling  surfaces  (Fig.  230). 
These  prisms  vary  from  1  inch  or  even  less  to  18  or  more  inches  in 
diameter,  and  range  up  to  100  or  even  150  feet  in  height.  Many 
excellent  and  well-known  examples  of  columnar  structure  are  exhibited 
on  the  coast-cliffs  of  the  Tertiary  volcanic  region  of  Antrim  and  the  west 
of  Scotland,  as  in  the  Giant's  Causeway  and  Fingal's  Cave.  In  many 
cases,  no  sharp  line  can  be  drawn  between  a  columnar  basalt  and  the 
beds  above  and  below,  which  show  no  similar  structure,  but  into  which 
the  prismatic  mass  seems  to  pass. 

Considerable  discussion  has  arisen  as  to  the  mode  in  which  this 
columnar  structure  has  been  produced.  That  it  is  a  species  of  jointing, 
due  to  contraction,  was  long  ago  pointed  out  by  Scrope,  and  is  now  gener- 
ally conceded,  though  the  conditions  under  which  it  is  produced  are  not 
quite  clear.  ^  Prof.  James  Thomson  showed  how  the  columnar  structure 
might  be  explained  as  a  phenomenon  of  contraction,  and  subsequently 
Mr.  Mallet  concluded  that  "  all  the  salient  phenomena  of  the  prismatic  and 
jointed  structure  of  basalt  can  be  accounted  for  upon  the  admitted  laws 
of  cooling,  and  contraction  thereby,  of  melted  rocks  possessing  the  known 
properties  of  basalt,  the  essential  conditions  being  a  very  general 
homogeneity  in  the  mass  cooling,  and  that  the  cooling  shall  take  place 
slowly,  principally  from  one  or  more  of  its  surfaces."  In  the  more 
perfectly  columnar  basalts,  the  columns  are  sometimes  articulated,  each 
prism  being  separable  into  vertebrae,  with  a  cup-and-ball  socket  at  each 
articulation  (Figs.  231  and  232).  This  peculiarity  was  traced  by  Mr. 
Mallet  to  the  contraction  of  each  prism  in  its  length  and  in  its  diameter, 

*  G.  P.  Scrope,  'Geology  and  Extinct  Volcanoes  of  Central  France,'  p.  92.  J.  Thomson, 
Brit,  Assoc,  1863,  sects,  p.  95.  R.  Mallet,  Proc.  Ray.  Soc.  1875  ;  Phil.  Mag,  ser.  4,  voL  i. 
pp.  122,  201.  T.  G.  Bonney,  Q.  J.  Geol.  Soc.  1876,  p.  140.  J.  Walther,  Jahrb.  Geol. 
Reichsanst.  1886,  p.  295.     J.  P.  Icldings,  Ainer.  Joum.  Sci.  xxxL  (1886),  p.  321. 

2  M 


530  GEOTECTONIC  {STRUCTURAL)  GEOLOGY  bookiv 

and  to  the  consequent  production  of  transverse  joints,  which,  as  the 
resultant  of  the  two  contracting  strains,  are  oblique  to  tha  aides  of  the 
prism,  but,  aa  the  obliquity  lessens  towards  the  centre,  assume  necessaril; 
when  perfect,  a  cup-shape,  the  convex  surface  pointing  in  the  same 
direction  as  that  in  which  the  prism  has  grown.  This  explanation,  how- 
ever, will  har<IIy  account  for  cases,  which  are  not  uncommon,  where  tht 
convexity  points  the  other  way,  or  where  it  is  sometimes  in  one  direction 
and  sometimes  in  the  other.'  The  remarkable  spheroids  (Fig.  94)  which 
appear  in  many  weathered  igneous  rocks  besides  basalts  may  be  due,  when 
they  are  not  the  result  of  weathering,  to  continued  contraction  witbio 
the  hexi^onal  or  [>olygonal  spaces  detined  by  the  columnar  joints  and 
cross-joints  of  a  cooling  mass.  The  contraction  of  these  blocks  wouH 
tend  to  the  development  of  successive  spheroidal  shellH,  which  might 
remain  mutually  adherent  and  invisible  in  a  fresh  fracture  of  the  rock,  yet 
might  make  their  presence  effective  during  the  complex  processes  d 
weathering."  After  some  exposure,  the  spheroids  of  basalt  begin  to 
appear,  aud  gradually  crumble  away  by  the  successive  formation  and 
disappearance  of  external  weathered  crusts  or  coats,  which  fall  off  into 


sand  and  clay.  Almost  all  augitic  or  hornblcndic  rocks,  with  manr 
granites  and  poqthyries,  exhibit  the  tendency  to  decompose  into 
rounded  spheroidal  blocks.  The  columnar  structure,  though  abundant 
among  modern  volcanic  rocks,  is  by  no  means  confined  to  these.  It  ii 
as  well  displayed  among  the  lavas  of  the  Lower  Old  Red  Sandstone,  tod 
of  the  Carboniferous  Limestone  in  central  Scotland,  fts  among  those  of 
Teitiary  age  in  Auvergiie  or  the  VivaraJs. 

As  already  stated,  prismatic  forms  have  been  snperinducod  upoa 
rocks  by  a  high  temperature  and  subsequent  cooling,  as  where  coal  tnd 
sandstone  have  been  invaded  by  basalt.  They  may  likewise  be  observed 
ti>  arise  during  the  consolidation  of  a  substance  from  aqueous  solutioa 
In  staich,  for  example,  the  columnar  striicturc  may  be  well  developed, 
aud  not  infiiH|iieiitly  radiates  from  eertain  centres,  as  in  basalt  and  othff 
igneous  rocks. 

>  Mr.  ikTn\K  jKiiuUd  this  out  {Ged.  May.  Svptemlwr  1875),  though  Mr.  UiUat  (tU 
Xc)Vitub«r  ISifi)  n-i>liud  that  in  such  caati  the  articnlatiouB  must  be  formed  just  atiant  Ik 
dividing  .surfuci-,  between  the  part  of  the  rock  whirli  cooleil  tmiii  abore  md  thit  <ririck 
cooltd  from  boloK.  See  also  on  this  subject  J.  I".  O'Keilly,  Traiu.  Roy.  Irith  Atai.  n"- 
(18791,11.61], 

'  Bonnev.  y.  J.  (/tol.  Soc.  1876,  p.  151.  The  [lerlilie  structure  ii  probibly  i  Bio*- 
scoplc  euuiple  of  the  ume  kind  of  c< 


PART  III  INCLINATION  OF  ROCKS  531 

3.  In  Foliated  (Schistose)  Rocks. — The  schists  likewise  possess  their 
joints,  which  approximate  in  character  to  those  among  the  massive  igneous 
rocks,  but  they  are  on  the  whole  less  distinct  and  continuous,  while  their 
effect  in  dividing  the  rocks  into  oblong  masses  is  considerably  modified 
by  the  transverse  lines  of  foliation.  These  lines  play  somewhat  the  same 
part  as  those  of  stratification  among  the  stratified  rocks,  though  with  less 
definiteness  and  precision.  The  jointing  of  the  more  massive  foliated 
rocks,  such  as  the  coarser  varieties  of  gneiss,  approaches  most  closely  to 
that  of  granite ;  in  the  finely  fissile  schists,  on  the  other  hand,  it  is  rather 
linked  with  that  of  sedimentary  formations.  Upon  these  differences 
much  of  the  characteristic  variety  of  outline  presented  by  cliffs  and  crests 
of  foliated  rocks  depends. 


Part  III.  Inclination  of  Rocks. 

The  most  casual  observation  is  sufficient  to  satisfy  us  that  the  rocks 
now  visible  at  the  earth's  surface  are  seldom  in  their  original  position. 
We  meet  with  sandstones  and  conglomerates  composed  of  water- worn 
particles,  yet  forming  the  angular  scarps  of  lofty  mountains ;  shales  and 
clays  full  of  remains  of  fresh-water  shells  and  land-plants,  yet  covered 
by  limestones  made  up  of  marine  organisms,  and  these  limestones  rising 
into  great  ranges  of  hills,  or  undulating  into  fertile  valleys,  and  passing 
under  the  streets  of  busy  towns.  Such  facts,  now  familiar  to  every 
reader,  and  even  to  many  observers  who  know  little  or  nothing  of  sys- 
tematic geolog}^  point  unmistakably  to  the  conclusion  that  most  of  the 
rocks  of  the  land  have  been  formed  under  water,  sometimes  in  lakes,  more 
frequently  in  the  sea,  and  that  they  have  been  elevated  into  land. 

But  further  examination  discloses  other  and  not  less  convincing  evi- 
dence of  movement  Judging  from  what  takes  place  at  the  present  time 
on  the  bottoms  of  lakes  and  of  the  sea,  we  confidently  infer  that  when 
the  strata  now  constituting  so  much  of  the  solid  framework  of  the  land 
were  formed,  they  were  laid  down  nearly  horizontally,  or  at  least  at  low 
angles  (an^,  p.  501).  When,  therefore,  we  find  them  inclined  at  all  angles, 
and  even  standing  on  end,  we  conclude  that  they  have  been  disturbed. 
Over  wide  spaces,  they  have  been  upraised  bodily,  with  little  alteration 
of  horizontality ;  but  in  most  places  some  departure  from  that  original 
position  has  been  effected. 

Dip. — The  inclination  thus  given  to  rocks  is  termed  their  Dip.  Its 
amount  is  expressed  in  degrees  measured  from  the  plane  of  the  horizon. 
Thus  a  set  of  rocks  half-way  between  the  horizontal  and  vertical  position 
would  be  said  to  dip  at  an  angle  of  45°,  while  if  vertical  they  would  be 
marked  with  the  angle  of  90°.  The  inclination  is  measured  with  an 
instrument  termed  the  Clinometer,  which  is  variously  made,  but  of  which 
one  of  the  simplest  forms  is  shown  in  Fig.  233.  This  consists  of  a  thin 
strip  of  boxwood,  two  inches  broad,  strengthened  with  brass  along  the 
edges,  and  divided  into  two  leaves,  each  6  inches  long,  hinged  together 
80  that  when  opened  out  they  form  a  foot-rule.     On  the  inside  of  one  of 


532 


GEOTECTOXir  (STRUCTURAL)  GEOLOGY 


BOOK  IT 


these  leaves,  a  graduated  arc  with  a  pendulum  is  inserted.  AMien  the 
instrument  is  held  horizontally,  the  pendulum  points  to  zero.  'When 
placed  vertically,  it  marks  90°.  By  retiring  at  a  right  angle  to  the 
direction  of  dip  of  a  group  of  inclined  heds,  and  holding  the  clinometer 


Fig.  233.— Cliiioiuet*fr— the  leaf  containing  the  pendulum  and  index. 

(Half  the  size  of  the  original.) 

before  the  eye  until  its  upper  edge  coincides  with  the  line  of  bedding,  irc 
readily  obtain  the  amount  or  angle  of  dip.  In  observations  of  this  nature 
it  is  of  course  necessary  either  to  place  the  clinometer  strictly  parallel 
with  the  direction  of  dip,  or,  if  this  be  impossible,  to  take  two  measure- 
ments, and  calculate  from  them  the  true  angle. ^  Simple  as  observation 
of  dip  is,  it  is  attended  with  some  liabilities  to  error,  against  which  the 
ol)server  should  be  on  his  guard.  A  single  face  of  rock  may  not  discloee 
the  true  dip,  especially  if  it  be  a  clean-cut  joint-face.  In  Fig.  234,  for 
example,  the  strata  might  be  supposed  to  be  horizontal ;  but  another 
side  view  of  them  (as  Fig.  235)  might  show  them  to  be  gently  inclined  or 
even  nearlv  vertical. 


^^s^^ 


Vi^.  234.— Api»arently  horizontal  strata  (B.) 

Again,  a  deceptive  surface  inclination  is  not  infrequently  to  be  seen 
among  thin -bedded  strata.  Mere  gravitation,  aided  by  the  downmrd 
pressure  of  sliding  detritus  or  "  soil-cap,"  suffices  to  bend  over  the  edgw 
of  fissile  strata,  which,  though  really  dipping  into  the  hill,  are  thus  mito 
to  appear  superficially  to  dip  away  from  it  (Fig.  236).  Similar  effecti^ 
with  even  proofs  of  contortion,  may  be  noticed  under  boulder  clay,  or  in 

^  In  Jukes'  '  Memoir  on  the  South  Staffordshire  Coal-Field,*  in  Memoirs  o/Gwl,  SutfV 
(2ikI  edit.  p.  *213),  a  formula  is  given  for  calculating  the  true  dip  from  the  apparent  dipiea 
in  a  cliff.  A  graphical  methoil  of  computing  the  true  dip  from  ^tbserrations  of  two  apptfCBt 
dip»  has  been  sui.'gested  by  Mr.  \V.  H.  Dalton,  Geol.  Mag.  z.  p.  332.  See  alio  Gimb'> 
'  Physical  Geology,'  1882,  p.  460.     A.  Marker,  Ged,  Mag,  1884,  p.  154. 


other  situations  where  the  rocks  have  been  bent  over  and  cnished  hy  a 
mass  of  ice. 

When  the  dip  is  outward  in  every  direction  from  a  central  point,  it 
ia  said  to  be  qtUl-qud-vfrsal  (A.  in  Fig.  238).     Strata  thus  affected  are 


thrown  into  a  dome-shaped  structure,  while  when  the  dip  is  toivards  a 
central  point,  they  have  a  basin-shaped  structure. 

Outcrop. — The  edges  of  strata  which  appear  at  the  surface  of  the 
ground  are  termed  their  Outcrop  or  Basset  If  the  strata  are  quite 
horizontal,  the  direction  of  outcroj)  depends  on  inequalities  of  the  ground 
and  variations  in  amount  of  denudation.  Perfectly  level  ground  lyin^^ 
upon  horizontal  beds  shows,  of  course,  no  outcrop,  for  the  surface  coin- 
cides with  a  plane  of  stratification.  But  occa. 
sional  water-courses  have  been  eroded  below 
the  general  level,  so  as  to  reveal  along  their 
sides  outcrops  of  the  strata.  The  remark- 
able sinuosities  of  outcroji  produced  by  tht- 
unequal  erosion  of  horizontal  strata  are  illus-  fib,  j:w.-i)«epiivrHn]wriici»idir. 
trated  in  Fig.  237,  where  A  is  a  map  of  a 

piece  of  ground  deeply  trenched  by  valleys,  and  B  that  of  an  area  com- 
paratively little  denuded.  In  both  cases  the  outcrops  are  seen  to  wind 
round  the  sides  of  the  slopes. 

Where  strata  are  inclined,  the  course  of  their  outcrop  is  regulated 
partly  by  the  direction  and  amount  of  inclination,  and  partly  by  the 
form  of  the  ground.  When  with  low  angles  of  dip  they  crop  mit,  that 
is,  rise  to  the  surface,  along  a  i)erfectly  level  piece  of  ground,  the  out- 
crop runs  at  a  right  angle  to  the  dip.  But  any  inequalities  of  the  surface, 
such  as  valleys,  ravines,  hills,  and  ridges,  will,  as  in  the  case  of  horizontal 
beds,  cause  the  outcrop  to  describe  a  circuitous  course,  even  though  the 
dip  should  remain  perfectly  steady  all  the  while.  If  a  Hue  of  precipitous 
gorge  should  run  directly  with  the  dip,  the  outcrop  will  there  be  coincident 
with  the  dip.  The  occurrence  of  a  gently  shelving  valley  in  that  position 
will  cause  the  outcrop  to  descend  on  one  aide  and  to  moiint  in  a  corre- 
sponding way  on  the  other,  so  as  to  form  a  V-shaped  indentation  in  its 
course.  A  ridge,  on  the  other  hand,  will  produce  a  deflection  in  the 
opposite  direction.     Hence  a  series  of  parallel  ridges  and  valleys,  running 


OEOTECTONIC  {STRUCTURAL)  GEOLOGY 


in  the  same  direction  aa  the  dip  of  the  strata  underneath,  causes  the  trat- 
crop  to  describe  a  wideljr  Bcrpeotinous  course. 

The  breadth  of  the  outcrop  depends  on  the  thickness  of  the  stntim 
and  on  the  angle  of  dip.  A  bed  one  foot  thick  inclined  at  an  angle  of  T, 
on  a  perfectly  level  piece  of  ground  would  have  an  outcrop  about  60  feet 
broad.  At  a  dip  of  5°  the  breadth  of  the  outcrop  would  be  a  little 
over  11  feet.  At  30°  it  would  be  reduced  to  2  feet,  and  the  dimi'nutioii 
would  continue  until,  when  the  bed  was  on  end,  the  breadth  of  the  out- 
crop would,  of  course,  exactly  coiift- 
spond  with  tlie  thickness  of  the  bed. 
It  is  further  to  be  observed  th|t 
among  vertical  rocks,  the  direction 
of  the  outcrop  necessarily  coire- 
aponda  with  the  strike^  and  continues 
to  do  so  irrespective  altogether  al 
any  irregularities  of  the  ground. 
The  lower  therefore  the  angle  of 
inclination,  the  greater  is  the  effect 
of  surface-inequalities  upon  the  Une 
of  outcrop ;  the  higher  the  angle, 
the  less  is  that  influence,  till  wheu 
the  beds  stand  on  end  it  ceaaea. 

Strike. — A  horizontal  linedrawn 
at  a  right  angle  to  the  dip  is  called 
the  Strike  of  the  rocks.  Fromwhst 
has  just  been  said,  this  line  muA 
coincide  with  outcrop  when  the  sur- 
face of  the  ground  is  quit«  level,  as  on 
the  beach  in  Fig.  QZB,  and  also  when 
the  beds  are  vertical.  At  all  other 
times,  strike  and  outcrop  are  not 
strictly  coincident,  but  the  latter 
wanders  to  and  fro  across  the  former 
according  to  changes  in  the  contour 
Fijt  Mr.— ainuoiw  outarom  cif  iinriionui  "triu  of  the  ground.  The  strike  may 
.i..pfndingonine<iiuiiii«ot-ura..r.  ,,g    ^  Straight    line,  or  may   curve 

ThewivybiM^^^inMiiMr^ejiii^pJiB^  su  le  pj^^j jiy  [„  every  direction,  according 
to  behaviour  of  the  dip.  A  set  of 
beds  dipping  westwards  for  half  a  mile  (o  to  b.  Fig.  238)  have  a  north  and 
south  strike  for  the  same  distance.  If  the  dip  changes  to  S.W.,  S.,  S.E, 
and  E.,  the  strike  will  bend  round  in  a  curving  line  (as  at  S),  In  the  case 
of  a  iia&-qu4-irrsal  dip  the  strike  forms  a  complete  circle  (as  at  A).  The  dip 
being  ascertained  gives  the  strike,  but  the  Rtrike  does  not  certainly  indicate 
tlie  direction  of  dip,  which  may  be  either  to  the  one  side  or  tie  other. 
Two  groups  of  strata,  dipping  the  one  east  and  the  other  west,  have  both 
a  north  and  south  strike.  Strike  may  be  conceived  as  always  a  level  Une 
on  the  plane  of  the  horizon,  so  that,  no  matter  how  much  the  ground  may 
undulate,  or  the  outcrop  may  vary,  or  the  dip  may  change,  the  strike  will 


PART  m  STRIKE  636 

remain  horizontal-  Hence  in  mining  operations,  it  is  commonly  Bpoken 
of  as  the  level-aniTse  or  leveliearing.  A  "  level  "  or  underground  roadway, 
driven  through  a  coal-seam  at  right  angles  to  the  dip,  will  undu1at«  in  it« 
trend  if  the  dip  changes  in  direction,  but  it  may  be  made  perfectly  level, 
and  kept  so  throughout  a  whole  coal-Seld  so  long  as  it  ia  not  interfered 
vrith  by  dislocations. 


Fig.  138.— Oeologteal  Hi 


Id  Fig.  238,  the  strihe  and  outcrop  are  coincident  on  the  flat  beach,  but  cease  to  be 
so  the  momeot  the  ground  begins  to  slope  up  into  the  coaHt-clifT.  This  is  seen  in  the 
eaatem  half  of  the  map,  where  the  lines  of  outcrop  slant  up  into  the  cliff  at  an  angle 
dependent  mainlj  on  the  amount  of  the  dip.  A  section  drawn  in  the  line  L  L'  would 
■how  the  geological  structure  represented  in  Fig.  230.     B;  noting  the  angles  of  dip  it  is 


Fig,  239.-8(1 


Fig.  iW. 


poadhle  to  estimate  the  thickness  of  a  series  of  beds,  and  how  far  beneath  the  aurToce 
U17  given  bed  might  he  expected  to  be  found.  If,  for  instance,  the  horizontal  distance 
Bcrosa  the  strike  between  beds  S  and  A  (Fig. -238)  were  found  to  be  SIX)  feet,  with  a 
mean  dip  of  15°,  the  actual  thickness  would  be  51-3  teet,  and  bed  A  would  be  found  at 
A  depth  of  G3'S  feet  below  the  outcrop  of  8.  If  the  same  development  of  strata  continues 
inland,  the  bed  a  shonld  be  found  at  a  little  more  than  200  feet  beneath  the  surface,  if  a 


536  GEOTECTONIC  (STRUCTURAL)  GEOLOGY  book  iv 


bore  were  sunk  to  it  iii  the  quarr}-  (Q).  If  the  total  depth  of  rock  between  a  mnd  6  be 
1000  feet,  then  e\n(leiitly,  if  the  strata  could  be  restored  to  their  original  mpproximately 
horizontal  jtosition,  with  bed  a  at  the  surface,  bed  b  would  be  covered  to  a  depth  of  1000 
feet.  It  will  be  noticed  also  that  as  the  angle  of  dip  increases,  the  outcrops  are  thereby 
brought  clos(*r  together.  AVliere  the  outcro])S  run  along  the  face  of  a  cliff  or  steep  bank 
(H)  they  must  likewise  1>e  drawn  together  on  a  map.  In  reality,  of  course,  these  raria- 
tions  take  place  though  the  same  vertical  thickness  of  rock  may  everywhere  intervene 
between  the  several  outcrojw. 

It  is  usually  desirable  to  estimate  the  thicknesses  of  strata,  especially  where,  as  in 
Fig.  239,  tliey  arc  exiiosed  in  continuous  section.  A  convenient  thougli  not  strictly 
accurate  rule  for  this  ])uri)Ose  may  be  applied  in  eases  where  the  angle  of  inclination  is 
less  than  45°.  The  real  thickness  of  a  mass  of  inclined  strata  may  be  taken  to  be  i^  of 
its  apjiarent  thickness  for  every  5"  of  dip.  Thus  if  a  set  of  beds  dips  steadily  in  one 
direction  at  5'  for  a  horizontal  s^Mice  of  1200  feet  measured  peqtendicularly  to  the  strike, 
their  actual  thickness  will  be  jVi  or  100  feet  If  the  dip  be  15*,  the  true  thickneas  »-ill 
l>e  ^V,  or  300  feet,  and  so  on.^ 


PaUT  IV.    CURVATURE.- 

A,  little  reflection  will  show  that  though,  so  far  as  regards  the  trifling 
portions  of  the  rocks  visible  at  the  surface,  we  might  regard  the  inclined 
surfaces  of  strata  as  i)arts  of  straight  lines,  they  must  nevertheless  be 


..^J^^^^'-^^''^^!^ 


..»«illi£^S 


^-^"^ 


Fig.  240.— Set'tion  of  inclined  strata. 


parts  of  large  curves.  Take  for  example  the  section  in  Fig.  240.  At  the 
left  hand  the  strata  descend  beneath  the  surface  at  an  angle  of  no  more 
tlian  15',  but  at  the  opposite  end  the  angle  has  risen  to  60".  There  being 
no  dislocation  or  abnipt  change  of  inclination,  it  is  evident  that  the  beds 
cannot  i)roceed  indefinitely  downward  at  the  same  angle  which  they  have 
at  the  surface,  otli(»rwise  they  woidd  run  away  from  each  other,  but  most 
])end  round  to  accommodate  themselves  to  the  difference  of  inclination. 
By  prolonging  the  lines  of  bedding  for  some  way  beneath  and  above  nm- 
level,  we  can  show  graphically  that  the  strata  are  necessarily  curyed  (Kg. 
241).  A  section  of  this  kind  brings  out  clearly  the  additional  fact  that  an 
upward  continuation  of  the  curved  beds  must  have  been  carried  away  by 
the  denudation  of  the  surface.  In  every  instance  therefore  where^  in 
walking  over  the  surface,  we  traverse  a  series  of  strata  which  gnuliiaUy, 
and  without  dislocations,  increase  or  diminish  in  inclination,  we  croM  part 
of  a  curvature  in  the  strata  of  the  earth  s  cnist.     The  foldings,  however, 

*  Maclareu's  *  Geology  of  Fife  and  the  Lothians,*  2ud  edit,  p:  xix.  For  tables 
for  estimating  dip  and  thickness  see  Jakes'  'Manual,'  p.  748;  Green's  'Phytiol 
Geolof^y,'  p.  400. 

^  A  useful  coni]>endiuni  of  information  regarding  geological  terms  for  the  dislocations  and 
cur^'utures  of  rocks  has  l>eeu  i»repared  by  M.  E.  de  Marjeriu  and  Professor  A.  Heim,  *  Les 
dislocations  de  I'ecorce  terrestre,  '  lb88,  Ziirich  (in  French  and  German). 


PART  IV  CURVATURE  OF  ROCKS  537 

can  often  be  distinctly  seen  on  clifis,  coast-line?,  or  other  exposures  of 
rock  (Fig.  342).  The  observer  cannot  long  continue  his  researches  in  the 
field  without  discovering  that  the  strata  composing  the  earth's  outer  cmst 


have  been  almost  everywhere  thrown  into  curves,  usually  so  broad  a 
gentle  as  to  escape  obsen'ation  except  when  specially  looked  for. 


If  the  inclination  and  curvature  of  rocks  urc  so  closely  connected,  a 
corresponding  relation  must  hold  between  their  stHke  and  curvature. 
In  fact,  the  prevalent  strike  of  a  region  is  determined  by  the  direction  of 


538  GEOTECTOXIC  (STRUCTURAL)  GEOLOGY  book  nr 

the  axes  of  the  great  folds  into  which  the  rocks  have  been  thrown.  If 
the  curves  are  gentle  and  inconstant,  there  will  be  a  corresponding  Tui- 
ation  in  the  strike.  But  should  the  rocks  be  strongly  plicated,  there  wiD 
necessarily  be  the  most  thorough  coincidence  between  the  strike  and  the 
direction  of  the  plication. 

Monoclines. — Curvature  occasionally  shows  itself  among  horizontal 
or  gently  inclined  strata  in  the  form  of  an  abrupt  inclination,  and  then 
an  immediate  resumption  of  the  previous  flat  or  gently  sloping  chancter. 
The  strata  are  thus  bent  up  and  continue  on  the  other  side  of  the  fold 
at  a  higher  level.  Such  bends  are  called  Monoclines  or  mono- 
c  1  i  n  a  1  folds,  because  they  present  only  one  fold,  or  one-half  of  a  fold, 
instead  of  the  two  in  an  arch  or  trough  (Fig.  265,  section  1).  The  meet 
notable  instance  of  this  structure  in  Britain  is  that  of  the  Isle  of  Wight 
(Fig.  243),  where  the  Cretaceous  rocks  (c)  on  the  south  side  of  the  isUnd 


t  c 

Fif?.  -243.— Section  of  a  Monoclinal  Fold,  Me  of  Wight. 

rapidly  rise  in  inclination  till  they  become  nearly  vertical,  while  the 
Lower  Tertiary  strata  (/)  follow  with  a  similar  steep  dip,  but  rapidly 
flatten  down  towards  the  north  coast  Probably  the  most  gigantic  mono- 
clinal folds  in  the  world  are  those  into  which  the  remarkably  horixontil 
and  undisturbed  rocks  of  the  Western  States  and  Territories  of  the 
American  Union  have  been  thrown.^ 

From  the  abundance  of  inclined  strata  all  over  the  world,  we  may 
readily  i>erceive  that  the  normal  structure  of  the  visible  part  of  the  earth's 
crust  is  one  of  innumerable  foldings  of  the  rocks.  Sometimes  more 
steeply,  sometimes  more  gently  undulated,  not  infrequently  dislocated 
and  displaced,  the  sedimentary  accumulations  of  former  ages  everywhere 
reveal  evidence  of  great  internal  movement.  Here  and  there,  the  moT^ 
mcnt  has  resulted  in  the  formation  of  a  dome-shaped  elevation  of  the 
strata,  wherein,  as  if  pushed  up  from  a  single  point,  they  slope  away  on 
all  sides  from  the  centre  of  greatest  upthrust,  with  a  qudrquA-versal  dip. 
Where  the  top  of  the  dome  has  been  removed,  the  successive  outcrops  of 
the  strata  form  concentric  rings,  the  lowest  at  the  centre,  the  highest  At 
the  circumference  (A  in  Figs.  238  and  239). 

Anticlines  and  Synelines. — But  in  the  vast  majority  of  cases,  the 
folding  has  taken  place,  not  round  a  point  but  along  an  azi& 
Where  strata  dip  away  from  an  axis  so  as  to  form  an  arch  or  saddle, 
the  structure  is  termed  an  Anticline,  or  anticlinal  axis  (Fig.  244). 
Where  they  dip  towards  an  axis,  forming  a  trough  or  basin,  it  is  called  a 
Syncline,  or  synclinal  axis   (Fig.  245).     An  anticlinal  or  synclinal 

^  See  Powell's  *Exploratiou  of  the  Colorado  River  of  the  West,*  and  '  Geology  of  the 
Uinta  Mountains,*  in  the  Re]>orts  of  the  United  States  Gec^raphical  and  Geological  Survey* 
Button's  *  High  Plateaux  of  UUh/  and  *  History  of  the  Grand  CafTon ' ;  Gilbert's  'GeolQ0 
of  the  Henry  Mountains.  *     Compare  Richthofen's  '  China,'  vol.  ii. 


ANTICLINES,  SYNCLINES,  INVERSIONS 


539 


ist  always  die  out  unless  abruptly  terminated  by 
ion.  In  the  case  of  the  anticline,  the  axis,  after 
ing  horizontal,  or  but  slightly  inclined,  at  last 
to  turn  downward,  the  angle  of  inclination  lessens, 


-Arch,  or  Anticline,  which  has  been  denuded  by  the  removal  of  beds,  as 
shown  by  the  dotted  line  a  c  above  the  axis  6. 

arch  then  ends  or  "  noses  out"  In  a  syncline,  the 
jntually  bends  upward,  and  the  beds,  with  gradually 
g  angles,  swing  round  it.  In  a  symmetrical  anticline 
line,  the  angle  of  slope  is  the  same  or  nearly  so  on 
de  (Figs.  244,  245).  But  a  difference  of  inclination 
sntly  to  be  observed.  The  Appalachian  coal-field,  for 
),  as  shown  by  H.  D.  and  W.  B.  Rogers,  presents 
uctive  series  of  plications,  beginning  with  symmetrical 


e 


txi 

e 


I 

8. 

5 


245.— Trough,  or  Syncline,  with  strata  (a  c)  rising  from  each  side  of  a 

central  axis  (b). 

ucceeded  by  others  with  steep  fronts  towards  the 
itil  at  last  these  steeper  fronts  pass  under  the  opposite 
the  arches,  giving  rise  to  a  series  of  inverted  folds 
16). 

Brsion. — Inverted  folds  occur  abundantly  in  regions 
t  plication.  The  Silurian  uplands  of  the  south  of 
i,  for  instance,  have  the  arches  and  troughs  tilted  in 
3Ction  for  miles  together,  so  that  in  one-half  of  each 
1  the  strata  lie  bottom  upwards  (Fig.  247).^  It  is 
9  mountain-chains,  however,  that  inversion  can  be 
[  the  grandest  scale.  The  Alps  furnish  numerous 
;  illustrations.  On  the  north  side  of  that  chain,  the 
iry  and  Tertiary  rocks  have  been  so  completely  turned 
r  many  miles  that  the  lowest  beds  now  form  the  tops 
hills,  while  the  highest  lie  deep  below  them.  Indi- 
mountains,  such  as  the  Glarnisch  and  some  in  the 

f.  Lapworth  has  worked  out  with  much  skill  the  inverted  anti- 

l  synclines  of  the  **  Moffat  Shales"  (Q.  J.  Ged.  Soc.  xxxiv.  (1878),  p.  240) ;  and 

ia  papers  on  the  ''Secret  of  the  Highlands"  {Oecd.  Mag.  1883). 


GEOTKCTUSir  (STRUCTURAL)  GEOLOGY 


Cantons  GlaruB  and  St.  Gall  (Figs.  248,  249),  present  8tupendonB«xaBi[JM 
of  inversion,  great  groups  of  strata  being  folded  over  and  over  eacb  otbar 

as  we  might  fold  carpets.' 


Wher-e  a  aeries  of  strata  has  been  so  folded  and  inverted  thtU  it« 
reduplicated  members  appear  to  dip  regularly  m  one  direction,  dw 
structure  is  termed  isoclinal  This  structure,  illustrated  on  a  smll 
scale  among  the  curved  Silurun  rocks  shown  in  Fig  '47  occurs  on  i 
grand  scale  among  the  Al[is,  where  the  folds  ha\e  sometimes  been  ao 
s'lueezed  together  that,  when  the  tops  of  the  arches  ha*e  been  worn  away, 
the  strata  could  scarcely  be  supposed  to  ha^e  been  really  inverted,  »« 
for  the  evidence  as  to  their  true  order  of  succesHion  supplied  by  their  in- 


cluded fossils.      The  extent  of  this  compression  in  the  Alps  has  been 
already  (p.  317)  referred  to.-     So  intense  has  been  the  plication,  and  » 

'  Till-  (iliinier  iloulJe  folil  lia*  tieen  the  liulyect  of  considerable  discasaiDD.  Aecofduf 
tn  Ileiiu  {' Mn-hniiibtitiis  der  Gebiiv^bililung '}  liie  wliole  of  the  rocks,  gchuta  inelDdid. 
renmiiieil  uiidlsturbril  until  the  time  of  tlie  ixut-eoeene  folding.  Vn«k,  however,  contOKls 
with  M-iileiit  prolnbility,  that  tlir  oWer  w.-hist'i  are  imcontonnBbly  averluii  by  IsUr  for 
ni:iti<His.  Hre  )l,  Vocek,  Jolirti.  flnif.  Jteifhmiiiil.  1879,  \<.  728;  1S84.  pp.  £33,  tlO: 
lW/..'«dl.  '.'«A  JWrft..  1880,  p.  189:  1881.  |>.  43.  A.  Heim,  Vrrk«ii,a.  (!f«l.  Rri*t 
I8,V0,  n  IM  ;  18N1,  [i.  204.  Bee  also  .Irrh.  .Sei.  I'hjia.  Sat.  CeiieTo,  Noreniber  ISSi 
]•.  24;  Lorj-,  null.  Af.  tii-l.  Fi'-nr'.  S""  n^T.  nL  (1882),  ]>.  14,  la  Fig.  249,  no  n»n  pU- 
cition  could  brill);  the  White  Jura  wliere  it  1ie«  comparatively  nndtiturbed  on  the  tip  't 
the  excewively  plicated  Eocene  beilii.  It  has  eridently  Iwen  pushed  over  Ihe  Utter,  the  te 
ofjniictiaiilietKecn  tlieniliciugn"thruat-p1iine"  (p.  551). 

-  See  alHO  F.  M.  Stapir,  '  Ziir  Hechaiiik  <ler  !i«hictenrRltungen,'  Xfuri  JaJui.  18^, 
p]i.  292,  702.  A  riiii'  Rerie.s  of  sections  illnHtnilitig  the  mrious  features  at  monntun  itiw- 
tiirc  may  lie  found  in  the  platrn  nci-oiupaiiyiiig  the  '  Mat^rianx  jiour  la  Carte  Grolopqia  dr 
la  Suisse.'  Kee  especially  IJi-miM>n  xv'i.  on  the  laiidnit  A/jiihy  Prof.  Renevier  ;  Llnun 
xxi.  by  K.  Fai-rc  iind  Sclianlt,  on  ranlM  de  I'cihi/,  &c.,  and  ixv.  l>y  A.  Heini  on  tb*  JTi)* 
Aljit  Mirrcn  StHif  niid  Rhine.     An  inteieBtrnf;  study  of  an  abnormal  syatcm  of  foWiUd 


PART  IV 


CRUMPLING  OF  ROCKS 


541 


great  the  subsequent  denudation,  that  portions  of  Carboniferous  strata 
appear  as  if  regularly  interbedded  among  Jurassic  rocks,  and  indeed 
could  not  be  separated  save  after  a  study  of  their  enclosed  organic 
remains. 

A  further  modification  of  the  folded  structure  is  presented  by  the 
fan -shaped  arrangement  {structure  en  iverUail^  Fikher- Fatten)  into  which 


tft 


V.^.r-^br  t 


Fig.  240.— Inversion  and  Thmst-plane  among  the  mountains  south  of  the  Lake  of  Wallenstadt,  Cantons 

Olanis  and  St.  Gall  (A.  Ueim). 

<,  Eocenp  ;  c,  Cretaceous ;  wj.  White  Jura  thrast  upward  on  the  left  hand  over  the  plicated  Eocene ;  \ 
b.j.  Brown  Jura ;  <,  Trias ;  *,  Schistose  rocks,  perhaps  metamorphosed  Paleeozoic  formations. 

higlily  plicated  rocks  have  been  thrown.  The  most  familiar  example  is 
that  of  Mont  Blanc,  where  the  sedimentary  strata  at  high  angles  seem  to 
dip  under  the  crystalline  schists  (Fig.  249). 

Crumpling. — In  the  general  plication  of  a  district  there  are  usually 
localities  where  the  pressure  has  been  locally  so  intensified  that  the  strata 
have  been  corrugated  and  crumpled,  till  it  becomes  almost  impossible  to 
follow  out  any  particular  bed  through  the  disturbed  ground.  On  a  small 
scale,  instances  of  such  extreme  contortion  may  now  and  then  be  found 


Fig.  250.— Fan -shaped  structure,  Central  Alps. 
ft  Upper  Jurassic  Limestone ;  j,  Brown  Jura  and  Lias  ;  f,  Trias ;  «,  Schistose  rocks. 

at  faults  and  landslips,  where  fissile  shales  have  been  corrugated  by  sub- 
siding heavy  masses  of  more  solid  rock  (Fig.  251).  But  it  is,  of  course, 
among  the  more  plicated  parts  of  mountain-chains  that  the  structure 
receives  its  best  illustrations.  Few  travellers  who  have  passed  the  upper 
end  of  the  Lake  of  Lucerne  can  have  failed  to  notice  the  remarkable 
cliffs  of  contorted  rocks  near  Fluelen.  But  innumerable  examples  of 
equal  or  even  superior  grandeur  may  be  observed  among  the  more  preci- 

fanlts  involving  Triassic,  Jurassic,  and  Cretaceous  rocks  in  the  south  of  France,  will  be  found 
in  M.  Bertrand's  monograph,  *  Le  Massif  d'Allauch,'  Bull.  Carte  Giol.  France,  iii.  No.  24 

/1AQ1N    n    9A2) 


OEOTECTONIC  (STRUGTUBAL)  GEOLOGY 


pitouB  valleys  of  the  Swiss  Alps.      Striking  illusbstions  of  the  same 
structure  may  be  found  in  aiiy  great  mountain  chain  (Fig.  252).    \o  n 


impressive  testimony  could  be  given  to  the  potency  of  the  force  by  which 
mountains  were  upheaved.  And  yet,  striking  as  are  these  colossal 
exaittpks,  involving  as  they  do  whole  mountain  masses  in  their  folds, 


their  eirect  upon  the  mind  is  even  heightened  when  we  discover  that  sach 
has  been  the  strain  to  which  solid  limestones  and  other  rocks  have  been 
subjected  that  even  their  liner  layers  have  been  intensely  puckerod.     Some 


DEFORMATION  AND  CRUSHING 


of  these  minor  crumpUnga  are  readily  visible  to  the  eye  in  hand-Bpecimens 
(Figs.  36,  253,  254).     But  in  many  foliated,  crumpled  rocks  the  puckering 


if  Alpine  limiMmxe, 


produced  br  gnt-t  Utenl  ci 


is  80  minute  as  to  be  best  seen  with  the  microscope  (Fig.  37).  Frequently 
the  puckerings  have  been  ruptured  and  a  fine  cleavage  or  jointing  has 
been  produced  (Auaweichungaclivage,  strain-slip  cleavage). 

It  may  often  be  observed  that  in  strata  which  have  been  intenssly 


Fig.  2M.— Cnituplwl  Trlas-i 

emaipled,  the  s&mo  bed  is  reduced  to  the  smallest  thickness  in  the  arms 
of  the  folds,  but  swells  out  at  the  bends  as  if  squeezed  laterally  into 
theee  loops.  This  appearance,  so  noticeable 
in  mountain  atructure,  may  be  seen  on  lower 
groands,  as  in  Pembrokeshire,  where  De  la 
Beche  has  shown  that  the  roofs  and  pave- 
ments of  coal-seams  are  brought  together,  the 
coal  itself,  as  having  least  resistance,  being 
thrust  into  the  loops  (a  a.  Fig.  255).' 

DefOFmatlon  and  Crushing. — During  the  ^i^ 
intense  shearing  movements  to  which  rocks  ( 
have  been  subjected,  their  individual  particles       ■ 

'  For  illiutratioM  of  this  structure  see  Heiin's  '  Mechaniami 
k  tanninolog?  tor  tha  diBciviit  psita  of  folds  is  proposed. 


:r  Gebirgsibildubg,'  where 


S44  tfEliTECTOXIC  (KTRUVTUHAL)  GEOLOGY  book  iv 

have  been  com  pressed,  elongated,  and  made  to  move  past  each  other,  us  in 
iiiatmo lively  shown  by  the  detbnuation  o£  pebbles  and  of  fossils  (p.  3U). 
The  most  itn|K>rtant  consequence  of  this  process  Is  the  production  of  the 
Bliear-Ktnictiire  already  noticeil  (p.  31G).  Massive  coarsely  crystalline  peg- 
niutites  muy  be  traced  through  successive  stages  wherein  the  compoDent 
ortboclase  and  felspar  are  more  and  more  crushed  and  drawn  out,  until 
in  the  end  the  rock  becomes  a  compact  finely  fissile  schist,  with  a  pecuhar 
thready  or  streaky  structure,  which  can  hardly  be  distinguished  from  the 
tiow-strnctui'e  of  a  rhyolite.  This  change  is  more  particularly  developed 
along  great  thmst-planes,  but  may  be  observed  throughout  a  mass  of  rock 
that  has  undergone  intense  shearing. 

In  many  cases  lenticular  "  eyes "  of  the  original  rock  have  been  left 
little  or  not  at  all  affected,  while  the  portions  between  them  have  been 
crushed  and  rolled  out  and  have  re- 
crystallized  more  or  less  completely  as 
true  schists  (Fig.  332).  Sections  shov- 
ing the  close  connection  between 
mechanical  crushing  and  the  production 
of  a  schistose  structure  may  be  seen 
abundantly  among  the  Scottish  High- 
lands.' Ill  the  Silurian  district  of 
Guldiileii  in  Norway  diabases  and  other 
igneous  rocks  exhibit  every  stage  in 
the  crushing  down  of  eruptive  matenal 
and  its  conversion  into  schists.  Similar 
structures  are  well  displayed  among 
the  schists  and  their  accompanimenta 
in  Anglesey. 

Not  only  are  the  individual  particle 
of  rocks  drawn  out  by  shearing,  but  in 
the  complicated  process  of  mountain- 
building,  larger  features  of  geological 
structure  likewise  undei^o  deformatioa 
The  anticlinal  anti  synclinal  folds  developed  in  the  earlier  stages  of  the 
process  arc  sometimes  bent  over  and  crushed  together,  so  as  to  be 
nearly  <ir  completely  effaced. 

Various  experiments  linve  lieen  devised  to  illustrate  the  fkcts  of 
mountain-structure.  By  a  combination  of  jiarallel  layers  of  different  nb- 
stiinces  ex|i0!te<l  to  lateral  compression  and  lension  it  is  possible  to  imitate 
many  of  the  features  of  that  striictuie  and  to  produce  very  instructive 
<liagrams.* 


(C.ni|«r-  fit. 


'  See  iiniirl.  Joiini.  Orot.  ,*f.  iliv.  (1888).  [i.  392. 
'  H*e  for  fxnmjilf,  A.  Kiivr*,  Xalun 
V.  (18S81,  )i.  337.     -MiiL-h  iiifomiali 
ilitaiii  Kangi'H.'  lSSr>. 


r.  103  ;  H.  M.  Cwlell,  TVbim,  /toy.  Soc.  JtfiR. 
will  alsn  In  Coutid  in  Mellird  Reade'i '  OiigiD  of 


PARTY  CLEAVAGE  546 


Part  V.  Cleavage. 

Cleavage-structure  having  been  described  at  p.  312,  we  have  to  notice 
here  the  manner  in  which  it  presents  itself  on  the  large  scale  among 
rock-masses.  The  direction  of  cleavage  usually  remains  persistent  over 
considerable  regions,  and,  as  was  shown  by  Sedgwick,^  corresponds,  on 
the  whole,  with  the  strike  of  the  rocks.  It  is,  however,  independent  of 
bedding.  Among  curved  rocks,  the  cleavage-planes  may  be  seen  traversing 
the  plications  without  sensible  deflection  from  their  normal  direction, 
parallelism,  and  high  angle.  They  must  thus  be  strictly  later  than  these 
plications.  But  their  general  coincidence  with  the  trend  of  the  axes  of 
folding  serves  to  indicate  a  community  of  origin  for  cleavage  and  folding, 
as  concomitant  though  not  absolutely  simultaneous  elfects  of  the  lateral 
compression  of  rocks.^  Among  curved  strata,  the  planes  of  cleavage 
sometimes  coincide  with,  and  are  sometimes  at  right  angles  to  the  planes 
of  bedding,  according  to  the  angles  of  the  folding  (Fig.    257).     The 


a  be 


Fig.  257.  —Curved  and  contorted  Deyonian  RockB,  near  Ilfracombe  (B.) 
Bedding  and  cleavage  planes  are  coincident  at  a  and  c,  bat  nearly  at  right  angles  at  h. 

persistence  of  cleavage-planes  across  even  the  most  diverse  kinds  of  rock, 
both  sedimentary  and  igneous,  was  first  described  by  Sedgwick.  Jukes 
also  pointed  out  that  over  the  whole  of  the  south  of  Ireland  the  trend  of 
the  cleavage  seldom  departs  10**  from  the  normal  direction  K  25''  N.,  no 
matter  what  may  be  the  differences  in  character  and  age  of  the  rocks 
which  it  crosses.  But  though  cleavage  is  so  persistent,  it  is  not  equally 
well  developed  in  every  kind  of  rock.  As  already  explained  (p.  313), 
it  is  most  perfect  in  fine-grained  argillaceous  rocks,  which  have 
been  altered  by  it  into  slates.  It  is  often  well  developed  in  felsites  and 
other  igneous  rocks,  which  then  furnish  good  fiags  or  even  slates.  It  may 
be  observed  at  once  to  change  its  character  as  it  passes  from  fine-grained 
rocks  into  others  of  a  more  granular  or  gritty  texture.  Occasional  traces 
of  distortion  or  deviation  of  the  cleavage -planes  may  be  observed  at  the 
contact  of  two  dissimilar  kinds  of  rock  (Fig.  258). 

A  region  may  have  been  subjected  at  successive  intervals  to  the 

*  *  On  the  Structure  of  large  Mineral  Masses/  Trans.  Geol.  Soc.  2nd  ser.  iii.  (1835) 
— an  admirable  memoir,  in  which  the  structure  of  a  great  cleavage  region  is  clearly  and 
graphically  described.  Phillips  gave  a  good  summary  of  our  knowledge  up  to  1856  in  his 
"Beport  on  Cleavage"  in  the  British  Assoc.  Rep.  for  that  year.  But  the  most  exhaustive 
memoir  on  the  subject  is  that  by  Mr.  A.  Harker  in  the  Reports  of  the  British  Association 
for  1885,  p.  818,  where  copious  references  to  the  bibliography  will  be  found.  See  also  papers 
by  the  Rev.  0.  Fisher  in  Qeol.  Mmj.  1884-85,  and  his  *  Physics  of  the  Earth's  Crust' 

s  Harker,  BrU.  Assoc.  Rep.  1885,  p.  852. 

2  N 


GEOTECTONIC  (STRUCTURAL)  GEOLOGY 


compression  that  has  produced  cleavage.  The  Silurian  rocks  of  the 
south-west  of  Ireland  were  upturned,  and  probably  cleaved,  before  tie 
deposition   of    the   Old   Red  Sandstone,   which    has   in   turn  been  well 


cleaved.'  Evidence  of  the  relative  date  of  cleavage  may  be  obtained 
from  unconformable  junctions  and  from  conglomerates.  An  ancleared 
series  of  strata,  lying  upon  the  denuded  edges  of  an  older  cleaved  eerie*, 
proves  the  date  of  cleavage  to  be  intermediate  between  the  periods  of 
the  two  groups.  Fragments  of  cleaved  rocks  in  an  uncleaved  con- 
glomerate show  that  the  rocks  whence  they  were  derived  had  already 
suffered  cleavage,  before  the  detritus  forming  the  conglomerate  wu 
removed  from  them.  An  intrusive  igneous  rock,  traversed  with  cleavage- 
planes  like  its  surrounding  mass,  points  to  cleavage  subsequent  to  iU 
intrusion  (Fig.  259).- 


),  riyiuDDtli  Sound,  butli  being 


Between  cleavi^e  and  foliation  there  is  in  many  cases  a  close  relation. 
Microscopic  examination  of  some  cleaved  rocks  shows  that  in  oii^iul 
clastic  sediment  a  micaceous  mineral  has  been  abundantly  developed, 
the  plates  of  which  are  ranged  along  the  planes  of  cleavage.  Thit 
mica  can  he  distinguished  from  original  mica-flakes  in  the  sediment 
It  may  be  observed,  in  many  cases,  to  impart  a  lustrous  silvery  or  wlkj 
sheen  to  the  cleavage-faces  of  a  slate,  yet  may  be  at  right  angles  to  Uw 
original  lamination  of  deposit.  Such  a  crystalline  rearrangement  it 
indeed  an  incipient  foliation.  It  is  the  same  structure,  further  developed 
and  intensified,  which  gives  their  distinctive  character  to  schists.  Tbs 
crystalline  metamorphosis  naturally  proceeds  along  the  linei  of  least 
resistance,  which  in  cleaved  rocks  are  the  cleavage -planes,  and  in 
uncleaved  sedimentary  rocks  are  the  planes  of  deposition.  Foliation,  at 
■  Do  U  Bwlie,  'Geol.  Observer,'  p.  620.  '  Ibid.  p.  821. 


PART  VI  DISLOCATION  647 

already  remarked  (p.  323),  may  sometimes  represent  stratification,  Bome- 
times  cleavage,  and  sometimes  divisional  planes  superinduced  by  shearing 
or  faulting.^ 

Before  passing  from  this  subject  it  may  be  well  to  note  bow  deceptive  is 
the  resemblance  of  cleavage-planes  to  bedding,  especially  on  weathered 
exposures  of  rock.  Even  experienced  observers  have  been  misled  by  this 
resemblance.  At  Llanberis,  for  example,  the  lower  portion  of  a  section 
consists  of  volcanic  tuff  and  the  upper  of  conglomerate,  llie  tuff  being 
compact  and  fine-grained,  has  undergone  such  decided  cleavage  that  at 
first  the  flags  into  which  it  is  divided  by  the  cleavage-planes  might  be 
mistaken  (as  they  have  in  fact  been)  for  bedding,  and  the  conglomerate 
would  then  be  regarded  as  a  much  younger  deposit  lying  unconformably 
on  tbe  tuff.  In  reahty,  however,  the  tuff  coincides  in  its  bedding  with  the 
conglomerate ;  they  are  parts  of  one  continuous  series,  but  the  coarse- 
grained conglomerate  has  been  only  slightly  affected  by  the  pressure  which 
induced  the  perfect  cleavage  in  the  tuff. 


Part  VI.  Dislocation. 

The  movements  which  the  crust  of  the  earth  has  undergone  have  not 
only  folded  and  corrugated  the  rocks,  but  have  fractured  them  in  all 
directions.  The  dislocations  may  be  either  simple  Fissures,  that  is, 
rents  without  any  vertical  displacement  of  the  mass  on  either  side,  or 
Faults,  that  is,  rente  where  one  side  has  been  moved  relatively  to  the 
other.-  It  is  not  always  possible,  in  a  shattered  rock,  to  discriminate 
between  joints  and  those  lines  of  division 
to  which  the  terra  fissures  is  more  usually 
restricted.  Many  so-called  fissures  may  be 
merely  enlarged  joints.  It  is  common  to 
meet  with  traces  of  friction  along  the 
walls  of  fissures,  even  when  no  proof 
of  actual  vertical  displacement  can  be 
gleaned.  The  rock  is  then  often  more 
or  less  shattered  on  either  side,  and  the 
contiguous  faces  present  rubbed  and  pol- 
ished surfaces  ("  slickensides "  p.  526). 
Mineral  deposits  may  also  commonly  be  observed  encrusting  the  cheeks  of 
a  fissure,  or  filling  up,  together  with  broken  fragments  of  rock,  the  space 
between  the  two  walls.  The  structure  of  mineral  veins  in  fissures  b 
described  in  Part  IX. 

'  9m  Sedgwick,  IVont.  0«of.  Soc.  (2),  iii.  p.  461.  Darwtn  on  foliation  uut  cleavage, 
'GMlogioal  ObHrvBtlons  in  South  America,'  1846,  p.  162.  A.  C.  Banisay,  'Geology  of 
North  Wales,'  Mem.  OeoL  Survey,  vol.  ill.  2ad  edit.  p.  233.  F.  M.  Stapff,  JVeuei  Jahii. 
18S2(L),  p.  82. 

*  The  gtndent  of  this  department  of  geology  will  ficd  in  the  joint  essa)-  bjr  M.  E.  de  Mar- 
jerie  and  ProCesBor  Hetm,  citeij  ou  p.  5S6,  a  valuable  haodbook  of  the  terms  lued  to  describe 
the  varloiu  structures  srisitig  from  raptures  of  the  terrestrial  crust 


GEOTECTONIC  {STRUCTURAL)  GEOLOGY 


Nature  of  Faults. — In  a  lai^  proportion  of  cases,  however,  there  hu 
been  not  only  fracture  but  dieplacement  The  rents  have  become  fault* 
aa  well  as  fissures.  The  movement  may  have  affected  only  one  aide  of  the 
fissure,  or  both  sides.  Sometimes  it  has  consisted  in  a  mere  vertical  subsid- 
ence of  one  side ;  in  other  cases  one  side  has  been  pushed  up,  or  while  one  sida 
has  moved  upward  the  other  has  sunk  downward,  or  both  sides  have  been 
shifted  up  or  down  from  their  original  position,  but  one  more  than  the 
other.  In  ordinary  faults  the  displacement  is  usually  vertical  or  nearly 
so.  But  in  some  regions  faults  have  been  produced  by  a  lateral  thisBt  of 
one  side  of  a  fissure  past  the  other  side.  This  structure  comes  out 
with  remarkable  prominence  in  the  gneJBS  district  of  western  Sutherland, 
where  dykes  crossed  by  such  lateral  thrusts  are  disrupted  and  drawn  out 
along  the  line  of  fissure  so  as  to  be  reduced  to  a  -^^  part  of  their  ordinary 
breadth.' 


Faults  on  a  small  scale  are  sometimes  sharply-defined  lines,  as  if  the 
rocks  had  been  sliced  through  and  fitted  together  again  after  being 
shifted.  In  such  cases,  however,  the  harder  portions  of  the  dislocated 
rocks  will  usually  I>e  found  slickensided.  More  frequently  some  disturb- 
ance has  occurred  on  one  or  both  sides  of  the  fault  (Fig.  261).  Some- 
times in  a  series  of  strata,  the  beds  on  the  side  which  has  been  pushed  up 
(or  side  of  upthrow)  are  bent  down  against  the  fault,  while  those  on  the 
opposite  side  (or  that  of  downthrow)  are  bent  up  (Fig.  263).  Most  com- 
monly the  rocks  on  both  sides  are  considerably  broken,  jumbled,  and 
crumpled,  so  that  the  line  of  fracture  is  marked  by  a  belt  or  watl-like  man 
of  fragmentary  rock,  known  as  "fault-rock."  Where  a  dislocation  has 
occurred  through  materials  of  very  unequal  hardness,  such  as  solid  lime- 
stone bands  and  soft  shales,  or  where  its  course  has  been  undulating,  ^a 
relative  shifting  of  the  two  sides  has  occasionally  brought  opposite  pro- 

■  S«f  Report  on  QMlogicil  Snrv«;  woik,  Quart.  JoHm.  Otti.  Son.  xliT.  (1888),  p.  SH 
mipotlm,  Fig.  331. 


FAVLTS 


a  together  ao  as  to  leave  wider  interflpacea  (Fig.  3 1 2).  The  actual 
breadth  of  a  fault  may  Tarj*  from  a  mere  chink  into  which  the  point  of  a 
knife  could  hardly  be  inserted,  up  to  a  band  of  broken  and  often  consolidated 
materials  many  yards  wide.  Where  a  fault  has  a  considerable  throw,  it 
is  sometimes  fianked  by  parallel  small  faulta  The  occurrence  of  these 
close  bother  will  obviously  produce  the  appearance  of  a  broad  zone  of 


Weit  or  Uveniock  Point  (B.) 


much  fractured  rock  along  the  trend  of  a  main  fissure.  A  line  of  dis- 
turbance may  consist  of  several  parallel  faults  of  nearly  equal  magnitude 
(Fig.  265,  section  3). 

Faults  are  sometimes  vertical,  but  are  generally  inclined.  The  largest 
faults,  or  those  with  the  greatest  vertical  ikrmo  or  displacement,  com- 
monly slope  at  high  ^gles,  while  those  of  only  a  few  feet  or  yards 
may  be  Inclined  aa  low  as  18°  or  20°.  The  inclination  of  a  fault  from 
the  vertical  is  called  its  hade.  In  Fig.  264,  for  example,  the  fault  at  B, 
being  vertical,  has  no  hade,  but  that  at  A  hades  at  an  angle  of  70°  from 
the  vertical  to  the  left  hand.  The  amount  of  throw  is  represented 
as  the  same  in  both  instances,  but  with  the  direction  of  throw  to 
opposite  quarters,  so  that  the  level  of  the  beds  is  raised  between  the  two 
faults  above  the  uniform  horizon  which  it  retains  beyond  them. 

The  effect  of  the  inclination  of  faults  is  to  give  the  appearance  of 
lateral  displacement.     In  Fig.  264,  for  example,  where  the  bade  of  one 


fault  is  considerable,  the  two  severed  ends  (c  and  d)  of  the  black  bed 
appear  to  have  been  pulled  asunder.  The  horizontal  distance  to  which 
they  are  removed  does  not  depend  upon  the  amount  of  vertical  displace- 
ment, but  upon  the  angle  of  hade.  A  small  fault  with  a  great  hade  will 
shift  strata  laterally  much  more  tlian  a  largi^  fault  with  a  small  hade.  It 
is  obvious  that  the  angle  of  hade  must  seriously  affect  the  value  of  a  coal- 
field.     If  the  black  bed  in  the  same  figure  be  supposed  to  be  a  coal-seam. 


550  GEOTECTONIG  {STRUCTURAL)  GEOLOGY  book  iv 

it  could  be  worked  from  either  side  up  to  c  and  dj  but  there  would  be  a 
space  of  barren  ground  between  these  two  points,  where  the  seam  never 
could  be  found  The  larger  the  angle  of  hade  the  greater  the  breadth  of 
such  ban*en  ground. 

Origin  of  Faults. — In  countries  where  the  rocks  have  not  undergone 
much  disturbance,  that  is,  where  stratified  formations  are  still  not  far 
removed  from  their  original  approximate  horizontality,  faults  are  probably, 
for  the  most  part,  due  to  mere  subsidence  of  the  crust  (Normal  Faults). 
Where,  on  the  other  hand,  rocks  have  been  much  plicated,  the  more 
gigantic  faults  have  been  produced  by  tangential  thrust,  whereby  one 
mass  of  rock  has  been  pushed  bodily  over  another  (Reversed  Faults, 
Thrust-planes).  In  some  cases,  both  lateral  thrust  and  subsidence  have 
been  concerned  in  the  origin  of  the  dislocations  of  a  much -fractured 
area. 

Normal  Faults. — In  the  vast  majority  of  cases,  faults  hade  in  the 
direction  of  downthrow,  or  in  other  words,  they  slope  away  from  the  side 
which  has  risen.  These  are  Nomial  Faults,  The  explanation  of  the 
structure  is  doubtless  to  be  found  in  the  fact  that  the  portion  of  the 
terrestrial  crust  towards  which  a  fault  hades  presents  a  less  area  of  base 
to  pressure  or  support  from  below  than  the  mass  with  the  broad  base  on 
the  opposite  side.  The  mere  inspection  of  a  faidt  in  any  natural  or 
artificial  section  suffices,  in  most  cases,  to  show  which  is  the  upthrow  side. 
In  mining  operations,  the  knowledge  of  this  rule  is  invaluable,  for 
it  decides  whether  a  coal-seam,  dislocated  by  a  faulty  is  to  be  sought  for 
by  going  up  or  down.  In  Fig.  264,  a  miner  working  from  the  left^  and 
meeting  with  the  fault  at  c,  would  know  from  it«  hading  towards  him  that 
he  must  ascend  to  find  the  coal.  On  the  other  hand,  were  he  to  work 
from  the  right,  and  catch  the  fault  at  d,  he  would  see  that  it  would  be 
necessary  to  descend.  According  to  this  rule,  a  normal  fault  never  brings 
one  part  of  a  bed  below  another  part,  so  as  to  be  capable  of  being  pierced 
twice  by  the  same  vertical  shaft. 

Reversed  Faults  are  those  in  which  lower  rocks  on  one  side  have 
been  pushed  over  higher  rocks  on  the  other.  In  these  cases,  the  same 
stmtum  may  be  pierced  twice  by  a  vertical  shaft.  The  hade  is  there- 
fore in  tlie  direction  of  upthrow.  Faults  of  this  kind  chiefly  occur  in 
regions  where  the  rocks  have  been  excessively  plicated,  and  especially 
where  one-half  of  a  fold  has  l)een  pushed  over  another  (Figs.  263  and 
265,  section  4).^  They  are  closely  connected  with  anticlinal  and  syn- 
clinal folding.  Thus,  a  monoclinal  fold  may  by  increase  of  movement  be 
develo|)ed  into  a  fracture  (Fig.  265).  Beautiful  examples  of  this  relation 
have  been  observed  by  Powell  and  others  among  the  little -disturbed 
formations  of  the  great  plateaux  of  Utah  and  Wyoming.  But  it  is  in 
mountainous  regions  that  they  are  chiefly  developed ;  they  become  there, 

^  If  faults  were  generally  due  to  rupture  from  compression  we  should  expect  Dm 
"  reversetl  "  to  be  tlie  ordinary  form.  The  normal  hade  of  faults  points  to  the  ezUtencc  of 
stresses  in  t)ie  crust  of  the  earth  which  are  from  time  to  time  relieved  by  disIocatifnL  But 
the  nature  of  these  stresses  and  the  manner  in  which  faults  arise  are  sUll  among  the  obieim 
problems  of  geolog}-. 


PART  VI  FAULTS  661 

indeed,  the  common  type  of  dislocation.     Many  excellent  examples  have 
been  adduced  from  the  plicated  rocks  of  the  Alps.^ 


Fig.  265. — Sections  to  show  the  relations  of  Monoclinal  folds  and  faults. 

1 ,  Monoclinal  fold :  2,Monoclinal  fold  replaced  by  a  single  nonnal  fault ;  3,  Monoclinal  fold  converted  into  a 
series  of  parallel  normal  faults  ;  4,  Monoclinal  fold  developed  by  increase  of  plication  into  a  reversed  fault 

Thrust-planes. — Under  this  name  the  Geological  Survey  of  Scotland 
has  described  a  remarkable  type  of  reversed  fault,  where  the  hade  is  so  low 
that  the  rocks  on  the  upcast  side  have  been  pushed  for  miles  horizontally 
across  the  rocks  on  which  they  lie  (see  Figs.  249,311,  328,  331,  334).2  Such 
a  structure  points  to  enormous  tangential  pressure,  under  which  the  very 
foundations  of  a  country  were  thrust  up  and  driven  over  younger  rocks. 
The  "  grande  faille  du  Midi/'  in  the  north  of  France  and  Belgium,  by  which 
the  Devonian  rocks  have  been  pushed  over  the  Carboniferous,  is  a  well- 
known  and  remarkable  example  of  this  structure.  In  some  cases  so  intense 
have  been  the  mechanical  movements,  that  extensive  metamorphism  has 
been  induced  by  them.  Along  the  thrust-planes  in  the  north-west  of  Scot- 
land, and  for  a  long  way  above  them,  the  rocks  that  have  been  pushed  for- 
ward have  undergone  enormous  shearing,  new  divisional  planes  have  been 
developed  in  them,  and  they  have  become  more  or  less  schistose,  the  new 
minerals  crystallizing  along  the  shearing-surfaces  approximately  parallel 
to  the  thnist-planes. 

Throw  of  Faults. — ^That  normal  faults  are  vertical  displacements  of 
parts  of  the  earth's  crust  is  most  clearly  shown  when  they  traverse  strati- 
fied rocks,  for  the  regular  lines  of  bedding  and  the  originally  flat  position 
of  these  rocks  afford  a  measure  of  the  disturbance.  In  Fig.  264,  the 
same  series  of  strata  occurs,  on  either  side  of  each  of  the  two  faults,  so 
that  measurement  of  the  amount  of  displacement  is  here  obviously  simple. 
The  measurement  is  made  from  the  truncated  end  of  any  given  stratum 
vertically  to  the  level  of  the  opposite  end  of  the  same  stratum  on  the 
other  side  of  the  fault.  Where  the  fault  is  vertical,  like  that  to  the  right 
in  Fig.  264,  the  mere  distance  of  the  fractured  ends  from  each  other  is 

*  See  Powell  in  the  works  cited  already  on  p.  538.  Heim,  *  Mechanismns  der  Gebirgs- 
bildung,*  Plate  XV.  Fig.  14.  Compare  C.  W.  Hayes,  Bull,  Ged,  Sac.  Amer.  ii.  (1891),  p. 
141. 

'  B.  X.  Peach  and  J.  Home,  Nature,  13th  Nov.  1884.  The  details  of  this  structure 
with  numerous  illustrations  will  be  found  in  the  Report  of  the  Geological  Survey,  Quart, 
Joum,  Oeol.  Soc.  xliv.  (1888),  p.  378.  M.  Bertrand  has  described  under  the  name  of  ''failles 
courbes  "  certain  curved  faults  which  affect  the  rocks  of  the  Jura  and  south  of  France,  but  do 
not,  he  thinks,  descend  into  the  cnist ;  and  he  cites  the  Mont  Faron  near  Toulon,  which,  he 
says,  one  cannot  climb  from  any  side  without  crossing  a  large  fault  that  brings  Jurassic 
down  upon  Triassic  rocks  (Bull,  Soc.  Giol.  France  (3),  xii.  (1884),  p.  452). 


552  OEOTECTOXIG  {STRUCTURAL)  GEOLOGY  book  it 

the  amount  of  displacement.  In  the  case  of  an  inclined  fault,  the  level 
of  the  selected  stratum  is  protracted  across  the  fissure  until  a  vertical 
from  it  viiW  reach  the  level  of  the  same  hed,  as  shown  by  the  dotted  linea. 
The  length  of  this  vertical  is  the  amount  of  vertical  displacement,  or  the 
throw  of  the  fault.  The  throw  of  faults  varies  from  less  than  an  inch  to 
several  thousand  feet. 

Unless  beds,  the  horizons  of  which  are  known,  can  be  recognised  on 
both  sides  of  a  fault,  exposed  in  a  cliff  or  other  section,-  the  fault  at  that 
particular  place  does  not  reveal  the  extent  of  its  displacement  It  would 
not,  in  such  a  case,  be  safe  to  pronounce  the  fault  to  be  large  or  small  in 
the  amount  of  its  throw,  unless  we  had  other  eWdence  from  which  to 
infer  tlie  geological  horizon  of  the  beds  on  either  side.  A  fault  with  a 
considerable  amount  of  displacement  may  make  little  show  in  a  difi^ 
while,  on  the  other  hand,  one  which,  to  judge  from  the  jumbled  and 
fractured  ends  of  the  beds  on  either  side,  might  be  supposed  to  be  a 
powerful  dislocation,  may  be  found  to  be  of  comparatively  slight  im- 
portance. Thus,  on  the  cliff  near  Stonehaven,  in  Kincardineshiiey  (me  of 
the  most  notable  faults  in  Great  Britain  runs  out  to  sea,  between  the 
ancient  crystalline  rocks  of  the  Highlands  and  the  Old  Red  Sandstones 
and  conglomerates  of  the  Lowlands  of  Scotland.  So  powerful  have  been 
its  effects  that  the  strata  on  the  Lowland  side  have  been  thrown  on  end 
for  a  distance  of  two  miles  back  from  the  line  of  fracture,  so  as  to  stand 
upright  along  the  coast-cliffs  like  books  on  a  library  shelf.  Yet  at  the 
actual  point  where  the  fault  reaches  the  sea  and  is  cut  in  section  by  the 
shore-cliff,  it  is  not  revealed  by  a  band  of  shattered  rock.  On  the  con- 
trary, no  one  would  at  first  be  likely  to  suspect  the  existence  of  a  fault 
at  all.  The  red  sandstone  and  the  reddened  Highland  schists  have  been 
so  compressed  and,  as  it  were,  welded  into  each  other,  that  some  care  is 
required  to  trace  the  demarcation  between  them. 

Dip-Faults  and  Strike-Faults. — The  same  fault  may  give  rise  to 
very  different  effects,  accoixling  to  variations  in  the  inclination  or  cur^'ature 
of  the  rocks  which  it  traverses,  or  to  the  influence  of  branch  faults 
diverging  from  it.  Faults  among  inclined  strata  may,  in  most  districts, 
be  conveniently  grouped  into  two  series,  one  running  in  the  same  general 
direction  as  the  dip  of  the  strata,  the  other  approximating  to  the  trend 
of  the  strike.  They  are  accordingly  classified  as  dip-faults  and  strike-faultSj 
which,  however,  are  not  always  to  l)e  sharply  marked  off  from  each  other, 
for  the  dip-faults  will  often  be  obser^'ed  to  deviate  considerably  from  the 
normal  direction  of  dip,  and  the  strike-faults  from  the  prevalent  strike,  so 
as  to  pass  into  each  other. 

A  dip-fault  produces  at  the  surface  the  effect  of  a  lateral  shift  of  the 
strata.  This  eifect  increases  in  proportion  as  the  angle  of  dip  lessen^ 
but  ceases  altogether  when  the  beds  are  vertical.  Fig.  266  may  be  taken 
as  a  plan  of  a  dip-fault  (//)  traversing  a  series  of  strata  which  dip 
northwards  at  20  .  The  beds  on  the  east  side  look  as  if  they  had  been 
pushed  horizontally  southwards.  That  this  apparent  horizontal  displace- 
ment is  due  really  to  a  vertical  movement,  and  to  the  subsequent  planing 
down  of  the  surface  by  denuding  agents,  will  be  clear,  if  we  consider 


PAST  Ti  FA  ULTS  663 

what  must  be  the  effect  of  the  vertic&I  asceut  or  descent  of  the  inclined 
beds  at  a  dialocation.  The  part  on  one  side  of  the  fracture  may  be  pushed 
up,  or,  vhat  is  equivalent,  that  on  the  other 
side  may  he  let  down.  If  the  strike  of  the 
beds  be  supposed  to  be  east  and  west,  then  a 
horizontal  plane  cutting  the  dislocated  strata 
will  show  the  portion  on  the  west  or  upthrow 
aide  of  the  fault  lying  to  the  north  of  that 
on  the  east  or  downthrow  aide.  The  effect 
of  denudation  has  usually  been  practically  to 
produce  such  a  plane,  and  thus  to  exhibit 
an  apparently  lateral  shift  This  surface 
displacement  has  been  termed   the  htave  of 

a  fault.     Its  dependence  upon  the  angle  of     " "         "DJi..[iuu  ' ' 

dip  of  the  strata   may  be  Been  by  a  com- 
parison of  Sections  A  and  B  in  Fig.  267.     In  the  former,  the  bed  a  b, 
which  may  be  supposed  to  be  one  of  those  in  Fig.  266,  dipping  north  at 
20*,  once  prolonged  above  the  present  surface  (marked  1^  the  horizontal 
line),  is  represented  as  having  dropped  from  w  b  to  e  d.      The  heave 


Fig.  MT.— aectioM  lo  >how 


■rlstion  nrhoiiiniitsl-lliplimine 


amounts  to  the  horizontal  distance  between  e  and  b,  the  throw  being  the 
vertical  distance  between  b  and  d.  But  if  the  angle  should  rise  to  50", 
as  in  B,  though  the  amount  of  throw  or  vertical  displacement  is  there  one- 
fourth  greater,  the  heave  or  horizontal  shift  diminishes  to  less  than  a  half 
of  what  it  is  in  A.  This  diminution  augments  with  increase  of  inclination 
till  among  vertical  beds  there  is  no  heave  at  all,  though  a  fault  with  a  hori- 
zontal thrust  will  cause  a  lateral  shift  even  in  vertical  strata  (see  Fig.  331). 
Strike-faults,  where  they  exactly  coincide  with  the  strike,  may  remove 
the  outcrops  of  some  strata  by  never  allowing  them  to  reach  the  surface. 
Fig.  268  shows  a  plan  (A)  and  section  (B)  of  one  of  these  faults,  /  /, 
having  a  downthrow  towards  the  direction  of  dip.  In  crossing  the  strike, 
we  pass  successively  over  the  edges  of  all  the  beds,  except  the  part 
between  the  asterisks,  which  is  cut  out  by  the  fault  as  shown  in  the 
section.  It  seldom  happens,  however,  that  such  strict  coincidence  between 
faults  and  strike  continues  for  more  than  a  short  distance.  The  direction 
of  dip  is  apt  to  vary  a  little  even  among  comparatively  undisturbed  strata, 
every  such  variation  causing  the  strike  to  undulate,  and  thus  to  be  cut 


(IHOTErTOSIC   .iTBCCTVRAL)  GEOLOQY 


iiKirii  III'  I<:mh  filfli<'|U'tly  by  the  line  of  dislocation,  which  may  neverthelm 
i-iiri  ((iiiMt  Htnu'Klit  Moreover,  an  increaee  or  diminution  in  the  throw  of 
ii  Mnkf  I'niilt  will  have  the  effect  of  bringing  the  dislocated  ends  of  tlie 
IiiiiIh  jiKitiiiHt  till!  line  of  dislocation.  In  Fig.  269,  for  instance,  which 
l'<i|ii'<ii<i^ii|ji  ill  (iluii  unother  Htrike-fault  (/),  we  see  that  the  amount  of  throw 


^»'^- 

s^^ms^^ 

-      ■G^—--~'^r^^ 

-    -   - ■  ■ 

.q-ii.' 


1  b,-d  V. 


>'iiiils  the  right  so  us  to  allow  lower  beds  BuccesBirely  Ia 
i>  Ado,  while  towards  the  K'ft  it  diminishes,  and  finally  diet 


'I'lifir  I'lrocts  Iwconu'  more  complicated  where  faults  trarerse  andu- 
biiiii^  uiid  contorted  strata.  Ttiti  counuction  between  folding  and  fracture 
hiiH  iilroady  licon  adverted  to  in  the  case  of  mouoclinal  bends.  It  soin«- 
ihiwn  ha))pens  that  the  {'lications  are  subsequently  fractured,  so  that  the 


fault  iiiiiy  a|i[ioar  to  Ik-  altenmioiy  a  downthrow  on  opposite  sides, 
jii-.ortiing  to  the  position  of  the  airhes  and  troughs  which  it  crosset. 
'I'his  stnii-tiire  may  1m  illustrated  by  a  plan  and  sections  of  a  dislocated 
autii'linc  and  synclinc,  which  will  aUo  show  clearly  how  the  apparently 
laifi-.d  displacement  of  outcrop  produeeti  by  dip-faults  is  due  to  vertical 
mnvt-niftit.  Kig.  liTO  represents  a  plan  of  strata  thrown  into  an 
anii.liiial  fold  .\.\  and  a  synclinal  fold  SS.  and  traversed  by  a  huUt  FF. 
hn\iiij;  an  upthrow  {»  »)  to  the  east.  A  dip-fault  shifts  the  ooterop 
towards  the  dip  on  the  upthrow  side,  and  this  will  be  observed  to  be  tk 
case  here.  On  the  west  side  of  the  fanlt,  the  black  bed  a,  dif^iif 
towards  the  south,  is  tnincated  by  the  fault  at  h,  and  the  portion  on  tk 
upthrow  side  is  shifter)  forwarxls  or  st>uthward.  Crossing  the  syncline** 
meet  with  the  same  bed  rising  with  a  contrary  dip,  and  as  the  apthro* 
of  the  f;udt  still  t-ontinuos  on  the  same  side,  the  portion  of  the  bed  oa  tk 
west  side  of  the  fault  must  be  sought  further  south.     The  effect  of  Ik 


PART  VI 


FA  ULTS 


556 


fault  on  the  syncline  is  to  widen  the  distance  between  the  two  opposite 
outcrops  of  a  bed  on  the  downthrow  side,  or  to  narrow  it  on  the  upthrow 
side.  On  the  southern  slope  of  the  anticline  A,  the  same  bed  once  more 
appears,  and  again  is  shifted  forwards,  as  before,  on  the  upthrow  side. 
Hence  in  an  anticline,  the  reverse  effect  takes  place,  for  there  the  space  be- 
tween the  two  outcrops  is  narrowed  on  the  downthrow  side.     A  section 


Fig.  270.— Plan  of  Anticline  (A)  and  Syncline  (S),  diHlocated  by  a  Fault  (F  F). 

along  the  east  or  upcast  side  of  the  fault  would  give  the  structure  repre- 
sented in  Fig.  271  (1);  while  one  along  the  downcast  side  would  be  as 
in  (2).  These  two  sections  illustrate  how  the  shifting  of  the  outcrops  at  the 
surface  can  be  simply  explained  by  a  mere  vertical  movement. 


r 


1 

A 


/v 


y    N 


y'   A 


Fig.  271.— Sections  along  the  Fault  in  Fig.  270. 
1,  Section  along  the  upcast  side  ;  2,  Section  along  the  downthrow  side. 

Dying  out  of  Faults. — Dislocation  may  take  place  either  by  a  single 
foult,  or  as  the  combined  effects  of  two  or  more.  Where  there  is  only  one 
fault,  one  of  its  sides  may  be  pushed  up  or  let  down,  or  there  may  be  a 
simultaneous  opposite  movement  on  either  side.  In  such  cases,  there  must 
be  a  gradual  dying  out  of  the  dislocation  towards  either  end ;  and  one  or 
more  points  where  the  displacement  has  reached  a  maximum.  Sometimes, 
as  may  be  seen  in  coal-workings,  a  fault,  with  a  considerable  maximum 
throw,  splits  into  minor  faults  at  the  terminations.     In  other  cases,  the 


nmt 


<iKOTK<:rosi(:  (stjiuctubal,  geology 


itfrniiootH  talc;  itlace  along  the  line  of  the  main  fissure.  Exceedingly  com- 
plji:uL4!'l  cxaiiii'leit  (NM:iir  in  some  coal-fields,  where  the  connected  faults 
\ii:\:i>uni  H<i  iiiinierouK  that  no  one  of  them  degerreB  to  be  called  the  nuin 
1)1'  |i<:iiiiiiK  ilisliicutioii.  Hy  u  series  of  branch-faults,  the  effect  of  a  main 
fault  iiiwy  I"!  iieutralistMi  or  reversed.  Suppose,  for  example,  that  a  main 
fuiilL  lit  itx  uasbsni  |iortion  throws  down  60  fathoms  to  the  north,  and 
tliiit:  lit  inti^t'viilH  thi'ue  faults  on  the  same  side  strike  off  from  it,  each 
liiiviiiK  '^  'lowntlifow  of  'J.5  futhoms  to  the  east;  the  combined  effect  of 
tlii-m)  hrutirh  fiiultM  will  he  to  reverse  the  throw  of  the  main  fault  towaidt 
itH  wi-ritorri  t-nil,  niid  [iriHlucc  a  downthrow  of  15  fathoms  to  the  soutL 

Qroups  of  Faults. — Tho  subsidence  or  elevation  of  a  large  mau  or 
liliick  of  riiik  linn  uNiiiilly  taken  place  by  a  combination  of  faults.     Detailed 


mill'!'  <f\  *t*iil  rii'Kl*.  siii'li  as  il 
t;iv.-n  iUiCiiin  I'll  a  siralo  of  «i 
iiwiorial  fi>r  tlio  study  of  \\ 
(vi'ii  n';ionl;itt'd  by  laulis. 
souu't  iiiii-#  ii*  a  Vi'mavkaMo  <• 
,-.';d-tivKl  ^^■pr^•*onI^^l  in   Fi; 


i>$i'  jiubli^heil  by  the  Geolt^cal  Snrrer  of 
1  inohes  (o  a  milo,  furnish  much  instnictin 
t>  >v:iy  in  whioh  the  crust  of  the  earth  hu 
111  nti^i  o.-ifC''.  dip-faults  are  predominant) 
ctt'nt.  a^  in  the  portion  of  the  South  Wain 
.  'I'-l.  In  other  places,  the  dislonticaf 
I  divide  the  ground  into  an  irrepilar  nrt- 


It  I'tti'ii  happens  that,  by  a  ^ucoe^sion  of  jmnllel  and  adjoining  EalU 
a  serio«  <>f  f^tiai.i  is  $ti  disUi-uioil  ih.ii  a  ^tihi  siratam.  whkh  mar  he  Mtf 
ihe  suriAio  on  I'ue  side,  is  «»rrii>i  <lown  Ity  a  serie«  of  etcp*  to  hM 


PAM  VI  FA  ULTS  667 

dietance  below.  Excellent  exampIeB  of  these  etep-faults  (Fig.  273)  are  to 
be  seen  in  the  coal-Relda  on  both  sides  of  the  upper  part  of  the  estuary  of 
the  Forth.  Instead,  however,  of  having  the  same  downthrow,  parallel 
faults  frequently  show  a  movement  in  opposite  directions.  If  the  mass 
of  rock  between  them  has  subsided  relatively  to  the  surrounding  ground, 
they  are  trough-faults  (Fig.  274),  and  enclose  wedge-shaped  masses  of  rock. 
It  will  be  observed  that  the  hade  of  these  faults  is  in  each  case  towards 


the  downthrow  side,  afld  that  the  wedge-shaped  masses  with  broad 
bottoms  have  risen,  while  those  with  narrow  bottoms  and  broad  tops  have 
sunk. 

The  faults  of  a  district  may  not  have  been  the  result  of  one  series  of 
movements,  but  of  a  long  succession  of  displacements,  or  of  renewed 
disturbance  after  prolonged  quiescence.  One  fault  sometimes  displaces 
another.  In  regions  of  reversed  faults  and  thrust-planes,  normal  faults 
have  sometimes  taken  place  long  after  the  first  dislocations.  In  north- 
western Scotland,  for  example,  the  t)lrus^planes  have  been  cut  across  and 
shifted,  exactly  as  if  they  had  been  ordinary  stratification-planes. 


Ftg.  ST4.— Trongh-Fi 


Detection  and  tracing  of  Faults. — As  a  rule,  faults  give  rise  to  little 
or  no  feature  at  the  surface,  so  that  tbeir  existence  would  commonly  not 
be  suspected.  They  comparatively  rarely  appear  in  visible  sections,  but 
ue  apt  rather  to  conceal  themselves  under  surface  accumulations  just  at 
those  points  in  a  ravine  or  other  natural  section  where  we  might  hope  to 
catch  them.  Yet  they  undoubtedly  constitute  one  of  the  most  important 
featores  in  the  geol<^eal  structure  of  a  district  or  country,  and  should  con- 
sequently be  traced  with  the  greatest  care.  In  the  majority  of  cases,  in 
eoontries  like  much  of  central  and  northern  Europe,  where  the  ground  is 
covered  with  superficial  deposits,  the  position  of  faults  cannot  be  seen, 
bat  must  be  inferred ;  though  it  must  be  admitted  that  geologists  have 


Iv 


&ftH  (IKdTErTOXir  {HTRUCTVEAL)  GEOLOGY  dooe  iv 

lw(>ii  {>M>iui  to  gnrut  reckiesanesa  in  this  respect,  introducing  fiinltx  for 
which  thi'jv-  vox  little  or  no  actual  evidence,  but  which  were  coavenieiit 
for  (lie  Dxiitiiiiiitiou  of  theoretical  views  of  the  structure  of  a  district. 
Kx]H'rii'iii-u  will  tcuch  the  student  that  the  mere  visible  section  of  a  fuih 
iin  HiDiKi  rliiror  shore  does  not  necessarily  afford  such  clear  evidence  of 
ilH  iiiiliiro  Diiil  flfucts  lis  may  be  obtained  from  other  parts  of  the  region, 
wlii'i-i<  it  docH  not  »how  itself  at  the  surface  at  all.  Li  fact,  he  might  be 
ili<i-i'i(  <-tl  !>>'  u  single  section  with  a  fault  exposed  in  it,  and  might  be  led  to 
ri>}:tiiil  lliut  fitull  as  an  important  and  dominant  one,  while  it  might  be 
(■lilt  »  tH'iMiidary  ili^locittion  in  the  near  neighbourhood  of  a  great  fractorci 
fur  wliii-li  tlio  I'viiioiice  would  be  elsewhere  obtainable,  but  which  mi^c 
\w\vt  l>(>  si>oi)  \X*v\t  The  actual  position  (within  a  few  yards)  of  a  lii^ 
l;tidl,  i(K  liiu*  acruss  the  countr}-,  its  effect  on  the  surface,  its  influence  on 


(;>' II I  optical  structure,  its  amount  of  vertical  displacement  at  different  puti 
»f  ils  course — all  this  infumiation  may  be  admirably  worked  out,  utd  jW 
tlii>  lu'liial  fviidiiru  may  never  l>e  seen  in  anyone  single  section  on  the 
ground.  A  viwilili'  exjiostire  of  the  fracture  would  be  interesting:  it 
stould  f^\e  the  exact  position  of  the  line  at  that  particular  plaee;  but  iC 
KixiM  not  1h>  uei'i'ssiiry  to  prove  the  existence  of  the  faidt,  nor  wonld  it 
pi'r1i:i]iH  furnish  luiy  additional  information  of  imi>ortaace.  The  existaKt 
of  uu  uTisccn  fault  may  usually  be  determined  by  an  examination  of  the 
gi<olii^ii-al  stnii-tnro  of  a  district.  An  abruptly  truncated  outcrop  i« 
always  sii};};(>»tivi-  of  fractiuv,  though  sometimes  it  may  be  due  to  utMO- 
forn)a)iK<  deiMisitiou  against  a  steep  declivity.  If  a  series  of  atnU  he 
diKiHivered,  in  a  watorn'oui^^e  or  other  exposure,  dipping  cootimioiuly  ii 
iiiii>  );i<ntT.)l  dinvtidii  at  angles  of  10"  or  more,  and  if,  at  a  short  dntane^ 
another  ixiittou  of  the  same  series  t>e  found  inclined  in  another  directioB. 
(hf  two  thus  striking  at  each  other,  a  fault  will  aliDOBl  always  be  reqoindu 


PART  VII  ERUPTIVE  ROCKS  669 

explain  their  relation.  If  all  the  evidence  obtainable,  from  the  sections 
in  water-courses  or  otherwise,  be  put  upon  a  map  (as  in  A,  Fig.  275),  it 
will  be  seen  that  a  dislocation  must  run  somewhere  near  the  points  marked 
//,  as  there  is  no  room  for  either  series  to  turn  round  so  as  to  dip  below 
the  other.  They  must  be  mutually  truncated.  The  completed  map  would 
represent  them  separated  by  a  fault  (f,  in  B).  The  upthrow  or  downcast 
side  of  the  dislocation  would  be  determined  by  the  observer's  knowledge 
of  the  order  of  superposition  of  the  respective  groups  of  strata. 

The  existence  of  a  fault  having  been  thus  proved  from  an  examination 
of  the  geological  structure  of  the  ground,  its  line  across  the  country  may 
be  approximately  laid  down — 1st,  by  getting  exposures  of  the  two  sets  of 
rock,  or  the  two  ends  of  a  severed  outcrop  on  either  side,  as  near  as  pos- 
sible to  each  other,  and  tracing  the  trend  of  the  dislocation  between ; 
2nd,  by  noting  lines  of  springs  along  the  supposed  course  of  the  fault, 
subterranean  water  frequently  finding  its  way  to  the  surface  along  such 
fissures ;  3rd,  by  attending  to  surface  features,  such  as  lines  of  hollow,  or 
of  ridge  rising  above  hollow,  the  effect  of  a  fault  often  being  to  bring 
rocks  of  unequal  resistance  together,  so  as  to  allow  the  more  durable  to 
rise  more  or  less  steeply  from  the  fracture.^ 


Part  VII.  Eruptive  (Igneous)  Rocks  as  Part  of  the  Structure 

OF  THE  Earth's  Crust i 

The  lithological  differences  of  eruptive  rocks  having  already  been 
described  in  Book  II.  (p.  154),  it  is  their  larger  features  in  the  field  that 
now  require  attention, — features  which,  in  some  cases,  are  readily  ex- 
plicable by  the  action  of  modern  volcanoes ;  and  which,  in  other  cases, 
bring  before  us  parts  of  the  economy  of  volcanoes  never  observable  in  any 
recent  cone,  by  revealing  deep-seated  rock-structures  that  lie  far  beneath  the 
upper  or  volcanic  zone  of  the  terrestrial  crust.  A  study  of  the  igneous 
rocks  of  former  ages,  as,  built  up  into  the  framework  of  the  crust,  serves 
to  augment  our  knowledge  of  volcanic  action. 

At  the  outset,  it  is  evident  that  if  eruptive  rocks  have  been  extruded 
from  below  in  all  geological  ages,  and  if,  at  the  same  time,  denudation  of 
the  land   has  been  continuously  in  progress,   many  masses  of  molten 


Fig.  270.— Extensively-denuded  Volcanic  District  (B.) 

material,  poured  out  at  the  surface,  must  have  been  removed.  But  the 
removal  of  these  superficial  sheets  would  uncover  their  roots  or  downward 
prolongations,  and  the  greater  the  denudation,  the  deeper  down  must 
have  been  the  original  position  of  the  rocks  now  exposed  to  daylight.  Fig. 
276,  for  example,  shows  a  district  in  which  a  series  of  tuffs  and  breccias 

*  See  *  Field  Gteology,'  by  the  author,  chapter  x. 


560  UEOTECTOXia  {STRUCTURAL)  GEOLOGY  book  it 


{hh)  traversed  by  dykes  {aa)  is  covered  unconformably  by  a  newer 
of  deposits  (rf).  Proi)erly  to  appreciate  the  relations  and  history  of  the 
rocks,  we  must  bear  in  mind  that  originally  they  may  have  presented 
some  such  outline  as  in  Fig.  277,  where  the  present  surface  (that  of  Kg. 
276)  down  to  which  denudation  has  proceeded  is  represented  by  the  dotted 
line  n  s}     We  may  therefore  a  priori  expect  to  encounter  different  leveb 


Fijr.  277.— Restoreil  outline  of  the  original  form  of  ground  in  Fig.  27fl  (A) 

of  eruptivity,  some  rocks  being  portions  of  sheets  that  solidified  at  the 
surface,  others  forming  parts  of  injected  sheets  or  of  the  pipe  or  colmnn 
-that  connected  the  superficial  sheets  with  the  internal  lava-reservoir. 
We  may  infer  that  many  masses  of  molten  rock,  after  being  driven 
so  far  upward,  came  to  rest  without  ever  finding  their  way  to  the 
surface.  It  cannot  always  be  affirmed  that  a  given  mass  of  inUiisiTe 
igneous  Vock,  now  denuded  and  exposed  at  the  surface,  was  ever  connected 
with  any  superficial  manifestation  of  volcanic  action. 

Now  there  will  obviously  }>e,  as  a  general  rule,  some  difference  in 
texture,  if  not  in  composition,  })etween  the  superficial  and  the  deep-seated 
masses,  and  this  difference  is  of  so  much  importance  in  the  interpretation 
of  the  history  of  volcanic  action  that  it  ought  to  be  clearly  kept  in  view. 
Those  portions  of  an  eruptive  mass  which  consolidated  at  some  depth  are 
generally  more  coarsely  crystid line  than  those  which  flowed  out  as  lava; 
they  are  likewise  destitute  of  tlie  cellular  scoriaceous  structure  and  the  ashy 
accompaniments  so  characteristic  of  suj>erficial  igneous  rocks.      Yet  even  if 
there  were  no  well-marked  i)etrographical  contrast  between  the  two  groupe^ 
it  would  manifestly  lead  to  confusion  if  no  distinction  were  drawn  between 
those  igneous  masses  which  reached  the  surface  and  consolidated  there, 
like  modern  lava-streams  or  showers  of  ashes,  and  those  which  never 
found  their  way  to  the  surface,  but  consolidated  at  a  greater  or  less  depth 
beneath  it.     There  must  be  the  same  division  to  be  drawn  in  the  case  of 
every  Jictive  volcano  of  the  present  day.     But  at  a  modem  volcano^  only 
the  materials  which  reach  the  surface  can  be  examined,  the  nature  and 
arrangement  of  what  still  lies  underneath  being  matter  of  inference.     In 
the  revolutions  to  which  the  crust  of  the  earth  has  been  subjected,  how- 
ever, denudation  has,  on  the  one  hand,  removed  superficial  sheets  of  lava 
and  tuff,  and  has  exposed  the  subterranean  continuations  of  the  erupted 
rocks,  and,  on  the  other  hand,  has  laid  open  the  very  heart  of  masses  which, 
though  eruptive,  seem  never  to  have  been  directly  connected  with  achul 
volcanic  outbursts.     All  subteiranean  intruded  masses,  now  revealed  at 
the  surface  after  the  removal  of  some  depth  of  overlying  rocky  may  be 
grouped  together  into  one  division  under  the  names  Plutonic,  Intra- 

^  De  la  Beche,  *GeoI.  Observer,*  p.  561. 


PART  vn  ERUPTIVE  ROCKS  661 

sive,  or  Subsequent.  On  the  other  hand,  all  those  which  came  up  to 
the  surface  as  ordinary  volcanic  rocks,  whether  molten  or  fragmental,  and 
were  consequently  contemporaneously  interstratified  with  the  formations 
which  happened  to  be  in  progress  on  the  surface  at  the  time,  may  be 
classed  in  a  second  group  under  the  names  Volcanic,  Interbedded,  or 
Contemporaneous. 

It  is  obvious  that  these  can  be  used  only  as  relative  terms.  Every 
truly  volcanic  mass  which,  by  being  poured  out  as  a  lava-stream  at  the 
surface,  came  to  be  regularly  interstratified  with  contemporaneous  accum- 
ulations, must  have  been  directly  connected  below  with  molten  matter 
which  did  not  reach  the  surface.  One  part  of  the  total  mass,  therefore, 
would  be  included  in  the  second  group,  while  another  portion,  if  ever 
exposed  by  geological  revolutions,  would  be  classed  with  the  first  group. 
Seldom,  however,  can  the  same  masses  which  flowed  out  at  the  surface  be 
traced  directly  to  their  original  underground  prolongations. 

It  is  evident  that  an  intrusive  mass,  though  necessarily  subsequent 
in  age  to  the  rocks  through  which  it  has  been  thrast,  need  not  be  long 
subsequent  Its  relative  date  can  only  be  certainly  aflBrmed  with  refer- 
ence to  the  rocks  through  which  it  has  broken.  It  must  obviously  be 
younger  than  these,  even  though  they  lie  upon  it,  if  .they  bear  evidence  of 
alteration  by  its  influence.  The  probable  geological  date  of  its  eruption 
must  be  decided  by  evidence  to  be  obtained  from  the  grouping  of  the 
rocks  all  around  Its  intrusive  character  can  only  certainly  determine 
the  limit  of  its  antiquity.  We  know  that  it  must  be  younger  than  the 
rocks  it  has  invaded ;  how  much  younger,  must  be  otherwise  determined. 
Thus,  a  mass  of  granite  or  a  series  of  granite  veins  {a  a,  Fig.  278)  is  mani- 
festly posterior  in  date  to  the  plicated  rocks  {h  b)  through  which  it  has 
risen.  But  it  must  be  regarded  as  older  than  overlying  undisturbed  and 
unaltered  rocks  (c),  or  than  others  lying  at  some  distance  («/),  which  con- 
tain worn  fragments  derived  from  the  granite. 


n  rx         a  o      a    b 

Fig.  278. — Section  showing  the  relative  age  of  an  Intrusive  Rock  (J5.) 

On  the  other  hand,  an  interbedded  or  contemporaneous  igneous  rock 
has  its  date  precisely  fixed  by  the  geological  horizon  on  which  it  lies. 
Sheets  of  lava  or  tuff  interposed  between  strata  in  which  such  fossils  as 
Culymene  Blumenbachii,  Leptcena  sencea,  Atrypa  reticularis^  Orthis  eleganiula, 
and  Feniamerus  Knightii  occur,  would  be  unhesitatingly  assigned  by  a 
geologist  to  submarine  volcanic  eruptions  of  Upper  Silurian  age.  A  lava- 
bed  or  tuff  intercalated  among  strata  containing  Sphenopteris  affinis, 
Lepidodendron  veltheimianum,  Leperditia,  and  other  associated  fossils,  would 
unequivocally  prove  the  existence  of  volcanic  action  at  the  surface 
during  the  Lower  Carboniferous  period,  and  at  that  particular  part  of 
the  period  represented  by  the  horizon  of  the  volcanic  bed.  Similar 
eruptive  material  associated  with  Ammonites^  BelemniieSj  PentacrinUes^ 
&c.,  would  certainly  belong  to  some  zone  in  the  great  Mesozoic  suite  of 

2  o 


562  GEOTECTOXrC  {STRUCTURAL)  GEOLOGY  book  iv 

formations.  An  interbedded  and  an  intrusive  mass  found  on  the  same 
platform  of  strata  need  not  necessarily  be  coeval.  On  the  contrary,  Uie 
latter,  if  clearly  intruded  along  the  horizon  of  the  former,  would 
obviously  be  posterior  in  date.  It  will  be  understood,  then,  that  the 
two  groups  have  their  respective  limits  determined  mainly  by  their 
relations  to  the  rocks  among  which  they  may  happen  to  lie,  though 
there  are  also  special  internal  characters  that  help  to  discriminate 
them. 

The  value  of  this  classiiication  for  geological  purposes  is  great  It 
enables  the  geologist  to  place  and  consider  by  themselves  the  granites, 
quartz -porphyries,  and  other  crystalline  masses,  which,  though  lying 
sometimes  perhaps  at  the  roots  of  ancient  volcanoes,  and  therefore 
intimately  connected  with  volcanic  action,  yet  owe  their  spedal 
characters  to  their  having  consolidated  under  pressure  at  some  depth 
within  the  earth's  crust ;  and  to  arrange  in  another  series  the  lavas  and 
tuffs  which,  having  been  thrown  out  to  the  surface,  bear  the  closest 
resemblance  to  the  ejected  materials  from  modern  volcanoes.  He  is 
thus  presented  with  the  records  of  hypogene  igneous  action  in  the  one 
group,  and  with  those  of  superficial  volcanic  action  in  the  other.  He 
is  furnished  with  a  method  of  chronologically  arranging  the  volcanic 
phenomena  of  past  ages,  and  is  thereby  enabled  to  collect  materials  for 
a  history  of  volcanic  action  over  the  globe. 

In  adopting  this  classification  for  unravelling  the  geological  structure 
of  a  region  where  igneous  rocks  abound,  the  student  will  encounter 
instances  where  it  may  be  difficult  or  impossible  to  decide  in  which 
group  a  particular  mass  of  rock  must  be  placed.  He  will  bear  in  mind, 
however,  that,  after  all,  such  schemes  of  classification  are  proposed  only 
for  convenience  in  systematic  work,  and  that  there  are  no  corresponding 
hard  and  fast  lines  in  nature.  He  will  recognise  that  all  crystalline  or 
glassy  igneous  rocks  must  be  intrusive  at  a  greater  or  less  depth  from  the 
surface  ;  for  every  contemporaneous  sheet  has  obviously  proceeded  from 
some  internal  pipe  or  mass,  so  that,  though  interbedded  and  contem- 
poraneous with  the  strata  at  the  top,  it  is  intrusive  in  relation  to  the 
strata  below. 

The  characters  by  which  an  eruptive  (igneous)  rock  may  be  dis- 
tinguished are  partly  lithological  and  partly  geotectonic.  The  litho- 
logical  characters  have  already  been  fully  given  (pp.  135,  154). 
Among  the  more  important  of  them  are  the  predominance  of  sOicates 
(notably  of  felsi>ars,  hornblende,  mica,  augite,  olivine,  &c.),  and  of 
disseminated  crystals  of  iron  oxides  (magnetite,  titaniferous  iron);  t 
prevailing  more  or  less  thoroughly  crystalline  structure ;  the  frequent 
presence  of  vitreous  and  devitrified  matter,  visible  megascopically  or 
microscopically ;  and  the  occurrence  of  porphyritic,  cellular,  pumiceoufl^ 
slaggy,  amygdaloidal,  and  fluxion  structures.  These  characters  are  never 
all  united  in  the  same  rock.  Tliey  possess  likewise  various  valnei 
as  marks  of  eruptivity,  some  of  them  being  shared  with  crystalline 
schists  which  were  certainly  not  eruptive.  On  t|ie  whole,  the  moflt 
trustworthy  lithological  evidence  of  the  eruptive  character  of  a  rock  is 


PART  VII  SECT,  i     PLUTONIC  PHASE  OF  ERUPTIVITY  563 


the  presence  of  glass,  or  traces  of  an  original  glassy  base.  We  do  not 
yet  certainly  know  of  any  natural  vitreous  substance,  except  of  an 
eruptive  natur&  The  occurrence  or  association  of  certain  minerals,  or 
varieties  of  minerals,  in  a  rock,  may  also  afford  presumptive  evidence 
of  its  igneous  origin.  Sanidine,  leucite,  olivine,  nepheline,  for  example, 
are,  for  the  most  part,  characteristic  volcanic  minerals ;  and  mixtures  of 
finely  crystallized  triclinic  felspars  with  dark  augite,  olivine,  and 
magnetic  iron,  or  with  hornblende,  are  specially  met  with  among 
eruptive  rocks. 

But  it  is  the  geotectonic  characters  on  which  the  geologist  must 
chiefly  rely  in  establishing  the  eruptive  nature  of  rocks.  These  vary 
according  to  the  conditions  under  which  the  rocks  have  consolidated. 
We  shall  consider  them  as  they  are  displayed  by  the  Plutonic,  or  deep- 
seated,  and  Volcanic,  or  superficial  phase  of  eruptivity.^ 


Section  1.    Plutonic,  Intrusive,  or  Subsequent  Phase  of  Eruptlvity. 

We  have  here  to  consider  the  structure  of  those  eruptive  masses 
which  have  been  injected  or  intruded  into  other  rocks,  and  have  con- 
solidated beneath  the  surface.  One  series  of  these  masses  is  crystalline 
in  structure,  but  with  felsitic  and  vitreous  varieties.  It  includes  most 
of  the  eruptive  rocks,  and  especially  the  more  coarsely  crystalline  forms 
(granite,  syenite,  quartz-porphyry,  granophyre,  liparite,  diorite,  &c)  The 
other  series  is  fragmental  in  character,  and  includes  the  agglomerates  and 
tuffs  which  have  filled  up  volcanic  orifices. 

After  some  practice,  the  field -geologist  acquires  a  faculty  of  dis- 
criminating, even  in  hand  -  specimens,  crystalline  rocks  which  have 
consolidated  beneath  the  surface,  from  those  which  have  flowed  out  as 
lavarstreams.  Coarsely  crystalline  granites  and  syenites,  with  no  trace 
of  any  vitreous  ground -mass,  are  readily  distinguishable  as  plutonic 
masses ;  while,  on  the  other  hand,  cellular  or  slaggy  lavas  are  easily 
recognisable  as  superficial  outflows,  or  as  closely  connected  with  them. 
But  it  will  be  observed  that  such  differences  of  texture,  though  furnishing 
useful  helps,  are  not  to  be  regarded  as  always  and  in  all  degrees  perfectly 
reliable.  We  find,  for  example,  that  some  lavas  have  appeared  at  or 
near  the  surface  with  so  coarsely  crystalline  a  structure  as  to  be  mistaken 
by  a  casual  observer  for  granite ;  while,  on  the  other  hand,  though  an 
open  pumiceous  or  slaggy  structure  is  certainly  indicative  of  a  lava  that 
has  consolidated  at  or  near  the  surface,  a  finely  cellular  character  is  not 
wholly  unknown  in  intrusive  sheets  and  dykes  which  have  consolidated 
below  ground.  Again,  masses  of  fragmentary  volpanic  material  are 
justly  regarded  as  proofs  of  the  superficial  manifestation  of  volcanism, 

^  As  already  stated  (p.  125),  a  chronological  basis  has  been  proposed  for  the  classifica- 
tion of  eruptive  rocks.  Some  writers  have  even  gone  so  far  as  to  suggest  that  different 
namra  should  be  given  to  eruptive  rocks  according  to  the  geological  formation  in  which  they 
occur,  as  Carbophyre,  Kohlephyre^  Triaphyre,  Juraphyre.  See  Th.  Ebray,  Bull,  Soc.  GM, 
France  (8),  iiL  p.  291. 


564  GEOTECTOSIC  {STRUCTURAL)  GEOLOGY  book  it 

and  in  the  vast  majority  of  cases,  they  occur  in  beds  which  were 
accumulated  on  the  surface,  as  the  result  of  successive  explosions.  Yet 
cases,  which  will  be  immediately  described,  may  be  found  in  many  old 
volcanic  districts,  where  such  fragmentary  materials,  falling  back  into  the 
volcanic  funnels,  and  filling  them  up,  have  been  compacted  there  into 
solid  rock  ;  they  may  occasionally  have  been  produced  by  explosions  of  lavt 
within  subterranean  caverns. 

The  general  law  which  has  governed  the  intrusion  of  igneous  rock 
within  the  earth's  crust  may  be  thus  stated :  Every  fluid  mass  impelled 
upwards  by  pressure  from  below,  or  by  the  expansion  of  its  own 
imprisoned  vapour,  has  sought  egress  along  the  line  of  least  resistanoeL 
That  line  has  depended  in  each  case  upon  the  structure  of  the  terrestrial 
crust  and  the  energy  of  eruption.  It  may  have  been  determined  bj 
an  already  existent  dislocation,  by  planes  of  stratification,  by  tbe 
surface  of  junction  of  two  unconformable  formations,  by  contemporu- 
eously  formed  cracks,  or  by  other  more  complex  lines  of  weakness 
Sometimes  the  intnided  mass  has  actually  fused  and  obliterated  some 
of  the  rock  which  it  has  invaded,  incorporating  a  portion  into  its  own 
substance.  The  shai)e  of  the  channel  of  escape  has  thus  determined  the 
external  form  of  the  intrusive  mass,  as  the  mould  regulates  the  form 
assumed  by  cast-iron.  This  relation  offers  a  very  convenient  means  of 
classifying  intrusive  rocks.  According  to  the  shape  of  the  mould  in 
which  they  have  solidified,  they  may  be  arranged  as — (1)  bosses  or 
amorphous  masses,  (2)  sheets,  (3)  veins  and  dykes,  and  (4)  necks. 


§  1.  Bosses. 

Bosses  or  amorphous  masses  consist  chiefly  of  crystalline,  coorse-tex- 
turcd  rocks.  Granite  and  syenite  are  the  most  conspicuous  examples,  hot 
various  quartz-porphyries,  felsites,  trachytes,  diorites,  gabbros,  diahssefl^ 
andesites,  dolerites,  etc.,  also  occur.  Where  rocks  assume  this  form  as 
well  as  that  of  sheets,  dykes,  and  contemix)raneous  beds,  it  is  commonly 
observed  that  they  are  more  coarsely  crystalline  when  in  large  amorphooi 
masses  than  in  any  other  form.  Pyroxenic  rocks  afford  many  examples 
of  this  charact<jristic.  In  the  basin  of  the  Forth,  for  instance,  while  the 
outflows  at  the  surface  have  been  fine-grained  basalts,  the  masses  con- 
solidated underneath  have  generally  been  coarse  dolerites  or  diabases.^ 

In  the  consolidation  of  an  igneous  rock,  the  more  basic  minerals  have 
generally  crystallized  out  first,  and  the  last  portions  of  the  mass  to  solidify 
have  not  infrequently  a  notably  more  acid  character  than  those  which 
solidified  first.  Hence  the  margin  of  an  eruptive  mass  may  show  a  more 
basic  composition  than  the  central  iK)rtions  which  cooled  more  slowly.  A 
reinarkiible  range  of  composition  may  thus  be  found  within  the  same  boo.' 
A<2:ain,  if  during  the  process  of  consolidation  a  rock  should  be  ruptured 
and  [>ortions  of  the  still  liquid  matter  bo  forced  into  the  rents^  theae 

^  Trans.  Roy.  Soc.  Edin.  xxix.  p.  493  (1879). 
-  Teall  aud  Dnkyns,  Quort.  Journ.  Oeol,  »»-,  1892,  p.  104. 


?ART  VII  SECT,  i  §  1  ERUPTIVE  BOSSES  665 

reins  or  squirts  will  generally  be  found  to  be  decidedly  more  acid  than 
ihe  rock  in  which  they  lie  (pp.  225,  262,  269). 

Granite. — It  was  once  a  firmly-held  tenet  that  granite  is  the  oldest 
)f  rocks,  the  foundation  on  which  all  other  rocks  have  been  laid  down. 
This  idea  no  doubt  originated  in  the  fact  that  granite  is  found  rising 
Tom  beneath  gneiss,  schist,  and  other  crystalline  masses,  which  in  their 
iurn  underlie  very  old  stratified  formations.     The  intrusive  character  of 
p:^nite,  shown  by  its  numerous  ramifying  veins,  proved  it  to  be  later  than 
it  least  those  rocks  which  it  had  invaded.     Nevertheless,  the  composition 
md   structure   of    gneiss   and   mica -schist  were   believed    to    be   best 
explained  by  supposing  these  rocks  to  have  been  derived  from  the  waste 
>{  granite,  and  thus,  though  the  existing  intrusive  granite  had  to  be 
"ecognised  as  posterior  in  date,  it  was  regarded  as  only  a  subsequent 
)rotrusion  of  the  vast  underlying  granitic  crust.     In  this  way,  the  idea 
>f  the   primeval  or   fundamental   nature   of  granite   held   its  ground. 
?rom  what  is  known  regarding  the  fusion  and  consolidation  of  rocks 
ante,  p.  300  et  seq.),  and  from  the  evidence  supplied  by  the  microscopic 
tructure  of  granite  itself  (p.  112),  it  appears  now  to  be  established  that 
pranite  has  consolidated  under  great  pressure,  in  presence  of  superheated 
vater,  with  or  without  liquid  carbon-dioxide,  fluorine,  &c.,  conditions  which 
►robably  never  obtained  at  the  earth's  immediate  surface,  unless,  perhaps, 
n  those  earliest  ages  when  the  atmosphere  was  densely  loaded  with  vapours, 
.nd  when  the  atmospheric  pressure  at  the  surface  must  have  been  enor- 
Qous  (p.  35).     Whether  the  original  crust  was  of  a  granitic  or  of  a  glassy 
haracter,  no  trace  of  it  has  ever  been  or  is  ever  likely  to  be  found.    There 
an  be  no  doubt,  however,  that  the  oldest  known  rocks  are  either  granites 
r  granitoid  gneisses  which  have  probably  been  formed  out  of  granite. 

The  presence  of  granite  at  the  existing  surface  is,  doubtless,  in  all 
ases  due  to  the  removal  by  denudation  of  masses  of  rock  under  which  it 
riginally  consolidated.  The  fact  that,  wherever  extensive  denudation  of 
n  ancient  series  of  crystalline  rocks  has  taken  place,  a  subjacent  granitic 
ucleus  is  apt  to  appear,  does  not  prove  granite  to  be  of  primeval 
rigin.  It  shows,  however,  that  the  lower  portions  of  crystalline  rocks 
ery  generally  assume  a  granitic  type,  and  it  suggests  that  if,  at  any  part 
f  the  earth,  we  could  bore  deep  enough  into  the  crust,  we  should  probably 
>me  to  a  granitic  layer.  That  this  layer,  even  if  general  round  the 
lobe,  is  not  everywhere  of  the  highest  geological  antiquity,  or  at  least 
as  consolidated  at  widely  different  periods,  is  abundantly  clear  from  the 
kct  that  in  many  cases  it  can  be  proved  to  be  of  later  date  than  fossili- 
irous  formations  the  geological  position  of  which  is  known ;  that  is,  the 
ranitic  layer  has  invaded  these  formations,  rising  up  through  them,  and 
>88ibly  melting  down  portions  of  them  in  its  progress.  Granite  invades 
id  alters  rocks  of  all  ages  up  to  late  Mesozoic  and  Tertiary  formations. 
[ence,  it  does  not  belong  exclusively  to  the  earliest  nor  to  any  one  geo- 
)gical  period,  but  has  rather  been  extruded  at  various  epochs,  and  may 
ran  be  in  course  of  extravasation  now,  wherever  the  conditions  required 
T  its  production  still  exist.  As  a  matter  of  fact,  granite  occurs  much 
ore  frequently  in  association  with  older^  and  therefore  lower,  than  with 


666 


flEOTECTOXIU  i^TIlUGTURAL)  GEOLOGY 


newer  and  higher  rocks.  But  a  little  reflection  shows  that  this  oaght  to 
be  the  case.  Granite,  having  a  deep-seated  origin,  must  rise  through  the 
lower  and  more  ancient  masses  before  it  can  reach  the  overlying  mon 
recent  formations.  But  many  protrusions  of  granite  would,  doubtlen, 
never  ascend  beyond  the  lower  rocks.  Subsequent  denudation  would  bt 
needed  to  reveal  these  protrusions,  and  this  very  process  would  remoTe 
the  later  formations,  and,  at  the  sanie  time,  any  portions  of  the  granitf 
which  might  have  reached  them. 

Granite  frequently  occurs  in  the  central  parts  of  mountam  chaini; 
sometimes  it  forms  there  a  kind  of  core  to  the  various  gneisses,  schisu, 
and  other  crystalline  rocks.  It  appears  in  large  eruptive  bosses,  which 
traverse  indifferently  the  rocks  on  the  lino  of  which  they  rise,  and  com- 
monly send  out  abundant  veins  into  them.  Sometimes  it  even  overUei 
schistose  and  other  rocks,  as  in  the  Piz  de  Graves  in  the  upper  Engadiixv 
where  a  wall-like  mass  of  granite,  with  syenite,  diorite,  and  altered  rocki, 
may  be  seen  resting  upon  schists.^  In  the  Alps  and  other  mounUm 
ranges,  it  is  found  likewise  in  large  bed-like  masses  which  run  in  the  ssdk 
general  direction  as  the  rocks  with  which  thoy  are  associated. 

Reference  has  already  been  made  (p.  157)  to  some  of  the  more  marked 
varieties  of  texture  and  structure  in  granite  bosses.  To  a  few  of  then 
further  and  more  detailed  remarks  may  be  appropriately  inserted  heic 
The  patches  or  enclosures  in  granite,  which  differ  in  colour,  texture,  tuxi 
composition  from  the  general  mass  of  the  rock,  may  be  grouped  in  tw) 
divisions ;  1st.  Angular  or  suliangular  fragments,  probably  in  moat  caiei 
derived  from  the  rocks  through  which  the  granite  has  been  protruded 
These  are  sometimes  tolerably  abundant  towards  the  outer  margin  of  i 
1k)ss.  They  usually  show  considerable  contact- metamorphiam,  due  no 
doubt  to  the  influence  of  the  eruptive  rock  in  which  they  are  enclosed 
2nd.  Globular  or  rounded  concretions,  due  to 
some  process  of  segregation  and  crystalliation, 
in  the  original  still  unconsolidated  graoitc- 
Examples  of  this  nature  occur  in  the  Cornish  and 
Devon  granite,  as  in  Fig,  379,  which  was  loaf 
ago  cited  by  I)e  la  Beche  as  sliowing  a  ceatnl 
cavity  (a),  not  quite  filled  with  long  crystals  i^ 
sciiorl  surrounded  with  an  envelope  of  quirti 
anil  schorl  (b),  outside  of  which  lies  a  seccDd 
envelope  (c)  of  the  same  minerals,  the  icbori 
predominating,  the  wliole  being  contained  in  t 
light  flesh-coloured  and  markedly  felspatliic 
granite  (d).  But  more  remarkable  concretionary 
forms  have  since  been  observed  in  many  granitM 
some  of  them  presenting  an  internal  radial  con- 
centric arrangement,  and  recalling  the  orbicular  structure  of  some  diniK* 
(Nn]ioleoiiite)  (Fig.  8).  Such  concretionary  aggregations  are  genenllf 
more  basic  than  the  surrounding  granite.^ 

'  fltuder,  '  G«olo>,-ie  iUt  a.'liiveii,'  i.  p.  280. 

'  BesiiUs  the  pnpers  of  Thlllips,  Briigger,  uid  H*lcb  cited  on  p.  159,  ««  a 


PART  VII  SECT,  i  §  1  ERUPTIVE  BOSSES  667 

Of  more  importance,  as  affecting  a  much  larger  proportion  of  a  granite 
boss,  are  the  differences  of  texture  and  of  structure  not  infrequently  trace- 
able from  the  margin  to  the  centre.  Like  most  intrusive  rocks,  granite  is 
apt  to  be  more  close-grained  at  its  contact  with  the  surrounding  strata 
than  in  the  centre  of  its  mass,  though  it  does  not  show  this  contrast  so 
strikingly  as  the  more  basic  rocks  such  as  gabbro,  diabase,  and  dolerite. 
Certain  characteristic  varieties  of  texture  and  even  to  some  extent  of  com- 
position may,  however,  be  recognised  in  many  granite  areas.  In 
particular  the  marginal  portions  not  infrequently  present  a  remarkable 
foliated  arrangement  which  simulates  the  structure  of  gneiss,  the  folia 
being  rudely  parallel  to  the  margin  of  contact  and  either  vertical  or 
dipping  at  high  angles  away  from  the  core  of  granite.  In  some  granite 
bosses  a  striking  gradation  can  be  traced  even  into  picrites  and  serpentines. 

A  detailed  study  has  been  made  by  Dr.  Charles  Barrois  of  the  granulites  {i.e. 
grauites  with  two  micas)  of  the  Morbihan  in  Brittany.  He  has  sho^'n  that  the  large 
bosses  measuring  some  hundreds  of  square  kilometres  present  certain  well-marked 
modifications  not  only  of  structure  but  of  composition  as  they  are  traced  from  the 
centre  to  the  periphery,  while  the  smaller  bosses  show  no  such  modifications  and  arc 
to  be  regarded  merely  as  apophyses  from  those  of  large  size.  The  modifications  along 
the  contact  do  not  arise  from  any  exchange  of  substance  between  the  granite  and  the 
surrounding  rock,  but  solely  from  the  influence  of  cooling  which  has  affected  the  orienta- 
tion of  the  minerals,  their  grouping  and  their  order  of  crystallization.  Where  the 
granite  has  risen  imrallel  to  the  strike  of  the  adjacent  strata  it  usually  passes  from  its 
ordinary  granular  into  a  porphyroid  stnicture,  with  its  large  constituents  arranged 
parallel  as  in  flow-structure  ;  where  on  the  other  hand  it  breaks  across  the  bedding  it 
has  assumed  a  finely  granular  massive  character  (aplite)  with  its  crystalline  constituents 
showing  regular  geometric  forms.  These  variations  are  thus  proved  in  this  particular 
instance  to  depend  on  the  influence  of  the  surrounding  envelope,  which  though  chemically 
inactive,  offers  considerable  diversity  as  a  conductor  of  heat  and  of  pressure.  The 
•crystallization  of  the  constituents  of  the  rock  took  place  progressively  from  the  outside 
inwards,  that  is,  from  a  mass  still  in  motion  across  a  magma  that  had  come  to  rest  and 
which  shows  now  no  trace  of  flow.  But  besides  this  marginal  band  of  "porphyroid 
granulite,"  the  external  portions  of  the  southern  flanks  of  the  bosses  present  a  i-emark- 
able  schistose  structure  which,  likewise  limited  to  a  peripheral  zone,  resembles  that  of 
gneiss,  both  fine-grained  and  glandular  (angen-gneiss).  Examined  in  detail  the  mica- 
flakes  of  this  gneissic  band  are  found  to  be  torn  and  drawn  out,  the  felspar  crystals 
deformed,  broken,  and  blunted,  indicating  the  powerful  mechanical  forces  which  have 
affected  the  rock.  These  cnished  constituents  have  subseciuently  been  re-cemented  by 
membranes  and  fibres  of  white  sericitic  mica,  sometimes  of  black  mica,  and  by  sheets  of 
flecondary  granular  quartz,  formed  out  of  the  triturated  debris  of  the  older  ingredients. 
Considering  the  gradual  passage  of  these  schistose  selvages  into  the  drdinary  granular 
rock,  and  the  further  fact  that  the  schistose  stnicture  occurs  only  on  the  soutliem  flanks 
of  the  granitic  bosses  of  the  Morbihan,  Dr.  Barrois  attributes  this  structure  to  a  power- 
ful lateral  pressure  which  has  acted  in  a  direction  from  south  to  north.* 

Relation  of  Granite  to  contiguous  Rocks. — From  an  early 
period  the  attention  of  geologists  has  been  given  to  the  evident 
mineralogical  change  which    has    taken  place  among  stratified  rocks  as 

the  Shap  granite  Harker  and  Marr,  Quart.  Journ.  Gcd.  Soc.  xlvii.  (1891),  p.  280  ;    on 
gradation  of  granite  into  basic  rocks,  Teall  and  Dakyns,  cited  on  p.  564. 
*  Ann.  Soc.  Geol.  Xord^  xv.  1887,  pp.  1-40. 


568  GEOTECTOXIC  (STBUCTURAL)  GEOLOGY  book  rr 

they  approach  a  mass  of  granite.  This  change  is  developed  within  a  ring 
or  areola  which  encircles  the  granite,  and  varies  in  breadth  from  a  few 
yards  to  two  or  three  miles.  The  most  intense  alteration  is  found  next 
the  granite,  while  along  the  outer  margin  of  the  areola  the  normal 
character  of  the  rocks  is  resumed.  In  some  cases,  however,  no  perceptible 
trace  of  alteration  can  be  detected  next  a  mass  of  granite.  Of  the  Euro- 
pean examples  of  contact-metamoq)hism,  those  of  Devon  and  Cornwall, 
Ireland,  Scotland,  the  Harz,  Vosges,  Pyrenees,  and  Norway  have  long 
been  known.  The  nature  of  the  mctamorphism  thus  superinduced  upon 
rocks  is  more  particularly  discussed  at  p.  605. 

Tlic  soutli-east  of  Ireland  supplies  an  admirable  illustration  of  the  relation  between 
granite  and  its  surrounding  rocks  (Fig.  280).  A  mass  of  granite  70  miles  in  length  and 
from  7  to  17  in  width  there  stretches  from  north-east  to  south-west,  nearly  along  the 
strike  of  the  Lower  Silurian  rocks.  These  strata,  however,  have  not  been  ujiraiaed  by  it 
in  such  a  way  as  to  exjwse  their  lowest  beds  dipping  away  from  the  granite.  On  the 
contrary,  they  seem  to  have  been  Contorted  prior  to  the  appearance  of  that  rock ;  at 
least  they  often  dip  towards  it,  or  lie  horizontally  or  undulate  upon  it,  apparently  with- 
out  any  reference  to  movements  which  it  could  have  produced.  As  Jukes  ^owed. 
the  Silurian  strata  are  underlain  by  a  vast  mass  of  Cambrian  rocks,  all  of  which  mnst 


u 

Fig.  280.— Section  across  part  of  the  granite  belt  of  the  south-east  of  Irelanil. 
a,  Granite ;  h  h,  patches  of  Lower  Silurian  rocks  lying  on  the  granite  at  various  distance  fttim  the 

main  I^wer  Silurian  area,  r  c. 

have  been  invaded  by  the  granite  before  it  could  have  reached  its  jiresent  horizon.  He 
infers  that  the  granite  must  have  slowly  and  irregularly  eaten  its  way  upward  thraogfa 
the  Silurian  rocks,  absorbing  much  of  them  into  its  own  mass  as  it  rose.  For  a  mile  or 
more,  the  stratified  beds  next  the  granite  have  been  altered  into  mica-schist,  and  are 
pierced  by  numerous  veins  from  the  invading  r(»ck.  Within  the  margin  of  the  granitic 
mass,  belts  or  rounded  irregular  patches  of  schist  (5  b)  are  enclosed  ;  but  in  the  oentnl 
tracts,  where  the  granite  is  widest,  and  where  therefore  we  may  suppose  the  deepest 
parts  of  the  mass  have  l)een  laid  bare,  no  such  included  patches  of  altered  rock  ooenr. 
From  the  manner  in  which  the  schistose  belt  is  disiK)sed  round  the  granite,  it  is  evident 
that  the  upi>er  surface  of  the  latter  rock,  where  it  extends  beneath  the  schists,  most  be 
very  uneven.  Doubtless  the  granite  rises  in  some  places  much  nearer  to  the  {veMnt 
surface  of  the  ground  than  at  others,  and  senrls  out  veins  and  strings  which  do  not 
a]>pear  above  ground.  If,  as  Jukes  supi>osed,  a  thousand  feet  of  the  schists  could  be 
restored  at  some  parts  of  the  granite  belt,  no  doubt  the  belt  would  there  be  entinlj 
buried  ;  or  if,  on  the  other  hand,  the  same  thickness  of  rock  could  be  stripped  off  some 
j»arts  of  the  band  of  schist,  the  solid  granite  underneath  would  be  laid  bare.  The  extoit 
of  granite  surface  exposed  must  thus  be  largely  determined  by  the  amount  of  denudati(», 
and  by  th(>  angle  at  which  the  up{>er  surface  of  the  granite  is  inclined  beneath  the 
schists.  Where  the  inclination  is  high,  prolonged  denudation  will  evidently  do  com- 
]»aratively  little  in  widening  the  belt.^  But  where  the  slope  is  gentle,  and  esjieciaUy 
where  the  surface  undulates,  the  removal,  for  some  distance,  of  a  comparatively  slight 
thickness  of  ro(ik,  may  uncover  a  large  breadth  of  underlying  granite.  Portions  of  the 
metamorphosed  rocks  left  by  denudation  ui)on  the  surface  of  the  granite  boas,  are  reli» 

^  See  Jukes's  *  Manual  of  Geology,*  8nl  ed.  p.  243. 


SECT,  i  §  1  ERUPTIVE  BOSSES  669 

3ep  cover  under  which  the  granite  no  doubt  originally  lay,  and,  being  tougher 
latter  rock,  they  have  resisted  waste  so  as  now  to  cap  hills  and  protect  the 
>elow,  as  at  the  mountain  Lugnaquilla  (L  in  Fig.  280),  which  rises  3039  feet 
)  sea. 

t  observations  by  Professor  Hull  and  Mr.  Traill,  of  the  Geological  Survey  of 
lave  shown  that  in  the  Moume  Mountains,  a  mass  of  granite  has  in  some  parts 
through  highly  inclined  Silurian  rocks,  which  consequently  seem  to  be  standing 
pright  upon  an  underlying  boss  of  granite.  The  strata  are  sharply  truncated 
ystalline  mass,  and  are  indurated  but  not  otherwise  altered.  The  intrusive 
*  the  granite  is  well  shown  by  the  way  in  which  numerous  dykes  of  dark  mela- 
cut  off  when  they  reach  that  rock.^  The  accompanying  diagram  (Fig.  281)  is 
m  one  of  the  sections  in  which  this  structure  is  portrayed  by  these  observers. 


Fig.  281.— Section  of  Slievenamaddy,  Moume  Mountaius. 

Silurian  strata  dipping  at  high  angles  ;  b  2>,  Dykes  of  basalt  (melaphyre),  cutting  these  strata 
incated  by  the  granite  c,  which  along  the  outer  margin  and  in  extruded  veins  passes  into  a 
porphyry,  d  d. 

9  Lower  Silurian  tract  of  the  south  of  Scotland  several  large  intrusive  bosses  of 
ccur  (Fig.  282).  The  strata  do  not  dip  away  from  them  on  all  sides,  but  with 
ceptions  maintain  their  normal  N.E.  and  S.W.  strike  up  to  the  granite  on  one 
resume  it  again  on  the  other.  The  granite  indeed  has  not  merely  pushed  aside 
so  as  to  make  its  way  j>ast,  but  actually  occupies  the  place  of  so  much  Silurian 
5  and  shale,  which  have  disappeared,  as  if  they  had  been  pushed  or  blown  out 
sen  melted  up  into  the  granite.  There  is  usually  a  metamorphosed  belt  of 
nile  in  width,  in  which,  as  they  approach  the  granite,  the  stratified  rocks 
thoroughly  schistose  character.  Numerous  small,  dark,  often  angular  patches 
tnts  of  mica-schist  may  be  observed  in  the  marginal  parts  of  the  granite. 
Uy  granite -veins  protrude  from  the  main  masses  ;  in  the  metamorphosed 
h  surrounds  the  Criffel  granite  area  in  Kirkcudbright,  hundreds  of  dykes  and 
arious  felsitic  or  elvanitic  rocks  occur  (see  p.  578).^ 

r  features  are  presented  by  the  granite  bosses  of  Devon  and  Cornwall,  which 
Q  through  Devonian  and  Carboniferous  strata.  The  Dartmoor  mass  is 
nstructive.  As  shown  by  the  early  work  of  De  la  Beche,  it  passes  across  the 
between  the  Devonian  and  Carboniferous  areas,  extending  chiefly  into  the 
that  it  cuts  across  strata  of  different  ages.  In  doing  so  it  has  risen  irresistibly 
he  crust,  without  seriously  affecting  the  general  strike  of  the  rocks.  It  cuts 
lands,  as  well  as  grits  and  shales  into  which  it  sends  veins.' 

nection  of  Granite  with  Volcanic  Rocks. — ^The  manner  in 

'xmtaX  Section  No.  22,  Geol.  Surv.  Ireland. 

ination  of  Sheet  9,  Geological  Survey  of  Scotland.     The  contact- metamorpilM|l  of 

.te  bosses  is  described  on  p.  606. 

I  Beche,  '  Report,  Devon  and  Cornwall,*  p.   165.     J.  A.  riiillips,  Q.  J,  Oeol,  Soc. 

498.     Compare  the  action  of  the  Tertiary  granites  of  Skye,   Traits.  Roy,  Soc. 

f.  (1888),  Fig.  56,  p.  170. 


B70  GEOTECTIWIC  (STRUCTURAL)  GEOLOGY  kwkit 

which  some  bosses  of  granite  penetrate  the  rocks  among  which  they  occur 
strongly  recalls  the  stnicture  of  volcanic  necks  or  pipes  (p.  584).  Tht 
granite  is  found  as  a  circular  or  ellipticnl  mass  which  seems  to  descend 
vertically  through  the  surrounding  rocks  without  seriously  disturbing  theoi, 
as  if  a  tube-shaped  opening  had  been  blown  out  of  the  crust  of  the  eartli, 
up  which  the  granite  had  risen.  Several  of  the  granite  masses  of  tbe 
south  of  Scotland,  above  referred  to,  exhibit  this  character  very  strikinglf 
(Fig.  2&2).  That  granite  and  granitoid  rocks  have  probably  ben 
associated  with  volcanic  action  is  indicated  by  the  way  in  which  they 
occur  in  connection  with  the  Tertiary  volcanic  rocks  of  Skye,  Mull,  ud 
other  islands  in  the  Inner  Hebrides.  Jukes  su^ested  muty  yean 
ago  that  granite  or  granitoid  masses  may  lie  at  the  roots  of  volcanoes,  ud 
may  be  the  source  whence  the  more  silicated  lavas  proceed' 


lamnrr  of  Fl«et,  Bnntluul. 

sini!  through  highly  In 

of  bUck  aDlhndttc  ini]  gnptolltlc  1 

<tutU'<l  llDeniiipil  Chi'iininlti'ili'nnrii  tfaelirltotincUnmrphisin. 

While  the  instances  are  few  where  any  satisfactory  connection  can 
actually  be  traced  between  granitic  ma-sses  and  true  lava-fonn  or  roloiiic 
rocks,  the  close  reUitionsbip  between  granite  and  the  crystalline  schiib 
has  long  been  recognised.  It  was  formerly  believed  by  many  geologisU 
that  some  granite  is  of  metamorphic  origin,  that  is  to  say,  may  havt 
been  produced  by  the  gradual  softening  and  recry stall ization  of  other 
rocks  at  some  (le])th  within  the  crust  of  the  earth.  As  gradatiou 
can  be  traced  from  gneiss  through  less  distinctly  crystalline  BchiAi 
into  unaltered  strata,  the  granite  into  which .  such  gneiss  seems  to 
pass  was  by  some  looked  upon  as  the  extreme  of  metamorphism,  the 
various  schists  and  gneisses  being  less  advanced  stt^a  of  the  procett. 
Profeitsor    Dana    has    described  a    scries    of  hornblendic,    hypersthenic, 

■  -  Mnnual  of  Geology,'  1in.\  e.l.  p.  93  ;  Qeikie.  Traru.  Oeol.  Soc  Editi.  ii.  p^  SOI  ;  Traai. 
Kail.  !<ae..  FaUb.  xxkt.  (1888),  p.  150  :  Jml.1,  l?nar(.  Joant.  Oeol.  Soc  xii.  p.  220 ;  Stjtr, 
Jahrb.  Gfol.  JUithiantl.  1S79.  p.  405,  uul  hU  '  Beitrag  ziir  Phjfalk  Am  ErapUoncii.' 


PART  VII  SECT,  i  §  1  ERUPTIVE  BOSSES  671 

augitic,  micaceous,  and  olivine  rocks  in  the  valley  of  the  Hudson  River, 
which,  as  varieties  of  granite,  syenite,  diorite,  norite,  &c,  he  describes  as 
masses  that  have  been  reduced  to  a  fused  or  plastic  condition  through 
metamorphic  action.^  The  tendency  of  modem  inquiry  is  to  regard 
granite  as  an  eruptive  and  not  as  a  metamorphic  rock,  and  to  look  upon 
the  gradations  between  it  and  various  schists  as  phases  in  the  deformation 
and  alteration  of  the  original  granite.  Many  cases  are  now  known  where 
under  great  mechanical  stresses  the  component  minerals  of  granite  have 
been  drawn  out,  as  in  the  fluxion  structure  of  lavas,  and  the  rock  has 
assumed  the  laminar  structure  of  gneiss.  Many  gneisses  are  almost 
certainly  only  varieties  of  granite  in  which  a  foliated  structure  has  been 
superinduced.^ 

Diorite,  &c. — On  a  smaller  scale  usually  than  granite,  other  crystal- 
line rocks  assume  the  condition  of  amorphous  bosses.  Diorite,  syenite, 
quartz-porph3nry,  gabbro,  and  members  of  the  diabase  and  basalt  family 
have  of  tea  been  erupted  in  irregular  masses,  partly  along  fissures,  partly 
along  the  bedding,  but  often  involving  and  apparently  melting  up  portions 
of  the  rocks  through  which  they  have  made  their  way.  Such  bosses  have 
frequently  tortuous  boundary-lines,  since  they  send  out  veins  into  or  cut 
capriciously  across  the  surrounding  rocks.  In  Wales,  as  shown  by  the 
maps  and  sections  of  the  Geological  Survey,  the  Lower  Silurian  formations 
are  pierced  by  huge  bosses  of  different  crystalline  rocks,  mostly  included 
under  the  old  term  "  greenstone,"  which,  after  running  for  some  way  with 
the  strike  of  the  strata,  turn  round  and  break  across  it,  or  branch  and 
traverse  a  considerable  thickness  of  stratified  rock.  In  central  Scotland, 
numerous  masses  of  doled te  or  diabase  have  been  intruded  among  the 
Lower  Carboniferous  formations.  One  horizon  on  which  they  are 
particularly  abundant  lies  about  the  base  of  the  Carboniferous  Limestone 
series.  Along  that  horizon,  they  rise  to  the  surface  for  many  miles, 
sometimes  ascending  or  descending  in  geological  position,  and  breaking 
here  and  there  abniptly  across  the  strata.^  There  can  be  little  doubt 
that  they  have  actually  melted  down  some  parts  of  the  stratified  rocks, 
particularly  the  limestone.*  Considerable  petrographical  differences 
occur  among  them,  which  may  perhaps  be  in  some  measure  due  to  the 
incorporation  of  such  extraneous  material  into  their  mass.  Gaps  occur 
where  these  intrusive  rocks  do  not  rise  to  the  surface,  but  as  they 
resume  their  position  again  not  far  off,  it  may  be  presumed  that  they  are 
really  connected  under  these  blank  intervals.  In  the  Inner  Hebrides 
huge  bosses  of  gabbro  occur  as  well  as  granophyre  and  other  acid  rocks 
in  the  midst  of  the  Tertiary  volcanic  series. 

Mr.  G.  K.  Gilbert  has  described,  under  the  name  of  **  laccolite,"  a 
structure  in  the  Henry  Mountains  in  Southern  Utah,  which  is  probably 

»  Amer.  Joum,  Sci.  xx.  (1880),  p.  219. 

'  See,  for  an  early  statement  of  this  view,  Dr.  Lehraann's  work  on  the  granulite  region  of 
Saxony,  cited  ant^j  p.  156.  The  gneisses  of  the  north-west  of  Scotland  are  believed  to  be 
ementially  crushed  and  foliated  eruptive  rocks. 

*  Trans.  Roy.  Soc.  Edin.  xxix.  p.  476. 

*  See  Dr.  Stecher's  papers,  quoted  postea,  pp.  601,  609. 


572  GEUTECTOXIC  {^TRUCTUBAL)  GEOLOGY  book  iv 

not  uncommon  in  denuded  volcanic  districts.  Lai^  bosses  of  tnchytic 
lava  have  risen  from  beneath,  but  instead  of  finding  their  way  to  tiie 
surface,  have  spread  out  laterally  and  pushed  up  the  overlj'ing  stratft  into 
a  dome-shaped  elevation  (Fig.  283).  Here  and  there,  Bmaller  sheets  pro- 
ceeding from  the  nioiu  masses  have  been  forced  between  the  beds,  or  veiiu 
liave  been  injected  into  fissures,  and  the  overlying  and  contiguous  stnts 
have  licen  considerably  metamorphosed.' 

Effects  on  Contiguous  Rocks.  —  The  contact  -  metamorpbisn 
around  bosses  of  diorite  and  other  rocks  includes  alteration  of  the 
texture  and  even  tlie  mineralogical  composition  of  the  rocks  througli 
which  intrusive  material  has  been  erupted.  The  amount  and  nature  of 
the  change  produced  vary  with  the  character  and  bulk  of  the  emptiTe 
mass,  as  well  as  with  the  susceptibility  of  the  surrounding  materials  to 
alteration.  Diorite,  diabase,  melaphjTe,  basalt,  felsite,  and  other  emptire 
rocks  are  not  infrequently  accompanied  by  considerable  metamoiphiflii 
of  the  adjacent  strata,  though  the  change  seldom  approaches  the  intensity 


of  that  around  largo  areas  of  gr.tnite.  These  phenomena  are  raanifested 
also  by  intrusive  sheets,  dykes,  veins,  ami  necks.  They  belong  to  tb 
scries  of  changes  embiitced  under  the  head  of  contact- metamorphism, 
and  are  grouped  together  for  <tesciiption  in  the  next  Part  (p.  597). 

Effects  on  the  Eruptive  Mass. — Allusion  has  been  made  abo\'e  to 
the  displacement  of  rocks  by  eruptive  bosses  as  if  the  original  mateiul 
that  filled  the  present  area  of  these  bosses  had  been  blown  out,  pushsd 
up,  or  melted  down  into  the  advancing  column  of  the  igneous  magma. 
If  any  serious  amount  of  material  were  incorporated  by  fusion  into  an 
eruptive  mass  we  should  expect  to  be  able  to  detect  some  change  in  du 
chemical  composition  or  crystalline  structure  of  the  rock  so  affected.  The 
observations  and  deductions  of  Dr.  Steelier  on  the  change  in  the  com- 
]>o.sition  of  intrusive  sheets  (postea,  p.  G09)  deser^'e  full  consideration,  for 

'  'Geology  urttieUvDrj-  MouiitaitiH,'  U.S.  GfOg.  and  Oral.  Surve}-,  Waahingtoo,  1B77. 
A  similar  structure  was  figured  auil  dMcribeii  by  C.  Mnolaren,  '  Oeol.  of  Fife  and  LotUaw,' 
1939,  pt>.  100.  101.  Tbe  gabbrw  of  Rkye  Iwve  lievn  injected  iu  tliin  my  into  th«  aliMti>  «( 
the  grrat  basal t-plal«au.  Tra«t.  Bui/.  .««.  JCdin.  iixi.  (1888),  p,  122.  8e«  ilM  J.  D. 
Dana,  Amtf.  Joiirn.  .'ki.  xlii.  (lSt>l),  i>.  79. 


\ 


\RT  VII  SECT,  i  §  2  INTRUSIVE  SHEETS  573 


/hey  appear  to  indicate  that  considerable  differences  may  be  induced  on 
in  igneous  mass  by  the  incorporation  into  its  substance  of  portions  of  the 
mrrounding  rocks. 

Connection  with  Volcanic  Action. — There  can  be  little  doubt 
:hat  in  regard  to  eruptive  masses,  particularly  of  the  dioritic,  gabbro,  and 
loleritic  or  basaltic  s^ies,  though  the  portions  now  visible  consolidated 
mder  a  greater  or  less  depth  of  overlying  material,  they  must  in  many 
5ases  have  been  directly  connected  with  superficial  volcanic  action.  Some 
>f  them  may  have  been  underground  ramifications  of  the  ascending  molten 
rock  which  poured  forth  at  the  surface  in  streams  of  lava,  though  these 
superficial  portions  have  been  removed  by  denudation.  Others  may  mark 
the  position  of  intruded  masses  which  were  arrested  in  the  unsuccessful 
sttterapt  to  open  a  new  volcanic  vent  The  gabbro  and  granophyre 
bosses  of  the  Inner  Hebrides  were  undoubtedly  a  part  of  the  general 
Tertiary  volcanic  phenomena  of  that  region. 

Connection  with  Crystalline  Schists. — In  some  regions  masses 
Df  diorite,  gabbro,  diabase,  &c.,  associated  with  crystalline  schists  have 
undergone  such  a  rearrangement  of  their  component  minerals  as  to  pass  into 
imphibolites  and  hornblende-schists.  These  changes  are  well  developed 
in  the  Saxon  Granulitgebirge  and  in  the  North  of  Scotland.  They  are 
Further  referred  to  at  pp.  182,  627. 

§  2.  Sheets,  Sills. 

Eruptive  masses  have  been  intruded  between  other  rocks,  and  now 
Gtppear  as  more  or  less  regularly  defined  beds.  In  many  cases,  it  will 
be  found  that  these  intrusions  have  taken  place  between  the  planes  of 
stratification.  The  ascending  molten  matter,  after  breaking  across  the 
rocks,  or  rather,  after  ascending  through  fissures,  either  previously 
formed  or  opened  at  the  time  of  the  outburst,  has  at  last  found  its  path 
of  least  resistance  to  lie  along  the  bedding-planes  of  the  strata.  Accord- 
ingly it  has  thrust  itself  between  the  beds,  raising  up  the  overlying 
mass,  and  solidifying  as  a  nearly  or  exactly  parallel  cake,  sheet,  or  sill. 

It  is  evident  that  one  of  these  intercalated  sheets  must  present  such 
points  of  resemblance  to  a  subaerial  stream  of  lava  as  to  make  it  occasion- 
ally a  somewhat  difficult  matter  to  determine  its  true  character,  more 
especially  when,  owing  to  extensive  denudation,  or  other  cause,  only  a 
small  portion  of  the  rock  can  now  be  seen.  Intrusive  sheets  are  marked 
by  the  following  characters,  though  these  must  not  be  supposed  to  be  all 
present  in  every  case.  (1)  They  do  not  rigidly  conform  to  the  bedding 
^f  the  rocks  among  which  they  are  intercalated,  but  sometimes  break 
icross  it,  and  run  along  on  another  platform.  (2)  They  catch  up  and 
involve  portions  of  the  surrounding  strata.  (3)  They  sometimes  send 
^eins  into  the  rocks  above  and  below  them.  (4)  They  are  connected 
irith  dykes  or  pipes  which,  descending  through  the  rocks  underneath, 
lave  been  the  channels  by  which  the  sills  were  supplied.  (5)  They  are 
commonly  most  close-grained  at  their  upper  and  under  surfaces,  and  most 
coarsely  crystalline  in  the  central  portions.     (6)  They  are  rarely  cellular 


674  GEOTECTDSW  (STBUCTURAL)  GEOLIHIY  Booiiv 

or  nmygdaloidal.     (7)  The  rocks  Imth  above  aiid  below  them  are  usuaUf 
hai'denod  and  otherwise  more  or  less  altered. 

Ah  s nrll-kiiowu Biid (from  ita  asaociiiticm  witli  tlie  Iluttonias  and  Weni^run  dupnlei) 
c1a»aica1  exain[>le  of  this  structure,  tlie  mural  esvari'nieut  cslled  SalisWiy  Crags  at  Edm- 
biirgli  may  be  described  (Kig.  28i].  Tliia  U  a  sill  of  crj-Btalliiic  diabase  (dolnitt), 
wliii'h  oad  bfi  trai'pd  for  n  ilijit«nn!  of  1500  yarls,  tyiliK  amoDjt  the  rrd  and  gny  und- 


atoilvs  shales,  atiil  iiiipuri'  liiiiCKtuiieH,  uliii-li  fonii  tije  l>a.i«  of  the  Carhonileroiu  sjatoD 
of  fentral  Seodaud.  As  the  general  dip  of  tho  rocks  ia  north -eaaterlj,  tho  rill  tOnuM  a 
lufty  clitf  racing  wvNt  and  south,  from  the  base  of  whieh  a  long  grassy  slope  of  dcbro 
litreti^hes  down  to  the  valley  in  front.  Ita  thii'kuesa  at  the  highest  part  ia  about  U 
feet,  hut  at  a  distance  of  650  yards  to  the  north  this  thickness  diminiabea  to  leas  thai 
a  half.  At  fiiiit,  the  iliahase  might  bo  taken  fur  a  conformable  sheet,  rcf^larly  ii- 
ler|w»ed  betweeu  tho  aedimeiitary  atruta.  But  au  examination  of  the  l>edH  on  whici 
it  rests  ahowH  that  it  tran)igre^vely  josses  over  a  succession  of  jilatfonns.  and  evrutaall)' 
comes  to  reat  at  the  cnat  end  on  strata  souienhat  lower  in  geologieal  position  than  tho* 
at  the  north  end.  Jlon^over,  another  |iiinillel  iiitruwive  sheet  intercalated  in  a  low»f 
portion  of  the  Kanditune  series  gi'adually  a]ij>msehes  the  rock  of  Salisbury  Crags.  Thtr 
are  lioth  tranagressive  orruas  tbu  »trata,  and  they  ajipear  to  unite  in  a  brge  niaas  albd 
Samson's  Kiha. 

On  the  we«t  front,  a  Urjfe  dyke-like  mass  of  the  diabasa  descends  vertically  throng 
the  sandstones,  and  has  bi-cn  regarded  as  not  improbably  a  jiipo  or  feeder,  up  trhich  tlic 
molten  rock  originally  rose  (Fig.  ^S4).     Along  the  sonthcm  focc  of  the  eticarpntnit, 


several  instniclive  exiiosm-es  show  the  behaviour  of  the  diabase  to  the  stnta  thnMgk 
which  it  has  made  its  way.  In  Kig.  2)>.>,  for  enaraple,  a  portion  of  the  undo'ljiag 
stiata  having  lieeii  carried  awuy,  tliu  dialiase  liaa  wedged  itsslf  belov  one  of  tbe 
remaining  broken  endii.     Again,  veins   and  thieads  of  tlie  emjjtive  rock  bar*  bMn 


Ill  BEcr.  i  §  2  INTRUSIVE  SHEETS  67ft 

1  into  fragments  of  the  strata  caught  up  in  its  mass  (Fig.  2S6).  The  atrata  in 
:  with  the  diabase  hare  been  much  hardened,  the  shales  being  cooverted  into  a 
f  porcellanit«,  and  the  saiidstoueB  into  quartzite.'  The  diabase  in  the  centre  of 
.  is  a  coarse-grained  rock,  in  which  the  component  minerals  can  readily  be  detected 

lens,  or  even  with  tlie  unassisted  eye.  But  as  it  approaches  the  sedimentaij 
boTG  aad  belon,  it  becomes  finely  crystAlline.  1  have  had  sections  cut  for  the 
M>pe,  showing  the  actual  junction  of  the  two  rocks  (Fig.  287).  In  these  it  is  intereat- 
obaerre  that  the  diabase,  for  about  the  eighth  of  an  inch  inwards  from  its  edg«, 
8  mainly  of  an  altered  glass  in  which  lie  well. formed  ciystals  of  triclinia  felspar 
merous  opaque  tuf[^  microlitae,  which  may  be  of  aiigite.  An  inch  back  from  the 
he  glass  and  the  microlites  have  alike  dtBap];eared,  and  the  rock  is  merely  a 
line  diabase,  though  finer  in  grain  than  in  the  central  iiortions  of  the  bed. 
ouB  st«ani'  or  gas.resicles  occur  in  the  vitreoos  part,  some  of  them  empty,  but 

filled  with  caleite  or  s  brown  ferruginous  earth.     There  can  be  little  doubt  that 


FlK,  S87. 

eud 

Bbide  (a)  fmbcdded 

1  Id  the  dliil 

MM  (6)  «f  8*li>l,iirr  Crap.,  and 

iajEc 

thnsdi  of 

■msWe 

Dikl 

HH  with  Sat 

iditot 

IP,  8«lisbiii 

omof 

the  d™»ii 

^f.  U>  >ind>toDe,  a  part  of  which  l> 

lies  tbe  rest  c 

.lide.    Th, 

jIiuI 

l«en  wrpentti 

liied. 

It  COUUll 

1  the  < 

on  of  flow. 

4bov 

e  the  dark. 

er  pirt  tlm  gl««.y  condition  raptdlj 

diabuc.    Tlie  r. 

ijr  uf  11 

IcIcundAui 

reous  structure  of  this  marginal  film  was  originally  tliat  of  the  whole  rock.  The 
m  of  tbe  glasny  crust  is  in  harmony  with  all  that  is  known  as  to  the  feeble 
il  conductivity  of  lava.  M'lien  the  rock  was  intruded,  it  was  no  doubt  a  molten 
ontaining  much  absorbed  vapour,  the  escape  of  which  at  its  high  temjierature  was 
ily  the  main  agent  iu  indurating  tbe  adjacent  strata.  In  a  number  of  slices  cut 
ifferent  parts  of  the  central  portion  of  the  diabase,  1  have  failed  to  detect  any  of 
Bin-holes  so  marked  in  the  outer  vitreous  edge.' 

Ir.  Sorly  has  observeil  iu  s|*cinien«  from  this  locality  sliced  by  him  for  microscopic 
■tion  that  the  Huid  cavities  in  the  quartz.grains  have  \xtn  emptied.— Address,  Q.  J. 
Soc,    ixxvi.    Address,    p.   82.      But  see  Dr.  Stecher's   papers  quoted   p.  601.     Thin 


gives  a 


>n  of  the  Carboniferous  sills  i 


t  the  Firth  of  Forth. 

na  of  the  most  remarkable  tiamples  of  an   intruHive  sheet  is  the  Whin  3i11  of 


576  GEOTECTONIC  {STRUCTURAL)  GEOLOGY  book  iv 

This  greater  closeness  of  texture  at  the  surfaces  of  contact  forms  one 
of  the  distinguishing  marks  of  an  intrusive  as  contrasted  with  a  contem- 
poraneous sheet  (pp.  573,  590).  Microscopic  examination  of  these  margiiial 
parts  from  many  of  the  intnisive  sheets  in  central  Scotland,  shows  that 
even  where  no  distinct  glass  remains,  the  rock  is  crowded  with  black 
opaque  microlites  arranged  in  a  delicate  geometric  network.  Back  from 
the  surface  of  contact,  the  microlites  disappear,  and  the  magnetite  or 
titaniferous  iron  assumes  its  ordinary  crystalline  and  often  indeterminate 
or  imperfect  contours.  These  bodies,  developed  along  the  marginal 
portions  of  the  intrusive  mass,  probably  belong  to  conditions  of  rapid 
cooling.^ 

Another  lithological  characteristic  of  the  intrusive,  as  compared 
with  the  interbedded  sheets,  is  the  considerable  variety  of  composition 
and  structure  which  may  be  detected  in  different  portions  of  the  same 
mass.  A  rock  which  at  one  place  gives  under  the  microscope  a  crystal- 
line-granular texture,  with  the  mineral  elements  of  diabase,  will  at  a 
short  distance  show  a  coarsely  crystalline  texture  with  abundant  ortho- 
clase  and  free  quartz — minerals  which  do  not  belong  to  normal  diabase — 
or  may  be  traversed  by  veins  of  fine-grained  siliceous  materiaL  These 
differences,  like  those  above  referred  to  as  noticeable  among  amorphous 
bosses,  seem  to  point  to  successive  stages  in  the  consolidation  of  a  molten 
magma  of  which  the  more  basic  constituents  separated  first.  But  some- 
times they  suggest  that  great  intrusive  sheets  have  here  and  there  in- 
volved and  melted  down  portions  of  rocks,  and  have  thus  acquired  locaUj 
an  abnormal  composition.- 

Effects  on  Contiguous  Rocks.  —  Admirable  examples  of  the 
alteration  produced  by  eruptive  masses  are  not  uncommonly  presented  at 
the  contact  of  intrusive  sheets  with  the  surrounding  rocks.  Induration, 
decoloration,  fusion,  the  production  of  a  piismatic  structure,  conversion  of 
coal  into  anthracite,  of  limestone  into  marble,  and  other  alterations,  may 
be  observed.     The  nature  of  these  changes  is  described  at  p.  597. 

Connection  with  Volcanic  Action. — Many  volcanic  rocks  occur 
in  the  form  of  intrusive  sheets,  as  felsite,  quartz -porphyry,  diorite, 
melaphyre,  diabase,  dolerite,  basalt,  trachyte,  and  others.  The  remarks 
a])ove  made  regarding  the  connection  of  intrusive  bosses  with  volcanic 
action  may  be  repeated  with  even  greater  definiteness  here.  Xntrusive 
sheets  abound  in  old  volcanic  districts,  intimately  associated  with  dykes 
and  surface-outflows,  thus  bringing  before  our  eyes  traces  of  the  under 
ground  mechanism  of  volcanoes.  They  frequently  occur  among  the  rocks 
that  lie  beneath  a  mass  of  ejected  lavas  and  tuffs,  or  traverse  the  lower, 
sometimes  even  the  upper  parts  of  the  volcanic  mass.  They  then  appear 
to  mark  some  of  the  later  stages  of  eniption  when  the  orifices  of  dischaige 
had  become  choked  up  and   the  subterranean  energy  only  sufficed  to 

Xortbumberland,  of  wliich  an  account  by  Messrs.  Topley  ami  I/ebour  wiU  be  found  in  Q.  /. 
Ucol.  Soc.  xxxiii.  (1877),  p.  406.     See  also  J.  J.  H.  Teall,  op.  cit.  1884. 

*  See  Fouiiue  and  Michel  I-^'vy,  *S^'ntht'8e  des  Mint'raux.' 

2  Tram.  Roy.  Soc.  JuUn.  xxix.  p.  492.  Clough,  QeoL  Mwj,  1880,  p.  438.  Sec  tbo 
J.  J.  H.  Teall,  Q.  J.  Ged.  SiK.  xl.  p.  247,  xlviii.  p.  104.     Stecber,  paper  cited  on  p.  541 


VEINS  AND  DYKES 


577 


ject  the  magma  between  the  bedding  of  the  rocks  below  ground 
it  not  to  impel  it  to  the  surface.  It  is  observable  that  lat«r  intruded 
asses  are  often  more  acid  than  the  lavas  previously  erupted,' 

Among  the  Palteozoic  and  Tertiary  volcanic  regions  of  Britain 
imerous  illustrations  of  such  sills  are  to  be  found.  Some  of  the  most 
riking  are  those  that  emerge  from  beneath  the  great  erupted  masses  of 
renig  and  Bwla  age  in  Xorth  Wales.  Admirable  examples  occur 
nong  the  Carboniferous  volcanic  rocks  of  the  basin  of  the  Forth.*  The 
ertiary  sills  injected  among  Carboniferous  and  Cretaceous  rocks  of 
ntrim  and  the  Jurassic  rocks  of  the  Inner  Hebrides  are  likewise  con- 
licuous  for  size  and  abundance.^ 


^  3    Veins  and  Dykes 

The  term  'vein  is  rather  vaguely  employed  by  geologists  It  is 
«d  as  the  designation  of  any  mass  of  mineral  matter  which  has  solidified 
itween  the  separated  walls  of  a  fissure      When  this  mineral  matter  has 


t^n  deposited  from  aqueous  solution  or  from  sublimation,  it  forms  what 
known   as  a  mineral-rein  (p.  633).     When  it  has  been  injected  in  a 

oIt«n  or  pasty  state  into  some  other  rock,  it  forms  an  eriiptivf  vein,  or, 
it  forms  a  vertical  wall-like  mass,  a  di/k-e.      When  it  forms  part  of  the 

neous  rock  in  which  it  occurs,  but  belongs  to  a  later  period  of  consolida- 

>  7VB.H.  Roy.  Sx-.  ft//".  «,vvv,  (J888),  p.  113,      Q.'arf.  J,„.n..  He,^.  .*<■.  xlviii.  (1892), 
Idnsa,  p.  177. 
"  See  TroHi.  Ron.  Hk.  E<li:i.  ixii.  p.  Hi.  '  Op.  cU.  iiiv.  (1888),  p.  111. 


578  GEOTECTONIC  (STRUCTURAL)  GEOLOGY  bcwk  it 

tiou  than  the  [lortioti  into  which  it  liaa  been  inject«il,  iC  has  been  called  » 
contemporaneous  tetit.  When  it  has  crystallized  or  segregated  out  of  the 
component  materials  of  some  still  unconsolidated,  colloid,  or  \Katy  rock,  it 
is  called  a  seffregation-rfiv. 


Eruptive  OF  loOnisive  Veins  and  Dykes  are  portions  of  once-melted, 

or  at  least  ])aaty  matter,  which  have  been  injected  into  rents  of  preWoiuly 
solidified  rocks.  When  traceable  sufficiently  far,  they  may  be  seen  to 
swell  out  and  mei^e  into  their  parent  mass,  while  in  the  opposite  direction 
they  may  become  attenuated  into  mere  threads.  Sometimes  they  run  !ca 
many  yards  or  miles  in  tolerably  straight  lines.  When  this  taJtes  place 
along  vertical  or  highly-inclined  stratification,  they  look  like  beds,  but 


Klg.     -0 


JKB.) 


thoy  are  of  cotu'se  really  intrusive  sheets.     They  may  frequently  be  found 
to  hruak  across  the  bedding  in  a  very  irregular  manner. 

So  r(i«k  pxliiliitH  more  iiistriKrtii'oly  tlisii  grnnite  the  numeroUB  vsriitiei  of  fons 
osaiiiiiFiI    by    Vi'ina.'      Tlini!   dixtiuct    kindn   of   graiiite   veins  may   be   obHrml. 


'  Oil  K 


>.  (lS7C),p.I04i 


VEINS  AND  DYKES 


679 


trnnona  of  the  ordinary  granite  si  tending  from  the  maiu  masses  into  the  surround- 
s  ajid  demonstrating  the  intrusive  character  of  the  granite  (Figs.  289,  290).  These 
in  breadth  from  several  feet  or  nanj  7>rds  down  to  iine  filanienta  or  tbreadi,  are 
markably  abundant  and  markedly  irre^lar  in  the  mauner  in  which  they  branch 
ertect.  Where  they  are  several  yards  broad  tbeir  texture,  at  least  in  the  central 
lay  not  sensibly  differ  from  that  of  the  main  granite  ttiaBS,  though  it  is  apt  to 
finer  especially  as  the  veins  diminish  in  breadth.  Round  some  bosses  of  granite 
.cent  rocks  are  injected  or  impregnated  with  such  an  abundance  of  minute  threads 
I  of  granite  substanee,  like  Isyera  or  leaves  parallel  with  the  stratification  or 
I,  that  tbey  are  said  to  be  "  granitized."  ' 

des  greater  closeness  of  teiture,  tliese  intmaive  veins  sometimes  jiresent  oou- 
e  differences  in  mineralogical  composition.      The  mica,  for  example,  may  be 


to  exceedingly  minute  and  not  very  abundant  Sakes,  and  may  almost  disappear, 
artz  also  occasionally  asnunics  a  subordinate  place,  and  the  rock  of  the  veins 
nto  one  of  the  varieties  of  tejaite,  quartz -porphyry,  elvauite,  aplite  or  eurite.' 
I  in  the  melaniorphosed  belt,  already  (p.  570}  described  as  encircling  an  intrusive 

granite,  that  eruptive  veins  are  typically  developed  and  most  readily  studied. 

wall,  for  example,  tlic  slates  around  the  granite  bosses  are  abuudantly  traversed 
*  or  dykes  of  granite  and  of  quartz 'poriihyry  {eliaiis),  which  are  most  numerous 
I  granite  (Fig.  291).  They  vary  in  width  from  a  few  inches  or  feet  to  GO  Tathoina, 
ntrat  portions  heing  commonly  more  crystalline  than  the  sides.     They  frequently 


chel  I/yy,  Bull.  f^oc.  UM.  F.f'-cf.  ii,  (1881),  p.  187,  an 
c  a  reference  ta  the  Bodegaag.  antt.  p.  1S9  ;  also  Huw< 
p.  2U. 


,d  /-,-(, 


I.  eo4. 


580  GEOTEGTONia  {STRUCTURAL)  GEOLOGY  book  iv 

enclose  angular  fragments}  of  slate  (p.  566).  In  the  great  grauite  region  of  Leiiuter, 
Jukes  traced  some  of  the  elvans  for  several  miles  running  in  parallel  bands,  each  only  a 
few  feet  thick,  with  intervals  of  200  or  300  yards  between  them.  Around  some  of  the 
granite  bosses  of  the  south  of  Scotland  similar  veins  of  fclsite  and  por|ihyry  abonnd. 
The  granite  of  the  Wahsatch  Mountains  in  Utah,  which  rises  through  the  Uf^ 
Carboniferous  limestones,  converting  them  into  white  marble,  sends  out  veins  of  granite- 
porphyry  and  other  cr^'stallinc  comi)ounds.  In  short,  all  over  the  world  it  is  commoa 
for  eruptive  ))088es  of  this  rock  to  have  a  fringe  of  intrusive  veins  (Apophyse*), 

(2)  Veins  in  the  granite  it«elf.  These  must  be  regarded  as  later  than  the  rock 
which  they  traverse,  but  they  may  represent  lower,  still  liquid  portions  of  the  granitic 
magma  which  have  been  forced  by  earth -movements  into  rents  in  the  partially  or  wholly 
solidified  granite.  They  are  generally  finer  in  grain  than  the  granite  around  them,  and 
differ  more  or  less  from  it  also  in  composition,  especially  in  their  greater  acidity 
(Fig.  30). 

(3)  Pegmatites.  These  are  distinguished  by  the  manner  in  which  their  componeut 
minerals,  notal>ly  the  quartz  and  fels^tar,  arc  intergrown  (see  \u  98).  Much  discusdon 
has  arisen  as  to  the  origin  of  such  veins.  They  evidently  cut  the  ordinary  grauite  and 
in  so  far  may  )3e  regarded  as  intrusive  veins.  But  it  is  difficult  to  conceive  that  they 
could  have  l>€en  injet^.ted  in  their  present  crystalline  condition.  They  may  have  hem 
squeezed  up  from  some  lower,  still  liquid  })art  of  the  granitic  magma,  but  their  remark- 
able crystalline  structure  would  seem  to  have  been  aftem-ards  superinduced  by  some 
process  of  segregation  or  re -arrangement  and  crystallization  of  their  materials. 

Many  other  eniptive  rocks  (diorite,  diabase,  melaphyre,  basalt,  &c. )  present  admir- 
able examples  of  intnisive  (even  pegmatitic)  veins.  These  are  generally  distinguished 
from  those  of  granite  by  the  much  less  metamorphism  with  which  they  are  attended. 

The  "Contemporaneous  Veins"  of  older  writers  included  those 
veins  in  crystalline  rocks  which  though  differing  sufficiently  from  the 
.surrounding  material  to  be  easily  distinguished,  resembled  it  so  closely 
as  to  indicate  that  they  were  probably  a  part  of  it  The  veins  above 
described  under  No.  2  are  examples.  But  they  are  not  confined  to 
granite,  since  they  may  not  infrequently  be  observed  in  sheets  of  gabbro, 
diorite,  dolerite,  diabase,  and  other  eruptive  rocks.  They  are  more 
])articularly  to  be  seen  in  sills  and  bosses.  They  run  as  straight^  curved, 
or  branching  ribands,  usually  not  exceeding  a  foot  in  thickness.  They 
are  finer  in  texture  than  the  rock  which  they  traverse.  Close  examinatiou 
of  thorn  shows  that,  instead  of  being  sharply  defined  by  a  definite  junction 
line  with  the  enclosing  rock,  they  are  welded  into  that  rock  in  such  a 
way  tliat  they  cannot  easily  be  broken  along  the  plane  of  union.  This 
wrldiiig  is  found  to  be  due  to  the  mutual  protrusion  of  the  component 
crystals  of  the  vein  and  of  the  surrounding  rock — a  structure  sometimes 
admirably  revealed  under  the  microscope.  Veins  of  this  kind  evidently 
point  to  some  process  whereby,  into  rents  formed  in  the  deeply  buried 
and  at  least  partially  consolidated  or  possibly  pasty  or  jelly-like  rnaff^ 
there  was  an  injection  of  similar  material  from  some  still  unsolidified  part 
of  the  mass  with  a  transfusion  or  exosmosis  of  some  of  the  crystallixing 
minerals  along  the  mutual  boundaries.  Such  veins  are  to  be  'distin- 
guished from  the  true  Segregation -veins,  which  are  irregular  bands 
usually  of  more  coarsely  crystalline  material  not  infrequently  to  be  seen 
in  intrusive  sheets,  wherein  the  constituent  minerals  have  ciystalli^ed  out 


:  SECT,  i  §  3 


SEGBEGA  TION-  VEINS 


uch  more  conspicuous  form  than  in  the  main  mass  of  the  sur- 
g  rock  along  certain  lines  or  sroiind  particular  centres.  These 
bably  due  to  some  kind  of  segregation  from  the  surrounding 
bough  the  coDditions  under  which  it  took  place  have  not  yet 
tisfactonly  explained '     Segregation  veins  occur  among  the  crya- 


WZ—P^njltteW 

n  Bswctalcd  with  MiMted  gnuLU.    BuWilnw  (Juuiy,  AberdMn. 

rjrgrtnitsofthem 

lu    pp  cosraepegnuttte  y*iru;  »t,foli«l*<l  granite  piMinglnM 

InlOff     IJ    TDU> 

qiurti.    Tlie  b  ick  patchu  In  p  uid  { in  nuti  of  ichorL. 

lalbly 


chiats  and  even  in  sedimentary  rocks  which  have  been  crushed  and 
rphosed,  as  in  the  felspathic  Torridon  Sandstone  of  Loch  Garron. 
ig  the  niai^n  of  segregation-veins  in  granite  a  foliat«d  structure 
ock  may  be  occasionally  observed,  as  in  some  of  the  large  granite 
near  Aberdeen  (Fig.  292).     Coarse  pegmatite  veins  abounding 


plates  of  muscovite,  black  tourmaline,  and  quartz,  with  occasional 
of  berj-1  and  other  minerals,  merge  into  the  surrounding  granite 
>r  a  few  inches  along  the  contact  has  a  foliated  structure  precisely 
ing  that  of  a  line  gneiss.  This  foliation  may  indicate  motion  of 
nite  mass  along  the  line  of  6ssure,  while  the  rock  itself  or  the 
DC  illaHtratianK  Me  Tmi.s.  Kui/.  fk-c.  Edin.  xiir.  (1838),  pp.  113,  115,  118,  ISl. 


5S2  GEOTECTONie  (STRUCTURAL)  GEOLOGY  booe  iv 

material  forced  up  into  the  fissure  was  still  capable  of  molecular  le- 
arrungement  It  is  in  veins  in  granite  that  the  remarkable  structure 
known  as  ijraphk  gnmiU  occurs.' 

Dykes  are  veins  of  eruptive  rock,  filling  vertical  or  highly-inclined 
fissures,  and  are  so  named  on  account  of  their  resemblance  to  willi 
(Scolice,  dykes).  Their  sides  are  often  as  parallel  and  perpendicular  u 
those  of  hiiilt  walls,  the  resemblance  to  human  worknaanship  being 
heightened  by  the  numerous  joints  which,  intersecting  each  other  along 
the  face  of  a  dyke,  remind  us  of  well-fitted  masonry.  Where  the  surround- 
ing rock  has  decayed,  the  dykes  may  be  seen  projecting  above  gFonnd 
exactly  like  walls  (Fig.  29i) ;  indeed,  in  many  parts  of  the  west  of 
Scotknd  they  are  made  use  of  for  enclosures.  The  material  of  the  dyket 
has  in  other  coses  decayed,  and  deep  ditch-like  hollows  are  left  to  mark 


t-iji  2M.-I.)k.. 


m,  Blir,  Flfc. 


their  sites.     The  const-lines  of  many  of  the  Inner  Hebrides  and  of  the 
Clyde  Islands  funiish   numerous  admirable  examples  of  both   kinds  of 

The  term  dyke  may  he  applied  to  some  of  the  wall-like  intnisiont 
of  quartz-jwi'phyry,  elvanite,  and  even  of  granite,  but  it  is  more  typically 
illustrated  among  the  l>asic  and  intermediate  igneous  rocks  such  as  bault, 
diabase,  aiidesite,  diorite,  Ac,  while  occnsionaily  dykes  may  be  observed 
of  even  tuft'  and  volcanic  agglomerate,-     Veins  have  been  injected  into 

'  For  an  alilo  iliHcuiision  of  Pe^matile  veiim  ate  Prof.  W.  C  Brnggsr'a  gre&t  work  '  Dit 
Mineniltsii  An  Hyeuilpegniatiti^iiiifte,'  in  Groth's  Ztitach.  KryaUUlagTaphie.  xn.  (1890)  \  M 
]■.  1\i  ct  nrq.  abiKlorii'al  rr*»m'  of  the  UiBCUsBinii  vtll  be  founil. 

"  Konu-  KHiitrkable  einiiiiili's  of  ^aniistone  -  ilykss  liava  been  describnl  from  virioiii 
riistrioli  of  Norlli  Anier[p!i.  ranging  from  h  nier*  fibn  to  eight  f«et  broad  and  varrlng  fron 
-.'00  yanls  tu  iipunrtls  uf  nine  uile^  in  l«ngth.      T)iey  Iisve  been  ucribed  to  the  iDBlli^of 


T.  i  g  3  DYKES  683 

ranching  cracks ;  dykes  IiavB  been  formed  by  the  welling 
liquid  or  plastic  rock  in  vertical  or  steeply  inclined  fissures, 
iously  there  is  no  easential  difference  between  the  two  forms 
,  Sometimes  the  line  of  escape  has  been  along  a  fault.  In 
jwever,  which  may  be  regarded  as  a  typical  region  for  this 
Ic^cal  stnicture,  the  vast  majority  of  dykes  rise  along  joints 
vliich  have  no  throw,  and  are  therefore  not  faults.  On  the 
le  dykes  may  be  traced  undeflected  across  some  of  the  largest 
a  midland  counties. 

lifTer  from  veins  in  the  greater  parallelism  of  their  sides,  their 
and  their  greater  regularity  of  breadth  and  persistence  of 
They  sometimes  occur  as  mere  plates  of  rock  not  more  than 
iwo  in  thickness,  at  other  times  they  attain  a  breadth  of  twelve 

more.  The  smaller  or  thinner  dykes  can  seldom  be  traced 
,  few  yards  ;  but  the  lai^er  examples  may  be  followed  sometimes 
liles.  Thus,  in  the  south  and  west  of  Scotland,  a  remarkable 
salt  and  andesite  dykes  can  be  traced  across  all  the  geological 
of  that  region,  including  the  older  Tertiary  basalt- plateau, 
larallel  to  each  other  in  a  general  north-west  and  south-east 
r  distances  of  twenty  and  thirty  miles,  increasing  in  numbers 

north-west,  and  they  have  been  assigned  to  the  great  volcanic 
Tertiary  time.  A  dyke  of  the  same  series  crosses  the  north  of 
jm  near  the  coast  of  Yorkshire  for  about  100  miles  inland.  A 
item  of  massive  pre-Cambrian  dykes  occurs  in  N.W.  Scotland. 

the  wall-like  form  is  predominant  among  dykes,  it  may 
■■  into  vein-like  ramifications  and  intrusive  sheets  (Figs.  284, 
I  molten  material  took  the  chan- 
appened  to  be  most  available, 
re  bent  off  at  an  angle  from  its 
end,  or  if  another  adjacent  fis- 
led  to  be  more  convenient,  the 
)ck  might  change  ita  course. 
e  the  ascending  lava,  under  the 

pressure  of    the  mass    below, 
)    main  fissure,  portions    of    it 

their    way  into    neighbownng 
ts,  and  enclose  wall-like  portions 

Mn  the  dyke,  as  in  Fig.  295,     '■'«■  -■"^--f""  "■  "y"'  i=  ">  ""nintt 
«tal  breadth  of  the  main  dyke,        mw.hiiT. 
he  sandstone  l)etween  the  two 

•out  thirty  feet,  the  sandstone  being  gently  inclined,  and  the 
closefl  between  the  arms  of    the  dyke  having  been   greatly 

be  kept  in  mind,  however,  that  irregular  expansions  and  con- 
dykes  may  sometimes  be  caused  by  subsequent  movements 
strial  crust.     The  dykes,  for  instance,  may  be  plicated  t<^ether 

Ml.  Ileal,  .'if.  .iHi'-int,  i.  (1^90),  p.  411. 


584  (iEOTECTONIC  {STRUCTURAL)  GEOLOGY  book  it 

with  the  rocks  among  which  they  have  been  intruded,  and  the  folds  may 
afterwards  be  pressed  in  such  a  way  as  to  give  rise  to  alternate  or  irregulariy 
distributed  enlargements  and  constrictions,  or  a  similar  effect  may  be 
produced  by  shearing  or  by  faulting.^  Mr.  Clough  has  found  that  in  a 
great  system  of  dykes  traversing  the  crystalline  schists  of  Argyllshire 
frequent  attenuations  of  the  dykes  are  produced  by  faults. 

In  internal  structure,  considerable  differences  may  be  detected  among 
dykes.  The  rock  may  appear  {a)  with  no  definite  structure  of  any  kind 
beyond  irregular  jointing ;  {b)  columnar,  the  prisms  striking  off  at  ri^t 
angles  from  the  walls,  and  either  going  completely  across  from  side  to 
side,  or  leaving  a  central  non-columnar  part  in  which  they  branch  and 
lose  themselves ;  when  the  side  of  a  dyke  having  this  structure  is  laid 
bare,  it  presents  a  network  of  polygonal  joints  formed  by  the  ends  of  the 
prisms  which,  if  the  dyke  is  vertical,  lie  of  course  in  a  horizontal  position, 
whence  they  depart  in  proportion  as  the  dyke  is  inclined :  occasionally 
the  prisms  are  as  well-foimed  as  in  any  columnar  bed  of  basalt ;  (c)  jointed 
parallel  with  the  walls,  the  joints  being  sometimes  so  close  as  to  cause  the 
rock  to  appear  as  if  it  consisted  of  a  series  of  vertical  plates  or  strata : 
this  platy  character  is  due  doubtless  to  contraction  in  cooling  between 
parallel  walls,  and  when  it  occurs  in  basalt-dykes  is  best  developed  near 
the  margins  ;  {d)  vesicular  or  amygdaloidal,  lines  of  minute  vesicles  having 
l)een  formed  parallel  with  the  walls,  and  attaining  their  greatest  number 
and  size  along  the  centre  of  the  dyke. 

As  a  nUe,  the  outer  parts  of  a  dyke  of  crystalline  rock,  like  the 
upper  and  under  surfaces  of  an  intrusive  sheet,  are  finer  grained  than 
the  centre.  Occasionally,  the  external  surface  has  a  vitreous  structure. 
Basalt  veins,  for  example,  have  not  infrequently  an  external  coating  or 
crust  of  glass  (tachylite,  hyalomelan,  &c.)  It  occasionally  happens  also 
that  the  central  portions  of  a  basalt  or  andesite  dyke  are  glassy,  of  which 
structure  several  cases  have  been  observed  in  Scotland ;  perhaps  in  these 
instances  the  dyke  has  opened  along  its  centre,  and  a  fresh  uprise  of  more 
glassy  material  has  risen  in  the  fissure.- 

Effects  on  Contiguous  Rocks. — These  are  similar  to  the  changes 
produced  by  intrusive  sheets  and  other  eruptive  masses.  Induration  is 
the  most  frequent  kind  of  alteration.  Eemarkable  examples  have  been 
observed  where  limestones  in  contact  with  dykes  have  had  a  saccharoid 
crystallization  of  the  calcite  superinduced  upon  them,  and  where  even 
new  crystalline  silicates  have  been  developed  (pp.  320,  602).' 

§  4.  Xecks. 

Under  this  term  are  included  the  fiUed-up  pipes  or  funnels  of  former 
volcanic  vents.  Every  series  of  volcanic  sheets  poured  out  at  the  surface 
must  have  been  connected  either  with  fissures,  or  with  orifices  probaUy 

^  Compare  the  structure  illustrated  by  Fig.  312.     See  al«o  Harker,  Oeol.  Mttg,  1889, 
p.  iV,\  aucl  the  account  of  the  pre -Cambrian  rocks  in  Book  VI.  Part  I. 
2  See  Proc.  Roy.  Phys.  S,>c.  Edin.  vol.  v.  1880,  p.  241. 
^  On  the  Mechanism  of  Dykes  see  Mallet,  Q.  J.  fieoi.  St^.  xxxii.  (1876),  p.  472. 


PART  VI 1  SBCT.  1 


VOLCAmC  NECKS 


opened  m  Imee  of  fissures  On  the  ceasalioa  of  the  eruptions,  these  onfices 
have  renamed  filled  with  lava  or  with  fragmentarj  matter  But  unless 
■ubsequent  denudation  has  removed  the  overlj  ing  cone  a  v  ent  lies 
buned  under  the  matenals  which  came  out  of  it  So  extensive,  however 
has  been  the  waste  of  the  surface  m  many  old  volcanic  regions  that 
the  vcnu  have  been  laid  bare  In  Fig  296  two  ^olcanlc  funnels  are 
represented,  one  of  them  still  buned  under  overlying  formations  the 
other  partiaUy  exposed  by  denudation  The  studj  of  volcanic  Necks 
brings  before  ua  some  of  the  more  deep  seated  phenomeni  of  \olcanic 
action,  that  cannot  usuall)    be  seen  at  a  modern  volcano 


Fig,  iW.-l>U«™"-»«tLun  lo»1m«  thr  Hlrarlutciin 

I,  TulTcune  with  built  plug  atm  bailed  unil«r  ■edlinenUry  iccuiiiiiliitloii* ;  i,  Tutt  ci>n«u'l  bualt 
plug  partljtllj  expotvd  by  il^nuJatinii, 

A  Neck  is  circular  or  elliptical  in  ground-plan,  but  occasionally  more 
irregular  and  branching,  and  may  vary  in  diameter  from  a  few  yaiiis 
(Fig.  297)  lip  to  two  miles,  or  even  more.  It  descends  into  the  earth 
perpendicularly  to  the  stratification  of  the  formation  with  which  it  is 
chronologically  connected.  Should  rocks  originally  horizontal  be  sul>- 
sequently  tilt«d,  a  neck  associated  with  them  would  of  course  be  tlirown 
out  of  the  vertical  {Fig.  296).  As  a  rule,  however,  the  vertical  descent 
of  necks  into  the  earth's  crust  appears  to  have  been  comparatively 
little  interfered  with.  In  external  form,  necks  commonly  rise  as 
cones  or  dome-shaped  hills  (Fig.  298).  This  contour,  however,  is  not 
that  of  the  original  volcanoes,  but  is  due  to  denudation.  Occasionally 
the  rocks  of  a  neck  have  been  so  worn  away  that  a  great  hollow, 
suggestive  of  the  original  crater,  occupies  their  site.  (Fintry  Hills, 
Stirlingshire.)' 

It  might  be  supposed  that  necks  should  always  rise  on  lines  of 
fissure.  But  in  central  Scotland,  where  they  abound  in  rocks  of 
Carboniferous  age,  it  is  quite  exceptional  to  find  one  placed  on  a  fault. 
As  a  rule,  they  seem  to  be  independent  of  the  structure  of  the  visible 
part  of  the  crust  through  which  they  rise. 

The  materials  filling  up  ancient  volcanic  orifices  may  be  (a)  some 

form  of  lava,  as  granophyre,  felsite,  gabbro,  diabase,  poqihyrite,  basalt ; 

>  For  •oiiie  atrihini;  vtcwx  of  denurled  volcniiic  ueckg  see  Cnptaia  Duttou's  Report  ou 

HoQDt  Tkylor  md  the  Ziifli  PlaWau,  6(A  .{»h,  Jifji.  C.K  ll/,J.  liurrry.  iaS4-S5,      Compnre 

alM  Tnou.  Roy.  tkK.  Kdi«.  vol,  hit.  (1888),  ]■-  100. 


OEOTKCTOXIi!  (STRUCTURAL)  GEOLOGY 


or  {!>)  the  fragmenUry  materials  which  fell  back  into  the  throat  of  tht 

volcano  anil  hnally  solidified  there      In  many  instances,  both  kinds  of 

rock  occur  m  the  same  nei.k   the  mam 

mass  consisting  of  agglomerate  or  tuff  "j  |  | 

with  1  ctntnl  pipe  or  iitimerous  vems 

of  iavi       iniong  the  Pal-eozoic  volcanic 

districtx    of    Bntaiii     necks    not   infre 

quentlj    ire  filled  with   some   eiliceous 

crystalline  rock    such   is  a  (|uartz-]>or 

phjrj    or  felsite.  e\en  where  tlie    aur 


roundiiig  hivas  ;ii-e  basic.  'Die  great  veiit 
of  the  Braid  Hills  near  Edinburgh,  I>c- 
longing  to  the  time  of  the  Lower  Old 
Kwl  Sandstone,  is  tilled  with  felsite-tiiff 
containing  70  per  cent  of  silica,  while 
the  lavas  which  flowed  from  it  are 
iwqihyrites  and  diabases  with  not  more 
than  Tifl  jjer  cent  of  this  acid.  Again, 
at  Liryo  in  Fife,  strings  of  quartz-felsite 
occur  in  one  of  the  Jiecks,  though  all  the  surrounding  lavaa  are  b&salts.* 
In    some    necks    composed    of    ern])tive    rock,  the  material   appean 


'  Xvcks  r>r  n^Ioruerate  niiil  fliie  tuff  ilIhuiiii) 
■uiic  ri'gi'iiis  nf  Srjillaiul,  ain\  an  liiiil  liare  in 
i>n<i  any  W  rvKanleil  av  typical  [i>t  tliiii  kiail  of 


:iiiiong  the  CarboDifennB  tad  Pcruiiu 
so  luuDf  aitminble  aectioni,  that  thew 
;ealo)dcal  atructun. 


PABT  VII  SECT,  i  g  4  VOLCANIC  NECKS  687 

arranged  in  successive  spherical  sWIb,  which  may  be  supposed  to  be 
due  to  the  protrusion  of  successive  portions  of  the  pngty  or  viscous  mass 
one  within  the  other,  the  outer  layers  thinning  away  over  the  crown 
of  the  (iomo  as  they  were  attenuated  by  the  ascent  of  fresh  material 
front  below.'  Or  we  may  suppose  that  the  top  of  the  plug  Bometimea 
solidified,  and  that  subsequent  emissions  of  lava  rose  through  rents  in 
the  crust,  and  flowed  down  the  outside  of  the  vent. 

The  fragmentary  materials  in  necks  consist  mainly  of  different  lava- 
form  rocks  imbedded  in  a  gravelly  pejiemwMke  matrix  of  more  finely 
comminuted  debris  of  the  same  rocks ;  but  tliey  also  contain,  sometimes 
in  abundance,  fragments  of  the  sti-ata  through  which  the  necks  have 
been  drilled.  When  Occasionally,  as  in  eome  of  the  Maare  of  the  Eifel, 
these  non-volcanic  fragments  constitute  most  of  the  debris  (p.  244), 
we  may  infer  that  after  the  first  gaseous  explosions,  tlie  activity  of 
the  vent  ceased,  without  the  rise  of  the  lava-column  or  its  ejection  in 
dust  and  fragments  to  the  surface  So  unchanged  are  many  of  the 
pieces  of  sandstone,  shale  limestone  or  other  stratified  rock  m  the  necks 
that  they  have  evidently  never  been  exfosed  to  any  high  temperature 
In  some  cases,  however  onsiderable 
alteration  is  displayed.  Dr  Heddlc 
from  observations  in  Fife  conclude  1 
that  the  altered  blocks  in  the  tulf  then  '' 

must  have  been  exposed  to  a  temperature       ^ 
of  between  600°  and  900   Fahr  J 

Among  the  numerous  \ents  of  fen        ' 
tral    Scotland,  pieces    of   fine   stratifie  1 
tuff    not    infreiiuently    appear    in    th 
agglomerates.     This  fact      ouplcd  with 

the  not  uncommon  occurrence  of  a  HK.■^■■l.-^l^lllclf^«l(.ull■llo^<■,lHEU*. 
tumultuous,    fractured,    and    highly -in-  •'"'f'- 

clineJ  bedding  of  the  tuff'  with  a  dip  c t»ff;u,p«rro«m.rk(wih.in».rddip: 
towards  the  centre  of  the  neck  (Figs.  h.ihiMnbiowuoiwn;  itn,  uuitdykw. 
29*1,  299),  appears  to   show   that   the 

pipes  were  partly  filled  up  by  the  subsidence  of  the  tuff"  consolidated  in 
beds  within  the  crater  and  at  the  upper  part  of  the  funnel.  Further 
in<lication  of  the  probable  subaerial  character  of  the  tuff  is  furnished  by 
abundant  pieces  of  enclosed  coniferous  wood,  which  may  have  belonged 
to  trees  or  brushwood  that  grew  upon  the  dry  slopes  of  the  cones ; 
for  these  fragments  are  seldom  to  be  seen  in  the  estuarine  and  marine 
strata,  out  of  which  the  necks  rise. 

It  is  common  to  find  among  necks  of  tuff  numerous  dykes  and  veins 
of  lava  which,  ascending  through  the  tuff,  are  usually  confined  to  it, 
though  occasionally  tbey  penetrate  the  surrounding  strata.  They  are 
often  beautifully  columnar,  the  columns  divei^ing  from  the  sides  of  the 
dykes  and  being  frequently  curved. 

■  Scroiw,  '  (ieolORf  and  Extinct  Voleanwi  ol  Centn)  France,'  2nd  eilition,  p.  63.  See 
B.  Reyer,  Jnhrb.  Ueol.  ItrMitimd.  xxii.  (187S),  ]>.  463  ;  nui  aolf,  p.  247  ;  A.  O.  Train. 
ftog.  Soe.  Ellin.  x\xv.  (IgSS),  )..  161.  ''  Trant.  Kos.  Soc.  Edin.  iiviil.  p.  187. 


!i88  GEOTECTOXir  (STRVfTUIlAL)  GEOLOGY  kkkt 

I'ratAs  of  subsidence  round  the  side.s  of  vents  mar  often  be  obwircd 
•Stratified  rocks,  through  which  a  volcanic  funnel  hu  been  optaii 
coinmouly  ili]>  into  it  all  round,  and  may  even  be  seen  on  edge,  sa  tf  tfaei 


liiid  bct-ii  di'agged  down  I>y  the  Kidtsidcnce  of  the  materials  in  the  vent 
ni^aiitifnl  examples  occur  along  the  shores  of  the  Firth  of  die  Forth, 

'    Tma'.  lt"!i.  Su(.  Eilia.  JL\i\.  y.  W9.    For  nu  excvllcut  eiamptc&ODi  Kew  ZeaUnd,  H 
Hr.il.Uv,  V.  J.  'ie.J.  S.K.  1860.  1..  24r.. 


PARTVii8ECT.ii§l     CONTEMPORANEOUS  LAVA-SHEETS  689 

(Figs.  300,  301).     The  fact  of  subsidence  beneath  modern  volcanic  cones 
has  been  already  referred  to  (pp.  231,  244). 

Effects  on  Contiguous  Kocks. — The  strata  round  a  neck  are 
usually  somewhat  hardened.  Sandstones  have  acquired  a  vitreous  lustre  ; 
argillaceous  beds  have  been  indurated  into  porcellanite ;  coal-seams  have 
been  fused,  blistered,  burnt,  and  rendered  unworkable.  The  coal-workings 
in  Fife  and  Ayrshire  have  revealed  many  interesting  examples  of  these 
changes,  which  may  be  partly  due  to  the  heat  of  the  ascending  column  of 
molten  rock  or  ejected  fragments,  partly  to  the  rise  of  heated  vapours, 
even  for  a  long  time  subsequently  to  the  volcanic  explosions.  Proofs  of 
a  metamorphism,  probably  due  to  the  latter  cause,  may  sometimes  be  seen 
within  the  area  of  a  neck.  Where  the  altered  materials  are  of  a  fragment- 
ary character,  the  nature  and  amount  of  this  change  can  best  be  estimated. 
What  was  originally  a  general  matrix  of  volcanic  dust  has  been  converted 
into  a  crystalline  and  even  porphyritic  mass,  through  which  the  dispersed 
blocks,  though  likewise  intensely  altered,  are  still  recognisable.  Such 
blocks  as,  from  the  nature  of  their  substance,  must  have  offered  most 
resistance  to  change — pieces  of  sandstone  or  quaitz,  for  example — stand 
out  prominently  in  the  altered  mass,  though  even  they  have  undergone 
more  or  less  modification,  the  sandstone  being  converted  into  vitreous 
quartz-rock.^ 

Section  ii.  Interbedded,  Volcanic,  or  Contemporaneous 

Phase  of  Eruptivity. 

Masses  of  igneous  materials,  ejected  to  the  surface  in  some  of  the 
forms  now  visible  in  modem  volcanoes,  possess  great  value  as  fixing  the 
geological  epoch  of  volcanic  eruptions.  It  is  evident  that,  on  the  whole, 
such  superficial  masses  must  agree  in  lithological  characters  with  rocks 
already  described,  which  have  been  extravasated  by  volcanic  efforts  with- 
out quite  reaching  the  surface.  Yet  they  have  some  well-marked  general 
characters,  of  which  the  most  important  may  be  thus  stated.  (1)  They 
occur  as  beds  or  sheets,  sometimes  lava-forra,  sometimes  of  fragmental 
materials,  which  conform  to  the  bedding  of  the  strata  among  which  they 
are  intercalated.  (2)  They  do  not  break  into  or  alter  overlying  strata. 
(3)  The  upper  and  under  surfaces  of  the  lava- beds  present  commonly  a 
scoriaceous  or  vesicular  character,  which  may  even  be  found  extending 
throughout  the  whole  of  a  sheet.  (4)  Fragments  of  these  upper  surfaces 
not  unusually  occur  in  the  immediately  overlying  strata.  (5)  Beds  of  tuff 
are  frequently  interstratified  with  sheets  of  lava,  but  may  also  occur  by 
themselves,  interstratified  among  ordinary  sedimentary  strata. 

§  1.  Crystalline,  or  Lavas. 

While  the  underground  course  of  a  protruded  mass  of  molten  igneous 
rock  has  widely  varied  according  to  the  shape  of  the  channel  through 

*  For  a  detailed  account  of  the  structure  of  some  volcanic  necks,  the  student  may  consult 
a  monograph  by  the  author  on  the  Carboniferous  volcanic  rocks  of  the  Basin  of  the  Forth, 
Trans.  Roy.  Soc,  Edin.  xxix.  p.  437. 


590  liEOTKfTOSH:  (STUfTVTVRAL)  GEOLOGY  book  n 

which  it  [inxeedetl  and  in  which,  as  in  a  mould,  it  solidified,  the  beluTiour 
uf  the  rock,  once  jiotirtd  out  at  the  surface,  has  been  much  more  unifonn. 
As  iu  modern  luva,  tho  erupted  mass  has  rolled  along,  varj-ing  in  thicknetu 
and  oth«r  minor  chanicters,  but  retaining  the  broad  general  aspect  of  ■ 
lenticular  lied  or  sheet.  A  comparison  of  such  a  bed  with  one  of  the 
intrusive  nheets  ulivaily  descrilicil  shows  that  in  several  important  litho- 
logical  characters  they  ditTer  from  each  other.  An  intrusive  sheet  a 
closest  in  grain  near  its  upper  ami  under  surfaces.  A  contemponneoiu 
bwl  or  true  lava-tiow,  on  the  contrary,  is  there  usually  most  open  and 
scoriitceoUA.  In  the  one  case,  we  rarely  see  vesicles  or  amygdale^,  in  tbf 
other  they  often  abound.  However  rough  the  upper  surface  of  an  intei- 
hedded  sheet  may  l>e,  it  never  sends  out  veins  into,  nor  encloses  portion; 
of,  the  suiieriucumlient  rocks,  which,  however,  sometimes  contain  portiom 
of  it,  anil  wnip  rixind  its  humniockii'  irregularities.     Occasionallv  it  nuv 


lilt  n-nl-  liav.r  W,a  tll-l  ill  «itli  ^\,.i  UI..I.-  !!.>■  eiui.tiun  ,.f  Hit  n.ii  a..i>. 

Ik'  ol>$t-rveil  lu  Iwr  full  of  rents,  which  have  l>een  tilled  up  with  sandstone 
01'  otlier  scJimriuary  material.  Thes«-  rents  were  formed  wliile  the  Ian 
wait  cofiliiig,  and  sand  vrm  subjieipiently  washed  into  them.  Exaraples  of 
thi.->  structui'i-  iilHiiinil  among  the  porph>Tites  of  the  volcauic  tracts  of  the 
.Siottisii  L"wer  Old  lied  Siindstone,  The  amygilaloidal  cavities  through- 
out an  interbcddeil  sheet,  hut  more  esjiecially  at  the  top,  often  present  an 
clon^r.'iii-d  fiirin.  .ind  are  even  pulled  out  into  tube-like  hollows  ia  onr 
geuer;d  iliiettiiui,  whii  h  was  obvinusly  the  line  of  movement  of  the  yet 
visL-ou*  in,iss  (pp.  lOi,  •I'l').  Some  kinds  of  rock,  when  occurring  in 
interbiddi-d  sheets,  are  iipt  to  assume  a  sjstem  of  columnar  jointing. 
lijtsiilt.  ill  partienlar.  is  ili.stiii^nished  by  the  frequency  and  perfection  of 
its  cuhiiuns.  The  tliants'  Causeway,  tlie  cliffs  of  Staffa,  of  Ar^tun  in 
Mull,  and  uf  Loch  StafKn  in  Skye,  the  Orgues  d'Expailly  in  Auvergne,  anil 
the  Kii'fiiliberg  of  Fulda  are  well-known  examples. 


PARTvn  SECT.ii  §  1     CONTEMPORANEOUS  LA  VA-SHEETS  591 

Interbedded  lavas  of  former  geological  periods,  like  those  of  recent 
date  (ante,  p.  239),  occur  under  two  tolerably  well-defined  conditions. 

1.  Lenticular  sheets  or  groups  of  sheets,  usually  of  limited  extent 
and  with  associated  bands  of  tuff,  form  the  more  frequent  type  among 
Palaeozoic  and  Secondary  formations.  A  single  interbedded  sheet  may 
occasionally  be  found  intercalated  between  ordinary  sedimentary  strata, 
without  any  other  volcanic  accompaniment.  But  this  is  unusual.  In  the 
great  majority  of  cases,  several  sheets  occur  together,  with  accompanying 
bands  of  contemporaneous  tuff. 

Ill  such  abundantly  volcanic  districts  as  central  Scotland,  the  necks  or  vents  of  erup- 
tion (p.  584)  may  frequently  be  detected  around  the  lavas  which  proceeded  from  them. 
The  thickness  of  an  interbedded  sheet  varies  for  different  kinds  of  lava.  As  a  rule,  the 
more  acid  rocks  are  in  thicker  beds  than  the  more  basic.  Some  of  the  thinnest  and  most 
persistent  sheets  may  be  observed  among  the  basalts,  where  a  thickness  of  not  more  than 
12  or  15  feet  for  each  sheet  is  not  uncommon.  Both  individual  sheets  and  groups  of 
sheets  possess  a  markedly  lenticular  character.  Tliey  may  be  seen  to  thicken  in  a 
particular  direction,  probably  that  from  which  they  flowed.  Thus  in  Linlithgowshire  a 
mass  of  lavas  and  tuffs,  reaching  a  collective  thickness  of  probably  2000  feet  in  the  Car- 
boniferous Limestone  series,  rapidly  dies  out,  until  within  a  distance  of  only  ten  miles  it 
dwindles  do^^oi  to  a  single  band  less  than  fifty  feet  thick.     On  the  other  hand,  beds  of 


Fig.  803.— Four  successive  flows  of  porjih yrite,  Lower  Carl:»onifcrou8,  East  Linton. 

tolerably  uniform  thickness  and  flatness  of  surface  may  be  found  ;  among  the  basalts, 
more  |)articularly,  the  same  sheet  may  be  traceable  for  mile.s  with  remarkable  regularity 
jf  thickness  and  parallelism  between  its  upper  and  under  surfaces  (p.  226).  The  por- 
phyrites  (Fig.  303)  and  trachytic  and  felsitic  lavas  are  more  irregular  in  thickness  and 
form  of  surface  (p.  222). 

Abundant  examples  of  this  tyj>e  of  volcanic  extrusion  may  be  studied  among  the 
Palieozoic  and  Tertiary  fonuations  of  Western  Euro^^e,  and  nowhere  on  a  larger  scale  than 
iu  the  British  Isles.  The  Cambrian  lavas  and  tuffs  of  Pembrokeshire,  and  those  of 
Lrenig  and  Bala  age  in  North  Wales,  the  Lake  District,  the  south  of  Scotland,  and  the 
louth-east  of  Ireland,  form  a  notable  record  of  volcanic  activity  iu  older  Palaeozoic  time. 
rhey  were  succeeded  by  the  great  out|K)uring8  of  the  Old  Red  Sandstone,  Devonian, 
CJarboniferous,  and  Permian  volcanoes.  But  the  volcanic  energy  gradually  diminished 
mtil  the  last  Carboniferous  and  Permian  eruptions  gave  rise  to  puys  like  those  of 
A>uvergne,  never  discharging  such  voluminous  floods  of  lava  as  those  of  earlier  periods, 
md  probably  in  many  cases  emitting  only  showers  of  ashes  and  stones.^  Tliere  appears 
U>  have  been  a  complete  quiescence  of  volcanic  activity  during  the  whole  of  the  Mesozoic 
iges  in  Britain.  But  the  subterranean  fires  were  rekindled  in  older  Tertiary  time,  and 
ijave  forth  the  great  basalt  sheets  of  Antrim  and  the  Inner  Hebrides. 

On  the  continent  of  Euroi>e  a  similar  long  record  of  volcanic  action  is  found,  with  a 
jorresponding  Mesozoic  quiescence.  Cambrian,  Silurian,  Devonian,  Carboniferous,  and 
Peimian  volcanic  rocks  have  l>een  found  in  France.      The  Permian  volt?anic  rocks  of 


1  Quart.  Journ,  Oeol.  iSoc,  (Auuiv.  Adtlress),  vol.  xlviii.  p.  147. 


592  GEOTEr'TifSIC  ^STRUCTURAL)  GEuLO^iY  book  it 


<i«;niiaiiy  lidve  long  }»^n  well  known  J     In  the  Tyntl  extensive  sheets  of  qoartz-ioq^iTrT 
of  Tria"»!»i«:  or  oMer  <Ut<f  with  ast^nriated  tutf«  occur.- 

Iiitei'iiedcled  (and  also  intrusive)  sheets  have  shared  in  all  the  sabs^ 
(jiient  curvature  and  faulting  of  the  formations  among  which  thev  lie. 
iliis  relation  is  well  seen  in  the  ^  loadstone  '*  or  diabase  beds  associated 
with  the  Carboniferous  Limestone  of  Derbyshire  (Fig.  304).* 

'1.  The  second  t^^pe  is  displayed  in  widespread  pUteanx  composed  of 
many  successive  sheets,  frequently  with  little  or  no  intercalation  of  tuff. 
It  occurs  even  among  Palaeozoic  formations,  but  attains  its  greatest  de- 
velopment among  the  volcanic  eruptions  of  Tertiary  time.  Instead  of 
mere  local  lenticular  patches,  these  sheets  lie  piled  over  each  other  some^ 
times  tr>  a  <leptli  of  several  thousand  feet,  and  frequently  cover  areas  of 
many  thousand  square  miles. 


ONE    MIIF 

Ki;:.  30*.  -Nrctinii  of  iuteroAlate*!  «lialjtt)ic  (toailtitime)  in  CArbcmiferou*  Liuiefitune,  Derbyihire  (£.) 
n  '/,  Tiia<lMtoii«?,  ill  two  Ijedri  ;  h  h,  lAmentonea  ;  r.  Millstone  ffit ;  ff,  Fuiltfl. 

Anioii^  the  Vahvo/jtic  rocks  of  Scotland  remnants  of  such  ancient  volcanic  platBtM 
(Kcur  in  thf  Old  Keel  Sandstone  (hills  of  lA>iiie)  and  Carboniferous  ayvtems  (Campoe 
Ft'lU  anrl  hills  above  Larp«\  where  they  consist  chiefly  of  consecutive  sheets  of  diflbmt 
iNirpliyritcs  and  dialtases  rising  into  long  terrace<l  tablelands.  The  regularity  of  thick- 
iK'ss  anrl  paralU'lisni  of  these  sheets  form  conspicuous  features  in  the  aoeneiy  of  tlw 
distrii'ts  in  whii.-h  thev  jx-i-ur. 

It  is  chieHy  btisiiltic  nnrks,  however,  that  in  all  {tarts  of  the  world  have  flowed  oot 
witliout  tlic  pnKliM.'tion  (if  pmminent  cones  and  craters,  and  now  build  up  vast  voletine 
platiaux.  The  fra^niicntary  ]^Iioceue  plateaux  of  the  British  Islands,  the  Fait>e  Idindb 
and  liflaiid  :  those  of  the  Indian  Deccan  and  of  Abyssinia,  and  the  more  recent  l«nlt 
Hoods  wliiili  have  closed  the  eventful  history  of  volcanic  action  in  North  America,  ue 
notalile  illustrations  of  this  tyjie  of  structure,  licds  of  tuft',  conglomerate,  gravel,  clij, 
•^liale,  or  other  stratified  intercalations  (K-casionally  sei>arate  the  sheets  of  basalt.  Ltyen 
of  la(.'U>trine  clays,  sometimes  full  of  leaves,  and  even  with  sufKciently  thick  masses  of 
ve;;etatii»n  to  form  l»aiids  of  lignite  or  coal,  may  also  here  and  there  be  detected.  But 
marine,  intercalations  are  rare  or  alisent.  There  can  be  no  doubt  that  these  widely  extended 
cherts  of  basalt  were  in  the  main  subaerial  outi>ourings,  and  that  in  the  hollows  of  their 
hardened  mu  laces  lay  lakes  and  smaller  i)ools  of  water  in  which  the  interstratified  sedi- 
mentary materials  were  laid  down.  The  singular  i>ersistence  of  the  basalt-beds  has  often 
been  n«»ticud.  The  same  sheet  may  l>e  followed  for  several  miles  along  the  magniticrDt 
elitt's  of  Skye  and  Mull.  Mr.  ClaRMice  King  l>elievcs  that  single  sheets  of  basalt  in  the 
Snake  Kiver  lava-tield  of  Maho  may  have  flowed  for  50  or  60  miles.^  Tlie  basalts,  how- 
ever, so  exactly  resemble  each  other  that  the  eye  may  be  deceived  unless  it  can  follows 
band  without  any  interruption  of  continuity. 

'  rwefcrences  to  the  intercalated  volcanic  ro<:ks  of  former  geological  periods  will  be  foud 
in  the  account  of  the  jreological  systems  in  Book  VI. 

-  K.  M(»jsisovics,  *  Die  Dolomit-riHe  von  Siidtirol/  1879. 

•  See  Section  18,  *  Hor.  Sec.  (Jeol.  Surv.  Great  Britain.' 

*  '(leolopcal  Kxploration  of  40th  Parallel,'  i.  p.  593.  See  also  C.  R  DuttOD,  Sattt/r, 
'17\h  NoveniUr  1 SS  \.  tjth  Ann.  R.p.  U.S,  O'coL  Surr.  1884-85,  ]>.  181,  and  Atk  Jnn,  Rty, U 
same  Survey,  1^.S2-.S3,  p.  85. 


PART  VII  SECT,  ii  §  2       INTERSTRATIFIED  TUFFS  693 

§  2.  Fragmental,  or  Tuffs. 

While  the  observer  may  be  in  doubt  whether  a  particular  bed  of  lava 
has  been  poured  out  at  the  surface  as  a  time  flow,  or  has  consolidated  at 
some  depth,  and,  therefore,  whether  or  not  it  is  to  be  regarded  as  evidence 
of  an  actual  volcanic  outbreak  at  the  locality,  he  is  not  liable  to  the  same 
uncertainty  among  the  fragmental  eruptive  rocks.  Putting  aside  the 
occasional  brecciated  structure  seen  along  the  edges  of  plutonic  intrusive 
masses,  he  may  regard  all  the  truly  fragmental  igneous  rocks  as  proofs  of 
volcanic  action  having  been  manifested  at  the  surface.  The  agglomerate 
found  in  a  volcanic  neck  could  not  have  been  formed  unless  the  vapours 
in  the  vent  had  been  able  to  find  their  way  to  the  surface,  and  in  so  doing 
to  blow  into  fragments  the  rocks  on  the  site  of  the  vent  as  well  as  the 
upper  part  of  the  ascending  lava-column.^  Wherever,  therefore,  a  bed  or 
a  series  of  beds  of  tuff  occurs  interstratified  in  a  geological  formation,  it 
points  to  contemporaneous  volcanic  eruptions.  Hence  the  value  of  these 
rocks  in  interpreting  the  volcanic  annals  of  a  region. 

The  fragmentary  ejections  from  a  volcano  or  a  cooling  lava-stream 
vary  from  the  coarsest  agglomerate  to  the  finest  tuff,  the  coarser 
materials  being  commonly  found  nearest  to  the  source  of  discharge. 
They  differ  in  composition,  according  to  the  nature  of  the  lavas  with 
which  they  are  associated  and  from  which  they  have  been  derived.  Thus, 
a  region  of  trachyte-lavas  supplies  trachyte-tuffs  and  trachyte-breccias  ; 
one  of  basalts  gives  basalt-breccias,  basalt-agglomerates,  basalt-tuffs ;  one 
of  obsidians  yields  pumiceous  tuffs  and  breccias.  The  fragmentary 
matter  ejected  from  volcanic  vents  has  fallen  partly  back  into  the  funnels 
of  discharge,  partly  over  the  surrounding  area.  It  is  apt,  therefore,  to  be 
more  or  less  mingled  with  ordinary  sedimentary  detritus.  We  find  it, 
indeed,  passing  insensibly  into  sandstone,  shale,  limestone,  and  other 
strata.  Alternations  of  gravelly  peperino-like  tuff  with  a  very  fine-grained 
"  ash  '*  may  frequently  be  obsei-ved.  Large  blocks  of  lava-form  rock,  as 
well  as  of  the  strata  through  which  the  volcanic  explosions  have  taken 
place,  occur  in  the  tuffs  of  most  old  volcanic  districts.  Occasionally  such 
ejected  blocks  or  bombs  are  found  among  fine  shales  and  other  strata,  the 
lamination  of  which  is  bent  down  round  them  in  such  a  way  as  to  show 
that  the  stones  fell  with  considerable  force  into  the  still  soft  and  yielding 
silt  or  clay  (Fig.  305).2 

Volcanic  tuffs  and  conglomerates  occur  in  interstratified  beds  without 
any  accompanying  lava,  much  more  commonly  than  do  interstratified 
sheets  of  lava,  without  beds  of  tuff;  just  as  in  recent  volcanic  districts,  it 
is  more  usual  to  find  cones  of  ashes  or  cinders  without  lava,  than  lava- 
sheets  without  an  accompaniment  of  ashes.  Masses  of  fine  or  gravelly 
tuff,  several  hundreds  of  feet  in  thickness,  without  the  intervention  of  any 
lava-bed,  may  be  observed  in  the  volcanic  districts  of  the  Old  Red  Sancl- 

'  It  is  conceivable  that  where  a  mass  of  lava  was  injected  into  a  subterranean  cavern, 
fragmentary  discharges  might  take  place  and  partly  fill  that  cavity  ;  but  such  exceptional 
are  probably  extremely  rare. 
«  See  OeoL  Mag.  I  (1864),  p.  22. 

2Q 


GEOTECTOXIC  (STSVCTURAL)  GKOLOOY 


Btone  and  Carboniferous  systems  in  Scotland.  These  funuBh  eridenee  of 
long^ontinued  voicanic  action,  during  which  fragmentarj-  materials  were 
showered  out  over  the  water-basins,  mingled  with  little  or  no  ordtnarr 


— EJrclnl  vulcuii 


1.  Pniyeur.  P 


sediment.  On  the  other  hand,  in  these  same  areas,  thin  scams  of  tnff 
interlaminatcd  with  sandstone,  shale,  or  limestone,  afford  indications  of 
feeble  intei-mtttent  volcanic  explosions,  whereby  light  shovers  of  dust  were 
discharged,  which  settled  down  quietly  amidst  the  sand,  mud,  or  limestont 
accumulating  at  the  time.  Under  these  latter  circumstances,  toffs  often 
become  fossiliferous ;  they  enclose  the  remains  of  such  plants  and  »ni"«l' 
as  might  be  lying  on  the  lake-bottom  or  sea-floor  over  which  the  shoven 
of  volcanic  dust  fell,  and  thus  they  form  a  connecting  link  between  aqueous 
and  igneous  rocks. 


-p  of  tliK  stntigraphic&l  evidence  for  former  coaditiou  of 


Ak  till 

volcanic  avtivitj'; 


of  the  1 


u  LililithgntrKhire  may  liere  be  given.  In  the  tint  of 
iiwne  (Fig.  30tj),  a  blark  shale  (1)  of  the  luna! 
I'arboiiaceous  tyi>e,  witli  renisina  of  trrrti'tml 
[ilunts,  [iv9  at  tlje  bottom.  It  is  covered  bj  ■  bol 
nf  nodular  Uluish.grey  tuff  (S),  coDtaining  black 
sliale  fragiDf-nts.  whence  we  omj  infer  that  ibe 
unilei'lyiiig  or  some  similar  shale  was  blown  out  fno 
(he  site  of  the  vent  that  furnished  this  dust  ind 
gravel.  A  N^oiid  black  sbaln  (3)  is  HUcctvdHl  I7  > 
second  thin  baud  of  tine  (lale  yellowiah  toff  (4^ 
BlaL'k  shale  (5)  again  supervenes,  containing  roanM 
fi-agmeuts  of  tutf,  perhajw  lapilli  intermittentl; 
ejected  from  the  neighbouring  vent,  and  paaoifl 
up  into  a  layer  of  tuff  (6),  which  maiks  hw 
I  the  volcanic  activity  gradually  incre^ard  tffiD. 
It  is  (evident  that,  but  for  the  proximity  of  ai 
^  active  volcanic  vent,  there  would  have  been  a  era- 

tiuuoua  deposit  of  block  shale,  tlie  conditioD*  of 
inn  having  remained  uncliangeil.  In  the  next  stratum  of  shale  (i],  llitn 
s>'am)i  anil  ixHhiles  of  clay -ironstone  accumulated  touiid  clecompoaiDg  organic  remaint  cm 
the  muddy  liotloni.  A  brief  voloanic  explmion  is  marked  by  the  thin  tulT-bed  (8),  afttr 
which  the  old  conditions  of  dei«sit  continued,  the  Iwtlom  of  the  water,  as  the  shall  ()) 
sliuwH.  Ivinj;  cll>wded  with  nslracod  cruHtaceans,  while  Ushes,  whose  co|>rolitcs  have  bet* 
left  in  the  mild,  haunteil  tlie  locality.  At  last,  however,  a  much  more  powerful  and  pco- 
lou^eil  vulcanic  explosion  look  jilace.  A  curse  sgglomerate  or  tuff  (10),  with  blocti 
sonietiiTics  tiearly  a  foot  iu  diameter,  was  then  throwu  out  and  overspread  the  lagiwD, 


METAMOEPHISM 


The  aecoiid  example  (Fig.  307}  brings  before  tbe  n 
kiuil,  ill  tbe  history  of  the  eanie  region.  At  the  bot- 
tom of  tlie  section,  a  pale  amygdaloidal,  somewhat 
■Itprtd  form  of  basalt  (A)  marks  tlie  upper  surface 
of  one  of  tbe  submarine  lavas  of  the  CarboitifFrou-s 
Limestone  period.  Directly  over  it  comes  a  bed  of 
limestone  (B)  15  feet  thick,  the  lower  layera  of  which 
are  mode  up  of  a  dense  growth  of  the  thin-stemmed 
coral,  Lithtatrolion  irreffviaTe,  which  overspread  the 
hardened  lava.  The  next  stratum  is  a  band  of  dark 
■hale  (C),  about  2  feet  thick,  followed  by  about  tbe 
same  tbickiiCBB  of  an  impure  limeatone  with  shale 
seams.  The  conditions  for  coral  growth  were  evi- 
dently not  favourable ;  for  tbe  deposit  of  IhLi 
argillaceous  limestone  was  arrested  by  the  precipita- 
tion of  a  dark  mud.  now  to  be  seen  in  the  form  of  3 
or  4  iuches  of  a  black  pyritous  shale  (E).  and  next 
hy  the  inroad  of  a  largo  quantity  of  a  dark  sandy 
mud,  and  drift  vegetation,  which  ha»  been  preserved 
OS  a  Bandy  shale  (F)  containing  CalamiUs,  Proditcti, 
ganoid  scales,  and  otiier  tracer  of  the  terrestrial  and 
marine  life  of  the  time.  Finally  a  sheet  of  lava, 
represented  by  the  uppermost  amygdaloid  (0),  over- 
spread the  area,  and  sealed  up  these  records  of  Pahv- 
owic  history.' 


volcanic  ejrisode  of  another 


Part  VIII.  Metamorpkism,  Local  and  Regional. 

At  ttie  outset  some  cuiition  must  be  employed  as  to  the  use  of  the 
terms  "  meUmorphism  "  and  "  metamorphic."  It  is  obvious  that  we  have 
no  right  to  call  a  rock  metamorphic,  unless  we  ciin  distinctly  truce  it  into 
an  unaltered  condition,  or  can  show  from  its  internal  composition  and 
■tnicture  that  it  has  undergone  a  definite  change,  or  can  prove  its  identity 
with  some  other  rock  whose  metamorphic  character  has  been  satisfactorily 
established.  Further,  it  must  be  remembered  that,  in  a  certain  sense,  all 
or  nearly  all  rocks  may  be  said  to  have  been  metamorphosed,  since  it  is 
exceptional  to  find  any,  not  of  very  modem  date,  which  do  not  show, 
when  closely  examined,  proofs  of  having  been  hardened  by  the  pressure 
of  superincumbent  rock,  and  altered  by  tbe  action  of  percolating  water  or 
other  daily  acting  agent  of  change.  Even  a  solid  crystalline  mass,  which, 
when  viewed  on  a  fresh  fracture  with  a  good  lens,  seems  to  consist  of 
unchanged  crystalline  particles,  will  often  betray  under  the  microscope 
unmistakable  evidence  of  alteration.  And  this  alteration  may  go  on 
until  the  whole  internal  organisation  of  the  rock,  so  far  at  least  as  we  can 
penetrate  into  it,  has  been  readjusted,  though  the  external  form  may  still 
remain  such  as  hardly  to  indicate  the  change,  or  to  suggest  that  any  new 
name  should  be  given  to  the  recomposed  rock.    Among  many  igneous  rocks. 


ra  of  Geol.  Survey,  Geologf  of  liilipburgh,'  pp.  4S,  9 


Traat.   Sog,  Sor. 


596  (iEOTEUTuNlC  {STRUCTURAL)  GEOLOGY  book  iv 

]>articularly  the  more  basic  kinds  (diabases,  basalts,  andesites,  diorites, 
olivine  rocks,  Sec),  alteration  of  this  nature  may  be  studied  in  all  stages.^ 

But  mere  alteration  by  decay  is  not  what  geologists  denote  by  meta- 
mor|)hisni.  The  term  has  been,  indeed,  much  too  loosely  employed :  but 
it  is  now  generally  used  to  expi-ess  a  change  in  the  mineralogical  or 
chemical  com])osition  and  in  the  internal  structure  of  rocks,  produced  at 
some  depth  from  the  surface,  through  the  operation  of  mechanical  move- 
ment, combined  with  the  influence  of  heat  and  heated  water  or  vapoiu*. 
A  metamorphic  rock  may  be  more  com])act  and  cr}'stal]ine  than  the 
parent  mass  from  which  it  has  been  derived,  like  which,  also,  when 
ex])osed  at  the  surface,  it  again  undergoes  alteration  by  weathering. 

Various  kinds  of  metamorphism  have  been  distinguished  by  special 
names ;  -  but  they  may  be  included  in  three  main  groups.  Ist,  change 
of  texture,  including  the  induration  and  other  minor  phenomena  of 
'' con  tact- metamorphism '' ;  2nd,  change  of  form,  including  all  para- 
m()q)hic  transformations,  such  as  the  conversion  of  a  pyroxenic  into  a 
hornblendic  rock,  and  the  alteration  of  a  clastic  into  a  crystalline  mass  by 
the  crystalliz!ition  of  its  original  constituents ;  3i*d,  change  of  substance, 
where  a  clicmical  change  has  been  su])erinduced  either  by  the  abstraction 
or  addition  of  one  or  more  ingredients,  as  in  the  remarkable  contact  zones 
round  ceitain  intrusive  bosses.  It  is  obvious,  however,  that  each  of  theK 
tlireo  kinds  of  metamorphism  may  be  included  in  the  changes  which  have 
been  superinduced  upon  a  given  mass  of  nx;k. 

TIio  conditions  that  appear  to  be  mainly  concerned  in  metamorphism 
have  been  already  stated  (p.  319).  It  may  Ije  added  here  that  these 
conditions  may  in  different  cases  be  supplied  :  1st,  by  the  action  of  heated 
subterranean  water  carrying  carbonic  acid  and  mineral  solutions  (p.  305) ; 
2nd,  by  the  action  of  hot  vapours  and  gases  upon  underground  rocks 
(j)p.  228,  305,  589);  3rd,  by  mechanical  movements,  particularly  those 
which  have  resulted  in  the  crushing  and  shearing  of  rocks  (p.  311);  4tfa, 
by  the  intrusion  of  heated  eruptive  rocks,  sometimes  containing  a  laige 
proportion  of  absorbed  water,  vapours,  or  gases  (pp.  230,  568,  572,  576); 
5  th,  occasionally  and  very  locally  by  the  combustion  of  beds  of  coal. 

^  See  Index,  svh  roc,  '*  Weatlierinjr. '* 

'  For  instance,  m*'0'SOfnatiai\  mftttsomaiu\  methi/ltmSf  methylotic,  and  mtt€tiehemic  ^^ilM 
to  olieinical  inetiiniorphism  or  alteration  of  constitution  or  substance  ;  metastasis,  indiotiaf 
changes  of  u  paraniorpliio  natun* ;  inetacrasi\  denoting  such  transformations  aa  the  coonr 
siou  of  mud  into  a  mass  of  mica,  quartz,  aud  other  silicxites  ;  macro^ructural  metamorpbMB, 
having  the  external  structure  (morphology)  changed,  a.s  where  an  amorphooB  condiUoB 
beiromes  scliistose ;  inirro- struct uriiiy  having  the  internal  structure  (bistology)  whoUf 
oh.in^'ed,  Avith  or  without  a  macro-Ntruetural  .ilteratiou  ;  mincnUofficaif  having  one  or  moR 
of  the  r^dniponent  minerals  changed,  with  or  without  an  alteration  of  the  chemical  conpoii* 
tion  of  the  rock  sls  a  wliole.  See  King  and  Rowney  '*  An  old  Chapter  of  the  Geokigkil 
lieconl,*'  1881  ;  Dana,  Amrr.  Jount.  JSci.  xxxii.  (1886),  p.  69.  Bonney,  ^tiart,  Jouhl 
f't?o/.  Sh\  (1886),  Address,  p.  30  et  seq.  (1.  H.  Williams,  BhU,  l\S,  Oeol.  Sure,  So,  ^ 
(1890),  ]).  43.  Various  terms  have  likewise  been  proiHMed  for  metamorphism  fhmi  fhc 
point  of  view  of  its  cause,  us  Dutloi'.ation-metiunorphism  (Lossen),  Mechanical  wuiaimoiifkim 
(Ileiin  nn<l  Baltzer),  Injnamiral  metamorphism  (Rosenbusch),  neapiny-vp  fm^awtorpkum 
{Stuiiuiujis  M.  Credner),  Pressure  metamorphism  (Bouney). 


PART  viii  S  i  LOCAL  METAMORPHISM  597 

When  the  term  "  metamorphism  "  was  originally  proposed  by  Lyell  it 
applied  to  rocks  having  a  schistose  or  foliated  structure  which  were 
regarded  as  altered  sediments.  For  many  years  afterwards  it  continued 
to  be  used  in  the  same  sense,  and  not  until  comparatively  recently  did 
geologists  recognise  that  rocks  originally  of  eruptive  origin  but  interposed 
amopg  sedimentary  strata,  were  necessarily  aifected  by  the  changes  which 
the  latter  underwent  in  the  processes  of  metamorphism.  It  is  now  well 
established  that  igneous  rocks  no  less  than  aqueous  have  been  metamor- 
phosed, and,  as  Lossen  has  pointed  out,  they  furnish  in  some  respects  even 
a  better  starting-point  from  which  to  attack  the  problem  of  metamor- 
phism, inasmuch  as  their  original  definite  mineral  aggregation,  chemical 
composition,  and  structure  furnish  a  scale  by  which  the  subsequent  muta- 
tions of  the  rocks  may  be  traced  and  measured.^ 

Metamoq)hism  is  manifested  in  two  distinct  phases.  1st,  Local  (the 
metamorphism  of  contact  or  of  juxtaposition),  where  the  change  has  been 
effected  only  within  a  limited  area,  round  some  eruptive  mass,  beyond 
which  the  ordinary  condition  of  the  altered  rocks  can  be  seen.  2nd, 
Kegional,  where  the  change  has  taken  place  over  a  large  tract  without 
reference  to  visible  eruptive  masses,  the  original  characters  of  the  altered 
rocks  being  more  or  less  completely  effaced.  Between  the  results  of  local 
and  regional  metamorphism,  no  sharp  line  can  be  drawn ;  they  insensibly 
graduate  into  each  other  and  may  arise  from  one  common  cause. 

§  i.  Local  Metamorphism  (metamorphism  of  contact 

or  juxtaposition). 

In  this  kind  of  alteration  two  fundamental  conditions  have  to  be 
considered  :  Ist,  the  nature,  mass,  temperature,  and  composition  of  the 
eruptive  rock  ;  and  2nd,  the  composition  and  structure  of  the  rocks  through 
which  the  intrusive  material  has  been  injected.  With  regard  to  the  first 
of  these  conditions,  it  is  obvious  that  a  large  intrusion  will  produce  more 
alteration  than  a  small  intrusion  of  the  same  rock.  The  areole  of  meta- 
morphism round  a  great  boss  of  granite  or  of  diorite  will  be  broader  and 
the  metamorphism  itself  more  intense  than  round  a  mere  vein  or  dyke. 
But  the  case  is  different  when  we  compare  intrusions  of  altogether  unlike 
materials.  The  temperature  of  granite  a])pears  to  have  been  comparatively 
low  (p.  308).  We  never  meet  with  cases  of  fusion  round  even  the  largest 
bosses  of  granite ;  carbonate  of  lime  is  not  deprived  of  its  carbonic  acid. 
Bat  the  injections  of  intermediate  and  basic  rocks  give  proofs  of  far  more 
elevated  temperatures.  Dykes  of  andesite  or  basalt  may  often  be  observed 
to  have  baked  argillaceous  rocks  into  porcellanite,  and  to  have  actually 
fused  the  rocks  in  contact  with  them.  But  in  these  instances  the  altera- 
tion is  confined  within  limits  of  a  few  inches  or  feet.  The  metamorphism 
induced  round  a  boss  of  granite,  on  the  other  hand,  may  extend  for  a 
breadth  of  a  mile  or  more.     Much  of  the  change  in  the  latter  case  may 

*  Jahrb,  Preuss.  Oeol,  Larideaanst.  1884,  p.  020.  See  also,  for  an  early  study  of  the  in- 
llnance  of  contact'inetaniorpliisni  on  augitic  igneoas  rockn,  Allport,  Q.  J.  Oeol,  Snc.  xxxii. 
(1876),  p.  418. 


598  aEOTECTuXIC  {STRUCTURAL)  GEOLOGY  book  iv 


Ije  ascribed  to  the  influence  of  the  mineralizing  agents  with  which  the 
granite  was  impregnated  (see  p.  308). 

With  respect  to  the  influence  of  the  nature  and  structure  of  the  altered 
rock  upon  the  metamorphism,  it  is  obvious  that  such  different  materiils 
as  shale,  sandstone,  coal,  and  limestone,  will  give  very  different  results  even 
if  exposed  to  the  same  amount  and  kind  of  metamorphic  energy.  ^AjkI 
much  will  depend  also  upon  the  relation  between  the  position  of  the 
intrusive  mass  and  the  stratification  of  the  rocks  affected.  As  stated  on 
p.  52,  heat  is  conducted  four  times  faster  along  the  planes  of  stratifica- 
tion than  across  them,  so  that  an  intnided  sheet  or  sill  should,  other 
things  being  equal,  produce  less  alteration  than  a  boss  which  breaks  across 
the  bedding. 

The  following  examples  of  the  nature  of  the  metamoiphism  of  contact 
are  arranged  in  progressive  order  of  intensity,  beginning  with  the  feeblest 
change,  and  ending  with  results  that  are  quite  comparable  with  the  great 
changes  involved  in  regional  metamorphism. 

Bleaching  is  well  seen  at  the  surface,  where  heated  volcanic  va]K)urs 
rise  through  tufl's  or  lavas  and  convert  them  into  white  clays  (p.  233). 
Decoloration,  however,  has  proceeded  also,  underneath,  along  the  sides  of 
dykes.  Thus  in  AiTan,  a  zone  of  decoloration  ranging  from  5  or  6  to 
25  or  30  feet  in  width,  runs  in  the  red  sandstone  along  each  side  of 
many  of  the  abundant  basalt-dykes.  This  removal  of  the  colouring  peroxide 
may  have  been  eff*ected  by  the  prolonged  escape  of  hot  vapours  from  the 
cooling  lava  of  the  dykes.  Had  it  been  due  merely  to  the  reducing  eflect 
of  organic  matter  in  the  meteoric  water  filtering  down  each  side  of  the  djke, 
it  ought  to  occur  as  frequently  along  joints  in  which  there  has  been  no 
ascent  of  igneous  matter. 

Ck)loration. — Kocks,  i)articularly  shale  and  sandstone,  in  contact  wth 
intrusive  sheets,  are  sometimes  so  reddened  as  to  resemble  the  burnt 
shale  from  an  ironwork.  Every  case  of  reddening  along  a  line  of  junction 
between  an  eruptive  and  non-eruptive  rock  must  not,  however,  be  set  down 
without  examination  as  an  eflect  of  the  mere  heat  of  the  injected  mas^ 
for  sometimes  the  colouring  may  be  due  to  subsequent  oxidation  of  iron  in 
one  or  both  of  the  rocks  by  water  percolating  along  the  lines  of  contact. 

Induration. — One  of  the  most  common  changes  superinduced  upon 
sedinifHtary  rocks  along  their  contact  with  intnisive  masses,  is  a  harden- 
ing of  their  substance.  Sandstone,  for  example,  is  converted  into  i 
coni])act  rock  which  breaks  with  the  lustrous  fracture  of  quartzite. 
Argillaceous  stnita  are  altered  into  flinty  slate,  Lydian-stone,  jasper,  or 
porcellanite.  This  change  may  sometimes  be  produced  by  mere  dry  heat, 
as  wheu  clay  is  baked.  But  probably,  in  the  majority  of  cases,  induration 
of  subterranean  rocks  results  from  the  action  of  heated  water..  The  most 
obvious  examples  of  this  action  are  those  wherein  the  percentage  of  silica 
has  been  increased  by  the  deposit  of  a  siliceous  cement  in  the  interstices 
of  the  stone,  or  by  the  re])la cement  of  some  of  the  mineral  substances  by 
silica.  This  is  specially  observable  round  eruptive  masses  of  granite  and 
diabase.^ 

'  Kayser,  on  contact -inetainor]>lii»ni  uround  the  diabase  of  the  Harz,  Z.  DeulMCk.  Geel, 


PABT  VIII  S  i 


LOCAL  METAMORPffISM 


Expulsion  of  Water. — One  effect  of  the  intrusion  of  molten  matter 
among  the  ordinary  cool  rocks  of  the  earth's  crust  has  doubtless  often 
been  temporarily  to  expel  their  interstitial  water.  The  heat  may  even 
have  been  occasionally  sufficient  to  drive  off  water  of  crystallization  or  of 
chemical  combinatiou.  Mr.  Sorby  mentions  that  it  has  been  able  to 
dispel  the  water  present  in  the  minute  fluid  cavities  of  quartz  in  a  sand- 
stone invaded  by  diabase.* 

Prismatic  Strueture. — Contact  with  eruptive  rocks  has  frequently 
produced  a  prismatic  structure  in  the  contiguous  masses.  Conspicuous 
illustrations  of  this  change  are  displayed  in  sandstones  through  which 
dykes  have  risen  (Fig.  308).-  Independently  of  the  lines  of  stratification, 
polygonal  prisms,  six  inches  or  more  in  diameter,  and  several  feet   in 


b}-.  nLBhopbrl){|;a,  GUigow. 


length,  starting  from  the  face  of  the  dyke,  have  been  developed  in  the 
sandstone.^ 

Same  of  the  luost  (lerfM^t  examples  of  aupcriuduc«l  iirUms  may  occ&sioually  be 
noticed  in  seaiiiH  of  coal  vliicli  liave  bwn  invaded  lij  intrusive  igneous  roclu,  la 
the  Kcottisii  roal-lields.  sheets  of  basalt  have  iMeu  forced  alou^  tlie  surfaces  of  omI- 
■caiDH,  and  even  along  their  centre,  so  as  t«  form  a  bed  or  sheet  in  the  middle  of  th« 
ooal'Seaui.  The  coat  in  tliese  liases  is  sometimes  tieautifiilly  columnar,  its  alender 
lieiagonal  and  iieiitagona)  jiriBin^  like  rows  of  stout  jiencilH.  diverging  from  the  snrfaca 
of  the  iutrnsive  slicet.' 

Oei.  »ii.  103,  where  annlyiiEii  showing  the  high  percentage  of  silica  are  given.  Hawes, 
Amtr.  Joum.  Sci.  January  1881.  Tlie  jihenoniena  of  iiietainor|>bi*in  roand  granite  are 
further  descril>ed  below,  p.  60[>  in;. 

'  q.  J.  (Jh>1.  Soc.  1880.     AnI,,  p.  67S. 

3  Sanibtone  altortil  by  iHualt,  nielaphyre,  or  allinl  rock,  Wildenstein,  near  Bttdingen, 
Upper  HeAse,  Snbtiberle,  near  KrIebiU,  Bobeniiu  ;  .Tobnadorf,  Dear  Zitlan,  Saiony  (tbe 
Qoader- sandstone  of  Gorisohstein,  in  8aion  Switzerland,  in  btnutifidly  columnar;  W. 
Keeping,  HeoL  Maij.  1870,  p.  437)  ;  BiabopbriggH,  upsr  Oloigow. 

'  Coal  and  lignite,  with  their  accompanying  I'lnys,  altered  by  basalt,  diabase,  mela- 
pbjre,  lie.,  Ayrslilre,  Scotland  ;  St.  Satuniin,  Aiivergne ;  Meissner,  Hemic  C^nel ; 
EtUngsbaoMn,  Vogelngebirge ;  Snlibacli,  Upper  PataCiiiale  of  Bavaria ;  PiinflciTchen, 
Hnngarjr :  by  tracliyt«,  Comraeutry,  Central  Prauce  ;  by  pbooolite,  Northern  Bavaria. 


600  GEOTECTONIG  (STRUCTURAL)  GEOLOGY  book  iv 

Other  examples  of  the  production  of  this  structure  have  been  described  in  dolomite 
altered  by  quartz  -  porphyry  (Canipiglia,  Tuscany) ;  fresh -water  limestone  altered  by 
basalt  (Oergovia,  Auvergno) ;  basalt -tuff  and  granite  altered  by  basalt^  (Mt,  Saint- 
Michel,  Lo  Puy). 

Calcination,  Melting,  Coking:.^ — By  the  great  heat  of  erupted 
masses,  more  especially  of  basalt  and  its  allies,  rocks  have  been  calcined 
and  partially  or  completely  melted.  In  some,  the  matnx  or  some  of  the 
component  minerals  have  been  melted ;  in  others  die  whole  rock  has 
buun  fused.  Among  granite  fragments  ejected  with  the  slags  of  old 
volcanic  vents  in  Auvergne,  some  present  no  trace  of  alteration,  others 
are  burnt  as  if  they  had  been  in  a  furnace,  or  are  partially  melted  so  as 
to  look  like  slags,  each  of  their  component  minerals,  however,  remaining 
distinct.  In  the  Eifel  volcanic  region,  the  fragments  of  mica-schist  and 
gneiss  ejected  with  the  volcanic  detritus  have  sometimes  a  crust  or  glaze 
of  glass.  Sandstones,  though  most  frequently  baked  into  a  compact 
quartzit^,  are  sometimes  changed  into  an  enamel -like  mass  in  which, 
when  the  rock  contains  an  argillaceous  or  calcareous  matrix  with 
dispersed  quartz-grains,  the  infusible  quartz  may  be  recognised  (Oberel- 
lenbach,  Lower  Hesse).  According  to  Bunsen's  observations,  volcanic 
tutr  and  ]>honolite  have  sometimes  been  melted  for  several  feet  on  the 
sides  of  the  dolerite  dykes  which  traverse  them,  so  as  to  present  the 
asj>ect  of  pitchstone  or  obsidian.^  Besides  complete  fusion  and  fluxion- 
structure  there  has  sometimes  l>een  also  a  production  of  ^  microscopic 
crystallites  in  the  fused  portions,  resembling  those  of  eruptive  rocka 

The  effects  of  eruptive  rocks  upon  carbonaceous  beds,  and  particularly 
upon  coal-seams,  are  among  the  most  conspicuous  examples  of  this  kind 
of  alteration.  In  a  coal-field  much  invaded  by  igneous  rocks,  seams 
of  coal  are  usually  found  to  have  suffered  more  than  the  other  strata, 
not  merely  because  they  arc  specially  liable  to  alteration  from  the 
j)roximity  of  heated  surfaces,  but  because  they  have  presented  lines 
of  more  easy  escape  for  the  igneous  matter  pressed  from  below.  The 
molten  rock  has  very  generally  been  injected  along  the  coal-seams: 
sometimes  taking  the  lower,  sometimes  the  upper  surface,  or  even,  as 
already  stated,  forcing  its  way  along  the  centre. 

The  alterations  produced  l)y  the  intrusion  vary  considerably,  accord- 
ing to  the  bulk  and  nature  of  the  eruptive  sheet,  the  thickness, 
composition,  and  structure  of  the  coal-seam,  and  probably  other  causes. 
In  some  cases,  the  coal  has  been  fused  and  has  acquired  a  blistered  or 

'  Naumunii,  *  Geogiiosiu,'  i.  p.  737. 

*  It  is  ^vorthy  of  obsen-atiou  that  clianges  of  the  kind  here  referred  to  occur  mo4 
conimonly  with  hasali-rncks,  mela]thyreR,  and  diabasus.  Trachyte  has  been  a  lens  frequent 
a^eut  of  alteration,  though  some  remarkable  exampIcA  of  its  influence  have  been  noted. 
Toulett  8tToi>c  {Oenl,  Trans.  Slid  ser.  ii.)  describe.s  the  alteration  of  a  trachyte  conglomentr 
>iy  trachyte  into  a  vitreous  maH.s.  Quartz-porphyry  and  diorite  occasionaUy  present  ezam^es 
of  calcination,  or  more  or  less  complete  fusion.  But  with  the  granitic  and  syeuitic  rocks 
changes  of  this  kiml  have  never  been  ol^erved.     Naumanu,  '  Geoguosie,'  i.  p.  744. 

^  Usually  tile  vitreous  band  at  the  margin  of  a  Imsalt  dyke  belongs  to  the  intruded 
rotk  and  not  to  that  tlirough  w^hich  it  has  risen  (see  "Basalt-glass,'*  ante,  pp.  171,  584;. 


PART  VIII J5  i  .  LOCAL  METAMORPHISM  601 

vesicular  texture,  the  gas  cavities  being  either  empty  or  filled  with  some 
infiltrated  mineral,  especially  calcite  (east  of  Fife).  In  other  examples, 
the  coal  has  become  a  hard  and  brittle  kind  of  anthracite  or  **  blind 
coal,"  owing  to  the  loss  of  its  more  volatile  portions  (west  of  Fife). 
This  change  may  be  observed  in  a  coal-seam  6  or  8  feet  thick,  even 
at  a  distance  of  50  yards  from  a  large  dyke.  Traced  nearer  to  the 
eruptive  mass,  the  coal  passes  into  a  kind  of  pyritous  cinder,  scarcely 
half  the  original  thickness  of  the  seam.  At  the  actual  contact  with 
the  dyke,  it  becomes  by  degrees  a  kind  of  caked  soot,  not  more  perhaps 
than  a  few  inches  thick  (South  Staffordshire,  Ayrshire).  Coal  altered 
into  a  prismatic  substance  has  been  above  (p.  599)  referred  to ;  it  has 
even  been  changed  into  graphite  (New  Cumnock,  Ayrshire,  see  Fig.  301). 
Striking  as  is  the  change  produced  by  the  intrusion  of  basalt  into 
coals  and  bituminous  shales,  it  is  hardly  more  conspicuous  than  the 
alteration  effected  on  the  invading  rock.  A  compact  crystalline  black 
heavy  basalt  or  diabase,  when  it  sends  sheets  and  veins  into  a  coal  or 
highly  carbonaceous  shale,  becomes  yellow  or  white,  earthy,  and  friable, 
loses  weight,  ceases  to  have  any  apparent  crystalline  texture,  and,  in 
short,  passes  into  what  would  at  first  unhesitatingly  be  pronounced  to  be 
mere  clay.  It  is  only  when  the  distinctly  intrusive  character  of  this 
substance  is  recognised  in  the  veins  and  fingers  which  it  sends  out,  and 
in  its  own  irregular  course  in  the  altered  coal,  that  its  true  nature  is 
made  evident.  Microscopical  examination  shows  that  this  "  white-rock  " 
or  "  white-trap "  is  merely  an  altered  form  of  some  diabasic  or  basaltic 
rock,  wherein  the  felspar  crystals,  though  much  decayed,  can  yet  be 
traced,  the  augite,  olivine,  and  magnetite  being  more  or  less  completely 
changed  into  a  mere  pulverulent  earthy  substance.^  Traces  of  the  glassy 
selvage  of  contact  may  still  sometimes  be  detected  in  these  altered  rocks. 
The  changes  in  the  constitution  of  an  igneous  mass,  owing  to  the 
surrounding  rocks,  is  referred  to  at  p.  571. 

^  The  following  analyses  show  the  composition  of  these  "white  rock-traps."  No.  I.,  by 
Henry,  is  from  the  South  Staffordshire  coal-field  ('The  South  Staffordshire  Coal- Field,'  in 
Mem.  Qeol.  Survey ^  p.  118);  No.  11.,  by  E.  Stecher,  is  from  Newhalls,  Queensferr}*, 
Linlithgowshire.  {THchemmlcs  Mittheil,  ix.  (1887),  p.  190.  Proc.  Roy.  Stw.  Ktiin.  1888, 
p.  172.  These  memoirs  of  Dr.  Stecher  give  an  account  of  the  contact  phenomena  round  the 
introslTe  diabases  of  the  Carl>oniferous  series  in  the  Imsin  of  the  Firth  of  Forth.) 


L 

II. 

Silica 

38-830 

36-8 

Alumina 

13-2.50 

22-95 

Lime 

3-925 

9-73 

Magnesia    . 

4180 

2-85 

Soda 

0-971 

0-5 

Potash 

0-422 

11 

Iron  protox. 

13-830 

4-08  FeO 

Iron  perox. 

4-335 

2-6    TiO. 

Carbonic  acid 

9-320 

11-9 
0-75  P2O5 

Water 

11010 

7-7 

100-073  100-96 


602  GEOTECTONIC  (STRUCTURAL)  GEOLOGY  book  iv 


Coke. 

BitiiineiL 

79-7 

20-3 

87-8 

12-2 

95*3 

4-7 

Tlie  ])aitalt  of  MoLssiier  (Lower  Hcsae)  ovcrlieH  a  thick  fltrstum  of  brown  coal  which 
shows  an  interesting  iwries  of  alterations.  Immediately  under  the  igueoiui  rock,  a  thin 
seam  of  impure  earthy  coal  ("letteu")  api)ears  as  if  completely  burnt.  The  nrxt 
underlying  stratum  has  been  altered  into  nietallic-lustred  anthracite,  passing  downwardi 
into  various  black  glossy  coals,  beneath  which  the  brown  coal  is  worthless.  The  de|«li 
to  which  the  alteration  extends  is  5'3  n)otres.^  Another  example  of  alteration  hu 
l>ecn  descril)ed  by  O.  vom  Rath  from  Fiinfkirchen  in  Hungary.'  A  coal-seam  \m 
there  ])een  invaded  by  a  basic  igneous  rock  (i)erha])s  diabase)  now  so  decomposed  that 
its  tnie  lithological  character  cannot  be  satisfactonly  detennined.  Here  and  there,  the 
intrusive  rock  lies  concordantly  with  the  stratification  of  the  coal,  in  other  places  it 
sends  out  fingers,  ramifies,  abniptl}'  ends  off,  or  occurs  in  detached  nodular  fragmenti 
in  the  coal.  The  latter,  in  contact  i^ith  the  inti-usivc  material,  is  converted  into 
prismatic  coke.  The  analysis  of  three  specimens  of  the  coal  throw's  light  on  the  nature 
of  the  change.  One  of  these  (A)  shows  the  ordinaiy  com]X)sition  of  the  coal  at  a 
distance  from  the  influence  of  the  intrusive  rock  ;  the  second  (B),  taken  from  a  distance 
of  about  0*3  metre  (nearly  1  foot),  exhibits  a  |)ai-tial  conversion  into  coke  ;  while  iu  the 
third  (C),  taken  from  immediate  contact  with  the  eniptive  mass,  nearly  all  the  volatile 
hydrocarbons  have  been  expelled. 

Axh.  Sulphur. 
A.     8-29  j>er  cent.  2*074 

K     9-73        „  1-112 

C.   45-96        „  0-151 

During  the  subterranean  distillation  arising  from  the  destruction  or  alteration  of  eoal 
and  bituminous  shales,  while  the  gases  cvolve<l  find  their  way  to  the  surface,  the  liquid 
prcNhicts.  (m  the  other  hand,  are  apt  to  collect  iu  fissures  and  cavities.  In  centnl 
Scotland,  where  the  coal -fields  have  been  so  abundantly  pierced  by  igneous  masses. 
IMftrohuim  and  asphaltum  arc  of  frequent  o<rcurrencc,  sometimes  in  chinks  and  veins  of 
sandstones  and  other  sedimentary  strata,  sometimes  in  the  cavities  of  the  igneous  roclu 
themselves.  In  West  Lothian,  intnisive  sheets,  traversing  a  group  of  strata  containing 
seams  of  coal  and  ()il-slial(>,  have  a  distinctly  bituminous  odour  when  freshly  broken,  and 
little  globuhw  of  petroleum  nmy  be  detected  in  their  cavities.  In  the  same  district,  the 
joints  and  fissures  of  a  massive  saii<lstone  are  filled  with  solid  brown  asphalt,  which  the 
(juanymen  manufacture  into  candles. 

Marmarosis. — The  conversion  of  ordinary  dull  granular  limestone 
into  crystalline  or  siiccliaroid  marble  may  not  infrequently  be  observed 
on  a  small  scale,  where  an  intnisive  sheet  or  dyke  hsus  invaded  the  rock. 
It  is  also  observable  as  a  general  phenomenon,  apart  from  the  appeannee 
of  visible  eruptive  rocks,  and  in  such  cases  serves  to  unite  local  and 
regional  metamorphism.  In  zones  of  contact-metamorphism  round  gnnite 
and  other  eruptive  bosses  many  minerals  have  crystallized  out  in  the 
altered  limestone,  such  as  tremolite,  zoisite,  and  garnet 

On(^  of  the  earliest  described  examples  of  this  change  is  that  at  Rathlin  Islandy  off  the 
north  coast  of  Ireland  (Fig.  309).  Two  basalt  dykes  (20  and  85  feet  thick  leapectivfly) 
ascend  tlK-re  through  chalk,  of  which  a  band  20  feet  thick  separates  them.  Bovn 
the  middle  of  this  central  chalk  band  nins  a  tortuous  d3'kc  one  foot  thick.  The  chftlk 
between  the  dykes  and  for  some  distance  on  either  side  has  l)een  altert-d  into  a  finely 

'   Moesia,  '  Geolopsche  Scliilderuug,  Meissner  und  HirschWrge,*  Marburg,  1867. 

-  (J.  voin  Kath,  X.  Jahrft.  18S0,  ]».  276.  Jn  the  alwve  analysis  the  bitumen  incloiIe« 
all  vol:ililo  eoiistitucnts  driven  off  by  heat,  hence  coke  and  bitumen  =  100.  Another 
iiistiiiice  is  (Irscribcd  by  (iiinibol  from  Miihrisch-Ostrau,  where  coal  is  coked  by  an  angitc- 
Horj.hyry,   Wrk.  (.'eoL  litichsunst.  1S74.  ]>.  f).*). 


111^ 


PART  vin  §  i  LOCAL  METAMORPHIBM  e08 

fCnniiUr  marble.'  On  the  east  side  of  the  great  intrusive  moss  of  Fair  Head  the  chalk 
is  likewise  mann«ri»ed.  Another  Binaller  but  inCennting  illustration  of  the  same  cliange 
occurs  >t  Camps  Quarry  near  Edinburgh,  The  dull  grey  Burdie 
House  limestone  (Lower  Carl)oniferaus),  full  of  valves  of  Ltprr- 
dilia  and  jiUnta,  baa  there  been  invaded  by  a  liasaltic  dyke, 
which,  sending  slender  veins  into  the  limestone,  has  encloeed 
portions  of  it  The  limestone  is  found  to  have  aoqoired  the 
gTkimlar  crystalline  character  of  marble,  each  little  gtsuule  of  f^_  sos.—Djka  of  NwiH 
calcite  having  its  own  orientation  of  cleavage  planes  (Fig.  310).       (a  o  o)  tnitnlng  chaJk 

(l>   b),  which   nwr   the 

ProducUon  of  New  Hlnerals. — One  of  the  results  <iykM  n  conviincd  into 
of  the  intnision  of  eruptive  rock  haa  been  the  de-  "j"^,'* /"uji,  "■"'"" 
velopraent  of  cryatalline  minerals  in  ordinary  sedi- 
mentary strata  near  the  line  of  coDtact.  The  new  minerals  have  usually 
an  obvious  alSnity  in  composition  with  the .  original  rock.  Bat  un- 
doubtedly silica  has  often  been  introduced  as  part  of  the  alteration, 
either  free  or  as  silicates.  Moreover,  a  certain  broad  order  of  succession 
in  the  appearance  of  these  new  minerals  may  be  observed  in  the  lai^r 
areas  of  contact-metamorphism.  On  the  outer  margin  of  the  ring  or 
areole  of  metamoriihism  the  internal  rearrangements  and  mineralogical 
re -combinations  show  themselves  in  many  argillaceous  rocks  by  the 
appearance  of  small  knots  or  concretions  which  are  replaced  further 
inward  by  recognisable  silicates,  such  as  chiastolite,  andalusite,  ptaurolite, 
or  kyanite,  while  towards  the  centre  the  dark  mica  which  appears  even 
in  the  outer  parts  of  the  ring  attains  a  marked  prominence,  often  accom- 
panied with  garnets  and  other  new  minerals. 

A  simple  but  tiiteresting  instance  of  tliia  kind  of  oontact-iuetamorphisni  was  dutcribeil 
many  years  ago  by  Hensluw,  near  Plaa  Xcwydd,  Anglcsea.  A  liasalt  dyke,  154  Feet  in 
breadth,  there  traversrii  strata  of  shale  and  argil- 
laceous limestone,  wbicli  are  altered  to  a  distante 
of  35  feet  from  the  intnuiive  rocks,  the  Itmeiitotie 
becoming  granular  and  crystalline,  and  the  shale 
being  hardeiieil,  lipre  and  there  i«rcellanizp<l. 
while  ita  shells  {Produeli,  Ac.),  though  nearly 
obliterated,  are  atill  traceable  by  tlieir  imjires- 
,  aions.  In  thealtered  fiMsitiferouH  shale  namerouH 
crystals  of  analcime  and  garnet  have  been 
develo[ied,  the  latter  yielding  as  much  an  20  [ler 
cent  of  time.'  Similar  phenomena  were  observed 
by  Sedgwick  along  tlie  edges  of  intruded  liasalt 
among  the  Oirlnniferous  limestones  and  slialen 
ofHighTeesdale.' 

In    Hesse    and    Tliuringerwald,    Zirkel   has 

bTbualt(t>).    JUgnlO«la»di»m*ler..        liaaalt,  where  the  i|Uartz-grainB  are  enveloped  in 
a  vitreous  matrix,  in  which  abundant  microscopic 

'  Conybeare,  Tram.  '/fot.  Sac.  iii.  p.  210  anri  plate  X. 
aianples  ot  manuroHis  is  the  alteration  of  the  (TrinKsic)  limes 
known  iUtuary  marble  {wejioalen,  p.  629). 

•  Vambrulge  Phii.  Trans,  i.  p.  a02. 


604  GEOTECTONW  {STRUCTURAL)  GEOLOGY  book  n- 

inicrolites  occur,  and  preH(uit  in  their  arrangement  evidence  of  a  fluxion-structure. 
This  glassy  constituent  probably  represents  the  argillaceous  and  other  materials  in 
which  the  quartz-grains  were  originally  iml)edded,  and  which  has  been  fused  and  made 
to  How  by  the  heat  of  the  basalt.* 

Among  localities  where  the  develo]>ment  of  new  minerals  in  proximity  to  eraptiTe 
rock  has  taken  ]>lace  on  the  most  extensive  scale,  none  have  been  more  frequently  or 
carefully  described  than  some  in  the  group  of  mountains  lying  to  the  east  and  80Uth-«Mt 
of  Botzen,  in  the  Tyrol  (Monzoni,  Predazzo).  Limestones  of  Lower  Triassic  (or  Permian) 
age  have  there  been  invaded  by  masses  of  monzonite  (a  rock  intermediate  between  syenite 
and  dioritc,  sometimes  containing  much  augite),  granite,  melaphyre,  diabase,  and  ortho* 
clase-iK)i'phyr}\  Tliey  have  l)ecome  coarsely-crystalline  marble,  portions  of  them  being 
completely  enveloj^ed  in  the  erujitive  rock.  But  their  most  remarkable  feature  is  that  in 
them,  and  in  the  eruptive  rocks  in  contact  with  them,  many  minerals  often  beautifnllj 
ciTstallized,  have  been  devclo])ed,  including  garnet,  idocrase,  gehlenite,  fassaite,  pistadtck 
spinel,  anorthite,  mica,  magnetic  iixin,  hiematite,  ajiatite,  and  serpentine.  Some  of  thoR 
mineials  occur  chiefly  or  only  in  the  eruptive  masses,  others  more  frequently  in  the  lime- 
stone, whicli  is  marked  by  a  lime-silicate  homstone  zone  along  the  junction.  But  theie 
arc  all  products  of  contact  of  the  two  kinds  of  rock.  Layers  of  carbonates  (oalcite,  abo 
with  bnu'itc)  alternate  with  lamiuie  and  streaks  of  various  silicates,  in  a  manner  strikingly 
similar  to  the  arrangement  found  in  limestones  among  areas  of  regional  metamoqihism, 
where  wu  visible  intrusive  rock  has  intluence<l  the  phenomena.' 

Production  of  Foliation. — This  is  the  most  complete  kind  of  meta- 
morphic  change,  for  not  only  are  new  minerals  developed,  but  the  whole 
texture  and  stnicture  of  the  rock  are  altered.  Eeference  has  been  already 
(p.  568  i^eq.)  made  to  the  striking  manner  in  which  foliation  has  been 
superinduci'd  upon  ordinary  sedimentary  rocks  round  large  bosses  (rf 
granite.  The  details  of  this  change  deserve  careful  consideration,  for  they 
possess  a  high  importance  in  relation  to  any  theory  of  metamorphism. 

In  some  cases  (and  probably  these  are  more  frequent  than  has  been 
suspected)  tliere  has  been  a  copious  injection  of  granitic  material  not 
merely  as  large  veins  or  dykes,  but  in  minute  threads  and  laminie  into  the 
surrounding  rock,  following  generally  the  more  marked  divisional  planes, 
sucli  as  those  of  bedding,  cleavage,  or  foliation.  This  impregnation  or 
granitization  has  been  strongly  insisted  upon  by  M.  Michel  L6vy  and  has 
ijeen  noticed  by  other  observers.^  Near  the  contact  of  the  micaceous 
schists  of  Saint  L^on  with  the  granite  which  pierces  them,  the  distin- 
guished French  geologist  found  that  the  eruptive  rock  has  been  injected 

^  X.  Jiihrh.  1872,  p.  7.  For  other  examples  see  Mohl,  Verhandi.  Geal.  Reithmmti, 
171,  p.  250  :  Iliissak,  Tsi'lunnak\s  Min.  MittML  1883.  ]>.  530. 

-  On  tint  Monzoni  region,  sec  Doelter,  Jahrh.  iHeol.  Reichsanstalt^  1875,  p.  207, 
where  a  l)ibliogni]>liy  of  the  locality  up  to  the  date  of  publication  "will  be  found.  Other 
]*apers  have  s^ince  a]ipeare(l,  of  which  the  following  dealing  with  the  phenomena  of  (xnitact- 
nietaniorphism  may  bo  nientioued.  G.  voni  liath,  Z,  l^etitttch.  Oeol.  Oes.  1875,  p.  348; 
'  Der  Monzoni  in  siidostlichen  TinO,*  Bonn,  1875;  Lemberg,  Z.  DeuUch.  Oeol,  Oes,  1877, 

^  Midiel  Levy,  Hull.  S>h\  fUol.  France,  ix.  (1881)  p.  187,  (1888)  p.  221.  CimpL  rmd. 
Internatiitnal  (reol.  ConffrcK.%  1888.  I  have  myself  studied  similar  cases  of  iigection  among 
the  .schistK  around  the  granites  near  I^iirg  in  Sutherland,  and  others  have  lately  been  irorked 
out  in  detail  l)y  Messrs.  Peach  and  Home  in  the  Geological  Survey  of  the  north^easteni  part 
of  the  same  conntv. 


PART  VIII  §  i  LOCAL  METAMORPHISM  606 

between  the  planes  of  the  schists  in  leaves  from  a  few  millimetres  to  one 
or  two  centimetres  thick,  the  rock  has  thus  a  ribboned  appeai'ance  from 
the  alternation  of  numerous  dark  micaceous  layers  with  the  finely  granular 
pink  or  white  veins  from  the  granite.  By  such  a  process  of  metamorphism 
and  injection  sedimentary  strata  have  acquired  a  structure  that  can  hardly 
be  distinguished  from  that  of  some  ancient  gneisses.^ 

Round  the  granite  bosses  of  Devon  and  Cornwall,  already  referred  to  {miU,  p.  569), 
the  Devonian  and  Carboniferoos  formations  have  undergone  remarkable  changes,  which 
have  long  been  cited  as  classic  examples  of  con  tact -metamorphism.  Fine  greywacke  and 
slate  have  been  converted  into  mica-schist  and  varieties  of  gneiss  (comubianite).  In 
some  cases  the  slates  become  indurated  and  dark  in  colour,  and  new  minerals  (schorl, 
chiastolite,  &c.)  are  develoj^ed  in  them.  The  volcanic  bands  intercalated  with  the 
sedimentary  series  likewise  undergo  alteration,  the  "greenstones,"  in  particular,  becoming 
much  more  coarsely  crystalline  as  they  approach  the  granite.  Each  boss  of  granite  is 
surrounded  with  its  ring  of  metamorphism,  which  varies  greatly  in  breadth  and  in  the 
intensity  of  alteration.^ 

In  the  Lake  District  of  the  north  of  England  excellent  examples  of  the  phenomena 
of  contact  may  be  observed  round  the  granite  of  Skiddaw.  The  alteration  here  extends 
for  a  distance  of  two  or  three  miles  from  the  central  mass  of  granite.  The  slate,  where 
unaltered,  is  a  bluish-grey  cleaved  rock,  weathering  into  small  flakes  and  ])encil-like 
fragments.  Traced  towards  the  granite,  it  flrst  shows  faint  sjwts,  which  increase  in 
number  and  size  until  they  assume  the  form  of  chiastolite  crystals,  with  which  the  slat« 
is  now  abundantly  crowded.  The  zone  of  this  chiastolite-slate  seldom  exceeds  a  quarter 
of  a  mile  in  breadth.  Still  closer  to  the  granite,  a  second  stage  of  metamorphism  is 
marked  by  the  development  of  a  general  schistose  character,  the  rock  becoming  more 
massive  and  less  cleaved,  the  cleavage-planes  being  replaced  by  an  incipient  foliation  due 
to  the  development  of  abundant  dark  little  rectangular  or  oblong  spots,  probably 
im])erfectly  crystallized  chiastolite,  this  mineral,  as  well  as  andalusite,  occurring  also  in 
large  crystals,  together  with  minute  flakes  of  mica  (sjwtted  schist,  Knotenschiefcr).  A 
third  and  flnal  stage  is  reached  when,  by  the  increase  of  the  mica  and  quartz-grains,  the 
rock  iiasses  into  mica-schist — a  light  or  bluish-grey  rock,  with  wonderfully  contorted 
foliation,  which  is  developed  close  to  the  granite,'^ there  being  always  a  sharp  line  of 
demarcation  between  the  mica-schist  and  the  granite.^ 

In  the  same  region  the  granite  boss  of  Shap  has  produced  some  interesting  changes  on 
the  andesitic  and  rhyolitic  lavas  and  tufis  associated  with  the  Lower  Silurian  strata. 
These  changes  have  been  studied  by  Messrs.  Harker  and  Man*,  who  describe  the  gradual 
alteration  of  the  andesitcs  by  the  development  of  brown  mica,  hornblende,  sphene,  and 
other  minerals.  The  amygdaloidal  cavities  had  been  fllle<l  with  secondary  products,  and 
the  rocks  had  thus  been  considerably  weathered  before  the  intrusion  of  the  granite,  for 
the  materials  filling  the  vesicles  partake  in  the  general  metamorphism.  By  the  gradual 
increase  of  the  brown  mica  and  the  production  of  a  marked  laminated  structure  indicated 

*  See  Michel  Levy  "sur  Toriginedes  Terrains  crystallins  primitifs,"  lniemalion<d  Getd. 
Ckmgress,  1888,  p.  59  ;  and  the  account  of  pre-Cambrian  rocks,  postfOy  Book  VI. 

'  De  la  Beche,  *  Report  on  Geology  of  Devon  and  Cornwall,*  Mem,  Oed.  &iirvei/f  1839, 
p.  268.  See  also  Forbes,  Trans,  Oeol,  Soc.  Cornwall,  ii.  p.  260,  and  Boase,  op,  cii.  iv.  (1832), 
p.  166.  The  microscopic  structure  of  the  unaltered  slates  of  Cornwall  has  been  described 
by  Allport,  Q.  J.  Oeol.  Soc.  xxxii.  (1876),  p.  407,  and  that  of  the  greenstones  by  J.  A. 
Phillips,  op,  cit,  zxxiv.  (1878).  Some  interesting  observations  on  the  metamorphism  of 
Cornish  and  other  slates  are  given  by  Sorby  in  his  Address  to  the  Geological  Society,  op.  cit. 
xzxvi.  (1880),  p.  81  et  seq, 

'  J.  C.  Ward,  Q.  Joum.  Oeol,  Soc  xxxii  (1876),  p.  1.  Compare  the  development  of 
andalusite  in  regional  metamorphism,  p.  627,  note. 


606  GEOTECTOKia  (STRUCTURAL)  GEOLOGY  book  iv 


by  tlic  jMrallcl  di8i>osition  of  the  mica-flakes,  these  lavas  aud  tufis  assume  the  aspect  of 
true  crystalline  schists.* 

Farther  north,  in  the  south-western  counties  of  Scotland,  several  large  masses  of 
tine-giained  granite  nse  through  the  Lower  Silurian  greywacke  aud  shale,  which,  around 
the  granite  for  a  variable  distance  of  a  few  hundred  yards  to  nearly  two  miles,  have 
undergone  great  alteration  (see  Fig.  282).  These  strata  are  ranged  in  steep  anticlinal  and 
synclinal  folds  which  nin  across  the  south  of  Scotland  in  a  general  north-east  and  south- 
west dircK;tion.  It  Is  observable  that  this  normal  strike  continues,  with  little  modificatiou, 
u])  to  the  granite,  whi<^h  thus  has  replaced  an  equivalent  area  of  sedimentary  rock  {me 
p.  570).  The  coarser  arenaceous  beds,  as  they  approach  the  granite,  are  clianged  into 
(|uartz-rock,  the  thin  siliceous  shales  into  Lydian-stone,  the  black  anthracitic  graptoHte- 
shales  into  a  compact  mass  charged  with  pyrites,  and  breaking  into  large  rough  blocbt. 
Strata  wherein  fels]>ar-grains  abound  have  l>een  altered  to  a  greater  distance  than  the 
mure  siliceous  beds,  and  show  a  giudation  through  spotted  schists,  with  an  increasing 
deveIo])ment  of  mica  and  foliation,  until  along  the  edge  of  the  granite  they  become  true 
mica-schist  and  even  a  tine  kind  of  gneiss.^  The  jiebbly  conglomerates  which  form  a 
marked  horizon  among  the  unaltei'ed  roi^ks,  are  traceable  in  the  metamoi^tliosed  areole  ss 
rocks  whicli,  at  first  sight,  might  be  taken  for  some  kind  of  poqihyritic  gneiss.  Their 
([uartz- lobbies  have  assumed  a  resinous  asiiect,  and  axe  enveloped  in  a  ctystalline 
micaceous  (loste.     The  metamorphism  of  the  Highlands  is  referred  to  on  pp.  625,  698. 

A  classical  region  for  the  study  of  contact-metamorphism  is  in  the  Harz,  where, 
round  the  granite  masses  of  the  Brocken  and  Kambcrg,  the  Devonian  aud  older  Palco- 
zoit.'  rocks  are  altered  into  various  Hinty  slates  and  schists  which  form  a  ring  round  the 
eruptive  rock.  Dykes  and  other  masses  of  a  crystalline  diabase  have  likowise  been 
erupted  through  the  greywackes  and  shales,  which  in  contact  and  for  a  Tarying 
distance  beyond,  have  been  converted  into  hard  siliceous  bands  (homstone)  and  into 
various  finely  foliated  masses  (lleckschiefer,  bandschiefer,  contactschiefer,  the  spilotite 
and  desmosite  of  Zincken).  The  limestones  have  their  carbon-dioxide  replaced  by  silica 
in  a  broad  zone  of  lime-silicate  along  the  contact.'  The  black  com]mct  limestone  of 
HoscnKle  becomes  a  whiie  saccharoid  marble,  charged  with  silicates  (rhombic  dodeca- 
hedrons of  garnet,  &c.)  and  with  its  carbonaceous  matter  segregated  into  abundant  veins. 
A  limestone  band  containing  ironstone  presents,  in  the  Spitzenberg  between  Altcnau  and 
Harzburg,  a  garnet iferous  magnetite  containing  well-j)reserved  crinoid  sterns.^ 

Round  the  syenite  of  Meissen,  in  Saxony,  the  dial)ases  when  they  come  within 
the  areole  of  contact-metamorphism  (>ass  into  actinolite- schists  aud  anthoikhyllite- 
schists.*'' 

The  French  Pyrenees  present  instructive  examples  of  the  effect  of  the  protrusion 
of  granite  and  other  eniptive  rocks  u[x)n  Cambiian  and  later  formations.     Fuchs  baa 


'   Harker  and  Marr,  Q.  J.  (ieoL  i^\  xlvii.  (1891),  p.  266. 

-  ,1.  llorne,  Mem.  (iful.  Sun\  Scolland^  Explanation  of  Sheet  9,  p.  22.  Brit.  Aane, 
1S92,  }>.  712.  The  microscopic  structure  of  the  altered  rocks  in  this  district  has  been  studkd 
by  Prof.  Bonuey  aud  Mr.  Allport,  Proc.  Roy.  Soc.  xlvi.  (1889),  and  Miss  M.  J.  Oardiner, 
V.  J.  (f'ed.  aSoc.  xlvi.  (1S90),  p.  569. 

•*  Ziuckeii,  Knrsten  nnd  r.  /><v/i^»,  ArchiVj  v.  p.  345  ;  xix.  p.  588.  Fuchs,  X.  Jahth. 
1862,  p}».  769,  929.  K.  A.  Losscn,  Z.  DextUrh.  iJeoL  Oes,  xix.  p.  509  (on  the  Tannas) : 
xxi.  p.  291  ;  xxiv.  p.  701.  Kayser,  op,  cit.  xxii.  p.  103.  The  memoirs  of  Lossen  fonu 
some  of  the  most  important  contributions  to  our  knowledge  of  the  phenomena  of  meta- 
morphism. 

*  K,  A.  Lessen,  Z.  Ikui^ch.  iieoL  ffes.  xxix.  1877,  p.  206.  Eri&uUr.  Geoi,  JS^tedal- 
Kod.  Prems,      Blatt,  llarzgerode  (1882). 

'•  K.  Dalmar,  Blatt  64  (Tauuel>erg)  KrlUuter.  Special-Kart,  ^iicAaei*  (1889) ;  A.  Saner, 
op.  cit.  Blatt  48  (Meissen). 


PART  VIII  §  i  LOCAL  METAMORPHISM  607 

traced  the  luetaiiiorphism  of  clay-slate  through  8i)otted  schists  (frucht-,  chiastolite-,  and 
aiidalusite-schists)  into  uiica-schist  and  gneiss.^  More  I'eceutly  the  region  has  been 
Mtudied  in  great  detail  by  Barrois,  who  distinguishes  three  successive  zones  in  the  meta- 
luorphic  areola  surrounding  the  granite.  On  the  outside  lies  the  zone  of  "goffered 
schists,"  in  which  a  puckered  structure  has  been  developed  without  any  new  mineral 
combination  of  the  elements  of  the  rock.  Next  come  the  chiastolite-schists,  with 
crystals  of  chiastolite,  tourmaline,  &c.,  which  become  more  and  more  micaceous  towards 
the  interior,  till  they  jniss  into  the  third  and  innermost  zone,  that  of  the  leptinolites, 
which  are  highly  micaceous  schists  with  small  crystals  of  chiastolite,  and  sometimes 
with  tourmaline,  rutile  and  triclinic  felspar.  Barrois  also  shows  that  round  the  masses 
of  kcrsaiitite  a  ring  of  chloritic  mica-schist  has  been  develoiied,  followed  outside  by  one 
of  spotted  schists.' 

^k)me  important  observations  have  been  made  by  Barrois  at  Gu^mene,  in  the  maritime 
de[»artment  of  Morbihan,  where  Lower  Silurian  strata  have  been  invaded  by  granite. 
Of  special  interest  are  the  effects  produced  upon  the  sandstones  (gres  h  soolithes),  which 
are  converted  into  micaceous  quartzites.  These  altered  rocks,  traced  farther  inwards, 
are  further  distinguished  by  the  development  in  them  of  sillimanite,  sometimes  in 
sufficient  abundance  to  imiMirt  a  foliated,  undulated,  gneissoid  structure.  At  the  con- 
tact with  the  eruptive  rock,  this  quartzite  shows  recr^'stallized  quartz,  black  mica, 
sillimanite,  cordierite,  and  a  good  many  crystals  of  orthoclase  and  plagioclase,  besides 
white  mica.  The  conglomerates  show  their  matrix  altered  into  a  mass  composed  of 
rounded  or  angular  grains  of  quartz  united  by  abundant  white  sericitic  mica,  and 
containing  some  crystals  of  zircon,  large  plates  of  muscovite,  and  yellow  granules  of 
limonite.' 

Another  admirable  locality  for  the  study  of  contact-metamorphism  is  the  eastern 
Vosges.  Kosenbusch,  in  describing  the  phenomena  there,  has  shown  that  the  unaltered 
clay -slates  are  grey,  brown,  violet,  or  black,  thinly  fissile,  here  and  there  curved,  crumpled, 
and  crowded  with  kernels  and  strings  of  quartz.**  Traced  towards  the  granite  of  Barr 
Andlau,  they  present  an  increasingly  pronounced  metamorphism.  First  they  assume 
a  spotted  appearance,  owing  to  the  development  of  small  dark  |K)ints  and  knots,  which 
increase  in  size  and  number  towards  the  granite,  while  the  ground-mass  remains  un- 
altered (knotenschiefer,  fnichtschiefer).  The  ground-mass  of  the  slate  then  becomes 
lighter  in  colour,  harder,  and  more  crystalline  in  apjicarance,  while  flakes  of  mica  and 
quartz -grains  make  their  apjiearance.  The  knots,  now  broken  up,  rather  increase  than 
diminish  in  size  ;  the  hardness  of  the  rock  rapidly  increases,  and  the  fissile  structure 
becomes  unrecognisable  on  a  fresh  fracture,  though  observable  on  a  weathered  surface. 
Still  nearer  the  granite,  the  knot-like  concretions  disap|)ear  from  the  rock,  which  then 
has  become  an  entirely  crystalline  mass,  in  which,  with  the  lens,  small  flakes  of  mica 
and  grains  of  quartz  can  be  seen,  and  which  under  the  microscoi>e  ap|)ears  as  a  thoroughly 
crystalline  aggregate  of  andalusite,  quartz,  and  mica.  The  projwrtions  of  the  ingredients 
vary,  but  the  andalusite  and  quartz  usually  greatly  prei>onderate  (andalusite-schist). 
Chemical  analysis  shows  that  the  unaltered  clay-slate  and  the  crystalline  andalusite- 
schist  next  the  granite   consist  essentially  of  similar  chemical  materials,  and   that 


^  y.  Jahrb.   1870,  p.  742  ;  see  also  Zirkel,  Zeitsch.   DeuUch.   Qeol.   Oes.   xix.   (1867). 
p.  175. 

^  '  Recherches  sur  les  Terrains  anciens  des  Asturies  et  de  la  Galice,'  quarto,  Lille,  1882. 
.  '  Ann,  Soc  OSol.   Aonif  xi.  (1884),  p.  103.      Compare  also  the  early  observation  of 
Puillon-Boblaye  regarding   trilobites  and  orthids   in  chiastolite   slates,   CompUs  rend.   vi. 
(1836),  p.  168,  confirmed  by  the  Comte  de  Limur,  Bidl.  Soc.  OM.  France  (3),  xiii.  (1885) 
|).  55. 

*  N,  Jahrb.   1875,  i>.  849.     *  Die    Steigerschiefer  und  ihre  Contact*Zone, '  Strassburg, 
1877.     Unger,  .V.  Jahrb.  1876,  p.  785. 


608  GEOTECTONIC  (STRUCTURAL)  GEOLOGY  book  it 


"  ]>i-()])ably  tlio  nietam()r|)lii.sm  lia8  uot  taken  place  by  the  addition  or  subtraction  of 
matU'Vy  but  by  another  and  still  unknown  process  of  molecular  transposition."^  In 
some  eases,  l>oric  acid  has  Iteen  supplied  to  the  Hchists  at  tlie  contact.'  Still  more 
striking,  perha]i8,  is  the  condition  of  the  rocks  at  Rothau  ;  they  have  l)oconie  homblendie, 
and  tlieir  included  corals  have  l>een  re])laced,  without  being  distorted,  by  crystab  of 
hornblende,  garnet,  and  axinite.' 

In  the  Christiania  district  of  southern  Norway,  singularly  clear  illustrations  of  the 
nietamorphism  of  sedimentary  rocks  round  eniptive  granite  have  long  been  known. 
Kjerulf  has  shown  that  each  lithological  zone  of  the  Silurian  formations,  as  it  approicbn 
the  granite  of  that  district,  assumes  its  own  distinctive  kind  of  metamorphism.  The 
liincKtones  become  marble,  with  cr}'stals  of  tremolite  and  idocrase.  Tlie  calcareoos  and 
marly  shales  are  changed  into  hard,  almost  ja8}>er}*,  shales  or  slates  ;  the  cement-stone 
ncnlules  in  the  shalt>s  appear  as  masses  of  garnet ;  the  sandy  strata  become  bard  silieeou 
scliists  (luilleflinta,  jasper,  hornstone)  or  quartzite  ;  the  nou -calcareous  black  clay-slates 
are  converted  into  chiastolite-schists,  or  grajihitic  schists,  but  often  show  to  the  eye  only 
trilling  alteration.  Other  shaly  beds  have  assumed  a  fine  glimmering  appearance ;  and, 
in  the  calcareous  sandstone,  biotite  has  been  develoi>ed.  In  spite  of  the  metamorphimi, 
however,  neitltt.>r  fossils  nor  stratification  have  been  quite  obliterated  from  the  altered 
rocks.  From  all  the  stratigi'aphical  zones  fossils  have  been  found  in  the  altered  belt, 
so  that  the  true  ]x>sition  of  the  metamorphosed  rocks  admits  of  no  doubt. ^  YnL 
W.  C  Hriigger  has  subjected  the  rocks  of  the  zones  of  contact -metamorphiam 
round  Christiania  to  a  searching  microscopic  examination,  and  has  published  a  hjghly 
im]N>rtant  and  interesting  memoir  on  the  subject.  He  describes  the  unaltered  and 
altered  conditions  of  tlie  more  conspicuous  stratigraphical  bands,  and  thus  proTidesnev 
material  for  the  investigation  of  contact-metamorphism.  Especially  interesting  are  his 
de.scri]>tions  of  tlie  distinctive  metamorphism  of  each  band,  the  reniarluibly  ?ariabk 
amount  of  alteration  even  in  the  same  band,  the  persistence  of  recognisable  graptolitM 
even  in  rocks  that  have  lKH.>ome  essentially  crystalline,  the  transformation  of  limettone 
into  marble,  of  whicli  a  fourth  or  fifth  j>art  is  composed  of  garnet,  partly  in  laige 
rhombic  dcwlecahedrons,  and  jwirtly  as  a  mould  enclosing  OrtJiis  caUigramma* 

One  further  European  example  may  be  cited  from  the  observations  of  F.  E.  Miiller. 
who  Inis  described  round  the  granite  of  the  Hennberg  near  Lehesten  in  the  Franken- 
wald  the  occurrence  of  knotted  scliists,  chiastolite-schists,  knotted  mica-schists,  and 
andalusitic  mica-rocks. •* 

Tlie  same  i)henomena  have  been  obsc*rved  in  many  other  jiarts  of  the  world.  One 
exani]>le  from  America  may  sufiice  t^  show  how  precisely  the  facts  collected  in  the  Old 
World  are  rei^-atetl  in  tlu^  New.  An  elaborate  examination  was  made  of  the  oontact- 
inetaniorphisni  of  the  granite  of  Albany,  New  Hampshire,  by  the  late  Mr.  G.  W. 
Hawes."  His  analyses  indicate  a  systematic  and  progressive  series  of  changes  in  the 
schists  us  th(>y  approach  the  granite.  The  rocks  are  dehydrated,  boric  and  sUidc  adds 
have  been  a<lded  to  them,  and  there  ap|>ears  to  have  l)een  also  an  infusion  of  alkali 
directly  on  the  contact.  He  regarderl  the  schists  as  having  been  impregnated  by  Ttn 
bot  vapours  ami  solutions  emanating  from  the  granite. 

Alteration  of  the  intrusive  Rock. — Reference  has  been  made  above 

(p.  571)  to  the  possible  alteration  of  composition  in  an  eruptive  maaa  bv 

*  Uuger,  op.  ct't.  p.  806. 

-  Rosenbiisch.  *  Die  Steigerschiefer,*  &c.,  p.  257. 

^  J//W.  tft'.s  MineSy  5"*  sor.  xli.  p.  31 S. 

^  '(Jcologie  Norwegeus,*  1880,  p.  75.  For  the  literature  of  the  Norwegian  locality  fee 
K  Keyer,  Juhrh.  O'col.  lUichsanst.  xxx.  (1880),  p.  '26. 

^  'Die  Silurischen  Ktagen  2  uiid  3  ini  Kristiania  Gebiet,*  Kristiania,  1882. 

•'  yeues  Jtthrb.  1882  (2),  p.  205.  "  Atiur.  Joum,  Sci.  xxi.  (1881),  p.  21. 


PART  viii  §  i  LOCAL  METAMORPHISM  C09 

fusing  into  itself  some  portion  of  the  rocks  through  which  it  is  intruded, 
and  also  to  the  remarkable  change  superinduced  upon  intrusive  sills  of 
diabase  by  contact  with  carbonaceous  strata.  Dr.  Stecher,  to  whom  I  sent 
a  carefully  collected  series  of  specimens  illustrative  of  the  intrusive  sheets 
of  the  basin  of  the  Firth  of  Forth  and  their  contact  phenomena,  has 
investigated  this  question  and  obtained  some  interesting  results.  He 
shows  that  along  the  edges  of  contact  with  the  sandstones  or  shales  these 
diabases  present  a  great  abundance  of  well-defined  crystals  of  olivine,  that 
as  the  rock  is  examined  progressively  further  from  the  contact  these 
crystals  become  more  or  less  corroded,  while  in  the  centre  of  the  sheet  they 
HO  entirely  disappear  that  the  rock  appears  as  a  diabase  without  olivine. 
He  finds  that  the  interior  parts  of  the  mass  are  more  acid  than  the 
exterior  parts  and  he  attributes  this  difference  to  the  incorporation  of 
silica  from  rocks  (sandstones,  v^c.)  broken  through  by  the  diabase.  The 
outer  olivine-bearing  selvage  he  regards  as  representing  the  original  com- 
position of  the  rock  at  the  time  of  its  extrusion,  and  he  thinks  that  the 
assimilation  of  acid  material  by  the  central  still  fluid  and  slowly  cooling 
portion  led  to  the  corrosion  and  re-solution  of  the  olivine  which  at  the  time 
of  extrusion,  as  proved  by  the  marginal  selvage,  was  already  perfectly 
crystallized  out.  In  some  of  the  rocks  he  found  a  surplus  of  silica  which 
had  crystallized  as  quartz.  Recognising  that  the  first  portion  to  take 
definite  crystalline  form  would  be  more  basic  than  the  still  liquid  portions, 
he  yet  concludes  that  this  will  not  account  for  the  observed  facts,  which 
in  his  opinion  point  to  an  actual  addition  of  silica.^  It  is  very  desirable 
that  similar  careful  chemical  and  microscopic  investigation  should  be 
undertaken  with  a  special  view  to  the  determination  of  the  difference  in 
chemical  constitution  between  the  peripheral  and  central  portions  of 
intrusive  masses,  and  to  ascertain  whether  any  such  difference  can  be 
traced  to  the  influence  of  the  rocks  through  which  these  masses  have 
been  erupted. 

Summai^  of  Facts. — The  foregoing  examples  of  the  alteration  super- 
induced ujx>n  stratified  rocks  in  proximity  to  granite  or  other  eruptive 
masses  might  be  largely  increased ;  but  they  may  suffice  to  establish  the 
following  deductions  in  regard  to  contact-metamorphism. 

1.  Groups  of  ordinary  sedimentary  strata,  likewise  eruptive  rocks 
associated  with  them,  where  they  have  been  pierced  by  granite  or  other 
plutonic  rock,  have  undergone  an  internal  change,  whereby  their  usual 
iithological  characters  have  been  partially  or  wholly  obliterated.  This 
alteration,  however,  is  not  always  observable  at  the  contact  of  intrusive 
masses,  and  we  do  not  yet  know  the  precise  conditions  that  have 
determined  its  development. 

2.  The  distance  to  which  the  change  extends  varies  within  wide 
limits,  being  in  some  cases  scarcely  traceable  for  more  than  a  few  feet,  in 
others  continuing  for  two  miles  or  more.  The  subterranean  surface  of  the 
plutonic  rock,  however,  being  unknown,  may  frequently  lie  nearer  the 
surface  of  the  ground  than  might  be  supposed.     Detached  minor  areas  of 

^  Stecher,  "  Contact  Erscheinungen  an  Schottlschen  Diabasen."  Tschermak*8  Afittheil.  ix. 
1887,  pp.  145-205. 

2  R 


610  GEOTEOTOMC  (STRUCTURAL)  GEOLOGY  book  n- 


metamorphism  may  thus  1)e  connected  with  eruptive  bosses  which  have 
not  yet  been  laid  bare  by  denudation. 

3.  As  the  alteration  increases  in  intensity  with  greater  proxinuty 
to  the  plutonic  rock,  it  must  be  regarded  as  a  result  of  the  pi-otrusion  of 
that  rock.  But  there  occur  exceptional  areas  or  bands  which  have  under- 
gone a  minor  degree  of  change  even  in  the  midst  of  highly  altered 
portions. 

4.  The  character  of  the  metamorphism  depends  fundamentally  upon 
the  nature  and  mass  of  the  invading  rock  and  on  the  composition  aod 
texture  of  the  materials  which  have  been  affected.  Sandstones  have 
been  changed  into  quartzite ;  siliceous  schists  into  hornstone,  Lydian- 
stone,  <^'c. ;  cl.ay- slates  into  s[)Otted  schists,  chiastolite- schists,  mica- 
schists,  «tc. ;  argillaceous  greywacke  and  grey wacke -slate  into  "knoten- 
schiefer,"  mica-slate,  and  gneiss ;  limestone  into  garnet,  hornblende,  and 
other  minerals.  Alternations  of  distinct  kinds  of  sedimentary  strata, 
such  as  slate  and  sandstone,  are  represented  by  distinct  alternating  meta- 
nioq)hic  bands,  such  as  quartzite  and  mica-schist. 

5.  In  some  cases,  the  transformation  of  a  thoroughly  clastic  rock  (day- 
slate,  greywacke,  greywacke-slate,  or  flagstone)  into  a  completely  crystalline 
one  (andalusite-schist,  mica-schist,  gneiss)  has  l)een  effected  with  little  or 
no  alteration  of  the  ultimate  chemical  composition  of  the  mass.  In  other 
cases  a  perceptible  altemtion  in  the  i)roportions  of  the  chemical  ingredients 
is  traceable.^  The  development  of  a  cr3'stalline  structure  can  be  followed 
through  intermediate  stages  from  ordinary  sedimentary  rock  to  thoroughly 
crystalline  schist,  remains  of  fossils  being  still  ol)servable  after  consider- 
able progress  has  been  made  towards  the  completion  of  a  cr}*stalline 
rearrangement. 

G.  Not  only  tloes  the  crystalline  character  increase  towards  the  limit 
of  contact  with  the  eruptive  rock,  but  it  is  not  infre([uently  accomi^anied 
with  a  progressive  development  of  foliation,  the  minemls,  more  especially 
tlie  mica,  crystallizing  in  folia  parallel  either  with  the  original  stratifica- 
tion of  the  clastic  mass  or  witli  cleavage  surfaces,  should  these  be  its 
dominant  divisional  planes.-  Along  the  line  of  contact  with  granite,  the 
foliation  is  sometimes  excessively  cnmipled  or  puckered. 

7.  The  phenomena  of  alteration  observed  round  intrusive  masses  of 
such  rocks  as  diabase  and  basalt  undoubtedly  point  to  the  heat  of  the 
eruptive  rock  as  their  prime  ciiuse.  Those  that  occur  round  the  deeper- 
seated  bosses  of  granitic  rocks  have  probably  involved  other  influences 
than  mere  heat ;  they  so  closely  resemble  those  of  regional  meta- 
morphism as  to  suggest  modifications  of  one  common  cause  for  them  both. 
In  any  case,  mere  dry  heat  would  probably  have  been  ineffective  for  the 

^  T1)is  is  speciaUy  noticeable  in  the  proportion  of  Ailica,  which  is  sonietinies  found  to  be 
largely  increased  in  the  altered  zone,  either  by  an  absolute  addition  of  this  acid,  or  by  sola- 
tion  and  removal  of  some  of  the  bases.  See  Kayser,  Z.  DfuUch.  Oeol.  Ces.  xxil,  p.  153.  Tlie 
development  also  of  such  minerals  as  tourmaline  suggests  that  boric  and  other  addi  have 
been  introduced  into  the  rocks. 

^  In  tlie  south  of  Scotland  the  foliation  round  the  granite  bosses  is  coincident  with 
stratification  ;  round  Skiddaw,  with  cleavage. 


PART  vm  §  ii  REGIONAL  METAMORPHISM  61 1 

production  of  the  more  marked  phases  of  the  contact-metamorphism  round 
gi*anite.  It  was  accompanied  by  the  co-operation  of  water,  either  already 
present  interstitially  in  the  sedimentary  rocks,  or  supplied  to  them  from 
the  eruptive  mass,  possibly  combined  with  various  mineralizing  agents 
and  acting  under  considerable  pressure.  Moreover,  the  intrusion  of 
large  bosses  of  eniptive  rock  not  improbably  gave  rise  to  mechanical 
movements  in  the  surrounding  parts  of  the  crust,  and  thereby  stimulated 
crystalline  re -arrangements,  such  as  have  undoubtedly  been  generated 
by,  crushing,  plication,  and  other  movements  in  areas  of  regional  meta- 
morphism. 

§  ii.  Regional  (Normal)  Metamorphism, — the  Crystalline  Schists. 

From  the  phenomena  of  metamorphism  round  a  central  boss  of 
eruptive  rock,  we  now  pass  to  the  consideration  of  cases  whefe  the  meta- 
morphism has  affected  wide  areas  without  visible  relation  to  eruptive 
matter.  It  is  clear  that  only  those  examples  are  here  admissible  in 
evidence  where  there  is  distinct  proof  that  what  are  called  metamorphic 
rocks  either  pass  into  masses  which  have  not  been  metamorphosed,  or 
present  characters  which  are  elsewhere  proved  to  have  been  produced  by 
the  alteration  either  of  stratified  or  of  massive  rocks. 

In  the  study  of  this  difficult  but  profoundly  interesting  geological 
problem,  it  is  desirable  to  begin  with  the  examination  of  rocks  in  which 
only  the  slightest  traces  of  alteration  are  discernible,  and  to  follow  the 
gradually  increasing  metamorphism,  until  we  arrive  at  the  most  perfectly 
developed  crystalline  schists.  It  is  the  earliest  stages  which  are  of  most 
importance,  for  it  is  there  that  the  nature  and  proofs  of  the  changes  can 
best  be  established.  As  already  remarked  (p.  597),  the  igneous  rocks, 
from  the  definiteness  of  their  original  structure  and  composition,  offer 
special  facilities  for  following  the  nature  and  extent  of  the  changes 
involved  in  the  metamorphism  of  a  region  or  a  large  series  of  rocks. 

The  extent  and  character  of  the  metamor{)hism  depend  in  the  first  ])lace 
upon  the  original  constitution  of  the  rock,  and  in  the  second  place  upon 
the  energy  of  the  metamorphic  agents.  Certain  rocks  resist  alteration.  Pure 
siliceous  sandstones,  for  example,  become  quartzites,  but  advance  no 
further,  though  occasionally,  under  intense  strain,  their  particles  are 
drawn  out  into  a  somewhat  schistose  arrangement.  But  where  felspathic 
elements  are  present,  particularly  where  they  are  the  chief  constituents, 
some  form  of  mica  almost  invariably  appears,  while  new  minerals  and 
structures  may  be  developed  in  progressively  increasing  abundance,  till  the 
rock  assumes  the  character  of  a  true  crystalline  schist. 

Possessing  characters  which  link  them  on  the  one  hand,  with  strati- 
fied, on  the  other,  with  eruptive  rocks,  the  Crystalline  Schists  present  a 
peculiar  type  of  structure  with  which  are  connected  some  of  the  most 
perplexing  problems  of  geology.  These  rocks  cover  extensive  areas  of 
the  surface  of  the  continents,  occurring  usually  wherever  the  oldest  forma- 
tions have  been  brought  to  light  But  they  everywhere  pass  imder 
younger  formations,  so  that  their  visible  superficies  is  probably  but  a  very 


012  OEOTECTOXIC  (STJiUCTUBAL)  GEOLOGY  book  iv 


small  part  of  their  total  extent.  In  the  northern  regions  of  Europe  and 
of  North  America,  they  spread  over  thousands  of  square  miles,  forming 
the  ta]>leland  of  Scandinavia,  the  Highhuids  of  Scotland,  and  a  great  pait 
of  Eastern  Canada  and  I^abrador.  They  likewise  coromonly  rise  to  the 
surface  along  the  axes  of  great  mountiiin-chains  in  all  quarters  of  the 
globe.  So  persistent  are  they,  that  the  belief  has  arisen  that  they  every- 
where underlie  the  stratified  formations  as  a  general  foundation  or 
platform.  Some  details  of  their  stnicture  will  be  given  in  the  description 
of  IVe-Cambrian  Hocks  in  Book  VI. 

The  most  distinctive  character  of  the  schists  is  undoubtedly  their 
foliation  (pp.  103,  175).  They  have  usually  a  more  or  less  conspicuous 
crystalline  structure,  though  occasionally  this  is  associated  with  traces,  and 
even  very  prominent  manifestations,  of  clastic  ingredients  (pp.  181,  627). 
Their  foliated  or  schistose  structure  varies  from  the  massive  type  of  the 
coarsest  gneiss  down  to  the  extremely  delicate  arrangement  of  the  finest 
talcose  or  micaceous  schist.  They  occur  sometimes  in  monotonous  uni- 
formity :  one  rock,  such  as  gneiss  or  mica-schist,  covering  vast  areas.  In 
otiier  places,  they  consist  of  rapid  alternations  of  various  foliated  masses 
— gneiss,  mica-schist,  clay-slate,  actinolite-schist,  and  many  other  species 
and  varieties.  Lenticular  seams  of  cr3'sta11ine  limestone  or  marble  and 
dolomite,  usually  with  some  of  the  minerals  mentioned  on  p.  151,  some- 
times strongly  graphitic,  not  unfrequently  occur  among  them,  esi)ecially 
where  they  contain  bands  of  seq)entine  or  other  magnesian  silicates. 
Thick  irregular  zones  of  magnetite,  hematite,  and  aggregates  of  bom- 
blendic,  pyroxenic,  or  chrysolitic  minerals  likewise  make  their  appear- 
ance. 

Another  characteristic  of  the  schists  is  their  usual  intense  crumpling 
and  plication.  The  thin  folia  of  their  different  component  minerals  are 
intricately  and  minutely  puckered  (Figs.  36,  37).  Thicker  bands  may  be 
traced  in  violent  plication  along  the  face  of  exposed  crags.  So  intense 
indeed  have  been  the  internal  movements  of  these  miisscs,  that  the  geo- 
logist experiences  great  and  often  insurmountable  difficulties  in  trying  to 
make  out  their  order  of  succession  and  their  thickness,  more  especially  as 
he  cannot  rely  on  the  banding  of  the  rocks  as  always  or  even  generally 
an  in<lication  of  consecutive  deposition.  Such  evidence  of  disturbance, 
though  usually  strongly  marked,  is  not  everywhere  equally  sa  Some 
areas  have  been  more  intensely  crumpled  and  plicated,  and  where  this  is 
the  case  the  rocks  usually  present  their  most  conspicuously  crystalline 
structure. 

A  further  eminently  characteristic  feature  of  the  schists  is  their  com- 
mon association  with  bosses  and  veins  or  bed-like  sheets  of  granite,  syenite, 
quartz -porphyry,  diorite,  gabbro,  or  other  massive  rocka  In  some  r^ons, 
indeed,  so  abundant  are  the  granitic  masses  and  so  coarsely  cry^alline  or 
granitoid  are  the  schists,  that  it  becomes  impossible  to  draw  satisfactoiy 
lx)un(1ary-lines  between  the  two  kinds  of  rock,  and  the  conviction  arises 
that  in  some  cases  they  represent  different  conditions  of  the  same  original 
material,  while  in  others  the  result  is  due  to  granitization  (p.  604). 

The  question  of  the  formation  and  geological  age  of  die  crystalline 


PART  VIII S  ii  REGIONAL  METAMORPHISM  613 

schists  has  given  rise  to  much  controversy.  Some  geologists  have  main- 
tained that  these  rocks  are  to  be  regarded  as  portions  of  the  early  crust 
of  the  globe  which  consolidated  from  a  molten  condition.  Others  have 
regarded  them  as  original  chemical  deposits  on  the  floor  of  a  primeval 
ocean.  These  writers,  justly  repudiating  the  exaggerated  views  of  those 
who*have  sought  by  metamorphic  (metasomatic)  processes  to  derive  the 
most  utterly  different  rocks  from  each  other  (for  example,  limestone  from 
gneiss  and  granite,  granite  and  gneiss  from  limestone,  talc  from  granite, 
&c.),  have  insisted  that  the  crystalline  schists,  in  common  with  many 
pyroxenic  and  hornblendic  rocks  (diabases,  gabbros,  diorites,  &c.),  as  well 
as  masses  in  which  serpentine,  talc,  chlorite,  and  epidote  are  prevailing 
minerals,  have  been  deposited  **for  the  most  part  as  chemically-formed 
sediments  or  precipitates,  and  that  the  subsequent  changes  have  been 
simply  molecular,  or  at  roost  confined  in  certain  cases  to  reactions  between 
the  mingled  elements  of  the  sediments,  with  the  elimination  of  water  and 
carbonic  acid."  To  support  this  view,  it  is  necessary  to  suppose  that 
the  rocks  in  question  were  formed  during  a  period  of  the  earth's  history 
when  the  ocean  had  a  considerably  different  relative  proportion  of  mineral 
substances  dissolved  in  its  (then  probably  much  warmer)  waters ;  they  are 
consequently  assigned  to  a  very  early  geological  period,  anterior  indeed 
to  what  are  usually  termed  the  Palaeozoic  ages.  It  becomes  further  need- 
ful to  discredit  the  belief  that  any  gneiss  or  schist  can  belong  to  one  of 
the  later  stages  of  the  geological  record,  except  doubtfully  and  merely 
locally.  The  more  thorough-going  advocates  of  the  pristine,  "  azoic,"  or 
"  eozoic,"  date,  of  the  so-called  "  Metamorphic  "  or  crystalline  schists,  do 
not  hesitate  to  take  this  stej),  and  endeavour,  by  ingenious  explanations, 
to  show  that  the  majority  of  geologists  (as  in  the  case  of  the  Alps, 
afterwards  referred  to)  have  mistaken  the  geological  structure  of  the 
districts  where  these  rocks  have  been  supposed  to  be  metamorphosed 
equivalents  of  what  elsewhere  are  Palaeozoic,  Secondary,  or  Tertiary  strata.^ 
Some  of  them  even  go  so  far  as  to  assert  that,  by  mere  mineral  characters, 
the  crystalline  rocks  of  contemporaneous  periods  can  be  identified  all  over 
the  world.  They  assume  that  in  the  supposed  chemical  precipitation, 
the  same  general  order  has  been  followed  everywhere  over  the  floor  of  the 
ocean.  Consequently  a  few  hand-specimens  of  the  crystalline  rocks  of  a 
country  are  enough  in  their  eyes  to  determine  the  geological  position  of 
these  formations.  Other  geologists,  recognising  that  the  more  crystalline 
members  of  the  series  of  schists  graduate  into  rocks  that  are  much  less 
crystalline,  and  even  into  what  are  recognisably  of  sedimentary  origin, 
likewise  that  they  include  and  pass  into  masses  that  were  certainly 
eruptive,  have  come  to  regard  the  schists  as  a  metamorphic  series  of 
sedimentary  and  igneous  rocks  owing  their  characteristic  foliated  structure 
to  some  subsequent  action  upon  them. 

One  of  the  chief  causes  of  difficulty  in  discussing  the  history  of  these 
rocks  has  lain  in  the  fact  that  the  crystalline  schists  are,  in  the  majority  of 
cases,  separated  from  all  other  geological  formations  by  an  abrupt  hiatus.^ 

^  See  Sterry  Hunt's  *Cliemical  E8say»,'  p.  382  sfq. 

*  Many  continental  geologiKts,  however,  belieTe  that  the  foliation  of  the  Bchists  ia  usually 


C14  OEOTKrwXIC  (STRUCTURAL)  GEOLOGY  book  iv 

Instead  of  [)assiiig  into  these  formations,  they  are  commonly  covered 
unconforma))ly  by  them,  and  have  usually  been  enormously  denuded 
before  the  deposition  of  the  oldest  overlying  rocks.  Hence,  not  only  is 
there  generally  a  want  of  continuity  between  the  schists  and  younger  fomiA- 
tions,  but  the  contrast  between  them,  in  regard  to  lithological  characters 
and  geotectonic  structure,  is  often  so  exceedingly  striking  as  naturally  to 
suggest  the  idea  that  the  schists  must  belong  to  a  far  earlier  period  than 
that  of  the  oldest  sedimentary  formations  of  the  ordinary  type,  and  to  a 
totiilly  different  order  of  physical  conditions.  Natural,  however,  as  this 
conclusion  may  be,  those  who  adopt  it  probably  seldom  realise  to  what  an 
extent  it  rests  upon  mere  assumption.  Starting  with  the  supposition 
that  the  crystalline  schists  are  the  residt  of  geological  operations  that 
preceded  the  times  when  ordinary  sedimentation  began,  it  assumes  that 
they  belong  to  one  great  early  geological  period.  Yet  all  that  can  logic- 
ally be  asserted  :is  to  the  age  of  these  rocks  is  that  they  must  be  older 
than  the  oldest  formations  which  overlie  them.  If  in  one  region  of  the 
globe  they  appear  from  under  Cretaceous,  in  another  below  Carboniferous^ 
in  a  third  below  Silurian  strata,  their  chronology  is  not  more  accuratelj 
dctinal)le  from  this  relation  than  by  saying  they  are  respectively  pre-Creta- 
ceous,  pre-Carboniferous,  and  pre-Silurian.  They  may  all  of  course  belong 
to  the  same  period  ;  but  where  they  occur  in  detached  and  distant  areas^ 
there  is  as  yet  no  method  whereby  their  synchronism  can  be  proved.  To 
assert  it  is  an  assum])tion  which,  though  in  many  cases  irresistible,  ought 
not  to  be  received  with  the  confidence  of  an  established  truth  in  geology. 

In  the  investigation  of  the  problem  of  the  crystalline  schists,  much 
assistiince  may  be  derived  from  a  study  of  the  localities  where  a  crystal- 
line and  foliated  structure  has  been  superinduced  upon  ordinary  sedi- 
mentary and  eruptive  rocks — where,  in  fact,  these  rocks  have  actually 
])een  changed  into  schists,  and  where  the  gradation  between  their 
unaltered  and  their  alteretl  condition  can  be  clearly  traced.  In  recent 
years  so  nuich  attention  has  been  given  to  these  transfonnations  that  our 
knowledge  of  metamorphic  processes  has  Ixjen  greatly  extended,  and  the 
problem  of  i-egional  metamorphism,  though  by  no  means  entirely  solved, 
is  at  least  much  more  clearly  understood  than  it  has  ever  been  before. 

There  is  now  a  general  agreement  among  geologists  that  a  funda- 
mental condition  for  the  production  of  extensive  mineralogical  alteration 
of  rocks  has  ))een  disturbance  of  the  terrestrial  crust,  involving  the  intense 
(compression,  crushing,  fracturing,  and  stretching  of  masses  of  rock. 
Compression,  as  we  have  seen,  may  give  rise  to  slaty  cleavage  (p.  312)l 
Hut  the  same  kind  of  force  has  resulted  in  a  further  change,  wherein 
chemical  reactions  have  been  set  up  and  new  minerals  have  been  formed. 
The  effects  of  pressure  and  of  movement  under  great  strain  in  quickening 
(ihemical  activity  are  now  clearly  recognised.  Not  only  have  the  original 
minerals  been  driven  to  re-arrange  themselves  "tiith  their  long  axes 
per])endicular  to  the  direction  of  the  pressiu*e,  but  secondary  minerals 
^nth  well-marked  cleavage  have  been  developed  along  the  same  lines  and 

})aralU'l  to  the  stratification  of  the  ini mediately  overlying  sedimentary  formations.     See,  for 
instance,  the  summary  given  by  M.  Michel  L^vy,  BuU,  Soc.  OM>  FYanee,  xvL  1888,  jk  102. 


PART  VIII  §  ii  REGIONAL  METAMORPHISM  615 

thus  a  distinct  foliated  structure  has  been  induced  in  what  were  originally 
amorphous  rocks. 

Still  more  marked  are  the  transformations  where  the  rocks  have  not 
merely  been  compressed,  but  where  they  have  been  crushed,  fractured,  or 
stretched.  The  extraordinary  manner  in  which  the  crust  of  the  earth 
has  been  fractured  in  some  areas  of  regional  metamorphism  has  been 
worked  out  in  great  detail  by  the  Geological  Survey  in  the  north-west 
of  Scotland.^  We  there  perceive  how  slice  after  slice  of  solid  rock  has 
been  pushed  forward  one  over  the  other,  how  those  accumulated  slices 
have  been  driven  over  others  of  similar  kind,  how  this  structure  has  been 
repeated  again  and  again,  not  only  on  a  great  scale  involving  mountain- 
masses  in  the  movement,  but  even  on  so  minute  a  scale  that  the  ruptures 
and  puckerings  cannot  be  seen  without  a  microscope  (p.  624). 

Such  dynamical  movements  could  not  but  be  accompanied  with  wide- 
spread and  very  marked  chemical  change.  Along  the  margins  of  faults 
or  planes  of  shearing,  where  the  rocks  have  been  ground  against  each 
other,  there  is  a  selvage  of  foliated  material  which  with  its  new  mineral 
combinations  gradually  passes  into  the  amorphous  rock  on  either  side.  In 
such  places  sericite,  biotite,  chlorite,  or  some  other  secondary  product  with 
its  cleavage  planes  ranged  in  one  common  direction,  shows  the  line  of 
movement  and  the  reality  of  the  chemical  re-combinations.  In  the  body 
of  a  mass  of  rock,  also,  subject  to  great  strain,  relief  has  been  obtained 
by  crushing  along  certain  planes,  with  a  consequent  greater  development 
of  the  secondary  minerals  along  these  planes,  and  the  production  of  a 
banded  or  schistose  structure  in  a  rock  that  may  have  been  originally  quite 
homogeneous.^ 

The  recognition  of  the  powerful  part  taken  by  mechanical  deformation 
in  producing  the  characteristic  structures  of  many  schistose  rocks  has 
not  unnaturally  led  to  some  exaggeration  on  the  part  of  geologists,  who 
were  thus  provided  with  what  appeared  to  be  a  solution  of  difficulties 
which  at  one  time  seemed  insuperable.  There  can  hardly  be  any  doubt 
that  the  theory  of  mechanical  deformation  has  been  too  freely  used  and 
has  been  applied  to  structures  to  which  it  cannot  properly  be  assigned. 
Among  the  coarser  gneisses,  for  example,  the  segregation  of  the  component 
minerals  in  more  or  less  parallel  lenticular  bands  is  a  structure  that 
seems  to  find  its  nearest  analogy  rather  among  the  segregation-veins  of 
eruptive  bosses  and  sheets  than  among  sheared,  cleaved,  and  foliated  rocks, 
such  as  undoubtedly  have  been  the  originals  of  many  schists.  There  is 
nothing  to  show  that  this  parallel  banding  is  not  an  arrangement  of  the 
materials  of  an  igneous  magma  before  final  consolidation. 

But  while  this  tendency  to  a  too  liberal  use  of  dynamical  causes  in 
explication  of  all  the  structures  of  the  crystalline  schists  must  be  admitted, 
we  are  now  furnished  with  ample  evidence  of  the  efficacy  of  mechanical 
movements  in  the  production  of  regional  metamorphism.  It  is  frequently 
possible  to  detect  portions  of  the  ori^nal  structures,  to  show  that  they 
belonged  to  certain  familiar  and  definite  types  of  sedimentary  or  eruptive 

*  Quart.  Journ,  Geol,  Soc,  xliv.  (1888)  p.  378. 
'  G.  H.  Williams,  Bull,  U.S,  OeoL  Surv.  No.  62  (1890),  pp.  202207. 


616  OEOTECTOXir    STRVrTVRAL,  GEOUjGY  kȣ  rr 

rocks,  and  to  trace  every  stage  of  transition  from  them  into  the  meet 
perfectly  developed  ciystalline  schist.  In  the  cnuhing  down  of  \ai^ 
masses  of  rock  during  ix)werful  terrestrial  movements,  lenticular  cores  of 
the  rocks  have  frequently  e6cai)ed  entire  destruction.  Sound  these  core& 
the  pulverised  material  of  the  rest  of  the  rock  has  been  made  to  Act. 
somewhat  like  the  tiow- structure  round  the  porphyritic  crystals  of  a 
cooling  lava.  And  successive  gradations  may  lie  followed  until  the 
cores,  Vjecoming  smaller  by  degrees,  pass  finally  into  the  general  recon- 
structed material.  That  this  structure  is  not  original,  but  has  been  saper- 
iuduced  upon  the  rocks  after  their  solidification  can  be  abundantly 
demonstrated.  Among  the  sedimentary  formations  the  elongation  tnd 
Hattcning  of  the  pebbles  in  conglomerates,  and  the  transition  from  griiB 
or  greywackes  into  foliated  masses,  prove  the  structure  to  have  been 
superinduced.  Among  eruptive  rocks  the  crushing  down  of  the  original 
minerals,  and  their  transformation  into  others  characteristic  of  foliated 
rocks,  afford  the  same  kind  of  proof.^ 

So  great  has  been  the  pressure  exerted  by  gigantic  earth-movement« 
upon  the  rocks  of  the  crust  that  even  the  most  solid  and  massive 
materials  liave  been  sheared,  and  their  component  minerals  hare  been 
made  to  move  upon  each  other,  giving  a  flow- structure  like  that 
artificially  ])roduced  in  metals  and  other  solid  bodies  {antty  p.  316). 
But  it  may  be  doubted  whether  this  motion  is  ever  strictly  molecolar 
without  rupture  of  the  constituent  minerals.  Microscopic  examination 
shows  that,  at  least  as  a  general  rule,  the  minerals  in  the  most  thoroughly 
iN^nt  and  cnished  rocks  have  been  broken  down.  It  is  observable  that 
under  the  eficcts  of  mechanical  strain  the  minerals  first  undergo 
lainellation,  twinning  being  developed  along  certain  planes.  This 
structure  increases  in  distinctness  with  the  intensity  of  the  strain  so  long 
as  the  niinenil  (such  as  felspar)  retains  its  cohesion,  but  the  limit  of 
endurance  is  soon  reached,  beyond  which  it  will  crack  and  separate  into 
fragmi;nts,  which,  if  the  movement  is  arrested  at  this  stage,  may  be 
cemented  together  by  some  secondary  mineral  filling  up  the  interspaces. 
Hut  should  the  i)rcssure  increase,  the  mineral  may  be  so  wholly  pulverised 
as  to  assume  a  finely  granular  structure  or  mosaic  of  interlocking  grainy 
which  under  the  infiuence  of  continual  shearing  may  develop  a  streaky 
arrangement,  as  in  flow-structure  and  foliation. ^ 

^  Oil  the  iiieclianical  defonnation  and  dynamical  metaiuorphisni  of  rocks  see  A.  Heim, 
•'  Uutc-rsui-lnnigen  iiber  den  Mechanisnius  der  Gebirgsbildung,'*  1878;  A.  Rotbpletz,  ZriUek, 
Deutsrh.  iieoL  desdl.  xxxi.  (1879)  p.  374  ;  H.  Reusch.  "Die  foMilien-fUhrendcn  krysUl- 
liuiscliiMi  Schiefer  von  Hergen,"  German  translation  by  Baldauf,  1883.  Settes  Jahrb,  (Beiligr- 
band;,  1887,  p.  50  ;  *•  Boinmebien  og  Karmoen,"  1888  ;  Hep.  Oed.  Con^rtn,  LtmeUm,  1891. 
I>.  Ui2  :  Jjehniann,  *'  Untersuchungen  iiber  die  £nt8tehung  der  altkrystalliniifchen  Schiefer/' 
1884  ;  J.  J.  H.  Twill,  h'enl.  Mag.  1886,  p.  481  :  G.  H.  Williania,  BuU,  U.S,  Geol,  Surrep, 
No.  t>2  (1890).  For  an  instance  of  the  nietamorphism  of  a  conglomerate  into  albite  schist 
sie  J.  E.  Woltr,  Bui/.  Mvs.  dnnp.  Zoo/.  Ilnnanf,  xvi.  No.  10,  p.  174  (1891).  The  Paper* 
on  tli«  Crystalline  Schists  by  Heini,  Lorj-,  Lehmann,  Michel  L^vy,  Lawson,  and  the  U.S. 
<  K'ol.  Survey  in  the  report  of  the  London  Session  of  the  International  Geological  Congren 
(l>iilili>]ied  in  1S91)  should  also  Ik;  consulted. 

-  Lehmann,  oj).  cit.  pp.  245.  -249  ;  G.  H.  Williams  null  U.S.  Oeol.  Survey,  No.62,  p.  47- 


PARTvmgii  REGIONAL  METAMORPHISM  617 

One  of  the  most  important  effects  of  this  mechanical  deformation 
and  trituration  under  gigantic  pressures  has  been  the  great  stimuhis 
thereby  given  to  chemical  reactions.  So  constant  and  so  great  have 
these  reactions  been,  and  so  completely  in  many  cases  have  the  ingredients 
of  the  rocks  been  recrystallized  in  fresh  combinations,  that  the  new 
structures  thus  produced  have  masked  the  proof  of  the  mechanical 
deformations  that  preceded  or  accompanied  them.  It  is  in  the  main  to 
the  light  thrown  on  the  subject  by  the  microscopical  investigation  of 
the  minute  structures  of  the  metamorphosed  masses  that  we  are  indebted 
for  the  recognition  of  the  important  part  played  by  pressure  and  stretch- 
ing in  the  production  of  the  more  essential  and  characteristic  features  of 
metamorphic  rocks.  Many  chemical  rearrangements  may  undoubtedly 
take  place  apart  from  any  such  dynamical  stresses,  but  none  of  these 
stresses  appear  to  have  affected  the  metamori)hic  rocks  without  being 
accompanied  by  chemical  and  mineralogical  readjustments. 

The  mineral  transformations  observable  in  regional  metamorphism 
"may  consist  (1)  in  the  breaking  up  of  one  molecule  into  two  or  more 
with  but  little  replacement  of  substance,  as  in  the  formation  of  saussurite 
from  labradorite ;  (2)  in  a  reaction  between  two  contiguous  minerals, 
each  supplying  a  part  of  the  substance  necessary  to  form  a  new 
compound  of  intermediate  composition,  more  stable  for  the  then  existing 
conditions  than  either,  as  in  the  formation  of  a  hornblende  zone  between 
crystals  of  olivine  or  hypei'sthene  and  plagioclase ;  or  (3)  in  more 
complicated  and  less  easily  understood  chemical  reactions,  like  the 
formation  of  garnet  or  mica  from  materials  which  have  been  brought 
together  from  a  distance,  and  under  circumstances  of  which  it  is  at  present 
impossible  to  state  anything  with  certainty."  ^  The  following  transforma- 
tions especially  deserve  attention. 

Micasizalion — the  production  of  mica  as  a  secondaiy  mineral  from  fel8|xii-s  or  other 
original  constituents.  One  of  the  most  common  forms  of  this  change  is  where  the  silky 
unctuous  sci'iciU  has  been  developed  from  orthoclase  (sericitization).  The  formation  of 
mica  is  one  of  the  most  conmion  results  of  the  mechanical  deformation  of  rocks,  and  is 
most  conspicuous  where  tlie  pressure  or  stretching  has  been  most  intense.  Massive 
orthoclase  rocks,  such  as  granite,  quartz- porphyry  or  felsite,  when  most  severely  crushed, 
pass  into  sericite  schist ;  felspathic  grits  and  slates  may  be  similarly  changed.^ 

Uralitizntion — the  conversion  of  pyroxene  into  comimct  or  fibrous  hornblende. 
This  change  may  not  be  a  mere  case  of  paramorphism  or  molecular  rearrangement,  but 
seems  generally  to  involve  a  certain  amount  of  chemical  rearrangement,  such  as  the 
surrender  of  i)art  of  the  lime  of  the  pyroxene  towards  the  formation  of  such  combinations 
as  epidote,'  and  the  higher  oxidation  of  the  iron."*     It  has  taken  place  on  the  most 

1  G.  H.  Williams,  Bull  U.S,  Oeol,  Survey,  No.  62,  p.  50.  ThU  admirable  essay,  with 
its  copious  bibliography,  will  well  repay  the  careful  peruMil  of  the  student.  I  am  indebted 
to  it  for  t  he  abstract  of  nietamor])hic  processes  above  given. 

^  See  especially  Lehmann's  **  Untersuchungen  iiber  die  Entstehung  der  altkrystal- 
linischen  Sohiefergesteine,"  where  the  development  of  sericite  as  a  result  of  mechanical 
deformation  is  well  enrorced. 

»  Roseiibusch,  "Mikrosk.  Phys."  2nd  edition  (1887).  p.  18.'>. 

*  J.vJ.  H.  Teall,  (/uart.  Journ.  Oeoi,  Soc.  xli.  (1885)  p.  137. 


/ 


618  GEuTEf 'TONIC  (STRUCTURAL)  GEOLOGY  boo£IT 


extensive  scale  among  the  crystalline  scliiats.  Rocks  which  can  be  shown  to  have  been 
ori^nally  eniptive,  such  a^  diabases,  have  been  converted  into  epidiorite,  and  where  the 
deformation  has  advanced  furtlier,  into  hornblende -schist  or  actiuolite-scbiat. 

Epidoti'^atioii — the  production  of  epidote  in  a  rock  from  reactions  between  two  or 
more  minerals,  es]>ecially  between  pyroxene  or  hornblende  and  plagioclase.  In  some 
cases  dia]>Hses  liavc  licon  converted  into  epidiorites  or  a^^^gates  of  epidote  and  quartz 
or  calcite.* 

SauHHuritiiation — the  alteration  of  plagioclase  into  an  aggregate  of  needles,  prumi, 
or  gi-ains  (chiefly  zoisite),  imbedded  in  a  glass-like  matrix  (albite),  by  an  exchange  of 
silica  and  alkali  for  lime,  iron,  and  water.  This  change  has  largely  affected  the  felspir 
of  coarse  gabbros  or  euithotides,  es^^ecially  in  distiicts  of  regional  metamorphism.' 

Albitiztttion — a  jtrocess  in  which,  while  the  lime  of  the  plagioclase  is  remoTed  or 
crystallizes  as  (;alcite,  instead  of  forming  a  lime-silicate  like  epidote  or  zoisite,  the  rat 
of  the  original  mineral  rccr^'stallizes  as  a  finely  granular  aggregate  or  mosaic  of  dev 
grains  of  albite.  Examples  of  this  change  may  be  found  in  association  with  the 
develojmient  of  saussurite.^ 

Chloritization — an  alteration  in  which  the  i)yroxene  (or  hornblende)  of  the  so-called 
"  greenstoiK's  "  has  l>een  changed  into  secondary  sultstances  (1)  more  or  less  fibroii»in 
structure  allied  to  ser])entine,  not  [tleochi'oic  but  showing  a  decided  action  on  polarized 
light ;  or  (2)  sc-aly.  ])le(X^liroie,  ]H)laming  so  weakly  as  to  appear  isotropic,  and  more  or 
less  resembling  chlonte.  This  alteration  is  rather  the  result  of  weathering  than  of 
metiimor]>hism  in  the  strict  sense. ^  Where  chloritization  and  epidotization  bare 
pro(H'cde(l  simultaneously  in  aluminous  pyroxene  or  hornblende,  the  result  is  an  aggrq^ 
of  shari)ly  tlelined  jwile  yellow  crystals  of  epidote  in  a  green  scaly  ma.ss  of  chlorite.* 

Srrpcntini'^iitioH — an  alteration  more  esi>ecially  noticeable  among  the  more  highly 
basic  igneous  rot^ks  in  which  olivine  has  been  a  pi-ominent  constituent.  Tlie  gndoil 
conversion  of  olivine  into  ser]H?ntine  has  l.>een  already  descrilied  (p.  75),  and  the 
(K'^urrenee  of  massive  S4»ri)entine  has  been  referi^ed  to  (p.  173). 

AHemtions  of  Titanic  Iron. — The  ilmenite  or  titaniferous  magnetite  of  diabases  and 
other  eruptive  rocks  undergoes  alteration  along  its  margins  and  ciueks  into  a  dull 
grey  substance  (leucoxene,  p.  71),  which  is  now  known  to  be  a  form  of  titanite  or  sphene. 
Th»'  grey  rim  frequently  jwisses  into  well-defined  aggi-egates  and  crystals  of  sphene.' 

Mifntuirosi'i,  or  the  altemtion  of  an  ordinaiy  dull  limejitonc  into  a  crystalline- 
gi'anular  marble  (p.  602)  may  be  again  referred  to  here  as  one  of  the  characteristic 
transformations  in  regional  metamorphism. 

IhtJuinitizntUm.     See  p.  321. 

frranUization.     See  j).  .'»79. 

PnxltK^thii  uf  Ncir  Mhurals. — In  tiucts  of  regional  metamorphism  a  number  of 
secondary  minerals  may  be  obs<!rve<l  to  have  crj'stallized  out,  and  to  be  characteristic  of 
the  s(•hi^tose  riK^ks.  Among  the  most  conspicuous  of  these  are  white  and  black  mica, 
garnet,  quart?,  epidote.  (iarnet  occurs  abundantly  a.s  a  constituent  of  mica-schist  and 
gneiss,  anil  lias  resulted  from  tb(>  alteration  of  lM)th  clastic  and  massive  rocks  (com]iare 
J).  OO.'O. 

A  few  illustrative  examples  of  regional  metamorphism,  culled  from 
different  quarters  of  the  globe,  and  various  geological  formatioDB,  may 
here  be  given.     The  subject  is  further  discuss^  in  Book  VI.  Part  I. 

1  A.  S<henck,  *'Die  Diabase  der  oberen  Ruhrthals,"  1884. 

-  Hagge,  •*  Mikrosko}»isehe  Untersuclmngen  iiber  Gabbro,"  Ac.  Kiel,  1871,  p.  51. 

•'•  Lossen,  Jahrb.  Prevss.  O'eol.  Lnndesanat,  1883,  p.  640  ;  1884,  pp.  525-530. 

■•  Kosfnbnscli,  "Mikroskopische  Physiographic,"  pp.  180-184. 

•'  C.  H.  Williams,  Dull.  U.S.  iieol.  Surv.  No.  62,  p.  56. 

^  A.  Cathrein,  Zeiisch.  KrysU  und  Mineral,  vi.  (1882)  p.  244. 


PART  VIII  S  ii  REGIONAL  METAMORPHISM  619 


Early  Stages  of  Metamorphisni.— In  1871  Zirkel  showed  that  some  of  the 
clay-slates  of  the  disturbed  Silurian  and  Devonian  tracts  of  central  Europe  contain 
minute  microscopic  needle-shaped  microlites.  Considerable  diversity  of  opinion  has 
arisen  as  to  the  nature  of  these  rudimentary  crystallizations.  They  have  been  regarded 
as  microlites  of  hornblende,  nitile,  epidote,  &c.  More  recently  they  have  been  care- 
fully isolated,  extracted,  and  analysed  by  E.  Kalkowsky,  who  regards  them  as 
staurolite,  constituting  from  two  to  five  per  cent  of  the  rock.*  The  whet -slate  of 
Belgium  has  been  found  by  Renard  to  be  characterised  by  the  presence  of  abundant 
garnets.  Microscopic  tourmaline  and  rutile  likewise  occur  among  clay-slates.  No  one 
would  class  as  metamorphic,  the  rocks  in  which  these  microlites  occur,  and  yet  the 
presence  in  them  of  microscopic  microlites  and  crystals  shows  that  they  have  undergone 
.some  of  the  initiatory  stages  of  metamorphism,  by  the  development  of  new  minerals. 
All  that  Ls  known  of  the  probable  origin  of  these  minerals,  negatives  the  supposition 
that  they  could  have  \>een  formed  in  the  original  sediment  of  the  sea-bottom  on  which 
the  organisms  entombed  in  the  de])osit8  lived  and  died.  For  their  production,  a 
temperature  and  a  chemical  compasition  of  the  water  would  seem  to  have  l>een  required, 
such  as  must  have  been  inimical  to  the  co-existence  in  the  same  water  of  such  highly 
organised  forms  of  life  as  brachiojiods  and  trilobites. 

One  of  the  most  marked  of  the  early  stages  of  regional  metamorphism  is  characterised 
by  the  appearance  of  fine  scales  of  some  micaceous  mineral  (muscovite,  biotite,  kc.) 
As  these  micaceous  constituents  increase  in  number  and  size,  they  ini]>art  a  silky  lustrous 
aspect  to  the  surfaces  on  which  they  lie  parallel.  In  many  cases,  these  surfaces  are 
probably  those  of  original  dejiosit,  but  where  rocks  have  l)eeu  cleaved  or  sheared,  the 
mica  ranges  itself  along  the  planes  of  cleavage  or  shearing.  Tlie  Cambrian  tuffs  of  South 
Wales,  of  which  the  bedding  still  remains  quite  distinct,  present  interesting  examples  of 
the  development  of  a  mica  along  the  laminae  of  deposit.*  The  Dingle  beds  of  Cork  and 
Kerry,  on  the  other  hand,  have  been  subjected  to  cleavage,  and  the  mica  appears  along 
the  cleavage  ))lanes,  which  have  a  lustrous  surface.  The  Torridonian  and  Cambrian 
sandstones,  quartzites  and  shales  of  north-west  Scotland  show  a  development  of  mica 
along  the  surfaces  of  the  shearing-planes. 

Ardennes. — As  far  back  as  1848,  Dumont  publi.she<i  a  description  of  the  Belgian 
Ardennes,  in  which  he  showed  that  a  zone  of  his  "terrains  ardennais  et  rhenan,"  had 
undergone  a  remarkable  n)etamor])hism.  Sandstones,  in  a[)proaching  this  zone,  were 
transformed,  he  said,  into  quartzites,  and  by  degrees  ftassed  into  rocks  characterised  by 
the  presence  of  ganiet,  hornblende,  and  other  minerals  ;  the  slates  (phyllades)  gradu- 
ated into  dark  rocks,  in  which  magnetite,  titanite,  and  ottrelite  had  l>een  developed. 
Yet  the  fossiliferous  character  of  the  strata  thus  metamorphosed  ha^l  not  been  destroyed. 
In  sjiecimens  showing  a  gradation  from  a  grit  to  a  conii>act  garnetiferous  and  hornblendic 
quartzite,  I^of.  Sandberger,  to  whom  they  were  submitted,  recognised  the  presence  of  the 
two  Devonian  shells,  Spiri/er  niacropterus  and  ChoncUs  sarcinulatus,  "The  garnets 
an<i  the  fossils  are  associated  in  the  same  specimen,"  he  wrote,  adding,  "who,  after  this, 
can  hesitate  to  admit  that  the  crystalline  schists  and  quartzites  of  the  Hundsriick  and 
Tannus  are  likewise  metamorphosed  Taunusian  rocks  ? "  ' 

In  1882  M.  Renanl,  fortified  with  the  resources  of  modern  i)€trography,  renewed  the 
examination  of  Dumont's  metamorphic  area  of  the  Ardennes,  and  conclusively  established 
the  accuracy  of  all  the  main  facts  noticed  by  the  earlier  observer.  Not  only  do  the 
geological  structure  of  this  region,  and  the  occurrence  of  recognisable  fossils,  show  that 
the  rocks,  now  transformed  into  more  or  less  crystalline  masses,  were  originally  parts  of 

*  Seue*  Jahrb.  (1879)  p.  382.  These  bodies  are  to  be  distinguiMhe<l  from  the  minute 
crystals  of  such  durable  minerals  as  zircon,  rutile,  &c.,  so  often  recognisable  as  clastic  grains 
in  sediments,  and  which  may  often  have  played  a  ]iart  in  the  sedimentation  of  more  than 
one  geological  period. 

'  Q.  J,  Oeol.  Soe.  xxxix.  (1883)  p.  310.  '  Xeues  Jahrb,  (1861)  p.  677. 


620  GEOTECTONIC  {STRUCTURAL)  GEOLOGY  book  iv 

the  oi-dinary  series  of  Devoniau  sandstones,  greywackes,  and  shales,  but  the  microscope 
comes  in  to  confirm  this  conclusion.  The  original  clastic  grains  of  quartz  and  tlie  diflfiised 
carbonaceous  material  of  the  unaltered  strata  can  still  be  recognised  in  their  m«t«mor- 
phosed  equivalents.  But  there  have  been  developed  in  them  abundant  new  minerals — 
garnet  (1  to  2  mm.),  hornblende,  mica,  titanite,  ai)atite,  bastonite,  ottreliteJ 

Dumont  appeare  to  have  believed  that  the  metamorphism  which  he  had  traced  so 
well  in  the  Ardennes  was  to  be  attributed  to  the  influence  of  underlying  masses  of 
eruptive  rocks,  though  he  frankly  admitted  that  the  metamorphism  is  less  marked  where 
eruptive  veins  have  made  their  appearance  than  where  they  have  not.'  M.  Rasard, 
however,  points  out  that  eruptive  rocks  are  really  absent,  and  that  the  association  of 
minerals  proves  that  the  metamorphosed  rocks  could  not  have  been  softened  by  a  high 
temperature,  as  supposed  by  Dumont,  otherwise  the  simultaneous  presence  of  graphite 
and  silicates,  with  protoxide  iron  bases,  such  as  mica,  hornblende,  &c.,  would  certainly 
have  given  rise  at  least  to  a  partial  production  of  metallic  iron.  He  connects  the 
metamorphism  with  the  mechanical  movements  which  the  rocks  have  undergone  along 
the  altered  zone.^  The  metamorphism  of  this  region  has  since  been  described  by  Professor 
Gosselet,  who  also  regards  it  as  due  to  dynamical  causes.^ 

Taunus. — A  similar  example  of  regional  metamorphism  extends  into  the  tracts  of  the 
Taunus  and  Hmulsriick.  In  1867  K.  A.  Lessen  published  an  elaborate  memoir  on  the 
structure  of  tlie  Taunus,  which  is  now  of  classic  interest  in  the  history  of  opinion 
regarding  metamorphism.'  He  showed  that  below  the  middle  Devonian  limestone,  the 
usual  lower  Devonian  slates,  greywackes,  and  quartzites  rise  to  the  surface,  but  that 
these,  traced  southwards,  pass  gradually  into  various  crystalline  schists.  Among  these 
schists,  he  distinguished  sericite-gneiss,  mica-schist,  phyllite,  knotted  schist,  angitf* 
schist,  sericite-lime-)>hyllite,  quai'tzite,  and  kiesel-schiefcr.  As  intermediate  grades 
between  these  crystalline  masses  and  the  ordinary  clastic  strata,  he  observed  quartz- 
conglomerates,  with  a  ciystalline  schistose  matrix,  or  with  albite  crystals,  and  quartzites 
with  sericite  or  mica.  He  concluded  that  while  these  crystalline  rocks  present  the 
most  complete  analogies  with  those  of  the  Alps,  Silesia,  Brazil,  &c.,  they  are  yet  so 
intimately  bound  up  alike  petrographically  and  stratigraphically  with  strata  containing 
Devonian  fossils,  and  into  which  they  pass  by  semi-crystalline  varieties,  that  they  must 
be  considered  as  of  Devonian  age.  Subsequently  K.  Koch  proposed  to  regard  the 
crystalline  schists  of  the  Taunus  as  Cambrian  (Huronian),®  and  they  have  been  indicated 
on  the  Geological  Survey  map  as  Cambrian  or  Silurian.     Hut  the  fact  that  a  conformable 


^  Renard  {Bull.  Mns.  Roy.  Beltjique,  i.  (1882)  p.  14)  estimates  the  components  of  one 
of  these  altered  rocks  to  be — 

Graphite      .......       4*80 

Apatite  .1-51 

Titanite 1-02 

Garnet  .......        4*14 

Mica 20-85 

Horul)len«li^  .....     37'62 

Quartz 30-62 

Water 1-32 


101-88 

-  Renard,  oj).  cit.  \k  34.  '  Op.  cit.  p.  37. 

^  See  his  great  Monograph  on  the  Ardennes,  Mhn.  Carte  Oeol.  France,  1888,  chap.  xxv. 

*  '  Geognostische  Be.schreibung  der  linksrlieinischeu  Fortsetzung  des  Taunus,'  kc,  Z. 
Dcutsch.  iied.  ties.  xix.  (1867)  p.  509,  (1885)  p.  29. 

"  See  Lossen'.s  reply,  Z.  Deutscii.  iJeol.  (Jes.  xxix.  (1877)  p.  341.  He  argues  convincingly 
against  the  supposition  that  these  can  be  original  chemical  deposits  of  Cambrian  age.  (See 
also  Renanl,  Bull.  Mus.  Roy.  Belg.  i.  p.  81,  note.) 


PARTViiigii  REGIONAL  METAMORPHISM  621 

sequence  can  be  traced  from  undoubted  fossiliferous  Devonian  strata  downwards  into 
these  crystalline  schists  makes  it  immaterial  what  stratigraphical  name  may  be  applied 
to  them.  They  are  almost  certainly  Devonian,  as  Lossen  described  them,  and  in  any 
case,  they  are  unquestionably  the  metamorphosed  equivalents  of  what  are  elsewhere 
ordinary  sedimentary  strata. 

Scandinavia  is  mainly  composed  of  crystalline  schists  which  have  been  assigned 
to  the  so-called  Archsean  system.  That  some  portions  of  them  cannot  be  of  so  ancient  a 
date  was  shown  some  years  ago  by  Tomebohm  in  the  uplands  of  Sweden.  More  recently 
similar  deductions  have  been  drawn  from  a  study  of  the  development  of  the  rocks  in 
Norway.  At  the  Hardanger  Fjord  the  following  order  of  succession  was  established  in 
1875  and  1877  by  W.  C.  Brogger :  »— 

Crystalline  schists  (diorite-schists,  hornblende-schists,  ganietiferous  mica- 
schists,  true  gneisses,  &c.),  the  whole  series  becoming  more  and  more 
crystalline  towards  the  higher  beds. 

Greenish  micaceous  schists  (phyllites).  This  and  the  overlying  group 
must  be  several  thousand  feet  thick. 

Impure  white  marble  (probably  orthoceratite  limestone)    .  30  ft. 

Hlue  quartz-sandstone' 100 

Black,  little  altered  alum-schist,  with  Dktyograptus  band  .  .150 

This  section  confirmed  the  early  conclusion  of  Naumann  that  the  great  series  of 
crystalline  schists  of  the  Norwegian  uplands  overlies  the  Silurian  stage  2  in  the 
Christiania  district.  Subsequently  H.  H.  Reusch  obtained  from  the  Bergen  district 
clear  proof  of  the  Silurian  age  of  certain  crystalline  rocks  in  that  part  of  Norway.  ^  He 
found  among  masses  of  mica-schist,  hornblende-schist,  gneiss,  and  other  crystalline  rocks, 
intercalated  bands  of  conglomerate  which,  while  obviously  of  clastic  origin,  have  under- 
gone enormous  compression,  the  pebbles  being  squeezed  flat  and  the  paste  having 
become  more  or  less  crystalline.  The  occurrence  of  such  bands  suggests  a  sedimentary 
origin  for  the  whole  series  of  deposits.  But  from  several  localities  he  obtained  fossils 
which  have  been  recognised  as  undoubtedly  Upper  Silurian.  Some  of  the  fossils  occur 
in  a  crystalline  limestone,  which  is  intercalated  in  a  dark  lustrous  phyllite.  But  they 
are  found,  as  casts,  most  abundantly  in  a  light-grey  lustrous  micaceous  schist,  which, 
under  the  microscope,  is  obsei*ved  to  be  composed  in  large  measure  of  quartz,  not  having 
a  fragmental  aspect,  with  mica,  rutile,  and  tourmaline.  The  fossils  recognised  comprise 
PhacopSf  CkilymenCj  several  undeterminable  gasteropods  and  brachiopods,  Cyathophyllumy 
HcUysUcs  catenulariay  FavosiUs,  RastriteSf  Moiwgraptus^  and  some  others. 

According  to  Reusch  the  sequence  of  rocks  is  continuous,  and  their  thickness  must 
be  at  least  16,000  feet.  If  we  suppose  that  the  fossiliferous  zones  have  been  brought  into 
an  older  series  by  plication  of  the  crust,  the  fact  remains  that  the  rock  in  which  most  of 
the  fossils  occur  is  itself  a  micaceous  schist,  like  those  among  which  it  is  imbedded,  and 
therefore  a  metamorphic  rock.  It  is  consequently  proved  that  some  at  least  of  the  meta- 
morphic  rocks  of  Norway  are  of  Silurian  age,  and  ai*e  associated  with  evidence  of  great 
mechanical  movements  in  the  crust  of  the  earth. 

The  Alps. — In  the  geological  structure  of  the  central  Alps,  crystalline  schists  play 

^  'Die  Silurischen  Etagen  2  uud  3  im  Kristiania  Gebiet,'  p.  352.  The  Swedish  work  of 
Tomebohm  is  referred  io  postea,  p.  711. 

'  *  Silurfossiler  og  Pressede  Konglomerater  i  Bergensskifrene,'  Christiania,  1882  ;  or  the 
same  work  translated  into  German  by  R.  Baldauf,  '  Die  fossilien-fiihreuden  krystallinischen 
Schiefer  von  Bergen  in  Norwegen,'  Leipzig,  1883.  In  the  year  1889  I  had  an  opportunity 
of  personally  going  over  Dr.  Reusch's  Bergen  region  and  of  collecting  fossils  from  the  rocks 
in  which  he  found  them.  There  can  be  no  doubt  that  he  has  demonstrated  that  the  meta- 
morphism  of  that  district  has  been  connected  with  powerful  dynamical  movements,  the  latest 
of  which  are  of  younger  date  than  the  Upper  Silurian  period. 


622  GEOTECTONIC  {STRUGTUBAL)  GEOLOGY  book  nr 


an  important  part.  Originally  these  rocks  were  regarded  as  one  series,  of  much  more 
ancient  date  than  the  ordinary  sedimentary  foimatious,  and  of  very  different  origin.  The 
discovery  of  Silurian,  Devonian,  Carboniferous,  and  Jurassic  fossils  in  various  schists  and 
altered  limestones  surrounding  the  central  gneiss,  led  to  the  belief  that  these  are  meta- 
morphosed sedimentary  rocks  of  Palceozoic,  Mesozoic,  and  even  of  older  Tertiary  date. 
This  belief  has  subsequently  been  attacked  by  several  able  observers,  who,  starting  with 
the  assumption  that  the  crystalline  schists  must  be  everywhere  of  great  relative  antiquity, 
have  endeavoured  to  show  that  the  fossiliferous  bands  intercalated  among  them  have  been 
brought  into  this  position  by  plication,  and  that  there  is  no  evidence  that  any  part  of 
the  schists  is  even  of  Palseozoic  age.^  Now  it  must  be  admitted  that  in  the  sections,  even 
as  drawn  by  those  who  adopt  this  explanation,  the  obvious  and  natural  interpretation  is 
that  which  has  been  so  generally  adopted — that  the  fossiliferous  beds  are  actuaUy  part 
of  the  crystalline  series  in  which  they  arc  imbedded.  If  the  apparent  order  is  deceptiTe, 
this  must  be  proved  by  those  who  maintain  it.  If,  however,  we  turn  to  their  writings 
we  find  a  good  deal  of  strong  assertion,  and  various  more  or  less  ingenious  attempts  to 
construct  sections  in  which  the  abnonnal  position  of  the  fossiliferous  beds  is  to  be 
accounted  for.  It  does  not  appear  to  l)e  realised  that  on  the  supposition  of  the  high 
antiquity  and  original  discordant  infrapositiou  of  the  schists,  the  chances  are  small  that, 
in  any  plication  of  the  mountains,  the  unconfonuable  fossiliferous  strata  would  become 
conformably  stratified  with  ancient  schists  even  at  one  locality.  But  when  we  look  at 
the  published  sections  of  the  Alps,  and  find  that  the  parallelism  between  the  schists  and 
the  enclosed  fossiliferous  Imnds  occurs  again  and  again  at  widely  se])arated  localities,  and 
that  in  fact  this  is  their  normal  position,  it  becomes  utterly  incredible  that  the  conform- 
ability  can  be  the  result  of  plication,  except  on  the  sujiposition  that  the  foliation  of  the 
schists  is  not  their  original  structure,  but  a  new  one  superinduced  upon  them  at  the  time 
of  the  plication  and  metamorphism  of  the  fossiliferous  strata.'*^ 

Let  us,  however,  grant,  for  the  sake  of  argument,  that  the  concordance  is  everywhere 
deceptive,  and  that  between  the  schists  and  the  fossiliferous  series  of  formations  there  is 
really  a  great  hiatus.^  When  the  fossil -bearing  intercalations  are  examined  they  are 
themselves  found  to  be  metamorphosed.  The  Jurassic  limestones  have  been  marmarized, 
and  the  shales  have  become  lustrous  sericite-sehists  in  which  belemnites  and  other  fossils 
are  recognisable.  The  Triassic  rocks  have  been  in  like  manner  rendered  crystalline,  and 
I)resent  Secondary [crj'stals  of  albite,  cpiaitz,  mica,  tourmaline,  garnet,  &c.  The  Carlwni- 
ferous  strata,  when  their  age  can  be  determined  hy  enclosed  fossils,  consist  of  dark 
anthracitic  bands,  whieli   have  undergone  less  alteration  than  the  adjacent  schists.* 

^  Consult  Lory,  'Description  goologique  du  Dauphim'' '  (1860),  part  i.  §§  40-42  ;  Compte 
rcrulu  Conijres  (Uoloijiqti.e  Infeniatiomily  Paris,  1881,  pp.  39-43  ;  Bull.  Soc.  Gkd.  France, 
3e  si'rie,  ix.  (1881)  pi).  652-679  ;  Fa\Te,  *  Recherches  grologi(pies  dans  les  parties  de  la  Savoie, 
&c.,  voisiues  du  Mt.  Blanc'  (1867),  chaps,  xxi.  xxiv.  xxv.  ;  A.  Miiller,  Alfm,  Soc  d^UiaL 
Nat.  Mle,  1865-70.  See  also  Sismonda,  Heal.  Acad.  :<ci.  Torin.  (2)  xxiv.  (1866)  p.  333; 
Sterry  Hunt,  *  Cliem.  Essays,'  pp.  283,  328.  Bonuey,  Aildress,  Qiuirl.  Journ,  Geol,  Soc. 
xlii.  (1886)  p.  38  ;  xlvi.  (1890)  p.  187,  and  other  papei-s  cited,  jxistea,  p.  624. 

2  See  this  structure  illustrated  by  that  of  north-west  Scotland,  postea^  p.  624. 

^  Professor  Lory  believed  tliat  in  the  Western  Alps  there  is  a  conforuiability  and  even 
gradation  between  the  true  crystalline  schists  and  the  Paljeozoic  and  Secondary  rocks.  He 
regarded  the  crystalline  character  of  the  latter  as  an  original  feature  dating  from  the  time  of 
deposition.  See  his  HsunU  in  the  Rei>ort  of  the  London  meeting  (1888)  of  the  Inter- 
national Geological  Congress,  and  the  views  of  M.  Michel  Lev}-  in  the  same  Report. 

^  It  is  well  known  that  carbonaceous  strata  can  be  recognised  across  zones  of  contact- 
metamorphism,  when  the  normal  character  of  the  ordinary  strata  above  and  below  them 
have  been  destroyed.  This  is  well  seen  in  the  case  of  the  black  graptolitic  shales  of  the  south 
of  Scotland,  and,  still  more  strikingly,  in  those  of  Christiauia.  See  Briigger's  memoir  cited 
on  p.  621. 


PART  VIII  §  ii  REGIONAL  METAMORPHISM  623 


But  the  extraordinary  way  in  which  many  of  the  plants  in  the  Alpine  Carboniferous 
rocks  have  been  distorted  indicates  the  enormous  shearing  which  these  rocks 
have  undergone.^  At  Vernayaz,  near  Martigny,  the  Carboniferous  strata  can  hardly  be 
separated  from  the  schists ; '  and,  indeed,  had  Carboniferous  plants  not  been  found  in 
them  the  idea  would  probably  never  have  occurred  to  any  one  to  draw  a  line  between 
them.  At  the  well-known  locality  of  Petit  Coeur,  the  plants  so  abundantly  and  admir- 
ably preserved  in  black  schist,  have  had  their  original  substance  replaced  by  a  white 
hydrous  mica.' 

A  detailed  investigation  of  the  geotectonic  and  {tetrographical  relations  of  these 
intercalated  Carboniferous  bands  was  carried  out  in  1882  by  the  late  Mr.  Stur,  Director  of 
the  Austro-Hungarian  Geological  Survey,  and  Baron  von  Foullon.*  On  the  northern 
boi-der  of  the  Styrian  Alps  near  Leoben  a  group  of  crystalline  schists  10,000  to  13,000 
feet  thick  reclines  steeply  (but  it  is  said  conformably)  against  gneiss.  It  consists  of 
phyllite-gneiss,  mica-schist,  and  chlorite-schist,  with  four  bands  of  dark  graphitic  schist 
and  one  or  two  seams  of  limestone.  The  plant-bearing  graphitic  schist  is  full  of  plant- 
remains  {Calamites  ramosuSj  Fecopteris  lonchiticay  Lepidodendron  phleginaria^  &c.)  The 
association  of  plants  and  the  occurrence  of  bands  of  graphite,  representative  doubtless  of 
former  beds  of  coal,  indicate  that  these  carbonaceous  rocks  belong  to  the  well-known 
Schatzler  group  of  the  lower  Coal-series  of  Silesia.  The  whole  succession  of  schists  of 
which  these  plant-bearing  beds  are  members,  forms  one  continuous  group,  which  Stur 
recognised  as  traceable  for  a  long  distance  on  the  northern  margin  of  the  central  range 
of  the  north-easteni  Alps.  He  insisted  that  this  group  of  schists  cannot  be  the  i-esult  of 
original  chemical  deposition,  but,  on  the  contrary,  that  it  is  shown,  by  a  great  series  of 
facts,  to  be  the  metamorphosed  equivalent  of  what,  elsewhere,  are  unaltered  Carboni- 
ferous strata.  The  distortion  of  the  fossils,  which  proves  that  the  rocks  have  behaved 
like  plastic  masses  under  the  strain  of  mountain -making,  the  alteration  of  their  substance 
into  anthracite  or  graphite,  and  its  replacement  by  micaceous  silicates,  are  evidence  of  a 
serious  metamorphism.  On  the  other  hand,  the  occurrence  of  unaltered  plant-bearing 
Carboniferous  rocks  elsewhere  in  the  Alps  shows  that,  as  usual,  the  metamorphism  has 
not  been  everywhere  equally  intense.  Stur  concluded  that  there  was  every  encourage- 
ment to  search  for  fossils  in  the  schist  envelope  of  the  central  Alpine  gneiss.* 

Baron  von  Foullon  describes  the  petrogi-aphical  characters  of  the  various  membei-s  of 
the  gi-oup  of  schists  in  which  the  plants  occur  near  Leoben.  As  to  the  thoroughly 
crystalline  chai-acter  of  the  phyllite-gneiss,  mica-schist,  &c.,  there  can  be  no  dispute. 
It  will  be  enough  here  to  refer  briefly  to  the  constitution  of  the  graphite-schist  in  which 
the  plants  occur.  Hand -specimens  present  a  dull  fracture,  on  which  none  of  the  com- 
ponents, except  the  graphite,  can  be  recognised,  though  sometimes  they  show  a  greenish 
fibrous  asbestiform  mineral.  In  thin  slices,  the  rock  is  seen  to  be  composed  of  quartz 
grains,  ehloritoid,  an  asbestos -like  substance,  and  a  mica,  with  abundant  ''clay -slate 
microlites,"  and  diffused  carbonaceous  matter.     It  resembles  the  mica-chloritoid -schists 

*  See  Heer's  *  Flora  Fossilis  Helvetin  *  (Steinkohlen  Flora),  plate  iv.  fig.  1  ;  v.  figs.  1,  3  ; 
viii.  figs.  1,  2  ;  xiii.  fig.  1,  &c. 

2  Favre,  *  Recherches  g^ol.'  ii.  p.  351.    The  same  fact  is  admitted  by  Lory  to  be  often 
true  elsewhere  {Bull,  Soc.  Oiol,  France,  ix.  (1881)  p.  663). 
'  Favre,  op,  cit.  iii.  p.  192. 

*  Jahrb.  Geol.  Reichsanst.  xxxiii.  (1883)  pp.  189,  207.  See  also  Toula,  Verh.  Qed. 
ReichsansL  1877,  p.  240. 

*^He  had,  many  years  before  this,  announced  his  belief  that  the  schistose  envelope 
(Schieferhulle)  of  the  Alps  probably  represents  Palseozoic  rocks.  Stache,  in  1874,  wrote 
that  "the  question  now  is  how  far  Cambrian  or  Silurian  rocks  are  represented."  Jahrb. 
Oeol.  Reichs.  1874,  p.  159.  In  1884  he  thought  that  the  epicrystalline  condition  of  the 
Silurian  rocks  in  the  Alps  might  be  due  to  original  crystalline  precipitation :  Z.  Deutsch, 
Qed.  Get.  1884,  p.  356. 


624  GEOTECTONIC  {STRUCTURAL)  GEOLOGY  book  iv 

of  the  Tauims.  Some  of  the  chloiitoid-schists  or  quartz-phyllites  associated  with  tliis 
]>laiit-l)eariug  Imnd  are  also  graphitic.  Petrographical  investigation  thus  concurs  with 
the  stratigraphical  eWdence  to  prove  that  a  tract  of  the  crystalline  schists  of  the  north- 
eastern Alps  consists  of  metamorphosed  Carboniferous  rocks. 

The  Silurian  rocks,  which  in  the  eastern  Alps  are  greywacke  and  slate,  become  more  and 
more  crystalline  as  they  are  followed  westwards.  The  Liassic  shales  become  micasized 
towards  the  central  mountains,  the  fossils  by  degrees  disappear,  and  the  limestones 
assuming  a  jointed  asfteot,  iinally  pass  into  a  completely  crystalline  condition.  In  the 
Vaud  Alps,  the  belemnites  of  the  middle  Oxfordian  shales  gradually  disappear  in  pro- 
]>ortion  as  the  rock  becomes  more  schistose,  till  at  the  Diablerets  it  is  an  almost  crystal- 
line sericitic  schist.^  The  Eocene  strata,  also,  under  intense  com)>ression,  have  aasnmed 
the  chai-acter  of  slates,  which  are  worked  for  economic  purposes.* 

So  far,  therefore,  from  being  entirely  a  pre-Cambrian  series,  the  crystalline  schists  of 
the  Alps  can  be  demonstrated  to  include  metamorphosed  Paleozoic  and  Secondary  rocks 
along  their  outer  border.  How  far  towards  the  central  mass  of  the  mountains  they  are 
of  Palfleozoic  age  has  yet  to  be  determined.  As  the  rocks  become  more  and  more  crystal- 
line ill  that  direction  it  may  not  always  be  possible  to  define  the  base  of  the  altered 
Palfleozoic  roi-ks.  That  there  is  a  nucleus  of  ancient  or  "Archroan"  gneisses  is  not 
disputed  ;  but  its  limits  must  be  proved  by  stratigraphical  evidence.' 

Scottish  Highlands. — This  region  consists  mainly  of  crystalline  schists  with 
bosses  of  granite,  ]K)r])hyry,  &c. ,  which,  stretching  through  four  degrees  of  latitude  and 
four  and  a  half  of  longitude,  must  cover  an  area  of  not  less  than  16,000  square  miles  at 
the  surface.  As,  however,  they  sink  beneath  later  formations,  and  are  prolonged  into 
Ireland,  their  total  area  must  be  still  more  extensive.  The  oldest  rocks  consist  nuunly  of 
a  remarkably  coarse  crystalline  gneiss  (Lewisian,  1  in  Fig.  311),  with  abundant  pegma- 
tite veins,  seen  in  Sutherland  and  Ross,  the  two  north  -  westerly  counties  of  Scotland. 
This  gneiss,  which  will  be  described  in  the  section  on  pre-Cambrian  rocks  in  Book 
^'I.,  is  unconformably  overlain  by  nearly  flat  bro>\Tii8h-red  (Torridonian)  sandstones, 
conglomerates  and  breccias  (2),  which  in  tuni  are  surmounted  unconformably  by 
inclined  beds  of  quartzite  (3,  4),  shales  (5),  calcareous  giit  (6),  and  limestones  (7),  the 
geological  age  of  which  is  fixed  by  the  occurrence  of  recognisable  fossils  in  them.  The 
(j^uartzite  is  full  of  anneli(le-bunx)wa ;  the  shales  contain  Oleuellus — the  distinctive 
trilobite  of  the  lowest  Cambrian  rocks  ;  the  limestone  has  yielded  Maelnrea^  Mureki- 
soniUj  Ojyhiletn,  Pleurotomaria,  OrthU,  Orthocenis,  Piloccras,  and  many  more  forms, 
indicating  Cambrian  and  jjossibly  the  very  lowest  Silunan  horizons.  The  strata  are 
generally  crowded  with  carlwnaceous  worm -casts  (the  so-called  "fucoids").  Along  their 
western  margin,  these  rocks  are  so  little  altered  that  they  do  not  in  any  way  deserve 
the  name  of  nietaniorphic.     Eastwards,  however,  they  jmss  under  vaiious  schists  and 


1  Renevier,  Bull.  Soe.  did,  France  (3),  ix.  p.  650  ;  xvii.  (1889)  p.  884. 

^  Loi-y,  Bull.  Soc.  diol.  France,  ix.  (1881)  p.  651. 

3  M.  Vacek  has  shown  an  unconformability  between  the  older  central  schists  and  the 
Silurian  gneiss,  diorite-schist,  mica-schist,  and  chloritoid-schist.  Jahrh.  Oeol,  ReichmruL 
xxxiv.  (1884)  p.  620.  The  Palaeozoic  and  Secondary  age  of  part  of  the  schists  of  the  Alps 
is  enforced  by  Heim,  *  Mechnnismus  der  Gebirgsbildung,'  1878  ;  Compte  rend.  Congria 
{ii(il.  International  Dmdon  (1888),  p.  16  ;  Naivrt,  xxxviii.  (1888)  p.  624  ;  Quart.  Joitm, 
tied.  «Sr>c.  xlvi.  (1890)  p.  236.  Grubenmann,  Mittheil.  Thurganiachen  Naturf.  GeaeUsek. 
Heft  viii.  (1888).  Baltzer,  *  Beitrage  zur  Geol.  Karte  der  Schweiz,*  No.  24  (1888).  The 
volumes  of  these  "  Beitrage  "  contain  ample  details  regartling  the  geological  stmctura  of  the 
Alps.  P.  Temiier,  Comptes  rend.  Acad.  France,  cxii.  (1891)  i>.  900.  Prof.  Bonney  holds 
that  the  crystalline  schists  of  the  Alps  are  older  than  the  Paleozoic  rocks.  See,  for  example, 
liis  Address  to  the  Geol.  Soc.  {Q.  J.  Ged.  Soc.  vol.  xlii.  1886,  \).  66),  and  the  same  Jonnial 
for  1889,  p.  67  ;  1890,  p.  187  ;  1892,  p.  390. 


REGIONAL  METAMURPHISM 


625 


gneisses  (S,  9,  10),  nbich  fann  a  riut  OTerlying,  thoroughly  cryBtelliue  series.  It  waa 
betierud  by  ilacculloch  and  Hsy  Cunningham  that  tlie  foBBilirerous  beds  tnity  underlie, 
and  are  older  than,  the  eastern  gneiss.  This 
view  waa  adopted  and  worked  out  in  Bonie  detail 
by  Murchison,  who  extended  his  generalisation 
over  the  whole  area  of  the  Higblands,  which  he 
regarded  as  composed  essentially  of  metamor- 
phosed Silurian  rocka  (see  p.  69S).  Other 
geologists  supported  Murchison,  whose 
opinions  met  with  general  acceptance.  Nico], 
however,  contended  that  the  overlying  or 
"newer  gneiss"  is  merely  the  old  gneiss 
brought  up  by  faulting.  Later  writers, 
particularly  Prof.  Lapworth,  Dr.  Callaway, 
and  Dr.  Hicks,  advanced  somewhat  ^ini- 
Ur  opinions  ;  but  the  difficulty  remained  of 
explaining  how,  if  the  "newer  gneiss"  in 
really  older  than  the  fossiliferous  strata,  it 
should  overlie  them  ao  conformably  as  to 
have  deceived  so  many  observers.  The 
problem  was  subscijuently  attacked  independ- 
ently by  Prof,  lapworth  and  by  the  Geo- 
logical Survey,  esfieciallj  by  MeasiH.  B.  N. 
Pea.;h,  J.  Home,  W.  Guim,  C.  T.  Clojigh,  L. 
Hinunan,  and  H.  M.  Cadell,  and  1  believe 
it  has  now  been  solved.  I  fully  ahared  Mur- 
chiHOn's  belief  in  a  continuous  upward  succes- 
BiDn  from  the  fosailiferoiis  Lower  Silurian  strata 
into  the  overlying  schists,  but  the  subse- 
quent detailed  investigation  of  the  ground  con- 
vinced me  that  this  belief  could  no  longer  be 
entertained. 

Tracing  the  unaltered  Cambrian  strata  east- 
wards from  where  they  lie  in  their  normal 
position  upon  the  Torridon  sandstone  and  old 
gneiss  below,  we  find  them  begin  to  undergo 
curvature.  They  are  thrown  into  N.N.E.  and 
S.S.W.  Htiticlinal  and  synclinal  folds  which 
become  increasiiigly  steeper  on  their  western 
^nts  until  they  are  disrupted,  and  the  eastern 
limb  of  a  fold  is  pushed  over  the  western. 
By  a  system  of  reversed  faults  (t  t  in  Fig.  311), 
a  single  group  of  strata  ia  made  to  cover  a 
great  breadth  of  ground  and  actually  ia  overlie 
higher  members  of  the  same  series.  The  moat 
extraordinary  dislocations,  however,  are  the 
Thrust- planes.  These  have  so  low  a  hade 
that  the  rocks  on  their  upthrow  aide  have  been, 
a«  it  were,  pushed  horizontally  westwards,  in 
sMne  places  for  a  distance  of  at  least  ten  miles. 
But  for  the  evidence'ef 


it  ttie  clear 
tutgnished  from   ordinary  stnitiflcation -planes. 


laultedanddenuded(dott«d]inesinFig.311).     Hereand  h 


these   hrust  planes  coi 

wh   h     h  y  ha 


y 


626  GEOTEGTONIG  (STRUGTURAL)  GEOLOGY  book  it 

displaced  Le\«isian  gneiss  may  be  seeu  capping  a  liill  of  quartzite  and  limestone  like 
an  ordinary  overlying  formation. 

The  general  trend  of  all  the  foldings  and  ruptures  is  N.N. K  and  S.S.  W.,  and  aa  the 
steeper  fronts  of  the  folds  face  the  west,  the  direction  of  movement  has  obviously 
been  from  the  opposite  quarter.  That  there  has  been  an  enormous  thrust  from  the 
eastwards,  is  fiirther  shown  by  a  series  of  remarkable  internal  re-arrangements  that 
have  been  superinduced  upon  the  rocks.  Every  mass  of  rock,  irrespective  of  litho- 
logical  character  and  structure,  is  traversed  by  striated  surfaces,  which  lie  approximately 
parallel  with  those  of  the  thrust -planes,  and  are  covered  with  a  line  parallel  lineation 
running  in  a  W.N.W.  and  E.S.E.  direction.  Along  many  zones  near  the  throst-planes, 
and  for  a  long  way  above  them,  the  most  perfect  shear-structure  has  been  developed 
(Fig.  256).  The  coarse  pegmatites  in  the  gneiss  have  had  their  pink  felspar  and 
milky  quartz  crushed  and  drawn  out  into  fine  parallel  laminse,  till  they  assmne  the 
aspect  of  a  rhyolite  in  which  fluxion-structure  has  been  exceptionally  well  developed. 
Hoi-nblende-rock  |)asses  into  hornblende -schist.  Sandstones,  quartzites,  and  shales 
l>ecome  finely  micaceous  schists.  The  annelide -tubes  in  the  quartzite  are  flattened  and 
drawn  out  into  ribbands.  New  minerals,  especially  mica,  have  been  abundantly 
developed  along  the  superinduced  divisional  planes,  and,  in  many  cases,  their  longer  axes 
are  ranged  in  the  same  dominant  direction  from  E.S.E.  to  W.N.W. 

The  whole  of  these  rocks  have  undergone  such  intense  shearing  during  their  west- 
ward displacement  that  their  original  charactei-s  have  in  many  cases  been  obliterated. 
Among  them,  however,  can  be  recognised  bands  of  gneiss  which  undoubtedly  belong  to 
the  underlying  Lewisian  series.  With  these  ai'e  intercalated  lenticular  strips  of 
Cambrian  quartzite  and  limestone.  In  some  areas  the  Torridon  sandstone  has  been 
heaped  on  itself,  sheared,  and  driven  westward  in  large  slices,  the  sandstones  passing 
int^  sericitic  schists  and  the  conglomerates  having  their  pebbles  flattened  and  elongated, 
while  the  matiix  has  become  full  of  secondary  mica.  Eastwards,  above  one  of  the  most 
marked  and  persistent  thrust-planes,  the  pi'evailiug  rock  is  a  flaggy  fissile  micaceous 
gneiss  or  gneissose  flagstone  ('^Moine  schist,"  p.  707).  All  these  rocks  have  a  general 
dip  and  strike  jtai-allel  with  those  of  the  Cambrian  strata  on  which  they  now  rest,  and 
ill  this  respect,  as  well  as  in  their  prevailing  lithological  characters,  they  present  the 
most  striking  contrast  to  the  rocks  that  uncouformably  underlie  the  quartzites  a  little 
to  the  west.  Whatever  may  have  been  their  age  and  original  condition,  they  have 
certainly  acquired  their  present  structure  since  Cam])rian  times. 

From  the  remarkably  constant  relation  between  the  dip  of  the  Cambrian  strata 
and  the  inclination  of  the  reversed  faults  which  traverse  them,  no  matter  into  what 
various  positions  the  two  structures  may  have  been  thrown,  it  is  tolerably  clear  that 
these  dislocations  took  place  before  the  strata  had  been  seriously  disturbed.  The 
l)ei*8istent  parallelism  of  the  faults,  folds,  and  prevailing  strike,  indicates  that  the 
faulting  and  tilting  were  parts  of  one  continuous  process.  The  same  dominant  north- 
easterly trend  governs  the  structure  of  the  whole  Highlands,  and  reappears  over  the 
Silurian  ti-acts  of  the  south  of  Scotland  and  north  of  England.  If,  as  is  probable,  it  is 
the  result  of  one  gieat  series  of  terrestrial  movements,  these  must  have  occurred  between 
the  middle  or  close  of  the  Cambrian  period  and  that  portion  of  the  Old  Red  Sandstone 
period  represented  by  the  breccias  and  conglomerates  of  the  Highlands.  Wlien  the  rocks 
were  undergoing  this  metamorphism,  there  lay  to  the  north-west  a  solid  ridge  of  old 
gneiss  and  Torridon  sandstone  which  oflei*ed  stmng  i-esistanoe  to  plication.  The  thrust 
from  the  eastwai-d  against  this  ridge  must  have  been  of  the  most  gigantic  kind,  for 
huge  slices,  hundi-eds  of  feet  in  thickness,  were  shorn  off"  from  the  quartzites,  lime- 
stones, red  sandstones,  and  gneiss,  and  were  pushed  for  miles  to  the  westward.  During 
this  process,  all  the  i*ocks  driven  forward  by  it  had  their  original  structure  more 
or  less  completely  etfaced.  New  planes,  generally  jMirallel  with  the  surfaces  of  move- 
ment, were  develoi>ed  in  them,  and  along  these  new  planes  a  re-arrangement  and  re- 


PABTvni§ii  REGIONAL  METAMORPHISM  627 

crystallization  of  mineral  constituents  took  place,  resulting  in  the  production  of  crystal- 
line schists.^ 

Much  remains  to  be  done  before  the  structure  of  the  Central  and  Southern  Highlands 
is  explained.  That  some  portions  of  the  rocks  may  belong  to  the  Lewisian  gneiss  is  not 
improbable.  But,  on  the  other  hand,  in  almost  all  parts  of  the  Highlands  east  of  the 
Great  Glen  traces  of  an  original  fragmental  or  clastic  origin  can  be  detected  among  the 
schistose  rocks.  Zones  of  argillaceous  shales  or  slates  passing  into  andalusite-8lates,*and 
'  of  fine  grit  full  of  well-rounded  fragments  of  quartz,  felspar,  or  other  ingredient,  occur. 
Bands  of  coarse  conglomerate  lie  on  different  horizons,  the  pebbles  (granite,  gneiss,  &c. ) 
being  enveloped  in  a  schistose  matrix.  Microscopic  investigation  likewise  reveals,  even 
among  crystalline  mica-schists,  traces  of  the  original  water-worn  granules  of  quartz  in 
the  sandy  mud  out  of  which  the  rocks  have  been  formed.  It  is  deserving  of  remark  that 
the  rocks  along  the  southern  margin  of  the  Highlands  are,  for  the  most  part,  so  little 
affected  as  closely  to  resemble  portions  of  the  unaltered  Silurian  series  of  the  south  of 
Scotland,  and  that  they  dip  towards  the  mountains,  becoming  more  highly  foliated  and 
crystalline  as  they  recede  from  the  lowlands.  It  is  also  noteworthy  that  zones  of 
graphitic  schist  can  be  traced  through  different  tracts  of  the  Highlands,  and  that  these 
schists  and  their  associated  strata  bear  a  close  resemblance  to  the  carbonaceous  bands 
associated  with  sedimentary  deposits,  such,  for  instance,  as  the  Silurian  anthracitic 
graptolite  zones  of  the  southern  counties.' 

Various  eruptive  rocks  traverse  the  Highland  schists,  and  afford  interesting  studies 
in  their  relation  to  the  problems  of  metamorphism.  Thus  in  Banffshire  and  Aberdeen- 
shire, large  masses  of  diorite,  diabase,  and  gabbro  cut  the  schists  in  places,  but  nm  on 
the  whole  parallel  with  the  general  strike  of  the  region.  Their  appearance,  though 
later  than  that  of  the  rocks  through  which  they  have  come,  was  earlier  than  the  regional 
metamorphism.  The  diorite  has,  in  many  places,  itself  undergone  great  alteration.  Its 
component  crystals  have  ranged  themselves  in  the  direction  of  the  prevalent  foliation, 
and  have  here  and  there  separated  into  distinct  aggregates,  the  felspar  forming  a  kind 
of  labrador-rock,  and  the  hornblende  assuming  the  structure  of  perfect  hornblende- 
schist.  Numerous  bosses  of  gianite  and  porphyries  likewise  occur,  traversing  the  dioiites 
and  schists  and  therefore  of  still  later  date.  In  the  course  of  the  Geological  Survey  of 
the  Southern  Highlands  Mr.  G.  Barrow  has  found  evidence  that  over  and  above  the 
effects  of  great  dynamical  movements  affecting  wide  tracts  of  country,  a  marked  amount 
of  metamorphism  may  be  traced  to  the  influence  of  eruptive  granites  and  gneisses.  He 
shows  that  a  vast  number  of  pegmatite  veins  which  traverse  the  schists  may  be  traced 
into  bosses  of  intrusive  granite  or  gneiss,  the  great  mass  of  which  is  concealed  below 
groimd.  He  finds  that  three  well-marked  zones  can  be  observed  in  the  schists,  of  which 
the  first,  lying  nearest  to  the  main  body  of  eruptive  material,  is  marked  by  an  abundance 
of  sillimanite,  the  next  by  kyanite,  and  the  outermost  by  staurolite.     He  has  followed  the 

*  Nature^  xxxi.  p.  80  ;  Qunrt.  Jaurn.  Oeol.  Soc.  xliv.  (1888)  p.  378.  For  further  details 
see  the  account  of  pre-Cambrian  rocks  in  Book  VI.  Part  I.  §  it 

^  It  is  important  to  note,  as  showing  the  relation  of  regional  to  contact-metamorpbism, 
that  every  stage  in  the  development  of  the  andalusite  can  be  traced  in  these  slates,  though  no 
eruptive  rock  appears  at  the  surface.  J.  Home,  Mineral,  Mag,  1884.  I  have  proposed  to 
class  the  metamorphic  rocks  of  the  Central  and  Southern  Highlands  by  the  name  of  Dal- 
radian,  for  convenience  of  reference  until  their  true  geological  position  shall  have  been  deter- 
mined. Address  to  Geol.  Soc.,  Quart,  Journ,  Oeol.  Soc.  (1891)  p.  75,  xnd  postea.  Book  VI. 
Part  I.  §  ii. 

'  Among  the  less  metamorphosed  rocks  that  form  the  southern  margin  of  the  Highlands 
some  zones  of  graphitic  schist,  together  with  chert  bands  and  courses  of  igneous  rocks, 
wonderfully  remind  the  geologist  of  the  similar  assemblage  of  rocks  in  the  south  of  Scotland. 
As  this  sheet  is  passing  through  the  press  Mr.  Peach  has  detected  radiolaria  in  these  cherts, 
occurring  much  in  the  same  way  as  they  do  in  the  radiolarian  cherts  of  the  southern  uplands. 


628  GEOTECTONIC  {STRUCTURAL)  GEOLOGY  book  iv 

same  band  of  altered  sedimentary  material  across  these  zones  which  are  thus  shown  to 
be  entirely  independent  of  the  original  structure  of  the  rocks.  These  observatioxis  which 
have  been  extended  over  many  hundred  square  miles  of  Forfarshire,  Perthshire,  and 
Aberdeenshire,  are  of  much  interest  and  importance  as  they  serve  to  connect  the  phe- 
nomena of  contact  and  regional  metamorphism  as  manifestations  of  one  great  prooeas.^ 

Greece. — In  the  Grecian  peninsula,  vast  masses  of  chlorite-schist,  mica-schist,  and 
gneiss  occur,  among  which  thick  zones  of  marble  are  interstratified.  At  several  places 
in  the  calcareous  zones  fossils  have  been  found  which,  though  not  well  preserved,  show 
that  the  rocks  belong  to  the  fossiliferous  series  of  formations,  and  are  not  pre-Cambrian. 
These  crystalline  rocks  in  north-eastern  Greece  lie  on  the  strike  of  normal  Cretaceous 
hippurite  limestones,  sandstones,  and  shales,  and  are  probably  of  Cretaceous  age.' 

Green  Mountains  of  New  England. — The  Lower  Silurian  strata,  which  to  the 
north  in  Vermont  are  comparatively  little  changed,  become  increasingly  altered  as  they 
are  traced  southwards  into  New  York  Island.  They  are  thrown  into  sharp  folds,  and 
even  inverted,  the  direction  of  plication  being  generally  N.N.K  and  S.S.W.  This 
disturbance  has  been  accompanied  by  a  marked  crystallization.  The  limestones  have 
become  marbles,  the  sandy  beds  quartzites,  and  the  other  strata  have  assumed  the 
character  of  slate,  mica-schist,  chlorite-schist,  and  gneiss,  among  which  homblendic, 
augitic,  hy[>ersthenic,  and  chrysolitic  zones  occur.  The  geological  horizon  of  these 
rocks  is  shown  by  the  discovery  in  them  at  various  localities  of  fossils  belonging  to  the 
Trenton  and  Hudson  River  subdivisions  of  the  Lower  Silurian  system  of  eastern  North 
America.  The  rocks  have  been  ridged  up  and  altered  along  a  belt  of  country  lying  to 
the  east  of  the  Hudson  and  extending  north  into  Canada.^ 

Menominee  and  Marquette  regions  of  Michigan. — One  of  the  most  luminous 
essays  yet  published  on  the  megascopic  and  microscopic  proofs  of  dynamic  metamorphism 
is  that  by  G.  H.  Williams  to  which  reference  has  already  been  made.^  The  author  proves 
that  a  series  of  pre-Cambrian  rocks  of  eruptive  origin  (greenstones,  tuffs,  agglomerates, 
kc.)  have  been  converted  into  perfect  schists.  The  various  stages  of  alteration  are 
minutely  detailed,  and  careful  drawings  are  given  of  the  microscopic  structures.  The 
deductions  arrived  at  by  the  author  have  far  more  than  a  mere  local  significance  ;  they  lay 
an  accurate  basis  for  the  study  of  similar  "greenstone-schists"  in  other  regions,  and 
show  how  the  original  eruptive  character  of  such  altered  rocks  is  to  be  recognised. 

It  may  be  useful  to  group  the  foregoing  and  a  few  other  examples  of  regional  meta- 
morphism in  stratigraphical  order,  that  the  student  may  see  over  how  wide  a  range  of 
the  geological  formations  such  transformation  has  taken  place. 

Tertiary. — Northern  and  Central  Italy. — Nummulitic  limestone  rendered  saccharoid, 
and  strata  (including  Miocene)  generally  more  indurated  in  proportion  to  tiie 
extent  to  which  they  have  been  folded  and  disturbed.  These  changes  which 
indicate  an  incipient  metamorphism  are  well  displayed  in  the  Apuan  Alps  and 
in  the  Apennines.* 

Cretaceous. — Greece. — Chlorite-schist,  mica-schist,  marble,  serpentine,  &c.,  believed 
to  be  altered  Cretaceous  sandstone,  shale,  limestone,  &c.  (see  above). 

^  G.  Barrow,  Quart.  Joxtm.Oeol,  Soc.  1893. 

*•*  M.  Neumayr,  JaJirh.  Geol.  Reichsanst,  xxvi.  (1876)  p.  249.  Z.  Deutsche  Oeol,  Oes, 
xxxiii.  pp.  118,  454.  A.  Bittner,  M.  Neumayr,  and  F.  Teller,  Denksch,  Akad.  WieH,  xl 
(1880)  p.  395.     This  essay  well  deserves  the  attention  of  the  student. 

^  See  Dana,  Amer.  Joum.  Set.  iv.  v.  vi.  xiii.  xiv.  xvii.  xviii.  xix.  xx.  ;  Q.  J,  OeoL  Soc. 
1882,  p.  397.  The  identification  of  the  so-called  Taconic  schists  of  New  England  with 
altered  Lower  Silurian  rocks  has  been  called  in  question  by  Sterry  Hunt,  but  the  strati- 
graphical  evidence  collected  by  A.  Wing,  Dana,  and  others,  and  the  testimony  of  the  fossils 
collected  by  Dana,  Dwight,  &c. ,  hatre  sustained  it.  In  the  Punjab  a  series  of  gneisses  and 
schists  overlies  infra-Triassic  rocks.     Wynne,  Oeol.  Mag.  1880,  p.  314. 

*  Bull.  U.S.  Oeol.  Survey,  No.  62,  1890. 

*  Lotti  and  Zaccagna,  BoU.  Camit.  Geol.  d' Italia,  1881,  p.  5.     Lotti,  ilnd,  p.  419,  BuU, 
Soc.  aiol.  France,  xvl  (1888)  p.  406. 


PART  VIII  §  ii  REGIONAL  METAMORPHISM  629 

Coast  range  of  California. — Strata  containing  Cretaceous  fossils  pass  into  jaspers, 
siliceous  slate,  (phthanites),  glaucophane-schist,  gametiferous  mica-schist,  serpen- 
tine, &c.^ 
Jurassic. — Alps. — Sericite-schists,  altered  limestones,  &c.  (p.  622). 

Sierra  Nevada  (California). — Clay-slates,  talcose  slates,  serpentine,  &c.,  {lassing 
into  rocks  containing  Jurassic  fossils.^ 
Trias, — Sierra  Nevada  (Spain). — Clay-slate,  micA-schists,  talc-schists,  and  limestones.' 
Carrara. — Mica-schist,  talc-schist,  marbles,  passing  down  into  limestones  contain- 
ing Encriniis  liliiformis,  Phylloceras,  Pentacrinus^  below  which  lie  gneissic  and 
other  schists  enclosing  OrthoceraSf  ActinocercLSy  and  evidently  of  Palaeozoic  age.* 
Alps. — Limestones,  dolomites,  and  gypsums  rendered  crystalline,  associated  with 
CAlc-mica-schist  and  other  varieties  oi  schist. 
Carboniferous. — Alps. — Graphite -schist,  phyllite-gneiss,  &a  (p.  622). 

Eastern  Brittany. — Carboniferous  shales  altered  into  crystalline  schists.* 
Devonian. — Tauuus. — A  large  series  of  crystalline  schists  (p.  620). 

Ardennes. — Crystalline  schists  with  garnet,  hornblende,  mica,  &c.  (p.  619). 
Siluriart. — Norway. — A  series  of  schists  resembling  those  of  Scotland,  lying  upon  and 
interstratilied  with  fossiliferous  beds  (p.  621). 
Green  Mountains  of  New  England.  — A  great  group  of  schists  and  limestones,  with 

fossils  in  some  beds  (p.  628). 
Northern  Alps. — Upper  Silurian  fossils  among  gneiss,  diorite-schist,  mica-schist, 
chloritoid-schist,  ic* 
Cambrian  and  Silurian. — Scotland. — A  great  series  of  ciystalline  schists  overlying 
quartzite  and  limestones  with  fossils  (p.  624). 
Saxon  granulite  tract. — Schists,  schistcee  conglomerates,  &c.^ 
South  Wales. — A  fine  foliation  of  the  tuffs,  representing  an  early  stage  of  regional 
metamorphism.  ® 
Pre- Cambrian,  {Archaean). — Scotland. — Sandstone  and  arkose  passing  into  lustrous 
crumpled  micaceous  schists  (p.  544).     Some  of  the  Archaean  gneisses  and  horn- 
blende-rocks of  Sutherland  have  had  a  new  schistosity  superinduced  in  them  by 
the  shearing  movements  that  altered  the  Cambrian  strata  (p.  627). 

Summary. — From  the  evidence  now  adduced  the  following  conclu- 
sions may  be  confidently  drawn. 

1.  There  are  wide  regions  in  which  crystalline  schists  (a)  overlie 
fossiliferous  strata,  or  (b)  contain  intercalated  bands  in  which  fossils  occur, 
or  (c)  pass  either  laterally  or  vertically  into  undoubted  sedimentary  strata. 

2.  These  schists  are  in  some  cases  the  metamorphosed  equivalents  of 
what  were  once  ordinary  sedimentary  deposits,  including  sometimes 
associated  igneous  rocks. 

3.  The  alteration  by  which  rocks  have  been  affected  in  regional  meta- 
morphism is  similar  in  its  stages  to  what  may  be  traced  in  local  metamor- 
phism round  bosses  of  granite,  but  has  attained  a  much  greater  development. 

4.  Regional  metamorphism  has  been  directly  connected  with  intense 
compression  or  tension,  and  is  usually  most  pronounced  where,  as  shown 

*  Whitney,  Oeol.  Surv.  California,  *  (Jeology,*  vol.  i.  p.  28.  G.  F.  Becker,  Atner,  Joum. 
Scu  xxxl  (1886)  p.  848  ;  *  Geology  of  the  QnicksUver  Deposits  of  the  Pacific  Slope,'  Mono- 
graph No.  xiii.  of  U.S,  Oeol.  Survey,  1888.  *  Whitney,  op.  cit.  p.  225. 

•  De  Vemeuil,  BuU.  Soc  QM.  France  (2),  xiii.  p.  708.  R.  von  Drasche,  Jahrb,  Oeol. 
Reichsanst.  xxiz.  (1879)  p.  93.  The  identification  of  these  rocks  with  Triassic  beds  is  a 
probable  conjecture. 

^  Coquand,  Bull.  Soc.  OSol.  France  (8),  iii.  p.  26  ;  iv.  p.  126.  Zaccagna,  Boll.  Com. 
Oeol.  Ital.  xii.  (1881)  p.  476.     Lotti,  op.  cit.  p.  419  and  plate  ix. 

»  Jannettaz,  BuU.  Soc,  Ofol,  France  (3),  ix.  (1881)  p.  649. 

'  M.  Vacek  and  Baron  Foullon,  Jahrb,  Oeol,  Reichsansf.  xxxiv.  (1884)  pp.  609,  635. 
G.  Stache,  Z,  Deutsch,  Oeol.  Oes,  1884,  p.  277. 

7  Lehmann's  work  cited  ante,  p.  156.  ^  Q.  J,  Oeol.  Soc,  xxxix.  (1888)  p.  810. 


630  GEOTECTONIG  (STRUGTURAL)  GEOLOGY  book  iv 

by  plication,  puckering,  shear- structure,  and  the  crushing  down  of  the 
component  minerals,  the  rocks  have  been  subjected  to  the  greatest 
mechanical  movement 

5.  The  d3aiamical  strain  has  been  generally,  perhaps  always,  accom- 
panied with  more  or  less  chemical  reaction,  not,  as  a  rule,  involving  the 
introduction  of  new  chemical  constituents,  but  consisting  chiefly  in  a 
recombination  of  those  already  present  in  the  rocks,  with  the  consequent 
development  of  new  crystalline  minerals. 

6.  This  chemical  and  mineralogical  rearrangement  has  probably  been 
superinduced  under  the  influence  of  moderate  heat,  and  in  presence  of 
water,  and  is  comparable  with  what,  on  a  feeble  scale,  can  be  achieved  in 
the  laboratory. 

7.  The  alteration  of  rocks  in  an  area  of  regional  metamorphism  is 
often  strikingly  unequal  in  degree  even  over  limited  areas,  being  apt  to 
attain  sporadically  a  maximum  intensity,  particularly  in  tracts  of  greatest 
shearing  or  plication,  while  in  other  areas,  the  original  clastic  or  crystal- 
line characters  may  be  easily  discernible. 

8.  The  nature  of  the  alteration  has  depended  first,  and  chiefly,  on  the 
original  character  and  structure  of  the  rocks  aflected  by  it :  and  secondly, 
on  the  natiu*e  and  intensity  of  the  metamorphic  activities.  Of  some 
rocks  (sandstone,  carbonaceous  shale,  coal),  the  original  condition  may  be 
recognisable  when  that  of  their  associated  strata  has  entirely  disappeared. 

9.  The  foliation  in  a  tract  of  regional  metamorphism  has  been 
developed  along  divisional  planes  which  guided  the  crystallization  or 
rearrangement  of  the  minerals.  In  some  cases,  these  planes  coincide  with 
those  of  original  deposit.  In  others,  they  may  represent  cleavage,  as 
pointed  out  by  Sedgwick  and  Darwin.  Or  they  may  indicate  the  planes 
along  which,  under  intense  pressure,  the  longer  axes  of  crystallizing 
minerals  would  naturally  range  themselves.  In  a  rock,  homogeneous  in 
chemical  composition  and  general  texture,  foliation  might  be  induced 
along  any  dominant  divisional  planes.  If  these  planes  were  those  of 
cleavage  or  of  shearing,  the  resultant  foliation  might  not  appreciably 
differ  from  that  along  original  bedding  planes.^  But  it  may  be  doubted 
whether  a  cleavage  foliation  of  clastic  sedimentary  strata  could  run  over 
wide  areas  without  sensible  and  even  very  serious  interruptions.  In  most 
large  masses  of  sedimentary  matter,  the  usual  alternations  of  different 
kinds  of  sediment  could  not  but  produce  distinct  kinds  of  rock  under  the 
influence  of  metamorphic  change.  Where  foliation  coincides  with  cleavage 
over  large  tracts,  it  will  almost  certainly  be  crossed  by  bands,  more 
or  less  distinct,  coincident  with  the  original  bedding  whether  of  sedi- 
mentary or  of  eruptive  rocks,  and  running  oblique  to  the  general 
foliation,  as  bedding  and  cleavage  do,  save  where  they  may  happen 
to  coalesce.  Where  a  massive  rock  of  generally  homogeneous  composi- 
tion, such  as  a  felsite  or  granite,  has  been  intensely  sheared,  a  re-arrange- 
ment or  re-crystallization  of  its  minerals  has  taken  place  along  the  planes 
of  shearing.     Such  a  rock  is  thus  transformed  into  a  schist     Even  rocks 

^  Jannettaz  poiuts  out  that  the  cleavage  of  the  slatea  in  the  Grenoble  Alps  is  parallel  to 
the  foliation  of  the  mica-8chists.     Bull.  Soc.  G(ol,  France  (3),  ix.  (1881)  p.  649. 


PART  IX  ORE  DEPOSITS  631 

of  much  more  varied  structure,  like  ArchsBan  gneisses,  have  been  subjected 
to  such  changes  from  shearing  as  not  only  to  lose  entirely  their  original 
structure,  but  to  acquire  a  new  foliation  parallel  to  the  shearing  planes. 

It  is  now  generally  agreed  that  gneisses  and  many  forms  of  schist 
have  been  formed  by  dynamical  action  from  deep-seated  masses  of  igneous 
rocks,  both  acid  and  basic  The  banding  of  these  rocks,  which  was 
formerly  regarded  as  evidence  of  aqueous  deposition,  is  no  doubt  generally 
due  to  an  original  segregation  of  the  component  minerals  of  still  luiconsoli- 
dated  igneous  rocks,  like  the  segregation-veins  described  on  p.  580,  though 
it  may  to  some  extent  have  resulted  from  the  re-arrangement  and  re-crystal- 
lization of  the  materials  of  such  rocks  under  intense  mechanical  strain.  The 
occurrence  of  lenticular  bands  or  bosses  of  amphibolite  in  gneiss  may  point  to 
dykes  of  some  homblendic  rock  by  which  the  original  granite  was  traversed 
before  the  development  of  the  foliated  structure.  A  similar  connection  can 
be  traced  between  masses  of  diorite,  gabbro,  &c.,  and  hornblende-schists, 
gabbro-schists,  &c.  The  granitoid  character  of  these  rocks,  under  the 
great  stresses  they  have  suffered  during  periods  of  terrestrial  disturbance, 
has  here  and  there  entirely  disappeared.  First  the  minerals  (especially 
the  felspars)  are  seen  to  have  ranged  themselves  with  their  long  axis 
in  one  general  direction.  Then  they  separate  into  layers  or  folia  in  the 
same  direction,  and  acquire  a  more  or  less  distinctly  foliated  struc- 
ture. Thus,  a  massive  diorite,  gabbro,  or  diabase  has  been  converted  into 
amphibolite-schist,  sometimes  with  bands  of  massive  labradorite.^ 

Part  IX.  Ore  Deposits.  ^ 

Metallic  ores  and  other  minerals  that  are  extracted  for  their  economic 
value  occur  in  certain  well-marked  forms  which  have  been  variously 
classified ;   but  for  the  purposes  of  the  geological  student  it  is  most 

^  The  idea  suggesteil  many  years  ago  by  Jukes  (*  Student's  Manual  of  Geology '),  that 
the  homblendic  bands  of  the  crystalline  schists  might  have  been  originally  eruptive  rocks, 
has  been  confirmed  by  more  recent  work.  See  Lehmann's  *  Entstehung  der  altkrystallin- 
ischen  Schiefergesteine ' ;  Allport,  Q,  J,  Oeol,  Soc,  xxxiu  (1876)  p.  425  ;  the  diorites  of  the 
north  of  Scotland,  anUt  p.  627,  and  paper  by  G.  H.  Williams,  cited  on  p.  617. 

Besides  the  works  already  cited  on  Metamorphism  the  student  may  consult  the  following : 
Delesse,  MSm,  Savans  Strangers,  xvii.  Paris,  1862,  pp.  127-222  ;  Ann.  des  MineSy  xii. 
(1857) ;  xiii.  (1858)  ;  'l^tudes  sur  le  Metamorphisme  des  Roches,*  Paris,  1869  ;  Durocher, 
*  Etudes  sur  le  Metamorphisme  des  Roches,'  Bull.  Soc,  Ofol.  France  (2),  iii.  (1846) ;  Daubree, 
Ann,  des  Mines,  5"^  serie,  xvi.  p.  155  ;  Bischof,  *  Chemical  Geology,*  chap,  xlviii.  ;  J.  Roth, 
Abhandlungen  Akad,  Berlin^  1871  ;  1880  ;  Gttmbel,  *  Oestbayerische  Grenzgebirge,'  1868  ; 
H.  Credner,  Zeitsch.  OesamnU,  Naturvoiss,  xxxii.  (1868)  p.  353  ;  N,  Jahrb,  1870,  p.  970  ; 
A.  Inostranzeff,  'Studien  iiber  metamorphosirte  Gesteine,'  Leipzig,  1879. 

3  The  best  English  work  on  this  subject  is  'Ore  Deposits,'  by  J.  A.  Phillips,  1884.  The 
following  works  on  ores  and  mining  may  also  be  consulted  :  B.  von  Gotta,  *  Die  Lehre  von 
Erzlagerstatten,'  1859-61  ;  A.  von  Groddeck,  *Die  Lehre  von  den  Lagerstatten  der  Erze,' 
1879  ;  W.  Forster,  'Treatise  on  a  Section  of  the  Strata  from  Newcastle-on-Tyne  to  Cross 
Fell*  ;  W.  Wallace,  'Laws  which  regulate  the  deposition  of  Lead  Ores,'  1861  ;  F.  von 
Sandberger,  '  Untersuchungen  Uber  Erzgange '  ;  numerous  valuable  papers  by  J.  W. 
Henwood  and  others  in  Trans,  Roy.  Oeol,  Soc,  Cormoall ;  G.  F.  Becker,  '  Geology  of  the 
Comstock  Lode,'  (/.S.  Oeol.  Survey,  Monographs  iii.  iv. ;  also  'Quicksilver  Deposits,'  8^ 


632  GEOTECTONIC  {STRUCTURAL)  GEOLOGY  book  it 

convenient  to  consider  them  from  the  point  of  view  of  geological  stractare 
and  history.  Thus  arranged,  they  naturally  group  themselves  into  three 
great  series;  1st,  those  qontemporaneously  deposited  among  stratified 
formations ;  2nd,  those  contemporaneously  formed  with  the  other  in- 
gredients of  crystalline  (massive  and  schistose)  rocks ;  3rd,  those  sub- 
sequently introduced  by  infiltration  or  otherwise  into  fissures,  caverns,  or 
other  spaces  of  any  kind  of  rock. 

1.  Contemporaneous  ores  of  stratified  rocks  have  been  deposited 
in  water,  together  with  the  sandstones,  limestones,  or  other  strata  among 
which  they  lie.  In  some  cases,  they  are  mere  mechanical  sediments,  such 
as  the  auriferous  gravels  of  California  and  Australia  (placer-works)  or  the 
stream-tin  deposits  of  Cornwall,  obviously  derived  from  the  disintegration 
of  older  rocks,  principally  veinstones,  in  which  the  ores  were  developed. 
In  other  cases,  they  result  from  the  accumulation  of  chemical  precipitates, 
as  in  the  modern  deposition  of  iron-ore  on  the  floors  of  lakes  and  beneath 
bogs.  These  precipitates  may  either  of  themselves  form  independent 
mineral  masses,  or  may  serve  as  impregnations  of  other  stratified  deposits, 
like  the  copper  ores  that  occur  so  abundantly  diffused  through  the  Kupfer- 
Schiefer  of  Saxony.  In  all  these  instances,  the  metalliferous  rocks  belong 
to  the  stratified  type  of  geological  structure  (p.  498  seq.)  They  -occur  in 
layers  varying  from  mere  films  up  to  beds  or  stratified  masses  of  great 
thickness.  In  some  cases,  they  retain  the  same  average  thickness  for  long 
<]istances,  in  others,  they  swell  out  or  die  away  rapidly,  or  occur  in  scattered 
concretions.    Organic  remains  are  commonly  associated  with  ores  of  this  type. 

2.  Contemporaneous  ores  of  crystalline  rocks  are  exemplified 
by  the  beds  of  iron-ore,  pyrites,  &c.,  that  so  frequently  occur  intercalated 
among  the  crystalline  schists.  They  lie  as  massive  sheets  or  thin  partings, 
and  usually  present  a  conspicuously  lenticular  character.  That  they  were 
formed  contemporaneously  with  the  layers  of  quartz,  mica,  felsj)ar,  horn- 
blende, or  other  minerals  among  which  they  lie,  and  owe  their  crystalline 
structure  to  the  same  process  that  produced  the  characteristic  foliation  of 
the  crystalline  schists,  may  usually  be  inferred  with  considerable  certainty, 
though  cases  not  infrequently  arise  where  it  is  difficult  or  impossible  to 
draw  any  line  between  this  type  and  that  of  true  subsequently-formed 
veins.  Besides  these  lenticular  ores  of  the  crystalline  schists,  the  massive 
rocks  also  contain  contemporaneously  crystallized  ores.  The  diffused 
magnetite  and  titaniferous  iron  of  the  basalts,  diabases,  &c.,  and  the 
occasional  separation  of  the  ore  in  the  layers  of  segregation-veins  in  these 
rocks  are  familiar  illustrations.  Large  included  masses  of  these  and 
other  ores  are  sometimes  available  for  mining  {ante,  p.  70). 

3.  Subsequently  introduced  ores  are  distinguished  by  the  contrast 
between  their  contents  and  structure  and  those  of  the  rocks  through  which 
they  pass.  They  have  been  deposited,  subsequent  to  the  consolidation 
of  these  rocks,  in  cavities  previously  opened  for  their  reception.  In 
certain  rocks  (limestones,  dolomites,  <fcc.),  intricate  channels  and  large 

Ann.  Rep.   U.S.   Geol.  Survey,    1886-87,    p.  965,   and  Monograph  xiii.  ;    R.  D.  Irring, 
*  Copper-bearing  rocks  of  Lake  Superior,'  Ann,  Rep.  U.S.  Ged.  Survey,  1881-82,  p.  98 
and  Monograph  v. ;  'Gites  Min^raux/  £.  Fuchs  et  L.  Delaunay,  Paris,  1893. 


PART  IX  Ji  i  MINERAL-VEINS  633 


irregular  caverns  have  been  dissolved  out  by  the  solvent  action  of  under- 
ground water ;  in  other  cases,  fissures  have  been  formed  by  fracture,  or 
the  rocks,  exposed  to  great  compression,  have  been  puckered  up  or  torn 
asunder,  so  that  irregular  spaces  have  been  opened  in  them.  Metallic  ores 
and  crystalline  minerals  introduced  by  infiltration,  sublimation,  or  other- 
wise, into  the  cavities  formed  in  any  of  these  ways,  may  be  grouped,  accord- 
ing to  the  shape  of  the  cavity,  into  veins  or  lodes,  which  have  filled  up 
vertical  or  highly  inclined  fissures,  and  stocks,  which  are  indefinite 
aggregations  often  found  occupying  the  place  of  subterranean  cavities. 

The  first  two  of  these  three  types  of  ore  deposits  do  not  require  special 
treatment  here.  The  stratified  type  has  the  usual  character  of  sediment- 
ary formations  (Book  IV.  Part  I.) ;  the  crystalline  type  forms  part  of  the 
structure  of  schistose  and  massive  rocks  (Book  11.  Part  II.  Sect  vii.  §§ 
2  and  3 ;  and  Book  VI.  Part  1.  §  i.) ;  the  third  type,  however,  from  its 
economic  importance  and  its  geological  interest,  merits  some  more  detailed 
notice. 

§  1.  Mineral- Veins  or  Lodes. 

A  true  mineral-vein  consists  of  one  or  more  minerals  deposited  within 
a  fissure  of  the  earth's  crust,  and  is  usually  inclined  at  from  10**  to  20** 
from  the  vertical.  The  bounding  surfaces  of  such  a  vein  are  termed 
walls,  and,  where  inclined,  that  which  is  uppermost  is  known  as  the 
hanging^  and  that  which  is  lowest  as  the  li/ing  or  foot  wall.  The  sur- 
rounding rock,  through  which  veins  run,  is  termed  the  country  or 
country -rock.  A  vein  may  coincide  with  a  line  of  fault  or  of  joint, 
or  may  run  independent  of  any  other  structural  divisions ;  in  all  cases  it 
is  independent  of  the  bedding  or  foliation  of  the  "country."  Cases 
occur  among  crystalline  massive  rocks,  however,  and  still  more  frequently 
among  limestones,  where  the  introduction  of  mineral  matter  has  taken 
place  along  gently  inclined  or  even  horizontal  planes,  such  as  those  of 
stratification,  and  the  veins  then  look  like  interstratified  beds.  Mineral- 
veins  are  composed  of  masses  or  layers  of  simple  minerals  or  metallic  ores 
alternating,  or  more  irregularly  intermingled  with  each  other,  distinct 
from  the  surrounding  rock,  and  evidently  the  result  of  separate  deposi- 
tion. They  are  in  no  respect  to  be  confounded  with  veins  of  rock 
injected  in  a  molten  condition  from  below,  or  segregated  from  a  surround- 
ing pasty  magma  into  cracks  in  its  mass.  But  they  are  commonly  most 
frequent  and  most  metalliferous  in  districts  where  eruptive  rocks  are 
abundant. 

Variations  in  breadth. — Mineral-veins  vary  in  breadth  from  a  mere 
paper-like  film  up  to  a  great  wall  of  rock  150  feet  wide  or  more.  The 
simplest  kinds  are  the  threads  or  strings  of  calcite  and  quartz,  so 
frequently  to  be  observed  among  the  more  ancient,  and  especially  more 
or  less  altered,  rocks.  These  may  be  seen  running  in  parallel  lines,  or 
ramifying  into  an  intricate  network,  sometimes  uniting  into  thick  branches 
and  again  rapidly  thinning  away.  Considerable  variations  in  breadth 
may  be  traced  in  the  same  vein.  These  may  be  accounted  for  by  unequal 
solution  and  removal  of  the  walls  of  a  fissure,  as  in  the  action  of  per- 


634 


GEOTECTONIC  {STRUCTURAL)  GEOLOGY 


BOOK  IT 


meating  water  upon  a  calcareous  rock ;  by  the  irr^oUur  opening  ol  a 
rent^  or  by  a  shift  of  the  walls  of  a  sinuous  or  irregularly  defined  fisBiiie.' 
In  the  last-named  case,  the  vein  may  be  strikingly  unequal  in  In^eadth, 
here  and  there  nearly  disappearing  by  the  convergence  of  the  walla,  and 

a 


Pig.  812.— Widening  of  a  flflsnre  by  relative  shifting  of  its  iride  (De  la  Becbe). 

then  rapidly  swelling  out  and  again  diminishing.  How  simply  this 
irregularity  may  be  accounted  for  will  be  readily  perceived  by  merely 

copying  the  line  of  such  an  uneven  fissure  on 
tracing  paper  and  shifting  the  tracing  along 
the  line  of  the  original  If,  for  example,  the 
fissure  be  assumed  to  have  the  form  shown  at 
a  b,  in  the  first  line  (Fig.  312),  a  slight  shifling 
of  one  side  to  the  right,  as  at  a'  V  in  the 
second  line,  will  allow  the  two  opposite  walls 
to  touch  at  only  the  points  o  o,  while  open 
«.    „,«    o  ..     ,   «  1     spaces  will  be  left  st  c  c  d.     A  movement  to 

Fig.  313.— Section  of  a  fisaure  nearly     ^  _x  •       i_  j-  •  u 

filled  with  one  mineral  (c  c),  bat    the  Same  extent  in  the  reverse  direction  wonkl 
with  a  portion  of  the  flssnre  (a  b)   give  rise  to  a  more  coutinuously  open  fissure, 

still  open  (B.)  ^^  j^    ^^^    ^^^^^  j.^^         ^j^^^    shif tlUgS    of   tUs 

nature  have  occurred  to  an  enormous  extent  in  the  fissures  filled  with 
mineral-veins,  is  shown  by  their  abundant  slickensides  (p.  526).  The 
polished  and  striated  walls  have  been  coated  with  mineral  matter,  which 
has  subsequently  been  similarly  polished  and  grooved  by  a  renewal  of 
the  slipping. 

Structure  and  contents. — A  mineral- vein  may  be  either  simple, 
that  is,  consisting  entirely  of  one  mineral,  or  compound,  consisting  of 
several ;  and  may  or  may  not  be  metalliferous.  The  minerals  are  usually 
crystalline,  but  layers  or  irregular  patches  of  soft  decomposed  earth,  day, 
<fec.,  frequently  accompany  them,  especially  as  a  layer  on  the  wall-face 
(Jluain).  The  non-metalliferous  minerals  are  known  as  gangue  or  vein- 
stones, the  more  crystalline  being  often  also  popularly  classed  as  spars. 
The  metal-bearing  minerals  are  known  as  ores.  The  commonest  vein- 
stones are  quartz  (usually  either  crystalline  or  crypto-crystalline,  with 
numerous  fiuid-inclusions),  calcite,  barytes,  and  fluorite.  The  presence 
of  silica  is  revealed  not  only  by  the  quartz,  but  by  the  hard  siliceous 
bands  so  often  observable  along  the  walls  of  a  vein.  These  can  often 
be  determined  to  be  portions  of  the  "  country "  which  have  been  in- 
durated by  the  deposition  of  silica  in  their  pores.  The  ores  are  some- 
times native  metals,  especially  in  the  case  of  copper  and  gold ;  but  for 
the  most   part  are  oxides,  silicates,  carbonates,  sulphides,  chlorides,  or 


MINERAL-VEINS 


635 


other  combiDations.  Some  of  the  contents  of  mineral-veuu  are  associated 
with  certain  minerals  more  usually  than  with  others,  as  galena  with 
blende,  pyrite  with  chalcopyrite,  gold  with  quartz,  magnetite  with  chlorite. 
Of  the  manner  in  which  the  contents  of  a  mineral-vein  are  disposed  the 
following  are  the  chief  varieties. 

(1)  Matsire. — Showing  no  definite  smugement  of  the  contents.  This  atructiire  is 
especially  cturact«rigtic  o(  reina  conusting  of  ■  single  mineral,  u  of  calcite,  qnarti,  or 
buytes.     Some  metalliferous  ores  (pyrites, 

limonite)  likewise  asaume  it. 

(2)  Banded,  comby,  in  parallel  (and 
sometimes  exactly  dnplicatad)  layen  or 
combs.  In  this  common  arraDgement,  each 
wall  (a  a,  Yig.  314)  may  be  coated  with  a 
layer  of  the  same  msteriat.  perhaps  some 
an  or  flucan  (fr  £),  followed  on  the  inside 
by  another  layer  [e  c),  perhapa  quartz,  then 
by  layers  of  calcite,  fluor-apar,  or  other  vein- 
atone,  with  strings  or  layers  of  ore,  to  the 
centre,  where  the  two  opposite  walla  may  be 
finally  united  by  the  last  zone  of  deposit  [>)• 
Even  where  each  half  of  the  vein  is  not 
strictly  a  duplicate  of  the  other,  the  same 
parallelism  of  distinct  layers  may  be  traced. 

(3)  Brecciated,  cantaiaitig  angular  fragments  of  the  surroundiiig  "country," 
cemented  in  a  matrix  of  veinstones  or  ores.  It  may  often  be  observed  that  these  frag- 
ments are  completely  enclosed  within  the  matrix  of  the  vein,  which  must  have  been 
partially  open,  with  the  matrix  still  in  course  of  deposit,  when  they  were  detached  from 
the  parent  rock.      I^rge  blocks  (riden)  may  be  thus  enclosed. 

(4)  Druay,  containing  or  made  up  of  cavities  lined  with  crystalline  mineral*.  The 
central  parts  of  veins  frequently  present  this  structure,  particularly  where  the  minerals 
have  been  deposited  from  each  side  towards  the  middle. 

(5)  Filamentous,  having  the  minerals  disposed  in  thread-like  veins  ;  thb^is  one  of 
the  commonest  structures. 

Metallic  ores  occur  under  a  variety  of  forms  in  mineral -veins.  Sometimes  they 
are  disseminated  in  minute  grains  or  fine  threads  (gold,  pyrites],  or  gathered  into 
irr^tlar  strings,  branches,  bunches,  or  leaf-like  expansions  (native  copper),  or  disposed 
in  layers  alternating  with  the  veinstones  parallel  with  the  walls  of  the  vein  (most 
metallic  ores),  or  forming  the  whole  of  the  vein  (pyrites,  and  occasionally  galena),  or 
lining  druay  cavities,  both  on  a  small  scale  and  in  Urge  ehamben  (hiemattte,  galena). 
Some  ores  are  frequently  found  in  association  (galena  and  blende),  or  are  noted  for 
containing  minute  proportions  of  another  metal  (argentifcroua  galena,  auriferous  pyrites). 

Successive  Infllllng  of  veins. — The  symmetrical  disposition  re- 
presented in  Fig.  3H  shows  that  the  fissure  bad  its  two  walls  coated 
first  with  the  layers  b  b.  Thereafter  the  still  open,  or  subsequently 
widened,  cleft  received  a  second  layer  (c  c)  on  each  face,  and  so  on  pro- 
greflsively  until  the  whole  was  filled  up,  or  until  only  cavernous  spaces 
(druses)  lined  with  crystals  were  left.  In  such  cases,  no  evidence  exists 
of  any  terrestrial  movement  during  the  process  of  successive  deposition. 
The  fissure  may  have  been  originally  as  wide  as  the  present  vein,  or  may 
have  been  widened  during  the  accumulation  of  mineral  matter,  so 
gradually  and  gently  as  not   to  disturb  the  gathering  layers.     But  in 


GEOTECTOmC  (STRUCTURAL)   GEOLOGY 


many  instances,  as  above  stated,  proofs  remain  of  a  series  of  disturbanoes 
whereby  the  formation  of  the  vein  was  accelerated  or  interrnpted.  Thns 
at  the  Wheal  Julia  Lode,  Cornwall,  the  central  zone  («  in  Fig.  315)  is 
formed  of  quartz-crystals  pointing  as  usual  from  the  sides  towards  the 
centre  of  the  vein,  hut  it  is  only  one  of  five  similar  zones,  each  of  which 
marlis  an  opening  of  the  Essure  and  the  subsequent  closing  of  it  by  a 
deposit  of  mineral  matter  along  the  walls.'  The  occurrence  of  different 
layers  on  the  two  walls  of  a  vein  may  sometimes  indicate  successive  open- 
ings of  the  fissure.  In  Fig.  316  the  fissure  at  one  time,  no  doubt, 
extended  no  farther  than  between  1  and  2.  Whether  the  band  of  copper 
pyrites  bad  already  filled  up  the  fissure,  previous  to  the  opening  which 
allowed  the  deposit  of  the  silica,  or  was  introduced  into  a  fissure  opened 
between  3  and  3  after  the  deposit  of  the  silica,  is  uncertain.' 


mm 


Lode.Oodolphl 

Bildet,Coni. 

-■.U(n.) 

tlHfc  of  Tela 

fp.  (jiiarti-crysUla  pointing  in- 

wnrd ;  c  c,  BjpitLlbnn  silica ;  d 

thick  lay<To(<^ 

pper-pyntw. 

Fig.  3tS.— Sertion  or  VHietil  Jull 
Ix«ie,  ComwUl.  sbowlng  r 
ccMivc  opentngi  of  llie  i 


The  occurrence  of  rounded  pebbles  of  slate,  quai-tz,  and  granite  in  the 
lodes  of  Cornwall  at  depths  of  GOO  feet  from  the  surface,  of  gneiss  in  the 
vein  at  Joacbimsthal  at  1150  feet,  and  of  Liassic  land  and  freshwater 
shells  at  270  feet  in  veins  traversing  the  Carboniferous  Limestone  of  the 
Mendip  Hills  and  South  Wales,  eeems  to  indicate  that  fissures  may 
remain  sufficiently  open  to  allow  of  the  introduction  of  water- worn  stones 
and  terrestrial  organisms  from  the  surface  even  down  to  considerable 
depths.8 

ConnectioD  of  veins  with  faults  and  cross-veins. — While  the  inter- 
spaces between  any  divisional  planes  in  rocks  may  serve  as  receptacles  of 
mineral  depositions,  the  largest  and  most  continuous  veins  have  for  the 
most  part  been  formed  in  lines  of  fault.  These  may  be  traced,  some- 
times in  a  nearly  straight  course,  for  many  miles  across  a  country,  and  as 
far  downward  as  mining  operations  have  been  able  to  descend.  Some- 
times veins  are  themselves  faulted  and  crossed  by  other  veins.  Like 
ordinary  faults   also,  they  are   apt   to  split  up  at  their  terminations. 

1  Dels  Bechs,  'Geol.  Obs.'  p.  098.  '  De  la  Beche,  cp.  eit.  p.  6B6. 

■  De  la  Beclie,  op.  cil.  p.  696.     Moore,  Q.  J.  Orol.  Soc.  xiiii.  483 ;  Brit.  Atioe.  1S69, 


PART  IX  §  i 


MINERAL-VEINS 


637 


These  features  are  well  exhibited   in  some  of  the  mining  districts  of 
Cornwall  (Fig.  317). 


y    .in*"' 


lU"*' 


Fig.  317.— Plan  of  Wheal  Fortune  Lode,  Cornwall  (A) 

{ I  m,  lodes  of  whicli  the  main  one  splits  up  towards  east  and  west,  traversing  elvan  dykes,  e  «,  but 
cut  by  faults  or  cross-courses,  ({  d.    Scale  one  inch  to  a  mile. 

The  intersections  of  mineral-veins  do  not  always  at  once  betray  which 
is  the  older  series.  If  a  vein  has  really  been  shifted  by  another,  it  must 
of  course  be  older  than  the  latter.  But  the  evidence  of  displacement 
may  be  deceptive.  In  such  a  section  as  that 
in  Fig.  318,  for  example,  a  cursory  examination 
might  suggest  the  inference  that  the  vein  d  e 
must  be  later  than  the  dyke  or  vein  a  by  by 
which  its  course  appears  to  have  been  shifted. 
Should  more  careful  scrutiny,  however,  lead 
to  the  detection  of  the  vein  crossing  the  sup- 
posed later  mass  at  c,  it  would  be  clear  that 
this  inference  must  be  incorrect.^  In  mineral 
districts,  different  series  or  systems  of  mineral -veins  can  generally  be 
traced,  one  crossing  another,  belonging  to  different  periods,  and  not  in- 
frequently filled  with  different  ores  and  veinstones.     In  the  south-west  of 


a- 


Fig.  818. — Deceptive  shifting  of  a 
Vein  (A) 


Fig.  819.— General  Map  of  Fissures  in  the  mineral  tracts  of  8.W.  England  (B.) 

Ungland,  for  example,  a  series  of  fissures  running  N.  and  S.,  or  N.N. W. 
^nd  S.S.K,  traverses  another  series,  which  runs  in  a  more  east  and  west 

*  De  la  Beche,  op.  ciL  p.  667. 


640  GEOTECTONIG  (STRUCTURAL)  GEOLOGY  book  iv 

stone  on  which  the  limestone  lies.  Lenticular  aggregations  of  ore  and 
veinstone  found  in  granite,  as.  in  the  south-west  of  England,  where  they 
are  known  as  Garbonas,  cannot  be  due  to  the  infilling  of  chambers  dis- 
solved by  subterranean  solution.  They  are  usually  connected  witli  true 
fissure-veins ;  but  their  mode  of  origin  is  not  well  understood. 

Stock-works  are  portions  of  the  surrounding  rock  or  "  country "  so 
charged  with  veins,  nests,  and  impregnations  of  ore  that  they  can  be 
worked  as  metalliferous  deposits.  The  tin  stock-works  of  Cornwall  and 
Saxony  are  good  examples.  Sometimes  a  succession  of  such  stock-works 
may  be  observed  in  the  same  mine.  Among  the  granites,  elvans,  and 
Devonian  slates  of  Cornwall,  tin-ore  has  segregated  in  rudely  parallel 
zones  or  '*  floors."  At  Botall&ck,  at  the  side  of  ordinary  tin  lodes,  floors 
of  tin-ore  from  six  to  twelve  feet  thick  and  from  ten  to  forty  feet  broad 
occur.  The  name  of  Fahlbands  has  been  given  to  portions  of  "  country  " 
which  have  been  impregnatecbwith  ores  along  parallel  belts. 

Origin  of  mineral-veins.— Various  theories  have  been  proposed  to 
account  for  the  infilling  of  mineral  veins.  Of  these  the  most  noteworthy 
are — (1)  the  theory  of  lateral  segregation, — which  teaches  that  the  sub- 
stances in  the  veins  have  been  derived  from  the  adjacent  rocks  by  a 
process  of  leaching,  or  solution  and  redeposit ;  and  (2)  the  theory  of  in- 
filling from  below, — according  to  which  the  minerals  and  ores  were 
introduced  (a)  dissolved  in  water  or  steam,  or  (h)  by  sublimation,  or  (c)  by 
igneous  fusion  and  injection. 

The  structure  and  characteristic  mineral  combinations  of  metalliferous 
veins  are  precisely  such  as  would  be  produced  by  deposition  from  aqueous 
solution.     There  can  hardly  be  now  any  doubt  that  the  contents  of  these 
veins    have  generally  been  deposited  by  water.     But  the  source  from 
which  the  metals  were  derived  is  not  so  obvious.     The  fact  that  the 
nature  and  amount  of  the  minerals,  and  especially  of  the  ores,  in  a  vein  so 
often  vary  with  the  composition  of  the  surrounding  rocks  shows  that  these 
rocks  have  had  an  influence  on  the  precipitation  of  mineral  matter  in  the 
fissures  passing  through  them,  if  they  were  not  themselves  the  source 
from  which  the  metals  were  obtained ;  for,   as  already  remarked,  the 
presence  of  the  heavy  metals  has  now  been  detected  in  rocks  of  almost 
every  kind  and  age.     On  the  other  hand,  in  some  volcanic  districts  at 
the  present  time,  various  minerals,  including  silica,  both  crystalline  and 
chalcedonic,  metallic  sulphides,  and  even  metallic  gold,  are  being  deposited 
in  fissures  up  which  hot  water  rises. ^    Each  of  these  modes  of  origin  may 
in  diflerent  cases  have  occurred.     It  is  almost  certain,  from  what  we  now 
know  of  the  diflusion  of  metallic  substances,  that  there  must  be  a  de- 
composition of  the  rocks  on  either  side  of  a  fissure,  perhaps  to  a  great 
distance,  and  that  a  portion  of  the  mineral  matter  abstracted  will  be  laid 
down  in  another  form  along  the  fissure-walls.     If,  on  the  other  hand,  the 
rocks  on  either  side  of  the  fissure  are  permeated  for  some  distance  by  hot 
ascending   waters,  holding   such    metalliferous   solutions   as   have  been 
detected  in  the  hot  springs  of  California  and  Nevada,  some  of  the  dis- 
solved mineral  substances  will  doubtless  be  deposited  in  the  fissure,  anoL 

1  See  J.  A.  PhUlips,  Q.  J,  Geol,  Soc,  xxxv.  p.  390. 


PART  X  UNCONFORMABILITY  641 

may  even  be   introduced    into  the  pores  and  cavities  of  the  adjacent 
rocks.  ^ 

Part  X.  Unconformability. 

Where  one  series  of  rocks,  whether  of  aqueous  or  igneous  origin,  has 
been  laid  down  continuously  and  without  disturbance  upon  another  series, 
they  are  said  to  be  confunmihle.     Thus  in  Fig.  322,  the  sheets  of  con- 


FiK.  32*i.— Unconfoniiabllity  among  horizontal  strata.     Llan  resting  on  Carboniferous 

LiineMtone,  Glamorganshire  {B.) 

glomerate  (p  h)  and  clays  and  shales  {c  d),  have  succeeded  each  other  in 
regular  order,  and  exhibit  a  perfect  ayjifartnabilili/.  They  oveiiap  each 
other,  however,  each  bed  extending  beyond  the  edge  of  that  below  it, 
and  thereby  indicating  a  gradual  subsidence  and  enlargement  of  the  area 
of  deposit  (p.  518).  But  all  these  conformable  beds  repose  against  an 
older  platform  a  a,  with  which  they  have  no  unbroken  continuity.  Such 
a  surface  of  junction  is  called  an  unconformubiliti/,  and  the  upper  are  said  to 
lie  unconformable  on  the  lower  rocks.  The  latter  may  consist  of  horizontal 
or  inclined  clastic  strata,  or  contorted  schists,  or  eruptive  massive  rocks. 
In  any  case,  there  is  a  complete  break  between  them  and  the  overlying 
formation,  the  beds  of  which  rest  successively  on  different  parts  of  the 
older  mass. 

It  is  evident  that  this  structure  may  occur  in  ordinary  sedimentary, 
igneous,  or  metamorphic  rocks,  or  between  any  two  of  these  great  series. 
It  is  most  familiarly  displayed  among  clastic  formations,  and  can  there 
be  most  satisfactorily  studied,  since  the  lines  of  bedding  furnish  a  ready 
means  of  detecting  differences  of  inclination  and  discordance  of  super- 
position. But  even  among  igneous  protrusions,  and  in  ancient  meta- 
morphic masses,  distinct  evidence  of  unconformability  is  occasionally 
traceable.  Wherever  one  series  of  rocks  is  found  to  rest  upon  a  highly 
denuded    surface  of  an    older   series,   the  junction    is   unconfonnable.- 

*  Heuwood,  Address  Roy.  Inst.  Ormwall,  1871.  J.  A.  Pliillips,  Phil.  Mag.  November 
1868,  December  1871,  July  1873,  March  1874  ;  *Ore  Depositjs'  1884,  p.  73.  J.  S.  New- 
berry, School  of  Mines  Quarterly ^  New  York,  March  1880.  J.  A.  Church,  *  The  Conistock 
Lode,'  4to,  New  York,  1879.  Sterry  Hunt,  'Chemical  and  Geological  Essays,"  1875, 
p.  i83.  Brough  Smyth's  'Goldfields  of  Victoria,'  Melbourne,  1869.  F.  Sandberger, 
'  Untersuchungen  iiber  Erzgiinge,'  part  i. 

*  The  occurrence  of  considerable  contemporaneous  erosion  between  undoubtedly  conform- 
able strata  belonging  to  one  continuous  geological  series  has  already  (pp  504-506)  been 
described. 

2  T 


642  GEOTECTONIC  {STRUCTURAL)  GEOLOGY  book.  Vf 

Hence,  an  uneven  irregularly-worn  platform  below  a  succesaion  of  mutu- 
ally conformable  rocks  is  one  of  the  most  characteristic  features  of  this 
kind  of  structure. 

It  has  already  been  pointed  out,  that  though  conformable  rocks  may 
usually  be  presumed  t«  have  followed  each  other  continuously  without 
any  great  disturbance  of  geographical  conditions,  we  cannot  always  be 
safe  in  such  an  inference.  But  an  unconformability  leaves  no  room  to 
doubt  that  it  marks  a  decided  break  in  the  continuity  of  deposit.  Hence 
no  kind  of  geological  structure  is  of  higher  importance  in  the  interpreta- 
tion of  the  history  of  the  stratified  fonnations  of  a  country.  In  ntre 
cases,  an  unconformability  may  occur  between  two  horizontal  groups  of 
strata.  On  the  left  side  of  Fig.  332,  for  instance,  the  beds  d  foUow 
horizontally  upon  the  horizontal  beds  {a).  Were  merely  a  limited  section 
visible,  disclosing  only  this  relation  of  the  rocks,  the  two  groups  a  and  d 
might  be  mistaken  for  conformable  portions  of  one  continuous  series. 
Further  examination,  however,  would  lead  to  the  detection  of  evidence 
that  the  limestone  [[  had  been  upraised  and  unequally  denuded  before  the 
deposition  of  the  overlying  strata  bed.  This  denudation  would  show 
that  the  apparent  contormability  was  merely  local  and  accidental,  the 
older  rock  having  really  been  upraised  and  worn  down  before  the  fonnft- 
tion  of  the  newer.  In  such  a  case,  the  upheaval  must  have  been  so 
uniform  over  some  tracts  as  not  to  disturb  the  horizontality  of  the  lower 
strata,  so  that  the  younger  deposits  lie  in  apparent  conformability  upon 
them. 

As  a  rule,  however,  it  seldom  happens  that  movements  of  this  kind 
have  taken  place  over  an  extensive  area  so  equably  as  not  to  produce  a 
want  of  coincidence  somewhere  between  the  older  and  newer  rocks. 
Most  frequently,  the  older  fonnations  have  been  tilted  at  various  angles, 
or  even  placed  on  end.  They  have  likewise  been  irregularly  aiid  often 
enormously  worn  down.  Hence  instead  of  lying  parallel,  the  younger 
beds  run  transgressively  across  the  upturned  denuded  ends  of  the  older. 
The  greater  the  disturbance  of  the  older  rocks,  the  more  marked  is  the 
unconformability.  In  Fig.  323,  the  lower  series  of  beds  (e)  has  been 
upturned  and  denuded  before  the  depo- 
sition of  the  upper  series  {a  b)  upon 
it.  In  this  instance,  the  upper  worn 
surface  of  the  limestones  {e}  has  been 
perforated  by  boring  mollusks  below  the 
sandy  stratum  (h). 

An   unconformability  forms   one  of 
Tig.  3^3.— uncnunirtiitbiiit)-  brtwcsn  hori-   the  great  breuks  in  the  geoli^cal  record. 
a-ntai  =n.i  inclined  .tot.,    inferipr    In  Fig.  221  (p.  518),  by  Way  of  ilinstra- 

Li'i!iMtMi*")ri^m(!'8om™t('B7'"     '''*'"'  "'"^  ®^^  ^*  *'"*'^  *''**■  *  ''Otal>le  histUS 

in  deposition,  and  therefore  in  geological 
chronology,  must  exist  between  the  older  conformable  series,  a  be,  and  the 
later  strata  by  which  these  are  covered.  The  former  bad  been  deposited, 
folded,  upheaved,  and  worn  down  before  the  accumulation  of  the  newer 
series  upon  their  denuded  edges.     These  changes  must  have  demanded  a 


PAKT  I  UNGONFOEM ABILITY  643 

considerable  lapse  of  time.  Yet,  looking  merely  at  the  structure  in  itself, 
we  have  evidently  no  means  of  fixing,  even  relatively,  the  length  of  interval 
marked  by  an  unconfonnability.  By  ascertaining,  from  some  other 
region,  the  full  suite  of  formations,  we  learn  what  members  of  the  succes- 
sion are  wanting.  In  this  way,  it  would  be  discovered  that  the  greater 
part  of  the  Carboniferous  system,  the  whole  of  the  Permian,  and  the 
Trias  and  the  Lias  are  absent  from  the  ground  represented  in  Fig.  323 
(compare  Fig.  221).  The  mere  violence  of  contrast  between  a  set  of  vertical 
beds  below  and  a  horizontal  group  above,  is  in  itself  no  certainly  reliable 
criterion  of  the  relative  lapse  of  time  between  their  deposition  ;  for 
obviously,  an  older  portion  of  a  given  formation  might  be  tilted  on  end, 
and  be  overlain  unconformably  by  a  later  part  of  the  same  formation.  A 
set  of  flat  rocks  of  high  geological  antiquity  may,  on  the  other  hand,  be 
conformably  covered  by  a  formation  of  comparatively  recent  date,  yet,  in 
8pit«  of  the  want  of  discordance  between  the  two,  they  might  have  been 
separated  by  a  large  portion  of  the  total  sum  of  geological  time.  Further 
examination  will  usually  suffice  to  show 
that  the  conformability  in  such  cases  is 
only  partial  or  accidental,  and  that  locali- 
ties maybe  found  where  the  formations  Fig.  3i4.— SMtion  ono«i  dtMpovB 
are  distinctly  unconformable.     From  the  '^"  *'™'     **^' 

centre  of  the  section  in  Fig.  324,  for  example,  the  two  groups  of  rocks 
might,  on  casual  examination,  be  pronounced  to  be  conformable.     Yet  at 


d,  PMt-TertUuy  Om 


short  distances  on  either  side,  proofs  of  violent  unconformability  are  con- 
spicuous.   It  sometimes  happens  that  more  than  one  imcouformability  may 


644  GEOTECTOXIC  {STRUCTURAL)  GEOLOGY  book  iv 

be  detected  in  the  same  section.  Thus  in  Fig.  325,  the  break  between  the 
quartzite  (q)  and  Old  Red  Sandstone  (s)  is  to  the  eye  mach  more  violent 
and  complete  than  that  between  the  sandstone  and  the  overlying  gravels 
and  clays  (d).  Yet  the  interval  separating  the  epoch  of  the  qnartzite 
from  that  of  the  sandstone  may  have  been  brief,  when  compared  with  the 
vast  lapse  of  time  that  intervened  between  the  nearly  flat  sandstones  and 
overlying  superficial  deposits.  It  is  by  the  evidence  of  organic  remains 
that  the  relative  importance  of  unconformabilities  must  be  measured,  as 
will  be  explained  in  Book  V. 

Paramount  though  the  effect  of  an  unconformability  may  be  in  the 
geological  structure  of  a  country,  it  must  nevertheless,  when  viewed  on 
the  large  scale,  be  merely  local.  The  disturbance  by  which  it  wag  pro- 
duced will  usually  be  found  to  have  affected  a  comparatively  circumscribed 
region,  l>eyond  the  limits  of  which  the  continuity  of  sedimentation  may 
have  been  undisturbed.  We  may,  therefore,  generally  expect  to  be  able 
to  fill  up  the  gaps  in  one  district  or  countr}''  from  the  more  complete 
geological  formations  of  another. 


BOOK   V. 

PALiEONTOLOGICAL  OEOLOGY. 

Pai^ontology  treats  of  the  structure,  affinities,  classification,  and  dis^ 
tribution  in  time  of  the  forms  of  plant  and  animal  life  imbedded  in  the 
rocks  of  the  earth's  crust.  Considered  from  the  biological  side,  it  is  a 
part  of  zoology  and  botany.  A  proper  knowledge  of  extinct  organisms 
can  only  be  attained  by  the  study  of  living  forms,  while  our  acquaintance 
with  the  history  and  structure  of  modem  organisms  is  amplified  by  the 
investigation  of  their  extinct  progenitors.  Viewed,  on  the  other  hand, 
from  the  physical  side,  palaeontology  is  a  branch  of  geology.  It  is 
mainly  in  this  latter  aspect  that  it  will  here  be  discussed. 

Palaeontology  or  Palaeontological  Geology  deals  with  fossils  or 
organic  remains  preserved  in  natural  deposits,  and  endeavours  to  gather 
from  them  information  as  to  the  history  of  the  globe  and  its  inhabitants. 
The  term  fossil,  meaning  literally  anything  "dug  up,"  was  formerly 
applied  indiscriminately  to  any  mineral  substance  taken  out  of  the 
earth's  crust,  whether  organised  or  not.  Ordinary  minerals  and  rocks 
were  thus  included  as  fossils.  For  many  years,  however,  the  meaning 
of  the  word  has  been  so  restricted  as  to  include  only  the  remains  or 
traces  of  plants  and  animals  preserved  in  any  natural  formation,  whether 
hard  rock  or  loose  superficial  deposit  The  idea  of  antiquity  or  relative 
date  is  not  necessarily  involved  in  this  conception  of  the  term.  Thus, 
the  bones  of  a  sheep  buried  under  gravel  and  silt  by  a  modern  flood,  and 
the  obscure  crystalline  traces  of  a  coral  in  ancient  masses  of  limestone, 
are  equally  fossils.^  Nor  has  the  term  fossil  any  limitation  as  to  organic 
grade.  It  includes  not  merely  the  remains  of  organisms,  but  also  what- 
ever was  directly  connected  with  or  produced  by  these  organisms.  Thus, 
the  resin  which  exuded  from  trees  of  long-perished  forests  is  as  much  a 
fossil  as  any  portion  of  the  stem,  leaves,  flowers,  or  fruit,  and  in  some 
respects,  is  even  more  valuable  to  the  geologist  than  more  determinable 
remains  of  its  parent  trees,  because  it  has  often  preserved  in  admirable 
perfection  the  insects  which  flitted  about  in  the  woodlands.    The  burrows 

^  The  word  "fossil"  is  sometimes  wrongly  usetl  as  synonymous  with  "petrified,"  and 
we  accordingly  find  the  intolerable  barbarism  of  "sub-fossil." 


646  PALjEOSTOLOGICAL  GEOLOGY  book  t 

or  trails  of  a  worm,  in  sandstone  or  shale,  claim  recognitioa  as  fosBflsy 
and  indeed  are  commonly  the  only  indications  to  be  met  with  of  the 
existence  of  annelide  life  among  old  geological  formatioiia.  The  drop> 
pings  (coprolites)  of  fishes  and  reptiles  are  excellent  fofisils,  and  tell  thcar 
tale  as  to  the  presence  and  food  of  vertebrate  life  in  andent  watefK  The 
little  agglutinated  cases  of  the  caddifi-worm  remain  as  foesils  in  formations 
from  which  perchance  most  other  traces  of  life  may  have  passed  away. 
Nay,  the  very  handiwork  of  man,  when  preserved  in  any  natural  manner 
is  entitled  to  rank  among  fossils ;  as  where  his  flint-implements  have  been 
dropped  into  the  prehistoric  gravels  of  river-valleys,  or  where  his  canoes 
have  l>een  buried  in  the  silt  of  lake-bottoms. 

The  term  fossil,  moreover,  sufiers  no  restriction  as  to  the  condition  or 
state  of  preservation  of  any  organism.  In  some  rare  instances,  the  tsit 
flesh,  skin,  and  hair  of  a  mammal  have  been  preserved  for  thousands  of 
years,  as  in  the  case  of  mammoth  carcases  entombed  in  the  frozen  mod- 
cliffs  of  Siberia.^  Generally,  all  or  most  of  the  original  animal  matter 
has  disappeared,  and  the  organism  has  been  more  or  less  ccNnpletely 
mineraliz^  or  petrified.  It  often  happens  that  the  whole  organism  has 
decayed,  and  a  mere  cast  in  amorphous  mineral  matter,  as  sand,  chj, 
ironstone,  silica,  or  limestone,  remains ;  yet  all  these  variations  most  be 
comprised  in  the  comprehensive  term  fo^iL 

Two  preliminary  questions  demand  attention  :  in  the  first  place,  how 
remains  of  plants  and  animals  come  to  be  entombed  in  rocks,  and  in  the 
second,  how  they  have  been  preserved  there  so  as  to  be  now  recognisable. 

§  i.  Conditions  for  the  entombment  of  organic  remains. — ^If  what 
takes  place  at  the  present  day  may  fairly  be  taken  as  an  indication  of 
what  has  been  the  ordinary  condition  of  things  in  the  geological  past, 
there  must  have  been  so  many  chances  against  the  conservation  of  either 
animal  or  plant  remains,  that  their  occurrence  among  stratified  forma- 
tions should  be  regarded  as  exceptional,  and  as  the  result  of  varioos 
fortunate  accidents. 

1.  On  Land. — Let  us  consider,  in  the  first  place,  what  chances  exist 
for  the  preservation  of  remains  of  the  present  fauna  and  flora  of  a  countiy. 
The  surface  of  the  land  may  be  densely  clothed  with  forest,  and  abund- 
antly peopled  with  animal  life.  But  the  trees  die  and  moulder  into  soil 
The  animals,  too,  disappear,  generation  after  generation,  and  leave  few 
perceptible  traces  of  their  existence.  If  we  were  not  aware  from 
authentic  records  that  central  and  northern  Europe  was  covered  widi 
vast  forests  at  the  beginning  of  our  era,  how  could  we  know  this  factf 
AMiat  has  become  of  the  herds  of  wild  oxen,  the  bears,  wolves,  and  other 
denizens  of  the  lowlands  of  primeval  Europe  f  For  unknown  ages,  too^ 
the  North  American  prairies  have  been  roamed  over  by  countless  herds 
of  buffaloes,  yet,  except  here  and  there  a  skull  and  bones  of  some  com- 
paratively recent  indi\idual,  every  trace  of  these  animals  has  disaj^peared 
from  the  surface. ^     How  could  we  prove  from  the  examination  of  the 

'  For  particulars  of  a  recent  exhumation  see  *Beitrage  zur  Kenntnin  des  RiUBBchen 
Rciches,'  Bd.  III.  (1887)  p.  175. 

-  See  J  ales  Marcou,  '  Lettres  sur  les  roches  du  Jura,*  p.  103. 


§  i.  1  THE  ENTOMBMENT  OF  ORGANIC  REMAINS  647 


soil  either  in  Europe  or  North  America  that  such  creatures,  though  now 
locally  extinct,  had  once  abounded  there  ?  We  might  search  in  vain  for 
any  superficial  relics  of  them,  and  should  learn  by  so  doing  that  the  law 
of  nature  is  everywhere  "  dust  to  dust" 

The  conditions  for  the  preservation  of  evidence  of  terrestrial  (includ- 
ing freshwater)  plant  and  animal  life  must,  therefore,  be  always  local,  and, 
so  to  say,  exceptional.  They  are  supplied  only  where  organic  remains 
can  be  protected  from  air  and  superficial  decay.  Hence,  they  may  be 
observed  in  lakes,  peat-mosses,  deltas  at  river-mouths,  caverns,  deposits 
of  mineral-springs  and  volcanoes. 

a.  Lakes. — Over  the  floor  of  a  lake,  deposits  of  silt,  peat,  marl,  &c.,  are  formed. 
Into  these,  the  trunks,  branches,  leaves,  flowers,  fruits,  or  seeds  of  plants  from  the 
neighbouring  land  may  be  carried,  together  with  the  bodies  of  vertebrates,  birds,  and 
insects.  An  occasional  storm  may  blow  the  lighter  debris  of  the  woodlands  into  the 
water.  Such  portions  of  the  wreck  as  are  not  washed  ashore  again,  may  sink  to  the 
bottom,  where  they  will,  for  the  most  part,  probably  rot  away,  so  that,  in  the  end,  only 
a  very  small  fraction  of  the  whole  vegetable  matter,  cast  over  the  lake  by  the  wind,  is 
covered  up  and  preserved  at  the  bottom.  In  like  manner,  the  remains  of  winged  and 
four-footed  animals,  swept  by  winds  or  by  river-floods  into  the  lake,  run  so  many  risks 
of  dissolution,  that  only  a  proportion  of  them,  and  probably  merely  a  small  proportion, 
is  preserved.  When  we  consider  these  chances  against  the  conservation  of  the  vegetable 
and  animal  life  of  the  land,  we  must  admit  that,  at  the  best,  lake-bottoms  can  contain 
but  a  meagre  and  imperfect  representation  of  the  abundant  life  of  the  adjacent  hills  and 
plains.  Lakes,  however,  have  a  distinct  flora  and  fauna  of  their  own.  Their  aquatic 
plants  may  be  entombed  in  the  gathering  deposits  of  the  bottom.  Their  mollusks,  of 
chaiacteristic  types,  sometimes  form,  by  the  accumulation  of  their  remains,  sheets  of 
soft  calcareous  marl  (pp.  139,  484),  in  which  many  of  the  undecayed  shells  are  preserved. 
Their  lacustrine  fishes,  likewise,  must  no  doubt  often  be  entombed  in  the  silt  or  marl. 

b.  Peat-mosses. — Wild  animals,  venturing  on  the  more  treacherous  watery  parts  of 
peat-bogs,  are  sometimes  engulphed  or  'Paired."  The  antiseptic  qualities  of  the  peat 
preserve  their  remains  from  decay.  Hence,  from  European  peat -mosses,  numerous 
remains  of  deer  and  oxen  have  been  exhumed.  Evidently  the  larger  beasts  of  the 
forest  ought  chiefly  to  be  looked  for  in  these  localities  (p.  478). 

e.  Delias  at  river-mouths. — It  is  obvious  that,  to  some  extent,  both  the  flora  and 
the  fauna  of  the  land  may  be  buried  among  the  sand  and  silt  of  deltas  (p.  401 ).  But 
though  occasional  or  frequent  river-floods  sweep  down  trees,  herbage,  and  the  bodies  of 
land-animals,  the  carcases  so  transported  run  every  risk  of  having  their  bones  separated 
and  dispersed,^  or  of  decaying  or  being  othen^'ise  destroyed,  while  still  afloat ;  and  even 
if  they  reach  the  bottom,  they  tend  to  dissolution  there,  unless  speedily  covered  up 
and  protected  by  fresh  sediment.  Delta-formations  can  therefore  scarcely  be  expected 
to  preserve  more  than  a  meagre  outline  of  a  varied  terrestrial  flora  and  fauna. 

d.  Caverns.  — These  are  eminently  adapted  for  the  preservation  of  the  higher  forms 

of  terrestrial  life  (pp.  368,  494).     Most  of  our  knowledge  of  the  prehistoric  mammalian 

fauna  of  Europe  is  derived  from  what  has  been  disinterred  from  bone-eaves.     As  these 

recesses  lie,   for  the  most  part,  in  limestone  or  in  calcareous  rock,   their  floors  are 

commonly  coated  with  stalagmite  from  the  drip  of  the  roof ;  and  as  this  deposit  is  of 

great  closeness  and  durability,  it  has  effectually  preserved  whatever  it  has  covered  or 

enveloped.     The  caves  have,  in  many  instances,  served  as  dens  for  predatory  beasts, 

like  the  hyeena,  cave -lion,  and  cave -bear,  which  sometimes  dragged  their  prey  into 

«  _^_^^^^^__^^^_^-^^-— ^-^— ^^— — ^.^^^^ 

^  Lower  jaws,  for  instance,  because  they  are  among  the  earliest  parts  of  the  skeleton  of 
a  floating  carcase  to  drop  off  are  not  infrequently  met  with  as  fossils. 


648  PALjEONTOLOGIGAL  GEOLOGY  book  v 

these  recesses.  In  other  cases,  they  have  been  merely  holes  whither  different  animals 
crawled  to  die,  or  into  which  they  fell  or  were  swept  by  inundations.  Under  what- 
ever circumstances  the  animals  left  their  remains  in  these  subterranean  retreats,  the 
bones  have  l)een  covered  up  and  preserved.  Still  we  nmst  admit  that,  after  all,  only 
a  small  fraction  of  the  animals  of  the  time  would  enter  the  caves,  and  therefore  that 
the  evidence  of  the  cavern-deposits,  profoundly  interesting  and  valuable  as  it  is,  pre< 
sents  us  with  merely  a  glimpse  of  one  aspect  of  the  life  of  the  land. 

e.  Deposits  of  mineral -springs. — The  deposits  of  mineral  matter,  resulting  from  the 
evai>oration  of  mineral  springs  on  the  surface  of  the  ground,  serve  as  receptacles  for 
occasional  leaves,  land-shells,  insects,  dead  birds,  small  mammals,  and  other  remains  of 
the  plant  and  animal  life  of  the  land  (pp.  366,  482). 

/.  Volcanic  deposits. — Sheets  of  lava  and  showers  of  volcanic  dust  may  entomb 
terrestrial  organisms  (pp.  201,  594).  It  is  obnous,  however,  that  even  over  the  areas 
wherein  volcanoes  occur  and  continue  active,  they  can  only  to  a  very  limited  extent 
entomb  and  prescr\'e  the  flora  and  fauna  of  the  land. 

2.  In  the  Sea. — In  the  next  place,  if  we  turn  to  the  sea,  we  find 
certainly  more  favourable  conditions  for  the  preservation  of  organic 
forms,  but  also  many  circumstances  which  operate  against  it. 

a.  Littoral  deposits, — While  the  level  of  the  land  remains  stationary,  there  can  be 
but  little  effective  entombment  of  marine  organisms  in  littoral  deposits ;  for  only  a 
limited  accumulation  of  sediment  will  be  formed  until  subsidence  of  the  sea-floor  taJces 
place.  In  the  trifling  beds  of  sand  or  gravel  thrown  up  by  storms  above  the  limits  of 
ordinary  wave-action  on  a  stationary  shore,  only  the  harder  and  more  durable  forms 
of  lifp,  such  as  the  stronger  gasteropods  and  lamellibranchs,  which  can  withstand  the 
triturating  effects  of  the  beach-waves,  are  likely  to  remain  unetfaced  (p.  454). 

h.  Deeper-water  terrigcnmis  deposits. — Below  tide-marks,  along  the  margin  of  land 
whence  sediment  is  derived,  conditions  are  more  favoumblc  for  the  preservation  of 
marine  organisms.  Sheets  of  sand  and  mud  are  there  laid  down,  wherein  the  harder  parts 
of  many  forms  of  life  may  be  entombed  and  protected  from  decay  (p.  456).  But  probably 
only  a  small  proiK)rtion  of  the  fauna  that  crowds  these  marginal  watere  of  the  ocean, 
with  perhaj^s  an  occasional  pelagic  sjjecies,  may  l)e  exi)ected  to  occur  in  such  deposits. 
Moreover,  for  the  entoniV)ment  and  preservation  of  the  remains  of  these  organisms,  there 
must  obviously  be  a  sufficiently  abundant  and  rapid  dejwsit  of  sediment,  combined  with 
a  slow  depression  of  the  sea-bottom.  Under  the  most  favourable  conditions,  therefore, 
the  organic  remains  actually  preserved  will  usually  represent  little  more  than  a  mere 
fraction  of  the  whole  assemblage  of  life  in  these  juxta-terrestiial  jiarts  of  the  ocean. 

c.  Abysmal  drposif^. — In  proportion  to  distance  from  land,  the  rat^  of  deposition  of 
sediment  on  the  sea-floor  miLst  become  feebler,  until  in  the  remote  central  abysses 
it  reaches  a  hardly  a[»preciable  mininium,  while  at  the  same  time,  the  solution  of  cal- 
careous organisms  may  become  marked  in  deep  water  (p.  457).  Except,  therefore,  where 
organic  dejwsits  such  as  ooze,  are  forming  in  these  more  j)ehigic  regions,  the  conditions 
must  be  on  the  whole  unfavourable  for  the  preservation  of  any  adequate  representation 
of  the  deep-sea  fauna.  Hard  enduring  olvjects,  such  as  teeth  and  bones,  may  slowly 
accumulate  and  be  protected  by  a  coating  of  jteroxide  of  manganese,  or  of  silicates,  such 
as  are  now  forming  here  and  there  over  the  deep  sea-bottom.  Yet  a  deposit  of  this 
nature,  if  raised  into  land,  would  supply  but  a  meagi-e  picture  of  the  life  of  the  sea. 

In  considering  the  various  conditions  under  which  marine  organisms  may  be  en- 
tombed and  pn^served,  we  must  take  into  account  certain  occasional  phenomena,  when 
sudden,  or  at  least  rapid  and  extensive,  destruction  of  the  fauna  of  the  sea  may  be 
caused.  (1)  Earthquake  shocks  have  been  followed  by  the  washing  ashore  of  vast 
quantities  of  dead  fish.^     (2)  Violent  storms,  by  driving  shoals  of  fishes  into  shallow 

^  C.  Forbes   Q.  J.  Geol.  S^tc.  xiv.  '1858)  p.  294. 


§  L  2  THE  ENTOMBMENT  OF  ORGANIC  REMAINS  649 

water  and  against  rocks,  produce  enormous  destruction.  Dr.  Leith  Adams  describes 
the  coast  of  part  of  the  Bay  of  Fundy  as  being  covered  to  a  depth  of  a  foot  in  some 
|»laces  with  dead  fish,  dashed  ashore  by  a  storm  on  the  24th  of  September,  1867.* 
(3)  Copious  discharges  of  fresh  water  into  the  sea  have  been  observed  to  cause  extensive 
mortality  among  marine  organisms.  Thus,  during  the  S.W.  monsoon  and  accompany- 
ing heavy  rains,  the  west  coasts  of  some  parts  of  India  are  covered  with  dead  fish  thrown 
ashore  from  the  sea.'  (4)  A  sudden  irruption  of  the  outer  sea  into  a  sheltered  and 
l^artially  brackish  inlet  may  cause  the  extinction  of  many  of  the  denizens  of  the  latter, 
though  a  few  may  be  able  to  survive  the  altered  conditions.'  (5)  Volcanic  explosions 
have  been  observed  to  cause  considerable  destruction  to  marine  life,  either  from  the 
heat  of  the  lava,  or  from  the  abundance  of  ashes  or  of  poisonous  gases.  (6)  Want  of 
oxygen,  when  fishes  are  crowded  together  in  frightened  shoals,  or  when,  burrowing  in 
sand  and  mud,  they  are  overwhelmed  with  rapidly  accumulating  detritus,  is  another 
cause  of  mortality.*  (7)  Shoals  of  fish  are  sometimes  driven  ashore  by  the  large 
predatory  denizens  of  the  deep,  such  as  whales  and  porpoises.  (8)  Too  much  or  too 
little  heat  in  shallow  water  leads  to  the  destruction  of  fish.  Large  numbers  of  salmon 
are  sometimes  killed  in  the  pools  of  a  river  during  dry  and  hot  weather.  (9)  Consider- 
able mortality  occasionally  arises  along  the  littoral  zone  from  the  effects  of  severe  frost. 
(10)  Various  diseases  and  parasites  affect  fish,  and  lead  directly  to  their  death,  or 
weaken  them  so  that  they  are  more  easily  caught  by  their  enemies.*  Such  phenomena 
suggest  probable  causes  of  death  in  the  case  of  fossil  fishes,  whose  remains  are  some- 
times crowded  together  in  various  geological  formations,  as  for  example,  in  the  Old  Red 
Sandstone. 

Of  the  whole  sea-floor,  the  area  best  adapted  for  preserving  organic 
exuviae  is  obviously  that  belt  in  which  life  is  most  varied  and  abundant, 
and  where,  along  the  margin  of  the  land,  fresh  layers  of  sediment,  trans- 
ported by  rivers  and  currents  from  the  adjacent  shores,  are  laid  down. 
The  most  favourable  conditions  for  the  accumulation  of  a  thick  mass  of 
marine  fossiliferous  strata  will  arise  when  the  area  of  deposit  is  under- 
going a  gradual  subsidence.  If  the  rate  of  depression  and  that  of  deposit 
be  equal,  or  nearly  so,  the  movement  may  conceivably  continue  for  a  vast 
period  without  producing  any  great  apparent  change  in  marine  geography, 
and  even  without  seriously  affecting  the  distribution  of  life  over  the  sea- 
floor  within  the  area  of  subsidence.  Hundreds  or  thousands  of  feet  of 
sedimentary  strata  may  conceivably  be  in  this  way  heaped  up  round  the 
continents,  containing  a  fragmentary  series  of  remains,  chiefly  forms  of 
shallow-water  life  which  had  hard  parts  capable  of  preservation. 

There  can  be  little  doubt  that  such  has,  in  fact,  been  the  history  of 
the  main  mass  of  stratifled  formations  in  the  earth's  crust.  These  piles 
of  marine  strata  have  unquestionably  been  laid  down  for  the  most  part 
in  comparatively  shallow  water,  within  the  area  of  deposit  of  terrestrial 
sediment.  Their  great  depth  seems  only  explicable  by  prolonged  and 
repeated  movements  of  subsidence,  sometimes  interrupted,  however,  as  we 
know,   by  other   movements   of   a  contrary   kind.     These  geographical 

*  Q.  J,  Oeol,  iSoc.  XX  ix.  p.  303. 

*  Denison,  tfp.  cU.  xviii.  p.  453.  Nature  (December  19, 1872,  p.  124)  gives  another  instance. 
'  Forchhanimer,  Et/in.  New,  Phil,  Jmtm.  xxxi.  p.  69.     Nature,  i.  p.  454  ;  xiii.  p.  107. 

*  Sir  J.  W.  Dawson,  (Jeoiogist,  ii.  (1869)  p.  216. 

*  For  fuller  reference.s,  see  an  interesting  paper  by  Prof.  T.  Rui>ert  Jones,  Oeol,  Mag. 
1882   p.  533. 


650  PAL^ONTOLOGIGAL  GEOLOGY  book  v 

changes  affected  at  once  the  deposition  of  inorganic  materials  and  the  suc- 
cession of  organic  forms.  One  series  of  strata  is  sometimes  abruptly 
succeeded  by  another  of  a  very  different  character,  and  we  not  uncommonly 
find  a  corresponding  contrast  between  their  respective  organic  contents. 

It  follows,  from  these  conditions  of  sedimentation,  that  representatives 
of  the  abysmal  deposits  of  the  central  oceans  are  not  likely  to  be  met 
with  among  the  geological  formations  of  past  times.  Thanks  to  the  great 
work  done  by  the  Challenger  Expedition,  we  know  what  are  the  leading 
characters  of  the  accumulations  now  forming  on  the  deeper  parts  of 
the  ocean -floor.  So  far  as  we  yet  know,  they  have  no  analogues 
among  the  formations  of  the  earth's  crust  They  differ,  indeed,  so  entirely 
from  any  formation  which  geologists  have  considered  to  be  of  deep-water 
origin  as  to  indicate  that,  from  early  geological  times,  the  present  great 
areas  of  land  and  sea  have  remained  on  the  whole  where  they  are,  and  that 
the  land  consists  mainly  of  strata  formed  of  terrestrial  debris  laid  down  at 
successive  epochs  in  the  surrounding  comparatively  shallow  seas. 

§  ii.  Preservation  of  organie  remains  in  mineral  masses. — ^The 
condition  of  the  remains  of  plants  and  animals  in  rock-formations  depends, 
first,  upon  the  original  structure  and  composition  of  the  organisms,  and 
secondly,  upon  the  manner  in  which  their  "  fossilization "  that  is,  their 
entombment  and  preservation,  has  been  effected. 

1.  Influence  of  original  structure  and  composition. 
— The  durability  of  organisms  is  determined  by  their  composition  and 
structure. 

The  internal  skeletons  of  most  vertebrate  animals  consist  mainly  of  phosphate  of 
lime  ;  in  saurians  and  fishes,  there  is  also  an  exo-skeleton  of  hard  bony  plates  or  of 
scales.  It  is  these  durable  ])ortions  that  remain  as  evidence  of  the  former  existence 
of  vertebrate  life.  The  hard  pai-ts  of  invertebrates  present  a  greater  variety  of  com- 
position. In  the  vast  majority  of  cases,  they  consist  of  calcareous  matter,  either 
calcite  or  aragonite.  The  carbonate  of  lime  is  occasionally  strengthened  by 
phosphate,  while  in  a  few  cases,  as  in  the  homy  brachiopods,  in  Conularia^  Serpula,  and 
some  other  forms,  the  phosphate  is  the  chief  constituent.^  Next  in  abundance  to  lime 
is  silica,  which  constitutes  the  frustules  of  diatoms  and  the  harder  parts  of  many 
protozoa,  and  is  found  also  in  the  teeth  of  some  molhisks.  The  integuments  of  insects, 
the  carapaces  of  Crustacea,  and  some  other  organisms,  are  composed  fundamentaUy  of 
chitin,^a  trans[)arent  homy  substance  which  can  long  resist  decomposition.  In  the 
vegetable  kingdom,  the  substance  known  as  cellulose  forms  the  essential  part  d 
the  framework  of  plants.  In  dry  air,  it  possesses  considerable  durability,  also  when 
thoroughly  water-logged  and  excluded  from  meteoric  influences.  In  the  latter  conditiou 
imbedded  amid  mud  or  sand,  it  may  last  until  gradually  petrified.' 

It  is  a  familiar  fact  that  in  the  same  stratum  different  organisms  occur  in  remarkably 
different  states  of  conservation.  This  is  sometimes  strikingly  exemplified  among  the 
mollusca.     The  conditions  for  their  preservation  may  have  been  the  same,  yet  some 

^  Logan  and  Hunt,  Amer.  Joum.  ScL  xvii.  (1854)  p.  235. 

-  According  to  C.  Schmidt,  the  composition  of  this  substance  is  C,  46*64  ;  H,  6*60; 
N,  6  '66  ;  0,  40  '20.  The  brown  chitin  of  Scottish  Carboniferous  scorpions  is  hardly 
distinguishable  from  that  of  recent  species. 

^  On  cellulose  and  coal,  see  C.  F.  Cross  and  E.  J.  Bevan,  Brit.  Assoc  1881,  Secta 
p.  603. 


§  il  2  FOSSILIZATION  651 

kinds  of  shells  are  found  only  as  empty  moulds  or  casts,  while  others  still  retain  their 
form,  composition,  and  structure.  This  discrepancy,  no  doubt,  points  to  original  dif- 
ferences of  composition  or  structure.  The  aragonite  shells  of  a  stratum  may  be  entirely 
dissolved,  while  those  of  calcite  may  remain.  ^  The  presence,  therefore,  of  calcite  forms 
only  does  not  necessarily  imply  that  others  of  aragonite  were  not  originally  present.  But 
the  conditions  of  petrifaction  have  likewise  greatly  varied.  In  the  clays  of  the  Mesozoic 
formations,  for  example,  cephalopods  may  be  exhumed  retaining  even  their  pearly  nacre, 
while  in  corresponding  deposits  among  the  Palsrazoic  systems  they  are  merely  crystalline 
calcite  casts. 

2.  Fossilization. — The  condition  in  which  organic  remains  have 
been  entombed  and  mineralized  may  be  reduced  to  three  leading  types. 

(1)  The  original  substance  is  partly  or  wholly  preserved, — Several  grades  may  be 
noticed  :  (a)  where  the  entire  animal  substance  is  retained,  as  in  the  frozen  carcases  of 
mammoths  in  the  Siberian  cliffs  ;  (6)  where  the  organism  has  been  mummified  by  being 
encased  in  resin  or  gum  (insects  in  amber) ;  (c)  where  the  organism  has  been  carbonized 
with  or  without  retention  of  its  structure,  as  is  characteristically  shown  in  peat,  lignite, 
and  coal ;  {d)  where  a  variable  portion  of  the  original  substance,  and  especially  the 
organic  matter,  has  been  removed,  as  happens  with  shells  and  bones  :  this  is  no  doubt 
one  of  the  first  steps  towards  petrifaction. 

(2)  The  original  substance  is  entirely  removed^  with  retention  merely  of  external 
form, — Mineral  matter  gathers  round  the  organism  and  hardens  there,  while  the  organ- 
ism itself  decays.  Eventually  a  mere  mould  of  the  plant  6r  animal  is  left  in  stone. 
Every  stage  in  this  process  may  be  studied  along  the  margin  of  calcareous  springs  and 
streams  (ante,  p.  482).  The  lime  in  solution  is  precipitated  round  fibres  of  moss,  leaves, 
twigs,  &c.,  which  are  thereby  incrusted  with  mineral  matter.  While  the  crust  thickens, 
the  organism  inside  decays,  until  a  mere  hollow  mould  of  its  form  remains.  Among 
stratified  rocks,  moulds  of  organic  forms  are  of  frequent  occurrence.  They  may  be  filled 
up  with  mineral  matter,  washed  in  mechanically  or  deposited  as  a  chemical  precipitate, 
so  that  a  cast  in  stone  replaces  the  original  organism.  Such  casts  are  particularly  common 
in  sandstone,  which,  being  a  porous  rock,  has  allowed  water  to  filter  through  it  and 
remove  the  substance  of  enclosed  plant-stems,  shells,  &c.  In  the  sandstones  of  the  Car- 
boniferous system,  casts  in  compacted  sand  of  stems  of  Lepidodendron  and  other  plants 
are  abundant.  It  is  obvious  that  in  casts  of  this  kind,  no^trace  remains  of  the  original 
structure  of  the  organism,  but  merely  of  its  external  form. 

(3)  The  original  substance  is  indecularly  replaced  by  mineral  matter  with  partial  or 
entire  preservation  of  the  internal  structure  of  the  organism. — This  is  the  only  true  petri- 
faction. The  process  consists  in  the  abstraction  of  the  organic  substances,  molecule 
by  molecule,  and  in  their  replacement  by  precipitated  mineral  matter.  So  gradual  and 
thorough  has  this  interchange  often  been,  that  the  minutest  structures  of  plant  and 
animal  have  been  perfectly  preserved.  Silicified  wood  is  a  familiar  example  (see  p.  864). 
-r  The  chief  substance  which  has  replaced  organic  forms  in  rocks  is  calcite,  either 
crystalline  or  in  an  amorphous  granular  condition.  In  assuming  a  crystalline  (or  fibrous) 
form,  this  mineral  has  often  observed  a  symmetrical  grouping  of  its  component  indi- 
viduals, these  being  usually  placed  with  their  long  axes  perpendicular  to  the  surface 
of  an  organism.  In  many  cases,  among  invertebrate  remains,  the  calcite  now  visible  is 
pseudomorphous  after  aragonite  (p.  122).  Next  in  abundance  as  a  petrifying  medium 
is  silica,  most  commonly  in  the  chalcedonic  form,  but  also  as  quartz.  It  is  specially 
frequent  in  some  limestones,  as  chert  and  flint,  replacing  the  carbonate  of  lime  in 
mollusks,  echinoderms,  corals,  &c.  It  also  occurs  in  irregular  aggregates,  in  which 
organisms  are  sometimes  beautifully  preserved.     It  forms  a  frequent  material  for  the 

^  See  ante^  pp.  122,  138,  and  authorities  there  cited. 


654  PAL^ONTOLOGICAL  GEOLOGY  book  v 

Ancient  woodland  surfaces  of  this  kind,  found  between  tide-marks,  and  even  below  low- 
water  line,  round  difi'ercnt  parts  of  the  British  coast,  unequivocally  prove  a  subsidence  of 
the  land  ('Submerged  Forests,'  p.  289).  Of  more  ancient  date  are  the  ''dirt-beds"  of 
Poi*tland  (Book  Yl.  Part  III.  Section  ii.  §  2),  which,  by  their  layers  of  soil  and  tree- 
stumps,  show  that  woodlands  of  cycads  sprang  up  over  an  upraised  sea-bottom  and  were 
buried  beneath  the  silt  of  a  river  or  lake.  Still  further  back  in  geological  history  come 
the  coal -growths  of  the  Carboniferous  period,  which,  with  their  ''under-clays"  or  soils, 
|)oint  to  wide  jungles  of  terrestrial  or  aquatic  plants,  like  the  modem  mangrove-swamps 
that  were  successively  submerged  and  covered  with  sand  or  silt  (Book  VI.  Part  II.  Sect, 
iv.  §  1). 

{b)  The  former  existence  of  lakes  can  be  satisfactorily  proved  from  beds  of  marl 
or  lacustiine  limestone  full  of  freshwater  shells,  or  from  fine  silt  with  leaves,  fruits,  and 
insect  remains.  Such  deposits  are  growing  abundantly  at  the  present  day,  and  they 
occur  at  various  hoiizons  among  the  geological  formations  of  past  times.  The  well- 
known  Nagelflue  of  Smtzerland — a  mass  of  conglomerate  attaining  a  thickness  of  more 
than  1000  feet — can  be  shown  from  its  fossil  contents  to  be  essentially  a  lacnstrine 
deposit  (Book  YI.  Part  lY.  Sect.  ii.  §  2).  Still  more  important  are  the  ancient 
Eocene  and  Miocene  lake-formations  of  North  America,  whence  so  rich  a  terrestrial  and 
lacustrine  flora  and  fauna  have  been  obtained  (Book  YI.  Part.  lY.  Sect.  i.  §  1). 

(c)  Old  sea -bottoms  are  vividly  brought  before  us  by  beds  of  marine  shells 
and  other  organisms.  Layers  of  water- worn  gravel  and  sand,  with  rolled  shells  of 
littoral  and  infra-littoral  species,  unmistakably  mark  the  position  of  a  former  shore-line. 
Deeper  water  is  indicated  by  finer  muddy  sediment,  with  relics  of  the  fauna  that  ]n«vails 
beneath  the  reach  of  waves  and  ground  -  swell.  Limestones  full  of  corals,  or  made 
up  of  crinoids,  point  to  the  slow,  continuous  growth  and  decay  of  generation  after 
generation  of  organisms  in  clear  sea-water. 

(d)  Variations  in  the  nature  of  the  water,  or  of  the  sea-bottom,  may  some- 
times be  shown  by  changes  in  the  size  or  shai>e  of  the  organic  remains.  If,  for  example, 
the  fossils  in  the  central  and  lower  parts  of  a  limestone  are  large  and  well-formed,  but 
in  the  upper  layers  become  dwarfed  and  distorted,  we  may  reasonably  infer  that  the 
conditions  for  their  continued  existence  at  the  locality  must  have  been  gradually 
impaired.  The  final  complete  cessation  of  these  favourable  conditions  is  shown  by  th« 
replacement  of  limestone  by  shale,  indicative  of  the  water  having  become  muddy,  and  by 
the  disappearance  of  the  organisms,  which  had  shown  their  sensitiveness  to  the  change. 

(e)  The  proximity  of  land  at  the  time  when  a  fossiliferous  stratum  was  in  the 
course  of  accumulation  may  be  sufficiently  proved  by  mere  lithological  characters,  as  has 
been  already  explained  ;  but  the  conclusion  may  be  further  strengthened  by  the  occurrence 
of  leaves,  stems,  and  other  fragments  of  terrestrial  vegetation,  with  remains  of  insects, 
birds,  or  terrestrial  manmials,  which,  if  found  in  some  numbers  in  certain  strata  inter- 
calated among  others  containing  marine  organisms,  would  make  it  improbable  that 
they  had  been  drifted  far  from  land  (see  p.  45t)). 

(/)  The  existence  of  different  conditions  of  climate  in  former  geological  periods 
is  satisfactorily  denionstrated  from  the  testimony  of  fossils.  Thus,  an  assemblage  of  the 
remains  of  jmlnis,  goui-ds,  and  melons,  with  bones  of  crocodiles,  tui-tles,  and  sea-snakes, 
proves  a  sub-tropical  climate  to  have  prevailed  over  the  south  of  England  in  the  older 
Tertiary  ages  (Book  YI.  Part  lY.  Sect.  i.  §  1).  On  the  other  hand,  the  extension 
of  a  cold  or  aixitic  climate  far  south  into  Europe  during  post -Tertiary  time,  can  be 
shown  from  the  existence  of  remains  of  arctic  animals,  even  in  the  south  of  England 
and  of  France  (Book  YI.  Part  Y.)  This  is  a  use  of  fossils,  however,*  where  great  caution 
must  be  obser\'ed.  We  cannot  affinn  that,  because  a  certain  species  of  a  genus  lives 
now  in  a  wann  part  of  the  globe,  every  species  of  that  genus  must  always  have  lived 
in  similar  circumstances.  The  well  -  known  examples  of  the  mammoth  and  woolly 
rhinoceros  that  lived  in  the  cold  north,  while  their  modem  representatives  inhabit  aomt 


§  iv.  2  FOSSILS  AS  GUIDES  TO  CHRONOLOGY  655 

of  the  warmest  regions  of  the  globe,  may  1)e  usefully  remembered  as  a  warning  against 
any  such  conclusion.  When,  however,  not  one  fossil  merely,  but  the  whole  assemblage 
of  fossils  in  a  group  of  rocks,  finds  its  modern  analogy  in  a  certain  general  condition 
of  climate,  we  may,  at  least  tentatively,  infer  that  the  same  kind  of  climate  prevailed 
where  that  assemblage  lived.  Such  an  inference  would  become  more  and  more  unsafe 
in  proportion  to  the  antiquity  of  the  fossils,  and  their  divergence  from  existing  forms.* 

As  an  illustration  of  this  application  of  the  evidence  of  fossils  in  the 
interpretation  of  ancient  conditions  of  geography  at  different  geological 
periods,  reference  may  be  made  more  especially  to  the  investigation  of 
the  various  basins  in  which  the  Jurassic  rocks  of  Eiu*ope  were  deposited. 
The  positions  of  the  seas  and  lands,  and  the  variations  of  climate  have 
been  ascertained  with  sufficient  definiteness  to  give  us  some  conception  of 
the  physical  geography  of  that  part  of  the  globe  during  early  Mesozoic 
time.  2 

2.  Geological  Chronology. — Although  absolute  dates  cannot 
be  fixed  in  geological  chronology,  it  is  not  difficult  to  determine  the 
relative  age  of  different  strata.  For  this  purpose  the  fundamental  law 
is  based  on  the  "order  of  superposition"  (pp.  523,  674) :  in  a  series  of 
stratified  formations,  the  older  underlie  the  younger.  It  is  not  needful 
that  we  should  actually  see  the  one  lying  below  the  other.  If  a  continu- 
ous conformable  succession  of  strata  dips  steadily  in  one  direction,  we 
know  that  the  beds  at  the  one  end  must  underlie  those  at  the  other, 
because  we  can  trace  the  whole  succession  of  beds  between  them.  Eare 
instances  occur,  where  strata  have  been  so  folded  by  great  terrestrial  dis- 
turbance that  the  younger  are  made  to  underlie  the  older.  But  this  in- 
version can  usually  be  made  clear  from  other  evidence.  The  true  order 
of  superposition  is  decisive  of  the  relative  ages  of  stratified  rocks. 

The  order  of  sequence  having  been  determined,  it  is  needful  to  find 
some  means  of  indentifying  a  particular  formation  elsewhere,  when  its 
stratigraphical  relations  may  possibly  not  be  visible.  At  first,  it  might 
be  thought  that  the  mere  external  aspect  and  mineral  characters  of  the 
rocks  ought  to  be  sufficient  for  this  purpose.  Undoubtedly  these  features 
may  suffice  within  the  same  limited  region  in  which  the  order  of  sequence 
has  already  been  determined.  But  as  we  recede  from  that  region,  they 
become  more  and  more  unreliable.  That  this  must  be  the  case  will 
readily  appear,  if  we  reflect  upon  the  conditions  under  which  sedi- 
mentary accumulations  have  been  formed.  The  markedly  lenticular 
nature  of  these  deposits  has  already  been  described  (p.  515).  At  the 
present  day,  the  sea-bottom  presents  here  a  bank  of  gravel,  there  a  sheet 
of  sand,  elsewhere  layers  of  mud,  or  of  shells,  or  of  organic  ooze,  all  of 
which  are  in  course  of  deposit  simultaneously,  and  will  as  a  rule  be 
found  to  shade  off  laterally  into  each  other.     The  same  diversity  of  con- 

*  See  NeumajT,  Nature,  xlii.  (1890)  pp.  148, 175.  This  author  specially  devoted  himself 
to  the  study  of  ancient  climates  as  indicated  by  fossils.  As  an  illustration  of  his  methods 
consult  his  essay  on  the  climatic  zones  of  Jurassic  and  Cretaceous  time,  Denksch.  Akad.  H'ien, 
xlTiL  (1883)  ;  also  the  same  work,  vol.  1.  (1885).  "  Fossil  plants  as  tests  of  Climate  "—the 
Sedgwick  Prize  Essay  for  1892.     By  A.  C.  Seward.     Cambridge,  C.  J.  Clay,  1892. 

*  See  especially  Neumayr,  Verk.  Oeol,  ReichsanaL  1871,  p.  54,  Jahrh.  Oeol.  Reichsanst, 
J[zviii  (1878),  and  hi?  essay  cited  in  the  foregoing  note. 


656  PAL.EOXTOLOGICAL  GEOLtXiY  book  v 

temporancous  deposits  has  obtained  from  the  earliest  geological  periods. 
Conglomerates,  sandstones,  shales,  and  limestones  occur  on  all  geological 
horizons,  and  replace  each  other  even  on  the  same  platform.  The  Coal* 
measures  of  Pennsylvania  are  represented  west  of  the  Rocky  Mountains 
by  thousands  of  feet  of  massive  marine  limestones.  The  white  Chalk  of 
England  lies  on  the  same  geological  horizon  with  marls  and  clays  in 
North  Germany,  with  thick  sandstones  in  Saxony,  with  hard  limestone  in 
the  south  of  France.  Mere  mineral  diameters  are  thus  quite  unreliable, 
save  \vithin  comparatively  restricted  areas. 

The  solution  of  this  problem  was  found,  and  was  worked  out  for  the 
Secondary  rocks  of  England,  by  William  Smith  at  the  end  of  last  century. 
It  is  supplied  by  organic  remains,  and  depends  upon  the  law  that  the 
order  of  succession  of  plants  and  animals  has  been  similar  all  over  the 
world.     According  to  the  order  of  supeqK)8ition,  the  fossils  found  in  any 
de|X)sit  must  be  older  than  those  in  the  dejKwit  above,  and  younger  than 
those  in  that  Ixilow.     This  order,  however,  must  be  first  accurately  deter- 
mined by  a  study  of  the  actual  stratigraphy  of  the  formations ;  for,  so  far 
as  regards  organic  structure  or  affinities,  there  may  be  no  discoverable 
reason  why  a  particular  species  should  precede  or  follow  another.     Unless, 
for  example,  we  knew  from  observation  that  Rhynrhoiidla  pleurodon  is  a 
shell  of  the  Carlwniferous  Limestone,  and  Rhjudumella  MraJisdra  is  a  shell 
of  the  Lias,  we  could  not,  from  mere  inspection  of  the  fossils  themselves^ 
pronounce  as  to  their  real  geological  position.^     It  is  quite  true  that^  hy 
practice,  a  palaeontologist  has  his  eye  so  trained  that  ho  can  make  shrewiZS 
approximations  to  the  actual  horizon  of  fossils  which  he  may  never  hav^ 
seen  before  (and  this  is  more  especially  true  in  regard  to  the  mammalia 
as  will  be  immediately  adverted  to),  but  he  can  only  do  this  by  availing 
himself  of  a  wide  experience,  based  upon  the  ascertained  order  of  appear 
ance  of  fossils,  as  determined  by  the  law  of  super|X)8ition.     For  geologic 
purposes,  therefore,  and,  indeed,  for  all  purposes  of  comparison  betwee^B 
the  faunas  and  floras  of  diflerent  periods,  it  is  absolutely  essential,  first  c^k 
all,  to  have  the  order  of  super|x>8ition  of  strata  rigorously  determiners 
Unless  this  is  done,  the  most  fatal  mistakes  may  be  made  in  paheontologica^ 

chronology.     But  when  it  has  once  been  done  in  one  typical  district,  th 

order  thus  esUiblished  may  l)e  held  as  proved  for  a  wide  region  wl 
from  paucity  of  sections,  or  from  geological  disturbance,  the  true 
sion  of  foimiations  cannot  be  satisfactorily  determined. 

The  order  of  supcqx>sition  ha\4ng  been  determined  in  a  great  seri<3« 
of  stratified  formations,  it  is  found  that  the  fossils  at  the  bottom  are  not 
([uite  the  same  as  those  at  the  top  of  the  series.     As  we  trace  the  forma- 
tions upward,  we  discover  that  species  after  s|>ecie8  of  the  lowest  platforms 
disiipi>eiirs,  until  |>erhaps  not  one  of  them  is  found.     With  the  cessation 

^  The  derivation  of  Home  forms  by  descent  from  others  may  be  inferred  with  more  or 
less  probability,  and  such  genetic  aflinities  may  famish  valuable  suggestions  to  the  p■ll^ 
ontologist.  But  that  the  risk  of  erroneous  interpretation  and  fanciful  dedaction  in  sach 
matters  is  real  and  serious  was  well  shown  in  the  discussion  of  the  presumed  derivation  of 
the  Oleuellidian  trilobites  from  the  Paradoxidian  forms,  until  it  was  shown  that  the  fomer 
were  really  the  precur^ors  of  the  latter. 


§  iv.  2  FOSSILS  AS  GUIDES  TO  CHRONOLOGY  667 

of  these  older  species,  others  make  their  entrance.  These,  in  turn,  are 
found  to  die  out  and  to  be  replaced  by  newer  forms.  After  patient  exam- 
ination of  the  rocks,  it  is  ascertained  that  every  well-marked  formation 
is  characterised  by  its  own  species  or  genera  (type-fossils,  Leitfossilien) 
or  by  a  general  assemblage  or  fades  of  organic  forms.  This  can  only,  of 
course,  be  determined  by  actual  practical  experience  over  an  area  of  some 
size.  The  characteristic  fossils  are*  not  always  the  most  numerous ;  they 
are  those  which  occur  most  constantly  and  have  not  been  observed  to 
extend  their  range  above  or  below  a  definite  geological  horizon  or  platform. 
For  the  determination  of  geological  chronology,  as  already  pointed  out,  it 
may  be  affirmed  as  a  general  principle  that  the  higher  and  more  special- 
ised the  type  of  organism  the  more  local  is  its  area  in  space  and  the  more 
limited  its  range  in  time.  Hence  mammalian  remains  have  a  special 
value  in  this  respect.^  But  some  invertebrate  groups  possess  great  im- 
portance as  fixing  stratigraphical  horizons ;  as  for  example  the  ammonites 
in  the  Jurassic  and  the  graptolites  in  the  Silurian  system. 

As  illustrations  of  type-fossils  characteristic  of  some  of  the  larger  subdivisions  of 
the  Geological  Record,  the  following  may  be  given.  Lepidodendra  and  Sigillaria  are 
typical  of  Old  Red  Sandstone  and  Carboniferous  de|)osits  ;  Graptolites  of  the  Silurian 
system ;  Trilobites  of  PalaBozoic  rocks  from  Cambrian  to  Carboniferous,  Cystideans  of 
the  older  Palaeozoic  rock-gi*oups.  Orthoceratites  are  Palaeozoic,  and  Ammonites  are 
Mesozoic  ;  Ichthyosaurs  and  Plesiosaurs,  Mesozoic  ;  Nummulites,  Palseotherium,  Anop- 
lotherium,  Hyopotamus,  and  Anthracotherium  belong  to  older  Tertiary,  and  Mastodon, 
Elepbas,  Hyaena,  Cervus,  and  Equus  to  younger  Tertiary  and  recent  time.  The 
occurrence  of  such  organisms  in  any  rock,  at  once  indicates  the  great  division  of 
geological  time  to  which  the  rock  should  be  assigned. 

The  type-fossils  of  a  system  or  formation,  having  been  ascertained  from 
a  sufficiently  prolonged  and  extended  experience,  serve  to  identify  that 
series  of  rocks  in  its  progress  across  a  country.  Thus,  as  we  trace  a  forma- 
tion into  tracts  where  it  would  be  impossible  to  determine  the  true  order 
of  superposition,  owing  to  the  want  of  sections,  or  to  the  disturbed 
condition  of  the  rocks,  we  can  employ  the  type-fossils  as  a  means  of 
identification,  and  speak  with  confidence  as  to  the  succession  of  the 
rocks.  We  may  even  demonstrate  that  in  some  mountainous  ground,  the 
strata  have  been  turned  completely  upside  down,  if  we  can  show  that  the 
fossils  in  what  are  now  the  uppermost  layers  ought  properly  to  lie  under- 
neath those  in  the  beds  below  them. 

Prolonged  study  of  the  succession  of  organic  types  in  the  geological 
past  all  over  the  world,  has  given  palaeontologists  some  confidence  in 
fixing  the  relative  age  of  fossils  belonging  even  to  previously  unknown 
species  or  genera,  and  occurring  under  circumstances  where  no  order  of 
superposition  has  been  made  out.  For  instance,  the  general  sequence  of 
mammalian  types  having  now  been  settled  by  the  law  of  superposition, 
the  horizon  of  a  maramaliferous  deposit  may  be  approximately  determined 
by  the  grade  or  degree  of  evolution  denoted  by  its  mammalian  fossils. 

^  Consult  the  papers  of  Prof.  Marsh  quoted  on  p.  653,  and  see  especially  the  plate  in  the 
second  paper  in  which  the  successive  mammalian  zones  in  the  Geological  Record  of  North 
America  are  given. 

2u 


658  PALjEOSTOLOGICAL  GEOLOGY  book  v 

Thus,  shoiild  remains  be  generically  abundant,  differing  from  those  now 
living  and  presenting  none  of  the  extreme  contrasts  which  are  now  found 
among  our  higher  animals,  should  they  embrace  neither  true  rumiiiantfi, 
nor  soli|)edes,  nor  proboscidians,  nor  apes,  they  might  vaXh  high  ]Ht>babil- 
ity  be  referred  to  the  Eocene  period.^  Reasoning  of  this  kind  must  be 
based,  however,  upon  a  wide  basis  of  evidence,  seeing  that  the  progress 
of  development  has  been  far  from  equal  in  all  ranks  of  the  animal  worid. 

Observations  made  over  a  large  part  of  the  surface  of  the  globe  have 
enabled  geologists  to  divide  the  stratified  part  of  the  earth's  crust  into 
systems,  formations,  and  groups  (p.  678).  These  subdivisions  are 
frequently  marked  off  from  each  other  by  lithological  characters.  But^ 
as  already  remarked,  mere  lithological  differences  afford  at  the  best  but 
a  limited  and  local  ground  of  separation.  Two  masses  of  sandstone, 
for  example,  having  exactly  the  same  general  external  and  internal 
characters,  may  belong  to  very  different  geological  periods.  On  tJie 
other  hand,  a  series  of  limestones  in  one  locality  may  be  the  exact 
chronological  equivalent  of  a  set  of  sandstones  and  conglomerates  at 
another,  and  of  a  series  of  shales  and  clays  at  a  third. 

Some  clue  is  accordingly  needed,  which  will  permit  the  divisions  of 
the  stratified  rocks  to  be  grouped  and  compared  chronologically.-  This 
fortunately  is  well  supplied  by  their  characteristic  fossils.  Each  forma- 
tion being  distinguished  by  its  own  assemblage  of  organic  remains,  it 
can  l)e  followed  and  recognised  even  amid  the  cnimplings  and  dislocations 
of  a  disturbed  region.  The  same  general  succession  of  organic  types  has 
been  observed  over  a  large  part  of  the  world,  though,  of  course,  with 
imjK)rtant  modifications  in  different  countries.  The  similarity  of  suc- 
cession has  been  called  homofaxis — ^a  term  which  expresses  the  fact  that 
the  order  in  which  the  leading  types  of  organised  existence  have  appeared 
upon  the  earth  has  been  similar  even  in  widely  separated  regions.* 

It  is  evident  that,  in  this  way,  a  method  of  comparison  is  furnished 
whereby  the  stratified  groups  of  different  parts  of  the  earth's  crust  can 
be  brought  into  relation  with  each  other.  We  find,  for  example,  that 
a  certain  grouj)  of  strata  is  characterised  in  Britain  by  certain  genera 
and  sj)ecies  of  corals,  brachiopods,  lamellibranchs,  gasteropods,  and 
cephalopods.  A  gi'oup  of  rocks  in  Bohemia,  diftering  more  or  less  from 
the  British  type  in  lithological  aspect,  contains  on  the  whole  the  same 
genera,  and  some  even  of  the  same  species.  In  Scandinavia,  a  set  of  beds 
may  1)0  seen,  unlike  perhaps  in  external  characters  to  the  British  type,  but 
yielding  many  of  the  same  fossils.  In  Canada  and  parts  of  the  northern 
United  States,  other  rocks  enclose  some  of  the  same,  and  of  closely  allied 
genera  and  sj>ecies.  All  these  groups  of  strata,  ha\ing  the  same  general 
facies  of  organic  remains,  are  classed  together  as  homotaxial,  that  is,  as 
having  been  deposited  diunng  the  same  relative  period  in  the  general 
progress  of  life  in  each  region. 

It  was  at  one  time  believed,  and  the  belief  is  still  far  from  extinct, 
that  groups  of  strata,  characterised  by  this  community  or  resemblance 

^  Gaudrv,  'Les  Enchainements  du  Monde  Animal,'  1878,  p.  246. 
-  Huxley,  <^.  J.  Oeol.  Soc,  xviii.  (1862)  p.  xlvi. 


§  iv.  2  HOMOTAXIS  659 

of  organic  remains,  were  chronologically  contemporaneous.  But  such  an 
inference  rests  upon  most  insecure  grounds.  We  may  not  be  able  to 
disprove  the  assertion  that  the  strata  were  strictly  coeval,  but  we  have 
only  to  reflect  on  the  present  conditions  of  zoological  and  botanical  dis- 
tribution, and  of  modem  sedimentation,  to  be  assured  that  the  assertion 
of  contemporaneity  is  a  mere  assumption.  Consider,  for  a  moment,  what 
would  happen  were  the  present  surface  of  any  portion  of  central  or 
southern  Europe  to  be  submerged  beneath  the  sea,  covered  with  marine 
deposits,  and  then  re-elevated  into  land.  The  river-terraces  and  lacus- 
trine marls  formed  before  the  time  of  Julius  Caesar  could  not  be  dis- 
tinguished by  any  fossil  tests  from  those  laid  down  in  the  days  of 
Victoria,  unless,  indeed,  traces  of  human  implements  were  obtainable 
whereby  the  progress  of  civilisation  during  2000  years  might  be  indi- 
cated. So  far  as  regards  the  shells,  bones,  and  plants  preserved  in  the 
various  formations,  it  would  be  absolutely  impossible  to  discriminate 
their  relative  dates;  they  would  be  classed  as  "geologically  contempo- 
raneous," that  is,  as  having  been  formed  during  the  same  period  in  the 
history  of  life  in  the  European  area  ;  yet  there  might  be  a  difference  of 
2000  years  or  more  between  many  of  them.  Strict  contemporaneity 
cannot  be  asserted  of  any  strata  merely  on  the  ground  of  similarity  or 
identity  in  fossils. 

But  the  phrase  "  geologically  contemporaneous  "  is  too  vague  to  have 
any  chronological  value  except  in  a  relative  sense.  To  speak  of  two 
formations  as  "contemporaneous,"  which  may  have  been  separated  by 
thousands  of  years,  seems  rather  a  misuse  of  language,  though  the 
phraseology  has  now  gained  such  a  footing  in  geological  literature  as 
probably  to  be  inexpugnable.  If  we  turn  again  for  suggestions  to  the 
existing  distribution  of  life  on  the  earth  (though  it  is  probable  that 
formerly,  and  particularly  among  the  earlier  geological  periods,  there 
was  considerably  greater  uniformity  in  zoological  distribution  than  there 
is  now),  we  learn  that  similarity  or  identity  of  species  and  genera  holds 
good,  on  the  whole,  only  for  limited  areas,  and  consequently,  if  applied 
to  wide  geographical  regions,  ought  to  be  an  argument  for  diversity 
rather  than  for  similarity  of  age.  If  we  suppose  the  British  seas  to  be 
raised  into  &vy  land,  so  that  the  organic  relics,  preserved  in  their  sands 
and  silts,  could  be  exhumed  and  examined,  a  general  type  or  common 
facies  would  be  found,  though  some  species  would  be  more  abundant  in 
or  entirely  confined  to  the  north,  while  others  would  show  a  greater 
development  in  the  opposite  quarter.  Still,  there  would  be  such  a  simi- 
larity throughout  the  whole,  that  no  naturalist  would  hesitate  to  regard 
the  organisms  as  those  of  one  biological  province,  and  belonging  to  the 
same  great  geological  period.  The  region  is  so  small,  and  its  conditions 
of  life  so  uniform  and  uninterrupted,  that  no  marked  distinction  can  be 
drawn  between  the  forms  of  life  in  its  different  parts. 

Widening  the  area  of  observation,  we  |)erceive  that  as  we  recede  from 
any  given  point  on  the  earth's  surface  the  existing  forms  of  life  gradually 
change.  Vegetation  alters  its  aspect  from  climate  to  climate,  and  with  it 
come  corresponding  transformations  in  the  characters  of  insects,  birds,  and 


660  PAL^OXTOLOGICAL  GEOLOGY  book  v 

wild  animals.  A  lake-bottom  would  preserve  one  suite  of  organisms  in 
England,  but  a  very  different  group  at  the  foot  of  the  Himalaya  Moun- 
tains, yet  the  deposits  at  the  two  places  might  be  absolutely  coeval,  even 
as  to  months  and  days.  If,  therefore,  in  the  geological  past  there  has 
been,  as  there  is  now,  a  grading  of  plants  and  animals  in  great  biological 
pro\'inces,  marked  off  by  differences  of  contour,  climate,  and  geological 
history,  we  must  conclude  that,  while  strict  contemporaneity  cannot  be 
predicted  of  deposits  containing  the  same  organic  remains,  it  may  actually 
be  true  of  deposits  in  which  they  are  quite  distinct^ 

If,  then,  at  the  present  time,  community  of  organic  forms,  except  in 
the  case  of  some  almost  world-wide  species,  obtains  only  in  restricted  dis- 
tricts, regions,  or  provinces,  it  may  have  been  more  or  less  limited  also  in 
past  time.  Similarity  or  identity  of  fossils  among  formations  geographi- 
cally far  apart,  instead  of  proving  contemporaneity,  may  be  compatible 
with  great  discrepancies  in  the  relative  epochs  of  deposit.  For,  on 
any  theory  of  the  origin  of  species,  the  spread  of  a  species,  still  more  of 
any  group  of  species,  to  a  vast  distance  from  the  original  centre  of  dis- 
persion, must  in  most  cases  have  been  inconceivably  slow.  It  doubtless 
occupied  so  prolonged  a  time  as  to  allow  of  almost  indefinite  changes  in 
physical  geography.  A  species  may  have  disappeared  from  its  primeval 
birthplace,  while  it  continued  to  flourish  in  one  or  more  directions  along 
its  outward  circle  of  advance.  The  date  of  the  first  appearance  and  final 
extinction  of  that  species  would  thus  differ  widely,  according  to  the 
locality  at  which  we  might  examine  its  remains. 

The  grand  march  of  life,  in  its  progress  from  lower  to  higher  forms, 
has  unquestionably  been  broadly  alike  in  all  quarters  of  the  globe.  But 
nothing  seems  more  certain  than  that  its  rate  of  advance  has  not  every- 
where been  the  siime.  It  has  moved  unequally  over  the  same  region.  A 
certain  stage  of  progress  may  have  been  reached  in  one  quarter  of  the 
globe  many  thousands  of  years  before  it  was  reached  in  another ;  though 
the  siime  general  succession  of  organic  types  might  be  found  in  each 
region.  At  the  present  day,  for  example,  the  higher  fauna  of  Australia 
is  more  nearly  akin  to  that  which  flourished  in  Europe  far  back  in  Meso- 
zoic  time  than  to  the  living  fauna  of  any  other  region  of  the  globe. 
There  seems  also  to  be  now  sufficient  evidence  to  warrant  the  assertion 
that  the  progress  of  terrestrial  vegetation  has  at  some  geological  periods 
and  in  some  regions,  been  in  advance  of  that  of  the  marine  fauna  (see  p. 
668).  Hence  arise  glaring  anomalies  in  the  attempts  to  group  the 
geological  formations  of  distant  countries  in  conformity  with  European 
standards.  As  Mr.  Blanford  has  well  remarked,  "in  instances  of  con- 
flicting evidence  between  ten-estrial  or  freshwater  faunas  and  floras  on  the 
one  side,  and  marine  faunas  on  the  other,  the  geological  age  indicated  by 
the  latter  is  probably  correct,  because  the  contradictions  which  prevail 

^  The  j)ret)ent  geogiaphical  distribution  of  plants  and  animals  has  a  profound  geolofi^cal 
interest,  but  cannot  be  properly  discussed  in  tliis  volume.  Tlie  student  will  find  it  lumin- 
ously treated  in  Darwin's  *  Origin  of  Species,  *  chaps,  xii.  and  xiii. ;  Lyell's  *  Principles  of 
Geology,'  chaps,  xxxviii.-xli.  ;  and  in  Wallace's  'Geographical  Distribution  of  Animals,'  2 
vols.  1876,  and  his  '  Island  Life,'  1880. 


§  iv.  3  IMPERFECTION  OF  THE  GEOLOGICAL  RECORD  661 

between  the  evidence  afforded  by  successive  terrestrial  and  freshwater 
beds  are  unknown  in  marine  deposits ;  because  the  succession  of  terres- 
trial animals  and  plants  in  time  has  been  different  from  the  succession  of 
marine  life ;  and  because  in  all  past  times  the  differences  between  the 
faunas  of  distant  lands  have  probably  been,  as  they  now  are,  vastly  greater 
than  the  differences  between  the  animals  and  plants  inhabiting  the  different 
seas  and  oceans.''^ 

Notwithstanding  such  exceptions,  it  may  be  asserted  that  in  every 
country  where  the  fossiliferous  geological  formations  are  well  displayed 
and  have  been  properly  examined,  a  similar  general  order  of  organic 
succession  can  he  made  out  among  them.  Their  relative  age  within  a 
limited  geographical  area  can  be  demonstrated  by  the  law  of  superposition. 
When,  however,  the  rocks  of  distant  countries  are  compared,  all  that  we  can 
safely  affirm  regarding  them  is  that  those  containing  the  same  or  a  repre 
sentative  assemblage  of  marine  organic  remains  belong  to  the  same  epoch 
in  the  history  of  biological  progress  in  each  area.  They  are  homotaxial ; 
but  we  cannot  assert  that  they  are  contemporaneous  unless  we  are  prepared 
to  include  within  that  term  a  vague  period  of  many  thousands  of  years. 

3.  Imperfection  of  the  Geological  Record.^ — Since  the 
statement  was  made  by  Darwin,  geologists  have  more  fully  recognised 
that  the  history  of  life  has  been  very  imperfectly  preserved  in  the  stratified 
parts  of  the  earth's  crust.  Apart  from  the  fact  that,  even  under  the  most 
favourable  conditions,  only  a  small  proportion  of  the  total  flora  and  fauna 
of  any  period  would  be  preserved  in  the  fossil  state,  enormous  gaps  occur 
where,  from  non-deposit  of  strata,  no  record  has  been  preserved  at  all.  It 
is  as  if  whole  chapters  and  books  were  missing  from  a  historical  work. 
But  even  where  the  record  may  originally  have  been  tolerably  full,  power- 
ful dislocations  have  often  thrown  considerable  portions  of  it  out  of  sight. 
Sometimes  extensive  metamorphism  has  so  affected  the  rocks  that  their 
original  characters,  including  their  organic  contents,  have  been  destroyed. 
Oftenest  of  all,  denudation  has  come  into  play,  and  vast  masses  of  strata 
have  been  entirely  worn  away,  as  is  shown  not  only  by  the  erosion  of 
existing  land-surfaces,  but  by  the  abundant  unconformabilities  in  the 
structure  of  the  earth's  crust. 

While  the  mere  fact  that  one  series  of  rocks  lies  unconformably  on 
the  denuded  surface  of  another,  proves  the  lapse  of  an  interval  l)etween 
them,  the  relative  length  of  this  interval  may  sometimes  be  demon- 
strated by  means  of  fossil  evidence,  and  by  this  alone.     Let  us  suppose, 

^  Mr.  Blanford,  in  his  suggestive  address  to  the  Geological  Section  of  the  British  Associa- 
tion at  the  Montreal  meeting,  from  which  the  above  quotation  is  taken,  gives  some  examples 
of  the  contradictions  involved  in  attempts  to  correlate  distant  deposits  by  means  of  land  and 
freshwater  faunas  and  floras.  The  Damnda  l>eds  of  India,  as  he  points  out,  contain  a  flora 
with  middle  Jurassic  affinities,  but  the  fauna  of  the  overlying  Panchet  beds  is  rather  Triassic 
or  even  Pennian.  Still  more  striking  is  the  example  funiished  by  the  Lower  Coal  measures 
of  New  South  Wales,  where  plants  which  botanists  unhesitatingly  pronounced  to  be  of 
Jurassic  types  are  found  in  the  same  stratified  deposits  with  undoubted  Carboniferous  Lime- 
stone marine  organisms  (OrthoceraSf  Canularia.  SpirifeVy  Fenestdlu^  &c.)  Mr.  Blanford 
has  returned  to  this  subject  in  his  presidential  addresses  to  the  Geological  Society.  Quart, 
Jaum,  xlv.  (1889)  p.  72,  xlvi.  (1890)  p.  104.  -  See  p.  674 


662  PALJEOXTOLOGIGAL  GEOLOGY  book  v 

for  example,  that  a  certain  group  of  formations  has  been  disturbed,  up- 
raised, denuded,  and  covered  imconformably  by  a  second  group.  In 
lithological  characters,  the  two  may  closely  resemble  each  other,  and  there 
may  be  nothing  to  show  that  the  gap  represented  by  their  unconfcMin- 
ability  is  of  an  important  character.  In  many  cases,  indeed,  it  would  be 
quite  impossible  to  pronounce  any  well-grounded  judgment  as  to  the 
length  of  interval,  even  measured  by  the  vague  relative  standards  of 
geological  chronology.  But  if  each  group  contains  a  well-preserved  suite 
of  organic  remains,  it  may  not  only  be  possible,  but  easy,  to  say  how 
much  of  the  known  geological  record  has  been  left  out  between  the  two 
sets  of  formations.  By  comparing  the  fossils  with  those  obtained  from 
regions  where  the  geological  record  is  more  complete,  it  may  be  ascer- 
tained, perhaps,  that  the  lower  rocks  belong  to  a  certain  platform  or  stage 
in  geological  history  which,  for  our  present  purpose,  we  may  call  D,  and 
that  the  upper  rocks  can,  in  like  manner,  be  paralleled  with  stage  H.  It 
would  be  then  apparent  that,  at  this  locality,  the  chronicles  of  three  great 
geological  periods,  E,  F,  and  G,  were  wanting,  which  are  elsewhere  found  to 
l)e  intercalated  between  D  and  H.  The  lapse  of  time  represented  by  this 
unconformability  would  thus  be  equivalent  to  that  required  for  the  accumu- 
lation of  the  three  missing  series  in  those  regions  where,  sedimentation 
having  been  more  continuous,  the  record  of  them  has  been  preserved. 

But  fossil  eWdence  may  be  made  to  prove  the  existence  of  gaps  which 
are  not  otherwise  apparent.  As  has  been  already  remarked,  changes  in 
organic  forms  must,  on  the  whole,  have  been  extremely  slow  in  the 
geological  past.  The  whole  species  of  a  sea-floor  could  not  pass  entirely 
away,  and  be  replaced  by  other  forms,  without  the  lapse  of  long  periods 
of  time.  If,  then,  among  the  conformable  stratified  deposits  of  former 
ages,  we  encounter  abrupt  and  important  changes  in  the  facies  of  the 
fossils,  we  may  Ije  certain  that  these  must  mark  omissions  in  the  rec(»rd, 
which  we  may  hope  to  fill  in  from  a  more  perfect  series  elsewhere.  The 
striking  palieontological  contrasts  between  unconformable  strata  are 
sufficiently  explicable.  It  is  not  so  easy  to  give  a  satisfactory  account  of 
those  which  occur  where  the  strata  are  strictly  conformable,  and  where 
no  eWdence  can  l)e  observed  of  any  considerable  change  of  physical  con- 
ditions at  the  time  of  deposit.  A  group  of  quite  conformable  strata, 
having  the  same  general  lithological  characters  throughout,  may  be 
marked  by  a  great  discrepance  between  the  fossils  of  the  upper  and  the 
lower  part.  A  few  species  may  pass  from  the  one  into  the  other,  or 
perhaps  every  species  may  be  different.  In  cases  of  this  kind,  when 
proved  to  be  not  merely  local  but  persistent  over  considerable  areas,  we 
must  admit,  notwithstanding  the  apparently  undisturbed  and  continuous 
character  of  the  original  deposition  of  the  strata,  that  the  abrupt  transi- 
tion from  the  one  facies  of  fossils  to  the  other  represents  a  long  interval 
of  time  which  has  not  been  recorded  by  the  deposit  of  strata.  Sir  A.  C. 
Kamsay,  who  called  attention  to  these  gaps,  termed  them  "  breaks  in  the 
succession  of  organic  remains."^  They  occur  abundantly  among  the 
European  Palaeozoic  and  Secondary  rocks,  which,  by  means  of  them,  can 

'  <^.  J.  (Jeol.  Soc,  XIX.  XX.  Presidential  Addresses. 


§  iv.  3         IMPERFECTION  OF  THE  GEOLOGICAL  RECORD  663 

be  separated  into  zones  and  sections.  But  though  traceable  over  wide 
regions,  they  were  probably  not  general  over  the  whole  globe.  There 
have  never  been  any  universal  interruptions  in  the  continuity  of  the 
chain  of  being,  so  far  as  geological  evidence  can  show.  The  breaks 
or  apparent  interruptions  no  doubt  exist  only  in  the  sedimentary  record, 
and  may  have  been  produced  by  geological  agencies  of  various  kinds, 
such  as  cessation  of  deposit  from  failure  of  sediment  owing  to  seasonal  or 
other  changes ;  alteration  in  the  nature  of  the  sediment  or  character 
of  the  water;  variations  of  climate  from  whatever  cause;  elevation 
or  subsidence  by  subterranean  movements,  bringing  successive  sub- 
marine zones  into  less  favourable  conditions  of  temperature,  &c. ;  and 
volcanic  discharges.  The  physical  revolutions,  which  brought  about 
the  breaks,  were  no  doubt  sometimes  general  over  a  whole  zoological 
province,  more  frequently  over  a  minor  region.  Thus,  at  the  close  of  the 
Triassic  period  the  inland  basins  of  central,  southern,  and  western  Europe 
were  effaced,  and  another  and  different  geographical  phase  was  introduced 
which  permitted  the  spread  of  the  peculiar  fauna  of  the  "  Avicula  contorta 
zone  "  from  the  south  of  Sweden  to  the  plains  of  Lombardy,  and  from  the 
north  of  Ireland  to  the  eastern  end  of  the  Alps.  This  phase  in  turn  dis- 
appeared to  make  way  for  the  Lias  with  its  numerous  "zones,"  each 
distinguished  by  the  maximum  development  of  one  or  more  species  of 
ammonite.^  These  successive  geographical  revolutions  must,  in  many 
cases,  have  caused  the  complete  extinction  of  genera  and  •species  possess- 
ing a  small  geographical  range.  Nevertheless,  it  must  be  admitted  that 
in  many  intances  where  fossil  species  have  a  wide  geographical  exten- 
sion, but  a  very  limited  stratigraphical  range,  such  as  the  Silurian 
graptolites  and  Jurassic  ammonites,  no  satisfactory  evidence  has  been 
adduced  to  connect  the  change  of  species  with  geographical  revolutions. 
There  may  be  some  biological  law  governing  such  organic  mutations, 
which  is  not  yet  perceived. 

It  is  abundantly  clear,  however,  that  the  geological  record,  as  it  now 
exists,  is  at  the  best  but  an  imperfect  chronicle  of  geological  history.  In 
no  country  is  it  complete.  The  lacunae  of  one  region  may  be  supplied 
from  another ;  yet  in  proportion  to  the  geographical  distance  between  the 
localities  where  the  gaps  occur  and  those  whence  the  missing  intervals 
are  supplied,  the  element  of  uncertainty  in  our  reading  of  the  record  is 
increased.  The  most  desirable  method  of  research  is  to  exhaust  the 
evidence  for  each  area  or  province,  and  to  compare  the  general  order  of 
its  succession  as  a  whole,  with  that  which  can  be  established  for  other 
provinces.  It  is,  therefore,  only  after  long  and  patient  observation  and 
comparison  that  the  geological  history  of  different  quarters  of  the  globe 
can  be  correlated. ^ 

*  Consult  on  this  subject  the  memoirs  on  Jurassic  Geography  of  the  late  Prof.  Neumayr, 
quoted  anUy  p.  655. 

'  For  an  example  of  the  working  out  from  fossil  evidence  of  the  history  of  the  various 
provinces  or  regions  of  a  large  area  of  the  earth's  surface  during  an  ancient  geological  period 
see  the  digest  given  by  Professor  Hyatt  of  what  is  known  of  the  Jurassic  tracts  of  Euroi»e, 
in  his  essay  on  the  '  Genesis  of  the  Arietidee.'  chapter  iv. 


664  PAL^OXTOLOGICAL  GEOLOGY  book  r 

4.  Subdivisions  of  the  Geological  Record  by  means 
of  fossil 8. — As  fossil  evidence  furnishes  a  much  more  satisfactonr  and 
widely  applicable  means  of  subdividing  the  stratified  rocks  of  the  earth's 
crust  than  mere  lithological  characters,  it  is  made  the  basis  of  the  geo- 
logical classification  of  these  rocks.     Thus,  a  particular  stratum  may  be 
ascertained  to  l>e  marked  by  the  occurrence  in  it  of  various  fosBils,  one  or 
more  of  which  may  be  distinctive,  either  from  occurring  in  no  other  bed 
above  and  below,  or  from  special  abundance  in  that  stratum.     These 
species  may,  therefore,  be  used  as  a  guide  to  the  occurrence  of  the  bed  in 
question,  which  may  be  called  by  the  name  of  the  most  abundant  speciea 
In  this  way,  a  geological  horizon  or  zone  is  marked  off,  and  geologists 
thereafter  recognise  its  position  in  the  geological  series.     But  befcwe  such 
a  generalisation  can  be  safely  made,  we  must  be  sure  that  the  species  in 
question  really  never  does  characterise  any  other  platform.     This  evi- 
dently demands  wide  experience  over  an  extended  field  of  observation. 
The    assertion   that  a  particular  species  or  genus  occiu^  only  on  one 
horizon,  or  within  certain  limits,  manifestly  rests  on  negative  evidence 
as  much  as  on  positive.     The  palaeontologist  who  makes  it  cannot  mean 
more  than  that  he  knows  the  species  or  genus  to  lie  on  that  horizon,  cnr 
within  those  limits,  and  that,  so  far  as  his  own  experience  and  that  of 
othere  goes,  it  has  never  l>een  met  with  beyond  the  limits  assigned  to  it. 
But  a  single  instance  of  the  occiurence  of  the  fossil  in  a  different  zone 
would  greatly  damage  the  value  of  his  generalisation,  and  a  few  such 
cases  would  demolish  it  altogether.     The  genus  Arethusina,  for  example, 
had  long  l>een  known  as  a  characteristic  trilobite  of  the  lower  zones  of  the 
third  or  highest  fauna  of  the  Bohemian  Silurian  basin.     So  abundant  is 
one  species  (A.  Konincki)  that  Barrande  collected  more  than  6000  speci- 
mens of  it,  generally  in  good  preservation.     But  no  trace  of  it  had  ever 
been  met  with  towards  the  upper  limit  of  the  Siliman  fauna.     Eventu- 
ally, however,  a  single  specimen  of  a  species  so  nearly  identical  as  to 
l)e   readily    pronounced    the    same    was    disinterred    from    the    upper 
Devonian   rocks  of  Westphalia — a   horizon   separated  from  the  upper 
limit  of  the  genus  in  Bohemia  by  at  least  half  of  the  vertical  height  of  the 
Upper  Silurian  and  by  the  whole  of  the  Lower  and  Middle  Devonian 
rock-groups.^     Such  an  example  teaches  the  danger  of  founding  too  much 
on   negative  data.     To  establish  a  geological  horizon  on  limited  fossil 
evidence,  and  then  to  assume  the  identity  of  all  strata  containing  the 
same  fossils,  is  to  reason  in  a  circle,  and  to  introduce  utter  confusion  into 
oiu"  interpretation  of  the  geological  record.     The  first  and  fundamental 
point  is  to  detemiine  accurately  the  superposition  of  the  strata.     Until 
this    is  (lone,   detailed    palseontological   classification  may  prove    to   be 
worthless. 

From  what  has  been  above  advanced,  it  must  be  evident  that,  even  if 
the  several  groups  in  a  series  or  system  of  rocks  in  any  district  or  country 
have  been  found  suscepti>)le  of  minute  subdivision  by  means  of  their 
characteristic  fossils,  and  if,  after  the  lapse  of  many  years,  no  discovery 
has  occurred  to  alter  the  established  order  of  succession  of  these  fossils, 

^  Barrande,  '  Reapparition  du  geure  Arethusina,'  Prague,  1868. 


§  V  PALAEONTOLOGY  AND  EVOLUTION  665 

nevertheless  the  sulxiivisions  may  only  hold  good  for  the  region  in  which 
they  have  been  made.  They  must  not  be  assumed  to  be  strictly  applic- 
able everywhere.  Advancing  into  another  district  or  country,  where  the 
petrogi-aphical  characters  of  the  same  formation  or  system  indicate  that 
the  original  conditions  of  deposit  must  have  been  very  different,  we  ought 
to  be  prepared  to  find  a  greater  or  less  departure  from  the  first  observed, 
or  what  we  unconsciously  and  not  unnaturally  come  to  look  upon  as  the 
normal,  order  of  organic  succession.  There  can  be  no  doubt  that  the 
appearance  of  new  organic  forms  in  any  locality  has  been  in  large  measure 
connected  with  such  physical  changes  as  are  indicated  by  diversities  of 
sedimentary  materials  and  arrangements.  The  Upper  Silurian  stages,  for 
example,  as  studied  by  Murchison  in  Shropshire  and  the  adjacent  counties, 
present  a  clear  sequence  of  strata  well  defined  by  characteristic  fossils. 
But  within  a  distance  of  sixty  miles,  it  becomes  impossible  to  establish 
these  subdivisions  by  fossil  evidence.  Again,  in  Bohemia  and  in  Kussia 
we  meet  with  still  greater  departures  from  the  order  of  appearance 
in  the  original  Siliman  area,  some  of  the  most  characteristic  Upper 
Silurian  organisms  being  there  found  beneath  strata  replete  with 
records  of  Lower  Silurian  life.  Nevertheless,  the  general  succession 
of  life  from  Lower  to  Upper  Silurian  types  remains  distinctly  trace- 
able. Still  more  startling  are  the  anomalies,  already  referred  to, 
where  the  succession  of  terrestrial  organisms  in  distant  regions  is  com- 
pared with  that  of  the  associated  marine  forms ;  as  where,  in  Australia, 
a  flora  with  Jurassic  affinities  and  a  Carboniferous  Limestone  fauna 
were  contemporaneous.  Such  facts  warn  us  against  the  danger  of  being 
led  astray  by  an  artificial  precision  of  palaeontological  detail  Even 
where  the  palseontological  sequence  is  best  established,  it  rests,  probably 
in  most  cases,  not  merely  upon  the  actual  chronological  succession  of 
organic  forms,  but  also,  far  more  than  is  usually  imagined,  upon  original 
accidental  differences  of  local  physical  conditions.  As  these  conditions 
have  constantly  varied  from  region  to  region,  it  must  comparatively 
seldom  happen  that  the  same  minute  palseontological  subdivisions,  so 
important  and  instructive  in  themselves,  can  be  identified  and  paralleled, 
except  over  comparatively  limited  geographical  areas.  The  remarkable 
"  zones  "  of  the  Lias,  for  instance,  which  have  been  recognised  over  central 
and  western  Eiu*ope,  cease  to  be  traceable  as  we  recede  from  their  original 
geographical  province. 

§  v.  Bearing  of  PalsBontological  data  upon  Evolution.  —  Since 
the  researches  of  William  Smith  at  the  end  of  last  century,  it  has  been 
well  understood  that  the  stratified  portion  of  the  earth's  crust  contains  a 
suite,  of  organic  remains  in  which  a  gradual  progression  can  be  traced, 
from  simple  forms  of  invertebrate  life  among  the  older  rocks  to  the 
most  highly  differentiated  mammalia  of  the  present  time.  Until  the 
appearance  of  Darwin's  *  Origin  of  Species'  in  1859,  the  significance  of 
this  progression,  and  its  connection  with  the  biological  relations  of  exist- 
ing faunas  and  floras  were  only  dimly  perceived,  though  Lamarck  had 
proposed  a  theory  of  development,  in  support  of  which  appeals  had  been 


666  PAL.-EONTOLOGICAL  GEOLOGY  book  t 

made  to  the  organic  succession  revealed  by  the  geological  reocmL 
Darwin,  arguing  that,  instead  of  being  fixed  or  but  slightly  alteraUe 
forms,  species  might  1k3  derived  from  others,  showed  that  processes  were 
at  work,  whereby  it  was  conceivable  that  the  whole  of  the  existiiig 
animal  and  vegetable  worlds  might  have  descended  from,  at  most,  a  very 
few  original  forms.  From  a  large  array  of  facts,  drawn  from  observatioiis 
made  uix)n  domestic  plants  and  animals,  he  inferred  that^  from  time  to 
time,  slight  peculiarities  due  to  differences  of  climate,  Sec,  appear  in  the 
offspring  which  were  not  present  in  the  ptarent,  that  these  peculiarities 
may  be  transmitted  to  succeeding  generations,  especially  where  from 
their  nature  they  are  useful  in  enabling  their  possessors  to  maintain 
themselves  in  the  general  struggle  for  life.  Hence  varieties,  at  first 
arising  from  accidental  circumstances,  may  become  permanent,  while  the 
original  form  from  which  they  sprang,  being  less  well  adapted  to  hold  its 
own,  perishes.  Varieties  become  species,  and  specific  differences  pass  in 
a  similar  way  into  generic.  The  most  successful  forms  are,  by  a  process 
of  "  natiu^l  selection,"  made  to  overcome  and  survive  those  that  are  less 
fortunate,  the  "  sun'ival  of  the  fittest "  being  the  general  law  of  nature. 
The  present  varied  life  of  the  globe  may  thus,  according  to  Darwin,  be 
explained  by  the  continued  accumulation,  perpetuation,  and  increase  of 
differences  in  the  evolution  of  plants  and  animals  during  the  whole  of 
geological  time.  Hence  the  geological  record  should  contain  a  m<Nne  or 
less  full  chronicle  of  the  progress  of  this  long  history  of  developments 

It  is  now  well  known  that  in  the  embryonic  development  of  animals, 
there  are  traces  of  a  progress  from  lower  or  more  generalised  to  higher 
or  more  specialised  types.  Since  Darwin's  great  work  appeared, 
naturalists  have  devoted  a  vast  amount  of  research  to  this  subject,  and 
have  sought  with  persevering  enthusiasm  for  any  indications  of  a  relation 
between  the  order  of  appearance  of  organic  forms  in  time  and  in 
embryonic  development,  and  for  e\'idence  that  species  and  genera  of 
plants  and  animals  have  come  into  existence,  in  the  order  which,  according 
to  the  theory  of  evohition,  might  have  been  anticipated. 

It  must  be  conceded  tliat,  on  the  whole,  the  testimony  of  the  rocks  is  in  favour  of 
the  floctrine  of  evolution.  That  there  are  difficulties  still  unexplained,  must  be  frankly 
granted.  Darwin  strongly  insisted,  and  ^nth  obvious  justice,  on  the  imperfection  of 
the  geological  record,  as  one  great  source  of  these  difficulties.  Objections  to  the 
develoi>ment  theory  may,  as  shown  by  Mr.  CaiTUthers,  l>e  drawn  from  the  observed 
order  of  succession  of  plants,  and  the  absence  of  transitional  forms  among  them. 
Ferns,  equisetums,  and  lycopods  appear  as  far  back  as  the  Old  Red  Sandstone,  not  in 
sinijile  or  more  generalised,  but  in  more  complex  stnictures  than  their  living  representa- 
tives. The  earliest  known  conifers  were  well -developed  trees,  with  woody  stmctare 
and  fruits  as  higlily  differentiated  as  those  of  the  li\nng  types.  The  oldest  dicoty- 
ledons yet  found,  tliose  of  the  Cretaceous  formations,  contain  representatives  of  the 
three  great  divisions  of  Apctahc,  Monopetala:,  and  Pohjpetal4jp,  in  the  same  deposit 
Tliese  *'  are  not  generalised  tyi)es,  but  differentiated  forms  which,  during  the  interven- 
ing epochs,  have  not  developed  even  into  higher  generic  groups.'*' 

Professor  A.   Agassiz  has  dra^vu  attention  to  the  parallelism  between   embryonic 

»  Camithers,  Oeol.  Mag.  1876,  p.  362. 


§  V  PALEONTOLOGY  AND  EVOLUTION  667 

development  and  palaeontological  history.  Taking  the  sea-urchins  as  an  illustrative 
group,  he  points  out  the  interesting  analogies  between  the  immature  conditions  of 
living  forms  and  the  appearance  of  corresponding  phases  in  fossil  genera.  He  admits, 
however,  that  no  early  type  has  yet  been  discovered  whence  star-fishes,  sea-urchins,  or 
ophiurans  might  have  sprung ;  that  the  several  ordei-s  of  echinoderms  appear  at  the 
same  time  in  the  geological  record,  and  that  it  is  impossible  to  trace  anything  like  a 
sequence  of  genera  or  direct  filiation  in  the  palaeontological  succession  of  the  echinids, 
though  he  does  not  at  all  dispute  the  validity  of  the  theory  which  regards  the  present 
echinids  as  having  come  down  in  direct  succession  from  those  of  older  geological  times.  ^ 
In  the  case  of  the  numerous  genera  which  have  continued  to  exist  without  interruption 
from  early  geological  periods,  and  have  been  termed  "  persistent  types,"  it  is  impossible 
not  to  admit  that  the  existing  forms  are  the  direct  descendants  of  those  of  former  ages. 
If,  then,  some  genera  have  unquestionably  been  continuous,  the  evolutionist  argues,  it 
may  reasonably  be  inferred  that  continuity  has  been  the  law,  and  that  even  where  the 
successive  steps  of  the  change  cannot  be  traced,  every  genus  of  the  living  world  is 
genetically  related  to  other  genera  now  extinct. 

Professor  A.  Hyatt,  who  has  closely  studied  the  CephaloiKxia,  regards  them  as 
furnishing  clear  evidence  of  evolution.  Returning  to  some  of  the  ideas  of  Lamarck  on 
development,  he  concludes  that  **the  efforts  of  the  orthoceratite  to  adapt  itself  fully  to 
the  requirements  of  a  mixed  habitat,  gave  the  world  the  Nautiloidea  ;  the  efforts  of  the 
same  type  to  become  completely  a  littoral  crawler,  developed  the  Ammonoidea."  He 
thinks  that,  on  the  whole,  the  observed  succession  of  the  organisms  in  time  coincides 
with  what  on  the  theory  of  evolution  it  ought  to  have  been.  "  The  straight  cones  pre- 
dominate in  Silurian  and  earlier  periods,  while  the  loosely  coiled  are  much  less  numer- 
ous, and  the  close-coiled  and  involute,  though  present,  are  extremely  rare."  He 
believes  that  traces  of  this  succession  may  be  found  in  the  structure  of  the  shells  them- 
selves. The  nautilus,  in  its  erabryological  development  and  subsequent  growth,  passes 
through  the  stages  of  the  nearly  or  quite  straight  shell,  then  of  a  slightly  curved  shell, 
and  then  of  a  completely  curved  shell,  the  spiral  being  continued  till  sometimes  the 
inner  whorls  are  entirely  enveloped  in  the  outer. ^ 

Neumayr,  from  a  prolonged  study  of  European  .Jurassic  and  Cretaceous  cephalo- 
]>ods,  concluded  that  "propagation,  filiation,  and  migration  are  sufficient  to  explain 
the  origin  of  the  whole  Jurassic  Ammonite  and  Belenmite  fauna  of  central  Europe. 
There  is  nothing  to  warrant  tlie  supposition  of  any  new  creation,  but  all  the  known 
facts  are  in  hai-niony  with  the  theory  of  descent. "  ^ 

Among  the  fossil  mammalia  many  indications  have  been  pointerl  out  of  an  evolution 
of  structure.  Of  these,  one  of  the  best  known  and  most  striking  is  the  genealogy  of  the 
horse,  as  worked  out  by  Professor  O.  C.  Marsh.*    The  original,  and  as  yet  undiscovered, 

*  Ann.  Mag,  Xat.  Hist.  Nov.  1880,  p.  369.  *•  Report  ou  Echinoidea,"  Challenger  Ex- 
pedition, vol.  iii.  p.  19. 

*  Science,  iii.  (1884)  pp.  122,  145.  For  an  elaborate  presentation  of  his  views  see  his 
essay  ou  the  'Genesis  of  the  Arietidae,'  Mem.  Mus.  Comparal.  Zool.  Harvard^  xvi.  (1889), 
where  also  full  references  to  the  literature  of  the  subject  treated  of  by  him  will  be  found. 

*  Jahrb,  Oeol.  Reichsanst.  xxviii.  (1878)  p.  78  ;  also  Abhandl.  Oeol,  ReichsansL  1873 ; 
SUzb.  K.  Akad.  Wi88,  Wicn,  Ixxi.  (1875)  p.  639.  Verh.  Oed.  Reichmnst,  1880,  p.  83  (in 
reply  to  the  an ti- Darwinian  views  of  T.  Fuchs,  op.  cit.  1879,  1880),  and  his  memoirs  already 
cited  on  p.  ,655.  W.  Hranco,  Z.  Detiisch.  Oed,  Oes.  xxxii.  (1880)  p.  596.  An  example 
of  the  tracing  of  pedigree  among  trilobites  was  supplied  by  R.  Hoemes,  Jahrh.  Geol, 
Reichsanst.  xxx.  (1880)  p.  651.  On  the  geological  history  and  affiliations  of  the  Palaeozoic 
invertebrates,  the  student  should  consult  Prof.  Gaudry's  '  Les  Enchainements  du  Monde 
Animal :  Fossiles  Priniaires,'  1883. 

*  Anier.  Journ.  Sci.  1879,  p.  499.  Consult  also  his  interesting  paper  ou  "Recent 
polydactyle  Horses,"  op,  cit.  xlii.  (1892)  p.  339. 


668 


pala:ontological  geology 


BOOK  T 


ancestor  of  our  modern  horse  had  fire  toes  on  each  foot.  In  the  oldest  known  equine 
ty|ie  (Eohippus — an  aninial  abont  the  sizt  of  a  fox,  belonging  to  the  early  part  of  the 
Eocene  period)  there  were  four  well-deyeloped  toes,  with  the  rudiment  of  a  fifth,  on 
each  fore-foot,  and  three  on  each  hind-foot.  In  a  later  part  of  the  same  geological 
period  appeared  the  Orohippus,  a  creature  of  about  the  same  sixe,  but  with  only  four 
toes  in  front  and  three  behind.  Traced  upwanls  into  younger  diyisious  of  the  Terdaiy 
series,  the  size  of  the  animal  increases,  but  the  number  of  digits  diminishea,  imtil  we 
reach  the  modem  Equus,  with  its  single  toe  and  rudimentary  splint-bones. 

Another  remarkable  example,  that  of  the  camels,  is  cited  by  Professor  £.  D.  Cope. 
The  succession  of  genera  is  seen  in  the  same  parts  of  the  skeleton  as  in  the  case  of  the 
horse.  The  metatarsal  and  metacarpal  bones  are  or  are  not  co-ossified  into  a  cannon 
bone  ;  the  first  and  second  superior  incisor  teeth  are  present,  rudimentary  or  wanting, 
and  the  premolar  number  from  four  to  one.  The  clironological  succession  of  gencfa  is 
given  by  Mr.  Cope  as  follows  : 

No  cannon  bone.  C!annon  bone  present. 


■^  /- 


Incisor  teeth  present. 


Incisors  1  and  2  wanting. 


4  premolars. 


3  premolars.        2  premolars.     1  {Hremolar. 


— '-.. 


Lower  Miocene 


Upper  Miocene  .  . 


Pliocene  and  receut. 


f 
I 


Poebrotherium. 
Protolabis. 
Procamelus. 


Pliaucheuia. 


Camelns. 


Auchenia. 


According  to  tliis  table,  tlie  Camelidte  have  gradually  undergone  a  consolidation  of 
the  lK)nes  of  the  feet,  with  a  great  reduction  in  the  number  of  the  incisor  or  premolar 
teeth.  Mr.  Cope  indicates  an  interesting  {Mirallel  between  the  palseontological  succes- 
sion and  the  embryonic  history  of  the  same  parts  of  the  skeleton  in  the  living  camel.* 
Among  the  Camivora,  as  M.  Gaudr>'  has  (tointed  out,  it  is  i)08sible  not  only  to  trace  the 
ancestry  of  existing  .sj)ecies,  but  to  discover  traits  of  union  between  genera  which  at 
present  seem  far  removed.*- 

It  is  not  necessary  here  to  enter  more  fully  into  the  biological  aspect 
of  this  wide  subject.  While  the  doctrine  of  evolution  has  now  obtained 
the  assent  of  the  great  majority  of  naturalists  all  over  the  globe,  even 
the  most  strenuous  upholder  of  the  doctrine  must  admit  that  it  is 
attended  with  pala?ontological  difficidties  which  no  skill  or  research 
has  yet  }>een  able  to  remove.  The  problem  of  derivation  remains 
insoluble,  nor  perhaps  may  we  hojie  for  any  solution  beyond  one  within 
the  most  indefinite  limits  of  correctness.^  But  to  the  palaeontologist,  it 
is  a  matter  of  the  utmost  importance  to  feel  assured  that,  though  he  may 
never  lie  able  to  trace  the  missing  links  in  the  chain  of  being,  the  chain 
has  been  unbroken  and  iKjrsistent  from  the  l>eginning  of  geological  time. 

It  was  remarked  above  (p.  660)  that,  while  the  general  march  of  life 
has  been  broadly  alike  all  over  the  world,  progress  has  been  more  rapid 
in  some  regions,  and  likewise  in  some  grades  of  organic  being,  than  in 
others.     The  evolution  of  terrestrial  plants  and  animals  appears  to  have 

^  Ainerimn  Xaturalisty  1880,  p.  172.  M.  Gaiidry  traces  an  analogous  process  in  the 
foot-bones  of  the  ruminants  of  Tertiary  time,  '  Les  Eucliainements  du  Monde  Animal,*  voL 
i.  p.  121. 


Op.  cit.  p.  210. 


*  A.  Agassiz,  Ann.  Mag,  Nat,  Hist,  1880,  p.  372. 


§  vi  FOSSIL-COLLECTING  669 

been  much  less  uniform  than  that  of  marine  life,  at  least  than  that  of  the 
marine  mollusca.  It  has  been  suggested  that  the  climatic  changes, 
which  have  had  so  dominant  an  influence  in  evolution,  would  affect  land- 
plants  before  they  influenced  marine  animals.  Certainly  a  number  of 
instances  is  known  where  an  older  type  of  marine  fauna  is  associated 
"with  a  younger  type  of  terrestrial  flora.  Besides  those  already  cited 
(p.  661),  reference  may  be  made  to  the  flora  of  Fiinfkirchen  in  Hungary, 
which,  though  Triassic  in  type,  occurs  in  strata  which  have  been  classed 
^vith  the  Palaeozoic  Zechstein ;  and  to  the  Upper  Cretaceous  flora  of  Aix 
la  Chapelle,  which,  with  its  numerous  dicotyledons,  has  a  much  more 
modern  aspect  than  the  contemporaneous  fauna.  In  the  Western 
Territories  of  North  America,  much  controversy  has  been  raised  as  to 
the  position  of  the  "  Laramie  series,"  its  rich  terrestrial  flora  having  an 
undoubted  Tertiary  facies,  while  its  fauna  is  Cretaceous.  According  to 
Fuchs,  the  most  important  turning-point  in  the  history  of  the  plant-world 
is  to  be  found  not,  as  in  the  case  of  the  terrestrial  fauna,  between  the 
Sarmatian  stage  and  the  Congeria-hofl^  but  on  an  older  horizon,  namely 
between  the  first  and  second  Mediten-anean  stage.^  Nor  is  this  inter- 
calation of  types  characteristic  of  other  peiiods  entirely  confined  to  the 
vegetable  world.  Examples  may  be  found  of  survivals  of  types  of 
terrestrial  animals  when  the  contemporaneous  marine  fauna  has  become 
distinctly  more  modern.  The  present  mammals  of  Australia  and  New 
Guinea  are  more  allied  to  forms  that  lived  in  Mesozoic  time  than  to  those 
now  living  in  other  countries.  The  remarkable  Miocene  mammalian 
fauna  of  Pikermi  has  been  found  to  lie  upon  strata  containing  Pliocene 
marine  shells. 

From  what  has  now  been  stated,  it  will  be  imderstood  that  the  exist- 
ence of  any  living  species  or  genus  of  plant  or  animal,  within  a  certain 
geographical  area,  is  a  fact  which  cannot  be  explained  except  by  refer- 
ence to  the  geological  history  of  that  species  or  genus.  The  existing 
forms  of  life  are  the  outcome  of  the  evolution  which  has  l)een  in  progress 
during  the  whole  of  geological  time.  From  this  point  of  \iew,  the 
investigations  of  palaeontological  geology  are  invested  with  the  pro- 
foundest  interest,  for  they  bring  before  us  the  history  of  that  living 
creation  of  which  we  form  a  part. 

§  vi.  The  collecting  of  Fossils. — Some  practical  suggestions  regard- 
ing the  search  for  fossils  may  be  of  service  to  the  student.  Any  sediment- 
ary rock  may  possibly  enclose  the  remains  of  plants  or  animals.  All 
such  rocks  should  therefore  be  searched  for  fossils.  A  little  i)ractice  will 
teach  the  learner  that  some  kinds  of  sedimentary  rocks  are  much  more 
likely  than  others  to  yield  organic  remains.  Limestones,  calcareous 
shales,  and  clays  are  often  fossiliferous ;  coarse  sandstones  and  con- 
glomerates are  seldom  so.  Yet  it  will  not  infrequently  l>e  found  that 
rocks  which  might  be  expected  to  contain  fossils  are  barren,  while  even 
coarse  conglomerates  may,  in  rare  cases,  yield  the  teeth  and  bones 
of  vertebrates  or  other  durable  relics  of  once  living  things.      The  peculi- 

'  B.  Weiss,  Nexus  Jahrb.  1878,  p.  180  ;  also  %,  Dtutsch,  Qed,  Oes.  xxix.  p.  252. 


670  PAL^ONTOLOGICAL  GEOLOGY  book  v 


arities  of  the  rocks  of  each  district  must,  in  this  respect,  be  discovered  by 
actual  careful  scrutiny. 

As  organic  remaius  usually  differ  more  or  less,  both  in  chemical  composition  and  in 
minute  texture,  from  the  matrix  in  which  they  are  imbedded,  they  weather  differ- 
ently from  the  surrounding  rock.  In  some  instances,  where  they  are  more  durable, 
they  project  in  relief  from  a  weathered  surface  ;  in  others  they  decay,  and  leave,  as 
cavities,  the  moulds  in  which  they  have  lain.  One  of  the  first  requisites,  therefore, 
in  the  examination  of  any  rock  for  fossils  is  a  careful  search  of  its  weathered  parts.  In 
the  great  majority  of  cases,  its  fossiliferous  or  non-fossiliferous  character  may 
thereby  be  ascertained. 

When  indications  of  fossils  have  been  obtained,  the  particular  lithological  characters 
of  the  i)art  of  the  rock  in  which  they  occur  should  be  noted.  It  will  often  be  found 
that  the  fossils  are  either  confined  to,  or  are  more  abundant  and  better  preserved  in, 
certain  zones.  These  zones  should  be  explored  before  the  rest  of  the  rock  is  examined 
in  detail.  Where  fossils  decay  on  exposure,  the  rock  containing  them  must  be  broken 
open  so  as  to  reach  its  fresher  portions.  Where  the  rock  is  not  disintegrated  in 
weathering,  it  must  likewise  be  split  up  in  the  usual  way.  But  where  it  crumbles  under 
the  influence  of  the  weather,  and  allows  its  fossils  to  become  detached  from  their  matrix, 
its  debris  should  be  examined.  Shales  and  clays  are  particularly  liable  to  this  kind 
of  disintegration,  and  are  consequently  deserving  of  the  fossil  -  collector's  closest 
attention,  since  from  their  decaying  surfaces  he  may  often  gather  the  organisms  of  past 
times,  as  easily  as  he  can  pick  up  shells  on  the  present  sea-shore. 

But  the  task  of  the  collector  does  not  end  when  he  has  broken  open  several  tons, 
perhaps,  of  fresh  rock,  and  has  searched  among  the  weathered  debris  imtil  he  can  no 
longer  meet  with  any  forms  he  has  not  already  found.  In  recent  years,  methods  have 
been  devised  for  enabling  him  to  extract  the  minuter  organisms  from  rocks.  Some  of 
these  methods  are  described  in  the  following  jwiges.*  They  show  that  a  deposit,  other- 
wise supposed  to  be  un fossiliferous,  may  be  rich  in  foraminifera,  entomostraca,  &c,  so 
that,  besides  the  abundant  fossils  readily  detected  by  the  naked  eye  in  a  rock,  there 
may  be  added  a  not  less  abimdant  and  varied  collection  of  microzoa. 

As  each  variety  of  rock  has  its  own  [peculiarities  of  structure,  which  may  vary  from 
district  to  district,  the  ap]>liances  of  the  fossil  collector  must  likewise  be  varied  to  suit 
local  retiuirenients.  The  following  list  compiises  his  most  generally  useful  accoutre- 
ments ;  but  his  own  judgment  will  enable  him  to  modify  or  supplement  them  according 
to  his  needs  : — 

List  of  Appliances  useful  in  Fossil-collecting. 

1.  Several  hammers,  varying  in  size  accoi-ding  to  the  nature  of  tlie  rocks  to  be 

examined.  Where  these  are  tough  and  haid,  a  hammer  weighing  2  lbs.  may 
be  needed.  A  small  trimming  hammer  (6  oz.)  for  reducing  the  size  of  specimens 
is  essential. 

2.  Several  chisels  of  different  sizes  and  shapes. 

o.  A  small  pick  weighing  1  lb.,  useful  for  loosening  blocks  of  rocks  from  their  bed. 

4.  A  small  trowel,  used  for  scooping  up  weathered  debris  of  shale.  &c. 

5.  A  gai-dener's  spade  with  circular  cutting  edge  ;  of  use  in  lifting  slabs  of  shale. 

C.  Pair  of  strong  pincers,  like  those  used  for  cutting  wire,  for  reducing  specimens 

which  might  go  to  pieces  under  a  blow  of  a  hammer. 
7.  A  collecting-bag  (canvas  or  leather). 

^  The  following  descriptions  of  methods  of  searching  for  fossil  microzoa  have  been  drawn 
up  from  notes  for  which  I  am  indebted  to  Mr.  James  Bennie,  Fossil  Collector  of  the 
Geological  Survey  of  Scotland,  who  has  been  singularly  successful  in  increasing  oar 
knowledge  of  the  minuter  forms  of  animal  life  in  the  Carboniferous  system. 


§  vi  FOSSIL-COLLECTING  671 

8.  A  supply  of  nests  of  pill-boxes  for  more  delicate  specimens. 

9.  Brown  and  softer  grey  wrapping  paper  (old  newspapers  are  serviceable). 

10.  Gummed  labels,  numbered  to  correspond  with  those  in  the  collecting- book. 

11.  Note -book  or  collecting -book,   in  which,    where  practicable,  each  specimen  is 

entered  under  its  number,  with  all  particulars  of  its  exact  locality,  geological 
horizon,  &c. 

12.  Fish-glue,  a  thin  solution  of  which  is  useful  to  preserve  specimens  that  may  be 

liable  to  crack  into  pieces. 

Weathered  Shale s. — The  heaps  of  shale  thrown  out  in  ciiuirrying  operations, 
afford  excellent  ground  for  fossil-hunting.  It  is  best  to  begin  at  the  bottom  of  a  heap, 
and  to  creep  slowly  along  the  same  level  for  a  dozen  yards  or  so,  where  the  ground  to 
be  examined  is  extensive  ;  then  to  return  along  a  baud  slightly  higher,  and  so  on 
backward  and  forward  until  the  top  is  reached,  which  may  be  searched  in  breadths  of 
a  yard  at  a  time.  In  this  way,  the  more  prominent  fossils  may  be  obtained.  Large  and 
thiu  fossils,  such  as  shells  of  Fectctij  Modioia^  &c.,  which  break  into  fragments  in 
weatherhig  must  be  sought  for  in  the  less  decayed  jwrts  of  the  shale.  When  found, 
the  matrix  around  them  should  be  reduced  to  the  desired  size  by  means  of  pincers. 
They  should  then  be  wrapped  up  in  a  box,  or,  at  least,  secured  against  injury  in  the 
homeward  transport,  and  as  soon  as  |M)ssible  thereafter  should  lie  dipped  in  a  thin 
solution  of  fish-glue  and  allowed  to  dry  slowly  in  the  air.  As  a  rule,  [particularly  where 
the  structure  of  a  fossil  is  well  preserved,  it  is  desirable  to  retain  also  the  surface  of 
rock  containing  its  impression,  which  not  infre([uently  atfoixls  evidence  of  structui-e 
that  may  be  less  distinctly  preserved  on  the  couuteri»art,  or  side  to  which  the  main 
portion  of  the  fossil  has  adhered. 

Some  fossils  of  great  delicacy,  such  as  fronds  of  Fenestella,  which  go  to  pieces  as  the 
rock  weathers,  may  be  extracted  by  an  ingenious  process  devised  by  Mr.  John  Young, 
Curator  of  the  Hunterian  Museum,  Glasgow  University.  If  the  shale  on  which  such 
organisms  lie  is  liable  to  go  to  pieces,  it  may  be  sutticiently  secured  for  transport  by 
being  coated  with  a  thin  solution  of  gimi,  which  is  allowed  to  dry  before  the  si)ecinien 
is  packed  up.  If  the  actually  exposed  face  of  the  Fenestciia  is  intendetl  to  be  exhibited, 
it  may  be  cleaned  from  the  gum  or  from  any  adherent  shale  by  being  rubbed  (juickly 
with  a  wet  nail-brush  and  wii)ed  with  a  clean  damp  sponge,  care  l)eing  taken  that  the 
gum  holding  down  the  lower  surface  of  the  fossil  is  not  softened,  and  that  the  shale  does 
not  get  too  wet.  If,  on  the  other  hand,  it  is  desirable  to  exjwse  the  face  of  the  frond 
that  adheres  to  the  shale,  this  may  be  effected  as  follows.  All  trace  of  any  gum  that 
may  have  been  used  should  be  carefully  removed.  The  sijeoimen  is  then  warmed  before 
a  fire,  and  a  thin  layer  of  asphalt  is  melted  over  it  by  means  of  a  hot  iron  rod.  If  the 
frond  to  be  lifted  is  large,  a  thick  strong  cake  should  be  formed  upon  the  specimen  by 
using  alternate  layers  of  strong  brown  pajier  and  asphalt,  the  i^i»er  always  forming  the 
outer  surface  of  the  cake.  When  the  cohesion  between  the  asphalt  and  the  specimen  is 
firm,  the  whole  is  then  placed  in  water,  when  the  shale  generally  crumbles  down  and 
can  be  removed,  leaving  the  Fenestella  adhering  to  the  asphalt.  In  this  way,  the 
jioriferous  surface,  which,  for  the  most  i»art,  clings  to  the  shale  when  the  rock  is  broken 
open,  is  laid  bare.  By  gently  brushing  the  specimen  with  water,  its  minute  structure 
may  be  revealed,  the  delicate  network  lying  on  the  asphalt  like  a  piece  of  lace  upon  a 
ground  of  black  velvet.  The  cake  of  asphalt  may  then  be  shajied  and  moimted  on  a 
wooden  tablet.^ 

But  in  most  cases  there  are  numerous  minuter  forms  which  escai)e  notice,  and  which 
must  be  searched  for  in  another  way.  To  secure  these,  a  little  shale  should  be  lifted 
vrith  a  trowel  from  the  most  weathered  parts  where  fossils  are  visible,  the  trowel  being 
gently  pushed  along  so  as  to  remove  only  the  superficial  layer,  where  the  fossils  are 


^  Mr.  Young  kindly  revised  for  me  this  account  of  his  asphalt-process. 


BOOK    VI. 

STRATIGKAPHICAL  GEOLOCIY. 

This  branch  of  the  science  arranges  the  rock?  of  the  earth's  cmst  in  tbe 
order  of  their  appearance,  and  interprets  the  sequence  of  events  ci 
which  they  form  the  records.  Its  province  is  to  cull  from  other  deput- 
ments  of  geologv'  the  facts  which  may  be  needed  to  show  what  has  been 
the  progress  of  the  planet,  and  of  each  continent  and  country  oo  iu 
surface,  from  the  earliest  times  of  which  the  rocks  have  preserved  any 
memorial.  Thus,  from  Mineralogy'  and  Petrography,  it  obtains  infcxma- 
tiori  regarding  the  origin  and  subsequent  mutations  of  minerals  and 
nxrkrf.  From  Dynamical  Geology,  it  learns  by  what  agencies  the  matmals 
of  the  earth's  crust  have  been  formed,  altered,  broken  or  uplieaTed. 
From  Geotec tonic  Geolog\',  it  understands  in  what  manner  these  materials 
have  Ijeeri  Vmilt  uj)  into  the  complicated  enist  of  the  earth.  From 
PalafTjntological  Geology,  it  receives,  in  well  determined  fossil  remains, 
A  cine  bv  which  to  follow  the  relative  chronoloirv  of  stratified  f<Hina> 
tions.  and  to  trace  the  grand  onwanl  march  of  organised  existence 
n\)t}n  the  planet.  Stratigraphical  geolog}"  thus  gathers  up  the  sum  of 
all  thiit  is  ascertainefl  by  other  departments  of  the  science,  and  makes  it 
sul>f*ervient  to  the  interpretation  of  the  geological  history  of  the  earth. 

The  leading  principles  of  stratigraphy  may  be  summed  up  as 
follows  : — 

1.  In  every  stratigraphical  research,  the  fundamental  requisite  is  to 
establish  the  true  or  original  order  of  .suj)erix)sition  of  the  strata.  Until 
this  is  accomplished  by  careful  study  of  the  actual  relations  of  the  rocks 
in  the  field,  it  is  imjx)ssil>le  to  arrange  relative  dates  and  make  out  the 
sequence  of  geological  history. 

2.  The  stratified  p<jrtion  of  the  earths  crust,  or  Geological  Record, 
may  Ini  sulxlivided  into  natural  groups  or  "  formations "  of  strata,  each 
marked  throughout  by  some  common  fades  of  organic  remains^  that  is 
by  the  fx:currence  of  some  characteristic  genera  or  species  or  a  general 
resemblance  in  their  palaeontological  type  or  character,^  or,  for  limited 
tracts  of  country,  by  some  common  lithological  features. 

'  The  ^tmlent  may  consult  an  interesting  paper  by  Prof.  E.  Renevier  {Arch,  ScL  Ph}fi. 


BOOK  VI  PRINCIPLES  OF  STRATIGRAPHY  675 


3.  Living  species  of  plants  and  animals  can  be  traced  downward  into 
the  more  recent  geological  formations ;  but  grow  fewer  in  number  as 
they  are  followed  into  more  ancient  deposits.  With  their  disappearance, 
we  encounter  other  species  and  genera  which  are  no  longer  living. 
These  in  turn  may  be  traced  backward  into  earlier  formations,  till  they 
too  cease,  and  their  places  are  taken  by  yet  older  forms.  It  is  thus 
shown  that  the  stratified  rocks  contain  the  records  of  a  gradual  progres- 
sion of  organic  types.  A  species  which  has  once  died  out  does  not  seem 
ever  to  have  reappeared. 

4.  When  the  order  of  succession  of  organic  remains  among  the 
stratified  rocks  of  a  district  or  country  has  once  been  accurately  determined 
on  the  basis  of  the  true  stratigraphical  order,  it  l>ecomes  an  invaluable 
guide  in  the  investigation  of  the  relative  age  and  stinictural  arrangements 
of  these  rocks  even  in  regions  beyond  that  in  which  the  organic  succession 
has  been  first  made  out.  Each  zone  or  group  of  strata,  being  characterised 
by  its  own  species  or  genera,  may  be  recognised  hy  their  means,  and  the 
true  succession  of  strata  may  thus  be  confidently  established  even  in  an 
area  such  as  that  of  the  Alps,  wherein  the  rocks  have  been  greatly 
fractured,  folded,  inverted,  or  metamorphosed. 

5.  The  relative  chronological  value  of  the  divisions  of  the  Geological 
Record  is  not  to  be  measured  by  mere  depth  of  strata.  While  a  great 
thickness  of  stmtified  rock  may  be  reasonably  assumed  to  mark  the 
jMissage  of  a  long  period  of  time,  it  cannot  safely  be  affirmed  that  a  much 
less  thickness  elsewhere  represents  a  correspondingly  diminished  period. 
The  tnith  of  this  statement  may  sometimes  be  made  evident  by  an  uncon- 
fomiability  between  two  sets  of  rocks,  as  has  already  been  explained. 
The  total  depth  of  both  groups  together  may  be,  say,  1000  feet.  Else- 
where we  may  find  a  single  unbroken  formation  reaching  a  depth  of 
10,000  feet  ;  but  it  would  be  utterly  erroneous  to  conclude  that  the 
latter  represents  ten  times  the  duration  indicated  by  the  two  former. 
So  far  from  this  being  the  case,  it  might  not  be  difficult  to  show  that  the 
minor  thickness  of  rock  really  denoted  by  far  the  longer  geological  interval. 
If,  for  instance,  it  could  be  proved  that  the  upper  j^art  of  l>oth  the 
sections  lay  on  one  and  the  same  geological  platform,  but  that  the  lower 
unconformable  series  in  the  one  locality  belonged  to  a  far  lower  and 
older  system  of  rocks  than  the  base  of  the  thick  conformable  series  in 
the  other,  then  it  would  be  clear  that  the  gap  marked  by  the  uncon 
formability  really  indicated  a  longer  period  than  the  massive  succession 
of  deposits. 

6-  Fossil  evidence  furnishes  the  chief  means  of  comparing  the  rela- 
tive chronological  value  of  groups  of  rock.  A  break  in  the  succession  of 
organic  remains  marks  an  interval  of  time  often  unrepresented  by  strata 
at  the  place  where  the  break  is  foimd.^  The  relative  importance  of  these 
breaks,  and  therefore,  prolmbly,  the  comparative  intervals  of  time  which 

NcU.  Geneva  (1884),  xii.  p.  297)  on  "Geological  Facies."  The  total  mean  depth  of  the 
fossiliferous  fonnations  of  Europe  has  been  set  down  at  75,000  feet,  or  upwards  of 
14  miles. 

*  See  atife,  p.  66*2,  and  the  classic  essays  of  the  late  Sir  A.  C.  Kanisay  there  cited. 


676  STRATIGRAPHICAL  GEOLOGY  book  vi 

they  denote,  may  be  estimated  by  the  difference  of  the  facies  of  the  fossils 
on  each  side.  If,  for  example,  in  one  case  we  find  every  species  to  be 
dissimilar  above  and  below  a  certain  horizon,  while  in  another  locality 
only  half  of  the  8])ecie8  on  each  side  of  a  band  are  peculiar,  we  natur- 
ally infer,  if  the  total  number  of  species  seems  large  enou^  to 
warrant  the  inference,  that  the  inter\'al  marked  bv  the  former  break 
was  very  much  longer  than  that  marked  by  the  latter.  But  we  may  go 
further,  and  compare  by  means  of  fossil  evidence  the  relation  between 
breaks  in  the  succession  of  organic  remains  and  the  depth  of  strata 
between  them. 

Three  series  of  fossiliferous  strata.  A,  C,  and  H,  may  occur  conform- 
ably above  each  other.  By  a  comparison  of  the  fossil  contents  of  all 
f)arts  of  A,  it  may  be  ascertained  that,  while  some  species  are  peculiar  to 
its  lower,  others  to  its  higher  portions,  yet  the  majority  extend  throughout 
the  group.  If  now  it  is  found  that,  of  the  total  number  of  species  in 
the  up|)er  portion  of  A,  only  one-third  ])asses  up  into  C,  it  may  be 
inferred  with  some  probability  that  the  time  represented  by  the  break 
between  A  and  C  was  really  longer  than  that  required  for  the  accumu- 
lation of  the  whole  of  the  group  A.  It  might  even  be  possible  to  dis- 
cover elsewhere  a  thick  intermediate  group  B  filling  up  the  gap  between 
A  and  C.  In  like  manner,  were  it  to  be  discovered  that,  while  the  whole 
of  the  group  C  is  characterised  by  a  common  suite  of  fossils,  not  one  of 
the  species  and  only  one  half  of  the  genera  pass  up  into  H,  the  infer- 
ence could  hardly  be  resisted  that  the  gap  between  the  two  groups  marks 
the  passage  of  a  far  longer  interval  than  was  needed  for  the  deposition  of 
the  whole  of  C.  And  thus  we  reach  the  remarkable  conclusion  that 
thick  though  the  stratified  formations  of  a  country  may  be,  in  some 
cases  they  may  not  represent  so  long  a  total  period  of  time  as  do  the 
gaps  in  their  succession, — in  other  words,  that  non-deposition  has  been 
in  some  areas  more  freciuent  and  prolonged  than  deposition,  or  that  the 
intervals  of  time  which  have  been  recorded  by  strata  have  sometimes  not 
been  so  long  as  those  which  have  not  been  so  recorded. 

In  all  speculations  of  this  nature,  however,  it  is  necessary  to  reason 
from  as  wide  a  liasis   of  observation  as  jx)ssible,  seeing  that  so  much  of 
the  evidence  is  negative.     Especially  needful  is  it  to  bear  in  mind  that 
the  cessation  of  one  or  more  species,  at  a  certain  line  among  the  rocks  of 
a  particular  district,  may  mean  nothing  more  than  that,  o^nng  to  some 
change  in  the  conditions  of  life  or  of  deposition,  these  species  were  com- 
I)elle(l  to  migrate,  or  became  locally  extinct,  at  the  time  marked  by  that 
line.     They  may  have  continued  to  flourish  abundantly  in  neighbouring 
districts  for  a  long   period    afterward.     Many  examples  of  this  obvious 
truth  might  be  cited.     Thus,   in  a  great  succession  of  mingled  marine, 
brackish- water,  and  terrestrial  strata,  like  that  of  the  Carboniferous  Lime- 
stone series  of  Scotland,  corals,  crinoids,  and  brachiopods  abound  in  the 
limestones  and  accompanying  shales,  but  grow  fewer  or  disappear  in  the 
sandstones,  ironstones,  clays,  and  bituminous  shales.     An  observer,  meet- 
ing for  the  first  time  with  an  instance  of  this  disappearance,  and  remem— 

ing  what  he  had  read  about  '^  breaks  in  succession,'^  might  be  tempteA- 


y* 


\ 


BOOK  VI  PRINCIPLES  OF  STRATIGRAPHY  677 

to  speculate  about  the  extinction  of  these  organisms,  and  their  replace- 
ment by  other  and  later  forms  of  life,  in  the  overlying  strata.  But 
further  research  would  show  him  that,  high  above  the  plant -bearing 
sandstones  and  coals,  lie  other  limestones  and  shales  charged  witli 
the  same  marine  fossils  as  before,  and  followed  by  still  further  groups  of 
sandstones,  coals,  and  carbonaceous  beds  and  yet  higher  marine  limestones. 
He  would  thus  learn  that  the  same  organisms,  after  being  locally  exter- 
minated, returned  again  and  again  to  the  same  area  when  the  conditions 
favourable  for  their  migration  reappeared  and  enabled  them  to  reoccupy 
their  former  haunts.  Such  a  lesson  would  probably  teach  him  how  largely 
the  fauna  entombed  and  preserved  on  any  particular  geological  horizon 
has  been  influenced  by  the  conditions  of  sedimentation,  and  that  he  should 
pause  before  too  confidently  asserting  that  the  highest  bed  in  which 
certain  fossils  can  be  detected,  marks  really  their  final  appearance  in  the 
history  of  life.  An  interruption  in  the  succession  of  fossils  may  be 
merely  temporary  or  local,  one  set  of  organisms  having  been  driven  to 
a  different  part  of  the  same  region,  while  another  set  occupied  their  place 
until  the  first  was  enabled  to  return. 

.The  remarkable  limitation  of  certain  species  to  a  restricted  vertical 
range  in  a  continuous  series  of  stratified  deposits,  as  in  the  case  of  the 
Silurian  graptolites  and  the  Jurassic  ammonites  already  cited,  affords  a 
valuable  basis  for  stratigraphical  arrangement  and  comparison.  The 
succession  of  these  species  has  been  in  some  cases  similar  over  such  wide 
geographical  areas  that  it  is  difficult  to  connect  this  organic  sequence 
with  any  physical  revolutions,  of  which  indeed  in  a  conformable  series  of 
sediments  there  may  be  little  or  no  trace.  As  already  suggested  there 
may  have  been  some  biological  law  that  governed  these  apparently 
rapid  extinctions  or  replacements  of  organic  forms,  but  which  is  not  yet 
perceived  or  understood. 

7.  The  Geological  Record  is  at  the  best  but  an  imperfect  chronicle  of 
the  geological  history  of  the  earth.  It  abounds  in  gaps,  some  of  which 
have  been  caused  by  the  destruction  of  strata  owing  to  metamorphism, 
denudation,  or  otherwise,  some  by  original  non  -  deposition,  as  above 
explained.  Nevertheless  it  is  from  this  record  that  the  progress  of  the 
earth  is  chiefly  traced.  It  contains  the  registers  of  the  births  and  deaths 
of  tribes  of  plants  and  animals,  which  have  from  time  to  time  lived  on 
the  earth.  Probably  only  a  small  proportion  of  the  total  number  of 
species,  which  have  appeared  in  past  time,  have  been  thus  chronicled, 
yet,  by  collecting  the  broken  fragments  of  the  record,  an  outline  at  least 
of  the  history  of  life  upon  the  earth  can  be  deciphered. 

It  cannot  be  too  frequently  stated,  nor  too  prominently  kept  in  view, 
that,  although  gaps  occur  in  the  succession  of  organic  remains  as 
recorded  in  the  rocks,  there  have  been  no  such  blank  intervals  in  the 
progress  of  plant  and  animal  life  upon  the  globe.  The  march  of  life 
has  been  unbroken,  onward  and  upward.  Geological  history,  therefore, 
if  its  records  in  the  stratified  formations  were  perfect,  ought  to  show  a 
blending  and  gradation  of  epoch  with  epoch,  so  that  no  sharp  divisions 
of  its  events  could  l)e  made.     But  the  record  of  the  historv  has  been 


678  STRATIGRAPHICAL  GEOLOGY  book  vi 


constantly  interrupted  :  now  by  upheaval,  now  by  volcanic  outbursts, 
now  by  depression,  now  by  protracted  and  extensive  denudation. 
These  interruptions  serve  as  natural  divisions  in  the  chronicle,  and 
enable  the  geologist  to  arrange  his  history  into  periods.  As  the  order 
of  succession  among  stratified  rocks  was  first  made  out  in  Europe,  and 
as  many  of  the  gaps  in  that  succession  were  found  to  be  widespread  over 
the  Europe;in  area,  the  divisions  which  experience  established  for  that 
portion  of  the  globe  came  to  be  regarded  as  typical,  and  the  names 
adopted  for  them  were  applied  to  the  rocks  of  other  and  far  distant 
regions.  This  application  has  brought  out  the  fact  that  some  of  the 
most  marked  geological  breaks  in  Europe  do  not  exist  elsewhere,  and,  on 
the  other  hand,  that  some  portions  of  the  record  are  much  more  com- 
plete there  than  in  other  regions.  Hence,  while  the  general  similarity 
of  succession  may  remain,  different  subdivisions  and  nomenclature  are 
required  as  we  i)ass  from  continent  to  continent. 

The  smallest  and  simplest  subdivision  of  the  Geological  Record  is  a 
stratum,  layer,  seam  or  bed.  As  a  rule  it  is  distinguishable  by 
lithological  rather  than  j)alaBontological  features.  Where  a  bed,  or 
limited  number  of  beds,  is  characterised  by  one  or  more  distinctive 
fossils,  it  is  termed  a  zone  or  horizon,  and,  as  already  mentioned,  is 
often  known  by  the  name  of  a  typical  fossil,  as  the  different  zones  in 
the  Lias  are  by  their  special  species  of  ammonite.^  Two  or  more  such 
zones,  united  by  the  occurrence  in  them  of  a  number  of  the  same 
characteristic  species  or  genera,  may  be  called  beds  or  an  assise, 
as  in  the  *'  Micraster  beds  or  assise  "  of  the  Cretaceous  system,  which 
include  the  zones  of  M.  cor-testudinarium  and  71/.  cor-anguinunu  Two  or 
more  sets  of  such  connected  beds  or  assises  may  be  termed  a  group  or 
stage  (Stage).  In  some  cases,  where  the  number  of  assises  in  a  stage  is 
large,  they  are  grouj)ed  into  sub-stages  {sous-^^iagf!s)  or  sub-groups.  Each 
sub-stage  or  sub-group  will  then  consist  of  several  assises,  and  the  stage 
or  group  of  several  sub-stages  or  sub-groups.  A  number  of  groups  or 
stages  constitutes  a  series,  section  {Ahtheilung\  or  formation,  and 
a  number  of  series,  sections,  or  formations  may  be  united  into  a  system.* 

'  Prof.  Gaudry  estimates  the  total  number  of  zones  in  the  European  geological  series  at 
114.  In  this  calculation  the  Jurassic  system  is  allowed  no  fewer  than  34;  the  Carboni' 
ferous  and  Permian  together,  10;  and  the  Cambrian  and  Silurian  together,  20  ('Enchaine* 
ments  du  Monde  Animal  :  Fossiles  Primaires,'  1883).  Professor  Lapworth  has  recognined 
20  distinct  graptolite  zones  in  the  Cambrian  and  Silurian  systems  {Ann,  Mag,  SaU 
Hist.  ser.  5,  vols.  iii.  iv.  v.  vi.  (1879-80),  see  especially  the  last  part  of  his  paper  in  vol. 
vi.  p.  \^^  seq.)     See  also /yoif^m,  p.  741. 

'•*  Compare  Ilebert,  Atni.  Sci.  Gtol.  xi.  (1881).  The  uuitication  of -geological  nomen- 
clature throughout  the  world  is  one  of  the  objects  aimed  at  by  the  **  International 
Geological  Congress,"  which  at  its  meeting  at  Bologna  recommended  the  adoption  of  the 
following  terms,  the  most  comprehensive  being  placed  first : — 

Divisions  of  setHmc atari/  formations.  CarrespundirKj  chronological  terms. 
Group.  Era. 

System.  %  Period. 

Series.  Epoch. 

Stage.  Age. 


680  STRATIRSAPIflCAL  GEOLOHY  noOKti 

Jurassic    Horn,   the  Cambriun    trilobitea,  as   if   these  adjectives  denoted 
simply  epochs  of  geological  time. 

The  Geological  Eecord  is  classified  into  five  main  divisions:  (1) 
Pfe-Cambrian,  also  called  Archiean,  Azoic  (lifeless),  Eozoic  (dawn  (rf 
life)  or  Proterozoic  (earliest  life);  (2)  Pttlseozoic  (ancient  life)  or 
Primary;  (3)  Mesozoic  (middle  life)  or  Secondary;  (4)  Cainozoic 
(recent  life)  or  Tertiary,  and  (5)  Post-Tertiary  or  Quaternarj'.  The 
Tertiary  and  Post-Tertiary  are  sometimes  grouped  together  as  Neozoic 
(new  life).  These  divisions  iire  further  ranged  in  systems,  each  system 
in  series,  sections,  or  formations,  each  formation  in  groups  or  stages,  and 
each  group  in  single  zones  or  horizons.'  The  accom|)anying  generalised 
table  exhibits  the  sequence  of  the  chief  subdivisions. 


Pakt  (.     Pre-Cambrian. 
g  i.  General  Characters. 

In  the  classification  of  the  materials  of  the  earth's  crust  enunciated 
by  Werner  the  term  "  transition  rocks  "  was  applied  to  a  large  series  of 
stratified  formations,  which,  underlying  the  well-known  fossiliferous  or 
Seconckry  deposits,  and  overlying  the  various  crystalline  masses  which 
were  regarded  as  the  most  ancient  or  Primary  \Mtt  of  the  earth's  surface 
were  believed  to  record  an  intennediate  period  of  terrestrial  history, 
Iwtween  the  time  when  any  such  crystalline  materials  as  granite  were  lud 
down  from  a  supposed  universid  ocean  and  the  time  when  ordinary  sediment 
nccumuLtted  and  entoml>ed  the  remains  of  the  earliest  animal  life.  Long 
after  the  theoretical  considenitions  that  led  to  its  adoption  had  been  proved 
to  be  fallacious,  this  tcrui  "transition"  continued  to  maintain  its  ground 
as  the  designation  of  the  most  ancient  stnitificd  rocks  underlying  the  Old 
Red  Sandstone,  and  containing  the  earliest  known  organic  remains.  Hm 
researches  of  Murchison  and  Sedgwick  eventually  showed  that  these 
venei-able  formations  contained  a  well-marked  succession  of  oi^nic  type^ 
whereby,  us  in  the  case  of  the  Secondary  rocks,  so  admirably  made  oat 
by  William  Smith,  they  could  be  gioujied  into  separate  systenu  i 
formations,  and  could  1>e  identified  in  all  j)arts  of  the  world.  The  t 
Cambrian  and  Silurian  (which  will  be  explained  in  later  pages)  1 
proposed  by  these  illustrious  pioneers  to  denote  the  oldest  kiK 
fossiliferous  formations,  and  soon  entirely  supplanted  the  older  i 
'■  tnirisition  "  and  "grauwacke."  The  Cambrian  system,  as  now  genet 
understood,  includes  the  lowest  aeries  of  Primary,  or  as  they  are  now  caUeiiL 
Palieozoic  dei)osits  (see  pusteii,  p.  719). 

But  it  has  been  well  established  that,  while  in  » 
of  the  Cambrian  system  is  sejtarated  by  h  strong  imconfoe 
rocks  of  older  date,  in  other  tracts  it  can  only  be  d  ' 
line,  beneath  which  lie  other  still  more  ancient  sediq 

'  On  the  clasaificatiati  of  tlie  (ieological  Bcoord  ste  Dr. 
1384.    Prof.  Renevier,  Bull.  Soc.  I'aml.  lilt  p.  SSa 


\o  temi 
b)  w^H 

ancnlJF     I 


PABT  I S  i  fUE-CAM^RIAX  ROOKS  OSl 

til  those  i>riiHev;tl  depuKits  therL'  ai-e  reuonls  of  duiiuilatioii  »nd  ileposi- 
tiuii,  af  Hltcrniite  stHlinieiitatioii  aiul  terrefitnul  niovoineiitM,  of  xtujx'iuloiirt 
iiiid  pi-oloiigcd  volctiiiic  iicthity,  and  of  distinct  though  Bcaiity  jiroofs  that 
plunt  and  animal  life  had  already  appeai'ed  u]M)n  the  fiice  of  the  );!ol>i.\ 
So  far  as  our  knowledge  yet  goes  there  is  no  meiina  of  uReeiiaiiiiiig  tho 
synchronisni  or  homotHxiri  of  these  formations  in  nidely  sejinnited  i'egioiii<. 
FiMsil  evidence  entirely  fails  here  as  a  guide,  and  mei'e  mineml  ehai-aetei-s 
are  only  reliuMe  n-ithin  comiwr.itively  limited  areas.  All  that  (uin  for  the 
present  W  attempted  is  to  determine  the  tnie  order  of  sGc|iU'net<,  tK-toiiii.- 
relntions  and  genenil  structure  of  the  several  distinct  formations  in  eiK-h 
cotinti-y  where  they  occur,  without  in  the  nieantinie  any  sei'ioiis  attemjit 
at  correlation. 

It  must  further  be  observed  that  these  oldest  stratified  rocks  have 
very  generally  undergone  more  or  less  alteration  during  the  numerous 
torrestiial  disturbances  of  geological  histoiy.  I'ying  as  they  do  at 
the  l»a»e  of  tlie  stratitied  jjiiit  of  the  eitrths  ciust,  they  have  shared  in 
the  movements  by  which,  during  the  la]***)  of  geological  time,  the 
fosniiifei-ous  rocks  have  Iteen  aHected.  p^rery  intruded  mass  of  igncoii.s 
rock,  every  volcanic  outbui-st,  every  agent  of  contact  or  of  regional 
metamutphisni  hud  firet  to  (lass  through  them  before  it  could  reach  the 
younger  iiK'ks  alwve.  Hence  not  only  have  they  usually  liiren  disloeatcil 
aiid  i>lii.-nted,  hut  they  have  Iwen  abundantly  iiivadc<l  by  intrusive  niHterials 
of  all  ages,  and  their  iiitenial  structure  has  fi'e<[uently  been  subj<<cted  to 
such  mechanical  stresses,  mth  accompanying  chemical  and  niineralogical 
readjustments  of  their  comiioncut  mateiials,  that  they  have  iKis^ed  into 
the  condition  of  s^^^hists.  In  this  highly  altereil  state  they  often  can- 
not !«  distinguished  fnim  still  more  ancient  whists,  the  tnu'  origin  of  " 
which  is  not  certainly  known.  In  some  rt-gions,  indeed,  whciv  the  older 
se<limentary  fonniitions  have  been  gi-eatly  disturK-d,  a  gradation  may  1>e 
traced  from  unmistakable  I'lilieoxoie  or  iMesozoie  seitiments  with 
recognisable  fossiU  into  thuntiighly  crystalline  and  foliated  schistit. 
Sometimes  this  tiiinsition  is  doubtless  due  to  an  nctiud  extensive  nieta- 
morphisni  of  the  sedimentary  rocks,  and  in  these  instances  there  may  It 
no  means  of  separating  the  schists  of  which  the  si^imeiitury  origin  >■ 
ascertainable  fmm  tboeo  where  It  is  not.  The  whole  may  1m  Paliriw-ii. 
or  ilcsozoic.  In  other  ewes,  there  aeeaa  rcaaon  to  believe  thrn  '**' 
gradation  is  mtber  due  to  exeod^^l^^^^Hfareby  uiiaVnt  a'^"^ 

<  and  Palfeozoic  or  MesoMM  •trateq^^^^^^^^B*'^  '''"'  '^v'^'^ 
1  direction  of  strike,  and  have  V^^^^^K^^Bf^forti*"^  "^  V^.  .." 
eries  have  been  enclosed  within  the  other,  ccfttaiderfiWc jTr"-™'  ■" 

I  phiam  having  at  the  same  time  been  KUiKn-indnonl  "/"'"  '^'  *  '     ' 

I         From  underneath  these  oldest  ledimcntary  nee*""    "" 
Ui  the  surface  a  remarkable  aiutcnibliige  of  t.ii'"*'"*^^ 

.  which  raii^o  from  anioqihomi  ma.-ue«  surli  ;t* )."~ 

'  ltje»  of   QfUTM  • 


082  STRATIGRAPHICAL  GEOLOGY  book  vi 

terpentines).  Though  sometimes  amorphous  over  considerable  spaces,  and 
then  not  to  be  distingiiished  from  ordinary  igneous  eruptive  masses,  they 
for  the  most  part  present  a  more  or  less  distinctly  schistose  or  foliated 
structure,  some  of  their  most  abundant  and  conspicuous  members  being 
gneisses,  often  so  coarsely  banded  as  to  pass  into  granite. 

The  infra-position  of  these  crystalline  rocks,  combined  with  their  jMre- 
valent  stratified  appearance,  naturally  led  to  their  being  regarded  as  the 
oldest  known  formation  on  which  all  the  later  portions  of  the  terrestrial  cnut 
rest.  But  recent  observations  have  proved  many  gneisses  to  be  originallj 
igneous  rocks,  sometimes  even  intrusive,  and  therefore  yoimger  in  date  than 
the  rocks  which  they  pierce  (pp.  186,  615).  Where  the  area  in  which  these 
ancient  mineral  masses  are  exposed  is  small,  and  especially  where  only 
the  gneissic  or  schistose  portion  of  them  is  seen,  the  oldest  fossiliferous 
rocks  may  lie  on  them  with  a  strong  unconformability.  The  contrast  in 
such  conditions  between  the  stratified  conglomerates,  sandstones,  and 
shales  of  the  Palaeozoic  series,  and  the  gnarled  crystalline  gneisses  below 
them  is  so  striking  as  to  have  suggested  the  idea  that  in  these  gneisses  we 
have  reached  the  lowest  and  earliest  part  of  the  earth's  crust.  Hence 
arose  such  names  as  Fundamental  gneiss,  Urgneiss  or  Urgebirge. 

Xo  portion  of  the  Geological  Record  has  in  recent  years  been  more 
diligently  studied  than  the  crystalline  schists,  which,  underlying  the  vast 
pile  <^f  fossiliferous  systems,  have  been  regarded  as  the  earliest  surviving 
chronicles  of  the  history  of  the  earth.  But  the  problems  presented  by 
these  rocks  are  so  many  and  so  difficult  that  comparatively  little  progresB 
has  been  made  in  the  endeavour  to  group  them  into  formations  or  systems 
comparable  with  those  of  the  fossilferous  series,  and  to  ascertain  the 
stages  of  geological  history  of  which  they  are  the  memorials.  The 
obstiicles  to  increase  of  knowledge  on  this  subject  arise  from  the  complica- 
tion and  obscurity  of  the  geotectonic  relations  of  the  rocks.  We  have  as 
yet  no  satisfactory  clue  to  their  chronological  sequence.  They  have 
undergone  so  many  disturbances  of  their  mass,  and  so  many  and  seriouB 
alterations  of  their  internal  structure,  that  it  is  often  quite  impossible  to 
be  certain  of  their  true  sequence.  Nothing  in  the  least  degree  analogous 
to  the  evidence  of  fossils  among  the  sedimentary  rocks  is  here  available. 
Whether  eventually  a  determinable  order  of  appearance  among  the 
minerals  of  these  ancient  rocks  may  be  ascertained  remains  still  uncertain. 
If  it  could  be  shown  that  certain  minerals,  or  groups  of  minerals,  came  into 
existence  at  particular  stages  in  the  formation  of  the  crystalline  schists,  a 
key  might  he  found  to  some  of  the  most  difficult  parts  of  this  branch  of 
geological  enquiry.  But  though  such  a  sequence  has  often  been  claimed  to 
exist,  no  satisfactory  proof  has  yet  been  adduced  that  it  has  been  asserted 
on  more  than  mere  local  observation.  Certainly  no  general  law  of  mineral 
sequence  in  geological  times  has  hitherto  been  established.^ 

'  The  l.ite  T.  S.  Hunt  was  one  of  the  main  exponents  of  the  view  that  the  crystalline 
pre-Cam]>rian  rocks  were  deposited  as  chemical  sediments  in  a  certain  definite  order,  and  that 
the  rocks  couhl  be  recognised  by  their  mineral  characters,  and  be  thereby  grouped  in  their 
proper  order  all  over  the  world.  See,  for  example,  his  essays  on  "Tlie  Taconic  Qaestion  in 
Geologj',"  and  on  "The  Origin  of  the  Crystalline  Rocks  "  in  vols.  i.  and  ii.  of  the  Trant,  Roy* 


PART  I  §  i  PRE-CAMBRIAN  ROCKS  683 

Thus  while  it  is  often  difficult  or  impossible  to  ascertain  the  original 
order  of  succession  among  the  crystalline  schists  of  a  particular  region,  it 
is  even  more  difficult  to  form  a  satisfactory  judgment  as  to  the  strati- 
graphical  relations  of  the  schists  of  two  detached  regions.  There  is  usually 
no  common  basis  of  comparison  between  them,  except  similarity  of  mineral 
character  and  structure.  But  as  it  can  be  shown  that  even  in  a  single 
area  the  crystalline  schists  may  sometimes  represent  the  results  of  many 
successive  opei*ations  continuing  through  a  long  series  of  geological 
periods,  it  is  obvious  that  the  task  of  correlating  these  rocks  in  distinct, 
and  especially  in  widely  separated,  areas  must  be  beset  with  almost 
insuperable  obstacles. 

Though  in  many  countries  a  complete  break  occurs  between  the  lowest 
gneisses  and  the  overlying  Palaeozoic  sedimentary  formations,  there  are 
other  regions  in  which  these  gneisses  are  intimately  associated  with  schists, 
limestones,  quartzites,  and  conglomerates.  The  real  character  of  this 
association  has  been  variously  interpreted,  but  on  any  explanation,  it  shows 
that  such  gneisses  cannot  be  older  than  certain  crystalline  masses  which  may 
be  regarded  as  probably,  if  not  certainly,  of  sedimentary  origin.  Hence, 
while  the  inference  from  one  series  of  sections  has  been  that  the  gneisses 
belong  to  an  early  condition  of  the  cooling  crust  of  the  globe,  from  another 
series  it  has  been  in  favour  of  these  gneisses  and  their  associated  sediment- 
ary materials  having  been  formed  after  the  crust  was  solidified,  and  after 
mechanical  and  chemical  sediments  had  begun  to  be  accumulated. 

Taking  the  widest  view  of  the  whole  series  of  pre-Palaeozoic  rocks,  with 
their  vast  piles  of  various  sedimentary  formations  above,  and  their  complex 
series  of  crystalline  massive  and  schistose  rocks  below,  we  encounter  a 
somewhat  serious  difficulty  in  the  attempt  to  group  the  whole  of  this 
varied  assemblage  of  mineral  masses  under  some  common  generally  applic- 
able stratigraphical  name.  Such  a  name  has  usually  been  held  to  imply 
that  the  rocks  which  it  designates  belong  to  one  well-defined  portion  of  the 
Geological  Record.  But  this  implication  is  one  which  every  geologist 
who  has  worked  among  these  ancient  rocks  would  earnestly  deprecate,  for 
he  has  in  some  measure  realised  how  vast,  varied,  and  long-continued  were 
the  geological  changes  of  which  they  are  the  memorials.  These  mutations 
include  many  transformations  of  the  earth's  surface,  many  disturbances  of 
its  crust,  with  enormous  denudation  and  sedimentation,  comparable  with,  if 
not  greater  than,  thpse  which  in  later  ages  were  repeated  again  and  again, 
even  after  the  older  fossiliferous  formations  were  laid  down.  So  similar 
have  been  the  results  that  it  is  now  difficult,  or  impossible,  to  discriminate 
between  the  more  ancient  and  the  more  recent  operations.  To  class  all 
the  crystalline  schists  and  the  great  piles  of  sedimentary  and  igneous 
materials  into  which  they  seem  to  pass,  by  one  general  name,  after  the  type 
of  "  Cambrian,"  "  Silurian,"  or  "  Devonian,"  may  be  convenient,  but  in 
the  present  state  of  our  knowledge  is  apt  to  lead  to  confusion,  by  placing 
together  masses  which  may  be  of  widely  different  geological  ages  and  of 

Snc,  Canada.  How  completely  this  artificial  system  breaks  down  when  tested  by  an  appeal 
to  the  rocks  in  the  field  has  been  well  shown  by  R.  D.  Irving,  1th  Ann.  Rep.  U.S,  Geol.  Survey 
(1888).  p.  383, 


684  STEATIGRAPHICAL  GEOLOGY  book  vi 

wholly  dissimilar  origin.  Various  terms  have  been  proposed  for  this  complex 
assemblage  of  rocks,  such  as  Primitive,  Proterozoic,  Azoic,  Agnotozoic  or 
Archaean.  But  from  the  data  adduced  in  Book  IV.  Pait  VIII.  regarding 
regional  metamorphism,  the  student  Avill  understand  how  full  of  uncer- 
tainty must  be  the  geological  age  of  many  areas  of  crystalline  schists. 
Mere  lithological  characters  afford  no  perfectly  reliable  test  of  relatiTe 
antiquity.  To  prove  that  any  region  of  crystalline  schists  may  be 
"Primitive,"  "Azoic,"  Or  "Archaean"  we  must  first  find  these  rocks 
overlain  by  the  oldest  fossiliferous  formations.  Where  no  evidence 
of  this  kind  is  available,  the  use  of  precise  terms,  which  are  meant  to 
denote  a  particular  geological  era,  is  undesirable.  There  seems  good 
reason  to  believe  that  the  asserted  "Archaean"  age  of  many  tracts  of 
schistose  and  granitoid  rocks  rests  on  no  better  basis  than  mere  supposi- 
tion, and  that  as  the  study  of  regional  metamorphism  is  extended,  the 
so-called  "Archaean"  areas  will  be  proportionately  contracted.^ 

Several  distinct  systems  of  mineral  masses  can  Y)e  shown  in  some  regions 
to  exist  beneath  the  base  of  the  Palaeozoic  formations,  differing  so  greatly  iu 
petrological  characters,  in  tectonic  relations,  and  probably  also  in  mode  of 
formation,  that  they  cannot,  without  a  very  unnatural  union,  be  arranged 
in  one  definite  stratigi-aphical  series.  For  the  present  it  seems  to  me 
least  objectionable  to  adopt  some  vague  general  term  which  nevertheleffi 
expresses  the  only  homotaxial  relation  about  which  there  can  be  no  doubt 
For  this  purpose  the  designation  "  Pre-Cambrian,"  already  in  use,  seems 
suitable.  The  rocks  which  I  would  embrace  under  this  epithet  may 
include  a  number  of  separate  systems  or  formations  which  have  little  or 
nothing  in  common,  save  the  fact  that  they  are  all  older  than  the  base  of 
the  Cambrian  rocks.  Until  our  knowledge  of  these  ancient  masses  is 
much  more  extensive  and  precise  than  it  is  at  present  I  think  it  would 
be  of  advantiige  to  avoid  the  adoption  of  any  general  terminolog}'  that 
would  involve  assumptions  as  to  their  definite  place  and  sequence  in  the 
geologiciil  record,  their  mode  of  origin,  their  relation  to  the  history  of 
plant  and  animal  life,  or  their  identification  in  difterent  countries. 

As  an  illustration  of  the  danger  of  such  assumptions,  I  may  refer  to 
the  history  of  the  investigation  of  the  Jjaurentian  rocks  of  Canada.  From 
the  early  observations  of  Sir  W.  Logan  and  Mr.  Alexander  Murray  these 
rocks  came  to  Ik?  regarded  as  types  of  the  oldest  gneisses  of  the  globe. 
They  were  looked  upon  as  probably  metamoiphosed  marine  sediments 
that  had  formed  the  solid  platform  on  which  the  whole  series  of  fossil* 
iferous  systems  of  North  America  had  been  deposited.  The  name  Lau- 
rentian  applied  to  them  was  transferred  to  similar  rock-masses  in  other 
parts  of  the  globe,  and  came;  to  be  accepted  as  the  designation  of  the  oldest 

^  Dr.  Barrois  thus  expresses  himself  on  this  subject  :  "  A  great  number  of  the  rocks  con- 
sidered to  be  Archaean  in  Brittany  are  only  nietamorpliosed  Cambrian  or  Silurian  rock«, 
having  merely  the  facies  of  primitive  rocks.  We  do  not  tliink  that  Brittany  can  be  the  only 
region  where  tliis  is  the  case  ;  on  the  contrary,  it  seems  to  us  probable  that  the  Palfeoxoic 
formations  are  destined  to  spread  more  and  more  over  geological  maps,  at  the  expense  of 
tlie  '  j»rimitive  formations,' by  assuming  gneissic  and  schistose  moditicatioris." — Ann.  Soc. 
Geol.  Xonf.  xi.  (1884).  ]k  139  {antf,  p.  612  ft  seq.) 


PART  I  §  i  PRE-CAMBRIAN  ROCKS  685 

known  zone  in  the  crust  of  the  earth.  But  eventually  it  was  discovered 
by  Mr.  Lawson  that  some  part,  at  least,  of  the  Laurentian  gneiss  is  essen- 
tially of  igneous  not  of  sedimentary  origin,  and  is  actually  intrusive  into 
what  are  undoubtedly  sedimentary  strata.  It  coidd  not,  therefore,  itself 
as  a  whole  be  the  oldest  rock  ;  and  all  the  generalisations  and  identifications 
founded  on  its  supposed  position  fell  to  t^e  ground.  The  term  Laurentian 
cannot  henceforth  have  more  than  a  local  significance.  It  serves  to  designate 
certain  ancient  crystalline  rocks  of  Canada,  but  a  geologist  would  not 
now  employ  it  to  denote  any  of  the  rocks  of  another  region,  even  though 
they  might  present  similar  general  lithological  characters.  We  must  in 
the  meanwhile  be  content  to  restrict  the  application  of  such  names  to  the 
regions  in  which  they  originated.  There  will  be  much  less  impediment 
to  the  progress  of  investigation  by  the  multiplication  of  local  names  than 
by  the  attempt  to  force  identifications  for  which  there  is  no  satisfactory 
basis.  Each  country  will  have  its  own  terminology  for  pre -Cambrian 
formations,  until  some  way  is  discovered  of  correlating  these  formations 
in  different  parts  of  the  globe. 

Although  where  the  stratigraphical  succession  is  most  complete  the 
gneisses  that  rise  from  under  the  oldest  sedimentary  rocks  have  been 
found  to  pierce  these  rocks,  and  thus  to  be  of  later  date ;  yet  in  most 
regions  no  such  proof  of  posteriority  is  to  be  seen.  The  coarse  banded 
gneisses  are  usually  the  foundations  on  which  the  stratified  fossiliferous 
formations  unconformably  rest.  There  is  thus  an  obvious  advantage  in 
treating  these  gneisses  first  in  an  account  of  pre-Cambrian  rocks.  I  shall 
here  follow  this  arrangement,  and  reserve  for  a  later  section  a  description 
of  the  sedimentary  and  igneous  formations  which  intervene  between  the 
gneisses  and  the  base  of  the  Cambrian  system. 

1.   The  lowest  gneisses  and  schists. 

It  has  often  l>een  remarked  that  one  of  the  most  singular  features 
about  the  oldest  known  crystalline  rocks  is  the  sameness  of  their  general 
mineral  characters  in  all  parts  of  the  earth.  Sedimentary  formations 
constantly  vary  from  country  to  country,  but  when  we  descend  beneath 
their  lowest  members  we  come  upon  a  wholly  different  group  of  rocks 
which  retain  vrith  remarkable  uniformity  one  general  type  of  structure 
and  composition.  These  rocks  include  massive  materials  such  as  granite, 
syenite,  gabbro,  diorite,  and  hornblende-rock.  But  even  in  these  a  tend- 
ency to  a  schistose  arrangment  can  usually  l>e  olwerved.  By  far  the 
most  generally  prevalent  structure  is  a  more  or  less  definite  foliation.  In 
the  coarse  varieties  it  is  marked  by  alternate  bands  of  distinct  mineral 
characters,  orthoclase,  plagioclase  (commonly  an  acid  variety),  quartz, 
hornblende  and  mica  (white  and  black)  being  universally  conspicuous. 
Such  rudely  foliated  rocks- are  known  as  coarsely-banded  gneisses,  and 
offer  gradations  into  masses  which  cannot  be  distinguished  from  ordinary 
eniptive  material.  The  banding  is  sometimes  strongly  marked  by  the 
separation  of  the  more  silicated  from  the  less  silicated  minerals,  as  where 
layers  of  felspar  or  of  quartz  alternate  with  others  of  hornblende, 
pyroxene  or  biotite. 


686  STRATIGRAPHICAL  GEOLOGY  book  ti 


While  the  foliated  stiiieture  and  the  arrangement  of  the  minerals  in 
})arallcl  bands  gives  a  bedded  aspect  to  these  rocks,  the  resemblance  of 
this  structure  to  the  true  Injdding  of  detrital  materials  is  probably  more 
appai'ent  than  real.     A  little  examination  shows  that  the  layers  are  uot 
persistent,  that  they  cross  each  other,  and  that  portions  of  one  may  be 
entirely  seimrated  and  enclosed  within  another.     Whatever  may   have 
been  their  origin  they  have  certaiidy  undergone  enormous  mechanical 
compression  and  deformation.     They  have  been  plicated,  rolled  out,  dis- 
located, and  crumpled  again  and  again.     Hence,  though  for  short  distances 
it  is  possible  to  8e|>arate  out  layers  or  bosses  of  felspathic,  homblendic, 
pyroxcnic,  peridotitic,  or  serpentinous  comjK)sition  from  the  general  body  of 
gneiss,  the  geologist  who  tries  to  fix  definite  stmtigraphical  horizons  by  this 
means  soon  abandons  the  attempt  in  desjmir,  and  conies  to  the  concludon 
that  no  seciuence  of  a  trustworthy  nature  can  be  established  in  the  body 
of  the  gneiss  itself. 

From  the  coarsest  gneisses  gi-adations  may  be  traced  to  fine  silky  schists ; 
and  this  not  only  on  a  laige  scale  in  tracts  capable  of  being  delineated  on 
a  map,  ])ut  on  so  small  a  scale  as  to  be  illustrated  even  in  hand-specimens. 
Such  transitions  seem  to  arise  from  the  ditt'erent  effects  of  mechanical  de- 
formation on  materials  that  offered  considerable  differences  in  lithological 
composition  and  structure.  Fine  talcose  schists,  for  example,  can  be  traced 
to  original  peridotites ;  hornblendic  and  actinolitic  schisU  to  such  rocks 
as  gabbro,  diorite,  or  dolerite. 

In  the  older  accounts  of  these  rocks  the  gneisses  are  described  as  pass- 
ing into  or  alternating  with  a  wholly  different  type  of  rocks,  among  which 
may  be  included  limestone  (sometimes  strongly  graphitic),  dolomite, 
(jUiirtzite,  graphite -schist,  mica -schist,  and  other  varieties  of  schistose 
material.  This  ai)parent  gradation  was  believed  to  mark  an  original 
transition  of  the  sedinuMit  out  of  which  the  gneiss  was  thought  to  have 
l>een  formed  into  the  calcareous,  argillaceous,  or  carbonaceous  sediment, 
which  was  the  earliest  condition  of  the  associated  limestones  and  schists 
It  was  thus  looked  upon  as  evidence  that  the  whole  crystalline  series 
represented,  in  a  metamorphosed  state,  an  ancient  accumulation  of  sedi- 
mentary materials.  The  existence  even  of  organic  remains  in  the  lime- 
stone was  insisted  upon,  and  the  so-called  Eoziton  was  cited  as  the  most 
ancient  relic  of  animal  life.^  But  there  is  now  everv  reason  to  believe 
such  gradations  to  be  generally  deceptive.  As  a  result  of  the  enormoos 
mechanical  compression  and  defonnation  which  these  ancient  rocks  have 
undergone,  igneous  and  aqueous  materials  have  been  so  plicated  and  crushed 
together  and  have  undergone  such  j)rofound  metamorpliism,  that  it  is 
sometimes  hardly  possible  to  trace  a  Ijoundary  between  them.  There 
seems  no  reason  to  look  upon  the  limestones,  argillitcs,  quartzites, 
and  schists  as  other  than  intenselv  altered  sediments,  which  in  theory, 
if  not  in  actual  practice  on  the  ground,  must  be  separated  from  the 
gneisses. 

Among  the  various  theories  which  have  been  pro]X)sed  to  account  for 
the  genesis  of  the  lowest  gneisses  and  schists,  three  deserve  particular  men- 

^  See  on  tliis  subject  ^os^tfrt,  p.  C94,  and  authorities  there  cited. 


PART  I  §  i  PRE'GAMBRIAN  ROCKS  687 

tion  here.  ( 1 )  These  rocks  are  by  some  geologists  believed  to  be  a  portion 
of  the  original  crust  which  solidified  on  the  surface  of  the  globe.  (2)  They 
are  by  others  held  to  be  ancient  sedimentary  rocks  in  a  metamorphosed 
condition,  and  in  some  parts  so  changed  as  to  have  been  actually  melted 
and  converted  into  intrusive  material.  (3)  They  are  believed  by  yet 
another  class  of  observers  to  be  essentially  eruptive  rocks,  and  to  be  com- 
parable with  the  deeper  seated  or  plutonic  portions  of  such  igneous  rocks 
as  may  be  seen  to  traverse  the  earth's  cnist. 

(1)  From  the  ubiquity  of  their  appearance,  the  persistence  of  their 
striking  lithological  characters,  and  especially  the  curious  apparent  blend- 
ing in  them  of  the  igneous  and  sedimentary  types  of  structiu*e,  the  idea 
not  unnaturally  arose  that  the  lowest  crystalline  rocks  represent  the  first 
crust  that  formed  on  the  surface  of  the  globe.  ^  These  rocks  have  been  sup- 
posed to  include  some  of  the  early  surfaces  of  consolidation  of  the  molten 
globe,  and  some  of  the  first  sediments  that  were  thrown  down  from  the 
hot  ocean  which  eventually  condensed  upon  the  planet.  Such  a  specida- 
tive  view  of  their  origin  may  seem  not  incredible  in  regions  where  these 
ancient  crystalline  rocks  are  covered  unconformably  by  the  oldest 
Palaeozoic  formations,  from  which  they  are  marked  off  by  so  striking  a 
contrast  of  structure  and  composition,  and  to  which  they  have  contributed 
80  vast  an  amount  of  detrital  material.  But  it  must  be  tested  by  the 
eWdence  of  the  rocks  themselves,  not  only  where  the  geological  record  is 
confessedly  incomplete,  but  where  it  is  comparatively  full.  Nowhere 
among  the  lowest  gneisses  is  any  structure  observable  which  can  be  com- 
pared with  the  superficial  portion  of  a  lava  that  cooled  at  the  surface. 
On  the  contrary  the  analogies  they  fiu'nish  are  with  deep-seated  and 
slowly-cooled  sills  and  bosses.  The  supposed  intercalation  and  alterna- 
tion of  limestone  and  other  presumably  sedimentary  materials  in  the  old 
gneisses  are  probably  all  deceptive.  In  some  regions  they  can  be  shown 
to  be  so,  and  it  can  there  be  demonstrated  that  the  gneisses  are  really 
eruptive  rocks  which  pierce  the  adjacent  sedimentary  or  schistose  masses, 
and  are  thus  of  younger  age  than  these.  If  this  relation  can  be  clearly 
established  in  regions  where  the  evidence  is  fullest,  it  is  obviously  safe  to 
infer  that  a  similar  relation  might  be  discoverable  if  the  geological  record 
were  more  complete,  even  in  those  parts  of  the  world  where  the  break 
between  the  lowest  gneisses  and  the  Palaeozoic  formations  seems  to  be 
most  pronounced.  At  least  the  possibility  that  such  may  be  the  case 
should  put  us  on  our  guard  against  adopting  any  crude  speculation  about 
the  original  crust  of  the  earth. 

The  present  condition  of  these  ancient  rocks  difters  much  from  that 
which  they  originally  possessed.  In  particular  they  have  undergone 
enormous  mechanical  deformation,  have  been  to  a  large  extent  crushed 
and  re-crystallized,  and  have  acquired  a  marked  schistose  structure.  But 
in  every  large  region  where  they  are  developed  we  may  obtain  evidence 
to  connect  them  with  plutonic  intrusions,  not  with  superficial  consolidation, 
and   to  show  that  many  of  their  essential  details  of  stiiicture  may  be 

*  See  Credner's  "Geologie,"  vi.   b.   Die  Fundamental    Formation;   ErstammgskriLste. 
Compare  also  Rosenbusch,  Xeues  Jahrb.  1889,  vol.  ii.  p.  81. 


(588  f^TRATIGRAPHICAL  GEOKJGY  book  vi 

})arallelcd  among  much  later  crystalline  schists  produced  from  the  meta- 
morphism  of  Palaeozoic  sediments  and  igneous  rocks. 

(2)  That  the  lowest  gneisses  of  Canada  and  other  regions  are  meta- 
morphosed sedimentary  rocks  was  believed  by  probably  most  geologists 
until  only  a  few  years  ago.     But  the  increased  attention  which  has  beoi 
given  to  the  study  of  the  subject  since  Professor  Lehmann's  great  work 
on  the  Saxon  gneisses  appeared  in  1884,  has  led  to  so  complete  a  revolu- 
tion of  opinion  that  this  belief,  at  least  in  its  original  form,  is  now  almost 
wholly  almndoned.     Those  who  still  hold  it  in  a  modified  shai)e  reoognise 
that  the  original  sediments  must  have  diiTered  considerably  from  those  of 
any  recognisably  sedimentary  formation,  and  were  prol>ably   deposited 
under  })eculiar  conditions.     They  admit  that  these  rocks  have  undeigone 
extreme  metamorphism,  and  that  the  alteration  of  them  has  been  carried 
so  far  as  to  reduce  them  in  some  places  to  an  amorphous  crystalline  con- 
dition   which    cannot   be  distinguished   from   that   of   normal    eruptive 
material.     It  has  been  maintained,  indeed,  that  the  Laiurentian  gneiaaes 
of  Canada  have  been  produced  by  the  actual  fusion  of  the  older  sedi- 
mentary prc-Cambrian  formations  and  the  absorption  of  these  rocks  into 
the  general  magma  of  eruptive  material  which  now  appears  as  gneiaB.^ 
The  intrusive  character  of  some  of  the  gneiss,  which  might  be  regarded 
as  proof  of  its  really  igneous  origin,  is  accounted  for  by  what  is  called 
an    "  acjuo- igneous   fusion"    of    some    jmrts  of    the    sedimentary  rocks 
and  their  intrusion  into  less  completely  metamorphosed  {)ortion8  of  the 
series. 

(3)  Probably  the  gi*eat  majority  of  geologists  now  adopt  in  some  form 
the  third  opinion,  that  the  oldest  or  so-ciilled  **  Archaean  "  gneisses  are  essen- 
tially eruptive  rocks,  and  that  they  should  l^e  compared  with  the  larger 
and  more  deeply-seated  bosses  of  intrusive  material  now  visible  on  the 
earth's  surface.  Whether  they  were  portions  of  an  original  molten  magma 
pi-otruded  from  beneath  the  crust,  or  were  produced  liy  a  re-fusion  of  already 
solidified  parts  of  that  crust  or  of  ancient  sedimentary  accumulations  laid 
down  upon  it,  must  be  matter  of  speculatiiHi.  In  the  gathering  of  actual 
fact  we  cannot  go  beyond  their  chanicter  as  eruj)tive  rocks,  which  is  the 
earliest  condition  to  which  they  can  be  traced,  and  we  must  consequently 
place  them  in  the  same  gi*eat  series  as  all  the  later  eruptive  materials 
with  which  geology  has  to  deal.  It  is  quite  true  that  they  have  been 
j)r()foun(lly  mollified  since  their  original  extrusion,  but  traces  of  their 
original  character  as  masses  of  mobile,  slowly  crystallizing  and  segregat- 
ing material  have  not  been  entirely  effaced. 

Looking  at  the  gneisses  as  a  whole,  ^\'ith  their  various  accompaniments, 
we  find  them  to  fonii  a  complex  assemblage  of  crystalline  rocks  which, 
though  generally  presenting  a  foliated  structure,  })Jiss  occasionally  into  the 
amorphous  condition  of  ordinary  eniptive  rocks.  In  composition  they 
range  from  granite  at  the  one  end  to  peridotites  and  serpentines  at  the 
other.  Hand-specimens  of  these  rocks  in  their  amorphous  or  unfoliated 
condition  do  not  differ  in  anv  essential  feature  from  the  material  of 
ordinary  intrusive  bosses  in  later  portions  of  the  terrestrial  crusty  and 

^  A.  C.  Lawson.  Annual  Report  Canadian  iitol.  Surv.  1887. 


PART  I  §  i  PRE-CAMBRIAN  ROCKS  689 

the  same  similarity  of  stnicture  is  borne  out  when  thin  slices  are  placed 
under  the  microscope. 

Perhaps  the  most  convincing  proof  of  the  really  eruptive  nature  of 
the  gneisses  is  to  be  found  in  those  tracts  where  they  have  undergone 
least  disturbance,  and  where  therefore  the  way  in  which  they  traverse 
the  adjacent  rocks  can  be  distinctly  perceived.  They  are  there  seen  to 
cross  many  successive  zones  of  sedimentary  material,  to  send  out  veins 
and  protrusions,  and  to  enclose  portions  of  the  adjacent  rocks,  while 
at  the  same  time  the  surrounding  masses  present  many  of  the  familiar 
features  of  contiict-metamorphism.  Sections  where  these  phenomena 
can  be  satisfactorily  observed  are  no  doubt  comparatively  rare,  for  in 
general  the  rocks  have  been  so  crushed  and  re-crystallized  that  their 
original  relations  have  been  destroyed.  It  is  in  consequence  of  these 
subse(|uent  movements  that  so  much  difficulty  has  l)een  found  in  de- 
termining the  igneous  nature  of  the  gneisses  and  their  intrusive  character 
with  reference  to  the  rocks  adjacent  to  them.  The  abundant  veins  which, 
as  in  ordinary  granite  bosses,  proceeded  from  the  original  gneiss  have 
been  compressed  into  long  parallel  bands  which  seem  to  alternate  with 
the  schists  among  which  they  were  injected,  while  portions  of  the  sur- 
rounding rock  enclosed  within  the  gneiss  have  had  a  foliation  super- 
induced upon  them  parallel  to  that  of  these  Imnds.  Any  one  who  first 
studied  the  older  rocks  where  such  structures  are  >'i8ible  might  easily  be 
deceived  into  the  belief  that  these  alternations  of  parallel  strips  of  gneiss 
and  schist,  or  gneiss  and  limestone,  really  represented  a  continuous  sequence 
of  sedimentary  material.  Nor  would  he  readily  perceive  his  mistake  until 
he  could  trace  the  junction-line  into  some  tract  where,  by  cessation  of  the 
deformation,  the  original  relation  of  the  two  gioups  of  rocks  could  be 
observed.  ^ 

It  is  not  difficult  to  obtain  conclusive  proof  that  in  the  complex  assem- 
blage of  rocks  constituting  the  lowest  gneiss  there  are  not  only  differences 
of  composition  and  structure,  but  also  differences  of  relative  age.  Some 
portions  of  the  series  can  be  distinctly  seen  to  have  been  intnided  into 
others.  True  dykes  can  be  traced  among  them  ]x)th  of  acid  and  basic 
composition.  In  the  north-west  of  Scotland,  for  example,  the  general 
body  of  gneiss  is  traversed  by  a  multiplicity  of  dykes,  cutting  across  the 
oldest  foliation  of  the  gneiss  in  a  general  north-westerly  direction.  A 
detailed  study  of  such  an  area  reveals  the  fact  that  the  fundamental 
rocks  represent  a  prolonged  series  of  igneous  protrusions.  As  this  com- 
plicated mass  of  eniptive  material  has  subsequently  undergone  profound 
alteration  by  dynamo -metamorphism,  the  difficulties  in  unravelling  its 
history  need  cause  no  surprise. 

Leaving  out  of  account  the  dykes  which  undoubtedly  mark  later 
injections  of  igneous  material,  and  confining  our  attention  to  the  general 
mass  of  gneiss  in  its  variations  from  an  amorphous  or  granitoid  condition 
through  the  coarse  banded  varieties  to  the  finer  schistose  types,  we  may 
pui*sue  the  history  of  these  puzzling  rocks  by  comparing  them  with  the 
larger  intrusive    bosses   and    sills  that  have   accompanied  the  volcanic 

*  See  A.  C.  Lawson,  Bufi.  Geol.  Soc.  Amer.  I  (1890)  p.  184. 

2  Y 


690  STRATIGRAPHICAL  GEOLOGY  book  yi 

eruptions  of  all  geological  periods.  In  these  deep-seated  and  slowly  cooled 
masses  of  igneous  material,  as  has  already  been  pointed  out  (p.  580),  we 
may  frequently  observe  eWdence  of  the  segregation  of  the  component 
minerals  in  bands  or  irregular  patches.  Such  a  segregation  seems  to  haTe 
taken  place  sometimes  after  the  erupted  rock  had  come  to  rest,  sometinies 
while  it  was  still  in  movement.  In  the  latter  case  the  layers  of  separated 
materials  may  sometimes  have  been  dragged  forward  so  as  to  acquire  a  some- 
what banded  or  streaky  structure.  How  far  the  characteristic  arrange- 
ments of  the  minerals  in  the  coarse  banded  gneisses  may  have  arisen  from 
a  process  of  this  kind  in  the  consolidation  of  originally  eruptive  materials, 
remains  still  an  open  question,  though  the  progress  of  research  favours 
the  idea  that  such  has  really  been  to  a  large  extent  their  source.^ 

It  is  certain,  however,  that,  besides  the  original  banded  and  probably 
segregated  structure,  the  gneisses,  as  the  result  of  much  mechanical  defonna- 
tion,  have  had  other  and  later  structures  superinduced  upon  them,  some- 
times at  successive  periods  of  disturbance.  The  most  massive  granitoid 
rocks  have  thus  been  crushed  down  under  great  strain,  and  have  re-crystal- 
lized as  fine  granulitic  gneiss  or  mica-schist.  Epidiorites  and  amphiboUtes 
have  by  a  similar  process  been  converted  into  hornblende-schists.  In  these 
cases  the  reconstructed  rocks  usually  exhibit  a  finely  schistose  structure, 
quite  distinct  from  that  of  the  parent  mass,  but  with  no  markedly  banded 
arrangement  Occasionally,  however,  in  the  re -crystallization  of  the 
materials,  segregation  into  more  or  less  definite  layers  or  centres  has  come 
into  play,  so  that  in  this  obviously  secondary  arrangement  a  certain  re- 
semblance may  be  traced,  though  on  a  small  scale,  to  the  much  coarser 
bands  in  the  earliest  remaining  condition  of  the  oldest  gneisses. 

There  is  yet  another  source  of  difficulty  in  judging  of  the  relative 
age  and  origin  of  various  structures  among  the  crystalline  schists. 
As  has  already  been  pointed  out  (p.  604),  it  is  now  well  established 
that  granite,  besides  l^reaking  through  the  old  rocks  and  forming 
huge  bosses,  as  well  as  abimdant  veins  among  them,  has  sometimes 
been  introduced  into  their  substance  in  such  a  way  that  they  seem  to 
be  penneated  by  the  granitic  material.  Minute  layers  and  lenticles 
of  this  material,  quite  uncrushed,  may  l>e  traced  between  the  folia- 
tion planes  of  granulitic  gneisses  and  different  schists.  But  where 
subsequent  movement  has  crushed  and  drawn  out  these  intei*calated 
layers,  younger  gneiss  is  produced  that  simulates  Avith  extraordinary 
closeness  some  aspects  of  the  most  ancient  and,  so  to  say,  original 
gneisses.-     This  transformation  apjjears  to  take  place  even  among  schists 

^  This  iuference  applies  more  particularly  to  the  coarsely  banded  gneisses  where  the 
individual  layers,  consisting  in  great  part  of  different  minerals,  resemble  some  of  the  segre- 
gation bands  in  eruptive  masses  (]>.  615).  Tliere  can  be  little  doubt  that,  as  already  re- 
marked, the  efficacy  of  mechanical  deformation  as  a  factor  in  the  production  of  gneisses  hu 
been  pushed  too  far.  It  will  account  for  the  crushed  granulitic  and  schistose  condition,  bnt 
hardly  for  the  coarsely  banded  stnicture,  where  the  layers  consist  of  very  different  minenl 
aggregates. 

^  See  observations  of  J.  Home  in  Oeolngical  Survey  Report,  Report  of  the  Science  and 
Art  Department  for  1892. 


PART  I  §  i  PRE'CAMBRIAN  ROCKS  691 

that  can  be  shown  to  have  been  originally  sedimentary  rocks.  So  that 
by  a  new  pathway  of  inquiry  we  are  brought  once  more  to  the  old 
doctrine  of  the  cycle  of  change  through  which  the  materials  of  the  earth's 
crust  pass.  The  most  ancient  gneisses  exposed  to  disintegration  on  the 
earth's  surface  have  furnished  materials  for  the  formation  of  sedimentary 
deposits,  which,  after  being  deeply  buried  within  the  earth's  crust,  crushed, 
plicated,  and  permeated  with  granitic  matenal,  present  once  more  the 
aspect  of  the  old  gneisses  from  which  they  were  in  the  first  instance 
derived. 

It  is  only  when  the  complex  tectonic  relations  of  the  several  masses 
composing  the  oldest  crystalline  rocks  are  closely  studied  that  we  can 
adequately  realise  how  hopeless  would  be  the  attempt  to  establish  any- 
thing of  the  nature  of  a  stratigraphical  sequence  among  them.  Where 
different  eruptive  materials  present  proofs  of  successive  intrusion,  we  have 
indeed  a  clue  to  their  relative  age ;  but  such  evidence  carries  us  but  a 
small  way.  The  gneisses  where  obviously  intrusive  are  indisputably  of 
eruptive  origin,  but  they  alternate  with  finely  schistose  bands  which  some- 
times seem  to  cut  them.  The  bedding  or  banding  of  the  rocks  afibrds  no 
guide  whatever  as  to  sequence.  It  has  been  so  folded  and  crumpled  that 
even  if  it  represented  original  stratification  it  could  probably  never  be 
unravelled.  But  there  is  every  reason  to  believe  that  it  bears  no  real 
analogy  to  stratification.  It  may  sometimes  represent,  as  already  stated, 
layers  of  segregation  and  fiow-structure  in  an  original  igneous  magma,  at 
other  times  pUnes  of  movement  in  the  crushing  of  already  consolidated 
material.  But  whatever  may  have  been  its  origin,  it  remains  now  in  an  in- 
extricable complexity.  Here  and  there,  indeed,  for  short  distances  some 
well-marked  band  of  rock  may  be  traced,  but  the  various  rock-masses 
generally  succeed  each  other  in  so  rapid  and  tumultuous  a  manner  as  to 
defy  the  efforts  of  the  field-geologist  who  would  patiently  map  them. 

As  a  rule,  only  where  the  earliest  type  of  gneiss  has  been  invaded 
by  subsequently  intruded  masses  can  a  successful  attempt  be  made 
to  disentangle  the  confused  structure.  Successive  systems  of  dykes 
may  thus  be  traced,  and  evidence  may  be  obtained  that  powerful  dynamic 
stresses  affected  the  rocks  between  some  of  these  intrusions.  The  dykes 
have  sometimes  been  crushed,  plicated,  and  disrupted  until  they  have  been 
reduced  to  isolated  patches  of  schist  irregularly  distributed  among  the 
reconstructed  gneiss.  And  through  these  involved  and  complicated 
masses  newer  groups  of  dykes  have  risen,  to  be  again  subjected  to 
mechanical  deformation. 

The  question  may  occur  to  the  student  whether  this  complex  system 
of  evidently  plutonic  igneous  rocks  was  ever  connected  with  any  super- 
ficial volcanic  activity.  No  such  connection  has  yet  been  definitely 
ascertained,  but  it  may  be  regarded  as  highly  probable.  If  the  most 
ancient  gneisses  with  their  dykes  and  bosses  were  the  deep-seated  portions 
of  the  successive  uprisings  of  the  igneous  magma  which  culminated  in 
volcanic  eruptions,  we  may  hope  eventually  to  discover  some  trace  of 
the  materials  that  were  thrown  out  to  the  surface  and  accumulated  there. 
In  some  of  the  overlying  pre -Cambrian  masses  of  sedimentary  rocks 


692  HTRATIGRAPHICAL  GEOLOGY  book  ti 

abundant  Ijiva^,  tuffs,  and  agglomerates  have  been  found,  indicating  the 
out[)ounng  of  volcanic  material  at  the  surface  during  the  dejxwition  of 
these  sediments.  The  vast  scale  of  these  volcanic  eruptions  may  be 
inferred  from  the  fact  that  in  the  I^ke  Superior  region  the  accumnlated 
materials  discharged  at  the  surface  attained  a  thickness  which  has  been 
estimated  at  more  than  six  and  a  half  miles.  It  may  be  eventuaUy  dis* 
covered  that  some  of  these  su}>erficial  manifestations  of  volcanic  action 
have  been  connected  with  bosses,  sills,  or  dykes  that  form  part  of  the  body 
of  the  gneiss  below. 

It  nmst  be  confessed  that  nuich  detailed  work  among  the  lower  gneisses 
in  all  parts  of  the  world  is  needed  before  the  many  problems  which  they 
present  are  solved.  But  the  following  conclusions  regarding  them  may  now 
be  regarded  as  certain : — these  rocks  are  in  the  main  various  forms  of 
original  eruptive  material,  ranging  from  highly  acid  to  highly  basic  ;  they 
form  in  general  a  conlplex  mass  Ixjlonging  to  successive  i>eriods  of  ex- 
trusion ;  some  of  their  coarse  structures  are  probably  due  to  a  process  of 
segregation  in  still  fluid  or  mobile,  probably  molten,  material  consolidat- 
ing Ixilow  the  surface  ;  their  gi-anulitized  and  schistose  characters,  and  their 
folded  and  cnimpled  structures  point  to  subsequent  intense  crushing  and 
deformation  ;  their  apparent  alternations  with  limestone  and  other  rocks, 
which  are  proliably  of  sedimentary  origin,  are  deceptive,  indicating  no 
real  continuity  uf  formation,  but  pointing  to  the  intrusive  nature  of  the 


gneiss. 


2.  Pre-Camhrian  sedimentarjf  find  rolaiuic  groups. 

In  ditierent  ])arts  of  the  worhl  enormous  masses  of  rock  are  now 
known  to  intervene  between  the  oldest  or  "  Archaean  "  gneisses,  and  the 
bottom  of  the  fossiliferous  series  of  formations.  It  was  in  Canada  that 
these  rocks  were  first  studied.  Logan  and  Murray  grouped  them  under 
the  general  name  of  Huronian,  and  they  were  believed  to  till  up  the  gap 
between  the  Liurentian  gneiss  on  the  one  hand,  and  the  Potsdam  saml- 
stone  or  base  of  the  fossiliferous  series  on  the  other.  Later  more  detailetl 
study  of  these  rocks  in  Canada  and  the  a<ljoining  regions  of  the  United 
States  has  shown  them  to  possess  even  a  greater  imiwrtance  than  their 
original  discoverers  imagined,  for  they  have  been  found  to  consist  of 
several  distinct  groups  or  systems,  att«iining  a  vast  thickness  and  present- 
ing a  lecord  of  stuj)endous  disturbances,  denudations  and  de|X)sitions  of 
sediment,  t<jgether  with  memorials  of  extensive  and  prolonged  volcanic 
action.  In  the  higher  nu?mbers  of  these  sedimentiiry  deposits,  distinct 
remains  of  animal  life  have  in  several  regions  been  found.  There  is 
thus  opened  out  the  possibility  of  the  ultimate  discovery  of  a  series  of 
fossiliferous  formations  even  below  the  base  of  the  Palaeozoic  series. 

Where  metamorphism  has  not  interfered  with  the  recognition  of  their 
original  characters,  these  ancient  sedimentary  rocks  present  no  structural 
feature  to  distinguish  them  from  the  detrital  accimiulations  of  higher  parts 
of  the  geological  record.  They  consist  of  clays  and  muds  hanlened  into 
shales  and  slates,  of  siinds  compacted  into  sandstones  and  quartzites,  of 


PART  I  §  i  PRE-C AMEBIAN  ROCKS  693 

gravels  and  shingles  solidified  into  conglomerates.  These  rocks  prove 
beyond  (juestion  that  the  processes  of  denudation  and  deposition  were 
already  in  full  operation  with  results  exactly  comparable  to  those  of 
Palaeozoic  and  later  time. 

Few  parts  of  the  stratified  crust  of  the  earth  present  greater  interest 
than  these  earliest  remaining  sediments.  As  the  geologist  lingers  among 
them,  fascinated  by  their  antiquity  and  by  the  stubbornness  with  which 
they  have  shrouded  their  secrets  from  his  anxious  scmtiny,  he  can  some- 
times scarcely  believe  that  they  belong  to  so  remote  a  part  of  the  earth's 
history  as  they  can  be  assuredly  proved  to  do.  The  shales  are  often  not 
more  venerable  in  appearance  than  those  of  Cambrian  or  Silurian  time,  and 
show  as  clearly  as  these  do  their  alternations  of  finer  and  coarser  sediment. 
The  sandstones  display  their  false-bedding  as  distinctly  as  any  younger 
rock,  and  one  can  make  out  the  shifting  character  of  the  currents  and 
the  prevalent  direction  from  which  they  brought  the  sand.  The  con- 
glomerates in  their  well-rounded  fragments  tell  as  distinctly  as  the  shingle 
of  a  modern  beach  of  the  waste  of  a  land-surface  and  the  pounding  action 
of  waves  along  a  shore. 

Not  only  are  these  structural  details  precisely  similar  to  those  of 
younger  detrital  rocks,  but  we  may  here  and  there  detect  the  remains  of 
the  pre-Cambrian  topography  from  which  these  primeval  sediments  were 
derived,  and  on  which  they  were  deposited.  Hills  and  valleys,  lines  of 
cliff  and  crag,  rocky  slopes  and  undulating  hollows  have  ])een  revealed 
by  the  slow  denudation  of  the  pre-Cambrian  strata  under  which  these 
features  were  gradually  buried.  To  this  day  so  marvellously  has  this 
early  land -surface  l)een  preserved  under  its  mantle  of  sediment  during 
the  long  course  of  geological  time,  that  even  yet  we  may  trace  its 
successive  shore-lines  as  it  gradually  settled  down  beneath  the  waters  in 
which  its  detritus  gathered.  We  may  follow  its  promontories  and  l>ays 
and  mark  how  one  by  one  the}'  were  finally  submerged  and  entombed 
beneath  their  own  waste.  ^ 

But  these  ancient  stratified  formations  do  not  consist  merely  of 
clastic  sediments.  They  include  important  masses  of  limestone  and 
dolomite,  sometimes  highly  crystalline,  but  elsewhere  assuming  much  of 
the  aspect  of  ordinary  grey  compact  Palaeozoic  limestone.  Sometimes 
they  contain  a  considerable  amount  of  graphite,  and  some  of  the  shales 
are  highly  carbonaceous.  In  other  places  they  are  banded  with  layers 
and  seams  or  nodules  of  chert,  in  a  manner  closely  similar  to  that  in  which 
the  Carboniferous  Limestone  of  Western  Europe  contains  its  siliceous 
material.  Sometimes  the  chert  bands  are  as  much  as  forty-five  feet  thick. 
The  general  character  of  these  mingled  carbonaceous,  calcareous  and 
siliceous  masses  at  once  reminds  the  observer  of  rocks  which  have 
undoubtedly  been  formed  by  the  agency  of  organic  life.  Moreover  there 
occur  extensive  deposits  of  iron-carbonate  associated  like  the  limestone 
with  chert,  and  again  recalling  the  results  of  the  co-oi>eration  of  plant 

*  These  features  are  admirably  displayed  in  Ross-sliire,  N.W.  Scotland,  where  the 
Lewisian  gneiss,  carved  into  hills  and  valleys,  has  been  buried  under  the  Torridon  Sand- 
stone {poatea,  p.  705). 


694  STRATIGRAPHICAL  GEOLOGY  book  vi 

and  animal  life.     The  large  amount  of  carbon  in  some  of  the  shales, 
points  likewise  in  the  same  direction. 

It  must  be  confessed,  however,  that  actual  traces  of  recognisable 
organic  forms  have  only  been  found  in  a  few  places.  Various  more  or 
less  determinable  patelloid  or  discinoid  shells,  fragments  of  what  appear 
to  have  been  trilobites  (like  Olendlus,  Olenoides  or  Paradoxides),  small  and 
rather  obscure  forms  like  Hyolithes^  and  others  like  StronuUop&ra^  indicate 
a  low  fauna  somewhat  like  that  of  the  Cambrian  system  above.  ^  Most  of 
these  fossils  have  been  detected  by  Mr.  Walcott  below  the  Olendlus  zone 
or  base  of  the  Cambrian  rocks  in  the  Grand  Canon  of  the  Colorado.  In 
the  Animikie  district  of  Lake  Superior,  fossil  tracks  and  shells  like  lAngvda^ 
and  some  obscure  forms  like  trilobites,  have  also  been  met  with.  More 
recently*  Dr.  Barrois  has  traced  a  band  of  graphitic  quartzite  for  a  long 
way  in  the  gneiss  of  Brittany,  and  has  detected  in  it  the  presence  (^ 
radiolarians,  belonging  to  their  most  primitive  group,  the  Monosphseridas.' 

lieference  may  be  made  here  to  the  controversy  regarding  the  tine 
nature  of  certain  curious  aggregates  of  calcite  and  serpentine,  which  were 
found  many  years  ago  in  some  of  the  limestones  associated  with  the  lower 
or  Laurentian  gneisses  of  Canada.  These  minerals  were  found  to  be 
arranged  in  alternate  layers,  the  calcite  forming  the  main  framework  of 
the  substance,  with  the  serpentine  (sometimes  loganite,  pyroxene,  &c) 
dis|X)sed  in  thin,  wavy,  inconstant  layers,  as  if  filling  up  flattened  cavities 
in  the  calcareous  mass.  So  different  from  any  ordinary  mineral  segrega- 
tion with  which  he  was  acquainted  did  this  arrangement  appear  to  Logan, 
that  he  was  led  to  regard  the  substance  as  probably  of  organic  origin.^ 
This  opinion  was  adopted,  and  the  structure  of  the  supposed  fossil  was 
worked  out  in  detail  by  Sir  J.  W.  Dawson  of  Montreal,*  who  pronounced 
the  organism  to  be  the  remains  of  a  massive  foraminifer  which  he 
called  Eozoon,  and  which  he  believed  must  have  grown  in  large  thick 
sheets  over  the  sea-bottom.  This  view  was  likewise  adopted  by  the  late 
Dr.  W.  B.  Carpenter,^  who,  from  additional  and  better  specimens, 
described  a  system  of  internal  canals  having  the  characters  of  those  in 
true  foraminiferal  structures.  Other  observers,  however,  notably  Pro- 
fessors King  and  Rowney  of  Galway,*^  maintained  that  the  "canal- 
system  "  is  not  of  organic  but  of  mineral  origin,  having  arisen  in  many 
cases  "from  the  wasting  action  of  carbonated  solutions  on  clotules  of 
*  flocculite '  or,  it  may  be,  saponite — a  disintegrated  variety  of  serpentine, 
and  in  others  from  a  similar  action  on  crystalloids  of  malacolite.     In  both 

1  C.  D.  Walcott,  IQthAnn.  Rep.  U.S.  Geoi.  Surv.  1890,  p.  552. 
-  Campt.  rend.  8th  August  1892. 

^  Rep.  Oed.  Surv.  Caiuula^  1858.  Amer.  Jourii.  Sci.  xxxvii.  (1864),  p.  272.  Q.  /. 
(f'eoi.  Soc.  xxi.  (1865)  p.  45.     Harrington's  'Life  of  Sir  W.  E.  Logan,'  1883,  pp.  365-878. 

*  Q.  J.  Gea.  JSoc.  xxi.  (1865)  p.  51  ;  xxiii.  (1867)  p.  257.  See  also  his  'Acadian 
Geology,'  2n(l  edit.,  *Dawn  of  Life,*  1875,  and  *  Notes  on  SjMicimens  of  Eozoon  Canadense,' 
Montreal,  1888. 

*  Proc.  Ii<nj.  Soc.  1864,  p.  545.  Q.  J.  Geof.  S^m:.  xxi.  (1865)  p.  59  ;  xxii.  (1866) 
p.  219. 

**  Quart.  Jonrn.  Oed.  Soc.  xxii.  (1866)  p.  185. 


PART  I  §  1  PRE'CAMBRIAN  ROCKS  695 

cases,"  according  to  Professor  King,  "  there  are  produced  residual  *  figures 
of  corrosion  *  or  arborescent  configurations,  having  often  a  regular  disposi- 
tion." The  regularity  of  these  forms  is  attribute  by  Messrs.  King  and 
Rowney  to  their  having  been  determined  by  a  mineral  cleavage.^  Pro- 
fessor Mobius  of  Kiel  ^  also  opposed  the  organic  nature  of  Eozoan,  main- 
taining that  the  supposed  canals  and  passages  are  merely  infiltration 
veinings  of  serpentine  in  the  calcite.  In  some  cases,  however,  the  "  canal- 
system  "  is  not  filled  with  serpentine  but  with  dolomite,  which  seems  to 
prove  that  the  cavities  must  have  existed  before  either  dolomite  or  ser- 
pentine was  introduced  into  the  substance.  It  may  be  admitted  that  no 
structure  precisely  similar  to  that  of  some  of  the  specimens  of  Eozoon  has 
yet  been  discovered  in  the  mineral  kingdom.'  But  it  must  also  be  con- 
ceded that  the  chances  against  the  occurrence  of  any  organism  in  rocks  of 
such  antiquity,  and  which  have  been  so  disturbed  and  mineralized,  are  so 
great  that  nothing  but  the  clearest  evidence  of  a  structiu*e  which  cannot 
be  other  than  organic  should  be  admitted  in  proof.  If  any  mineral 
structure  could  be  appealed  to,  as  so  approximately  similar  as  to  make  it 
possible  that  even  the  most  characteristic  forms  of  Eozoon  might  be  due 
to  some  kind  of  mineral  growth,  the  question  would  be  most  logically 
settled  in  a  sense  adverse  to  the  organic  nature  of  the  substance.** 

The  opinion  of  the  organic  nature  of  Eozomi  has  been  supposed  to 
receive  support  from  the  large  quantity  of  graphite  found  throughout 
the  older  rocks  of  Canada  and  the  northern  parts  of  the  United  States. 
This  mineral  occurs  partly  in  veins,  but  chiefiy  disseminated  in  scales  and 
laminae  in  the  limestones  and  as  independent  layers.  Sir  J.  W.  Dawson 
estimates  tbe  aggregate  thickness  of  it  in  one  band  of  limestone  in  the  Ottawa 
district  as  not  less  than  from  20  to  30  feet,  and  he  thinks  it  is  hardly  an 
exaggeration  to  say  that  there  is  as  much  carbon  in  the  ^*  Laurentian  " 

^  Prof.  W.  King,  Qe^yl.  Mag,  1883,  p.  47.  See  the  views  of  these  writers,  summarised 
in  their  work,  'An  old  Chapter  in  the  Geological  Record  with  a  new  Interpretation,' 
London,  1881,  where  a  full  bibliography  will  be  found. 

'  *  Palseontographica,*  xxv.  p.  175  ;  Nature^  xx.  p.  272.  See  replies  by  Carpenter  and 
Dawson,  Nature j  xx.  p.  328.  Amer,  Joum,  Sci.  (3)  xvii.  p.  196  ;  also  Amer.  Joum,  Sci, 
(8)  xviii.  p.  117.     See  also  A.  G.  Nathorst,  Jieiies  Jahrb.  1892,  i.  p.  169. 

^  The  nearest  resemblance  to  the  ' '  canal-system  "  of  Eozoon  which  I  have  seen  in  any 
undoubtedly  mere  mineral  aggregate  is  in  the  structure  known  as  micropegmatite,  where, 
in  the  intergrowth  of  quartz  and  orthoclase,  arborescent  divergent  tube-like  ramifications  of 
the  one  mineral  are  enclosed  within  the  other  (see  Fig.  5).  Mr.  Rudler,  who  called  my 
attention  to  the  resemblance,  showed  me  a  remarkable  micropegmatite,  brought  from  tbe 
Desert  of  Sinai  by  Pi-ofessor  Hull,  in  which  the  Eozoonal  arrangement  is  at  once  suggested. 

*  Whitney  and  Wads  worth  in  their  *  Azoic  System '  {BulL  Mus.  Comp.  Zool.  Harvard^ 
1884,  pp.  528-548)  give  a  summary  of  the  controversy,  and  decide  against  the  organic 
origin  of  Eozoon.  From  the  zoological  side  also  Roemer  and  Zittel  decline  to  receive 
Eozoon  as  an  organii^m.  In  the  pre-Cambrian  rocks  of  Bohemia  and  Bavaria  specimens  were 
some  years  ago  obtained  showing  a  structure  like  that  of  the  Canadian  Eozoon.  They 
were  accordingly  described  as  of  organic  origin,  under  the  respective  names  of  Eozoon 
hohemicum  and  E,  havaricum.  But  their  true  mineral  nature  appears  to  be  now  generally 
admitted.  The  original  '  Tudor  specimen '  of  Eozoon  figured  by  Dawson  has  recently  been 
re-examined  by  Mr.  J.  W.  Gregory,  who  decides  against  its  organic  origin.  Quart,  Joum, 
Oeol,  Soc,  xlvii.  (1891)  p.  348. 


696  STHATIGHAPHICAL  GEOLOGY  '  book  n 

as  in  e(|uivalent  areas  of  the  Carboniferous  system.  He  compares  some  of 
the  pure  bands  of  graphite  to  l>eds  of  coal,  and  maintains  that  no  other 
source  for  their  origin  can  be  imagined  than  the  decomposition  of  carbon- 
dioxide  by  living  plants.^ 

An  important  and  interesting  feature  of  the  pre-Gimbrian  rocks  is  the 
occurrence  among  them  of  abundant  proofs  of  extensive  and  long-con- 
tinued volcanic  action.  Sheets  of  lava  having  an  aggregate  thickness  of 
many  thousand  feet  are  interstratified  with  coarse  and  thick  volcanic  con- 
glomerates and  tuffs.  The  eniptive  rocks  include  lx)th  basic  and  add 
varieties,  for  among  them  are  found  dialjases,  melaphyres  (often  highly 
amygdaloidal),  porphyrite,  gabbro,  quartzless  and  quartziferous  porphyry, 
rhyolitic  felsite,  augite-syenite,  and  granite.  Some  further  details  re^od- 
ing  these  masses  will  be  given  in  subsequent  pages.  In  the  Lake  Superior 
region  the  amygdaloidal  dial>ases  and  the  conglomerates  are  largely 
impregnated  with  native  copper. 

While  in  some  regions  the  original  characters  of  pre-Cambrian 
rocks,  sedimentiu-y  and  eiTiptive,  are  as  easily  determina])le.  as  those  of 
any  ordinary  Palaeozoic  series,  in  othere  they  have  l>een  more  or  less 
effaced  l)y  subsequent  geological  revolutions.  Gradations  can  sometimes 
l»e  ti-aced,  as  in  the  Penokee  district  of  Wisconsin,  fi-om  greywackes  and 
slates  through  every  stage  of  increasing  meUimorphism  into  mica-schists 
which  present  every  ap|>earance  of  complete  original  crystallization.^ 
The  limestones  have  j>jissed  into  the  condition  of  marbles ;  the  iron 
ores,  j>r<jlMibly  originally  cjirbonates,  have  become  oxidized  into  limonite, 
hainiatite  and  magnetite,  while  the  ore  has  l>een  concentrated  into  separate 
niJLsses.  The  "  gi*eenstones "  have  iwssed  into  the  condition  of  true 
schists.'*  Some  of  these  metamorphosed  areas  })resent  so  many  |X)int«  of 
rescml)lance  to  the  lower  gneisses  alreaily  described  that  it  is  not  at  all 
surprising  that  they  should  have  been  confounded,  and  that  their  true 
relations  should  only  have  been  made  out  after  much  controversy  and 
lon^i-continued  detailed  study. 

A  great  deal  of  discassion  has  arisen  as  to  the  true  relations  of  these 
])re-Cain]Hian  stratified  and  eruptive  rocks  to  the  coarse -crystalline 
Iwnuled  gneisses  above  (lescri])ed.  In  some  sections  a  complete  and 
strong  unconformability  occurs  between  the  two  series,  and  no  doubt  can 
there  exist  as  to  the  enormous  break  that  separates  them.  In  other 
regions,  however,  the  lower  gneisses  are  so  involved  with  schists,  lime- 
stones, and  conglomerates  that  no  satisfactory  separation  of  them  has 
been  made,  while  in  some  places  the  gneiss  actually  crosses  these  rocks 
intrusively.  Each  country  or  district  may  present  its  own  phase  of 
the  problem.  At  present  we  have  no  means  of  determining  the  true 
correlation  of  the  pre-Cambrian  rocks  in  separate  and  especially  in  dis- 
tant areas.  If  we  admit  that  the  lowest  gneisses  with  their  accompani- 
ments form  an  eruptive  assemblage  of  which  the  component  portions 

^  But   compare   the  a<lvocacy  of  an  opi)o.site  opinion   by  Whitney  and  Wadswoith* 
'Azoic  System,'  p.  539. 

-  r..  D.  Irving  ami  C.  R.  Van  Hpse,  lOtk  Ami.  licp.  U.S.  Goyf.  Syrr.  1890,  p.  434. 
^  G.  H.  Williams.  Bid/.  U.S.  GevL  Sun\  No.  02.  1S90. 


PART  I  §  i  PRE-CAMBRIAN  ROCKS  697 

may  belong  to  widely  different  periods  of  time,  it  is  quite  conceivable 
that  a  certain  group  of  sedimentary  formations  may  be  found  in  one 
district  to  lie  unconfonnably  on  these  gneisses,  and  in  another  to  be 
pierced  by  some  of  their  younger  members. 

There  is  likewise  some  difficulty  in  fixing  the  upper  limit  of  the 
pre-Cambrian  formations.  Where  the  Cambrian  rocks  lie  on  them  uncon- 
formably  the  obvious  stratigraphical  break  forms  a  convenient  line  of 
division.  But  in  some  countries  a  thick  mass  of  conformable  sedimentarv 
rocks  underlies  the  Olenelltts-zone  which  has  been  taken  as  the  base  of 
the  Cambrian  system,  and  in  these  instances  the  line  of  separation 
becomes  entirely  arbitrary.  Sections  of  this  nature  are  of  great  value, 
inasmuch  as  they  impress  upjon  the  geologist  that  the  artificial  character 
of  the  divisions  by  which  he  classes  the  geological  record  is  not  confined 
to  the  fossiliferous  formations,  but  marks  also  those  of  the  pre-Cambrian 
series.  Unconformabilities,  even  where  wide-spread,  cannot  be  regarded 
as  universal  phenomena,  and  though  of  infinite  service  in  classification, 
should  be  employed  with  the  full  consciousness  that  the  blanks  which 
they  represent  do  not  indicate  any  world-wide  inten^uption  of  geological 
continuity,  but  may  at  any  moment  be  filled  up  by  the  evidence  of 
more  complete  sections. 

With  regard  to  the  comparative  value  of  the  pre-Gimbrian  rocks  in 
the  chronology  of  geological  history  no  precise  statement  can  be  made. 
But  various  circumstances  show  that  they  must  represent  an  enormous 
period  of  time.  We  shall  see  in  succeeding  pages  that  from  the  general 
character  of  the  Cambrian  fauna  it  must  be  regarded  as  certain  that  life 
had  existed  on  the  earth  for  a  long  series  of  ages  ]>efore  that  fauna 
appeared,  in  order  that  such  well-advanced  grades  of  organisation  should 
then  have  been  reached.  One  of  the  most  interesting  chapters  of 
geological  history  would  be  supplied  if  some  adequate  account  could  be 
given  of  the  stages  of  this  long  pre-Cambrian  evolution. 

But  the  mere  thickness  and  variety  of  the  pre-Cambrian  formations, 
together  with  their  unconformabilities  and  other  structural  features, 
suffice  to  prove  that  they  represent  an  enormous  chronological  interval. 
In  North  America,  where,  so  far  as  at  present  known,  they  are  most 
extensively  developed,  they  are  estimated  to  attain  a  thickness  of  more  than 
65,000  feet,  or  upwards  of  twelve  miles,  <and  have  been  regarded  there 
as  chronologically  quite  equal  to  the  whole  of  the  rest  of  the  geological 
record.  Even  when  we  eliminate  the  bedded  volcanic  rocks  from  the 
computation  and  reduce  the  remaining  sedimentjiry  series  to  the  lowest 
allowable  dimensions,  an  enormous  mass  of  stratified  material  remains, 
which,  even  if  it  had  been  uninterruptedly  dejwsited,  would  have  required 
a  period  of  time  comparable  to  probably  more  than  that  taken  by  the 
whole  of  the  Palaeozoic  systems.  But  we  know  that  the  deposition  was 
not  continuous.  Both  in  North  America  and  in  Europe  there  is  clear 
evidence  from  marked  unconformabilities  that  it  was  broken  by  epochs  of 
upheaval  and  by  long  periods  of  extensive  denudation.  It  is  evident,  there- 
fore, that  we  must  assign  to  the  records  of  pre-Cambrian  time  a  far  more 
important  chronological  value  than  has  generally  been  apportioned  to  them. 


698  STRATIGRAPHICAL  GEOLOGY  BOOK  vi 

If,  as  already  stated,  it  is  impossible  in  the  present  state  of  science  to 
find  any  satisfactory  basis  for  the  correlation  of  the  oldest  gneisses  in 
distant  and  disconnected  regions,  it  is  not  more  practicable  to  establish  a 
basis  of  correlation  for  the  pre -Cambrian  stratified  formations.  The 
evidence  of  fossils  hardly  as  yet  exists,  and  mere  lithological  characters  are 
in  such  circumstances  of  little  value.  All  that  can  be  done  at  present  is 
to  work  out  the  succession  of  rocks  in  each  well-defined  geographical  and 
geological  area,  giving  local  names  to  the  stratigraphical  groups  or 
systems  that  may  be  established,  and  trusting  to  future  research  for  some 
method  of  possibly  ascertaining  the  parallelism  of  these  divisions  in 
different  parts  of  the  world.  Hence  in  the  following  summary  of  the 
characters  of  the  pre-Cambrian  rocks  in  the  Old  World  and  in  the  New 
no  attempt  will  be  made  to  adopt  any  general  terminology,  but  in  each 
country  the  names  and  divisions  adopted  there  will  be  given. 

§  ii.  Local  Development. 

Britain. — Much  attention  has  been  given  in  recent  years  to  the  pre-Cambrian  rocks 
of  the  British  Isles  and  a  vohiminous  literature  has  arisen  concerning  them.  Rocks, 
however,  liave  been  claimed  as  pre-Cambrian  which  are  certainly  eruptive  masses  oJ 
later  date  than  parts  of  the  Lower  Silurian  series.  Others  have  been  assigned  to  a 
similar  position,  though  their  relations  to  the  older  Palaeozoic  rocks  cannot  be  seen ; 
while  others  again  cannot  properly  be  disjoined  from  the  lower  (K>rtion  of  the  Cambrian 
system.  In  the  confusion  which  has  thus  been  introduced  it  will  be  most  satisfactoiy 
to  restrict  attention  to  those  rocks  and  areas  about  the  true  relations  of  which  there 
appears  to  be  least  room  for  dispute. 

In  no  |>art  of  the  Eurojiean  area  are  rocks  of  pi-e-Cambrian  age  more  admirably 
displayed  than  in  the  north-west  of  Scotland.  Their  position  there,  previously 
iudicAted  l)y  Macculloch  ^  and  Hay  Cunningham,'-'  was  first  definitely  established  by 
Murchison,^  who,  with  Nicol  as  his  earlier  colleague,  showed  that  an  ancient  gneiss  is 
unconfornmbly  overlain  with  a  thick  mass  of  dull  red  sandstones  above  which  lie  (also 
unconfonnably,  as  was  eventually  discovered)  quartzites  and  limestones  containing  fossils 
which  he  referred  to  the  Lower  Silurian  system.  He  regarded  the  red  sandstones  ss 
probably  Cambrian,  and  after  pro|)osing  the  terms  Fundamental  and  Lewisian  for  this 
imderlying  gneiss,  he  finally  adopted  instead  of  them  the  term  Laurentian,  believing 
that  the  rocks  so  designated  by  him  in  this  country  were  equivalents  of  those  which 
had  been  studie<i  and  described  by  his  friend  Logan  in  Canada.*    More  recently  the 


^   '  A  Description  of  the  Western  Islands  of  Scotland,*  1819. 

-  *  Geoguostical  Account  of  the  County  of  Sutherland,'  Highland  Soc.  TVans.  viiL 
(1841)  p.  73. 

3  Brif.  Amoc.  1885,  Sect.  p.  85  ;  1857,  Sect.  p.  82  ;  1858,  Sect.  p.  94  ;  Quart.  Jaun, 
Oeol.  Soc.  xiv.  (1858)  p.  501  ;  xv.  (1859)  p.  353  ;  xvi.  (1860)  p.  215  ;  xvii  (1861)  p.  171. 
Nicol,  Quart.  Journ.  tied.  Soc.  xiii.  (1857)  p.  17  ;  xvii.  (1861)  p.  85;  BrU.  Assoc,  1858, 
Sect.  p.  96  ;  1859,  Sect.  p.  119. 

*  In  the  elucidation  of  the  true  relations  of  the  rocks  to  each  other  in  the  N.W.  of 
Scotland  later  geologists  have  taken  part,  more  especially  Dr.  Hicks,  Prof.  Bonney,  Mr. 
Hudleston,  Dr.  Callaway,  and  above  all.  Professor  Lapworth  and  the  officers  of  the  Geo- 
logical Survey.  Tlie  literature  of  the  subject,  up  to  1888,  will  be  found  condensed  in 
the  Report  by  the  Geological  Survey,  in  Quart.  Journ.  Oeol.  Soc,  vol.  xliv.  (1888)  p.  378. 
The  more  important  announcements  since  that  date  will  be  referred  to  in  the  sequel. 


PART  I  §  ii  PRE-CAMBRIAN  ROCKS  699 

officers  of  the  Geological  Survey  have  discovered  the  Olaiellus-zoue  in  strata  inter- 
mediate between  the  quartzites  and  the  limestones.^  These  formations  are  thus  shown 
to  be  of  Cambrian  age.  The  base  of  the  Cambrian  series  in  the  north-west  of  Scotland 
lies  at  the  bottom  of  the  quartzite  which  reposes  with  a  strong  uuconformability,  some- 
times on  the  red  sandstones,  sometimes  on  the  gneiss.  Hence  these  last  two  distinct 
groupe  of  rock  are  now  definitely  proved  to  be  pre -Cambrian.  As  they  differ  so 
strongly  from  each  other  their  respective  limits  can  be  easily  followed,  and  as  they 
extend,  over  a  united  area  of  hundreds  of  square  miles  in  the  north-west  of  Scotland 
they  afford  abundant  opportunities  for  the  most  detailed  examination.  The  rocks  of 
this  region  may  be  arranged  in  descending  order  as  in  the  following  table : — 

/  Limestones  of  Durness  with  numerous  fossils  indicating  Cam- 
I      brian  and  possibly  lowest  Silurian  horizons  (p.  730). 
Cambrian.  ^  Serpulite    grit    and    "Fucoid    beds,"   with    Salterdla    and 

I      0lenellu8  =  OlBneWvLS  zone. 
\  Quartzites  with  abundant  worm-burrows. 


[Unconformability.  ] 


Pre- Cambrian.  « 


Dull  red  sandstones,  shales  and   conglomerates  attaining  a 
I  <{      thickness  of  at  least  8000  or  10,000  feet,  the  upper  limit 


"E  I      being  lost  by  denudation  and  unconformability. 


[Strong  unconformability.] 


{Coarse  gneisses  and  schists  derive^  from  a  complex  aggregate 
of  eruptive  rocks  of  different  ages  by  mechanical  deforma- 
tion. In  one  area  there  appears  to  be  a  group  of  still  more 
ancient  and  sedimentary  rocks  through  which  the  gneisses 
have  been  intruded. 

Lewisiax. — The  oldest  gneisses  of  Scotland  form  the  Isle  of  Lewis  with  the  rest  of 
the  Outer  Hebrides,  and  extend  in  an  interrupted  band  on  the  mainland  from  Cape 
Wrath  at  least  as  far  as  Loch  Duich.  For  this  important  and  well-defined  group  of 
rocks  the  name  Lewisian,  formerly  proposed  by  Murchison,  seems  most  appropriate. 
As  originally  studied,  it  was  thought  to  be  a  comparatively  simple  formation.  Its 
foliation -planes,  like  those  of  other  similar  rocks,  were  supposed  to  mark  layers  of 
deposit,  and  to  show  that  the  rocks  were  metamorphosed  sediments.  It  was  believed 
to  have  been  thrown  into  sharp  anticlinal  and  synclinal  folds,  of  which  the  axes  ran  in 
a  general  north-westerly  direction.  The  detailed  mapping  of  the  region  by  the 
Geological  Survey,  however,  has  shown  that  the  apparent  bedding  is  wholly  deceptive, 
and  that  the  seeming  simplicity  gives  place  to  an  extraordinarily  complex  structure. 
Instead  of  being  altered  sediments,  the  rocks  have  been  ascertained  to  consist  essentially 
of  eruptive  masses,  varying  from  an  extremely  basic  to  a  markedly  acid  type,  and 
belonging  to  successive  periods  of  extrusion.' 

As  a  whole  the  gneiss  is  considerably  more  basic  than  the  typical  rocks  to  which 
this  term  was  originally  given.  It  commonly  consists  of  plagioclaae  felspar  with  py- 
roxene, hornblende,  and  magnetite,  sometimes  with  blue  opalescent  quartz,  and  sometimes 
with  black  mica.  These  predominant  minerals  are  sometimes  distributed  quite  without 
structure,  so  that  the  rock  appears  as  a  syenite,  diorite,  gabbro,  peridotite,  picrite, 

*  Brit.  Assoc.  1891,  Sect.  p.  633.  Peach  and  Home,  Quart,  Jaum,  (j€ol»  Hoc.  xlviii. 
(1892)  p.  227. 

'  For  details  regarding  the  gneiss  of  N.W.  Scotland,  and  the  remarkable  geological 
structure  of  that  region  see  the  report  of  the  Geological  Survey,  <^uart.  Joum,  GeoL  Soc 
xliv.  (1888)  p.  378,  where  the  work  of  Messrs.  Peacb«  Home,  Gunn,  Clough,  Hinxman,  and 
Cadell  is  summarised. 


PRE-CAMBRIAN  ROCKS 


701 


the  influenee  of  meclianical  derormation,  they  appear  as  nidely  parallel  and  puclteitd 
baniU  (FJK.  327).  But  as  we  paa9  into  the  more  thoroughly  foliated  jiortioiis  of  tlie 
gneiss,  the  original  character  of  the  |»gtn«tit«a  i»  found  to  he  more  and  more  affected, 
until  it  Uecomea  no  longer  recognisable  in  the  ao<|iiired  RchiHtoge  Htructiire,     The  dark 


buic  i>ortiou3  of  the  original  niasx  ]>as8  into  nidely  foliated  ba.sic  gncixseii,  and  the 
grey  [legniatitea  sliaile  into  the  more  qiiartioae  hands  associated  with  iheiu.  Thua  the 
derivation  of  the  gneisses  from  amorphous  igneous  rotka  may  be  regarded  as  estalilialied 
beyond  dispute. 

Aa  illuatrative  of  (lie  concluaion  that  while  them  seenia  good  reasou  to  believe  that 
the  wgregated  or  coarsely -liaudud  structure  indicates  a  ae|nratioa  and  crystal lizatiou  of 
■lateriala  out  of  a  still  unconsolidated  igneous  magma,  the  predomiuaut  foliation  Htruc- 
tnrea  which  traverse  these  bands  were  produced  by  [owerful  meclianical  niovenjcnts, 
socb  a  section  as  that  represented  iu  Fig.  328  may  he  cited.  Tlie  mineral  bands  have 
there  been  violently  jilicated,  and  have 
b«eDcut  through  byasuccessionof  thriiat- 
pUnea  ((  t),  by  which  they  have  been 
pushed  fomard  and  piled  over  each  other. 
The  foliation  thua  sui>erinduced  rollotrs 
the  direction  of  movement,  and  ci'ossea 
indiscriminately  tlie  boundaries  of  the 
ditTereut  nggregatea  of  original  materiala. 
Viewed  from  a  little  distance  the  darker 
and  lighter  cnim[ilerl  layers  form  a  strik- 
ing feature  on  many  coast  cliffs,  but  they 
are  seen  to  l>e  abruptly  truncated  above  i 
and  lielow  by  thruat  -  planes  parallel  t« 
which  the  gneiss  has  sometimea  been 
crushed  and  rolled  out  into  flaggy  sheets 
(Fig  32.1  Th.„„o,.,,...r,,,t,™.„  „,„„_«„., i,.,..„  „„„, ,„„.™. . 
Similar  to  tliose  so  ahundaiitly  develojied  isrtiisl  surfnwHf  aevcnl  hunaml  ■iiunr'  vanl>. 
in  the  younger  or  eastern  gneisses  already 

(p.  625)  referre<l  to.  Tliuy  seem  to  make  it  certain  that  after  the  consolidation  of  the 
complex  assemblage  of  igneous  rocks  and  the  production  of  their  pegmatites,  a  series 

1  Figs.  328.  331,  334  are  taken  hy  permiasion  of  ! 
trmn  the  Beimrt  of  the  Geological  Survey  pnliliihe.!  in 
for  Auguat  1398. 


STBATIGRAPHWAL  GEOLOGY 


of  ]iov'rrful  mechanical 


cnunpled,  crushed,  and  sheared  the  whole  iw. 

distinct  foliatioti.      Portions  of  one  kind  of  mmteiul,  nd  ai 

have  be«n  inralrad  m  dil- 


and  |>rt>ducrd  in 

dark  hornblende,  have  been  leparated  froni  the  reat, 

tintt  lumps  in  another  vtrictj'  sncli  as  grvj  qnarUiMe  gneiM. 


Ptg.  3 


the  thrnat-plum 


The  detailpd  inreatigationii  of  the  Geological  Surrey  hare  further  ahown  that,  afta 
the  liret  foliation  liail  been  supprinduced,  a  new  wriea  of  igneous  prolmaions  invaded  lU 
gneisses,  cliieHy  in  the  fomi  of  dykes.  The  earliest  and  moat  conspicnoiui  of  these  ui 
eitraonlinarily  abundant  baaalt-rocks,  running  as  long  [umllel  bands  in  a  general 
W.X.U'.  auil  E.S.E.  direction.  The  latest  are  dykes  of  granite  or  syenite,  while  pnib- 
ably  of  intermediate  date,  are  certain  highly  basic  dykes,  among  which  peridotite  and 
picrite  ore  charaoteriatie.  The  evidence  as  to  the  relative  dates  of  these  igDcous  inttn- 
sionH  beiuf;  tolerably  elear,  we  have  here  ])roors  of  a  long  interval  of  aitbtemneaii 
activity,  during  which  the  magma  that  was  lirst  injected  into  the  gneias  iu  such  basic 
form  as  liasalt  larted  progressively  with  tta  more  hasic  constituents  until  it  became  in 
the  end  quili-  acid.     It  is  interesting  to  iin'l,  even  among  the  most  ancient  rocb(>f 


i-riwXMkJyWAjJSHkfM^. 


Loeh  LaiTonl. 


liiitain,  a  secjueuce  of  cni|itive  materials,  like  that  uliieh  ap]>ears  so  markedly  unMt 
thi)  I'alwoioic  and  Tertiary  volcanic  phenomena  (ji.  '2*<2). 

Atttr  tliB  iigei-tion  of  these  various  eruptive  materials,  the  whole  region  of  the  nortli- 
weat  of  Scotland  wus  once  more  Hulijected  to  powerful  dynamic  movements,  whereby  ill 
the  Toi-ks  Hi-re  i>rufi>undly  airit-tiil.  The  resultn  of  these  oi>erations  arc  found  partljin 
vurti<.<>)l  linen  or  luuds  of  rii|ituce  or  crushing,  along  which,  sonietiniet  for  a  breadth  of 
500  feet  or  inoro,  thr  rocks  have  Iweii  crushird  or  sheared,  jiartly  in  thrust-pUnes  whid 
are  often  nearly  Hat.     In  some  iustances  the  intrusive  dykes  remain  quite  diatinct,  bit 


PART  I  §  ii 


PRE'CAMBRIAN  ROCKS 


703 


have  ac<|uired  a  more  or  less  distinct  foliated  structure,  the  planes  of  foliation  being 
parallel  to  those  which  traverse  the  surrounding  gneiss  (Fig.  330).  But  the  alterations 
produced  by  these  enormous  terrestrial  stresses  are  most  strikingly  displayed  by  some  of 
the  more  basic  dykes. 

Along  the  central  portions  of  one  of  the  basalt  or  dolerite  dykes,  the  massive  rock 
may  be  observed  to  have  been  broken  into  oblong  lenticles  round  which  the  more 
crushed  material  passes  into  hornblende-schist,  while  the  outer  portions  of  the  dyke  like- 
wise become  entirely  schistose  (Fig.  332).  So  great  has  been  the  metamorphism  that  the 
augite  for  the  most  part  has  been  changed  into  hornblende.  The  felspars  have  assumed 
an  opaque  granular  condition,  and  the  rock  becomes  a  diorite.  The  peridotite  and 
picrite  dykes  have  been  converted  into  soft  talcose  schists,  the  veins  and  belts  of  granite 
into  granitoid  gneiss.  Such,  too,  has  been  the  compression  that  in  some  cases  dykes 
of  50  or  60  yaixis  in  breadth  are  reduced,  where  one  of  these  crush'lines  crosses  them 


Fig.  331. — Ground-plan  showing  deflection  and  disruption  of  dykes  in  the  LewiNian  gneisK  of 

N.W.  Scotland. 
TT,  Thrust-plane,  DD,  Dyke,  deflecte<l  about  J  mile  and  much  compreBsed.    The  dotte<l  lines  show  the 
strike  of  tlie  gneiss  and  its  displacement  by  the  thrust-plane ;  the  line  parallel  lines  in  the  dyke  and 
in  the  gneiss  mark  the  direction  of  the  newer  achistosity  develoi^ed  by  the  tlinist-movement, 
which  was  in  the  direction  of  the  arrow. 


obliquely,  to  a  thickness  of  no  more  than  four  feet,  while  the  horizontal  displacement 
sometimes  amounts  to  a  quarter  of  a  mile  (Fig.  331).  Besides  foliation  produced  jiarallel 
to  the  vertical  or  highly  inclined  lines  of  movement,  a  similar  structure  has  been 
superinduced  in  the  gneiss  parallel  to  the  gently  inclined  thrust- planes. 

The  influence  of  these  movements,  not  only  on  the  amorphous  dykes  and  veins,  but 
on  the  general  body  of  the  already  foliated  gneiss  itself,  has  been  profound.  Where  the 
change  has  been  most  complete  a  new  foliation  has  completely  obliterated  the  original 
structure.  From  this  extreme  every  gradation  may  be  traced  back  to  the  first  schistose 
structure,  and  thence  into  the  original  amorphous  condition.  In  many  cases  this  new 
foliation  ha.s  been  produced  nearly  or  quite  along  the  planes  of  the  old  structure.  But 
everywhere  examples  may  be  observed  where  (as  in  Fig.  333)  the  alternate  bauds  of  lighter 
and  darker  material  are  traversed  obliquely  by  the  newer  structure,  which  may  be  perfect 
in  the  dark  more  basic  bands  and  hardly  develo{>ed  in  the  grey  more  quartzose  parts. 

It  is  obvious  that  the  various  terrestrial  movements  indicated  by  the  complex 


STHATICllAfHIVAL  GEOLOGY 


K  TI 


]«ni]l<'I  \M  tliat  u 


ttliioli  till'  IjpwUinii  Kii.'isn  li.i 
M'liist,  i|ii.irti;-v'litst.  ^rii)i1iili 


>  must  represeut  a  iirotracted  jwriod  of 
((eologicnl  time.  Bnt  there  is  deuion- 
Htmtivepvideuco  that  the  whole  of  tlmu 
liiiil  lieeu  com|iletc(l,  and  that  the  rockt 
ill  ubicli  they  took  Jilnce  at  a  )pnt 
ilf|ilh  liiid  beeu  v\j>08m1  at  the  BUrfaee 
liy  vast  deiiiuktion  before  tho  next 
itieluWr  (if  the  jire -Canibriaii  wrin 
WHS  foniiiil.  The  Torridon  sandftoiw 
lies  with  tlie  iiLoHt  complete  oncua- 
fnniiabilitr  nii  the  old  glieim,  covering 
uUIlv  its  ilykfs.  (rnuh-liuvs  and  thiuM- 
|il.iiivii,  1>y  nut  one  of  wliieh  is  it  ia  tbc 
least  ih'tcrpe  slfi'deil.  It  is  of  rouiK 
iniiMWailile  to  fonii  niiy  ii<le<|tlate  eMi' 
['i']>lioii  (if  the  Icuglli  of  time  rleuoted 
by  tbiH  iiui'iiiiruntiability.  But  IIm 
iiiDii'  tbe  fjn.bjgist  tries  to  ix-alis*  whit 
Ihc  di-iiuiliitiuii  of  the  old  gurisK  in- 
vtilves,  the  more  ini|ire»sed  will  he  Iw 
n'ilh  tbr  vastuess  of  the  {leriod  whirli 

Oi'er  nearly  the  whole  of  tlie 
Lpwisinii  gneiss,  so  fiir  as  it  lia^  teen 
studied  on  the  maiidainl,  no  tnce  bu 
been  round  of  any  rocka  sutc  vhii 
(•ivbiibly  had  an  emptire  origin.  In 
one  ilintrict.  honi-ver,  which  inclndn 
tbc  pictnnmiine  valley  of  Luob  Mane. 
a  reniarknbli'  ^iu|.  of  rocks  o«nr> 
wliieli.  thou);b  their  exact  n-lationmrt 
nut  irithoilt  some  doubt,  apjiear  tu 
iiiilti'iiti'  a  setliineutarj'  aeriet  thn>ii(tk 
lesc  rocks  consist  cliifHy  iif  fitw  min- 
■ne.     The  grnjihitic  iiiati-rial  occurs  in 


PART  I  §  ii  PRE'CAMBRIAN  ROCKS  705 

having  generally  a  saccharoid  texture,  and  sometimes  full  of  the  usual  minerals  found 
in  a  mai'ble  in  a  zone  of  contact-metamorphism.  The  line  of  junction  of  this  group  of 
rocks  with  the  gneiss  is  well  defined,  but  does  not  distinctly  show  any  intrusion  of 
the  latter,  appearing  rather  to  have  resulted  from  movement  with  concomitant  crushing. 
If  these  strata,  so  similar  in  many  respects  to  the  undoubted  altered  sedimentary  piasses 
of  the  central  Highlands,  are  eventually  proved  to  be  truly  of  sedimentary  origin  they 
will  possess  a  high  interest  as  the  oldest  geological  formation  yet  known  in  Britain 
or  in  Europe.^ 

In  some  portions  of  the  north-west  of  Scotland,  especially  in  the  north  of  Sutherland, 
the  surface  of  the  gneiss  has  been  reduced,  after  prolonged  denudation,  to  a  kind  of  level 
platform  on  which  the  Torridon  Sandstone  has  been  deposited.  But  further  south  that 
surface  presents  a  singularly  imeven  character  rising  into  heights  8000  feet  above  the 
sea  and  sinking  into  hollows  that  descend  below  sea-level.  In  the  rugged  mountainous 
ground  between  Lochs  Maree  and  Broom  this  primeval  land-surface  is  impressively 
displayed,  for  the  thick  mantle  of  red  sandstone  under  which  it  was  buried  and  preserved 
has  been  irregularly  stripped  oflf,  and  the  details  of  the  pre-Torridonian  topography  can 
easily  be  traced. 

ToRRiDONiAN. — From  Cape  Wrath,  at  the  extreme  north-west  end  of  Scotland,  south- 
wards for  more  than  100  miles,  there  stretches  a  broken  belt  of  singular  conical  or 
pyramidal  hills,  rising  sometimes  to  more  than  3000  feet  above  the  sea,  and  presenting 
alike  in  their  form  and  colouring  a  striking  contrast  to  the  rest  of  the  scenery  of  that 
region.  They  are  built  up  of  nearly  horizontal  or  gently  inclined  strata  of  reddish-brown 
or  chocolate-coloured  sandstones  and  conglomerates,  which  lie  with  a  violent  unconform- 
ability  on  the  gneisses  above  described,  and  are  in  turn  covered  uncouformably  by  the 
quartzites  which  form  the  base  of  the  Cambrian  system.  Where  most  fully  developed, 
in  the  south-west  of  Ross-shire,  these  strata  are  between  8000  and  10,000  feet  thick.  They 
have  doubtless  been  derived  from  the  waste  of  the  Lewisian  rocks,  though  pebbles 
occur  in  them  which  have  not  been  identified  with  any  material  in  the  older  formation. 
Some  of  the  conglomerates  are  so  coarse  as  to  deserve  the  name  of  boulder  -  beds. 
Sometimes,  indeed,  where  the  component  blocks  are  large  and  angular,  as  at  Gairloch, 
they  remind  the  observer  of  the  stones  in  a  moraine  or  in  boulder-clay.*  Some  of  the 
sandstones  arc  in  large  measure  composed  of  pink  felspar  derived  from  such  rocks  as  the 
pegmatites  of  the  surrounding  gneiss.  An  occasional  thin  band  may  be  found  among 
them  consisting  largely  of  grains  of  magnetite  and  zircon,  whence  we  learn  at  what  an 
ancient  epoch  in  geological  history  heavy  and  durable  grains  were  separated  out  from 
the  more  ordinary  sediment  (see  p.  129).  In  the  highest  visible  i>ortion  of  these  sand- 
stones a  group  of  shales  occurs,  and  another  more  important  group  with  thin  bands  of 
impure  limestone  fonns  a  prominent  feature  near  the  base  of  the  series  in  the  west  of 
Ross -shire.  These  strata  may  yet  yield  recognisable  fossils,  but  hitherto  except 
Bome  tracks  and  other  obscure  markings  no  trace  of  organic  forms  has  been  met  with 
in  them. 

Messrs.  Peach  and  Home  have  detected  near  Loch  Inver  a  baud  of  tine  volcanic  tuflF 
among  the  red  sandstones,  showing  the  contemporaneous  activity  of  some  volcanic  vent 
in  that  district.  Small  vesicular  pebbles  of  porphyrite  found  among  the  contents  of  the 
conglomerates  may  jierhaps  indicate  the  outflow  of  lavas. 

The  strata  now  under  consideration  are  abundantly  displayed  among  the  mountains 
that  surround  Loch  Torridon,  one  of  the  most  picturesque  inlets  in  the  north-west  of 
Scotland.  Hence  they  were  called  by  Nicol  the  Torridon  Sandstone.  They  were 
originally  sup|)osed  to  be  Old  Red  Sandstone,  and  to  represent  the  lower  sandstones  and 
conglomerates  of  that  system  in  the  East  of  Sutherland  and  Ross.  After  the  discovery  of 
what  were  believed  to  be  Lower  Silurian  fossils  in  the  Durness  limestones,  Murchison 
Assigned  these  sandstones  to  the  Cambrian  system.     But  the  recent  detection  of  the 

1  See  BrU.  Assoc.  1891,  Sect.  p.  634.  *  Nature  xxii.  (1880)  p.  402. 


STRATI6BAPSICAL  QEOLOQY  boot  ti 

Olenelloa'ZDne  among  the  strata  which  unconfonnablf 
oratlie  tbem  proves  that  thej  moBt  be  of  still  older 

I  date.      They  are  now  classed  ai  TorridoDiau  in  tb* 

g,  j)re-Cambriau  fomiatioDS  or  sjitems  of  Britain. 

I  The  intorral  between  the  deposition  of  the  higbeit 

^  lisible  (Kirticin  of  tlic  Torridonian  series  and  ths  ban 

5  of  the  Cambrian  formations  must  hail's  been  of  pco- 

^  longed  duration.    For  not  only  had  the  red  saudstonM 

;,-  bei-n  upraised,  but  they  liad  been  profonndly  trenched 

I  by  denudation.     .So  vast  and  unequal  was  the  etnttov 

I  that  wliile  at  one  [>1bog  the  lower  quartzites  are  Kcn 

I  reposing  on  3000  or  4000  feet  of  Torridon  sandst«ie, 

-  at  another  only  a  few  miles  distant  they  rent  directly 

g  on  the  Lewistan  gneixs,  the  intervening  maBBivegnnp 

S  of  strata  haviug  been  entirely  l)ared  away.' 


Hut  besides  Ihe  soliil  areas  of  pre-CambriBli  rocki 
ill  the  north-vest  of  Scotland  there  arrextensivetncti' 
where  these  rocks  do  not  i-emain  in  their  originsl 
{iflsitions,  but  have  beeu  pushed  into  their  pment 
places  by  great  snbterranesn  disturbances,  and  have 
actually  been  shoved  over  strata  of  recognisably  CMi- 
brian  age.  In  the  aeroaut  already  given  (pp.  (i24-27) 
of  the  structure  of  tliat  region  it  was  shown  that  by 
tlii'se  earth -nioveinelits  slice  aflet  slice  of  the  Lewiain 
gneiss  and  of  the  Torridon  sandstone  has  been  shoni 
from  the  masi;  of  these  formations  below  ground,  his 
iM-en  piled  one  on  the  other,  snd  has  been  driven  w«st- 
wiird  over  the  Cambrian  strata  which  originally  liy 
aliove  llieui  ;  that  the  roc'ks,  subjected  to  sneh  enor- 


1  Tliias 


ucture  is  shown  both  in  Figs.  311  and  334. 


PART  I  §  ii  PRE-CAMBRIAN  ROCKS  707 

mous  pressure,  dislocation  and  deformation,  haye  undergone  serious  metamorphism  ; 
and  that  finally  by  a  gigantic  rupture  and  thrust  a  thick  series  of  gneissose  flagstones 
("Moine  schists")  have  been  brought  forward.  By  way  of  further  explanation  of 
this  extraordinary  structure  the  annexed  sections  are  given  (Figs.  334,  335).  It  will 
be  seen  what  an  enormous  body  of  gneiss  has  here  been  displaced  and  pushed  over 
the  Cambrian  strata,  which  in  turn  have  been  cut  into  slices  and  piled  up  above  and 
against  each  other.  Among  the  alterations  of  the  Torridon  sandstones  one  of  the 
most  interesting  is  the  production  of  pegmatitic  veins  in  them,  like  those  which  traverse 
eruptive  rocks.  These  strata  have  been  crushed  and  stretched  in  such  a  manner  that 
ruptures,  often  lenticular  in  form,  have  been  produced  in  them.  In  the  cavities  thus 
caused  there  has  been  a  deposition  of  quartz  and  of  quartz  and  pink  felspar  (Fig.  335). 

With  regard  to  the  rocks  which  have  been  thus  displaced  and  metamorphosed,  it  is  ex- 
tremely difficult  to  form  a  satisfactory  opinion  as  to  the  probable  source  and  original  con- 
dition of  many  of  them.  Portions  of  the  Lewisian  gneiss  can  be  recognised,  and  in  the 
west  of  Inverness-shire  this  rock  probably  constitutes  a  large  proportion  of  the  reconstructed 
schistose  series  which  has  been  thrust  westward  over  the  Cambrian  limestones  and  quartz- 
ites.  The  Torridon  sandstones  also  can  occasionally  be  identified,  and  they  may  consti- 
tute a  not  inconsiderable  proportion  of  the  * '  upper  gneiss  "  of  Western  Ross-shire.  Possibly 
other  sedimentary  material,  such  for  instance  as  any  which  succeeded  the  Durness  lime- 
stones, may  have  been  involved  in  the  gigantic  crushing  movements  that  produced 
the  younger  or  eastern  schists.  As  the  detailed  work  of  the  Geological  Survey  advances 
the  sources  from  which  these  schists  have  been  derived  may  be  more  fully  known.  But 
the  great  fact  has  been  abundantly  established  that  the  movements  which  pushed  the 
rocks  into  their  present  positions  and  imparted  to  them  their  existing  foliation  took 
place  after  Cambrian  time,  and  before  the  period  of  the  Old  Red  Sandstone.  We  have 
thus  a  notable  example  of  extensive  regional  metamorphism  during  the  Palaeozoic  ages. 

In  the  central,  southern,  and  eastern  Highlands  of  Scotland,  that  is,  throughout  the 
hilly  ground  east  and  south  of  the  line  of  the  Great  Glen,  an  important  series  of 
metamorphic  rocks  is  largely  developed,  the  true  stratigraphical  position  of  which  is  not 
yet  certainly  kno^vn.  They  consist  in  large  proportion  of  altered  sedimentary  strata, 
now  found  in  the  form  of  mica-schist,  graphite -schist,  andalusite-schist,  phyllite,  schistose- 
grit,  greywacko  and  conglomerate,  quartzite,  limestone,  and  other  rocks,  together  with 
epidiorites,  chloritic  schists,  hornblende-schists,  and  other  allied  varieties  which  probably 
mark  sills,  lava-sheets  or  beds  of  tuff,  intercalated  among  the  sediments.  The  total 
thickness  of  this  assemblage  of  rocks  must  amount  to  many  thousand  feet.  Some  of  its 
members  are  so  persistent  as  to  form  recognisable  horizons,  and  to  afford  a  basis  for  some 
approximation  to  a  stratigraphical  arrangement  of  the  whole.  In  Perthshire,  for  example, 
the  following  groups  in  descending  order  have  been  mapped  by  the  Geological  Survey  : — 

Dark  schist  and  limestone  (Blair  Athol). 

Quartzite  (Ben-y-Gloe). 

Graphite-schist. 

Calcareous  sericite-schist,  and  sericite-schist  with  bands  of  quartzite.     On  this  horizon 

occurs  a  great  mass  of  epidiorite  and  bomblende-schist. 
Gametiferous  mica-schist  and  schistose  pebbly  grits. 

Limestones  (Loch  Tay).     Hornblende-schists  occur  above  and  below  this  horizon. 
Gametiferous  mica-schists,  schistose  grits,  with  pebbly  bands  and  thick  bands  of  "  green 

schists."     Homblendic  sills  begin  to  appear  in  this  group. 
Massive  grits  with  schists  and  conglomerate  containing  pebbles  sometimes  as  large  as  a 

pigeon's  egg.     (Ben  Ledi,  Loch  Achray,  &c. ) 
Zone  of  slates  ( Aberfoyle). 
Pebbly  greywacke  and  grit  with  black  shales  and  limestone  below  (Pass  of  Leny). 

The  Loch  Tay  Limestone  has  now  been  traced  completely  across  the  country  from  the 
Moray  Firth  through  the  Grampian  Mountains  to  the  west  of  Argyllshire,  and  some  of 
the  other  zones  have  been  followed  for  many  miles.  The  metamorphosed  condition  of 
the  rocks  varies  considerably,  not  only  according  to  their  composition,  but  even  along  the 


708  STRATIGRAPHICAL  GEOLOGY  bootyi 

line  of  strike  of  the  same  group.  On  the  whole  the  alteration  appears  to  be  most  intense 
in  the  Central  Highlands,  and  to  become  less  as  the  rocks  recede  from  that  area  towards 
the  north-east  and  south-west.  One  of  the  most  singular  and  instructive  instanoea  of  this 
variation  is  that  which  has  recently  been  mapped  by  Mr.  J.  B.  Hill  of  the  Geologiotl 
Survey  in  the  district  of  Loch  Awe.  A  series  of  grits,  phyllites,  and  limestones,  resembling 
ordinary  Palaeozoic  sediments,  has  there  been  followed  by  him  north-eastwards,  and  has 
been  found  to  pass  along  the  strike  into  the  thoroughly  crystalline  schists  of  the  Oentnl 
Highlands.  Mr.  Barrow  of  the  Geological  Survey  has  found  the  metamorpbism  in 
Forfarshire  to  be  probably  connected  with  the  protrusion  of  large  bodies  of  granite  which 
often  passes  into  a  variety  of  gneiss.  After  the  great  terrestrial  movements  by  which  the 
rocks  were  folded  and  metamorphosed,  large  bodies  of  eruptive  material,  notably  granite, 
invaded  the  schists  and  produced  extensive  metamorpbism,  as  already  stated  (p.  027). 
The  change  is  most  intense  near  the  granite,  where  sillimanite  embedded  in  quartz  is  a 
conspicuous  mineral  in  the  schists.  A  little  farther  away  comes  a  band  in  which  kyanite 
is  often  abundant,  while  at  a  still  greater  distance  the  predominant  mineral  is  staoro- 
lite.  These  three  successive  zones  of  contact-metamorphism  can  be  found  passing  throng 
the  same  band  of  aluminous  schistose  material  as  it  recedes  from  the  eruptive  rock. 

At  present  no  definite  opinion  can  be  expressed  as  to  the  stratigraphical  poaition  of 
this  important  group  of  metamorphic  rocks,  which  forms  the  greater  part  of  the  High- 
lands  of  Scotland.  On  the  one  hand,  it  is  conceivable  tliat  they  may  all  be  pre-Torridonian. 
They  may  be  of  the  age  of  the  Loch  Maree  limestones  and  mica-schists  above  referred  to 
(p.  704 ) ;  or  they  may  represent  some  part  of  the  vast  interval  denoted  by  the  unoonfonna* 
bility  between  the  Lewisian  gneiss  and  theTorridon  sandstone ;  or  again  they  may  posaibly 
include  that  sandstone  and  the  sedimentary  deposits  which  conformably  succeeded  it,  and 
which  are  absent  in  the  North-west  Highlands.  On  the  other  hand,  they  may  include, 
as  Murchison  believed,  representatives  of  the  quartzites  and  limestones  of  Durness,  and 
even  of  later  sedimentary  formations  which  may  have  succeeded  these  strata,  bat  of 
which,  as  we  now  know,  no  trace  remains  in  the  North-west  Highlands.^  It  is  thns still 
an  open  question  whether  the  metamorphic  rocks  which  constitute  the  main  part  of  the 
Scottish  Highlands  are  of  pre-Cambrian  or  of  Cambrian,  or  even  possibly  in  part  of 
Silurian  ago.  Tliey  are  not  confined  to  Scotland,  but  spread  over  many  hundreds  of  square 
miles  in  the  north  and  west  of  Ireland.  As  it  is  convenient  to  avoid  periphrasis  by 
having  a  short  name  to  designate  so  important  a  series  of  rocks,  I  have  proposed  to  cdl 
them  provisionally  Dalradiany  after  the  old  Celtic  kingdom  of  Dalriada,  w^hich,  origin- 
ally fixed  in  the  north  of  Ireland,  subsequently  extended  into  the  south-west  of  Scotland, 
and  finally  gave  the  name  of  Scotland  to  the  kingdom  which  l>ears  that  appellation.'  I 
have  little  doubt,  however,  that  before  long  it  will  be  }K>ssible  to  make  out  satisfactorily 
the  structure  of  the  central  and  southeni  Highlands,  and  to  show  the  presence  and 
areas  of  Lewisian,  Torridonian,  Cambrian,  and  even  Lower  Silurian  rocks  in  tliat  region. 

In  the  north  and  west  of  Ireland  crystalline  schists  and  eruptive  rocks  cover  a  large 
area ;  but  as  the  rocks  which  unconformably  overlie  them  are  not  of  higher  antiquity  than 
the  Carboniferous  and  Old  Red  Sandstone  there  is  no  absolute  proof  in  that  country  of 
their  i)re-Canibrian  age.  There  cannot,  however,  be  any  doubt  that  it  is  the  Dalradian 
series  of  limestones,  quartzites,  phyllites,  mica-schists,  epidiorites,  granites,  and  other 
crystalline  rocks,  which  crosses  from  Scotland  and  spreads  across  the  northern  and 
western  counties  of  Ireland.  The  Irish  develojmicnt  of  these  rocks  is  similar  to  their 
grouping  in  Scotland,  some  of  the  bands  of  quartzite,  conglomerate,  limestone,  phyUite, 
and  mica-schist  being  prolwibly  continuations  of  similar  bands  on  the  Scottish  mainland 

^  Along  the  Highland  border  the  remarkable  band  of  cherts  and  igneous  rocks  referred 
to  on  p.  627  may  not  improbably  show  the  presence  there  of  the  radiolarian  cherts  and  vol- 
canic zone  at  the  base  of  the  Lower  Silurian  series  of  the  Southern  Uplands. 

'  Presidential  Address,  Quart.  Joum.  OeoL  Soc,  xlviL  (1891)  p.  75. 


FART  I  §  ii  PRE-CAMBRIAN  ROCKS  709 

and  in  the  islands  of  Argyllshire.^  Bat  there  are  also  scattered  areas  of  coarsely-banded 
gneisses  which  present  the  closest  resemblance  to  parts  of  the  Lewisian  gneiss  of  Scot- 
land. The  best  areas  for  the  study  of  these  rocks  lie  near  Pettigoe  and  Ballyshannon 
(Donegal),  from  Erris  Head  to  Blacksod  Point  (Mayo),  in  the  Slieve  Gamph  or  Ox 
Mountains  stretching  from  Castlebar  beyond  Sligo  to  Manor  Hamilton,  and  in  the 
western  part  of  the  County  of  Galway.  The  relations  of  the  Dalradian  series  to  the 
gneisses  and  granitoid  rocks  have  not  yet  been  accurately  determined.  But  there  is  reason 
to  believe  that  the  former  rests  with  a  violent  unconformability  upon  the  latter.  Near 
Castlebar,  Mr.  A.  M 'Henry,  of  the  Geological  Survey,  has  recently  found  at  the  base 
of  the  Dalradian  schists  a  coarse  conglomerate  made  up  largely  of  fragments  of  the 
gneisses  and  granites  on  which  it  rests. 

In  England  and  Wales  many  isolated  areas  hav^been  described  as  pre-Cambrian 
pn  evidence  which,  as  already  stated,  cannot  be  considered  satisfactory.^  The  areas 
where  in  my  opinion  the  most  satisfactory  evidence  of  pre-Cambrian  rocks  can  be  pro- 
duced are  Anglesey,  the  Caer  Caradoc  and  Longmynd  area  and  the  Malvern  Hills.  Of 
these  areas  by  much  the  most  important  is  the  first  named.  In  Anglesey  the  OleTielhu-zone 
has  not  been  discovered,  but  the  fossils  found  indicate  Tremadoc  and  possibly  even  Menevian 
horizons  in  the  Lower  Cambrian  series.'  The  basement  strata  are  conglomerates,  and 
they  evidently  lie  with  a  marked  unconformability  on  certain  crystalline  schistose  rocks. 
It  was  the  belief  of  Sir  A.  C.  Ramsay  that  the  latter  were  metamorphosed  portions  of  the 
Cambrian  system,  and  they  were  so  represented  on  the  Geological  Survey  maps.  But  a  re- 
examination of  the  ground  leads  to  the  conclusion  that  they  had  acquired  their  present 
crystalline  characters  before  the  Cambrian  strata  were  laid  down  upon  them  ;  and  as  these 
strata  belong  to  a  low  part,  if  not  the  base,  of  the  Cambrian  system,  it  becomes  manifest 
that  the  schists  must  be  of  pre-Cambrian  age.^ 

Two  groups  of  schistose  rocks,  which  differ  considerably  in  petrographical  characters, 
have  been  detected  in  Anglesey.  One  of  these,  consisting  mainly  of  coarse  gneisses, 
abounding  in  hornblende,  garnets,  and  brown  mica,  and  with  coarse  pegmatite  veins, 
presents  a  close  resemblance  to  portions  of  the  Lewisian  series  of  N.W.  Scotland. 
The  other  group  occupies  a  much  larger  area,  and  is  composed  of  flaggy  chloritic  schists, 
green  and  purple  phyllites  or  slates,  quartzite,  grit,  and  other  more  or  less  recognisably 
clastic  rocks.  The  resemblance  of  these  masses  to  the  Dalradian  series  of  Scotland  and 
Ireland  is  striking.  The  quartzites  of  Holyhead  contain  annelide  burrows.  The  exact 
stratigraphical  relations  of  the  two  crystalline  groups  to  each  other  have  not  yet  been 
satisfactorily  determined.  There  was  probably  an  original  unconformability  between  them, 
like  that  referred  to  as  occurring  in  the  west  of  Mayo.°    It  may  be  regarded  as  a  well- 

^  The  fullest  account  of  these  Irish  metamorphic  rocks  will  be  found  in  the  Memoirs  of 
the  Geological  Survey  of  Ireland  ;  see  especially  those  on  Sheets  1,  2,  5,  6,  and  11  (Inishowen, 
Co.  Donegal)  ;  3,  4,  6,  9,  10,  11,  15,  and  16  (N.W.  and  Central  Donegal)  ;  22,  23,  80,  and 
31  (S.W.  Donegal) ;  31  and  32  (S.E.  Donegal).  See  also  Harkness,  Quart.  Jounu  OeoL  Soc 
xvii.  (1861)  p.  256  ;  Callaway,  op.  cU,  xU.  (1885)  p.  221. 

*  There  is  now  a  voluminous  literature  on  this  subject ;  only  some  of  the  more  im- 
portant papers  will  be  here  cited. 

*  Prof.  Hughes,  Quart.  Joum.  GeoL  Soc.  xxxvi.  (1880)  p.  237  ;  xxxviii.  (1882)  p.  16. 

*  Prof.  Hughes,  op.  cit.  xxxiv.  (1878)  p.  137,  xxxv.  (1879)  p.  682  ;  Brit,  Assoc.  1881, 
Sects,  p.  643  ;  Proc.  Camb.  Phil.  Soc.  iii.  pp.  67,  69,  341.  Prof.  Bonney,  Quart.  Joum.  Qeoi. 
Soc.  xxxv.  (1879)  pp.  300,  321  ;  Oeol.  Mag.  1880,  p.  125.  Dr.  Hicks,  Quart.  Joum.  Oeol. 
Soc.  xxxiv.  (1878)  p.  147  ;  xxxv.  (1879)  p.  295 ;  Oeol.  Mag.  1879,  pp.  433,  628.  Dr.  CalUway, 
Quart.  Joum.  Geol.  Soc.  xxxvii.  (1881)  p.  210,  xl.  (1884)  p.  567.  Prof.  J.  F.  Blake,  op.  cit. 
xliv.  (1888)  p.  463  ;  Brit.  Assoc.  1888  (Report  on  Microscopic  Structure  of  Anglesey  Rocks). 

*  Quart.  Joum.  Oeol.  Soc  xlvii.  (1891)  Address,  p.  82.  Mr.  Blake  has  proposed  the 
name  of  ''  Monian  System  "  for  the  pre-Cambrian  rocks  of  Anglesey.  In  the  Address  just 
quoted  I  have  given  reasons  for  my  inability  to  adopt  this  term. 


710  STRATIGRAPHICAL  GEOLOGY  bookvi 

established  fact  in  British  Geology  that  early  in  the  Cambrian  period  there  existed  at 
least  one  tract  of  old  crystalline  rocks  above  water  in  the  north-west  of  Wales. 

On  the  borders  of  Shropshire  and  Wales  a  ridge  of  ancient  rocks  rises  up  from  rmda 
Silurian  strata  which  lie  upon  it  unconformably.  Part  of  this  ridge  consists  of  emptifs 
material  which  was  formerly  believed  to  be  of  later  date  than  the  sedimentary  rocks 
immediately  around.  But  the  main  portion  of  the  high  ground  is  formed  of  a  thick 
series  of  evidently  very  old  grits,  slates,  and  other  clastic  deposits,  which,  though  lisidly 
any  trace  of  organic  remains  had  been  found  in  them,  were  assigned  to  the  Gambrisn 
system.  More  recent  researches,  however,  have  shown  the  presence  of  the  OUnellus-MOM 
in  this  district  at  the  base  of  a  group  of  strata  which  are  thus  definitely  proved  to  be 
lower  Cambrian.^  From  this  important  horizon  it  is  possible  to  work  backward  sjid  to 
show  that  underlying  these  basement  parts  of  the  Cambrian  system  a  remarkable  group 
of  igneous  rocks  comes  to  the  surface.  Tlie  investigations  of  Mr.  Allport  and  Dr. 
Callaway  have  shown  that  these  rocks  include  both  lavas  and  fragmental  ejections  yaiying 
from  coarse  breccias  to  fine  tuffs.  The  lavas  are  generally  felsitic  in  character,  showing 
true  rhyolitic  structures,  but  there  occur  also  bands  of  diabase  which  may  possibly  be 
sills.  Tliere  is  thus  clear  evidence  of  a  copious  ejection  of  volcanic  materials  in  this  pert 
of  England  before  the  oldest  Cambrian  formations  were  laid  down.' 

Though  the  evidence  is  not  perhaps  conclusive,  it  seems  to  point  to  an  nnconfenn- 
ability  between  the  base  of  the  Cambrian  system  and  this  volcanic  group,  which  would 
thus  probably  be  of  pre-Cambrian  date.  The  relation  of  the  volcanic  masses  to  the 
great  thickness  of  ancient  sedimentary  strata  constituting  the  Longmynd  ridge  has  not  yet 
been  satisfactorily  determined,  though  there  are  indications  that  the  volcanic  gronp  liei 
at  the  bottom.  Dr.  Callaway  has  proposed  the  name  Uriconmn  for  that  group,  and  Lemf- 
myiidian  for  the  thick  series  of  sedimentary  strata  lying  to  the  westward.  Those  nsmei 
may  be  provisionally  accepted.  The  Longmyndian  rocks  have  generally  been  assigned 
to  the  Cambrian  system,  and  they  may  possibly  still  be  shown  to  belong  to  that  part  of  tiie 
geological  record.     The  Uriconian  volcanic  group,  however,  is  probably  pre-Cambrian. 

In  other  parts  of  England  and  Wales  isolated  areas  have  been  described  as  containing 
l>re-Cambrian  rocks.  Of  these  the  district  of  St.  David's  in  Pembrokeshire  has 
attracted  tlie  largest  share  of  attention,  chiefly  through  tlie  labours  of  Dr.  Henry  Hicks, 
who  in  that  small  area  has  endeavoured  to  establish  the  existence  of  three  distinct  p^^ 
(Cambrian  formations.  At  the  base,  under  the  name  of  * '  Dimetian, "  he  places  what  he  con- 
siders  to  be  granitoid  and  gneissic  rocks  with  l)auds  of  impure  limestone  or  dolomite, 
schists  and  dolerito.  Above  these  he  distinguishes  as  "Arvonian  "  a  group  com  posed  essenti- 
ally of  rhyolitic  felstones,  breccias,  and  tuffs,  marking  volcanic  eruptions  of  an  acid  type, 
while  at  the  top  he  describes,  by  the  designation  *'  Pebidian, "  a  series  of  tuffs  and  slates.' 
After  a  careful  study  of  the  ground  I  came  to  the  conclusion  that  there  is  no  trace  of 
pre-Cambrian  rocks  at  St.  David's.  I  regard  the  so-called  ** Dimetian"  as  a  granite 
which  has  invaded  the  Cambrian  rocks  ;  the  "Arvonian  "  includes  the  quartz-porphyries, 
which  appear  as  apophyses  of  the  granite  ;  while  the  "  Pebidian  "  is  an  interesting  gronp 
of  basic  lavas  and  tuffs  which  form  here  the  lowest  visible  part  of  the  Cambrian  system 
(referred  to  at  pp.  727,  728).    A  similar  group  of  breccias  and  tuffs  underlies  the  Cambrian 

»  Lapworth,  Oeol.  Mag.  1888,  p.  484. 

2  S.  Allport,  Quart,  Joiim.  Oeol.  Soe.  xxxiii.  (1877)  p.  449.  C.  Callaway,  qp.  ct^zzriiL 
p.  652,  xxxiv.  (1878)  p.  754,  xxxv.  (1879)  p.  643,  xxxviii.  (1882)  p.  119,  xlii.  1886)  p. 
481,  xlvii.  (1891)  p.  109  ;  Oeol.  Mag.  1881,  p.  348  ;  1884,  p.  362  ;  1885,  p.  260.  J.  F. 
Blake,  Qmrt.  Journ.  Oeol.  Soc.  xlvi.  (1890)  p.  386. 

3  Quart.  Journ.  Oeol.  Soc.  xxxi.  (1876)  p.  167,  xxxiii.  (1877)  p.  229,  xxxiv.  (1878)  p.  IM, 
xxxv.  (1879)  p.  285,  xl.  (1884)  p.  507.  My  account  of  the  so-called  pre-Cambrian  rock* 
of  St.  David's  will  be  found  in  Qiuxrt.  Journ.  Oeol.  Soc.  xxxix.  (1883)  p.  261.  Prof.  Uoyd 
Morgan  has  since  confinned  my  main  conclusions,  op.  cit.  xlvi.  (1890)  p.  241.  Compare 
also  J.  F.  Blake  op.  cit.  xl.  (1884)  p.  294. 


PART  I  §  ii  PRE-CAMBRIAN  ROCKS  711 


slates  of  Llanberis,  and  has  likewise  been  claimed  as  pre-Cambrian,  but  it  can  be  shown 
to  pass  up  continuously  into  the  Cambrian  strata.  In  the  Malvern  Hills  a  core  of 
gneissose  and  schistose  rocks  is  doubtless  of  pre-Cambrian  age,  fragments  derived  from 
it  being  found  at  the  base  of  the  overlying  unconformable  Cambrian  strata.^  From  the 
plains  of  Leicestershire  rises  an  insular  area  of  rocky  hills  (Charnwood  Forest)  composed 
of  slates,  tuffs,  and  various  crystalline  rocks,  which  by  the  Geological  Survey  have  been 
coloured  as  altered  Cambrian.  Messrs.  Bonney  and  Hill,  who  have  fully  described  these 
rocks,  regard  them  as  of  pre-Cambrian  date,  and  show  to  what  a  large  extent  they  are 
composed  of  volcanic  agglomerates  and  tuffs.*  No  conclusive  evidence,  however,  has  been 
adduced  that  these  rocks  are  pre-Cambrian.  The  slates  resemble  some  of  the  Cambrian 
slates  of  Wales,  and  the  volcanic  rocks  maybe  compared  with  those  which  in  that  principal- 
ity lie  at  the  base  of  the  Cambrian  system.  Another  protuberance  of  ancient  rocks  rises  in 
Central  England  from  beneath  the  coal-field  of  eastern  Warwickshire.  In  this  instance 
a  definite  age  can  be  assigned  to  one  portion  of  the  rocks,  for  they  contain  Upper  Cam- 
brian fossils.'  Beneath  these  strata,  and  apparently  in  conformable  sequence  with  them, 
lies  a  well-marked  volcanic  group.  The  occurrence  of  this  group  in  the  position  which  it 
occupies  affords  support  to  the  belief  that  the  volcanic  rocks  elsewhere  conjectured  to  be 
pre-Cambrian  really  belong  to  the  Cambrian  system.  At  the  Lizard  Point  in  Cornwall 
a  series  of  eruptive  and  schistose  rocks  occurs,  the  true  relations  of  which  have  not  yet 
been  fixed.  They  may  be  pre-Cambrian.  They  include  coarse  gneisses  which  rise  as 
islets  near  the  coast. 

On  the  continent  of  Europe  numerous  isolated  areas  of  schists  and  other  ancient  rocks 
have  been  assigned  to  a  pre-Cambrian  or  Archsean  series.  In  the  older  descriptions  of 
these  tracts  an  order  of  succession  was  often  given,  the  foliation  being  assimied  to  represent 
consecutive  layers  of  deposition.  But  we  now  know  that,  in  the  great  majority  of  cases, 
the  foliation  is  entirely  independent  of  original  structure,  so  that  the  former  attempts 
to  establish  a  stratigraphical  order  among  the  gneisses  and  schists,  and  to  compare  that 
order  in  different  countries,  cannot  be  accepted.  All  that  can  be  attempted  here  is  to  give 
a  summary  of  the  general  characters  of  the  most  ancient  rocks  of  each  region  referred  to. 

SoaTidinavia  exhibits  the  largest  continuous  tract  of  pre-Cambrian  rocks  in  Europe.^ 

*  J.  Phillips,  'Geology  of  the  Malvern  Hills,'  Mem.  Oeol.  Surv.  ii.  part  1  ;  HoU,  Quart. 
Jouni.  Gecl,  Soc.  xxl  p.  72  ;  Rutley,  op.  cil.  xliii.  (1887)  481  ;  Callaway,  p.  525,  ap,  cU. 
xlv.  (1889)  p.  475. 

»  Quart.  Joum.  Oeol.  Soc.  xxxiii.  (1877)  p.  754,  xxxiv.  (1878)  p.  199,  xxxvi.  (1880)  p. 
337,  xlvii.  (1891)  p.  78. 

3  Lapworth,  Oeol.  Mag.  (1886)  p.  321  ;  T.  H.  Waller,  op.  cit.  p.  323 ;  Rutley,  p.  657. 

■*  In  the  older  literature  consult  Keilhau,  'Gaea  Norvegica,'  iii  (1850).  Kjerulf, 
*Ud8igt  over  det  Sydlige  Norges  Geologi,'  Christiania,  1879  (translated  into  German  by 
Gurlt,  and  published  by  Cohen,  Bonn,  1880).  A.  E.  Tomebohm,  "Die  Schwedischen 
Hochgebirge,"  Schtced.  Akad.  Stockholm,  1873.  "Das  Urterritorium  Schwedens,"  Irenes 
Jahrb.  1874,  p.  131.  Karl  Pettersen,  "Geologiske  Undersogelser  inden  Tromso  Amt,"  &c., 
Norske  Videnskab.  Skrift^  vi.  44  ;  vii.  261.  For  more  recent  work  see  Reusch's  important 
monograph  on  the  fossiliferous  crystalline  schists  of  Bergen,  quoted  on  p.  621,  also  his 
instructive  essay  '  Biimmeloen  og  Karmden,'  1888  ;  his  papers  in  the  *  Aarbog  for  1891 '  of 
the  Geological  Survey  of  Norway  {Norges  Oeologiske  Under8i>gelse)  ;  his  *  Greologiske  lagtta- 
gelser  fra  Trondhjems  Stift,'  Christiania  vidensk.  selsk.  forhandl.  1891  ;  and  his  paper  on 
crystalline  schists  of  Western  Norway,  Compt.  rend.  Congrka  Oiol.  Internat.  1888  (1891), 
p.  192.  T.  Dahll,  0.  A.  Corneliussen,  and  H.  Reusch,  *  Det  nordlige  Norges  geologi,'  Norges 
Oeolog.  UndersHg.  1892;  C.  H.  Homan,  *Selbu,'  Norges  Otdog.  Undersog.  1890;  and 
Tomebohm,  Nature,  1888,  p.  127.  It  is  to  be  hoped  that  Professor  Briigger  may  be  able 
to  attack  the  problem  of  the  schistose  rocks  of  Norway,  and  that  we  may  have  from  him 
such  a  detailed  study  of  them  as  he  has  given  us  in  his  memoirs  on  the  Christiania  district. 


712  STRATIGRAPHIGAL  GEOLOGY  book  n 

Although  these  rocks  have  been  more  or  less  minutely  examined  throughout  the  whole 
extent  of  the  peninsula,  and  have  been  described  in  many  papers  and  memoirs,  the 
published  descriptions  of  them,  thougli  often  excellent  from  the  lithological  point  of 
view,  were  almost  entirely  written  before  the  recent  revolution  in  the  views  of  geologists 
regarding  metamorphism,  and  are  therefore  without  that  knowledge  of  the  true  meaning 
of  structural  characters  and  that  detailed  study  of  the  tectonic  relations  of  the  rodks 
which  the  present  condition  of  the  science  demands.  There  can  be  no  doubt  that  the 
older  crystalline  rocks  of  Scandinavia  are  a  prolongation  of  those  which  farther  to  the 
south -w^cst  rise  out  of  the  Atlantic  in  the  Highlands  of  Scotland  and  the  hills  of  the 
north  and  west  of  Ireland.  And  there  seems  every  probability  that  the  broad  featnrei 
of  geological  stnicture  which  have  been  ascertained  to  prevail  in  the  British  area  will  be 
found  to  extend  also  into  Norway  and  Sweden.^ 

Wide  tracts  of  western  Norway  consist  of  coarse  banded  gneisses  (QrundQeldet, 
Urbcrget),  which  present  the  closest  resemblance  to  the  Lewisian  series  of  Sutherland  and 
Ross,  but  with  a  wider  range  of  petrographical  diversity.  They  include  rod  and  giej 
gneisses,  banded  and  streaked  granulites,  epidote- gneiss,  cordierite- gneiss,  granites^ 
syenites,  gabbros,  diorites,  labradorite-rocks,  garnet-rocks,  amphibolites,  peridotites^ 
serpentines,  &c.  The  general  assemblage  of  these  rocks  suggests  that  they  represent  a 
complex  series  of  acid  and  basic  eruptive  masses.  With  them  is  intimately  associated 
another  group  of  rocks,  of  which  conspicuous  members  are  quartzite,  limestone^  mica- 
schist,  quartz -scliist,  and  others  which  point  with  more  or  less  clearness  to  a  sedimentary 
origin.  ThLs  gi-oup  is  usually  quite  crystalline,  and  is  certainly  older  than  some  portions 
of  the  gneisses  which  can  be  seen  to  pierce  it.  It  contains,  however,  bands  of 
amphibolite,  which  may  represent  sills  intruded  between  its  component  layers.  Thus  at 
Rukedal  (Soutliem  Nom'ay)  a  mass,  3900  feet  thick,  of  quartzite,  quartz-schist,  and 
interbedded  seams  of  hornblende-schist,  lies  upon  a  group  of  hornblende-schists  and 
grey  gneiss  traversed  by  abundant  granite  veins.  Thin  bands  of  limestone  occasionally 
occur  in  the  gneiss,  as  near  Christiansand,  w^herc  they  have  yielded  many  minenda, 
especially  vesuvianite,  coccolite,  scapolite,  phlogopite,  chondrodite,  and  black  spinel. 
Apatite  with  magnetite,  titaniferous  iron,  haematite,  and  other  ores  forms  a  marked 
feature  of  tlie  Norwegian  pre-Carabrian  series.  The  most  important  mineral  masses  in 
an  industrial  sense  are  tliick  l>eds  and  lenticular  masses  of  iron-ore  (Dannemors, 
Filipstad,  &c.) 

Of  obviously  later  date  than  the  coarse  gneisses  with  their  accompaniments  is  another 
series  of  crystalline  schists  which  spreads  over  vast  tracts  of  country  in  Scandinavia. 
Among  these  rocks  mica-schists,  phyllites,  quartz-schists,  clay-slates,  quartzites,  and 
schistose  conglomerates  are  conspicuous,  and  indicate  that  a  large  proportion  of  the  whole 
mass  is  probably  of  clastic  origin.  But  there  are  also  included  chloritic  and  hornblende 
schists,  amphibolites,  gneisses,  and  many  other  rocks  which  wei-o  probably  of  eruptive 
origin,  whether  injected  as  sills  or  thrown  out  contemporaneously  with  the  sedimentation 
of  the  schists  as  tutfs  and  lavas.  In  many  respects  this  imi)ortant  series  of  schists  bears 
a  close  resemblance  to  the  "younger  gneiss  "  and  Dalradian  rocks  of  Scotland.  But  its 
actual  stratigraj)hy  has  not  yet  been  accurately  elucidated.  That  some  portion  of  it 
may  be  jire-Cambrian  seems  suflBciently  probable.  But  its  true  relations  are  complicated 
by  the  discovery  of  Silurian  fossils  in  some  portions  of  the  scries  and  by  the  apparrat 
gradation  of  comjiaratively  unaltered  fossiliferous  Silurian  strata  into  the  schistose 
condition.  Dr.  Hans  Reusch  of  the  Geological  Survey  of  Norway  has  shown  that  among 
the  cr}\stalline  schists  to  the  south  of  liergen  bands  of  fine  mica-schist  or  phyllite  with 

^  As  the  result  of  two  journeys  in  Norway  from  Bergen  to  Hammerfest  1  was  convinced 
of  tills  general  parallelism,  but  the  determination  of  the  detailed  stratigraphy  of  the  conntrj 
will  be  a  task  of  incredible  labour  demanding  from  the  Scandinavian  geologists  many  yeais 
of  patient  application. 


PART  I  §  ii  PRE-CAMBRIAN  ROCKS  713 


layers  and  nodules  of  limestone  contain  fossils  probably  of  Upper  Silurian  age.*  I  have 
had  an  opportunity  of  visiting  the  district  described  by  him,  have  collected  fossils  from 
all  the  localities  which  he  enumerates,  and  can  entirely  confirm  the  account  which  he 
gives  of  the  thoroughly  metamorphic  character  of  the  rocks  among  which  the  fossiliferous 
bands  occur.  The  phyllites  are  intercalated  among  white  quartzites,  quartzite  con- 
glomerates, green  schists,  homblendic  and  actinolitic  schists  and  gneisses.  But  for  the 
occurrence  of  the  fossils,  a  geologist  would  naturally  class  the  rocks  as  probably  of  pre- 
Cambrian  age.  But  the  corals,  graptolites,  and  other  organic  remains  make  it  quite 
certain  tliat  the  crystalline  schists  in  which  they  occur  underwent  their  great  meta- 
morphism  not  earlier  than  some  part  of  the  Upper  Silurian  period.  It  will  be  an 
extremely  difficult  and  laborious  task  to  disentangle  the  complications  of  these  Nor- 
wegian rocks,  and  to  determine  which  are  of  pre-Cambrian  and  which  of  Palieozoic  age. 
Dr.  Reusch,  summing  up  what  is  known  regarding  the  distribution  of  fossils  among  these 
strata,  believes  that  a  more  or  less  continuous  belt  of  Cambrian  and  Silurian  rocks,  usually 
in  an  extremely  metamorphosed  condition,  can  be  traced  along  the  axis  of  the  Scandi- 
navian peninsula  from  near  Stavanger  to  the  North  Cape.*  That  in  this  region  there  were 
gigantic  terrestrial  movements  with  concomitant  faults,  over-thrusts  and  metamorphism 
after  Lower  Silurian  times,  is  abundantly  evident.  In  southern  Norway  and  in  Sweden 
enormous  masses  of  crystalline  schists  actually  overlie  the  oldest  fossiliferous  rocks,  as 
will  be  described  in  later  p«tges  (p.  769). 

In  the  east  and  south  of  Norway  a  thick  mass  of  reddish  and  greyish  felspathic 
sandstone,  known  there  as  SparagmUtf  intervenes  between  the  oldest  gneisses  (Urberget) 
and  the  base  of  the  Cambrian  series.  It  is  associated  with  quartzite  and  shales,  and 
sometimes  becomes  strongly  conglomeratic.  It  recalls  the  Torridon  sandstone  of 
Scotland.  Probably  a  large  mass  of  strata,  belonging  to  distinct  geographical  periods, 
has  been  grouped  together  under  the  common  name  of  sparagmite.  The  older  sparag- 
mite  which  underlies  the  Olenellus-zone  is  probably  pre-Cambrian.  In  western  and 
northern  Norway,  where  the  crushing  and  metamorphism  have  been  so  intense,  the 
sparagmite  is  not  recognisable,  though  it  may  in  an  altered  condition  extend  through 
these  regions. 

In  southern  and  central  Sweden  three  or  four  groups  of  stratified  formations,  attain- 
ing a  united  thickness  of  many  thousand  feet,  have  been  recognised  as  intermediate 
between  the  old  gneiss  and  the  lowest  portions  of  the  Cambrian  system.  Their  relations 
to  each  other  have  not  been  very  satisfactorily  determined,  some  of  them  having  only  a 
local  development.     They  are  distinguished  by  the  following  names  : — 

Visingso  group. — Sandstones,  red  and  green  shales,  limestone,  and  conglomerates.  300 
metres.     Visingso  on  Lake  Wettem. 

AlmesSkra  group  (near  Lake  Wettem)  and  Dala  Sandstone.  — Red  and  white  sandstones 
and  quartzites,  sparagmite,  red  shales,  and  rarely  limestone.  The  Dala  Sandstone  is 
believed  by  Tomebohm  to  spread  over  an  area  of  7150  square  kilometres.  It  attains 
a  thickness  of  sometimes  nearly  900  metres,  and  contains  in  the  south  two  well- 
marked  sheets  of  diabase. 

Dalsland  group.  — Seen  in  Dalsland  only,  and  composed  of  an  upper  group  of  shales  or 
slates  lying  on  a  quartzite  series,  below  which  lies  a  lower  shaly  series  followed  by 
a  thick  group  of  sandstones  and  coarse  conglomerates.  The  total  thickness  according 
to  Tomebohm  is  1900  metres. 

Central  Europe. — From  Scandinavia,  a  great  series  of  crystalline  schists  presumed  to 
be  pre-Cambrian  ranges  through  Finland  '  into  the  north-west  of  Russia,  re-appearing 

'  '  Silurfossiler  og  pressede  Konglomerater  i  Bergensskifrene,'  1882;  translated  into 
German  by  R.  Baldauf  with  the  title  '  Die  fossilien-fuhrenden  krystallinischen  Schiefer  von 
Bergen,'  Leipzig,  1883. 

*  See  his  sketch-map  of  Norway  and  Finland  (Geologisk  Kart  over  de  Skandinaviske  Lande 
og  Finland),  Christiania,  1890. 

'  The  petrographical  characters  of  the  vast  area  of  ancient  gneiss  in  Finland  are  now 


714  STRATIGRAPHICAL  GEOLOGY  bookti 

in  the  north-east  of  that  vast  empire  in  Petcliora  Land  dovm  to  the  White  Sea,  and 
rising  in  the  nucleus  of  the  chain  of  the  Ural  Mountains,  and  still  farther  sonth  in 
Podolia.  In  Central  Europe,  similar  rocks  appear  as  islands  in  the  midst  of  mofe  recent 
formations.  Among  the  Carpathian  Mountains,  they  protrude  at  a  number  of  pointi. 
Westwards  of  the  central  jwrtion  of  the  Alpine  chain,  they  rise  in  a  more  continnoiii 
belt,  and  show  numerous  mineralogical  varieties,  including  gneiss,  mica-schist,  and  muiy 
other  schists,  as  well  as  limestone  and  serpentine.  Some  of  these  rocks  are  certainly 
altered  sedimentary  deposits,  others  are  probably  crushed  igneous  rocks.  The  protogine 
of  the  Alps  has  been  shown  by  Michel  L^vy  to  be  intrusive.  It  behaves  to  the  sur- 
rounding schists  as  some  parts  of  the  Lauren tian  gneiss  of  Canada  do  to  the  sdiisii 
next  to  that  rock. 

Pre-Cambrian  rocks  rise  to  the  surface  in  a  number  of  detached  areas  in  France, 
particularly  in  Brittany,  the  Cotentin,  the  central  plateau,  Morvan,  Cevennea,  the 
Pyrenees,  the  Dauphiny  Alps,  and  the  Vosges.  In  Brittany  they  have  recently  been 
carefully  studied  by  Dr.  Barrois,  who  describes  them  as  largely  composed  of  mioa-schiBtB, 
passing  often  into  gneiss  and  into  quartzite,  and  including  chlorite-schists,  amphibolitc^ 
talcose  and  sericitic  schists,  sei'pentines,  eclogites,  and  pyroxenites.^  Extensive  msifi 
of  granitoid  and  granulitic  gneisses  with  mica-schists,  amphibolites  and  other  crjrstaUine 
rocks  form  the  foundation  of  the  great  central  plateau  of  France.  In  Brittany,  in  the 
central  plateau,  as  well  as  in  other  regions  of  France,  thick  masses  of  slates  and  phyllitei 
occur  which  by  some  writers  have  been  placed  in  the  pre-Cambrian  series.  In  the 
Cotentin  they  are  represented  by  the  "Phyllades  de  St.  L6" — a  thick  series  of  hsid 
lustrous  slates  or  phyllites,  among  which  tracks  of  annclides  (!)  have  been  found.  Bj 
other  geologists,  however,  these  rocks  are  placed  in  the  Cambrian  system. 

A  large  area  of  ancient  crystalline  schists  extends  southward  from  Dresden  throng 
Bavaria  and  Bohemia  between  the  valley  of  the  Danul)e  and  the  headwaters  of  the 
Elbe.  Two  well-marked  groups  have  been  recognised — (a)  red  gneiss,  containing  pink 
orthoclase  and  a  little  white  ]X)tash-mica,  covered  by  (6)  grey  gneiss,  containing  irhite 
or  grey  felsi)ar,  and  abundant  dark  magnesia-mica.  According  to  Giimbel  the  fonner 
(called  by  him  the  Bojan  gneiss)  may  be  traced  as  a  distinct  formation  associated  with 
granite,  but  with  very  few  other  kinds  of  crystalline  or  schistose  rocks,  while  the  latter 
(termed  the  Hercynian  gneiss)  consists  of  gneiss  with  abundant  interstratifications  of 
many  other  schistose  rocks,  graphitic  limestone,  and  serpentine.  The  Hercynian  gneisB 
is  overlain  by  mica-schists,  above  which  comes  a  vast  mass  of  argillaceous  schists  and 
shales.  In  Bohemia,  these  overlying  crystalline  clay-slates  and  schists  (**  Etage  A"  of 
Barrande)  graduate  upw^ard  into  undoubted  clastic  rocks  known  as  the  Pribram  Shales, on- 
conformably  over  which  come  conglomerate^}  and  sandstones  lying  at  the  base  of  the 
fossiliferous  series.^  The  same  gradation  occurs  around  the  granulite  tract  of  Saxony, 
where  the  outer  schists  may  be  merely  metamorphosed  Palaeozoic  sedimentary  rocks.' 


being  carefully  mapped  and  described  by  the  Geological  Survey  of  that  country  under  K.  A. 
MoWrg.  Each  Hheet  of  the  map,  of  which  twenty-oue  have  been  published  up  to  the  present 
time  (July  1893),  is  accompanied  by  an  explanatory  pamphlet. 

^  Ann.  Soc.  (J^ol.  Nurdy  x.  xiv.  xvi. 

*  For  descriptions  of  the  pre-Cambrian  rocks  of  Saxony  see  Credner,  Zeiiwh.  DeuUdi. 
(f'eoL  Oes.  1877,  j).  757  ;  explanations  accompanying  the  slicets  of  the  Geological  Sanrey 
Map  of  Saxony,  particularly  sections  Geringswalde,  Geyer,  Glauchau,  Hohenstein,  Penig, 
Rochlitz,  Schwarzenl>erg,  Waldheira,  WiesenthaL  Bavaria  and  Bohemia:  Giimbel, 
*  Geognostische  Beschreibung  des  Ostbayerischen  Grenzgebirges,'  Gotha,  1868  ;  Jokely,  Jakr. 
(Je/il.  ReichmnstalU  vi.  p.  355  ;  viii.  pp.  1,  516  ;  Kalkowsky,  *  Die  Gneissformation  de« 
Eulengebirges'  (Habilitationschrift),  Leipzig,  1878  ;  NeuesJahrb.  1880  (i.)  p.  29.  F.  Katxer, 
'Geologie  von  Bdhmen,*  1892. 

^  Lehmann,  *  Entstehung  der  altkrystallinischen  Schiefeiigesteine,*  1884. 


PART  I  §  ii  PRE-CAMBRIAN  ROCKS  716 

In  the  central  Pyrenees  pre-Cambrian  granites,  with  associated  well -stratified  masses 
of  gneiss,  mica-schist,  limestone,  &c.,  are  said  to  occur,  but  possibly  some  at  least  of  these 
rocks  are  altered  Cambrian  slates.^  In  Asturias  and  Gallicia,  Barrois  has  investigated 
a  great  series  of  schists  regarded  by  him  as  pre-Cambrian,  and  divisible  into  two  im- 
portant groups — a  lower,  composed  essentially  of  mica-schists,  and  an  upper,  consisting 
of  green  chloritous,  amphibolitic,  talcose  or  micaceous  schists,  with  subordinate  bands 
of  quartzite,  serpentine,  and  cipoline.* 

America. — In  North  America  the  pre-Cambrian  rocks,  which  cover  an  area  estimated 
at  more  than  2,000,000  square  miles,  from  the  Arctic  Ocean  southwards  to  the  great 
lakes,  have  been  studied  in  detail  for  a  longer  period  than  those  of  any  other  region,  and 
in  many  respects  they  may  serve  as  the  type  with  which  those  of  other  parts  of  the  globe 
may  be  compared.  They  were  first  mapped  and  described  by  Logan  and  Murray  in 
Canada,  and  were  divided  by  these  observers  into  two  distinct  divisions.  The  lower 
of  these,  named  Laurentian  from  its  extensive  development  among  the  Laurentide 
mountains,  was  described  as  consisting  chiefly  of  coarse  red,  grey,  and  banded  fel- 
spathic,  homblendic,  micaceous,  and  pyroxcnic  gneisses  with  pegmatites,  and  included 
zones  of  limestone.  The  upper  group,  called  Huronian  from  its  exposures  in  the 
Lake  Huron  district,  was  recognised  as  being  composed  mainly  of  quartzites,  felsites, 
diorites,  diabases,  syenites,  various  coarse  and  fine  fragmental  volcanic  rocks  (agglo- 
merates and  tufls),  clay-slates,  and  other  bedded  materials  that  passed  into  schists. 
Though  the  Huronian  series  was  found  along  the  line  of  junction  to  dip  below  the 
Laurentian,  this  position  was  believed  to  be  due  to  disturbance,  no  doubt  being  enter- 
tained that  the  former  series  was  the  younger  of  the  two. 

Since  the  days  of  these  two  great  pioneers  of  American  pre-Cambrian  geology  the 
subject  has  been  attacked  by  many  able  observers.  The  Geological  Surveys  of  Canada 
and  the  United  States,  as  well  as  those  of  some  of  the  States  of  the  Union,  particularly 
Michigan,  Wisconsin,  and  Minnesota,  have  examined  the  rocks  over  many  hundred 
square  miles,  and  have  published  voluminous  reports  concerning  them.  Unfortunately 
as  many  of  the  districts  were  worked  out  independently,  considerable  variety  of  nomen- 
clature and  diversity  of  view  have  arisen.  At  i)resent  it  is  hardly  {wssible  to  reconcile 
these  conflicting  opinions,  though  there  can  be  little  doubt  that  before  long  a  general  con- 
currence will  be  arrived  at  regarding  the  main  features  of  pre-Cambrian  geology  in  this 
important  region.  The  table  on  the  next  page  gives  the  subdivisions  which  appear  to  be 
best  established  in  the  Lake  Superior  and  Lake  Huron  territory.' 

According  to  the  general  consensus  of  opinion  among  the  present  geologists  of  the 
United  States  and  of  Canada,  the  pre-Cambrian  rocks  of  those  countries  may  be  divided 
into  two  great  series.  At  the  base  lies  a  vast  mass  of  gneisses,  schists,  and  eruptive 
rocks,  which,  known  as  the  "  Fundamental  Complex/'  is  regarded  as  the  oldest  of  the 
whole.  Above  tliis  ancient  series  comes  another  enormous  succession  of  rocks  comprised 
under  the  general  name  of  "  Algonkian,'*  but  consisting  of  several  distinct  formations, 
separated  from  each  other  by  unconformabilitics,  as  shown  here  in  the  table.* 

1  Garrigou,  BuU.  Soc,  O^ol.  France,  i.  (1873)  p.  418. 

*^  Ann.  Soc.  Giol.  Nwd,  iL  (1882). 

'  In  compiling  this  table  I  have  been  indebted  to  Mr.  C.  R.  van  Hise  of  the  United 
States  Geological  Survey  for  information  kindly  supplied  by  him,  also  to  his  paper  in  the 
Am/er.  Joum.  Sci.  and  to  Mr.  Lawson's  *  Report  on  the  Rainy  Lake  Region'  in  the  Annual 
Report  of  the  Canadian  Geological  Survey  for  1887. 

*  Out  of  the  large  amount  of  literature  which  has  grown  up  concerning  the  pre-Cambrian 
rocks  of  North  America  the  following  works  may  be  cited : — W.  £.  Logan,  *  Geology  of 
Canada,*  1863  ;  AnntuU  Reports  of  the  Geological  Survey  of  Canada^  particularly  Mr. 
Lawson's  Report  on  Rainy  Lake  above  cited  ;  Oeological  and  Natural  History  Survey 
of  Minnesota,  vol.  ii.  Geology,  by  N.  H.  Winchell  and  W.  Upharo,  1888,  and  Annual 
Reports  for  1887,  1888,  1891  ;  Oeological  Survey  of  Wisconsin,  Final  Reports,  vols.  i.  it 


716 


STRATIGRAPHICAL  GEOLOGY 


BOOK  n 


Table  of  the  Sequence  of  the  tre-Cambrian  Formations  of  the  United  States 

AND  Canada. 

Detrital   rocks  derived  in  large  measure  from  the  de- 
gradation of  the  volcanic  series  below,  15,000  feet. 
Keweenawan     [Nipigon- 1  Sheets  of  basic  and  acid  lavas,  with  intercalated  manes 
of  W.  Ontario].  |      of  sandstone  and  conglomerate,  especially  towards  tlie 

upper  part.     Said  to  reach  a  thickness  of  35,000  feet 
or  more  than  6^  miles  (?). 


'Nipijron- 1 


8.^ 


Upper  (original)  Huron-"^ 
ian  [Animikie  and 
Upper  Kaministiquia 
of  W.  Ontario,  Ani- 
mikie and  Upper  Ver- 
milion of  N.  Minne- 
sota, Upper  Marquette 
of  Michigan]. 

Lower  Huronian  [Kee- 
watin.  Lower  Kamin- 
istiquia, Ontario, 
Lower  Vermilion  of 
N.  Minnesota,  Lower 
Marquette,  Felch 
Mountain  iron  -  bear- 
ing series,  Menominee 
of  Michigan]. 


[Unconformability.  ] 

Quartzites,  carbonaceous  and  argillaceous  shales,  slates, 
conglomerates  and  ferruginous  rocks  with  intrusiTe 
greenstones,  at  least  12,000  feet  Traces  of  orgsn- 
isms  occur  in  this  series. 

[Unconformability.  ] 


Limestones,  quartzites,  phyllites,  slates,  mica-Bchists, 
green  chloritic  schists,  schistose  conglomerates, 
jas|ierH,  iron-ores,  diabase  and  quartz-porphyry  lavas, 
volcanic  agglomerates  and  tuffs  with  acid  and 
intrusions.     Prolmbly  more  than  5000  feet. 


B 

6 

■S   K 

c  '^ 

a 


Coutchiching. 


Laurentiau. 


[Unconformability.  ] 

Quartz-biotite  mica-schists  and  fine  grey  gneisses  of  re- 
L  markably  uniform  character,  estimated  by  Lawson  to 
I  be  more  than  20,000  feet  thick  in  some  places,  but 
J  elsewhere  thinner  and  disappearing. 
A  Hornblende-granites  and  syenites,  coarse  granitic  gneisses 
I  and  biotite  gneisses,  some  of  which  have  been  intruded 
r  into  the  quartz-biotite  schists,  and  even  into  the  base  of 
j      the  group  above  tliem. 

Mr.  Lawson,  iu  bis  remarkable  essay  on  the  Geology  of  the  Rainy  Lake  region,  has 
brought  forward  conclusive  proof  that  the  Laurcntian  gneisses  invade  and  alter  hia 
Coutchiching  schists,  and  even  penetrate  in  some  places  into  his  Keewatin  series  above. 
He  believes  that  these  gneisses  arose  from  the  fusion  of  the  basement  or  floor  on 
which  the  overlying  formation  rested,  portions  having  been  absorbed  into  the 
magma,  and  finally  ap])earing  with  it  as  gneiss.  More  recently  Messrs.  Pumpelly  and 
Van  Hise  have  found  on  the  north  shore  of  Lake  Huron  clear  evidence  that  the  base  of 


iii.  iv.  by  T.  C.  Chamberlin,  R,  D.  Irving,  C.  E.  Wright,  E.  T.  Sweet,  T.  B.  Brooks, 
&c.  ;  Geological  Survey  of  Michigan,  1873  (T.  B.  Brooks),  1881,  vol.  iv.  (C.  Rominger), 
1891-92,  containing  a  sketch  of  the  geology  of  the  iron,  gold,  and  copper  districts  by  M. 
E.  Wadsworth  ;  Second  Geohgical  Survey  of  Pennitylmnia,  summary  volume  on  Archsan 
Rocks  by  J.  P.  liCsley,  1892;  Annual  lieports  of  the  Uniteii  States  Oeological  Survey^ 
especially  the  5th  and  7th,  containing  memoirs  by  R.  D.  Irving,  and  the  10th  containing  a 
joint  memoir  by  R.  D.  Irving  and  C.  R.  van  Hise,  and  monograph  V.,  on  the  copper-bearing 
rocks  of  Lake  Superior  by  R.  D.  Irving ;  Bull.  U.S.  Geol.  Smr.  No.  23,  T.  C.  Chamberlin 
and  R.  D.  Irving  ;  A.  C.  Lawson,  Bull.  Geol.  *S<>c.  Amer.  i.  (1890)  pp.  163, 175  ;  A.  Winchell, 
op.  cit.  i.  p.  357,  ii.  p.  85  ;  N.  H.  Winchell,  Proc.  Ame.r.  Assoc,  xxxiii.  (1885)  ;  J.  D. 
Whitney  and  M.  E.  Wadsworth,  *The  Azoic  System,'  Bull.  Mus.  Camp.  ZooL  Harvard, 
1884.  C.  R.  van  Hise,  Amer.  Journ.  Sci.  xli.  (1891)  117;  R.  Pumpelly  and  C.  R.  van 
Hise,  op.  cit.  xliii.  (1892)  p.  224.  The  literature  of  American  pre-Cambrian  geology  has 
recently  been  exhaustively  discussed  by  C.  R  van  Hise  in  Bull.  U.S.  Geol.  Sure,  No.  86, 
'Correlation  Papers — Archaean  and  Algonkian,'  1892. 


PART  I  §  ii  PRE-CAMBRIAN  ROCKS  717 

the  Lower  Huroiiian  rocks  is  marked  by  a  coarse  conglomerate  lying  with  a  complete 
unconformability  upon  and  made  up  out  of  the  schists,  granites,  and  pegmatites  of  the 
fundamental  complex.^ 

India. — In  India,  the  oldest  known  rocks  are  gneisses  which  underlie  the  most 
ancient  Palaeozoic  formations,  and  appear  to  belong  to  two  periods.  The  older  or 
Bundelkund  gneiss  is  covered  unconformably  by  certain  "transition"  or  "submeta- 
morphic  "  rocks,  which,  as  they  approach  the  younger  gneiss,  become  altered  and  inter- 
sected by  granitic  intrusions.  The  younger  or  peninsular  gneiss  is  therefore  believed  to  be 
a  metamorphic  series  unconformable  to  the  older  gneiss.  In  the  western  Himalayan  chain 
there  are  likewise  two  gneisses — ^a  central  gneiss,  probably  Archsean,  and  an  upper  gneiss 
formed  by  the  metamorphism  of  older  Palseozoic  rocks  into  which  it  passes,  and  which  lie 
unconformably  on  the  older  gneiss  and  contain  abundant  fragments  derived  from  it.' 

China. — Pre-Cambrian  rocks  are  extensively  developed  in  northern  China,  forming 
the  fundamental  masses  round  and  over  which  the  later  rocks  have  been  laid  down. 
According  to  Richthofen,  the  oldest  portions  of  the  series  are  mica-gneisses  and  gneiss- 
granites  with  hornblende-schists,  mica-schists,  &c.,  having  an  N.N.W.  strike  and  steep 
inclination.  Apparently  of  later  date  are  some  chlorite-giieisses  and  hornblende-gneisses 
with  intercalations  of  mica-gneiss  and  granulite,  but  without  gneiss-granite,  seen  in 
north  Tshili  and  north  Shansi,  and  marked  by  a  persistent  W.S.W.  and  E.N.E.  strike. 
These  rocks  are  succeeded  unconformably  by  a  great  series  of  groups  which  may  belong 
to  distinct  periods.  They  consist  of  mica-schists,  crystalline  limestones,  black  quartz- 
ites,  liomblende-schists,  coarse  conglomerates  and  green  schists.  With  some  of  these 
grou|)s  are  associated  granite,  pegmatite,  syenite,  and  diorite.  The  whole  series  under- 
went great  plication  and  denudation  before  the  deposition  of  the  older  Palaeozoic  forma- 
tions (Sinisian).* 

Australasia. — In  the  South  Island  of  New  Zealand,  the  most  ancient  Palaeozoic 
rocks  are  underlain  by  vast  masses  of  crystalline  foliated  rocks  traceable  nearly  cout 
tinuously  on  the  west  side  of  the  main  watershed.  The  geological  relations  of  these 
masses  have  not  yet  been  satisfactorily  defined,  and  it  does  not  appear  to  be  established 
whether  any  j>ortion  of  them  are  undoubtedly  ])re- Cambrian.  They  are  divided 
by  Sir  J.  Hector  into  two  series,  of  which  the  lower  consists  of  gneiss,  granite,  &c., 
with  an  overlying  mass  of  hornblendic,  micaceous,  and  argillaceous  schists  (prob- 
ably metamorphosed  Devonian) ;  while  the  upper  consists  of  argillaceous  slates  and 
schists,  which  are  regarded  as  probably  altered  Silurian  or  even  Carboniferous  rocks.*  In 
Canterbury  there  is  a  central  zone  of  micaceous,  talcose,  and  graphitic  schists,  overlain 
by  chlorite  and  hornblende-schists,  and  lastly  by  a  quartritic  zone  interleaved  with 
schists. °  Crystalline  schists  and  gneisses  form  the  nigged  mountainous  ground  of 
south-western  Otago.  The  centre  of  this  province  is  occupied  by  a  broad  band  of  gently 
inclined  mica-schists  and  slates.    These  rocks  are  the  main  gold-bearing  series  of  Otago.' 

In  Australia,  large  areas  of  granite  and  of  crystalline  schists  occur,  but  their  precise 
relations  have  not  yet  been  worked  out.  Some  of  these  rocks  have  been  described  by 
Selwyn,  Ulrich,  R.  L.  Jack,  R.  A.  F.  Murray,  and  others,  as  probably  including 
metamorphosed  Paheozoic  formations.  But  there  are  not  improbably  portions  of  them 
referable  to  a  pre-Cambrian  series. 

1  Anier.  Journ,  ^Sci.  xluL  (1892)  p.  224. 

*-*  Medlicott  and  Blanford,  *  Manual  of  Geology  of  India,'  pp.  xviii.  xxvi.  But  there  are 
younger  Indian  schistose  rocks,  from  which  these  must  be  distinguished.  In  the  Himalayan 
region  there  is  a  series  of  gneisses  and  schists  below  which  lie  comparatively  unaltered  beds 
of  supra-Triassic  age. 

••»  Richthofen,  'China,'  ii.  (1882). 

*  '  Handbook  of  New  Zealand,'  by  J.  Hector,  M.D.,  Wellington,  1888. 

^  Haast's  *  Geology  of  Canterbury,'  p.  252. 

«  Hutton's  'Geology  of  Otago,'  p.  81. 


718  STRATIGRAPHICAL  GEOLOGY  book  vi  paw  n 


Part  II.    Paleozoic. 

It  has  been  shown  in  the  foregoing  pages  that  though  the  stratified 
pre-Cambrian  rocks  are  generally  separated  by  an  unconfomiability  from 
formations  of  later  age,  such  a  break  does  not  always  occur,  and  that 
in  its  absence  no  sharp  line  of  division  can  be  drawn  by  way  of  upward 
limit  to  the  pre-Cambrian  series.     It  is  obvious  that  the  physical  con- 
ditions of  sedimentation   underwent  no  universal  interruption   at   the 
close  of   pre-Cambrian   time,  that  these  conditions   had  already  been 
established  long  before  the  Cambrian  period,  and  that  they  were  con- 
tinued in  some  regions  into  that  period  \^dthout  a  break.     Moreover,  it 
has  now  been  ascertained  beyond  doubt  that  plant  and  animal  life  had 
already  appeared  upon  the  earth  during  pre-Cambrian  time.     Hence  the 
term  Palaeozoic,  or  Primary,  which  has  hitherto  been  used  to  denote  the 
older  fossiliferous  systems  that  terminate  downward  at  the  base  of  the 
Cambrian  rocks  is  no  longer  stnctly  accurate,  unless  it  is  extended  so  as 
to  include  the  very  oldest  strata  in  which  organic  remains  have  been 
found.     Geologists  have  agreed  to  fix  the  kise  of  the  Cambrian  system 
at  the  Oknellus-zoney  already  referred  to.     It  is  quite  evident,  however, 
that  at  any  moment  a  new  series  of  fossils  may  be  discovered  below  that 
horizon,  and  it  >vill  then  be  matter  for  consideration  whether  such  a  series 
should  be  included  in  the  Cambrian  fauna  or  be  made  the  palaeontologieal 
basis  for  the  designation  of  a  still  older  geological  system.    In  the  present 
meagre  state  of  our  knowledge  regarding  these  ancient  rocks,  it  seems  the 
most  prudent  course  to  take  in  the  meantime  the  platform  of  the  OUnellus- 
zone,  which  has  now  been  recognised  in  many  parts  of  the  globe,  as  the 
Cambrian  basement,  and  to  fix  there  provisionally  the  downward  limit  of 
the  Palaeozoic  series  of  systems.      That  series  will  thus  include  all  the 
older  sedimentary  formations  from  the  bottom  of  the  Cambrian  to  the 
top  of  the  Permian  system.     The  strata  embraced  under  the  comprehen- 
sive designation  of  Palajozoic  consist  mainly  of  sandy  and  muddy  sediments 
>vith  occasional  intercalated  zones  or  thick  masses  of  limestone.     They 
seem  everywhere  to  bear  witness  to  comparatively  shallow  water  and  the 
proximity  of  land.     Their  frequent  alternations  of  sandstone,  shale,  con- 
glomerate, and  other  detrital  materials,  their  abundant  rippled  and  sun- 
cracked  surfaces,  marked  often  with  burrows  and  trails  of  worms,  as  well 
as  the  prevalent  character  of  their  organic  remains,  show  that  they  must 
generally  have  been  deposited  in  areas  of  slow  subsidence,   bordering 
continental  or  insular  masses  of  land     From  the  character  of  the  organ- 
isms preserved  in  them,  the  Palaeozoic  rocks,  as  far  as  the  present  evidence 
goes,  may  be  grouped  into  two  main  divisions — an  older  and  a  newer : — 
the  former,  or  Silurian  facies  (from  the  base  of  the  Cambrian  to  the  top 
of  the  Silurian  system),  distinguished  more  especially  by  the  abundance 
of  its  graptolitic,  trilobitic,  and  brachiopodous  fauna,  and  by  the  absence 
of  vertebrate  remains ;  the  latter,  or  Carboniferous  facies  (from  the  top  of 


SECT,  i  §  1  CAMBRIAN  SYSTEM  719 

the  Silurian  to  the  top  of  the  Permian  system),  marked  by  the  number 
and  variety  of  its  fishes  and  amphibians,  the  disappearance  of  graptolites 
and  trilobites,  and  the  abundance  of  its  cryptogamic  terrestrial  flora. 


Section  i.     Cambrian  (PrimoFdial  Silurian). 

§  1.  General  Characters. 

In  those  regions  of  the  world  where  the  relations  of  the  pre-Cambrian 
to  the  oldest  unmetamorphosed  Palaeozoic  rocks  are  most  clearly  exposed 
and  have  been  most  carefully  studied,  it  is  seldom  that  any  conformable 
passage  can  be  traced  between  these  two  great  rock-groups,  though,  as 
already  stated,  occasional  examples  of  such,  a  gradation  occur.  More 
usually  a  marked  uiiconformability  and  strong  lithological  contrast  have 
been  observed  between  the  two  series,  the  younger  frequently  abounding 
in  pebbles  derived  from  the  waste  of  the  older.  Such  a  break  points  to 
the  lapse  of  a  vast  interval  of  time  during  which  the  pre-Cambrian  rocks, 
after  suffering  much  crumpling  and  metamorphism,  were  ridged  up  into 
land  and  were  then  laid  open  to  prolonged  denudation.  These  changes 
seem  to  have  been  more  especially  prevalent  in  the  northern  part  of  the 
northern  hemisphere.  At  all  events,  there  is  evidence  of  extensive  up- 
heaval of  land  in  the  north-west  of  Europe  and  across  the  northern  tracts 
of  North  America  and  Northern  China^  prior  to  the  deposit  of  the  earliest 
remaining  portions  of  the  Palaeozoic  formations.  These  strata,  indeed, 
were  derived  from  the  degradation  of  that  northern  land,  the  extent  and 
height  of  which  may  be  in  some  measure  realised  from  the  enormous 
piles  of  sedimentary  rock  which  have  been  formed  out  of  its  waste.  To 
this  day,  much  of  the  land  in  the  boreal  tracts  of  the  northern  hemisphere 
still  consists  of  pre-Cambrian  gneiss.  We  cannot  affirm  that  the  primeval 
northern  land  was  lofty ;  but,  if  it  was  not,  it  must  have  been  subjected 
to  repeated  renewals  of  elevation,  to  compensate  for  the  loss  of  height 
which  it  suffered  in  the  denudation  that  provided  material  for  the  deep 
masses  of  Palaeozoic  sedimentary  rock. 

The  earliest  connected  suite  of  deposits  in  the  Palaeozoic  series  re- 
ceived the  name  "  Cambrian,"  from  Sedgwick  who  with  great  skill  un- 
ravelled the  stratigraphy  of  the  most  ancient  sedimentary  rocks  of  North 
Wales  (Cambria).  When  the  peculiar  brachiopodous  and  trilobitic  fauna 
of  Murchison's  Silurian  system  was  found  to  descend  into  these  rocks,  the 
term  Primordial  Zone  or  Primordial  Silurian  was  applied  to  them  by 
Barrande  in  Bohemia.  For  many  years,  however,  they  yielded  so  few 
fossils  that  their  place  as  a  distinct  section  of  the  geological  record  was 
disputed.  Eventually  by  the  labours  of  Barrande  in  Bohemia ;  Hicks 
in  South  Wales ;  Brogger,  Linnarsson,  and  others  in  Scandinavia  ;  Schmidt 

^  Tlie  vast  erosion  of  the  pre-Palseozoic  land  is  nowhere  more  impressively  shown  than  in 
Northern  China,  where,  as  Richthofen  has  pointed  out,  the  oldest  gneisses  are  surmoiuited 
by  thousands  of  feet  of  sedimentary  material  (Sinisian  formation),  in  the  uppermost  parts  of 
which  Primordial  fossils  are  found.     'China,*  vol.  ii. 


720  STRATIGRAPHICAL  GEOLOGY  book  vi  paw  u 

in  the  Baltic  provinces  of  Russia ;  Billings,  Mathew,  Walcott^  and  others 
in  Canada  and  the  United  States,  as  well  as  various  workers  in  other 
countries — such  a  distinctive  fauna  has  been  brought  to  light  as  serves' to 
characterise  a  series  of  deposits  at  the  base  of  the  Palaeozoic  formations. 
This  assemblage  of  fossils,  Barrande's  first  or  Primordial  fauna,  is  now  by 
common  consent  more  commonly  known  as  Cambrian.  The  use  of  the 
terms  Cambrian  and  Silurian  will  be  more  fully  referred  to  in  later 
jmges. 

Rocks. — ^The  rocks  of  the  Cambrian  system  present  considerable 
uniformity  of  lithological  character  over  the  globe.  They  consist  of  grey 
and  reddish  grits  or  greywackes,  quartzites  and  conglomerates,  with 
shales,  slates,  phyllites  or  schists,  and  sometimes  thick  masses  of  lime- 
stone. Their  false-bedding,  ripple-marks,  and  sun-cracks  indicate  deposit 
in  shallow  water  and  occasional  exposure  of  littoral  surfaces  to  desiccation. 
Sir  A.  C.  Ramsay  suggested  that  the  non-fossiliferous  red  strata  may  have 
been  laid  down  in  inland  basins,  and  he  speculated  upon  the  probability 
even  of  glacial  action  in  Cambrian  time  in  Britain.^  As  might  be 
expected  from  their  high  antiquity,  and  consequent  exposure  to  the 
terrestrial  changes  of  a  long  succession  of  geological  periods,  Cambrian 
rocks  are  usually  much  disturbed.  They  have  often  been  thrown  into 
plications,  dislocated,  placed  on  end,  cleaved,  and  metamorphosed.  In 
Wales  they  include  towards  their  base  an  interesting  volcanic  group 
consisting  of  felsitic  and  diabase-tuffs,  and  olivihe-diabase  in  interbedded 
sheets,  through  which  eruptive  acid  rocks  (quartz-felsites,  &c.)  have  risen. 

Life. — Much  interest  necessarily  attaches  to  Cambrian  fossils,  for 
excepting  the  few  and  obscure  organic  remains  obtained  from  pre- 
Carabrian  strata,  they  are  the  oldest  assemblage  of  organisms  yet  known. 
They  form  no  doubt  only  a  meagre  representation  of  the  fauna  of  which 
they  were  once  a  living  jmrt  One  of  the  first  reflections  which  they 
suggest  is  that  they  present  far  too  varied  and  highly  organised  a  suite 
of  organisms  to  allow  us  for  a  moment  to  sup])ose  that  they  indicate  the 
first  fauna  of  our  earth's  surface.  Unquestionably  they  must  have  had 
a  long  series  of  ancestors,  though  of  these  still  earlier  forms  such  sli^t 
traces  have  yet  been  recovered.-  Thus,  at  the  very  outset  of  his  study 
of  stratigraphical  geology,  the  observer  is  confronted  with  a  proof  of  the 
imperfection  of  the  geological  record.  When  he  begins  the  examination 
of  the  Cambrian  fauna,  so  far  as  it  has  been  preserved,  he  at  once 
encounters  further  evidence  of  imperfection.  Whole  tnbes  of  animals, 
which  almost  certiiinly  were  represented  in  Cambrian  seas,  have  entirely 
disappeared,  while  those  of  which  remains  have  been  preserved  belong  to 
different  and  widely  separated  divisions  of  invertebrate  life. 

The  prevailing  absence  of  limestones  fiom  the  Cambrian  deposits  of 
western  Eiu'ope  is  accomjxanied  by  a  failure  of  the  foraminifera,  corals, 

1  Q.  J.  Oeoi,  Soc,  xxvii.  (1871)  p.  250  ;  Proc.  Roy.  Soc.  xxiii.  (1874)  p.  334  ;  Brit. 
Asaoc.  1880,  I^esidential  Address. 

2  Richthofen  has  suggested  that  in  Cliina  possibly  some  of  the  deep  jmrts  of  his  "  Sinisian  " 
formation  (which  in  its  higher  parts  yields  Primordial  fossils)  may  yet  reveal  traces  of  still 
older  faunas. 


BICT.  i  %  I 


CAMBRIAN  SYSTEM 


721 


and  other  calcareous  organiamB  which  abound  in  the  limestones  of  the 
next  great  geological  series.'  The  character  of  the  general  sandy  and 
muddy  sediment  must  have  determined  the  distribution  of  life  on  the 
floor  of  the  Cambrian  sea  in  that  region,  and  doubtless  has  also  affected 
the  extent  of  the  final  preservation  of  organisms  actually  entombed. 
Ill  North  America,  on  the  other  hand,  where  thick  sheets  of  Cambrian 
limestone  occur,  the  conditions  of 
sedimentation  have  been  far  more 
favourable  for  the  preservation  of 
organic  forma  ;  hence  the  known 
Cambrian  fauna  of  this  region 
exceeds  in  numerical  value  that  of 
Europe. 

The  plants  of  the  Cambrian 
period  have  been  scarcely  at  all 
preserved.  No  vestige  of  any  land 
plant  of  this  age  has  yet  been 
detected.  That  the  sea  then  pos- 
sessed its  sea-weeds,  can  hardly  be 
doubted,  and  various  fucoid-like 
markings  on  slates  and  sandstones 
{f.ff.  the  so-called  fucoida  of  the 
"fucoid-beds"  of  N.W.  Scotland, 
and  of  the  "fucoidal  sandstone" 
of  Scandinavia)  have  been  referred 
to  the  vegetable  kingdom.  The 
genus  Eirpkijioii^  from  Sweden,  and 
others  from  the  Potsdam  sand 
stone  of  North  America,  have  been    •'is- »wi.-oi™-iiii'(( 

J  'i_    J  1       .  rrw.  trorth)  th«  cluLractTlHtic  flpriu?i  of  Ihe  lowest 

described  as  plants.     There  seems         c»iNbri«h»(niu.(jx 
to  l>e  little  doubt,  however,  that 

of  these  various  markings  some  are  tracks,  probably  of  worms,  others 
are  merely  imitative  wrinldes  and  markings  of  inorganic  origin.^  It  is 
not  certain  that  any  of  them  are  truly  plants.  What  has  been  i-egiirded 
aa  an  undoubted  organism  occurs  in  abundance  in  the  Cambrian  rocks 
of  the  south-east  of  Ireland,  and  is  named  OUIhainiu  (Fig.  338).  For 
many  years  it  was  considered  to  be  a  sertularian  zoophyte,  subsequently 
it  was  referred  to  the  calcareous  algte ;  but  its  true  grade  seems  still 
uncertain.' 

'  In  the  Baltic  baajn  aome  banilt  of  limeatons  occur  in  the  compantivel;  thin  Knva  of 
Cambrian  strata.      In  ScDtlsnd  the  Camhriaa  syiteni  includes  aoine  ISOO  feet  of  llmeHloiie. 

'  Si«  G.  J.  Hiiide,  Gtol.  Mag.  1888,  p.  387  ;  the  "fiicoids"  of  the  ■' fuooid-beda  "  of 
N.W.  Scotland  are  andoubtedly  vorm-cast*. 

*  See  A.  G.  Nutborat'K  esfiay,  "Nouvelles  obKrvations  lur  den  traces  tl'Animaui,  etc." 
4ta,  Stockholm,  1886. 

''  Its  claim  to  be  consiiteivd  organic  has  even  been  disputed,  but  firom  the  nunner  iti 
which  it  occurs  on  saccesBive  thin  laminae  ot  deposit  I  cannot  doubt  that  it  is  really  Of 
otgKnic  origin. 

3.1 


STEATIGRAPHICAL  GEOLOGY 


BOOK  VI  PAKT  n 


Among  tho  animal  organisms  of  the  Cambrian  rocks  the  most  lowly 
forms  yet  detected  are  hexactinellid  sponges,  J'rotospongia^  (Fig-  338X 
Leplomi/lus,  Tradii/um.  The  hydrozoa  appear  in  the  earliest  forms  of  tlie 
tribe  of  graptolites  which  played  such  an  important  part  in  Siluriui 
time.  Of  the  Cambrian  types,  Dklij<'<fniplus  (JHctyonewa)  is  one  of  the 
moat  characteristic  fossils  of  the  primordial  zone  of  Scandinavia,  and 
other  forms  are  doubtfully  referred  to  PhyllograpUis,  CliTOacngraptui, 
and  DiidyloUlUej!.  Casts  which  are  regarded  as  those  left  by  medusae 
on  the  soft  mud  of  the  sea-shore,  have  been  noticed  in  Scandinavia.  The 
Actinozoa  of  the  Cambrian  period  occur  in  a  number  of  early  type*  of 


nviiliK,  HhIl  (,V);  3,  (^mocorrphf  (f)  Williiiuiiml. 
^..^^^^,  ..,  nf^nntttUH  {iriiK^u}*,  Halt,  (eularffd);  fl,  MierodiKui 
Lffiimtu*  Oirluwii,  Itrll,  (piilnninl) :  S,  Ertnnye  cenuloa,  Bilt. : 
-   '  unlminKHickHdindKiUiiieil):  ll.DJkeloceiihiltH 


corals  referred  to  Jrclixocnat/itts,'  Hfkwiplii/Uum  and  Spirofijaihus.  The 
Ki-hinodermata  are  represented  by  crinoids  {Df^iidrocrinus),  cystideans 
(Proloei/stilfs,  Fig.  338,  KoeyslUes),  and  star-fishes  (Palxasieriita,  Fig.  339). 
The   crinoids  reached   their  culmination   in   a  variety  of   forms  during 


'  For  a  deHcriptiou  of  the  cliarocter  at  tli 

vi.  (18801  p.  362. 

-  Where  tiat  otherwiae  ststeil  the  Hgnraa  ire  of  the  unturnl  slie, 

=  Hiiide,  QiKirf.  Jniim.  Gtnl.  Soe.  x\v.  (188B)  p.  125. 


sponge,  see  SotlAS,  Q.  J,  GeoL  Sec 


SECT,  i  S  1 


CAMBRIAN  SYSTEM 


Palaiozoic  time.  Though  still  eaonnously  abundant  in  individuals  on 
some  parts  of  the  present  sea-floor,  they  are  but  poorly  represented  there 
compared  with  the  profusion  of  their  genera  and  species  in  the  earlier 
pci'iods  of  the  earth's  history.  Paheozoic  crinoids  were  distinguished  by 
the  vaulted  arrangement  of  accurately  fitting  plat«s,  by  which  their 
viscera  were  completely  enclosed,  after  the  manner  of  the  sea-urchins. 
The  cyatideana  were  so  named  from  the  bag-like  form  in  which  the 
iwlygunal  plates  enclosing  them  are  arranged. 

That  annelides  existed  during  the  Cambrian  period  is  shown  by  their 
frequent  trails  and  burrows  (Arenicoliles,  Fig.  338,  Ormiaaa,  Scoliihua, 


FtK.  *S«,-Orou 

porCkinbruiiFo»i]H. 

.lite-  aiJv.i 

VHpsi,,    HiQl 

HUH,  Salt.  iJ,01.lhiiiii>anti 

HrmU,  Bolt.  (>ii<l  cnl«Be<l  t)  I  «,  D^l""  I>il«ului,  H»ri)> 

J'taHoiites,  Ac.)  But  the  most  abundantly  preserved  forma  of  life  are 
Crustacea,  chiefly  I>elonging  to  the  extinct  order  of  trilobites  (Figs.  336, 
337).  It  is  a  suggestive  fact  that  these  organisms  appear  oven  here,  as 
it  were,  on  the  very  threshold  of  authentic  biological  history,  to  have 
reached  their  full  stnictural  development  Some  of  them,  indeed,  were  of 
dimensions  scarcely  ever  afterwards  equalled,  and  already  presented  great 
variety  of  form.  Individuals  of  the  species  Piiradoxtdea  Davidit  are  some- 
times nearly  two  feet  long.  But  with  these  gianta  were  mingled  other 
types  of  diminutive  size.  It  is  not«worthy  also,  as  Dr.  Hicks  has  pointed 
out,  that  while  the  trilobites  bad  attained  their  maximum  size  at  this 
early  period,  they  were  represented  by  genera  indicative  of  almost  every 


724 


STRATIGRAPHICAL  GEOLOGY  BOOKViPAurn 


stage  of  development,  "  from  th(:  little  Agnodtts  with  two  rings  in  tke 
thorax,  and  Mkrodiscvs  with  foui-,  to  Erinnijs  with  twenty-four,"  while 
blind  genera  occurred,  together  with  those  having  the  largest  eyes,*  In 
the  lower  portions  of  the  system  the  genus  Olfnelivs  (Fig.  336)  is 
especially  distinctive.  Other  characteristic  Cambrian  genera  (Fig.  337) 
besides  those  already  mentioned  are  Flulimia,  Ellipsoapkaius,  Conoeorypie 
(Vonofepkali/es),  Jnomocure,  Ap'a^os,  Pti/ckc^irut,  Solenopteura,  DikeloMphalMt, 


I,  Couiilurk  Hcniirnyi,  Italt. 
HniikliiHini,  lll<-k» ;  i,  Hjn 
IlJrliK  {riilarKiHl). 

Ohnus,  Olenoides,  and  Anopoleniis.  Phyllopod  crustaceans  likewise  occur 
{HymenocarU,  Fig.  339,  Aristoxot),  and  there  are  likewiae  representatives 
of  the  living  order  of  oatracode  {Lfperdttm). 

lu  striking  contrast  to  the  thoroughly  Palreozoic  and  long  extinct 
'  Q.  J.  Qeol.  Soe.  iiviii.  p.  \1i. 


SECT,  i  §  2  CAMBRIAN  SYSTEM  725 

order  of  trilobites,  the  brachiopods  appear  in  genera  of  the  simple  non- 
articulated  group  which  are  still  familiar  in  the  living  world ;  but  the 
more  highly  organised  articulate  division  is  also  represented.  Lingula 
and  Discimi  (Fig.  338),  which  appear  among  these  ancient  rocks,  have 
persisted  with  but  little  change,  at  least  in  external  form,  through  the 
whole  of  geological  time  and  are  alive  still.  Other  genera  are  LingtdeUii , 
(Fig.  339),  Acrotreta,  Oholella  (Fig.  338),  Kutorgiiui^  lAnnarsmfiia^  Orihis 
(Fig.  339),  and  Orthisina,-  Every  class  of  the  true  mollusca  had  ite 
representatives  in  the  Cambrian  seas.  The  lamellibranchs  occurred  in 
the  genera  Ctenodonia  (Fig.  339),  Palssarca  (Fig.  339),  Davidia^  Modiolopsis^ 
and  FordUla.  The  gastcropods  were  present  in  the  heteropod  genus 
Belleroplwn  (Fig.  339),  so  characteristic  of  Palaeozoic  time,  also  in  Scenella, 
Stenotheca^  Platyceras^  and  Pleurotomaria.  The  pteropods  were  represented 
by  the  genera  HyolWies  or  Theca  (Fig.  338)  HyolUhellus,  Salterella  and 
Canularia  (Fig.  339),  the  cephalopods  by  Ortlwceras  (Fig.  339). 

Taking  palaeontological  characters  as  a  guide  in  classification,  and 
especially  the  distribution  of  the  trilobites,  geologists  have  grouped  the 
Cambrian  rocks  in  three  divisions — the  lower  or  Olenellus  group,  the 
middle  or  Paradoxidian,  and  the  upper  or  Olenidian. 

§2.    Local    Development 

BritaixL^ — The  area  in  which  the  fullest  development  of  the  oldest  known  Pabeozoic 
rocks  lias  yet  been  found  is  undoubtedly  the  principality  of  Wales.  The  rocks  are 
there  of  gi*eat  thickness  (12,000  feet  or  more),  they  have  yielded  a  fauna  wliich,  though 
.somewliat  scanty,  is  suflScient  for  purposes  of  stratigraphical  correlation,  and  tliey 
jwssess  additional  importance  from  the  fact  that  they  were  the  first  strata  of  sucli 
antitpiity  to  be  worked  out  stratigrapliically  and  palaiontologically.  As  already  stated, 
they  were  called  Cambrian  by  Sedgwick,  from  their  extensive  development  in  North 
Wales  (Cambria),  where  lie  originally  studied  them.  Their  true  base  is  nowhere  seen. 
Professor  Hughes,  Dr.  Hicks,  Professor  Bonney  and  others  believe  that  a  conglomerate 
and  grit  generally  mark  the  base  of  the  Cambrian  series.^  According  to  Sir  A.  C. 
Ramsay,  on  the  other  hand,  the  base  of  the  Cambrian  series  is  either  concealed  by  over- 
lying foiTiiations  or  by  the  metamorphism  which,  in  his  opinion,  has  converted  portions 
of  the  Cambrian  series  into  various  crystalline  rocks.  Both  in  Pembrokeshire  and 
Carnarvonshire  the  lowest  visible  slates,  shales,  and  sandstones  are  intercalated  with 
and  ]>ass  down  into  a  volcanic  series  (felsites,  diabases,  and  tuffs)  the  base  of  which  has 
not  been  found.^  In  certain  localities,  as  in  Anglesey,  Cambrian  strata  are  seen  to  lie  un- 
confomiably  on  prc-Cambrian  schists,  and  there  not  only  the  basement  volcanic  group  but 
some  of  the  lowest  members  of  the  fossiliferous  series  are  wanting.  There  is  then  not 
only  an  unconformable  junction,  but  an  overlap. 

*  See  Sedgwick's  Memoirs  in  Quart,  Jmtm,  Qtfji.  Sue.  vols.  i.  ii.  iv.  viii.,  and  his  *  Synopsis 
of  the  Classification  of  the  British  Palaeozoic  Rocks,'  4to,  1855  ;  Murchison's  *  Silurian 
System'  aud  'Siluria' ;  Clalter's  'Cat.  of  Cambrian  and  Silurian  Fossils,*  with  preface  by 
Sedgwick,  1873  ;  Ramsay's  *  North  Wales,'  Geological  Survey  Memoirs^  vol.  iii.  ;  and  papers 
by  Salter,  Harkness,  Hicks,  Hughes,  and  others  in  the  Quart,  Jnurn.  Oeol.  Soc.  and  Oeol. 
M(t(j.y  to  some  of  which  reference  is  made  below.  J.  E.  Marr,  in  his  'Classification  of  the 
Cambrian  and  Silurian  Rocks,'  gives  a  bibliography  of  the  subject  up  to  1883. 

^  Q.  J.  (ieof.  Soc.  xxxiv.  p.  144  ;  xl.  (1884)  p.  187.  For  references  to  the  literature  of 
the  subject  see  the  same  Journal,  xlvii.  (1891)  Ann.  Address,  p.  90  seq. 


726  STBATIGRAHPICAL  GEOLOGY  BOOKViPABin 

Starting  from  the  volcanic  group  at  the  base  the  geologist  can  trace  an  apwmrd 
succession  through  thousands  of  feet  of  grits  and  slates  into  the  Silurian  systom. 
Considerable  diversity  of  opinion  has  existed  as  to  the  line  where  the  upper  limit  of  the 
Cambrian  division  should  be  drawn.  Murchison  contended  that  this  line  should  be 
placed  below  strata  where  a  trilobitic  and  brachiopodous  fauna  begins,  and  that  these 
strata  cannot  be  separated  from  the  overlying  Silurian  system.  He  therefore  indadsd 
as  Cambrian  only  the  barren  grits  and  slates  of  Harlech,  Llanberis,  and  the  Longmynd. 
Sedgwick,  on  the  other  hand,  insisted  on  carrying  the  line  up  to  the  base  of  the  Upper 
Silurian  rocks.  He  thus  left  these  rocks  as  alone  constituting  the  Silurian  system,  and 
massed  all  the  Lower  Silurian  rocks  in  his  Cambrian  system.  Murchison  worked  oat 
the  stratigraphical  order  of  succession  from  above,  chiefly  by  help  of  organic  remains. 
He  advanced  from  where  the  superposition  of  the.rocks  is  clear  and  undoubted,  and  for 
the  first  time  in  the  history  of  geology,  ascertained  that  the  '* Transition-rocks"  of  the 
older  geologists  could  be  arranged  into  zones  by  means  of  characteristic  fossils,  as  satis- 
factorily as  the  Secondary  formations  had  been  classified  in  a  similar  manner  by  William 
Smith.  Year  by  year,  as  he  found  his  Silurian  types  of  life  descend  farther  and 
farther  into  lower  deposits,  he  pushed  backward  the  limits  of  his  Silurian  system.  In 
this  he  was  supported  by  the  general  consent  of  geologists  and  palaeontologists  all  over 
the  world.  Sedg>vick,  on  the  other  hand,  attacked  the  problem  rather  from  the  point 
of  stratigraphy  and  geological  structure.  Though  he  had  collected  fossils  from  many  of 
the  rocks  of  which  he  had  made  out  the  true  order  of  succession  in  North  Wales,  he 
allowed  them  to  lie  for  years  unexamined.  Meanwhile  Murchison  had  studied  the  pro- 
longations of  some  of  the  same  rocks  into  South  Wales,  and  had  obtained  from  them  the 
copious  suite  of  organic  remains  which  characterised  his  Lower  Silurian  formations. 
Similar  fossils  were  found  abundantly  on  the  continent  of  Europe  and  in  America. 
Naturally  the  classification  proposed  by  Murchison  was  generally  adopted.  As  he 
included  in  his  Silurian  system  the  oldest  rocks  then  known  to  contain  a  distinctive 
fauna  of  trilobites  and  brachiopods,  the  earliest  fossiliferous  rocks  were  everywhere 
classed  as  Silurian.  The  name  Cambrian  was  regarded  by  geologists  of  other  countries 
as  the  designation  of  a  British  series  of  more  ancient  deposits  not  characterised  by 
lK?culiar  organic  remains,  and  therefore  not  capable  of  being  elsewhere  satisfactorily 
recognised.  Barrande,  investigating  the  most  ancient  fossiliferous  rocks  of  Bohemia, 
distinguished  by  the  name  of  the  ' '  Primordial  Zoue  "  a  group  of  strata  forming  the 
lowest  member  of  the  Silurian  system,  and  containing  a  peculiar  and  characteristic  suite 
of  trilobites.  Murchison  adopted  the  term,  grouping  under  it  the  lowest  dark  slates 
which  in  Wales  and  the  border  English  counties  contained  some  of  the  same  early  forms 
of  life. 

Subsequent  investigations,  by  the  late  Mr.  Salter  and  Dr.  Hicks,  brought  to  light, 
from  the  Primordial  rocks  of  Wales,  a  nmch  more  numerous  fauna  than  they  were 
supposed  to  possess,  and  one  in  some  degree  distinct  from  that  in  the  undoubted  Lower 
Silurian  rocks.  Thus  the  question  of  the  proi)er  base  of  the  Silurian  system  was  re- 
opened, and  much  controversy  arose  as  to  the  respective  limits  and  relative  stratigraphical 
value  of  the  formations  to  be  included  under  the  designations  Cambrian  and  Silurian. 
No  such  marked  break,  either  i)al{eontological  or  stratigraphical,  had  been  found  as  to 
atford  a  clear  line  of  division  between  two  distinct  "systems."  Those  who  followed 
Murchison  contended  that  even  if  the  line  of  division  were  drawn  at  the  upper  limits 
reached  by  the  primordial  fauna,  the  Cambrian  could  not  be  considered  to  be  a  system 
as  well  defined  and  important  as  the  Silurian,  but  that  it  ought  rather  to  be  regarded  as 
the  lower  member  of  one  great  system  comprising  the  primordial,  and  the  second  and 
third  faunas,  so  admirably  worked  out  by  Barrande  in  Bohemia.  To  this  system  they 
maintained  that  the  name  Silurian,  in  accordance  with  prioiity  and  justice,  should  be 
assigned.  Unfortunately  a  disagreement,  which  was  not  settled  during  the  lifetime  of 
Sedg\vick  and  Murchison,  bequeathed  a  dispute  in  which  personal  feeling  played  a  large 
|»art.     And  though  the  fires  of  controversy  have  died  out,  it  cannot  be  said  that  the 


BECT.  i  §  2 


CAMBRIAN  SYSTEM 


727 


questions  in  debate  have  been  left  in  a  wholly  satisfactory  footing.  For  myself  I  repeat 
what  I  have  said  in  previous  editions  of  this  text -book,  that  the  most  natural  and 
logical  classification  is  to  group  Barrande's  three  faunas  as  one  system  which  in  accord- 
ance with  the  laws  of  priority  should  be  called  Silurian.  But  as  this  arrangement  has 
not  been  generally  adopted  in  this. country  I  retain  the  Cambrian  in  the  position  which 
has  here  been  usually  assigned  to  it.^ 

The  Cambrian  rocks  of  Britain  vary  widely  in  mineralogical  composition,  thick- 
ness, and  area  of  exposure  in  the  different  districts  where  they  rise  to  the  surface.  In 
North  Wales,  where  they  cover  the  widest  extent  of  ground,  they  consist  of  purple, 
reddish-grey,  green,  and  black  slates,  grits,  sandstones,  and  conglomerates,  with  a  volcanic 
group  at  the  bottom,  the  whole  attaining  a  thickness  of  probably  more  than  12,000  feet. 
In  Western  England  this  enormous  mass  of  sedimentary  material  has  dwindled  down  to 
a  fourth  or  less,  consisting  at  the  base  of  quartzite  and  sandstone,  and  in  the  upper 
part  of  shales.  In  the  East  of  Ireland,  rocks  assigned  to  the  Cambrian  system  resemble 
on  the  whole  the  Welsh  type.  In  the  north-west  of  Scotland,  on  the  other  hand,  the 
Cambrian  strata,  about  2000  feet  thick,  consist  of  quartzites  below,  graduating  upwards 
into  massive  limestones.  The  following  grouping  of  the  British  Cambrian  rocks  has 
been  made : — 


Wales 

(ranging  up  to  12,000  feet  or 

more). 

(Tremadoc  Slates. 
Lingula  Flags 
{LingudlUy  OienuSf  &c. ) 
Middle  or  Para-  (  Meneviau  Group  {Para- 
doxides  Zones.    \  doxides). 

{Harlech  and  Llanberis 
group  and  basement  vol- 
canic rocks  ("  Pebidian  " 
of  Dr.  Hicks,  p.  710), 
bottom  not  seen. 


ellus  Zones. 


Western  England 
(about  8000  feetX 

Shineton  Shales  [Dictyo- 
graptus  {Dictynema) 
OfeiiH^y  &c.) 
Conglomerates  and  lime- 
stones (Comley)  with 
ParadoxideSf  &c. 
Thin  quartzite  passing 
up  into  green  flags,  grits, 
shales,  and  sandstone 
(Comley  Sandstone)  con- 
taiuuig  Olendlun. 


N.W.  Scotland 
(2000  feetX 

A  thick  mass  of  lime- 
stone divisible  into 
seven  groups  with 
ArchafocyqthuSf  Mclc- 
lurea,  Ophilrtay  Mnr- 
chisonia,  Orthoceras, 
and  vast  quantities  of 
annelid  castings. 
Shales  with  Oien^liis, 
Scdterella. 

Quartzites,  with  anne- 
lid burrows  (p.  699). 


Lower." — In  South  Pembrokeshire  the  lowest  visible  Cambrian  rocks  are  of  volcanic 
origin.     They  consist  of  fine  tuffs,  and  silky  schists  with  sheets  of  olivine-diabase  and 


^  After  the  first  edition  of  this  work  was  written,  ui  which  the  future  merging  of 
Cambrian  and  Silurian  into  one  great  system  was  regarded  as  probable,  M.  Hebert  thus  ex- 
pressed himself :  '*  I  adopt  the  opinion  of  M.  Barrande,  based  as  it  was  on  such  thorough 
and  prolonged  research,  that  there  is  one  common  character  in  his  three  first  faunas  which 
unites  them  into  one  great  whole.  To  these  faunas  and  the  beds  containing  them  I  assign 
the  name  Silurian,  because  the  Silurian  fauna  was  the  first  to  be  determined  ;  and,  further, 
I  am  of  opinion  that  the  Cambrian  group  ought  not  to  appear  in  our  nomenclature  as  of  equal 
rank  with  the  Silurian  group,  of  which  it  is  merely  a  subdivision." — Bidl,  Soc,  OM,  France 
(3)  xi.  (1882)  p.  34.  F.  Schmidt,  also,  would  prefer  to  regard  the  Cambrian  as  only  part 
of  one  system  extending  up  to  the  overlying  unconformable  Devonian  rocks.  Q.  J,  Oeol, 
Soc.  xxxviii.  (1882)  p.  515.  My  friend  Prof.  De  Lapparent  has  followed  the  same  principle, 
making  the  Silurian  system  range  from  the  base  of  the  primordial  zone  to  the  base  of  the 
Devonian  rocks.     *  Traits  de  G^logie,'  3rd  Edit  (1893).     See  aho  postea,  p.  737. 

^  The  chief  authority  on  the  fossils  of  the  Lower  Cambrian  rocks  is  the  monograph  by 
C.  D.  Walcott,  "The  Fauna  of  the  Lower  Cumbrian  or  Olendlus  Zone,"  published  in  the 
10/^  Ann.  Rep.  U.S.  Oeol.  Surv.  (1890).  This  work  contains  figures  and  descriptions  of 
this  the  oldest  known  distinct  assemblage  of  organisms,  and  gives  a  bibliography  of  the  sub- 
ject  up  to  the  year  of  publication.  Some  of  the  other  more  important  memoirs  will  be  cited 
in  subsequent  pages. 


728  STRATIGRAPHICAL  GEOLOGY  book  vi  paw  n 

andesite,  aud  intnisive  quartz-porphyries.^  It  is  this  volcauic  group  which  Dr.  Hieki 
has  proposed  to  class  as  a  pre-Camhrian  formation  under  the  name  of  '*  Pebidian."  In 
Carnarvonshire  the  Llanberis  Slates,  which  form  the  lowest  member  of  the  Cambrian 
sedimentary  series,  arc  interleaved  at  their  base  with  bands  of  volcanic  tuffs  and  nft 
uix}n  a  mass  of  quartz-felsite  which  is  the  lowest  rock  visible  in  the  district.' 

The  Olenellus  zone  which  is  the  characteristic  feature  of  the  lower  Cambrian  group 
has  not  yet  been  certainly  established  in  Wales.'  It  was  first  detected  in  the  British 
Isles  by  Prof.  Lapworth,  who  in  1885  found  fragments  of  Olenellus  on  the  flanks  of  Ca«r 
Cai-adoc  in  Shropshire,  associated  with  Kutargina  cingvJaia,  Linnarsaoma  aitgiUtUis, 
ITyolithellus  and  Ellipsocephalus.*  It  has  been  found  by  the  officers  of  the  Geologkal 
Survey  in  the  west  of  Koss-shire,  where  the  following  lower  Cambrian  strata  may  be 
traced  in  a  narrow  strip  of  country  for  a  distance  of  more  tlian  100  miles :  ^ — 

Base  of  Durness  limestones  with  Salterella, 

Band  of  quartzite  and  gnt  (Serpulite  grit)  with  abundant  Scdterdla  McuxuUoekU 
and  occasionally  thin  shales  with  Oletidliis, 

Calcareous  and  doloniitic  shales  (*'  Fucoid  beds ")  with  numerous  worm-casts 
usually  flattened  and  resembling  facoidal  impressions.  Oteiidhis  occurs  in  bands 
of  dark  blue  shale. 

Quartzites,  in  two  divisions,  the  upper  crowded  with  worm-burrows,  the  lower  be- 
coming pebbly  at  the  base  and  resting  uncouformably  on  pre-Cambrian  rocks 
(Torridonian  or  Lewisiau). 

Middle. — This  group  api>ears  to  be  most  fully  develoi>ed  in  South  Wales,  where  it 
was  first  studied  by  Dr.  Hicks,  and  found  to  jrield  a  number  of  characteristic  fossils. 
He  has  divided  it  into  two  groui)8,  the  Solva  below  and  Menevian  above.  From  the 
lower  group  a  number  of  trilobites,  including  the  typical  genus  ParadoxideSt  have  been 
obtained,  also  Plutonia^  MicrodiseuSf  Agnostus,  Conocoryphc.  Tliere  occur  likewise 
annelides  {AreiiicolUes)^  brachiopods  {IHseina^  Lingulell^),  pteropods  {Theca),  and  a 
sponge  {Protospongia), 

The  name  Menevian  was  j)roposed  by  J.  W.  Salter  and  Dr.  Hicks  for  a  series  of 
sandstones  and  shales,  with  dark-blue  slates,  flags,  and  grey  grits,  which  are  seen 
near  St.  David's  (Mcncvia),  where  they  attain  a  depth  of  about  600  feet.  They  pass 
conformably  into  the  Lower,  and  also  into  the  Ui)jier  grouj).  They  have  yielded 
upwards  of  50  sjK'cies  of  fossils,  among  which  trilobites  are  sj)ecially  prominent 
Paradoxides  is  the  tyjiical  genus,  while  Agnostus  and  Cwiocoryphc  are  of  frequent 
occurrence.  Sponges  {Pi'oiospongia)  and  annelide-tracks  likewise  occur.  The  raollnsca 
are  represented  by  brachioiK)ds  of  the  genera  Di^cina,  Liiigtdella,  Obvlellaj  and  Orthis; 
and  by  pteropods  (Ci/riothcca,  Thrca),  An  entoniostracan  {EiUamis)  and  cystidean 
(ProtoajstHes)  have  also  been  met  with. 

Ui'PER. — This  highest  section  of  the  system  has  long  been  divided  in  Wales  into  two 
well-marked  gioups  of  strata,  the  Lingula  Flags  below  and  the  Tremadoc  Slates  above. 
As  already  stated,  its  characteristic  iMilajontological  feature  is  the  prevalence  of  trilobites 
of  the  genus  OUniiS. 

Linyula  Flags. — These  strata,  consisting  of  bluish  and  black  slates  and  flags,  with 
bands  of  grey  flags  and  sandstones,  attain  in  some  |)arts  of  Wales  a  thickness  of  more 
than  5000  feet.  They  received  their  name  from  the  vast  numbers  of  a  lingula 
{LingulcUa  Davisii)  in  some  of  their  layers.     They  rest  conformably  upon,  and  pass 

^  Quart.  Jouni.  Geol.  Sttr.  xxxix.  (1883)  p.  294,  C.  Lloyd  Morgan,  op.  cit.  xlvi.  (1890) 
p.  241. 

-  Oji.  cit.  xlvii.  (1891).     Presidential  Address,  p.  90,  and  authorities  there  cited. 

^  Dr.  Hicks  believes  that  it  exists  there,  Oeol.  Ma//.  1892,  p.  21. 

*  Lapworth,  Oeai.  Mag.  1888,  p.  484  ;  1891,  p.  529. 

'  Brit.  AssiK.  Rep.  1891,  p.  633.  Peach  and  Home,  Qiuirt.  Journ,  Oecl.  Soc.  xlviii. 
(1892)  p.  227. 


SECT,  i  §  2  CAMBRIAN  SYSTEM  729 

do^^ii  into,  the  Menevian  group  below  them,  and  likewise  gradutite  into  the  Tremadoc 
group  above.  They  are  distinguished  by  a  characteristic  suite  of  oi^nic  remains.  The 
trilobites  include  the  genera  Olcnus,  Agnostus^  AnopoleiuiSf  Conocoryphe,  Dikelocephaltu, 
ErinnySt  and  Paradoxidea,  Early  forms  of  phyllopods  {Hymenacaria)  and  heteropods 
(Bdlerophon)  occur  in  these  strata.  The  brachiopods  include  species  of  Lingulella 
{L.  Dam8ii)y  Diacina,  Oholella,  KiUorginc^  and  Orthis,  The  pteroijods  are  represented 
by  species  of  Theca,  Several  annelides  {Cniziana)  and  polyzoa  {Fenestella)  likewise 
occur. 

A  subdivision  of  the  Lingula  Flags  into  three  sub-groujw  has  been  proposed  by  Mr. 
T.  Belt,  in  descending  order  as  follows  : ' — 

3.  Dolgelly  slates,  about  600  feet,  well  seen  at  Dolgelly,  consist  of  soft  and  hard 
blue  slates  and  contain  Protoapongia,  Lingulellay  Orthia  lentic^daria^  Olenua 
acaraba»oidea,  0.  apintdoaua^  Agnoahia  triatctus^  Conocoryphe  abdiUi, 

2.  Ffentiuiog  flags,  about  2000  feet,  well  Keen  at  Ffestiniog,  consist  of  hard  sandy 
micaceous  flagstones,  and  have  yielded  Lingvldla  Dariaii^  Olenua  viicnirua, 
Hymeiwcaria  vermicauda,  Bdlerophon  cambrenaia. 

1.  Maentwrog  flags  and  slates,  about  2500  feet,  best  seen  at  Maentwrog  in 
Merionethshire,  consist  of  grey  and  yellow  flagstones,  and  grey,  blue^  and  black 
slates,  and  contain  among  their  somewhat  scanty  fossils,  Olenua  ccUaracUa, 
0.  gibboauaf  Agnoatua princepa  {piai/onnis),  A.  nodoaua. 

Tremadoc  Slates. — This  name  was  given  by  Sedgwick  to  a  group  of  dark  grey  slates, 
about  1000  feet  thick,  found  near  Tremadoc  in  Carnarvonshire,  and  traceable  thence  to 
Dolgelly  in  Merionethshire,  and  reappearing  l>eyond  the  eastern  side  of  Wales  at  the 
Wrekin,  in  Shropshire.'  Their  importance  as  a  geological  fonnation  was  not  recognised 
until  the  discovery  in  them  of  a  remarkably  abundant  and  varied  fauna,  which  now 
numbers  more  than  80  species,  including  early  forms  of  crinoids,  star-flshes,  lamelli- 
branchs,  and  cephalopods.  The  trilobites  embrace  some  genera  {OlenuSy  Agnoslua, 
Conocoryphe f  Dikelocephalua^  &c. )  found  in  the  Lingula  flags,  but  include  also  new  forms, 
{Angelinay  AsaphuSf  Cheirurus,  Neseureltiaf  XiobCy  Ogygia^  PailoeeplMlua).  The 
phyllopods  are  represented  by  Ceratiocaris  and  Lingulocaria.  The  same  genera,  and  in 
some  cases  s|)ecies,  of  brachiopods  appear  whicli  occur  in  tlie  Lingula  flags,  Orthia  lenticu- 
laris  and  Lingnlella  Daviaii  being  common  fonns.  Dr.  Hicks  has  described  12  species 
of  lamellibranclis  from  the  Tremadoc  rocks  of  Ramsey  Island  and  St.  David's,  belonging 
to  the  genera  Ctenodontay  Palamrcay  Glyptarca^  Davidia,  Modiolopaia.  The  cephalojKxiH 
are  represented  by  Orthoccras  aericeum  and  Cyrtoctraa  prsccox  ;  the  pterojKxls  by  Theca 
JJavidiiy  T.  opcrculcUa,  and  Conularia  Homfrayi :  tlie  echinoderms  by  a  beautiful  star- 
fish {Palfeaaterina  raitiseyenaia)  and  by  a  crinoid  {Dendrocrinua  cambrenaia).^  Careful 
analysis  of  the  fossils  suggests  a  sejmration  of  the  Tremadoc  sub-grou])  into  two  divisions. 
The  most  characteristic  forms  of  the  lower  division  are  Niobe  Homfrayi^  N.  menapiensia^ 
Psilocephalua  innotatuSf  Angelina  Scdgwiekii^  Aaaphua  ajinia,  and  more  ]mrticularly 
Dictyograptua  flabelli/ormia  {Dictyoncma  80ciale\  which  is  a  characteristic  fossil  of  the 
uppermost  Cambrian  rocks  in  Scandinavia  and  Russia.  Tlie  upper  division  contains 
Aaaphua  Homfrayi ^  Conocoryphe  depresaa^  and  otlier  fossils  having  a  general  lower 
Silurian  facies. 

It  is  at  the  top  of  the  Tremadoc  strata  that  the  uj)per  limit  of  the  Cambrian  or 
Primordial  formations  is  now  drawn  in  Britain.  The  late  Sir  A.  C.  Ramsay  was  of 
opinion  that  though  no  visible  uuconformability  could  be  seen  at  this  horizon,  neverthe- 
less there  was  evidence  on  a  large  scale  of  the  transgi-essive  superposition  of  the  Arenig 
rocks  upon  the  Treniailoc  Slates  and  Lingula  flags  below  them.* 

1  (f'fiJ.  Mag.  (1867)  p.  538. 

3  Callaway,  Q.  J.  iieol.  Soe.  xxxiii.  (1877)  p.  652.  Lapworth,  op,  cit.  (1888)  p.  486, 
(1891)  p.  533. 

^  Hicks,  Quart.  Journ,  (weol.  Sih\  xxix.  p.  39. 

*  Mem.  (r'eol,  Surv.  vol.  iii.  '  Geology  of  North  Wales,'  p.  250. 


7  30  STRA  TIGRA  PHICA  L  GEOLOG  Y  book  vi  pabt  n 

There  appeara  to  be  more  satisfactory  proof  of  a  distinct  palsontological  break  at 
this  stage  of  the  geological  record  in  Britain,  or  at  least  >>etween  the  lower  and  npper 
|>art  of  the  Tremadoc  sub-group.  Up  to  the  present  time  rather  more  than  aerenty 
species  of  fossils  have  been  chronicled  from  the  Tremadoc  Slates.  Of  these  so  far  as  ire 
know  at  present  only  eighteen  pass  up  into  the  Arenig  group  above.  As  these  survinqg 
si>ecics  possess  a  special  interest,  in  that  they  connect  by  a  link  of  continued  oi^ganic  life 
two  great  geological  j>eriods  of  such  remote  antiquity,  they  are  here  named — Arewieolita 
liiuaris,  Asaphvs  affinis,  A.  Hainfrayi,  Calyrtieiie  Blumenhachiiy  CheiruruB  Fredenei^ 
Ogygia  peltafa,  0.  sculatriXy  0.  Selun/niiy  LingtUa  pctalon,  L.  Davisii,  L.  Uyng,  Ortku 
Carausiiy  0.  lenticulariSy  0.  Afenapiae,  Conularia  Homfrayiy  Theca  simplex^  BeHeraphtm 
muUistriatuSf  Orthoccras  sericcum.^ 

In  the  north-west  of  Scotland,  the  discovery  of  the  Olenellus-zone,  already  referred 
to,  has  given  a  definite  geological  horizon  from  which  to  work  out  the  stratigraphioal 
succession  above  and  below.  It  has  conclusively  proved  that  the  thick  mass  of  Torridon 
sandstone,  formerly  classed  as  Cambrian,  must  now  be  relegated  to  the  pre-Cambrian 
series  {anti'^  p.  699).  Above  the  quartzite  and  shales  which  include  the  Olenellus-zone 
there  lies  a  series  of  limestones  which  attain  an  aggregate  thickness  of  about  1500  feet 
Their  original  upper  limit,  however,  cannot  now  be  ascertained,  for  it  has  been  concealed 
by  the  great  dislocations  which  have  so  complicated  the  structure  of  that  region  (see  Kgs. 
311, 334 ).  We  cannot  tell  what  additional  thickness  of  limestone  may  have  been  aocumn- 
lated  in  the  north-west  at  the  time  when  only  mud,  silt,  and  sand,  were  deposited  over 
the  southern  ]»arts  of  the  British  area,  nor  Ijy  what  kind  of  sediment  the  limestones  were 
succeeded.  The  limestones  are  most  fully  develoj»ed  around  Durness  in  the  extreme 
north-west  of  Sutherland,  where  they  have  yielded  a  large  number  of  fossils.  The  fades 
of  these  fossils,  however,  is  so  iKJculiar  that  it  has  not  yet  been  j^ssible  by  their  means 
to  correlate  tlie  rocks  containing  thom  with  the  Cambrian  formations  of  Wales.  The 
limestones  are  so  crowded  with  worm-casts  that,  as  Mr.  Peach  has  ^minted  out,  nearly 
every  particle  of  their  mass  must  have  passed  througli  the  intestines  of  worms.  Hence 
they  are  obviously  of  detrital  origin,  and  were  j)n)l>ably  formed  in  chief  part  by  small 
pelagic  animals.  Only  one  coral  has  been  found  in  them.  The  most  abimdant  fossils 
are  chambered  sliells  [OrfhoceriUites^  Lihiiks,  Nautilus) ;  next  in  number  are  gasteropoda 
(chieHy  Maclurea  and  Pleurotomaria)^  while  the  lamellibranchs  and  brachiopods  come 
last.  The  bivalves  have  their  valves  still  united,  and  the  lamellibranchs  retain  the 
[)Ositions  in  whieli  they  lived.  "All  the  apecimoiis  show  tliat  every  oj)en  sjtace  into 
which  tlie  calcareous  mud  could  gain  access,  and  the  worms  could  crawl,  is  traversed  by 
worm-casts.  In  the  case  of  the  OrthoirratUeSj  they  seem  to  have  lain  long  enough  un- 
covered by  sediment  to  allow  the  septa  to  be  dissolved  away  from  the  siphuncles  which 
they  held  in  place  ;  many  of  these  siphuncles  are  now  found  isolated."  Sponges  of  the 
genus  CaUilhinm  are  scattered  through  the  calcareous  sediment,  and  likewise  the  doubt- 
ful but  characteristic  Cambrian  fonns,  known  as  Archnroci/athus  which,  once  referred  to 
the  sponges,  are  now  thouglit  to  be  more  probably  allied  to  the  madrepores.  The  general 
assemblage  of  fossils,  as  was  originally  j)ointed  out  by  Salter,  is  of  a  distinctly  North 
American  type,  and  docs  not  resemble  that  found  in  the  slates,  flags,  and  grits  of 
Wales.  The  conditions  of  deposit  must  have  been  so  entirely  different  that  a  great 
contrast  in  the  organisms  of  the  two  areas  of  sedimentation  could  not  but  occur. 
Whether  or  not  the  contrast  further  arose  from  some  geographical  cause,  such  as  a  land- 
Iwrrier,  that  comj)letely  sejiarated  the  areas  remains  uncertain.  The  Durness  limestones, 
as  regards  their  fossil  contents  and  lithological  character,  may  be  com^mred  with  the 
Potsdam  sandstone  and  Calciferous  group  of  the  United  States  and  Canada.  They  repre- 
sent the  Middle  and  Upi»er  Cambrian,  jiossibly  part  of  the  Lower  Silurian  formations.* 

*  Tliis  list  is  compileil  from  Mr.  Etheridge's  "  Fossils  of  the  British  Islands,"  vol.  i. 
(1888). 

-  B.  N.  Peach,  Quart,  Jcnirn.  Utd.  Sor.  xliv.  (1888)  p.  407. 


SECT,  i  §  2  CAMBRIAN  SYSTEM  731 

In  the  south-east  of  Ireland  masses  of  purplish,  red,  and  green  shales,  slates,  grits, 
quartzites,  and  schists  occupy  a  considerable  area  and  attain  a  depth  of  apparently 
several  thousand  feet  without  revealing  their  base,  though  in  Wexford  they  may  possibly 
rest  on  pre-Cambrian  rocks.  Their  top  is  covered  by  unconformable  formations  (Lower 
Silurian  and  Lower  Carboniferous).  They  have  yielded  Oldhamia^  also  numerous  burrows 
and  trails  of  annelides  {Histioderma  hibemicumy  Arenicolites  didyviuSy  A,  aparftuSf 
Haughtonia  poscild).  In  the  absence  of  fossil  evidence  it  is  imi)ossible  to  bring  these 
strata  into  correlation  with  those  of  Wales.  Some  portions  of  them  have  been  consider- 
ably metamorphosed.  On  the  Howth  coast  they  appear  as  slates,  schists,  and  quartzites, 
and  include  there  some  remarkable  breccias,  as  well  as  single  blocks  of  stone  scattered 
through  the  slates.^ 

Continental  Europe. — According  to  the  classification  adopted  by  M.  Barrande,  the 
fauna  of  the  older  Paleeozoic  rocks  of  Europe  suggest  an  early  division  of  the  area  of  this 
continent  into  two  regions  or  provinces, — a  northern  province,  embracing  the  British 
Islands,  and  extending  through  North  Germany  into  Scandinavia  and  Russia,  and  a 
central-European  province,  including  Bohemia,  France,  Spain,  Portugal,  and  Sardinia. 

Passing  from  the  British  type  of  the  Cambrian  deposits,  we  encounter  nowhere  in 
the  northern  part  of  the  continent  so  vast  a  depth  of  stratified  deposits  ;  on  the  contrary, 
one  of  the  most  singular  contrasts  in  Palaeozoic  geology  is  that  presented  by  the  develop- 
ment of  these  formations  in  Wales,  and  in  the  north  of  Europe.  The  enormous  masses 
of  sediment,  thousands  of  feet  thick,  and  with  such  uniformity  of  lithological  character, 
which  record  the  oldest  Palaeozoic  ages  in  Wales,  are  represented  in  the  basin  of  the 
Baltic  by  only  a  few  hundred  feet  of  sediments,  which  show  strongly  separated  litho- 
logical subdivisions.  Again,  while  the  English  and  Welsh  rocks  have  been  much 
disturbed  and  even  metamorphosed,  those  in  the  eastern  i)art  of  the  Baltic  basin 
remain  over  wide  tracts  hardly  altered  from  their  original  condition  of  level  sheets  of 
sand  and  clay. 

In  Scandinavia  the  Cambrian  system  lies  with  a  strong  unconformability  on  pre- 
Cambrian  rocks.  The  so-called  "Primordial  zone"  of  tliis  region  appeal's  to  be  every- 
where characterised  by  uniformity  of  lithological  comjx>sition  as  well  as  of  fossil  contents, 
consisting  mainly  of  black  shales  with  concretions  or  thin  seams  of  fetid  limestone. 
In  Scania  the  following  grouping  of  the  Cambrian  system  has  been  made,  the  whole 
thickness  of  strata  being  about  400  Norwegian  feet  (120  metres). 

3.  Oleuus  group.  About  200  feet  of  bituminous  fissile  alum-sbales,  with  nodules 
and  layers  of  fetid  limestone.  The  following  zones  in  descending  order  are 
noted  by  S.  A.  Tullberg — (k)  zone  with  Acerocare  ecome,  (i)  IHctyimema 
fiabeUifonne,  {h)  Cydognaih'us  viycropygus^  (f/)  Pdtura  scaraba^oides,  {/)  Eury- 
care  camvricome^  (e)  Paraboltna  spiniUimif  (d)  Ceratopyge  sp.,  (c)  Olmus  (the 
special  zone  of  this  genus  of  which  it  has  many  species),  (6)  Leperditia  sp., 
(a)  Agnostus  pisi/ormis. 

Professor  Brogger  has  abbreviated  this  subdivision  by  making  two  chief 
zones,  a  higher  with  Pdtura,  CydognathuSf  &c.,  and  a  lower  with  (Menus  (in 
the  strict  sense)  Parabolina,  Eurycare,  &c. 

2.  Paradoxides  group.  About  160  feet  of  sandy  shales,  alum  shales,  with  three  bands 
of  limestone,  the  lowest  (IJ  feet),  known  as  the  "  Fragnientenkalk,"  the  middle 
as  the  '  *  Exsulanskalk,"  and  the  highest  (2  to  3  feet)  the  ' '  Andrarumskalk. "  Mr. 
Tullberg  divides  the  group  into  the  following  zones  in  descending  order, 
{m)  Agtutatus  Isevigatvs,  {I)  Paradoxides  Forchhammeri.  (This  is  the  horizon 
of  the  Andrarum  limestone,  which  contains  an  abundant  fauna,  including  many 
species  of  Agiwstus  and  other  trilobites. )  (k)  Agnosdis  Lundgreni,  (i)  Para- 
doxides DavidtSy  (h)  Conocoryphe  spqualis,  (</)  Agnostus  rex,  (f)  Agnostvs 
intermediusj  (e)  Mxcrodiscus  scanicuSf  (d)  Conocoryphe  exsvlans,  (r)  Agnostus 
atamiSj  {b)  ' '  Fragmentenkalk  "  with  Paradoxides  iUandicus,  (a)  Black  alum- 
shale  with  Lingvldla^  Acrotreta,  Oboldla^  &c. 

'  Quart.  Jourti,  (Jed.  Soc,  xlvii.  (1891).     Presidential  Adilress,  p.  104. 


732  STRATJGRAPHICAL  GEOLOGY  book  vi  pabtii 

Professor  BKigger  recogiiises  two  chief  Imnds  the  higher  marked  by  Para- 
doxidas  Forchhammerif  the  lower  by  P.  iiUnidicus  I\  Tessiftij  P.  Davidis,  kc 
1.  Olenellus  group,  cousistiug  of  two  thin  bands  of  strata,  (b)  Phosphate  limeftone 
and  Randy  shale  with  Lingulelia^  Arrothr/ej  &c.,  (^0  Sandy  shales  iiossing  into 
sandstone  (greywacke-shale)  with  OUmUus  Kjendji,  EflipsocephcUns  Narden- 
skioidij  Arionfilns  priinwvuSt  Hyolithfs^  &cJ 

In  the  Christiaiiia  district  the  lowest  stage  of  the  Cambrian  series  is  90  Norwegian 
feet  thick  and  is  conijxised  of  conglomerates,  sandstones,  and  dark  shales  with  limestone. 
It  includes  the  OlcncUus  zone  and  that  of  Paradoxidts.  It  is  surmounted  by  an  upper 
stage  (ir>0  feet)  comi)OAed  of  black  slates  (alum -shales)  and  fetid  limestone,  with  Oiemu, 
&c.  This  up]>er  or  Olenua  stage  has  been  grouped  by  Briigger  into  the  following  fire 
membei-s  in  ascending  order  :  (a)  Zone  ol  Agnosliis pisiformiSy  Olrnus  trurveatus  ;  (6)  -Aim- 
Itoliiia  sjnnulosa  beds  ;  (c)  Eurycare  latum  beds  ;  {d)  shales  with  bands  and  nodules  of 
limestone,  Peltiira  scarabseoidr^  ;  («)  Dictyograptm  shales  with  Dictyograptu$  {Dieiyo- 
iir,nia)  flahelli/ormts,'^ 

Though  the  Scandinavian  Cambrian  series  is  so  nnich  thinner  than  that  of  Wales, 
it  contains  the  three  distinctive  life-platfonns  recognisable  in  Britain,  and  apjiears  thus 
to  be  a  full  ]>alieontological  and  homotaxial  e(piivalent  of  the  much  fuller  development 
of  sedimentary  material  in  Britain.  The  Cambrian  type  of  Southern  Sweden  undergoes 
considerable  modification  as  it  i>asse8  eastwards,  into  the  Baltic  provinces  of  Russia. 
The  black  shales  so  characteristic  in  Scandinavia  thin  away,  and  the  distinctive  para- 
doxidian  and  olenidian  divisions  disappear.  A  group  of  strata,  traceable  from  the  S.E. 
of  Lake  Ladoga  for  a  distance  of  about  330  miles  to  near  Baltisch})ort  on  the  Gulf  of 
Finland,  with  a  visible  thickness  of  not  more  than  100  feet  (but  pierced  to  a  depth  of 
000  feet  more  in  artesian  wells)  consists  of  three  subdivisions  ;  (a)  Blue  clay  composed 
of  a  lower  set  of  iron-sandstones  (300  feet)  resting  on  granite  and  an  upper  blue  clay 
(300  feet),  formerly  noted  only  for  some  obscure  fossils  (PlatysoUnites^  Pander,  probably 
fragments  of  cystideans)  but  now  known  to  include  the  OUiieUiis-zone  ;  {b)  Ungalite 
grit  (50  to  60  feet)  containing  Obolus  Ajtollinis  {Ungiila,  Eichw.)  Schmidiia  celata,  kc  ; 
(c)  Z>/(^i/o«<;>/?rt-shales  (al>out  20  feet)  with  Dictifo<jraptiuf  [JJictyonenia)  flnbciliformis.* 
The  n?cent  researches  of  Schmidt  have  clearly  shown  the  relations  between  these  soft  and 
seemingly  not  very  old.deiwsits  and  the  Cambrian  system  of  the  rest  of  Europe.  The 
lower  sandstone,  blue  clay  and  a  fucoidal  sandstone  lying  immerliately  above  the  latter 
form  an  unequivocally  Lower  Cambrian  group,  for  they  have  yielded  OlendlM  MickwUzi, 
Sceiiflla  disHnoides,  Mickwitzia  Dwnilifera,  Obohlla,  Discina,  VolboriheUn  (doubtfully 
referred  to  the  orthoceratites),  Plntysolcnitcs  and  Mrdnsiteft.  Schmidt  jwints  out  that  a 
complete  break  occurs  between  the  top  of  the  fucoid  sandstone  and  the  liase  of  the 

^  S.  A.  Tiillberg,  Afliand.  Sitriges  iieol.  UiuletsHkn.  ser.  C.  No.  50  (1882).  W.  C. 
Brog^'er,  <rVi)/.  Far.  SforUiofm  FOrhandl.  No.  101,  vol.  viii.  (1886)  p.  196. 

-  For  Scandinavian  Cambrian  rocks  see  Angelin,  '  Paheontologica  Suecica,'  1851-54. 
Kjenilf,  'iGeolo^ie  des  Siid.  iind  Mittl.  Norwegen,'  1880.  Dalill,  Viden.tk.  Sflnk.  FOrhamU, 
1867.  Natliorst,  Knngl.  Vtt.  Ahid.  FUrJuiniU.  1869,  p.  64,  and  'Sveriges  Geologi.' 
Torell,  Acta  Uulrenf.  Lundy  1870,  p.  14,  Kongl.  VH.  Akad.  FiirhamU.  1871,  No.  6. 
Linuarssou,  Srensk.  Vet.  Akad.  Handl.  1876,  iii.  No.  12:  'Om  Agnostus-Arterna,*  &c., 
Sceriges  (r't'ttf.  Undersfikn.  ser.  C  No.  42,  1880.  *  De  undre  Paradoxides  lageren  vid  And- 
ranini,'  op.  n't.  ser.  C.  No.  54,  1883  ;  ^W.  Mag.  1869,  p.  393  :  1876.  p.  145.  TuUberg, 
'Skiines  Graptoliter,*  Sren'gea  tied.  Undersokn.  ser.  C.  Nos.  50,  55  (1882,  3)  ;  X.  DetiUcK 
iieol.  (,'('.«.  XXXV.  (1883),  p.  223.  W.  C.  Brcigger,  Xyf.  Mag.  1876  ;  Oed.  Firen.  Stockholm 
Fnrhandl.  1875-1876,  1886,  p.  18.  '  Die  Silurischen  Etagen  2  und  3  im  Kristiauia  Gebiet, 
1S82.'  Luudgren  in  text  to  Angelin's  Geol.  Maji  of  Sweden,  X.  Jahrb.  1878.  Lapwortb, 
<tVo/.  Mag.  1881,  p.  260;  1888,  p.  484.  Marr,  C^.  J.  iieol.  Sue.  xxxviii.  (1882)  p.  813. 
*  riasxification  of  the  Cambrian  and  Sihirian  Rocks,'  1883,  pp.  72-100. 

•*  F.  Schmidt,  Qv/tri.  .fnarn.  UeU.  Sttc.  xxx>iii.  (1882)  p.  516. 


SECT,  i  §  2  CAMBRIAN  SYSTEM  733 

Ungulite  sandstone,  and  that  this  hiatus  represents  the  Paradoxidian  and  Olenidiaii 
groups,  while  the  Dictyonema- shales  form  the  characteristic  uppermost  zone  of  the 
system.^ 

In  Central  Europe,  Cambrian  rocks  appear  from  under  later  accumulations  in  Belgium 
and  the  north  of  France,  Sjiain,  Bohemia,  and  the  Thuringer  Wald.*  The  most  im- 
portant in  France  and  Belgium  is  that  of  the  Ardennes,'  where  the  princiiial  rocks  ai'e 
grit,  sandstone,  slates,  and  schistose  quartzites  or  quartz-schists  (quartzo-phyllades  of 
Dumont),  with  bands  of  whet-slate,  quartz-jwrphyry,  diabase,  diorite,  and  pori)liyroid. 
According  to  Dumont  these  rocks,  comprehended  in  his  'Terrain  Ardennais/  can  be 
grouped  into  three  great  subdivisions— Ist  and  lowest  the  '*Systeme  Devillien,"  pale 
and  greenish  ([uartzites  with  shales  or  phyllades,  containing  Oldhamia  radiata  and 
annelide  tracks  {Nereites) ;  2nd,  the  "Syst^me  Revinien,"  phyllades  and  black  pyritous 
quartzites  from  which  IMctywjraptus  fldbfUifwnnis  {IHctyoiieina  sociale\  and  wonn- 
burrows  have  been  obtained;  3ixi,  the  "Systeme  Salmien,"  consisting  mainly  of 
qiiartzose  and  schistose  strata  or  quartzo-phyllades,  and  yielding  Diciyograptiis  fiabelH- 
fonnis.  Chondrites  aiUiquus  and  Lvigula.  Tlie  Devillian  and  Keviniau  divisions  are 
united  by  Gosselet  into  one  series  composed  of  (o)  Violet  slates  of  Fumay  ;  (6)  Black 
pyritous  shales  of  Revin  ;  (c)  magnetite  slates  of  Deville  ;  (rf)  Black  pyritous  shales  of 
Bogny.  These  rocks  have  been  greatly  disturbed.  They  are  covered  uuconformably  by 
Devonian  and  later  formations.  In  the  north-west  of  France  extending  through  the  old 
provinces  of  Brittany,  the  west  of  Normandy  and  the  north  of  Poitou,  a  great 
isolated  mass  of  ancient  rocks  rises  out  of  the  plains  of  Secondary  formations,  and  the 
pre-Cambrian  rocks  already  refeiTed  to  are  there  succeeded,  with  a  more  or  less  distinct 
nnconformability,  by  a  thick  series  of  sedimentary  grou|)s  which  are  now  considered  to 
be  of  Cambrian  age.  In  western  Brittany  the  pre-Cambrian  green  silky  schists  known 
as  the  '*  Phyllades  de  Douamencz,"  which  are  believed  to  be  about  3000  metres  thick, 
are  followed,  perhaps  uuconformably,  by  purple  conglomerates,  sometimes  530  metres 
thick,  and  ()assing  up  into  red  shales  which  have  a  vertical  depth  of  2500  metres,  and  are 
surmounted  by  the  Ores  Armoricain  or  bottom  of  tlie  Silurian  system.  In  these  strata 
Scolithns  and  TigiUites  occur,  but  recognisable  fossils  are  exti-emely  rai-e,  and  no  trace  ha.s 
yet  been  found  here  of  the  more  typical  Cambrian  forms.  In  the  Itasiu  of  Kennes  con- 
siderable bands  of  limestone,  sometimes  magnesian,  together 'with  quartzites,  con- 
glomerates, and  greywackes  occur  in  the  Cambrian  series.  In  the  region  of  the  Sarthe 
iMisement  conglomerates  are  followed  by  grey  shales  with  thick  bands  of  siliceous  and 
magnesian  limestone,  above  which  lies  a  series  of  sandy  rocks  containing  Lingula  cntmena 
and  passing  under  the  ( Jres  Armoricain.*  In  southern  France  from  the  Cambrian  rocks 
which  flank  the  isolated  pre-Cambrian  axis  of  upper  Languedoc  the  most  satisfactory 
fossil  evidence  has  recently  been  obtained,  showing  the  existence  there  of  lK)th  the 
Paradoxidian    {ParadoxideSy   Conocoryphe)  and   Olenidian   divisions  of  the   Cambrian 

'  Mhn.  Acad.  Imp.  Sci.  St.  Pfterabovrfj,  xxxvi.  (1889)  No.  2. 

'  The  student  will  find  a  useful  compendium  on  the  correlation  of  the  Cambrian  and 
Silurian  rocks  of  western  Europe  by  S.  Tcimquist  in  Gfoli>g.  Ftiten.  Stockholm  FdrhandL  xi. 
(1889)  p.  299. 

*  Dumont,  '  Memoires  sur  les  Teqtiins  ArdeunaLs  et  Rh^nan,'  1847-48.  Dewalque, 
*  Prodrome  d'une  Description  Geol.  de  la  Belgique,'  1868.  Mourlon,  'G^logie  de  la 
Belgique,'  1880.  Gosselet,  'Esquisse  G^ol.  du  Nord  de  la  France,  &c.,'  1880,  and  his 
great  Monograph,  *  L'Ardenne,'  Mhti.  Carte  OM.  detaill.  4to,  1888. 

*  The  (pre-Cambrian)  phyllades  of  Brittany  and  the  (Cambrian)  purple  conglomerates 
and  red  shales  which  succeed  them  were  exhaustively  treated  by  Hubert,  Bull,  Soc,  OM.  France, 
(3)  xiv.  (1886)  p.  713.  See  also,  Tromelin  et  Lebesconte,  Btdl.  Soc  (/4ol.  France,  iv.  (1876) 
p.  583  ;  Tromelin,  Assoc.  Fran^aise  (1879),  p.  493,  Lebesconte,  Bull.  Soc.  OM.  France  (3) 
X.  p.  55,  xix.  (1891)  p.  15,  Guillier,  op.  cit.  (3)  ix.  p.  374  ;  BarroLs  op.  cit.  v.  (1877)  p.  266, 
Carte  OM.  France,  Redon  sheet. 


734  STRA  TIGRA PHIGAL  GEOLOGY  book  vi  pabt  n 

system.  *  Among  the  French  Pyrenees,  narrow  strips  and  patches  of  strata  have  been 
detected  wliich,  lying  below  fassiliferous  Lower  Silurian  rocks,  are  believed  to  be 
Cambrian.^ 

In  various  i^arts  of  Spain,  indications  of  the  presence  of  Cambrian  rocks  are  famisbed 
by  Primordial  fossils.  In  the  province  of  Seville  the  highest  beds  have  yielded 
ArchfeocyalhuSy  and  in  the  province  of  Ciudad-Keale,  Primordial  trilobites  {Eiiipto 
eep/ialus)..  But  it  is  in  the  Asturias  that  the  most  abundantly  fossiliferous  rocks  of  this 
age  occur.  They  are  grouped  by  Barrois  into  (a)  Slates  of  Rivadeo,  blue  phyllades  and 
green  slates  and  quartzites,  in  all  about  3000  metres,  and  (6)  Paradoxides  beds  of  La 
Vega  (50  to  100  metres)  composed  of  limestones,  slates,  iron-ores,  and  thick  beds  of 
green  quartzite.  In  the  upper  ])art  of  (6)  a  rich  I^mordial  fauna  occurs,  compriaing  a 
eystideau  (Trochoq/stites  bohemicus)  and  trilobites  of  the  genera  Paradoxides,  2  special 
Conocoryphe  {Conoeephaliles)^  3  species,  and  ArionelluSy  1  species." 

In  the  Thuringer  Wald  certain  phyllitcs,  clay-slates,  quartzites,  &c.,  passing  op 
into  strata  containing  Silurian  fossils  are  referred  to  the  Cambrian  system.  The 
(juartzites  have  yielded  some  indistinct  fossils  i-eferred  to  Davidia  and  Lingula.*  Bat  it 
is  in  Bohemia  that  the  central  European  type  of  the  Cambrian  system  is  best  developed 
The  classic  researches  of  Ban*ande  have  given  to  the  oldest  fossiliferous  rocks  of  that 
country  an  extraordinary  interest.  At  the  base  of  the  Bohemian  geological  formatioiii 
lie  the  slates  which  BaiTande  placed  as  his  Ktage  A  (Pi-zibram  schists),  and  which  are  no 
doubt  pre-Cambrian.  They  are  overlain  by  vast  masses  of  conglomerates,  quartritesy 
slates,  and  igneous  rocks  (Etage  B),  which  have  been  more  or  less  metamorphosed,  and 
are  singularly  barren  of  organic  remains,  though  some  of  them  have  3ielded  traces  of 
annelides  {Arenicolitcs).  They  jiass  up  into  certain  grey  and  green  fissile  shales,  in 
which  the  earliest  well-marked  fossils  occur.  The  organic  contents  of  this  Etage  C  or 
Primordial  zone  (300  to  400  metises  thick)  fonn  what  Barrande  termed  his  Primordial 
fauna,  which  yielded  him  40  or  more  species,  of  which  27  were  trilobites,  belonging  to 
the  characteristic  Cambrian  genera — Paradoxides  (12),  Agnostiia  (5),  ConuKorypht  (4), 
Ellipsoccphalm  (2),  Uydrocepiialus  (2),  Arionellm  (1),  Sao  (1).  Not  one  of  these  genera, 
save  Agnostics  (of  which  four  species  appear  in  the  second  fauna),  were  found  by  Barrande 
higher  than  his  Primordial  Zone.  Among  other  organisms  in  this  Primordial  fauna, 
tlie  brachiopods  are  represented  by  siMJcics  of  Orthis  and  Orbicula,  the  i)teroj>od8  by  Thaa^ 
and  the  echinodcrms  by  cysjideiins.  It  is  worthy  of  note  that  the  fossil  contents  of  the 
zone  on  the  opjiosite  sides  of  tlie  little  Bohemian  -basin  were  found  by  the  same  great 
j)ioneer  to  be  not  quite  the  same,  only  eight  si>e(;ies  of  trilobites  being  common  to  both 
\)e\Uj  while  no  fewer  than  27  species  were  detected  by  him  only  on  one  or  other 
side.  The  Olenidian  trilobites  which  characterise  the  upper  Cambrian  group  were  not 
observed  by  liim  in  Boheraia.*"^  More  i^ecent  researches  have  modified  some  of  the  stiati- 
graj)liical  details  of  his  work,  the  geological  structure  of  the  country  having  been  found 
to  b<'  nmch  less  simple  than  he  supj>osed.  But  the  fundamental  grouping  which  he 
estiiblished  remains  much  as  he  left  it.  A  jjortion  of  his  Stage  B,  the  whole  of  his 
Primordial  zouo,  (Stage  C),  and  a  jjart  of  the  base  of  his  Stage  D  (Lower  Silurian),  have 

^  J.  Bergeron,  Bidi.  S)c.  f#VrV.  Frnnce.f  xvi.  (1888)  p.  282,  '^tude  g^ologique  du  massif 
ancien  au  su<l  du  Plateau  central,'  1889. 

-  J.  Caralj),  '  Etudes  geol.  sur  les  hauts  massifs  des  Pyrenees  centrales,'  1888,  p.  452. 
E.  Jacquot,  Bidl.  Soc.  GtoL.  Franr^,  1890,  p.  640. 

^  Barrande,  BiUl.  *S(/f.  Oeol.  France  (2)  xvi.  p.  543.  Maci>her80u,  Xeues  Jahrb,  1879, 
p.  930.     Barrois,  M^ni.  S^^r,  Oeol.  Nordy  ii.  (1882)  p.  168. 

"*  H.  Loretz,  Jahrb.  Preuss.  Gejtf.  Ijindesanst.  1881,  p.  175.  Marr,  Oed,  Mag.  1889i 
p.  411. 

^  See  his  colossal  work,  *  Systenie  Silurieu  de  la  Bohenie,'  published  in  successive  parts 
and  volumes  from  1852  up  to  his  death  in  1883  ;  also  Marr,  Quart,  Journ,  Oeol,  Soc,  xxxvL 
(1880).     ' 


SECT,  i  §  2  CAMBRIAN  SYSTEM  736 

been  grouped  together  by  Dr.  Katzer  in  four  members  as  the  Cambrian  development  in 
Central  Bohemia  thus :  (a)  Basement  conglomerates,  {b)  Paradoxides  shales,  (c)  Quartz- 
gre3n*'acke  group,  {d)  Diabase  and  red  iron -ore  group.*  The  Olenellus-zone  has  not  been 
noticed. 

In  Sardinia  a  characteristic  assemblage  of  Cambrian  fossils  has  been  described  by 
Pix)f.  G.  Meneghini,  comprising  three  species  of  Paradoxides^  six  of  Conocephalites^  five 
of  Arwmocaref  five  of  OlennSj  as  well  as  other  forms.* 

North  America. — During  the  last  decade  a  large  amoimt  of  attention  has  been  paid 
by  the  geologists  of  the  United  States  and  of  Canada  to  the  study  of  stratigraphy  and 
fossil  contents  of  the  Cambrian  rocks  of  North  America,  and  the  result  of  their  labours 
has  been  to  show  that,  whether  as  regards  extent  and  thickness  of  strata,  or  variety  and 
abundance  of  organic  remains,  these  rocks  surpass  in  importance  the  corresponding 
European  series.  The  European  types  of  sedimentation  are  replaced  by  a  varied  assem- 
blage of  materials,  among  which  limestone  plays  a  large  part ;  and  this  change,  as  might 
he  expected,  is  accompanied  by  a  remarkable  contrast  in  the  general  facies  of  the  fossils. 
Nevertheless,  the  leading  type-genera  of  Europe  have  lieen  found  in  their  usual  sequence, 
so  that  it  has  been  possible  to  subdivide  the  American  Cambrian  system  into  three  groups 
which  can  be  broadly  correlated  with  the  threefold  arrangement  adopted  in  Europe. 

From  the  straits  of  Belle  Isle  tlie  Cambrian  formations  of  North  America  run  through 
Newfoundland  and  Nova  Scotia  into  New  Brunswick.  From  the  eastern  coast  of  Gasj^e 
they  stretch  along  tlie  right  bank  of  the  St.  Lawrence  to  Lake  Ontario.  In  several 
approximately  parallel  bands  they  range  througli  the  north-eastern  states  of  the  Union, 
spreading  out  more  widely  in  the  north  of  New  York  State,  and  in  Vermont  and 
Eastern  Massachusetts.  They  rise  along  the  A])palachian  ridge,  striking  through 
Pennsylvania,  Maryland,  Virginia,  Tennessee,  and  Georgia,  down  into  Alabama,  to  a 
distance  in  the  eastern  part  of  the  continent  of  about  2000  miles.  In  the  heart  of  the 
continent,  again,  they  rise  to  the  surface,  and  flanking  the  vast  pre-Cambrian  region  of 
the  north,  extend  over  a  wide  area  between  Lake  SujHirior  and  the  valley  of  the 
Mississippi  in  the  States  of  Michigan,  Wisconsin,  and  Minnesota.  An  isolated  tract  of 
them  is  found  in  Missouri,  and  another  in  Texas.  The  gi-eat  terrestrial  movements 
which  ridged  up  the  Rocky  Mountains  and  their  offshoots  have  brought  the  Cambrian 
rocks  once  more  to  the  surface  from  under  the  vast  pile  of  younger  formations  beneath 
which,  during  a  large  part  of  geological  time,  they  lay  bijried.  Hence  along  the  axes 
of  these  elevations  of  the  terrestrial  crust  they  can  be  traced  in  many  lines  of  outcrop 
from  Arizona  northwards  through  Utah,  Colorado,  Nevada,  Wyoming,  Dakota,  and 
Montana,  whence  they  strike  far  northward  into  the  Dominion  of  Canada. 

In  thickness  and  lithological  character  the  Cambrian  rocks  of  North  America  exhibit 
considerable  variation  as  they  are  traced  across  the  continent,  and  these  changes  afford 
interesting  evidence  of  the  geograjthical  conditions  and  geological  revolutions  of  the 
region  in  the  early  ages  of  Palfeozoic  time.'  In  Newfoundland,  where  the  three  grou{» 
of  the  system  have  been  recognised,  the  total  depth  of  strata  measured  by  A.  Murray 
was  about  6000  feet,  of  which  the  Lower  division  forms  only  about  200  feet.    In  Western 

^  F.  Katzer,  *Daa  jiltere  Palseozoicum  in  Mittelbiihmen,'  Prague,  1888;  ^Geologie  von 
Biihmen,*  Prague,  1892,  p.  804. 

'  Memorie  per  serv.  alia  descriz.  della  Cart.  Oeol.  d*lUUia^  III.  part  2  (1888). 

'  Among  writers  on  the  Cambrian  palaeontology  of  North  America  a  high  place  must 
be  assigned  to  James  Hall,  £.  Billings,  C.  D.  Walcott,  and  G.  F.  Mathew.  Mr.  Walcott  has 
devoted  himself  to  the  subject  with  untiring  enthusiasm  and  much  skill.  His  most  im- 
portant memoirs  will  be  found  in  the  Bulletins  of  the  U.S.  Geological  Survey,  Nos.  10  (1884), 
30  (1886),  81  (1891),  and  in  the  10th  and  12th  Annual  Reports  (1890).  He  gives  a  full 
bibliography.  Of  great  importance  also  are  the  memoirs  on  the  Cambrian  rocks  and  fossils 
of  Canada,  by  Mr.  Mathew,  published  in  the  Trans.  Roy.  Sac.  Canada^  from  the  fint 
volume  (1882)  onwards. 


736  STRATIORAPHirAL  GEOLOGY  BOOK^iPJUrru 


Wniioiit  aij'l  Kai«t<rni  New  York  the  total  de[»th  of  the  by!«tem  swciiui  to  l«  aboat  7000 
fi'et ;  and  of  this  <aeat  niawt  of  Kedinieutary  iiiaterial  the  lower  diTuion  mar  ooe^i|r 
jierhafM  at  much  as  5000  feet.^  0%'er  the  central  |iarts  of  the  continent  west  of  tlie  fine 
of  tin;  Min«i?<Mii|ii  thf  thi''knehM  dinjiuii^heM  to  1(.h>j  feet  or  less  :  bnt  again  to  the  vert  of 
thi?  lOx'kv  Mountains  it  increases  to  7000  fe*'t  or  more  in  Xevada.  while  in  British 
Oiliimhia  it  ri>es  to  10,000  feet. 

In  the  north -eaHtern  n'^onr»  the  sMliments  were  chiefly  muddy,  and  are  now  ie< 
(ire.vnt<'il  hy  thick  maHses  of  .tliale  with  a  little  sandstone  and  limestone.  The  liair- 
HtoncH  incp'ase  in  nuiiil^'r  and  thicknesA  southwurds  in  Vermont,  where  a  considerable 
niHss  iti  calcareous  material  lien  in  the  lower  group  U-low  several  thousand  feet  id 
Hliah.'.  Still  further  Houth  the  lower  gniup  consists  larg«'Iy  of  sandi^tonea.  which  aie 
followt-rl  hy  sandy,  dolomitic,  and  ]iurely  calcareous  limestones.  In  Nevada,  where  a 
thick nchs  of  7700  feet  han  lieen  assisted  to  the  Cambrian  system,  the  liniwtoneB  «e 
V2:t()  feet  in  a^;^re«^te  thickness.- 

It  will  1m'  seen,  thenffore.  that  the  neareftt  Euro{ieau  {larallel  to  the  combination  of 
thif'k  arcnac4'ou.<i  with  thick  calcareous  accumulations,  which  distinguishes  the  Cambrian 
Hysti'm  of  North  America,  is  to  be  found  in  the  north-west  of  Scotland.  In  thid 
connection  it  is  int'-n-stin^;  to  note  that  the  gcMicral  fa<.*ies  of  the  Scottish  Cambrian 
foHsilh,  HO  distinct  from  that  of  the  rockn  of  Wales  and  the  rest  of  Kurojie,  and  so  modi 
mop-  akin  to  that  of  the  Unitc<l  States  and  Canada,  is  accompanied  by  a  markedly 
North  American  tyjN*  of  wniinK'ntarA'  material. 

The  foll(iwin;jc  table  p^vvA  the  latest  classification  of  the  Cambrian  system  of  North 
America  :^ — 

''  SiiiidstoncN  r)f  N.  and  E.  siiles  of  A«liron«laok  Mountaiuii  of  New  York  and 
adjacent  parts  nf  (..anada.  On  the  same  horizon  lie  the  limestones  S.  of 
Adiroudacks  ami  Duti'hess  ('ounty,  New  York  ;  aud  the  Khaies  of  Tennessee, 
(tcorKin,  and  Alal>ama.  In  the  west  come  the  sandstones  of  the  Cpper 
Mississippi  Valley,  S.   Dakota,  Wyomiujf,  Montana,  and  Colonuio,  the 


O    et 


^  I  sandstones  ami  calcareous  l>e«ls  of  N.  Arizona,  and  the  limestones  and 
<<liales  of  Nevaila.  In  the  far  north-east  are  the  black  shales  at  the  to]* 
of  the  New  Hriiiiswirk  antl  ^'ajK;  Hreton  Island  sections,  and  the  shales 
and  saudstoMcH  of  ( 'onceiitioii  Bay,  Newfoundland  (Belle  Isle). 

-^  ^  '-  I  Shales  and  slates  of  hiistcrn  Massiurhusetts  f  Hrnintree*.  New  Brunswick  iSt. 

X'  §  !       .John/,  an«l  I-iistiMU  Newfoumlland  (Avalon,'.      With  these  typical  rocks 

<^   -cH  j       are  rorr»"late»l  part  of  the    limestones  of  Dutchess  County,  New  York 

-i  a  ^  1       (Stissiii^')  and  the  central  parts  of  the  Tennessee  and  Alabama  sections 

?  :=  S  I       '<'oos.'i;.  with  limestones  in  central  Nevatla  and  British  Columbia  (Mount 

5  rt  .2  1.      St«'phen). 

/  The  tyjucal  locality  is  in  western  Vermont  wliere  shales  anil  limestones  are 
I  <levelo|K'(l.  With  these  are  paralleled  the  quartzite  of  W.  8lo]^>e  of  Green 
I  Mountains  and  Appalachian  chain  in  I*ennsylvania,  Virginia,  Tennessee, 
(ieor>;ia.  und  Alabama ;  the  shales  and  interljedded  limestones  and  slates 
of  S.  Vermont  and  New  York  southward  to  Alabama  ;  the  limestone. 
santlstone,  ami  shale  of  Straits  of  Belle  Isle  (Labrador),  N.W.  Coast  of 
\fwfoiunllan«l  aud  peninsula  of  Avalon  (Plai'cntia) ;  the  basal  series  of 
Ilanfiinl  BnK)k  Section,  Caton's  Island,  &c..  New  Brunswick  ;  the  shales 
and  limestoues  of  E.  and  S.  Massachusetts  (Attlel)orough) ;  the  lower 
jiortion  of  the  Eureka  aud  Hi};hlan<l  ranges,  Nevada  (Prosi>ect)  j  a  iK)rtion 
of  the  Wasatch  Cambrian  Se<'.tion  (Cotton woo<l)  and  the  base  of  the 
L     (-astle  Mountain,  British  Columbia. 


.5  "«■ 

^    r. 

c  ::; 
1,  -. 


'  Walcott  hiui  found  Olenellus  about  2000  feet  below  the  summit  of  the  series,  bnt  Ik 
hesitates  to  assume  that  it  can  really  range  through  such  an  enormous  thickness  of  strata, 
10th  Ann.  lirjt.  C.S.  (^'eof,  Snrr.  p.  583.  See  his  Intersection  in  12th  Ann.  Hep,  (189*i) 
fdate  xlii. 

-  A.  Hague,  Ann.  Rrp.  C.S.  finil.  Surv.  1881-82.  Walcott,  Monogr.  U.S.  Oed.  Syrr. 
vol.  viii.  (1884). 

»  C.  D.  Walcott,  IMl.  r.S.  iie^i.  Svn-.  No.  81  (1891),  p.  360. 


SECT,  n  SILURIAN  SYSTEM  737 

A  large  assemblage  of  fossils  has  been  obtained  from  the  Cambrian  rocks  of  Nortli 
America.  The  fauna  of  the  Olenellus-zone  has  been  fully  described  in  a  separate  mono- 
graph ))y  Mr.  Walcott.  The  middle  group  in  New  Brunswick  (St  John)  has  also 
yielded  an  abundant  fauna  which  has  been  described  by  Mr.  Mathew.^ 

South  America. — In  the  northern  ^^art  of  the  Argentine  Republic  a  representative 
of  the  Upi>er  Cambrian  or  Olenus  group  has  been  found  by  Lorcutz  and  Hyeroninms. 
It  includes  species  of  the  genera  LinguUiy  OboluSj  Orthis,  HyolUhes^  Arioncllus^  Agnostus, 
and  Olenus.' 

China. — Baron  yon  Richthofen  has  brought  to  light  a  succession  of  undisturbed  strata 
(his  *  Sinisian  formation ')  which  in  Leao-tong  and  Corea  attain  a  thickness  of  many 
thousand  feet.  In  the  higher  parts  of  this  series  he  found  a  characteristic  assemblage  of 
Primordial  trilobites  :  Conocoryphe  (Conocephaliles)  (4  sp. ),  Anamocare  (6),  Liostractts  (3), 
Dorypyge  {Olenoides?),  AgnostxLs  (1),  with  the  brachiojKxls  Lingulclla  (2)  and  OrUiis  (1).' 

India. — In  the  Salt  Range,  among  shales  (Xeobolus  beds)  underlying  magnesian  sand- 
stones and  sliales  >vith  pseudomorphs  of  salt,  and  overlying  pur])le  sandstones,  with  a 
group  of  beds  of  rock -salt  and  gypsum,  Cambrian  fossils  have  been  detected.  They 
include  a  number  of  brachiopods  {Lingula^  DavulsoneUa^  NeoboluSy  Ac.)  and  two  tri- 
lobites, one  of  which  has  been  deteimined  to  be  a  ConoccphalitcSy  nearly  related  to  C. 
fonnosiis  from  the  St.  John's  group  (p.  736),  while  the  other  is  pi*ol>ably  an  Oltiius.* 

Australia. — In  the  south-east  of  this  continent  and  in  Tasmania  traces  of  the  exist- 
ence of  a  Cambrian  fauna  have  recently  been  detected.  Mr.  R.  Etheridge  jun.,  has 
described  from  that  region  forms  of  ConocephaliteSf  Asaphusj  Dikclocephdliis  and  Ophileta, 
and  some  species  belonging  to  the  family  of  Archieocyathime.^ 

Section  ii.     Silurian. 

Miirchisoii  was  the  first  to  discover  that  the  so-called  "Transition 
rocks "  or  "  Grauwacke "  of  early  geological  literature  were  capable  of 
suMivisioii  into  distinct  formations  characterised  by  a  peculiar  assemblage 
of  organic  remains.  As  he  found  them  to  be  well  developed  in  the  region 
once  inhabited  by  the  British  tribe  of  Silures,  he  gave  them  the  name  of 
Silurian.®  From  the  base  of  the  Old  Red  Sandstone,  he  was  able  to  trace 
his  Silurian  types  of  fossils  into  successively  lower  zones  of  the  old 
"  Grauwacke."  It  was  eventually  found  that  similar  fossils  characterised 
the  older  sedimentary  rocks  all  over  the  world,  and  that  the  general  order 
of  succession  worked  out  by  Murchison  could  everywhere  be  recognised. 
Hence  the  term  Silurian  came  to  be  generally  employed  to  designate  the 
rocks  containing  the  first  great  fauna  of  the  Geological  Record. 

The  controversy  regai'ding  the  respective  limits  of  the  Cambrian  and 
Silurian  formations  {ante,  p.  726)  survived  the  lifetime  of  the  two  great 

'  Walcott,  10th  Ann.  Report  U.S.  (Jeol,  Sarv.  (1890),  where  plates  and  descriptions  of 
the  fossils  will  be  found.  See  also  his  papers  in  Bitll.  U.H^.  (ie4)l.  Svrr.  Nos.  10  and  30.  For 
the  fossils  of  the  St.  John  division  consult  the  papers  of  G.  F.  Mathew,  quoted  on  p.  73r>. 

2  E.  Kayser,  "  Beitriige  zur  Geol.  u,  Palaeont.  d.  Argentinischer  Republik.   Part  II.  (1876). 

^  Richthofen,  *  China,'  vol.  iii.  (1882).  W.  Dames  compares  this  Chinese  Cambrian 
fauna  with  that  of  the  Andrarumskalk  of  Scandinavia :  op.  cit.  p.  32  {ante^  p.  731).  Mr. 
Walcott  inclines  to  believe  that  the  fossils  rather  point  to  a  Middle  Cambrian  fauna  {Bull. 
U.S.  (rV/>/.  Stirr.  No.  81,  1891,  p.  379). 

*  Pdlxontolitfjia  Indka^  ser.  13,  vol.  i.  (1887)  p.  750. 

*  Proc.  Hoij.  Soc.  Tasmania^  1882-83,  p.  151  ;  Tratix.  Roy.  Soc.  South  Ausiraluif  xiiL 
(1890)  p.  10.  «  Phil.  Miig.  (3)  vii.  (1835)  p.  47. 

3  B 


738  STRATIGRAPHICAL  GEOLOGY  book  vi  part  ii 

antagonists.  Professor  Lapworth  in  1879  proposed,  as  a  compromise, 
that  the  lower  half  of  Murchison's  Silurian  system,  which  Sedgwick  had 
claimed  as  Cambrian,  should  be  detached  from  both  and  erected  into  a 
distinct  system  under  the  name  "  Ordovician."  ^  I  consider  that  this  pro- 
posal, which  was  honestly  intended  to  obviate  confusion  and  to  promote 
the  progress  of  the  science,  is  fair  to  neither  of  these  fathers  of  English 
geology,  and  is  especially  unjust  to  Murchison.  The  division  of  "  Lower 
Sihirian"  has  the  claim  not  only  of  priority,  but  of  having  been  established 
and  of  having  had  its  component  members  defined  by  the  author  of  the 
Silurian  system  in  the  early  years  of  his  investigation.  The  primordial 
fauna  which  Barrande  had  shown  to  underlie  the  Lower  Silurian  rocks  of 
Bohemia  was  hardly  known  to  exist  in  Britain  during  Murchison's  life,  and 
certainly  was  not  then  ascertained  to  have  the  stratigraphical  significance 
and  wide  geographical  diffusion  which  have  now  been  proved.  It  is  uni- 
versally admitted  that  this  fauna  marks  a  distinct  section  of  the  geological 
record  to  which  by  common  consent  the  name  Cambrian  is  given.  The 
upper  limit  of  this  fauna  is  likcAvise  recognised.  So  that  it  is  not  a 
question  of  fact  but  of  nomenclature  which  is  in  dispute.  With  the 
modification  of  the  accepted  base-line  at  the  top  of  the  Tremadoc  Slates, 
I  shall  continue  to  employ  the  terminology  proposed  by  the  illustrious 
author  of  the  "  Silurian  System  "  as  being  quite  adequate  for  the  most 
recent  requirements  of  investigation.^ 

§  1.     General   Characters. 

Rocks. — The  Silurian  system  consists  usually  of  a  massive  series  of 
greywackes,  sandstones,  grits,  shales,  or  slates,  with  occasional  bands  of 
limestone.  The  arenaceous  strata  include  pebbly  grits  and  conglomer- 
ates, which  are  specially  apt  to  occur  at  or  near  any  local  base  of  the 
formation,  where  they  rest  unconformably  on  older  rocks.  Occasional 
zones  of  massive  conglomerate  occur,  as  among  the  Llandovery  rocks  of 
Britain.  The  argillaceous  strata  are  in  some  regions  (Livonia,  &c.)  mere 
soft  clays  :  most  commonly  they  are  hard  fissile  shales,  but  in  some  areas 
(Wales,  &c.),  where  they  have  been  subjected  to  intense  compression, 
they  appear  as  hard  cleaved  slates,  or  even  as  crystalline  schists.  In 
Europe,  the  limestones  are,  as  a  rule,  lenticular,  as  in  the  examples  of  the 
Bala,  Aymostry,  and  Dudley  bands,  though  in  the  basin  of  the  Baltic, 
some  of  the  limestones  have  a  greater  continuity.  In  North  America,  on 
the  other  hand,  the  Trenton  limestones  in  the  Lower,  and  the  Niagara 
limestone  in  the  Upper  Silurian  division  are  among  the  most  persistent 
formations  of  the  eastern  United  States  and  Canada,  while  in  the 
Western  Territories  vast  masses  of  Siliurian  limestone  constitute  nearly 
the  whole  of  the  system.     Easily  recognisable  bands  in  many  Silurian 

^  (AvV.  Mag,  1879,  \k  13. 

-  The  reader  who  would  peruse  a  weighty  and  dispassionate  examination  of  this  disputed 
question  in  geologioal  nomenclature  should  turn  to  the  essay  l>y  the  venerable  Professor  J. 
D.  Dana  on  *'.SedK^vick  and  MuR-hison  ;  Cambrian  and  Silurian"  {Amer,  Journ,  Sd,  xzxix. 
ISOO,  i>.  167).  With  tlie  conclusions  of  his  examination  of  the  whole  question  I  most 
thoroughly  agi'ee. 


SECT,  ii  g  1 


SILURIAN  SYSTEM 


tracts,  especially  in  the  north-west  of  Europe,  are  certain  dark  anthracitic 
shales  or  schisU,  which,  though  Homotimes  only  a  fen  feet  thick,  can  be 
followed  for  many  leagues.  As  they  usually  contain  much  decomposing 
iron-disulpbide,  which  produces  an  efflorescence  of  alum,  they  are  known 
in  Scandinavia  as  the  aJum-slatea.  In  Scotland,  they  are  the  chief  reposi- 
tories of  the  Silui'ian  graptolites.  Their  black,  coal-like  aspect  has  led 
to  much  fruitless  mining  in  them  for  coal.  In  the  northern  part  of  the 
State  of  New  York,  a  series  of  beds  of  red  marl  with  salt  and  gypsum 
occurs  in  the  Upper  Silurian  series.  In  the  Salt  Bange  of  the  Punjab 
the  group  of  saliferous  strata  occurs  which  has  been  already  alluded  to  in 
the  account  of  the  Cambrian  rocks.  These  salt-bearing  deposits  are  the 
oldest  yet  discovered.  In  Styria  and  Bohemia,  important  beds  of  oolitic 
hiematite  and  siderite  are  interstratified  ivith  the  ordinary  greywackas 
and   shales.       Occasionally   sheets    of    various    eruptive    rocks    (felsites. 


Fin.  S-ML— Group  of  Silurtan  Oraiitiilitfii. 
«,  Monf>«ni[itii.  priwlon,  Bronn  (WenlocH) ;  \  Phifllogmptiia  typii-.  Hill  (Lower  Arenig) :  c.  Diplo- 
KiaptixB  foUnm,  Hi>,  (Lluidovery):  .1,  SutrtUa  prngriiiui.  Birr  (Umiilover;) ;  (.  DiiljriiiOfirapCuri 
Murclilsoni,  Bwk.   (Llandrtlo):  /.   Mont«npttu  HrdKwIckil,   Port).  (UudoTPiT) :   u,  Dlcruo- 
gnpliii  ranioaua,  Ilill  (Lludeilo) ;  A,  Tstngimptui  Ilickiii,  H'>|>k.  (Lowrr  Arenig). 

diabases,  diorites,  &c.)  occur  contemporaneously  imbedded  in  the  Silurian 
rocks  (Wales,  Lake  District,  S.  Scotland,  S.E.  Ireland,  &c),  and,  with 
their  associated  tuffs,  represent  the  volcanic  ejections  of  the  time. 

As  a  rule,  Silurian  rocks  have  suffered  from  subsequent  geological 
revolutions,  so  that  they  now  appear  inclined,  folded,  contort«d,  broken, 
and  cleaved,  sometimes  even  metamorphosed  into  crystalline  schists.  In 
certain  regions,  however  (Basin  of  the  Baltic,  New  York,  &c),  they  still 
remain  nearly  in  their  original  undisturbed  positions. 

LiFK — The  general  aspect  of  the  life  of  the  Silurian  period,  so  far  as 
it  has  been  preserved  to  us,  may  be  gathered  from  the  following  summary 
published  by  Bigsby  in  1868 — plants  82  species;  amorphozoa  136 
foraminifera  25;  ctelenterata  507;  echinodermata  500;  annelida  154 
cirripedes  8;  trilobita  1611;  entomostraca  318;  polyzoa  441 
brachiopoda  1650;  monomyaria  168;  dimyaria  541;  faeteropoda  358 


740  STBATIGKAPHICAL  GEOLOGY  book  vi  part  n 

gasteropoda  895;  cephalopoda  1454;  pisces  37;  class  uncertain  12; 
total  8897  species.  Barrande  in  1872  published  another  census  in  which 
some  variations  are  made  in  the  proportions  of  this  table,  the  total 
number  of  species  being  raised  to  10,074,  which  has  subsequently  been 
still  further  increased.  ^ 

The  plants  as  yet  recovered  are  chiefly  fucoids.  In  many  cases 
they  occur  as  mere  impressions,  which  are  often  probaWy  not  of  vegetable 
origin  at  all,  but  casts  of  the  trails  or  burrows  of  wonns,  Crustacea,  &c* 
Among  the  most  abundant  genera  are  Bufhotrephis,  ArUiraphyc^ 
FalxoplnjcuSy  and  Xematophj/cus  (Carruth.)  But  in  the  Upper  Silurian 
rocks  )>eautifully  preserved  sea- weeds  like  the  living  Gelidiwn  or 
Plocamium  occur,  such  as  the  Chmulrites  rens'unilis  (Salt.)  of  the  Ludlow 
rocks  of  Edinburghshire.  Traces,  however,  of  a  higher  vegetation  have 
been  discovered,  which  are  of  special  interest  as  being  the  earliest  known 
remains  of  a  land-flora.  Many  years  ago  certain  minute  bodies  {Pachytheca) 
in  the  Ludlow  bone-bed  were  regarded  as  lycopodiaceous  spore-cases,  but 
some  doubt  has  been  cast  on  their  organic  gi*ade.  More  recently.  Dr.  Hicks 
obtained  from  the  Denbighshire  grits  of  N.  "Wales  other  spores  and  like- 
'wise  dichotomous  stems,  probably  lycopodiaceous.-  Tnie  lycopods 
{Siujenaria)  have  been  met  with  in  the  U})per  Silurian  rocks  of  Bohemia. 
From  the  Clinton  limestone  of  Ohio  portion  of  a  lepidodendroid  tree 
(Glffptodenflron.  eatomnse)  has  been  obtained.  The  Cincinnati  group  of 
strata,  at  the  top  of  the  Lower,  and  the  Lower  Helderberg  at  the  top  of 
the  Upper  Sihirian  formations  of  eastern  North  America,  have  yielded  a 
microcosmical  re})resentation  of  the  Carlxjniferous  flora.  The  genera 
noted  include  PsilophijUm,  Calamophyms^  Annularia,  Proiostigma,  Sigillaria^ 
and  Sphmophylhnn.^  From  the  meagi*e  evidence  as  yet  collected,  it  would 
ap})ear  that  the  land  of  the  Silurian  ])eriod  had  a  cryi)togamic  vegetation 
in  which  lyco})0(ls  and  ferns  no  doubt  played  the  chief  pjirt.** 

In  the  fauna  of  the  Silurian  rocks,  the  most  lowly  organisms  known 
are  foraminifera,  of  which  several  genera,  including  the  still  living  genus 
Saccffrinniita,  have  been  detected.  Certain  layers  of  chert,  widely  spread 
over  the  south  of  Scotland,  have  yielded  upwards  of  a  dozen  genera  with 
more  than  twenty  species  of  radiolaria.^  The  Silurian  seas  possessed  repre- 
sentatives }x)th  of  the  calcareous  and  of  the  siliceous  sponges  of  modern 
times.  Under  the  former  grou}>  may  be  ])laced  the  genus  Archxocyaihns 
which  occurs  in  the  Canil>rian  system,  and  the  genera  Astrxoqtongia  and 
Ainphispfmffia  of  the  Upper  Silurian  rocks  ;  under  the  latter  group  come 

^  Natliorst  (Knnyl.  Scensk.  Vet.  Akad.  Hand/,  xviii.  (1881)  Las  imitated  some  of  these 
markings  by  caiisiug  crustacea,  aunelids,  ami  mollusks  to  move  over  wet  mud  and  gjiwnin, 
and  lias  thus  iihown  the  higli  probability  that  they  are  not  plants.  (See  Gfol.  Alag,  1882, 
pj).  22,  485  ;  188-J,  pp.  33,  192,  286.)  Nathorst's  ojnnion,  adverse  to  tlie  ])lant  nature  of 
tlie  markings,  is  strongly  opposed  by  Sajwrta  in  his  *  A  propos  des  Algues  Fossiles,*  1882. 

-  i^.  J.  iicd.  ,Sor.  1881,  p.  482  ;  1882,  p]).  97,  103. 

•'  L.  Lescpiereux,  Jw/^v.  Jomn.  Sci.  (3)  vii.  j>.  31  :  J*ror.  Amer.  Phil,  ihc,  xvii.  p.  IdS. 

■*  The  student  will  lind  a  valuable  compendium  of  information  by  L.  F.  Ward  regarding 
the  fossil  lloras  of  past  time  all  over  the  world  in  the  8th  Ann.  Rej).  V,K  Ofol.  Svrr,  i>»rt 
ii.  1889. 

^  a.  J.  llinde,  .471?*.  3Iaff.  Xat.  Hifft.  1890,  p.  40. 


SBCT.  ii  ^  1 


SILURIAN  SYSTEM 


741 


.4sli/logpoiiffiti  and  Prota'hilleum.  Of  the  puzzling  genera  BecepfaculiUs  and 
Ischudites,  the  true  relationships  have  not  yet  been  determined.  Nidulilts, 
too,  though  a  common  fossil,  is  still  a  subject  of  uncertainty  as  to  its 
organic  grade,  the  latest  view  being  that  it  may  be  related  to  the  polyzoa. 
•Some  of  the  most  plentiful  and  characteristic  denizens  of  the  Silurian 
seae  were  undoubtedly  the  various  hydrozoan  genera  united  under  the 
common  name  of  graptotites  (Fig.  340).'  Among  the  monoprionidian 
forms,  or  those  with  a  single  row  of  colls,  the  genera  Moiwgraptm  (of 
which  upwards  of  40  species  have  been  found  in  Britain),  Ritstrites  and 
Cr/rloffTujilui  are  characteristic  of  Upper  Silurian  rocks.  The  diprionidian 
forms,  or  those  with  two  rows  of  cells,  are  equally  characteristic  of  the 
lower  subdivision  of  the  Silurian  system,  and  are  richest  in  genera,  of  which 
some  of  the  commonest  are  Dicelhp-aptuf,  DidifmogmpUia,  and  Telragrapius. 


V\n.  .141.— Gnjup  nr  Loiter  Miliiruin  Tril..W1»«. 
]..ni»'iim  OirloH.  Halt.  A);   2,  Wymena  bifvlisjiiuitii.  Pnrtl, ;    3,  OmifU  ItucMi.  BroiiEii,  (|):    4, 
AwiiliiiH  tyniiiiiiis.  Mnreh.  (A)i  .'.,  Ampyi  uuilus,  Mun'li.  i\t:   %\,  .T^Udi  bipoluMi,  Silt.  ;  T. 
.lciiln»|.l.  Jaih.ili,  Suli. :  s,  Trtnuelrm  Lliiyai).  Mnreli, 

Graptolites  were  formerly  supposed  to  belong  exclusively  to  Silurian 
rocks ;  but  it  has  already  been  pointed  out  that  they  descend  into  the 
Cambi-ian  system.  Nevertheless  it  was  in  Silurian  time  that  they  reached 
their  muximum  development.  A  few  genera  {Diplog^'aptim,  Climncograptus, 
MmUlei)  occur  both  in  the  Lower  and  Upper  Silurian  stratA,  though  the 
species  are  not  persistent.  Through  the  researches  chiefly  of  Professor 
Lapworth  it  has  been  ascertained  that  the  ^'ertical  range  of  the  species  of 
grajitolites  is  comparatively  limited,  and  hence  that  these  fossils  may  be 
used  to  mark  definite  paheontological  horizons.     He  enumerates  twenty 

I  Tht  student  slioiilil  commit  Professor  Upworlli'H 
DistriliiLtion  of  llie  Rlinbilopliora"  (.Inn-  -V"-!-  -Vur.  h 
187f,  l^^f^O)  in  wtiirli  the  {^logil^a1  "igniHrance  of  tbe  |[nptoIit«ii  ia  fully  (liBciuaed. 


742  STRATIGIiAPHIGAL  GEOLOGY  book  vi  part  n 

recognisable  graptolite  zones,  one  in  the  Upper  Cambrian,  eight  in  the 
Lower  Silurian,  and  eleven  in  the  Upper  Silurian  formations,^  The 
peculiar  form  Siroinatapora  and  several  allied  genera  are  now  referred  to 
the  Hydrozoa. 

Corals  must  have  ^vanned  on  those  parts  of  the  Silurian  sea-floor  on 
which  calcareous  accumulations  gathered,  for  their  remains  are  abundant 
among  the  limestones,  particularly  in  the  upper  division  of  the  system. 
Among  the  tabulate  forms  are  the  genera  FavositeSy  so  characteristic 
in  the  Upper  Silurian  limestones  of  Euroi)e  and  America,  ChseteteSj 
Thtcui^  Hahjsites  or  chain  coral,  Sijrin^opora^  and  Teiradium,  The  rugose 
corals  are  likewise  abundant,  some  conspicuous  genera  being  Stauria^ 
Cf/atliaxonia,  CyathophyUum,  Zaphrentis,  Petraia,  '^mphymay  Siromhodes, 
Ptyrliophi/Uum,  and  Acervulami  (Fig.  345).  The  echinoderms  were  re- 
presented by  star-fishes  {Palxaster,  Pala'asterina,  Palwoconia,  Lejndaster), 
brittle-stars  (Protaster,  Eucladki\  many  forms  of  crinoids  {Acfinocrinm, 
Cyathocnnus,  Glypfttcmnis,  Eucalyptocnniis,  TfLrocmius,  &c.),  and  particularly 
by  species  of  cystideans  {Eclwwspifuerites^  Sph^roniies,  Pleurocystiies, 
Uemkosimte^).  The  annelides  of  the  Silurian  sea-bottom  comprised 
representatives  of  both  the  tubicolar  and  errant  orders.  To  the  former 
belong  the  genera  Cormdites,  Ortonia,  Conchmlites,  Serpulifea,  and  also  the 
still  living  geiuis  H^jnivrbis.  The  errant  forms  are  known  chiefly  by  their 
burrows  or  trails,  which  appear  in  immense  profusion  on  the  surfaces  of 
shales  and  sandstones  {AreniadifeSy  Xereites,  Scolithiis,  &c.),  but  also  by 
their  jaws,  which  occur  in  great  nimibers  in  the  Wenlock  and  Ludlow 
rocks.  2 

The  Crustacea  of  the  period  have  been  abundantly  preserved  and 
form  some  of  the  most  familiar  and  distinctive  fossils  of  the  system. 
Undoubted  cirripcdes  have  been  found  in  the  Silurian  rocks  of  Britain, 
Bohemia,  and  North  America  {7'urrdcpa.%  Anatifopsis).  Small  astracods 
abound  in  certain  shales,  some  of  the  most  frequent  genera  being  EntoDm, 
Ba/rkhiiij  Primitia^  LepndUia,  Aristozoe,  Orozncj  Callizoe,  The  phyllopods, 
which,  as  we  have  seen,  made  their  appearance  in  Cambrian  times, 
continue  to  occur  on  scattered  horizons,  and  generally  not  in  great 
numl)ers,  throughout  the  Lower  and  Upper  Silurian  rocks  :  characteristic 
genera  are  Curf/iM'ariSy  Pellocani^,  Ducinocaris,  Ceratiocnri%  Diciyocuri% 
Cri/pfoairis,  and  Aplychopsiii.  But  by  far  the  most  prolific  order  is  that  of 
the  trilobites  (Figs.  341,  345),  which,  beginning  in  the  Cambrian,  attained 
its  maxinmm  development  in  the  Siliu-ian,  waned  in  the  Devonian,  and 
became  extinct  in  the  Carboniferous  ]>eriod.  According  to  the  census  of 
Barrande  in  1872  there  were  then  1579  known  species,  but  this  number 
has  since  been  greatly  increased.  With  a  few  exceptions  the  Cambrian 
genera  did  not  survive  into  Silurian  time  (p.  730).  They  were  succeeded 
by  many  new  genera  which  continued  to  live  through  most  of  the  Silurian 
period.     In  the  lower  division  of  the  system,  characteristic  genera  are 

1  Op.  rif.  V.  (1S80)  p.  197.  0.  Jaekel  {Zdtsch.  Deutsrh.  Grt^l.  Ges.  1889,  p.  658)  has 
recently  i>ropose<l  to  distinguish  the  nionogi'aptidai  in  two  groups,  Pnstiograptvs  charac- 
terising the  older  and  Pomntotjrafitvs  the  later  parts  of  the  Upper  Silurian  series. 

-  O.  J.  Hinde,  Q.  J.  Grol.  S^k:  18S0,  p.  368  :  Rihamj.  Scmsk.  Vet.  Akad.  Handl,  vi.  (1882). 


BECT.  ii  §  1 


SILURIAN  SYSTEM 


T43 


jUffliiut,  Asaphus,  j4mphi(nt,  Ampyz,  Barrandia,  Ckaimops,  Cyhele,  Uarpts, 
Offijgia,  Pkicopa-rm,  Itemopleuridea,  and  Triniicleus;  gome  geoera  are  common  U> 
both  the  lower  and  upper  dirisiong  (but  usually  with  speciflc  distinctions), 
such  as  Acidagpis,*  BrmtUiis,'  Calymene,  Ulieirvrus,  Ct/phaspis,  DalvtatiUes, 
Eiicrinurus*  Homedtmotus*  Ulienus,  Lidua,  Pkacops,*  '  and  f^liafrexoeku!'. 
Proelas  is  confined  to  the  upper  division.  Towards  the  top  of  the  systeni 
eurypterids  make  their  appearance,  and  continue  to  occupy  a  prominent 
place  until  the  Carboniferous  period.  The  Silurian  genera  are  Pterygoid, 
Euryplerus,  Slimonia,  Stylonurus,  and  Hemiaspia. 

The  polyzoa  of  Silurian  times  have  been  tolerably  well  preserved,  and 
present  many  peculiarities  of  structure.  One  of  the  most  abundant 
genera  is  FentsUUa,  which  ranges  from  Lower  Silurian  to  Permian  rocks ; 
another,  PiUodidi/a,  ascends  into  the  Carboniferous  system.  Other  genera 
are  Betepora,  Paleschara,  and  Hippolhoa.  So  abundant  are  the  brachiopotls  ' 
(many  hniitlreds  of  species  being  known),  and  so  characteristic  on  the 


^^1 diJ 


whole  are  the  species  of  them  occurring  in  certain  Silurian  zones  or 
bands,  that  these  fossils  must  be  regarded  as  of  8j)ecial  value  for  purposes 
of  stratigraphical  comparison.^  The  old  and  still  living  genera  Dixiiui, 
Liiiy)it(i,  and  Crania  are  found  on  different  horizons  In  the  Silurian  scries. 
Characteristic  types  are  Acrotrda,  Afnjpa,  LeplaiM,  Mervitlla,  Orikii 
(Figs.  342,  343),  Pentamerus  (Fig.  3ii),' Pm-amlnmtes,  Bkifnchonrlla  (Fig. 
346),  Sijtitonotreia,  Spirifer,  Slrietlandima,  Slnphmena  (Fig.  346),  and 
Tri/i/f.iia.  Some  of  these  are  particularly  distinctive  of  certain  zones. 
Thus,  from  the  abundance  of  Peniameri  in  them,  certain  strata  received 
the  name  of  the  "  Pentamerus  beds  "  (Fig.  344).  Orihis  is  most  abundant 
in  species  in  the  lower  part  of  the  Silurian  system ;  Pentnmerns,  Bhim- 
diiindla,  Spirlffr,  Ouindet,  and  Terdimliiln,  occur  in  the  upper.  The 
'  Thnse  genera  ninrlted  witli  *  are  more  ch»racl«riatic  of  the  Upper  than  of  the  Ixiwer 

'  For  iiii  neeoiiot  if  tlie  inteninl  nmmgenients  of  some  Silurinn  hrarliiftpodii  bdiI  n  list  of 
the  Upper  Silurian  sptcitm  of  Englaml,  «ee  DaTidnon.  finJ.  Mag.  1S81,  pp.  1, 100, 145,  28P. 


744  STRATIGRAPHICAL  GEuLOGY  book  vi  fakt  n 


lamelliViranchs  have  been  less  abundantly  preserved  ;  some  of  their  most 
frequent  genera  are  the  monomyarian  Amionydiia  (Fig.  343)  and  Pteriwa 
and  the  dimyarian  Ctenodmiia,  Modidopsis,  Goniaphara,  Orthonota  (Fig.  346X 
Cieidophorus  (Fig.  343),  Palxarciiy  and  Redonia  (Fig.  342).  Cardinlainiemg^ 
(Fig.  346)  is  a  characteristic  shell  of  the  highest  Upper  Silurian  rocka. 

Of  the  gasteropoils  of  the  Silurian  seas  upwards  of  1300  species  have 
been  named  ;  some  of  the  more  frequent  genera  are  Acroctdui^  C^onema^ 
Enmnphahis  (Fig.  346),  Helicotomu,  Holapa^a,  HohtpeUa,  MurchisomOy  Ophileta, 
riatusclmma,  Plevroh/miriaj  liaphisi^miay  Trochus  (Fig.  346),  and  SubidUes, 
Some  heteroiKxl  foniis  occur,  e.g.  Bellerophon  and  Maclnrea ;  but  pteropodB 
are  more  frequent,  being  represented  sometimes  abundantly  by  the  genera 
TenfaaUites  (regarded  by  some  as  an  annelide),  HifolWies  (or  Theca\  Camur 
laria,  and  Pterotheai.  That  the  salt  watei^s  of  the  Silurian  era  swarmed 
with  cephalopods  may  be  inferred  from  the  fact  that  according  to 
Barrande's  census  no  fewer  than  1622  species  had  then  been  described. 
They  are  all  tetrabranchiate.  Some  of  the  most  abimdant  forms  are 
straight  shells,  of  which  Orthoceras  (Figs.  342,  346)  is  the  type.  This 
characteristically  Palaeozoic  genus  abounded  in  the  Silurian  period,  when 
many  of  its  individuals  attained  a  great  size.  Barrande  has  described 
upwards  of  550  species  from  the  Imsin  of  Bohemia.  Of  Cyrtoceixts^  in 
which  the  shell  was  curved,  the  same  small  area  has  yielded  more  than  330 
species.  Phragrnocems  (Fig.  346)  likewise  possessed  a  curved  shell,  but  with 
an  aperture  contracted  in  the  middle.  In  Ascoceras  the  shell  was  globular 
or  flask-shaped,  with  curiously  curved  septa;  in  Lituites  (Fig.  346)  it  was 
curled  like  that  of  Nautilus.  The  two  latter  genera  occur  in  SUurian 
rocks,  but  while  Lituites  never  outlived  the  Silurian  period,  Nautiltts  is 
still  a  living  denizen  of  the  sea. 

The  first  traces  of  vertebrate  life  make  their  appearance  in  the 
Sihirian  system.  They  consist  of  the  remains  of  fishes,  the  most 
<leterniinal)le  of  which  are  the  plates  of  })laco(lerms  (Pferaspi^,  Cephalaspi^ 
Auchnuispis^  Sruphaspis).  The  bone-bed  of  the  Ludlow  rocks  has  also 
yielded  certain  curved  spines  (Chichus\  which  have  been  referred  to  a 
cestraciont,  and  some  shagieen-like  plates  which  have  been  supposed  to 
be  scales  of  })lacoi(l  fishes  (Sphagodus^  lliflodus),  and  bodies  like  jaws  with 
teeth  which  were  called  PlectnKlus,  but  which  are  now  known  to  be 
lateral  shield-spines  of  a  cephalaspidean  fish  {Eukera.'^ns).  It  is  probable 
that  some  of  these  remains  have  been  incorrectly  determined,  and  may 
belong  to  crusUiceans  or  annelides.  The  Upper  Silurian  rocks  have 
yielded,  both  in  p]uro])e  and  North  America,  great  numbers  of  minute 
tooth-like  bodies  which  were  named  "Conodonts"  by  their  discoverer. 
Pander,  and  were  supposed  to  be  the  teeth  of  such  fishes  as  the  lamprey, 
which  possessed  no  other  hard  parts  for  preservation.  These  Ixxiies  have 
l>een  also  referred  to  different  divisions  of  the  invertebrata,  but  palaeontolo- 
gists now  regard  them  as  probably  in  most  cases  the  jaws  of  annelids.^ 

Satisfactory  evidence  of  the  occurrence  of  fishes  in  rocks  of  Silurian 
age  is  supplied  by  Mr.  Walcott,  who  has  described  from  the  Lower 
Silurian    rocks   of  Canon    City,   Colorado,  a    number   of   fish    remains, 

^  Zittel  and  Rohon,  .SV/;/>.  fi«yr.  Akad.  Munich,  1886,  p.  108. 


SECT,  ii  §  I 


SILURIAN  SYSTEM 


among  which  he  has  been  able  to  identify  dermal  plates  and  scales 
belonging  to  genera  like  Aslerolepis  and  Hdoplyckius,  which  play  so 
important  »  part  in  the  Devonian  fauna.'  According  to  Dr.  J.  V. 
Rohon,  all  the  so-called  "Conodonta"  are  not  annelidian,  but  include 
undoubted  teeth  of  fishes  with  recognisable  dentine,  enamel,  and  pulp- 


in  hinunti'mia.  Mi'Co)' ;  r,  Lliiguli  liiDKliialinft.  P*oilerO'> 
d,  mnifihonivui  cniullt,  Sbjr. ;  t,  OrUils  plhstd,  Hbjr. :  /,  Onhia  cillli^ninii,  Dalm. :  i,  Cnnla  <lt- 
vsrliimi,  McCuy;  V  Trl|ilviila  (t)  niuwoyuii,  Div. ;  i,  Alryp*  (?)  UHdti,  Bflllopi  ({);J,  Atrjia 
nurKiiHiIIa,  Dalm.;  k,  DIkIiu  oblongnta,  ^rtl. ;  I.  Aiiibonychtu  prl§a,  Forll.;  m,  Fmlnna 
billiiitKiaiH,  Malt. ;  »,  Khynchonella  lUini,  »>lt. :  ».  ClfUriiihomi  onlli,  McCoy. 

cavity.      He  descriljes  from  the  Ulauconite  Sand  of  St  Petersburg  forms 
belonging  to  two  new  genera  named  by  him  FaUeodua  and  Archodus.^ 
Up  to  the  present  time  no  trace  has  been  detected  of  any  vertebrate 

I  B,iU.  (lai.  Sor.  Amrricn,  iiL  (1893)  p.  153, 
•  ,1.  V.  Rohon,  Bull.  Acad.  Imp.  SH.  -S.  PlUrAoHT^,  xxxiiL  1890,  p.  26B, 


746  STRATIGRAPHIGAL  GEOLOGY  book  vi  part  n 

land -animals  of  Silurian  age.  In  Sweden,  France,  Scotland,  and  the 
United  States,  however,  the  discovery  of  remains  of  arachnid  and  insect 
life  in  Silurian  rocks  may  herald  the  ultimate  detection  of  higher 
forms  of  life.  From  the  Upper  Silurian  strata  of  the  island  of  Gothland 
a  true  scorpion  has  been  discovered,  which  appears  to  differ  in  no 
essential  respect  from  recent  forms,  except  in  the  walking  limbs,  which 
are  dumpy  in  form,  and  terminate  in  a  single  claw.  One  of  the  breathing 
stigmata  on  the  second  ventral  scute  shows  clearly  that  the  animal  was 
an  air-breather.^  Subsequently  a  still  more  perfect  example  of  the  same 
genus  (Palie/>ph/m€us)  was  described  from  the  Upper  Silurian  rocks  of 
Lesmahagow,  Lanarkshire  (Fig.  347).  The  presence  of  a  poison-gland 
and  sting  at  the  extremity  of  the  tail  shows  that,  like  their  modem 
representatives,  these  ancient  animals  preyed  on  other  denizens  of  the 
land.  Soon  after  the  European  discoveries,  the  finding  of  a  scorpion 
in  the  "Waterlime"  (Upper  Silui-ian)  of  New  York  was  announced.^ 
These  specimens  lifted  the  veil  that  had  concealed  from  us  all  evidence 
of  the  terrestrial  fauna  of  this  ancient  period  of  geological  history. 
If  there  were  scorpions  on  the  land,  there  were  almost  certainly  other 
land-animals  on  which  they  lived  Mr.  Peach  has  suggested  that  they 
may  have  fed  partly  on  marine  crustacean  eggs  left  bare  by  the  tides.' 
But  that  insects  already  existed  has  been  made  known  by  the  discovery 
of  a  true  insect-wing  in  the  Lower  Silurian  (probably  Caradoc)  sandstone 
of  Jurques,  Calvados."*  It  measures  about  IJ  inch  long,  and  is  dis- 
tinguished by  the  length  of  the  anal  nervure  and  the  small  breadth  of 
the  axillary  area.  It  is  a  primeval  form  of  Blatta,  and  has  been  named 
by  M.  Brongiiiart  Falwoblaitijia.  We  may  be  confident  that  these  are 
not  the  only  relics  of  the  Silurian  terrestrial  fauna  that  have  been 
preserved,  and  we  may  hope  that  still  more  remarkable  treasures  are 
3'et  to  1)0  unearthed  from  their  primeval  resting-places. 

§  2.    Local  Development. 

Britain."'— Ill  the  ty])ical  area  where  Murchison's  discoveries  were  first  made,  he 

fouiul  the  Silurian  rocks  divisible  into  two  great  and  well-marked  series,  which  he 

termed  Lower  and  Upi>er.     This  classification  has  been  found  to  hold  good  over  a  large 

part  of  the  world.     The  subjoined  table  shows  the  aiTaugoment  and  nomenclature  of 

the  various  subdivisions  of  the  Silurian  svsteni : — 

Fe«»t. 

i6.   Ijudlow  group      .     approximate  average  thickness  1900 

5.   Wenlock  group    .  ..  1600 

4.   Llandovery  grou]^  ,.  3000 

3.   liala  and  Caradoc  group  ,.  6000 

2.   Llandeilo  group  .  .,  8000 

^1.   Arenig  group  ,,  4000 

19,500 


^  (J.  Liudstr.im,  Cnmpteft  ren(f.  xcix.  (1884)  ;  T.  Thorell  and  G.  Lindsti'om,  K.  Strnd: 
IV/.  AL'tf.  IIondL  xxi.  No.  9  (1885). 

2  W.  P.  Whitfield,  Science,  vi.  (1885)  p.  87. 

3  B.  N.  Peach,  Suture,  xxxv.  (1885)  p.  295  ;  Tmns.  Roy.  .^>r.  Eiiin.  xxx.  (18S2). 

*  Ch.  Brongniart.  Compte.s  rctuf.  xcix.  (1884)  p.  1164  :  Oef»l.  Mag.  (1885)  p.  481. 

*  See    Murchison's    'Silurian   System.*   and    'Siluria':    Sedgwick's    'Synopsis'  (cited 


SECT,  ii  §  2  SILURIAN  SYSTEM 


a.  Lower  Silurian. 

The  typical  subdivisions  iii  Wales  and  Shropshire  will  first  be  described,  and 
afterwards  the  development  of  the  series  in  other  parts  of  Britain. 

1.  Arenig  Group. — These  rocks  consist  of  dark  slates,  shales,  flags,  and  bands 
of  sandstone.  They  are  abundantly  developed  in  the  Arenig  mountain,  where,  as 
originally  described  by  Sedgwick,  they  include  masses  of  associated  volcanic  rocks.  In 
their  abundant  suite  of  organic  remains  new  genera  of  trilobites  make  their  appearance 
(j-Eglvia^  Barrandia^  Calyineiic^  ffomalonotus,  Ulanopsis,  IlU^iu^j  PhacojiSy  Placopariay 
Trinucleus).  Pteropods  are  represented  by  species  of  Conul^iria  and  Theca ;  brachiopods  by 
Lingula,  Lingulella,  Obolclla,  Disciiuif  Siphonotretay  and  Orfhis ;  lamellibranchs  by 
Palxarca  and  Rihriria ;  gasteropods  by  Ophilcta  and  Pleurotonaria ;  hoteropods  by 
BcJlcrophon  and  Maclurea ;  and  cephalopods  by  Orthoceras,  But  the  most  abundant 
organisms  are  the  graptolites,  of  which  no  fewer  than  twenty  genera  have  been  found  in 
the  Arenig  rocks  of  Britain.  In  the  lower  part  of  the  group  the  genus  Tctragraptiis  is 
especially  characteristic,  for  it  is  not  at  present  known  to  occur  on  any  higher  or  lower 
horizon.  Here  lies  the  lowest  Silurian  graptolitic  zone,  that  of  Tdragraptus  hryonoides. 
The  genera  Loganograptus,  Cloiiogi-aptus^  SchizograjytuSj  and  Dichograptua  are  probably 
also  peculiar  to  ihe  same  strata,  as  well  as  the  species  IHdymograptu-s  ext^nsuSy  D. 
pennatulus^  and  the  only  known  examples  of  Rftiograjitus,  The  ui>iier  i>art  of  the 
Arenig  grouj)  (zone  of  Didymograptus  bifidus)  is  esi)ecially  marked  by  the  presence  of 
PhyUogiaptuSy  in  association  with  forms  of  Dichograptua  like  D.  bijidus.  Species 
peculiar  to  it,  besides  the  last-named,  arc  D.  minutus  and  some  forms  of  Diplograpta, 
such  as  Climacograptiis  confcrtus.^ 

Dr.  Hicks  has  proposed  to  construct  a  sci)arate  group  under  the  name  of  '*  Llanvini,"' 
by  taking  the  upper  part  of  the  Arenig  and  lower  j^ortion  of  the  Llandeilo  rocks,  making 
a  total  thickness  of  about  2000  feet  of  strata  near  St.  David's  in  South  Wales. ^  It  is  in 
this  group  of  strata  that  the  trilobites  Acidaspis^  Barrandia,  lUasnuSj  and  Pfut<^ops  make 
their  earliest  appearance.  Sir  A.  C.  Ramsay  believed  that  in  North  Wales  there  is  an 
unconformable  overlap  of  the  Arenig  upon  the  Tremadoc  and  older  beds  ;  but  in  South 
Wales  there  does  not  appear  to  be  any  break.' 

A  remarkable  feature  in  the  history  of  the  Arenig  rcx-ks  in  Wales  was  the  volcanic 
action  during  their  formation,  whereby  various  felsitic  or  rhyolitic  lavas,  with  almndant 
discharges  of  fine  ashes  and  coarser  agglomerates,  were  erupted  over  the  sea-bottom  and 
interstratified  with  the  contemj>oraneously  dei)Osited  sediments,  while  more  l>asic  sills 
were  subsequently  injected  under  the  volcanic  sheets.  Some  of  the  more  inii)ortant 
Welsh  mountains  consist  mainly  of  these  ancient  volcanic  materials — Cader  Idris.  the 
Arans,  Arenig  Mountain,  and  others.* 

2.  Llandeilo  Group. — These  dark  argillaceous  and  occasionally  calcareous  flag- 
stones, sandstones,  and  shales  were  first  described  by  Murchison  as  occumng  at 
Llandeilo,  in  Carmarthenshire.  They  reajipear  near  St.  David's,  on  the  coast  of  Pem- 
brokeshire, and  at  Builth,  in  Radnorshire.  In  the  lower  sulnlivision  of  them  a  seam  of 
limestone  occurs,  while  intercalated  igneous  rocks  are  specially  noticeable  in  the  upi>er 
sul)di\ision.     It  was  at  one  time  believed  that  graptolites  were  almost  confined  to*  this 


p.  725) ;  Kamsay's  '  North  Wales '  in  Memoirs  of  (iecJ.  Sure.  vol.  iii.  ;  Etheridge,  Adilres>«, 
Q.  J.  Oeitl.  St)c.  1881  ;  numerous  local  memoirs  in  receut  volumes  of  the  Q,  J.  O'ei^f,  Sx\ 
and  Oertf.  Mag.,  particularly  by  Hicks,  Ward,  Hughes,  Keeping,  Lapworth,  &c. 

^   Lapworth,  Ann.  Mag.  yat.  Hint.  vol.  vi.  (1880)  p.  197. 

-  Pop.  S^u'encf  Per.  (1881)  p.  289.  »  'Geology  of  N.  Wales,'  Mem,  Gft*f.  Sun-,  iii. 

*  For  descriptions  of  the  Arenig  lavas  and  tuffs  consult  the  *  Geology  of  N.  Wales ' 
already  cited  :  also  G.  A.  Cole  and  C.  V.  Jennings,  Quart.  Jonrn.  (iefd.  Soc.  xlv.  (18'<9\ 
Oeoi.  Mag.  (1890)  p.  447  ;  Jennings  and  G.  J.  Williams,  Quart.  Journ.  (r'eof.  Soc.  xlvii. 
(1891)  p.  374.     Op.  cit.  Presidential  Address,  p.  105. 


748  STRATIGUAPHWAL  GEOLOGY  book  vi  FART  U 

groii|».  These  fossils,  now  known  to  range  from  the  Cambrian  to  the  top  of  the  SiluriAn 
system,  occur  abundantly  in  the  Llanrleilo  rocks,  and  yiresont  there,  a  trauflitioual 
eharactt'r  U'tween  the  Arenig  types  below  and  those  in  tlie  Camdoc  or  Bala  rocks 
above.  In  the  lower  i)ortionH  of  the  group  the  most  abundant  genus  is  Didym4}grajitu$, 
I).  Murchisniii  being  the  characteristic  species  (and  serving  to  mark  a  graptolitic  zone) 
accomjMinied  ]>y  many  of  tlie  Ai-enig  si»e<.*ies,  together  with  new  forms  of  Cryptograptm 
ami  GJossofjrajttUH.  In  the  middle  ]»art  of  the  grouj)  tlie  D.  Mnrcliisonl  liecomes  very  rare 
and  is  associated  >\ith  Ifiphpffraptus/ofiaeevs  and  CHinactMjrnptus  Scharenbergi.  lu  the 
Upi)er  Llandeilo  rocks  graptolites  of  the  type  of  Cnrjitotjrapfiis  trkornis  and  Climaco- 
grap/us  Sckoroiffcnji  are  abundant,  also  S]HH.'ies  of  Carnugruptus  witli  Uiceilograptiu 
sedans  (zone  of  Co-utHfrnptiia  gracilis).  Trilobites  are  characteristic  fossils  of  the  groap, 
upwanls  of  fifty  sjwcies  belonging  to  eighteen  or  twenty  genera  lieing  known.  Tlicse  in- 
<'lude  characteristic  forms  which  do  not  range  beyond  the  gnmp,  Asaphus  tyrannut, 
Cahjynene  ramhrtiisisy  Trinurh'ivi  Lloytlii^  and  T.factts  being  found  in  the  lower  sub- 
division, an<l  Jinrrandia  Cordaiy  Chrirurtis  Scdgwickii^  and  Ogygia  Biichii  in  the  upper. 
The  phyllo|MKl  Pdtocaris  uptychvides  is  also  ]>eculiar.  The  bracliioixxis  include  the 
genera  Arn^irta^  Cnmia,  IHscina^  JA^ttsna,  LiiigidUf  Orthis^  JlhynckwicUa^  and  Stro- 
phouieiifi,  some  of  which  here  make  their  fii"st  api^arance.  The  lamellibitmclw  are  re- 
l>resente«l  by  species  of  Cardiola{C.  intcrrvpta)  aimX  M(nliohq>si8  (J/,  cxpan/ia,  M,  inflaUi), 
the  gasterojMMls  by  Cffc/ononn,  Em>mph<ihts^  Murrhistmia^  PUurotviiiaria^  Raphistonui, 
and  Turhti,  the  heterojjods  by  JivUrrophony  Etr.u1ioini>haliai^  and  Machirea^  the  ptero]H}d8 
by  Contilaria  and  Thrca,  the  <.'ei)halopods  by  Ciirttyccras,  Orthactyras^  and  Eadoccras, 

•"{.  Caradoc  and  l^ala  (Jroup.- -Under  this  name  were  placed  by  Murchison  the 
thick  yellowish  and  grey  s^mdstones  of  Caer  CaradiK*  in  Shroiwhire,  and  the  Horderley 
and  May  Hill  Sandstone.  It  was  afterwards  ascertained  that  the  grey  and  dark  slates, 
grits,  and  s;indstones,  describe<l  by  Sedgwick  as  occuning  ixmnd  Dala  in  Merionethshirv 
and  reganled  by  him  as  the  higher  pjirt  of  his  Cambrian  system,  were  n»ally  slightly 
diffen'nt  lithologiiral  develo]»ments  of  the  same  stratij^iajihical  division.  In  the  Shroi»- 
shire  area,  some  <»f  the  rocks  are  so  shelly  as  to  become  strongly  calcareous.  In  the 
Bala  district,  tin*  strata  contain  two  limestones  sej)arated  by  a  sjindy  and  slaty  gri:>up  of 
rocks  1400  feet  thick.  Tin*  lower  or  Bala  limestont^  (25  feet  thick)  has  been  traceil  as  a 
variable  band  over  a  largf  nrea  in  North  Wales.  It  is  usually  identified  with  the 
C<»niston  limestone  of  the  WestmonOand  region.  The  upper  or  llirnant  limestone  ':10 
feet)  is  more  lo«al.  Biinds  of  volcani(^  tuHs  and  large  be<ls  of  various  felsitic  lavas  ocrur 
amoijg  th«'  Bala  lu^ds,  and  j»rove  the  coutemjioraneous  ejection  of  volcanic  products. 
These  attain  a  thi(rkness  of  several  thousand  feet  in  the  Snow<lon  regitm.^ 

A  large  suite  of  fossils  Ims  been  obtained  fn>m  this  group.  The  siM)nges  arc  repre- 
sented by  Sp/iH'i'ospfunjiif,  Ar<tnfhosjnjngiif,  and  other  g«!nera.  The  graptolites  ai*e  strongly 
ditfen-ntiated  from  those?  of  the  Arenig  rocks  by  tlie  entire  absence  of  THchograptidie 
and  Phyllt)graptid!e.  The  Diplogiaptidie.  feebly  represented  in  the  Arenig  and  Lower 
Llanrleilo  groups,  are  now,  as  Professor  T^apworth  ])oints  out,  the  dominant  forms,  oixjur- 
riiig  in  swarms  in  evi*ry  zone.  The  two  genera  l>iplogr<ij>fiis  and  CUmncograpfvn  arc 
especially  abundant.  The  f»>llowing  successive  zones  each  marke<l  by  the  prevalence  of 
its  own  species  of  gi'aptolite  have  been  observed  by  Professor  La]>wortli  in  ascending 
order:  (1)  Zone  of  Climdcogrdpfvs  irilsoiu\  (2)  Zone  of  J>irra)W(fr<ipfus  Cfingani,  (3) 
Zone  of  P/nn'fujntpf US  I iiimris,  :4)  Zone  of  DiccHoijriiptiis  cumphinatHSy  (5)  Zone  of  Vic^l- 
/ourifpfus  anaps.  The  same  observer  remarks  upon  the  extraordinary  extinction  of 
families,  genera,  and  sjiecies  of  graj>tolites  during  the  period  of  the  Camdoc-Bala  rocks. 

'  For  accounts  of  the  voleiinic  phenoiiiena  of  the  Caradoc-Bala  series  of  Wales,  see  A. 
('.  Ramsay's  '  (Jeology  of  North  Wales,'  forming  vol.  iii.  of  the  General  Memoirs  of  the 
(leojoiirjeal  J<urvey  ;  Harker's  '  Bala  Volcanic  Series  of  Ciernarvoiishire,'  being  the  S««lgwick 
Prize  Kssay  for  ISSS  ;  F.  \l\\\\i^\\  t^uart.  Jmnn,  ficoi.  So<\  xxxv.  :1879)  p.  508;  W.  W. 
Watts,  „jK  tit.  xli.  '.ISsrO  p.  r»:J2  ;  and  vol.  xlvii.  (ISOl)  Presidential  Address,  p.  117. 


8ECT.  ii  §  2  SILURIAN  SYSTEM  749 

"The  entire  families  of  the  Dicranogi^aptidae,  Leptograptidse,  and  La^iograptidte,  dis- 
appear from  sight  altogether.  The  only  families  that  survive  into  the  Llandovery 
rocks  are  those  of  the  Diplograptidai  and  Rctioliti<l»,  and  these  only  in  a  very  de- 
generate form."  Yet  it  is  remarkable  that  it  was  during  Caradoc  time  that  the 
Dicranograptidie  and  Leptograptidfle  attained  their  highest  develojiment.* 

To  the  conditions  that  allowed  the  de]>08ition  of  limestone  bands  in  this  group  we 
doubtless  owe  the  presence  of  upwards  of  40  sj^ecies  of  corals  (Fig.  345)  belonging  to 
Alveolites,  CyathophyVumf  Favositcs^  Jlalysiies,  Heliolites,  Moniiculipora,  Oniphyma, 
Petraux,  &c.  The  echinoderms  are  i-epresented  by  encrinites  of  the  genera  Actinocnnus, 
Oyathocrinus,  and  Glyptocrinus,  by  no  fewer  than  16  sjwcies  of  cystideans  {Echhwsphm- 
rites f  SphxroniteSy  Agelacrinites^  Hemicosmites,  &<•.),  and  by  star-fishes  of  the  genera 
J*almtster,  Protaster,  and  Stenaster  ;  the  annelides  by  Serpulites^  and  numerous  burrows 
and  tracks ;  the  trilobites  by  species  of  Aciilasjm  (7  sjiecies),  Ampyx  (H),  Asnphus  (6), 
Calymaie  (5),  Chtintrus  (6),  Cyhele  (2),  Encrimiriis  (3),  Hoiiialonotxis  (4),  Illtenus  (9), 
Lichas  (5),  Phacajts  (15),  KanopleuHdes  (7),  TrinuclcMs  (6) ;  the  ostracods  by  Beyrichia, 
Leperditia,  Cytherc^  Primitin,  and  Entomis  \  the  polyzoa  by  Feiw^tella^  Glauconome, 
Ptilodictya,  an<l  Retepora  ;  the  bi-achiopods  by  Atrypii^  Rhynchonella,  Mcristella,  Lepteena 
(10  si)ecies),  Orthis  (nearly  40),  Strophomaui  (17),  Crania ^  Discina,  and  Linefula  ;  the 
lamellibranchs  by  Ctcnodonta  (17  species),  Orthonota  (5),  Moiliolopsis  (15),  Ptcrinea  (6), 
Amhonychia  (8),  Palxarea  (5);  the  gasteropods  by  Murchisonia,  Plcvrotoniaria^ 
Raphistomiiy  Cyclonema^  Eivomphalns^  Holopaea  and  Uolopella  ;  the  pterojMxis  by  Tenia- 
culiteSj  Conularia,  Theca  ;  the  heteroixKis  by  species  of  Bellerophou^  EccuHomphalus 
and  Maclurea  ;  and  the  cephalo]K>ds  by  the  genera  Orthoeeras  (between  30  and  40 
species),  Cyrtoceras,  Lifuite^,  &c. 

The  Lower  Silurian  rocks,  typically  developed  in  Wales,  extend  over  much  of 
Britain,  though  largely  buried  under  more  recent  formations.  They  rise  into  the  hilly 
tracts  of  Westmoreland  and  Cumberland,-  where  they  consist  of  the  following 
subdivisions  in  descending  order  : — 

C-oniston  Limestone  series  with  the  Ashgill  "j 
shales  aV>ove  the  limestone  and  the  Dufton  ] 
shales  below  it        .  .  .  .  .J 

Borrowdale  volcanic  series  (green  slates  and 
l>orphyries) :  tufl's  and  lavas  without  ordi- 
nary sedimentary  strata  except  at  base, 
12,000  ft.      .      '  . 

Skiddaw  Slates,  10,000  or  12,000  ft,  base  \  _  f  Arenig group,  with  perhaps Tre- 
not  seen  .         .         .  .  |  ~  \      niadoc  slates  and  LiiigulaFlags. 

A]»art  from  the  massive  intercalation  of  volcanic  rocks,  these  strata  j>resent  con- 
siderable lithological  and  palteontological  differences  from  the  typical  subdivisions  in 
Wales.  Tlie  Skfddaw  slates  are  black  or  dark-grey,  argillaceous,  and  in  some  beds 
sjindy  rocks,  often  much  cleaved,  though  seldom  yielding  workable  slates,  sometimes 
soft  and  black,  like  Carboniferous  shale.  As  a  nde,  they  are  singularly  unfossiliferous, 
but  in  some  of  their  less  cleaved  and  altered  iH>rtions,  they  have  yielded  about  40  species 
of  graptolites  ;  Lingula  brevis,  traces  of  annelides,  a  few  trilobites  {^gliiuif  Agnostus, 
Asftphu»,  kc),  some  j>hyllopods  {Caryocaris),  and  remains  of  plants  (?)  {Bnthotrqth is,  &c. ) 
According  to  Professors  Nicholson  and  Jjapworth  they  may  he  provisionally  divide<l  into 
two  groups,  the  lower  consisting  of  dark  flagstones  and  shales  distinguished  by  species 
of  Trf  rag  rapt  US,  DidyinograptuSf  PhyllograptuSy  Diplogra^itus,  Loganograpfiis,  Temno- 
graptus,  Schizograpfifs,  Ctenograptus,  />w*/io//rrtjt/^«j>,  and  the  u])j)er  made  uj»  of  black  shales 

'  Lapworth,  A»».  Mag.  yat.  Hist.  v.  (1880)  p.  358  sefj. 

'  Sedgwick's  "Three  Letters  addressed  to  W.  Wordsworth,"  1843  ;  J.  C.  Ward,  'Cieologj'' 
of  the  North  Part  of  the  English  Lake  District'  {fy'eofagicat  Surrey  Memoir)  1876  ;  Nichol- 
son, 'Essay  on  the  Geology  of  Cumberland  an<l  Westniorelaml,'  1868.  See  also  papers  by 
Harkness,  Nicholson,  Hughes,  ^Larr  and  others  in  ^.  J.  fteol.  «Sf>r.  and  Vw/.  Mag. 


Bala  beds. 

Part  of  Bala,  whole  of  Llandeilo, 
and  perhaps  j)art  of  Arenig 
groujjs. 


750  STEATIGRAPHICAL  GEOLOGY  book  vi  part  n 

and  miulstoiied,  containing  some  of  tlie  same  and  some  different  species  of  Didymoffrapttu 
and  PhyUoijraptiLSj  and  siMJcies  of  Trigonograptits,  TrkhograptuSf  Giossograpttis,  Diplo- 
(jraptus,  and  Climacoffraptus.  The  Skiddaw  slates  have  been  invaded  by  granite  and  other 
eru]itive  rocks,  and  display  around  these  a  well-marked  contact-metamorphism  (pi  605). 

Tpwards  the  close  of  the  long  period  represented  by  the  Skiddaw  slates,  volcanic 
action  manifested  itself,  first  by  intermittent  showers  of  ashes  and  streams  of  lava,  which 
were  interstratified  with  the  ordinary  marine  sediment,  and  then  by  a  more  powerfiil 
and  continuous  series  of  explosions,  whei'eby  a  huge  volcanic  mountain  or  group  of  cones 
was  piled  up  above  the  sea-level.  Tlie  vast  pile  of  volcanic  material  (estimated  at  some 
12,000  fe«t  in  total  thickness)  consists  entirely  of  lavas  and  ashes  without  the  interstrati- 
tication  of  ordinary  sediment  except  at  the  base  and  the  top.  The  lower  lavas  are  varieties 
of  andesite,  which  are  also  met  with  in  the  central  and  higher  parts  of  the  Borrowdale 
volcanic  series,  while  rhyolitic  felsites  were  specially  poured  out  towards  the  close  of  the 
volcanic  pericxi.  Enormous  quantities  of  fine  volcanic  ashes  were  likewise  dischai'ged. 
The^e  various  volcanic  rocks  form  the  pictures^iue  hills  of  the  Lake  District*  The  length 
of  time  occupied  by  this  volcanic  episode  in  Cumbrian  geology  may  be  inferred  from  the 
fact  that  all  the  Llandeilo  and  a  large  pait  of  the  Bala  beds  are  absent  here.  The  volcanic 
island  slowly  sank  into  a  sea  wherein  Bala  organisms  flourished.  In  some  places  a 
group  of  shales  occasionally  300  feet  thick,  and  known  as  the  Dufton  shalea,  overlies  the 
Borrowdale  scries,  and  contains  among  other  characteristic  species  Slrophomena  aepansa, 
Lcptxna  sericfu^  Trinuchus  coiicoUricus,  Honialonotus  bisufcatus,  lllssnua  Bownuinnu 
The  most  marked  rock  of  the  overlying  series  is  the  Coniston  limestone,  which  has 
yielded  such  familiar  Bala  species  as  Favosites  fibrosa^  HclioliUs  interstincius^  Cyhele 
verrucosa,  Lcptxna  scriccd^  Orthis  Actonia^  0.  hiforata,  0.  calligranuna,  0.  elegantula, 
0.  porcatOj  and  Strophoinena  rhomhcdddlis.  These  organisms  and  their  associates, 
gathering  on  the  submerged  flanks  of  the  sinking  volcano,  before  the  eruptions  had 
finally  ceased,  formed  there  the  bed  of  limestone  which  is  now  traceable  for  many  miles 
through  the  Westmoreland  hills,  like  the  Bala  limestone  of  North  Wales,  which  it 
probably  represents.  This  Coniston  limestone  has  an  overlying  conformable  group  of 
argillaceous  strata  (Ashgill  shales)  containing  Trinudciis  amccutriciis,  Phacops apic%iJatv.Sy 
P.  mucronitiusj  Stropho/ncna  sUnriana,  and  other  Lower  Silurian  fossils.  Not  far  to  the 
east,  at  the  base  of  the  great  Pennine  escarpment,  contemiK)raneous  volcanic  rocks  in 
the  Coniston  series  are  well  developed.'-  But  the  enormous  volcanic  group  of  Westmore- 
land and  CumlxTland  dies  out  rapidly  in  that  direction,  for  in  the  Craven  district  it  is 
rei)resento(l  by  a  scries  of  sandstones,  grits  and  slates  (often  green),  probably  10,000 
feet  thick,  which  jwisses  up  conformably  into  the  Coniston  limestone  series.' 

The  Southern  Uplands  of  S  c  o  1 1  a  n  d  are  formed  almost  wholly  of  Lower  and  Upper 
Silurian  strata  which  have  been  thrown  into  innumerable  plications,  often  overthrust 
and  revei-sed.  The  working  out  of  this  complicated  structure  has  been  made  i)osaible 
chiefly  by  the  evidence  furnished  by  certain  zones  of  gi-aptolitic  shales,  as  has  been  well 
worked  out  by  Professor  Lapworth.  The  following  table  exhibits  in  descending  onier 
the  subdivisions  which  have  been  established,  with  some  of  their  characteristic  fossils.^ 

^  Ou  the  volcauic  geology  of  this  region  consult  J.  C.  Ward  in  the  work  above  cited ; 
Presidential  Address  to  Geological  Society,  Quart.  Jnurn.  Oeol.  Soc,  1891,  p.  137,  and  authors 
there  given. 

-  Harkness,  Q.  J.  OcoL  Soc.  xxi.  (1865)  p.  235.  Nicholson,  Oeoi.  Mag.  1869,  p.  218. 
Tliis  '' Crossfell  inlier"  has  been  described  by  Messrs.  Nicholson,  Marr,  and  Barker, 
Quart.  Jour/i.  f.ieU.  Sftc.  xlrii.  (1891)  p.  500. 

'*  Hughes,  (rVu/.  Mag.  iv.  (1867)  p.  346.  This  area  had  previously  been  de^ribed  by 
Sedgwick,  Trantf.  O'eoL  /<or.  (2)  iii.  p.  1  ;  and  by  Phillips,  Q.  J.  GeoL  Soc.  viii.  p.  35. 

"*  See  Lai»worth,  O'rcd.  Mag.  1889,  i)p.  20,  59.  The  prolongation  of  the  remarkable 
volcanic  zone  over  the  greater  part  of  the  Southern  Uplands  has  been  detected  by  Mr.  B.  N. 
Peach  in  the  course  of  the  Geological  Survey. 


SECT,  ii  §  2 


SILURIAN  SYSTEM 


751 


LeiuIhillB  and  N.E.  port  of  r«ifiuo. 


Pale  Handv  shales  and  flag- 
stones with  occaftional  bands 
()f  grit  and  seams  of  black 
shale  with  Upper  Hartfell 
graptolites  (Lowther 
Shales). 


Mot&t  and  MUtral  port  of  region. 


o 

8 


■5 


Grey  wackes  and  shales  passing 
north-eastwards  into  a  thick 
group  in  which  the  Lower 
Hartfell  black  graptolitic 
shale  loses  its  lithological 
character.  The  grey  wackes 
are  often  pebbly,  and  con- 
tain some  thin  limestone 
(Wrae,  Winkstone)  with 
Caradoc  fossils. 


Greywackes  and  shales,  in- 
cluding the  Glenkiln  Black 
Shales  with  their  distinctive 
graptolites  and  bands  of  red 
nodular  chert,  with  courses 
of  re<l  and  green  mudstone, 
massive  grey  and  black 
cherts  and  occasional  black 
shales  containing  Upper 
Llandeilo  graptolites. 


mm 

5 


Slaggy  diabases,  tuffs,  and 
agglomerates  only  seen  on 
the  crests  of  the  anticlines 
where  revealed  by  denuda- 
tion. 


Not  seen. 


Green    and  grey   mudstones 
with  black  shales,  forming 
the  Upper  Hartfell  Shales 
and  divided  into : 
S.   Zone  of  Dicellograptus 
anctpsy  I>iplograptti9 

trutuntu8fC1imacograptU8 
tcalaris. 
2.   Mudstone  (unfossilifer- 
ous). 

1.  Zone  of  Dicellograptus 
complanatiu,  Dictyotuma 
moffdUnsis. 

Band  of  black  shales  about  50 
feet  thick  forming  the  Lower 
Hartfell  Shales  and  contain- 
ing the  following  zones : 
8.  Zone    of   PTeuroffniptus 
liruaritt  with  Leptograp- 

t%u    /oliaeeus,     tlimaco- 
graptus  tubtUiferus. 

2.  Zone  of  Dicrantjgnipttu 
Clingani,  with  D.  ramo- 
«tts,  Clinuicogr(iptv8  wu- 
dtttusj  C.  bicomis,  Dicello- 
graptus Forckhtimnieri. 

1.  Zone  of  ClitnacograptuB 
Wilsoni^  with  Crypto- 
graptus  tricorniSf  Diplo- 
graptus  rugosus,  Lasio- 
gnipttu  Harknessit  Clima- 
cograptus  Schartnbergi. 


Group  of  grits  and  green 
shales  with  black  and  grey 
cherts  and  several  bands  of 
black  graptolitic  shale  form- 
ing the  Glenkiln  Shales. 
The  cherts  contain  more 
than  20  species  of  radio- 
laria.  The  black  (Glenkiln) 
shales  are  marked  by  the 
occurrence  of  Dulymograp- 
tus  auperstes,  Cccnogruptus 
graeUiSf  Dicellogmptus  $eX' 
tan*,  D.  divaricatus,  Diplo- 
graptus  mucronatus,  and 
other  forms. 


A]mhire  and  8.W.  part  of  rrglou. 


Fine  tuffs  or  volcanic  mud- 
stones  are  generally  the  only 
indications  of  the  volcanic 
group  in  this  district  But 
much  of  the  mat«rial  of  the 
ordinary  greywackes  and 
shales  has  probably  been 
derived  fh>m  the  denudation 
of  the  volcanic  rooks. 


Not  seen. 


Green  mudstones  and  shales 
(Drummuck)  with  Stauro- 
ce}iKalus  globicepa,  Trinuc- 
leus,  Ampkus,  Dicellograptus 
anceps,  Diplograptus  trun- 
eatus. 

Grey  and  dark  flagstones  and 
shales  (Whltehouse)  with 
Ampyx,  Asavhus,  DictUo- 
graptus  complanatus,  Diplo- 
graptus aocialis,  D.  foliaceuSy 
D.  quajdrimucronatus,  Lepto- 
graptus  Jlaccidiu,  CUmaco- 
graptus  tubul\ftrus. 

FUgs,  shales,  and  griU  (Ard- 
well)  with  Dicellograptus 
Fordthammeri,  Dieranograp- 
tus  ramosusy  Climacograptus 
caudatus,  C.  Scharenbergi, 
Cryptograptus  tricornis,  Dip- 
lograptus rugosus,  Umo- 
graptus  Harkne^i. 


Grits,  flags,  and  shales  (Bal- 
clatchie)  with  Dicrunograp- 
tus  rectus,  Glossogniptus 
Hicksii,  Climacograptus  tri- 
cornis, &c. 

Massive  conglomerate  with 
pebbles  from  the  cherts  and 
volcanic  group  below  (Gir- 
van). 

Shales  with  Didymogniptus 
superstrs,  Dicellograikus  sex- 
tuns,  Diplograptus  eugljff^us, 
CUithrograptus. 

Limestone  (Stinchar,  Craig- 
head) with  Madurea  iMgani, 
Ofkileta  compacta,  LejitKna 
sericea,  and  many  other 
Llandeilo-Caradoc  fossils. 

Thick  conglomerate  with  some 
sandstones  containing  Or- 
this  a>nfinis,  Ac. 

Red  and  green  mudstones  with 
nodules  and  bands  of  red 
chert  and  jasper  containing 
radiolaria. 

Volcanic  group,  shiggy  dia- 
bases and  porphyrias  with 
breccias  and  agglomerates 
and  traversed  by  gabbros, 
serpentines,  and  other  in- 
trusive rocks  (BalUntrae 
and  lower  part  of  Stinrhar 
valley). 


Black  shales  and  limestones 
(Ballantrae,  Lendalfoot) 
with  Phyllograptus  typus, 
Tetmgraptus  bryonoides,  T. 
quadribrachiatus,  Didymo- 
gniptus exttnsus,  D,  bifidus, 
&c.,  and  forms  of  Dictyo- 
namif  LinyuJa^  and  OboleUa. 


752  STRATIGRAPHICAL  GEOLOGY  BOOKViPABrn 

In  the  uort)i-east  of  Ireland  a  broad  belt  of  Silurian  rocks,  crossing  from  the  sontL- 
west  of  Scotland,  runs  from  the  coast  of  Down  into  the  heart  of  the  counties  of  Bot- 
cominon.  and  Longford.  It  is  marked  by  the  same  graptolitic  zones  that  oocnr  in 
Scotland.  The  Glcnkiln  shales  >\ith  their  typical  Llandcilo  graptolites  are  found  to 
the  south  of  Belfast  Lough,  while  the  Hartfell  shales  with  their  Caradoc  feesila  have 
also  been  observed.^  Tlie  richest  fossiliferous  localities  among  the  Irish  Silurian 
rocks  are  found  at  the  Chair  of  Kildare,  Portrane  near  Dublin,  Pomeroy  iu  Tyrone,  and 
Lisbellan  in  Fermanagh,  where  small  protusions  of  the  older  rocks  rise  as  oases  among 
the  surrounding  later  formations.  Portlock  brought  the  northern  and  western  localities 
to  light,  and  Murchison  ]K)inted  out  that,  while  a  number  of  the  trilobites  {Trinudeus, 
Phacitps,  CaJynwnc,  Ilieemis),  as  well  as  the  simjile  plated  Orthidse,  LfjfUente^  and 
Strophomcnmy  some  spiral  shells,  and  many  Ortliocerata,  are  sj^ecifically  identical  with 
those  from  the  typical  Caradoc  and  liala  beds  of  Shropshire  and  Wales,  yet  they  are 
ass<xjiate<l  with  iwculiar  forms,  first  discovered  in  Ireland,  and  very  rare  elsewhere  in 
the  British  Islands.  Among  these  distinctive  fossils  he  cited  the  trilobites,  Jle^itapleu- 
rid^.s.  Harps,  Amphion,  and  BroiUau%  with  smooth  forms  of  Asapfms  {Isotelus),  which, 
though  abundant  in  Irtiland  and  America,  had  seldom  Wen  found  in  Wales  or  England, 
and  never  on  the  continent.'  In  the  south-east  of  Ireland  a  large  tract  of  Silurian 
rocks  extends  through  the  counties  of  Wicklow,  Wexford,  and  Watcrford.  In  this  aret 
also  the  Llundcilo  and  Caradoc  gi-aptolitic  zones  occur.  Even  as  far  south  as  the 
southern  coast-line  of  Waterford  black  shales  continue  the  ])hysical  aspect  of  the  Glen- 
kiln  shales,  and  contain  some  of  the  same  gi-a])tolites.  We  have  thus  evidence  that 
the  black  carbonaceous  mud  in  which  these  graptolites  lived  spread  over  the  sea-floor  for 
a  distance  of  at  least  300  miles. 

b.    Upjyer  Silurian. 

Wales  and  Shropshire. — This  series  of  rocks  occurs  in  two  very  distinct  lithological 
tyi>es  in  the  British  Islands.  So  great  indeed  is  the  contrast  between  these  types,  that  it 
is  only  by  a  comparison  of  organic  remains  that  the  whole  has  been  grouped  together  as 
the  deposits  of  one  g«M)l()gical  ])eriod.  In  the  original  Shro})shire  ivgion  de^scribed  by 
Murchison,  and  from  which  his  tyj»e  of  the  system  was  taken,  the  strata  are  comjiara- 
tivcly  tiat,  soft,  an<l  unaltered,  consisting  mainly  of  somewhat  incoherent  sandy  rondstone 
and  shale,  with  occasional  bands  of  limestone.  But  as  these  rocks  are  followed  into  North 
Wales,  tliey  are  found  to  swell  out  into  a  vast  series  of  grits  and  shales,  so  like  ))ortions 
of  the  hard  altere<l  Lower  Silurian  rocks  that,  save  for  the  evidence  of  fossils,  they  would 
naturally  be  groujMMl  as  part  of  that  more  ancient  series.  In  Westmoreland  and  Cum- 
berland, and  still  farther  north  in  the  bonier  counties  of  Scotland,  also  in  the  south-west 
of  Ireland,  it  is  the  North  Welsh  type  which  j>revails.  This  type,  therefore,  is  really  the 
j>rcvalent  one  in  Britain,  «'xtenfling  over  many  hundreds  of  wjuare  miles,  while  the  original 
Shropshire  type  haitlly  s])reads  beyond  the  border  district  biitween  Kngland  and  W'ales, 
Taking  lirst  the  original  tract  of  Siluria  (W.  England  and  E.  and  S.E.  Wales),  we 
find  a  (le(;ided  unconformability  seimrating  the  Lower  from  the  Upper  Silurian  dejiosits. 
In  some  places  the  latter  steal  across  the  edges  of  the  fonner,  group  after  group,  till 
they  lie  directly  ui)on  the  Cambrian  rocks.  Indeed,  in  one  district,  between  the  Long- 
mynd  an<l  Wenlock  Edge,  the  l>ase  of  the  Uj>per  Silurian  rocks  is  found  witliiu  a  few 
miles  to  jwss  from  the  Caradoc  grouji  across  to  the  Longmyndian  rocks.  It  is  evident, 
therefore,  that  in  the  Welsh  region  very  great  disturbance  and  extensive  denudation 
preceded  the  commencement  of  the  deposition  of  the  Upjwr  Silurian  rocks.  As  Sir  A. 
C.   Ramsiiy  has  ])ointed  out,  the  ai-ea  of  Wales,  previously  covered  by  a  wide  though 

^  W.  Swauston,  Tnnis.  Belfast  ^at.  Field  Club,  187t>-77.  Lapworth,  Ann.  Afa*/.  yat. 
Hist.  iv.  (1879)  p.  424. 

-  '  Siluria,'  p.  174.  The  upper  portion  of  the  Pomeroy  section  has  yielded  Llandover>' 
graptolites,  so  that  the  strata  there  are  partly  Lower  and  partly  Upper  Silurian. 


8.  Ludlow  group. 


SECT,  ii  §  2  SILURIAN  SYSTEM  753 

shallow  sea,  was  ridged  tip  into  a  series  of  islands,  round  the  margin  of  which  the 
conglomerates  at  the  base  of  the  Upper  Silurian  series  began  to  be  laid  down.  This 
took  place  during  a  time  of  submergence,  for  these  conglomeratic  and  sandy  strata  are 
found  creeping  up  the  slopes  and  even  capping  some  of  the  hills,  as  at  Bogmine,  where 
they  reach  a  height  of  1150  feet  above  the  sea.  The  subsidence  probably  continued 
during  the  whole  of  the  interval  occupied  by  the  deposition  of  the  Upper  Silurian  strata, 
which  were  thus  piled  to  a  depth  of  from  3000  to  5000  feet  over  the  disturbed  and 
denuded  platform  of  Lower  Silurian  rocks. 

Arranged  in  tabular  form,  the  subdivisions  of  the  Upper  Silurian  rocks  of  Wales  and 
the  adjoining  counties  of  England  are  in  descending  order  as  follows  : — 

Base  of  Old  Red  Sandstone. 

'^Tilestones. 

Downton  Castle  Sandstone,  90  feet. 
Ledbury  Shales,  270  feet. 
U^r  Ludlow  Rock,  140  feet. 
Aymestry  Limestone,  up  to  30  or  40  feet. 
^  Lower  Ludlow  Rock,  350  to  700  feet 

(Wenlock  or  Dudley  Limestone,  800  feet       .  )       /  n*.  k*  i.  »»• 
Wenlock  Shale,  up  to  2300  feet  .         .         .  f       J  ^°^^'^8'"'« 
Woolhope  or  Rirr  Limestone  and  Shale,  150  j  =  (North Vides. 
leei    .         .         *         .         .         .         »  J 
TTarannon  Shales,  1000  to  1500  feet. 
1.   Llandovery  group.  -|  Upper  Llandovery  Rocks  and  May  Hill  Sandstone,  800  feet. 

(^  Lower  Llandovery  Rocks,  600  to  1500  feet. 

1.  Llandovery  Group. — The  most  marked  lithological  character  of  this  group  in 
Britain  is  the  occurrence  of  conglomerates  which  indicate  the  terrestrial  disturbance 
and  extensive  denudation  that  followed  the  close  of  the  deposition  of  the  Lower  Silurian 
rocks. 

(a)  Lower  Llandovery. — In  North  Wales,  the  Bala  beds,  about  fivemilii  S.K  of  Bala 
Lake,  begin  to  be  covered  with  grey  grits,  which  gradually  expand  southwards  until  they 
attain  a  thickness  of  1000  or  even  1500  feet.  These  overlying  rocks  are  well  displayed 
near  the  town  of  Llandovery,  where  they  contain  some  conglomerate  bands,  and  where 
Mr.  Aveline  detected  an  unconformability  between  them  and  the  Bala  group  below 
them.  Elsewhere  they  seem  to  graduate  downwards  conformably  into  that  group. 
They  cover  a  considerable  breadth  of  country  in  Cardigan  and  Carmarthenshire,  owing 
to  the  numerous  undulations  into  which  they  have  been  thrown,  and  they  extend  as  far 
as  Haverford  West  in  Pembrokeshire.  A  marked  change  is  now  visible  in  the  fossil 
contents  of  the  rocks,  as  compared  ¥dth  those  of  the  Lower  Silurian  subdivisions. 
Thus  the  familiar  Lower  Silurian  types  of  trilobites  become  few  or  extinct,  such  as 
Agtwstus,  Ampyx,  Asaphus,  Ogyffw,  Remoplcurides^  TrinueleuSy  and  their  places  are 
taken  by  species  of  Acidaspis^  Enerinurus^  Phacops^  ProituSj  and  other  genera.  A 
still  more  striking  contrast  occurs  among  the  types  of  graptolites.  The  families  of 
the  Dicranograptidffi,  Leptograptidse,  and  Lasiograptidae  wholly  disappear,  and  the 
forms  which  now  take  their  place  and  distinguish  the  Upper  Silurian  rocks  belong 
to  the  Monograptidse  which  gradually  exclude  the  Diplograptidse,  until  before  the 
higher  parts  of  the  system  are  reached  they  are  the  sole  representatives  of  the 
graptolites.  Four  graptolitic  zones  have  been  recognised  in  the  Llandovery  group, 
viz.  in  ascending  order:  (1)  Diplograptus  ocumiTuUus,  (2)  Diplograptua  vesiculosna, 
(3)  Monograptus  gregarius,  (4)  MoiwgraptM  spinigems.  Besides  these  species, 
MoTwgraptus  Unuis^  M.  aUenucUuSf  M,  Eisingeri^  M,  lobiferus,  and  RotstrUus  pere- 
grinus  are  common  Llandovery  forms.  Other  characteristic  fossils  are  Orthis  elegan- 
tukif  Stricklandinia  {Pentamerm)  lens,  Meristella  craasa^  and  Calymene  Blumenbachii, 
From  the  abundance  of  the  peculiar  brachiopods  termed  Peniamerus  in  the  Lower,  but 
still  more  in  the  Upper  Llandovery  rocks,  these  strata  were  formerly  grouped  together 
under  the  name  of  ''Pentamerus  beds."    Though  the  same  species  are  found  in  both 

3C 


754  STRATIGRAPHICAL  GEOLOGY  book  ti  r 


divisions,  PeiUamenu  obUm^us  is  chiefly  characteristic  of  the  apper  groiip  and 
tirely  infrequent  in  the  lower,  while  Striekiamdinia  {Ptntamenu)  letu  mbomids  in  tba 
lower,  bat  appears  more  sparingly  in  the  upper.  The  genus  aaeendi  into  tlie  Wcnloek 
and  Ladlow  gronps,  and  is  specially  distinctiTe  of  Upper  Silurian  roclok 

fb)  Upper  Llandovery  and  May  HUl  SandaUme. — ^This  sub-group  baa  lecicivcd  tha 
name  of  May  Hill  Sandstone  from  the  locality  in  Glouoesterahire  where,  as  fifst  ahowm 
by  Mnrchison,  it  is  well  displayed.  Sedgwick  pointed  out  that  it  forms  oircr  a  widt 
region  the  natural  base  to  the  Upper  Silurian  series,  for  it  rests  unconfonDab^  ob.  all 
older  rocks.  It  consists  of  grey,  yellow  and  brown  ferruginous  sandstonea  and 
conglomerates,  sometimes  calcareous  from  the  abundance  of  shells,  which  are  apC  to 
weather  out  and  leave  cssts.  Where  the  organisms  have  been  moat  crowded  togetlier 
the  rock  even  passes  into  a  limestone  (Pentamerus  limestone,  Xorbuiy  limestone^  Hollies 
limestone).  The  lower  membera  are  usually  strongly  conglomeratic,  the  pebblea  being 
derived,  sometimes  in  great  part,  from  Lower  Silurian  rock^  Appearing  on  the  eoast 
of  Pembrokeshire  at  Marloes  Bay,  this  sub-group  ranges  acnJs  South  Wake  until  it  is 
ovcrlapfied  by  the  Old  Red  Sandstone.  It  emerges  again  in  Carmarthenshixe,  and  troids 
north-eastward  as  a  narrow  strip  at  the  base  of  the  Upper  Silurian  seiiaik  from  a  few 
feet  to  1000  feet  or  more  in  thickness,  as  far  as  the  Longmynd,  where,  as  a  marked 
conglomerate  wrai>ping  round  that  ancient  Cambrian  ridge,  it  disappears.  In  the  oonrse 
of  this  long  tract  it  passes  successively  and  unconformably  over  Lower  UandoTay, 
Caradoc,  Llandeilo,  Cambrian,  and  pre-Cambrian  rocks. 

Among  the  fossils  are  some  traces  of  fucoids :  sponges  {Cliona,  a  burrowing  form 
like  the  modem  Cliona) ;  species  of  Monoffraptus  (M.  Hisingtri^  if.  intermediut, 
M,  crenularis\  JiastriUs  {R,  pereffrinus),  Diployraptus,  {D,  JIughen),  Cephalograpiut 
((7.  eomeia) ;  a  number  of  corals  {Petraiay  Helioiitcs,  FavosiUSf  HalytiUM^  Syringo- 
para,  kc.)  ;  a  few  crinoids  and  the  earliest  known  sea-urchins  {Paimehinug) ;  the  ^enus 
TentaculiUs  u  particularly  abundant ;  a  number  of  trilobites,  of  which  I%aetfp§  Siokeni, 
P.  Weaveri,  Encrinurus  puncUUus,  Calymene  Blununbachii,  ProHus  Siokeni,  and 
llUenua    Thcmsoni   are   common ;    numerous    brachiopods,   as  Atrypa   lumitpkerioa, 

A,  reiiciUaris,  Pentamerus  ohUniguSy  Strickland inia  lyrata,  S.  lens,  LqtUgna  trans- 
versalis,    Orthis  calligramina,    0.   elcganttila,    0.   reversa,    Strophomena  eompressOy   S. 

pecten,  and  Lingu/a  parallela ;  lamellibranchs  of  the  mytiloid  genera  Orthcnota,  Myliltu, 
and  Modiolopsis^  with  forms  ori*teHiua,  Ctenodonta,  and  Lyrodesma  ;  gasteropods, 
particularly  the  genera  Acroculia,  Paphistoma,  Murchisonia,  Pleurotomaria,  CyeUmema, 
Holomlla  ;  heteropods,   especially  the  species  BcUerophon  dilaiatus,  B.  trilobatus,  and 

B.  r.trinatus;  aud  cephalopods,  chiefly  Orthocerata^  with  some  forms  of  Actinoceras, 
CyiiocerciSy  Trctoceras,  and  Phragjtioceras,  and  the  old  species  Lituiles  comu-arietis. 

(c)  Tarannon  Shale. — Above  the  Upper  Llandovery  beds  .comes  a  very  persistent 
band  of  fine,  smooth,  light  grey  or  blue  slates,  which  has  been  traced  from  the  mouth 
of  the  Conway  into  Carmarthenshire.  These  strata,  termed  the  "paste-rock"  by 
Sedg\iick,  have  an  extreme  thickness  of  1000  to  1500  feet.  Poor  in  organic  remains, 
their  chief  interest  lies  in  the  fact  that  the  persistence  of  so  thick  a  band  of  rock 
between  what  were  supposed  to  be  continuous  and  conformable  formations  should  have 
been  unrecognised  until  it  was  proved  by  the  detailed  mapping  of  the  Geological  Survey. 
The  occurrence  of  certain  species  of  graptolites  affords  a  ]>alseoutological  basis  for  placing 
on  this  horizon  a  considerable  mass  of  slaty  and  gritty  strata  in  Cardiganshire,  and  for 
identifying  these  and  the  typical  Tarannon  Shales  with  their  probable  equivalents  in 
the  Lake  District  and  in  Scotland.  The  following  graptolitic  zones  in  ascending  order 
have  been  determined  in  the  Tarannon  rocks:  {1)  Jiastritcs  maximus,  {2)  Mcmogrtqitus 
exiguiis,  (3)  Cyrtograptus  Grays.  Other  common  species  are  Mmograptus  galaensis,  M. 
primlon^  M.  riccartonensis,  and  Jietiolites  gcinitzianus. 

2.  Wen  lock  Group. — This  suite  of  strata  includes  the  larger  part  of  the  known 
UpjKir  Silurian  fauna  of  Britain,  as  it  has  yielded  more  than  160  genera  and  600 
species.     In  the  typical  Silurian  area  of  Murchison,  it  consists  of  two  limestone  bands 


8BCT,  ii  §  a  SILURIAN  SYSTEM  7S6 

(Woolhope  and  Wenlock),  separated  b^  a  thicL  man  of  ahole  (Wenlock  Shale).     Ths 
foHowiDg  aub-graup«  in  MC«tiding  orier  an  reoogniMd  :^ 

(a)  WoolKopt  Limatotu. — In  tlis  original  typical  Upper  Silurian  tract  of  Shropshiro 
and  the  adjacent  conntJM,  the  Upper  Llandovery  rockt  are  overlain  by  a  local  gronp  of 
grey  shales  containing  nodular  limestone,  which  here  and  there  swells  out  into  beds 
haviTig  an  aggregate  thickness  of  30  or  40,  but  at  Malvern  as  much  as  150  feet.  Tlieee 
strata  are  well  displayed  in  the  picturesque  valley  of  Woolhope  in  Herefordahire,  which 
lies  upon  a  worn  qui-quA-versal  dome  of  Upper  Silurian  strata,  rising  in  the  midst  of 
the  surrounding  Old  Red  Sandstone.      They  are  seen  likewise  to  the  north-west,  at 


Fig.  144.— Group  or  PcDiamerl  from  Llandovarr  and  Wa 
lu  oblonKoa.  Sby.  :  It,  F.  gulHtns,  Dalm. ;  c.  P.  KnIghUl.  Sby     .  .    . 
la.  Hby.  ());  /,  P.  EnLghUI  (snull  siMelincn);  },  P.  UngulCsr,  Sb/. ;  l>,  F. 


F  oblongni  Bby     t  F 


Prestcign,  Nash  Scar,  and  Old  Radnor  in  Radnorshire,  and  to  the  east  and  sonth,  in  the 
Malvern  Hills  (where  they  include  a  great  thickness  of  shale  below  the  limestone),  and 
May  Hill  in  Gloucestershire.  Among  the  common  fossils  of  these  strata  may  be  men- 
tioned Itlmnua  {Bumaaltu)  barrieniia,  Homaltnwtva  tUtphinoeephalvt,  Phaeopi  taudaUu, 
Eittriauriu  punelalua,  Aadaapa  Brightii,  Alrypa  rftimlaru,  Orlhit  ealligramma, 
Strophomena  imirex,  S,  euglypha,  Leplana  trantvenatii,  Shyndumella  bortalit,  S. 
IViUani,  Suomphatia  iculplua,  Orthoeenu  annuiaivm.  * 

It  is  a  feature  of  the  older  Paleeozoic  limestones  to  occur  in  a  very  lenticular  form, 
■welling  in  some  places  to  a  great  thickness  and  rapidly  dying  out,  to  t«appear  again 


766  STRATIGRAPHICAL  GEOLOGY  book  ti  pabt  n 

perhaps  some  miles  away  with  increased  proportions.  This  looal  chftracteor  is  well 
exhibited  by  the  Woolhope  limestone.  Where  it  disappears,  the  shales  underneath  and 
intercalated  with  it  join  on  continuously  to  the  overlying  Wenlock  shale,  and  no  line 
for  the  Woolho])e  sub-group  can  then  be  satisfactorily  drawn.  The  same  disoontinnity 
is  strikingly  traceable  in  the  Wenlock  limestone  to  be  immediately  referred  ta 

(6)  Wenlock  Shale. — This  sub-group  consists  of  grey  and  black  shales,  traceable  from 
the  banks  of  the  Severn  near  Coalbrook  Dale  across  Radnorshire  to  near  Carmarthen 
— a  distance  of  about  90  miles.  The  same  strata  reappear  in  the  protrusions  of  Upper 
Silurian  rocks  which  rise  out  of  the  Old  Red  Sandstone  plains  of  Qloucesterahire, 
Herefordshire,  and  Monmouthshire.  In  the  Malvern  Hills,  they  are  estimated  by 
Professor  Phillips  to  reach  a  thickness  of  640  feet,  but  towards  the  north  they  thicken 
out  to  more  than  2000  feet.  On  the  whole,  the  fossils  are  identical  with  those  of  the 
overlying  limestone.  The  corals,  however,  so  abundant  in  that  rock,  are  here  com- 
paratively rare.  The  brachiopods  {JAngulct,  Leptsena,  Orthis,  Strophomena,  Atrypa, 
Bhynchomlla,  Spiri/er)  are  generally  of  small  size— Or^m  bUoba,  0,  hyhridei,  and  the 
large  flat  0.  rustiea  being  characteristic.^  Of  the  higher  moUusca,  thin-shelled  forms 
of  Orihoctras  are  specially  abundant.  Among  the  trilobites,  Encrinurus  punelatus, 
E,  variolariSf  Calyirune  Blumenbackii,  C,  tuberculosa,  Phacops  eaudatus,  P.  longi' 
eaudalus  are  common.  Distinctive  species  of  graptolites  characterise  the  shales  of  this 
group.  At  the  base  lies  the  zone  of  Cyrtograptus  Murchisoni,  with  MofwgraptuB  priodon, 
M.  Halliy  M.  vomerinuSf  M.  eolonus  and  RetioliUs  geinitzianus.  Higher  up  comes  the 
zone  of  Cyrtograptus  Linnarssoni  and  still  higher  that  of  Monograptus  testis.  The  most 
abundant  Wenlock  si)ecies  in  Britain  are  M,  voinerinus,  M.  rieearUmensis,  and  if. 
priodon^  which  last  does  not  api)ear  to  reach  the  Lower  Ludlow  rocks.* 

(c)  Wenlock  Limestone. — This  is  a  thick-bedded,  sometimes  flaggy,  usually  more  or 
less  concretionary  limestone,  grey  or  |)ale  pink,  often  highly  crystalline,  occnrring  in 
some  places  as  a  single  massive  bed,  in  others  as  two  or  more  bands  separated  by  grey 
ehales,  the  whole  forming  a  thickness  of  rock  ranging  from  100  to  300  feet.  As  its 
name  denotes,  it  is  typically  developed  along  Wenlock  Edge  in  Shropshire,  where  it 
nins  as  a  prominent  ridge  for  fully  20  miles  ;  also  between  Aymestry  and  Ludlow.  It 
likewise  ai)j)eai's  at  the  detached  areas  of  Upper  Silurian  strata  above  referred  to,  being 
specially  well  seen  near  Dudley  (whence  it  is  often  spoken  of  as  the  Dudley  limestone), 
Woolhope,  Malvern,  May  Hill,  and  Usk  in  Monmouthshire. 

A  distinguishing  characteristic  of  the  Wenlock  limestone  is  the  abundance  and 
variety  of  its  corals,  of  which  no  fewer  than  24  genera  and  upwards  of  80  species  have 
been  described.  The  rock  seems,  indeed,  to  have  been  formed  in  part  by  massive 
sheets  and  bunches  of  coral.  Characteristic  species  are  Haly»ites  catenularia,  Heliolites 
intersfijvctiis,  U.  tubulatuSy  Alveolites  Labech^iy  Fat>osites  aspera,  F,  Jibrosa,  F.  goUandica, 
Cccnitis  juniper imis,  Syringoporafascicularis,  Omphyma  stiblurbinalum.  The  crinoids  are 
also  sj)e(;ially  abundant,  and  often  beautifully  preserved,  Periechocrinus  monili/ortnis  being 
one  of  the  most  fre<|ueut ;  others  are  Crotalocrinus  rugosiis,  Cyathocrinus  gonioda^yluSy 
and  Marsupiocrinns  cmlatus.  Several  cystidcans  occur,  of  which  one  is  Pseudocrinites 
quadrifasciatuji.  More  than  30  species  of  annelides  have  been  found.  The  crustaceans 
include  numerous  trilobites,  one  of  the  most  abundant  being  the  long-lived  Calymene 
Blumenbackii,  which  ranges  from  the  Llandeilo  flags  (i)ossibly  from  a  still  lower  horizon) 
up  to  near  the  top  of  the  Upper  Silurian  fonnations.     It  occurs  abundantly  at  Dudley, 

^  As  an  example  of  the  small  size  but  extraordinary  abundance  of  brachiopods  in  this 
formation  reference  may  be  made  to  the  fact  that  a  cartload  of  the  shale  from  Buildwas  was 
found  by  careful  washing  to  contain  no  fewer  than  4800  specimens  of  one  species  {Ortkis 
bUob(i)y  besides  a  much  greater  bulk  of  other  brachiopods,  amounting  together  to  10,000 
specimens  at  least ;  while  from  seven  tons  weight  of  the  shale  at  least  25,000  specimens  of 
Orthis  bilofxt  were  obtained. — Davidson  and  Maw,  OeoL  Mag.  1881,  p.  101, 

-  Lapworth,  Ann.  Mag.  Nat.  Hist.  v.  (1880)  p.  369. 


awai.  ii  §  2  SILURIAN  SYSTEM  767 

where  it  noeived  the  nune  of  the  "Dudley  Locust."  Other  common  forms  tre 
Enerinunu  punelalut,  E.  mnaloru,  i%ieiipa  eaudatxu,  P.  Dotsningim,  P.  StoteHi, 
Illmitia  {BuTnattia)  barrienns,  Somalojuituii  dtlphinMipkaliu,  and  Chtirunia  bimu- 
CTtmatus.  One  of  the  moat  remarkable  reatnrea  in  the  croituean  fauna  is  the  filBt 
appearance  of  the  meroatomata,  which  are  repreeented  by  Evryptena  punctatia, 
Semia^u  lumidtii,  and  PUrygotu*  probUmalicua.  The  brachiopods  continue  to  be 
abundant,  about  20  genera  and  100  species  haTing  up  to  this  time  been  enumerated. 
Among  typical  speciea  may  bo  noted  Atrypa  retieularia,  Whitfieldia  (MeruOtlla)  t\tmula, 
Spiri/tr  fUvaiua,  S.  pliattdlm,  Shynehonella  borealU  (very  common),  R.  euneata,  S. 
tyilsoni,  Orthu  cUganlula,  O.  hybrida,  Slropkomena  r/iomboidalit,  and  FetUanKnu 
gaUaita,     The  laroellibranchs  are  abandant  and  are  represented  by  species  of  Avieula, 


»n{l);  IT,  Cyt>h«*piii  n 


fig.  345.— Upper  ailuiiu  < 
inn,:  A.PtychophyUumiHUIliCiim,  Schloth.  ());  c, 
biiia,  LoDL :  t,  Oentiocarl*  paplUo,  Halt.  ()) ;  /,  Hom 
"  :Coy  i  h^  Phacope  Downinglte,  llui 


■niptiTTu  lubtDiblnatoni, 


Pterinea,  Cardiola,  and  CuculUUa,  with  Qrammyiia  cingalata,  Orthonota  amygdiJina, 
and  some  speciea  of  Modioloptis  and  CUnodanla.  The  gasteropoda  are  marked  by  speciea 
of  Euompfutlia,  Murchinrtia,  Holopeila,  Aeroeutia,  Cycltnuma,  Tlie  cephalopoda  are 
confined  to  five  genera,  LUuitta,  Adinocera$,  Cyrloartu,  OrChix^nu,  and  Phragmaxrat  i 
of  these  the  orthoceratites  are  by  far  the  most  abundant  both  in  species  and  individuals, 
OrAoecrat  atinulatum  being  the  most  common  form.  The  pteropods  appear  in  the 
beautiful  and  abundant  C^ularia  Sowtrbyi,  and  the  heteropods  in  the  common  and 
charspteristic  Beflrroplum  wfnUidimni, 

3.   Ludlow  Group.— This  group  consists  eaaentially  of  shales,  with  occasionally  • 
calcareous  band  in  the  middle.     It  graduates  downward  into  the  Wenlock  group,  so  that 


768  STRATIGRAPHICAL  GEOLOGY  book  vi  pabt  n 


when  the  Wenlock  limestone  disappears,  the  Wenlock  and  Ludlow  shales  form  one 
continuous  argillaceous  formation,  as  they  do  where  they  stretch  to  the  sonth-west 
through  Brecon  and  Carmarthen.  The  Ludlow  rocks,  typically  seen  between  Ludlow 
and  Aymestry,  appear  likewise  at  the  detached  Silurian  areas  from  Dudley  to  the  month 
of  the  Severn.  They  were  arranged  by  Mnrchison  in  three  sub-groups — Lower  Ladlow 
Rock,  Aymestry  Limestone,  and  Upper  Ludlow  Rock. 

(a)  Lotoer  Ludlow  Rock, — This  sub-group  consists  of  soft  dark  grey  to  pale  greeniah- 
brown  or  olive  sandy  shales,  often  with  calcareous  concretions.  Much  of  the  rock, 
however,  presents  so  little  fissile  structure  as  to  get  the  name  of  mudstone,  weathering 
out  into  concretions  which  fall  to  angular  fragments  as  the  rock  crumbles  down.  It 
becomes  more  sandy  and  flaggy  towards  the  top.  From  the  softness  of  the  shales,  this 
zone  of  rock  has  been  extensively  denuded,  and  the  Wenlock  limestone  rises  up  boldly 
from  under  it.     It  attains  a  thickness  of  750  feet  at  Malvern. 

An  abundant  suite  of  fossils  is  contained  in  these  shales.  Eight  species  of  star-fishes 
have  been  found,  belonging  to  the  genera  Protaster  (like  the  brittle-stars  of  the  British 
seas),  FalseodiscuSf  and  PalaBocoma,  The  graptolites  which  played  so  conspicuous  a 
part  in  the  marine  fauna  of  Cambrian  and  Silurian  time  now  appear  for  the  last  time. 
They  are  restricted  entirely  to  the  genus  Monograptua,  of  which  if.  Nilssoni,  M,  colonua, 
M,  leintipardinensiSf  M.  Salweyi^  M*  hofiemicus,  M,  scanicuSf  M,  pricdon  (var.  Itideiuis)^ 
and  M.  Roemeri  are  especially  characteristic  The  distinctive  graptolitio  zone  of  this 
part  of  the  Sihirian  series  has  been  named  that  of  Monograptua  Nilsaoni,  and  is  the  last 
of  the  long  series. 

A  few  corals  occur  in  the  Lower  Ludlow  rock,  all  of  species  that  had  already 
appeared  in  the  Wenlock  limestone,  but  the  conditions  of  deposit  were  evidently 
unfavourable  for  their  growth.  The  trilobites  are  less  numerous  than  in  older  groups  ; 
they  include  the  venerable  Calymene  BluineTibachii ;  also  PhcLCops  caueUUus,  P,  con- 
strictuSf  P.  Doumingiagf  Acidaspis  coronaius^  Cheirurus  bimucroncUuSf  JEncrinurus  punc' 
tatiSy  Lichas  anglicuSy  ffomalonotua  delphinocephalus,  H,  Knightii,  and  Cypha^s 
megalops.  But  other  forms  of  crustacean  life  occur  in  some  number.  As  the  trilobites 
began  to  wane,  numerous  phyllopods  appeared,  the  genus  CercUiocaris  being  represented 
by  nine  or  more  species.  Still  more  remarkable,  however,  was  the  increasing  import- 
ance of  the  merostoinatous  crustaceans  {Eurypterus,  UemiaspiSy  Pterygotua),  Though 
brachiopods  are  not  scarce,  hardly  any  seem  to  be  peculiar  to  the  Lower  Ludlow  rock, 
uearly  all  of  the  known  species  occurring  in  the  Wenlock  group.  RhynchoneUa  JFilsoni, 
Spiri/er  exporrcctus^  S.  crispus^  S.  bijugosuSf  Strophornena  euglypha,  S.  rhomboidulis, 
Atri/pa  reticularis^  IHscina  MorHni,  Lingula  lata,  and  L.  Leurisii  are  not  infrequent. 
Among  the  more  frequently  recurring  species  of  lamellibranchs  the  following  may  be 
named — Cardiola  iiUemipta^  C.  striata,  Ctetiodonta  sulcata,  Oravimysia  eingulata, 
Modiol apsis  gradata,  M.  Nilssoni,  Orthonota  amygdalinxa,  0.  rigida,  0,  semisulcata^  and 
a  number  of  species  of  Pterinea,  Among  the  gasteropods  not  uncommon  species  are 
Cyclonema  corallii,  Euomphalus  alatus,  Holopella  gregaria,  Loxonema  tdnuosa,  and 
Murchisonia  Lloydii.  The  old  heteropod  genus  Bellerophon  is  still  represented 
{B.  expaiisxis).  The  cephalopods  abound,  the  genus  Orthoceras  being  the  prevalent  type 
(0.  angulatum,  0.  annulaium,  0.  bullatuni,  0.  ludeiise,  0.  subundulcUum,  0,  tracheali), 
but  with  species  of  Exosiphonites,  Lituites,  and  Phragmoceras,  The  numbers  of  straight 
and  curved  cephaloi>ods  form  one  of  the  distinguishing  features  of  the  zone.  At  one 
locality,  near  Leintwartline  in  Shropshire,  which  has  been  prolific  in  Lower  Ludlow 
fossils,  particularly  in  star-fishes  and  eurypterid  crustaceans,  a  fragment  of  the  fish 
Scaphaspis  {Pteraspis)  ludcnsis  was  discovered  in  1859.  This  is  the  earliest  trace 
of  vertebrate  life  yet  detected  in  Britain.  It  is  interesting  to  note  that  this  fish  does 
not  stand  low  in  the  scale  of  organisation,  but  has  affinities  with  our  modem  sturgeon. 

(6)  Aymestry  Limestone — a  dark  grey,  somewhat  earthy,  concretionary  limestohe  in 
beds  from  1  to  5  feet  thick.     Where  at  its  thickest  (from  30  to  40  feet)  it  forms  a 


SECT,  ii  §  2  SILURIAN  SYSTEM  759 

conspicuous  feature,  rising  abore  the  soft  and  denuded  Lower  Ludlow  shales.  Owing 
to  the  easily  removable  nature  of  some  fuUers'-earth  on  which  it  lies,  it  has  here  and 
there  been  dislocated  by  large  landslips.  It  is  still  more  inconstant  than  the  Wenlock 
limestone.  Though  well  developed  at  Aymestry  in  Herefordshire,  it  soon  dies  away  into 
bands  of  calcareous  nodules,  which  finally  disappear,  and  the  lower  and  upper  divisions 
of  the  Ludlow  group  then  come  together.  The  organic  remains  at  present  known  are 
for  the  most  part  identical  with  Wenlock  forms.  It  is  evident  that  the  organisms 
which  flourished  so  abundantly  in  the  clear  water  wherein  the  Wenlock  limestone  was 
accumulated,  continued  to  live  outside  the  area  of  deposit  of  the  Lower  Ludlow 
rock,  and  reappeared  in  that  area  with  the  return  of  the  conditions  for  their  existence 
during  the  deposition  of  the  Aymestry  limestone.  The  most  characteristic  fossil  of 
the  latter  rock  is  the  FerUamerus  Knightii ;  other  common  forms  are  Hhynchondla 
WiUoniy  Lingula  Lewini,  Strophomma  euglypha,  Atrypa  reticulariSf  Btllerophcn 
dilcUattis,  PUrinea  Soteerbyi,  with  many  of  the  same  shells,  cords,  and  trilobites  found 
iu  the  Wenlock  limestone.  Indeed,  as  Murchison  has  pointed  out,  except  in  the  loss 
number  of  species  and  the  occurrence  of  some  of  the  shells  more  characteristic  of  the 
Upper  Ludlow  zone,  there  is  not  much  palseontological  distinction  between  the  two 
limestones.^ 

(e)  Upper  Ludlow  Rock. — In  the  original  Silurian  district  described  by  Murehison, 
the  Aymestry  limestone  is  covered  by  a  calcareous  shelly  band  full  of  Rhynehonella 
navicular  sometimes  30  or  40  feet  thick.  This  layer  is  succeeded  by  grey  sandy  shale 
or  niudstone,  often  weathering  into  concretions,  as  in  the  Lower  Ludlow  zone,  and 
assuming  externally  the  same  rusty-brown  or  greyish  olive-green  hue.  Its  harder 
beds  are  quarried  for  building  stone ;  but  the  general  character  of  the  deposit,  like 
that  of  the  argillaceous  portions  of  the  Upper  Silurian  formations  as  a  whole,  in  the 
typical  district  of  Siluria,  is  soft,  incoherent,  and  crumbling,  easily  decomposing  once 
more  into  clay  or  mud,  and  presenting,  in  this  respect,  a  contrast  to  the  hard,  fissile, 
and  often  slaty  shales  of  the  Lower  Silurian  series.  Many  of  the  sandstone-beds  are 
crowded  with  ripple-marks,  rill-marks,  and  annelid-trails,  indicative  of  the  shallow 
littoral  waters  in  which  they  were  deposited.  One  of  the  uppermost  sandstones  is 
termed  the  ''Fucoid  Bed,"  from  the  number  of  its  cylindrical  seaweed-like  stems.  It 
likewise  contains  numerous  inverted  pyramidal  bodies,  which  are  believed  to  be  casts  of 
the  cavities  made  in  the  muddy  sand  by  the  rotary  movement  imparted  by  tides  or 
currents  to  crinoids  or  seaweeds  rooted  and  half  buried  in  it.'  At  the  top  of  the 
Upper  Ludlow  rock,  near  the  town  of  Ludlow,  a  brown  layer  occurs,  from  a  quarter  ot 
an  inch  to  three  or  four  inches  in  thickness,  full  of  fragments  of  fish,  Pterygotus, 
and  shells.  This  layer,  termed  the  ''Ludlow  Bone-bed,"  is  the  oldest  from  which 
any  considerable  number  of  vertebrate  remains  has  been  obtained.  In  spite  of  its 
insignificant  thickness,  it  has  been  detected  at  numerous  localities  from  Ludlow  as  far 
as  Pyrton  Passage,  at  the  mouth  of  the  Severn — a  distance  of  45  miles  from  north  to 
south,  and  from  Kington  to  Ledbury  and  Malvern — a  distance  of  nearly  30  miles  from 
west  to  east ;  so  that  it  probably  covers  an  area  (now  largely  buried  under  Old  Red 
Sandstone)  not  less  than  1000  square  miles  in  extent.  Yet  it  appears  never  to  exceed, 
and  usually  to  fall  short  of,  a  thickness  of  1  foot.  Fish  remains,  however,  are  not 
confined  to  this  horizon,  but  have  been  detected  in  strata  above  the  original  bone-bed 
at  Ludlow. 

A  considerable  suite  of  organic  remains  has  been  obtained  from  the  Upper  Ludlow 
rock,  which,  on  the  whole,  are  the  same  as  those  in  the  zones  underneath.  Some 
minute  globular  bodies,  doubtfully  referred  to  the  sporangia  of  a  lyoopod  {Paehyiheea*), 
occur  with  some  other  plant  remains  {Pachyaporangiuntf  AetinQphyllum^  Chondrites — a 

1  'Siluria,'  p.  130.  *  Op,  cU,  p.  188. 

*  See  Q,  J,  Oeol.  Sac,  xxxviii.  (1882)  p.  107.  Mr.  Carmthers  suggests  that  they  are 
possibly  the  remains  of  an  animal  rather  than  a  plant. 


760  STRATIGRAPHICAL  GEOLOGY  book  vi  part  n 


beautiful  seaweed).  Corals,  as  might  be  supposed  from  the  muddy  character  of  the 
deposit,  seldom  occur,  though  Murchison  mentions  that  the  encrusting  form  ^fMmUi 
fibrosus  may  not  infrequently  be  found  enveloping  shells,  OycUmema  eoraUH  and 
Mnrchisonia  corcUlii  being,  as  their  names  imply,  its  favourite  habitats.  All  the  corals 
of  the  Ludlow  group  are  also  Wenlock  species.  Some  annelides  {SerpuliUs  iangianmut. 
Corn  uliUs  seiy^darius^  and  Trctchyderma  eoriaecum)  are  not  uncommon.  The  cmatacea 
are  represented  in  the  Upi)€r  Ludlow  rock  by  ostracods  {Beyrichia  Kloedeniy  LeperdiHa 
marginatay  EtUomis  tuberoaa),  phyllopods  {CeratiocariSf  Dictyoeari8\  and  more  especially 
by  eurypterids  {EurypUruSy  ffemiaspiSy  PtcrygotuSy  Slimonia,  StyUmums^  Him&mUh 
pteriis).  The  trilobites  have  still  further  waned  in  the  Upper  Ludlow  rock,  though 
Homalonotus  Knightiiy  Encrinurus  punctcUnSy  Phacops  Doumingim,  and  a  few  others 
still  occur,  and  even  the  persistent  CaJymene  Blumenbaehii  may  occasionally  be  foand. 
Of  the  brachiopods,  the  most  abundant  forms  in  this  group  are  Lingula  minima,  L.  kUa, 
DUcina  nigatay  Rhynchonella  Wilsonij  Stropkamena  filosay  and  Chcnetts  tiriaUUa,  The 
most  characteristic  lamellibranchs  are  Ortiumoia  amygdalina,  Goniophora  eymbm/armiSy 
PUrinea  lineatay  P,  retroJUxa ;  some  of  the  commonest  gasteropods  are  Murekimmia 
coralliiy  Platysdiisma  helieUeSy  and  Holopella  obsolrta.  The  orthoceratites  are  specifically 
identical  with  those  of  the  Lower  Ludlow  rock,  and  are  sometimes  of  laige  size, 
Orthoeerns  bullatum  being  specially  abundant.  The  fish -remains  consist  of  bones, 
teeth,  shagreen-like  scales,  plates,  and  fin-spines.  They  include  some  plagiostomoiis 
(placoid)  forms  {Thelod\is\  shagreen-scales  {Sphagodus)y  and  some  ostracosteans  {Ctpkal^ 
aspis  {C.  ornaluSy  C  3furchUoni),  Auchenaspia  {A,  SdU€ri)y  Ptertupit  {P.  Bankfii), 
Scaphaspis  {S.  ludensis)y  and  Eukeraspia  {Pleetrodus)  {E,  mirahilis).  Some  of  the 
spines  described  under  the  name  of  Onchus  are  probably  crustacean. 

(d)  Tilestoiiesy  DounUon  Castle  Stone  and 'Ledbury  Shales, — Above  the  Upper  Ludlow 
shales  and  mudstones  lies  a  group  of  fine  yellow,  red,  and  grey  micaceous  sandstones  from 
80  to  100  feet  thick  which  have  long  been  quarried  at  Down  ton  Castle,  Herefordshire. 
At  Ledbury  these  sandstones  are  surmounted  by  a  group  of  red,  purple,  and  grey  marls, 
shales,  and  thin  sandstones,  having  a  united  thickness  of  nearly  300  feet.  Originally 
the  whole  of  these  flaggy  ui)per  jwrts  of  the  Ludlow  group  were  called  **  Tilestones  **  by 
Murcliison,  and,  being  often  red  in  colour,  were  included  by  him  as  the  base  of  the  Old 
Red  Sandstone,  into  which  they  gradually  and  conformably  ascend.  They  point  to  a 
gradual  change  of  physical  conditions,  which  took  place  at  the  close  of  the  Silurian 
period  in  the  West  of  England  and  brought  in  the  peculiar  deposits  of  the  Old  Red 
Sandstone.  There  is  every  reason  to  believe  that  for  a  long  time  the  marine  sedimenta- 
tion of  Upi)cr  Silurian  type  continued  to  prevail  in  some  areas,  while  the  probably  lacus- 
trine type  of  the  Old  Red  Sandstone  had  already  been  established  in  others,  and  that 
by  the  breaking  dowii  or  submergence  of  the  barriers  between  these  different  areas,  marine 
and  lacustrine  conditions  alternated  in  the  same  region.  The  Tilestones  are  the  records 
of  this  curious  transitional  time.* 

V^egetable  remains,  some  of  which  seem  to  be  fucoids,  but  most  of  which  are  prob- 
ably terrestrial  and  lycopodiaceous,  abound  in  the  Downton  sandstone  and  passage-beds 
into  the  Old  Red  Sandstone.  The  eurypterid  genera  still  continue  to  occur,  together 
with  phyllojKxls  {Ceratiocaris)  and  vast  numbers  of  the  ostracod  Beyrichia{B,  Kloedeni). 
Prevalent  shells  are  Lingula  cornea  and  Platyschitfina  hclkites.  The  Ludlow  fishes  are 
also  met  with. 

In  the  ty]>ical  Silurian  region  of  Shropshire  and  the  adjacent  counties,  nothing  can 
be  more  decided  than  the  lithological  evidence  for  the  gradual  disappearance  of  the 
Silurian  sea,  with  its  crowds  of  graptolites,  trilobites,  and  brachiopods,  and  for  the  gradual 
introduction  of  those  geographical  conditions  which  brought  about  the  deposit  of  the 

*  On  these  passage-beds  see  Symonds,  *  Records  of  the  Rocks,'  1872,  pp.  183-215 ; 
Q.  J.  (Jml.  Sue.,  xvi.  (1860)  p.  193  ;  Roberts  and  Randall,  op,  cit,  xix.  (1863)  p.  229;  alio 
the  remarks  made  ou  the  corresiwnding  strata  in  Scotland,  posteay  pp.  764,  799. 


erlitlni  dliljmii.  Ddm. :  b.  atmphomeiw  uiUqiuU.  Hby. ;  <;,  Llngnla  Lewlill,  Bbr, ;  d,  LepbBM 
tnnavcimllB,  Dilm.  :  i.  RbytichnnFlLi  borralii,  8chtoth. ;  /  RhynchoDilUi  Wltwnl,  EibT .:  a,  CWiUoU 
lnUmIptl^  Brad.:  k.  Ambonychii  unUniaUU,  McCoy ;  ^  llaJlt>)o|»liNUHOtil,  HU. ; /onhoDota 

j_.._.    <.^..  .  ..   r..-i...^ ._, ,^  g^j,_ .  j_  Euociptiloi  rngonu,  SOf. ;  m,  Tiwpliai 

im,  Sby.  (0:  s,  OrUKKCiu  unolitam,  Bbj.  (U ;  p. 


jmTKiliillILt,  Bby.  .  .. 
Lltnlta  glguitaiu,  8 


Utulla* 


762 


STRATIGRAPHICAL  GEOLOGY 


BOOK  TI  PART  n 

Old  Hed  Saudatone.  The  fine  gre;and  oUv«-ooloured  mada,with  their  oeeuioiul  nma 
of  limeatoDe,  are  Buoceeded  by  bright  red  cIajb,  sandstoiiw,  cornetoncB,  mud  oonglonier- 
ates.  The  evidence  from  fossils  ie  equally  explicit.  Up  to  thetopof  the  Ludlov  rocks, 
the  abuadaiit  Siloridn  fnuna  coDtiDues  in  hardly  diminished  numbera.  But  as  soon  u 
the  red  strata  begin  the  organic  remains  rapidly  die  out,  tutil  at  last  only  the  fish  and 
the  large  eurypterid  crustaceans  continue  to  occur. 

Turning  now  from  the  interesting  and  extremely  important,  though  limited,  area  in 
which  the  original  type  of  the  Upper  Silurian  rocks  is  developed,  vre  obMrre  that, 
whether  traced  northwards  or  •outh-wett- 
wardg,  the  soft  mudstones  and  thick  lime- 
stones give  way  to  hard  date*,  grits,  and 
flagstones,  among  which  it  ia  aesreely 
possible  sometimes  to  discriminate  what 
representa  the  Wenlock  bvm  what  may 
be  the  oqniTalent  of  the  Ludlow  group. 
It  is  in  Denbighshire  and  the  adjacent 
counties  that  this  change  becomes  most 
marked.  The  Tarannon  shale  above  de- 
scribed passes  into  that  region  of  North 
Wales,  where  it  forms  the  baae  of  the 
Upper  Silurian  formations.  It  ia  oovered 
by  a  series  of  grits,  flags,  sandatonea,  mud- 
stones,  and  shales,  which  in  some  places 
are  at  least  3000  feet  thick.  These  ore 
overlain  by  and  pass  latenlly  into  bard 
shales,  and  are  believed  to  Tepresent  the 
true  Wenlock  group,  perhaps  even  tome 
portion  of  the  Ludlow  rocka.  The  iodb 
of  Cyrtograpliia  JtfureAinmi  which  marks 
the  lower  part  of  the  Wenlock  group  is 
found  in  Denbighshire,  and  ^vea  a.  recog- 
nisable horizon.  It  is  evident,  however, 
that  ill  apiCc  of  tlie  wide  extent  over  which 
these  Upper  Silurian  rocks  of  North  Wales 
are  spraad,  and  the  great  thickness  which 
n™X  Uj.pfr  tbcyattain,  they  do  not  present  an  adequate 
'  1^  stratigrsphical  equivalent  for  the  complete 

Buccesaion  in  the  original  Siloriau  district- 
Inatead  of  passing  up  conformably  into  the  base  of  the  Old  Red  Sandstone,  as  at  Ludlow, 
they  are  covered  by  that  fonuation  tin  conformably.  In  fact  they  have  been  upturned, 
crumpled,  faulted,  and  cleaved  before  tlie  deposition  of  those  portions  of  the  Old  Red 
Sandstone  (Upper)  which  lie  upon  them.  Tliese  great  physical  changes  took  place  in 
Denbighsliire  when,  ao  far  as  tlie  evidence  goea,  there  was  entire  quiescence  in  the 
Shropshire  diatrict  ;  yet  the  distance  between  the  two  areas  waa  not  more  than  about 
60  miles.  These  subterranean  movementa  were  doubtless  connected  with  those  more 
widely  extended  upheavals  that  converted  the  floor  of  the  Silurian  sea  into  •  aeries  of 
isolated  basins,  in  which  the  Old  Red  Sandstone  was  laid 'down  (pp.  777,  799). 

In  Westmoreland  and  Cumberland  a  vaat  masa  of  hard  slates,  grits,  and 
flags,  was  identified  by  Sedgwick  aa  of  Upper  Silurinn  age.  These  form  the  varied 
ranges  of  hills  in  the  soiithem  [lart  of  the  Lake  District,  from  near  Shap  to  Duddon 
mouth.  The  following  are  the  local  aubdi visions,  with  the  conjectural  equivalents  in 
Siiuria :  '— 

'  For  papers  on  the  Upv>er  Silurian  rocks  of  the  Lake  District  see  Harknesa  and  Nichol- 


II »  LviimahB^w.  Laiibi 


SECT,  ii  §  2 


SILURIAN  SYSTEM 


763 


iTirVhv  ifnnr  maM  f^^^  bedt  of  hud  nndttone,  maasive  and  concretionary  or  flaggy' 
Hav  Fell  FTaffT  -!     *°*^  mlcaceoua  (Phacopt  DowningUt,  P.  caudatus,  Ceratiooarit 
(2000  teetU         I     ^^**>^^^^^t  Linffula  «>m«a,  Orthi$  Ivnata^  OrtKonota  aniygdalina. 

Calcareous    beds    (Bkfiuikonetta    navicula  abundant)   probably 
equivalent  to  the  Aymestry  Limestone. 
Bannisdale  Flags  J  Sandstone  and  Shale,  vrlth  star-flshes  {PrcfoMUr). 


=  Upper 
Ludlow  Group 


(5200  feet). 


CSoniston  Grits 

(upwards  of  4000 

feet). 


O>ni8ton  Flags 
(2800  feet). 


Stockdale  Shales 
(200-450  feet). 


Dark  blue  flags  and  grits  of  great  thickness. 

{MonoQTaptu*  UintwardinentU   ranges  through   the   Bannisdale ^ 
Flags  and  M.  ootoniM  and  M,  Salweyi  also  occur.) 

Flass  and  greywacke  generally  unfossiliferous,  but  containing 
Monogroj^u*  oo/oniM,  if.  hohemicut^  M.  Roemeri^  Cardiola  inter' 
rvpta,  Orikooerat  angiUaiun^  0.  prinutvumt  Ceratioeari*  Mur- 
ckuoni. 

Dark  grey  coarse  flags  divided  hy  Sedgwick  into  stages  which  axe 
charact«rised  by  Mr.  Marr  as  follows : 

Upper  Ck)ldwell  Beds  (lower  part  of  zone  of  Monoffraphu  hohemi- 
cum)  with  M.  colontw,  M.  Boemsri,  SpirorbU  Lewisii,  Ceraiioeari$ 
Murchiaoni,  Kncrinvrus  minctoftt*,  Pkiuops  Stokuil,  Cardiola 
inUrrupiat  Pterinea  tub/bueatat  Orthooeraa  prinutvum,  0.  dimidi- 
a/iim,  0.  ndmndukUumy  0.  litdenae. 

Middle  Coldwell  Beds  (zone  of  Phacops  obtuHeaudattu)  with  Car- 
diola interrupta,  Orthoeeraa  tubantiMlare,  0.  anguUUmi^  0.  line- 
eUvm,  0.  inbrUxUuvi, 

Lower  C!oldwell  Beds  (zone  ot  Monograplu$  NiUnoni^ 

Brathay  Flags  (zone  of  Cfriograptu$  Murchi»oni)^  fossils  chteflv< 
graptolites  including  Monograptut  priodouy  M.  Domeriniw,  it. 
culUlluSf  RetiolUts  ffetnitzianna^  Aptfchopsi$,  Cardiola  interrupta, 
Orthocerat  primmvum.    Thickness  more  than  1000  feet. 

Upper  pale  grten  and  purple  shales  with  badly  pc«senred\ 

fossils,  67  feet. 
Lower  pale  shales  (65  feet)  with  zones  of  Monograptue  eri$pu$ 

and  Ii.  turriculattu. 
pper  blue  mudstones  with  two  bands  of  black  and  blue 


»Midd]« 
f  Ludlow  Group 


= Lower 
^Ludlow  Group 


Wenlock 
Group. 


''%. 


4 

] 


t 


a 

<  = 

u 
o 
« 


s 


4/ 

M 
CD 


s  Llandovery 
Group. 


graptolitic  shale,  the  upper  of  which  contains  Monograptm 
Mpinigenu,  the  lower  if.  Clingani. 

Middle  blue  mudstones  with  tliree  bands  of  dark  graptolitic 
shale,  the  highest  being  the  zone  of  Monograpttu  convolutut. 
(with  M.  gregariue,  if.  Clingani^  Rattrites  peregrintu  ana 
many  other  graptolites^  the  middle  being  the  zone  of  Mono-  V  •■ 
grapttu  argenteus  (with  M.  gregarivs^  M.  Ifptothrca,  and  ten  / 
other  species ;  lUutrites  peregrintis,  and  three  other  species  ; 
Diplograpttis  tamariecuSf  D.  Hughe$iij  Climaoograpttu  nor- 
mcUig,  and  other  fossils);  and  the  lower  band  being  the  zone 
of  MonograptueJlmJbritUtu,  M.  gregarius,  if.  tenuU^  and  other 
species ;  Bastritee peregrinust  Dijdograptm  tamarisnuj  Petalo- 
graptve  ovaius,  Cllnutcograptus  norrnaiis. 

Lower  calcareous  shales = zone  of  Dimorphograptus  cor\fertvt, 
with  Monograpttu  revolittuti,  M.  tenuis,  Dipiograpttu  veti- 
euhsvs,  &c.,    resting    on   a    thin    limestone  with  Atrifpa 
flexuoea. 

In  some  places  beneath  these  shales  a  conglomeratic  band  occurs  that  forms 
their  base  and  lies  nuconformably  on  Lower  Silurian  strata. 

In  the  northern  part  of  the  Lake  District  a  great  anticlinal  fold  takes  place.  The 
Skiddaw  slates  arch  over  and  are  succeeded  by  the  base  of  the  volcanic  series  above 
described.  But  before  more  than  a  small  portion  of  that  series  has  appeared,  the  whole 
Silurian  ar^  is  overlapped  unconformably  by  the  Carboniferous  Limestone.  It  is 
necessary  to  cross  the  broad  plains  of  Cumberland  and  the  south  of  Dumfriesshire  before 
Silurian  rocks  are  again  met  with.  In  this  intervening  tract,  a  synclinal  fold  must  lie, 
for  in  the  south  of  Scotland  a  broad  tract  of  Upper  Silurian  strata  is  now  known  to 
form  the  greater  part  of  the  pastoral  uplands  which  stretch  from  the  Irish  Sea  to  the 
North  Sea.  Its  northern  limit  where  it  rests  conformably  upon  and  passes  down  into  the 
Caradoc  group,  extends  from  a  little  south  of  Port  Patrick  north-eastwards  to  near  Dunbar. 
The  strata  throughout  this  region  have  been  thrown  into  innumerable  folds  which  are 

son,  Quart.  Joum.  OeU.  Soc.  xxiv.  (1868)  p.  296 ;  xxxiii.  (1877)  p.  461.  H.  A.  Nichol- 
son,  op.  cit.  p.  521  ;  xxviil.  (1872)  p.  217,  *An  Essay  on  the  Geology  of  Cumberland  and 
Westmoreland,'  1868.  Nicholson  and  Lapworth,  Brit.  Assoc  1875,  sects,  p.  78.  Oeol.  Sur- 
vey Memoirs,  Explanations  of  Sheet  98,  S.K  and  N.E.  1872  (Aveline  and  Hughes).  Marr, 
Quart.  Joum.  Ged.  Soc.  xxxiv.  (1878)  p.  871  ;  Oeol.  Mag.  1892,  p.  534  ;  Marr  and  Nichol- 
son, Quart.  Joum.  Oeol.  Soc.  xliv.  (1888)  p.  654. 


764 


HTRATIGRAPHICAL  GEOLOGY 


BOOK  TI  PAST  n 


often  reversed.  The  result  of  this  distnrbaoce  has  been  to  oompress  the  roeks  into 
highly  ineliucd  positions,  and  to  keep  the  same  group  at  the  surface  over  a  great  breadth 
of  ground,  so  that  in  spite  of  their  steep  angles  of  dip  the  strata  are  made  to 
occupy  as  much  space  on  the  map  as  if  they  were  almost  flat.  Here  and  there  where 
the  anticlines  are  more  pronounced  and  denudation  has  proceeded  &r  enough,  long  boat- 
sha])ed  in  Hers  of  Lower  Silurian  rocks  have  been  laid  bare  underneath  the  upper  series 
of  formations.  In  this  way  the  Llandeilo  volcanic  group  can  be  traced  by  oocasioDal 
exposures  for  some  90  miles  to  the  north-eastward  from  the  Ayrshire  coast  where  it  is 
most  largely  developed.  By  far  the  larger  part  of  the  Uplands  is  formed  of  rocks  which, 
from  the  researches  of  Professor  Lapworth  among  their  graptolitic  contents,  are  now 
known  to  l>e  the  general  equivalents  of  the  Llandovery  group.  Wenlock  and  Ludlow 
rocks  occur  on  both  sides  of  the  Uplands.  Towards  the  north-east  the  general  litho- 
logical  characters  of  the  Upper  Silurian  are  comparatively  uniform — thick  masses  of  grey- 
wacke  and  shale,  with  pebbly  layers  and  well  -  marked  bands  of  graptolitic  black 
shale.  This  uniformity  is  accompanied  by  a  corresponding  monotony  in  the  organic 
remains,  which  consist  almost  wholly  of  graptolites,  confined  for  the  most  part  to  the 
zones  of  black  shale,  in  which  they  are  thickly  crowded.  But  towards  the  south-west  in 
Carrick  (Ayrshire)  there  is  a  much  greater  diversity  of  sedimentation,  thick  masses  of  con- 
glomerate, limestone  and  calcareous  shale  being  conspicuous.  In  that  district  accord- 
ingly there  is  so  marked  a  contrast  in  the  abundance  and  variety  of  the  organic  remains, 
that  the  strata  may  be  comi)ared  with  the  more  fossiliferous  deposits  of  the  original  and 
typical  Silurian  region.  The  following  table  shows  the  succession  of  strata  which  follow 
continuously  those  given  in  the  table  on  p.  751.^ 


2 

is 

o 


2 
o 

o 

s 


PentlAnds  and  northern  part  of 
region. 


Yellow  and  brown  niudgtonex, 
Hhalefl  and  sandMtonefl  paas- 
inj;  up  into  base  of  Ix)wer 
Old  Red  HandHtonefl,  with 
many  Liidlow  foHHils  (/>/>- 
twna  tnumvirrmliJi,  (Jrthoyiota 
nmygditlina,  I'UityftchUmw 
helicitfM,  Orthorenis  Maclar- 
cni,  Beyrkhin  KloetJftii,  Cer- 
atuicnrix,  DiHyncarin,  Eury- 
j)teriui,  PieryqntuM,  Sliynonia, 
Stylonnruji,  &c.) 


Blue,  K^py,  and  bromi  RhiileH, 
jjroywackeH  and  flajjgy  ^rits 
with  »ome  Wenlock  YosailH 
{Motunjraptui  wnw^riwiw,  M. 
cnlonns,  M.  prindon,  Ht- 
tioliUs  geitiitzvtMts). 


Thick  Rroiip  of  Kritj*  and  vx^y- 
wack»»s,  with  ^rey  KhaleH 
and  flaK«t/itiP8  (Queensberry 
KritH,  Gala  Kronp)  the  upper 
portion  containing  Retiftliirs 
(jrin  itzianwt,  Monogntptus 
priiHhm,  the  h)wer  jK)rtion 
yielding  M.  frigun^,  M.  crLt- 
pvA,  rrotnvirgnlarui,  Croitsn- 


Central  part  of  regiuo. 


Af^hli*  and  wmtlMni  part  of 
ragim. 


Flaggy  shales,  grey  ffrit«,  and 
conglomerate  (StraitonX 
with  Beyrichia  Kloedeni^ 
Pterygottu^  CeratiooariSj  &c 


Thick  group  of  greywackes, 
grits,  and  grey  shales 
(Hawick  group  of  Roxburgh- 
shire, &c.,  Ardwell  group  of 
DumfriesHhire  and  Gallo- 
way), with  Protonrit^tlarla, 
CrDssopotlia,  and  Monograp- 


Blue,  grey,  and  yellow  flag- 
stones and  shales,  wiUi 
MoHograptns  wfrnerinus, 
Cardiola,  &c. 

Purple  sandstones  and  cal- 
careous bands  (Penkill, 
Dailly),  with  Cfftiographu 
Gmyte,  JUtiolite*  ifieinitiU 
anus,  and  Wenlock  fossils. 


Purple  shales  and  mndstones, 
grey  and  green  fla^;stone8, 
and  grits  witii  Monogmpiut 
exigutu,  M.  gidaen9i$.  Pro- 
tovirgnlariu,  OoMOjwrfia, 
Cnuriana,  Ac  (=Stockdale 
shales  of  Lake  District). 

Limestone  (Camregan)  with 
Pentamerus  dblongwL 


^  See  Lai)Worth,  Quart.  Joum.  (ieol.  Soc.  xxxiv.   (1878),  xxxviii.  (1882)  ;  <?eoi.  if«^. 
1889,  pp.  20,  59  ;  Ann.  Mag.  Nat.  Uist.  1879,  1880. 


8BCT.  ii  §  2 


SILURIAN  SYSTEM 


765 


PwitUndi  and  northani  pArt  of 
nffioo. 


a 

s 

•«» 

z 

8 


8 
o 

o 
•a 


Greywackes,  flagstoueg,  and 
Mlialeii,  with  occajdonal 
bands  of  conglomerate,  some 
of  which  contain  fhtgmenta 
of  rocks  like  those  of  the 
Highlands.  Thin  leaves  of 
black  shale  in  this  group 
(Queensberry  in  part,  Dal- 
veen  and  Haggis  •  rock 
groups  of  Geological  Sur- 
vey) contain  Birkhill 
graptolites. 


Owtnlput  of  ragkm. 


Grey  waek%  and  shales  includ- 
ing the  black  gniptolitic 
Birkhill  shales  which  form 
two  bands  separated  by  al- 
ternations of  grey  and  green 
shales,  and  are  sub-divided 
as  follows : — 


I 
I 


f'8.  Zone  of  Bastrite*  maH- 
miw,  Monograptiu  tuv' 
ricukUu*,  &c. 

2.  Zone   of   Monographu 

Jduigerutf  M.  dUtatu, 
c. 
1.  Zone   of  Monoarapiiu 
Clinganif  with  M.cnnu- 
faH#,  M.  Sedgwicki,  Pe- 
.    taloffmptv*  cometa. 


I 


[Z,  Zone   of  iiotutarapXyu 
vrtgariHSy  with  si.  Jim- 

Diplograptiu/(Uium,  Ra- 
$trUe*  pertgrinuit,  &c. 

2.  Zone  of  Diplograptus 
vetieulotus^  with  Afono- 
graptia  cyphus,  M. 
tenuU. 

1.  Zone  of  Diplograj^us 
aeuminatus  with  IHmor- 
nkofpxiptus  eiongatu*  f 
Monograplus  atten  ualM«, 

,    M.  tenui*. 


Aynhln  mmI  Mmtbera  put  of 
rtsloii. 


Greywackes,      shales,      and 

Iuartz-conglomerates,  with 
fonograptits  turrievlatus 
and  other  Upper  Birkhill 
graptolites. 

Limestone  (Woodland  Point), 
with  PeiOamerus  leiu,  &c. 


Sandstones,  shalm,  and  con- 
glomerates, with  the  grap- 
tolites of  the  Lower  BirkhUl 
zones. 

Sandstones,  grits,  shales,  and 
conglomerates,  with  Meris- 
teila  anffustifronM^  Diplo- 
graptus (tcuminatus  and 
other  I^wer  Birkhill  grap- 
tolites. The  conglomerates 
contain  the  earliest  traces 
of  fhigments  of  rocks  like 
those  of  the  Highlands  in 
this  region. 


Silurian  rocks  cover  lai^ge  continaous  tracts  in  the  north-east  and  sonth-east  of 
Ireland,  while  at  many  places  in  the  interior  of  the  island,  even  to  the  western  coast, 
they  rise  up  in  isolated  areas  from  under  younger  formations.  It  is  evident  that,  except 
where  Cambrian  and  pre-Cambrian  rocks  appear,  they  spread  across  the  whole  country, 
though  now  so  largely  concealed  by  the  Carboniferous  formations.  The  Scottish  type  of 
sediments  and  of  fossils  is  prolonged  into  Down  and  the  other  counties  in  the  north-east 
and  east.  As  already  stated,  the  Glenkiln  shales  with  their  characteristic  graptolites, 
traced  to  the  south-western  coast-line  of  Scotland,  reappear  in  full  force  on  the  Irish 
shore,  and  strike  inland  along  the  same  persistent  south-easterly  line.  They  are  found 
as  far  south  as  the  southern  coast  of  County  Waterford  and  as  far  west  as  the 
flanks  of  the  Slieve  Bemagh  Mountains  in  County  Clare.  In  like  manner  the  Hartfell 
or  Caradoc-Bala  shales  with  their  distinctive  graptolites  are  found  in  County  Down,  and 
probably  occur  in  \nany  other  districts,  while  the  Llandovery  group  of  Birkhill  has  been 
recognised  not  only  in  Down,  but  in  Tyrone,  Fermanagh,  and  other  counties.  Abundant 
evidence  of  contemporaneous  volcanic  action  has  been  obtained  from  the  Silurian  rocks 
of  the  east  of  Ireland.^  Upper  Silurian  rocks  representing  the  Llandovery  and  Wenlock 
formations  attain  an  enormous  development  in  the  west  of  Ireland.  In  the  picturesque 
tract  between  Lough  Mask  and  KiUary  Harbour,  where  they  reach  a  thickness  of  more 
than  7000  feet,  they  consist  of  massive  conglomerates,  sandstones,  and  shales,  with 
Llandovery  and  Wenlock  fossils  and  intercalated  felsites,  diabases  and  tuffs.  Again, 
in  the  Dingle  promontory  of  County  Kerry,  Upper  Silurian  strata  full  of  Wenlock  fossils 
contain  the  most  impressive  proofs  of  contemporaneous  volcanic  action  ;  agglomerates, 
tuffs,  and  volcanic  blocks  being  intermingled  with  the  foesiliferous  strata,  which  are 
further  se|)aratod  by  thick  sheets  of  nodular  felsitic  lavas.' 

^  QuarL  Jourru  Geol,  Soc.  xlvii.  (1891)  Presidential  Address,  p.  150,  and  authorities 
there  cited. 

'  Op,  cif.  p.  159,  and  authorities  cited.  Consult  on  Irish  Silurian  rocks  the  £!xplana- 
tions  to  the  one-inch  Sheets  of  the  Geological  Surrey. 


766  STRATIGRAPHICAL  GEOLOGY  book  vi  pam  n 

Baain  of  the  Baltic,  Busiia  and  Boandiiiavia.^— The  broad  hollow  which,  mnniiig 
from  the  mouth  of  the  English  Channel  across  the  plains  of  northern  Germany  into  the 
heart  of  Russia,  divides  the  high  grounds  o^the  north  and  north-west  of  Enrope  from 
those  of  the  centre  and  south,  separates  the  European  Silurian  region  into  two  distinct 
areas.  In  the  northern  of  these  we  find  the  Lower  and  Upper  Silurian  formations 
attaining  an  enormous  development  in  Britain,  but  rapidly  diminishing  in  thickness 
towards  the  north-east,  until  in  the  south  of  Scandinavia  and  the  Gulf  of  Finland,  they 
reach  only  about  ^th  of  that  depth.  Along  the  Baltic  shores,  too,  they  have  on 
the  whole  escaped  so  well  from  the  dislocations,  crumplings,  and  metamorphisms  so  con- 
spicuous along  the  north-western  European  border,  that  to  this  day  they  remain  over 
wide  spaces  nearly  as  horizontal  and  soft  as  at  first.  In  the  southern  European  area, 
Silurian  rocks  appear  only  here  and  there  from  amidst  later  formations,  and  almost  eveiy.- 
where  present  proofs  of  intense  subterranean  movement.  Though  sometimes  attaining 
considerable  thickness  tliey  are  much  less  fossiliferous  than  those  of  the  northern  part 
of  the  region,  except  in  the  basin  of  Bohemia,  where  an  exceedingly  abundant  series  of 
Silurian  organic  remains  has  been  preserved. 

In  Russia,  Silurian  rocks  must  occupy  the  whole  vast  breadth  of  territory  between 
the  Baltic  and  the  flanks  of  the  Ural  Mountains,  beyond  which  they  spread  eastward 
into  Asia.  Throughout  most  of  this  extensive  area  they  lie  in  horizontal  undisturbed 
beds,  covered  over  and  concealed  from  view  by  later  formations.  Along  the  southern 
margin  of  the  Gulf  of  Finland,  they  appear  at  the  surface  as  soft  clays,  sands,  and 
unaltered  strata,  which,  so  far  as  their  lithological  characters  go,  might  be  supposed  to 
be  of  late  Tertiary  date,  so  little  have  they  been  changed  during  the  enormous  lapse  of 
ages  since  Lower  Palaeozoic  time.  The  great  plains  bounded  by  the  Ural  chain  on  the 
east,  by  the  u[>lands  of  Finland  and  Scandinavia  on  the  north,  and  by  the  rising  groimds 
of  Germany  on  the  south-west,  have  thus  from  a  remote  geological  antiquity  been 
exempted  from  the  ten-estrial  corrugations  that  have  affected  so  much  of  the  rest  of 
Europe.  They  have  been  alternately,  but  gently,  depressed  as  a  sea-floor,  and  elevated 
into  steppes  or  plains.  But  along  the  flanks  of  the  Ural  Mountains,  the  older  Palaeozoic 
rocks  have  been  upheaved  and  placed  on  cud  or  at  a  high  angle  against  the  central 
portions  of  that  chain  ;  and,  according  to  the  observations  of  Murchison,  Keyserling  and 
De  Verneuil,  have  been  partially  metamorphosed  into  chlorite-schists,  mica-schists, 
quartzites  and  other  crystalline  rocks.  To  the  north-west  also,  over  a  vast  region  in 
Scandinavia,  they  have  been  subjected  to  gigantic  displacements  and  great  regional 
metamorphism  (p.  621). 

Taking  first  their  unaltered  condition,  we  find  them  well  exposed  along  the  southern 
shores  of  the  Gulf  of  Finland,  in  the  Baltic  provinces  of  Russia,  where,  according  to  F. 
Schmidt,  they  form  with  the  Cambrian  groups  below  them  one  cohtinuoiis  and  con- 
formable series,  and  are  callable  of  arrangement  as  in  the  subjoined  table  :  * — 


^  Consult  Angelin's  *  Palteontologica  Suecica'  (1854);  Kjerulf,  *  Norges  Geologi, '  1879 
(or  'Geologie  des  Siidl.  Norvegen '  (Gurlt),  1880)  ;  Linnarsson,  Svejisk,  Vet,  AkcuL 
viii.  No.  2 ;  Zeitsch.  Deutsch.  Geol.  Oesell.  xxv.  675  ;  Geol.  Mag.  1876,  pp.  145,  240, 
287,  379  ;  (ieol,  Fiireningeiis  Stockholm  Fiirhandl.  1872-74,  1877,  1879  ;  a  Tomquist, 
Kong.  Vet.  Akml.  Forhandl.  1874,  No.  4  ;  Ged.  FOrcn.  Stockholm  Fitrhandl,  1879 ; 
Luudgren,  yeues  Jahrb.  1878,  p.  699  ;  Brogger,  *  Die  Silurischen  Etagen  2  und  3  im 
Kristiania  Gebict,'  1882;  F.  Schmidt,  Q.  J.  Oeol.  Soc.  1882,  p.  514;  J.  E.  Marr,  Quart. 
Jouni.  Gcol.  Soc.  1882,  p.  313;  A.  G.  Nathorst,  *  Sveriges  Geologi,*  part  i.  1892,  and 
papers  cited  below. 

2  Mem.  Ac.  Imp.  St.  PUersh.  (7)  xxx.  (1881)  No.  1  ;  Q.  J.  GtoU  S.  xxxviii.  1882, 
p.  514.  ' 


ascT.  ii  §  2 


SILURIAN  SYSTEM 


767 


a 
a 

9 


>» 


»» 


I 


>) 


M 


s 
C 


f  stage  K.  Upper  Oesel  Zone  (50  or  60  ft  =  Ludlow  Group) — grey  limestones 
and  mails,  yellow  limestones :  Spirifer  elevcUus,  Chonetes  striaUUoy 
Beyriekia  tuberctdata,  PUrinea  retrq/Uxa  ;  an  abundant  eurypterid 
fauna  and  fish  remains  (Onchus^  Ptuhyl^pis), 
I.  Lower  Oesel  Zone  (60  ft.  =  Wenlock) — chiefly  dolomites  with  marls  : 
Orthoceras  annuUUuin^  Btunnphalus  funattu,   Spirtfer  erispuSf 
Orthis  degantula,  Leptmna  trantwrsalU, 
H.  Pentamems-eethonus  Zone — in  the  east,  dolomites  ;  in  the  west,  grey 
coral  limestone,  with  Pentamenia  esthonua  {oblongus)^  Syringopwra 
bifurc€Ua,  Favosites  goUandica,  Holy  sites  (5  sp. ) 
'8.  Raikilll  Beds  (100  ft.)— coral-reefs  and  flagstones:   Leperditia 

Keyaerlingiiy  Phacops  elegans, 
2.  Borealis  Bank  (40  fL)---consisting  almost  entirely  of  agglomerated 
G.  -(  shells  of  Pentamerus  borealis. 

1.  Jorden  Beds  (20-30  It.) — thin  calcareous  flagstones  and  marls  : 
Leperditia  Hisingeri,  Orthis  Davidsoni,  Strophamena  pedeUy 
Rhynchonella  affims, 

F.  (1)  Lyckholm  and  (2)  Borkholm  Zones  (100  fL  =  Middle  Bala  or 
Caradoc),  contain  the  most  abundant  fauna  of  all  the  stages: 
Phacops  {Chasmops)  nuurourct,  Cheirurus  oetddbatus^  Encrinwrus 
muUisegmentaiuSf  BeUerophon  hUdbatus,  Strophomena  expansa, 
Orthis  vespertilio,  0.  ActcnisBf  0.  insularis, 

E.  Wesenberg  Zone  (SO  ft.  =  Bala  or  Caradoc) — hard  yellowish  lime- 
stone, with  marly  partings :  Leptsena  sericea^  Strophomena  deltoidea^ 
Orthis  testudiTiariOj  Phacops  Hiesxkotoskii,  P.  wesenbergensis, 
Encrinurus  Seebachif  CybeU  bremcavda, 

D.  Jewe  Zone  (100  ft. ),  consisting  of  a  lower  or  Jewe  band  and  an  upper 

or  Kegel  band  :   Cheirurtu  pseudohemicraniumf  Hemicosmites  ex- 

traneuSf   Lichas    dejUxa^    L,    illtenoideSf    Chasmops    btuxulentOf 

Strophomena  Asmusii. 

'  3.  Itfer  Beds  (20-30  ft. ) — hard  limestone  with  siliceous  concretions  ; 

fauna  nearly  same  as  in  C.  2,  but  with  some  peculiar  trilobites, 

and  some  forms  belonging  to  Stage  D. 

2.  Kuckers  Shale  (Brandschiefer),  consisting  of  bituminous  marls  and 
limestones  (30-60  ft.) :  Phacops  exilisj  P.  {Chasmops)  Odini, 

C.  -{  Cheirurus  spinulosuSf  Pleurotomaria  elliptical  Porambonites 

teretiory  Orthis  lynx,,  Echinospharites  aurantium. 

1.  Echinosphserite  Limestone,  kc.  (20-50  ft.  =  uppermost  Orthocera- 
tite  Limestone  of  Sweden) — Echinosphserites  auraniium^  and 
Orthoceras  regulare  are  the  most  characteristic  fossils,  with 
numerous  trilobites. 

'3.  Orthoceratite  (Vaginaten-)  Limestone  (3-20  ft.  =  Orthoceras 
limestone  of  Scandinavia) — hard  grey  limestone  crowded  with 
Orthoceras  commune  and  0.  vaginatum  ;  also  Phacops  scierops, 
Cheirurus  omatuSf  Asaphus  heroSf  Ampyx  nasutus^  &c. 

2.  Glauconite    Limestone   (12-40    ft.) — Megalaspis  planilimbata, 
B.  -l  Cheirurus  davtfrons,  Asaphus  expansus^  Porambonites  reticu' 

lotus,  Orthis  parva, 
1.  Glauconite  Sand  (Greensand),  lying  directly  on  the  Cambrian 
Dictyonema  shale  (1-10  ft.  =  Ceratopyge  Stage  of  Scandinavia) 
— Obolus  siluricus,  Siphonotreta,  Lingula  ;    **  conodonts  "  of 
Pander. 


In  Scandinavia  the  following  general  order  of  succession  has  been  established  : — 


Limestones  and  marls  (50-60  ft.  in  Gothland)  with  Ludlow  fossils. 
Limestones  and  shales  (150  ft.  in  Gothland)  with  Wenlook  fossils  {Monograptus 

ludensiSf  M.  colonus,  Retiolites  geinitzianus). 
Marls  and  shales  (with  Llandovery  forms)  apparently  unconformable  on  all 

older  rocks. 


768 


STRATIGRAPHICAL  GEOLOGY 


BOOK  TI  PAST  n 


Brachiopod  shales  {Trinucleiu,  Staurocephabia). 

Trinucleus  shales  and  limestones. 

Middle  graptolite  shales  (Llandeilo  species  of  Didymograptua,  Diplograpius, 
ClimacograptuSf  and  other  genera)  which  pass  laterally  into  limestone,  and 
are  in  different  districts  represented  by  the  Chasmops  limestone. 

Lower  graptolite  shales  (Arenig  species  of  PhyllograpluSy  Dichoffraptus,  Didy- 
mograptuSf  and  other  genera)  passing  into  the  Orthoceras  limestone,  which 
is  recognisable  over  a  large  part  of  southern  Scandinavia. 

Ceratopyge  limestone  {DiceUocephalus,  Agnostus,  Nidbe^  Amphion^  Obolua) 
and  other  fossils  like  those  found  at  the  base  of  the  Arenig  and  in  the 
Tremadoc  group. 


n 


» 
> 

a 


a. 


0 

a 


U 

w^ 

m  s 


In  Scania,  the  Silurian  series  has  been  subdivided  into  graptolitic  zones  as  in  the 
subjoined  table  :  ^ — 

A.  Upper  Group — Cardiola  shales,  with  limestone  and  sandstone. 

B.  Middle  Group,  with  the  following  zones  in  descending  order :  (a)  CyrUh- 
graptus  Carruthersi;  {b)  C,  rigidus;  (c)  C.  MurchUmU  ;  (rf)  Monograptug 
riccartanensi8 ;  {e)  Cyrtog.  Lapworihii;  (/)  C7.  (?)  spiralis;  {g)  C.  Oraym. 

C.  Lower  Group,  composed  of  the  following  zones  in  descending  order  :  (a) 
Monograpius  cometa  ;  (6)  Grey  unfossiliferous  shales ;  (c)  C^halograptu* 
cometa  ;  {d)  Mon.  leptotheca ;  (e)  M.  gregarius ;  (/)  M.  eyphus, 

'  D.  Upper  Group,  composed  of  the  following  zones  in  descending  order  :  (a) 
Diplograpt^ts,  sp.  ;  (6)  Phacops  mucroncUa ;  (c)  Staurocephalus  davi- 
frons  ;  (d)  Unfossiliferous  nmrly  shales  ;  (c)  Niobe  lata ;  (/)  Unfossili- 
ferous shales  ;  {g)  Diplograptus  quadrimucronatus  ;  {h)  Trinudeus,  sp. ; 
(i)  Calymene  diiatata  ;  (k)  Unfossiliferous  shales. 

E.  Middle  Group — Graptolite  shales,  with  zones  of  (a)  Climacograptus  rugosus; 
{b)  C  styloidens ;  (c)  Black  unfossiliferous  shales  ;  (d)  Limestone  band, 
with  Ogygia^  sp. ;  {e)  Dicranograptus  Clingani ;  (/)  Climacograptus 
VassB ;  {g)  Unfossiliferous  shales  ;  {h)  Comograptus  gracilis ;  (»)  Thin 
apatitic  band  :  {k)  Diplograptus  putillus  ;  (/)  Qlossograptus  ;  (m)  Oymno- 
graptits  Linnarssoni ;  (n)  Qlossograptus;  (o)  Didymograptus  geminus 
{Afnrchi^oni). 

F.  Lower  Group,  composed  of  the  zones  of  (a)  PhyUograptus^  sp. ;  (6)  Ortho- 
ceras limestone  ;  (c)  Tetragraptus  shales  (lower  graptolite  shales)  ;  {d) 
Ceratopyge  limestone. 

The  island  of  Gothland  has  long  been  celebrated  for  its  development  of  Upper 
Silurian  rocks.  According  to  Lindstrom*  the  following  subdivisions  are  there  trace- 
able : — 

H.  Cephalopoda  and  Stromatopora-Limestone  (20-30  feet)  with  PhragmoceraSf 
AscocemSj  Qlossoccras. 

Megalomus-Limestone  (8-12  feet),  with  Mcgalomtis  OUlandicus^  Trimertlla. 

Criuoidal  and  Coral  conglomerate  (20  feet),  a  limestone  made  up  of  stems 
of  crinoids,  corals,  and  other  fossils.  Among  the  crinoids  are  species  of 
Crota^^crinus,  Enallocrinus,  Barrandeocrinus,  Cyathocrinus  ;  there 
occur  also  Sjnrifer  ^hmidti  ^  Pentavierus  conchidium.  This  band  lies 
somewhere  about  the  horizon  of  the  Aymestry  Limestone. 

Pterygotu«-clay  or  marl  (1-2  feet)  with  abundant  fragments  of  Pterygotus 
oniliensisj  also  Phnsganocaris^  i^rophomena,  Eatonia,  ConiUaria,  &c. 

D.  Limestone,  oolite  and  marly  bands  (50  feet)  with  numerous  lamellibranchs  ; 
species  of  Pterinen^  At^iculopertcn,  and  Grammysia^  also  Orthis  basalis, 
0.  hiforala^  and  Atrypa  Angdini^  Lkhas,  Cydx^nema  ddicatulum,  &;c. 

Younger  marly  shales  and  sandstone  (100  feet),  with  a  large  and  varied 
assemblage  of  fossils  like  those  of  the  Weulock  Shale  {Phacops  Dotcningisty 
P.  vulgaris^  Homaioiiotits  Knightly  Strophtnuena  euglypha,  Orthis  biloboy 
Stnphoinena  Walmstedtiy  Rhytiehoiieliu  WHsoiU,  Orthoceras  anntUatumf  0. 
gregariumy  Mono(jraptus  ludensis,  M.  colonuSj  Reliolit^s  geinilxianus,  kc. 


G. 
F. 


R 


J4 

'3 


c. 


^  S.  A.  Tullberg,  '  Skanes  Graptoliter, '  Sveng.  Oeol,  UndersHkn.  ser.  c.  No.  50, 
1882-83. 

'^  Neu£s  Jahrb,  1888,  i.  p.  147,  and  F.  Schmidt,  op,  cit,  1890,  ii.  p.  249.  Morchison, 
Quart.  Jouni.  Geol.  *S(>c.  1847. 


> 


hi 


)» 


»» 


SECT,  ii  §  2  SILURIAN  SYSTEM  769 

'  B.  Stricklandinia-marl  (8  feet)  with  Ueliolitea,  Plasfiwpora,  ffalysitts,  BronUua 
platyaclinf  Calymene  papiUaaa,  C,  frontosa^  Orthis  Damdaoni^  0,  Lovfni, 
and  especially  the  abandant  Stricklandinia  lyraia. 
Older  red  marly  shales  (thickness  unknown  and  not  seen  in  place)  with 
some  40  species  of  fossils,  among  which  are  Favositea  goUandica^  F, 
Forbesi,  Holy  sites,  PUumoporaf  Arachnophyllum  diffiuens,  &c. 

In  the  Christiania  district,  according  to  Kjerulf,  the  following  subdivisions  can  be 
established : — 

y.  Compact  grey,   often  bituminous  limestone,   with  abundant 
Orthoceras  cockUatum  and  Chonetea  striaUUa, 
Staire  8    -I  ^'  ^^y*  somewhat  bituminous  limestone,  with  shales  and  clays. 
^     '    >  a.   Fissile  green  or  grey  marly  shales  containing  the  last  grapto- 

lites.     This  and  the  two  overlying  members  have  a  united 
depth  of  835  Norwegian  feet  at  Ringerige. 
Stages  6  &  7.  Coral  limestone  and  Pentamerus  limestone. 
'Stage  5.  Calcareous  sandstone,  with  RhynchontUa  diodotUa  and  shales,  150  to 
370  feet 
4.  Shales  and  marls,  with  nodules  and  short  beds  of  cement-stone  {Tri- 

nucleus,  (Jhasmops),  700  feet. 
3.  Graptolite  shales,  Limestone  in  two  or  more  bands  (Orthoceras-, 
Asaphus-,  Megalaspis-limestone),  250  feet  in  places,  resting  upon 
the  alum-shales  of  the  Primordial  zone.^ 

lu  Easter  and  Wester  Gothland  patches  of  Silurian  strata  are  met  with  preserved 
in  horizontal  sheets  under  an  overlying  capping  of  diabase.  But  when  the  rocks  are 
traced  into  the  western  parts  of  Norway  and  through  the  central  regions  where 
the  boundaries  of  Norway  and  Sweden  meet,  the}'  present  a  remarkably  different 
development  from  that  just  described.  According  to  the  researches  of  Kjerulf,  Dahll, 
Tornebohm,  Brogger,  and  Keusch,  vast  masses  of  quartzite,  mica-slate,  gneiss,  horn- 
blende-schist, clay-slate,  and  other  crystalline  rocks  can  be  seen  reposing  upon  recog- 
nisable Silurian  strata  in  numerous  natural  sections.  Not  improbably  these  Scandinavian 
metamorphic  rocks,  like  those  occupying  a  similar  position  in  Scotland,  will  be  found 
to  include  portions  of  different  pre-Cambrian  systems  which,  together  with  the  Cambrian 
and  Silurian  strata,  have  been  subjected  to  such  great  disturbance  as  to  have  had  a 
new  crystalline  structure  superinduced  upon  them.  Enormous  displacements  and  lateral 
thnists  have  driven  the  crystalline  rocks  over  the  fossiliferous  strata,  as  in  Scotland,  but 
the  details  of  this  structure,  which  has  been  recognised  by  Tornebohm,  have  still  to 
be  worked  out.  As  regards  the  date  of  these  great  earth -movements  and  metamorphism, 
it  is  important  to  remember  that,  as  already  stated  (p.  712),  Upper  Silurian  fossils  have 
been  found  by  Reusch  at  Bergen  in  the  crystalline  schists  themselves,  as  well  as  in  the 
limestones  intercalated  in  and  underlying  them.^ 

Western  Europe. — The  researches  principally  of  Gosselet  and  Malaise  have  demon- 
strated that  a  considerable  part  of  the  strata  grouped  by  Dumont  in  his  ''Terrain 

Rh^nan,"  and  generally  supposed  to  be  of  Devonian  "age,  must  be  relegated  to  the 

^ — — . 

^  Professor  Brogger  has  further  subdivided  Stage  3  as  follows,  in  ascending  order :  3a, 
(a)  Shales  and  limestones  with  Symphyswrus  incipUns,  (/3)  Ceratopyge  shales,  (7)  Ceratopyge 
limestone  ;  36,  Phyllograptus  shales  ;  3c:,  (a)  Megalaspis  limestone,  (/3)  Expansus-shales, 
(7)  Orthoceras  limestone,  the  whole  stage  having  a  thickness  of  about  47  metres  in  the 
Christiania  district  — '  Die  SiL  Etagen,'  p.  28. 

3  See  Dahll,  FOrh,  Vedensk-Selskab,  ChrUtiania,  1867.  Kjerulf,  *Geologie  des  Slid, 
u.  Mit  Norwegen,'  1880.  Tornebohm,  Bihang  K.  Svtnsk.  vet  Akad.  Uandl,  i.  No.  12 
(1873)  ;  GeoL  For.  Stockholm  F&rhand,  vL  (1883)  p.  274  ;  xiii.  (1891)  p.  37  ;  xiv.  (1892) 
p.  27 ;  Nature,  xxxviii.  (1888)  p.  127.  Brogger,  '  Die  Silurischen  Etagen  2  und  8  im 
Kristianiagebiet,'  1882,  p.  352.  Pettersen,  Tromsii  Museums  Aarsh^t,  vi.  (1888)  p.  87. 
F.  Svenonius,  Neues  Jahrb,  1882  (i.)  p.  181.     Nathorst,  'Sveriges  Geologi,'  p.  141. 

3d 


770 


STRATIGRAPHIGAL  GEOLOGY 


BOOK  VI  PART  n 


f           f 

3   o 

• 

V 

e« 

.     •  ( 

f- 

h^\ 

O 

S  "  i 

ai  ' 

^^y 

Silurian  series.^  Though  almost  concealed  by  younger  formations,  the  Silurian  rodu 
that  are  laid  bare  at  the  bottom  of  the  valleys  of  the  Ardennes  can  be  paralleled  in  a 
general  way  as  under  : — 

Equivalents  of  the  Ludlow  rocks  seen  in  the  valley  of  the  Fnette 
between  Fosse  and  Malonne,  containing  Monograpiua  colontu,  M. 
yif88onif  JUtiofites  geiriiizianus,  OrihoceraSy  Cardiola  interrupta, 
&c. 

Brown  sandy  shales  of  Naninne,  with  Cyrtoffraptua  Murehisoniy 
Monograptus  hohemicusy  M,  Nil^soni,  M.  priodorif  M,  vomerinus, 
RetiolUes  geinitzianuSj  Cardiola  interrupta,  OrthoeeraSy  kc, 

Quartzites  and  sandstones  of  Grand-Manil,  with  Monograpius  hoht- 
micuSy  M.  galaensis  %  M.  priodoriy  M.  protens,  M,  mboonicug. 

Shales  overlying  the  eurites  of  Grand-Manil,  and  containing  Clitnaco- 
graptus  normalia,  C.  raUangularis,  Dimorphograptus  elongatus, 
D.  Swanstoniy  Diplograpttis  modesttiSy  Monograptus  gregariua,  M. 
leptothecay  M.  tenuis, 

Schistes  de  Gembloux ;  pyritous  black  and  greenish  shales,  which  at 
Grand-Manil,  in  the  valley  of  the  Orneau,  have  yielded  Calymene 
incerta,  Trinucleus  seiiformxSy  lUttnue  Boumuinni,  Bellerophon 
bilobatus,  Strophomena  rhomboidaliSy  Orthis  testudinariay  O.  vesper- 
tiliOy  0.  calligrammay  0.  ActoniWy  Climacograptua  caudatus,  C. 
8ti/hideu8,  C,  ttU>uliferu8, 

The  horizon  of  the  Llaudeilo  rocks  is  doubtfully  represented  at 
Sart- Bernard. 

Graptolitic  shales,  with  Climacograptus  aniennariu^y  C  Scharen- 
bergiy  Dichograptus  octohrachiatuSy  Didynwgraptua  jfurchisoni,  D. 
nanuSy  Diplograptus  /oliaceuSy  D.  tricomiSy  PhyUograptus  angusti- 
folius^  P.  typusy  Tetragraptus  bryonoideSy  &c 

Upper  Cambrian  horizons  are  represented  at  Spa  and  elsewhere  by 
Dictyonevia  sociaie. 

The  Silurian  rocks  of  Belgium  comprise  several  contemporaneously  erupted  masses 
of  porphyrite  and  of  diabase,  as  w^ell  as  beds  of  porphyroid,  arkose,  and  earite. 

Silurian  rocks  have  been  detected  in  many  parts  of  the  old  Palaeozoic  ridge  of  the 
north-west  of  France.  According  to  De  Tromelin  and  Lebesconte,^  the  order  of  suc- 
cession in  Ille-et-Vilaine  is  as  under  : — 


5 


u 
o 

i 


^ 


t 
< 


a; 


4*         I 


2 


{  White  limestone  of  Erbray  {Ccdymene  B/nmenbou'Miy  Harpes  venvJoatLs). 
Ampelitic  (carbonaceous)  limestone  of  Briasse  (Monograptus  priodon, 
M.  Jlxsingeriy  M.  cdonusy  M,  vovierimiSy  M.  jacxtluni). 
I  Sandy  and  fenuginous  nodules  of  Martigne-Ferchaud,  Thourie,  &c. 
L      {Cardiola  interrttpta,  Monograptiis  priodan). 

(  Ampelitic  (carbonaceous)  shales  of  Polign6  {Monograptus  crassttSy  M, 

■{      .  Halliy    M.  priodoHy    M.  jaculvmy   M,   convolulnSy   M,   conlinenSy 

JJiplograptns  pabneuSy   Cepludograptvs  fdiumy  Retiolitee  geinitzi- 

anus). 

'I    "^  Phtanites  of  Aujou  {Monograptus  amvolutus,  M.  crenularis,  M,  Mn- 

ferus,  M.  sublobiferusy  M.  Sedginckiy  M.  cyphuSy  M,  crispuSy  M. 

Cliaganiy  Cephxdograptus  folium y  Dij^lograptus  Hughesiy  RastriUs 

peregrinvs^  R.  Litinwfj. 

^  Gosselet,  'Esquisse  Geologique  du  Nord  de  la  France,' p.  34.  'L'Ardenne,' Jf*^. 
Carte  Giol.  France  (1888)  p.  137.  Mourlon,  '  Geol.  de  la  Belgique,*  p.  40  ;  Malaise,  Mim. 
Couronn.  Acad.  Roy.  BelgiquCy  1873  ;  Bull.  Acad.  Roy.  Belg.  xx.  (1890)  p.  440.  C.  Barrois, 
Ann.  Hoc.  O^.  Nonly  xx.  (1892)  p.  75  ;  in  this  work  references  are  given  to  the  literature 
of  French  Silurian  geolog>'. 

^  De  Tromelin  and  Lebesconte,  Bull.  Sac.  GSol.  France  (1876),  p.  586  ;  Assoc,  Franf, 
(1875)  ;  Bull.  Soc.  Linn.  Normandie  (1877),  p.  5.  See  also  Dalimier,  '  Stratigraphie  des 
Terrains  primaires  dans  la  presqu'ile  de  CJotentin,'  Paris  (1861) ;  BvU.  Soc  Oiol.  IVanee 
(1862),  p.  907  ;  De  Lapparent,  Bull.  Soc.  GSol.  France  (1877),  p.  569;  Barrois,  Ann,  Soc. 
GSol.  Nordy  iv.  vii.  and  the  memoirs  cited  below. 


SECT,  ii  §  2 


SILURIAN  SYSTEM 


771 


0 


^ 

3 


a 


a 

3 


Slates  of  Riadan  (Trinueleua). 

Sandstones  (May,  Thonrie,  Bas-Pont,  Saint-Germain  de  la  Bouexiere, 

&c.)>   containing  Trinucleiu   Ool^usH^   Calymene  Bayani^   Orthis 

reduXf  0.  hudleigkenais,  0,  pvlvinatay  0.  valpyana,  0,  Berihoai^ 

yitcleoapira   Vicaryi,  LingtUa  Moriereiy   Pseudarca  typa,   Diplo- 

grapius  foliaenu,  D,  anguatifolius. 
Slates  of  La  Couy^re  {Orihis  Berthosi), 
Nodular  shales  of  Guichen,  &c.  {Caiyniene  Tristaniy  Placqparia  Tour- 

nemineit  AcicUupis  Btichii), 
Slates  of  Angers  {Ogygia  Dernnaresii^  Didymograptus  Murchisoniy  I), 

euodusy  D.  nanuSf  D,  furciUatus). 
Shales  of  Laill6  and  Sion  {Placoparia  Zippei^  Ilyolithea  cinctus), 
Armorican  sandstone  (Gr^  Armoricaiu),  containing  Asaphu^  armori- 

canuSfLingula  Letueuri,  L.  Hawktiiy  L,  SaUerif  Dinobolua  Brimonti, 

Lyrodeama  armoricana,  annelides. 
Red  shales  and  conglomerates  without  fossils. 

In  Normandy,  where  the  first  French  graptolites  were  found,  some  of  the  species 
characteristic  of  the  uppermost  groups  of  Brittany  have  been  obtained.  Silurian  fossils 
have  also  been  detected  southwards  in  Maine  and  Anjou,  and  still  more  abundantly 
from  the  ridge  of  old  rocks  which  forms  the  high  grounds  of  Langiiedoc  where  the 
following  section  has  been  determined.^ 

Shales  and  ampelitic  orthoceratite  limestones  (200  metres)  with  Cardiola  inter- 
Tuptttj  MonograptM priodon,  M.  bohemtctiSf  M.  colonuSf  M.  Roemeri,  M,  NilMoni. 
This  zone  evidently  corresponds  with  that  of  the  English  Wenlock  group. 

Alternations  of  shales  and  white  cystidean  limestones. 

Shales  with  Orthis  Actonim, 

Green  shales  with  concretions  (gftteaux)  formed  around  large  trilobites,  Asaphus 
Foumeti,  lUamus  Lebeseontei,  Didymograptus  euodus.  These  strata  are  prob- 
ably of  Llandeilo  age. 

Sandstone  and  grit  like  the  Gr^  Armoricain,  about  50  metres  thick,  containing 
CruzianUf  Vexillum^  Lingida  Lestieuri,  Dinoholus  Brinwnti. 

Shales  with  calcareous  nodules  (150  metres)  containing  BeUerophon  Oehlerti, 
Agnostusy  Caiyniene^  IlUenuSt  MegalcupiSf  Didymograptvs  balticuSf  D,  peiina- 
tuluSj  D.  nitidua,  D.  hijidus^  D.  indentus,  Tetragraptus  serra,  T.  quadri- 
brachiattts.  These  strata  and  the  overlying  sandstone  represent  the  British 
Arenig  rocks. 

Recent  researches  in  the  Pyrenees  have  revealed  a  great  development  of  fossiliferous 
rocks  which  from  their  graptolites  may  be  paralleled  with  the  English  and  Scottish 
Tarannon  sub-group.*  Three  zones  with  Monograptus  vomerinuSf  M.  Beekiy  and  if. 
crctssus  are  well  developed,  and  are  compared  by  Dr.  Barrois  with  the  British  zones  of 
Rastrites  maximuSj  Monograptus  exiguus  and  Cyrtograpttts  Graya  respectively.  The 
same  observer  remarks  that  these  graptolitic  faunas  of  the  Pyrenees  present  more  resem- 
blance with  others  found  in  the  south  of  Europe  than  with  those  in  the  original  typical 
regions  of  Britain  and  Scandinavia.  The  specific  types  are  generally  the  same  as  those 
of  Bohemia.^  Silurian  rocks  have  been  recognised  at  various  points  on  the  Spanish 
tableland,  a  lower  quartzite,  with  Cruzianay  Lingular  &c.,  being  surmounted  by  shales 
containing  Calymene  Tristani,  kc.  Graptolite-bearing  schists  occur  in  the  province  of 
Minho  in  the  west  of  Portugal.* 

^  Rouville,  'Monographie  G^ol.  de  Cabri^res,  Herault '  (1887).  Bergeron,  '^tude 
Geol.  du  Massif  ancien  au  sud  du  Plateau  Central '  (1889).  Barrois,  Ann,  Soc,  Oioi.  Nord, 
XX.  (1892)  p.  85.     F.  Freeh,  Zeitsch.  Deuisch.  Oeol,  Oes,  (1887)  p.  860. 

'  Caralp,  *  Etudes  geoL  sur  les  hauts  Massifs  des  Pyrenees  centrales,'  Toulouse,  1888, 
p.  453. 

s  Barrois,  Ann.  Soc,  GM,  Nord  (1892),  p.  127.  On  the  Silurian  rocks  of  the  Astnrias 
see  Barrois,  M^,  Soc,  GM,  Nord,  1882. 

*  J.  F.  N.  Delgado,  Comm,  Trabal,  Geol,  Portugal^  XL  fasc.  iL  (1892). 


772 


STRATIGRAPHICAL  GEOLOGY 


BOOK  TI  PABT  U 


850  ft 


if 


G. 


1000 


>> 


Central  and  Southern  Europe. — It  is  a  remarkable  fact  in  the  Palieozoio  geology  of 
the  European  continent  that  while  the  general  facies  of  the  fossils  continues  tolerably 
uniform  in  the  north-west  and  north  throughout  the  Silurian  territory  first  described, 
that  is,  from  Ireland  across  the  Baltic  basin  into  Russia,  a  great  contrast  is  to  be  noted 
between  this  northern  facies  and  that  of  central  and  southern  Europe.  The  P3rrenean 
exemplification  of  the  southern  type  has  just  been  alluded  to.  But  it  is  in  Bohemia 
that  this  type  is  most  abundantly  developed  and  most  excellently  preserved.  Out  of 
the  many  thousands  of  species  obtained  in  that  country  very  few  are  found  also  in  the 
north.  Among  the  forms  common  to  the  two  regions  graptolites  are  especially 
prominent,  more  than  a  dozen  of  the  characteristic  Upper  Silurian  species  of  Britain 
being  also  found  in  the  southern  province.^ 

In  the  im|)ortant  Silurian  basin  of  Bohemia,'  so  admirably  worked  out  by  Barrande, 
the  formations  are  grouped  as  in  the  subjoined  table  : — 

'  Stage  H.'"*  Shales  with  coaly  layers  and  beds  of  quartdte 
{Phacops  fecundusy  TentaciUiUa  elegatu),  with 
species  of  Zej7^«na,  OrthoceraSf  LituUesyChnicUiles, 
&c. 

Argillaceous  limestones  with  chert,  shales,  and  cal- 
careous nodules         .         .         .         . 

Numerous  trilobites  of  the  genera  IkUmanitegf 
BronteiUt  Pkacops,  Proistus,  HarpeSf  and  Caly- 
mene  ;  Atrypa  reticularis ^  Pentamerus  linguifer. 

Pale  and  dark  limestone  with  chert  Harpes,  Lichas, 
Phacops^  Atrypa  reticularis,  Pentamenia  galeatus, 
Favosites  gotlandica,  F.  Jtbrosa,  Tentcuiulites. 

Shales  with  calcareous  nodules,  and  shales  resting 
on  sheets  of  igneous  rock  (300  ft.),  lying  with  a 
slight  unconformability  on  the  group  below       450-900 

A  very  rich  Upper  Silurian  fauna,  abundant 
cephalopods,  trilobites,  kc. ;  Halysites  catenularia, 
graptolites  in  many  species,  such  as  are  found  in 
the  Birkhill  group  of  Britain. 

Yellow,  grey,  and  black  shales,  with  quartzite  and 
conglomerate  at  base,  divided  by  Barrande  into 
live  bands  numbered  Drfl  to  Drf5,  the  first  being 
further  separated  into  three  members  T>dl  a,  /3, 
and  y.  Jkll  a  and  /9  may  perhaps  be  paralleled 
with  the  Welsh  Treraadoc  group,  Ddl  7  with 
the  Arenig  rocks,  Dd  2,  8,  4,  and  5  with  the 
Bala-Caradoc  rocks  ..... 

Abundant  trilobites  of  genera  Trinudeiis, 
OgygiOf  Asaphus,  IlUenuSy  RemopleurideSf  kc 

Shales,  sometimes  with  porphyries  and  conglom- 
erates      ........ 

Paradoxid^f  EUipsoc-ephalusy  AgnostuSf  Arion- 
ellusy  and  other  genera  of  trilobites  referred  to 
above  (ante,  p.  723). 

Grits,  shales,  and  conglomerates. 


at 

c 

m 

I. 


00 

00 


»» 


>i 


F. 


E. 


>» 


a 


)) 


D. 


3000  ft 


5  i 


»» 


c. 


300 


*f 


ft 


B. 


A.  Green  schists,  grits,  breccias,  tuff's,  and  homstones 
resting  on  gneiss. 


^  Marr,  Quart,  Joum,  Oeol.  Soc.  1880,  p.  603. 

'^  See  Barrande's  magnificent  work,  *Systeme  Silurien  de  la  Boh^me.'  F.  Katzer, 
'Geologic  von  Bohmen,'  1892,  p.  791.     J.  E.  Marr,  Quart,  Joum.  Oeol,  Soc.  1880,  p.  591. 

^  Stages  F,  G,  H  are  classed  as  Devonian  by  Kayser  and  other  German  geologists. 
(Kayser,  Zdlsch.  Deutsch.  Oeol.  Oes,  xxix.  (1877)  pp.  207,  629,  notices  the  occurrence  of 
Bohemian  Upper  Silurian  fossils  in  the  Rhenish  Lower  Devonian  rocks.)  Barrande 
defended  his  classification  :    Verh.  K.  Oeol,  Racks,  1878,  p.  200. 


SECT.  ii§2  SILURIAN  SYSTEM  .773 

< 

Small  though  the  area  of  the  Silurian  basin  of  Bohemia  is  (for  it  measures  only  100 
miles  in  extreme  length  by  44  miles  in  its  greatest  breadth),  it  has  proved  extraordinarily 
rich  in  organic  remains.  Barrande  has  named  and  described  several  thousand  species 
from  that  basin  alone,  the  greater  number  being  peculiar  to  it.  Some  aspects  of  its 
organic  facies  are  truly  remarkable.  One  of  these  is  the  extraordinary  variety  and 
abundance  of  its  straight  and  curved  cephalopods,  of  which  18  genera  and  two  sub- 
genera, comprising  in  all  no  fewer  than  1127  distinct  species,  were  determined  by 
Barrande.  The  genus  Orthoeercu  alone  contained  in  his  census  554  species,  and 
Cyrtocerds  had  830.^  Of  the  trilobites,  which  appear  in  great  numbers  and  in  every 
stage  of  growth,  as  many  as  42  distinct  genera  were  noted,  comprising  850  species  ;  the 
most  prolific  genus  being  BrotUeus,  which  included  46  species  entirely  confined  to  the 
Srd  fauna  or  Upper  Silurian.  AddaapU  had  40  species,  of  which  six  occur  in  the  2nd 
and  34  in  the  Zrd  fauna.  ProHtus  also  numbered  40  species,  which  all  belong  to  the  Brd 
fauna,  save  two  found  in  the  2nd,  Other  less  prolific  but  still  abundant  genera  are 
Dalmanites,  Phacopa,  and  Illanus,  The  2nd  fauna,  or  Lower  Silurian  series,  was  found 
by  Barrande  to  contain  in  all  32  genera  and  127  species  of  trilobites ;  while  the  ^rd 
fauna,  or  Upper  Silurian  series,  contained  17  genera  and  205  species,  so  that  generic 
types  are  more  abundant  in  the  earlier  and  specific  varieties  in  the  later  rocks.  ^ 

Reference  may  be  made  here  to  the  famous  doctrine  of  *'  Colonies  "  propounded  and 
ably  defended  by  the  illustrious  Barrande.  Drawing  his  facts  from  the  Bohemian  basin 
he  believed  that  while  the  Silurian  strata  of  that  region  presented  a  normal  succession 
of  organic  remains,  there  were  nevertheless  exceptional  bands,  which  containing  the 
fossils  of  a  higher  zone,  were  yet  included  on  different  horizons  among  inferior  portions 
of  the  series.  He  termed  these  precursory  bands ''  colonies,"  and  defined  the  phenomena 
as  consisting  in  the  partial  co-existence  of  two  general  faunas,  which,  considered  as  a 
whole,  were  nevertheless  successive.  He  supposed  that,  during  the  later  stages  of  his 
second  Silurian  fauna  in  Bohemia,  the  first  phases  of  the  third  fauna  had  already  api>eared, 
and  attained  some  degree  of  development,  in  a  neighbouring  but  yet  unknown  region. 
At  intervals,  corresponding  doubtless  to  geographical  changes,  such  as  movements  of 
subsidence  or  elevation,  volcanic  eruptions,  &c.,  communication  was  opened  between  that 
outer  region  and  the  basin  of  Bohemia.  During  these  intervals,  a  greater  or  less  number 
of  immigrants  succeeded  in  making  their  way  into  the  Bohemian  area,  but  as  the 
conditions  for  their  prolonged  continuance  there  were  not  yet  favourable,  they  soon  died 
out,  and  the  normal  fauna  of  the  region  resumed  its  occupancy.  The  deposits  formed 
during  these  partial  interruptions,  notably  graptolitic  schists  and  calcareous  bands, 
accompanied  by  igneous  sheets,  contain,  besides  the  invading  species,  remains  of 
some  of  the  indigenous  forms.  Eventually,  however,  on  the  final  extinction  of  the 
second  fauna,  and,  we  may  suppose,  on  the  ultimate  demolition  of  the  physical  barriers 
hitherto  only  occasionally  and  temporarily  broken,  the  third  fauna,  which  had  already 
sent  successive  colonies  into  the  Bohemian  area,  now  swarmed  into  it,  and  peopled  it  till 
the  close  of  the  Silurian  period.* 

The  general  verdict  of  palaeontologists  has  been  adverse  to  this  original  and 
ingenious  doctrine.  The  apparent  intercalation  of  younger  zones  in  older  groups  of  rock 
has  been  accounted  for  by  such  infoldings  of  strata  as  have  already  been  described  in  this 
volume  and  by  the  effects  of  faults.  It  has  been  shown  that  not  only  are  the  zones 
repeated,  but  that  when  they  reappear  they  bring  with  them  their  minute  palieontologi- 
cal  subdivisions  and  their  peculiar  lithological  characters.'* 

1  *Sy8t  Silur.'  ii.  suppt.  p.  266,  1877. 

*  Op.  cit.  i.  suppt.  "Trilobites,"  1871. 

•  The  doctrine  of  colonies  is  developed  in  the  *  Systeme  Silurien  du  Centre  de  la  Boheme,* 
1852,  i.  p.  73  ;  *  Colonies  dans  le  Bassin  Silurien  de  la  Boheme,*  in  Bidl.  Soc,  Oiol.  France 
(2nd8er.)  xvii.  (1859)  p.  602  ;  'Di^fense  des  Colonies,'  Prague,  i.  (1861),  ii.  (1862),  iu.  (1865), 
iv.  (1870).  V.  (1881).  *  See  J.  E.  Marr,  Q.  /.  Oeol,  Soc  1880,  p.  605 ;  1882,   p.  313. 


774  STRATIGRAPHICAL  GEOLOGY  book  vi  pam  n 


Silurian  rocks  appear  in  a  few  detached  areas  in  Germany,  but  the  only  compsjatiTely 
large  tract  of  them  occurs  in  Thuringia  and  the  Fichtelgebii^ge.  They  present  a  great 
contrast  to  those  of  Bohemia  in  their  comparatively  nnfosslliferous  character,  and  tbe 
absence  of  any  one  continuous  succession  of  the  whole  Silurian  system.  In  the 
Thuriugcr  Wald,  a  series  of  fucoidal-slates  (perhaps  Cambrian)  passes  up  into  slates, 
greywacke^,  kc,  with  Lingula,  Diaeina,  CcUymene^  numerous  graptolites  and  other 
fossils.  These  strata  (from  1600  to  2000  feet  thick)  may  represent  the  Lower  S|ilarisii 
groups.  They  are  covered  by  some  graptolitic  alum-slates,  shales,  flinty  slates,  and 
limestones  {FavosiUs  gotlandiea,  Cardiola  inUmiptay  Tentaculites  acuariua,  kc.\  which 
no  doubt  represent  the  Upper  Silurian  groups,  and  pass  into  the  base  of  the  DeTonian 
S3rstenL^  The  graptolites  include  many  species  found  in  the  Stockdale  shales  of  the 
Lake  District,  so  that  the  Llandovery  group  is  well  represented  in  this  part  of  the 
continent.^  Among  the  Harz  mountains  certain  greywackes  and  shales  containing  land- 
plants  (lycopods,  &c.),  trilobites  (DalmaniteSf  &c.),  graptolites,  &c.,  are  regarded  as  of 
intermediate  age  between  true  Upper  Silurian  and  Lower  Devonian  rocks.' 

Among  the  Alps,  the  band  of  ancient  sedimentary  rocks,  which,  flanking  the 
crystalline  masses  of  the  central  chain,  has  been  termed  the  ^'greywacke  zone,"  has  in 
recent  years  been  ascertained  to  contain  representatives  of  the  Silurian,  Deyoman, 
Carboniferous,  and  Permian  systems.^  In  the  eastern  Alps,  a  belt  of  clay-slate  and 
greywacke,  with  limestone,  dolomite,  magnesite,  ankerite,  and  siderite  runs  fnmi 
Kitzbiihel  in  the  Tyrol  as  far  as  the  south  end  of  the  Vienna  basin.  About  twenty 
species  of  fossils  {OrthoceraSf  Atrypa.  Cardiola,  &c.)  found  at  Dienten,  near  Werfen, 
belong  apparently  to  the  substage  e^of  Barrande's  Stage  E.  In  this  band,  the  strata 
have  been  changed  into  crystalline  schists  (p.  624).  As  the  fossils  are  Upper  Silurian, 
a  large  part  of  the  a(^'acent  unfossiliferous  schistose  rocks  may  represent  older  parts  of 
the  Silurian  system  ;  but  no  Lower  Silurian  fossils  have  yet  been  found  in  them  in 
the  northern  Alps. 

In  the  southern  Alps  (Carinthia),  above  the  older  Palseozoic  masses  which  have 
not  yet  yielded  fossils,  the  following  subdivisions  have  been  given  by  Stache  in 
descending  oi-der : — 

Limestones   (1000  to  1500  feet)  with   Silurian   fomis  of  Pentaments,  Spitifer^ 

RliynchaneUa  and  Atrypa,  and  Silurian  and  Devonian  corals  =  Stages  F,  O,  H, 

of  Barrande. 
Dark  clay-slates  and  sandstones  with  plant-remains,  yellow  and  red  crinoid-shales 

=  Stage  F,  in  parts  Onondago  group  (?). 
Limestone  with  orthoceratites,  gasteropods,  lamellibranchs,  trilobites  (Kokberg). 

About  100  species  occur  in  the  lower  or  dark  Orthoceras  limestone.     These 

rocks  appear  to  represent  Stage  E  of  Bohemia,  and  the  Ludlow  and  Wenlock 

groups  of  England. 
Graptolite-schista  with  DiplograptVrS  folium^  D.  pristis,  &c.=  Stage  D  and  base  of 

E  (Tarannou  group). 
Oreywacke-slate  and  sandstone  {Strophomtna  gnindi^j  Or^Aw)  =  upper  part  of  Stage 

D  ;  perhaps  Bala  beds.* 

In  the  southern  half  of  Sardinia,  Silurian  rocks  (in  part,  at  least.  Upper)  have  been 

^  Richter,  Zeitsch.  Dtutsch.  Geol.  OeselL  xxL  p.  369  ;  xxvii.  p.  261. 

-  Marr,  Oeol.  May.  1889,  p.  414.     Tomquist,  Oeol.  F6r,  Stockholm  Ffhrhandl.  ix.  (1887). 

'  Lessen,  Zeitach.  Deutsch,  Geol.  Ges.  xx.  p.  216  ;  xxii.  p.  284  ;  xxix.  612. 

*  Von  Hauer,  'Geologic,*  p.  216.  Stache,  Jahrb.  Geol.  Reichsanstalt,  xxiiL  p.  175; 
xxiv.  136,  334  ;  Verh.  Geol.  Reichs.  1879,  p.  216.  Stache  divided  the  greywacke  zone  of 
the  eastern  Alps  into  five  pre-triassic  groups  :  1,  Quartzphyllite  group  ;  2,  Kalkphyllite 
group  ;  3,  Kalkthonphyllite  group  ;  4,  Group  of  the  older  greywackes  (Silurian  and 
Devonian) ;  5,  Group  of  the  Upper  Coal  and  Permian  rocks. 

*  VerharuU.  Geol.  IteichsansL  1884,  p.  26  ;  Zeitach.  DetUach.  Geol.  Ges,  1884,  p.  277. 


SECT,  ii  §  2  SILURIAN  SYSTEM  775 

divided  into  three  zones,  the  lowest  6f  which  contains  important  metalliferous  lodes. ^ 
Among  these  rocks  Meneghini  recognises  two  chief  graptolitio  horizons,  one  probably 
representing  the  Tarannon  sub-group  (with  Monograptua  antennulariuSf  comp.  Beehi,  M, 
Ocniiy  comp.  conthwns,  M,  hemipridiSy  comp.  jaculum)  the  other  (with  M,  colontis,  M. 
LamarmoraBy  M,  muUuliferus,  comp.  vornerinus)  answering  to  the  Wenlock  group. 

In  the  south-west  of  Russia  (Podolia)  and  in  Gallicia,  an  Upper  Silurian  area  occurs 
in  which  there  is  almost  perfect  palseontological  agreement  with  the  Silurian  rocks 
of  the  basin  of  the  Baltic,  but  a  great  contrast  to  those  of  Bohemia,  with  which  it  has 
only  a  few  brachiopods  in  common.^ 

North  America.' — In  the  United  States  and  Canada,  Silurian  rocks  spread  con- 
tinuously over  a  vast  territory,  from  the  mouth  of  the  St.  Lawrence  south-westwards 
into  Alabama  and  westwards  by  the  great  lakes.  They  almost  enciccle  and  certainly 
underlie  all  the  later  Paleozoic  deposits  of  the  great  interior  basin.  The  rocks  are 
most  typically  developed  in  the  State  of  New  York,  where  they  have  been  arranged  as 
in  the  subjoined  table  : — 

'(4)  Water-lime  {TentaculiteSf  Eurypierus,  and  Pteryyolua)  Onondago  salt  group, 
consistiDg  of  red  and  grey  marls,  sandstones  and  gypsum,  with  large 
impregnation  of  common  salt,  but  nearly  barren  of  fossils. 

(3)  Niagara  shale  and  limestone  {Hcdytites^  FavosiieSj  Calymene  Blumenbachii, 
Homalonotus  delphinocephcUus,  Leptana  transversalut,  &c,  ;  also  fish- 
remains  {Onchus,  Olypiaapis)  in  the  shale  in  Pennsylvania.  The  Niagara 
Limestone  may  be  paralleled  with  the  Wenlock  Limestone. 

(2)  Clinton  group  {Pentamerus  oblonguSf  Atrt^pa  reticularis,  Monograpttus  din- 
tonensis,  Retiolites  venostts,  &c.)     This'^^roup  may  represent  the  Tarannon 
shales. 
.(I)  Medina  group  with  Oneida  conglomerate  {Modiolopsis  orthonota), 

(5)  Cincinnati  (Hudson  River)  group  {Syringopora,  HalysiteSy  Pt^nea  demtssa, 
Leptvna  sericea,  Climacograptus  bicarnisj  C  ti/piccUi/t,  Diplograptus  pristis, 
D,  puiUlus).     This  group  corresponds  to  the  Caradoc  rocks  of  Britain. 

(4)  Utica  group — Utica  shale  (Ijepiograptus  flacciiluSy  Diplograptus  mucronatus(1), 
D,  quadrimueronatua,  &c.)  The  shales  of  Nonnan's  Kiln,  near  Albany, 
on  the  Hudson  River,  have  yielded  a  large  series  of  graptolites  resembling 
the  assemblage  that  characterises  the  Glenkiln  shales  of  Scotland. 

(  Trenton  limestone.     )  «  .       ,  ,  .         r....    .... 

-:   .  (3)  Trenton  )  Black     River    lime-  (  Tnnucleu.  concentru^m,  Orihis  testud.nana, 
'  trrouD     1      stonsb  i       Murchisonui,  ConuMruiy  OrthoreraSf  Cyrto- 

*^  *    (  Birdseye  limestone.    )      *'*^'^»  ^^* 

(2)  Chazy  group^Chazy  limestone  (Maclurea  magruij  M.  Logani,  Orthoceras, 
lUtenus,  Asaphus), 

(1)  Calciferous  group  {LingtUella  acuminata,  Leptana,  Canocardium,  OphUeta 
rompactOj  Orthoceras  primigenium,  Amphion,  BathyuruSj  Asaphns,  Cono- 
I'oryphe,  TetragraptuSt  PhyUagraptus,  Didyinograptus,  Clonograptus, 
TjOganograptuB,  Diplograptus,  &c.)  This  group  answers  to  the  Welsh 
Arenig  rockH.* 

^  Meneghini,  Mem.  R,  Acad,  Lined,  1880. 

3  F.  Schmidt,  '  Die  Podolisch-galizische  Silurforniation,'  St.  Petersburg,  8vo,  1875. 

*  See  especially  the  Memoirs  of  the  Geological  Survey  of  Canadfi,  numerous  monographs 
of  Prof.  James  Hall,  of  Albany  ;  Walcott,  Monogr.  U.&  Geol,  Surv,  viiL  (1884). 

*  Remains  of  ganoid  fishes,  like  Uoloptychius  and  AsteroUpis,  and  of  a  chimseroid  fish, 
have  been  found  in  what  seems  to  be  a  representative  of  the  Trenton  group  in  Colorado. 
C.  D.  Walcott,  BuU.  Geol.  Soc.  Amer,  iii.  (1892)  p.  153. 

*  According  to  researches  by  Mr.  Selwyn,  the  so-called  Quebec  group  as  defined  by 
Logan  embraces  three  totally  distinct  groups  of  rock,  belonging  res]iectively  to  Archsean, 
Cambrian,  and  Lower  Silurian  horizons ;  and  in  the  fossiliferoos  belt  of  Logan's  Quebec 
group  are  included — in  a  folded,  crumpled  and  faulted  condition — portions  of  subdivisions 
that  lie  elsewhere  comparatively  undisturbed,  and  embrace  strata  even  lower  than  the 
Potsdam  formation.     Trans,  Rtty,  Soc  Canada,  vol.  L  sect.  iv.  p.  1  (1882). 


776  STRATIGRAPHICAL  GEOLOGY  book  n  past n 


It  is  interesting  to  obsenre  the  number  of  genera  and  even  of  species  common  to  the 
Silurian  rocks  of  America  and  Europe,  and  the  close  parallelism  in  their  order  of 
appearance.     Not  a  few  of  the  widely  diffused  forms  occur  in  Arctic  America,  so  that  a 
former  migration  along  shallow  northern  waters  between  the  two  continents  is  rendered 
highly  probable.     Among  these  common  species  the  following  may  be  ennmerated  as 
occurring  in  the  Upper  Silurian  rocks  of  New  York,  the  coasts  of  Barrow  Straits  within 
the  Arctic  Circle,  Britain,  and  the  Baltic  basin  :  Str&matopora  concentriea,  Matynia 
oatenulariay  FavosiUs  gollandiea,  Orthis  eUgantula,  Atrypa  reticularis.     The  genera  of 
graptolites  appear  to  have  followed  the  same  order  of  appearance  and  to  hare  reached 
their  full  development  and  final  decline  at  corre8i)onding  stages  of  the  Silurian  period 
on  each  side  of  the  Atlantic.    Among  the  Crustacea,  trilobites  were  the  dominant  order, 
represented  in  ea^h  region  by  a  similar  succession  of  genera,  and  even  to  some  extent  of 
species.     And  as  these  earlier  forms  of  articulates  waned,  there  appeared  among  them 
about  the  same  epoch  in  the  geological  series,  the  eurypterids  of  the  Water-lime  of  New 
York  and  of  the  Ludlow  rocks  of  Shropshire  and  Lanarkshire. 

Asia. — Silurian  rocks  have  been  recognised  over  a  large  jtart  of  the  surface  of  the 
globe.  They  have  been  found,  for  example,  running  through  the  Cordilleras  of  Sontk 
America  on  the  one  hand,  and  among  the  older  rocks  of  the  Himalaya  chain  on  the 
other.  The  Salt  Range  of  the  Punjab  contains  thick  masses  of  bright  red  marl,  with 
beds  of  rock-salt,  gyjMum,  and  dolomite,  over  which  lie  purple  sandstones  and' shales. 
Tliese  saliferous  rocks  have  been  already  (p.  737)  referred  to  as  containing  Cambrian 
fossils,  but  it  is  not  yet  known  whether  they  include  any  representatives  of  the  Silurian 
system.^  In  the  regions  of  the  Northern  Punjab  and  Kashmir  traces  of  Silurian  organic 
remains  have  been  discovered  ;  while  in  the  north  of  Kumaun  such  fossils  have  been 
found  in  considerable  quantities. 

From  the  province  of  Sze  Chueu,  in  Western  China,  Richthofen  has  obtained  numerous 
fossils  which  show  the  presence  there  of  Middle  and  Upper  Silurian  rocks.  Among  the 
sj>ecies,  some  are  the  same  as  those  that  occur  in  Western  £uro]>e,  such  as  OrihiM  calli- 
grainiiuij  Leptmiia  sericca^  Sjyirifer  radiatus,  Atn/pa  reticulariSy  FarosUts  fibrosa,  Hetio- 
lites  intr.rsdncliiSj  Halysilcs  catcnulariay  and  othens.' 

Australasia. — In  Australia  the  existence  of  the  Silurian  system  has  been  proved  by 
the  discovery  of  a  considerable  number  of  characteristic  fossils,  among  which  are  numerous 
graptolites  of  the  genera  ClimacofjraptuSy  Cienograpticsy  DichograptuSy  Dicranot^raptuSy 
l>idijvnKjraptuiiy  JJiplograptu^,  Monograptu^y  Logntiograptiis,  Phyllograpfi/Xy  HetiofiUXy 
and  Trtragntptus,  with  species  of  Siphonotreta  and  JfymenocariSy  which  occur  in  the 
Lower  Silurian  series  of  Victoria — an  enormous  series  of  sedimentary  deposits,  estimated 
by  Mr.  Sclwyn  to  be  not  less  than  35,000  feet  thick — also  many  Upper  Silurian  fossils 
from  New  South  Wales  and  Victoria,  including  such  world-wide  species  as  FavosiUs 
gotfdudic/i,  Ileliolitcs  inf^rsfinctus,  Ciilyniciic  Blununhachii^  Encrinvrvs  pundeUvSj 
Entom  is  t ubrroHa,  J'/i/tcops  cuudatus,  Atnfpa  ret ku Inn's,  Strophomeiia  jiccteny  Peniamcrus 
Knight li,  P.  ob/ongus,  Whitjieldift  {Mvristclla)  iuviidti,  Orthocrrasihcx.^  Near  Bathurst 
and  elsewlierc,  tlic  Upper  Silurian  rocks  of  New  South  Wales  have  been  much  altered, 
sandstones  ])assing  into  qnartzites,  slates  into  gneiss  and  hornblendic  schists,  and  the 
coral-limestones  into  crystalline?  marbles  with  total  obliteration  of  fossils.* 

^  A.  B.  Wynne,  Mem.  (JeoL  Sun\  India,  xiv.  See  also  PalxonL  Indica,  ser.  18,  vol. 
i.  (1887)  p.  750  :  Medlicott  and  Blanford,  *  Manual  of  the  Geology  of  India,*  1879. 

-  Hichthofen's  'China,'  vol.  iv.  pp.  37,  50,  where  descriptions  of  the  fossils  are  given 
by  Kayser  and  Lindstroni. 

^  McCoy,  '  Prodronnis  of  Paleontology  of  Victoria  : '  L.  G.  de  Koninck,  *  Recherches  sur 
les  Fossiles  Paleozoiques  de  la  Nouvelle-Galles  du  Sud,'  Brussels,  1876  ;  R.  Etheridge  jun., 
*  Catalogue  of  Australian  Fossils  ; '  W.  B.  Clarke,  '  Remarks  on  the  Sedimentary  Formations 
of  New  South  Wales,'  4th  edit.  ;  C.  S.  Wilkinson,  '  Notes  on  the  Geology  of  New  South 
Wales,'  Sydney  (1882).  *  C.  S.  Wilkinson,  op,  cii. 


SECT,  iii  DEVONIAN  SYSTEM  777 

In  New  Zealand  some  dark  slates  and  crystalline  limestones  which  form  the  mass  of 
Mount  Arthur,  and  from  which  a  few  graptolites,  &c.,  have  been  obtained,  are  referred 
to  the  Lower  Silurian  series.  They  are  much  disturbed  by  homblendic  and  syenitic 
eruptive  rocks.  To  the  Upper  Silurian  series  are  assigned  some  fossiliferous  rocks  from 
which  Calymene  Blumenbachii,  Spiri/er  radiatiiSy  Stricklandinia  lyrata,  &c.,  have  been 
procured  (Baton  River  series).  A  great  part  of  the  so-called  metamorphic  schists  arc 
probably  Upper  Silurian  rocks.  ^ 


Section  ill.     Devonian  and  Old  Red   Sandstone. 

In  Wales  and  the  adjoining  counties  of  England,  where  the  typical 
development  of  the  Silurian  system  was  worked  out  by  Murchison,  the 
abundant  Silurian  marine  fauna  disappears  in  the  red  rocks  that  overlie 
the  Ludlow  group.  From  that  horizon  upwards  in  the  geological  series, 
we  have  to  pass  through  some  10,000  feet  or  more  of  barren  red  sand- 
stones and  marls,  until  we  again  encounter  a  copious  marine  fauna  in  the 
Carboniferous  Limestone.  It  is  evident  that  between  the  disappearance 
of  the  Silurian  and  the  arrival  of  the  Carboniferous  fauna,  very  great 
geographical  changes  occurred  over  the  site  of  Wales  and  the  west  of 
England.  For  a  prolonged  period,  the  sea  must  have  been  excluded,  or 
at  least  must  have  been  rendered  unfit  for  the  existence  and  development 
of  marine  life,  over  the  area  in  question.  The  striking  contrast  in  general 
facies  between  the  organisms  in  the  Silurian  and  those  in  the  Carboniferous 
system,  proves  how  long  the  interval  between  them  must  have  been. 

The  geological  records  of  this  interval  are  still  only  j)artially  un- 
ravelled and  interpreted.  At  present  the  general  belief  among  geologists 
is  that,  while  in  the  west  and  north  of  Europe  the  Silurian  sea-bed  was 
upraised  into  land  in  such  a  way  as  to  enclose  large  inland  basins,  in  the 
centre  and  south-west  the  geographical  changes  did  not  suffice  to  exclude 
the  sea,  which  continued  to  cover  that  region  more  or  less  completely. 
In  the  isolated  basins  of  the  west  and  north,  a  peculiar  type  of  deposits, 
termed  the  Old  Red  Sandstone,  is  believed  to  have  accumulated,  while 
in  the  shallow  seas  to  the  south  and  east,  a  series  of  marine  sediments 
and  limestones  was  formed,  to  which  the  name  of  Devonian  has  been 
given.  It  is  thus  supposed  that  the  Old  Red  Sandstone  and  Devonian 
rocks  represent  different  geographical  areas,  with  different  phases  of  sedi- 
mentation and  of  life,  during  the  long  lapse  of  time  between  the  Silurian 
and  Carboniferous  periods.  A  somewhat  similar  contrast  between  the 
lithological  and  palaeontological  characters  of  the  corresponding  forma- 
tions in  different  parts  of  the  United  States  and  Canada,  shows  that  in 
America  also  this  geological  period  was  marked  by  geological  changes 
which  produced  distinct  geographical  conditions  in  adjacent  regions. 

That  the  Old  Red  Sandstone  of  Britain  does  represent  the  prolonged 
interval  between  Silurian  and  Carboniferous  time  can  be  demonstrated  by 
innumerable  sections,  where  the  lowest  strata  of  the  system  are  found  gradu- 
ating downward  into  the  top  of  the  Ludlow  group,  and  where  its  highest 

^  Hector,  '  Handbook  of  New  Zealand,'  p.  37. 


«  • 


*  TfhATI'^hJLrHI'iAL  vZ-Jl^^J 


r*^,£.*.-     W*;  t:skfii^f\  •'ipj^^'fe  that  tli«  ricii  SCiztkl  fkza 

at  rh^  '>^^  of  th-i  Jjcvilow  efr^k     Wt  shoajd  be  bri 

tjtytrr'j  *A  .Si'^ihACi  r»frk*  \fnuy^*ir  than  the  1a:«c  f.4  :bc«e  in 

M  Vj/unuA*:  khowt^l  Uj  «!rzL-t  in  bis  ^^age  H.  or  f->r  a  E^pv-^ 

hpih'.'iV*  in  r'^:]c*  which  are  nevfrrth*:!^*  regard€»i  as  SCnrian. 

X^nu*^\  fy/vif-r  fievfnian  may  fjartlv  rejrt^srtt  sctne  <4  tiie 

'yf  .Sil-jriafi  Jif«r.     ^in  the  other  hand,  the  npp^r  parts  ocjiiie 

ityitt/rfn  ffjj;rht  in  ^%'era]  re^jjectis  be  claimed  as  fairly  bekmgji^  to  tke 

CarV/uifero'ijii  ftv»t4;m  aU^ve. 

J.  Jl  Juke*  ]tT«*\ftfff^\  a  n^ilation  of  the  Devonian  problem,  the  effect 
of  whi':h  Mould  l/e  u»  turn  the  whole  of  the  Devonian  rocks  into  Lower 
f. ar Wi if eroijii,  and  U9  pla/;e  them  above  the  Old  Ked  Sandstoiie,  vlikk 
would  thijM  \Hifu,uiH  the  hhAh  representative  in  Europe  of  the  inteml 
Ift'Xwfj'M  Siltjrian  ami  Carljoniferons  time.-  In  the  following  descriptioiis 
an  a/r^'ount  will  fintt  b^;  given  of  the  Devonian  type  and  then  of  the  Old 
\U^\  SandifUine. 


I.  DEVONIAN  -nPE. 

$  1.  (leiieral  Characters. 

lt<><:Ks. — Throu^^hout  central  and  western  Euroi)e,  the  Devonian 
HynUifn  jin;,H<fntK  ;i  rcniHrkable  p-rsistence  of  petrographical  characters. 
indicating  prol);ibIy  the  f^revalcnce  of  the  same  kind  of  physical  conditions 
ovrrr  thr;  anfa  during  the  jKjrifxl  when  the  rocks  were  accumulated.  The 
lowf^r  division  consiHtH  mainly  of  sandstones,  grits,  and  gre^^wackes,  with 

'  A':<'onling  to  Kayncr  arid  I^'yrich  the  limefltones  of  the  Hercynian  series  in  the  Han 
Mu\  Nahhaii,  ioi^t'WuT  with  IkuramlK'H  Up|;>er  Silurian  StageM  F,  G,  H,  in  Bohemia,  are  to  he 
r«^^u(l<'(l  us  truly  Dttvoniuri,  and  as  )>einK  the  deeper- water  equivalents  of  the  arenaceoos 
Ht'.rinH  of  the  normal  J»wer  Devonian  Heries  on  the  Rhine.  {AbhaiulL  Geol.  Spedaikarif 
I'rriiHHrn,  II.  II«ft  4,  1H7H.     Z,'U»h.  heuUrh.  <ieol.  Oes.  xxxiii.  (1881),  p.  628.) 

'■'  S«M'.  hlN  pniH»rH  in  Journ.  Roy.  (ietil,  S(K,  IreUind  (1865),  i.  pt.  1,  new  ser.,  and  Qwui. 
.foil  I'll.  fJi'ol.  Sim'.,  xxii.  (1860),  and  his  pamphlet  on  'Additional  Notes  on  Rocks  of  North 
I)»von,'  9n\  (1867).  Thu  "I)<;voniaii  qnestion/'  a.s  it  has  been  called,  has  evoked  a  large 
numlM'i  of  pap<'rN,  of  whi(;h,  iM'HideK  thoKc  cite<l  in  Rul>8e(j[uent  I)age8,  the  following  may  he 
«nnniirat«'«l :  Prof.  Hull,  (I  J.  tJeol.  Sitr,  xxxv.  (1879)  p.  699;  xxxvL  (1880)  p.  255. 
A.  ('liam|Hirnowm«,  f^i-ol.  Maij.  v.  2nd  Ser.  (1878)  p.  193  ;  vi.  (1879)  p.  125  ;  viiL  (1881) 
p.  4 1 0.  Tho  K^'iK'ral  V(*rdict  hnn  boen  adverse  to  the  explanation  of  the  stractnre  of  North 
Ih'von  propoHi^l  by  Jukes. 


8ESCT.  iii.  I.  §  1  DEVONIAN  SYSTEM  779 

slates  and  phyllites.  These  rocks  attain  a  great  development  on  the 
Khine,  where  they  form  the  material  through  which  the  picturesque 
gorges  of  the  river  have  been  eroded.  In  the  central  zone,  limestones 
predominate,  often  crowded  with  the  corals  and  mollusks  of  the  clearer 
water  in  which  they  were  laid  down,  and  in  some  cases  actually  repre- 
senting former  coral-reefs.^  The  upper  series  is  more  variable  :  being  in 
some  tracts  composed  of  sandstones  and  shales,  in  others  of  shales  and 
limestones,  but  everywhere  presenting  a  more  shaly  thin-bedded  aspect 
than  the  subdivisions  beneath  it.  Considerable  masses  of  diabase,  tuff 
(schalstein),  and  other  associated  volcanic  material  are  intercalated  in  the 
Devonian  system  in  Devonshire  and  in  Germany.  As  a  rule,  the  rocks 
have  been  subjected  to  more  or  less  disturbance,  and  have  in  some  places 
been  plicated  and  cleaved,  and  even  metamorphosed  into  schists,  quartzites, 
&c.  In  some  districts,  they  have  been  invaded  by  large  masses  of  granite 
and  other  eruptive  rocks. 

Among  the  economic  products,  the  most  important  in  £iux)pe  are  the 
ores  of  iron,  lead,  tin,  copper,  &c.,  which  occur  in  veins  or  lenticular 
masses  through  the  Devonian  rocks  (Devon  and  Cornwall,  Harz,  &c.) 
In  North  America  the  Devonian  rocks  of  Pennsylvania  contain  bands 
of  "  sand-rock  "  charged  with  petroleum. 

Life. — An  abundant  cryptogamic  flora  covered  the  land  during  the ' 
ages  that  succeeded  the  Silurian  period.  As  the  remains  of  this  vegeta- 
tion are  chiefly  preserved  in  the  Old  Ked  Sandstone  facies  of  deposits,  it 
is  described  at  p.  793.  But  the  true  Devonian  rocks  contain  remains  of 
marine  vegetation,  of  which  HcUiserites  is  a  frequent  sea-weed  in  the 
Lower  Devonian  rocks  of  the  Rhine.  The  fauna  of  the  Devonian  rocks 
is*  unequivocally  marine.  Among  the  more  lowly  forms  of  life  are  some 
of  which  the  true  zoological  grade  has  been  the  subject  of  much  un- 
certainty. Of  these,  the  fossil  known  as  Calceola  sandalina  (Fig.  349)  has 
been  successively  described  as  a  lamellibranch,  a  hippurite,  and  a  brachio- 
pod ;  but  is  now  regarded  as  a  rugose  coral  possessing  an  opercular  lid. 
The  Pleurodictywm  prohlematicum^  a  well-known  form  of  the  Lower 
Devonian  beds,  is  now  classed  with  the  Favositkiee  among  the  perforate 
corals.  The  puzzling  genus  Stromatopora  occurs  in  some  of  the  limestones 
as  abundantly  and  much  in  the  same  way  as  reef-building  corals  do  in  a 
modern  coral-reef.  The  curious  ReceptaculUe^i,  already  (p.  741)  referred 
to,  is  a  well-known  Devonian  fossil.  The  last  graptolites  are  met  with 
in  the  Devonian  system.  They  are  of  the  simple  type  so  characteristic 
of  the  Upper  Silurian  rocks,  and  have  chiefly  been  found  in  the  Hercynian 
formation  of  the  Harz.^  The  corals  of  the  Devonian  seas  were  both 
r  abundant  in  individuals  and  varied  in  their  sp>eciflc  and  generic  range. 
Not  a  single  species  is  common  either  to  the  Silurian  system  below  or 

1  Dupont,  Bull.  Acad.  Roy.  Belgique  (3)  ii.  ;  Comptes  rend.  Feb.  18,  1884.  The 
frequent  singularly  lenticular  character  of  Palaeozoic  limestones  is  explicable  on  the  Msump- 
tion  that  in  many  cases  they  grew  np  in  patches  after  the  manner  of  modem  coral-reefs. 
The  interrupted  bands  of  shale  in  the  Belgian  Devonian  limestones  are  regarded  by  M. 
Dupont  as  representing  the  lagoons  that  were  filled  up  with  muddy  sediment 

^  E.  Kayser,  Abhandl.  Specialharte  Preusaen,  II.  Heft  4,  1878. 


780  STRATIGRAPHICAL  GEOLOGY  book  vi  pam  n 

the  Carboniferous  above.  Among  the  rugose  forms,  the  genera  Cyatko- 
pkyllum,  Acervidana,  and  CyUiphyllwm  are  characteristia  The  tabulate 
kinds  belong  chiefly  to  the  important  genera  of  Fatontes,  AlveolUea,  and 
Heliolites.  Calceola  and  Fleurodidyum,  already  referred  to,  are  important 
Lower  Devonian  corals,  while  Phillipsastrxa  is  of  great  consequence 
among  the  coral-reefs  of  the  Upper  Devonian  rocks.     Of  the  echinoderms 


«.  E«(1ierla  nienibniiiBPci,  Pnchl.,  nat.  >lie  und  inignillF 

(Lower  Old 

Re.1  S.iid.Mne):  b.  Entonii. 

(Cj-prHipa)  -emlo.ilrii.ta,  Siin.lb.,  nagninM  (Uppej 

(Liiwer  UU  R*.l  ttandntoiie) ;  d.  It«>'notiu  .nullcu,,  I 

.(r«w8rl>: 

Re<1  SindatDui') :  i,  rhwopi 

r,Clol.lf.(L., 

n.  (Ixiwer  Uevo 

by  far  the  most  abundant  repi-esentatives  are  ci-inolds,  which  occur  in 
great  profusion  in  the  limestones,  sometimes  forming  entire  beds  of  rock. 
They  belong  chiefly  to  two  families — the  Cyatlioi-rinidte,  simple  peduncu- 
late forms  with  five  branching  arms,  and  Ciipressocrinid-se  (wholly  Devonian) 


SECT.  iii.  L  §  1  DEVONIAN  SYSTEM  781 

having  five  arms  which  when  folded  up  form  a  p>entagonal  pyramid, 
the  accurate  fitting  of  which  recalls  the  ambulacra  of  sea-urchins.  The 
Cystideans  appear  to  have  died  out  in  the  Devonian  period.  True  star- 
fishes also  occur  {Helianthastery  Astropeden,  Codaster). 

The  known  crustacean  fauna  of  the  Devonian  period  indicates  a 
striking  diminution  in  number  both  of  individuals  and  of  species  of  trilo- 
bites  (Fig.  348).  Most  of  the  genera  so  abundant  and  characteristic  among 
the  Silurian  rocks  are  now  absent,  the  most  frequent  Devonian  forms 
being  species  of  PhacopSj  Cryphasus,  DalmanUes^  Homalonotus,  and  Bronteus. 
But  some  other  Silurian  genera  still  survived,  especially  AcidaspiSy  Calymeney 
Cheirurus,  Harpes,  LichaSy  and  Proetus,  The  ostracods  are  chiefly  repre- 
sented by  the  genus  Entomis  (Cypridina),  which  occurs  in  enormous 
numbers  in  some  Upper  Devonian  shales  ("  Cypridinen-schiefer  "),  The 
phyllopods,  eurypterids,  and  myriapods  appear  chiefly  in  the  Old  Red 
Sandstone,  and  are  noticed  on  p.  794  (Fig.  348,  a,  c,  d). 

Among  the  mollusca  of  the  Devonian  rocks  remains  of  the  pteropod 
TentaculUes  are  not  imcommon.  The  brachiopods  (Fig.  349)  now  reached 
perhaps  their  maximum  development^  whether  as  regards  individual  abund- 
ance or  number  of  specific  and  generic  forms ;  more  than  60  genera  and 
1100  species  having  been  described.  They  compose  three-fourths  of  the 
known  Devonian  fauna.  While  all  the  families  of  the  class  are  represented, 
the  most  abundant  are  the  SpiriferidsBy  including  the  genera  Spirifer 
(especially  broad-winged  species),  Cyrtiay  Aihyris  {Spirigera)y  UnciteSy 
and  Atrypa  {A,  retiaUaris  ranging  from  the  Upper  Silurian  through  the 
Devonian  system),  and  the  Bhyrtchondlid^  {BhynchonelUiy  Camarophoriay  Pen- 
tamerus).  The  Strophomenids  or  Orthids,  so  abundant  in  the  Silurian  rocks, 
are  now  represented  by  a  waning  number  of  forms,  including  the  genera 
OrthiSy  Strophomenay  StreptorhynchuSy  and  Leptama,  The  Productids  made 
their  appearance  in  Silurian  times,  but  were  more  abundant  in  the  De- 
vonian seas,  where  their  most  frequent  genera  were  Chonetes  and  Pro- 
ductiiSy  both  of  which  attained  their  maximum  development  in  the 
Carboniferous  period.  One  of  the  most  characteristic  and  largest 
Devonian  brachiopods  is  Stringocephalus — a  genus  allied  to  Terebratulay 
but  entirely  confined  to  this  geological  system  (Fig.  349,  a).  Another 
characteristic  terebratula-like  form  is  Rensseleria, 

The  known  Devonian  lamellibranchs  belong  chieflv  to  the  genera 
PUnneay  CardioUiy  Megalodoriy  GrammysiOy  Cucullasay  CurUmotuSy  LadTuiy  and 
Avicidopecien ;  Pterinea  being  specially  abundant  in  the  lower,  Cucullssa 
and  Curtonoias  in  the  upper  subdivision  of  the  system.  The  most  im- 
portant genera  of  gasteropods  are  EuomphdltLSy  Murchisaniay  Loxonerruiy 
MacrocheiluSy  Acroculia  (Capulus)  and  Pleurotomariay  with  the  heteropods 
Bellerophon  and  Porcellia.  The  cephalopods  embrace  representatives  of 
both  the  tetrabranchiate  families  of  Nautilids  and  Ammonitids.  Among 
'  the  Nautilids  are  the  genera  Clymeniay  an  especially  abimdant  form  in  some 
of  the  Upper  Devonian  shales  and  limestones,  GyroceraSy  OrthoceraSy  Cyrlo- 
ceraSy  HercoceraSy  and  Gcmphoceras*  The  great  family  of  the  Ammonites 
had,  in  the  Devonian  waters,  representatives  of  the  more  abundant  coiled 
^  forms,  in  the  characteristic  genus  GoniaiiUSy  and  of  the  straight  forms,  in 


788 


STBATIGRAPHWAL  GEOLOGY  sooK  Ti  part  n 


Badrites.  Other  Devonian  genera  are  Anarcedes,  Apft^ites,  Jidecerm, 
Gtphyroctras,  Mimoceras,  PinaeUei,  ProlecaniUs,  Sporadocerai,  and  Tonwcerag. 
In  the  Devonian  rocks  of  Central  Europe,  scanty  remains  of  the  great 
fish  fauna  of  the  Old  Red  Sandstone  have  been  found,  more  eqiecioUj  in 
the  Eifel,  but  seldom  in  such  a  state  of  preservation  as  to  warrant  ^eir 
being  assigned  to  any  definite  place  in  the  zoological  scale.  Professor 
Beyrich  has  described  from  Gerolstein  in  the  Eifol  an  undoubted  speciefl 


.■.;/i,A  MpRBlod, 


of  Pleriddhijs,  which,  as  it  canDot  be  certainly  identified  with  any  known 
form,  he  has  named  P.  rheitanws.  A  Coccosteus  has  been  described  by  F. 
A,  Roemer  from  the  Harz,  and  more  recently  one  has  been  cited  from 
Bicken  near  Herbom  by  Von  Koenen  ;  but,  as  Beyrich  points  out,  there 
may  be  some  doubt  as  to  whether  the  latter  is  not  a  Plerichthys.^ 
Ctenacanlhtts,  seemingly  undistinguishable  from  the  C.  '  ' 
'  ZtitKh.  DfvtKh.  Gml.  OatU.  xiix.  751. 


SECT,  iil  L  §  2  DEVONIAN  SYSTEM  783 

Barrande's  £tage  G,  has  also  been  obtained  from  the  Lower  Devonian 
"  Nereitenschichten  "  of  Thuringia.^  Two  sharks  (FcUsedaphus  dewniensis 
and  Byssacanihus  Gosseleti)  have  been  obtained  from  the  Belgian  and 
north  of  France  area.  The  characteristic  HoloptycJiius  nobilissimus  has 
been  detected  in  the  Psammite  de  Condroz,  which  in  Belgium  forms 
a  characteristic  sandy  portion  of  the  Upper  Devonian  rocks.  These 
are  interesting  facts,  as  helping  to  link  the  Devonian  and  Old  Eed  Sand- 
stone types  together.  But  they  are  as  yet  too  few  and  unsupported  to 
warrant  any  large  deduction  as  to  stratigraphical  correlations  between 
these  types.     The  fishes  of  the  Old  Red  Sandstone  *are  noticed  on  p.  796. 


§  2.  Local   Development. 

Britain.^ — The  name  "  Devonian  "  was  first  applied  by  Sedgwick  and  Murchison  to 
the  rocks  of  North  and  South  Devon  and  Cornwall,  whence  a  suite  of  fossils  was  obtained 
which  Lonsdale  pronounced  to  be  intermediate  in  character  between  Silurian  and  Car- 
boniferous. The  passage  of  these  strata  into  Silurian  rocks  has  not  been  satisfactorily 
determined,'  but  they  clearly  graduate  upward  into  Carboniferous  strata.  Considerable 
difference  exists  between  the  development  of  the  Devonian  rocks  in  the  north  and  south 
of  Devonshire.  In  the  former  area  they  consist  of  sandy  and  muddy  materials  in  the 
form  of  sandstones,  grits,  and  slates.  In  South  Devonshire  on  the  other  hand  they 
include  thick  masses  of  limestone  and  abundant  volcanic  intercalations  in  the  form  of 
tuffs  (schalstein)  and  lavas  (diabase,  &c. )  With  these  lithological  contrasts  there  is  a 
corresponding  difference  in  the  abundance  and  variety  of  organic  remains,  the  calcareous 
rocks  of  Plymouth  and  Torquay  being  the  chief  rejMJsitories  of  fossils.  Yet  even  at  the 
best  the  Devonian  rocks  of  this  classical  region,  though  they  served  as  the  type  formations 
of  the  same  geological  age  elsewhere,  are  much  less  clearly  and  fully  developed  than  those 
of  the  Rhine  country  and  other  parts  of  the  continent.  It  is  rather  from  the  sections 
and  fossil  collections  of  central  Europe  than  from  those  of  England  that  the  stratigraphy 
and  palaeontology  of  the  Devonian  system  are  to  be  determined. 

This  system  has  long  been  grouped  into  three  divisions,  each  more  or  less  distinctly 
marked  off  by  its  pala?outological  characters.  In  Devonshire  and  West  Somerset  these 
divisions  are  arranged  as  follows  : — 

1  Op.  cit,  423. 

3  Sedgwick  and  Murchison,  Trans,  Oeol.  Soc.  2nd  ser.  v.  p.  633.  Sedgwick,  Q,  J.  Oeol, 
Soc  viii..  p.  1.  Lonsdale,  Proc,  Oeol,  Soc,  iii.  p.  281.  R.  A.  Godwin- Austen,  Trans, 
Oeol.  Soc.  (2)  vi.  p.  433.  J.  W.  Salter,  Q.  J.  Oeol.  Soc.  xix.  p.  474.  T.  M.  Hall,  op.  cit. 
xxiii.  p.  371.  Etheridge,  Q.  J.  Oeol.  Soc.  xxiii.  (1867)  568,  where  a  copious  bibliography 
up  to  date  will  bejound  ;  also  op.  cit.  xxxviL  Address,  p.  178.  A.  Champemowne  and  W. 
A.  K  Ussher,  Q.  J.  Oeol.  Soc.  1879,  p.  682.  A.  Champemowne,  op.  cit.  1889,  p.  369. 
W.  A.  K  Ussher,  Oeol.  Mag.  1881,  p.  441,  Quart.  J&tim.  Oeol.  Soc.  1890,  p.  487.  R 
Kayser,  yeues  Jahrb.  1889,  i.  p.  189.  The  Devonian  rocks  of  Cornwall  and  Devon  have 
undeigone  much  crumpling  and  have  suffered  considerable  metamorphism.  Their  fossils  are 
often  singularly  distorted,  and  mica  has  been  almost  ever}' where  abundantly  developed  in 
their  argillaceous  and  calcareous  portions.  Much  of  the  so-called  ''slate"  or  "killas"  of 
these  districts  is  a  lustrous  phyllite.  On  distortion  of  the  fossils,  see  D.  Sharpe,  Q.  J. 
OeoL  Soc.  iiL 

'  The  recent  discovery  by  Mr.  Fox  and  Mr.  Teall  of  radiolarian  cherts  at  the  Lizard  in 
Cornwall,  and  the  tracing  of  these  cherts  eastward  into  the  Silurian  tract  of  Gorran  may 
famish  a  base-line  for  determining  the  relations  of  Silurian  and  Devonian  rocks  in  the 
south-west  of  England. 


784 


STKATIGRAPHICAL  GEOLOGY 


BOOK  VI  PART  n 


M 

Oi 


Northern  Type 

'  Pilton  group.     Slates  and  grits  with 

calcareous  se&ma  {Spiir\fer  Vemeuili^ 

Athyris      concentrica,     Produclits 

prmlonguSy  &c. 
Baggy  group.    Sandstones  with  Cucul- 

Iwaf  Blates  with  lAwjula^  Discina. 
Pickwell  -  Down  group.     Red,  green, 

grey,  and  purple  slates  and  grits, 

generally  uufossiliferous. 
Morte  slates,  uufossiliferous,  passing 

down  into  the  slates  below. 


Q 

Q 

i-i 


f  Ilfracombe  slates  ;  grey  silvery  slates 
with  lenticular  impure  fossiliferous 
limestone,  resting  on  grits  and  slates 
of  Combe  Martin  {Cyathophyllum 
cespitosuiiif  &c.) 


r  Hangman  grits   and  slates   {NaticUy 
.   I      Myalinu). 

M  J  Lynton  group,  grits  and  calcareous 
§  J  slates  {Spirtfer  hystericusy  Chonetes 
^   I      sarcinalatiis,  &c.) 

L  Foreland  grits  and  slates. 


Southern  Type. 

Slates  near  Ashburton  with  Spir\fer 
Vemeuiliy  &c. 

Slates  of  Livaton  with  Clymenia, 

Bed  and  green  slates  with  Potidonia 
venusta  and  abundant  EntomtM 
(Cypridina)  serratostritUa  (  =  Cyp- 
ridinen-schiefer). 

Red  and  grey  slates  with  Yolcanic 
tuffs. 

Chudleigh  limestone  with  QoniatiUs 
intunuscensy  O,  lobattiSt  O,  acutus, 
G.  simpleXy  Cardiola  re^rostriatOy 
HhynehoneUa  cuboides,  R,  cicu- 
minatay  Atrypa  reticutaris,  ^ir\fer 
bifiduSf  Productus  subcuntleatus,  &c. 

Torquay  and  Plymouth  limestones 
passing  laterally  into  slates  and 
volcanic  rocks  {Stringocephaltts 
Burtiniy  UncUes  gryphiut,  FavogiUs 
polymorpha,  &c.) 

Slates  and  limestones  of  Hope's  Noae 
{Atrypa  reticularis^  Kayaeria  lens^ 
Spirtfer  speciosus,  S.  eurtxUus^ 
Rhynchonelia  procuboides,  he.  = 
Calceola  beds). 

Slates  and  greywackes  (Cookington, 
Warberry,  Meadfoot)  with  Pleuro- 
dictyum  problematicum,  Homalo- 
notusy  Spir\fer  cuUrijugaiua^  S. 
hystericusy  Pterinea  cosUUay  &c. 


Lower. — The  clay-slate  of  Looe,  Cornwall,  has  yielded  a  species  of  PlercispiSy  also 
PUurodictyum  problemalieicm.  The  lower  gritty  slates  and  limestone  bands  of  North 
Devon  contain,  among  other  fossils,  Favosites  (Pachypora)  cervicorniSy  Cyathophyllum 
hdiantJmdeSy  Pctraia  celtiaiy  Pleurodictyuni  probleniaticinrij  Cyathocrinus  (two  species), 
Hoinalonotus  (two  siMJcies),  Phacops  IticiniatiiSy  Feiicstella  anliquay  Atrypa  reticulariSy 
Orthis  arcuaUty  Spirifcr  caiialiferuSy  S.  lasvicostuSy  Pterinea  spinoaay  &c.  The  recent 
researches  of  Mr.  Ussher  and  Professor  Kayser  have  brought  the  Lower  Devonian  rocks  of 
South  Devon  into  closer  palteontological  relations  with  their  equivalents  on  the  continent. 
Among  the  species  noted  by  these  observers  are — PUurodictyum  problemaiicum,  Spirifer 
hystcriciiSy  S.  paradoxus,  S.  macropterus,  S.  cultrijugatuSy  Strophomena  rhomhoidaliSy 
Ithyiichonella  daUid^nsiSy  Civonetcs  sardnulatay  C.  semiradiatay  Pterinea  costatUy 
Honialonotiis  gignSy — an  assemblage  which  resembles  that  in  the  Coblenzian  stage  of 
Rhineland. 

Middle. — It  is  in  this  division  that  limestones  are  best  developed  and  fossils 
are  most  abundant.  Some  of  the  limestones  of  South  Devon  are  made  up  of  corals,  and 
from  their  lenticular  or  sporadic  occurrence  suggest  that  they  were  accumulated  as  reefis. 
Large  masses  of  limestone  rapidly  die  out  laterally  and  are  replaced  by  slates.  In  the 
Ashprington  district  a  thick  group  of  volcanic  rocks  consisting  of  breccias  and  tufis 
(schalstein)  and  diabasic  lavas  appears  entirely  to  take  the  place  of  the  limestones. 
These  volcanic  ejections  are  traceable  for  many  miles,  sometimes  dwindling  down  and 
giving  j)lace  to  limestones  or  slates,  and  again  swelling  out  into  considerable  masses.^ 
They  appear  to  have  been  discharged  from  numerous  small  vents  across  the  area  of  south 
Devonshire,  but  no  trace  of  any  similar  material  has  yet  been  detected  in  the  northern 
part  of  the  county. 


Champeruowne  ou  the  Ashprington  Volcanic  Series,  Quart,  Joum,  OeoL  Soc,  1889,  p.  869. 


SECT.  iii.  I.  §  2  DEVONIAN  SYSTEM  785 

The  palfleontological  evidence  makes  it  abundantly  clear  that  the  limestones  of 
Torquay  and  Plymouth  represent  the  great  Middle  Devonian  limestones  of  France, 
Belgium,  and  Germany — the  Calcaire  de  Givet,  and  the  Stringocephalen-Kalk  and 
Calceola-Kalk  of  the  Eifel.  Near  Torquay  shaly  limestones  occur  containing  fossils 
that  place  them  on  the  horizon  of  the  Eifelian  group  or  the  Calceola  beds  of  the  continent, 
that  is,  the  lower  division  of  the  Middle  Devonian  rocks.  Among  these  fossils  are 
Atrypa  reticulariSy  A,  asperaj  A,  desqtuimatay  Kayseria  lauty  LepUena  interstricUiSf 
PeiUamerus  galecUxiSy  Rhynchonella  procuboides,  Spiri/er  curvatus,  S,  speeiosuSy  Strepto- 
rhyiichiis  umbraculum,  Productus  subaculcatuSy  Phacops  latifronSy  Cydthophyllum 
heterophylltiniy  C.  damnonienae,  C,  heliarUhoideSy  CyUiphyllum  vesiculoaum,  CaXceola 
sandalinay  FavosiUs  Goldfussiy  HelioliUs  poroaay  StronuUopora  eoneentrica.  The  massive 
limestones  yield  the  characteristic  fauna  of  the  Givet  or  Stringooephalus  limestone 
including  the  corals  CyoUhophyllum  helianthoidcs,  C,  dam^wnitnaCy  CystiphyUum 
vesiculosumy  AlveoliteSy  FavosUes  polymorphay  Striatopora  derUiculatay  Amphipora 
ramosay  Heliolites  porosa,  FavosUes  Ooldfussiy  Stromatopora,  JUceplaculites  Neptuniy 
Stringocephalus  Burtiniy  Utieites  gryphuSy  Terebratula  Whidborneiy  T.  juveniSy  Cyrtina 
heUroclUay  Spirifer  undiferuSy  Hhynchotullaparallelopipcda,  R.procuhoideSy  R,  pugnuSy 
R.  lummatonieHsiSy  Penlatnerus  brevirostriSy  Leptmna  irUerstrialis,  Productus  sub- 
aculccUiiSy  Cypricardinitty  ProHtuSy  BronUuSy  &c.^ 

Upper. — In  South  Devon  Upper  Devonian  rocks  are  now  known  to  be  well 
developed  and  to  present  palteontological  representatives  of  the  several  zones  which  have 
been  established  in  this  division  on  the  continent.  Three  such  zones  have  been  recognised. 
Ist,  Massive  limestones  which  pass  down  continuously  into  those  of  Middle  Devonian 
age.  They  contain  Rhyncfumella  citboideSy  R.  acumiruUay  Atrypa  reticularis,  Athyris  eon' 
cetUrica,  Spirifer  bifiduSy  S,  lineaiuSy  Productus  subaculecUus,  IValdheimia  Whidborneiy 
Meristaplebeiay  CoTwcardiuMy  HarpeSy  Stromalopora  Hilpschiiy  Actinostroma cUUhratum{'{) 
kc.  2ud,  Goniatite  beds  which,  overlying  and  passing  down  into  the  limestones,  are  marked 
by  the  presence  of  numerous  goniatites  {O.  iHtuuieseens,  O.  co$nplanaluSy  G.  viultilobalusy  O. 
aciUuSy  G.  simplex),  with  Cardiola  retrostriatay  Myalina  sp.,  Sanguinolaria,  BacirilcSy 
Alveolites,  3rd,  Cypridina-slates,  containing  ostracods  {Entomis  or  Cypridina  seirato- 
striata)  and  Clymenias  (C.  laevigata  and  other  species).  These  three  zones  may  be 
paralleled  re8|)ectively  with  the  Frasnien  and  Fammenien  group  of  the  Franco- Belgian 
area  and  with  the  Goniatite  (Adorf,  Iberg)  limestone,  Cypridina  slates  and  Clynienia 
limestone  of  the  Eifel  and  Rhine. 

In  North  Devon  this  palseontological  grouping  has  not  been  so  satisfactorily  made  out ; 
but  in  that  region  there  is  an  insensible  gradation  upwards  through  various  sandy  and 
muddy  sediments  into  the  Culm  or  Carboniferous  system.  The  micaceous  flaggy  sandstones 
of  Baggy  Point  contain  Cueullasa  trapezium,  C.  I/ardingiiy  Avicula  damnonieiisiSy  Liiigula 
squami/omiis,  Discituiy  Rhynchonella  lalicostay  Strophalosia  productoides,  Spirifer 
disjundusy  &e.  The  greenish  slates  and  calcareous  bands  of  Pilton  near  Barnstaple  have 
yielded  some  characteristic  fossils  of  the  uppermost  part  of  the  Devonian  system,  such 
sis  Pctraia  celticay  Cyathocrinus  pinnatuSy  Spirifer  Vemeuiliy  Athyris  eoneentrica y  Strep- 
torhynchics  crenislriay  Productus  pralongus,  Strophalosia  productoideSy  S.  eaperatay 
Jlhy}ichonellii  pleurodony  and  Chonetes  hardrensis.  Remains  of  land-plants  are  found  in 
the  Upper  Devonian  rocks  of  North  Devon  {Sagenaria  (Knorria)  veWicimianay  ArchsBo- 
pteris  {PalsRopteris)  hibernica).  The  higher  red  and  yellow  sandy  portions  of  these  rocks 
shade  up  insensibly  at  Barnstaple  in  North  Devon  into  strata  which  by  their  fossils  are 
placed  at  the  base  of  the  Carboniferous  Limestone  series.  But  in  no  other  British 
locality  save  in  Devonshire  can  such  a  passage  be  observed.  In  all  other  places,  the 
Carboniferous  system,  where  its  true  base  can  be  seen,  passes  down  into  the  red  sandy 
and  marly  strata  of  the  Up[)er  Old  Red  Sandstone  without  marine  fossils. 

Central  Europe. — A  large  tract  of  Devonian  rocks  extends  across  the  heart  of  Europe 

*  Ussher,  (^uart.  Joum.  Oeol.  Soc.  1890,  p.  501.    R  Kayser,  NeuesJahrb,  1889,  i.  p.l86. 

3  £ 


786 


STRATIQRAPHICAL  GEOLOGY 


BOOK  TI  PART  U 


from  the  north  of  France  through  the  Ardennes,  the  south  of  Belgium,  Rhenish 
Prussia,  Westphalia  and  Nassau.  But  that  the  same  rocks  have  a  much  wider  spread 
under  younger  formations  which  cover  them  is  sho^vn  by  their  reappearance  far  to  the 
west  in  Brittany,^  and  to  the  east  in  the  Harz  and  the  Thuringer  Wald.  They  present 
a  much  clearer  sequence  of  strata  than  their  British  equivalents,  for  they  can  be  seen 
in  manyfplaces  to  pass  down  into  Silurian  strata  as  well  as  to  graduate  upward  into  the 
Carboniferous  system.  In  the  Belgian  and  Eifelian  tracts  they  have  been  subdivided  as 
under : — 

Belgium  Mid  the  North  of  France.  Rhl&eland.' 


K 

a,  J 

5  ' 


^Famennien,  consisting  of  two  facies,  one 
sandy,  the  other  shaly. 
(b)  Psaumites  du  Condros  (CondnisienX  in 
which  six  zones  are  distini^ished  (Cuctd- 
laea  Ilurdingii,  Spiri/er  Kcmeai/i,  Rhyn- 
chonella  Dumonti,  Orthiscrenistria,  Phaocrps 
lati/rons,  ArchafopterU  hibtmica,  Sphen- 
opterU  JIaccidaj  &c. 

(a)  Schistes  de  Famenne,  divisible  into  four 
xones  (1)  that  of  i>pirifer  disUins,  (2)  of 
JthynchoiuUa  letiensi*,  (8)  of  Rhynchonella 
Dutiumti,  (4)  of  Rhynchonella  Omaliusi. 

Frasnien,  varying  in  composition  and  organic 
contents  in  different  parts  of  the  Devonian 
basins.     In  the  Diuant  basin  it  consists  of 

(b)  HchiBtemic  M&taigne(GoniatitesretrorsuSf 
Card ium pal inat-um,  Camarophoria  tutnida^ 
Bact rites  subconicus,  Entomis  [Cypridina] 
serralo -striata). 

(a)  Calcaires  et  schistes  de  Frasne,  shales 
and  lenticular  limestones,  sometimes  of 
great  thickness,  with  abundant  fMsils 
(Bron teusjtabelli/er,  Goniatitei intumescenSy 
Spinfer  Vertieuili,  Sp.  pachyrhyn^us^  Sp. 
orbelianua,  Spirigera  coiuxntrioa^'^'iAJSnpa 
reticularis,  Rhynchoiulla  cnboide*,  Pemn; 
merus  brevirostris,  Camaropfioria  formotfa , 
\.  Receptaculites  Neptuni). 

^Givetien. — The  great  limestone  of  the  middle 
Devonian  series,  well  seen  at  Givet,  above 
Dinant  on  the  Meuse,  400  metres  thick. 
Among  the  abundant  characteristic  fossils 
are  Spiri/ei'  mediottxtns,  Sp.  undi/crtm.  String- 
ocephalns  Burtini,  Uncitcs  gryphus,  Megalodon 
cucullatus,  Murchisonia  corotuUa,  M.  bilineatu, 
Cyathophylluvi  quadriijemiunin,  Heliolites 
porosa. 

In  the  basin  of  Namur  the  conglomerate  of 
Pairy-Bony  lies  bolow  the  limesti^me,  and  con- 
tains a  biind  of  sandstone  with  plants  (Lepido- 
dendron  ga^p Uinum). 
Eif61ien,  Shales  (Schistes  de  Couvin),  with 
Calceola  sandolina,  Pliactyps  latifrons,  Bronteiis 
Jlnbelli/er,  Spiri/mr  curvatti.t,  Sp.  aubcusjndaiui, 
Sp.  ehgans,  Spirigera  conrentrion,  Pentamenis 
galratics,  Stnyphalosia  productoides,  &c. 


Younger  group  of  Cypridina  shales,  with  En- 
tmnis  (Cypridina)  srrra/aMriato,  A  vicHla  (Posi- 
donia)  ventunta^  Phacopt  eryptopkthalmtUf  and 
limestones  (Kramenzelkalk)  with  numerous 
Clymeni&s  (C.  Isevigata,  C.  undvkUa,  C.  striata^ 
&c.)j  and  Goniatites. 


Brachiopod  limestone  directly  OTerlying  the 
Middle  Devonian  limestone,  and  containing 
Rhynchonella  cuboides,  R.  pugntu,  R,  oevm- 
tnata,  Spiri/er  Vemeuili,  Camarophoria  /or- 
mom.,  Prodtictus svJbac^eatvM^Goniatite* iniuwt- 
eifcens.  Iberg  limestone  of  Hiux,  Adoif  lime- 
stone of  Waldeck,  shales  of  BUdesheini  in  the 
Eifel,  with  Goniatites  intumesoens,  Rhynekon- 
ella  cuboides,  and  Cardiola  retroatriata.  The 
prevalence  of  this  Rhynchonella  has  led  to 
the  group  being  called  the  "Cuboides  beds,** 
and  the  Goniatite  has  given  the  name  of, 
"  Intumescens  beds." 


(b)  Stringocephalus  group,  consisting  of  the 
great  Eifel  limejitone  with  underlying  crinoidal 
be<ls  (Stringocephalus  Burtini,  UncitesgryphuSy 
Spiri/rrundntus,  Productrtssubaculeatus, Penta- 
wienw  goleatus,  Atrypartticvlaris,  Macrockeilv* 
aratlutus,  Pleurotomaria  delphinuloides^  Mvr- 
chisoniii  bilineata,  Megalodon  cuciUlalum,  and 
many  corals  and  crinoids). 

(a)  Calceola  group,  —  marly  limestones  with 
Athyris  concentrioa,  Camarophoria  micro- 
rhyticha,  Atrypa  reticularis,  Merita  pldftia, 
Spiri/er  speciosus,  S.  curvatus,  Pentamtrus 
gaUatns,  Rhynchonella  parallelopipedaj  Orthis 
strintula,  Calceola  sanaalina,  Cf^hophyllum 
heliantoides,  Cystiphyllum  vesiculosum,  aeluh 
Hies  porosa,  Alveolites,  Favosite*,  Stromato- 
jtora,  Phacops  Schlotheimi,  Ac,  resting  upon 
impure  shaly  ferruginous  limestone  and  grey- 
wacke,  marked  by  an  abundance  of  Spiri/er 
cultrijugatvs,  Rhynchonellaorbignyanaf  Atrypa 
reticularis,  Phacops  lati/rons,  &c. 


^  A  ridge  of  Devonian  rocks  stretches  eastward  under  the  south  of  England  (where  its 
existence  has  been  proved  by  well-borings  at  London),  and  no  doubt  joins  the  Devonian 
area  of  the  Boulonuais. 

-  See  especially  Gosselet's  *  Esquisse  G^ologique, '  and  his  great  memoir  on  the  Ardennes 
already  cited. 

'  See  the  series  of  elaborate  papers  by  E.  Kayser  in  the  Zeitschrift  Deutsch,  Qeol,  Oesdl. 
vols.  xxii.  (1870)  to  xxxiii.  (1881),  and  Abhand.  Qeol.  Specialkarte  Preusaen^  Band  II.  Heft 
4,  1878  ;  Jahrh,  Preuss.  Ge<d.  Landesanst.  1881,  and  subsequent  volumes.  Prof.  F.  von 
Saudberger  has  published  a  valuable  paper,  *  Ueber  die  Entwickelung  der  unteren  Abtbeilung 
des  Devoniscben  Systems  in  Nassau,'  Wiesbaden,  1889,  in  which  he  compares  the  formations 
with  those  of  other  countries. 


2] 


SECT.  HI  I.  §  2  DEVONIAN  SYSTEM  787 

Bdginm  and  Um  North  of  Pnunc*.  Rhiii«I«iid. 

^Coblenzien,  compoHed  of  grey wacke,  sand-  Coblenz  group  (Spirifer  sandstone) divisible  into 

stones,  shales,  and  conglomerate,  having  a  the  three  following  sub-groups : — 

united  thickness  of  sometimes  7000  or  800O  (c)  Upper  greywacke  and  slate  (Coblenz, 

feet,  and  divisible  into  five  sub-groups  as  Ems,  Daleiden)   with    (XeHOcrinMs  deea- 

under  : —  dactylus^  Spiryfer  auHcuicUiM,  S.  curvatutt 

5.  Greywacke  of  Hierges  with  S.  panidoxus,  Atrypa  reticularU,  Chonetes 

(b)  Zone  of  Spirifer  eultriJvgatuSf  CcUeeola  dilatata,    Homalonotus    Uevioavda,    Cry- 

aandaliiM.  phanu  laciniatus. 

(a)  Zone  of  Spirifer  arduennertsUy  Pterinea  (fe)  Coblenz   quartzite    probably    on    the 

Uneata.  horizon  of  the  Bumot  conglomerate  in 

4.  Red  slates  of  V^ireux  and  conglomerate  of  the  EifeL 

Bumot.               y  (a)  Greywacke   with  Stropovuna   laUoosta, 

3.  BUck  sandstone  Iff  Vireux  (AhrienX  Orthia  circulariSy  Spirifer  dunmsis,  Homa- 

2.  Greywacke  of   Xontigny    with  Spirifer  Umotus  ornottu,  H.  crastioauda. 

paradoxujt,  Atl^'ris  undaia^  Strophomena  Slates    (Hundsriick,    Taunns)  with    numerous 

depreaaa.  (HuiidsriickienX  trilobites  (Uomalonottu  ornatuSf  Phacopa  Fer- 

1.  Sandstone  of  Anor  (Taunusien).  dinandi,   Crypkaeus,  Dalmanites,    Orthocenu^ 

Gedinnien,   comprising   an  upper   group   of  Goniatite*,  he.) 

shales  and  sandi^tones  and  a  lower  group  of  Taunus  quartzite,  Siegen  grauwacke  {Spirifer 

fossilif«rous  shales,  quartzo-phylUideH,  quartz-  pHmtevut,  S.  hystericus,  Rensseltria,  kc.) 

ites,  and  conglomerates.    The  fossils  in  the  Sandstones,   sUites,  phyllites,  arkoses,  ending 

lower  group  comprise  Jkdmaniies,  Homalo-  downwards  in  conglomerates. 
iwtus  Koenuri,  Primitia  Jonetii,  TentaeuliUs 
grandis,  T.  irregularis,  Spirifer  Mercuri,  Orthis 
Verneuili,  Pterinea  ovatis,  6lc.  The  base  of 
the  Devonian  system  lies  unconformably  on 
Cambrian  rocks. 

In  the  I^a  r  z,  according  to  the  researches  of  F.  Roemer  ^  and  K.  A.  Lossen,'  the 
Devonian  system,  which  is  there  largely  developed,  consists  of  (1)  a  lower  group  of 
quartzites,  ^grey  wackes,  flinty  slates,  clay -slates,  and  associated  bands  of  diabase  (Taunus 
quartzite,  Hundsriick  shales,  &c.,  resting  upon  the  graptolitio  Wieda  shales  and  Tanne 
greywacke  ;  (2)  a  middle  group  jsomposed  of  (a)  Calceola-beds  {Spirifer  cuUrijugcUvs, 
Calceola  sandalina)  and  {h)  Stringocephalus  limestone  (consisting  of  a  lower  crinoidal 
band  and  a  massive  limestone  ;  and  (3)  an  upper  group  consisting  of  (a)  Cuboides-beds, 
limestones  and  marls,  (6)  Goniatite  shales,  (c)  Cypridina  shales.  The  eastern  part  of  the 
region  consists  mainly  of  greywackes  and  slates  which,  with  their  associated  igneous  rocks 
attaining  a  great  thickness  in  the  Wieda  slates,  contain  a  number  of  simple  graptolites 
and  in  the  limestones  underneath  yield  abimdant  trilobites  belonging  to  genera  familiar 
in  the  Upper  Silurian  rocks  {Dalmanites,  CryphxuSy  Phacops,  Bronteus,  Acidaspis). 

Representatives  of  the  Devonian  system  reappear  with  local  petrographical  modifica- 
tions, but  with  a  remarkable  persistence  of  general  palseontological  characters,  in 
Extern  Thuringia,  Franconia,  Saxony,  Silesia,  the  north  of  Moravia,  and  East  Gallicia. 
Among  the  crumpled  formations  of  the  Styrian  Alps,  the  evidence  of  organic 
remains  has  revealed  the  presence  of  Upper  Devonian  rocks  with  abundant  Clymenias, 
Middle  Devonian  limestones  with  the  characteristic  StringocephcUus  and  numerous 
corals,  and  Lower  limestones  and  slates  with  cephalopods  and  brachiopods.'  Perhaps 
in  other  tracts  of  the  AIjm,  as  well  ad  in  the  Carpathian  range,  similar  shales,  lime- 
stones, and  dolomites,  though  as  yet  unfossiliferous,  but  containing  ores  of  silver,  lead, 
mercury,  zinc,  cobalt,  and  other  metals,  may  be  referable  to  the  Devonian  system. 

To  the  west  of  central  Europe  the  system  has  been  recognised  by  its  fossils  in 
the  Boulonnais,  where  its  middle  and  upper  members  (Givetian,  Frasnian,  Famennian) 
are  well  ex]K)8ed.  In  Normandy  and  Maine,  sandstones  (with  Orthis  Monnieri), 
are  followed  by  limestones  (with  ffomalonotuSy  Cryphm%is^  Pha/copSt  &c),  and  by  upper 
greywackes  and  shales  (with  Phurodiclyum  probJematicum),*  In  Brittany  also, 
Devonian  strata  are  found,   including  representatives  of  the  Famennian  groups  with 

^  '  Versteinemngen  des  Harzgebirges,'  1843  ;  *Rheinisch.  Uebergangsgebirge, '  1844. 
«  *Geologisch.  Uebersichtskarte  Harz,*  1881. 

'  G.  Stache,  ZeiUch,  Deutsch,  OeoL  Ges,  1884,  p.  358  ;  Freeh,  op.  cit.  1887,  p.  660 
(and  authors  there  cited) ;  1891,  p.  672. 

*  Oehlert,  Bull.  Soc,  OSol.  France^  xvii.  (1889)  p.  742. 


788  STRATIGRAPHICAL  GEOLOGY  book  vi  part  n 


Cypridinas  and  Goniatitos,  shales  and  limestones  with  Eifelian  cephalopods,  JHewro- 
diclyum  problematiaim  and  Spirifer  cuUrijtigatuSj  and  a  series  of  greywackes,  sandstonfis, 
and  shales  with  Chondet  aarcinuUUa,  Phaeops  lati/rons,  kc.^  In  this  region  lies  the 
limestone  of  Erbray  (Loire  Inf^rieure)  so  fully  described  by  Barrois  who,  from  its 
abundant  corals,  numerous  brachiopods  and  gasteropods,  and  its  trilobites  of  the  genera 
Calyynene,  Phacopa,  DcUmanites,  Proetus^  ffarpes,  BronUiis  and  Cheirurua^  places  it  in  the 
Gedinnian  group  at  the  base  of  the  Lower  Devonian  series,  and  compares  it  with  the 
Hercynian  limestones  of  the  Harz.'  In  the  remarkable  oasis  of  ancient  rocks  which 
has  been  already  referred  to  as  forming  a  conspicuous  feature  among  the  younger 
formations  ofLanguedoc  representatives  of  the  three  great  divisions  of  the  I>evonian 
system  have  recently  been  worked  out  by  F.  Freeh. ^  Again,  the  central  Silurian  zone  of 
the  Pyreneesis  flanked  on  the  north  and  south  by  bands  of  Devonian  rocks  (with 
broad-winged  spirifers  and  other  characteristic  fossils),  which  have  been  greatly  disturbed 
and  altered.  In  the  Asturias,  according  to  Barrois,  a  mass  of  strata  about  3280  feet 
thick  contains  representatives  of  the  three  divisions  of  the  Devonian  series,  and  has 
yielded  an  abundant  fauna,  numbering  upwards  of  180  species,  among  which  the  corals 
and  brachiopods  are  specially  abundant.^ 

Throughout  central  Euroi)e  there  occurs,  in  many  parts  of  the  Devonian  areas, 
evidence  of  contemporaneous  volcanic  action  in  the  form  of  intercalated  beds  of  diabase, 
diabase-tuff,  schalstein,  &c.  These  rocks  are  conspicuous  in  the  ''greenstone"  tract  of 
the  Harz,  in  Nassau,  Saxony,  Westphalia,  and  the  Fichtelgebirge.  Here  and  there,  the 
tuff-bands  are  crowded  with  organic  remains.  It  is  also  deserving  of  remark  that  over 
considerable  areas  (Ardennes,  Harz,  Sudeten-Gebirge,  &c.)  the  Devonian  sedimentary 
formations  have  assumed  a  more  or  less  schistose  character,  and  appear  as  quartzo- 
phyllades,  quartzites,  and  other  more  or  less  crystalline  rocks  which  were  at  one  time 
supposed  to  belong  to  the  ''Archsean"  series,  but  in  which  recognisable  Devonian  fossils 
have  been  found  {anUy  p.  619).  At  numerous  places,  also,  they  have  been  invaded 
by  masses  of  granite,  quartz-porphyry,  or  other  eruptive  rocks,  round  which  they  present 
the  characteristic  phenomena  of  contact-metamorphism  (pp.  605,  606).  These  changes 
may  have  led  to  the  subsequent  development  of  the  abundant  mineral  veins  (Devon, 
Cornwall,  Westphalia,  &c. ),  whence  large  quantities  of  iron,  tin,  copper,  and  other  metals 
have  been  obtained. 

Russia. — In  the  north-east  of  Europe  the  Devonian  and  Old  Red  Sandstone  types 
appear  to  be  united,  the  limestones  and  marine  organisms  of  the  one  being  interstratified 
with  the' fish-bearing  sandstones  and  shales  of  the  others.  In  Russia,  as  was  shown  in 
the  great  work  'Russia  and  the  Ural  Mountains,*  by  Murchison,  De  Verneuil  and 
Keyserling,  rocks  intermediate  between  the  Upi>er  Silurian  and  Carboniferous  Limestone 
formations  cover  an  extent  of  surface  larger  than  the  "British  Islands.'  This  wide 
development  arises,  not  from  the  thickness,  but  from  the  undisturbed  horizontal 
character  of  the  strata.  Like  the  Russian  Silurian  deposits,  they  remain  to  this  day 
nearly  as  flat  and  unaltered  as  they  were  originally  laid  down.  Judged  by  mere 
vertical  deptli,  they  present  but  a  meagre  representation  of  the  massive  Devonian 
greywacke  and  limestone  of  Germany,  or  of  the  Old  Red  Sandstone  of  Britain. 
Yet,  vast  as  is  the  area  over  which  they  constitute  the  surface -rock,  it  probably 
forms  only  a  small  |)ortion  of  their  total  extent;  for  they  rise  up  from  under  the 

^  Barrois,  Ann.  Soc.  Gktl.  Nordf  iv.  xvi. 

'^  'Faune  du  Calcaire  d'Erbray,'  Mhn.  Soc,  iUol,  yard,  iii.  (1889). 

3  Zeitsch.  Deutuch.  Geol.  Ges.  xxxix.  (1887)  p.  402. 

*  "  Recherches  sur  les  Terrains  auciens  des  Asturies,"  &c.,  M^m,  Soc.  OSol.  Nord,  ii. 

*  Besides  the  great  work  of  these  three  pioneers  the  student  will  find  much  recent 
information  regarding  Russian  geology  in  the  Mimrires  du  Comiti  Giologique  of  Russia. 
See  for  Devonian  data  T.  Tschemyschew,  vols.  i.  iii.  (a  detailed  memoir  on  the  lower, 
middle,  and  upper  divisions  of  the  system  in  the  Ural  region). 


SECT.  iii.  I.  §  2  DEVONIAN  SYSTEM  789 


newer  formations  along  the  flank  of  the  Ural  chain.  It  would  thus  seem  that  they 
spread  continuously  across  the  whole  breadth  of  Russia  in  Europe.  Though  almost 
everywhere  undisturbed,  they  afford  evidence  of  terrestrial  oscillation  immediately 
previous  to  their  deposition,  for  they  gradually  overlap  Upper  and  Lower  Silurian 
rocks. 

The  chief  interest  of  the  Russian  rocks  of  this  age,  as  was  first  signalised  by  Murchison 
and  his  associates,  lies  in  the  union  of  the  elsewhere  distinct  Devonian  and  Old  Red 
Sandstone  types.  In  some  districts,  these  rocks  consist  largely  of  limestones,  in  others 
of  red  sandstones  and  marls.  In  the  former,  they  present  mollusks  and  other  marine 
organisms  of  known  Devonian  species  ;  in  the  latter,  they  afford  remains  of  fishes,  some 
of  which  are  specifically  identical  with  those  of  the  Old  Red  Sandstone  of  Scotland. 
The  distribution  of  these  two  palseontological  facies  in  Russia  is  traced  by  Murchison 
to  the  lithological  characters  of  the  rocks,  and  consequent  original  diversities  of  physical 
conditions,  rather  than  to  differences  of  age.  Indeed,  cases  occur  where,  in  the  same 
band  of  rock,  Devonian  shells  and  Old  Red  Sandstone  fishes  lie  commingled.  In  the 
belt  of  the  formation  which  extends  southwards  from  Archangel  and  the  White  Sea, 
the  strata  consist  of  sands  and  marls,  and  contain  only  fish  remains.  Traced  through 
the  Baltic  provinces,  they  are  found  to  pass  into  red  and  green  marls,  clays,  thin  lime- 
stones, and  sandstones,  with  beds  of  gypeum.  In  some  of  the  calcareous  bands  such 
fossils  occur  as  Orthis  striatula,  Spiri/erina  prisca,  LepUsna  productoides,  Spirifer 
Anossofiy  S,  Archiacit  S,  Vemeuili,  jRhynehonella  euboides,  Spirorbis  omphaloideit 
and  Orihoceras  suh/'umforme.  The  lower  parts  of  the  series  contain  Osteolepia^  DipteruSj 
DiplopUruSy  and  Asterolepis  {Homoeleua\  while  in  the  higher  beds  Holoptyehius^ 
OlyptosteiiSf  and  other  well-known  fishes  of  the  •  Upper  Old  Red  Sandstone  occur. 
Followed  still  farther  to  the  south,  as  far  as  the  watershed  between  Orel  and  Woronesch, 
the  Devonian  rocks  lose  their  red  colour  and  sandy  character,  and  become  thin-bedded 
yellow  limestones,  and  dolomites  with  soft  green  and  blue  marls.  Traces  of  salt 
deposits  are  indicated  by  occasional  saline  springs.  It  is  evident  that  the  geographical 
conditions  of  this  Russian  area  during  the  Devonian  period  must  have  resembled  those 
of  the  Rhine  basin  and  central  England  during  the  Triassic  period.  There  is  an 
unquestionable  passage  of  the  uppermost  Devonian  rocks  of  Russia  into  the  base  of  the 
Carboniferous  system,  but  a  complete  break  between  them  and  the  highest  Silurian 
strata.  The  lowest  parts  of  the  British  Old  ^Red  Sandstone,  containing  Pterygotua^ 
CfphalaspiSj  Pterasfns,  &c. ,  are  wanting.  Devonian  rocks  have  been  recognised  in  other 
parts  of  the  vast  Russian  empire,  across  Siberia  in  the  Altai  mountains,  in  Asia  Minor, 
and  in  Africa. 

North  America. — The  Devonian  system,  as  developed  in  the  Northern  States,  and 
eastern  Canada  and  Xovia  Scotia,  presents  much  geological  interest  in  the  union  which 
it  contains  of  the  same  two  distinct  petrographical  and  biological  types  found  in  Europe. 
Traced  along  the  Alleghany  chain,  through  Pennsylvania,  into  New  York,  the  Devonian 
rocks  are  found  to  contain  a  characteristic  suite  of  marine  organisms  comparable  with 
those  of  the  Devonian  system  of  Europe.  But  on  the  eastein  side  of  the  great  range 
of  Silurian  hills  in  the  north-eastern  States,  we  encounter  in  New  Brunswick  and 
Nova  Scotia  a  succession  of  red  and  yellow  sandstones,  limestones,  and  shales  nearly 
devoid  of  marine  organisms,  yet  full  of  land-plants,  and  with  occasional  traces  of  fish 
remains.     The  marine  type  is  well  developed  above  the  Silurian  seriei  in  Nevada. 

The  marine  or  Devonian  type  has  been  grouped  in  the  following  subdivisions  by  the 
geologists  of  New  York  : — 

iCatskill  Red  Sandstone,  with  fish  remains  {Holoptychius). 
Chemung  group  {Spirifer  VerneuUi). 
Portage  group  {Ooniatiies^  CardioUt^  Clymenia). 
Genesee  group  [RhynehoneUn  of.  cuboicUs), 

Middle     f  Hainilton  group  {PhacopSy  Hovicdonotusy  Orypftmtu). 
Devonian.  \  Marcellus  group  {Ooniatites). 


Lower 
Devonian. 


790  STRATIGRAPHICAL  GEOLOGY  book  vi  pabt  n 

Corniferous    litnestoue   {Spir\fer    acuminatus,    &    gregariiu,   Dal- 

manitesj  ProSlus). 
Onondaga  limestone,  Schoharie  grit,  Canda-galli  grit.     (This  and  the 

Corniferous  limestone  are  bracketed  together  as  the  Upper  Helder> 

berg  group. ) 
Oriskany  sandstone  {Spififer  arenosus,  Renssderia  ovoides). 
Lower  Helderberg  group  consisting  of 

(c)  Upper  Pentamerus  limestone  {Pentamerus  p9eyd<hgaleaius\ 

(6)  Delthyris  limestone  {MeristeUa  Isttns), 

(a)  Lower  Pentamerus  limestone  {Pentamerus  gaUaius), 

In  the  Lower  Devonian  series,  traces  of  terrestrial  plants  {Pgihphyton,  CaulopUrig, 
kc)  have  been  detected,  even  as  far  west  as  Ohio.  Corals  (cyathophylloid  fonns,  with 
FavosUeSf  Syringopora^  kQ.)  abound  especially  in  the  Corniferous  Limestone,  perhaps 
the  most  remarkable  mass  of  coral-rock  in  the  American  Palseozoic  series,  and  fnnn 
which  Hall  has  made  a  magnificent  collection  of  specimens.  Among  the  brachiopods 
are  species  of  PentameniSf  Stricklandinia,  Rhynchonella,  and  others,  with  the  charac- 
teristic European  form  Spiri/er  cultrijugaius^  and  the  world-wide  Atrypa  rtHetdaru. 
The  trilobites  include  the  genera  DalmaniteSy  Pro^tnSy  and  Phacops.  Remains  of 
fishes  occur  in  the  Corniferous  group,  consisting  of  ichthyodorulites  and  teeth  of 
oestraciont  and  hybodont  placoids,  with  plates,  bones,  and  teeth  of  some  peculiar  ganoids 
{MacropetalichthySf  Onychodua), 

In  the  Marcellus  shale,  Hamilton  beds,  and  Genesee  shale  remains  of  land-plants 
occur,  but  much  less  abundantly  than  among  the  rocks  of  New  Brunswick.  BrachiopodB 
are  cs{)ecially  abundant  among  the  sandy  beds  in  the  centre  of  the  formation.  They 
comprise,  as  in  Europe,  many  broad-winged  spirifers  {S,  mucronatuSy  &c.),  with  spedes 
of  ProdudrUSy  Chonetcs,  Athyris^  kc.  The  earliest  American  Goniatites  have  been  noticed 
in  these  beds.  Newberry  has  described  a  gigantic  fish  {Dinichthys)  from  the  Black  Shale 
of  Ohio. 

The  Portage  and  Chemung  groups  have  yielded  land-plants  and  fucoids,  also  some 
crinoids,  numerous  broad-winged  spirifers,  with  Avicula  and  a  few  other  lamellibranchs. 
These  strata,  in  the  New  York  region,  consist  of  shales  and  laminated  sandstones,  which 
there  attain  a  maximum  thickness  of  upwards  of  2000  feet,  but  die  out  entirely  towards 
the  interior.  They  are  covered  by  a  mass  of  red  sandstones  and  conglomerates — the 
Catskill  group,  which  is  2000  or  3000  feet  thick  in  the  Catskill  Mountains,  and  thickens 
along  the  Appalachian  region  to  5000  or  6000  feet.  These  red  arenaceous  rocks  bear  a 
striking  similarity  in  their  lithological  and  biological  characters  to  the  Old  Bed  Sand- 
stone of  EurojKj.  As  a  whole  they  are  unfossiliferous,  but  they  have  yielded  some  ferns 
like  those  of  the  Upper  Old  Red  Sandstone  of  Ireland  and  Scotland  {ArcksBvptcris),  some 
characteristic  genera  of  fish,  as  HoloptychUis  and  BoOiriolepiSy  and  a  large  lamellibranch 
closely  resembling  the  Irish  AnodorUa,  The  Old  Red  Sandstone  development,  found  on 
the  ca.steru  side  of  the  crystalline  ridge  which  runs  southwards  from  Canada  far  into  the 
States,  is  described  at  p.  803. 

Asia. — Froiu  south-western  China,  Richthofen  brought  a  series  of  marine  fossils 
which  show  the  presence  there  of  strata  probably  referable  to  Middle  and  Upper  Devonian 
horizons.  Out  of  28  species  named  by  Kayser,  no  fewer  than  13  are  cosmopolitan,  in- 
cluding such  familiar  forms  as  Rhyiichonella  cvhoideSf  R.  pugnus,  Pentamerus  galeaius, 
Atrypa  reticularis  (var.  desq%minata)y  Merista  plebeia,  Spirifer  Vemetiili,  Orthis 
striutula,  Productus  subaculealuSf  Strophalosia  prodxictoidea,  Aulopora  iuhm/ormis,^ 

Australasia. — In  New  South  Wales,  the  presence  of  Devonian  rocks  was  determined 
by  W.  B.  Clarke  from  the  evidence  of  fossil  remains.  The  thickness  of  strata  (sand- 
stones, quart/ites,  conglomerates,  shales  and  limestones)  is  in  some  places  estimated  at 
not  less  than  10,000  feet,  passing  down  into  Silurian  and  upwards  into  Carboniferous 
strata.     Among  the  numerous  fossils  are  many  forms  familiar  in  corresponding  strata  in 

^  Richthofen,  *  China,'  vol.  iv,  p.  76. 


SECT.  iii.  n.  §  1  OLD  RED  SANDSTONE  79 1 


Europe  and  America,  such  as  Cyathophyllum  damnonienM,  Favosites  reticvXtUay  F,  fibrosa^ 
F.  Ooldfusaiy  HeliolUes  poroad,  Chonetta  laguesnana  (hardrensis),  Orthis  striatulat 
Rhynchanella pleurodon,  R.  pugniis,  Atrypa  reticularis^  Spirifer  Vemeuili.^  In  Victoria 
certain  limestones  found  at  Bindi  on  the  Tambo  river  and  elsewhere  have  yielded  char- 
acteristically Middle  Devonian  fossils,  including  Favosites  OoldfusHy  Spirifer  lavieostalus, 
Clumetes  australiSy  and  a  placoderm  fish.  With  these  rocks  are  associated  contempora- 
neous felsitic  lavas  and  tuffs.  Other  strata  are  referred  to  the  Upper  Devonian 
series.* 

Devonian  rocks  play  an  important  part  in  the  structure  ofNewZealand.  To  the 
lower  part  of  the  system  are  assigned  quartzites,  cherts,  and  limestones,  which  in  the 
South  Island  at  Rcefton  have  yielded  Spirifer  vespertilio  and  ffomalonotus  expansus. 
To  the  Upper  Devonian  series  should  probably  be  referred  the  enormously  thick  Te 
Anau  group  of  ' '  greenstone-breccias,  aphanite-slates,  diorite-sandstones,  with  great  con- 
temporaneous flows  and  dykes  of  diorite,  serpentine,  syenite,  and  felsite."  These  rocks 
form  important  mountain  ranges  in  the  South  Island,  and  at  Reefton  are  the  matrix  of 
the  auriferous  reefs.  They  rest  unconformably  on  the  Lower  Devonian  and  pass  up 
into  the  Maitai  series  (Carboniferous).' 


II.  OLD  RED  SANDSTONE  TYPE. 
§1.    General    Characters. 

Under  the  name  of  Old  Red  Sandstone,  is  comprised  a  vast  and  still 
imperfectly  described  series  of  red  sandstones,  shales,  and  conglomerates, 
intermediate  in  age  between  the  Ludlow  rocks  of  the  Upper  Silurian  and 
the  base  of  the  Carboniferous  system  in  Britain.  These  rocks  were  termed 
*^  Old  **  to  distinguish  them  from  a  somewhat  similar  series  overlying  the 
Coal-measures,  to  which  the  name  "  New  "  Red  Sandstone  was  applied. 
When  the  term  Devonian  was  adopted  it  speedily  supplanted  that  of  Old 
Red  Sandstone,  inasmuch  as  it  was  founded  on  a  type  of  marine  strata  of 
wide  geographical  extent,  whereas  the  latter  term  described  what  appeared 
to  be  merely  a  British  and  local  development.  For  the  reasons  already 
given,  however,  it  is  desirable  to  retain  the  title  Old  Red  Sandstone  as 
descriptive  of  a  remarkable  suite  of  deposits  to  which  there  is  little  or 
nothing  analogous  in  typical  Devonian  rocks.  The  Old  Red  Sandstone 
of  Europe  is  almost  entirely  confined  to  the  British  Isles.  It  was  de- 
posited in  separate  areas  or  basins,  the  sites  of  some  of  which  can  still  be 
traced.  Their  diversities  of  sediment  and  discrepance  of  organic  contents 
point  to  the  absence,  or  at  least  rare  existence,  of  any  direct  communica- 
tion between  them.  It  was  maintained  many  years  ago  by  Fleming  and 
still  more  explicitly  by  God  win- Austen,  and  was  afterwards  enforced  by 
A.  C.  Ramsay,  that  these  basins  were  lakes  or  inland  seas.  The  character 
of  the  strata,  the  absence  of  unequivocally  marine  fossils,  the  presence  of 
land-plants  and  of  numerous  ganoid  fishes,  which  have  their  modem 
representatives  in  rivers  and  lakes,  suggest  and  support  this  opinion, 
which  has  been  generally  adopted   by  geologists.*     The  red  arenaceous 

^  See  the  authors  cited  on  p.  776,  note. 

^  R.  A.  F.  Murray,  'Victoria — Oeology  and  Physical  Geology/  1887. 

'  Hector,  'Handbook  of  New  Zealand,'  p.  36. 

*  For  a  history  of  opinion  on  this  subject  see  Trans,  Royal  Soc,  Edin,  xxviii.  1869,  p.  846. 


792  STRATIGRAPHWAL  GEOLOGY  book  vi  past  u 

and  marly  strata  which,  H-ith  their  fish-remains  and  land-plants,  oocupy  a 
depth  of  many  thousand  feet  between  the  top  of  the  Silurian  and  the  base 
of  the  Carboniferous  systems,  are  regarded  as  the  deposits  of  a  series  of 
lakes  or  inland  seas  formed  by  the  uprise  of  portions  of  the  Silurian  sea- 
floor.  The  length  of  time  during  which  these  lacustrine  basins  must  hare 
existed  is  shown,  not  only  by  the  thickness  of  the  deposits  formed  in 
them,  but  by  the  complete  change  which  took  place  in  the  marine  fauna 
between  the  Silurian  and  Carboniferous  periods.  The  prolific  fauna  of  the 
Wenlock  and  Ludlow  rocks  was  driven  away  from  western  Europe  by  the 
geographical  revolutions  which,  among  other  changes,  produced  the  lake- 
basins  of  the  Old  Red  Sandstone.  When  a  marine  population — crinoids, 
corals,  and  shells — once  more  overspread  that  area,  it  was  a  completely 
different  one.  So  thorough  a  change  must  have  demanded  a  long  interval 
of  time. 

Rocks. — As  shown  by  the  name  of  the  type,  red  sandstone  is  the 
predominant  rock.  The  colour  varies  from  a  light  brick-red  to  a  deep 
chocolate-brown,  and  occasionally  passes  into  green,  yellow,  or  mottled 
tints.  The  sandstones  are  for  the  most  part  granular  siliceous  rocks, 
wherein  the  component  grains  of  clear  quartz  are  coated  and  held  to- 
gether by  a  crust  of  earthy  ferric  oxide.  In  no  part  of  the  geological 
record  is  the  prevalence  of  this  red  material  more  marked  than  in  the 
Old  Red  Sandstone.  The  conditions  that  led  to  the  precipitation  of  this 
oxide  in  such  quantity  are  not  yet  well  understood.^  Scattered  pebbles  of 
quartz  or  of  various  crystalline  rocks  are  frequently  noticeable  among  the 
sandstones,  and  this  character  aflbrds  a  passage  into  conglomerate.  *  The 
latter  rock  forms  a  conspicuous  feature  in  many  Old  Red  Sandstone  dis- 
tricts. It  varies  in  thickness  from  a  mere  thin  bed  up  to  successive 
massive  beds,  having  a  united  thickness  of  several  thousand  feet.  The 
pebbles  vary  much  in  composition  and  size.  They  consist  of  quartz, 
quartzite,  greywacke,  granite,  syenite,  quartz-porphyry,  gneiss,  felsite,  or 
any  durable  material,  and  their  varying  nature  serves  to  distinguish 
some  bands  of  conglomerate  from  others.  They  are  of  all  sizes  Up  to 
blocks  eight  feet  or  more  in  length.  They  are  sometimes  tolerably  angular, 
{)articularly  where  the  conglomerate  rests  u})on  schists  or  other  rocks  which 
weather  into  angular  blocks.  In  the  upper  Old  Red  Sandstone,  thick 
accumulations  of  sul)angular  conglomerate  or  breccia  recall  some  glacial  de- 
posits of  modern  times.  For  the  most  part  the  stones  in  the  conglomerates 
are  well  rounded,  sometimes  indeed  remarkably  so,  QXfioAwhen  they  are 
a  foot  or  more  in  diameter.  The  larger  blocks  are  usually  angular 
fragments  that  have  been  derived  from  rocks  in  the  immediate  neighbour- 
hood. The  smaller  rounded  stones  have  often  come  from  some  distance ; 
at  least  it  is  imi)ossible  to  discover  any  near  source  for  them.  Bands  of 
red  and  gieen  clay  or  marl  occur,  in  which  seams  and  nodules  of  corn- 
stone  may  not  infrequently  be  observed.     Here  and  there,  too,  the  sand- 

^  Hec  jxhstra,  p.  797.  Mr.  I.  ('.  Russell  in  a  memoir  already  cited,  on  the  siiboerial  decay 
of  rocks  and  tlie  origin  of  the  red  colour  of  certain  formations,  concludes  that  in  the  majority 
of  cases  the  ferric  oxide  was  de}>osited  during  tlie  subaerial  decay  of  the  rocks  from  which 
the  se<liment  was  derived.     Bull.  U.S.  Oeol.  Surr.  No.  52  (1889). 


SECT.  iii.  II.  §  1  OLD  RED  SANDSTONE  793 

stones  assume  a  flaggy  character,  and  sometimes  pass  into  fine  grey  or 
olive-coloured  shales  and  flagstones.  Organic  remains  occur  in  some  of 
these  grey  beds,  but  are  usually  absent  from  the  red  strata,  though  in 
some  of  the  conglomerates  teeth,  scales,  and  broken  bones  of  fishes  are 
not  uncommon.  In  the  north  of  Scotland,  peculiar  very  hard  calcareous 
and  bituminous  flagstones  are  largely  developed,  and  have  yielded  the 
chief  part  of  the  remarkable  ichthyic  fauna  of  the  system.  In  Scotland, 
also,  contemporaneously  erupted  diabases,  porphyrites,  felsites,  and  tuffs 
play  an  important  part  in  the  petrography  of  the  Old  Red  Sandstone, 
seeing  that  they  attain  a  thickness  in  some  places  of  more  than  6000 
feet,  and  form  important  ranges  of  hills.  They  point  to  the  existence  of 
extensive  volcanic  eruptions  from  numerous  vents  in  the  lakes  or  inland 
basins  in  which  the  sediments  were  accumulated. 

Life. — No  greater  contrast  is  to  be  found  between  the  organic  con- 
tents of  any  two  successive  groups  of  rock  than  that  which  is  presented 
by  a  comparison  of  the  Upper  Silurian  and  Old  Red  Sandstone  systems 
of  Western  Europe.  The  abundant  marine  fauna  of  the  Ludlow  period 
disappeared  from  the  region.  As  soon  as  the  red  rocks  begin,  the  fossils 
rapidly  die  out.  Some  traces  of  the  aquatic  plants  that  grew  in  the 
fresh-wuter  lakes  have  been  detected.  An  abundant  fossil,  originally  re- 
ferred to  the  vegetable  kingdom  and  named  Parka  by  Fleming,  was  after- 
wards considered  to  be  more  probably  the  egg-packets  of  the  large  crus- 
taceans which  abounded  in  these  waters.  More  recently,  however,  this 
organism  has  been  carefully  studied  by  Sir  J.  W.  Dawson  and  Prof.  D.  P. 
Penhallow,  who  have  come  to  the  conclusion  that  it  represents  what  were 
aquatic  plants  \vith  creeping  stems,  linear  leaves  and  sessile  sporocarps 
bearing  two  kinds  of  sporangia.^  On  the  land  that  surrounded  the  lakes 
or  inland  seas  of  the  period,  there  grew  the  oldest  terrestrial  vegetation 
of  which  more  than  mere  fragments  are  known.  It  has  been  scantily 
preserved  in  the  ancient  lake-bottoms  in  Europe ;  more  abundantly  in 
( jasp^  and  New  Brunswick.  The  American  localities  have  yielded  to 
the  long- continued  researches  of  Sir  J.  W.  Dawson  more  than  100 
species  of  land-plants.  They  are  almost  all  acrogens,  lycopods  and  ferns 
being  largely  predominant.  Among  the  distinctive  forms  the  following 
may  be  mentioned  :  Psilophyton  (Fig.  350),  Arthrostigvia,  Leptophleum,  and 
Pmtotaxites,  Forty-nine  ferns  include  the  genera  Palxopieris  (Cydopieris), 
Neuropteris,  Splienopteris,  and  some  tree-ferns  {Psaronius,  Cauiopteru), 
Lcpidodendroid  and  sigillaroid  plants  abound,  as  well  as  calamites. 
Higher  forms  of  vegetation  are  represented  by  a  few  conifers  (Dadoxi/lon, 
Orm/yxi/hn,^  &c.)  From  a  locality  on  Lake  Erie,  Dawson  describes  a  frag- 
ment of  what  he  believes  to  be  dicotyledonous  wood,  not  unlike  that  of 
some  modern   trees — the  most  ancient  fragment  of  an  angiospermous 

*  Trans.  Roy.  Sik.  Canada,  ix.  (1891)  sect.  iv.  pp.  8-16. 

*  Mem.  Oeol.  Survey  Canada,  1871  ;  op.  cit.  1873.  Q.  J,  Oeol,  Soc,  1881,  p. 
299.  .'Acadian  Geology,'  2nd  edition.  Prototajcites,  included  by  Dawson  among  the 
Conifers,  is  relegated  by  Mr.  Camitbers  to  the  AlgsB  under  the  name  of  Nematophycus 
— a  genus  also  found  in  the  Upper  Silurian  rocks  of  N.  Wales.  {Month.  Microscopical 
Joum.  1872.) 


STRATIGRAPBICAL  GEOLOGY 


BOOK  TI  PAST  n 


exogen  yet  discovered.     So  abundant  are  the  vegetable  remiunB  tlut  in 
some  layers  they  actually  form  thin  aeams  of  coaL 

The  interest  of  this  flora  is  heightened  by  the  discovery  of  the  fact 
that  the  primeval  forests  were  not  without  the  hum  of  insect  lifet  The 
most  ancient  known  relics  of  ineect  forms  have  been  recovered  from  the 
Devonian  strata  of  New  Brunswick.'  They  include  both  orthopterons 
and  neuropterous  wings,  and  have  been  regarded  by  Mr.  Sciidder  of 
Boston  as  combining  a  remarkable  union  of  characters  now  found  in 
distinct  orders  of  insects.  In  one  fragment  he  observed  a  structure  vhicb 
he  could  only  compare  to  the  atridulating  organ  of  some  male  (hthoptera. 
Another  wing  indicates  the  existence  of  a  gigantic  Ephemera,   with  a 


spread  of  wing  extending  to  five  inches.  The  continued  existence  of 
scorpions  during  this  period  has  been  established  by  the  discovery  of  two 
genera  (Pal«ophoiteus  and  Fivscorpim)  in  the  Lower  Helderberg  rocks  of 
New  York. 

The  existence  of  myriapods  in  the  forests  of  this  ancient  period  has 
been  shown  by  Mr.  B.  N.  Peach,  who  finds  that  the  so-called  Kampe- 
curis,  previously  regarded  as  a  larval  form  of  isopod  crustacean,  really 
contains  two  genera  of  chilognathous  myriapods  differing  from  other 
'  For  s  synopais  or  all  known  species  of  bnsil  insecta  up  to  tha  year  1890,  see  Stitt' 
U.S.  Oe«l.  Sarv.  No.  71,  1881. 


OLD  RED  SANDSTONE 


796 


known  forms,  foesll  and  recent,  in  their  less  differentiated  structure, 
each  body  segment  being  separate,  and  supplied  with  only  one  pair  of 
walking  legs.'  There  were  also  pulmoniferous  shells,  of  which  one  species 
{SlTOphUes  grandteva,  Dawson)  occurs  in  the  plant-beds  of  St.  John,  New 
Brunswick. 


The  water-basins  of  the  Old  Red  Sandstone  might  be  supposed  to  have 
been,  on  the  whole,  singularly  devoid  of  life ;  for  remains  of  it  have  been 
■  Proe.  Phyt.  Sx.  Edin.  vii.  (1882)  p.  179, 


STBATIGRAPHICAL  GEOLOGY 


BOOK  TI  PAST  n 


but  meagrely  preserted.  Nevertheless,  in  some  basiiw  at  least  (Caithnett, 
Horaj-  Firth),  it  must  have  been  exceedingly  abundant,  as  is  sbown  by  the 
extraordinary  profusion  of  the  foaeils.  The  fauna  consists  almost  vbc^y 
of  fishes  (Figs.  351,  352).  Among  tbeae  the 
P/fl-fwpwsurvived  for  a  while  from  Upper  SUtuian 
times.  With  it  there  lived  other  forms  (S^jiAtupu, 
HiAafpU)  and  genera  of  the  allied  family  of  the 
Cephalaspidie.  The  ancient  order  of  Dipnoi  which 
still  survives  in  a  few  forms  in  some  Airican  and 
Australian  rivers  {Proiaplenis,  Cfmtodtts),  was  re- 
presented in  the  lakes  of  the  Lower  Old  Ked 
Sandstone  by  the  abundant  Diptena,  and  in  those 
of  the  Upper  by  PhuReropleurim.  But  the  ganoids 
were  the  most  ^■aried  order  in  these  watery 
being  represented  by  a  niunber  of  families. 
Besides  those  which  lingered  on  from  the  Upper 
Silurian  period  there  now  appeared  the  striking 
group  of  the  Asteroiepids  of  which  Ast<rolepit 
and  Pleritklhys  (Fig.  352),  are  characteristic 
genera.  BnthriiAfput  appears  to  be  confined  to 
the  Upper  Old  Red  Sandstone,  where  it  some- 
times occurs  with  other  genera  crowded  together 
on  the  surfaces  of  the  layers  of  stone,  as  if  the 
ntricMii>-ac»rnui<u>.  ak.  fishes  had  been  killed  suddenly  and  had  been 
covered  over  with  sediment  where  they  died. 
The  family  of  the  Coccosteide  includes  the  type  genus  Coreosfeits  and  the 
gigantic  !{omi4eiii  {J sierdrjns).  This  latter  form  appears  to  have  been  the 
largest  fish  of  the  period  in  the  European  area,  its  massive  cuii-ase-like  head- 
shiekl  itometimcs  measiiring  twenty  inches  in  length  by  sixteen  in  breadth. 
The  sul>-order  of  Acantho<lians,  marked  by  their  strong  fin-spines,  attained 
a  great  development  in  the  waters  of  this  period  ;  among  their  genera 
are  Mfmwinlhux  (.-imiitli'iika),  Cknraran(h>L%  Isdinaeaitthus  (Diphcatitkiu), 
lihaiHnacantkiis.  The  sub-order  Cmssnp/eri/gUla'j  so  remarkable  for  the 
central  scaly  lol)o  of  their  fins,  and  represented  at  the  present  time  by 
PiAi/jitertis,  swarmed  in  the  waters,  some  of  the  most  characteristic  genera 
I)eing  TrinHdii'plrrHS,  Gymplichiiis,  Olitplolejiis,  Osteiilfpis,  Tkursius,  and 
Jjiptoptnas  which  are  found  in  the  Lower  Old  Red  Sandstone  of  Scotland, 
and  Jloli'pti/fJiuis  which  is  a  characteristic  fish  of  the  Upper  division  of  the 
system.  Of  the  sturgeon  tribe  there  were  some  small  representatives 
belonging  to  the  genus  Chnrdfph}  The  Dhiiclilhi/s  already  referred  to  as 
oeciH-ring  in  the  Devonian  rocks  of  North  America  was  probably  one  of 
the  largest  and  most  formidable  of  these  early  fi.shes.  Its  head  alone 
encased  in  strong  plates  attained  a  length  of  3  feet,  and  was  armed  with 
a  {mwurful  apparatus  of  teeth. 

A  few  einypterids  occur,    especially  of  the  genera  Evryptervs  and 
Plnijgot'is  (Fig.  348).     The  sj^eiea  of  the  former  are  smalt,  but  one  of 

'  Trni|iiiiir,  «eo/.  Miuj.    1888,  p.    507.      W.  Lnhest,  Ann.  Soc.  OtoL  Brig,  iv,  (1888)  p. 
\Vi.     Wliiteaves,  fJa«M.  Xat.  x.  Nos.  1,  2  (1881). 


SECT.  iii.  II.  §  2  OLD  RED  SANDSTONE  797 

the  latter,  P.  anglicus  is  found  in  Scotland,  which  must  have  had  a  length 
of  five  or  six  feet. 

§  2.  Local  Development. 

Britain. — Murehison,  who  strongly  advocated  the  opinion  that  the  Old  Red  Sand> 
stone  and  Devonian  rocks  represent  different  geographical  conditions  of  the  same  period, 
and  who  had  with  satisfaction  seen  the  adoption  of  the  Devonian  classification  by 
Continental  geologists,  endeavoured  to  trace  in  the  Old  Red  Sandstone  of  Britain  a 
threefold  division,  like  that  which  had  been  accepted  for  the  Devonian  system.  He 
accordingly  arranged  the  formations  as  in  the  subjoined  table  : — 


s   v   S 

2  3 

o 


%' 


ellow  and  red  sandstones  and  conglomerates  {Pterichthya  major j 
Ualoptychius  nobiliaaimus,  &c. )  =  Dnra  Den  beds. 


^  [  Grey  and  bine  calcareous  and  bituminous  flagstones,  limestones,  and 
^  \  red  sandstones  and  conglomerates  {DipteruSf  OsteoUpia,  Aaterol^pis, 
jg   i      AcanlhodeSf  Pterichthya^  &c.)  =  Caithness  flags. 

»  J  Red  and  purple  sandstones,  grey  sandy  flagstones,  and  coarse  con- 
I   I      glomerates  {Cephalaapis^  JPteraspiSf  Pterygotus)  =  Arbroath  flags. 


It  is  important  to  observe  that  in  no  district  can  these  three  subdivisions  be  found 
together,  and  that  the  so-called  "middle"  formation  occurs  only  in  one  region — the 
north  of  Scotland.  The  classification,  therefore,  does  not  rest  upon  any  actually  ascer- 
tained stratigraphical  sequence,  but  on  an  inference  from  the  organic  remains.  The 
value  of  this  inference  will  be  estimated  a  little  farther  on.  All  that  can  be  affirmed 
from  the  stratigraphical  evidence  of  any  district  in  Britain  is  that  a  great  physical  and 
palseontological  break  can  generally  be  traced  in  the  Old  Red  Sandstone,  dividing  it 
into  two  completely  distinct  series.  ^ 

As  a  whole,  the  Old  Red  Sandstone,  where  its  strata  are  really  red,  is,  like  other 
masses  of  red  de{K>sits,  singularly  barren  of  organic  remains.  As  above  remarked,  the 
physical  conditions  under  which  the  precipitation  of  iron  oxide  took  place  are  not  easily 
explained.  They  were  evidently  unfavourable  for  the  development  of  animal  life  in  the 
same  waters.  Sir  A.  C.  Ramsay  has  connected  the  occurrence  of  such  red  formations 
with  the  existence  of  salt  lakes,  from  the  bitter  waters  of  which  not  only  iron  oxide  but 
often  rock-salt,  maguesian  limestone,  and  gypsum  were  thrown  down.^  He  points 
also  to  the  presence  of  land-plants,  footprints  of  amphibia,  and  other  indications  of 
terrestrial  surfaces,  while  truly  marine  organisms  are  either  found  in  a  stunted  condition 
or  arc  absent  altogether.  Where  the  strata  of  the  Old  Red  Sandstone,  losing  their  red 
colour  and  ferruginous  character,  assume  grey  or  yellow  tints  and  pass  into  a  calcareous 
or  argillaceous  condition,  they  not  infrequently  become  fossiliferous.  At  the  same 
time,  it  is  worthy  of  i*emark  that  the  red  conglomerates,  which  might  be  supposed  little 
likely  to  contain  organic  remains,  are  occasionally  found  to  be  full  of  detached  scales, 
plates,  and  bones  of  fishes. 

The  Old  Red  Sandstone  of  Britain,  according  to  the  author's  researches,  consists  of 
two  subdivisions,  the  lower  of  which  passes  down  conformably  into  the  Upper  Silurian 

1  Q.  J.  Ged.  Soc.  vol.  xviii.  (1860)  p.  312. 

'■^  ProfesHor  Gosselet  contends  that  the  precipitation  of  iron  might  quite  well  have  taken 
place  in  the  sea,  and  he  cites  the  case  of  the  Devonian  basin  of  Dinant,  where  the  same 
beds  are  in  one  part  red  and  barren  of  organic  remains,  and  in  another  part  of  the  same 
area  are  of  the  usual  colours,  and  are  full  of  marine  fossils.  But  the  red  colour  of  the  Old 
Red  Sandstone  is  general,  and  is  accompanied  with  other  proofs  of  isolation  in  the  basins  of 
deposit  (see  j).  792). 


98  STBATIGRAPHICAL  GEOLOGY  book  ti  p 


dferprjntA.  the  opfi^r  fthadiDg  off  in  the  Mme  maoiiMr  into  tbe  base  of  tke 
fljftt«m,  «hi>  th^j  an  aepftnteti  from  ea^h  <»tlier  bj  an  imcoafonBabiHtT. 

1.    L^jWke.  —  R««i   undstonea,   conglooMrates,   ilasstooca,  Ubi 
rock  A,    pa^oj^   in   »om«    places  confonnabtj  dovn   into   Upper  Sthmaa 
eUrvher^  resting  onconformablT  on  Dal  radian  or  other  older  rocki — DifieruM,  C% 

In  a  memoir  on  the  Old  Red  Sandstone  of  Westrm  Europe,  the  aathor  ha»  |innwd 
■hort  name:^  for  the  different  detached  basins  in  which  the  Lover  Old  Red 
was  accnmalate^i.*  The  most  sonthrrlj  of  these  the  Welsh  Lake}  fies  in  the  Sfli 
region  extending  from  Shropshire  into  South  Wales.  Here  the  nppenost  parts 
of  the  Silurian  «tTatem  grafluate  into  red  strata,  not  less  than  lO.OOO  feet  thick,  wkidi 
in  turn  pass  up  conformably  into  the  base  of  the  Carboniferoas  wptam.  This  Tsft 
accumulation  of  red  rocks  consists  in  its  lower  portions  of  red  and  green  aiiales  aad 
flagstones,  with  some  white  sandstones  and  thin  comstoaes ;  in  the  ceotiml  aad  diicf 
dirisiou,  of  rerl  and  green  spotted  sandj  marls  and  clavs,  with  red  aaodstoBcs  aad 
comstones ;  in  the  higher  parts,  of  grey,  red.  chocolate-cokxired,  and  jelkyw  aaad- 
stones,  with  bands  of  conglomerate.  No  unconformability  has  yet  been  proTed  la  any 
(art  of  this  beries  of  rocks,  though,  from  the  obserrations  of  De  la  Beebe  and  Jokes,  it 
may  be  susjiected  that  the  higher  strata,  which  graduate  upwards  into  the  GsrbonifcnMi 
formations,  are  sefjarated  from  the  underlying  portions  of  the  Old  Red  Sandstone  bj  a 
distinct  discordance,* 

Although,  as  a  whole,  barren  of  organic  remains,  these  red  rocks  hare  here  and 
there,  more  i>articularly  in  the  calcareous  zones,  yielded  fragments  of  fishes  and 
crustaceans.  In  their  lower  and  central  portions  remains  of  the  fishes  Cepkala^iA, 
l/idymaxpviy  Sraphaspis,  FUraspii,  and  CyaihoApU,  hare  been  found,  together  with 
crustaceans  of  the  genera  Stylonurus,  Pterygolus,  Preardurus,  and  obacnre  traces  of 
j»lants.  Tlic  upper  yellow  and  red  sandstones  contain  none  of  the  cephalaspid  fiahes,- 
which  are  there  replaced  by  PUrichihys  and  Holoplyckius,  associated  with  distinct 
inipreshions  of  land-plants.  In  some  of  the  higher  parts  of  the  Old  Red  Sandstone  of 
South  Wales  and  Shropshire,  S'.rjmla  and  Conuhiria  occur,  but  these  are  exceptional 
case:<,  aud  [K>int  to  the  a^lvent  of  the  Carboniferous  marine  fauna,  which  doubtless 
exi.Ht«r<l  outside  the  British  area  Ijefore  it  spread  over  the  site  of  the  Old  Red  Sandstone 
baiins  (see  p.  801;. 

It  is  in  Scotland^  that  the  Old  Red  Sandstone  shows  the  most  complete  and 
varied  development,  alike  in  physical  structure  and  in  organic  contents.  Throughout 
that  country  the  system  is  found  to  consist  of  two  well-marked  groups  of  strata, 
separated  from  each  other  by  a  strong  unconformability  and  a  complete  break  in  the 
succession  of  organic  remains.  Each  subdivision  occurs  in  distinct  basins  of  deposit 
The  most  important  basin  of  the  Lower  Old  Re<l  Sandstone  occupies  the  central  valley, 
between  the  base  of  the  Highland  mountains  and  the  Uplands  of  the  southern  counties 
(Lake  Caledonia).  On  the  north-east,  it  presents  a  series  of  noble  cliff-sections  along 
the  coast  line  from  Stonehaven  to  the  mouth  of  the  Tay.  On  the  south-west  it  ranges 
by  the  island  of  Arran  and  the  south  of  Cantyre  across  St.  George's  Channel  into 
Ireland,  where  it  runs  almost  to  the  western  seaboard,  flanked  on  the  north,  as  in 
Scotland,  by  hills  of  crj'stalline  rocks,  and  on  the  south  chiefly  by  a  Silurian  belt.     In 

^  Tram.  Roy.  Soc.  Edin.  vol.  xxviii.  (1879). 

2  De  la  Beche,  M^m.  Ueol.  Sure.  vol.  L  (1846)  p.  50.  J.  B.  Jukes,  'Letters,  &c'  (1871) 
p.  508  ;  letter  to  A.  C.  Ramsay,  dated  1857.  Symonds,  *  Records  of  the  Rocks'  (1872); 
Hughes,  Jirit.  AssffC.  Rep.  (1875)  sects,  p.  70. 

*  See  Agassiz,  *  Poissons  du  Vieux  Gres  Rouge,'  Hugh  Miller's  *01d  Red  Sandstone,' 
and  *  Footprints  of  the  Creator ' ;  J.  Anderson's  *  Dura  Den  * ;  Explanalions  Geol.  Sure. 
^Scotland,  sheets  14,  15,  23,  24,  32,  33,  34  ;  author's  memoir  cited  on  previous  page,  and 
j>apers  referred  to  in  subsequent  notes. 


BBCT.  iii.  11.  §  2  OLD  RED  SANDSTONE  799 

this  basin  abundant  volcanic  action  manifested  itself  across  the  whole  breadth  of 
Scotland  and  in  the  north  of  Ireland.  Another  distinct  and  still  larger  basin  (Lake 
Orcadie)  of  the  lower  subdivbion  lies  on  the  north  side  of  the  Highlands,  but  only 
a  portion  of  it  emerges  above  the  sea  in  the  north  of  Scotland.  Skirting  the  slopes  of 
the  mountains  along  the  Moray  Firth  and  the  east  of  Ross  and  Sutherland,  it  stretches 
through  Caithness  and  the  Orkney  Islands  as  far  as  the  south  of  the  Shetland  group, 
and  may  possibly  have  been  at  one  time  continued  as  far  as  the  Sogneijord  and  Dalsfjord 
in  Norway,  where  red  conglomerates  like  those  of  the  north  of  Scotland  occur.  There 
is  even  reason  to  infer  that  it  may  have  ranged  eastwards  into  Russia,  for,  as  already 
stated,  some  of  its  most  characteristic  organisms  are  found  also  among  the  Devonian 
strata  of  that  country.  Several  distinct  contemporaneous  volcanic  centres  have  been 
detected  in  this  basin.  A  third  minor  area  of  the  Lower  Old  Red  Sandstone  (Lake 
Cheviot)  lay  on  the  south  side  of  the  Southern  Uplands,  over  the  east  of  Berwickshire 
and  the  north  of  Northumberland,  including  the  area  of  the  Cheviot  Hills,  where  a 
copious  volcanic  series  has  been  preserved.  A  fourth  (Lake  of  Lome)  occupied  a  basin 
on  the  flanks  of  the  south-west  Highlands,  which  is  now  partly  marked  by  the 
terraced  volcanic  hills  of  Lome.  There  is  sufficient  diversity  of  lithological  and 
palieontologioal  characters  to  show  that  these  several  areas  were  on  the  whole  distinct 
basins,  separated  both  from  each  other  and  from  the  sea.  The  interval  between  the 
Lower  and  Upper  Old  Red  Sandstone  was  so  protracted,  and  the  geographical  changes 
accomplished  during  it  were  so  extensive,  that  the  basins  in  which  the  late  parts  of  the 
system  were  deposited  only  partially  correspond  with  those  of  the  older  lakes. 

In  the  central  basin,  or  Lake  Caledonia,  both  divisions  of  the  Old  Red  Sandstone  are 
typically  seen.  The  lower  series  of  deposits,  attaining  a  maximum  depth  of  perhaps 
20,000  feet,  everywhere  presents  traces  of  shallow-water  conditions.  The  accumulation 
of  so  great  a  thickness  of  sediment  can  only  be  explained  on  the  supposition  that  the 
subterranean  movements,  which  at  first  ridged  up  the  Silurian  sea- floor  into  land,  enclos- 
ing separate  basins,  continued  to  deepen  these  basins,  until  eventually,  enormous  masses 
of  sediment  had  slowly  gathered  in  them.  This  massive  series  of  deposits  passes  down 
conformably  in  Lanarkshire  into  Upper  Silurian  rocks  ;  elsewhere  its  base  is  concealed 
by  later  formations,  or  by  the  unconformability  with  which  different  horizons  rest  upon 
the  older  rocks.  Covered  unconformably  by  every  rock  younger  than  itself,  it  consists 
of  reddish-brown  or  chocolate-coloured,  grey,  and  yellow  sandstones,  red  shales,  grey 
flagstones,  coarse  conglomerates,  with  occasional  bands  of  limestone  and  comstone.  The 
grey  flagstones  and  thin  grey  and  olive  shales  and  ^^calmstones"  are  almost  confined  to 
Forfarshire,  in  the  north-east  part  of  the  basin,  and  are  known  as  the  "  Arbroath  flags." 
One  of  the  most  marked  lithological  features  in  this  central  Scottish  basin  is  the 
occurrence  in  it  of  extensive  masses  of  interbedded  volcanic  rocks.  These,  consisting  of 
diabases,  porphyrites,  felsites,  and  tuffs,  attain  a  thickness  of  more  than  6000  feet,  and 
form  important  chains  of  hills,  as  in  the  Pentland,  Ochil,  and  Sidlaw  ranges.  They  lie 
several  thousand  feet  above  the  base  of  the  system,  and  are  regularly  interstratified 
with  bands  of  the  ordinary  sedimentary  strata.  They  point  to  the  outburst  of 
numerous  volcanic  vents  along  the  lake  or  inland  sea  in  which  the  Lower  Old  Red 
Sandstone  of  central  Scotland  was  laid  down  ;  and  their  disposition  shows  that  the 
vents  ranged  themselves  in  lines  or  linear  groups,  parallel  with  the  general  trend  of  the 
great  central  valley.  The  fact  that  the  igneous  rocks  are  succeeded  by  thousands  of 
feet  of  sandstones,  shales,  and  conglomerates,  without  any  intercalation  of  lava  or  tuff, 
proves  that  the  volcanic  episode  in  the  history  of  the  lake  came  to  a  close  long  before 
the  lake  itself  disappeared.^  As  a  rule,  the  deposits  of  this  basin  are  singularly  unfos- 
siliferous,  though  some  portions  of  them,  particularly  in  the  Forfarshire  (Arbroath)  flag- 
stone group,  have  proved  rich  in  remains  of  crustaceans  and  fish.  Nine  or  more  speciea 
of  crustaceans   have   been   obtained,   chiefly   eurypterids,   but  including  one   or  two 

^  Presidential  Address,  Quart.  Joum,  Oeol.  Soc,  1882,  p.  62  aeq. 


800  STRATIGRAPHICAL  GEOLtjGY  BOOKTiPAnn 

phyllofiods.  The  Urge  pterrgotus  (P.  anglicu*.  is  especially  chanctoristie,  and  Bmt 
have  attained  a  great  dze,  for  some  of  the  indiTiduaU  indicate  a  length  of  6  feet,  vith  a 
brearlth  of  1^  feet.  There  occur  also  a  smaller  species  {F.  Manor),  two  Emrjfpitri,  and 
three  sfiecies  of  Slylonunu.  Upwards  of  twenty  s|jecies  of  fishes  have  been  ohtaiaed, 
chiefly  from  the  Arbroath  flags,  belonging  to  the  sub-orders  Aoanikodidm  and  Otiraeodei 
'Fig.  351).  One  of  the  most  abundant  forms  is  the  little  Meaaeamikua  {Aeanthtda) 
SfUcJulli,  Another  common  fish  is  Isehnaeanihus  {DipUuantkus)  graciliL,  There  oonir 
also  CUmatius  Kutigrr^  C.  reticulatuSt  C.  uneinatu*,  C.  Macnieoli,  C.  grandit^  C  gruciliM, 
Partxui  incurxiu,  C^phalaspU  Lyellii,  and  Pterasjn»  Mitckelli.  Scxne  (^  the  aandafcones 
and  shales  are  crowded  with  indistinctly  preserved  vegetation,  occasionally  in  safficknt 
quantity  to  form  thin  lamime  of  coaL  The  egg -like  imjireasions  known  as  Farta 
decipuiru  and  referred  to  on  p.  793,  also  abound  in  some  layers.  In  Forfarshire,  the 
surfaces  of  the  shaly  flagstones  are  now  and  then  covered  with  linear  grass-like  plants, 
like  the  sedg}'  vegetation  of  a  lake  or  marsh.  In  Perthshire,  certain  layers  oocnr,  chiefly 
made  up  of  compressed  stems  of  Pnlqphyton  (Fig.  350;.  The  adjoining  land  was  donbt- 
less  clothed  with  a  flora  in  large  measure  lycopodiaceous. 

The  Old  Red  Sandstone  of  the  northern  basin  (Lake  Orcadie)  is  typically  developed 
in  Caithness,  where  it  consists  chiefly  of  the  well-known  daik-grey  bituminoos  and 
calcareous  flagstones  of  commerce.  It  rests  unconformably  upon  various  crystalline 
schists,  granites,  &c.,  and  must  have  been  de|K)sited  on  the  very  uneven  bottom  of  a  MtiVing 
baiiin,  seeing  that  occasionally  even  some  of  the  higher  platforms  are  found  resting 
against  the  more  ancient  rocks.  The  lower  zones  consist  of  red  sandstones  and  con* 
glomerates,  which  graduate  upward  into  the  flagstones.  Other  red  sandstones,  however, 
HU[>erveue  in  the  higher  {larts  of  the  system.  The  total  deftth  of  the  series  in  Caithness 
has  been  estimated  at  upwards  of  16,000  feet.  Murchison  was  the  first  to  attempt  the 
correlation  of  the  Caithness  flagstones  with  the  Old  Red  Sandstone  of  the  rest  of  Britain. 
Founding  upon  the  absence  from  these  northern  rocks  of  the  cephalas|»dean  fishes 
characteristic  of  the  admitted  Lower  Old  Red  Sandstone  in  the  south  of  Scotland  and  in 
Wales  and  8hro[(shire,  ui>on  the  presence  of  numerous  genera  of  fishes  not  known  to 
occur  elsewhere  in  the  true  Lower  Old  Red  Sandstone,  and  ujion  the  discovery  of  t 
PUirijfjolus  in  the  l>asemeiit  red  sandy  group  of  strata,  he  concluded  that  the  massive 
flagstone  series  of  Caithness  could  not  be  classed  with  the  Lower  Old  Red  Sandstone, 
but  must  )>e  of  younger  date.  He  sup[K>sed  the  red  sandstones,  conglomerates,  and 
shales  at  the  base,  with  their  Ptcriffjotus,  to  represent  the  true  Lower  Old  Red  Sand- 
stone, while  the  great  flagstone  series  with  its  distinctive  fishes  was  made  into  a 
middle  division  answering  in  some  of  its  ichthyolitic  contents  to  the  Middle  Devonian 
rocks  of  the  Continent.  This  view  was  accepted  by  geologists.  I  have,  however, 
endeavoured  to  show  that  the  Caithness  flagstones  belong  to  the  Lower  Old  Red  Sand- 
stone, and  that  there  is  no  evidence  of  the  existence  of  any  middle  division.  It  appears 
to  me  tliat  tlie  discrepance  in  organic  contents  between  the  Caithness  and  the  Arbroath 
flags  is  by  no  means  so  strong  as  Murchison  supf>osed,  but  that  several  species  are 
crimmon  to  both.  In  i>articular,  I  find  that  the  characteristically  Lower  Old  Red  Sand- 
stone* and  U[)i»er  Silurian  crustacean  genus  I^rt/yof  us  w:i:m'H^  not  merely  in  the  basement 
zone  of  the  Caithness  flags,  but  also  high  up  in  the  series.  The  genera  AcanUwdes 
{MesticMiUhus)  and  DiplacaiUhwt  {Ischnacanthiis)  appear  both  in  Caithness  and  in 
Forfarshire.  Parcxus  iivctirvus  occurs  in  the  northern  as  well  as  the  southern  basin. 
The  admitt4;d  paheontological  distinctions  are  probably  not  greater  than  the  striking 
lithological  differences  between  the  strata  of  the  two  regions  would  account  for,  or  than 
the  contrast  between  the  ichtliyic  faunas  of  adjacent  but  disconnected  water -basins  at 
tlic  present  time. 

More  than  sixty  s[>ecies  of  fislies  have  been  obtained  from  the  Old  Red  Sand- 
stont;  of  the  north  of  Scotland.  Among  these,  the  genera  AcanUiodes,  AderolrpiSj 
CliciracaiUhiiHy  ChcirolcpiSf  Coccostcus,  JJijflaainthuSj  DiploptentSy  DipUrus,  Olyj^epUj 


SECT.  iii.  II.  §  2  OLD  RED  SANDSTONE  801 

• 

OsteolapiSy  and  Plerichthys  are  specially  characteristic.  Some  of  the  sliales  are  crowded 
witli  the  little  phyllojKjd  crustacean  Estheria  membranacea.  Land -plants  abound, 
esi)ecially  iu  the  higher  groups  of  the  flagstones,  where  foi-ms  of  Fsilophyton,  Lepido- 
dendroHy  Stigmaria^  Sigillaria,  CalamiteSy  and  Cyclopteris,  as  well  as  other  genera,  occur. 
In  the  Shetland  Islands,  traces  of  abundant  contemporaneous  volcanic  rocks  have  been 
observed.'  These,  with  the  exception  of  two  trifling  examples  in  the  region  of  the  Moray 
Firtli,  are  the  only  known  instances  of  volcanic  action  in  the  Lower  Old  Red  Sandstone 
of  Lake  Orcadie.  In  the  other  two  Scottish  basins,  those  of  the  Cheviot  Hills*  and  of 
Lome,^  volcanic  action  long  continued  vigorous,  and  produced  thick  piles  of  lava,  like 
those  of  Lake  Caledonia. 

2.  Upper. — This  division  consists  of  yellow  and  red  sandstones,  conglomerates, 
marls,  &c.,  passing  up  conformably  into  the  base  of  the  Carboniferous  system,  and 
resting  unconformably  on  the  Lower  Old  Red  Sandstone  and  every  older  formation. 
Among  its  distinctive  fossils  are  Holoptychius,  Bothriolepis  {PtericJUhys)  majors  &c. 

Below  the  Carboniferous  system  there  occur  in  Scotland  certain  red  sandstones,  deep- 
red  clays  or  marls,  conglomerates,  and  breccias,  the  sandstones  passing  into  yellow  or 
even  white.  The^e  strata,  wherever  their  strat {graphical  relations  can  be  distinctly 
traced,  lie  unconformably  upon  every  fonnation  older  than  themselves,  including 
the  Lower  Old  Red  Sandstone,  while,  on  the  other  hand,  they  pass  up  conformably 
into  the  Carboniferous  rocks  above.  As  already  remarked,  they  were  deposited  in 
basins,  which  only  partially  corresponded  witli  those  wherein  the  Lower  Old  Red 
Sandstone  had  been  laid  down.  Studied  from  the  side  of  the  underlying  formations, 
they  seem  naturally  to  form  part  of  the  Old  Red  Sandstone,  since  they  agree  with  it  in 
general  lithological  character,  and  also  in  containing  some  distinctively  Old  Red  Sandstone 
genera  of  fislies,  such  as  Ptrrichthys  and  Holoptychiu^ ;  though,  approached  from  the 
upper  or  Carboniferous  direction,  they  might  rather  be  assumed  as  the  natural  sandy 
base  of  that  system  into  which  they  insensibly  graduate.  On  the  whole,  they  are 
remarkably  barren  of  organic  remains,  though  in  some  localities  (Dura  Den  in  Fife, 
Lauderdale)  they  have  yielded  a  number  of  genera  and  species  of  fishes,  crowded  pro- 
fusely tlirough  the  pale  sandstone,  as  if  the  individuals  had  been  suddenly  killed  and 
rapidly  covered  over  \^ith  sediment  (see  j).  648).  Among  the  characteristic  organisms 
of  the  Scottish  Upper  Old  Red  Sandstone  are  Bolhriolepis  (PtericMhys)  inajor^  Ilolopty- 
chilis  nohiliasinmSj  H.  AncUrsoniy  Olyptopomusj  Oiyptoltemus  and  Phaneropleuron. 

In  the  Upi)er  Old  Red  Sandstone  of  the  Firth  of  Clyde,  Bothriolepis  {PUridUhys)  umjw 
and  Holoptychiiis  occur  at  the  Heads  of  Ayr,  while  a  band  of  marine  limestone,  lying  iu 
the  red  sandstone  series  in  Arran,  is  crowded  with  ordinary  Carboniferous  Limestone 
shells,  sucli  as  Prodtictus  gigarUeiis,  P,  semireticvlatuSj  P.  p%n\ttatiiSf  Choiietcs  hardrensis, 
Spirifcr  lineatns^  &c.  These  fossils  are  absent  from  the  gi*eat  series  of  red  sandstones 
overlying  the  limestone,  and  do  not  reappear  till  we  reach  the  limestones  in  the  Lower 
Carboniferous  series  ;  yet  the  organisms  must  have  Wen  living  during  all  that  long 
interval  outside  of  the  Upper  Old  Red  Sandstone  area  (p.  828).  Not  only  so,  but  they 
must  have  been  in  existence  long  before  the  fonnation  of  the  thick  Arran  limestone, 
thougli  it  was  only  during  the  comparatively  brief  interval  represented  by  that  limestone 
that  geographical  changes  permitted  them  to  enter  tlie  Old  Red  Sandstone  basin  and 
settle  for  a  wliile  on  its  floor.  The  higher  parts  of  the  Upi)€r  Old  Red  Sandstone  seem 
thus  to  have  been  contemiwraneous  with  a  Carboniferous  Limestone  fauna  which,  liaving 
appeared  beyond  the  British  area,  was  ready  to  si)read  over  it  as  soon  as  the  conditions 
became  favourable  for  the  invasion.     It  is,  of  course,  obvious  that  such  an  abundant 

*   Trans.  Roy.  Soc.  Edin.  xxviii.  (1878)  p.  345.     Presidential  Address,  </uart.  Journ,  (rV/*/. 
,Sbc.  xlviii.  (1892)  p.  94.     Peach  and  Home,  Trans.  Roy.  iSoc.  EiUnr.  xxxii.  (1884)  p.  359. 

'  C.  T.  enough,  *  Cheviot  Hills,'  Ue^,  Sun\  Menu  Sheet  108  N.E.  (1888) :  J.  J.  H.  Teall, 
Oeol.  Mag.  1883. 

*'  Presidential  Address,  (^irt.  Journ.  Ueol.  Soc.  xlviii.  (1892)  p.  95. 

3  F 


802  STRATIGKAPHICAL  GEOLOGY  book  vi  pabt  it 

and  varied  fauna  as  that  of  the  Carboniferous  Limestone  cannot  have  come  suddenlr 
into  existence  at  tlie  period  marked  by  tlie  base  of  the  limestone.  It  must  have  had  a 
long  [)revious  existence  outside  the  present  are*  of  the  deixjsit. 

In  the  north  of  Scotland,  on  the  Lowlands  bordering  the  Moray  Firth,  and  again  in 
the  island  of  Hoy,  one  of  the  Orkney  group,  yellow  and  red  sandstones  (with  interbedded 
diabase  and  tuff),  containing  characteristic  Upper  Old  Red  Sandstone  fishes,  lie  uncon* 
formably  u(K)n  the  Caithness  flags. ^  In  these  northern  tracts,  the  same  relation  ia  thus 
traceable  as  in  the  central  counties,  between  the  two  divisions  of  the  system. 

Turning  southward  across  the  border  districts  into  the  north  of  England,  we  find 
the  red  sandstones  and  conglomerates  of  the  Upi>er  Old  Ked  Sandstone  lying  uncon- 
formably  on  Silurian  rocks  and  Lower  Old  Red  Sandstone.  Some  of  the  breeciated 
conglomerates  have  much  resemblance  to  glacial  detritus,  and  it  was  suggested  by 
Ramsay  that  they  have  been  connected  mt\\  contemporaneous  ice-action.-  Such  are  tlu» 
breccias  of  the  I^mmennuir  Hills,  and  those  which  show  themselves  here  and  there 
from  under  the  overlying  mass  of  Carboniferous  strata  that  flanks  the  Silurian  hills  of 
Cumberland  and  Westmoreland.  Red  conglomerates  and  sandstones  appear  inter- 
ruptedly at  the  base  of  the  Carboniferous  rocks,  even  as  far  as  Flintshire  and  Anglesey. 
They  are  commonly  classed  as  Old  Red  Sandstone,  but  merely  from  their  position  and 
lithological  character.  No  organic  remains  have  Ixjen  found  in  them.  They  may  there- 
fore, in  part  at  least,  belong  to  tlie  Carboniferous  system,  having  been  deposited  on 
difierent  successive  horizons  during  the  gradual  depression  of  the  land.  In  Devonshire, 
at  Barnstaple,  Pilton,  Man\'ood,  and  Baggy  Point,  certain  sandstones,  sliales,  and  lime- 
stones (already  referred  to  in  the  account  of  the  Devonian  rocks)  graduate  upward  into 
the  base  of  the  Carboniferous  system,  and  api>ear  to  represent  the  Upper  Old  Red  Sand- 
stone of  the  rest  of  Britain.  They  contain  land-j^lant^  and  also  many  marine  fossils, 
some  of  which  are  common  Carboniferous  fonus.  They  thus  indicate  a  transition  into 
the  geographical  conditions  of  the  Carboniferous  period,  as  is  still  more  clearly  illustrated 
by  the  corrcjjponding  strata  in  Scotland. 

Tlie  Old  Red  Sandstone  attains  a  great  development  in  the  south  and  south-west  of 
Ireland.  The  thick  *' Dingle-Beds"  and  "Glengariff  gi*its"  pass  down  into  Upper 
Silurian  strata,  an<l  no  doubt  represent  the  Lower  Old  Red  Sandstone  of  Scotland. 
Thoy  are  succeeded  in  Kerry  by  red  sandstones  which  cover  th<jm  unconfomiably,  and 
respinblo  the  ordinary  Uj)per  Old  Red  Sandstone  of  Sc^otland.  In  Cork  and  the  south- 
east of  Ireland  tliey  are  followwl  by  the  pale  sandstones  and  shaly  flagstones  knoAvn  as 
the  "  Kiltorcan  beds,"  with  aj>i>arently  a  perfect  conformability.  The  Kiltorcan  beds 
(which  pass  up  conformably  into  the  Carboniferous  Slat^)  have  yielded  a  few  fishes 
{Bothrioirpis,  CoccosU-us,  PtcrkhthySy  Olyptohpis),  some  crustaceans  {Bclinurus^  Piety- 
goti(s)j  a  fresh-water  lamellibranch  {AiwdoiUu  Jukesh'),  and  a  number  of  ferns  and  other 
land-plants  {Palmopteris^  SphcnoptcriSy  Sagcnaria  {Cnc.hAti(ji}ia)j  Kiwrria,^ 

Norway,  &c.  — On  the  continent  of  Europe  the  Old  Red  Sandstone  type  can  hardly 

^  Trans.  Jioi/.  See.  Edin.  xxviii.  (1878)  p.  405  ;  Quart.  Journ.  Ged.  Sm:.  xlviii.  (1892) 
Presitlcntial  Address,  p.  100. 

-  Tlie  examples  of  supposed  glacial  striie  in  the  pebbles  in  these  breccias  may  be  merely 
frictioiial  markings  connected  with  faults  or  internal  movements  of  the  rocks.  But  the 
forms  of  the  pebbles,  their  moraine-like  unstratifie<l  or  rudely-stratifictl  accumulation,  and 
tlie  occun*eiice  of  aggregated  lumjis  of  breccia  in  the  midst  of  fine  sandstone  strongly  remind 
one  of  the  familiar  features  of  tnie  glacial  deposit*.  Compare  H.  Reusch,  on  similar  evidence 
from  the  Palaiozoic  rocks  of  Norway,  Xonjes  Oeol.  Undersog.  Aarbog.  1891. 

^  Prof.  Hull,  Q.  J.  Oeol.  Soc.  xxxv.  xxxvi.  ;  Trans.  Roy.  Duhlin  Soc.  (new  ser.)  L  p. 
135,  1880  ;  Rrplanations  of  the  6W.  Survey y  Ireland^  sheets  167,  Ac,  187,  &c. ;  J.  Nolan, 
</  J.  GeU.  Soc.  1880,  p.  629  ;  Kinahan,  Trans.  Oeol.  Soc.  EiUn.  1882,  p.  152.  A  recent 
personal  examination  has  convinced  me  that  the  south  of  Ireland  formed  another  of  the  basins- 
iii  which  the  Lower  Old  Red  Sandstone  was  accumulated. 


SECT.  iii.  II.  §  2  OLD  RED  SANDSTONE  803 


be  said  to  occur.  Some  outliers  of  red  sandstone  and  conglomerate  (p.  799)  in  northern 
and  western  Norway -reach  a  thickness  of  1000  to  1200  feet.  Near  Christiania,  they 
follow  the  Silurian  strata  like  the  Old  Red  Sandstone,  but  as  yet  have  yielded  no  fossils, 
so  that,  as  they  pass  up  into  no  younger  formation,  their  geological  horizon  cannot  be 
certainly  fixed.  The  Devonian  rocks  of  Russia  have  been  above  referred  to  as  presenting 
a  union  of  the  two  types  of  this  part  of  the  geological  series.  The  extension  of  the  land 
of  the  Old  Red  Sandstone  i)eriod,  with  its  characteristic  flora,  far  north  within  the  Arctic 
circle  is  indicated  by  the  discoveries  made  at  Bear  Island  (lat.  70**  30'  N.)  between  the 
coast  of  Norway  and  Spitzbergen.  Certain  seams  of  coal  and  coaly  shale  occur  at  that 
locality,  underlying  beds  of  Carboniferous  Limestone  and  overlying  some  3'ellow  dolomite, 
calcareous  shale,  and  red  shales.  They  have  been  assigned  by  Heer  to  the  Carboniferous 
series,  but  are  regarded  by  Dawson  as  unquestionably  Devonian.  They  may  be  correlated 
with  the  Upper  Old  Red  Sandstone  of  Biitain.  Heer  enimierates  eighteen  species  ;  only 
three  are  i)eculiar  to  the  locality,  while  among  the  others  are  some  i^idely-diffused  forms  : 
Calamitcsradialus  (transitionis),  Paleeopteris  rocineriana,  Sphenopteris  Schimperif  Cardio- 
pt4:ris  frondosay  Lepidodendron  veltheimianum  and  three  other  species,  Knorria  im- 
bricata,  and  Sagenaria  (Cyclostigma)  kiltorkcnsis.^  In  Spitzbergen  itself,  according  to 
the  researches  of  Nathorst,  the  so-called  "Heckla-Hook  formation"  contains  a  large 
assemblage  of  fish-remains,  shells,  and  plants,  which  prove  it  to  be  the  equivalent  of 
part  of  the  Scottish  Old  Red  Sandstone. 

North  America. — It  is  interesting  to  observe  that  in  North  America  representatives 
occur  of  the  two  divergent  Devonian  and  Old  Red  Sandstone  types  of  Europe.  The 
American  Devonian  facies  has  already  been  referred  to.  On  the  eastern  side  of  the 
ancient  pre-Cambrian  and  Silurian  ridge,  which,  stretching  southwards  from  Canada, 
separated  in  early  Palaeozoic  time  the  great  interior  basin  from  the  Atlantic  slopes,  we 
find  the  Devonian  rocks  of  New  York,  Pennsylvania,  and  the  interior  represented  in 
New  Brunswick  and  Nova  Scotia  by  a  totally  different  series  of  deposits.  The  contrast 
strikingly  recalls  that  presented  by  the  Old  Red  Sandstone  of  the  north  of  Scotland  and 
the  Devonian  rocks  of  North  Germany.  On  the  south  side  of  the  St.  Lawrence,  the 
coast  of  Gasi>e  shows  rocks  of  the  so-called  '*  Quebec  group  "  unconformably  overlain  by 
grey  limestones  with  green  and  red  shales,  attaining,  according  to  Logan,  a  total  thick- 
ness of  about  2000  feet,*  and  in  some  bands  replete  with  Upper  Silurian  fossils.  They 
are  conformably  followed  by  a  vast  arenaceous  series  of  deposits  termed  the  Gaspe  Sand- 
stones, to  which  the  careful  measurements  of  Logan  and  his  colleagues  of  the  Canadian 
Geological  Survey  assign  a  depth  of  7036  feet.  This  formation  consists  of  grey  and 
drab -coloured  sandstones,  with  occasional  grey  shales  and  bands  of  massive  con- 
glomerate. Similar  rocks  reappear  along  the  southern  coast  of  New  Brunswick,  where 
they  attain  a  depth  of  9500  feet,  and  again  on  the  opposite  side  of  the  Bay  of  Fundy. 
The  researches  of  Sir  J.  W.  Dawson,  already  referred  to,  have  made  known  the  remark- 
able flora  of  these  rocks.  Some  of  the  same  plants  have  been  met  with  in  the  Devonian 
rocks  to  the  west  of  the  Archaean  ridge,  so  that  there  can  be  little  doubt  of  the  con- 
temporaneity of  the  dejwsits  on  the  two  sides.  Besides  the  abundant  vegetation,  a  few 
traces  of  the  fauna  of  the  period  have  been  recovered  from  the  Old  Red  Sandstone. 
Among  them  are  the  remains  of  several  small  crustaceans,  including  a  minute  shrimp- 
like EurypUrmy  and  the  more  highly  organised  AmphipeltiSf  with  the  snail  {Slrophiies) 
referred  to  on  p.  795.  That  the  sea  had  at  least  occasional  access  to  the  inland  basins 
into  which  the  abundant  terrestrial  vegetation  was  washed,  is  proved  by  the  occurrence 
of  marine  organisms,  such  as  a  small  annelid  (Spirorbis)  adhering  to  the  leaves  of  the 
plants,  and  (in  Gasj*  and  Nova  Scotia)  by  the  occasional  appearance  of  brachiopods, 
especially  Lingular  Spirifery  and  Chonetea,^ 

^  Heer,  Q.  J.  Oeol.  Soc.  xxviii.  p.  161.     Dawson,  op,  cii.  xxix.  p.  24. 

2  'Geology  of  Canada,'  p.  393. 
^  Dawson's  '  Acadian  Geolog}','  chaps.  xxL  and  xxii. 


804  STBATIGRAPHICAL  GEOLOGY  book  vi  pabt  n 


Section  iv.  Carboniferous. 

§  1.  General  Characters. 

This  great  system  of  rocks  has  received  its  name  from  the  seams  of 
coal  which  form  one  of  its  distinguishing  characters  in  most  parts  of  the 
world.  Both  in  Europe  and  America  it  may  be  seen  passing  down  con- 
formly  into  the  Devonian  and  Old  Red  Sandstone.  So  insensible  indeed 
is  the  gradation  in  many  consecutive  sections  where  the  two  systems 
join  each  other  that  no  sharp  line  can  there  be  drawn  between  them. 
This  stratigraphical  passage  is  likewise  in  many  places  associated  with  a 
corresponding  commingling  of  organic  remains,  either  by  the  ascent  of 
undoubted  Devonian  species  into  the  lower  parts  of  the  Carboniferous 
series,  or  by  the  appearance  in  the  Upper  Devonian  beds  of  species  which 
attained  their  maximum  development  in  Carboniferous  times.  Hence 
there  can  be  no  doubt  as  to  the  true  place  of  the  Carboniferous  system  in 
the  geological  record.  In  some  places,  however,  the  higher  members  of 
this  system  are  found  resting  unconformably  upon  Devonian  or  older 
rocks,  so  that  local  disturbances  of  considerable  magnitude  occurred  be- 
fore or  at  the  commencement  of  the  Carboniferous  period.  It  is  deserving 
of  notice  that  Carboniferous  rocks  are  very  generally  arranged  in  basin- 
shaped  areas,  many  of  which  have  been  wholly  or  partially  overspread 
unconformably  by  later  formations.  This  disposition,  so  well  seen  in 
Europe,  and  particularly  in  the  central  and  western  half  of  the  continent, 
has  in  some  cases  been  caused  merely  by  the  plication  and  subsequent 
extensive  denudation  of  what  were  originally  wide  continuous  sheets  of 
rock,  as  may  be  observed  in  the  British  Isles.  But  the  remarkable 
small  scattered  coal-basins  of  France  and  central  Germany  were  probably 
from  the  first  isolated  areas  of  deposit,  though  they  have  suffered,  in 
some  cases  very  greatly,  from  subsequent  plication  and  denudation.  In 
Russia,  and  still  more  in  China  and  western  North  America,  Carboniferous 
rocks  cover  thousands  of  square  miles  in  horizontal  or  only  very  gently 
undulating  sheets. 

KocKS. — The  materials  of  which  the  Carboniferous  system  is  built 
up  differ  considerably  in  different  regions ;  but  two  facies  of  sedimenta- 
tion have  a  wide  development.  In  one  of  these,  the  marine  type,  lime- 
stones form  the  prevailing  rocks,  and  are  often  visibly  made  up  of 
organic  remains,  chiefly  encrinites,  corals,  foraminifera,  and  mollusks. 
According  to  Dupont's  researches  in  the  Carboniferous  Limestone  of 
Belgium  there  are  two  main  types  of  limestone:  (1)  the  massive  lime- 
stones formed  by  reef-building  corals  and  coralloid  animals,  and  disposed 
in  fringing  reefs  or  dispersed  atolls,  according  to  their  nearness  to  or 
distance  from  the  coast  of  the  time;  and  (2)  the  detritic  limestones, 
consisting  either  of  an  aggregation  of  crinoid  stems  or  of  coral-debris, 
and  often  stretching  in  extensive  sheets  like  sandstone  or  shale.^  The 
limestones  of  both  types  assume  a  compact  homogeneous  character,  with 
black,    giey,    white,   or   mottled    colours,   and    are   occasionally    largely 

1  Bull.  Acoil.  Roy,  Iklg.  (3)  v.  1883,  No.  2. 


SECT,  iv  S^  1  CARBONIFEROUS  SYSTEM  805 


quarried  as  marble.  Local  developments  of  oolitic  structure  occur 
among  them.  They  also  assume  in  some  places  a  yellowish,  dull,  finely 
granular  aspect  and  more  or  less  dolomitic  composition.  They  occur  in 
beds,  sometimes  as  in  cerili*al  England,  Ireland,  and  Belgium,  piled  over 
each  other  for  a  depth  of  hundreds  of  feet,  and  in  Utah  for  several  thou- 
sand feet,  with  little  or  no  intercalation  of  other  material  than  limestone. 
The  limestones  frequently  contain  irregular  nodules  of  a  white,  grey,  or 
black  flinty  chert  (phtanite),  which,  presenting  a  close  resemblance  to  the 
flints  of  the  chalk,  occur  in  certain  beds  or  layers  of  rock,  sometimes 
in  numbers  sufficient  to  form  of  themselves  tolerably  distinct  strata.^ 
These  concretions  are  associated  with  the  organisms  of  the  rock,  some  of 
which,  completely  silicified  and  beautifully  preserved,  may  be  found  im- 
bedded in  the  chert.  Dolomite,  usually  of  a  dull  yellowish  colour, 
granular  texture,  and  rough  feel,  occurs  both  in  beds  regularly  inter- 
stratified  with  the  limestones  and  also  in  broad  wall-like  masses  running 
through  the  limestones.  In  the  latter  cases,  it  is  evident  that  the  lime- 
stone has  been  changed  into  dolomite  along  lines  of  joint ;  in  the  former, 
the  dolomite  may  be  due  to  contemporaneous  alteration  of  the  original 
calcareous  deposit  by  the  magnesian  salts  of  sea- water  as  already  explained 
(pp.  321,412).  Traced  to  ^  distance,  the  limestones  are  often  found  to  grow 
thinner,  and  to  be  separated  by  increasing  thicknesses  of  shale,  or  to  be- 
come more  and  more  argillaceous  and  to  pass  eventually  into  shale.  The 
shales,  too,  are  often  largely  calcareous,  and  charged  with  fossils  ;  but  in 
some  places  assume  dark  colours,  become  more  thoroughly  argillaceous, 
and  contain,  besides  carbonaceous  matter,  an  impregnation  of  pyrites  or 
marcjisite.  Where  the  marine  Carboniferous  type  dies  out,  the  shales  may 
pass  even  into  coal,  associated  with  sandstones,  clays,  and  ironstones.  In 
Britain,  abundant  contemporaneous  volcanic  rocks  are  preserved  in  the 
Carboniferous  Limestone  series. 

The  second  facies  of  sedimentation  points  to  deposit  in  shallow 
lagoons,  which  at  first  were  replenished  from  the  sea,  but  afterwards 
api)ear  to  have  been  brackish  and  then  fresh,  or  in  lakes  into  which 
coarse  and  fine  detritus  as  well  as  vegetation  and  animal  remains  were 
washed  from  neighbouring  land.  The  most  abundant  strata  of  this  type 
are  sandstones,  which,  presenting  every  gradation  of  fineness  of  grain  up 
to  pebbly  grits,  and  even  (near  former  shore-lines)  conglomerates,  are 
commonly  yellow,  grey,  or  white  in  colour,  well-bedded,  sometimes 
micaceous  and  fissile,  sometimes  compact ;  often  full  of  streaks  or  layers 
of  coaly  matter.  Besides  the  existence  of  pebbly  grits  and  conglom- 
erates pointing  to  comparatively  strong  currents  of  transport,  there  occur 
in  different  parts  of  the  Carboniferous  system,  scattered  pieces  and  even 
blocks  of  granite,  gneiss,  quartzite,  or  other  durable  material  which  lie 
imbedded,  sometimes  singly  sometimes  in  groups,  in  limestone,  sandstone, 
and  in  coal.  Various  explanations  have  been  proposed  to  account  for 
these  erratics,  some  writers  having  even  suggested  the  action  of  drifting 
ice.";     The  stones  were  most  probably  transported  by   floating  plants. 

^  Renanl,  op.  cit.  (2)  xlvi.  p.  9. 
2  For  remarks  on  the  climate  of  the  Carboniferous  p>erio<l  see  postfeaj  p.  809. 


806  STEATIGRAPHICAL  GEOLOGY  book  vi  part n 


Seaweeds  with  their  rootlets  wrapt  round  loose  blocks  might  easily  be 
torn  up  and  drifted  out  to  sea  so  as  to  drop  their  freight  among  corals 
and  crinoids  living  on  the  bottom.  But  more  usually  trees  growing  on 
the  land  would  envelop  soil  and  stones  among  "their  roots,  and  if  blown 
down  and  carried  away  by  storms  and  floods  might  bear  these  with  theuL^ 

Next  in  abundance  to  the  sandy  sediment  came  the  deposits  of  mud 
now  forming  shales.  These  occur  in  seams  or  bands  from  less  than  an 
inch  to  many  yards  in  thickness.  They  are  commonly  black  and  carbon- 
aceous, frequently  largely  charged  with  pyritous  impregnations,  sometimes 
crowded  with  concretions  of  clay-ironstone.  Coal  occurs  among  these 
strata  in  seams  varying  from  less  than  an  inch  up  to  several  feet  or  yards 
in  thickness,  but  swelling  out  in  some  rare  examples  to  100  feet  or  more. 
A  coal-seam  may  consist  entirely  of  one  kind  of  coal.  Frequently,  how- 
ever, it  contains  one  or  more  thin  layers  or  "  partings  "  of  shale,  the 
nature  or  quality  of  the  seam  being  alike  or  different  on  the  two  sides  of 
the  parting.  The  same  seam  may  be  a  cannel-coal  at  one  part  of  a 
mineral  field,  an  ordinary  soft  coal  at  a  second,  and  an  ironstone  at  a 
third.  Moreover,  in  Britain  and  other  countries,  each  coal-seam  iM 
usually  underlain  by  a  bed  of  fire-clay  or  shale,  through  which  rootlets 
branch  freely  in  all  directioi;^.  These  fireclays,  as  their  name  denotes, 
are  used  for  pottery  or  brick-making.  They  appear  to  be  the  soil  on 
which  the  plants  of  the  coal  grew,  and  it  was  doubtless  the  growth  of  the 
vegetation  that  deprived  them  of  their  alkalies  and  iron,  and  thus  made 
them  industrially  valuable.  In  the  small  coal-basins  of  central  France 
the  coal  is  dispersed  in  banks  and  isolated  veins  all  through  the  Carboni- 
ferous strata.  Clay-ironstone  occurs  abundantly  in  some  coal-fields,  both 
in  the  form  of  concretions  (spha?rosidcrite)  and  also  in  distinct  layers  from 
less  than  an  inch  to  eighteen  inches  or  more  in  thickness.  The  nodules 
have  generally  been  formed  round  some  organic  object,  such  as  a  shell, 
seed-cone,  fern-frond,  &c.  Many  of  the  ironstone  beds  likewise  abound 
in  organic  remains,  some  of  them,  like  the  "  mussel-band  "  ironstone  of 
Scotland,  consisting  almost  wholly  of  valves  of  Jnthrawsia  or  other  shell 
converted  into  carbonate  of  iron. 

The  mode  of  origin  of  coal  cannot  be  closely  paralleled  by  any  modern 
formation,  and  various  divergent  views  have  been  expressed  on  the  sub- 
ject. There  seem  to  have  been  two  distinct  modes  of  accumulation,  (1) 
by  growth  in  sUu,  and  (2)  by  drifting  from  adjacent  land.  It  is  possible 
that  in  some  coal-fields  both  these  processes  may  have  been  successively 
or  simultaneously  in  operation,  so  that  the  results  are  commingled. 

1.  In  those  c^ses  where  the  evidence  points  to  growth  in  siiu^  the 
coal-seams  have  been  laid  down  with  tolerable  uniformity  of  thickness 
and  character  over  considerable  areas  of  ground,  and  they  now  appear  as 
regular  layers  intercalated  between  sheets  of  sediment  and  for  the  most 
part  rest  on  fireclay  or  shale,  into  which  the  stigmaria  rootlets  may  fre- 

'  For  accounts  of  these  travelled  stones  in  Carboniferous  rocks  see  especially  D.  Stur, 
Jahrh.  Ocol.  Reichsanst.  xxxv.  (1885)  p.  613,  and  the  authorities  cited  by  him  ;  also  W.  S. 
CJresley,  OeoL  Mag,  1885,  p.  553  ;  Quart.  Joum.  Geol.  Soc.  xliii.  (1887)  p.  734  ;  V.  Ball, 
op.  cit.  xliv.  (1888)  p.  371. 


'§1 


CARBONIFEROUS  SYSTEM 


■(uently  be  seen  to  ramify  as  in  the  {Kwition  of  growth.'  The  nearest 
analogy  to  these  conditione  ia  probably  furnished  by  cypress  swamps^ 
or  the  mangrove  swamps  alluded  to  already  {p.  481),  where  masses  of 
arborescent  vegetation,  with  their  roots  spreading  in  salt  water  among 


marine  organisms,  grow  out  into  the  sea  as  a  belt  or  fringe  on  low  shores, 
and  form  a  matted  soil  which  adds  to  the  breadth  of  the  land.     The  coal- 

'  For  argiiTn«nU  in  aupport  of  the  view  thit  conl  was  formed  of  pliDlf  in  niu  «ee  Logui, 
Tmm.  fieol.  .'be  vi.  (1842)  p.  491,  Newberry,  Amrr.  Jmm.  ,%i.  «iii.  (1857)  p.  218, 
■n™i.  Snrv.  Ohio,'  vol.  ii.  Oeotogy,  p.  126  ;  OUnibi'l,  SiUb.  Ba^r.  Akad.  1888. 

>  For  in  account  of  the  gubnierged  luiils  of  the  MiHiaaippi,  see  Ljrell'i  '  Second  Vitit  tn 
the  United  SUtea,'  chap,  xiiiii. 


808  STRA  TIGRA  PHICA  L  GEOLOG  Y  book  vi  part  ii 


growths  no  doubt  also  flourished  in  salt  water  ;  for  such  shells  as  Avieulo- 
pecien  and  Goniatites  ai'e  found  lying  on  the  coal  or  in  the  shales  attached 
to  it.  Each  coal-scam  represents  the  accumulated  growth  of  a  period 
which  was  limited  either  by  the  exhaustion  of  the  soil  underneath  the 
vegetation  (as  may  bo  indicated  by  the  composition  of  the  fire-claysX  or 
by  the  rate  of  the  intermittent  subsidence  that  aflected  the  whole  area  of 
coal-gi*owths.  Though  the  vegetation  in  these  coal-flelds  may  have 
grown  as  a  whole  in  situ,  there  may  also  have  been  considerable  trans- 
port of  loose  leaves,  branches,  trunks,  &c.,  after  storms,  and  also  during 
times  of  more  rapid  subsidence.  From  the  fact  that  a  succession  of  coal- 
seams,  each  representing  a  former  surface  of  terrestrial  vegetation,  can  be 
seen  in  a  single  coal-field  extending  through  a  vertical  thickness  of 
10,000  feet  or  more,  it  is  clear  that  the  strata  of  such  a  field  must  have 
been  laid  down  during  prolonged  and  extensive  subsidence.  It  has  been 
assumed  that,  besides  depression,  movements  in  an  upward  direction  were 
needful  to  bring  the  submerged  surfaces  once  more  up  within  the  limits 
of  plant  growth.  But  this  would  involve  a  prolonged  and  almost  incon- 
ceivable sea-saw  oscillation ;  and  the  assumption  is  really  unnecessary  if 
we  suppose  that  the  downward  movement,  though  prolonged,  was  not 
continuous,  but  was  marked  by  pauses,  long  enough  for  the  silting-up  of 
lagoons  and  the  spread  of  coal-jiuigles.^ 

2.  The  researches  of  Grand'  Eury,  Fayol,  and  others  in  the  small 
coal -basins  of  central  France  have  shown  that  in  these  regions  much 
vegetable  matter  was  washed  down  from  adjacent  land.-  The  coal  is 
irregularly  distributed  among  the  strata,  and  it  is  associated  w4th  beds 
of  coarse  detritus  and  other  evidence  of  torrential  action.  Numerous 
trunks  of  calamododcndra,  sigillarije,  and  other  trees  imbedded  in  the 
sandstones  and  shales  verticallv  and  at  all  angles  of  inclination  bear 
witness,  like  the  "  snags ''  of  the  Mississippi,  to  the  currents  that  trani^- 
ported  them.  The  basins  in  which  the  accumulated  detritus  and 
vegetation  were  entombed  seem  to  have  been  small,  but  sometimes 
comparatively  deep  lakes  lying  on  the  surface  of  the  crystalline  rocks 
that  formed  an  uneven  land-surface  during  the  Carboniferous  period  in 
the  heart  of  France.  But  there  is  evidence,  even  in  these  basins,  of  the 
growth  of  coal-plants  in  sifu,  and  of  the  gradual  subsidence  of  the  alluxial 
floors  on  which  they  took  root.  (Jrand'  Eury  has  shown  the  existence 
of  tree-trunks  with  their  roots  in  place  on  many  successive  levels,  and 
has  further  ascertained  that  these  trees,  as  they  were  enveloped  in  sediment, 
pushed  out  rootlets  at  higher  levels  into  the  silt  that  gathered  round  thera. 

It  would  thus  appear  that  no  one  hypothesis  is  universally  applicable 

'  See  a  statt'iiieut  of  the  oscillatiou  theory  as  far  back  as  1849  by  M.  Virlet  d'AouM. 
/Jul/.  Sor.  (KuL  IVctncc  (2)  vi.  j).  616. 

-  For  the  detrital  origin  of  coal,  see  Grand*  Eury,  Ann.  dts  Mines,  1882  (i.)  p|>. 
99.*292  ;  M^m.  »Syr.  ('Vol.  France,  3«  Si'r.  iv.  1887;  *Geol.  et  Paleontol.  du  bassin  HouUK-r 
du  Gard,'  1891.  Fayol,  *  t^tiides  siir  le  Terrain  Hoiiiller  de  Comnientry,' part  1.  Bi'lL 
.S('C.  Jit'hi.stiif  Miii.  ser.  2,  vol.  xv.  and  Atlas  (1887).  Bull.  Soi\  Gtol.  France,  3"  scr.  xvii. 
(1888)  :  Ji.  Renault,  'Flore  Fossile  de  Comnientry,'  lUiU.  Site,  Hist.  Sat,  d^Autun  (1891). 
A.  dr  Lapparent,  Itci\  (^ncst.  ikicn.  July  1892. 


6ECT.  iv  g  1  CARBONIFEROVa  SYSTEM  809 

for  the  explanation  of  the  origin  of  coal,  but  that  growth  on  the  epot 
and  transport  from  neighbouring  land  have  both  in  different  regions 
contemporaneously  and  at  successive  periods  come  into  play. 

In  this  place  reference  may  most  conveniently  be  made  to  the  probable 
climate  in  which  these  geological  changes  took  place.  The  remarkable 
profusion  of  the  vegetation  of  the  Carboniferous  period,  not  only  in  the 
Old  \\^orld  but  in  the  New,  suggested  the  idea  that  the  atmosphere  was 
then  much  more  charged  with  carbonic  acid  than  it  now  is.  Undoubtedly 
there  has  been  a  continual  abstraction  of  this  gas  from  the  atmosphere 
ever  since  land-plants  began  to  live  on  the  earth's  surface,  and  it  is 
allowable  to  infer  that  the  proportion  of  it  in  the  air  in  Palteozoic 
time  may  have  been  somewhat  greater  than  now.  But  the  difference 
could  hardly  have  been  serious,  otherwise  it  seems  incredible  that  the 
numerous  insects,  labyrinthodonts  and  other  air-breathers,  could  have 
existed.  Most  probably  the  luxuriance  of  the  flora  is  rather  to  be 
ascribed  to  the  warm  moist  climate  which  in  Carboniferous  times  appears 
to  have  spread  over  the  globe  even  into  Arctic  latitudes.  On  the  other 
hand,  evidence  has  been  adduced  to  support  the  view  that  in  spite  of  the 
genial  temperature  indicated  by  the  vegetation  there  were  glaciers  even 
in  tropical  and  sub-tropical  regions.  Coarae  boulder-conglomerates  and 
striated  atones  have  been  cited  from  various  parts  of  India,  South  Africa, 
and  IJisterii  Australia,  as  evidence  of  ice-action.  There  appears,  how- 
evei",  to  be  some  element  of  doubt  as  to  the  interpretation  of  the 
facts  adduced.  It  may  be  matter  for  consideration 
whether  the  bouldcr-bcds  could  not  be  accumulateil 
by  torrential  waters,  and  whether  the  striated  sur 
faces  on  the  stones  might  not  have  been  pi-oduced 
by  internal  movements  in  the  rocks,  like  slickensidc 
(p.  -y2(i)} 

LlF*l — Each  of  the  two  facies  of  sedimentation 
above  described  has  its  own  characteristic  oi-ganic 
types,  the  one  series  of  strata  presenting  us  chiefly 
with  the  fauna  of  the  sea,  the  other  mainly  with 
the  flora  of  the  land.  The  marine  fauna  is  spe- 
cially rich  in  criiioids,  corals,  and  brachiopods, 
which  of  themselves  constitute  entire  beds  of  lime-  sm,-(ii  i.    f    .  - 

stone.  Among  the  lower  forms  of-  life  the  fora-  '  criiiuij, 
minifera  are  well  represented.  The  genera  include  cyaihaerinu>piaiiii<>,MiL]cr: 
AmjiltisUgimi,  Archxodiii(u»,  Ctiinaaimminu,  Endolkyru,  •'•  <"')"•  ■""'  ''"'  "I'P" 
Luffemi,  Sitccammina,  Fvsttlina,  TTOchtimmvut,  and  Sth(!itain';'r,..iieTVi'h" 
{■'ulmlina.  Some  of  these  genera  exhibit  a  wide  cuiumii -inintii  ahiiwinE 
geographical  range  ;  Sacriimmiiui,  for  example,  forms       «"*"'  ™'»i- 

'  TliH  glaci.il  origin  of  the  phenomeua  in  qTiealion  baa  l)eeii  ablj-  advoc»t«l  by  Mr.  W. 
T.  Blnnfonl,  'Mauual  of  Geology  of  India,'  Address  lo  Geological  Section  of  British  Asaoci- 
alioii.  JIoiitTfnl ;  aiid  H.  F.  Blaaford,  ^arl.  Joiin.  Geo!.  .Sue  xxxi.  (1875)  p.  519. 
Sullierlaiid,  op.  /-il.  iivi,  p.  514;  W.  Waageii,  JnJirb.  Oeol.  nrichsaniU.  xxivii.  (1887) 
p.  113.  A.  Julieii  has  advocated  the  glacial  origin  of  the  coane  Carboniferous  breetiaa  of 
Central  France.      Gmpl.  mul.  CMvii.  (1893)  p.  25.=i. 


STRATIGKAPHICAL  GEOLOGY 


BOOK  TI  PAST  n 


beds  of  limestone  in   Briuin  and  Belgium,  and  Fxmdina  plays  a  stfll 
more   important  part  in  the  CarboniferoiiB   Limestone   of    the   Fegian 


from  Russia  to  China  and  Jajian,  as  woU  as  in  North  America ;  one 
sjwcics  of  J'lilrnlhii  {f.  piiLrofrocliiis)  extends  from  Ireland  to  Russia  on 
the  one  side  and  to  \orth  jVmerica  on  the  other. 
As  already  noticed,  siwcies  of  organisms,  with 
a  wide  geographical  extension,  have  also  a  long 
^eologieal  ntnge,  and  this  is  more  specially 
exemplified  in  such  lowly  grades  of  existence 
us  the  foraminifera.  I'rochtimmina  inreiin,  for 
instance,  is  found  through  the  whole  Carboni- 
ferous Limestone  scries  of  England,  reappears 
in  the  Magnesian  Limestone  of  the  Permian 
system,  and  occui-s  not  only  in  Britain  but  in 
(rormany  and  Russia.^  The  corals  (Fig.  353) 
arc  roprcBcnte<l  by  tabulate  {FavoHles,  MieJitUnia, 
AlveolUes,  Cliivkfes),  and  still  more  by  rugose 
foi'ms  {Ampkxus,  Z'iphretUis,  Cyaibopkyilvm, 
.tttlophyllum,  ClisiophyUum,  Lilhostrotioa,  Loiu- 
:  diilein,  Pkillipfxisfrxa).  The  Echinoderms  were 
more  abundant  and  varied  in  this  than  in  any 
other  geological  period.  Thus  among  the 
<f  Carboniferana  nnd  Permiim  ForuniuiFan,'  PaltroiUof. 


r.   l^ocarillum  ■liruir 

h,    Avieuloiiniti^n    mbUibiitua, 
PhilL.aho'TliK;™!. 


8KCT.  iv  g  I  CARBONIFESOVS  SYSTEM  811 

urchins  of  the  Carboniferous  seas  were  species  of  Archxoddans, 
I'alxchiiius,  and  Udeloniles.  The  biastoids  or  pentremites,  vhich  now 
took  the  place  in  Carboniferous  waters  that  in  Silurian  times  had  been 
tilled  by  the  cystideans,  attained  their  maximuni  development.  But 
it  was  the  order  of  crinoids  that  chiefly  swarmed  in  the  seas  where 
the  Carboniferous  Limestone  was  laid  down,  their  separated  joints  now 
mainly  comjiosing  solid  masses  of  rock  several  hundred  feet  in  thickness. 
Among  their  most  conspicuous  genera  were  Plaiycrinus,  Adinocrinus, 
CgaOuxrinus  (Fig.  354),  I'otfrtQcriims,  and  lihodnrr'mus.    Tubicoiar  annelides 


abounded,  some  of  the  species  being  solitary  and  attached  to  shells,  corals, 
Ac,  others  occurring  in  small  clusters  and  some  in  gregarious  masses  form- 
ing beds  of  limestone.  The  chief  genera  are  Sptrorbis,  Serpu/Ues,  Ortonia, 
Vcrmil'ta.  Polyzoa  abound  in  some  portions  of  the  Carboniferous  Lime- 
stone, which  were  almost  entirely  composed  of  them,  the  genera  Fenestella, 
Veriopora,  RhombopnTu,  Sulcoretepora,  VhicuUiTw,  Polypora,  and  Ghuconome 
being  frequent  Of  the  brachiopods  (Fig.  355)  some  of  the  most  common 
forms  are  Producius  (the  most  characteristic  genus),  Spiri/er,  RhjMhoneUa, 
Athyris,  Choneies,  Orthis,  Terebralvia,  Lintjvla,  and  Discina}  Among  these 
are  species  that  appear  to  range  over  the  whole  world,  such  as  Produdus 
semirftiriiltifus,  o&talus,  lougisptnus,  pustuloxus,  writ,  aculeatus,  undatui  ; 
Streploikyncltns  creniitria ;  Spiri/er  Hneaius,  glaber ;  Athyrxs  ghUmlaris  ;  and 
Terebraiula  hasfata.  The  higher  molluaks  now  begin  to  preponderate 
over  the  brachiopods.  The  lamellibranchs  (Fig.  356)  include  forms 
of  Anaiiopeclen,  Posidonamya,  Leda,  Nuctda,  Sanguinolites,  Leplodomus, 
•^hixdvs,  Edim}idia,  Anlhracosia,  Modiola,  and  Conocarditim.  The  gastero- 
pods  (Fig.  357)  are  represented  by  numerous  genera,  among  which 
EiKrmphalxis,  Natiea,  Pleurolomaria,  Macrocheilus,  and  Loxonema  are  fre- 
quent. The  genus  Bellerophon  is  represented  by  many  species,  among 
which   B.    Urei  and  B.  decussfUus   are  frequent.      The  most   abundant 

'  Productui  la  almost  wholly  Carboniferoua,  and  Id  tha  speclM  /*.  giganttui  of  tha 
CBrbonif«rou3  Lituestone  Teached  tbe  mniimnm  »iza  attained  by  the  brachiopods,  Bome 
iDdividttaU  measuiing  eight  inches  acrOH.  Other  genera  had  already  eilsted  a  long  time  ; 
■ome  even  ot  the  species  were  of  ancient  date — Orihu  retupijiala  of  the  Carbonlferoiu 
Limeitoae  and  the  Devonian  0,  atriaiula  and  Strophonuiia  dtprata  had  survived,  according 
to  Gosseiet,  from  tbe  time  of  the  Bala  beds  of  tbe  Loner  Silurian  period.  (Qaaaelet,  Xt^itm, 
p.  lis.) 


STHATIGRAI'HICAL  GEOLOGY 


BOOK  \i  ViXt  n 


pteiopod  genus  is  Coniilaria  (Fig.  358),  which  often  attains  a  length  of 

several  inches  Of  the  cephalopods  (Fig.  359),  the  most  abundant  ind 
widely  distributed  are  forms  of  Orlhoceras,  Gyrioceraa,  NaMm, 
Utidles,  and  Gotmtites. 

The  Crustacea  present  a  facies  very  distinct  from  t 
the  previoiia  Palseozoic  formations.  Trilobites  now  a 
wholly  disappear,  only  four  genera  of  small  forma  {PnUmi^ 
Gnffithides,  PhUlipsia,  BrachymeUipus)  being  left.  But  other 
Crustacea  are  abundant,  especially  ostracods  (Batnfw,  (7)rpri- 
deUiiia,  Cythe-re,  Kirkhyii,  Lejierditia,  Bfyrichta),  which  crowd 
niiiny  of  the  shales  and  sometimes  even  form  seams  of  lime- 
stone. Some  schizopod  forms  are  met  with  {PaUeocaris)  and 
a  few  macrura  occur  not  infrequently,  particularly  Anlhra- 
CiriM^n  f  ■>  pai^moH  (Fig.  360)  and  Falxoa-iingon  {Crangopsis),  also  several 
PUr  i-od        phjUopods   {DUhyromris,   Cerutioearin,  Estheria,  Leaia),    with 

cun  lariiq  a  1  the  larger  merostomatous  Euri/pleiii^  and  king-crabs  {Pirst- 
wuhi-i,  lielbiwu.'i).      The  Carboniferous   Limestone   of   the 

Bntmh  Isles  has  supplic<l  somewhere  slwut  100  genera  of  fishes,  chiefly 


reprcwjuted  by  toeth  and  spines  {I'ftniiiwnhi',  Cu'-hlunhn'.  ('liiilotliu<,  Petalixliit, 
Vhimiiw,    Ilhi-.i'ihi.-,    Cfi-iioji/i/rhiiis,    A'c.)       Some  of  these  were  no  donbt 


selachians  which  lived  solely  in  the  sea,  but  many,  if  not  all,  of  the 
ganoids  pn;il)ably  mignited  Ixitwecn  salt  and  fresh  water ;  at  least  their 


8ECT.  iv  §  1  CARBONIFEROUS  SYSTEM  813 

remains  are  found  in  Scotland  not  only  in  marine  limestones,  but  also 
in  strata  full  of  land-plants,  cyprids,  and  other  indications  of  estuarine  or 
Huviatile  conditions.'    Some  of  the  fishes  met  with  in  the  plant-bearing 


type  of  the  Carboniferous  system  are  mentioned  on  p.  820,  together  with 
the  air-breathers  and  other  terrestrial  organisms. 

It  is  deserving  of  remark  that  in  the  marine  type  of  the  Carbon- 
iferous systetn  considerable  differences   may   be   observed   between   the 


fossils  of  the  limestones  and  the  shales  even  in  the  same  quarry.  The 
limestones,  for  example,  may  be  crowded  with  the  joints  of  crinoids, 
corals  of  various  kinds,  producti  and  other  brachiopode,  while  the  shales 
above  them    may   contain  few  of  these  organisms,   but  afford  polyzoa, 


Mb-  SM.-Csrbi 


la  (»fl«r  Tni.|u«ir). 


Vonulmi'i,  horny  brachiopods  {LtTUfulu,  Disdtta),  many  lamellibrancha, 
especially  pectens,  aviculopectens,  nuculas,  le<las,  and  gasteropode 
(I'Unrvtumai-in,  Lonmemi,  JSellerophoii,  &f.)  It  is  evident  that  while  some 
organisms  flourished  only  in  clear  water,   such  as  that  in  which  the 


814 


STBATIGRAFHWAI.  GEOLOGY 


BOOK   V'l  PAST  U 


limestones  acciunulated,  others  abouuded  on  a  muddy  bottom,  although 
some  seem  to  have  lived  in  either  situation,  if  we  may  judge  from  finding 
their  remains  indifferently  in  the  calcareous  and  the  muddy  deposit& 

The  second  phase  of  sedimentation,  that  of  the  c<hi1  -  awamps,  is 
marked  by  a  very  characteriBtic  suite  of  organic  remains.  Meet  abondant 
of  these  are  the  plants,  which  possess  a  special  interest,  inasmuch  as  they 
form  the  oldest  terrestrial  flora  that  has  been  abundantly  preserved.' 

This    flora    is    marked    by   a 

singular  monotony  o(  character 
all   over   the  world,   from  the 
Bquator  into  the  Arctic  Circle, 
the  same  genera,  and  sometiiiies 
even  the  same  species,  appear- 
ing to  have   ranged   over  the 
whole  surface  of  the  globe.     It 
consisted    almost    entirely    of 
V    vascular  cryptogams,  and   pre- 
1   eminently  of  Equisetacete,  Lyco- 
;    podiacea;,  and  Ferns.      Though 
'eforable    to   existing    groups, 
ihe  plants  presented  many  re- 
liable differences  from  their 
^   living  representatives.      In  par- 
'  ticular,  save  in  the  case  of  the 
,'  ferns,  they  much  exceeded  in 
size  any  forms  of  the  present 
vegetable  world  to  which  they 
T  ^[(^     V'~  f     ean  be  assimilated.    Our  modern 

\,::-^;'  J^^     fl  horse-tails    had    their   allies  in 

huge  trees  among  the  Carboni- 
ferous jungles,  and  the  familiar 

„„.;,:,«;^;::'=;;::;:k.„.       ci«i>-n.o»  ot  o,,,  miu,  „o»  . 

low  creeping  plant,  was  repre- 
sented by  tall-stemmed  Lcpidodendra  that  rose  fifty  feet  or  more  into  the 
air.  The  ferns,  however,  present  no  such  contrast  to  forms  stiU  living. 
On  the  contrary,  they  often  recall  modem  genera,  which  they  resemble 
not  merely  in  general  aspect,  but  even  in  their  circinnate  vernation  and 
fi-ucttfication.  With  the  exception  of  a  few  tree-ferns,  they  seem  to  have 
been  all  low-growing  plants,  and  perhaps  were  to  some  extent  epiphytic 

'  Uiit)ieCsrbonireTousflara,couBultA.  BrongniaH, ' Prodrome d'uue Histoire des V^plaDi 
fossiks,' 182S  ;  Liudlcyand  Uuttou,  '  Fossil  Flora  of  Great  Britaio,' 1831-37.  C.  G.  Weio. 
'  Fosailc  Flora  J,  jilagateQ  Steinkohl  ini  3aor-RheiQ-GBb,'  Bonn.  1869-72.  '  Die  Flora  A 
Stuiakolileti  Formation,'  Berlin,  1881.  WilluiDisou's  Meinolra  'On  the  Organisation  of  Ibr 
Plants  of  the  Coal  Measures,'  Phil.  Trana.  cUlL  (1872),  and  subsequent  t-olumes.  Zeiller, 
on  the  Cnrbouiferous  flora  of  Valencieiiues,  Auluu,  aad  Brire,  in  tbe  aeries  of  voliune* 
entitled  '  fitudes  des  Giles  Mincraui  de  la  France,'  iiuUlisbed  by  the  Ministry  of  Public 
Works  ;  Zeiller  aud  Renault  OL  Fo^il  Flora  of  Commeiitr)-,  Biill.  Soc.  Indutl.  iTin.  S. 
Eaeane,  2  vols,  with  Atlas,  1888-90.    R.  Kiiiston,  Tmni.  R.  S.  fi/in.  xiiv.  tt  teq. 


CARBONIFEROUS  SYSTEM 


Fig.  3M.— A,  AnnoUrii  ipb«nnpliylloidM :  b,  AitnophTllltfa. 


%lf;  STKATIGRAPHICAL  GEoUjGY  book  n  past  u 

upon  the  larger  vegetation  of  the  lagoons     Some  of  tlie  more  cohudod 
genera   are    ^yph^noft^ru^   XeuropftrU,   Cttrioptrrif,    Oiomicpieriiif   Ptoifteru, 

AUihopkr'i^} 

Among  the  Equisetaceae,-  the  genus  CalimiUjt  is  speciaUr  abundant 
It  usually  occurs  in  fragments  of  jointed  and  finely- rihbed  stems. 
From  the  roundefl  or  blunted  base  of  the  stem,  other  stems  budded,  and 
numerous  rootlets  proceeded,  whereby  the  plants  were  anchored  in  the 
mud  or  sand  of  the  lagoons,  where  they  grew  in  dense  thicketa.  Accord- 
ing to  Sir  J.  Dawson  they  seem  to  have  fringed  the  great  jon^es  of 
Sigillaria?,  and  to  have  acted  as  a  filter  that  cleared  the  w«ter  of  its 
sediment  and  prevented  the  vegetable  accumulations  of  the  ooal-swamps 
from  admixture  with  muddy  sediment.  To  the  foliage  erf  Calamites 
different  generic  appellations  have  been  attached  (Fig.  366).  The  mune 
AsttrophjUites  (Calarnocladus)  is  given  to  jointed  and  fluted  stems  with 
verticils  of  slim  branches  proceeding  from  the  joints  and  bearing  whorls 
of  long,  narrow,  pointed  leaves.  In  SpIienopInfUum  the  leaves  were  fewer 
in  numlicr  and  wedge-shaped ;  in  Annularia,  the  close-set  leaves  were 
united  at  the  base.  Calamodendron  is  believed  by  some  botanists  to  be 
the  cast  of  the  pith  of  a  woody  stem  belonging  to  some  unknown  tree,  by 
others  it  is  regarded  as  only  a  condition  of  the  preser\'ation  of  CalamUt^, 
Some  fruits,  supposed  to  l>elong  to  the  calamaries,  have  been  met  with. 
Poth^rit^i  has  been  referred  to  AsterocalamiieSy  Stachannularia  seems 
attached  to  Annularvt^  while  others  known  as  Calanwstackfs  and 
Mtirrostadif/s,  are  proljably  the  fructification  of  calamites. 

The  Lycopods  (Fig.  367)  were  represented  by  numerous  species  of  the 
^^enus  Le/nd<xletuJ roily  distinguished  by  the  quincuncial  leaf-scars  on  its 
clichotomous  stem.  Its  branches,  closely  covered  with  pointed  leaves, 
Ik^fc  at  th<'ir  ends  cones  or  spikes  {Lep'vlodrohus)  consisting  of  a  central 
axis,  round  which  were  placed  imbricated  scales,  each  carrying  a  spore- 
case.  Other  conspicuous  genera  were  Uhtdendron,  Knorria,  LepuIapfMoi, 
Ho  Ion  i'  I ,  C  [i/rloclif/Iia . 

Among  the  most  remarkable  trees  of  the  Carlx)niferous  forests  were 
the  Sigilhirioids,  which  are  believed  to  have  licen  akin  to  the  Lepido> 
(lendra.  The  genus  Higilhiria  was  distinguished  by  the  great  height  (50 
feet  or  more)  of  its  trunk,  which  sometimes  measiu'ed  fixe  feet  in  diameter. 
Its  stem  was  fluted  (Fig.  368),  and  marked  by  parallel  perpendicular 
lines  of  leaf-scars,  but  as  it  grew  these  external  markings  were  lost.  The 
bascj  of  the  stem  passes  into  the  roots  known  as  Stigmariiiy  the  pitted  and 
tuberculed  stems  of  which  are  such  common  fossils  (Figs.  368  B,  369). 
There  can  be  little  doubt,  however,  that  StigmarUi  was  a  form  of  root 
common  to  more  than  one  kind  of  tree.  The  genus  Cordaites  belonged 
U)  a  type  of  tree  which  by  some  botanists  has  been  placed  among  the 
cycads,  by  others  among  the  conifers.  It  attained  a  great  profusion  in 
the  time  of  the  (.-oal -measures.  Shooting  up  to  a  height  of  20  or  30  feet, 
it  carried  narrow  or  broad,  parallel- veined  leaves,  somewhat  like  those  of 

'  For  an  essay  on  tlie  mori)liolofO'  aud  classitioation  of  the  Carboniferous  ferns  see  D. 
Stur,  SH:h.  AhnL  Wien.  Ixxxvi.  (1883). 

-  On  CarhonifcTOUs  Calaiuaries,  consult  Weisis,  Abh.  (Jeol,  S^itciaikarU  Prtusant,  v. 


JBCT.  iv  §  1  CAEBONIFESOVS  SYSTEM  817 

I  Yucca,  which  were  attached  by  broad  bases  at  somewhat  wide  distances 


to  the  stem,  and  on  their  fall  left  prominent  leaf-scars.     It  bore  catkins 


Fill,  xa A,  aiKiiinri* 


B,  MlKHIuia  lUiii  ttrni'tiuttiie 


(Anlholilhits)  which  ripened  into  berries  not  unlike  those  of  Yews  {Cardio- 
earjftts)  (Fig.  371).     Both  of  these  forms  of  fructification  occur  in  great 


STRATIGRAPHICAL  GEOLOGY  BOOKTiPABin 


abundance  in  some   bands   of   Bhale       True   Conifene  were   probably 
abundant  on  the  drier  ground  for  their  ateme  {Dadoxyhn,  jirattearioxf^on. 


Pinites)  have  been  met  with  particularly  in  the  tuffs  of  ancient  volcanic 
cones,  on  which  they  no  doubt  grew  and  in  sandstone,  where  they  occur 
as  drift-wood,   perhaps  from  higher  ground   (Fig.  370).     It  should  be 


ilfc,   ,^_^-i 

SiSfRSt^iii,.  W  , 

F5?«*-T 

'^^il^bH^H 

3;:Mi^ 

remembered  that  the  flora  preserved  in  the  Carboniferous  rocks  is 
essentially  that  of  the  low  grounds  and  sn-amps.  The  fruit  known  as 
Trigmuairpas  is  supposed  to  be  coniferous,  somewhat  like  the  fruit  of  the 


SECT,  iv  ^  1 


CARBONIFEROUS  SYSTEM 


living  Salisbima.  That  true  monocotyledons  existed  in  the  Carboniferous 
period  was  until  recently  supposed  to  be  proved  by  the  discovery  of  a 
number  of  spikes,  referred  to  the  living  order  of  Axoidore  (Pothoeiia),  in 
the  lower  part  of  the  Carboni- 
ferous system  of  Scotland;  but 
Mr.  R.  Kidston  has  shown  that 
the  specimens  are  almost  certainly 
the  fructification  of  Bamia,  a  genus 
of  Calamite.' 

The  animal  remains  in  the 
coal-bearing  part  of  the  Carboni- 
ferous rocks  are  comparatively 
few.  As  already  stated,  in  cer- 
btin  bands  of  shale,  coal,  and 
ironstone  in  the  lower  half  of  the 
Coal-measures,  undoubted  proofs 
of  {the  presence  of  the  sea  are 
afforded  by  the  occurrence  of  some 
of  the  familiar  shells  of  the  Car- 
boniferous Limestone.  But  towards  the  upper  part  of  the  Coal-measures, 
where  these  marine  forms  almost  entirely  disappear  (among  their  last 
representatives  being  species  of  Lingula  and  LHseina),  other  mollusks, 
that  were  probably  denizens  of  brackish  if  not  of  fresh  water,  occur 
in  abundance.     Among  the  more  frequent  are  Anlhraanasa,  Anthracosia, 


toatta  of  Stnpwilua  «un 


and  AiUhracoptera.  Crustaceans  are  chiefly  represented  by  Beyrichiu  and 
Esthervi,  Imt  large  eurypterid  forms  likewise  occur.  Fishes  are  found 
frequently,  remains  of  the  larger  kinds  usually  appearing  in  scales, 
teeth,  fin-spines,  or  bones,  while  the  smaller  ganoids  are  often  preserved 
'  .Inn.  Mag.  Xat.  HUL  M*r  1S83,  p.  297. 


JtWA-."        f.rM\3UJa. 

:r.«  V,  •.!>:  ji->«r  ->trj«>  'i  Mr.  ?*na*iw  -j«»  wtr*  izmw.  ^  s*  !?» 
«m:s^U.'      7KI.FIA.W    £«rtfOaa«t  k««  bc^ 

373',     '"rrr   i   im  '"  "i^i       'r    in  Tiifiaj.  jimb 

fomu  '!<  fpi4er     {"-Ai^r-Mi  ■.      MTrnpodL  «t 

wfaieh  cp'TArd'-  'itf  tO  q>wK»  !kt«  b«eB  dcMr- 

minol.  w«r«  r«iir>!:ienu4  br  fsrwiH  [Jibi  i«ii^ 

miIliIi«d«A    'AV-^nv.'.    .irdtimimK  Jmim*.    fijAr 

■^no'.     TrtK  inarvu    Hkevwe  flhtol   ihroit^ 

rbctK  dntat  jun^I^-'.  ar>i  during  the   ba  ^ 

y«An  tb«  nnmlitr  of  fpmi»  deun«d  k*s  been 

7  -o  Urge  that  it'i  t>>er  tb»i   239   fpeeie»  of 

.^^   >.y  'frtifjpUTa,  109  'rf  ii«DiTipc<>n.  17  <rf  hcHiptoK. 

andll  of  o:4e»pterahaT«be«iobtainaL  TVia^ 

(•■K  J-!    '•?-».  .!..-..  -..TTwr.,    [jj^j^  remain*  have  li>t«n  bill  MantilT  pftaeiicd. 

'X^'w'f'v'V.-',.  K.^.-    *"•=  know  thai  t  her  included  uKienl  fonnf  of 

-'.-.  :  inai-flv,  nuekmach.  cri-ket,  and  beetle.       It  u 

rirfuarkalile  that  from  *vjme  Miol-fields  hardlr 

a  tiri;.'!'-   tra^:':  'if  iii.4(^t  lif>;  ha^   lie«n  otiiaine'l.  while  in  others  great 

iiiitriUr.f   of    ^j<i:i:imi:iin    h»\e    Iieeri    lir'.tu^bt    to   light.       A    remarkable 

vari'^ty    of    foiTdn    ba.t    l«en    fouiKl    in    the  .Saarbnick    Coal-field;    hut 

|f;rlM{rt  th<:  ;:i'i^at<;.tt  riumlier  of    indiiidual  g})ecimenE    has    come  from 

thai  of  ('orniiictit.rv,  which  uji  Ui  the  end  of  the  year  H^^l  is  computed 

Ui    havt:    fiirriii-hi^l    not    Ickk    than    130<J    Iridinduab.       Some    of    the 

jti-*<;<it  wirii;  of  cotiHidentble  *izi:.     Thii*  the  neuropterous  Arduf/^Sv.- 

from    th'-    Ihirlij'ithin;    <>«l-ticld    had    a    sjtread    of    wing   of    perhap' 

(oiirt<;';ii   iii'ln;.t  or  iriorc  ;  and  a  »i>eci«s  of  lUdt/fjiifura  (D.  ifrnvi)  had 

a   wiii^^  iiliiiiit   Iw'i'lvc  inches  in  length.      Others   were    remarkable  for 

thi!  vividmrHM  of  their  colouring  {lln/dia),  the  markings  of  which  are  «till 

rrco;{(ii>-al>li:  ill  th(;  foNHil  H[iecimetiK.     Oiii'  of  the  most  singular  featurei- 

yi-A,  •i\i->i:rvi:i\  (inioiij!  thoitc  ancient  ingectji  is  the  union  in  the  same  indi- 

viihiul  'if  lyjH-s  of  struvture  which  are  now  entirely  distinct.     M.  Ch. 

Iti'iiii;:iiiart  hiiM  ri:ci.-ntly  Hhown  that  wings  which  were  admittedly  neuro- 

piiioiiH,  aii'J  wi:ti:  referred  to  the  genus  IHftyni'vrn,  were  really  attacfae<l 

Ui  lnHli'--"  whirli  arc  uiKiueHtioiiably  orthoptcrous.- 

I    l!,-'l.   r.X  ilr.l.  Hart.  .V...  71,  1S»1. 

"  I'll.    KroiiKiiiiirt.    lUUI.   S,k.    lif-jl.    Ff-iiic!  (»;.  il.    ]>.  143  ;  aluo  ScoddcT,  Otel.   Mag. 
IKKl.  [..  lil'l);  Mm..  /fc«/»H.  .Sue.   AW.  Uul.  iiL  (1883)  p.  213;  Proc.  Aatr.  Atad.  1884. 


SECT,  iv  §  1  CARBON IFERO  US  SYSTEM  82 1 

The  Labyrinthodonts  which  appeared  in  Carboniferous  times  as  the 
magnates  of  the  vertebrate  world  had  a  salamander-like  body  with 
relatively  weak  limbs  and  a  long  tail.  Sometimes  the  limbs  seem  to 
have  been  undeveloped,  so  that  the  body  was  serpent-like.  The  head 
was  protected  by  bony  plates,  and  there  was  likewise  a  ventral  armour 
of  integumentary  scales.  The  British  Carboniferous  rocks  have  yielded 
about  20  genera  (Anihracosaurus,  Laxomnia,  OphiderpetoUy  Fholiderpeton, 
Pteraplax,  UrocardyliLs,  &c.)  These  were  probably  Huviatile  animals  of 
predaceous  habits,  living  on  fish,  Crustacea,  and  other  organisms  of  the 
fresh  or  salt  waters  of  the  coal-lagoons.  The  larger  forms  are  believed  to 
have  measured  7  or  8  feet  in  length ;  some  of  the  smaller  examples, 
though  adult  and  perfect,  do  not  exceed  as  many  inches.^  The  coal-field 
of  Bohemia,  which  may  be  in  part  Permian,  has  likewise  furnished  a 
considerable  number  of  genera  and  species  of  Labyrinthodonts  and  fishes.* 
The  terrestrial  fauna  obtained  from  the  interior  of  fossil  trees  in  the 
Coal-measures  of  Nova  Scotia  includes  land -shells  of  which  several 
genera  are  now  known  (Dendropupa,^  Pupa,  Anihracopupa,  Zanites,  and 
Dawsonella). 

Fossil  plants  do  not  serve  so  well  for  purposes  of  geological  classifica- 
tion as  fossil  animals  (pp.  652,  660,  668).  In  the  Saxon  Coal-field, 
however,  Geinitz  (1856)  distinguished  five  zones,  each  characterised  by 
its  own  facies  of  vegetation.  1st.  The  Culm  with  Lepidodendron  veithei- 
mianum,  CiUamites  transUioniSy  followed  by  the  remaining  four  zones, 
which  comprise  the  productive  coal-measures;  viz.  2nd,  the  zone  of 
Sigillarias  ;  3rd,  the  zone  of  Calamites ;  4th,  the  zone  of  Annularia ;  and 
5th,  the  zone  of  Ferns.*  More  recently  Grand*  Eury  has  subdivided  the 
Carboniferous  system  of  central  France  into  the  following  members, 
according  to  the  succession  of  vegetation  :  •** — 

Supra-Carbouiferous  Flora,  simpler  and  less  lich  than  that  below,  showing  a 
passage  into  the  Pennian  flora  above,  characterised  by  a  rapid  diminution  oi  Alelhopteris, 
Odontopteris  xenopteroidcs^  DictyopteriSy  AnmUaria,  Sphenophyllum,  The  Calamites  are 
represented  by  abundant  individuals  of  C.  variaiis  and  C.  Suckotcii,  also  AsterophylliUs 
equisctiforniis  ;  the  ferns  by  Pecopteris  q/atheoide^,  P.  JiemitelioideSy  Odontopteris  minora 
O.  Schlotheimii,  several  species  of  Neuropt^rU,  Ac.  ;  the  Sigillarias  by  ^S'.  Brardiiy  S, 
spinulosa,  and  Stiginaria  ficoides ;  Cordaites  by  numerous  narrow-leaved  forms  ;  the 
Calamodendra  by  a  prodigious  abundance  of  some  species,  e.g,  Calamodendron  histrialum, 
Calamites  cruciafus,  Arthropitus  subcotnmunis  ;  the  conifers  by  Walchia  piniformis  a.nd 
some  others. 

p.  167  ;  Bidl.  U.S.  Oeol.  Surv,  Nos.  31  and  71.  H.  Woodward,  Q.  J.  Geol,  Soc.  1872,  p. 
60.  The  student  interested  in  the  study  of  fossil  insects  will  find  Mr.  Scudder's  Bibliography 
of  the  subject,  Bull.  U.S.  Oeol.  Surv.  No.  71  (1890),  a  valuable  book  of  reference. 

»  MiaU,  Brit.  Assoc.  1878,  1874. 

^  C.  Feistmantel,  Arcliiv.  Xaiurtc.  Landesdurcliforsch.  Bohmen.  v.  No.  3  (1888),  p.  55  ; 
A.  Fritsch,  *  Fauna  der  Gaskohle  Bohmens,*  1879  and  subsequent  years. 

5  J.  W.  Dawson,  Phil.  Trans,  vol.  173(1882),  p.  621. 

*  'Geognost.  Darst.  Steink.  Sachsen/  1856,  p.  83;  'Die  Steinkohlen  Deutschlands,' 
1865,  i.  p.  29. 

^  '  Flore  Carbouifere  du  Departement  de  la  Loire  et  du  Centre  de  la  France,*  Cyrille 
Grand*  Eury,  Mem.  Sav.  Ktrangers,  xxiv.  (1877). 


822  STRATIGRAPHICAL  GEOLOGY  book  vi  part  n 

Upper  Coal  Flora  (properly  so  called). — Calami tes  often  abundant — C.  inter- 
ruptns,  C.  Svckomiit  C.  cannseformiSy  AsterophylliUs  hippuroides,  Maerostaehya  iv^undi- 
buliformis  (very  common),  AnniUaria  hrevifoliay  and  A.  l<mgifolia  (common  through- 
out), Sphenophyllum  oblongi/olium.  Ferns  richly  developed,  particularly  of  the  genera 
Pecopteris  {P.  nnita,  argutaf  polymorphay  and  especially  Schlotheimii) ;  OcUnUopieria  {0. 
reichiana,  Brardiiy  mixoneura,  xenopteroides,  the  last  extremely  abundant) ;  CaulopUrut 
macrodiscuSj  Alethopteris  Grandini  in  great  profusion,  Callipteridiura  {C.  ovatum,  giffOSy 
densifoliay  common).  Lepidodendra  have  almost  disappeared  ;  Sigillarise  are  not  un- 
common {S.  rhitydolcpiSy  S.  Brardii\  with  Stigjnariopsis  and  Syringodendron.  Cor- 
daUes  occurs  in  great  abundance ;  the  conifers  are  represented  by  WaZchia  pinifcrmis 
and  a  few  other  species.  Calamodendra  occur  in  great  abundance,  especially  CcUamiUs 
cruciatiis. 

Upper  Coal  Flora — (Lower  Zone,  Flore  du  terrain  houiller  9ou8-8up6rieure), — 
Calamites  and  Asterophyllites  abundant  in  individuals  and  species  {C,  Suckotoii,  CisUiy 
cann«formiSy  varianSy  approximaiuSy  A.  rigidvSy  grandiSy  hippuroides),  Annularia 
radiatay  Sphenophyllum,  Among  the  ferns  there  are  few  true  sphenopterids,  but  Neurop- 
teris  is  common  (iV.  flexuosay  auricvlata)y  also  OdoiUoptcris  {0.  reichianay  SMothtimi%)y 
Pecopteris  {P.  arborcscensy  pulchrUy  candollianay  villosOy  oreopteridia,  crenulalcL,  cupi- 
doideSy  elegans)y  Canlopteris,  Psaronins.  Lepidodendra  are  few  (X,  Stemhergiiy  elegansj 
Lepidostrobus  aub-variabilis,  Lepidophloios  lar^cinvSy  Kncrria  Selloniy  Lepidophyilum 
majus).  Sigillarioid  forms  are  likewise  on  the  wane  when  compared  with  their  profhsion 
below  {Sigillaria  ellipiica,  Candolliiy  tesselkUay  elegansy  grasianUy  Brardiiy  spinulosa: 
Syringodendron  cyclosligmay  distans  ;  Stigmaria  Jicoides  AhwxLdL&iit),  Cordaites,  however, 
now  becomes  the  dominant  group  of  plants,  but  with  a  somewhat  different  facies  finom 
that  which  it  presents  in  the  middle  Coal-measures  (C.  borassifolius,  C.  prindpaliSy 
Dadoxylon  Brandlingiiy  Cardiocarpus  emargiiuUnSy  Gulbieri,  major,  ovatus),  CcUa- 
mites  crucicUvs  makes  its  appearance,  also  Walchia  piniformis. 

Middle  Coal  Flora — (Upper  Zone,  Sttpm-tnoyeniie). — Calamites  numerous  (C, 
Sitekowiiy  Cistii,  cannseformiSy  ramosus ;  Asterophyllites /oliosuSy  longifoliiiSy  grandiSy 
rigidus ;  Annvlaria  minvtay  brcvifolUi ;  Sphenophyllum  saxifrag8efolium,y  Schlolheimiiy 
trimcalum,  majus.  Ferns  represented  by  Sphenopteris  {S.  latifoliay  irregvlariSy  trifolio- 
lata,  cristata,  &c.)  Prepecopieris  (maximum  of  this  genus),  Pecopte^ris  {P.  abbreviaiay 
villosa,  Gistii,  07'copteridia,  Ac),  CaulopteriSy  NcnropteriSy  and  other  genera.  Lepido- 
dendra are  not  infrequent  {Lepidodendron  axuleatiwi,  Sternbergit,  rJeganSy  rimosum  ; 
Lepidostrobus  variabilis;  Lepidophloios  laricinuSy  Lepidophyilum  majus)y  and  various 
Lycojwdites.  Tlie  proportion  of  Sigillaria  is  always  large  {S.  Cortei,  intermediay  Silli- 
manni,  fessellata,  ajclostigmay  altemaiiSy  Brongniarti,  Stigmuria  Jicoides,  minor).  Pseu- 
dosigillaria  is  abundant,  especially  P.  monostigma.  Cordaites  appears  in  some  places 
abundantly  {C.  borassifolius,  Artisia  transivrsOy  Cladiscus  scJinorrianvs),  and  its  fruits 
are  numerous  and  varied  (Cardioearptis  cmargiiiatvSy  orbicularis,  ovalus). 

Middle  Coal  Flora  (properly  so  called),  characterised  above  all  by  the  dominant 
place  of  the  Sigillarioids,  which  now  surj)ass  the  lepidodendroids  and  form  the  main 
mass  of  the  coal-seams.  The  genus  Sigillaria  here  attains  its  maximum  development  {S. 
Groeseriy  angusta,  scutellatay  intermediay  elongata,  nofatUy  altemans,  ntgosa,  reni/ormisy 
leopoldinay  and  many  more  ;  Psciidosigillaria  striatal,  rimosay  monostigma  ;  Stigmaria 
Jicoides,  minor).  Lepidodendroids  are  large  and  frecjuent  {Lepidodendron  aetUeeUumy 
of)otHUnm^  eaudaiuniy  rimosum,  Stembergiiy  elcgans ;  LepidoplUoios  laricinus ;  Uloden- 
dron  majuSy  minus;  Haionia  tvbcrculatay  tortvosa,  regularis ;  Lepidophyilum  majus; 
Lejn'dosfrobus  variabilis).  The  ferns  are  abundant  and  varied  ;  the  Sphenopterids 
include  many  si)ecies,  of  which  Sphenopteris  Hoeainghausii  and  tenella  are  common  (also 
S.  Bronniy  Schlotheimii,  tenuifolia,  rigida,  furcafu,  elcgans);  Alethopteris  is  very 
j>lentiful  {A.  lonchitica,  Scrlii,  Mantclliy  heterophylla) ;  also  Lonehopteris  Bricii  and  L. 
Jidh/ii ;  Prcpecojyteris,  PccopferiSy  Afegaphyton,  Neuropterts  {N,  flex^tosay  Loshii,  tenni/oHUy 


SECT,  iv  §  1  CARBONIFEROUS  SYSTEM  823 

gigantea\  CydopteriSj  AulaeopUria.  The  calamites  are  widely  diffused  and  abundant, 
especially  Calamites  dubiits,  undukUus,  ramosua,  deeorcUtis,  Steinhaueri  ;  AsUrophylliUs 
subhippuroideSy  grandiSf  lomgifolius ;  Volkmannia  binneyana ;  SphenopkyUv/m  seems 
here  to  reach  its  maximum,  characteristic  species  being  8.  emarginatum,  Mxi/ragmfolium, 
erosumy  cUrUaium,  trunaUum,  Schlotheimii.  Some  coals  and  shales  abound  with 
CardiocarpuSf  also  TrigonocarpuSf  and  Ndggeraihia, 

Middle  Coal  Flora  —  (Lower  Zone,  Flore  houUUre  aaus-moyenne), — Lepido- 
dendroids  are  characteristically  abundant  and  varied  {Lepidodendron  aculeatumf  ob<na- 
tunij  crenatum,  Haidingeri,  undulatumy  longi/olium  ;  and  Lepidaphloioa  larieintia,  irUer' 
mediuSf  crassicavlis ;  Ulodendron,  abundant  in  England,  U.  dichotomumf  punetatunif 
majtiSf  minusy  &c.  ;  Halonia  tartuoaa,  regulariSy  &c. )  Sigillariolds  are  numerous  {Sigil- 
laria  oculata,  elegans,  setUelkUa,  eUmgatay  mamUlariSf  alveolariSy  ren^ormia  ;  Stigmaria 
ficoideSy  minor y  sUlkUa,  reticulata;  DietyoxyloUy  Lyginodendron).  Calamites  abound 
{Calamites  cannae/ormiSy  Suckowiiy  Cistii,  deeoratuSy  approximatus ;  Aatercphyllites 
subhippuroidesy  longi/olius ;  Volkmannia  polystadiya).  Ferns  likewise  form  a  notable 
part  of  the  flora,  especially  sphenopterids  {Spkenopteris  IcUi/oli^h  eunUi/oliay  elegans, 
dissectayfuTcatay  Oravenhorstiiy  nervoaay  muricaiay  obtusilobaj  trifoiiata) ;  also  Frepecopteria 
sUesiacay  oxyphyllay  Olockeriy  dentata ;  MegaphyUm  mAJJus ;  Feeopteris  ophiodermatica 
and  other  similar  forms.  The  nenropterids  become  abundant  {Neuropteris  heterophylla, 
Loshiiy  giganteay  tenui/olia ;  Cyelopteris  obliqua ;  Alethopteris  lonehiHcay  &c.)  The 
abundant  Cordaites  of  the  higher  measures  are  absent,  though  the  fruit  Carpolithes 
occasionally  occurs. 

Infra  Coal-measure  Flora — (Millstone  grit,  Vitage  infra-houiller)y  cha- 
racterised essentially  by  lepidodendroids  and  stigmarias. — Lepidodendron  cuuleatumy 
ohcvatumy  crenatumy  brevi/oliumy  eaudatum,  carinatumy  rimosum^  volkmannianum  ;  Ulo- 
dendron punctatumy  ellipticumy  majus  ;  ffalonia  tuberculosa  ;  Lepid^yphloios  intermediuSy 
Inricinus,  Sigilldria  is  not  very  common,  but  S,  oculatay  aZveolata  (Stem. ),  Knorriiy 
trigonay  mininuiy  and  other  species  occur.  The  ferns  are  more  varied  than  in  older 
parts  of  the  system,  sphenopterids  being  the  dominant  tyi>e8  {Spkenopteris  distanSy 
elegatis,  tridactyliteSy  fnrcaJtay  dissectay  rigiday  divaricainy  lineariSy  CLCuiiloba,  Ac.) 
The  genus  Feeopteris  is  represented  by  a  few  species.  Neuropteris  is  comparatively  rare 
{N.  Loshiiy  tenuifolia)y  Alethopteris  appears  in  the  widespread  species  A,  Umckiticay  and 
a  few  others.  Calamites  are  not  relatively  abundant  {Calamites  undnlatusy  SteinhaueHy 
comm.uniSy  cannsgformis,  Cistii;  Asterophyllites /oliosuSy  &c.) 

Flora  of  the  Upper  Greywacke. — Lepidodendroids  are  the  prevalent 
forms  {Lepidodendron  carinatumy  polyphyllumy  volhmannianumy  rugosum,  caudatumy 
aculeatumy  obovatum  ;  Halonia  tetrasHchay  regularis  ;  Ulodendron  ovalcy  eommutatum). 
Stigmaria  in  several  species  occurs,  sometimes  abundantly ;  but  SigUlaria  is  rare  {S.  un- 
dulcUUy  Voltziiy  costatay  subeleganSy  venostty  Ouerangeriy  vemeuillana).  Calamites  are  not 
infrequent  {C.  Roemeriy  Voltziiy  eannmformiSy  &c.)  The  ferns  are  chiefly  sphenopterids 
{Spheiwpteris  dissectOy  eleganSy  Oeradorfiiy  distanSy  tridactylitesy  schistorum ;  Cyelopteris 
tenuifoliay  Haidingeriy  flabellata  ;  Frepecopteris  aspera^  svhdentata  ;  Neuropteris  hetero- 
phyllay  Loshil). 

Flora  of  the  Culm,  characterised  by  the  abundance  of  lepidodendroids  of  the 
type  of  L.  veltheimianum  (with  Knorria  imbricata)y  by  the  number  otBomia  transitumis, 
associated  with  Calamites  Roemeriy  Stigmaria  ficoides  (and  other  species),  and  by  the 
abundance  of  the  paleeopterid  ferns  {Falmopteris  Machanetiy  antiqua,  dissectay  {Spkeno- 
pteris) affinis  (Fig.  364) ;  Cardiopteris  frondosa  ;  Rliodea  divariciUa,  eleganSy  moraviea; 
Spkenopteris  Goppertiy  Sckimperiy  &c.) 

Carboniferous  Limestone  Flora. — The  palseopterid  ferns  reach  a  maxi- 
mum {Fals^teris  insBquilateniy  lindseseformiSy  polymorphay  frondosa).  Sphenopterid 
forms  are  found  in  Spkenopteris  bifiday  tanceolata,  con/crti/olia.  The  old  genus 
Cyclostigma  here  disappears  {C.  minutay  Nalhorstii).     The  more  characteristic  lepido- 


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SECT,  iv  §  2  CARBONIFEROUS  SYSTEM  826 


3.  Coal-measures^ 


1.   Carboniferous 
Limestone'* 
series 


^Retl  and  grey  sandstones,  clays,  and  sometimes  breccias,  with  occa- 
sional seams  and  streaks  of  coal  and  Spirorbis  limestone  {Cy there 
inflataf  Spirorbis  pusiUtis  {carbonarius). 

Middle  or  chief  coal-bearing  series  of  sandstones,  clays,  and  shales, 
with  numerous  workable  coals  {Anthracosiay  Anthracomj/a,  Bey- 
richiaj  Estheria,  Spirorbis,  Ac.) 

Gannister  beds,  flagstones,  shales,  and  thin  coals,  with  hard  siliceous 
(gannister)  pavements  {Orthoceras,  Ooniatites,  Posidonomya^  Avi- 
ctdopecUn,  Lingula,  &c.) 
2.  Millstone  Grit — Grits,  flagstones,  and  shales,  with  thin  seams  of  coal. 

''Yoredale  group  of  shales  and  grits,  passing  down  into  dark  shales 
and  limestones  {Ooniatites,  AvicuXopecten,  Posidonomya,  LinguiUy 
DisdnOy  &c.) 

Thick  (Scaur  or  Main)  limestone  in  south  and  centre  of  England 
and  Ireland,  p«Msing  northwards  into  sandstones,  shales,  and  coals 
with  limestones  (abundant  corals,  polyzoa,  brachiopods,  lamelli- 
branchs,  &c.) 

Lower  Limestone  Shale  of  south  and  centre  of  England  (marine  fossils 
like  those  of  overlying  limestone).  The  Calciferous  Sandstone 
group  of  Scotland  (marine,  estuarine,  and  terrestrial  organisms), 
probably  represents  the  Scaur  Limestone  and  Lower  Limestone 
Shale,  and  graduates  downward  insensibly  into  the  Upper  Old  Red 
Sandstone. 

1.  Carboxifeuous  Limestone  Series  and  local  equivalents. — In  the  south-west 
of  E  n  g  1  a  u  d,  and  in  South  Wale  s,  the  Carboniferous  system  passes  down  conformably 
into  the  Old  Red  Sandstone.  The  jjassage  beds  consist  of  yellow,  green,  and  reddish 
sandstones,  green,  grey,  red,  blue,  and  variegated  marls  and  shales,  sometimes  full  of 
terrestrial  plants.  They  are  well  exposed  on  the  Pembrokeshire  coasts,  marine  fossils 
being  there  found  even  among  the  argillaceous  beds  at  the  top  of  the  Red  Sandstone 
series.  They  occur  with  a  thickness  of  about  500  feet  in  the  gorge  of  the  Avon  near 
Bristol,  but  show^  less  than  half  that  depth  about  the  Forest  of  Dean.  At  their  base 
there  lies  a  bone-bed  containing  abundant  jwilatal  teeth.  Not  far  above  this  horizon, 
j)lant- bearing  strata  are  found.  Hence  these  rocks  bring  before  us  a  mingling  of  terres- 
trial and  marine  conditions.  In  Yorkshire,  near  Lowther  Castle,  Brough,  and  in 
Ravenstonedale,  alternations  of  red  sandstones,  shales,  and  clays,  containing  Stigmaria 
and  other  plants,  occur  in  the  lower  part  of  the  Carboniferous  Limestone.  Along  the 
eastern  edge  of  the  Silurian  hills  of  the  Lake  district,  at  the  base  of  the  Pennine  escarp- 
ment and  round  the  Cheviot  Hills,  a  succession  of  red  and  grey  sandstones,  and  green 
and  red  shales  and  marls  with  plants,  underlies  the  base  of  the  Carboniferous  Limestone. 
It  is  highly  prolmble,  however,  that  these  red  strata  form  merely  a  local  base,  and 
occur  on  many  successive  horizons  ;  so  that  they  should  be  regarded  not  as  marking  any 
{particular  period,  but  rather  as  indicating  the  recurrence  or  |)ersistence  of  certain  peculiar 
littoral  conditions  of  deposit  during  the  subsidence  of  the  land  (p.  516).  Farther  north, 
in  the  southern  counties  of  Scotland,  the  Upper  Old  Red  Sandstone,  with  its  character- 
istic fishes,  graduates  upward  into  reddish  and  grey  sandstones  with  Carboniferous 
plants. 

In  Devon  and  Cornwall  a  type  of  the  Carboniferous  system  is  found,  which,  though 
it  does  not  occur  elsewhere  in  Britain,  has  been  ascertained  to  reappear  and  to  have  a 
wide  extension  in  central  Europe.  It  presents  a  thick  series  of  well-bedded  grits,  sand- 
stones, shales,  often  dark  grey,  and  occasional  thin  limestones,  and  passes  down  con- 
formably into  upper  Devonian  strata.  Though  much  contorted  and  faulted,  like  the 
Devonian  formations  of  the  same  region,  this  arenaceous  and  shaly  series  has  yielded 
a  sufficiently  large  number  of-  recognisable  fossils  to  show  its  geological  position.  The 
plants  resemble  generally  those  found  in  the  Calciferaus  Sandstone  series  of  Scotland. 
The  animal  remains  include  species  of  Orthoceras,  OoniatiUSj  Posidonomya  (P.  Beeheri) 
ChantUSy  Spirf/er  {S.  Urei)^  Fhillipsiay  &c.,  an  assemblage  that  also  points  to  a  pasition 


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SECT,  iv  §  2  CARBONIFEROUS  SYSTEM  827 

well  for  the  corresponding  limestone  series  of  Belgium.  The  fossils  commonly  stand 
out  on  weathered  surfaces  of  the  rock,  but  microscopic  investigation  shows  that  even 
those  portions  of  the  mass  which  appear  most  structureless  consist  of  the  crowded 
remains  of  marine  organisms.  The  limestone  has  been  derived  entirely  from  the 
organisms  of  the  sea-floor,  either  growing  up  into  a  solid  mass  after  the  manner  of  coral- 
reefs,  or  spreading  over  the  bottom  in  sheets  of  crinoid  detritus,  or  coral  sand,  mixed 
with  the  remains  of  foraminifera,  moUusks,  &c.  Diversities  of  colour  and  lithological 
character  occur,  whereby  the  bedding  of  the  thick  calcareous  mass  can  be  distinctly 
seen.  Here  and  there,  a  more  markedly  crystalline  structure  has  been  superinduced  ; 
while  along  lines  of  principal  joints  the  rock  on  either  side  for  a  breadth  of  20  or  SO 
fathoms  is  occasionally  converted  into  yellowish  or  brown  dolomite  or  **dunstone"  (see 
p.  321).  In  Derbyshire,  sheets  of  contemporaneous  lava,  locally  termed  '*toadstone," 
are  interpolated  in  the  Carboniferous  Limestone.  Other  evidences  of  contemporaneous 
volcanic  action  have  been  noted  in  the  Isle  of  Man  ^  and  in  Devonshire,'  but  it  is  in 
Scotland,  as  will  be  immediately  referred  to,  that  the  most  remarkable  proofs  of 
abundantly  active  Carboniferous  volcanoes  have  been  preserved. 

In  the  Carboniferous  areas  of  the  south-west  of  England  and  South  Wales,  the  limits 
of  the  Carboniferous  Limestone  are  well  defined  by  the  Lower  Limestone  Shale  below, 
and  by  the  Farewell  Rock  or  Millstone  Grit  above.  In  the  Pennine  area,  however,  the 
massive  limestone  is  succeeded  by  a  series  of  shales,  limestones,  and  sandstones,  known 
as  the  Yoredale  Group.  These  oover  a  large  area  and  attain  a  great  thickness.  In 
North  StafToi-dshire  they  are  2300  feet  thick.  In  Lancashire,  they  attain  still 
greater  dimensions,  Mr.  Hull  having  there  found  them  to  be  no  less  than  4500  feet 
thick.  Both  the  lower  or  main  (Scaur)  limestone  and  the  Yoredale  group  pass  north- 
wards into  sandstones  and  shales  with  coal  seams.  In  Northumberland,  the  Carboni- 
ferous Limestone  series  has  been  grouped  into  the  following  subdivisions  :  * — 

Upper  Calcareous  group,  from  the  base  of  the  Millstone  grit  to  the  Great  Lime- 
stone, 350-1200  feet. 

Lower  Calcareous  group,  from  the  Great  Limestone  to  the  bottom  of  the  Dun  or 
Kedesdale  Limestone,  1300-2500  feet. 

Carbonaceous  group,  Scremerston  coals,  from  the  Dun  Limestone  to  the  top  of  the 
Fell  Sandstone,  800-2500  feet. 

Fell  Sandstone,  500-1600  feet. 

Tuedian  or  Cement  Stone  group,  500-1500  feet. 

Basement  conglomerate. 

These  subdivisions  are  not  all  fully  develoi)ed  in  any  one  district,  but  the  average  thick- 
ness of  the  whole  is  at  least  as  great  as  in  districts  farther  south. 

Traced  northwards  into  Scotland,  the  Carboniferous  Limestone  series  undergoes  a 
still  further  petrographical  and  palseontological  change.  Its  massive  limestones  dwindle 
down,  and  are  replaced  by  thick  courses  of  yellow  and  white  sandstone,  dark  shale,  and 
seams  of  coal  and  ironstone,  among  which  only  a  few  thin  sheets  of  limestone  are  to  be 
met  with.  Scottish  geologists  have  divided  the  lower  half  of  their  Carboniferous  system 
into  two  well-marked  series — the  Calciferous  Sandstones  and  the  Carboniferous  Lime- 
stone. The  Calciferous  Sandstone  series  is  composed  of  two  groups  of  strata — the 
lower  of  which,  or  Red  Sandstone  group,  consists  of  red,  white,  and  yellow  sand- 
stones, with  blue,  grey,  green,  and  red  marls  or  clays,  while  the  upper  or  Cement-stone 
group   is   made   up   of  white   and  yellow  sandstones,    blue,    grey,   green,  and   black 

*  J.  Home,  Trans.  Qeol,  Soc,  £din.  ii.  (1874)  p.  332  ;  B.  Hobson,  Qitart,  Joum. 
Oeol.  Soc.  xlvii.  (1891)  p.  432.      Vn  Lioar  Manninagh,  Douglas,  January  1892,  p.  337. 

»  De  la  Beche,  'Report  on  the  Geology  of  Cornwall,'  &c.  (1839)  p.  119;  F.  Rutley, 
*  The  Eruptive  Rocks  of  Brent  Tor,*  Mem,  Oeol,  Surr.  (1878). 

=»  See  G.  Tate's  *  History  of  Alnwick,'  vol.  ii.  (1869)  p.  441  ;  H.  MUlcr,  Brit,  Auoe, 
(1886)  sects,  p.  675  ;  and  'Geology  of  Otterbonme,'  &c.  Mem,  Oeol.  Sure.  (1887). 


828  STRATIGRAPHICAL  GEOLOGY         book  vi  part  n 

shales  and  marls,  thin  coals,  seams  of  limestone  and  cement -stone,  and  abandaat 
volcanic  rocks.  The  red  sandstones  pass  down  into  the  Upper  Old  Red  Sandstone, 
from  which  they  differ  in  the  less  intensity  of  their  colour,  in  the  frequent  grey  and 
purplish  tints  they  assume,  in  the  absence  of  the  deep  brick-red  marls  so  marked  in  the 
Upper  Old  Red  Sandstone,  and  in  the  occurrence  of  carbonaceous  streaka  and  tree- 
tiniuks,  roots,  and  twigs.  In  the  west  of  Scotland  there  occur  among  the  red  sandstones 
(some  of  which  contain  Upper  Old  Red  Sandstone  fishes)  bands  of  limestone  full  of 
true  Carboniferous  Limestone  corals  and  brachio|)ods.  Hence  it  is  evident  that  the 
Carboniferous  Limestone  fauna  had  already  appeared  outside  the  British  area  before  the 
final  cessation  of  the  peculiar  conditions  of  sedimentation  of  the  Old  Red  Sandstone 
pei-iod.  It  was  not,  however,  until  these  conditions  had  disappeared  that  the  sea 
began  to  invade  the  lakes  and  creep  over  the  sinking  land  of  this  part  of  Britain,  and 
to  bring  with  it  the  abundant  Carboniferous  Limestone  fauna.  The  Calciferous  Sand- 
stones of  Scotland  represent  a  phase  of  sedimentation  contemporaneous  with  the 
deposition  of  the  Lower  Limestone  Shale  and  the  Scaur  Limestone  of  the  CarbonifiBrous 
Limestone  series  of  England. 

One  of  the  most  singular  features  of  the  Lower  Carboniferous  rocks  of  Scotland  is 
the  ])rodigious  abundance  of  the  intercalated  volcanic  rocks.  So  varied,  indeed,  are  the 
chaiucters  of  these  masses,  and  so  manifold  and  interesting  is  the  light  they  throw  upon 
volcanic  action,  that  the  region  may  be  studied  as  a  typical  one  for  this  claaa  of 
phenomena.  (See  Book  IV.  Part  VIL  Sect  i.)  Inland  sections  are  abundant  on  the 
sides  of  the  hills  and  in  the  stream -courses,  while  along  the  sea-shore  the  rocks  have 
been  admirably  exposed.  T^'o  great  phases  or  types  of  volcanic  action  during  Carboni- 
ferous time  may  be  recognised  :  (1)  Plateaux,  whei'e  the  volcanic  materials  were 
discharged  so  copiously  that  they  now  form  broad  tablelands  or  ranges  of  hills,  some- 
times many  hundreds  of  square  miles  in  extent  and  1500  feet  or  more  in  thickness  ;  (2) 
l^ys,  where  the  ejections  were  often  confined  to  the  discharge  of  a  small  amount  of 
fragmentary  materials  from  a  single  independent  vent,  and  where,  when  lavas  and  more 
copious  showers  of  ash  were  thrown  out,  they  generally  covered  only  a  small  area  roimd 
the  volcano  which  discharged  them.^ 

The  Plateau  ty|>e  of  cruj)tion  was  specially  developed  during  the  deposition  of  the 
Calciferous  Sandstones.  It^  lavas  consist  of  augite-olivine  rocks  (picrites,  limburgites), 
basalts,  jwrphyrites,  and  trachytes,  while  its  necks  or  vents  are  filled  with  agglomerates, 
felsitos,  and  in  East  Lothian,  phonolites.'^  Sheets  of  tuff  are  intercalated  among  the 
bedded  lavas.  The  Puy  type  was,  on  the  whole,  of  later  date,  reaching  its  chief  develop- 
ment during  the  time  of  the  Carboniferous  Limestone.  Its  lavas  are  mostly  basalts  of 
various  types,  together  with  j)icrites,  diabases,  and  porphyrites.  Tuffs  and  agglomerates 
arc  abundant,  not  infrequently  containing  organic  remains. 

While  the  scattered  vents  of  the  puys,  with  their  associated  lavas  and  tuffs,  occur  on 
many  horizons,  the  plateau  lavas  occupy  a  tolerably  definite  position  in  the  Calciferous 
Sandstones,  though  sometimes  confined  to  the  lower  part  of  that  group,  sometimes 
ascending  to  thejvcry  base  of  the  Carboniferous  Limestone  series.  This  volcanic  zone 
forms  an  important  feature  in  the  geology  of  southern  Scotland.  Composed  of  nearly 
horizontal  sheets  of  porj>hyrite,  diabase,  and  basalt,  it  extends  from  the  Clyde  islands  on 
the  west  to  Stirling  on  the  east,  and  sweeps  in  high  tablelands  through  Rewfrewshire 
and  Ayrshire.  It  reappears  in  East  Lothian,  and  presents  there  some  interesting  and 
remarkably  fresh  trachytic  lavas.  Even  far  to  the  south,  in  Berwickshire,  Roxburgh- 
shire, and  Kirkcudbright,  volcanic  sheets  occujiy  the  same  ix>sition,  and  extend  across 
into  the  English  border. 

The  upper  subdivision  of  the  Calciferous  Sandstones,  known  as  the  Cement-stone 

^  Presidential  Address,  Quart.  Journ.  Oed,  Sac.  (1892)  p.  105  ;  Trans,  Roy,  iSoc  Edin, 
xxix.  p.  437. 

*  F.  H.  Hutch,  Trans.  Hoy.  !<oc.  Eilin,  (1892)  and  Presidential  Address  just  cited. 


SECT,  iv  §  2  CARBONIFEROUS  SYSTEM  829 

group,  consists  of  two  sections  differing  from  each  other  in  lithological  character,  and 
pointing  to  distinct  conditions  of  deposit.     The  lower  section  is  made  up  of  thin-bedded 
white,  yellow,  and  green  sandstones,  grey,  green,  blue,  and  red  clays  and  shales,  with 
thin  bands  of  pale  argillaceous  limestone  or  cement-stone.    Seams  of  gypsum  occasionally 
appear.     These  strata  are,  on  the  whole,  singularly  barren  of  organic  remains.     They 
seem  to  have  been  laid  down  with  great  slowness,  and  without  disturbance,  in  enclosed 
basins,  wliich  were  not  well  fitted  for  the  support  of  animal  life,  though  fragmentary 
plants  serve  to  show  that  the  adjoining  slopes  were  covered  with  vegetation.     They 
underlie  the  volcanic  zone  in  Stirlingshire  and  the  Lothians,  and  overlie  it  in  Berwick- 
shire.    The  upper  section  is  chiefly  developed  in  the  basin  of  the  Firth  of  Forth,  where, 
overlying  the  volcanic  zone,  it  presents  an  entirely  distinct  lithological  aspect  and  is 
abundantly  fossiliferous.    It  there  usually  consists  of  yellow,  grey,  and  white  sandstones, 
with  blue  and  black  shales,  clay-ironstones,  limestones,  '*  cement-stones,"  and  occasional 
seams  of  coal.     The  sandstones  form  excellent  building  stones,  the  city  of  Edinburgh 
having  been  built  of  them.     Some  of  the  shales  are  so  bituminous  as  to  yield,  on  distilla- 
tion, from  thirty  to  forty  gallons  of  crude  petroleum  to  the  ton  of  shale  ;  they  have  con- 
sequently been  largely  worked  for  the  manufacture  of  mineral-oils.     The  limestones  are 
usually  dull,  grey  or  yellow,  and  close-grained,  in  seams  seldom  more  than  a  few  inches 
thick,  and  graduate  by  addition  of  clay  and  protoxide  of  iron  into  cement-stone  ;  but 
occasionally  they  swell  out  into  thick  lenticular  masses  like  the  well-known  limestone 
of  Burdie  House,  so  long  noted  for  its  remarkable  fossil  fishes.     This  limestone  appears 
to  be  mainly  made  of  the  crowded  cases  of  a  small  ostracod  crustacean  (Leperditia 
Okeni,  var.  acoto-hurdigaleims).     The  coal-seams  are  few  and  commonly  too  thin  to  be 
workable,  though  one  of  them,  known  as  the  Houston  coal,  has  been  mined  to  some 
extent  in  Linlithgowshire.     The  fossils  of  the  Cement-stone  group  indicate  an  alterna- 
tion  of  fresh   or  brackish   water  and  marine   conditions.      They   include   numerous 
plants,  of  which  tlie  most  abundant  are  Sphtnopteris  ajinis  (Fig.  364),  Lepidodendr&n 
(two  or  three  species),  Lepidostrobus  variabilis  (Fig.  367,  6),  Araucarioxylmi.     Ostracod 
crustaceans,  chiefly  the  Leperditia  above  mentioned,  crowd  many  of  the  shales.     With 
these  are  usually  associated  abundant  traces  of  the  pi-esence  of  fish,  either  in  the  form 
of  coprolites,  or  of  scales,  bones,  plates,  and  teeth.     The  following  are  characteristic 
species  :  ElmiichUiys  strioUUus,  E.  Robisani,  Rhadinichthys  omatissimvSy  Nematoptychivs 
Oreenockiif  Eurynotvs  creiialus  (Fig.  868),  Rhizodtis  Hibberti,  Megalichthys  sp.,  Oyra- 
canthns  tvberculatnSj  Callopristodua  {Ctenopiychim)  peclirmtns.     At  intervals  throughout 
the  group,  marine  horizons  occur,  usually  as  shale  bauds  marked  by  the  presence  of  such 
distinctively  Carboniferous  Limestone  species  as  Spirorbis  carbonarins,  IHscina  nitida, 
Lingula  squami/ormiSy  Bellerophon  dectissatuSj  and  Chrthocrras  cyiindraceum.^ 

The  Cement -stone  group  of  the  basin  of  the  Firth  of  Forth  contains  a  great 
number  and  variety  of  associated  volcanic  masses  of  the  puy  type.  At  the  time  when 
it  was  deposited,  the  region  of  shallow  lagoons,  islets,  and  coal-growths  was  dotted  over 
with  innumerable  small  active  volcanic  vents.  The  eruptions  continued  into  the  time 
of  the  Carboniferous  Limestone,  but  ceased  before  the  deposition  of  the  Millstone  Grit.* 

The  Carboniferous  Limestone  series  of  Scottish  geologists,  probably  representing 
the  upper  part  of  the  Carboniferous  Limestone  series  or  Yoredale  group  of  England, 
consists  mainly  of  sandstones,  shales,  fire-clays,  and  coal-seams,  with  a  few  com- 
jjaratively  thin  seams  of  encrinal  limestone.     The  thickest  of  these  limestones,  known 

*  For  descriptions  of  the  Calciferous  Sandstone  group,  see  Moclaren,  *  Geology  of  Fife  and 
the  Lothians '  ;  also  the  explanations  to  accompany  the  Maps  of  the  Geological  Survey  of 
Scotland,  particularly  those  on  Sheets  14,  22,  23,  32,  33,  and  34.  T.  Brown,  Trans.  Roy. 
Soc.  Edin.  xxii.  (1861)  p.  385  ;  Kirkby,  Q.  Oeol.  Soc.  xxxvi.  p.  559. 

*  For  an  account  of  these  Puys  see  Presidential  Address,  Quart,  Joum.  Oeol.  Soc.  1892, 
p.  125  ;  Tram.  Roy.  Soc.  Edin.  xxix.  p.  437.  Some  of  the  vents  are  represented  in  Figs. 
297-301,  303-307  of  this  text-book. 


830  STRATIGRAPHICAL  GEOLOGY  BooKViPABrn 

as  the  Hurlet  or  Main  limestone,  is  usually  about  6  feet  in  thickness,  but  in  the  north 
of  Ayi-shire  swells  out  to  100  feet,  which  is  the  most  massive  bed  of  limestone  in  any 
part  of  the  Scottish  Carboniferous  system.  One  of  a  group  of  limestone  beds  at  the  base 
of  the  series,  it  lies  upon  a  seam  of  coal,  and  is  in  some  places  associated  with  pyiitous 
shales,  which  have  been  largely  worked  as  a  source  of  alum.  This  superposition  of  a 
bed  of  marine  limestone  on  a  seam  of  coal  is  of  frequent  occurrenoe  in  Scotland.  AboTe 
these  lower  limestones  comes  a  thick  mass  of  strata  containing  many  valuable  ooal- 
seams  and  ironstones  (Lower  or  Edge  Coals).  Some  of  these  strata  are  full  of  terrestrial 
plants  (Lepidodendron,  Sigillariay  Stigmaria,  SpJienopteriSy  Aleihopteris) ;  others,  par- 
ticularly the  ironstones,  and  the  shales  associated  with  the  limestones  and  ironstones, 
contain  marine  shells,  such  as  Liriffula,  Discinay  Lcda^  MycUinay  EuomphalUB. 
Numerous  remains  of  fishes  have  been  obtained,  more  especially  from  some  of  the  iron- 
stones and  coals  {Gyracanthus  fomwsus  and  other  fin-spines,  MtgoUichthys  Sihberii, 
jRhizodus  Hibbertiy  with  species  of  ElonichthySy  AcaiUhodcSy  CtenoptyehiuSy  kc,)  TtAniMna 
of  labyrinthodonts  have  also  been  found  in  this  group  of  strata,  and  have  been  detected 
even  down  in  the  Burdie  House  limestone.  The  highest  division  of  the  Scottish 
Carboniferous  Limestone  series  consists  of  a  gix)up  of  sandstones  and  shales,  with  a  few 
coal-seams,  and  three,  sometimes  more,  bauds  of  marine  limestone.  Although  these 
limestones  are  each  only  about  2  or  3  feet  thick,  they  have  a  wonderAil  persistence 
throughout  the  coal-fields  of  central  Scotland.  As  already  mentioned  (p.  615),  they 
can  be  traced  over  an  area  of  at  least  1000  square  miles,  and  they  probably  extended 
originally  over  a  considerably  greater  region.  The  Hurlet  limestone,  with  its  under- 
lying coal,  can  also  be  followed  across  a  similar  extent  of  country.  Hence  it  is  evident 
that,  during  certain  epochs  of  the  Carboniferous  period,  a  singular  uniformity  of  con- 
ditions prevailed  over  a  large  region  of  deposit  in  the  centre  of  Scotland. 

A  distinguishing  feature  of  the  Carboniferous  Limestone  series  of  Scotland  is  the 
abundance  of  its  intercalated  volcanic  rocks  of  the  puy  type.  They  are  well  developed 
in  the  basin  of  the  Forth  and  in  North  Ayrshire.  The  lavas  and  tuffs  are  interbedded 
among  the  ordinary  sedimentary  strata,  and  the  tuffs  are  sometimes  full  of  plants  or  of 
marine  shells,  crinoids,  kc.^ 

The  difference  between  the  lithological  characters  of  the  Carboniferous  Limestone 
series,  in  its  typical  development  as  a  great  marine  formation,  and  in  its  arenaceous 
and  argillaceous  prolongation  into  the  north  of  England  and  Scotland,  has  long  been  a 
familiar  example  of  the  nature  and  application  of  the  evidence  furnished  by  strata  as  to 
former  geographical  conditions.  It  shows  that  the  deeper  and  clearer  water  of  the 
Carboniferous  sea  spread  over  the  site  of  Yorkshire,  Derbyshire,  and  Lancashire  ;  that 
land  lay  to  the  north,  and  that,  while  the  whole  area  was  undergoing  subsidence,  the 
maximum  movement  took  place  over  the  area  of  deeper  water.  The  sediment  derived 
from  the  north,  during  the  time  of  the  Carboniferous  Limestone,  seems  to  have  sunk  to 
the  bottom  before  it  could  reach  the  great  basin  in  which  foraminifers,  corals,  crinoids, 
and  mollusks  were  building  up  the  thick  calcareous  deposit.  Yet  the  thin  limestone 
bands,  which  run  so  |)ersistently  among  the  Lower  Carboniferous  rocks  in  Scotland, 
prove  that  there  were  occasional  episodes  during  which  sediment  ceased  to  arrive,  and 
when  tlie  same  species  of  shells,  corals,  and  crinoids  spread  northwards  towards  the  land, 
forming  for  a  time,  over  the  sea-bottom,  a  continuous  sheet  of  calcareous  ooze,  like  that 
of  the  deeper  water  farther  south.  These  intervals  of  limestone-growth  no  doubt  point 
to  times  of  more  rapid  submergence,  perhaps  also  to  other  geographical  changes,  whereby 
the  sediment  was  for  a  time  prevented  from  spreading  so  far.  It  is  further  deserving  of 
remark  that  the  fossils  in  these  thin  upi)er  limestones  in  Scotland,  though  specifically 
identical  with  those  in  the  thick  lower  limestones  and  in  the  massive  Carboniferous 
Limestone  of  central  and  south-western  England,  are  often  dwarfed  forms,  as  if  the 
conditions  of  life  were  much  less  favourable  than  where  the  thicker  sheets  of  calcareous 


See  the  papers  cited  already,  p.  828. 


SECT,  iv  §  2  CARBONIFEROUS  SYSTEM  831 

material  were  accumulated.  The  corals,  for  instance,  are  generally  few  in  number  and 
small  in  size,  and  the  large  Produdua  [P.  ffiganteus)  is  reduced  to  a  half  or  third  of  the 
dimensions  it  attains  in  its  best  development. 

Viewed  as  a  whole,  the  Carboniferous  Limestone  series  of  the  northern  part  of  the 
British  area  contains  the  records  of  a  long-continued  but  intermittent  process  of  sub- 
sidence. The  numerous  coal-seams,  with  their  under-days,  may  be  regarded  as  surfaces  of 
vegetation  that  grew  in  luxuriance  on  wide  marine  mud- flats.  They  mark  pauses  in  the 
subsidence.  Perhaps  we  may  infer  the  relative  length  of  these  pauses  from  the  comparative 
thicknesses  of  the  coal-seams.  The  overlying  and  intervening  sandstones  and  shales  indi- 
cate a  renewal  of  the  downward  movement,  and  the  gradual  infilling  of  the  depressed 
area  with  sediment,  until  the  water  once  more  shoaled,  and  the  vegetation  from  adjacent 
swamps  spread  over  the  muddy  flats  as  before.  The  occasional  limestones  serve  to  mark 
epochs  of  more  prolonged  or  more  rapid  subsidence,  when  marine  life  was  enabled  to 
flourish  over  the  site  of  the  submerged  forests.  But  that  the  sea,  even  though  tenanted 
in  these  northern  parts  by  a  limestone-making  fauna,  was  not  so  clear  and  well  suited 
for  the  development  of  animal  life  during  some  of  these  submergences  as  it  was  farther 
south,  seems  to  be  proved  by  the  paucity  and  dwarfed  forms  of  the  fossils,  as  well  as  by 
the  admixture  of  clay  in  the  stone. 

Ireland  presents  a  development  of  Carboniferous  rocks,  which  on  the  whole  follows 
tolerably  closely  that  of  the  sister  island.  In  the  northern  counties,  the  lowest  members 
are  evidently  a  prolongation  of  the  type  of  the  Scottish  Caloiferous  Sandstones.  In  the 
southern  districts,  however,  a  very  distinct  and  peculiar  facies  of  Lower  Carboniferous 
rocks  is  to  be  observed.  Between  the  Old  Red  Sandstone  and  the  Carboniferous  Lime- 
stone there  occurs  in  the  county  of  Cork  an  enormous  mass  (fully  5000  feet)  of  black 
and  dark-grey  shales,  impure  limestones,  and  grey  and  green  grits,  which  have  been  so 
affected  by  slaty  cleavage  as  to  have  assumed  more  or  less  perfectly  the  structure  of  true 
cleaved  slates.  To  these  rocks  the  name  of  Carboniferous  Slate  was  given  by  Griffith. 
They  contain  numerous  Carboniferous  Limestone  species  of  brachiopods,  echinodemn, 
kc,  as  well  as  traces  of  land-plants  in  the  grit  bands.  Great  though  their  thickness  is 
in  Cork,  they  rapidly  change  their  lithological  character  and  diminish  in  mass,  as  they 
are  traced  away  from  that  district.  In  the  almost  incredibly  short  space  of  15  miles, 
the  whole  of  the  5000  feet  of  Carboniferous  Slate  of  Bantry  Bay  seems  to  have  disappeared, 
and  at  Kenmare  the  Old  Red  Sandstone  is  followed  immediately  and  conformably  by  the 
Limestone  with  its  underlying  shale.  This  rapid  change  is  probably  to  be  explained,  as 
Jukes  suggested,  by  a  lateral  passage  of  the  slate  into  limestone  ;  the  Carboniferous 
Slate  being,  in  part  at  least,  the  equivalent  of  the  Carboniferous  Limestone.  Between 
Bandon  and  Cork  the  Carboniferous  Slate  is  conformably  overlain  by  dark  shales  con- 
taining Coal-meaaure-fossils,  and  believed  to  be  true  Coal-measures.  Hence  in  the  south 
of  Ireland,  the  thick  calcareous  accumulations  of  the  limestone  series  appear  to  be  replaced 
by  a  corresiK)nding  depth  of  argillaceous  sedimentary  rocks.  ^ 

The  Carboniferous  Limestone  swells  out  to  a  great  thickness,  and  covers  a  large  part 
of  Ireland.  It  attains  a  maximum  in  the  west  and  south-west,  where,  according  to  Mr. 
Kinahan,^  it  consists  in  Limerick  of  the  following  subdivisions  : —  p    .    * 

TT          /n          \  T '       J.                ( Bedded  limestone    .....       240 
Upper  (Burren)  Lmiestone       .  |  ^^^^^  ^^^ 20 

TT         /n  1   \  T  •       4.                   (  Liraeatones  and  shales     ....     1000 
Upper  (Calp)  Limestone  .  |  ^^^^^  ^^^ ^^ 

{FencateUa  limestone        ....  1900 

Lower  cherty  zone  .....  20 

Lower  shaly  limestones  ....  280 

Lower  Limestone  Shales  ..........  100 

3600 

^  J.  B.  Jukes,  Memoirs  (Jeol.  Survey y  Ireland.  Explanation  of  Sheets  194,  201,  and 
202,  p.  18  ;  Explanation  of  Sheets  187, 195,  and  196,  p.  35.       '  ' Geology  of  Ireland,'  p.  72. 


832  STRATIGRAPHICAL  GEOLOGY  bookvipabth 

The  chei-t  (phtanite)  bands  which  form  such  marked  horizons  among  these  limestones 
are  countcr])arts  of  others  found  abundantly  in  the  Carboniferous  Limestone  of  England 
and  Scotland.  Portions  of  the  limestone  have  a  dolomitic  character,  and  sometimes  are 
oolitic.  Great  sheets  of  porphyiite,  basalt,  and  tuff,  representing  volcanic  eruptioDS  of 
contemporaneous  date,  are  interpolated  in  the  Carboniferous  Limestone  of  Limerick.^  Ab 
the  limestone  is  traced  northwards,  it  shows  a  similar  change  to  that  which  takes  place 
in  the  north  of  England,  becoming  more  and  more  split  up  with  sandstone,  shale,  and 
coal-seams. - 

2.  Millstone  Grit. — This  name  is  given  to  a  group  of  sandstones  and  grits,  with 
shales  and  clays,  which  runs  persistently  through  the  centre  of  the  Carboniferous  system 
from  South  Wales  into  the  middle  of  Scotland.  In  South  Wales,  it  has  a  depth  of  400 
to  1000  feet ;  in  the  Bristol  coal-field,  of  about  1200  feet.  Traced  northwards  it  is  found 
to  be  intercalated  with  shales,  fire-clays,  and  thin  coals,  and,  like  the  lower  members  of 
the  Carboniferous  system,  to  swell  out  to  enormous  dimensions  in  the  Pennine  region. 
In  North  Staffordshire,  according  to  Mr.  Hull,  it  attains  a  thickness  of  4000  feet,  which 
in  Lancashire  increases  to  5500  feet.  These  massive  accumulations  of  sediment  were 
deix)sited  on  the  north  side  of  a  barrier  of  more  ancient  Palaeozoic  rocks,  which,  during 
all  the  earlier  part  of  the  Carboniferous  period,  seems  to  have  extended  across  central 
England,  and  which  was  not  submerged  until  jmrt  of  the  Coal-measures  had  been  laid 
down.  North  of  the  area  of  maximum  deposit,  the  Millstone  Grit  thins  away  to  not 
more  than  400  or  500  feet.  It  continues  a  comparatively  insignificant  formation  in 
Scotland,  attaining  its  greatest  thickness  in  Lanarkshire  and  Stirlingshire,  where  it  is 
known  as  the  *'  Moor  Rock."  In  Ayrshire  it  does  not  exist,  unless  its  place  be  repre- 
sented by  a  few  beds  of  sandstone  at  the  base  of  the  Coal-measures. 

The  Millstone  Grit  is  generally  barren  of  fossils.  When  they  occur,  they  are  either 
plants,  like  those  in  the  coal-bearing  strata  above  and  below,  or  marine  organisms  of 
Carboniferous  Limestone  species.  In  Lancashire  and  South  Yorkshire,  indeed,  it  eon- 
tains  a  band  of  fossiliferous  calcareous  shale  undistinguishable  from  some  of  those  in 
the  Yoredale  group  and  Scaur  limestone. 

3.  Coal-Mkasukes. — This  division  of  the  Carboniferous  system  consists  of  numerous 
alternations  of  grey,  white,  yellow,  sometimes  reddish,  sandstone,  dark-grey  and  black 
shales,  clay-ironstones,  fire-clays,  and  coal-seams.  In  South  Wales  it  attains  a  maximum 
depth  of  about  12,000  feet ;  in  the  Bristol  coal-field,  about  6000  feet.  But  in  these 
dLstricts,  as  in  most  of  the  Carboniferous  areas  of  Britain,  we  cannot  be  sure  that  all 
the  Coal-measures  originally  deposited  now  remain,  for  they  are  generally  unconformably 
covered  by  later  formations.  Palaeontological  considerations,  to  be  immediately 
adverted  to,  render  it  probable  that  the  closing  part  of  the  Carboniferous  period  is  not 
now  represented  in  Britain  by  fossiliferous  strata.  Towards  the  end  of  the  Carboni- 
ferous i>eriod,  possibly  also  within  early  Permian  time,  the  Carboniferous  strata  were 
in  many  if  not  most  districts  of  Britain  upheaved  so  as  to  be  exposed  to  denudation.  In 
some  areas  the  denudation  was  so  great  that  the  Permian  rocks,  as  in  the  case  of  the 
Magnesiau  Limestone  of  Durham,  sweep  across  the  denuded  edges  of  the  Coal-measures, 
Millstone  grit,  and  even  the  higher  parts  of  the  Carboniferous  Limestone.  But  these 
disturbances  and  erosion  were  not  universal  within  the  British  region,  for  we  find  that 
over  parts  of  South  StatTordshire  these  strata  are  followed  with  apparent  conformability 
by  the  Pennian  sandstones. 

In  North  Staffordshire,  the  depth  of  Coal-measures  is  about  5000  feet,  which  in  South 
Lancashire  increases  to  8000.  These  great  masses  of  strata  diminish  as  we  trace  them 
eastwards  and  northwards.  In  Derbyshire,  they  are  about  2500  feet  thick,  in  Northumber- 
land and  Durham  about  2000  feet,  and  about  the  same  thickness  in  the  Whitehaven 
coal-field.     In  Scotland,  they  attain  a  maximum  of  over  2000  feet, 

1  Presidential  Address,  Quart.  Jouni.  GeoL  ^h'.  1892,  p.  145,  and  authors  there  cited. 
-  Hull's  '  Physical  Geology  and  Geography  of  Ireland,'  2nd  edit.  (1891)  p.  49. 


SECT,  iv  §  2 


CARBONIFEROUS  SYSTEM 


833 


The  Coal-measures  are  susceptible  of  local  subdivisions  indicative  of  different  and  vari- 
able conditions  of  deposit.     The  following  table  shows  the  more  important  of  these  : — 


Glamorqanshirb. 

Feet. 

Upper  series :  sand- 
stones, shales,  &c, 
with  26  coal -seams, 
more  than  .         .     8400 

Pennant  Grit :  hard, 
thick-bedded  sand- 
stones, and  15  coal- 
seams         .  8246 

Lower  series :  shalra, 
ironstones,  and  84 
coal-seams  .    450  to  850 


Millstone  Grit. 


South  Lancashire. 

Feet, 

Upper  series:  shales, 
red  sandstones,  Spir- 
orbis  limestone,  iron- 
stone and  thin  coal- 
seams     .     1600  to  2000 

Middle  series:  sand- 
stones, shales,  clays, 
and  thick  coal-seams. 
The  chief  repository 
of  coal    .     8000  to  4000 

Lower  or  Gannister 
series :  flagstones, 
shales,  and  thin  coals 

1400  to  2000 


Millstone  Grit. 


900 


Central  Scotland. 

Feet. 

Upper  red  sandstones 
and  days,  with  Spir- 
orbis  limestone;  in 
Fife  upwards  of    . 

True  Coal-measures : 
sandstones,  shales, 
fire  •  clays,  with 
bands  of  black -baud 
ironstone,  and  nu- 
merous seams  of 
coal.  Thickness  in 
Lanarkshire  up- 
wards of  .  2000 


Moor  Rock,  or  Millstone 
Grit. 


The  numerous  beds  of  compressed  vegetation  form  the  most  remarkable  feature  of 
the  Coal-measures.  As  already  stated,  coal-seams  in  Britain  are  usually  underlain  by 
fire-clay  {mur  of  the  Belgian  coal-fields),  which,  traversed  in  all  directions  by  rootlets, 
and  free,  or  nearly  free  of  alkalies  and  iron,  appears  to  have  been  the  soil  on  which  the 
plants  that  formed  the  coal  grew.  A  coal-seam  accordingly  marks  there  a  former  sur- 
face of  terrestrial  vegetation,  and  the  shales,  fissile  micaceous  sandstones,  and  other  strata 
that  overlie  it  show  the  nature  of  the  sediment  under  which  it  was  eventually  buried. 

The  Coal-measures  of  Britain  have  not  yet  been  very  precisely  subdivided  into 
palseontological  zones.  The  lower  portions  or  Gannister  beds  of  Lancashire  contain 
at  least  70  species  of  undoubtedly  marine  fossils,  including  species  of  OoniatiUs  {G. 
Lister i) J  Orthoceras^  Nautilus^  Edmondia,  Posidonia,  Sanguinolites^  AmciUopecten  {A,  papy- 
raceus),  Lingula  {L.  aqvami/ormis),  Discinay  ProductuSy  Spirifer,  &c.  Other  horizons 
with  marine  fossils  have  been  observed  in  England  and  Scotland  even  in  the  upper  Coal- 
measures.'  The  middle  and  upper  divisions  are  characterised  by  the  prevalence  of 
species  of  Antkraama,  AiUhracopUra,  and  Anthracomya.  These  shells  are  not  met 
with  in  association  with  the  more  typical  marine  fauna,  but,  on  the  contrary,  are  mingled 
with  a  peculiar  assemblage  of  fishes  and  reptiles,  annelids  and  crustaceans,  such  as  might 
be  supposed  to  inhabit  brackish  or  fresh  water,  together  with  abundant  remains  of 
terrestrial  vegetation.'  Some  of  the  more  characteristic  fishes  are  Slrepsodus  sauroides 
(Fig.  372),  Rhizodopsia  sauroideSy  Megalichthys  Uibhertiy  Chtirodns  granulos^ts  (Fig.  872), 
Janassa  linguiformiSy  Sphenacanthus  hyhodoides  (Fig.  861),  PUuracanthus  IsBvissimus, 
Ct^noptychius  apiealis.  Some  species  range  from  bottom  to  top  of  the  Coal-measures — 
e.g.  Callopristodits  (Ctenoptychius)  pecHiiaitis  and  Gyracanthns  formosus.* 

Little  has  yet  been  done  in  working  out  the  stratigraphical  distribution  of  the  Coal- 
measure  flora  of  Britain,  but  some  recent  progress  in  this  direction  has  been  made  by  Mr. 
Kidston,  who  believes  the  Coal-measures  to  be  divisible  into  Upper  (Radstock,  Somer- 
set), Middle  (South  Staffordshire,  part  of  Yorkshire),  and  Lower  (part  of  Yorkshire, 
Northumberland,  Scotland).*  The  late  D.  Stur,  correlating  the  Coal-measures  of  this 
country  with  those  of  central  Europe  mainly  by  means  of  the  plants,  regarded  the  Coal- 
measures  of  Wales  and  the  west  of  England  generally  as  equivalent  to  the  higher  series 

»  J.  W.  Kirkby,  Quart.  Joum.  Oeol.  Soc,  xliv.  (1888)  p.  747. 
«  Wheel  ton  Hind,  QuaH.  J&um.  Qtd,  Soc,  xlix.  (1898)  p.  259. 

'  My  friend  Dr.  Traquair  has  been  kind  enough  to  furnish  me  with  information  on  this 
subject,  which  he  has  so  carefully  studied. 

*  Trans.  Roy.  Soc.  Edin.  xxxv.  (1890-91)  pp.  63,  891,  419  ;  xxxvii.  (1898)  p.  807. 

3  H 


834  STRATIGRAPHIGAL  GEOLOGY  book  vi  pabt  n 


of  Germany,  those  of  central  and  northern  England  and  Scotland  as  equivalent  to  the 
lower  series,  both  of  these  series  being  represented  in  Lancashire.^  From  plant-remains 
obtained  from  the  recent  boring  through  the  chalk  at  Dover,  Zeiller  regards  the  Goal- 
measures  there  as  belonging  to  the  upper  part  of  the  middle  Coal-measures  of  France.' 

On  the  Continent  of  Europe  the  Carboniferous  system  occupies  many  detached  areas 
or  basins — the  result  partly  of  original  deposition,  partly  of  denudation,  and  partly  of 
the  spread  and  overlap  of  more  recent  formations.  There  can  be  no  doubt  that  the 
English  Carboniferous  Limestone  once  extended  continuously  eastward  across  the  north 
of  France,  along  the  base  of  the  Ardennes,  through  Belgium,  and  across  the  present 
valley  of  the  Rhine  into  Westphalia.  From  the  western  headlands  of  Ireland  this 
calcareous  formation  can  thus  be  traced  eastward  for  a  distance  of  750  English  miles 
into  the  heart  of  Europe.  It  then  begins  to  pass  into  a  series  of  shales  and  sandstones, 
which,  as  already  remarked,  represent  proximity  to  shore,  like  the  similar  strata  in  the 
north  of  England  and  Scotland.  In  Silesia,  and  still  much  farther  eastwards,  in 
central  and  southern  Russia,  representatives  of  the  Carboniferous  Limestone  or  Culm 
appear,  but  interstratified,  as  in  Scotland,  with  coal-bearing  strata.  Traces  of  the 
same  blending  of  marine  |ind  terrestrial  conditions  are  found  also  in  the  north  of  Spain. 
But  over  central  France,  and  eastwards  through  Bohemia  and  Moravia  into  the  region 
of  the  Carpathians,  the  Coal-measures  rest  directly  upon  older  Pakeozoic  groups,  most 
commonly  upon  gneiss  and  other  crystalline  rocks.  These  tracts  had  no  doubt  remained 
above  water  during  the  time  of  the  Carboniferous  Limestone,  but  were  gradually  de- 
pressed during  that  of  the  Coal-measures. 

France  and  BelginnL — In  Belgium  and  the  north  of  France  the  British  type  of 
the  Carboniferous  system  is  well  developed.'    It  comprises  the  following  subdivisions : — 
Zone  of  the  gas-coals  {Charbons  d  gaZy  rich  bituminous  coals,  with  28  to  40 
per  cent  of  volatile  matter),  containing  47  seams  of  coaL     Pecopteris  nerfx>mi, 
P.    dentatOf   P,    abbreoiatay    Alethopieris   Serliiy    Neuropteris    heierophyUa^ 
Sphenopteria  irregulariSy  S.  macilenlOy  S.  corallcideSy  &  herbacea,  &  /urcatti, 
Calamites  Suckowiiy  Annularia  radiatay  Sphencphyllum  erosuMy  SigUUtria 
tesseUatOy  S.  fnamillarisy  S.  rimosay  S.  laticostay  Dorycordaitea, 
Zone  of  the  **  Charbons  gras*'  (18  to  28  per  cent  volatile  matter),  soft  caking 
coal  (21  seams),  well  suited  for  making  coke.     Sphenopteris  nummuluria,  & 
macilentay  S.  chasrophylloideSy  S,  artemisifolioy  S.  ?ierbacea,   S.  irregvlarisy 
Neuropteris  giganleay  Alethopteris  Serliiy   A.   txdiday    Calamites   Suckowiiy 
SphenophyUum  emarginatuviy  Sigillaria  polyplocOy  S.  riwosay   S.   laticostay 
Trigonocarpus  Ndggerathii. 
Zone  of  the  *^  Charbons  demi-gras"  (12  to  18  per  cent  volatile  matter),  29  seams 
^  "{      of  coal,  chiefly  fitted  for  smithy  and   iron -work   purposes.       ^hencpUris 
convexifoliuy  S.  Ilaminghausiy  S.  trichomarwicUSy  S.  furcatOy  &  SchiUingtiiy 
S.  irregularisy  Lonchopteris  rugosOy  Calcimites  Suckowiiy  Annularia  radiatay 
Sigillaria  mamUlariSy  S.  elcgansy  S.  piriformis,  S.  eUipticOy  S,   scuicUatay 
S.  Groeseri,  S.  Uevigatay  S.  rvgosa,  Halonia  tortuosa. 
Zone  of  the  **  Charbons  maigres."     Lean  or  poor  coals  (20  to  25  seams),  only  fit 
for  making  bricks  or  burning  lime  (9  to  12  per  cent  volatile  matter).       Peco- 
pteris Loshiiy  P.  pennaeformisy  Neuropteris  hetcrophyllcLy  Alethopteris  lonehitica, 
SphenophyUum  saxifragm/oliumy  Annuluria  radiatay  Sigillaria  conferta,  S. 
Candolliy  S   Voltziiy  Calcimites  Suckoirii,  L^ndodendron  rhodeanumy  L.  pus- 
tulatumy  Lepidophloios  laricinus. 
Zone  of  Productvs  carbonarius.     Ooniatites  diademay  O.  atratuSy  Spir\fer  meso- 
goniusy  S.  glabery  S.  trigonalisy  Streptorhynchvs  crenistriay  Producttu  semi- 
.     reticulatus,  P.  mnrginalisy  Axnctila  papyraceUy  Schizodus  sidcatus. 

^  Jahrb.  k.  k.  Gecl.  Reichsanst.  1889. 

'  Compt.  rerul.  Oct.  24,  1892.  The  details  of  this  Dover  boring,  which  has  proved  the 
existence  of  coal-bearing  strata  beneath  the  south-east  of  England,  are  given  by  Lorienx, 
Ann.  Minesy  ser.  9,  vol.  ii.  (1892)  p.  227.  Bertrand  has  discussed  the  relations  of  this 
Dover  coal-field  to  those  of  northern  France  and  Belgium,  op.  cit.  iii.  (1893)  p.  1. 

^  On  the  Carboniferous  rocks  of  this  area  see  De  Koninck,  *  Descriptions  des  Animaux 


o 

w 

a 


a> 

a 


(C 

m 

I 

CO 

2 

% 

a 
• 

o 


SECT,  iv  §  2 


CARBONIFEROUS  SYSTEM 


835 


»    t-i  4 

=  o 


Sandstones  or  quartzites  passing  into  conglomerates,  separated  from  the  Carboni- 
ferous Limestone  below  by  carbonaceous  shales  with  some  thin  coal-seams  ; 
chiefly  developed  towards  the  north-east  (Li^,  Aix-la-Chapelle). 


o 

s 

ao 


a 
o 

■«-> 

a 
o 


5 


Thickness 
in  metres 
in  area  of 
the  Ham- 
bre. 

Limestone  of  Vis^.  Often  poor  in  fossils,  distin- 
guished by  Productus  giganteus  ....       50 

Limestone  of  Limont  (Napoleon  marble  of  Boulon- 
nais).  Fossils  numerous  :  Productus  uncUit%M,  P, 
itemireticuUUus,  Spiri/er  glabefy  S.  dupHcicostuSy 
RhifnchoneUa  pleurodon^  Terebraiula  sodxulus        .       10^ 

Limestone  of  Haut  Banc,  compact  or  oolitic  in  south 
part  of  Sambre  basin,  with  Productus  subUevU ; 
but  in  north  part  of  that  basin,  as  well  as  on  the 
Meuse  and  in  the  Boulonnais,  Productus  cora 
replaces  P,  svhUevis .40' 

Dolomite  of  Namur,  well  developed  between  Namur 
and  Lii'ge,  and  extending  into  the  Boulonnais 
(Uure  dolomite),  alternating  with  grey  limestone, 
containing  Chonetes  comoides        ,  '      .         .         .       40  ^ 

Limestone  of  Bachant,  grey,  bluish-black,  or  black, 
with  cherts  (phtanites).  Productus  cora  (and 
sometimes  P.  giganteus)y  Spirifer  tticomisy  Den- 
talium  priscutn^  EuompfuUits  cirroideSy  Nautilus 
sulcatuSf  Orihoceras  munsterianum        ...       35 

Limestone  of  Waulsort,  grey,  often  dolomitic  ;  only 
seen  in  area  of  the  Meuse.  Spirifer  cuspidatus^ 
Conocardium  aliformt ......         0 

Limestone  of  Anseremme,  grey  and  blue-veiued  lime- 
stone and  dolomite.  Productus  semireticul^usy 
Spirifer  mosquensiSy  &  cuspidcUwt^  Orthis  resu- 
piriata         ........         8 

Limestone  of  Dinant,  only  found  in  the  Meuse  area. 
Productus  semireticulaiuSy  P,  Flemingiiy  Pecten 
intermedius  .......         0 

Limestone  of  Ecaussines  ("petit  granite"),  crinoidal 
limestone.  Phillipsia  gemmuliferay  Productus 
semircticulatusy  Spirifer  mosquensis,  Strcptorhyn- 
chv^  crenistriay  Orthis  Afiehelinit  Strophomena 
rhomboidalis        .......       25 

Limestones  and  shales  of  Avesnelles,  black  limestone  }- 

(16  metres),  resting  upon  argillaceous  shales  (40 
metres).  Among  the  numerous  fossils  of  the 
limestone  are  Productus  FUmingiU  P-  Hibertif 
Chonetes  vaHolaris,  Rhynchondla  pleurodony  Spir- 
ifer mosquensiSy  Euomphalus  equalise  Pecten 
Soiaerbyi      .         .         .         .         .         .         .         .       50 . 


Thickness 
in  metres 
in    area 
of  the 
Meu«e. 


250 


150 


100 
100 


60 


100 


^  258      I      760  metres. 

The  base  of  these  strata  passes  down  conformably  into  the  Devonian  system,  with 
which,  alike  by  ])alwontological  and  petrographical  characters,  it  is  closely  linked. 
The  Carboniferous  rocks  of  the  north  of  France  and  of  Belgium  have  undergone 
considerable  disturbance.  A  remarkable  fault  ('Ha  grande  faille"  of  this  region)  result- 
ing from  the  rupture  of  an  isoclinal  syncline,  and  the  consequent  sliding  of  the  inverted 

Fossiles  du  Terrain  Carbonifere  de  la  Belgique'  (1842-67).  Gosselet's  ^Esquisse,'  aln;ady 
cited,  and  his  'L'Ardenne'  (1888),  chaps,  zxii.  and  xxiii.  Mourlon's  'G^ologie.'  Boulay, 
*  Terrain  Houiller  du  Nord  de  la  France  et  ses  Veg^taux  fossiles,*  Lille  (1876).  Dnpont, 
Bull.  Soc.  Roy.  Belg.  (1883). 


836  STRATIGRAPHIG A L  GEOLOGY  book  vi  part  n 

side  over  higher  beds,  runs  from  near  Li^  westwards  into  the  BoulomuuB,  with  a 
general  but  variable  hade  towards  the  south.  On  the  southern  side  lie  lower  Devonian 
strata,  below  which  the  Carbonifei-ous  Limestone,  and  even  Coal -measures  are  made 
to  plunge.  Bores  and  pits  near  Liege  at  the  one  end,  and  in  the  Boulonnaia'  at 
the  other,  have  reached  workable  coal,  after  piercing  the  inverted  Devonian  rocks.  By 
continuing  the  boring  the  same  coals  are  found  at  lower  levels  in  their  normal  positions. 
Besides  this  dominant  dislocation  many  minor  faults  and  plications  have  taken  place  in 
the  Carboniferous  area,  some  of  the  coal-seams  being  folded  zig-zag,  so  that  at  Mons  a 
bed  may  be  (>erforated  six  times  in  succession  by  the  same  vertical  shaft,  in  a  depth  of 
350  yards.  At  Charlcroi  a  series  of  strata,  which  in  their  original  horizontal  position 
occupied  a  breadth  of  8}  miles,  have  been  compressed  into  rather  less  than  half  that 
space  by  being  plicated  into  twenty-two  zig-zag  folds. 

Southwards  the  plateau  of  crystalline  rocks  in  central  France  is  dotted  with'more  than 
300  small  Carboniferous  basins  which  contain  only  portions  of  the  Coal-measurea.  The 
most  important  of  these  basins  are  those  of  the  Roannais  and  Beai\jolais,  St.  Etienne, 
Autun,  Com  men  try,  Gard,  and  Brive.  It  would  appear,  however,  that  some  of  the 
surrounding  slates  are  altered  representatives  of  the  lower  parts  of  the  Carboniferous 
system,  for  Carboniferous  Limestone  fossils  have  been  found  in  them  between  Roanne 
and  Lyons,  and  near  Vichy.'  Even  as  far  south  as  Montpellier,  beds  of  limestone  full 
of  Product  us  gUjanteus  and  other  characteristic  fossils  are  covered  by  a  series  of  work- 
able coals.  Grand'  Eury,  from  a  consideration  of  the  fossils,  regards  the  coal-basins  of 
the  Roannais  and  lower  part  of  the  basin  of  the  Loire,  as  belonging  to  the  age  of  the 
"culm  and  uj)per  greywacke,"  or  of  strata  immediately  underlying  the  true  Coal- 
measures.  But  the  numerous  isolated  coal-basins  of  the  centre  and  south  of  France  he 
refers  to  a  much  later  age.  He  looks  on  these  as  containing  the  most  complete  develop- 
ment of  the  upper  coal,  proi)erly  so-called,  enclosing  a  remarkably  rich,  and  still 
little  -  known,  flora,  which  serves  to  fill  up  the  palieontological  gap  between  the 
Carboniferous  and  Permian  jieriods.'  Some  of  these  small  isolated  coal -basins  are 
remarkable  for  the  extraordinary  thickness  of  their  coal-seams.  In  the  most  important 
of  their  number,  that  of  St.  Etienne,  from  15  to  18  beds  of  coal  occur,  with  a 
united  thickness  of  112  feet,  in  a  total  depth  of  2500  feet  of  strata.  In  the  basin 
near  Chalons  and  Autun  the  main  coal  averages  40,  but  occasionally  swells  out  to  130 
feet,  and  the  Coal-measures  are  covered,  apparently  conformably,  by  Permian  rocks, 
from  which  a  remarkable  series  of  saurian  remains  has  l)een  obtained.  Other 
Carboniferous  areas  appear  in  the  north-west  of  France,  where  representatives  of  the 
Carboniferous  Limestone  and  the  coal-bearing  series  above  it  are  found.  The  Carboni- 
ferous Limestone  is  also  well  develoj>ed  westward  in  the  Cantabrian  mountains  in  the 
north  of  Spain,  where  it  likewise  is  surmounted  by  coal-bearing  strata.* 

Germany.'^ — The  Coal-measures  extend  in  detached  basins  north-eastwards  from 

^  For  the  Boulouuais,  see  Godwin -Austen,  Q.  J.  (Jeol.  Soc.  ix.  p.  231  ;  xii  p.  38  ; 
Barrois,  Proc.  Gcol.  Ass(k.  vi.  No.  1  ;  Report  of  meeting  at  Boulogne,  BuJi,  Soc.  GM. 
FraihcCf  svt.  3,  viii.  p.  483  ;  Rigaux,  ^fSm.  Soc,  Sa'.  Boulogjie,  vol.  xiv.  (1892)  ;  *  Notice 
Geol.  sur  le  Bas  Boulonnais,'  Boulogne-sur-mer,  1892. 

'^  Murchison,  (^.  J.  Oeol.  Soc,  vii.  (1851)  p.  13  ;  Julien,  Comptes  Rendus^  Ixzviii.  p.  74. 

3  Grand'  Eury,  'Flore  Carbonifere. '  Bertrand,  Rull.  Soc.  OSd.  France,  xvi/(1888)  p. 
517  ;  Fayol,  p.  968  et  seq.,  Memoirs  cited  ante,  p.  808  ;  G.  Mouret,  *  Bassin  Houiller  de 
Brive,'  1891. 

"*  The  coal-field  of  the  Asturias  is  described  by  Barrois,  *  Recherches  sur  les  Terrains 
anciens  des  Asturies,'  p.  551.  Zeiller  {Man.  Soc.  OSol.  XirrU,  i.  1882)  refers  the  plants 
to  tlie  Middle  and  Upi>er  Coal-measures  of  France. 

^  Geinitz,  'Die  Steinkohlen  Deutschlands,'  Munich,  1865  ;  Von  Dechen,  ' Erltoterangen 
zur  Geol.  Karte  der  Rheinprov.'  ii.  (1884);  C.  E.  Weiss,  '  Fossile  Flora  der  jiingsten 
Stcinkohlenformation  und  des  Rothliegenden  im  Saar-Rhein  Gebiete,'  1869-72. 


SECT,  iv  §  2  CARBONIFEROUS  SYSTEM  837 

Central  France  into  Germany.  One  of  the  most  important  of  these,  the  basin  of  Pfalz- 
Saarbriicken,  lying  unconformably  on  Devonian  rocks,  contains  a  mass  of  Coal- 
measures  believed  to  reach  a  maximum  thickness  of  not  less  than  20,000  feet,  and 
divided  into  two  groups  : — 

2.  Upper  or  Ottweiler  beds,  from  6500  to  11,700  feet  thick,  consisting  of  red 
sandstones  at  the  top,  and  of  sandstones  and  shales,  containing  20  feet  of 
coal  in  various  seams.  Pecopteris  arborescenSf  Odontopteris  obttua,  AtUhra- 
cosia,  Bstheriay  Leaia  ;  fish-remains. 

1.  Lower  or  nimiu  coal-bearing  (Saarbriicken)  beds,  5200  to  9000  feet  thick,  with 
82  workable  and  142  unworkable  coal-seams,  or  in  all  between  850  and  400 
feet  of  coal.  Abundant  plants  of  the  middle  and  lower  zone  of  the  upper 
coal  flora. 

The  Franco- Belgian  Coal-field  is  prolonged  across  the  Rhine  into  Westphalia.  The 
Carboniferous  Limestone  here  dwindles  down  as  a  calcareous  formation,  and  assumes  the 
*'  Culm  "  phase,  passing  up  into  the  '*  flotzleerer  Sandstein  "  or  Millstone  Grit — a  group 
of  sandstones,  shales,  and  pebbly  beds  some  3000  feet  thick,  but  without  coal-seams. 
These  barren  measures  are  succeeded  by  the  true  Coal-measures  about  10,000  feet  thick, 
with  90  workable  seams  of  coal,  having  a  united  thickness  of  more  than  250  feet. 

Southern  Oermany,  Bohemia. — Carboniferous  rocks  occur  in  many  scattered  areas 
across  Germany  southwards  to  the  Alps  and  eastwards  into  Silesia,  including  repre- 
sentatives both  of  the  lower  or  Culm  phase  and  of  the  Coal-measures.  The  Culm  rocks 
reappear  in  the  Harz,  where  they  are  traversed  by  metalliferous  veins  and  enclose  small 
patches  of  Coal-measures.  The  same  structure  extends  into  Thuringia,  the  Fichtel- 
gebirge.  Saxony,  and  Bohemia,  the  Culm  yielding  Carboniferous  Limestone  fossils,  as 
well  as  Lepidodendrorif  &c.,  and  containing  sometimes,  as  in  Saxony,  workable  coals. 
This  union  of  fossils  characterises  the  ^ries  of  shales,  sandstones,  greywackes,  and 
conglomerates  which  forms  the  German  Culm.  The  abundant  fauna  of  the  Carboniferous 
Limestone  is  reduced  to  a  few  moUusks  {Productus  anliqu^iSy  P.  lalissimus,  P,  semire- 
ticukUus,  Po»idonomya  Becheri,  GonicUUes  sphxricas,  Orthoeeras  striatulumy  &c. )  The 
Posidonomya  particularly  characterises  certain  dark  shales  known  as  *'  Posidonia 
schists."  Of  the  plants,  typical  species  are  CalamiUa  transitionia,  Lcpidodendron 
veWieimianum,  Stigmaria  Jtcoides^  Spheiiopteris  distaiiSj  Cyclopteris  tenuifolia.  This 
flora  bears  a  strong  resemblance  to  that  of  the  Calciferous  Sandstones  of  Scotland. 
True  Coal-measures,  however,  alsa  occur  in  these  regions,  though  to  a  smaller  extent 
than  the  lower  parts  of  the  system.  One  of  the  most  extensive  coal-fields  is  that  of 
Silesia,^  where  the  seams  of  coal  are  both  numerous  and  valuable,  one  of  them  attaining 
a  thickness  of  50  feet  It  is  noteworthy  that  in  the  Coal-measures  of  eastern  and 
southern  Germany  horizons  of  marine  fossils  occur  like  those  so  marked  in  the 
corres})onding  strata  of  Britain. 

The  coal-field  of  Pilsen  in  Bohemia  occupies  about  300  square  miles.  It  consists 
mainly  of  sandstone,  passing  sometimes  into  conglomerate,  and  interstratified  with 
shales  and  a  few  seams  of  coal  which  do  not  exceed  a  total  thickness  of  20  feet  of  coal. 
In  its  up|)er  ]>art  is  an  important  seam  of  shaly  gas-coal  (Plattel,  or  Brettelkohle), 
which,  besides  being  valuable  for  economic  purposes,  has  a  high  paheontological  interest 
from  Dr.  Fritsch's  discovery  in  it  of  a  rich  fauna  of  amphibians  and  fishes.  The 
plants  above  and  below  this  seam  are  ordinary  typical  Coal -measure  forms,'  but 
these  animal  remains  present  such  close  affinities  to  Permian  types,  that  the  strata 
containing  them  may   belong  to    the   Permian   system   (pp.   846,  850).      What  are 


»  D.  Stur,  AbhatuU.  k,  k,  Oeol.  Reichmnst.  (1877). 

^  From  the  coal-field  of  Central  Bohemia  C.  Feistinantel  enumerated  278  s{)ecies  of 
plants,  of  which  137  were  ferns  {Sphenopteris,  Neuropteris,  OdontopteriSy  CycUheUea^ 
AUthopteriSf  Megaphylon^  kc.)  Archiv,  NcUurw.  Landeadurchforsch.  Bdhmen,  y.  No.  8, 
1883.     For  the  amphibian  remains,  see  Fritsch's  '  Fauna  der  Gaskohle.' 


838  STRATIGRAPHICAL  GEOLOGY  book  vi  pabt  n 

believed  to  be  true  Permian  rocks  in  the  Pilscn  district  seem  to  overlie  the  ootls 
unconformably. 

Alps,  Italy. — The  Carboniferous  strata  of  the  Alps  have  been  already  (p.  622)  referred 
to  in  connection  with  the  metamorphism  of  that  region.  In  the  western  part  of  the 
chain  they  occur  embedded  in  or  associated  with  a  great  series  of  reddish  sandstones^ 
conglomerates  and  red-greenish  shales  or  slates,  which  occasionally  become  quite  ciys- 
talliue,  and  cannot  indeed  be  satisfactorily  separated  from  what  have  been  regarded  as 
the  primitive  schists  of  the  mountains.  To  these  strata  the  name  of  '*  Ycrmcano"  has 
been  given.  That  they  are  partly,  at  least,  of  Carboniferous  age  is  shown  by  the 
characteristic  flora,  amounting  to  upwards  of  60  species,  which  the  dark  carbonaceous 
bands  have  yielded.^ 

In  Italy  the  Carboniferous  and  Permian  rocks  are  so  closely  related  and  so  nmilar 
that  it  is  doubtful  to  which  system  some  of  the  intermediate  portions  shonld  be 
assigned.  At  Monte  Pizzul  in  the  Camic  Alps,  the  lower  strata  contain  Produdui 
gigarUeus  and  P.  scmireticulcUiis^  while  the  highest  are  characterised  by  numerons  forms 
of  F^istditia^  FeneslelUif  kc.^  In  other  parts  of  the  same  region  lower  strata  of  the  age 
of  the  Culm  of  Germany  have  been  described  by  Stur  and  Stache. 

Bussia. — Over  a  vast  region  of  the  east  of  Europe  Carboniferous  limestones, 
sandstones,  shales,  and  thin  coal-seams  are  spread  out  almost  horizontally.  They 
unite  the  marine  and  terrestrial  types  of  sedimentation  so  characteristic  of  the 
north  of  Britain.  In  the  central  provinces  of  Russia,  the  Moscow  basin  or  coal-field  oi 
Tula,  said  to  occupy  an  area  of  13,000  sqiiare  miles,  lies  conformably  on  the  Old  Bed 
Sandstone  or  Devonian  system,  and  contains  limestones  full  of  Carboniferous  Limestone 
fossils,  and  a  few  poor  seams  of  coal.  In  the  south  of  the  empire,  the  coal-field  of  the 
Donetz,  covering  an  area  of  11,000  square  miles,  (contains  60  seams  of  coal,  of  which  44, 
having  a  united  thickness  of  114  feet,  are  worl<able.  Agahi,  on  the  flanks  of  the  Ural 
Mountains,  the  Carboniferous  Limestone  series  has  been  upturned  and  contains  some 
workable  coal-seams.  It  would  appear,  therefore,  that  this  particular  type  of  mingled 
marine  and  terrestrial  strata  of  Carboniferous  age,  occupies  a  vast  expanse  under  later 
formations  in  the  east  of  Euroj)e.  Since  so  much  of  the  Russian  development  of  the 
Carboniferous  system  consists  of  limestone,  it  is  interesting  to  find  that  it  contains  many 
of  tlie  familiar  fossil  8i)ecies  of  the  Carboniferous  Limestone  of  Western  Europe.  Thus 
in  the  Ural  region,  according  to  Prof  Tschemyschew,  the  Carboniferous  S3rstem  may  be 
divided  into  five  zones,  of  which  the  lowest,  a  limestone  containing  Prodvctus  gigarUeus^ 
P.  HlrlaiuSy  Chonctcs  papilUmacca^  &c.,  and  the  next  a  limestone  with  Spir^fer  mostmensis^ 
may  be  regarded  as  corresponding  to  the  typical  Carboniferous  Limestone  of  the  west 
The  three  U[)j)er  zones,  viz.  those  of  {a)  Syringopora  parallels j  Spirifer  8tri(Uu$,  &c,  (6) 
Product lUH  corOy  and  (c)  Spirifer  fasciger  and  Conocardium  uralicum^  are  probably 
equivalent  to  the  Millstone  Grit  and  Coal-measures.*  One  of  the  most  abundant  and 
persistent  organisms  of  the  upper  zones  is  the  foraminifer  Fustdina.  The  upper  Car- 
boniferous rocks  on  the  west  side  of  the  Urals  shade  upwards  into  the  base  of  the 
Pennian  system,  and  show  a  commingling  of  Carboniferous  and  Peimian  fossils. 

Even  as  lar  north  as  Spitsbergen  a  characteristic  Carboniferous  flora  has  been  ob- 
tained, comprising  26  species  of  plants,  half  of  which  are  new,  but  among  which  we 
recognise  such  common  forms  as  Lepidendron  Stcmhergii  and  Cordaites  borass^olius.^ 

^  For  an  essay  on  these  rocks,  see  L.  Milcb's  *■  Beitrage  zur  Kenntniss  des  Verrucano,' 
Leipzig,  1892.  The  metamorphism  of  Carboniferous  and  Permian  rocks  in  the  Alps  of 
Savoy  is  described  by  P.  Termier,  Bull.  Carle  GSol.  France,  ii.  (1891)  p.  367. 

■^  A.  Tommasi,  Boll.  Soc.  Ocol.  Ital.  viii.  p.  564  ;  C.  F.  Parona  and  L.  Bozzi,  op,  cU,  ix. 
pp.  56,  71. 

«  Ann.  Soc.  Oid.  Nord,  xvii.  (1890)  p.  201.  Nikitin,  Mem.  Com.  Okl.  Russ.  v.  (1890), 
No.  5. 

*  Heer,  Flora  Fossilis  Arctica,  iv.  (1877)  p.  4. 


SECT,  iv  §  2  CARBONIFEROUS  SYSTEM  839 

Africa. — The  sea  in  which  the  brachiopods,  corals,  and  crinoids  of  the  Carboniferous 
Limestone  lived  extended  across  the  Mediterranean  basin  into  Africa.  Species  of  Pro- 
dtictuSy  AthyriSy  Spirifery  Streptorhynchus,  Orthis,  Cyathophyllumt  Ac,  have  been 
obtained  in  the  western  Sahara  between  Morocco  and  Timbuetoo.^  The  red  sandstones, 
which  extend  into  the  peninsula  of  Sinai  and  thence  into  Palestine,  have  yielded  stems 
of  Lepidodendron  and  Sigillaria,  and  an  intercalated  limestone  contains  Orthia  Michelini 
and  Streptorhynchus  crenistria,^  A  number  of  characteristic  brachiopods  of  the  Carboni- 
ferous Limestone  have  also  been  obtained  from  the  hills  in  the  Egyptian  desert  to  the 
west  of  the  Gulf  of  Suez,  such  as  Rhynchonella  pleurodoUy  Productus  semireticultUus, 
Spirifcr  stricUus.^  In  Southern  Africa  the  existence  of  Carboniferous  rocks  has  long 
been  known.  Above  certain  slates  and  sandstones  (Bokkeveldt)  containing  fossils  with 
Devonian  affinities  come  the  quartzites  of  Cape  Colony,  enclosing  Lepidodendron  and 
other  Carboniferous  plants.  These  are  unconformably  overlain  by  the  "  Dwyka 
Conglomerate,"  probably  in  great  part  of  volcanic  origin,  and  the  Ecca  mudstones  and 
sandstones,  some  4000  feet  thick.  After  another  great  unconformability  come  the 
Kimberley  shales  and  the  **  Karoo  Beds,"  which  have  been  compared  with  the  Permian 
and  Trias  rocks  of  Europe.* 

Asia. — The  Carboniferous  system  is  extensively  developed  in  Asia.  In  China,  where 
it  covers  an  area  of  many  thousand  miles,  forming  a  succession  of  vast  tablelands,  it 
has  been  found  by  Richthofen  to  be  composed  of  three  stages :  1st,  a  massive  brown 
bituminous  limestone,  which  from  its  foraminifera  {Fusulinay  Fusulinella^  Lingulinaj 
Endothyraf  Vdlviilinaf  Climcuxtmmina)  is  obviously  the  equivalent  of  the  Carboniferous 
Limestone  of  Europe.  It  is  covered  by  (2nd)  productive  Coal-measures  with  both  bi- 
tuminous and  anthracitic  coals,  and  containing  a  characteristic  Coal-measure  flora, 
among  which  are  numerous  ferns  of  the  genera  SphenopUriSy  PalaMpUris,  Cyclopteris, 
NeuropteriSf  Callipter^dium,  OycUheUea,  &c.,  also  species  of  CatamiteSy  Sphenophyllu/m, 
Lepidodendron  (including  L,  Sterribergii)^  Stigmaria  {S.  ficoides)^  CordaiteSy  and  others. 
3rd,  Upi)er  Carboniferous — sandstones,  conglomerates  and  thin  limestones,  containing 
marine  fossils,  among  which  are  the  cosmopolitan  brachiopods  mentioned  on  p.  811.* 

AustralaBia. — In  Australia,  important  tracts  of  true  Carboniferous  rocks,  with 
coal-seams,  range  down  the  eastern  colonies,  and  are  specially  developed  in  New  South 
Wales,  where  they  are  divisible  into  :  1st,  Lower  Carboniferous — sandstones,  conglomer- 
ates, limestones,  shales,  much  disturbed  in  some  places,  traversed  by  valuable  auriferous 
quartz-reefs,  and  yielding  abundant  plant-remains  (Lepidodendron  veltheimianumy  L. 
nothuniy  species  of  Bomiay  Sphenopteris,  CalamiteSy  Rhacopteris,  Ac.)  2nd,  Upper  or 
Permo-Carboniferous,  including  a  series  of  coal-bearing  strata,  both  below  and  above 
which  are  thick  masses  of  calcareous  conglomerates  and  sandstone  abounding  in  marine 
fossils.  The  coal-seams  are  sometimes  30  feet  thick,  and  among  the  plants  associated 
with  them  are  five  species  of  Olossopteris,  also  species  of  PhylloUiecay  Annularia,  and 
NoggercUhiopsis.  The  genus  Olossopteris  was  formerly  believed  to  be  entirely  Mesozoic, 
and  its  occurrence  with  true  Carboniferous  organisms  was  for  a  time  denied.  There 
can  now  be  no  doubt,  however,  that  it  appears  among  strata  in  which  are 
found  the  widespread  and  characteristic  Carboniferous  Limestone  forms  LUhostrotion 
basaUiformCy  L.  irregulare.y  Fenest^Ua  plebeia,  Athyris  Rayssii,  Orthis  Michelini^  0.  resu- 
pincUay  Prodiictus  aculeatuSy  P.  eoray  P.  longispinuSy  P.  punetcUuSy  P.  aemireticulcUtUy 
and  many  more."    Prof.  T.  "W.  E.  David,  in  summarising  our  knowledge  of  the  coal- 

1  G.  Stache,  Denksch,  Acad.  Wiu.  Wien,  xlvi.  (1893). 

2  R.  Tate,  Quart,  Joum,  OtoL  Sac,  xxvii.  (1871)  p.  404. 
»  J.  Walther,  ZeiUch,  Deutsch,  Oeoi.  Oes,  (1890)  p.  419. 

*  A.  H.  Green,  Quart.  Joum.  Oeol.  Soc.  xliv.  (1888)  p.  240. 

*  Richthofen,  *  China,*  vols.  ii.  and  iv. 

^  See  the  papers  by  W.  B.  Clarke,  R.  Etheridge  jun. ,  De  Koninck  and  Wilkinson  cited 
on  p.  776. 


840  STRATIQRAPHICAL  GEOLOGY  book  vi  PAinn 


bearing  rocks  of  New  Sooth  Wales,  gives  a  thickness  of  11,150  feet  to  the  Upper  or 
Pcrmo-Carboniferous  series,  and  11,300  feet  to  the  Lower  Carboniferous.  The  prodnotiTe 
Coal-measures  lie  in  the  upper  series.  In  descending  order  these  are :  the  Newcastle 
group,  Tomago  or  East  Maitland  group,  and  Greta  group.  The  Permo-Carboniferoiis 
series  is  separated  by  an  unconformability,  and  a  strong  break  in  tiie  flora,  from  the  lower 
division,  in  the  top  of  which  sheets  of  andesitic  dolerite  with  tuffs  occur. ^  Among  the 
marine  strata  of  the  Lower  Coal -measure  series  R.  D.  Oldham  found  ooarse  con- 
glomerates, which  he  compared  with  those  of  India  as  probably  indicative  of  glacial 
transport.^ 

In  New  Zealand  the  rocks  assigned  to  the  Carboniferous  system  consist,  in  the  upper 
part,  of  fine  clay -slates,  becoming  calcareous  and  passing  down  into  true  limestones  at 
the  base,  from  which  Spiri/er  hisiUcatuSf  S.  glaheTf  Productus  tnuchytkaerua^  &c.,  have 
been  obtained.  They  are  thus  probably  Lower  Carboniferous  ;  and,  though  they  do  not 
yield  coal,  they  are  geologically  important  from  the  large  share  they  take  in  the 
structure  of  the  great  mountain-ranges,  and  from  the  occasional  abundant  development 
in  them  of  contemporaneous  igneous  rocks,  which  are  associated  with  metalliferous 
deposits.* 

North  America. — Rocks  corresponding  in  geological  position  and  the  general  aspect 
of  their  organic  contents  with  the  Carboniferous  system  of  Europe  are  said  to  cover 
an  area  of  more  than  200,000  square  miles  in  the  United  States  and  British  North 
America.  The  following  table  shows  the  subdivisions  which  have  been  established 
among  them : — 

Coal-measures, — a  series  of  sandstones,  shales,  ironstones,  coals,  ^,  varying 
from  100  feet  in  the  interior  continental  area  to  4000  feet  in  Pennsylvania, 
and  more  than  8000  feet  in  Nova  Scotia.  The  plant  remains  inclade  forms 
of  Lepidodendrorif  Sigillaria,  StigmariOf  CalamUes,  ferns,  and  coniferous 
leaves  and  fruits.  The  animal  forms  embrace  in  the  marine  bands  species  of 
Spiri/eTy  ProductiUj  BeUerophon,  NaiUilus,  &c.  Among  the  shales  and  car- 
bonaceous beds  numerous  traces  of  insect  life  have  been  obtained,  comprising 
species  related  to  the  may-fly  and  cockroach.  Spiders,  scorpions,  centipedes, 
limuloid  crabs,  and  land-snails  like  the  modem  Pupa  have  also  been  met 
with.  The  tish  remains  comprise  teeth  and  ichthyodorulitesof  selachian  genera, 
and  a  number  of  ganoids  {Euryl^pia,  Cvclacanthus,  MegcUichthySf  Rhizodus, 
&c. )  Sevei*al  labyriuthodonts  occur,  aud  true  reptiles  are  represented  by 
one  saurian  genus  found  in  Nova  Scotia,  the  Sosaurus. 

In  the  Western  Territories  the  Upper  Carboniferous  rocks  consist  of  a 
massive  group  of  limestone  2000  feet  thick,  resting  on  Lower  Carboniferous 
("  Weber  Quartzite"  of  King),  estimated  at  6000  to  10,000  feet,  but  with 
no  coals. 

Millstone  Grit, — a  group  of  arenaceous  and  sometimes  conglomeratic  strata, 
with  occasional  coal-seams,  only  25  feet  thick  in  some  parts  of  New  York, 
but  swelling  out  to  1500  feet  in  Pennsylvania. 


^  Trans.  Amlral,  Assoc.  Soc.  vol.  ii.  (1890)  pp.  459-465.  O.  Feistmantel,  Mem.  Oeol. 
Sun\  N.S.  Wales,  Palo'mtology,  No.  8,  1890,  p.  37.  The  Carboniferous  and  Permo- 
Carboniferous  corals  of  New  South  Wales  are  described  by  K  Etheridge  jun.,  op.  eU.  Na  5, 
1891.  For  recent  information  on  the  Australian  Coal-fields,  see  papers  by  Walker, 
Robertson,  &  Cox,  Trans.  Fed.  Inst.  Min.  Eng.  ii.  (1891)  pp.  268,  821  ;  iv.  (1893)  p.  83. 
For  a  detailed  account  of  the  Permo-Carboniferous  rocks  and  fossils  of  Queensland,  see  R 
L.  Jack  and  E.  Etlieridge  jun.,  *The  Geology  and  Paleontology  of  Queensland,'  1892, 
chaps,  vi. -xxii. 

'-^  Rec.  Geol.  Surv.  India,  xix.  part  i.  p.  39. 

*  Hector's  *  Handbook  of  New  Zealand,'  1883,   p.   35.     F.  W.  Button,  QuarU  Joum. 
Geol.  Soc.  1885,  p.  200. 


Vi 

O 
u 

!~ 

'5 
o 

cS 

O 


SECT.  V  §  1 


PERMIAN  SYSTEM 


841 


In  the  Mississippi  basin,  where  the  sub-Carboniferons  groups  are  best 
developed,  they  present  the  following  subdivisions  in  descending  order : — 

Chester  group. — Limestones,  shales,  and  sandstones,  sometimes  600  feet. 

St.  Louis  group. — Limestones  with  shale,  in  places  250  feet. 

Keokuk  group. — Limestone  with  chert  layers  and  nodules. 

Burlington  group. — Limestone,  in  places  with  chert  and  homstone,  25  to 
200  feet 

Kinderhook  group. — Sandstones,  shales,  and  thin  limestones,  100  to  200 
feet,  resting  on  the  Devonian  black  shale. 
The  sub-Carboniferous  groups  are  mainly  limestones,  but  contain  here  and  there 
remains  of  the  characteristic  Carboniferous  land  vegetation.  Crinoids  of 
many  forms  abound  in  the  limestones.  A  remarkable  polyzoon,  Archimedes^ 
occurs  in  some  of  the  bands.  The  brachiopods  are  chiefly  represented  by 
species  of  Spiri/er  and  Produetus  ;  the  lamellibranchs  by  MycUina, 
SchizoduSf  Aviculopeden^  Nticula,  Pinnay  and  others  ;  the  cephalopods  by 
Orttiocerasy  NatUUuSf  OoniatiteSf  Oyroceras,  kc.  The  European  genus  of 
trilobite,  PhUlipsia,  occurs.  Numerous  teeth  and  fin-spines  of  selachian 
fishes  give  a  further  point  of  resemblance  to  the  European  Carboniferous 
Limestone.  Some  of  the  rippled  rain-pitted  beds  contain  amphibian  foot- 
prints— the  earliest  American  forms  yet  known.  Lai^e  deposits  of  g}'psum 
occur  in  this  stage  in  Nova  Scotia. 


<0 

z 

V 

O 
en 


Tlie  highest  members  of  the  Carboniferous  system  in  the  United  States  are  usually 
barren  of  coal.  The  characteristic  Lepidodendra  and  Sigillariie  disappear  and  their 
place  is  taken  by  plants  with  Permian  afllnities  (Pennsylvania,  Ohio,  W.  Virginia), 
whilst  in  Illinois,  Texas,  and  New  Mexica,  Permian  reptiles  occur  in  this  part  of  the 
system.  In  these  regions  no  definite  upper  limit  to  the  system  can  be  found,  as  it  shades 
upwards  into  strata  which  may  represent  the  Permian  series  of  Europe.^ 


Seetion  v.  Pepmian  (Dyas). 

§  1.  General  Characters. 

The  Carboniferous  rocks  are  overlain,  sometimes  conformably,  but  in 
Europe  for  the  most  part  unconformably,  by  a  series  of  red  sandstones, 
conglomerates,  breccias,  marls,  and  limestones.  These  used  to  be 
reckoned  as  the  highest  part  of  the  Coal  formation.  In  England  they 
received  the  name  of  the  "  New  Red  Sandstone  "  in  contradistinction  to 
the  "  Old  Red  Sandstone  "  lying  beneath  the  Carboniferous  rocks.  The 
term  "  Poikilitic  "  was  formerly  proposed  for  them,  on  account  of  their 
characteristic  mottled  appearance.  Eventually  they  were  divided  into 
two  systems,  the  lower  being  taken  as  the  summit  of  the  Palaeozoic  series 
of  formations,  and  the  upper  as  the  basement  of  the  Mesozoic.  This 
arrangement,  which  is  mainly  based  on  the  difference  between  the 
organic  remains  of  the  two  divisions,  is  generally  adopted  by  geologists.^ 

Following  the  usual  grouping,  we  remark  that  the  portion  of  the  red 

'  See  Report  to  the  International  Geological  Congress,  London,  1888,  by  J.  J.  Stevenson. 
Full  details  of  the  N.  American  Carboniferous  system  are  given  in  Correlation  Papers — 
Devonian  and  Carboniferous,  by  H.  S.  Williams,  Bull.  U.S.  Oeol.  Survey,  No.  80  (1891). 

^  Some  writers,  however,  still  contend  that  the  red  rocks  of  Europe  between  the  summit 
of  the  Carboniferous  and  base  of  the  Jurassic  system  form  really  one  great  series,  the  break 
between  them  being  merely  local  See,  for  example,  H.  B.  Woodward,  Oeol.  Mag.  1874, 
p.  385  ;  'Geology  of  England  and  Wales,'  2nd  edit.  (1887)  p.  207,  and  authorities  cited 
by  him. 


842  STRA  TIGBA  PHICAL  GEOLOG  Y  book  ti  r 


strata  classed  as  Pakeozoic  has  received  the  name  (^  ^  Permian,^  firom  its 
wide  development  in  the  Russian  province  of  Perm,  where  it  was  studied 
by  Murchison,  De  Vemeuil,  and  Kejserling.  In  Germany,  where  it 
exhibits  a  well-marked  grouping  into  two  great  series  of  deposits^  the 
name  "  Dyas/'  proposed  by  Geinitz»  has  on  that  account  been  to  some 
extent  adopted.  In  North  America,  where  no  good  line  of  sabdividoD 
can  be  made  at  the  top  of  the  Carboniferous  system,  the  term  ^Permo- 
Carboniferous''  has  been  used  to  denote  the  transitional  beds  at  the 
top  of  the  Palaeozoic  series,  and  this  name  has  been  proposed  for  use  also 
in  Europe  and  in  Australia. 

In  Europe  two  distinct  types  of  the  system  can  be  made  out.  In  <Hie 
of  these  (Dvas)  the  rocks  consist  of  two  great  divisions:  (1)  a  lower 
series  of  red  sandstones  and  conglomerates,  and  (2)  an  upper  group  of 
limestones  and  dolomites.  In  the  other  (Russian  or  Permian)  the  strata 
are  of  similar  character,  but  are  interstratified  in  such  a  way  as  to 
present  no  twofold  petrographical  subdivision. 

Rck:k8. — The  prevailing  materials  of  the  Permian  series  in  £urope 
are  undoubtedly  red  sandstones,  passing  now  into  conglomerates  and 
now  into  fine  shales  or  *'  marls."  In  their  coarsest  forms,  these  detrital 
def>osit8  consist  of  conglomerates  and  breccias,  composed  of  fragments  <^ 
different  crystalline  or  older  Palaeozoic  rocks  (granite,  diorite,  gneiss, 
mica-schist,  qiiartzite,  greywacke,  sandstone,  &c.),  that  vary  in  size  up 
to  }>lo€ks  a  foot  or  more  in  diameter.  Sometimes,  these  stones  are  well 
rounded,  but  in  many  places  they  are  only  partially  so,  while,  here  and 
there,  they  are  quite  angular,  and  then  constitute  brecciaa  The  pebbles 
are  held  together  by  a  brick-red  ferruginous,  siliceous,  sandy,  or  argilla- 
ceous cement.  The  sandstones  are  likewise  characteristically  brick-red 
in  colour,  generally  with  green  or  white  layers  and  spots  of  decoloration. 
The  "marls,"  showing  still  deeper  shades  of  red,  and  passing  occasionally 
into  a  kind  of  livid  purple,  are  crumbling  sandy  clay-rocks,  sometimes 
merging  into  more  or  less  fissile  shales.  Of  the  argillaceous  beds  of  the 
system  the  most  remarkable  are  those  of  the  Marl-slate  or  Kupferschiefer 
— a  brown  or  black  often  distinctly  bituminous  shale,  which  in  certain 
parts  of  Germany  is  charged  with  ores  of  copper.  The  limestone,  so 
characteristic  a  feature  in  the  "  Dyas  "  development  of  the  system,  is  a 
compact,  well-bedded,  somewhat  earthy,  and  usually  more  or  less  dolomitic 
rock  (Zechstein).  It  is  the  chief  repository  of  the  Permian  invertebrates. 
With  it  are  associated  bands  of  dolomite,  either  crystalline  and  cavernous 
( Ranch wacke)  or  finely  granular  and  crumbling  (Asche) ;  also  bands 
of  gypsum,  anhydrite,  and  rock-salt.  In  certain  localities  (the  Harz, 
Bohemia,  Autun)  seams  of  coal  are  intercalated  among  the  rocks,  and 
with  these,  as  in  the  Coal  measures,  are  associated  bituminous  shales  and 
nodular  clay-ironstones.  In  Germany,  France,  the  south-west  of  England, 
and  the  south-west  of  Scotland,  the  older  part  of  the  Permian  system 
contains  abundant  contemporaneous  masses  of  eruptive  rock,  among 
which  occur  diabase,  melaphyre,  porphyrite,  and  various  forms  of  quartz- 
por])hyry. 

Some  of  the  breccias  in  the  west  of  England  contain  striated  stones. 


8BCT.  V  §  1  PERMIAN  SYSTEM  843 

which,  according  to  Sir  A.  C.  Ramsay,  indicate  the  existence  of  glaciers 
in  Wales  during  the  Permian  period.^ 

The  Permian  system  in  the  greater  part  of  Europe,  from  the  prevalent 
red  colour  of  its  rocks,  the  association  of  dolomite,  rock-salt,  saliferous 
clays,  gypsum,  and  anhydrite,  and  the  remarkably  impoverished  and 
stunted  aspect  of  its  fauna,  has  evidently  been  deposited  in  isolated  basins 
in  which  the  water,  cut  off  more  or  less  completely  from  the  sea,  under- 
went concentration  until  chemical  precipitation  could  take  place.  Look- 
ing back  at  the  history  of  the  Carboniferous  rocks,  we  can  understand 
how  such  a  change  in  physical  geography  was  brought  about  The  Car- 
boniferous Limestone  sea  having  been  by  upheaval  excluded  from  the 
region,  wide  lagoons  occupied  its  site,  and  these,  as  the  land  slowly  went 
down,  crept  over  the  old  ridges  that  had  for  so  many  ages  been  prominent 
features.  The  downward  subterranean  movement  was  eventually  varied 
by  local  elevations,  and  at  last  the  Permian  basins  came  to  be  formed. 
As  a  result  of  these  disturbances,  the  Permian  rocks  overlap  the  Carboni- 
ferous, and  even  cover  them  in  complete  discordance,  the  denudation  of 
the  older  formation  having  been,  in  some  places,  enormous  before  the 
Permian  strata  were  laid  down.* 

In  Southern  Europe  and  thence  eastwards,  abundant  evidence  of  open 
seas  is  supplied  by  limestone  containing  a  rich  fauna  of  foraminifera, 
gasteropods,  orthoceratites,  and  early  precursors  of  the  ammonites. 

LiFK — The  conditions  under  which  the  Permian  rocks  of  the  greater 
part  of  Europe  were  deposited  must  have  been  eminently  unfavourable  to 
life.  Accordingly  we  find  that  these  rocks  are  on  the  whole  singularly 
barren  of  organic  remains.  So  great  is  the  contrast  between  them  and 
older  formations,  that  instead  of  such  rich  faimas  as  those  of  the  Silurian, 
Devonian,  and  Carboniferous  systems,  they  have  yielded  only  somewhere 
about  300  species  of  organisms.    ^ 

The  flora  of  the  older  Permian  rocks  presents  many  points  of  resem- 
blance to  the  Carboniferous.'  According  to  Grand*  Eury  upwards  of  50 
species  of  plants  are  common  to  the  two  floras.  Among  the  forms  which 
rise  into  the  Permian  rocks  and  disappear  there,  are  Catamites  Sudcomiy  C. 
approxijfiaius,  Asterophyllites  eguisetifarmiSj  A,  rigidus,  Pecopteris  degans, 
Odontopteris  Schlotheimii^  SigUlaria  Brardli  (and  others),  Siigmaria  ficoides^ 
Cordaites  borassifolius,  &c.  Others,  which  are  mainly  Permian,  are  yet 
found  in  the  highest  coal-beds  of  France,  e,g.  Calamites  gigas,  Calamodendron 
striatum,  Arthropitus  ezonata^  TsBniopteris  abnormiSy  Walchia  piniformis,  &c. 
But  the  Permian  flora  has  some  distinctive  characters ;  such  as  the  variety 
and  quantity  of  the  ferns  united  under  the  genus  Callipteris,  which  do 
not  occur  in  the  Coal-measures,  the  profusion  of  tree-ferns  {Psaronivs,  of 
which  24  species  are  described  by  Goppert,  Proiopteris,  CauhpteriSy  &c) 

*  Quart.  Joum.  Oeol.  Soc.  1865,  p.  185. 

'''In  some  places,  the  whole  of  the  Carboniferous  system  has  been  worn  away  down  to 
the  Carboniferoiw  Limestone,  upon  which  the  Permian  sandstones  and  conglomerates  have 
been  directly  deposited.  The  discordance,  however,  sometimes  disappears,  and  then  the 
Carboniferous  and  Permian  rocks  shade  into  each  other. 

»  See  Ooppert's  •  Die  Fossile  Flora  der  Permischen  Formation,'  Cassel,  1864-65. 


844 


STRATIGBAFHICAL  GEOLOGY 


BOOK  VI  PABT  n 


of  BqaisttiUs,  and  of  the  conifers  fFcUchia  piniformis  and  ff.  filidfonms, 
and  Uie  occurrence  of  species  of  Qingko.  The  most  characteristic  plant* 
throughout  the  German  Permian  groups  are  OdonUypteru  obtiuiloba,  Caitiplent 
am/erta,  Catamites  gigas,  and  IfaUhia  pim/ormis.  The  last  representatiTes 
of  the  ancient  tribes  of  the  lepidodendra,  sigillariolda,  and  calamites  are 
found  in  the  Permian  syslem.  Cjcade  now  make  their  appearance  and 
increase  in  importance  in  the  succeeding  geological  periods.  Among 
their  Permian  forms  are  the  genera  PteropkyUwn,  and  Maiitiloia,  In  extn' 
European  Permian  areas  a  commingling  of  Palieozoic  and  MesoEoic  types 
of  vegetation  has  been  observed,  forms  of  Voltzia,  Pterepkyllmn,  and 
Glossopleris  being  there  prominent 

The  impoverished  fauna  of  the  Permian  rocks  of  central  Europe  is 
found  almost  wholly  in  the  limestones  and  brown  shales,  the  red  con- 


glomerates and  sandstones  being,  as  a  rule,  devoid  of  organic  contents. 
A  few  corals  {Stenopora,  Polyc«Ua)  and  polyzoa  (Ferusfdlii,  Polypt^a, 
Syiwclwiia,  Acaiillwcladia)  occur  in  the  limestones,  the  latter  sometimes 
even  in  continuous  masses  like  coral-reefs,  as  in  the  dolomite-reef  of  S.E. 
Thuringia.  The  echinoderms  are  few,  the  chief  crinoids  being  species  of 
Cytiilwcrinux.  Among  the  brachiopods,  of  which  some  30  species  are 
known,  the  most  conspicuous  are  forms  of  PnidiiMvs,  Camarophoria,  Spirifer, 
Strojihalofia  (Fig.  374),  and  AidoxUges.  Lamellibranchs  are  more  numerous, 
characteristic  genera  in  the  German  limestone  being  j4i:inus  (Fig.  374), 
AUm-isnid,  Sohmya,  Schizodus,  Edmondia,  Arm,  Avicula,  Bakevellia  (Fig. 
374),  I'ecteiK  Among  the  few  gasteropoda,  forms  of  ChemnUzia,  Tmiw, 
Murchisonm,  Pleurotomaria,  and  OAi/on  have  been  recorded.  An  occasional 
NatUUiis,  Orlhoceras,  or  Cyrtoceras  represents  the  rich  cephalopodan  founa 
of  the  Carboniferous  Limestone. 

It  is  not,  however,  from  the  sites  of  the  brackish  inland  seas  of 


v§l 


PERMIAN  SYSTEM 


western  and  central  Europe  that  we  may  obtain  the  beat  conception  of 
the  animal  life  of  Permian  time.  If  we  pass  southwardH  into  die  Alpa 
and  the  Mediterranean  basin,  or  eastwards  into  the  Urahan  region  and 
thence  into  India,  we  find  that  while  some  of  the  European  forms  ext«nd 
into  these  areas,  they  are  accompanied  by  many  hundreds  of  other  species. 


One  of  the  most  remarkable  features  in  this  richer  and  more  varied  fauna 
is  the  great  number  of  the  cephalopoda  and  the  affinities  which  many  of 
them  present  to  the  Ammonites  so  characteristic  of  Mesozoic  time. 
Among  the  Permian  genera  of  this  type  are  Adrianiies,  Arcestet,  Med- 
lieollia,  Papanoceras,  Slaeheocerag,  Thaiassiieems,  and  Waagenouras.  They 
are  associat«d  with  many  forms  of  Ortkoceras,  Gyroceras,  and  Navtilus — 
a  blending  of  Palieozoic  and  Mesozoic  types  which  is  much  less  clearly 
shown  in  central  and  western  Europe. 


Fishes,  which  are  proportionately  better  represented  in  the  Permian 
rocks  than  the  invertebrates,  chiefly  occur  in  the  marl-slate  or  Kupfer- 
schiefer,  the  most  common  genera  being  Pntwonwcun  {Fig.  375),  which  is 
specially  characteristic,  Plalysmnui  (Fig.  376),  Pygoptervs,  Acanlhodes, 
AcmlepU,  and  Amblyplerus. 

Amphibian   life  appears  to  have  been  abundant  in  Permian  times, 


846  STEATIGRAPHIGAL  GEOLOGY  book  vi  past  n 

for  some  of  the  sandstones  of  the  system  are  covered  with  footprints, 
assigned  to  the  extinct  order  of  Labyrinthodonts.  Occasional  skulls  and 
other  bones  have  been  met  with  referable  to  Archegosaurus,  Ltpidoiosamvs, 
Zygosaurus,  &c.  The  remains  of  comparatively  few  forms,  however,  had 
been  found  until  the  remarkable  discoveries  of  Dr.  Anton  Fritsch  in  the 
basins  of  Pilsen  and  Eakowitz  in  Bohemia.  The  strata  of  these  localities 
have  been  already  (p.  837)  referred  to  as  containing  an  abundant  and 
characteristic  coal-flora,  yet  with  a  fauna  that  is  as  decidedly  like  that 
of  known  Permian  rocks.  According,  therefore,  as  we  give  preference 
to  the  plants  or  the  animals,  the  strata  may  be  ranked  as  Carboniferous 
or  as  Permian.  Of  the  numerous  Saxon  and  Bohemian  species  of 
amphibians,  Prof.  Credner  in  Dresden  and  Dr.  Fritsch  in  Prague  have 
published  elaborate  descriptiona  Among  the  genera  are  ProtrUon  (Bran- 
chiosaurus),  a  form  resembling  an  earth-salamander  in  possessing  gills,  and 
of  which  the  largest  specimen  is  only  about  2h  inches  long),  Sparodus, 
Hi/lanovms^  Dawsonia,  MelanerpetoUy  Dolkhosoma,  Ophiderpeton,  Macrortierum^ 
UrocordyluSy  lAmnerpetony  HylopUsiony  Sedeya^  Microbrachis,  Diplospotidylus^ 
Nyrania,  and  Dendrerpeton,  Some  of  these  forms  were  remarkably  small 
The  adult  Protritonidse,  for  instance,  were  only  from  2^  to  6^  inches  long. 
Other  types,  however,  attained  a  much  larger  size,  Palagosirefij  for  instance, 
being  estimated  to  have  had  a  length  of  45  feet^  From  the  correspond- 
ing strata  of  Autun  in  Central  France,  M.  Gaudry  has  also  described 
some  interesting  forms — Actinodan,  ProtrUon^  EuchirosauruSy  a  larger  and 
more  highly  organised  type  than  any  previously  known  from  the  Palaeozoic 
rocks  of  France,  but  inferior  to  another  subsequently  found  at  Autun, 
which  he  has  named  StereorhachiSy  and  which  was  distinguished  by  com- 
pletely ossified  vertebrae  and  other  proofs  of  higher  organisation  that 
connect  it  with  the  Theriodonts  of  Eussia  and  Southern  Africa  and  with 
the  Pelycosaurians  of  the  United  States. ^  Various  other  anomodont 
reptiles  have  been  met  with,  referable  to  a  number  of  genera  {NaosauraSy 
Clepsydrops),  Of  still  higher  grade  were  other  types  to  which  the  names 
Proterosaums  and  PalmoIuiMeria  (Rhynchocephalia)  have  been  given. 

§  2.  Local  Development. 

Britain.'— In  England  on  a  small  scale,  a  rei^resentative  is  to  be  found  of  the  two 
contrasted  tyi)e8  of  the  European  Permian  system.     On  the  east  side  of  the  island,  from 

1  A.  Fritsch,  *  Fauna  der  Ga«kolile  und  der  Kalksteine  der  Perraformation  Bohraens,' 
Prag,  1881.  See  also  H.  Credner  on  StegocephaJi  from  the  Rothliegende  of  Dresden, 
Z.  DcuUch.  Oeol.  Oes.  1881-86;  E.  D.  Cope,  Anier.  Nat.  xviii.  (1884). 

-^  Gaudry,  Bull.  Soc.  GSol.  France,  vii.  (3  s^r.)  p.  62  ;  ix.  p.  17  ;  xiiL  p.  44  ;  xlv.  pp.  480, 
444.     *Les  Enchainements  du  Monde  Animal,'  1883,  Arch.  Mus.  Nat.  Paris,  x.  (1887). 

»  Sedgwick,  Trans.  Geol.  Soc.  (2)  iii.  (1836)  p.  37  ;  iv.  383  ;  De  la  Beche,  'Geology  of 
Cornwall,  Devon,'  &c.  p.  193;  Murchlson,  'Siluria,'  p.  308;  W.  King,  'Monograph  of 
the  Permian  Fossils,'  Palwantog.  Soc.  1850  ;  Hull,  *  Tiiassic  and  Permian  Rocks  of  Midland 
Counties  of  England,*  in  Mem.  Geol.  Surv.  1869  ;  Q.  J.  Oeol.  Soc,  xxv.  171  ;  xxix.  p.  402 ; 
xlviii.  p.  60  ;  Ramsay,  op.  cit.  xxvii.  p.  241  ;  Kirkby,  op.  cU.  xiii.  xvi.  xviL  xx.  ;  E.  Wilson, 
op.  cit.  xxxii.  p.  533  ;  D.  C.  Davies,  op.  cit.  xxxiii.  p.  10  ;  H.  B.  Woodward,  Oeol.  Mag. 
1874,  p.  385  ;  *  Geology  of  England  and  Wales,'  p.  210  ;  T.  V.  Holmes,  Q.  J.  OtoL  Soc. 
xxxvii.  p.  286  ;  W.  T.  Aveline,  H.  H.  Howell  in  various  Memoirs  Oeoi,  Surv, 


SECT.  V  §  2  PERMIAN  SYSTEM  847 

the  coast  of  Northumberland  southwards  to  the  plains  of  the  Trent,  a  true  *'Dyas" 
development  is  exhibited,  the  Magnesian  Limestone  and  Marl  Slate  forming  the  main 
feature  of  the  system  ;  on  the  west  side  of  the  Pennine  chain,  however,  the  true  Permian 
or  Russian  faciei  is  presented.  The  system  is  in  this  country  most  nearly  complete  in 
the  north-western  and  south-western  counties  of  England.  Arranged  in  tabular  form 
the  rocks  of  the  western  and  eastern  areas  may  be  grouped  as  follows  : — 

W.  or  England.  E.  of  England. 

Red  sandstones,  clays,  and  gypsum    .  600  ft.  50-100  ft. 

Magj.»u»n  Limestone       •         ;         ;}  10-30..  600 


Lower  red  and  variegated  sandstone,  \ 

reddish   brown   and   purple   sand-  I  3000  100-260 

stones  and  marls,  with  calcareous  |  " 


it 


if 


conglomerates  and  breccias    .         .  J 

Lower  Sandstone. — This  subdivision  attains  its  greatest  development  in  the  vale 
of  the  £den,  where  it  consists  of  brick -red  sandstones,  with  some  beds  of  calcareous 
breccia,  locally  known  as  "brockram,"  derived  principally  from  the  waste  of  the  Car- 
boniferous Limestone.  These  red  rocks  extend  across  the  Solway  into  the  valleys  of  the 
Nith  and  Annan  in  the  south  of  Scotland,  where  they  lie  unconformably  on  the  Lower 
Silurian  rocks,  from  which  their  breccias  have  generally  been  derived,  though  near 
Dumfries  they  contain  some  "brockram."  The  breccias  have  evidently  accumulated  in 
small  lakes  or  narrow  Qords.  In  the  basin  of  the  Nith,  and  also  in  Ayrshire,  numerous 
small  volcanic  vents  and  sheets  of  diabase,  picrite,  olivine-basalt,  porphyrite  and  tutf 
are  associated  with  the  red  sandstones,  marking  a  volcanic  district  of  Permian  age.  The 
vents  rise  through  Coal-measures  as  well  as  more  ancient  rocks.  Similar  vents  in 
Fifeshire,  also  piercing  Coal-measures,  have  been  referred  to  the  same  volcanic  period. 
In  Devonshire  similar  rocks  mark  the  outpouring  of  lavas  in  the  early  part  of  the 
Permian  period.^  But  these  volcanic  phenomena  were  on  a  feeble  scale.  They  are 
interesting  as  marking  the  close  of  the  long  continuance  of  volcanic  activity  during 
Paleozoic  time.  Neither  in  Britain  nor  throughout  most  of  the  Continent  has  evidence 
been  found  of  renewed  eruptions  during  the  long  lapse  of  the  Mesozoic  ages.^ 

In  central  England,  Staffordshire,  and  the  districts  of  the  Clent  and  Abberley  Hills, 
the  Permian  system  contains  some  remarkable  brecciatod  conglomerates  which  attain  a 
thickness  of  400  feet.  They  have  been  shown  by  Ramsay^  to  consist  in  large  measure 
of  volcanic  rocks,  grits,  slates,  and  limestones,  which  can  be  identified  with  rocks  on 
the  borders  of  Wales.  Some  of  their  blocks  are  three  feet  in  diameter  and  show  distinct 
striation.  These  Permian  drift-beds,  according  to  Ramsay,  cannot  be  distinguished  by 
any  essential  character  from  modem  glacial  drifts,  and  he  had  no  doubt  that  they  were 
ice-borne,  and,  consequently,  that  there  was  a  glacial  period  during  the  accumulation  of 
the  Lower  Permian  deposits  of  the  centre  of  England. 

Like  red  rocks  in  general,  the  Lower  Permian  beds  are  almost  barren  of  organic 
remains.  Such  as  occur  are  indicative  chiefly  of  terrestrial  surfaces.  Plant  remains 
occasionally  appear,  such  as  Ullmannia  (supposed  to  be  of  marine  growth),  Lqndoden- 
dron  dilatatum^  CalamiUSf  StenCbergia,  Dadoxylon,  and  fragments  of  coniferous  wood. 
The  cranium  of  a  labyrinthodont  {Daayccpa)  has  been  obtained  from  the  Lower  Permian 
rocks  kt  Kenilworth.  Footprints,  referred  to  members  of  the  same  extinct  order,  have 
been  observed  abundantly  on  the  surfaces  of  the  sandstones  of  Dumfriesshire,  and  also 
in  the  vale  of  the  Eden.  ^ 

Magnesian  Limestone  Group. — This  subdivision  is  the  chief  repository  of  fossils 

1  Oeol,  Mag.  (1866)  p.  248  ;  QmH,  Joum.  Gtol.  Soc.  (1892),  Presid.  Address,  p.  147, 
and  authorities  cited. 
'-'  Op,  cU.  p.  162. 
'  Q,  J.  Qeol,  Soc,  xi.  p.  181. 


848  STRATIGRAPHICAL  GEOLOGY  book  vi  paw  n 

in  the  Permian  system  of  England.  Its  strata  are  not  red,  but  consist  of  a  lower  itme  of 
hard  brown  shale  with  occasional  thin  limestone  bands  (Marl  Slate)  and  an  upper  thid^ 
mass  of  dolomite  (Magnesian  Limestone).  The  latter  is  the  chief  feature  in  the  Dyai 
development  of  the  system  in  the  east  of  England.  Corresponding  with  the  Zechatein 
of  Gennauy,  as  the  Marl  Slate  does  with  the  Kupferschiefer,  it  is  a  very  yariable  rock 
in  lithological  characters,  being  sometimes  dull,  earthy,  fine-grained,  and  foasOifennu, 
in  other  places  quite  crystalline,  and  composed  of  globular,  reniform,  botryoidal,  or 
irregular  concretions  of  crystalline  and  frequently  internally  radiated  dolomite.  It  is 
divisible  in  Durham  into  three  sections — 1st,  Lower  compact  limestone,  about  2O0  feet 
thick  ;  2nd,  Middle  fossiliferous  and  brecciform  limestone,  150  feet ;  3rd,  Upper  yellow 
concretionary  and  botryoidal  limestone,  250  feet.  The  Magnesian  Limestone  runs  as  a 
thick  i)ersi8tent  zone  down  the  east  of  England.^  In  southern  Yorkshire  it  is  split 
up  by  a  central  zone  of  marls  and  sandstones  with  gypsum.  It  is  represented  on  the 
Lancashire,  Cheshire,  and  Cumberland  (Penrith)  side  by  bright  red  and  variegated 
sandstones  covered  by  a  thin  group  of  red  marls,  with  numerous  thin  courses  of  lime- 
stone, containing  SchizoduSy  BakevelUa^  and  other  characteristic  fossils  of  the  Magnesian 
Limestone.  Murchison  and  Harkness  have  classed  as  Upper  Permian  certain  red  sand- 
stones with  thin  partings  of  red  shale,  and  an  underl3ring  band  of  red  and  green  marls 
and  gypsum.  These  rocks,  seen  at  St.  Bees,  near  Whitehaven,  resting  on  a  magnesian 
limestone,  have  not  yet  yielded  any  fossils. 

The  Magnesian  Limestone  group  of  the  north  of  Elngland  has  yielded  about  150 
species  belonging  to  some  70  genera  of  fossils— a  singularly  poor  fauna  when  contrasted 
with  that  of  the  Carboniferous  system  below.  The  brachiopods  include  Produettu 
harridiis,  Cairuirophoria  humblet&iunisiSf  C  Schlotheimii,  Slrophalosia  Ooldfuasi,  Lin- 
gula  Crcdnerij  and  Terrbratula  elaivgata.  Of  the  lamellibrauchs  Axinus  {Schxsodtu) 
Schlotheimiij  BakevcUui  tumida^  B.  antiqiui,  B.  ceraiophaga^  MytUus  aqvamostts,  and 
Area  striaia  are  characteristic.  The  univalves  are  represented  by  10  genera  and  26 
species,  including  PJcurotoituiria  and  Turbo  as  common  genera.  Nine  genera  of  fishes 
have  been  obtained  chiefly  in  the  Marl  Slate,  of  which  PalsBoniscus  and  FUUysomus  are  the 
chief.  These  small  ganoids  are  closely  related  to  some  which  haunted  the  lagoons  of  the 
Carboniferous  i>eriod.  Some  reptilian  remains  have  been  obtained  from  the  Marl  Slate, 
particularly  ProterosauriM  Speiieri  and  7*.  Hvxlcyi^  while  Lcpidotosaunis  Duffii  has  been 
found  in  the  Magnesian  Limestone. 

Fine  sections  are  ex]x)sed  on  the  south  coast  of  Devonshire  of  coarse  breccias  and  red 
sandstones,  which  have  been  assigned  by  some  writers  to  the  Trias,  by  others  to  the 
Pennian  series.  They  rest  unconfonnably  on  Devonian  strata,  and  have  been  derived 
from  the  degradation  of  these  rocks.  At  many  places  in  the  interior  to  the  west  of 
Exeter  bands  of  basic  amygdaloidal  lavas  are  intercalcated  in  them,  like  the  volcanic 
sheets  in  the  Permian  sandstones  of  Scotland.  Owing  to  the  apj)arent  passage  of  these 
red  strata  upwards  into  others  which  graduate  into  the  base  of  the  Lias,  and  are 
undoubtedly  TriavSsic,  the  whole  series  of  red  sediments  has  not  unnaturally  been  re- 
gardwl  as  referable  to  the  Trias.  The  resemblance  of  the  lower  {>arts  of  the  series  to 
Permian  rooks,  coupled  with  the  occurrence  of  volcanic  bands  in  them,  has  more  recently 
been  held  to  justify  the  sejwration  of  these  lower  breccias  and  sandstones  from  the  rest 
as  representatives  of  the  Permian  series  of  the  Midlands.^ 

GhBrmany,'  &c. — The  "  Dyas  "  type  of  the  system  attains  a  great  development  along 

^  In  borings  at  Middlesboro'  beds  of  salt  and  gypsum  have  been  found  at  a  depth  of 
more  than  1300  feet  from  the  surface,  and  below  a  mass  of  limestone  67  feet  thick,  which 
is  believe*!  to  be  the  Magnesian  Limestone. 

-  Hull,  Quart.  Jaurn.  Otoh  Soc.  xlviii.  (1892)  ji.  60  ;  A.  Irving,  op.  cU.  xliv.  (1888) 
and  xlviii.  p.  68. 

^  H.  B.  Geiuitz,  *  Die  animalischen  Ueberreste  der  Dyas,'  1861-62,  Suppl.  1880-82  ;  *Znr 
Dyas   in   Hessen,'  Fesisch,    Ver.  /.   Xaturk,   CasseU    1886;   Geinitz   and  Gutbier,    *  Die 


SECT.  V  §  2  PERMIAN  SYSTEM  849 


the  flank  of  the  Harz  Mouutains,  also  in  the  Rhine  province,*  Thuringia,  Saxony, 
Bavaria,  and  Bohemia.  On  the  south  aide  of  the  Harz  it  is  grouped  into  the  following 
subdivisions : — 

C  r  Anhydrite,  gypsum,  rock-salt,  marl,  dolomite,  fetid  shale,  and  lime- 
2;  -j  stone.  The  amorphous  gypsum  is  the  chief  member  of  this  group ; 
O     \     the  limestone  is  sometimes  full  of  bitumeu. 


£ 

— *     1 

V 


<§ 


!2  ^  J  Crystalline  granular  {Rauchicacke)  and  fine  powderj*  {Aache)  dolo- 
j§  'B  \      mite  (sometimes  150  feet  thick,  with  gypsum  at  the  bottom). 

(  Zechstein-liniestoue,  an  argillaceous  thin-bedded  compact  limestone 
^  15  to  30  (sometimes  even  90)  feet  thick. 

I    -  Kupferschiefer — a  black  bituminous  shale  not  more  than  about  2  feet 
^  thick. 

^Zech^tein-conglomerate,  and  calcareous  sandstone. 

c^C  \  Red  sandstones  {Kreuznach)^  red  shales  {Monzig\  with  sheets  of 
1=^  a,  I      mclaphyre  and  masses  of  quartz-porphyry  conglomerate  (Sotern). 
(  Sandstones  and  conglomerates  lying  on  black  shales  with  poor  coal- 
I     I      seams  {LeUich), 

o     I  Sandstones  and  shales,  with  some  seams  of  coal  resting  on  red  and 
L     grey  sandstones,  with  bands  of  impure  limestone  (Cusel). 


The  name  **  Rothliegende,"  or  rather  *'Rothtodtliegende"  (red-layer  or  red-dead- 
layer),  was  given  by  the  miners  because  their  ores  disappeared  in  the  red  rocks  below 
the  copjier-bearing  Kupferschiefer.  The  coarse  conglomerates  have  been  referred  by 
Ramsay  to  a  glacial  origin,  like  those  of  the  Abberley  Hills.  They  attain  the  enormous 
thickness  of  6000  feet  or  more  in  Bavaria.  One  of  the  most  interesting  features  of  the 
formation  is  the  evidence  of  the  contemporaneous  out]:>ouring  of  great  sheets  of  quartz- 
jMjrphyry,  granite-jwrphyry,  porphyrite,  and  melaphyre,  with  abundant  interstratifi- 
<;ations  of  various  tuffs,  not  unfrequently  enclosing  organic  remains.  From  the  very 
nature  of  its  comi)onent  materials,  the  Rothliegende  is  comparatively  baiTcn  of  fossils  ; 
a  few  ferns,  calamites,  and  remains  of  coniferous  trees  are  found  in  it,  j»articularly  in 
tlie  lower  jwirt  of  the  group,  w*here  they  form  thin  seams  of  coal. 

The  plants,  all  of  terrestrial  growth,  on  the  whole  resemble  geneiically  the  Carboni- 
ferous flora,  but  seem  to  be  nearly  all  8i)ecifically  distinct.  They  include  forms  of 
Calamites  {C.  gigas),  Asterophyllites^  and  ferns  of  the  genera  Callipieris  (C.  conferia), 
Sphrnopteris,  AUthopteris,  NeuroptcriSf  Odontopteris^  with  well-j»reserved  silicified  stems 
of  tree-ferns  {PsaronitiSj  Tnbicaulis).  The  conifer  Wakhia  (  JV.  pinifonnis)  is  si)ecially 
<:liaracteristic.  Fish  remains  occur  sparingly  {Amhlypttrus,  Paiseonisciis^  Acanthode8\ 
while  labyrinthodouts  have  been  met  with  in  the  Dresden  district  in  considerable 
number  and  variety. 

The  Zechstein  group  is  characterised  by  a  suite  of  fossils  like  those  of  the  Magnesian 
Limestone  group  of  England.  The  Kupferschiefer  contains  numerous  fish  {Palseonixtut 
Frckskbeni^  Platysortius  gibbasiis,  &c.)  and  remains  of  plants  (coniferous  leaves  and  fruits 
and  sea-weeds,  Ullnuinniay  &c.).  This  deiK)sit  is  believed  to  have  been  laid  down  in 
some  enclosed  sea-basin,  the  waters  of  which,  probably  from  the  rise  of  mineral  springs 
connected  with  some  of  the  volcanic  foci  of  the  time,  were  so  charged  with  metallic 
salts  in  solution  as  to  become  unfit  for  the  continued  existence  of  animal  life.  The  dead 
fish,  plants,  &o.,  by  their  decay,  gave  rise  to  reduction  and  precipitation  of  these  salts 

Versteinerungen  des  Zechsteinsgebirge,'  &c.  1848-49;  C.  K  Weiss,  'Fossile  Flora  der 
jiiugst.  Steinkohlenf.  und  des  Rothliegend.'  &c.  1869-72.  Much  recent  information  will  be 
found  in  the  publications  of  the  Geological  Surveys  of  Prussia,  Saxony,  and  Alsace- 
Lorraine.  See,  for  example,  £.  W.  Benecke  and  L.  van  Wervecke,  Mitth.  Oeol.  Landesanst, 
Elaass-Lothr.  iii.  part  i.  (1890). 

^  For  an  account  of  the  Pennian  development  in  this  region,  see  especially  H.  von 
Dechen,  'Geolog.  und  Palajont.  Ubersicht  der  Rbeinproviuz  und  der  Provinz  Westfalen,* 
Bonn  (1884),  p.  291. 

3  I 


860  STBATIGRAPHIGAL  GEOLOGY  book  vi  part  n 

as  sulphides,  which  thereupon  enclosed  and  replaced  the  organic  forms,  and  permeated 
the  mud  at  the  bottom.  This  old  sea-floor  is  now  the  widely-extended  band  of  copper- 
slate  which  has  so  long  and  so  extensively  been  worked  along  the  flanks  of  the  Harz. 
After  the  formation  of  the  Kupferschiefer  the  area  must  have  been  once  more  covered 
with  clearer  water,  for  the  Zechstein  Limestone  contains  a  number  of  organisms,  among 
which  Productus  horriduSy  Spirifcr  undtilatvs,  Strophalosia  GoldfusH,  Terebratula 
clongaia,  Camarophoria  Schlotheiiniif  Schizodus  obsniruSy  and  FenesUUa  retiformis  are 
common.  Renewed  imfavoui-able  conditions  are  indicated  by  the  dolomite,  gypsmn, 
and  rock  -  salt  which  succeed.  Reasoning  upon  similar  phenomena  as  developed  in 
England,  Ramsay  has  connected  them  with  the  abundant  labyrinthodont  footprints  and 
other  evidences  of  shores  and  land,  as  well  as  the  small  number  and  dwarfed  forms  of 
the  shells  in  the  Magnesian  Limestone,  and  has  speculated  on  the  occurrence  of  a  long 
"continental  period"  in  Europe,  during  one  epoch  of  which  a  number  of  salt  inland 
seas  existed  wherein  the  Pennian  rocks  were  accumulated.  He  compares  these  deposits 
to  what  may  be  supposed  to  be  forming  now  in  jmrts  of  the  Caspian  Sea. 

Some  of  the  deposits  of  the  Zechstein  in  Germany  have  a  great  commercial  value. 
The  beds  of  rock-salt  are  among  the  thickest  in  the  world.  At  Sjierenberg,  near  Berlin, 
one  has  been  pierced  to  a  depth  of  nearly  4000  feet,  yet  its  bottom  has  not  been  reached. 
Besides  rock-salt  and  gypsum  there  occur  with  those  deposits  thick  masses  of  salts  of 
potash  (Carnallite)  and  magnesia  (Kieserite)  and  other  salts. 

In  Bohemia  (i)p.  821,  837,  846)  and  Moravia,  where  the  Permian  system  is  exten- 
sively develoijcd,  it  has  been  divided  into  three  grouj)s.  (1)  A  lower  set  of  conglomerates, 
sandstones,  and  shales,  sometimes  bituminous.  These  strata  contain  difi^sed  copper 
ores,  and  abound  here  and  there  in  remains  of  land-plants  and  fishes.  (2)  A  middle 
group  of  felspathic  sandstones,  conglomerates,  and  micaceous  shales,  with  vast  numbers 
of  silicified  tree-stems  {AraucarUc^^  Fsaronius),  (3)  An  upper  group  of  red  cla3rs  and 
sandstones,  >\'ith  bituminous  shales.  Eruptive  rocks  (melaphyre,  porphyrite,  kc)  are 
associated  with  the  whole  formation.  The  Zechstein  is  here  absent.  In  place  of  the 
marine  shells,  criuoids,  and  corals  so  characteristic  of  that  formation,  the  Bohemian 
Permian  strata  liave  yielded  the  remarkable  series  of  amphibian  remains  already  alluded 
to,  together  witli  abundant  traces  of  the  land  of  the  i)eriod,  such  as  remains  of  ortho- 
pterous  insects,  scorpions,  millipedes,  and  a  rich  terrestrial  flora  {Sphenopteris^Neuropteris^ 
Odantoplcris,  Pccoptcrisj  AlcthopteriSy  Calliptcri^  coufcrta,  SchizqpteriSf  CalatniteSt 
AsterophylUtafy  Sphenophyllum,  Lcpidodendron^  Sigillaria,  JVakhia^  Araucaryoxylon), 

Vosges. — In  this  region  the  following  succession  of  strata  has  been  assigned  to  the 
Permian  system  : — 

4.  Kohlbiichel  group  of  red  arkoses,  felsi)athic  sandstones,  shales,  conglomerates, 

breccias,  and  dolomite,  500  to  600  feet,  with  intercalated  sheets  of  mela- 

pbyres  and  tuffs. 
3.   Variegated  tufls  and  marls  of  Meisenbuckel. 
2.   Dark  shales,  limestones,  and  dolomites  of  Heisensteiu. 
1.   Arkose  and  shale  {Callipteris  conferia\  with  conglomerate  (sometimes  150  feet 

thick),  containing  blocks  of  porphyry,  gneiss,  quartz,  &c.,  filhng  up  hollows 

of  the  crystalline  schists  on  which  they  lie  unconfonnably. 

The  existence  of  volcanic  action  during  Pennian  time  in  this  region  is  shown  by  the 
presence  of  interstratified  basic  lavas,  and  by  the  great  quantity  of  fragments  of 
quartz  -  porphyry  in  the  conglomerates,  which  have  been  compared  to  volcanic 
agglomerates. ' 

*  Beiiecke  and  Van  Wervecke,  Miith.  Oeol.  Landesanst.  Elsass-LoUi.  vol.  iii.  (1890) 
p.  45  ;  Velain,  Bull.  Soc.  Gk^l.  France^  ser.  3,  xiii.  ;  Eck,  '  Qeogn.  Karte  d.  Umg.  von 
Lahr. '  ( 1884 )  :  '  Geogn.  Karte  v.  Schwartzwald. '  (1887).  A  full  bibliography  for  Alsace  and 
Ix)rraine  will  be  found  in  Ablh.  Oeol.  Sjjecialkfwt.  r.  Elsass-Lothringen^  vol.  i.  (1875)  and 
vol.  for  1887. 


SECT.  V  §  2  PERMIAN  SYSTEM  861 

France. — Permian  rocks  occur  in  many  detached  areas  in  France.  In  the  central 
plateau  they  are  found  most  fully  developed,  resting  u|>on  and  passing  down  into  the 
higher  parts  of  tlie  Carboniferous  system.  They  have  been  carefully  studied  in  the 
district  of  Autun,  where  the  lower  part  of  the  Permian  system  is  represented  by  a 
mass  900  to  1000  metres  thick  of  alternations  of  sandstone  and  shale  more  or  less 
rich  in  hydrocarbons,  with  thin  bands  of  magnesian  limestone.  No  marine  fossils  occur 
in  these  strata,  even  the  magnesian  limestone  containing  only  fresh-water  organisms. 
From  the  distribution  of  the  fossils  a  threefold  stratigraphical  subdivision  of  the 
whole  series  has  \yeen  made.  1st,  A  lower  group  at  least  150  to  200  metres  thick, 
lying  conformably  upon  the  Coal-measures,  and  containing  numerous  ferns  {Peeopteris, 
abundant),  SigiUarise^  SyHngodendrUy  Cordaites,  a  profusion  of  JFalchia,  large  num- 
bers of  seeds  or  fruits,  cyprids  crowded  in  some  layers  of  shale,  an  amphipod  {Necto- 
teison)y  a  number  of  fishes  {Pal«<miscu8f  Amhlypteras^  Acanthodes^  PleurcuMtith'us)^  and 
the  amphibians  and  reptiles  already  referred  to  {Actinodon^  EuchirosauruSf  Stereorhaehis). 
2nd,  A  middle  group  about  300  metres  thick,  showing  a  cessation  of  the  character- 
istically Carboniferous  species  of  plants,  and  an  increasing  prominence  of  typically  Per- 
mian forms.  Numerous  species  of  Pecopteris  still  occur,  but  Calliptcris  makes  its  appear- 
ance {C.  confcrttty  C,  gigantm).  Walchia  ( JF.  piniformiSj  W.  hyjmoidcs),  CalamiteSy 
Sphenopfiyllum,  Calamodendron,  and  fruits  abound.  The  animal  remains  resemble  those 
of  the  lower  group,  but  with  the  addition  of  Profritan  and  Pleuroneura.  3rd  An  upper 
group  locally  known  as  that  of  the  *'Bogliead,"  from  a  workable  band  of  bituminous 
shale  or  coal.*  The  thickness  of  this  group  is  about  500  metres,  the  upper  portion 
consisting  of  red  sandstones  \vithout  fossils.  Tlie  flora  is  now  markedly  Permian. 
Pecopterid  ferns  are  rare,  and  are  specifically  distinct  from  those  in  the  group  below. 
There  is  an  abundance  and  variety  of  Callipteris^  together  with  Sigillariay  abundant 
Walchia  and  AsterophyllUeSy  Piceites,  Sphcnophyllumy  Carpolithcsy  Ac.  The  fauna  is 
generally  similar  to  that  in  the  middle  group,  but  less  varied.^ 

In  the  extreme  south  of  France,  between  Toulon  and  Cannes,  Permian  rocks  re- 
appear, and  though  occupying  but  a  limited  area,  constitute  some  of  the  most  pictur- 
esque features  along  the  Mediterranean  shores  of  the  country.  They  consist  of  lower 
massive  conglomerates,  with  intercalations  of  shale,  containing  Walchia  and  CcUlipteris, 
followed  by  shales,  marls,  red  sandstones,  and  conglomerates.  But  their  distinguishing 
feature  is  the  enormous  mass  of  volcanic  materials  associated  with  them.  The  lower 
conglomerates,  besides  their  fragments  of  gneiss  derived  from  the  pre-Cambrian  rocks 
of  the  district,  contain  abundant  pieces  of  quartz-porphyry,  of  which  rock  also  there 
are  massive  sheets,  which  rise  up  into  the  well-known  group  of  hills  forming  the 
Estercl  between  Cannes  and  Frejus.  Besides  these  acid  outbursts  in  the  older  part  of 
the  formation,  sheets  of  melaphyre  are  found  in  the  upjjer.part,  while  dykes  of  nodular 
felsite,  pitchstone,  and  melaphyre  traverse  the  series.^ 

*  '*  Boghead,"  so  named  from  a  place  in  Linlithgowshire,  Scotland,  where  the  substance 
was  first  worked  for  making  gas  and  oil.  The  so-called  *'  Boghead  "  of  Autun  has  been 
ascertained  to  contain  a  large  quantity  of  the  remains  of  gelatinous  fresh -water  algae 
mingled  with  the  pollen  of  CordaiUs  ;  B.  Renault  and  E.  Bertrand,  Soc,  Hist,  Nat. 
Autun,  1892. 

3  E.  Roche,  Dull,  Soc.  GSol.  France,  ser  3,  ix.  (1880)  p.  78.  See  also  the  series  of 
*  fitudes  des  Gites  Min^raux,*  published  by  the  Ministry  of  Public  Works  in  JYance,  par- 
ticularly the  volumes  by  Delafond  on  the  Autun  basin,  and  by  Mouret  on  that  of  Brive ; 
likewise  the  Memoirs  by  Grand'  Eury  already  cited ;  Bergeron,  *  l^tude  G^ologique  du 
Massif  au  sud  du  Plateau  Central/  and  Bull.  Soc.  Q(ol,  France,  3  s^r.  vol.  xvi  Reinach, 
ZeUsch.  Deutsch.  Oeol,  Oes.  (1892)  p.  23,  gives  a  careful  comparison  of  the  French  central 
plateau  Permian  rocks  with  those  of  the  Saar  and  Nahe. 

'  F.  Walleraut,  *iinde  Strat  Petrog.  des  Maures  et  de  I'Esterel,'  1889.  Carte  Deiaill. 
Oiol.  France,  Feuille  d'Antibes. 


862  STRATIGRAPHICAL  GEOLOGY  book  vi  part n 


Westwards  in  the  region  of  the  Pyrenees,  and  in  various  parts  of  the  Iberian  peninsula, 
rocks  believed  to  be  Permian  have  been  recognised.  They  frequently  present  thick 
masses  of  conglomerate,  sometimes  resting  upon  Carboniferous  rocks,  sometimes  on  for- 
mations of  older  date. 

Alps.  ^ — On  both  sides  of  the  Alpine  chain  a  zone  of  conglomerates  and  sandstones, 
which  intervenes  between  the  Trias  and  older  rocks  of  the  region,  has  been  referred  to 
the  Permian  system.  The  conglomerates  (Vernicano  conglomerate)  are  made  up  of  the 
dstritus  of  schistose  rocks,  jwrphyries,  quartz,  and  other  materials  of  the  <»ntral  core 
of  the  mountains.  They  sometimes  contain  sheets  of  porphyry,  and  occasionally,  as  at 
Botzen,  they  are  replaced  by  vast  masses  of  quartz-jwrphyry  and  other  volcanic  rocks, 
>nth  tuffs  and  volcanic  conglomerates,  indicating  vigorous  volcanic  action.  An  inter- 
calated zone  of  shales  in  the  lower  conglomeratic  and  volcanic  part  of  the  series  in  the 
Val  Trompia  has  yielded  Walchia  pini/ormist  W.  filicifomiis,  SchizopUria  fascieuUUa, 
Spheihoptcris  irUladylUcs^  &c.,  and  serves  to  mark  the  Permian  age  of  the  rocks.  East- 
wards, at  Fiinfkirchen,  in  Hungary,  in  a  corresponding  position  below  the  Verrucano 
conglomerate,  a  group  of  younger  Permian  plants  has  been  found,  including  species  of 
Baicra,  Ulltnannia^  Voltzia,  Schizolepis,  and  CarpolUhcs,  nearly  half  of  which  occur  also 
in  the  German  Kupferschiefer.  Above  the  conglomerate  or  the  porphyry  comes  a 
massive  red  sandstone  called  the  "Groden  Sandstone,"  containing  carbonised  plant- 
remains.  But  the  most  distinctive  and  interesting  feature  in  the  Alpine  development 
of  the  Permian  system  is  found  in  the  upper  jwrtion  of  the  series  in  the  southern 
region  of  Tyrol  and  Carinthia.  The  red  Groden  sandstone  is  there  succeeded  by  beds  of 
gypsum,  rauchwackc,  and  dolomite,  above  which  comes  a  bituminous  limestone  known, 
from  tlie  abundance  of  species  of  Bellero})hony  as  the  "  Bellerophon  Limestone."  This 
calcareous  member  is  highly  fossiliferous.  It  contains  an  abundant  marine  fauna, 
which  includes  ten  species  of  BeUcrophotiy  and  species  of  Nautilns^  Natiea,  Pcden, 
Aviculopectetiy  AH^ulaf  Bakevelliaj  Schizodvs,  Spirifcr  (7  species),  Spirigera^  Sirepto- 
rhynchxis  Orthi^,  Strophmncna,  Leptsena,  ProdnctiiSj  and  Fusnlina.  Nearly  all  these  are 
peculiar  species,  but  tlie  Schizodus,  Bakcvellia^  and  Kiitkn  connect  the  assemblage  with 
tliat  of  the  Zechstein. 

It  is  interesting  to  trace  in  this  Bellerophon  Limestone  an  indication  of  the 
distribution  of  the  more  oi)eu  sea  of  Permian  time  in  the  European  area.  While  the 
Zechstein  was  in  course  of  deposition  in  isolated  Caspian-like  basins  across  the  centre 
of  the  Continent,  calcareous  sediments  were  accumulated  on  the  floor  of  a  wider  sea 
which,  lying  to  tlie  south,  stretched  over  the  site  of  the  present  Mediterranean,  east- 
wards across  Russia  and  the  heart  of  Asia.  A  portion  of  this  sea-floor  has  been 
detected  in  Sicily,  wliere  near  Palermo  M.  Gemniellaro  has  described  the  abundant 
fauna  found  in  its  limestones.  Foraminifera  {Fusiilina)  abound  in  these  rocks,  but 
their  most  remarkable  feature  is  the  number  and  variety  of  their  cephalopods,  which, 
besides  Palieozoic  types  {Goniatitcs,  OrthoceratUeft),  comprise  many  new  forms  (17 
genera  and  54  species)  akin  to  the  tribe  of  Mesozoic  Ammonites  {Adrianitcs^ 
AgaihiceraSy  Cifdohtbus,  DarcvclUeSy  Gastrioceras^  Medlicottia,  ParapronoriUSf  Popano- 
ccra^y  StacJicocerasy  JFaageiioceras),  also  gasteropods  {Bellerophon j  Plenrotomaria^  &c.) 
and  braehiopods.^ 

Russia.^ — The  Permian  system  attains  an  enormous  development  in  Eastern  Eurojie. 

1  £.  Suess,  Sitzb.  Akiid.  IFien,  Ivii.  (1868)  pp.  230,  763  ;  G.  Stache,  Zeitsch,  Deulsch, 
OeoL  frt^a.  xxxvi.  (1884)  p.  367  ;  Jahrh.  k.  k.  Ocol.  Reichmnst.  xxvii.  (1877)  p.  271, 
xxviii.  (1878)  p.  93  (giving  the  fauna  of  the  Bellerophon  Limestone);  Verhand.  Jt.  k. 
Oeol.  Rcichsanst.  (1888)  p.  320;  E.  Mojsisovics,  'Die  Doloniit-Riffe  von  Siidtirol  und 
Venetieii '  (1879),  chap.  iii. ;  Fraas,  'Scenerie  der  Alpen.' 

-  Prof.  Gemniellaro,  '  La  Fauna  dei  Calcari  con  Fusulina,'  &c.  Palermo,  1887-89. 

•'  See  for  the  earliest  descriptions  *  Russia  and  Ural  Mountains,'  Murchison,  De  Vemeuil, 
an<l  Keyserling,  4to,  2  vols.  1845. 


SECT.  V  §  2  PERMIAN  SYSTEM  853 


Its  nearly  horizontal  strata  cover  by  far  the  largest  part  of  European  Russia.  They  lie 
conformably  on  the  Carboniferous  system,  and  consist  of  sandstones,  marls,  shales, 
conglomerates,  limestones  (often  highly  dolomitic),  gypsum,  rock-salt,  and  thin  seams 
of  coal.  In  the  lower  and  more  sandy  half  of  this  scries  of  strata  remains  of  land - 
plants  {Calamitcs  gigas,  CyclopUris^  Pecopteris^  &c.),  fishes  {PalsoniseHs)^  and  labyrin- 
thodonts  occur,  but  some  interstratified  bands  yield  Productus  Cancrini  and  other 
marine  shells.  The  rocks  are  over  wide  regions  impregnated  with  copper-ores.  The 
upper  half  of  the  series  consists  of  clays,  marls,  limestones,  gypsum,  and  rock-salt, 
with  numerous  marine  mollusca  like  those  of  the  Zcchstein  {Productus  Cancrini^ 
P.  honidifs,  Camarophoria  Schfotheimii),  but  with  a  rather  more  abundant  fauna, 
and  with  intercalated  bands  containing  land-plants. 

Milch  attention  has  been  given  in  recent  years  to  these  rocks,  which  have  now  been 
brought  into  closer  comparison  with  those  of  other  regions.  As  developed  on  the 
western  slope  of  the  Ural  Mountains,  they  have  been  found  to  consist  of  the  following 
groups  of  strata  : — 

Red  clays  and  marls,  with  intercalated  sandntones  and  limestones,  almost 
wholly  uufossiliferous,  but  with  a  few  lamellibranchs  resembling  Unio  {Anthra- 
cosia)  castor  and  C.  umbonatus.  This  thick  group  may  possibly  be  partly  or 
wholly  Triassic. 

Copper- bearing  sandstone,  permeated  with  oxide  and  sulphide  of  copper,  and 
containing  species  of  Calamites  {yigas),  J^phenoptert's  {ioUtta,  erosa),  CalUptccis 
{obliqiui^  co7i/erta)y  NOggtrathiaf  Dadooyylon^  Knorrin,  &c. 

Marls,  sandstones,  and  conglomerates  with  ill-preserved  plants  (which  seem  to  be 
on  the  whole  like  those  of  the  Artinsk  group  below),  rnio  castor^  U.  umbonatus, 
C  (roUi/u^ianaj  ArchegosauruSj  Acrolcpis,  while  some  of  the  sandy  marls  contain 
a  characteristically  marine  fauna,  Pnxitictus  Cafwrini,  P.  koninckianus,  Athyri.s 
jtcctini/era^  and  Spiri/er  lineatvs. 

GyiKseous  limestones  and  dolomites. 

Artinsk  group  of  sandstones,'  conglonierates,  shales,  marls,  limestones,  and 
dolomites,  stretching  from  the  Arctic  Ocean  to  the  Kirj^iz  Steppes,  and  lying 
conformably  on  the  Carboniferous  Fusulina  Limestone.  This  group  contains  a 
remarkably  abundant  and  varied  assemblage  of  fossils.  The  plants  include  species 
of  i.'alnmiteSy  NOggerathiaj  i^phenopteris^  Chhntoptcris,  &c.  The  fauna  comprises 
a  number  of  common  Carboniferous  shells  such  as  Productus  semircticvlatus^ 
[\  coraj  P.  iongispinuSf  P.  sc^bricuiu^y  Strcptorhynchus  crenistria,  but  with  these 
are  found  many  new  types  of  cephalopods  like  the  ammonoid  forms  above  alluded 
to  as  occurring  in  the  Bellerophou  Limestone  of  the  Tyrol  {Ayathiceras^  Cw'astrw- 
ceraSy  MedUcottiay  PopanoceraSj  Pronorites).  About  300  sjiecies  of  fossils  have 
been  found  in  the  group,  of  which  a  half  also  occur  in  the  Carlx)niferous  system, 
and  only  about  a  sixth  in  the  Permian  above.  ^ 

Asia. — The  type  of  sedimentation  found  in  the  east  and  south  of  Europe  extends  into 
Asia.  In  the  valley  of  the  Araxes  a  limestone  occurs  containing  Productus  horridus, 
Athyris  subtilita,  and  a  number  of  the  ammonoid  forms  above  referred  to  ;  while  in 
Bokhara  other  limestones  occur  at  Darwas  which  from  their  cephaloi>ods  {Pronorites^ 
PopanoceraSj  &c.)  probably  represent  the  Artinsk  group  of  Russia.  The  same  character 
of  deposits  and  of  paleontology  is  still  more  extensively  developed  in  the  Salt  Range  of 
the  Punjab.  In  this  region  the  ancient  Palaeozoic  sediments  with  their  saliferous  deposits 
are  overlain  by  a  remarkable  limestone  which  has  yielded  a  large  assemblage  of  fossils. 
At  the  base  of  this  deposit  comes  a  coarse  conglomerate  and  sandstones  followed  by  the 
well-known  Productus  Limestone.^  The  lower  portions  of  the  limestone  abound  in 
Fusulina  with  Carboniferous  brachiopods  {Productus  cora^  P.  semircticulatusy  P.  linecUuSy 

*  A.  Krasnopolsky,  M^n,  Com.  GioL  Russ.  xi.  (1889)  No.  1  ;  A.  Karpinsky,  Verhand. 
k.  Mirt.  iiesell.  St.  Petersboitrg,  ix.  (1874)  p.  267  ;  M^m.  Acad.  St,  PHersUniry,  1889  ; 
T.  Tschernyschew,  Mrvu  Com.  OSoi.  Russ.  iii.  (1889)  No.  4. 

f  I  *  W.   Waagen,  Mem.  (ieol.   Surv.   India^  'Salt  Range  Fossils,*  vol.  i.  Productus  Lime- 
stone, 1879-88. 


864  STRATIGRAPHICAL  GEOLOGY  book  vi  part  n 


Athyrk  Royssii,  Spiriftr  strUUus),  The  cephalopods  are  numerous  and  include  the 
ammonoid  types  {CydolobuSj  ArcesUs,  MedlicoUia^  Popanoceras,  Xenodiaeus),  as  well  as 
many  Nautili,  Orthoceratites,  and  Gyroceratites.  The  gasteropoda  include  forms  of  Bel- 
Icrophon^  EuomphaluSy  Holopcllat  Phasiandlat  and  Pleurotoviaria.  Lamellibranclis  are 
abundantly  represented  by  such  genera  as  AllorisTna^  Schizodus,  Avicula,  AvictUcpedem, 
and  PecUn,  but  also  with  others  of  a  distinctly  Mesozoic  character,  as  Lima^  Luchut^ 
Loripes,  Cardinia,  Astartef  and  Myophoria.  Yet  with  these  evidences  of  a  newfer  fades 
of  molluscan  life  it  is  interesting  to  notice  the  extraordinary  variety  and  abundance  of 
the  brachiopods,  including  ancient  genera  such  as  Prodmctus  (20  species),  ChoMUSf 
AthyriSf  Orthis,  Lepteena,  and  SirepUyrhynchus^  mingled  with  a  number  of  new  genera 
first  met  with  here  {Heiniptychina,  NotothyriSf  LyttoniUy  Oidhatnia,  &c.)  Though  the 
general  aspect  of  this  fauna  is  so  unlike  that  of  the  Permian  rocks  of  central  Europe, 
the  appearance  of  a  number  of  Zechstein  species  links  the  limestones  of  northern  IndsA 
with  the  European  tract.  Among  these  are  Caniarophoria  humbldonensiSf  StrophdUaia 
cxcavafa^  S.  horresccnSf  Spiri/eriiui  cristata. 

This  oceanic  type  of  deposit,  however,  does  not  seem  to  extend  southwards  across 
the  Indian  peninsula.  South  of  the  line  of  the  Narbada  River  a  totally  different  series 
of  sedimentary  formations  occurs.  In  that  southern  region  the  lower  and  midiUe  Mesosoic 
marine  rocks  of  other  countries,  and  probably  also  the  upper  part  of  the  Palaeozoic  series, 
are  represented  by  a  vast  thickness  of  strata,  chiefly  sandstones  and  shales,  which  are 
probably  almost  entirely  of  fluviatile  origin.  To  this  great  fresh-water  accumulation 
the  name  of  Gondwana  system  has  been  given  by  the  Geological  Survey  of  India.  The 
lower  parts  of  the  system  (Talchir  and  Damuda  series)  may  perhaps  be  paralleled  with 
the  Permian  rocks  of  Europe.  The  exceedingly  coarse  Talchir  conglomerates  contain 
blocks  which  sometimes  show  smoothed  and  striated  faces,  and  have  been  compared  with 
those  of  the  boulder-clay  as  evidences  of  ancient  glacial  action  iu  India.  Among  the 
overlying  sandstones  and  carbonaceous  layers  ferns  i^OanganwptcriSy  Glossopteri^ 
NeuropUris)  and  Voltzia.  are  found.  The  Damuda  series,  estimated  to  be  10,000  feet 
thick,  QowifLms  Glossopteris,  Gnngamoptcris,  Schizojieuray  Kertebrariai  and  Arehegosaurus. 
The  Pancliet  series  which  succeeds  is  more  probably  Triossic,  while  the  upper  sub- 
divisions apjiear  to  be  of  Jurassic  age.* 

Australia. — The  "Upper  Coal-measures"  (Newcastle  series)  of  New  South  Wales 
have  been  classed  as  Pennian.  They  consist  of  shales,  sandstones,  and  conglomerates, 
with  abundant  plant -remains  {Ghssoptcris,  Gangamopteris,  VertebrarMj  Phyllotheca, 
SpJwnoptcris),  but  with  no  marine  shells.  This  group  of  coal-bearing  strata  comprises 
nearly  all  the  scams  of  coal  in  the  Newcastle  coal-field,  the  lowest  of  which  is  from 
eight  to  fifteen  feet  thick.     Another  seam,  near  Jamberoo,  is  twenty-five  feet  thick.* 

In  Victoria  certain  sandstones  and  conglomerates  (Bacchus  Marsh,  Grampian)  have 
been  compared  with  those  of  the  Talchir  series  of  India  as  possibly  indicating  glacial 
action.  They  contain  GanganwpUris  and  Glossopteris.^  In  Queensland  a  much  fuller 
development  of  Upper  Palaeozoic  rocks  has  been  ascertained.  A  great  thickness  of 
stratified  dejiosits  comprising  four  or  five  distinct  formations  has  been  named  Permo- 
Carboniferous.  In  its  higher  portions  (Bowen  series)  it  consists  of  an  upper  fresh-water 
series  with  plants  {Sph4:)ioptcris^  GlossopteH^\  and  a  lower  marine  series  containing  a 
fauna  which  includes  the  genera  Fcnestella,  Di^lasvui^  Spirifcr  (stricUiiSf  trigatialis,  &c), 
Derby ia,  ProduHu^  {cora^  &c.),  Strophalosia^  Chonctcs^  Aviculopectcn,  Platyschisma,  Monr- 
lonid,  Bdlcrophoiiy  PorccUia,  (yrthoccras,  GoniaUiies,^ 

*  Medlicott  and  Blauford,  *  (Jeology  of  India.  * 

-  C.  S.  Wilkinson,   'Notes  on  Geology  of  New  South  Wales,'  Sydney,  1882,  p.  51. 
0.  Feistmantel,  Mem.  Geol.  Sure,  N.S.   Wales^  Palwontdogyy  No.  3  (1890),  p.  38. 
^  R.  A.  F.  Murray,  'Geology  and  Phys.  Geog.  of  Victoria,'  1887,  p.  84. 

*  K.  L.  Jack  and  R.  Etberidge  jun.  '  Geology  and  Palaeontology  of  Queensland  and  New 
Guinea,'  1892,  chaps,  vi.-xxii. 


SECT.  V  §  2  PERMIAN  SYSTEM  855 


Africa. — In  the  south  of  this  continent  a  group  of  rocks  occurs  which  present^  some 
of  the  lithological  and  palseontological  ty[)es  of  southern  India  and  south-eastern 
Australia.  At  their  base  is  a  remarkable  conglomerate  (Dwyka)  which  lies  unconform- 
ably  on  the  Carboniferous  quartzite  and  has  been  com])ared  with  the  conglomerate  of  the 
Talchir  series,  but  it  presents  many  of  the  characters  of  a  volcanic  conglomerate.^  It  is 
Murmoimted  by  a  series  of  clays  or  mudstones  and  sandstones,  at  least  4000  feet  thick, 
containing  plant-remains,  among  which  Glossopteris  is  said  to  have  been  recognised. 
This  series  is  unconformably  surmounted  by  the  **  Kimberley  shales,"  which  i>ass  up 
into  the  *'  Karoo  beds."    The  latter  are  generally  regarded  as  Triassic. 

North  America. — The  Permian  system  is  hardly  represented  at  all  in  this  part  of 
the  globe.  In  Kansas  certain  red  and  green  clays,  sandstones,  limestones,  conglomerates, 
and  beds  of  gypsum  lie  conformably  on  the  Carboniferous  system,  and  contain  a  few 
genera  and  species  of  moUusks  {Bak'cvelliaj  Mtjalinay  &c.)  which  occur  in  the  £uro])ean 
Permian  rocks.  It  ha.s  been  urged,  however,  that  the  upper  part  of  the  Api)alachian 
coal-field  should  be  regarded  as  belonging  to  the  Permian  system.  These  strata,  termed 
the  **Upi>er  Barren  Measures,"  are  upwards  of  1000  feet  thick.  At  their  base  lies  a 
massive  conglomeratic  sandstone,  above  which  come  sandstones,  shales,  and  limestones, 
with  thin  coals,  the  whole  becoming  very  red  towards  the  toj*.  Professors  W.  M. 
Fontaine  and  I.  C.  White  have  shown  that,  out  of  107  plants  examined  by  them  from 
these  strata,  22  are  common  to  the  true  Pennsylvaiiian  Coal-measures  and  28  to  the 
Permian  rocks  of  Eurojje  ;  that  even  where  the  si)ecics  are  distinct  they  are  closely  allied 
to  known  Permian  forms  ;  that  the  ordinary  Coal-measure  flora  is  but  jjoorly  represented 
in  the  **  Barren  Measures,"  while  on  the  other  hand  vegetable  Xyytos  api)ear  of  a 
distinctly  later  time,  forms  of  PecopteriSy  CalUptrridium^  and  Saportten  foreshadowing 
characteristic  plants  of  the  Jurassic  period.  These  authors  likewise  point  to  the 
indications  furnLshed  by  the  strata  themselves  of  important  '^lianges  in  the  ]>hysical 
condition  of  the  American  area,  and  to  the  remarkable  jmucity  of  animal  life  in  these 
beds,  as  in  the  red  Permian  rocks  of  Euroi>e.  The  evidence  at  present  before  us  seems 
certainly  in  favour  of  regarding  the  upper  i)art  of  the  Ap])alachian  coal-fields  as  re- 
jnesenting  the  reptiliferous  beds  overlying  the  Coal-measuros  at  Autim  and  their 
c<iuivalents.*''  In  Nova  Scotia  also  a  similar  ujmard  ])assage  has  been  observed  from 
true  Coal-measures  into  a  group  of  reddish  strata  containing  Permian  types  of 
vegetation. 

Passing  to  tlie  we^iteni  regions  of  the  continent,  we  find  that  the  vegetation  which 
succeeded  that  of  the  Carl)oniferous  jwriod  spreatl  far  to  the  west,  and  that  it  has  been 
entombed  among  marine  sediments.  The  Permian  deposits  traced  in  that  direction 
undergo  a  change  somewhat  similar  to  that  shown  by  the  Carboniferous  system,  though 
on  a  much  feebler  scale.  In  the  so-called  "Wichita  beds"  of  Texas,  consisting  of  red 
and  mottled  clays,  sandstones,  and  concretionary'  limestones  resting  on  Coal-measures,  a 
series  of  i»lant  and  animal  remains  has  been  discovered,  which  throws  much  light  upon 
the  extension  of  the  Permian  flora  and  fauna  in  North  America.  The  plants  are 
essentially  the  same  as  those  found  al)ove  the  Coal -measures  of  Western  Virginia.  They 
include  SphcnophijUum,  Aniniiaria,  JVakhiay  OdcmtojitrriSy  Callipteris  con/crta,  Callip- 
tcridium  (4  si>ecies),  PecoptcrU  (8  si)ecies),  and  OoniopUrU.^  The  animal  -  remains 
comprise  some  Carboniferous  sj^ecies,  but  also  distinctively  Permian  types,  especially 
some  of  the  ammonoid  cephalopods,  which  are  now  known  to  have  so  wide  a  range  in 
the  Old  World.  The  cephalojxKls  already  ol)tained  include  sjKJcies  of  ihrUioceraa, 
Nautilus,  jyaagcnoccrcut,  Medlicottia,  Popanocera.%  the  gasterojKKis  are  represented  by 
^lecie^oi  EnomphaluSy  BcUcrophon^  and  Murchhoitia,  and  other  organisms  have  been 

^  A.  H.  Green,  Quart.  Journ.  Geol.  Soc.  xliv.  (1888)  p.  239. 

2  ••  On  the  Permian  or  Upper  Carboniferous  Flora  of  W.  Virginia  and  S.W.  Pennsyl- 
vania," Second  (itd.  Sttru.  Penn.  Rfporty  p.p.  1880. 

«  I.  C.  White,  Bull,  Amer.  iitU.  Soc.  iii.  (1892)  p.  217. 


856  STRATIGRAPHIGAL  GEOLOGY  book  vi 


detected.^  There  have  also  been  obtained  from  those  strata  and  the  **Clep8ydroiJ» 
shales  "  of  Illinois  a  number  of  fish,  stcgocephalous  amphibia,  and  rhynchocephalous 
reptiles.-* 

Spitsbergen. — The  Pennian  sea  appears  to  have  extended  far  within  the  Arctic  circle, 
for  al)ov(!  the  Carboniferous  rocks  of  Spitzbergen  there  occurs  a  group  of  strata  which 
contains  Permian  forms  {ProdiKtus^  Streptorhynchus,  Rrtzia^  Psfudomonotis,  BaketrHw, 
&c.).3 

Part  111.  Mesozoic  or  Secondary. 

Though  no  geologist  now  admits  the  abrupt  lines  of  division  which 
were  at  one  time  believed  to  mark  off  the  limits  of  geological  systems 
and  to  bear  witness  to  the  great  terrestrial  revolutions  by  which  these 
systems  were  supposed  to  have  been  terminated,  nevertheless  the  influence 
of  the  ideas  which  gave  life  to  these  banished  beliefs  is  by  no  means 
extinct.  The  threefold  division  of  the  stratified  rocks  of  the  terrestrial 
crust  int<i  Primary,  Secondary,  and  Tertiary,  or,  as  they  are  now  called, 
Palaeozoic,  Mesozoic,  and  Cainozoic,  is  a  relic  of  those  ideas.  This  three- 
fold arrangement  is  retained,  however,  not  because  each  of  these  great 
periods  of  geological  time  is  thought  to  have  l>een  separated  by  any  marked 
geological  or  geographical  episode  from  the  period  which  preceded  or 
that  which  followed  it,  l)ut  because,  classification  and  sulxlivision  being 
necessary  in  the  acquisition  of  knowledge,  this  grouping  of  the  earth's 
stratified  formations  into  three  great  series  is  convenient.  In  our  survey 
of  the  older  members  of  these  formations  we  have  come  to  the  end  of 
the  first  series  of  fossiliferous  systems,  and  are  about  to  enter  upon  the 
consideration  of  the  second.  But  we  find  no  indication  in  the  rocks  of 
any  general  break  in  the  continuity  of  the  processes  of  sedimentation 
and  of  life  which  we  have  seen  to  be  recorded  among  the  Palaeozoic 
rocks.  On  the  contrary,  so  insensibly  do  the  Palaeozoic  formations  in 
many  places  merge  into  the  Mesozoic,  that  not  only  can  no  sharp  line 
be  drawn  between  them,  but  it  has  even  l>een  proposed  to  embrace  the 
strata  at  the  top  of  the  one  series  and  the  base  of  the  other  as  parts  of  a 
single  continuous  system  of  deposits. 

Xevertheless,  when  we  look  at  the  Mesozoic  rocks  as  a  whole,  and 
contrast  them  \Wth  the  Palaeozoic  rocks  below  them,  certain  broad 
distinctions  readily  present  themselves.  Whereas  in  the  older  series 
mechanical  sediments  form  the  prevalent  constituents,  piled  up  in  masses 
of  greywackc,  sandstone,  conglomerate,  and  shale  often  many  thousands 
of  feet  in  thickness,  in  the  newer  series  limestones  play  a  much  more 
conspicuous  part.  Again,  while  in  the  Palaeozoic  formations  a  single  kind 
of  sediment  may  continue  monotonously  persistent  for  many  hundreds  or 
even  thousands  of  feet  of  vertical  dei)th,  in  the  Mesozoic  series,  though 
thick  accumulations  of  one  kind  of  material,  especially  limestone,  are 
locally  developed,  there  is  a  much  more  general  tendency  towards  frequent 
alternations  of  difTerent  kinds  of  sedimentary  material,  sandstones,  shales, 

^  C.  A.  White,  Amcr.  Nat,  (1889)  p.  109,  Bull.  C.S.  O'eol.  Sun.  No.  77  (1891). 
-  K.  1).  Cope,  Pror.  Amer.  Phil.  .S<>r.  xvii.  (1877-78)  pp.  182,  505. 
^  B.  LuiKlgri'ii,  BIIkuvj.  fi^inink:  Vcf.  Ahnl.  J/tuidl.  xiii.  (1887). 


PART  III  MESOZOIC  OR  SECONDARY  857 

and  limestones  succeeding  each  other  in  rapid  interchange.  Another 
contrast  between  the  two  series  is  supplied  by  the  very  different  extent 
to  which  they  have  suffered  from  terrestrial  disturbances.  Among  the 
Palaeozoic  rocks  it  is  the  rule  for  the  strata  to  have  been  thrown  into 
vai-ious  inclined  positions,  to  have  been  dislocated  by  faults,  and  in 
many  regions  to  have  been  crumpled,  pushed  over  each  other,  and 
even  metamorphosed.  The  exceptions  to  this  rule  are  so  few  that  they 
are  always  signalised  as  of  special  interest.  Among  the  Mesozoic  rocks, 
on  the  contrary,  the  original  stratification-planes  have  usually  been  little 
deranged,  faults  are  generally  few  and  trifling,  and  it  is  for  the  most  part 
only  along  the  flanks  or  axes  of  great  mountain-chains  that  extreme 
dislocation  and  disturbance  can  be  observed.  A  further  distinction  is  to 
be  found  in  the  relation  of  the  two  series  to  volcanic  activity.  We  have 
seen  in  the  foregoing  chapters  that  every  period  of  Palaeozoic  time  has 
been  marked  somewhere  in  the  Old  World  by  volcanic  eruptions,  that  in 
certain  regions,  such  as  that  of  the  British  Isles,  there  has  been  an  abundant 
outpouring  of  volcanic  material  again  and  again  in  successive  geological 
I>eriods  within  the  same  limited  area,  and  thus  that  masses  of  lava  and 
tuff  thousands  of  feet  in  thickness,  and  sometimes  covering  hundreds  of 
square  miles  in  extent,  have  been  thrown  out  at  the  surface.  But  in  the 
European  area,  with  some  trifling  excei)tions  at  the  beginning,  the  whole 
of  the  Mesozoic  ages  appear  to  have  been  unbroken  by  volcanic  erup- 
tions. The  felsites,  rhyolites,  porphyrites,  diabases,  basalts,  and  other 
lavas  and  eruptive  rocks  so  plentiful  among  the  Primary  formations 
are  generally  absent  from  the  Secondary  series. 

But  perhaps  the  most  striking,  and  certainly  the  most  interesting, 
contrast  between  the  rocks  of  the  older  and  the  newer  series  is  8ui)plied 
in  their  respective  organic  remains.  The  vegetiible  world  undergoes  a 
remarkable  transformation.  The  ancient  preponderance  of  cryptogamic 
forms  now  ceases.  The  antique  types  of  Sigillaria,  Stigmaria,  Lepido- 
dendron,  Calamites,  and  their  allies  (lisapj)ear  from  the  land,  and  their 
places  are  taken  by  cycads  and  conifers,  while  eventually  the  earliest 
monocotyledons  come  as  the  vanguard  of  the  rich  flora  of  existing  time. 
Nor  are  the  changes  less  marked  in  the  animal  world.  Such  ancient  and 
l^ersistent  types  as  the  graptolites  and  trilobites  had  now  wholly  vanishe<l. 
The  crinoids,  that  grew  so  luxuriantly  over  the  sea-floor  in  older  time, 
now  flourished  in  greatly  diminished  numl)ers,  while  the  urchins,  which 
had  previously  occupied  a  very  sul)ordinate  position,  took  their  place  as 
the  most  conspicuous  grouj)  of  the  Echinoderms.  The  brachiopods, 
which  from  the  remotest  time  had  filled  so  prominent  a  place  among 
the  mollusks,  now  rapidly  diminished  in  number  and  variety.  Among 
the  cephalopods  the  Palaeozoic  type  of  the  Orthoceratites  was  suc- 
ceeded by  the  Mesozoic  type  of  the  Ammonites.  But  perhaps  the 
most  distinctive  feature  of  the  fauna  was  the  variety  and  abundance 
of  reptilian  life.  The  labyrinthodont  amphibians  were  replaced  by 
many  new  orders,  such  as  the  Ichthyosiuirs,  Plesiosfiurs,  Ornithosaurs, 
Deinosaiu's,  and  Crocodiles.  It  was  in  Mesozoic  time  also  that  the 
first  mammals  made  their  appearance  in  marsupial  forms,  which  remainetl 


868  STRATIGRAPHICAL  GEOLOGY        book  vi  pabt  ni 

the  highest  types  that  were  reached  before  the  beginning  of  the  Caino- 
zoic  periods. 

The  Mcsozoic  formations  have  been  grouped  in  three  great  divisioiiSy 
which,  though  first  defined  in  Europe,  are  found  to  have  their  repre- 
sentative series  of  rocks  and  fossils  all  over  the  world.  The  oldest  of 
these  is  the  Trias  or  Triassic  system,  followed  by  the  Jurassic  and 
Cretaceous. 

Section  i.  Triassic. 

It  has  been  already  mentioned  that  the  great  mass  of  red  rocks, 
which  in  England  overlie  the  Carboniferous  system,  were  formerly 
classed  together  as  New  Red  Sandstone,  but  are  now  ranged  in  two 
systems.  Wo  have  considered  the  lower  of  these  under  the  name  of 
Pennian.  The  general  facies  of  organic  remains  in  that  division  is  still 
decidedly  Palaeozoic.  Its  brachiopods  and  its  plants  connect  it  with  the 
Carboniferous  rocks  below.  Hence  it  is  placed  at  the  close  of  the  long 
series  of  Palaeozoic  formations.  When,  ho^^^ver,  we  enter  the  upper 
division  of  the  red  rocks,  though  the  general  lithological  characters 
remain  in  most  of  Europe  very  much  as  in  the  lower  group,  the  fossils 
bring  before  us  the  advent  of  the  great  Mesozoic  flora  and  fauna.  This 
group  therefore  is  put  at  the  base  of  the  Mesozoic  or  Secondary  series, 
thougli  in  some  regions,  as  in  England,  no  very  satisfactory  line  of 
demarcation  can  alwavs  be  drawn  between  Permian  and  Triassic  rocks. 
The  term  Trias  was  suggested  by  F.  von  Alberti  in  1834,  from  the  fact 
that  in  Suabia,  and  throughout  most  of  Germany,  the  group  consists 
of  three  well-marked  subdivisions.^  But  the  old  name.  New  Red  Sand- 
stone, is  familiarly  retained  by  many  geologists  in  England.  The  word 
Trias,  like  Dyas,  is  unfortunately  chosen,  for  it  elevates  a  mere  local 
character  into  an  importance  which  it  does  not  deserve.  The  threefold 
subdivision,  though  so  distinct  in  Germany,  disappears  elsewhere. 

§  1.  General  Characters. 

As  the  term  Trias  arose  in  Germany,  so  the  development  of  the 
Triiis.sic  rocks  in  that  and  adjoining  parts  of  Europe  has  been  accepted 
as  the  normal  type  of  the  system.  There  can  be  little  doubt,  however, 
that  though  this  type  is  l)est  known,  and  has  been  traced  in  detached 
areas  over  the  centre  and  west  of  Europe,  from  Saxony  and  Franconia  to 
the  north  of  Ireland,  and  from  Basle  to  the  Germanic  plain,  reap|>earing 
even  among  the  eastern  Suites  of  North  America,  it  must  be  looked  upon 
as  a  local  phenomenon.  This  assertion  commends  itself  to  our  accept- 
ance, when  we  reflect  upon  the  nature  of  the  strata  of  the  central 
European  Triassic  basins.  These  rocks  consist  for  the  most  part  of 
bright  red  sandstones  and  clays  or  marls,  often  ripple-marked,  sun-cracked, 

^  '  Beitrag  zii  eiuer  Monojjraphie  des  Bunten  Sandsteins,  Muschelkalks,  und  Kenpers 
und  die  Verbiudung  dieser  Gebilde  zu  einer  Formation,'  Stuttgart,  1834,  p.  324.  Thirty 
years  later  the  same  observer  published  his  '  Ueberblick  iiber  die  Trias,'  1864,  and  gave  a 
s3nioi>9is  of  the  Triassic  literature  of  that  interval. 


SECT,  i  %  1 


TBIASSIC  SYSTEM 


rain-pitted,  and  marked  with  animal  footprinte.  They  contain  layers, 
nodules,  or  veininga  of  gypsum,  beds  (and  scattered  casts  of  crystals)  of 
rock-salt,  and  bancls  or  massive  beds  of  limestone,  often  dolomitic.  Such 
an  association  of  materials  points  to  isolated  basins  of  deposit,  or  salt- 
lakes  or  inland  seas,  to  which  the  outer  sea  found  occasional  access,  and 
in  which  the  water  underwent  concentration,  until  its  gypsum  and  salt 
were  thrown  down.  That  the  intervals  of  diminished  salinity,  during 
which  the  sea  renewed,  and  perhaps  maintained,  a  connection  with  the 
basins,  were  occasionally  of  some  duration,  is 
shown  by  the  thickness  and  fossiliferous  nature 
of  the  limestones. 

It  is  evident,  however,  that  in  this,  as  in  all 
other  geological  periods,  the  prevalent  type  of 
sedimentation  must  have  been  that  of  the  open 
sea.  The  thoroughly  marine  or  pelagic  equi- 
valents of  the  red  rocks  of  the  basins  have  now 
been  traced  over  a  far  wider  portion  of  the 
earth's  surface.  In  the  Alps  and  thence  east- 
ward  through  the  Carpathian  Mountains  and 
southern  Russia  into  the  heart  of  Asia  and 
northern  India,  as  well  as  southward  into  Italy 
and  Sjmin,  the  deposits  of  the  open  Triassic 
sea  are  well  <leveloped.  Masses  of  limestone 
and  dolomite,  attaining  sometimes  a  thickness 
of  several  thousands  of  feet,  are  there  replete 
with  a  characteristically  marine  fauna.  The 
same  fauna  has  been  detected  over  a  wide 
region  of  the  north  of  Asia  from  Spitzbergen 
to  Japan,  the  western  regions  of  North  and 
South  America,  in  New  Zealand,  and  in  Southern 

Life. — The  flora  of  the  Triassic  period  ' 
appears  to  have  been  closely  similar  to  that  of 
the  Permian.  It  consisted  mainly  of  ferns  (some  of  them  arborescent), 
equisetums,  conifers,  and  cycads.  Among  the  ferns,  a  few  Carboni- 
ferous genera  {Spkenopteris,  PecopterU,  CyclfipUris)  still  survive,  together 
ivith  Gli'ASitptms,  Txnwpleris,  Cauhpleris,  and  other  old  genera,  but 
new  forma  have  appeared  {Ammopleris,  Acroslichiles,  Clathrnpleris,  Lepi- 
iliipkris,  MmamipUru,  NenrnptfTidium  (CrenialopUris),  Sagenopteris).  The 
earliest  undoubted  horse-tail  reeds  occur  in  this  system.  Here 
they  are  represented  by  the  two  genera  EqniseluM  (Fig.  377)  and 
SfhizoMura.  The  latter  genus  died  out  in  the  Jurassic  period,  but 
the  former  is  still  represented  by  twenty-five  living  species.  The 
conifers  are  represented  by  VoUsia,  the  cypress-like  or  spruce-like  twigs 
of  which  are  specially  characteristic  organisms  of  the  Trias  (Fig.  378), 
and  by  Albertiti.  But  the  most  distinctive  feature  in  the  flora  of  the 
earlier  Mesozoic  ages  was  the  great  development  of  cycadaceous  vege- 
tation.    The  moat  abundant  genus  is  PUrophyllHm ;  others  are  Nilssoim, 


llrangn.  ((). 


660 


STRATIGRAPHIOAL  GEOLOGY 


BOOK  VI  PABT  HI 

Zamites,  Podozamitex,  PiilophyHum,  Otozamiles.  So  typical  are  these  plants 
that  the  Mesozoic  formations  have  been  classed  ae  belonging  to  the  "Age 
of  Cycads."  Calcareous  aigte  {Gynrporella,  &c)  abounded  in  the  open 
seas  of  the  time  and  contributed  to  the  growth  of  limestone  reefs. 

The  fauna  is  exceedingly  scanty  in  the  red  sandy  and  marly  stj^ta 
of  the  central  European  Trias,  and  comparatively  poor  in  forms,  though 
often  abundant  in  individuals,  in  the  calcareous  zones  of  the  same  region. 
From  the  Alpine  development,  a  much  more  varied  suite  of  organisms 
has  been  disinterred.  Some  of  the  Alpine  limestones  are  full  of  forami- 
nifera  (Orbiilmt,  Glvhitffrina),  others  contain  numerous  calcareous  sponges 


{Kndi'i,  I'trliccllik",  teroudla,  &c.)  Comls  abound  in  some  localities  in 
the  same  rocks,  occasionally  forming  tnie  reefs.  Echinoderms  are 
plentiful  among  the  limestones,  particularly  crinoid-stems,  of  which 
these  rocks  ai-c  in  some  cases  almost  wholly  composed.  One  of  the 
most  characteristic  fossils  of  the  Muschelkalk  is  the  Eiif-riiius  lilH/onnii: 
(Fig.  379).  Species  of  urchins  {C'iiiari.-i)  are  common  in  the  Alpine 
Trias.  An  abundant  fossil  in  some  of  the  upper  Triassic  and  Rhsetic 
shales  is  the  little  jjhyllopod  EsVifria  (Fig.  379, 1).  Long-tailed  decapods, 
like  our  li\ing  shrimps  and  prawns,  were  well  represented  {Pendants,  ^gtr, 
Pfiiiijiln/j;  Ac.)  The  brachiopods,  while  shon-ing  some  resemblances  to 
those  of   PaliPozoic   time,   present   on   the  whole   a   great  contrast  to 


ig  1 


TSIASSIC  SYSTEM 


these  in  their  comparatively  diminished  numbers,  and  in  the  final  dis- 
appearance of  some  of  the  ancient  genera.  Thus  Athyris  and  Relzia, 
which  survived  from  Upper  Silurian  into  Triasaic  time,  then  dieappeared ; 
Cyiiina,  which  began  in  the  Devonian  period,  hkeFise  died  out  in  the 
Triassic  seas,  while  ita  contemporary  Spiriferiiia  continued  to  flourish 
until  the  time  of    the  Lias.     Although    species   of  Spiriferina,  Athyris, 


and  RehUi  are  common,  the  two  most  conspicuous  genera  of  brachiopods 
are  Terebrafula  and  RhyiKhoneUa,  and  they  continued  to  hold  this  posi- 
tion during  the  whole  of  the  Mesozoic  ages.      i^Ci-.,v,ir"  * 

^Vhile  the  brachiopods  were  waning,  the  lamellibrancha  were  taking  a 
more  prominent  place  in  the  moUuscan  fauna,  and  in  the  Triassic  seas 
they  had  already  established  the  predominance  which  they  hare 
maintained  down  to  the  present  day.     One  of  the  most    distinctively 


862  STRATIGRAPHICAL  GEOLOGY         book  vi  pxbt  m 


Triassic  genera  is  Myiyplu>riay  of  which  there  is  a  great  abundance  and 
variety  of  species.  Fecten,  Daonelkiy  UinnUeSy  Mmotis,  Lima,  GrervilUa, 
Anoplophora,  Aiicuki,  Cardium,  Carditay  Megalodorij  Nitcuki,  Cassianella^ 
Pulladra  (Fig.  379,  c),  likewise  occur  throughout  the  system.  Among 
gasteropods  we  find  representatives  of  some  Palaeozoic  types  (Naticapsis, 
Loxouemay  MiicrocJieilvSy  Murchmnia\  together  with  genera  characteristic  of 
Secondary  time,  and  some  of  which  even  continue  to  live  now  {Turriiella, 
Cerithiunij  C/iemnitzUi,  Solarium), 

In  no  feature  is  the  contrast  between  the  palseontological  poverty 
of  the  German,  and  the  richness  of  the  Alpine  Trias  so  marked  as  in  the 
development  of  cephalopods  in  the  respective  regions.  In  the  former 
area  the  nautili  are  represented  chiefly  by  a  few  species  of  NatttUuf 
{N.  bidorsatus,  Fig.  379,  e),  and  the  Ammonites  by  species  of  CeraiiUs 
(0.  nodosus,  Fig.  379,  a;  C.  sernipartitus).  In  the  Alpine  limestones, 
however,  there  occurs  a  profusion  of  cephalopod  forms,  among  which  a 
remarkable  commingling  of  Palaeozoic  and  Mesozoic  tyi)e8  is  noticeable. 
The  genus  OrthoceniSy  so  typical  of  the  Palaeozoic  rocks,  has  never  yet  been 
met  with  in  the  German  Tiiassic  areas;  but  it  appears  in  the  Alpine 
Trias  in  species  which  do  not  differ  much  from  those  of  the  older 
formations.  Associated  with  it  are  many  forms  of  the  ancient  and 
still  surviving  type  of  the  Nautilus,  It  is  especially  interesting  amid 
these  examples  of  the  persistence  of  primeval  forms  to  notice  the  advent 
of  the  earliest  precursors  of  types  which  played  a  conspicuous  part  in  the 
animal  life  of  later  periods.  Thus  the  family  of  the  Belemnites,  which 
appeared  so  prominently  among  the  denizens  of  the  Mesozoic  seas,  had 
its  earliest  known  forms  in  the  open  waters  of  Triassic  time  (AuJa^eocera^s, 
Afradiles).  Though  the  earliest  Ammonites  had  appeared  long  before,  it 
was  not  until  Triassic  time  that  this  gi'cat  order  began  to  assume 
the  importance  which  it  maintained  all  through  the  Mesozoic  ages.  So 
long  as  only  the  German  type  of  the  Trias  had  been  studied  this  early 
development  was  not  known.  But  now  besides  the  Ctratite.%  which  also 
ranged  into  the  opener  Triassic  waters,  we  have  become  acquainted  with 
a  remarkable  variety  of  ammonoid  types  {Arcestes,  Didijmiks,  HalofiteSy 
Tropih'-<y  lihuhdoceratyy  PtyehiteSy  Sagecera^y  TrachyceraSy  Piiuicoceras,  Lobite^, 
Cladisrites,  Megaphyllites). 

The  fislies  of  the  Triassic  period  include  teeth  and  spines  of  elasmo- 
branchs  (Ilybodus,  AcriHliLs)^  scales,  teeth,  or  exoskeletons  of  ganoids 
{GyrokpUj  Dapediu.%  Semiojwtiis,  Lepidotus,  NeplirotuSy  Saurichthys,  En- 
gnathus)  and  teeth  of  the  dipnoan  genus  Ceratodus. 

One  of  the  distinctive  palaeontological  features  of  the  Trias  is  the 
remarkable  assemblage  of  amphibian  and  reptilian  remains  found  in 
it.  The  ancient  order  of  Labyrinthodonts  still  flourished ;  numerous 
prints  of  tlieir  feet  have  been  observed  on  surfaces  of  sandstone  beds, 
and  the  bones  of  some  of  them  have  been  found  {Trematosaurus,  Masto- 
donsaurus,  S:c.)  The  rhynchocephalous  reptiles,  which  are  now  almost 
extinct,  first  appear  in  Permian,  and  are  well  represented  in  Triassic  rocks. 
Bones,  and  sometimes  even  nearly  entire  skeletons,  of  several  have 
been  discovered,  the  most  important  genera  being  TeUrpetanj  Hyperoda- 


BECT.  i  §  1  TRIASSIC  SYSTEM  863 

pedoiiy  and  Bhynchosaurus.  It  is  noteworthy  that  while  these  various  forms 
are  by  no  means  abundant  in  the  Triassic  system  generally,  they  have 
been  obtained  in  considerable  numbers  from  one  or  two  localities.  In 
Britain  the  most  prolific  deposit  for  them  is  the  pale  sandstone  of  Elgin, 
in  the  north  of  Scotland,  formerly  believed  to  be  Upper  Old  Red  Sand- 
stone. This  rock  contains  the  remains  chiefly  in  the  form  of  empty 
casts.  Besides  the  small  lizard,  Telerpetan,  described  by  Man  tell  in  1852, 
as  well  as  the  larger  possibly  allied  form  Ilyperodapedoriy  the  sandstone  has 
recently  yielded  a  number  of  new  forms  of  Anomodonts  which  present 
a  curious  resemblance  to  those  found  in  the  South  African  deposit  to  be 
immediately  referred  to.  These  skulls  and  skeletons  have  been  skilfully 
worked  out  and  described  by  Mr.  E.  T.  Newton  of  the  Geological  Survey.^ 
One  of  them,  Gardonid,  was  nearly  allied  to  Dicynodon  (Owen),  Geikia  was 
closely  related  to  Ptychognathus,  while  Elginia  was  a  remarkable  many- 
homed  animal  distantly  allied  to  Pareiasaunis  (Owen).  The  South 
African  formation,  to  which  allusion  has  been  made,  is  known  as  the 
"  Karoo  beds,"  which,  extending  over  a  vast  region  in  the  south  of  the 
continent,  have  furnished  an  interesting  assemblage  of  vertebrate  remains. 
Among  these  there  occur  Labyrinthodonts  (Micr&pholiSy  Petraphryney  Sauro- 
stenion)y  Anomodonts  (TapinocephaluSy  Pareiasaurus,  Anihodon),  and  a 
large  number  of  genera  belonging  to  a  remarkable  carnivorous  order, 
the  Theriodonts,  distinguished  by  having  three  sets  of  teeth,  like  those 
of  carnivorous  mammals  (Lycosaunis,  Tujrisuchu^^y  Cyiwdracoriy  &c.)  There 
were  like>vise  examples  of  Dicynodonts,  characterised  by  having  no  teeth, 
or  by  a  single  tusk-like  pair,  the  jaws  being  probably  prolonged  into  a 
horny  beak.  The  limbs  of  these  creatures  were  well  developed,  and  the 
animals  probably  walked  on  the  land  (DicynodoUy  Oudeimhtiy  &c.)  ^  The 
earliest  deinosaurs  yet  known  occur  in  this  system  {TliecodontosauruSy 
TeratosauruSy  Pal^osauruSy  Cladyodony  Plakosaurus  {Zandodon)y  Amnwrnumny 
AiichisauruSy  &c.)  ^  These  long-extinct  types  of  reptilian  life  pre- 
sented characters  in  some  measure  intermediate  between  those  of  the 
ostriches  and  true  reptiles,  and  their  size  and  unwieldiness  gave  them  a 
resemblance  to  the  elephants  and  rhinoceroses  of  modern  times.  They 
appear  to  have  walked  mainly  on  their  strong  hind  legs,  the  prints  of 
their  hind  feet  occurring  in  great  abundance  among  the  red  sandstones 
of  Connecticut.  Many  of  them  had  three  bird-like  toes,  and  left  foot- 
prints quite  like  those  of  birds.  Others  had  four  or  even  five  toes,  and 
attained  an  enormous  size,  for  a  single  footprint  sometimes  measures 
twenty  inches  in  length. 

The    ichthyosaurs   and    plesiosaurs,    which    played    so    foremost    a 

1  Phil.  Trans.  1893. 

^  Owen's  'Catalogue  of  Fossil  Reptilia  of  South  Africa,*  Brit.  Museum,  1876. 

•  See  on  deinosaurs  of  the  Trias,  Huxley,  Q.  J.  Ged.  Soc.  xxvi.  32.  In  the  year  1877, 
a  slab  of  the  **  Stubensandstein  *'  near  Stuttgart  was  obtained,  in  which  were  twenty-four 
individuals  of  **a  mailed  bird-lizard,"  named  Aetosaurus,  probably  a  deinosaur  with 
lacertiliau  characters.  0.  Fraas,  Jahrh,  Ver.  Nat.  WUrlemberg,  xxxiiL  (1877).  For  the 
Triassic  deinosaurs  of  Connecticut  see  Marsh,  Amer.  Joum.  Sci.  xxxvii.  (1889)  p.  831  ; 
xlii.  (1891)  p.  267  ;  xliii.  (1892)  p.  542  ;  xlv.  (1893)  p.  169. 


864  STRATIGRAPHWAL  GEOLOGY         book  vi  fart  m 

part  in  the  reptilian  life  of  Mesozoic  time,  had  their  Triassic  forerunnerB 
{IchthyosauruSy  Nothosauru.%  SimosauruSy  Neusticosaurus),  Of  higher  grade 
were  the  earliest  types  of  crocodiles,  the  remains  of  which  have  been 
detected  in  Triassic  rocks.  They  belong  to  an  extremely  generalised 
type,  and  appear  to  have  been  widely  distributed.  Staganolepis  occurs 
among  the  other  reptilian  remains  at  Elgin, ^  while  Phytosaurus  (BeJodm) 
has  been  obtained  in  Germany,  India  and  North  America. 

It  has  been  supposed  that  evidence  of  the  existence  of  Triassic  birds 
is  furnished  by  the  three-toed  footprints  above  referred  to.  But  prob- 
ably these  are  mostly,  if  not  entirely,  the  tracks  of  deinosaurs,  the 
absence  of  two  pairs  of  prints  in  each  track  being  accounted  for  by  the 
bird-like  habit  of  the  animals  in  the  use  of  their  hind  feet  in  walking. 
One  of  the  most  noteworthy  facts  in  the  palaeontology  of  the  Trias  is  the 
occurrence  in  this  system  of  the  first  relics  of  mammalian  life.  These 
consist  of  detached  teeth  and  lower  jaw-bones,  referred  to  small  marsupial 
animals  allied  to  the  Myrmecobuis,  or  Banded  Ant-eater  of  New  South 
Wales.  The  European  genus  is  Microlesfes  {Hypsiprymiwpsis).  In  the 
Trias  of  North  Carolina  an  allied  form  has  been  described  under  the  name 
of  Dromatherium. 

§  2.  Local   Development. 

Britain.- — Triassic  rocks  occupy  a  large  area  of  the  low  plaius  in  the  centre  of 
England,  ranging  thence  northwards  along  the  flanks  of  the  Carboniferous  tracts  to 
Lancaster  Bay,  and  south waixis  by  the  head  of  the  Bristol  Channel  to  the  south-east  of 
Devonshire.    They  have  been  arranged  in  the  following  subdivisions  : — 

P,     . .    3        (  Penarth  beds. — Red,  green,  and  grey  marls,  black  shales,  and  **  White 
Kliajtic.         -^      Lias  "  (20  feet  or  less  up  to  150  feet). 

(  Upper  Keuper  or  New  Red  Marl. — Red  .and  grey  shales  and  marls, 

I      with  beds  of  rock-salt  and  gypsum  (800  to  3000  feet). 
U])p(;r  Trias  J  Lower  Keu^ier  Sandstone. — Thinly  laminated  micaceous  sandstones 
or  Keuper.    \      and  marls  (waterstoues),  passiug  downwards  into  white,  brown, 

!      or  reddish  sandstones,  with  a  base  of  conglomerate  or  breccia  (150 

I      to  250  feet). 

Ui)i>er  Mottled  Sandstone. — Soft  bright  red  and  variegated  sandstones, 

without  pebbles  (200  to  700  feet). 
Pebble-beds.  —  Harder   reddish  -  brown    sandstones   with   quartzose 
-j      pebbles,  passing  into  conglomerate  ;    with   a  base  of  calcareous 

breccia  (60  to  more  than  1000  feet). 
Lower  Mottled  Sandstone. — Soft  bright  red  and  variegated  sandstone, 

without  iiebbles  (80  to  650  feet). 

^  On  the  Crocodiliau  remaius  of  the  Elgin  Sandstone  see  Huxley,  Quart.  Joum.  Geol.  Sik, 
1859  ;  Mem.  fJeol.  Surv.  Monograph  iii.  1877. 

-  See  E.  Hull,  "  Permian  and  Triassic  Rocks  of  England,"  (Jcological  Survey  Memoirs, 
1869  ;  H.  B.  Woodward,  (Jeol.  Mag.  1874,  p.  385  ;  ''Geology  of  East  Somerset  and  Bristol 
Coal-fields,"  Mem.  Owl.  JSunri/j  1876  ;  Ussher,  Q.  J.  Geol.  Soc.  xxxii.  367  ;  xxxiv.  459; 
ffei>l.  Man.  1875,  p.  163  ;  Proc.  Somerset.  Arch,  Nat.  Hist.  Soc.  xxxv.  (1889) ;  Etheridge, 
Q.  J.  Oeol.  Sim:  xxvi.  174  ;  A.  Irving,  Oeol.  Mag.  1874,  p.  314  ;  1887,  p.  309  ;  Qtiart, 
Joum.  Hcol.  Sk.  1888,  p.  149  ;  W.  T.  Aveline,  op.  cit.  1877,  p.  380  ;  J.  G.  Goodchild, 
Trails.  Camherl.   Westmorel.  Assoc,  xvii.  (1891-92). 

^  The  term  **Rha?tic"  is  derived  from  the  Rhwtiau  Alps,  where  the  rocks  so  named  are 
well  (leveloi>ed.     "Bunter  "  and  "Kneper"  are  terms  borrowed  from  Germany,  the  first  was 


Lower  Trias 
or  Buuter 
(1000  to 
2000  feet). 


SECT,  i  §  2 


TRIASSIC  SYSTEM 


865 


Like  the  Permian  red  rocks  below,  the  sandstones  and  marls  of  the  Triassic  series 
are  almost  barren  of  organic  remains.  Extraordinary  differences  in  the  development  of 
their  several  members  occur,  even  within  the  limited  area  of  England,  as  may  be  seen 
from  the  subjoined  table,  which  shows  the  variations  in  thickness  from  north-west  to 
south-east : — 


Lancashire 

Leicentershire 

and  W. 

Staffordshire. 

and  Warwick- 

 _                 __  . 

CheHhire. 

shire. 

Feet. 

Feet. 

Feet. 

T,             \  Red  marl         .... 
euper.   j  Lo^g^  Keuper  sandstone  . 

3000 

800 

700 

450 

200 

150 

'  Upper  mottled  sandstone  . 

500 

50-200 

absent 

Bunter.  - 

Pebble-beds     .... 

500-750 

100-300 

0-100 

^  Lower  mottled  sandstone  . 

200-500 

O-lOO 

absent 

Hence  we  observe  that,  while  towards  the  north  -  west  the  Triassic  rocks  attain  a 
maximum  depth  of  5200  feet,  they  rapidly  come  down  to  a  fifth  or  sixth  of  that  thick- 
ness as  they  pass  towards  the  south-east.  South-westwards,  however,  they  swell  out  in 
Devon  and  Somerset  to  probably  not  less  than  2500  or  3000  feet.  ^  Recent  borings  in 
the  south-eastern  counties  show  the  Trias  to  be  there  generally  absent.^  The  main 
source  of  supply  of  the  sediment  which  formed  the  material  of  the  Triassic  deposits 
probably  lay  towards  the  north  or  uorth-west.  The  pebble-beds,  besides  local  materials, 
contain  abundant  rolled  pebbles  of  quartz,  which  have  evidently  been  derived  from  some 
previous  conglomerate,  probably  from  some  of  the  Old  Red  Sandstone  masses  now 
removed  or  concealed.  The  Trias  rests  with  a  more  or  less  decided  unconformability  on 
the  rocks  underneath  it,  so  that,  although  the  general  physical  conditions  as  regards 
climate,  geography,  and  sedimentation,  which  prevailed  in  the  Permian  period,  still  con- 
tinued, terrestrial  movements  had,  in  the  meanwhile,  taken  place,  whereby  the  Permian 
sediments  were  generally  upraised  and  exposed  to  denudation.  Hence  the  Trias  rests 
now  on  Permian,  now  on  Carboniferous,  and  sometimes  even  on  Cambrian  rocks.  More- 
over, the  upper  parts  of  the  Triassic  series  overlap  the  lower,  so  that  the  Keuper  groups 
repose  successively  on  Permian  and  Carboniferous  rocks. 

The  Bunter  series  is  singularly  devoid  of  organic  remains.  The  rolled  fragments  in 
the  i)ebble-beds  have  yielded  fossils  at  Budleigh  Salterton,  on  the  southern  coast  of 
Devonshire,  proving  that  Silurian  and  Devonian  rocks  were  exposed  within  the  area 
from  which  the  materials  of  these  strata  were  derived.  The  peculiar  quartzites  of  the  Bud- 
leigh Salterton  pebbles  do  not  seem  to  have  come  from  any  British  rocks  now  visible,  but 
rather  to  have  been  derived  from^the  north-west  of  France.'  A  marked  characteristic 
of  the  Bunter  series  in  central  England  is  its  capacity  for  holding  water,  whence  it  is  an 
im|>ortant  source  of  water-supply. 

At  the  base  of  the  Keuper  series,  in  the  region  of  the  Mendip  Hills,  a  remarkable 
littoral  breccia  or  conglomerate  occurs.  Over  Carboniferous  Limestone  it  consists  mainly 
of  limestone,  and  is  precisely  like  "  brockram "  (p.  847),  but  in  the  slaty  tracts  of 
Devonshire,  the  fragments  are  of  slate,  porphyry,  granite,  &c.  Its  matrix  being  sometimes 
dolomitic,  it  has  been  called  the  Dolomitic  conglomerate  ;  but  it  occasionally  passes  into 

taken  by  Werner  from  the  variegated  (German,  bunt)  colours  of  the  strata,  the  second  is  a 
local  miner's  term.  *  Ussber,  Q,  J,  QeoL  Soc,  xxxiL  892. 

^  Red  strata  in  the  deep  boring  at  Richmond  are  believed  by  Prof.  Judd  to  be  Triassic. 
Mr.  Whitaker  regards  as  Trias  similar  rocks  found  under  Kentish  Town  and  Crossness  near 
London. 

'  For  an  account  of  their  included  fossils  see  Davidson,  PaUsontograpK  Soc,  188L 

3  K 


866  STRATIGRAPHIGAL  GEOLOGY         book  vi  pabt  m 

a  magnesian  limestone.  It  represents  the  shore  deposits  of  the  Trias  salt-lake  <Mr  inland 
sea,  and,  as  it  lies  on  many  successive  horizons,  we  see  that  the  conditions  for  its  fonna- 
tibn  persisted  during  the  subsidence  by  which  the  Mendips  and  other  land  of  this 
region  were  gradually  depressed  and  obliterated  under  the  red  sandstones  and  maris 
(see  Figs.  219,  220,  221).^  The  Dolomitic  conglomerate  averages  20  feet  in  thicknns, 
but  here  and  there  rises  into  cliffs  40  or  50  feet  high.  It  has  yielded  two  genera  of 
deinosaurs  {Paleeosaurus,  Thecodoniosaurus).^  Some  geologists  have  regarded  this  band 
of  rock  as  an  English  representative  of  the  German  Muschelkalk.  But  the  manner  in 
which  it  ascends  along  what  was  the  margin  of  the  Triassic  land  shows  it  to  be  a  local 
base  occupying  successive  horizons  in  the  red  rocks.  There  is  no  equivalent  of  the 
Muschelkalk  in  Britain,  unless  the  middle  division  of  the  Devonshire  Trias  can  be  so 
regarded.' 

The  lower  Keuper  group  is  composed  of  red  and  white  sandstones  with  occasional 
lenticular  bands  of  coarser  material,  and  like  the  corresponding  strata  in  the  Bunter 
group,  is  generally  uufossiliferous,  but  has  furnished  many  amphibian  footprints.  The 
surfaces  of  the  sandstone-beds  are  likewise  impressed  with  rain-drops  and  are  marked 
with  desiccation-cracks  and  ripple-marks,  suggestive  of  flat  shores  exposed  to  the  air. 

In  the  upper  Keu|)er  group  the  sediments  were  generally  muddy  and  now  appear  as 
red  and  variegated  marls  with  occasional  partings  of  sandstone  or  bands  of  dolomite  or 
of  g}'psum.  Among  these  strata  are  beds  of  rock-salt  varying  from  a  few  inches  to  more 
than  100  feet  in  thickness.  The  marly  character  of  the  up{)er  Keuper  is  a  distinguishing 
feature  of  the  group  from  the  south  of  Scotland  to  the  south  of  Devonshire,  and  from 
Antrim  to  the  east  of  Yorkshire.  Throughout  this  wide  area  cubical  casts  of  salt 
(chloride  of  sodium)  are  not  infrequent,  though  this  substance  is  only  workable  at  a 
few  places  (Antrim,  Cheshire,  Middlesbrough^).  The  salt  is  chiefly  obtained  by  dis- 
solving the  material  underground  and  pumping  up  the  br^ne,  very  little  being  now 
actually  mined.  The  rock-salt  as  it  occurs  intercalated  in  the  marls  is  a  crystalline  sub- 
stance, usually  tinged  yellow  or  red  from  intermixture  of  clay  and  peroxide  of  iron,  but  is 
tolerably  pure  in  the  best  parts  of  the  beds,  where  the  proportion  of  chloride  of  sodium 
is  as  much  as  98  per  cent.  Through  the  bright  red  marls  with  which  the  salt  is  inter- 
stratified  there  run  thin  seams  of  rock-salt,  also  bauds  of  gypsum,  somewhat  irregular 
in  their  mode  of  occuiTence,  sometimes  reaching  a  thickness  of  40  feet  and  upwards. 

The  paucity  of  organic  remains  in  the  English  Keuper  indicates  that  the  conditions 
for  at  least  animal  life  must  have  been  extremely  unfavourable  in  the  waters  of  the 
ancient  Dead  Sea  wherein  these  red  rocks  w:ere  accumulated.  The  land  possessed  a 
vegetation  which,  from  the  fragments  yet  known,  seems  to  have  consisted  in  large 
measure  of  cypress -like  coniferous  trees  [Voltzia,  Walchia)^  with  calamites  on  the  lower 
more  marshy  grounds.  The  red  marl  group  contains  in  some  of  its  layers  numerous 
valves  of  the  little  crustacean  Estheria  minuta^  and  a  solitary  species  of  lamellibranch, 
PuUastra  a/rnicola.  A  number  of  teeth,  spines,  and  sometimes  entire  skeletons  of  fish 
have  been  obtained  {Dipteronotus  cyphus,  Pala^niscus  supcrsUs^  Hybodus  Kfuperiy 
Acrodus  7)Lutunu.%  Sphcnonchus  minimus,  &c.)  The  bones,  and  still  more  frequently  the 
footprints,  of  labyrinthodont  and  even  of  saurian  reptiles  occur  in  the  Keuper  beds 
— Labyrinthodon  (4  species),  Cladyodon  Lloydii,  Hypcrodapedon,  Falxosaurus,  Zaiiclodon 
{TertUosaurics),  Tliecodoiitomurus,  MyiichoiwsaurnSy  and  footprints  of  Cheirothcriunu  The 
remains  of  the  small  marsupial  Microlcst-es  have  likewise  been  discovered  in  the  highest 
beds  sometimes  taken  as  the  base  of  the  Rhwtic  series. 

At  the  top  of  the  Keuper  marl  certain  thin-bedded  strata  fonn  a  gradation  upwards 

^  De  la  Beche,  Mem.  (Jeid.  Survey,  i.  p.  240.  H.  B.  Woodward,  "Geology  of  East 
Somerset  and  Bristol  Coal-fields,"  Mem.  Oed.  Survey,  1876,  p.  53. 

-  Etheridge,  Q.  J.  Geol.  Soc.  xxvi.  174.  ^  Ussher,  op.  cU.  xxxiv.  p.  469. 

*  T.  Hugh  Bell  on  salt  deposits  of  Middlesbrough,  Proc.  develund  IfuL  Engin, 
Session  1882-83. 


SECT,  i  §  2  TRIASSIG  SYSTEM  867 

into  the  base  of  the  Jurassic  system.  As  their  colours  are  grey,  blue,  and  black,  and 
contrast  with  the  red  and  green  marls  below,  they  were  formerly  classed  without 
hesitation  in  the  Jurassic  series.  Egerton,  however,  showed  that,  from  the  character 
of  the  fish  remains  found  in  the  "bone- bed"  of  the  black  shales,  they  had  more 
palseontological  affinity  with  the  Trias  than  with  the  Lias.  Subsequent  research, 
particularly  among  the  Rhsetian  Alps  and  elsewhere  oti  the  Continent,  brought  to  light 
a  great  series  of  strata  of  intermediate  characters  between  the  previously  recognised  Trias 
and  Lias.  These  results  led  to  renewed  examination  of  the  so-called  beds  of  passage  in 
England  (Penarth  beds),^  which  were  found  to  be  truly  representative  of  the  massive  * 
formations  of  the  Tyrolese  and  Swiss  Alps.  They  are  therefore  now  known  as  H  h  se  t  i  c 
(sometimes  as  I  n  fr  a-Lias),  and  are  usually  classed  as  the  uppermost  member  of  the 
Trias,  but  offering  evidence  of  the  gradual  approach  of  the  physical  geography  and  char- 
acteristic fauna  and  flora  of  the  Jurassic  period. 

The  Rhsetic  (Penarth)  beds  occur  as  a  continuous  though  thin  band  at  the  top  of  the 
Trias,  throughout  the  British  area.  They  extend  from  the  coast  of  Yorkshire  across 
England  to  Lyme  Regis  on  the  Dorsetshire  shores.^  They  occur  in  scattered  patches  up 
the  west  of  England,  and  on  both  sides  of  the  Bristol  Channel,  and  they  may  be  detected 
even  in  the  north  of  Scotland.  Their  thickness,  on  the  average,  is  probably  not  more 
than  50  feet,  though  it  rarely  increases  to  150  feet.  In  the  south-west  of  England,  they 
consist  of  the  following  subdivisions  in  descending  order  : — 

White  Lias — composed' of  an  upper  hard  limestone  (Sun-bed  or  Jew-stone,  6  to  18 
inches)  with  Modiola  minima  and  Ostrea  liassica  ;  and  a  lower  group  of  pale 
limestones  (10  to  20  feet)  with  the  same  fossils  and  Cardium  phiUipianum 
{rfueiicum),  Monotia  decustata.  The  Cotham  Stone  or  Landscape  Marble  (4  to 
8  inches)  is  a  hard  compact  limestone,  with  dendritic  markings,  lying  at  the  base 
of  these  calcareous  strata.  At  Aust  it  has  yielded  elytra  of  Coleoptera,  wings 
of  insects,  and  scales  and  perfect  specimens  of  the  fishes  Legnonotus  cothamentis, 
PholidophoruM  Higginai, 

Black  paper-shales  (10  to  15  feet),  finely  laminated  and  pyritous,  with  selenite  and 
fibrous  calcite  ("beef")  and  one  or  more  seams  of  ferruginous  and  micaceous 
sandstone  (bone-bed)  containing  remains  of  fish  and  saurians.  Some  of  the  shales 
yield  Avicula  (CassianeUa)  coniortcL,  Cardium  phiUipianum  (rhseticum),  Pecten 
valoniensis  {  =  Avicula  contorta  zone). 

Green  and  grey  Marls  (20  to  30  feet),  with  alabaster,  celestine,  and  sometimes 
pseudomorphs  of  rock-salt ;  generally  unfossiliferous,  but  yielding  Microlestes. 
TheSe  Marls  form  properly  the  top  of  the  Trias,  the  bone- bed  above  serving  as  a 
convenient  base  for  the  Rheetic  beds. 

A  bone-bed  similar  to  that  in  the  foregoing  section  reappears  on  the  same  hoiizon  in 
Hanover,  Brunswick,  and  Franconia.  Among  the  reptilian  fossils  are  some  precursors 
of  the  great  forms  which  distinguished  the  Jurassic  period  {Ichthyosaurus  a.nd  Flesio- 
saurus).  The  fishes  include  Acrodus  minimus^  Ceratodus  alius  (and  five  other  species), 
Hyhodus  minoTy  Nemacanthvs  monilifer,  &c.  Some  of  the  lamellibranchs  (Fig.  380)  are 
specially  characteristic ;  such  are  Cardium  phiUipianum  {rhasticum)^  Avicula  {Cassianclla) 
cont(yrtay  Pecten  valonieivtiSy  and  Pullastra  arenicola  (Fig.  379). 

*■  So  named  from  their  being  well  developed  in  the  clifla  of  Penarth  on  the  Glamorgan- 
shire coast.  Bristow,  Brit.  Assoc.  1864,  sects,  p.  50  ;  Oeol.  Surv.  Vertical  Sections,  sheets 
47,  48. 

'  Strickland,  Proc,  Oeol,  Soc.  iii.  part  ii.  p.  585  ;  H.  W.  Bristow,  Geol.  Mag.  i.  (1864) 
p.  236  ;  T.  Wright,  Quart.  Joum,  Qed.  Soc.  xvi.  p.  374  ;  C.  Moore,  op.  cit.  xvi.  p.  483  ; 
xxiii.  p.  459  ;  xxxvii.  pp.  67,  459  ;  W.  B.  Dawkins,  xx.  p.  396  ;  E.  B.  Tawney,  xxii  p. 
69  ;  P.  B.  Brodie,  p.  93  ;  F.  M.  Burton,  xxiii.  p.  315  ;  W.  J.  Harrison,  xxxii.  p.  212  ; 
P.  M.  Duncan,  xxiii.  p.  12  ;  J.  W.  Davis,  xxxvii.  p.  414  ;  E.  Wilson,  xxxviii.  p.  451  ;  H. 
B.  Woodward,  "  Geology  of  E.  Somerset  and  Bristol  Coal-fields,"  Mem,  Oeol.  Survey,  p. 
69 ;  Proc.  Oeol.  Assoc,  x.  (1888). 


STBATIGBAPHICAL  GEOLOGY 


BOOE  TI  PARI  m 


Centnl  Europe. — The  Trias  is  Oae  of  the  uio»C  compactly  distributed  geologic^ 
rormations  of  Europe.  ]ta  maia  area  extends  as  a  great  bssin  from  Basel  down  to  the 
pUins  of  Uanover,  trarersed  along  its  centra  by  the  course  of  the  EUne,  and  stretching 
from  the  flaake  of  the  old  high  grounds  of  Saxon;  and  Bohemia  on  the  east  actnas  the 
Voages  Mouutsius  into  Fisnce,  &tid  across  tbs  Uoaelle  to  the  flanks  of  tbe  Ardennes. 
This  muat  have  been  a  great  inland  sea,  oat  of  which  the  Hsrz  Monntsins,  and  the  high 
grounds  of  the  Eifel,  Hunsdruck,  and  Taonus  probably  rose  as  islands.  To  the  west- 
ward of  it,  the  Palseozoic  area  of  the  north  of  France  and  Belgium  had  been  niacd  np 
into  land.'  Along  the  margin  of  this  land,  red  conglomerates,  sandstones,  and  clsjs 
were  deposited,  which  now  appear  here  and  there  reposing  unconformable  on  the  older 
formstiona.  Traces  of  what  were  probably  other  basins  occur  esstwsrd  in  the  Csxpathisn 
district,  in  the  west  and  south-east  of  France,  and  over  the  eastern  half  of  the  Spanish 
peninsula.     But  these  areas  have  been  considerably  obscured,  sometimes  by  dislocalian 


I.  UerrsiiC' 


'111)  contorts,  Portlock : 


and  denudation,  sometimes  by  the  overlap  of  mare  recent  accumulations.  In  the  region 
between  Marseilles  and  Nice,  Triosaic  rocks  cover  a  considerable  area.  They  contain 
feeble  representatives  of  the  Oris  bigarri  or  Bunter  beds,  and  of  the  ilamta  iria^a  or 
Keugier  division,  separated  by  a  calcareous  zone  believed  to  be  the  equivalent  of  the 
Muaclielkalk  of  Germany.  Their  hif-hest  platform,  the  Rhsetic  or  In/ra-tiaa,  contoius 
s  shell-bed  abounding  in  Avieula  eoidorii,  and  is  traceable  throughout  Provence,' 

III  the  great  German  Triasaic  Ijasiu*  the  de{)ositg  are  as  shown  in  the  subjoined 
table  :— 

irt,  rose  into  pesks  16,000  to  20,000  feet 


ii.  p.  100.     Dieulofait,  Ann.  Sd.   OteL  I 


■  This  land,  accoriliDE  to  MM.  Corn, 
high  !  {Ann.  fioi:  Oiol.  Xord,  iv.) 

=  Htbert,  Ball.  Soc  Olol.  Fnintt  (! 
p.  337. 

>  E.  Weis.s  Zeitach.  Deutick.  Oeol.  6a.  xii.  (1869)  p.  837  ;  C.  W.  GUmbel,  'Qei^Daa- 
tiiche  Besclireibung  des  Kunigreichs  Bayem,'  Hi,  (1879)  chap.  iv.  ;  F.  Boemer,  '  Geolc^e 
von  Oberschleaieu,'  1870,  p.  122  ;  E.  W.  B*necte,  '  Uber  die  Trias  in  Elaass-Lothringen 
und  Luxemburg,'  Abh.   Gevl.  SpecUiikarU  Elsats-Lolhr.    i.  port  iv.   (1S77)  ;    O.  Meyer, 


SECT,  i  §  2 


TRIASSIC  SYSTEM 


869 


4 
1 

Si 


a, 

i4 


M 

'3 


V. 


Rhsetic  (Rhat,  Infra-Lias). — Grey  sandy  clays  and  fine-grained  sandstones, 
containing  Equisetum,  Atplenites,  and  cycads  (Zdmites,  Pterophyllum), 
sometimes  forming  thin  seams  of  coal — Cardium  phiUipianum  {rhmtxcuin)^ 
Avicula  {CaasianeUa)  contortaj  Estheria  mintUaf  Nothoaaurus,  TremoUo- 
saurus,  Betodotiy  and  MieroleaUs  antiqutu.^ 

Kenpermergel,  Oypskeuper. — Bright  red,  green  and  mottled  marls,  with  an 
underlying  set  of  beds  of  gypsum  and  rock-salt.  In  some  places  where 
sandstones  appear  they  contain  numerous  plants  {Equisetum  columnare^ 
PterophyUum^  &c.),  and  labyrintbodont  and  fish  remains '  (800  to  1000 
feet). 

Lettenkohle,  Kohlenkeuper. — Orey  sandstones  and  dark  marls  and  clays,  with 
abundant  plants,  sometimes  forming  thin  seams  of  an  earthy  hardly  work- 
able coal  (Lettenkohle),  about  230  feet.  The  plants  include,  besides  those 
above  mentioned,  the  conifers  Araucarioxylon  thuringicum,  Voltzia  hetero- 
phylia,  kc.  A  few  shells  have  been  obtained  from  this  group,  especially 
fh)m  a  band  of  dolomite  at  its  upper  limit  {Lingula  ienuissimay  Myophoria 
Ooldfusaiy  M,  iranaveraa,  Anophp?iora^  OervUlia).  Some  of  the  shales  are 
crowded  with  small  phyllopod  Crustacea  {Estheria  minuta,  also  Bairdia), 
Remains  of  fish  (AcrodxiSf  BybodHSj  Ceratodus)  and  of  the  Mastodon- 
aaurus  Jmgeri  and  Nothosaurua  have  been  obtained. 

Upper  Limestone,  capable  of  subdivision  into  two  groups,  a  lower  hard 
encrinite  limestone  (Trochitenkalk)  and  an  upper  group  of  thin  limestone 
with  argillaceous  partings,  known  as  the  Noilosus  group  from  the  abun- 
dance of  Ceratites  nodosua  (200  to  400  feet).  In  some  regions  a  third  still 
higher  group  of  dolomites  and  limestones  is  called  the  Trigonus  group  from 
the  prevalence  in  it  of  Trigonodus  Sandbergeri,  The  upper  Muschelkalk 
is  by  far  the  most  abundantly  fossiliferous  division  of  the  German  Trias. 
Among  its  fossils,  Nautilus  bidorsatus,  Lima  striaia,  Myophoria  vulgaris, 
Trigonodus  Sandbergeri,  and  Terebratula  vulgaris  are  specially  character- 
istic, with  Encrinus  lilii/ormis  in  the  lower  and  Ceratiles  nodosus  in  the 
upper  part  of  the  rock.  Some  parts  of  the  lower  limestones  are  almost 
wholly  made  up  of  crinoid  stems. 

Middle  Limestone  and  Anhydrite,  consisting  of  dolomites  with  anhydrite, 
gypsum,  and  rock-salt.  Nearly  devoid  of  organic  remains,  though  bones 
and  teeth  of  saurians  have  been  found  (200  to  400  feet). 

Lower  Limestone  (Wellenkalk),  consisting  of  limestones  and  dolomites 
(Wellendolomite),  with  in  the  upper  part  bands  of  porous  limestone  known 
as  Schaumkalk  (160  to  500  feet).  This  zone  is  on  the  whole  poor  in 
fossils,  save  in  the  limestone  bands,  some  of  which  form  a  lower  zone  full 
of  Encrinus  liliiformisy  while  a  higher  zone  is  characterised  by  Myophoria 
orbicularis.  ■  The  upper  portion  of  the  limestone,  however,  is  high))' 
fossiliferous,  and  has  yielded  a  number  of  brachiopods  {Spiriferina  fragilis^ 
S.  hirsuta,  Athyris  trigonella,  Terebratula  vulgaris^  T.  angusta,  numerous 
lamellibranchs,  especially  the  widespread  genus  Myophoria  {M.  vulgaris^ 
eUgan-Sf  cardissoides),  OervUlia  costataj  Monotis  Alherti,  and  some  am- 
monites (Beneckeiaf  Ceratites,  kc.) 


MittheU.  Com,  Oeol.  Landes-Untersuch.  i.  part  i.  (1886)  ;  H.  BUcking  and  K  Schumacher, 
op.  cit.  ii.  part  ii.  (1889) ;  K  W.  Benecke  and  L.  van  Wervecke,  op.  cit.  iii.  part  i.  (1890) ; 
and  the  Jahrbuch  of  the  Prussian  Geological  Survey.  Detailed  measured  sections  of  the 
Muschelkalk  and  Lettenkohle  in  Franconla  are  given  by  F.  v.  Sandberger,  Verh.  Phys.  Med. 
Oes.  Wurzburg,  xxvi.  (1892)  No.  7.  S.  Passarge,  '  Das  Roth  im  ostlichen  Thttringien,' 
Jena,  1891. 

*  The  AiHcula  contorta  zone  (see  Dr.  A.  von  Dittmar,  *Die  Contorta-Zone,'  Munich, 
1864)  ranges  from  the  Carpathians  to  the  north  of  Ireland  and  from  Sweden  to  the  hills  of 
Lombardy.  In  northern  and  western  Europe,  it  forms  part  of  a  thin  littoral  or  shallow- 
water  formation,  which  over  the  region  of  the  Alps  expands  into  a  massive  calcareous  series, 
that  accumulate<l  in  a  deeper  and  clearer  sea.  It  is  well  developed  also  in  northern  Italy. 
See  Stoppani,  '  Geologic  et  Pal^ontologie  des  Couches  a  Avicula  Contorta  en  Lombardie,' 
MUan,  1881. 

'  It  is  deserving  of  notice  that  while  in  the  pelagic  or  Alpine  fa'cies  of  the  Trias  fish- 


870 


STRATIGRAPHICAL  GEOLOGY 


BOOK  VI  PART  m 


(  Upper  (Roth). — Bed  and  green  mark,  with  gypsum  in  the  lower  part,  and 
sometimes  beds  of  rock-salt  (250  to  300  feet).  Occasional  bands  of  dolo- 
mite {RhizocoraUium  dolomite  of  Thnringia),  yield  a  number  of  fossils 
{RhizocoraUtum  jenensej  probably  a  sponge,  Myophoria  coetata^  M.  vulfforist 
OerviUia  sodaXiSy  Myacites  maciroides,  the  Ammonite  Beneckeia  tetiuis). 
The  Myophoria  is  specially  characteristic.  The  plants  of  this  stage  con- 
sist chiefly  of  VoUziOf  with  ferns  and  horse-tails  {Anomcpteris,  Equiadum). 

Middle. — Coarse-grained  sandstones  (1000  feet),  sometimes  incoherent,  with 
wayboards  of  i?5^rta -shale  ;  amphibian  footprints  and  remains  of  laby- 
rinthodonts. 

Lower. — Fine  reddish  argillaceous  false-bedded  sandstone  (Gr^  des  Vosges) 
several  hundred  feet  thick,  often  micaceous  and  fissile,  with  occasional 
interstratifications  of  dolomite  and  of  the  marly  oolitic  limestone  called 
"  Rogenstein."  Fossils  extremely  scarce-;  Estheria  minuta  occurs  in  some 
layers. 

The  Bunter  division,  in  the  north  and  centre  of  Germany,  lies  conformably 
on  and  passes  insensibly  into  the  Zechstein.  Except  in  the  dolomite  beds  of 
the  tRoth,  it  is  usually  barren  of  organic  remains.  The  plants  already 
known  include  Equiaetum  aretutceum,  one  or  two  ferns,  and  a  few  conifers 
{Albertia  and  Vdtzia),  The  lamellibranch  Myophoria  costaia  is  found  in 
the  upper  division  all  over  Germany.  Numerous  footprints  occur  on  the 
sandstones,  and  the  bones  of  labyrinthodonts  as  well  as  of  fish  have  been 
obtained. 


In  the  Vosges,  the  Bunter  (Gr^s  bigarre,  Vosgian)  consists  of  (1)  a  lower  coarse  red 
unfossiliferous  sandstone  (Gres  des  Vosges)  resting  conformably  on  the  red  Permian 
sandstone  and  marked  by  the  frequent  crystalline  condition  of  its  quartz-grains  (crystalline 
sandstone,  p.  132) ;  also  by  its  quartz- conglomerates,  which  occasionally  reach  a  thickness 
of  more  that  1600  feet ;  (2)  an  upper  series  of  red  sandstones,  surmounted  by  marls, 
forming  the  Gr^  bigarre^  and  containing  among  other  fossils  VbUzia,  Albertia,  JBquut- 
turn .  arenaceumy  Myophoriay  Nothosaurua  Schimperiy  Mcnodon  pliecUuSy  Odoniosaurus 
VoUziiy  Mastodonsaurus  wasleiiensis.  The  Muschelkalk  in  the  same  region  is  a  compact 
grey  limestone  caimble  of  subdivision  into  three  zones,  as  in  Germany,  while  the  Keuper 
(Marnes  irisees)  presents  a  characteristic  assemblage  of  bright  red  and  green  mottled 
argillaceous  marls. ^ 

Scandinavia.'-' — Though  fragmentary  remains  of  the  terrestrial  flora  that  clothed 
the  land  which  surrounded  the  German  Triassic  inland  sea  not  infrequently  occur,  it  is 
on  the  north  side  of  the  basin  that  the  moat  abundant  traces  have  been  recovered  of 
the  vegetation  of  this  period.  Above  reddish  saliferous  rocks,  presumably  Triassic, 
there  come  in  southern  Sweden  certain  light  grey  and  yellow  strata,  which,  from  the 
occurrence  of  AviciUa  contorta  and  other  fossils  in  them,  are  assigned  to  the  Rhsetic 
stage,  though  jwssibly  their  higher  members  may  be  Jurassic.  They  attain  in  some 
places  a  thickness  of  500  to  800  feet,  and  cover  about  250  square  miles.  They  have 
been  divided  into  a  lower  fresh-water  group,  with  workable  coal-seams,  but  no  marine 
fossils,  and  an  upi)er  marine  group,  with  only  poor  coals,  but  with  numerous  marine 


remains  are  on  the  whole  scarce,  and  only  occur  in  numbers  at  a  few  places,  they  are  widely 
distributed  and  tolerably  abundant  throughout  the  German  Trias.  See  O.  Jaekel,  Abhand, 
Geol.  Spevialkart.  Ehaas-Lothr,  iii.  Heft  iv.  (1889). 

^  Henecke,  Abhandl.  Specialkarte  Elsass-Lothrinyeny  1877;  Lepsius,  Z.  Deutsck,  Oeol, 
Oes.  1875,  p.  83. 

2  See  Hcbert,  Ann.  Sci.  O^l.  1869,  No.  1  ;  Bull.  Soc,  GM,  France  (2),  xxvii.  (1870), 
p.  366  ;  Memoirs  of  the,  Geoloyictd  Survey  of  Sweden^  especially  Nathorst  "Cm  Floran 
Skanes  Kolfcirande  Bilduingar,"  1878,1879;  E.  Erdmann,  *'Beskrifning  till  Kartbladet 
Helsingborg,"  1881,  p.  42;  G.  Lindstrum,  op,  cii.  "Kartbladet  Engelholm,"  1880  ;  also 
Nathorst,  '^Bidrag  till  Sveriges  fossila  Flora,"  K,  Vet.  Akad.  Handl,  Stockholm,  xiv.  xvi.  ; 
Lundgren,  Geol,  FOren.  StockJwlvi  Fiirh.  1880, 


SECT,  i  §  2  TRIASSIC  SYSTEM  871 

organisms  {Ostrea,  Pecten,  Avieula,  &c).  In  the  coal-bearing  strata  clay-ironstones 
occur,  and  seams  of  fireclay  underlie  the  coals.  Nathorst  and  Landgren  have  brought 
to  light  150  species  of  plants  from  these  beds — a  larger  number  than  the  whole  of  the 
Triassic  flora  of  other  countries.  At  Bjuf  they  include  36  species  of  ferns,  36  cycads, 
15  conifers,  and  1  monocotyledon.  The  sabjoined  grouping  of  the  Swedish  Triassic 
rocks  has  been  given  by  Lundgren : — 

Arieten-Lias. 
Cardiuia  Lias. 
Younger  Rhaetic.  Zone  of  Nilssonia  polynwrpha, 

iPuUastra  bed. 
Zone  of  ThaunuUqpUris  SchtnkL 
Zone  of  JSquigetum  gracUe, 
Zone  of  LepidopUrU  Ottonis, 
Older  Rhaetic   .     Zone  of  CamptopUrU  spiralis, 

Keuper. 

Alpine  Trial. ^ — In  the  western  Alps,  certain  lustrous  schists,  with  gypsum, 
anhydrite,  dolomite  and  rock-salt,  lie  underneath  the  Jurassic  series,  and  are  referred  to 
the  Trias.  On  the  Italian  side,  they  swell  out  to  great  proportions,  reaching  a  thickness 
of  more  than  13,000  feet  along  the  line  of  the  Mont  Cenis  Tunnel.  Traced  through 
Piedmont,  they  are  found  to  play  an  important  part  in  the  structure  of  the  northern 
A|>ennines,  where  they  contain  the  celebrated  statuary  marbles  of  Carrara  (p.  629). 
They  have  undergone,  in  these  mountainous  tracts,  extensive  metamorphism,  the 
original  shales  or  marls  being  changed  into  lustrous  schists,  and  the  limestones  into 
crystalline  marbles.  But  even  in  this  altered  condition  Triassic  fossils  have  been 
found  in  them. 

Already  in  Triassic  time  a  notable  distinction  had  been  established  between  the 
geographical  conditions  of  the  regions  now  marked  by  the  eastern  and  western  Alps. 
The  liue  of  division  between  the  two  areas  may  be  said  to  coincide  generally  with  that 
ancient  line  of  N.E.  and  S.W.  disturbance  known  as  the  ' ' Rhine-Ticino  fault."  To 
the  west  the  Triassic  deposits  point  to  varying  conditions  of  lagoons  and  inland  seas. 
Eastward,  however,  the  corresponding  deposits  attain  an  enormous  development,  and  are 
now  recognised  as  presenting  a  record  of  the  deeper  water  or  pelagic  conditions  of  the 
Triassic  period.  As  Mojsisovics  has  remarked,  what  England  and  North  America  are 
for  the  Palseozoic  formations  in  general,  what  Bohemia  is  for  the  Silurian  system,  what 
the  Jura  Mountains  are  for  the  Jurassic  deposits,  the  eastern  Alps  are  for  the  Trias.' 
Special  interest  attaches  to  the  Trias  of  the  eastern  Alps  from  the  great  thickness  of  its 
limestones  and  their  thoroughly  marine  fauna,  with  a  commingling  of  Palaeozoic  and 
Mesozoic  types  intercalcated  between  the  Permian  and  Jurassic  systems.  It  would 
appear  that  during  the  deposition  of  these  limestones  the  central  core  of  crystalline  and 
Palaeozoic  rocks  of  the  Alpine  chain  rose  as  an  island  that  stretched  from  the  Engadine 
eastward  into  Austria.     North  of  this  old  insular  tract  the  Triassic  strata  are  on  the 

^  See  F.  von  Richthofen,  *  Geognostische  Beschreibung  der  Umgegend  von  Predazzo,'  &c. 
Gotha,  1860  ;  Giimbel,  *Geog.  Beschreib.  des  Bayerisch.  Alpen,*  1861  ;  Stur,  'Geologie  der 
Steiermark,'  1871  ;  E.  von  MojsiBOvics,  Jahrb.  Oeol.  Reichsanstalt^  Vienna,  1869,  1874, 
1875,  1880  ;  Abhandl.  OeoL  Reichsanstali,  vi.  (1875)  p.  82  ;  Verhandl.  Geol.  ReUhsanstalt, 
1866,  1875,  1879;  and  'Dolomitriffe  Siidtirols  und  Venetiens,'  1878;  E.  Siiss,  'Die 
Enstehung  der  Alpen,'  1875  ;  also  memoirs  by  Von  Hauer,  Laube,  Silss,  Stache,  Star,  Toula, 
Bittner,  and  others  in  the  Jahrb,  Oeol,  Reichsanstalt ;  Von  Hauer's  *  Geologie,'  p.  358  et 
seq.  ;  Miss  M.  Ogilvie,  Quart  Joum,  Geol,  Soc,  xlix.  (1893)  p.  1.  The  fossils  are  described 
by  Benecke,  Oeol.  Palasontol.  Beitr.  vol.  ii.  ;  Mojsisovics,  Abhandl.  k.  k,  Oeol,  Reichsanst, 
vii.  X. ;  Bittner,  op.  cU.  vol.  xiv.  ;  G.  L.  Laube,  Denksch.  Akad.  Wien^  xxiv.-xxx. ;  numerous 
other  memoirs  are  cited  by  Mojsisovics  in  his  *  Dolomitriffe.  * 

'  '  Die  Dolomitriffe,'  p.  39. 


872  STRATIGRAPHICAL  GEOLOGY         BOOKTiPABTm 

whole  somewhat  sandy,  the  accumulation  of  limestone  there  having  been  freqaentlj 
interrupted  by  inroads  of  sand  or  silt  On  the  south  side  the  deposition  of  limestone 
and  dolomite  went  on  more  continuously,  though  interfered  with  occasionaUy  by  sub- 
marine volcanic  eruptions.  Some  of  the  dolomite  masses  may  have  been  coral-reefs ; 
Mojsisovics  even  believes  that  in  the  conglomeratic  portions  he  can  detect  traces  of  the 
breaker-action  by  which  the  reefs  were  ground  down,  while  the  thin  marls  were  deposited 
in  lagoons,  or  in  the  inner  channels  between  the  reefs  and  the  land.  But  it  is  speciaUy 
deserving  of  notice  that  corals  were  not  the  only  agents  in  the  accumulation  of  reef-like 
masses  in  this  region.  Alike  in  the  dolomites  and  the  massive  limestones  calcareous 
sea-algae  occur  so  abundantly  as  to  show  that  they  grew  up  into  wide  reefs,  which, 
judging  from  what  is  known  of  the  distribution  of  such  organisms  at  present,  show  that 
the  Tiiassic  sea  in  these  tracts  did  not  exceed  200  fathoms  in  depth.  Though  organisms 
of  higher  grade  are  often  associated  with  these  reef-building  plants,  they  occur  most 
frequently  in  the  thin-bedded  marls  and  shales  at  definite  horizons  in  the  series  of 
strata. 

Having  regard  to  the  lithology  and  palaeontology  of  the  Alpine  Trias,  Mojsisovics 
proposed  some  years  ago  to  regard  the  system  in  the  eastern  Alps  as  pointing  to  the 
existence  of  two  great  marine  **  provinces."  The  larger  of  these  lay  over  the  sites  of 
North  and  South  Tyrol,  Lombardy,  and  Carinthia,  and  stretched  far  to  the  east.  To  this 
area  the  able  Austrian  investigator  gave  the  name  of  the  "Mediterranean  province." 
To  the  other,  which  occupied  a  limited  tract  on  the  north-east  slopes  of  the  Austrian 
Alps,  extending  from  the  Salzkammergut  into  Hungary,  he  gave  the  designation  of 
"Juvavian  province"  (from  the  old  Roman  name  of  Salzburg).  Though  the  Triassic 
deposits  of  these  two  regions  were  geologically  contemporaneous,  they  enclose  remarkably 
different  assemblages  of  organic  remains,  insomuch  that  the  palaeontological  zones  which 
can  be  determined  in  the  one  have  not  been  found  to  hold  good  in  the  other.  In  no 
respect  is  this  independence  more  strongly  shown  than  in  the  great  contrast  presented 
by  the  Ammonites  of  the  two  areas.  The  Juvavian  province  has  yielded  a  Triassic 
cephalopodous  fauna  far  outrivalling  in  variety  and  interest  that  of  any  other  tract 
It  was  for  a  long  time  believed  that  the  cephalopods  were  quite  distinct  in  the  two 
regions,  PhylloceraSt  Didymit^'s^  Haloriks,  Tropitcs,  Hhahdocera^,  and  Coehloceras  being 
regarded  as  the  dominant  and  distinctive  genera  of  the  Juvavian  province,  while 
Lytoccras^  Sa{fcceras,  and  Ptychitcs  were  equally  characteristic  of  the  Mediterranean 
province.*  The  progress  of  research,  however,  has  shown  that  the  so-called  Juvavian 
province  can  no  longer  be  strictly  maintained,  for  the  type  of  rocks  and  fossils  on  which 
it  was  based  have  been  found  in  the  midst  of  the  Mediterranean.  Nevertheless  it 
remains  true  that  the  peculiar  lithological  and  pal.Tontological  features,  as  well  as  the 
complicated  structure,  of  the  district  of  the  Salzkammergut  have  up  to  the  present  time 
interposed  very  great  difficulties  in  the  way  of  the  institution  of  any  exact  comparison 
between  the  Triassic  succession  in  that  area  and  in  other  parts  of  the  Alpine  region. 
The  subjoined  table,  compiled  from  the  results  of  the  latest  researches,  shows  the  con- 
trasted gi'ouping  of  the  Triassic  formations  on  the  two  sides  of  the  eastern  Alps,  and 
their  distinction  from  those  of  the  German  inland  sea,  between  which  and  the  Alpine 
basins  there  seem  to  have  been  only  occasional  and  brief  intervals  of  connection  :  * — 

*  Mojsisovics  has  recently  modified  his  previously  published  opinions  regarding  the 
order  of  the  Triassic  formations  in  the  Salzkammergut,  Sitztu  Akad.  Wien,  1892,  p.  780. 
The  views  of  this  observer,  however,  regarding  the  succession  of  the  strata  are  not  every- 
where accepted  among  the  geologists  of  Austria.  For  a  recent  critique  on  this  subject  see 
A.  Bittner,  Jahrh.  k.  k.  Geol.  Beichsanst.  vol.  xlii.  (1892)  p.  387. 

'^  In  the  i)reparation  of  my  account  of  the  Alpine  Trias  I  have  been  greatly  aided  by 
Miss  M.  M.  Ogilvie,  whose  intimate  acquaintance  with  this  geological  system  in  the  eastern 
Alps  is  well  shown  in  her  paper  already  cited.  The  table  on  next  page  has  been  entirely 
drawn  up  by  her. 


TSTASSIO  SYSTEM 


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874  STRATIGRAPHICAL  GEOLOGY         book  vi  part  in 

1.  Banter. — The  base  of  the  Alpine  Trias  shades  down  into  the  Permian  fonnationi 
(Bellerophon  limestone,  Groden  sandstone),  and  consists  of  the  gronp  of  red  sandy 
micaceous  shales  known  as  the  Wcrfen  beds  (from  Werfen  in  the  Salzburg),  which  form 
a  tolerably  persistent  horizon.  Among  the  fossils  in  the  upper  part  are  NaiiceUa  costaUtf 
Turbo  rcct^costatus,  Trigonia  costcUay  Monotis  aurUay  and  the  ammonites  TirolUes 
{CeratUcs)  cassianuSy  Dalmatiniis  idrianus,  D,  muchianus,  Trachffeercts  Idecanum, 
Noriles  capHlciisis,  Some  of  these  organisms  occur  so  abundantly  as  to  form  entire 
beds.  Corals,  echinoderms,  and  brachiopods  (except  Lingula)  are  absent.  In  the  lower 
part  of  the  group  Monotis  Clarai  is  especially  abundant.  The  presence  here  of  Triffonia 
costatay  a  characteristic  form  of  the  German  Roth,  serves  to  mark  the  relation  of  the 
Werfen  beds  to  the  Triassic  series  of  the  German  area. 

2.  Muschelkalk. — It  is  above  the  position  of  the  Werfen  beds  that  the  Alpine 
Trias  begins  to  manifest  great  lithological  differences,  not  only  in  the  two  provinces  on 
the  northern  and  southern  sides  of  the  Alps,  but  even  within  the  confines  of  each  province. 
The  general  character  of  these  differences  is  expressed  in  the  foregoing  table.  Yet, 
with  some  notable  exceptions,  the  pala^ontological  zones  can  be  distinguished.  The 
lower  Muschelkalk  of  the  eastern  Alps  consists  in  its  inferior  portion  of  sedimentary 
deposits  which  are  largely  argillaceous,  while  the  upper  part  is  composed  of  limestones 
and  dolomites  arranged  in  lenticular  reef-like  masses.  The  lower  argillaceous  division 
varies  in  its  palaeontological  character.  Mojsisovics  distinguishes  three  facies,  the  lowest 
in  which  lamellibranchs  predominate  (Recoaro),  and  which  shows  a  close  litho- 
logical and  palreontological  relation  to  the  German  Muschelkalk,  followed  by  one  with 
brachiopods  and  land-plants,  and  that  by  a  third  with  cephalopods  (Dont,  Val  Infema 
and  Brags).  The  calcareous  group  sometimes  resembles  in  lithological  character  the 
German  Wellenkalk,  but  in  certain  places  it  assumes  the  aspect  of  reefs.  Among  the 
most  important  fossils  of  the  Alpine  Lower  Muschelkalk  some  are  common  to  this  stage 
in  Germany,  such  as  Spiriferina  Meiiizeliy  S,  hirstUa,  Khyiichonella  securtata^  Terebrci^la 
vulgaris,  T.  angusta,  Myophoria  vulgaris,  Pecten  dis^Ues,  Emerinus  graeiliSy  CeraMUs 
trijuklosiis.  But  there  remains  a  large  number  of  peculiar  forms,  especially  the  abundant 
ammonites  {Plychites,  Tnichyceras,  numerous  species,  Lytoctras).  The  Upper  Muschel- 
kalk is  generally  a  dark  grey  to  black  limestone,  but  sometimes  (Salzkammergut)  is  red 
and  like  a  marble.  Among  the  typical  fossils  are  Daonella  Sturi,  D.  parthancnsis, 
Orthoccras  campanile.  Nautilus  Pichleri,  Ptychitcs  gihhus,  Arccst^  BramantH,  jEgoctras 
mrgalodlHcus,  Ceratitcs  {Tra<:?iyccras)  trinodosns,  and  other  genera. 

3.  Noric  Stage. — It  was  at  the  close  of  the  deposition  of  the  Alpine  Muschelkalk 
and  the  befjinning  of  the  Noric  stage  that  the  two  great  biological  provinces  above 
referred  to  were  finally  established.  The  general  grouping  of  the  formations  in  each  area 
and  the  striking  difference  they  present  even  within  the  same  area  are  best  understood 
from  tlie  inspection  of  such  a  table  as  that  given  above.  On  the  southern  side  of  the 
Alps  two  groups  in  tliis  stage  have  been  recognised  :  (1)  the  Buchenstein  beds,  consisting 
of  flaggy  and  nodular  limestones,  with  hornstone  concretions.  These  strata  have  not  yet 
been  found  in  the  northern  Alps.  Among  their  fossils  are  Orthoccras  Bockhi,  Areestes 
trompiamts  and  other  species,  Ptychitcs  angusto-iunbilicatus,  Sageceras  Zsigmondyi, 
Lyf^eras,  cf.  tcengcnense,  Trachyceras  Curionii,  T,  Reitzi  and  other  species,  Spiriferina 
Menfzdi,  Damiclhi  Taramcllii,  and  other  species.  (2)  The  Wengen  beds  comprise  aU  the 
strata  lying  between  the  Buchenstein  l)eds  and  the  base  of  the  St.  Cassian  group.  Their 
most  important  material  consists  of  a  dark  sandstone  with  shaly  partings,  derived  chiefly 
from  volcanic  detritus.  In  South  Tvrol  and  in  Carinthia  sheets  of  lava  and  tuff  lie  at 
the  base  of  this  group,  and  thicken  out  round  the  centres  of  eruption.  With  these  inter- 
bedded  ifjneous  rocks  are  associated  bosses  and  dykes  of  augite- porphyry  and  melaphjrre. 
A  characteristic  feature  of  the  Wengen  beds  is  the  great  development  of  reefs  formed 
by  calcareous  algii'  {Gyroporella,  including  Diplopora),  and  built  up  into  enormous 
masses  of  limestone  and  dolomite  with  corals,  large  Naticas,'and  Chemnitzias.     Among 


SECT,  i  §  2  TRIASSIC  SYSTEM  875 

the  characteristic  fossils  of  the  Wengen  beds  are  Trachycercu  Arehelaus,  and  numerous 
other  species,  Arcestes  triderUinuSy  Pinacoeeras  daonicumf  Malobia  Lommeli,  with  in  some 
places  remains  of  land-plants — EquiMtiUs  arenaceus,  Calamites  arenac&uSy  Neuropteria 
several  species,  SagenopUris,  PecopUriSf  ThinnfehUa^  PUrophyllum^  TeeniopteriSf  VoUzia, 

4.  Carinthian  Stage.  — The  geographical  distribution  of  the  two  marine  provinces 
lasted  beyond  the  early  part  of  this  stage.  The  separation  between  these  areas  gradually 
disappeared,  and  some  of  their  peculiar  ammonites  began  to  migrate  from  the  one  territory 
to  the  other.  In  the  southern  area  Mojsisovics  has  noted  three  distinct  Carinthian 
groups :  (1)  the  St.  Cassianbeds,  consisting  of  brownish  calcareous  marls,  limestones,  and 
oolites.  This  group  has  long  been  celebrated  for  the  astonishing  abundance  and  variety 
of  its  organic  remains.  The  Echinoderms  are  particularly  prominent.  Abundant  also  are 
the  species  of  HcUohia  (Daanella)  {H,  casaiana  and  ff.  Richthofeni).  Corals  abound  in  the 
neighbourhood  of  the  dolomite-reefs,  and  the  coral  banks,  like  the  beds  of  echinoderms, 
can  be  traced  laterally  into  these  reefs.  The  St.  Cassian  beds  are  represented  in  other 
parts  of  the  Alps  by  fossiliferous  limestones  (Marmolata  and  Esino  limestones  in  South 
Tyrol  and  Lombardy,  Wetterstein  limestone  in  North  Tyrol)  and  nearly  unfossiliferous 
dolomites  (Schlern  dolomite  in  South  Tyrol,  **Erzfuhrende  Dolomit"  of  Carinthia)  of 
the  "  reef- type  "  of  Mojsisovics.  Out  of  the  large  series  of  fossils  the  following  may  be 
mentioned  here: — Trachyceras  aon,  species  of  Arcestes^  LobiteSf  Orthoeeras,  Nautilus, 
BactrUeSf  GervUlia  angusta,  Koiiinckina  Leanardi,  Rhynchonella  semiplecta,  Encrinus 
cassianus,  Peniacrinus  propinquus,  Cidaria  doraata.  (2)  The  Raibl  beds  mark  the  close 
of  the  separation  of  the  two  provinces,  for  they  range  from  the  one  into  the  other.  They 
consist  of  dark  bituminous  marly  strata,  with  lenticular  beds  and  thick  reef-like  masses 
of  limestone,  and  frequently  with  gypsum  and  rauchwacke.  Their  fauna,  distinguished 
by  the  large  number  of  littoral  lamellibranchs,  includes  Trigonia  Kefersteini,  Cardita 
Gumbeli,  Corhula  Rosthonii,  Halobia  rugosa,  GervUlia  bipartila,  Afcgalodus  carinthicunia, 
Chemnitzia eximia.  Nautilus  Wulfeni,  Tra^ckyceras  aonoides.  The  Limz  sandstones,  which 
belong  to  this  horizon,  have  yielded  numerous  land-plants  comprising  many  species  of 
Pterophy Hum  and  forms  of  EquiaetiteSy  CalamiteSy  NeuropteriSy  AlethoptcriSy  &c.  (3)  The 
beds  comprising  the  zone  of  Avicula  exilis  and  Turbo  solitarius  show  a  return  of  the 
dolomitic  condition  of  earlier  parts  of  the  system.  These  conditions  had  already  set  in 
during  the  deposition  of  the  Raibl  beds,  but  they  reached  their  full  development  during 
the  accumulation  of  the  next  group,  when  masses  of  dolomite  ranging  up  to  nearly  4000 
feet  in  thickness  were  laid  down.  This  group  of  rocks,  though  placed  by  Mojsisovics 
in  the  Carinthian  stage,  is  by  other  authors  considered  to  be  Rhaetic.  In  North  Tyrol 
it  is  known  as  the  Main  Dolomite  (Hauptdolomit),  in  the  Salzkammergut  as  the  lower 
part  of  the  Dachstein  limestone,  which  forms  an  important  feature  in  the  scenery  of  the 
district.  These  rocks  everywhere  present  a  great  contrast  to  the  strata  below  them  in 
their  poverty  of  organic  remains.  Some  of  their  most  prominent  fossils  are  casts  of 
Afcgalodus  {M.  GUmbeliy  M.  complanatuSy  M.  Mojsvdri,  &c.),  and  remains  of  calcareous 
algae  {Gyroporella).  The  bituminous  Seefeld  beds  of  the  North  Tyrol  have  yielded  many 
fishes  (SemionotuSy  LcpidotuSy  Pholidophorua)  and  remains  of  plants. 

Until  recently,  according  to  Mojsisovics,  the  order  of  superposition  of  the  rocks  in 
the  Hallstadt  area  was  misinterpreted.  He  now  believes  that  the  Hallstadt  marble 
does  not  form  a  continuous  mass  overlying  the  Zlambach  beds,  but  that  the  latter, 
instead  of  underlying  the  Hallstadt  rock,  actually  lie  within  it  He  has  grouped  a 
section  of  the  Hallstadt  series  as  a  separate  stage  under  the  name  of  *' Juvavian."  It 
consists  at  the  base  of  red  and  variegated  lenticular  seams  of  limestone  with  Sagenitea 
Giebeli.  Then  follow  red  lenticular  limestones  with  gasteropods  (zone  of  Cladiaeitea 
ruber).  It  is  here  that  the  Zlambach  beds  come  in  with  their  Ch4yristoceraa  Haxieri,  They 
are  succeeded  by  gi*ey  limestone  with  Pinacoceras  MdUmichiy  and  this  by  seams  of 
limestone  carrying  Cyrtoplcuritea  bicrenatys.^    This  whole  series,   comprising  several 

1  Mojsisovics,  Sitzb,  Akad.  IKt«n,  1892,  p.  769. 


876  STRATIGRAPHICAL  GEOLOGY         book  vi  part  in 


palffiontological  zones,  is  regarded  by  Mojsisovics  as  the  equivalent  in  time  of  the  Main 
Dolomite. 

5.  Rhaetic  Stage. — Two  distinct  facies  of  this  stage  are  developed  in  the  eastern 
Alps,  but  the  unity  of  the  deposits  over  the  whole  region  is  shown  by  the  presenee  of 
the  characteristic  Aricttla  contorta.  The  Kossen  beds  are  a  marly,  highly  fcenlifefoiig 
group  of  strata,  marking  probably  the  shallower  water,  while  the  upper  Dachstein  lime- 
stone into  which  they  merge  may  indicate  the  opener  sea.  Siiss  has  distinguished  a 
series  of  "facies"  in  this  gi'oup,  the  lowest  (Swabian)  marked  by  the  preponderanoe  of 
lamellibranchs,  the  next  (Carpathian)  by  the  abundance  of  Terebratvla  grtgaria  and 
Plicatula  iniusstriaia  ;  the  Hauptlithodendron-limestone — a  thick  mass  of  ooral  lime- 
stone ;  the  Kbssen  facies  includes  the  dark  brachiopod  limestones  with  shaly  partings, 
while  the  Salzburg  facies  is  recognised  by  the  prominence  of  its  cephalopods  {CharisUh 
ceras  Marshij  uEgoceriis  planorboides). 

The  Kossen  beds  are  most  fully  developed  in  the  northern  Alps,  more  particnlarly 
in  Bavarian  and  North  T}To1,  thinning  out  towards  Salzkammergut,  while  the  dolo- 
mitic  facies  of  Dachstein  limestone  predominates  in  the  southern  Alps,  the  fossiliferous 
marly  facies  only  appearing  in  the  Lombardy  Alps.  Tlie  occurrence  of  the  fossiliferous 
Rhflptic  beds  in  the  Alps  gave  not  only  the  first  clue  to  the  identity  in  time  of  the 
Triassic  l)ed8  in  Aljnne  and  extra-Alpine  regions,  but  it  has  proved  of  the  greatest 
imi)ortance  in  tracing  the  zonal  parallelism  of  the  Triassic  succession  within  the  Alps 
themselves.  As  has  been  said,  a  great  thickness  of  wholly  un fossiliferous  dolomitic  and 
gypsiferous  rock  sometimes  occurs  in  the  western  Alps,  and  it  would  be  impossible  to 
assign  a  Triassic  age  to  any  }>art  of  this  series  were  it  not  for  the  presence  of  well-known 
Rhii'tic  fossils  in  the  beds  immediately  succeeding  them.  Again,  the  same  fossils  give 
undoubted  evidence  of  the  gradual  submersion  of  the  island  of  older  crystalline  and 
Palaeozoic  rock  in  the  Triassic  sea  of  the  eastern  Alps.  Rhaetic  fossils  are  found  on  the 
Radstiidter  Taucr  and  on  the  Stubey  Mountains  in  the  central  chain  of  the  Alps. 

The  intrusive  volcanic  rocks  of  the  celebrated  districts  of  Predazzo  and  Monzoni  in 
South  Tyrol  are  referre<l  by  some  authors  to  Lower,  by  others  to  Upper  Triassic  time. 
At  Predazzo  tliere  is  a  core  of  orthoclase  porphyry  and  tourmaline  granite  with  an 
envelope  of  syenite,  by  which,  among  the  now  familiar  phenomena  of  contact-meta- 
morphism,  the  Triassic  limestones  have  been  in  places  converted  into  marble.  Similar 
phenomena  are  presented  at  Monzoni,  where  a  central  lx)ss  of  augite -syenite,  traversed 
by  veins  of  gabbro,  melaphyre,  &c.,  cuts  across  the  Triassic  strata  (fl/rte,  p.  604). 

The  Triassic  rocks  of  the  Alps  have  i>articipated  in  the  great  earth -movements  to 
which  this  chain  of  mountains  owes  its  structure,  and  they  consequently  present  remark- 
able cases  of  dislocation,  inversion,  and  even  of  metamorphism.  Thus  the  Triassic 
fonnations  of  the  Radstjidter  Tauer  in  the  Tyrol  cannot  be  se|)arated  from  the  calc-mica 
schist  of  that  district,  and  Professor  Siiss  regards  this  schist  as  an  altered  Triassic  lime- 
stone. ^ 

Spitzbergen. — Since  the  Alpine  type  of  the  Trias  has  been  recognised  as  that  of  the 
open  sea,  it  has  been  traced  far  and  wide  over  the  Old  World,  northwards  into  the 
Arctic  circle,  eastwards  across  Asia  to  Australasia,  and  along  the  eastern  borders  of  the 
Pacific  Ocean.  In  northern  Siberia,  at  the  mouth  of  the  River  Olenek,  and  in  Spitz- 
bergen, Triassic  strata  have  been  found  with  a  characteristic  marine  fauna,  including  the 
following  genera  of  cephalopods :  DinarUcSy  CeratUcs^  SihiriUSj  ProsphingittSj  Popano- 
crrns,  MmuyphyUitcSy  Xeiiodisctts,  Mcckoceras^  llungarites^  Ptychiles,  Pl^uroriauUius, 
Nfuttilus,  and  AtractUcs;  also  species  of  Psetidmtionoti^,  Oxyt^ma,  Avitula,  PecUn,  Gtr- 
tilh'a,  CiirdUa,  Lingula,  Spiriferina^  and  KfiynchonelJa^  together  with  remains  of  fish 
and  reptiles  {Acrodvs  spitzbcrgensis.  Ichthyosaurus  polaris^  I.  Nordenskioldii).^ 

^  Anzriger  Aknd.  UVen,  No.  xxiv.  20th  Nov.  1890. 

-  A.  E.  XordenskioM,  Gecl.  Mag.  1876,  p.  741  ;  A.  Bittner  and  A.  Teller,  Mhn.  Acad. 
St.  Petersfnni rg ,  vol.  xxxiii.  ;  Mojsisovics,  Verhandl.  k.  k.  OeoL  ReUhsanst.  1886,  No.  7. 


SECT,  i  §  2  TRIASSIC  SYSTEM  877 

Asia. — The  Trias  has  a  wide  extension  in  this  continent.  In  the  old  district  of  Mysia. 
Asia  Minor,  dark  shales  and  limestones  enclose  undoubted  Triassic  forms  such  as  ArccsUSy 
Nautilus,  and  Halobia.^  Strata  with  CeratUes  and  Orthoceras  occur  in  Beloochistan, 
and  in  the  Salt  Range  of  the  Puigab.  In  northern  Kashmir  and  western  Tibet  a  well- 
developed  succession  of  Triassic  formations  appears  among  the  Himalayan  ranges,  some- 
times exceeding  4000  feet  in  thickness.  It  contains  many  of  the  same  species  of  fossils 
as  occur  in  the  Alpine  Trias.  Some  of  its  forms  are  Ammonites  flortdus.  A,  diffusus 
Halobia  Lommeli,  Monotis  scUinaria^  Megalodus  triqueter.  The  researches  of  Mr 
Griesbach  have  added  much  to  our  materials  for  a  comparison  between  these  Himalayan 
Triassic  rocks  and  their  representatives  in  Europe.  At  the  base  of  these  formations  in 
the  Himalayan  regions  lies  a  group  of  strata,  the  Otoceras  beds,  with  a  cephalopodan  fauna 
poor  in  s|)ecies  but  rich  in  individuals  (XenodiscuSy  MeckoceraSy  Otoceras^  Prosphingites). 
These  are  followed  by  another  lower  Trias  member,  with  a  large  assemblage  of  cephalopods 
resembling  that  of  the  Ceratite  beds  of  the  Salt  Range,  which  are  regarded  by  Waagen  as 
homotaxial  with  the  Bunter  sandstone  of  Europe.  The  horizon  of  the  Muschelkalk  is 
represented  by  rocks  in  which  there  is  a  blending  of  the  palaeontological  characters  of 
the  Arctic  and  Mediterranean  types  of  this  formation.  Three  upper  Triassic  groups  have 
been  recognised.  Of  these  the  lowest,  consisting  of  black  Daonella  limestone,  contains 
forms  of  ArcesteSy  ErUojiioceras,  and  ArpadiUs,  the  middle  contains  small  ammonites 
of  the  genera  Sibirites,  HeraclUes,  and  Halorites,  while  the  highest  group  may  be 
compared  with  the  zone  of  TropUes  svhbulUUuSy  at  the  base  of  the  Carinthian 
stage  of  the  eastern  Alps.^  The  freshwater  Karharbari  beds,  near  the  base  of  the 
Gondwana  series  of  peninsular  India,  contain  a  Bunter  assemblage  of  plants, 
including  Voltzia  hUerophylla  and  Albertia  (near  A.  speciosa) ;  ^  also  several  cycads 
(Olossoza mites f  Zamia)  and  a  nimiber  of  ferns  (Neuropteris,  Oangamopteris,  GlossopteriSy 
Sagenopteris),  It  has  been  already  observed  that  some  of  these  types,  which  were 
believed  to  be  exclusively  Mesozoic,  occur  in  Australia  associated  with  a  Carboniferous 
Limestone  fauna  {ante,  p.  839).  The  Talchir  group  contains  boulder-beds  that  may 
indicate  glacial  action  in  Triassic  or  Permian  time.  The  Damuda  group,  which 
comprises  nearly  all  the  coal-fields  of  the  Indian  peninsula,  contains  a  remarkable  flora, 
distinguished  by  the  abundance  of  ferns  {GlossopteriSy  Oangamopteris,  SagenopteriSy 
Tasniopteris,  &c.),  and  by  its  mingled  PalsBOzoic  and  Mesozoic  characters.  The  Panchet 
group,  crowning  the  lower  Gondwdna  system,  is  composed  of  sandstones  with  bands  of 
red  clay,  the  whole  having  a  thickness  of  1800  feet,  and  yielding  the  Rh«tic  ferns 
Pecopteris  concinna  and  Cyclopteris  pachyrhachis,  the  Triassic  and  Rhaetic  genus  of  horse- 
tail Schizomura ;  the  labyrinthodonts  Oonioglyptus  and  Paehygoniay  allied  to  Triassic 
forms,  together  with  Dicynodoriy  Epicampodon,  kQ.* 

Australia. — In  New  South  Wales  a  group  of  yellowish -white  sandstones  (Hawkes- 
bury  beds)  about  1000  feet  thick  lies  unconformably  upon  the  coal -bearing  strata  referred 
to  the  Permian  period.  This  group  forms  the  picturesque  cliffs  around  the  coast  of  Port 
Jackson,  and  has  furnished  the  building-stone  for  the  principal  public  buildings  in 
Sydney.  It  has  yielded  a  large  number  of  plants  [Phyllothecay  SphenopteriSy  Neuropteris, 
Thinnfeldia — common,  Odo^dcpteris,  Alethopteris,  MacrotasniopteriSf  Podozamites,  and 
Walchia) ;  also  the  fishes  Palaoniscus  antipodeuSj  Myriolepis  Clarkei,  Cleithrolepis 
granulaiusy  and  labyrinthodonts,  but  no  marine  shells.  At  Gosford,  near  the  base  of 
the  Hawkcsbury  beds,  in  a  thin  seam  of  grey  shale,  a  large  collection  of  fishes  has  been 
obtained.  The  animals  seem  to  have  lived  in  some  land-locked  lake  or  estuary,  and 
to  have  been  killed  in  large  numbers  by  the  sudden  silting  up  of  the  water  with 

1  Neumayr,  SUzb.  Akad,  Wien,  1887. 

2  Mojsisovics,  SUzb.  Akad,  Wien,  ci.  (1892)  p.  372. 

'  Medlicott  and  Blanford's  'Geology  of  India,'  pp.  xlvi.  114.     C.  L.  Griesbach,  Mem, 
Oeol,  Sure.  India,  vol.  xxiii. 
*  'Geology  of  India,'  p.  131. 


878  STRATIGRAPHICAL  GEOLOGY         book  vi  PABf  m 


coarse  sand  aud  gravel.  They  belong  to  at  least  six  genera,  four  of  which  ooonr  in  tbe 
European  Trias.  Of  these  four,  two  {Dietyopyge  and  Semionotiu)  are  typically  Triaane, 
whUe  the  third  {Bel&norhynchus)  commonly  ranges  to  the  Lias,  and  the  fourth  (Pkolido- 
phorus)  is  best  developed  in  the  Jurassic  system.  The  iifth  genus  {Fristiaamus)  is  new, 
but  scarcely  higher  in  rank  than  SemionotitSf  while  the  sixth  {Cleithrolepis)  has  only  been 
definitely  recognised  in  the  Stromberg  beds  of  South  Africa,  the  age  of  which  may  be 
Triassic  or  Lower  Jurassic.^  On  the  Hawkesbury  sandstones,  perhaps  unconformably, 
lies  a  group  of  shales  (Wianamatta  beds)  with  abundant  plants,  chiefly  ferns,  sometimes 
aggregated  into  thin  seams  of  coal  {ThinnfMia,  Odojitopteris,  Pecqpieri$,  MaeroUati' 
optcrisy  Phyllotliecaf  and  Unio  and  Unionella).  These  two  groups  of  strata  are  with 
some  hesitation  referable  to  the  Trias.* 

New  Zealand.  — Under  the  name  of  Trias,  Sir  J.  Hector  groups  a  great  thickness  of 
strata  divisible  into  three  series.  (1)  The  Oreti  series — a  thick  mass  of  green  and  grey 
tuff-like  sandstones  and  breccias,  with  a  remarkable  conglomerate  (50  to  400  feet  thick) 
containing  boulders  of  crystalline  rocks  sometimes  5  feet  in  diameter,  found  both  in  the 
North  and  South  Islands  ;  fossils,  chiefly  Permian  and  Triassic,  but  with  a  PcrUacriwu 
like  a  Jurassic  species.  (2)  Above  these  beds  lies  the  Wairoa  series,  containing  Monatit 
saliiutrla,  Halobia  Lomvicliy  &c.,  and  also  plants,  as  Daminarcu,  Olossopteris,  ZamUu, 
kc.  (3)  The  Otapiri  scries,  which,  from  the  commingling  of  fossils  nearly  allied  to 
Jurassic  species  with  others  which  are  Triassic  and  some  even  Permian,  and  from  the 
presence  of  many  forms  identical  with  those  of  the  Rhsetic  formations  of  the  Alps,  is 
assigned  to  the  Upper  Trias  or  Rhsetic  division.^ 

Africa. — In  South  Africa  the  "Karoo  beds"  spread  over  a  wide  area  of  country, 
consisting  of  nearly  horizontal  incoherent  sandy  materials,  from  which  the  remarkable 
assemblage  of  amphibian  and  reptilian  remains  already  referred  to  has  been  obtained. 
The  similarity  of  the  fossils  in  these  rocks  and  in  those  which  are  assigned  to  the 
Triassic  series  in  India  and  Australia  deserves  to  be  specially  remarked. 

North  America. — Kocks  which  are  regarded  as  equivalent  to  the  Euro^iean  Trias 
cover  a  large  area  in  North  America.  On  the  Atlantic  coast,  they  are  found  in  Prince 
Edward's  Island,  New  Bmnswick,  and  Nova  Scotia  ;  in  Connecticut,  New  York,  Penn- 
sylvania, and  North  Carolina ;  in  Honduras  and  along  the  chain  of  the  Andes  into  Brazil 
aud  the  Argentine  Republic.  Spreading  also  over  an  enomious  extent  of  the  western 
territories,  they  cross  the  Rocky  Mountains  into  Califoraia  and  British  Columbia.  They 
consist  mainly  of  red  sandstones,  passing  sometimes  into  conglomerates,  and  often 
including  shales  and  impure  limestones.  But  an  important  distinction  may  be  dravm 
between  the  system  as  develoi)ed  in  the  eastern  and  central  parts  of  the  continent,  on 
the  one  hand,  aud  along  the  Pacific  slope  on  the  other.  In  the  former  wide  region,  the 
rocks,  evidently  laid  down  in  inland  basins,  like  those  of  the  same  period  in  central 
Europe,  are  remarkably  barren  of  organic  remains.  Their  fossil  contents  include  i*emains 
of  terrestrial  vegetation,  with  footprints  and  other  traces  of  reptilian  life,  but  with 
hardly  any  indications  of  the  presence  of  the  sea.  This  is  the  German  type  of  the 
system. 

The  fossil  plants  of  the  Triassic  rocks  in  the  valley  of  the  Connecticut  and  New 
Jersey  present  a  general  facies  like  that  of  the  European  Triassic  flora.  Among  them 
are  horse-tails  {Eqicisctum,  Schizoncura),  cycads  (Pierophyllum  (some  European  species), 
Zamites^  Otozamites,  Spheiwzaynites,  Nilssoniu  polymorplia^  Dioonitcs),  ferns  {Pecopteris^ 


^  A.  S.  Woodward,  Mem.  Ged.  Surv.  iV..S.   WalcSj  PalA  ontology ^  No.  4  (1890),  p.  54. 

-  C.  S.  Wilkinson,  *  Notes  ou  Geology  of  New  South  Wales,'  Sydney,  1882,  p.  53. 
O.  F%istmautel,  Mem.  Oeol.  Sarv.  N.8.  Wales,  PalAontoiogy,  No.  3  (1890)  ;  R.  Etheridge 
juu.  op.  cit.  No.  1  (1888). 

^  *Haudl)Ook  of  New  Zealand,'  p.  33.  F.  W.  Hutton,  Quart.  Jounu  Oeol,  JSoc,  (1885) 
p.  202. 


SECT,  ii  §  1  JURASSIC  SYSTEM  879 

NeiiropUris,  TaBniopteris,  ClathrqpUris)  and  conifers  {CheiroUpis),^  In  Virginia,  where 
two  distinct  Mesozoic  floras  have  been  preserved,  the  older  appears  to  be  not  more 
ancient  than  the  Rhsetic  stage.  So  abundant  is  the  vegetable  matter  in  the  sandy 
strata  of  the  series  as  to  form  seams  of  workable  coal,  one  of  which  is  sometimes  26 
feet  thick.  The  plants  include  species  of  Equisetum^  Schizomura^  IfacrotaniopteriSf 
AcrostichUes,  CladophlehiSf  LonchopUriSf  ClathropUriSy  PUrophyllum^  Ctenophyllumy 
PodozamiteSf  OycaditeSy  Zamiostrobus,  Baiera,  CheiroUpis,  kc.  Again  in  North  Carolina 
a  coal-bearing  formation  occurs  with  a  similar  flora,  41  per  cent  of  the  plants  being  also 
found  in  Virginia.* 

The  fauna  of  the  North  American  Triassic  rocks  is  remarkable  chiefly  for  the  num- 
ber and  variety  of  its  vertebrates.  The  labyrinthodonta  are  represented  by  footprints, 
from  which  upwards  of  fifty  species  have  been  described.  Saurian  footprints  have  like- 
wise been  recognised  ;  in  a  few  cases  their  bones  also  have  been  found.  Some  of  the 
vertebrates  had  bird -like  characteristics,  among  others  that  of  three- toed  hind  feet, 
which  produced  impressions  exactly  like  those  of  birds  (p.  864).  But,  as  already 
remarked,  it  is  by  no  means  certain  that  what  have  been  described  as  "  omithichnites  " 
were  not  really  made  by  deinosaurs.  The  small  insectivorous  marsupial  {DromatJurium) 
above  referred  to,  found  in  the  Trias  of  North  Carolina,  is  the  oldest  American  mammal 
yet  known. 

On  the  Pacific  slope,  however,  a  very  different  development  of  the  Trias  occurs.  The 
Alpine  or  pelagic  type  of  the  system  is  here  seen.  The  strata  are  estimated  to  attain  a 
thickness  of  sometimes  as  much  as  14,000  or  15,000  feet.  Like  the  Alpine  formations, 
they  include  a  mingling  of  such  Palaeozoic  genera  as  Spirifer^  Orthoceras,  and  OonicUiteSy 
\\ith  characteristically  Secondary  forms  as  ammonites  {CeratiUs  Haidingerij  Ammonites 
aiisseanuSj  kc.)  and  bivalves  of  the  genera  Halobia,  Monotis,  Myophoria,  kc. 

Section  ii.  Jurassic. 

This  great  series  of  fossiliferous  rocks,  first  recognised  by  William 
Smith  in  the  geological  series  in  England,  received  originally  the  name 
of  "  Oolitic "  from  the  frequent  and  characteristic  oolitic  structures  of 
many  of  its  limestones.  Lithological  names  being,  however,  objection- 
able, the  term  "Jurassic,"  applied  by  the  geologists  of  France  and 
Switzerland  to  the  great  development  of  the  rocks  among  the  Jura 
Mountains,  has  now  been  universally  adopted  to  embrace  both  Lias  and 
Oolites. 

§  1.  General  Characters. 

Jurassic  rocks  have  been  recognised  over  a  large  part  of  the  world. 
But  they  do  not  present  that  general  uniformity  of  lithological  character 
so  marked  among  the  Palaeozoic  systems.  The  suite  of  rocks  changes  as 
it  passes  from  England  across  France,  and  is  replaced  by  a  distinctly 
difterent  type  in  Northern  Grermany,  and  by  another  in  the  Alps.  If  we 
trace  the  system  farther  into  the  Old  World  we  find  it  presenting  still 
another  aspect  in  north-western  India,  while  in  America  the  meagre 
representatives  of  the  European  development  have  again  a  facies  of  their 

^  J.  S.  Newberry,  Monographs  of  U,S,  Oeol.  Survey,  vol.  xiv.  (1888)  and  Anier.  Joum, 
Sci,    xxxvi.  (1888)  p.  342. 

^  W.  M.  Fontaine,  Monogr.  U.S.  Geol.  Surv.  vol.  vi.  (1883).  The  youuger  Mesozoic 
flora  of  Virginia  is  probably  Neocomian  {pottea,  p.  923). 


STRATIGBAPHICAL  GEOLOGY 


BOOK  TI  PABT  HI 


own.     Hence  no  generally  applicable  petrographical  choracten  can  be 
assigned  to  this  part  of  tbe  geological  record. 


>,  Llndl.  siHi  Huti 


JuraiMlc  F<m>  (Ijymti  Oolite). 
:  i,  TienloptariM  miioT,  Llndl.  ind  Hatt.  (t);  t,  PccoptcA 
iiicl  nuig.) ;  d,  Phlrbopleria  polypodioidw.  BnmgiL  <iul.  ■!» 


The  flora  of  the  Jurassic  period,  so  far  as  known  to  us,  was 
essentially  gymnospennous.^  The  Palieo- 
zoic  forms  of  vegetation  traceable  Up  to  the 
close  of  the  Permian  system  are  here 
absent.  Equisetums,  so  common  in  the 
Trias,  are  still  abundant,  one  of  them 
{E.  aTeitaceam)  attaining  gigantic  propor- 
tions. Fenis  likewise  continue  plentiful, 
some  of  the  chief  genera  being  AUlhupUris, 
SpheiwpUru,  I'hkbiipteris,  Oleandrtdtum,  and 
r«nwp/«rM  (Figs.  381,  382).  The  cycads 
(Fig.  3S3),  however,  are  the  dominant 
forms,  in  species  of  ZamiUs,  PftrophyUuiiif 
AnomoxamiUs,  NUmonta  (^PUrozamiUs),  Dto- 
onitef,  Podosamiiti,  Sj^hetwinimiUs,  Glossoza- 
miies,  OtozamiUs,  CycadiUs,  Bvddandia 
(Ctutkraria),  BemetlUfs,  ManitUia  {CycadUes 
and  CycadiAdea),  Zamiostrobus  (fiycadeostrobus), 
Beania,  Cffeadospadix,  Cycadinoearptis.  ffU- 
luiiiistmui  is  by  some  botanists  placed  with 

'■''"  the  cycada,  by  othei-s  with  the  dicotyledons 
or  ^vith  the  monocotyledons.     Conifera  also 

c  llora  of  Britnia,  up  to  the  tap  ot  the  Portluidiui  itage. 


BBCT.  ii  S  I  JURASSIC  SYSTEM  881 

are  found  in  some  numberB,  particularly  Araucarians  of  the  genera 
Ptuhjfphyllum  (fVitlt-Aia)  and  Araucaria ;  also  Pintles,  Pence,  Braehyphyllum, 
and  Tkiiyites.  This  flora  appears  to  have  flourished  luxuriantly  even  as 
fjir  north  ae  Spitzbergen,  where  the  large  number  of  cycads  givea  an 
almost  tropical  aspect  to  the  JurasBic  vegetation  of  this  Arctic  island.' 

The  Jurassic  fauna  ^  presents  a  far  more  varied  aspect  than  that 
of  any  of  the  preceding  syBt«ms.  Owing  to  the  intercalation  of  fresh- 
water, and  sometimes  even  terrestrial,  deposits  among  the  marine  forma- 
tions, traces  of  the  life  of  the  lakes  and  rivers,  as  well  as  of  the  land 
itself,  have  been  to  Bome  extent  embalmed,  besides  the  preponderant 
marine  forms.  The  conditions  of  sedimentation  have  likewise  been 
favourable  for  the  preservation  of  a  succession  of  varied  phases  of  marine 


Fig.  S83.-.Junulc  Cf ca<l>  (Lower  Ooliln), 

'I.  WIllUiniHoniiL  {Zimli}  i^gu,  Can  (1) ;  t>,  Otoainitnt  linmilituii,  Undl.  ind  Uut(.  (i); 

c.  Will  lam  Kiiis  hutiiU,  Bun.  (nut.  niu  and  nuK.) 

life.  Professor  Phillips  directed  attention  to  the  remarkable  ternary 
arrangement  of  the  English  Jurassic  series.^  Argillaceous  sediments  are 
there  succeeded  by  arenaceous,  and  these  by  calcareous,  after  which  the 
argillaceous  once  more  recur.  These  changes  are  more  or  less  local  in 
their  occurrence,  but  five  repetitions  of  the  succession  are  to  be  traced 
from  the  top  of  the  lias  to  the  top  of  the  Portlandian  stage.  Such  an 
alternation  of  sediments  points  to  interrupted  depression  of  the  sea- 
bottom.*     It  permitted  the  growth  and  preservation  of  different  kinds  of 

comprises  between  60  and  70  genen  uid  ^bout  SCO  sj>«eies-~daiibtleu  ■  mere  fngment  of 
the  whale  ttoni  of  the  period. 

1  O.  Heer,  K.  Snenak.   Yet.  Akad.  Bandl.  liv.  No.  5,  p.  I. 

'  The  total  Jomaslc  fanoB  ol  Britain  u|i  to  the  lop  of  the  PartUodian  stage  was 
aatimatcd  in  1882  to  incluile  450  gunera  aod  4297  species,  which  is  likewise  but  a  small 
proportion  of  the  whole  original  fauna.      Etheridge,  <J.  J.  Otel.  Soc.  1882,  Addreiw. 

»  'Geolog;  of  Oifordshire,'  *c.  p.  893.  *  Aitfe,  p.  621. 

3  L 


STRATIGRAPHICAL  GEOLOGY 


BOOK  TI  FAST  HI 


marine  organisms  in  succession  over  the  same  areas, — at  one  time  sand- 
banks, followed  by  a  growth  of  corals,  with  abundant  sea-urchins  and 


GoMf. :  b,  MonUlTBltU  dixpar,  l^iill,  ;  c.  CoiuoMrli  tmdDitu,  H 


shells,  and  then  hy  an  inroad  of  fine  mud,  which  destroyed  the  corals, 
but  in  which,  as  it  sank  to  the  bottom  the  abundant  ceph&lopods  and 
other  mollusks  of  the  time  were  admirably  preserved 


IB  bsultirnnnlH,  Gold 


Fig.  >M.— Liu  CrJnoldi. 


A  characteristic  feature  of  the  Jurassic  fauna  is  the  abundance  of  its 
beds  or  banks  of  coral.     During  the  time  of  the  Corallian  formation,  in 


8ECT.  ii  §  I  JURASSIC  SYSTEM  883 

[wrticular,  the  greater  part  of  Europe  appears  to  have  been  submerged 
beneath  a  coral  sea.  Stretching  through  England  from  Dorsetshire  to 
Yorkshire,  those  coral  accumulations  have  been  traced  across  the  Con- 
tinent from  Normandy  to  the  Mediterranean,  over  the  east  of  France, 
through  the  whole  length  of  the  Jura  Mountains,  and  along  the 
flank  of  the  Swabian  Alps.  The  corals  belonged  to  the  genera  Isaslrxa, 
Thamnastrsai,  T}ieeosmilia,  Calavwphyllia,  Monllivallia,  &c.  (Fig.  384),  In 
the  Jurassic  seas  generally  echinoderms  were  abundant,  particularly 
crinoida  of  the  genera  Peniacrinus,  Exlracrimu  (Fig.  385),  and  Apiocrinus. 
Among  these  the  multiplication  of  identical  or  nearly  identical  parts 
reaches  a  climax  in  the  Extracrinus  hiareus,  which  is  estimated  to  have 
possessed  no  fewer  than  600,000  distinct  ossicles.  There  were  likewise 
several  forms  of  star-fishes,  but  it  is  in  the  great  profusion  of  echinoids 
that  the  echinoderms  now  begin  to  be  distinguished 
Among  these  the  genera  Acruiolmia,  Cidaris  (Fig 
386),  Uemicidajis,  JHeJiitiobrissus,  Hemipedina,  Pseudo  ^ 
diadema,  Clypeus,  Pygaster,  and  Pygurus  were  con  1b3e3es5kS' 
spicuous.  Polyzoa  of  creeping,  foliacoous  and 
dendroid  types  abound  on  many  horizons  in  the 
Jurassic  system.  They  include  some  extinct  forms,  f*s-  aso.— JurMsLc  urthin. 
but  also  some  {Diastopora,  Aledo)  which  have  aur-  Cfi^n.  norigemna,  Phiii. 
vived  to  the  jn-esent  time.     They  occur  plentifully  "" 

in  the  Pea-grit  beds  of  the  Inferior  Oolite  near  Cheltenham,  and 
Forest  Marble  near  Batb,  and  still  more  abundantly  near  Metz  and 
near  Caen.'  The  bracbiopods  continue  to  decrease  in  importance 
compared  to  the  prominence  they  enjoyed  in  Paleeozoic  time.  So  far 
as  known,  they  are  chiefly  species  of  Jtliyiic/wmlla  and  lerebratvla  (Fig. 
387).     The  last  of  the  ancient  group  of  Spiri/ers  (Spiri/erimi)  and  of  the 


Pig.  Ml.-Oolltlc  Bni-hlopada. 

a,  Rliyni^hoaFlU  Bpinou,  Bchloth.  (1),  Loner  OnliU ;  b.  Terebntula  PbiUlpiil.  Unr.  (i), 

Lower  OuUU ;  e,  UljynchonclU  plnxui',  itueoi.  Middle  Oolite. 

genus  Leplxna  (Koaitickella,  Fig.  388)  disappear  in  the  Lias,  while  fFald- 
heimia,  a  still  living  genus,  now  takes  its  place.  Among  the  lamellibranchs 
(Figs.  389-392)  some  of  the  more  abundant  genera  are  Aoicula,  Pseudo- 
moiiutis,  Aucelta,  Posidonomya,  Gtnnllia,  Odrea,  Grypkxa,  Ezogyra,  Lima, 
Pecten,  Pinna,  AslarU,  Cardinia,  Cardivm,  Gresstya,  Hippopodium,  Modida, 

'  F.  D.  LoDge,  Geol.  Mag.  ISSl,  p.  23.     The  genus  AUcto  Hems  to  nnge  bock  Co  Lower 
SUniian  timcB. 


884  8TEATIGRAPHICAL  GEOLOGY         BooKviFAWni 

Myaciien,  Ci/prina,  Isocardia,  Phdadomya,  G^monij/a,  and  Trigonia.  Some 
of  these  genera,  particularly  the  tribe  of  oysters,  are  specially^  character- 
istic :  GrypkaM,  for  example,  occurring  in  such  numbers  in  some  of  die 
Lias  iimeetODes  as  to  suggest  for  these  strata  the  name  of  "  Gryphite 
Limestone,"  and  again  in  the  so-called  "Gryphite  Grit"  of  the  Inferior 
Oolite.  Different  species  of  Tii^imia,^  a  genus  now  restricted  to  the 
Australian  seas,  are  likewise  distinctive  of  horizons  in  the  middle  and 
upper  part  of  the  system.  Many  of  the  most  abundant  gasteropoda  (Fig. 
393)  belong  to  still  living  genera,  as  I'Uurolomaria,  Cerilhium,  and  Natva. 
But  the  most  important  element  in  the  molluscan  fauna  was  undoubtedly 
supplied  by  tlie  cephalopods.  In  paiticular,  the  tetrabranchiate  tribe  of 
Ammonites  attained  an  extraordinary  exuberance,  both  in  number  of 
individuals  and  in  variety  of  form  (see  Figs.  405-409).  These  organisms 
possess  a  great  im|)ortance  to  the  geologist,  for  their  limited  vertical 
runge  makes  them  extremely  valuable  in  marking  successive  life-zones. 
The  whole  Jurassic  system  has  been  divided  into  a  series  of  platforms. 


ciich  chai-acterised  by  some  predominant  species  or  group  of  Ammonites. 
The  ammonoid  families  which  had  previously  existed  seem  to  have  in  great 
measure  died  out,  and  a  new  and  still  richer  series  took  their  place  al 
the  close  of  the  Triassic  periotl.  The  old  comprehensive  genus  Ammomlff 
has  now  been  broken  up  into  many  families  and  genera.  In  the  older  part 
of  the  Jurassic  system  the  genera  AriHilfs,  ^Egoetras,  Amaltheus,  Lytocmxx, 
Phylloi-rriiA,  and  Slfphmoreraa  are  characteristic.  Higher  up,  besides  some 
of  these  genera,  we  find  Coxmoceras,  Harjiom'itf,  and  Aspidoffras,  and  in  the 
upper  parts  I'erisphiiirfrs  and  Oppellia.  The  dibranchiate  division  was 
likewise  represented  by  species  of  cuttle-fish  (Tfudopnis,  Bflotfvthis,  Septa. 
but  particularly  Bflemuitrs,  Fig.  394).  The  Belemnites  are  the  pre- 
ponderating type,  and  like  the  Ammonites,  though  in  a  less  degree, 
their  specific  forms  serve  to  mark  life-zones. 

No  contrast  can  be  more  marked  than  between  the  crustacean  fauna 

'  Tliic  genuB  afforih  an  iiiatructlve  eiiim])le  of  the  remarkable  eliangea  of  form  wliich 
same  genera  of  shells  have  undergone.  See  Ljcett's  idonogrsph  on  Trigonia,  Palteoxto- 
ffraph.  Sifc. 


JUSASSIC  SYSTEM 


of  the  Jurassic  and  that  of  the  older  systems.    The  ancient  trilobites  and 
eiirypterids,  as  remarked  by  Phillips,  are  here  replaced  by  tribes  of  long- 


tailed    ten-footed   lobsters  and   prawns,  and  of   representatives  of  our 
modern  crabs  (jEger,  Eryon).^ 

Here  and  there,  particularly  in  the  Jurassic  series  of  England  and 

'  For  an  accaunt  of  the  Jurassic  decapods  at  North  Germiny  fee  O.  Krause,  ZeUtch. 
Dttilmh.  (ienl.  Ou.  1891,  p.  171. 


686  STBATIGRAPHICAL  GEOLOGY         BOOKTipABfm 

Switzerland,  thin  bands  occur  containing  the  remains  of  termtrial 
ineecta  (Fig.  395).  The  neuropMrous  forms  predominate,  incladiog  renuuns 
of  dragon-flies  and  mayflies.  There  are  also  cockroaches  and  graesht^ipen. 
The  elytra  and  other  remains  of  numerous  beetles  have  been  obtuned 
belonging  to  still  familiar  types  (Curculioittdx,  Elateridx,  MdoUmihidm). 
A  wing  (Palxonlina  oolUica)  disinterred  from  the  Stoneafield  Slate  im 


(HyCilus)  Bowerbyin*,  D'Orb.  (1). 


originally  believed  to  be  the  oldest  known  trace  of  a  butterfly,  but  it  is 
now  considered  to  bolong  to  the  hemiptera.  A  few  dipterous  insects 
have  been  detected  even  as  low  down  as  the  Lias  towards  the  base  of  the 
system.^ 

In  no  department  of  the  animal  kingdom  was  the  advent  of  Mesozoic 
time  more  marked  than  among  the  fishes.  The  Palfeozoic  types,  with 
their  heterocercal   tails,  nearly  died  out     The  sharks  and  rays  were  well 


ie  lAnielllbnnchg. 


represented  by  species  of  Acrodus  and  IIyhodu.%  while  the  ganoids  appeared 
in  numerous,  mostly  homocercal  genera,  such  as  Daptdius,  ^ckmodw, 
McKodtm,  Gyrodus,  LejAdotus,  Pk<^idophorvs,  Fackydianrms,  Catums,  lq>l(h 
Ifpis,  Megalunit!.^ 


■  A.  Cl.  Butler,  Gfd.  Mag,  i.  (1873)  p.  2  ;  1 
?  '/eol.  San.  No.  71  (1891).  p.  175.  and  auth 
-  For  a  list  of  Liaasic  flshea,  see  memoir  by  H.  K  Soviv^e, 


r.  (1874)  p.  446.     Scndder,  BM, 


JURASSIC  SYSTEM 


The  most  impreaaive  feature  in  the  life  of  the  Jurassic  period  was  the 
abundance  and  variety  of  the  reptilian  forms.  Meeozoic  time,  as  ah-eady 
remarked,  has  been  termed  the  "Age  of  Reptiles,"  and  it  was  especially 
during  the  Jurassic  period  that  the  maximum  development  of  reptilian 


FIr.  S»2.— IIpp«r  C 
n  (Ontna)  vlrgnlt,  D'Orb. ;  b,  Oitna  deltoidea,  Sby.  (1) ;  < 

rUtiilura,  Sbf.  ({);  t,  Trtgtmti  glbbnu,  ftby.  (f);/,  C 

types,  with  the  final  disappearance  of  the  ancient  order  of  labyrintho- 
donts  and  the  rise  and  growth  of  new  orders  of  reptiles  which  have  long 
since  been  extinct,  was  reached.  The  first  true  turtles  seem  to  have 
made   their   appearance   during  this   period.     Numerous  fragments  of 


lacertilians  have  been  obtained  The  bones  of  various  crocodilian  genera 
occur,  such  as  Tdeosaurus,  SleneosaUTVS,  Mysinosaunu,  and  Goniopholis. 
Teleosaurus,  found  in  the  Yorkshire  Lias  and  the  Stonesfield  Slate,  was  a 
true  carnivorous  crocodile,  measuring  about  18  feet  in  length,  and  ii 
judged  by  Phillips  to  have  been  in  the  habit  of  venturing  more  freely 
to  sea  than  the  gavial  of  the  Ganges  or  the  crocodile  of  the  Nile.     CM 


STRATIGBAPHICAL  GEOLOGY 


BOOK  TI  PAST  in 


the  long-extinct  reptilian  types,  one  of  the  moat  remarkable  was  that  of 
the  enaliosaiiTB  or  sea-lizards.     One  of  these,  the  Ichthi/osaunig  (Fig.  396,  a), 


was  a  creature  with  a  fish-like  body,  two  pairs  of  strong  s^t'imming 
paddles,  probably  a  vertical  tail-fin,  and  a  head  joined  to  the  body 
without  any  distinct  neck,  but  furnished  with  two  large  eyes,  having  a 


ring  of  bony  plates  round  the  eyeball,  and  with  teeth  that  had  no 
distinct  sockets.  Some  of  the  skeletons  of  this  creature  exceed  24  feet 
in  length.     Contemporaneous  with  it  was  the  Plesiosaums  (Fig.  396,  b), 


c:r.ii§l 


JURASSIC  SYSTEM 


distinguished  by  its  long  neck,  the  larger  size  of  its  paddles,  tbe  smallel- 


is 


size  of  its  head,  and  the  insertion  of  ite  teeth  in  special  sockets,  as 


in   tho   higher 


Fig.  3B7.— Jurassic  PI 
8di>liOKnathus(PtFroiliirtyLut)cn 
Ooliit  (Middle  Oolitd). 

hollow  and  air-filled.^ 


STRATIGRAPHICAL  GEOLOGY         BOOETiPAKiin 


These  creatures  seem  to  have  haunted  the 
Bballow  Liassic  seas,  cuid,  TaiTing  in 
species  with  the  ages,  to  have  aurviTed 
till  towards  the  close  of  Meaonnc 
time.'  The  genus  Pliosauras,  rel&ted 
to  the  last-named,  was  distinguishable 
from  it  by  the  shortness  of  its  neck 
and  the  proportionately  large  siee  of 
its  head.  Another  extraordinary-  rep- 
tilian type  was  that  of  the  pteroeaurians 
or  flying  reptiles,  which  were  likewise 
peculiar  to  Mesozoic  time.  These  huge, 
winged,  bat  -  like  creatures  had  large 
heads,  teeth  in  distinct  aockete,  eyes  like 
the  IckUiyosaurus,  one  finger  of  each  fore- 
foot prolonged  to  a  great  length,  for  the 
purpose  of  supporting  a  membrane  for 
flight,  and  bones,  like  those  of  birds, 
The  best-known  genus,  Pterodactylus  {Seaphogruiihvs, 


'  On  the  rliatribution  of  the  PUsiosaars  see  a  table  by  O.  F.  Wliidboroe,  Q.  J.  OeoL  Soc 
1881,  p.  480. 

'  Sea  Mnriih  oh  wings  of  Pterodaotyles,  Amfr.  /oum.  Set.  April  1882.  The  remirk- 
ahle  speeinieu  of  RhampharkyncKvn  {R.  ilSmteri]  from  the  Solenhofen  Slate,  deacribcil 
b]'  this  Buthor  (Figs.  3S9,  400,  401),  possessed  a  long  tail,  tbe  last  siiteeD  ahort  Tertft>TC 
of  which  supported  a  peculiar  caudal  membrane  which,  kept  in  an  nprlght  poaition  by 
flexible  spines,  mast  have  been  an  efficient  instrument  for  steering  the  flight  of  the 
creature.  I  ant  iudebted  1^  the  kindness  of  Prof.  Marah  for  the  three  flg(u«a  which 
Uluatrate  this  structure. 


■.  ii  §  1 


JURASSIC  SYSTEM 


Fig.  397),  had  a  short  tail  and  jaws  furnished  from  end  to  end  with 
long  teeth.  Others  were  Dimorj^iodfm,  distinguished  especially  by  long 
anterior  and  short  hinder  teeth,  and  by  the  length  of  its  tail,  and 
Ehampkorkynchus  (Figs.  398,  399,  400,  401),  also  possegsing  a  long  tail, 


with  a  caudal  membrane  and  having  formidable  jaws,  which  may  have 
terminated  in  a  homy  beak.  These  strange  harpy-like  creatures  were 
able  to  fly,  to  shuffle  on  land,  or  perch  on  rocks,  perhaps  even  to  dive  in 
search  of  their  prey.  The  long  slender  t«etb  which  some  of  them 
possessed  probably  indicate  that  the  creatures  lived  on  fish.     Lastly,  the 


892  STRATIGRAPHICAL  GEOLOGY         BOOKVlPABTm 

most  colossal  living  beings  of  Mesozoic  time,  and,  indeed,  so  far  as  we 
know,  of  any  time,  belonged  to  the  ancient  order  of  Deinosaiirs,  which 
now  attained  their  maximum  development.  In  these  animals,  which 
appeared  in  the  earliest  Mesozoic  ages,  ordinary  reptilian  characters 
(as  already  remarked)  were  united  to  others,  jMirticularly  in  the  hinder 
part  of  the  skeleton,  like  those  of  birds.  It  was  during  the  Jurassic 
l)eriod  that  the  Deinosaurs  reached  their  culmination  in  size,  variety,  and 
abundance.  The  most  important  European  Jurassic  genera  are  Comjh 
sognathxLs,  Megalosaums  (Fig.  398),  and  Cetiosaurus.  In  ConipsogtMikm, 
from  the  Solenhofen  Limestone,  the  bird -like  affinities  are  strikingly 
exhibited,  as  it  possessed  a  long  neck,  small  head,  and  long  hind  limbs  on 
which  it  must  have  hopped  or  walked.  The  Megalosaurus  of  the  Stones- 
field  Slate  is  estimated  to  have  had  a  length  of  25  feet,  and  to  have 
weighed  two  or  three  tons.  It  frequented  the  shores,  of  the  lagoons, 
walking  probably  on  its  massive  hind  legs,  and  feeding  on  the  moUusks, 
fishes,  and  perhaps  the  small  mammals  of  the  district.  Still  more  gigantic 
was  the  Cetiosaunis,  which,  according  to  Phillips,  probably  reached,  when 
standing,  a  height  of  not  less  than  10  feet  and  a  length  of  50  feet.  It 
seems  to  have  been  a  marsh-loving  or  river-side  animal,  living  on  the 
ferns,  cycads,  and  conifers  among  which  it  dwelt. 

But  these  monsters  of  the  Old  World  were  surpassed  in  dimensions  by 
some  discovered  in  the  Jurassic  formations  of  Colorado.  Of  these,  BronU)- 
saurus  was  distinguished  by  its  relatively  short  body,  long  neck  and  tail, 
and  remarkably  small  head.  Its  legs  and  feet  were  massive,  with  solid 
bones,  and  made  footprints  each  measuring  about  a  square  yard  in  area. 
Its  length  is  estimated  at  50  feet  or  more,  and  its  weight,  when  alive, 
at  more  than  20  tons.  In  habit  it  was  more  or  less  amphibious, 
probably  feeding  on  aquatic  plants  or  other  succulent  vegetation.  The 
small  head  and  brain  and  slender  neural  cord  indicate  a  stupid,  slow- 
moving  reptile.^  Stegosauni;<  had  a  renuirkably  small  skull  with  short 
massive  jaws,  very  short,  powerful  fore-limbs,  'with  comparatively  long 
and  slender  hind -limbs.  But  its  most  singular  character  was  the 
possession  of  numerous  dermal  spines,  some  of  great  size  and  power,  and 
many  bony  plates  of  various  sizes  and  shapes,  some  of  them  more  than 
3  feet  in  diameter.  Thus  armed  as  well  as  protected,  it  must  have  been 
one  of  the  most  uncouth  monsters  that  haunted  the  waters  of  the  time. 
Yet  it  was  itself  herbivorous,  and  appears  to  have  been  more  or  less 
aquatic  in  habit. *^  But  the  most  colossal  of  all  these  forms,  and,  indeed, 
the  most  gigantic  creature  yet  known,  was  that  to  which  Professor 
Marsh  has  given  the  name  of  Athntosauius.  It  was  built  on  so  huge  a 
scale  that  its  femur  alone  is  more  than  8  feet  high,  the  corresponding 
bone  of  the  most  gigantic  elephant  looking  like  that  of  a  dwarf,  when 
put  beside  this  fossil.  The  whole  length  of  the  animal  is  supposed  to 
have  been  not  much  short  of  100  feet,  with  a  height  of  30  feet  or  more. 
Contemporaneous  with  these  huge  creatures,  however,  there  existed  in 
Jurassic  time  in  North  America  diminutive  forms  having  such  strong 

^  Marsh,  Ainer.  Joum.  Sci.  xxvi.  (1883)  p.  81. 
"^  Marsh,  op.  cit.  xix.  (1880)  p.  258. 


SECT,  ii  §  1  JURASSIC  SYSTEM  893 

avian  affinities  that  their  eeperate  bones  cannot  be  distinguished  from 
those  of  birds.  Professor  Marsh,  who  has  brought 
these  interesting  forms  to  light,  regards  them  as 
having  1>een  in  some  cases  probably  arboreal  in 
habit,  with  possibly  at  first  no  more  essential  differ- 
ence from  the  birds  of  their  time  than  the  absence 
of  feathers.' 

The  oldest  known  bird,  Archseopteryx  (Fig.  402), 
comes  from  the  Solenhofen  Limestone  tn  the  Upper 
Jurassic  series — a  rock  which  has  bteen  especially 
prolific  in  the  fauna  of  the  Jurassic  period.  This 
interesting  organism,  which  was  rather  smaller  than 
a  crow,  united  some  of  the  characters  of  reptiles 
with  those  of  a  true  bird.  Thus  it  possessed  bicon- 
cave vertebrse,  a  well-ossified  broad  Bternum,  and 
a  long  lizard-like  tail,  each  vertebra  of  which  bore 
a  pair  of  quill-feathers.  The  three  wing-fingers 
were  all  free  and  each  ended  in  a  claw,  and  there 
apptear  to  have  been  four  toes  to  each  foot,  as  in 
most  of  our  common  birds.  The  jaws  carried  tnie 
teeth,  as  in  the  toothed  birds  found  in  the  Creta- 
ceous rocks  of  Kansas.'  Kemains  of  birds  have 
likewise  been  obtained  from  the  Upper  Jurassic 
rocks  (Atlantosaurufl- beds)  of  Wyoming  Territory 
in  Western  America.     The  best  preserved  of  these  _ 

has  been  named  by  Marsh  Laopfetyr,  which  he  it^i^iorhiMhiw'vi^i'nMMi 
believes  to  have  possessed  teeth  and  biconca^'e  uoiitr  cmuiiii  •'nrrniiiy 
vertebne.*  f™'-  ■'"'■ 

The  most  highly  organised  animals  of  which  the  remains  hti\e  been 
discovered  in  the  Jurassic  system  arc  small  marsupials.  Two  horizons 
in  England  have  furnished  these  interesting  relics — the  Stonesfield 
Slate  and  the  Purbeck  beds.  The  Stonesfield  Slate  has  yielded  the 
remains  of  five  genera — Amphiitflus,  AmpliUfj'teii,  and  riuiscololherium 
(Fig.  4().1),  probably  insectivorous,  the  latter  being  related  to  the  living 
American  opossums ;  Ampkitherium,  resembling  most  closely  the  Aus- 
tralian Myrmecobius ;  and  Sierengvathus,  which  Owen  was  disposed  to  think 
was  rather  a  placental,  hoofed,  and  herbivorous  form.  Higher  up  in  the 
English  Jurassic  series  another  interesting  group  of  mammalian  remains 
has  1)een  obtained  from  the  Purbeck  lieds,  whence  upwards  of  twenty 

'  For  Prof.  Marah's  deKripliom  of  Jurasaiu  DeiiiomHr*  ate  Amer.  Joum.  Sei.  ivl, 
(1878)  p.  411 ;  iTii.  (1879)  p.  86  ;  ivia  (1880) ;  xix.  (1880)  p.  253  ;  xii.  (1881)  p.  117  ; 
«ii.  (1881)  p.  340  ;  xiiii.  (1882)  p.  81  ;  xxvi.  (1883)  p.  81  ;  iivii.  (1684)  p.  161 ;  ixxW. 
(1887)  p.  413  ;  xixvii.  (1889)  pp.  323.  331  ;  iiiii.  (1890)  p.  416 ;  ilii.  (1891)  p.  17B  ; 
xliv.  (1892)  p.  347. 

>  Ore  Msrsb,  Amer.  Journ.  SH.  Not.  1881,  p.  337;  (-'eol.  Mag.  ISSl,  p.  48fi ;  Ctrl 
Vogt,  Jim,  Sci.  Sept.  1879;  Seele;,  Qtet.  Mag.  1881,  pp.  300,  454  ;  W.  Damea,  aUd>. 
Btrlin  Akad.  ixxriiL  (1882)  p.  817  ;  Oeal.  Mag.  1882,  p.  568  ;  1881,  p.  418. 

>  Amtf.  Jo'im.  Sci.  iii.  (1881)  p.  341  ;  ilso  xxii.  p.  337. 


STUATIGRAPHICAL  GEOLOGY 


BOOK  TI  PABX  in 


Bpociee  have  been  exhumed  belonging  to  eleven  genera  {Spalatioth^iiiim, 

Amhlolkerium,  Peralesies,  Achifrodon,  Kurtodon,  Ptvamvs,  Stylodon,  Botoden, 
Triconndim,    Triacanihodon,   Fig.    404),   of  which    some    appear    to    hare 


been  insectivorous,  with  their  closest  living  representatives  among  the 
Australian   phnlangers   and  American  opossums,  while  one,  Plagiauiax, 


Oliddle  Janoic)^ 


resembling   the   Australian    kangaroo-rats    (Hypsiprymnus),   is   held   by 
Owen   to  have   been  a  carnivorous  form.i      A  still   more  varied  and 

'  See  Falconer,  Q.  J.  Oeol.  Soc.  xiiL  261  ;  iviii.  318  ;   Owen,  "  Honognph  o{  Hennk 
Mammals,"  J'alisonlograpli.  Soc  1S71 ;  '  Extinct  Miuimals  of  AaBtnU*,'  1S77. 


SECT,  ii  §  1  JURASSIC  SYSTEM  895 

abundant  assemblage  of  mammalian  remains   has   been  exhumed  from 
the  Jurassic  rocks  of  the  western  regions  of  the  United  States  (p.  9 1 9). 

Geographical  Distribution. — The  Jurassic  system  covers  a  vast 
area  in  Europe.  Beginning  at  the  west,  remnants  of  it  occur  in  the  far 
north  of  Scotland.  It  ranges  across  England  as  a  broad  band  from  the 
coasts  of  Yorkshire  to  those  of  Dorset.  Crossing  the  Channel,  it  encircles 
with  a  great  ring  the  Cretaceous  and  Tertiary  basin  of  the  north  of 
France,  whence  it  ranges  on  the  one  side  southwards  down  the  valleys  of 
the  Saone  and  Ehone,  and  on  the  other  round  the  old  crystalline  nucleus 


Fig.  40S.— Marsupial  from  the  Stoneatleld  Slate. 
Phascolotherium  Bncklandi,  Broderip  :  a,  teeth,  nuifniifled  ;  h,  Jaw,  nat.  size. 

of  Auvergne  to  the  Mediterranean.  Eastwards,  it  sweeps  through  the 
Jura  Mountains  (whence  its  name  is  taken)  up  to  the  high  grounds  of 
Bohemia.  It  forms  part  of  the  outer  ridges  of  the  Alps  on  both  sides, 
rises  along  the  centre  of  the  Apennines,  and  appears  here  and  there  over 
the  Spanish  peninsula.  Covered  by  more  recent  formations,  it  underlies 
the  great  plain  of  northern  Germany,  whence  it  ranges  eastwards  and 
occupies  large  tracts  in  central  and  eastern  Eussia.  Some  years  ago, 
Neumayr,  following  up  the  early  generalisation  of  L.  von  Buch,  showed 
that  three  distinct  geographical  regions  of  deposit,  marking  diversities  of 


Fig.  404.— Marsupials  fh>m  the  Purbeck  Beds. 

a,  Jaw  of  Plagiaulax  minor,  Fklconer  (f)  ;  b,  same  (nat.  size) ;  c,  molar  (f) ; 
d,  Triconodon  mordax  (Triacanthodon  serrula)  Owen  (nat.  sizeX 

climate,  can  be  made  out  among  the  Jurassic  rocks  of  Europe.^  (1)  The 
Mediterranean  province,  embracing  the  Pyrenees,  Alps,  and  Carpathians, 
with  all  the  tracts  lying  to  the  south.     One  of  the  biological  characters 

1  Neumayr,  **  Jura-Studien,"  Jahrb.  Oeol,  ReichsanstaU,  1871,  pp.  297,451  ;  Verhandl. 
Oeol,  ReuJuanst.  1871,  p.  165;  1872,  p.  54;  1873,  p.  288.  "  Uber  dimatische  Zonen 
wahrend  der  Jura- und  Kreidezeit,"  Denkach.  Wien.  Akad.  xlvil  (1883)  p.  277.  'Die 
geograpbische  Verbreitung  der  Juraformation,'  op,  cU.  1.  (1885)  p.  57.  In  tbese  memoirs 
tbe  student  will  find  mucb  interesting  speculation  regarding  zoological  distribution,  organic 
progress  and  vicissitudes  of  climate  during  tbe  Jurassic  and  Neocomian  periods.  The  last 
memoir  contains  two  suggestive  maps  of  Jurassic  geography. 


896  STKATIGRAPHICAL  GEOLOGY  BooKTiPAarn 

j"  '• 

!;'  of  this  area  was  the  great  abundance  of  Ammonites  belonging  to  thi 

i;  groups   of    Heterophiflli   (Phylloeeras)  and   Fimf/riati  {Lifioceras),    and  thi 

•  presence  of  forms  of   Terehratuh  of  the  family  of  T,  diphya  (jatiilor) 

(2)  The  central  European  province,  comprising  the  tracts  to  the  nortli 
of  the  Alpine  ridge,  including  France,  England,  Germanj,  and  th< 
Baltic  countries,  and  marked  by  the  compai-ative  rarity  of  the  Ammonitef 
just  mentioned,  which  are  replaced  by  others  of  the  genera  Aspidocercu 
and  Oppellia,  and  by  abundant  reefs  and  masses  of  coraL  (3)  The  boreal 
or  Russian  j)ro\'ince,  comprising  the  middle  and  north  of  Kussia,  Petschora, 
Spitzbergen,  and  Greenland.  The  life  in  this  area  was  less  varied  than 
in  the  others ;  in  particular,  the  widely  distributed  species  of  Oppellia  and 
*  Jlapidoceras  of  the  middle-European  province  are  absent,  as  well  as  large 

masses  of  corals,  shoAving  that  in  Jurassic  times  there  was  a  perceptible 
j-  diminution  of  teniperatiu*e  towards  the  north. 

I.  Neumayr  subsequently  extended  these  three  provinces  into  homoiozoic 

i  zones  or  belts  stretching  round  the  globe,  and  showing  the  probable  dis- 

I  tributioii  of  climate  and  life  during  Jurassic  and  early  Cretaceous  times. 

j  (1)  The  Boreal  Zone  descends  as  far  as  lat.  46'  in  North  America,  whence 

!  it  bends  north-eastwards,  coming  as  high  as  lat.  63"  in  Scandinavia ;  but 

j.  then  taking  a  remarkable  bend  towards  the  south-east  across  Russia,  the 

i;  Kirghiz  Steppes  and  Turkestan  into  Tibet,  about  lat.  29'  N.  and  long. 

?■  85    E.     This  curious  projection  is  explained  by  the  fact  that  the  fauna  of 

'  the  Jurassic  rocks  of  Tibet,  Kashmir  and  Neiwil,  though  peculiar,  has 

!:  greater  affinities  with  that  of  the  boreal  than  with  that  of  more  southern 

j'  zones.     The  boreal  zone  is  divisible,  as  far  as  yet  known,  into  three 

1  provinces,  the  Arctic,  Russian  and  Himalayan.     (2)  The  North  Tem|>erate 

Zone  reaches  to  about  lat.  33"  in  North  Americii.     In  Euroi)e  its  limits 

are  more  precisely  defined.     It  extends  from  Lisbon  across  the  S})ani8h 

tableland  to  the  west  end  of  the  Pyrenees,  thence  across  the  south  of 

!  France  and  along  the  north  side  of  the  Alps  to  the  north  end  of  the 

'  Carpathians,  bending  southward  so  as  to  keep  to  the  north  of  the  Black 

Sea  and  Caucasus,  and  then  striking  south-eastwards  into  the  Himalaya 
chain,  where  it  is  nearly  cut  ofl'  by  the  extension  of  the  Boreal  Zone  just 
mentioned.  In  this  zone  four  provinces  have  been  recognised — the  middle 
European,  Caspian,  Punjab,  and  Californian.  (3)  The  Equatorial  Zone 
extends  southwards  to  the  southern  end  of  Peru,  and  does  not  include 
the  extreme  southern  coasts  of  South  Africa  and  Australia,  which  with 
the  remaining  part  of  South  America,  lie  in  the  South  Temperate  Zone. 
In  the  Ixjuatorial  Zone,  seven  provinces  are  more  or  less  clearly  defined : 
the  Alpine,  Mediterranean,  Crim-Caucasian,  Ethiopian,  Columbian,  Carib- 
bean (?),  and  Peruvian.  The  South  Temi)erate  Zone  is  allowed  four 
provinces :  the  Chilian,  New  Zealand  (?),  Australian  and  Cape. 

By  carefully  collecting  and  collating  the  evidence  furnished  by  the 
discovery  of  Jurassic  rocks  in  all  parts  of  the  world,  Neumayr  believed 
himself  warranted  to  give  a  sketch  of  the  probable  geographical  distri- 
bution of  sea  and  land  during  the  Jurassic  period,  and  even  to  reduce  the 
data  to  the  form  of  mai)s.  He  thought  there  was  sufficient  proof  of  the 
existence  of  three  great  oceans  partly  coincident  with  those  still  existing 


SECT,  ii  §  2  JURASSIC  SYSTEM  897 

— the  Arctic  Ocean,  the  Pacific  Ocean,  and  the  Antarctic  Ocean.  A 
central  Mediterranean  stretched  across  the  narrow  part  of  the  American 
Continent,  and  traversing  what  is  now  the  North  Atlantic,  swept  all  over 
central  and  southern  Europe,  the  present  Mediterranean  Sea,  and  the 
north  of  Africa.  It  joined  the  Arctic  Ocean  in  the  Russian  plain,  sent 
various  arms  into  Asia,  and  passing  across  central  India  stretched  south- 
wards to  the  Antarctic  Ocean.  A  long  and  wide  branch  extended  between 
Africa  and  a  supposed  mass  of  land  connecting  southern  Africa,  Mada- 
gascar, and  southern  India.  The  chief  terrestrial  areas  of  the  period, 
according  to  Neumayr,  were  the  African-Brazilian  continent,  extending 
across  the  southern  Atlantic  ;  the  Chinese- Australian  continent,  extending 
from  the  north  of  China  over  the  south-east  of  Asia  to  Tasmania  and 
New  Zealand ;  the  Nearctic  continent,  extending  from  south-eastern 
Greenland  and  Iceland  across  the  North  Atlantic  to  the  Gulf  of  Mexico  ; 
the  Scandinavian  island,  the  European  Archipelago,  consisting  of 
numerous  insular  tracts  dotted  over  the  Jurassic  sea  from  Ireland  on  the 
west  to  southern  Russia  on  the  east ;  the  Turanian  island,  lying  to  the 
Ciist  of  the  Caspian ;  and  the  Ural  island,  on  the  site  of  the  Ural 
Mountains.  But  much  of  this  geography  rests  on  slender  evidence.  One 
of  the  most  remarkable  facts  pointed  out  by  Neumayr  is  the  extent  of 
the  overlap  of  upper  Jurassic  rocks  upon  lower  members  of  the  system. 
He  showed  that  the  Lias  was  not  deposited  over  an  enormous  part  of  the 
earth's  surface,  which  nevertheless  sank  beneath  the  sea  wherein  later 
parts  of  the  Jurassic  series  were  laid  down. 

§  2.  Local  Development. 

Britain.* — The  stratigraphical  succession  of  the  Jurassic  rocks  was  first  worked 
4)Ut  in  England  by  William  Smith,  in  whose  hands  it  was  made  the  foundation  of  strati- 
graphical  geology.  The  names  adopted  by  him  for  the  subdivisions  he  traced  across 
the  country  have  passed  into  universal  use,  and,  though  some  of  them  are  uncouth 
English  provincial  names,  they  are  as  familiar  to  the  geologists  of  other  countries  as  to 
those  of  England. 

The  Jurassic  formations  stretch  across  England  in  a  var^'ing  band  from  the  mouth  of 
the  Tees  to  the  coast  of  Dorsetshire.     They  consist  of  sands,  sandstones  and  limestones 

*  For  British  Jurassic  rocks  the  student's  attention  may  be  si>ecially  called  to  Phillij^s' 

*  (reolog)'  of  Oxford  and  the  Thames  Valley '  ;  Tate  au<l  Blake's  *  Yorkshire  Lias  '  ;  Hudle- 
ston's  *'  Yorkshire  Oolites,'*  in  Oed.  Moff.  1880-84,  and  Pntc.  (Jeol.  Assoc,  vols.  iii.  to 
V.  ;  Memoirs  published  by  the  Palaeontographical  Society,  particularly  Morris  and  Lycett's 

*  Mollusca  from  Great  Oolite ' ;  Davidson's  *  Tertiary,  Oolitic,  and  Liassic  Brachipoda '  ; 
Wright's  *  Oolitic  Echinodennata '  and  *  Lias  Amnionities  '  ;  Owen's  *  Mesozoic  Reptiles  '  ; 

*  Mesozoic  Mammals,'  *  Wealden  and  Purbeck  Reptiles'  ;  Hudleston's  *  British  Jurassic 
Gasteropotla *  ;  Buckman's  'Inferior  Oolite  Ammonites.'  The  Memoirs  of  the  (Jedogiral 
Survey  comprise  some  imj)ortant  works  on  this  subject,  such  as  Hull's  *  Geology  of  Chelten- 
ham '  ;  Judd's  *  Geology  of  Rutland,'  Ac.  ;  H.  B.  Woodward's  *  Jurassic  Rocks  of  England 
and  Wales  (Yorkshire  excepte<l)  *  ;  C.  Fox-Strangway's  'Jurassic  Rocks  of  Yorkshire,'  Ac 
Further  information  will  be  found  in  the  Address  by  Mr.  Etheridge,  Q,  J.  Geol.  »Soc.  1882  ; 
in  Wood  want's  *  Geology  of  England  and  Wales  *  ;  and  in  other  memoirs  cited  below.  See 
also  Opjwl's  *  Juraformation  Englands,  Frankreichs  und  Deutschlands,' 1856  ;  Quenstetlt's 
*Der  Jura,'  1Si)S. 

3  M 


1! 


J! 

3 

r 

1 1 


:j 

4. 
t 


898 


STRA  TIGRA  PHICA  L  GEOLOG  Y 


BOOK  VI  PART  ni 


interstratitied  with  softer  clays  and  shales.  Hence  they  give  rise  to  a  characteriatic  type 
of  scenery, — the  more  durable  and  more  porous  beds  standing  out  as  long  ridges,  some- 
times even  with  low  cliffs,  while  the  clays  underlie  the  level  spaces  between.  Arrauged 
in  descending  order,  the  following  subdivisions  of  the  English  Jurassic  system  are 
generally  recognised : — 


FonnationH 
or  Series. 


Groupii  or 
.Stages. 


O     Q     CC 


A 


Purbeckian 


Cup 

;:5  P-t 


0.05 


<v  c  cS 
•~3  C3  :— 

goo 


CO 


Portlaudian 
l^  Kimeridgiau 

Coral  lian 
Oxfordian 


Sub-groups  or  Sub-staKHK. 

'  Upper  fresh -water  beds 
Middle  marine  beds 
Lower  fresh -water  beds 
Portland  Stone  ...... 

Portland  Saiid.s  ...... 

Kimeridge  Clay  ...... 

Coral  Rag,  Coralline  Oolite,  and  Calcareous  Grit 
Oxford  Clay  and  Kellaways  Rock 


Maximnui 

thickntvMv. 

Feet. 

.      360 

.  70 
.  150 
.     600 

.  2fi0 
.     600 


/Combra.sh.     This  forms  a  persistent  band  at  the 

/^      ^        Tj  *i   I       top  of  the  lower  or  variable  (marine  and  estuar- 
Great  or  Bath  F       .     v 

me)  group 


o 
O 


o 


< 


25 

160 
180 


p.  y.  <      me)  group      ....... 

uoiiie  group     g^adford  Clay  and  Forest  Marble 

\  Great  or  Bath  Oolite,  with  Stouesfield  Slate  . 

Fuller's  Earth      Fuller's  Earth 150 

{  Cheltenham  beds  (thick  estuariue  series  of  York- 
)      shire,  representing  the  whole  succession  up  to 
j      the  base  of  the  Combrash)        ....      270 
{  Northampton  Sands  (**  Dogger  *'  of  Yorkshire)       .     160 
Midford  Sands  (pa.ssage  l)edH) 

C  Upper  Lias         ..........     400 

-!  Marlstone  ..........      850 

(Lower  Lias 900 


Inferior  Oolite 


I 


Although  these  names  appear  in  tabular  order,  as  expressive  of  what  is  the  predomi- 
nant or  normal  succession  of  strata,  considerable  differences  occur  when  the  rocks  are 
traced  across  the  country,  especially  in  the  Lower  Oolites.  Thus  the  Inferior  Oolite 
consists  of  marine  limestones  and  marls  in  Gloucestershire,  but  chiefly  of  massive  estua- 
riue sandstones  and  shales  in  Yorkshire.  These  differences  help  to  bring  before  us  some 
of  the  geographical  features  of  the  British  area  during  the  Jurassic  period. 

The  Lias/  consists  of  three  stages  or  groups,  well  marked  by  physical  and  palaeonto- 
logical  characters.*'^  In  the  Lower  member,  numerous  thin  blue  and  brown  limestones, 
with  partings  of  dark  shale,  are  surmounted  by  similar  shales  with  occasional  nodular 
limestone  bands.  The  Middle  Lias  consists  of  argillaceous  and  ferruginous  limestones 
(Marlstone)  with  underlying  micaceous  sands  and  clays.  In  some  of  the  midland 
counties,  but  more  esj)ecially  in  Yorkshire,  this  subdivision  is  remarkable  for  contain- 
ing a  thick  series  of  beds  of  earthy  carbonate  of  iron  (Ironstone  series),  which  has  been 
extensively  worked  in  the  Cleveland  district.  The  Upj>er  stage  is  composeil  of  clays 
and  shales  with  nodules  of  limestone,  surmounted  by  sandy  deposits,  which  are  iter- 
haps  best  classed  with  the  Inferior  Oolite.  In  Yorkshire  it  consists  of  about  240  feet 
of  grey  and  black  shale,  in  the  upper  part  of  which  lies  a  dark  band  full  of  pyritous 
"  doggers  "  (ironstone  concretions)  and  blocks  of  jet,  which  are  extracted  for  the  manu- 


^  This  word,  now  so  familiar  in  geological  literature,  was  adopted  by  William  Smith, 
who  found  it  given  l)y  the  Somerset  quarrymen  to  the  **  layers  "  of  argillaceous  limestone 
forming  a  part  of  the  series  of  rocks  to  which  the  term  is  now  applied. 

-  The  English  Lias  is  fully  described  by  Mr.  H.  B.  Woodward  in  his  monograph  in  the 
Memoirs  of  the  Ueolngical  Sui'vey  above  cited. 


BiCT.  u  §  2  JURASSIC  SYSTEM  899 

facture  of  ornaments.     This  jet  appears  to  have  been  originally  water-logged  fragments 
of  coniferous  wood.^ 

These  three  stages  are  subdiyided  into  the  following  zones  according  to  distinctive 
species  of  Ammonites,  though  the  zones  are  not  so  definite  in  nature  as  in  palreonto- 
logical  lists  :  '** — 


o 

Cm 

TS  .2  i 


2.  Zone  of  Ammonites  (Stephanoceras)  communis. 
!•  „  (Harpoceras)  serpentinus. 


2.  ,,  (Amaltbeus)  spinatus. 
1*                  „  ,,  margaritatus. 

^  10.  „  (iijgoceras)  Henleyi. 

9.  ,,  (Amaltbeus)  Ibex. 

8.  fj  (JBgoceras)  Jamesoni. 

7.  „  (Arietites)  raricostatus. 

6.  „  (Amaltbeus)  oxynotus. 

5.  „  (Arietites)  obtusus. 

4.  „  „         Turneri. 

3.  „  „  Bucklandi. 
2.  ,.  (iSgoceras)  angulatns. 
1.  „  M         planorbis. 

restiug  conformably  on  Avicula  contorta  beds  (p.  867). 

The  organic  remains  of  the  British  Lias  now  include  nearly  300  genera  and  more 
than  six  times  that  number  of  species.  The  plants  comprise  leaves  and  other  remains 
of  cycads  {PaltBOzamia,  OtozamiUs),  conifers  {Pinites,  ClathropteriSf  Pence),  ferns  {Ale- 
thopteris,  &c. ),  and  mares'  tails  {EquisetiUs),  These  fossils  serve  to  indicate  the  general 
character  of  the  flora,  which  seems  now  to  have  been  mainly  cycadaceous  and  coniferous, 
and  to  have  presented  a  great  contrast  to  the  lycopodiaceous  vegetation  of  Palfeozoic 
times.  The  occurrence  of  land-plants  dispersedly  throughout  the  English  Lias  shows 
also  that  the  strata,  though  chiefly  marine,  were  deposited  within  such  short  distance 
from  shore,  as  to  receive  from  time  to  time  leaves,  seeds,  fruits,  twigs,  and  stems  from 
the  land.  Further  evidence  in  the  same  direction  is  supplied  by  the  numerous  insect 
remains,  which  have  been  obtained  principally  from  the  Lower  Lias.  These  were,  no 
doubt,  blown  off"  the  land  and  fell  into  shallow  water,  where  they  were  preserved  in  the 
silt  on  the  bottom.  The  Neuroptera  are  numerous,  and  include  several  species  of 
Lihellula.  The  coleopterous  forms  comprise  a  number  of  herbivorous  and  lignivorous 
beetles  {Elater,  BuprestiUs,  &c.)  There  were  likewise  representatives  of  the  ortho- 
pterous,  dipterous,  and  palseodictyopterous  orders.  These  relics  of  insect  life  are  so 
abundant  in  some  of  the  calcareous  bands  that  the  latter  are  known  as  insect-beds.^ 
With  them  are  associated  remains  of  terrestrial  plants,  cyprids,  and  moUusks,  some- 
times marine,  sometimes  apparently  brackish-water.  The  marine  life  of  the  period 
has  been  abundantly  preserved,  so  far  at  least  as  regards  the  comparatively  shallow  and 
juxta-littoral  waters  in  which  theLiassic  strata  were  accumulated.^  Foraminifera  abounded 

1  C.  Fox-Strangways,  Mem,  Oeol.  Survey,  "Scarborough  and  Whitby"  (1882),  p.  21. 

^  Wright  on  Liassic  Ammonites,  Pal«ontograph.  Soc.  and  Q,  J.  Oeol,  Soc.  xvi.  374  ;  C. 
H.  Day,  op.  cit,  xix  p.  278  ;  Etheridge,  op,  ci^  xxxviiL  (Address).  As  the  zones  are  not 
generally  defined  by  lithological  features  they  cannot  be  satisfactorily  mapped.  On  the 
maps  of  the  Geological  Survey  the  base  of  the  Middle  Lias  is  perhaps  not  drawn  uniformly 
at  one  palseontological  horizon  ;  but  it  generally  corresponds  with  the  base  of  the  Margaritatus 
zone.     (See  Judd,  'Geology  of  Rutland,*  pp.  45,  89.) 

»  Brodie,  Proc,*  Oeol,  Soc,  1846,  p.  14  ;  Q.  J.  Oed.  Soc,  v.  31  ;  *  History  of  Fossil 
Insects,'  1846.  See  Scudder,  BiM,  U.S,  Oeol.  Sure.  No.  71  (1891),  pp.  98-236,  for  a  list 
of  all  known  Mesozoic  insects,  and  references  to  the  authorities  for  the  description  of  each 
species. 

^  See  R.  Tate,  "Census  of  Lias  Marine  Invertebrata,"  Oeol,  Mag.  viii.  p.  4. 


STRATlGHAPmCAL  GEOLOGY         book  vi  PAnm 


uii  some  of  the  aea-battDuia,  the  eonern  Criakllnria,  DenialtHa,  UargiuHlina,  Fretnii- 


SECT,  ii  §  2  JURASSIC  SYSTEM  901 

valtia,  SeptastrsMy  &c. )  The  crinoids  were  represented  by  thick  growths  of  Extraerinu» 
aud  Pcniucrinus,  There  were  brittle-stars,  star-fishes,  and  sea-urchins  {Ophioglypha, 
[/raster,  Luidia,  Hemipedina,  Cidaris,  Acrosalenia) — all  generically  distinct  from  those 
of  the  Palwozoic  periods.  The  annelides  were  represented  by  Serpula,  Vennilia,  and 
Ditrupa.  Among  the  Crustacea,  the  more  frequent  known  genera  are  Eryon  (entirely 
Liassic),  Olyphma  (from  Lower  Lias  to  Kimeridge  clay),  and  Eryma.  The  brachio- 
l>ods  are  chiefly  Hhynchcnella,  WcUdheimiay  Spiriferiiia,  Thectdivm^  and  Terebratvla. 
Spiriferina  is  the  last  of  the  Spirifers,  and  with  it  are  associated  the  last  forms  of  Leptsma. 
of  which  five  Liassic  species  are  known  from  English  localities  (Fig.  388).  Of  the  lamel- 
libranchs  a  few  of  the  most  characteristic  genera  are  Pectcn^  Lima^  Avicvla,  Oryph^a, 
ffervillia^  Ostrea,  PliccUula,  Mytilus,  Cardiniot  Leda^  CyprUardia,  Astarte,  Pleuromya^ 
Hippopodium^  and  Pholadomya,  Gasteropoda,  though  usually  rare  in  such  muddy  strata 
as  the  greater  part  of  the  Lias,  occasionally  occur,  but  most  frequently  in  the  calcareous 
zones.  The  chief  genera  are  CerUhiwrn^  TurhOj  Trochits,  PUurotomaria,  Chemnitzta,  and 
Turritella.  The  cephalopoda,  however,  are  the  most  abundant  and  characteristic  shells 
of  the  Lias  ;  the  family  of  the  Ammonites  numbers  upwanls  of  300  species  in  the  British 
Lias.  Many  of  these  are  the  same  as  those  that  have  been  found  in  the  Jurassic  series  of 
Germany,  and  they  occupy  on  the  whole  the  same  relative  horizons,  so  that  over 
central  and  western  Europe  it  has  been  possible  to  group  the  Lias  into  the  various 
zones  given  in  the  table  (p.  899).  Of  the  genus  Nautilus  about  ten  species  have  been 
found.  The  dibranchiate  cephalopods  are  represented  by  more  than  60  species  of  the 
genus  Bclemniies. 

From  the  English  Lias  numerous  species  of  fishes  have  been  obtained.  Some  of 
these  are  known  only  by  their  teeth  {Acrodns),  others  by  both  teeth  and  spines 
{Hybodus).  The  ganoids  are  frequently  found  entire  ;  DapediiiSy  Pholidophorus,  ^Ech- 
modus t  PachycarmuSf  EugncUhus,  and  Leptolcpis  are  the  most  frequent  genera.  But 
undoubtedly  the  most  remarkable  palaeontological  feature  in  this  group  of  strata  is  tlio 
number  and  variety  of  its  reptilian  remains.  The  genera  Ichthyosaurus,  Plesiosavrus, 
Dimorphodon^  ScelidosauruSf  Teleosaurus,  and  Steneosaurus  have  been  recovered,  in 
some  cases  the  entire  skeleton  having  been  found  with  almost  every  bone  still  in  place. 
The  two  genera  first  mentioned  are  especially  frequent,  and  more  or  less  perfect  skeletons 
of  them  are  to  be  seen  in  most  public  museums. 

The  Lias  extends  continuously  across  England  from  the  mouth  of  the  Tecs  to  tlie 
coast  of  Dorsetshire.  It  likewise  crosses  into  South  Wales.  Interesting  patches 
occur  in  Shropshire  and  at  Carlisle,  far  removed  from  the  main  mass  of  the  formation. 
A  considerable  development  of  the  Lias  stretches  across  the  island  of  Skye,  and  skirts 
adjoining  tracts  of  the  west  of  Scotland,  where  the  shore-line  of  the  i)eriod  is  partly 
traceable ;  while  small  portions  of  the  lower  division  of  the  formation  are  exposed 
on  the  foreshore  of  the  east  of  Sutherland,  near  Dunrobin.  In  the  north  of  Ireland, 
also,  the  characteristic  shales  appear  in  several  places  from  under  the  Chalk  escarpment. 

The  Lower  Oolites  lie  conformably  upon  the  top  of  the  Lias,  with  which  they  are 
connected  by  a  general  similarity  of  organic  remains,  and  by  about  46  species  which 
pass  up  into  them  from  the  Lias.  In  the  south-west  and  centre  of  England  they  chieliy 
consist  of  shelly  marine  limestones,  with  clays  and  sandstones  ;  but,  traced  northwards 
into  Northampton,  Rutland,  and  Lincolnshire,  they  contain  not  only  marine  limestones, 
but  a  series  of  strata  indicative  of  deposit  in  the  estuary  of  some  river  descending  from 
the-  north,  for,  instead  of  the  abundant  cephalopods  of  the  truly  marine  and  typical 
series,  we  meet  with  fresh- water  genera  such  as  Cyrena  and  Unio,  estuarine  or  marine 
forms  such  as  Ostrea  and  Modiola,  thin  seams  of  lignite,  thick  and  valuable  deposits  of 
ironstone,  and  remains  of  terrestrial  plants.  These  indications  of  the  proximity  of  land 
become  still  more  marked  in  Yorkshire,  where  the  strata  (800  feet  thick)  consist  chiefly 
of  sandstones,  shales  with  seams  of  ironstone  and  coal,  and  occasional  horizons  containing 
marine  shells.  It  is  deserving  of  notice  that  the  Cornbrash,  at  the  top  of  the  Lower 
Oolite  in  the  typical  Wiltshire  district,  though  rarely  20  feet  thick,  runs  across  the 


STRATIGSAPHWAL  GEOLOGY 


BOOK  VI  PABT  tn 


country  from  Devonshire  to  Linoolnshire  and  Yorksbire.     Thna  a  diatiiiGtIj'  dsfimd 


y^.  4(M.-Mlddli>snd  Lower  Liis  Ammoiiitea. 
■',  Aii>niantti!ii(Ainii1th«i«)itiarRariUtut,  UaDt.<();  b,  A.  <A.)  apiiutiii,  Bra£.  (1);  c,  A.  < J^fooou} 
IMivo'i,  Sh.v.  (J);  .(,  A.  (.E.)  cspricorau*.  Schloth.  («;  r,  A.  (.E.)  JwiiCToni,  Sbjr.  Q);/,  A.  («.) 
brvrt'iaiiB,  Mby.  (1). 

series  of  lifds  of  an  estuarinc  cliaracter  is  in  tlip  north  homotsmll;  representAtiTc 
or  the  uiaritic  formations  of  tlie  soutli'Weat.     At  the  close  of  the  Lower  Oolitic  period 


■.ii§« 


JURASSIC  SYSTEM 


g03 


the  eatuary  of  tlie  northern  tract  wm  submerged,  and  marine  deposits  were  laid  down 
ncroM  England 

Tbe  English  Loner  0»ht«e  show  considerable  local  variation  in  their  subdivisions. 
They  are  typically  developed  in  the  aouth-westem  counties,  but  the  IJniMtones  and 
^.lajrs  pass  laterally  into  rands.  The  lowest  group,  that  ol  tbe  Midford  Sands,  is 
sometimes  placed  with  the  Lias.  It  conaiats  of  yellow  micaceous  sands,  with  aome 
tniicretionary  sandstone  and  sandy  limestone,  sud  ranges  from  25  to  200  feet  in 
thickness.    A  ferruginous  limestone  at  the  top  containa  so  many  Ammonites,  Belemnit««, 


—Upper  Lisa  AimiiDDitea. 

ia,  Bb)F.  (j) :  h.  A.  (Lytocerii>Juren.iii,  Ziaten  (A) ;  ' 

k«  {)):  d,  A.  (1-liylJocenu>  heteraidiylliu,  Sby.  <!). 


und  Nautili,  tbat  it  has  been  called  the  " Ceplialopoda  bed." 
may  be  recognised  in  this  group,  viz. : — 


Among  the  other  charaeteristic  foesils  are  AmmoiiiUi  aalenns,  A.  kirciniit,  A.  radiauM, 
A.  mriabilia,  BrUmaitts  eamprcama,  B.  irregnfarii,  Orealya  ahdada,  Trigonia  fomwta, 
fi'frvittia  Hartmanni.  Rhi/nekonella  cynoerphala,  K.  plkatetln,  &c 

The  Inferior  Oolite  (B^ocian)  attains  its  maximum  development  in  tbe  neighbonr- 
liood  of  Chelteuliam,  where  it  has  a  thickness  of  264  feet,  and  conaists  of  calcareous  free- 
Estone  and  ragatone  or  grit.  It  presents  a  tolerably  copious  suite  of  invertebrate  reniaina, 
which  resemble  generically  those  of  the  Lias.  The  corals  include  species  of  Isattrma, 
Jltmlliialtin,  and  other  genera.  The  crinoids  are  represented  by  Penlatrirna  ;  the  star- 
lishea  by  specica  of  Aampcdeii,  Gmiadtr,  SolaMer,  and  SuUasUr  ;  the  Bsa-invhins  by 


904 


STRATIGRAFHWAL  GEOLOGY  book  vi  paiw  in 


Hpecies  oi  AcToaaleitin,  Cidarii,  Htmipediiui,  Clyptus,  PggaHer,  ftc.  The  predoi 
of  Ilhymhoiidla  and  Tercbratula  over  the  rest  of  the  hrachiopods  becomes  atOl  mort 
marked.  Liiaa,  Oatrea,  PecUn,  Pinna,  Aitarle,  CiKatlKO,  Jfyadla,  Mtfiiiiu,  Pkola- 
domya,  Trlgoniii  are  the  most  common  genera  of  lamellibrauchs.  The  gasteropoda  are 
abundant,  esjiecially  ill  the  genera  Pltunlomaria,  Alaria,  Trochtu,  Turbo,  Iferinma, 
Cerilkiiim,  and  pMudomelanin.  Ammonites,  Nautili,  and  Belenmitei  are  of  freqnenl 
occuvreliLt.  Paliuontologically  tlie  iDferJor  Oolite  has  been  subdivided  into  the  following 
XOI1C9  in  descending;  order  : ' — 

Zone    ot    Ammonitea    (CosmocerBs)    Parkinsoui    (.-1.   >ubra4ialif,    TmbmtHla 

globala,  BJigaekiiadta  tvbttiroJiaira,  £c.) 
Zone    of    Ammonites    (StephanocBras)    liumplirieBianus    {A.     Blagttevi,    A. 

ilarUnaii,  Wablheimia  carinala,  Ac) 
Zone  of  AmmoniteH  (Harpoceras)  Murchisonie,  with  sub-zone  of  A.  Soteerbgi 

in  npiwr  )>art  [A.  conattat,  TerehnUnla  lini/mn,  T.  simplex,  T.  piicala,  ftc.) 


TI  n  I"  t  It  i  I'f  the  (jronp  are  subject  to  Kreat  variations  in  thickness  and 
litholo;,  al  I  a  1  t  Tlie  tliitk  marine  scries  of  ChvlteiiliBin  is  reduced  in  a  distance 
of  ;tO  or  -10  tndes  to  a  thickness  of  a  few  feet.  Tlie  liincitoliea  jiass  into  sandj  strata, 
.to  tliiit  in  jMrlM  of  Xorthamptonsbirc  the  whole  of  the  formations  between  the  Upivr 
LiaK  Clay  and  the  (Jn-at  Oolite  consist  of  sands  with  beds  of  ironstone,  known  as  the 
Ncnlliuini'toii  (iaud.  The  higher  portiuiiK  uf  the  sandy  series  contain  estuarine  shells 
[Oi/irim)  and  remains  of  terrestrial  plants.  In  Yorkshire  the  Great  Oolite  series 
disd]ii>PHrs  (unless  its  ujij«r  [art  is  represented  by  the  Upper  Eatuarine  aeries  of  that 
ili.'ili'icl)  u'liilu  the  Inferior  Oolites  swell  out  into  a  gi'ent  thickness  and  are  composed  of 
the  fiilhnvinj;  HulMjiviiiions  in  descending  older  ;  -— 

■  On  the  AiiinioniteH  of  these  lones,  see  S.  S.  Huckiiiaii,  Q.  J.  tlid.  £<k.  18H1,  p.  588. 
'  rhillips'  MiealotDor  Yorkshire,'  Undleston,  Ural.  May.  1S80,  p.  246;  1682,  |>.  116^ 


SECT,  ii  §  2  JURASSIC  SYSTEM  905 


a 

c3 


'o 

O 

u 
o 


Feet. 
'  Upper  Estuarine  neries,  shales  and  sandstones  resting  on  a  thick  sand- 
stone (Moor  Grit) more  than       200 

Scarborough  or  Grey  Limestone  series,  consisting  of  grey  calcareous  and 
siliceous  bauds  with  shaly  partings  {Belenin,  yiijitnteus^  Amm.  hmn- 
pkriesiamtSf  Amm.  Blagdeni,  &c.) 3-100 

Middle  Estuarine  series,  chiefly  shales,  with  three  or  four  beds  of  sand- 
stone full  of  plant-remains.     This  is  the  chief  coal-bearing  zone  of 
the  Lower  Oolites.     A  few  thin  coal-seams  occur,  onlv  two  of  which 
have  been  found  worth  working;  none  of  them  exceed  18  inches 
°  -(      or  2  feet  in  thickness 50-100 

Millepore  bed,  a  ferruginous  or  calcareous  grit  i>a.'^ing  into  a  sandy 
limestone  {Ammonites  Sowcrhyi) 10-40 

Lower  Rstuariue  series,  consisting  of  an  upper  group  of  false -bedded 
ferruginous  sandstones  with  carbonaceous  matter,  separated  by  sonje 
ironstone' bands  from  a  lower  group  of  carbonaceous  shales  and  sand- 
stones with  thin  coal-seams 300 

Dogger — ferruginous  sandstone  and  sandy  ironstone  passing  down  into 
the  Jurensis-beds  (Midford  Sands)  {Ceromya  hajociana^  Amm,  Mi*r- 
chisoiuf,  A.  aalfnns,  &c.) 40-95 

A  tolerably  abundant  fossil  flora  has  been  obtaine«l  from  these  Yorkshire  l>ed». 
With  the  exception  of  a  few  littoral  fucoids,  all  the  ])lants  are  of  terrestrial  forms. 
Among  them  are  more  than  50  species  of  ferns  {P€cap(4^rut,  SphenoptcriSt  PhkbopteriSy  and 
Tamiopteris  being  characteristic).  Next  in  abundance  come  the  cycads,  of  which  above 
20  species  are  known  {(Hozamites,  Zamin,  PterophyUum,  CycadiUs).  Coniferous  remains 
are  not  infrequent  in  the  form  of  stems  or  fragments  of  wood,  as  well  as  in  occasional 
twigs  with  attached  leaves  {ArauearU4:s,  Brachyphylhnn,  Thuyites,  Pence,  JFakhta, 
CryptomeriteSy  Taxiles). 

The  Fuller's  Earth  is  an  argillaceous  deix)sit  which,  extending  from  Dorsetshire  to 
the  neighbourhood  of  Bath  and  Cheltenham,  attains  a  maximum  depth  of  nearly  150  feet, 
but  dies  out  in  Oxfordshire,  and  is  absent  in  the  eastern  and  north-eastern  counties. 
Among  its  more  abundant  fossils  are  Ammonites  subcojUractufi,  Ooniomya  literatOy  Ostrcn 
acuminata,  RhynchoneVa  variaiis,  and  WaWicimia  ornithocephala  ;  but  most  of  its 
fossils  occur  also  in  the  Great  and  Inferior  Oolite.  The  conditions  for  marine  life  over 
the  muddy  bottom  on  which  this  deposit  was  laid  down  would  ap})ear  to  have  been 
unfavourable.  Thus  few  gasteropods  are  known  from  the  Fuller's  Earth.  The  beds  of 
economic  fuller's  earth  are  worked  at  Midfoixl  and  VVellow  near  liath  ;  their  detergent 
properties  are  due  to  physical  characters  rather  than  chemical  composition. 

The  Great  Oolite  (Bathonian)  consists,  in  Gloucestershire  and  Oxfordshire,  of  three 
sub-groups  of  strata :  {a)  lower  sub-group  of  thin-bedded  limestones  with  sands,  known  as 
the  Stonesfield  Slate  ;  (6)  middle  sub-group  of  shelly  and  yellow  or  cream-coloureil,  often 
oolitic  limestones,  with  partings  of  marl  or  clay — the  Great  Oolite  proper ;  (r)  upper 
sub-group  of  clays  and  shelly  limestones,  including  the  Hradfortl  Clay,  Forest  Marble, 
and  Conibrash.  These  subdivisions,  however,  cease  to  be  recognisable  as  the  beds  are 
traced  eastward.  The  Bradford  Clay  of  the  upper  sub-group  soon  disappears,  and  the 
Forest  Marble,  so  thick  in  Dorsetshire,  thins  away  in  the  north  and  east  of  Oxfordshire, 
the  horizon  of  the  group  being  represented  in  Bedfonlshirc,  Northamptonshire,  and 
Lincolnshire,  by  the  ** Great  Oolite  Clays"  of  that  district.  The  Cornbrash,  however, 
is  remarkably  persistent,  retaining  on  the  whole  its  lithological  and  italteontological 
characters  from  the  south-west  of  England  to  the  borders  of  the  Humber.  The  lime- 
stones of  the  middle  sub-group  can  be  traced  from  Bnulfoni-on-Avon  to  Lincolnshire. 
The  lower  sub-group,  including  the  Stonesfield  Slate,  is  locally  develoi)ed  in  parts  of 

Proc.  lieoL  Assoc,  iii.  iv.  v.  C.  Fox-Strangways,  "Geology  of  Scarborough  aud  Whitby," 
Mem,  iieol.  Sitn\  1882.  The  fullest  account  of  the  Jurassic  rocks  of  Yorkshire  will  l)e 
found  in  the  volumes  by  Mr.  Fox-Strangways  in  the  series  on  'The  Jurassic  Rocks  of 
Britain,*  in  the  Memoirs  of  the  0>ot.  .Vi/nvy  (1892). 


906  STRATIGRAPHICAL  GEOLOGY         book  vi  PABrm 

Gloucestershire  and  Oxfordshire,  and  passes  into  the  **  Upper  Estuarine  series"  of  the 
Midland  counties.* 

The  fossils  of  the  Stonesfield  Slate  arc  varied  and  of  high  geological  interest 
Among  them  are  about  a  dozen  species  of  ferns,  the  genera  Peeopteris^  SphenopUris^  and 
Tsenioptcris  being  still  the  prevalent  forms.  The  cycads  are  chiefly  species  of  PiaisBO- 
zamia,  and  the  conifers  of  Thuyites,  With  these  drifted  fragments  of  a  terrestrial 
vegetation  there  occur  remains  of  beetles,  dragon-flies,  and  other  insects  which  had 
been  blown  or  washed  off*  the  land.  The  waters  were  tenanted  by  a  few  brachiopods 
{Rhynchon^lla  coiicinna  and  TerebrcUula)^  by  lamellibranchs  (Gervillia  acuta,  Phaladom^ 
ncuticmtn,  Linui,  Oslrea  gregaria,  PecteUy  Astarte^  Modiola^  Trigoniay  &c.),  by  gasteropods 
[Natk/j,  NeritUi  Pat^Ua^  TrochuSj  &c.),  by  a  few  ammonites  {A,  gracilis)  and  belemnites 
{B.  fusi/armis,  B.  bessiniis),  and  by  elasmobranch  and  ganoid  fishes,  of  which  abont  50 
species  are  known  {Ceratodus^  Oanodus,  Hyhodus^  LepidoluSt  Pholidaphorus,  Pyenodus, 
&c.)  The  reptiles  comprise  representatives  of  turtles,  with  species  of  PUsiosaunUt 
CetiosauruSf  TehosauruSj  MegalosauruSt  and  BhamphoccphaJus,  But  the  most  important 
organic  relics  from  this  geological  horizon  are  the  marsupial  mammalia  already  referred  to. 

The  fauna  of  the  Great  Oolite  proper  is  distinguished,  among  other  character- 
istics, by  the  number  and  variety  of  its  corals  (including  the  genera  Imstrmay  CyaUuh 
pJiorUy  Thaynnasfrsea).  The  ecliinoderms,  which  rank  next  to  the  ammonitei 
in  stratigraphical  value,  are  well  represented.  Among  the  regular  echinoids  the 
most  frequent  forms  are  Ucmicidaris^  AcrosaUnia^  Psendodiadema,  and  Cidarii. 
The  irregular  echinoids  are  represented  by  species  of  Ediiiwhrissus,  Clypeus,  Pygunu, 
kc. ;  the  asteroids  by  Astropecten  and  Goniastcr ;  the  crinoids  by  Apiocrinus,  MUUri- 
crinius,  and  Pentncrimis.  Polyzoa  are  abundant  {Diasiopora^  Hdcropora).  The 
l)racliioix)ds  are  represented  by  species  of  Terebriitulay  Bhynchanella,  Waldhcimia,  Tat- 
braUlla,  Cranui,  &c.  Of  the  whole  British  Jurassic  lamellibranchs,  numbering  aboot 
100  genera  and  nearly  1400  species,  more  than  half  the  genera,  and  about  one-fifth  of 
the  species,  are  found  in  the  Great  Oolite.  S{)ecially  conspicuous  are  the  genera  PteUn, 
Lima,  Ostrm,  Ai^ula,  AstartCy  Modiola^  Phol^idoniyay  Trigmiia,  Cardium,  Area, 
Tancredia.  The  characteristic  gasteropods  of  the  Great  Oolite  include  ActsBonina, 
Xcrinxa,  Ncrilaj  Purpuroidea,  Brachyfrema,  Paiella.  Species  of  ammonite  peculiar  to 
the  Great  Oolite  are  Am,  arhu^igcrits,  A.  discus  (passes  to  Cornbrash),  A.  gracilis^  A. 
micromphalusy  A.  morisea,  A.  subcontract  us,  and  A.  Waierhousei,  Characteristic  like- 
wise are  Nautilus  Babcri,  N.  dispansus,  N.  subcoufradus,  Bchmnites  aripisfillwi,  and 
B.  b(^.HsiHus.  Of  the  fishes,  the  genera  most  abundant  in  species  are  Mesodon,  Ganodus^ 
IIlfbodii.%  and  Strephodioi,  with  Acrodus,  Lepidotus,  Pholidoph&rus,  kc.  The  reptilian 
remains  in('lu<l(?  the  crocodilians  Tekosaurus  and  StencosauruSy  Plcsiosaurus,  the  pter- 
osaur Jihumphifccphalus,  and  the  dcinosaurs  Megalosaurus,  Cdiosaurus,  and  CarduxUm, 

The  Forest  Marble  varies  gi-eatly  in  thickness  and  lithological  character.  In  the 
north  of  Dorsetshire  it  is  estimated  to  be  450  feet  thick,  but  it  rapidly  diminishes  north- 
wards, the  liniostone  bands  being  usually  not  more  than  30  feet  thick.  It  lies  sometimes 
on  the  Great  Oolite,  sometimes  on  the  Fuller's  Earth.  Its  lower  portion  becomes  a  grey 
marly  clay  near  liradford-on-Avon,  about  10  feet  thick,  and  this  argillaceous  band  has 
been  soparately  designated  the  Bradford  Clay.  The  Forest  Marble  contains  a  moch 
diminisiied  fauna.  Among  the  forms  peculiar  to  it  are  the  echinoderms  Apiocrinus 
rlrgaiis,  AstrojM'ctcn  JIurleyi,  A.  Phillipsii,  Hcmiciiiaris  alpinn.  The  Bradford  Clay  of 
Wiltshire  has  long  been  well  known  for  its  pear-encrinites  (Apiocrinus  Parkinson i),  which 
arc  foiiiKl  at  the  bottom  of  the  clay  with  their  base  attached  to  the  top  of  the  Great 
Oolite  limestone. 

The  Cornbrash  (a  name  given  by  W,  Smith)  consists  of  earthy  limestones,  which 
when  freshly  liroken  are  blue  and  compact,  but  which  under  the  influence  of  the  weather 
break  up  into  rubbly  material.     It  varies  from  5  to  40  feet  in  thickness,  yet  in  spite  of 

1  Judd's  "Geology  of  Rutland,"  Metru  Geol.  Stirv, 


SECT,  ii  §  2  JVBASSJC  SYSTEM  907 

this  insigniliuant  development  it  is  one  of  the  most  peraistent  bands  in  the  English 
Jurassic  sygtem.  It  U  rich  in  echinodemis,  limellibranchg,  and  guteropods.  Among 
its  common  and  characteristic  species  are  Bchinobruaut  clnnictUarii,  Botatypw  deprtatu, 
OlyjAaa  roitrata,  Hippolkoa  Smilhii,  SinnUa  gradua,  Lima  rigidida,  (Mrea  fiabellaide», 
Pedtn  vagaaa,  Cardium  latum,  Ltda  rottralia,  Myaeila  vni/ormis,  Trigonia  camiopt, 
Adaonina  scarbuTgengia,  CeriUlla  eosUUa.  Its  ammonites  are  A.  diKiia  and  A.  maero- 
cepkatat,  tlie  last-nnmed  ranging  up  into  the  Kellaways  Rock  and  Oxford  Clay.' 

The  Great  Oolite  series  iu  the  north-east  of  Scotland  coDsisU  mainly  of  sundstones 
and  shales,  vith  some  coal-seams  which  were  formerly  worked  at  Brora  in  Sutherland- 
la  Skye  and  Raasay  the  fonnation  consists  of  a  very  thick  eatuariue  series,  with  abundant 
oysters,  Trigonias,  Anoniiaa,  Cyrenaa,  Hydrobias,  Cyprids,  and  remains  of  tanJ-plants. 

The  Middle  or  Oxford  Oolitks  are  composed  of  two  distinct  groups :  (1)  the 
Oxfordian,  and  (2)  the  Corallian, 

(1)  Oifordian,  divisible  into  two  sub-groups:  (ii)  a  lower  division  of  calcareous 
abundantly  fossiliferous  sandstone,  known,  from  a  place  in  Wiltshire,  aa  the  Kellaways 
Rock  (Calloviao).  This  rock-di vision,  from  6  to  more  than  80  feet  thick,  may  be  traced 
from  Wiltshire  through  Bedfordshire  to  Lincolnshire,  and  attains  a  considerable  import- 
ance in  Vorkshirc.     It  contains  about  200  species  of  fossils,  of  which  one-third  are  found 


in  lower  parts  of  the  Jurassic  series,  and  nearly  the  same  proportion  [lasses  upward  into 
higher  zones.  Among  its  characteristic  forms  are  Arioila  ina^iMlma,  Oryph/ea  bilo- 
Ma,  Lima  -iwttUa,  Os/rta  archelypa,  0.  striata,  Anatiiia  veTiioMata,  Cardium  iiibdii- 
aimile,  CirrUi  Imvis,  Liieina  lyrata,  Triganin  wmpUinaln,  T.  paudcotlata,  Alaria  arsinoe, 
C-'rilkiiiia  aibrctialvm,  Flfurotomaria  annoaa.  The  distinctive  ammonite  of  this  stage 
is  A.  eallovientia,  which  gives  its  name  to  a  zone.  Xumcroiia  other  species  of  ammonites 
occur,  including  A.  itwdiolarii,  A.  goiorrianiia,  A.  aurUuliii,  A.  Bakeria,  A.  Baiiffieri, 
A.  Eugeaii,  A.  fitxUoatalwA,  A.  fiudmsHa,  A.  golialhui,  A.  lalaadian-aa,  A.  Lantdalei, 
A.  pUiimla,  A.  latrUua,  A.  Drmoiti  ;  alao  Ancyloetraa  eallovii^K,  Navtiliui  r^allorirnsii, 
and  BehmHiUt  OiBeaii. 

{b)  The  Oxford  Clay— so  called  from  the  name  of  the  county  through  which  it  pastes 
in  its  coiirsa  from  the  coast  of  Dorsetshire  to  tliat  of  Yorkshire — consists  mainly  of  layers 
of  stiff  hlue  and  brown  clay,  attaining  a  thickness  of  from  300  to  600  feet.  From  the 
nature  of  its  msterial  and  the  .conditiona  of  its  dcgiosit,  this  rock  Is  deflcieut  in  same 
forms  of  life  which  were  no  doubt  abundant  in  neighbouring  arems  of  clearer  water. 
Thus  there  are  no  corals,  hardly  any  species  of  echinoderms,  no  polyzoa,  and  less  than  a 
dozen  s|i«cica  of  brachiopodg.  Some  lamallibranchs  are  abundant,  particnlarly  llryphaa 
•iitalatii  and  Oslrfa  (both  forming  sometimes  wide  oyster-beds),  Linia,  Aviaiia,  PteUn, 
AatarU,  Trigonia  {rliireltaln,  eloiigala,  irregiilarit),  Nutula  (A',  nuda.  If.  Phitlipni) — 
'  Etheridge,  Q.  J.  Oeol.  Sor.  1882,  Address,  p.  202. 


908  STRATIGRAPHICAL  GEOLOGY         book  vi  pakt  m 

the  whole  Laving  a  great  similarity  to  the  assemblages  in  the  clayey  beds  of  the  Lower 
Oolite.  The  gasteropods  are  not  so  numerous  as  in  the  calcareous  beds  below,  but  belong 
mostly  to  the  same  genera.  The  ammonites,  especially  of  the  Omati,  Deniati,  FUxuoti, 
and  Arnmti  groups,  are  plentiful, — A.  corcUUus,  A,  Duneani,  A,  Elisabethm  {Jcuon),  A. 
Lamberti,  A,  oculaliis,  A.  omatuSy  A,  athleta  being  characteristic.  Two  ammonite 
zones  have  been  determined  in  this  part  of  the  group,  viz.  : — 

Zone  of  Amni.  cordatus  {A,  Lamherti,  &c.) 

,,  ,,      Jason  {A,  orncUus^  *4.  athleta^  &c.) 

The  l)clemnites,  which  also  are  common,  include  B,  kasUUus  (found  all  the  way  from 
Dorsetshire  to  Yorkshire)  and  B,  puzosmmts.  The  fishes  include  the  genera  AspitU^ 
rhyiichuSf  Hybodus^  Ischyodus^  and  Lepidotus,  The  reptilian  genera  Ichthifosaurvf, 
Marmnosaurus,  PlcsiosauruSy  Flioaaurus,  and  MegaJosaurua  have  been  noted. 

(2)  Coral lian,  traceable  ^ith  local  modifications  from  the  coast  of  Dorset  to  York* 
shire.  The  name  of  this  group  is  derived  from  the  numerous  corals  which  it  contains. 
According  to  the  exhaustive  researches  of  Messrs.  Blake  and  Hudleston,^  this  group 
when  complete  consists  of  the  following  subdivisions  : — 

t>.  Supra-t'orallian  beds — clays  and  grits,  imrluding  the 
Upper  Calcareous  Grit  of  Yorkshire,  and  the  Sands- 
foot  clays  and  grits  of  Weymouth. 

.'».  Coral  Rag,  a  rubbly  limestone  composed  mainly  of 

masses  of  coral  (sub-zone  of  Cidaris  JlorUjemvia).      }'2k)ne  of  Amm.  plicatilis. 

4.  Coralline  Oolite,  a  massive  limestone  in  YorkHhire, 
but  dying  out  southwards  and  reappearing  in  the 
form  of  marl  and  thiu  limestone. 

3.   Middle  Calcareous  Grit,  probably  i)eculiar  to  Yorkshire. 

2.   Lower  or  Hambletou  Oolite,  not  certainly  recognise<r 

out  of  Yorkshire.  }-       ,,  ,,     perarmatus. 

1.   Lower  Calcareous  Grit. 


} 


The  corals  are  found  in  their  positions  of  growth,  forming  massive  coral -banks 
in  Yorkshire  {ThamnaMraea^  Isaslreca,  ThecosmiUa,  lihabdojthyllia  (Fig.  384),  kc] 
Numerous  sea-urchins  occur  in  many  of  the  be<ls,  particularly  Cidaris  floriganmn 
( Fifj.  386),  also  Pygunis,  Pycjastcrj  Hemicidaris,  kc.  Brachiopods  are  comparatively 
infrequent.  The  laniellibranchs  are  still  largely  represented  by  species  of  Avkuia, 
Liiiut,  (Js/rca,  Pcctcu,  and  Grifpheca  {Ostrca  gregaria  being  si>ecially  numerous).  Nearly 
all  tile  species  of  gasteropods  are  peculiar  to  or  characteristic  of  the  Corallian  stage.  Tin- 
•listinctive  ammonites  are  A.  ancrpsafbitSj  A.  bitbeaniof,  A.  Bergen^  A.  cadoneasis^  A. 
(/'cijuc/i.Sj  A.  rn^u'llciiSi's,  A.  p/icatilui,  A.  pcrarmatus,  A.  pstudo-cardatu^y  A.  retroflcxmt, 
A.   JVUliamsonL 

The  Ui'i'KU  or  Puiitland  Oolites  bring  before  us  the  records  of  the  closing  epochs 
of  the  lon^  Jurassic  jieriod  in  England.  They  are  divisible  into  three  groups:  (1) 
Kimeridgian,  at  the  btise  ;  (2)  Portlandian,  and  (3)  Purbeckian. 

(1)  Kimeridgian,  so  named  from  the  clay  at  the  base  of  the  Upi)er  Oolites, 
well  developed  at  Kimeridge,  on  the  coast  of  Dorsetshire,  whence  it  is  traceable  con- 
tinuously, save  where  covered  by  the  Chalk,  into  Yorkshire.  It  consists  of  dark  bluish- 
grey  shale  or  clay,  which  in  Dorsetshire  is  in  i>art  bituminous  and  can  be  burnt. 
According  to  31  r.  J.  F.  Blake  it  may  be  subdivided  into  two  sub-groups : — 

{<>)  rp])t'r  Kimeridgian,  consisting  of  paper-shales,  bituminous  shales,  cement 
stone,  and  clays,  ch.aracterised  hy  a  comparative  paucity  of  species  of  fossils 
but  an  infinity  of  individuals  ;  perhaps  650  feet  thick  in  Dorsetshire,  but 
thinning  away  or  disappearing  in  the  inland  counties.  This  zone  is  fairly 
conii>aral)le  with  the  *' Virgulian  suh-stage  "  of  foreign  authors. 


1    (i 


On  the  Corallian  Rocks  of  England,"  Q.  J,  (Jeol.  Soc.  xxxili.  p.  260. 


SECT,  ii  S  2 


JURASSIC  SYSTEM 


909 


(h)  Lower  Kimeridgian,  blue  or  sandy  clay  with  calcareous  "doggers,"  represent- 
ing the  "  Astartian  sub-stage  "  of  foreign  geologists.  This  is  the  great  re- 
l»ository  of  the  fosHils  of  this  group.* 

Amou^  the  more  common  fossils  are  numerous  foraminifera  {Pulviilimt  pulchella, 
llohulina  Mihisteri)^  also  Serpula  tetragona,  I>iscitia  lalissiviaj  Rrogyra  virgula  (Fig. 
392),  E.  nana,  Adarte  supracorallina,  Thraeia  depressa,  Corhula  Deshaycsiiy  Cardium 
Mriatulum  (Fig.  392).  Upwards  of  20  species  of  ammonite  occur  only  in  this  stage  ; 
among  them  are  A.  accipitris^  A,  altcmans^  A.  Bcaugrandi,   A.  flexuosus.  A,  Kapjii, 

A.  lallerianus,  A.  laiUabilis,  A.  Thunnanni,  A.  triplex.     Among  the  belemnites  are 

B.  abbreviatus,  B.  cxceniricusj  B.  cxplaiuUu^,  B.  nitidxis.  The  Kimeridge  Clay  derives 
its  chief  palaeoutological  interest  from  the  fact  that  it  has  supplied  the  largest  number 
of  the  Mesozoic  genera  and  species  of  reptiles  yet  found  in  Britain.  The  huge  deinosaurs 
are  well  represented  by  Bothriospondylus^  Cetiosaurus,  CryptodracOj  (jrigantosaurus, 
Iguniu/don  {Camptosaurus),  Megalosaurus,  OnLosaurus  ;  the  pterosaurs  by  Pterodadylus  ; 
the  plesiosaurs  by  Plesiosaurus  and  Pliosaitru^  ;  the  ichthyosaurs  by  Jclithyosauriaf  and 
f>])hUialmosaurus;  chelonians  hy  Enaliochclys  and  Pclohatochelys ;  auvl  crocodilians  by 
iJakosauriis,  StciieomuruSy  and  Teleosaiiruit.'^ 

In  the  sea-ijliffs  of  Speeton,  Yorkshire,  a  thick  group  of  clays  occurs,  the  lower  i>art 
of  which  contains  Kimeridgian  fossils,  while  the  higher  portions  are  unmistakably 
Cretaceous  (p.  939).  Traces  of  a  representative  of  the  Kimeridge  Clay,  and  possibly 
of  the  Portlandian,  above,  are  found  even  as  fur  north  as  the  east  coast  of  Cromarty 
and  Sutherland,  at  Eathie  and  Helmsdale. 

(2)  Portlandian,  so  named  from  the  Isle  of  Portland,  where  it  is  typically 
developed.  This  group,  resting  directly  on  the  Kimeridge  Clay,  consists  of  tw«> 
divisions,  the  Portland  Sand  and  Portland  Stone.  At  Portland,  according  to  Mr. 
J.  F.  Blake,  it  presents  the  following  succession  of  beds  in  descending  order  :  •'  — 

Shell  limestone  (Roach),  containing  casts  of  Cerithium  portlamHcum  (very 
abundant),  S(/icerbya  Dukei,  Buccinvm  naticoideay  and  casts  of  Trigonia. 

"Whit-bed" — Oolitic  Freestone,  the  well-known  Portland  stone  {Ammonites 
giganteus). 

"Ciirf,"  another  calcareous  stone  {Ostrea  solitoria). 

*'  Base-bed,"  a  building  stone  like  the  whit-bed,  but  sometimes  containing 
irregular  bands  of  flint. 

Limestone,  *' Trigonia  bed  "  {Trigonia  giftbosa,  Fig.  392,  Pernn  mytiloides). 
■{  Bed  (3  feet)  consisting  of  solid  flint  in  the  upper  and  rubbly  limestone  in  the 
lower  flat. 

Baud  (6  feet)  containing  numerous  flints  {Serpida  gm'dialis,  Ostren  midti- 
fur  mis). 

Thick  series  of  layers  of  flints  irregidarly  spaced  [Ammonites  hyloniensis,  Tri- 
gonia gibbosa,  T  incurra). 

Shell-bed  abouniling  in  small  oysters  and  serpula*  {Ammonites  pseudiMjigas, 
A.  triplejTy  Plexirotomaria  rugata,  P.  Bozetiy  Cardium,  dissimile^  Fig.  392, 
Trigonia  gitihosa,  T.  incurca,  Pleuromya  tellina). 

'  Stitr  blue  marl  without  fossils  (12  to  14  feet). 
Liver-coloured  marl  and  sand  with  nodules  and  bauds  of  cement  stone — 26  feet 

{Mytilus  autissiodorensis,   Pecten  soti<lus,   Cyprinn   implirata.  Ammonites 

biplex,  &c.) 
Oyster-Vjed  (7  feet)  composed  of  Exogyra  brunt rutana. 
Yellow  sandy  beds — 10  feet  {Cyprina  implicata,  Area), 
Sandy  marl  (at  least  30  feet)  passing  down  into  Kimeridge  Clay  {Ammonites 

biplex,  Lima  hAoniensis,  Pecten  Morini,  Avicula  tpctaria^  Trigonia  incurva, 

T.  mnricata,  T.  Pellati,  Rhynchonella  portlandica,  Discina  hnmphriesiuna). 


o 

32 


O 


at 

*j 
u 
o 

Oh 


Among    Portlandian    fossils  a  single    species  of  coral    {Isastra'a  oblonga)   occurs; 

1  J.  F.  Blake,  "  On  the  Kimeridge  Clay  of  England,"  Q.  J.  Oeol.  Soc,  xxxi. 
^  Etheridge,  Q.  J.  (Jeol.  So<\  1882,  Address,  p.  221.  ^  Q.  J,  Oeol.  S,>c.  xxxvi.  i>.  189. 


910  STRATIGRAPHICAL  GEOLOGY  book  vi  pakt  in 

echinoderms  are  scarce  {Acrosalenia  KOnigi,  &c.),  there  are  also  few  brachiopods.  The 
most  abundant  fossils  are  lamellibranchs,  the  best  represented  genera  being  Trigtmia 
( r.  gibbosa,  T.  iiicurva)^  Astarte,  MytUus,  Peden^  Linuiy  Pema,  Ostrea,  Cfyprina,  Lucima 
{L.  portlandica\  Cardium  {C.  dissimile),  Pleuromya,  The  most  frequent  gasteropod  is 
Cerithiuia  p&rllaiidicicm.  The  ammonites  include  A.  gigaiUeus,  pseiuiogigas,  bohmieimt, 
gravesiamis,  pectincUus.  Fish  are  represented  by  Gyrodus,  Hyhodus,  iKhyodus,  and 
PyaioduSf  and  some  of  the  older  Jurassic  reptilian  genera  {Steneosaurus,  Plenomturtu^ 
Pliomurusy  Cetiosaunis,  Megalosaurus)  still  appear,  together  with  the  crocodile 
Goiiiopholib:^ 

(3)  Purbeckian. — This  group,  so  named  from  the  Isle  of  Purbeck,  where  be«t 
developed,  is  usually  connected  with  the  foregoing  formations,  as  the  highest  zone  of  the 
Jurassic  series  of  England.  But  it  is  certaiuly  separated  from  the  rest  of  that  aeries  by 
many  peculiarities,  which  show  that  it  was  accumulated  at  a  time  when  the  phyaical 
geography  and  the  animal  and  vegetable  life  of  the  region  were  undergoing  a  remarkable 
change.  The  Portland  beds  were  upraised  before  the  lowest  Purbeckian  strata  were 
deposited.  Hence,  a  considerable  stratigraphical  and  palsontological  break  is  to  be 
remarked  at  this  line.  The  sea-floor  was  converted  partly  into  land,  partly  into  shallow 
estuaries.  The  characteristic  marine  fauna  of  the  Jurassic  seas  nearly  disappeared  from 
the  area.  Fresh-water  and  brackish-water  forms  characterise  the  great  series  of  strata 
which  reaches  up  to  the  Xeocomian  stage,  and  might  be  termed  the  Purbeck-Wealden 
series. 

The  Purbeckian  group  has  been  divided  into  three  sub-groups.  Of  these,  the  lowest 
(95  to  160  feet)  consists  of  fresh-water  limestones  and  clays,  with  layers  of  ancient  aoO 
("dirt  beds")  containing  stumps  of  the  trees  which  grew  in  them  ;  the  middle  com- 
prises 50  to  150  feet  of  strata  with  some  marine  fossils,  while  the  highest  (50  to  60  feet) 
shows  a  return  of  fresh -water  conditions.  Among  the  indications  of  the  presence  of  the 
sea  is  an  oysteV-bed  {Ostrea  distorts)  12  feet  thick,  with  PecUn,  Modiola,  Aviciila,  7%rada, 
&c.  Tlie  fresh-water  bands  contain  still  living  genera  of  lacustrine  and  flnviatile  shells 
{Pafudina,  Li/nnaxtj  Planorbis,  Pkysa^  Valrata,  Unio,  Cyrena),  Numerous  fishes, 
placoid  and  ganoid,  haunted  these  Purbeek  waters.  Many  insects,  blown  oflf  from  the 
a<.ljac'ent  land,  sank  and  were  entombed  and  preserved  in  the  calcareous  mud.  These 
include  coleopterous,  orthopterous,  hemipterous,  neuropterous,  and  dipterous  forms 
(Fig.  395).  Remains  of  several  reptiles,  chiefly  chelonian,  biit  including  the  Portlandian 
crooodile  Goniopholis,  and  likewise  some  interesting  dwarf  crocodiles  {7%criosuckiis  is 
computed  to  have  been  only  18  inches  long),  have  also  been  discovered.  The  most 
remarkable  organisms  of  this  group  of  strata  are  the  mammalian  forms  already  noticed 
(p.  893),  which  occur  almost  wholly  as  lower  jaws,  in  a  stratum  about  5  inches  thick, 
lying  near  the  base  of  the  Middle  Purbeek  sub-group,  these  being  the  portions  of  the 
skeleton  that  would  be  most  likely  first  to  drop  out  of  floating  and  decomposing 
carcases. 

The  zone  of  Bckmnitds  lateralis  in  the  Speeton  Clay  of  the  Yorkshire  coast  and 
the  Spilsby  Sandstone  of  Lincolnshire,  are  considered  by  Prof.  A.  Pavlow  and  Mr. 
G.  W.  Lami>high  to  represent  in  part  the  Purbeek  and  Portland  beds  of  the  southern 
districts.- 

France  and  the  Jura. — The  Jurassic  system  is  here  symmetrically  developed  in 
the  form  of  two  great  connected  rings.  The  southern  ring  encloses  the  crystalline  axis 
of  the  centre  and  south  ;  the  northern  and  larger  ring  encircles  the  Cretaceous  and 
Tertiary  basin  and  opens  towards  the  Channel,  where  its  separated  ends  point  across  to 
the  continuation  of  the  same  rocks  in  England.  But  the  structure  of  the  two  areas  is 
exactly  opposite,  for  in  the  southern  area  the  oldest  rocks  lie  in  the  centre  and  the 
Jurassic  strata  dip  outwards,  while  in  the  northern  region  the  youngest  formations  lie 

^  J.  F.  Blake,  op.  cit.  and  Etheridge,  op.  cit,     Damon's  *  Geology  of  Weymouth,'  1884. 

^  Bull.  Soc.  Imp.  des  Sat.  MoscoUj  1891. 


SECT,  ii  §  2  JURASSIC  SYSTEM  911 

in  the  centre  and  the  Jurassic  beds  dip  inward  below  them.  Whei*e  the  two  rings 
unite  in  the  middle  of  France  they  send  a  tongue  down  to  the  Bay  of  Biscay.  On  the 
eastern  side  of  the  country  the  Jurassic  system  is  copiously  developed,  and  extends 
thence  eastwards  through  the  Jura  Mountains  into  Germany. 

The  subdivisions  of  the  Jurassic  system  in  the  north  and  north-west  of  France 
belonging  to  what  has  been  termed  the  Anglo- Parisian  basin,  resemble  generally  those 
established  in  England.  But  in  the  southern  half  of  the  country,  and  generally  in  the 
Mediterranean  province,  the  facies  departs  considerably  both  lithologically  and  palo;- 
ontologically  from  the  English  type,  more  particularly  as  regards  the  Upper  Jurassic 
rocks.  The  following  table  gives  in  descending  order  a  summary  of  the  distribution  of 
the  Jurassic  system  in  France  :  ^ — 

10.  Portland ian,  separated  into  two  sub-stages.  At  the  base  lie  sands  and  clays, 
equivalents  of  the  Portland  sands,  or  **  Bonouian  "  with  AmmonUes  {Slephanoceras) 
porUandicwnif  A .  gigaa^  and  Oatrea  eaqmnsa.  Higher  up  come  sands  and  calcareous 
sandstones  corresponding  to  the  Portland  stone,  with  Trigonia  gibbosa  and  Peri- 
sphincles  tranntarius.  The  Purbeckian  is  marked  by  Corhula  inflexa.  The  stage  is 
best  developed  along  the  coast  near  Boalogne-sur-mer,  where  it  is  composed  of  about 
75  feet  of  clays,  sands,  and  sandstones,  with  Acrosaleiiia  Koenigif  Penia  Bauchardi, 
Echincbrissua  BrodieL,  Cardium  Pdlatij  Trigonia  radUUa^  T.  gibbosa^  T,  incurva, 
&c.  At  the  top  lies  a  bed  of  limestone  containing  Cyrena  Pdlati^  Cardium  dissimiU, 
and  covered  by  a  travertin  with  Cypris^  which  may  represent  the  Purbeck  beds. 
Far  to  the  south,  in  Charente,  some  limestones  containing  Portlandian  fossils  are 
covered  by  others  with  Corbula  infiexa^  Phyaa,  PiUudina^  &c.,  possibly  Purbeck. 
Fresh -water  limestones,  gypsiferous  marls  and  dolomites  (about  200  feet),  and 
containing  Corbula  forbesianaj  Physa  loealdiana^  Vol  rata  hdicoides,  Trigonia  gibbosa^ 
&c.,  occur  in  the  Jura,  round  Pontarlier  and  near  Morteau,  in  the  valley  of  the 
Doubs. 

The  Upper  Jurassic  rocks  of  southern  France  and  the  southern  flank  of  the  Alps, 
or  what  has  been  termed  the  Mediterranean  basin,  present  a  facies  so  different  from 

.  that  which  was  originally  studied  in  England,  northern  France,  and  Germany  that 
much  difliculty  was  for  many  years  experienced  in  the  correlation  of  the  deposits, 
and  much  discussion  has  aiisen  on  the  subject.  From  the  researches  of  Oppel, 
Benecke,  Hebert,  and  later  writers,  the  true  meaning  of  tlie  southern  facies  is  now 
better  understood.  It  api^ears  that  the  divisions  ranging  above  the  Oxfordian  are 
represented  in  the  southern  area  by  a  singularly  uniform  series  of  limestone^i, 
indicative  of  long  unbroken  deposition  in  deeper  water,  and  unvaried  by  those 
oscillations  and  occasional  terrestrial  conditions  which  are  observable  farther  north. 
The  name  of  Tit  h  on  ian  was  given  by  Oppel  to  this  more  uniform  suite  of  strata, 
which  were  marked  by  the  mixed  character  of  their  cephalopods,  and  by  their 
peculiar  perforated  brachiopods  of  the  type  of  Terebratula  diphya  {janitor),'- 
Around  Grenoble,  the  m'assive  limestones  resting  upon  some  marls  with  species 
belonging  to  the  zone  of  Ammonites  tenuilofjatus,  contain    TerebrditUa   diphya 


^  For  a  detailed  account  of  the  development  of  the  Jurassic  rocks  of  France,  see  De 
Lapparent's  *  Geologic,'  3rd  edition  (1893);  also  A.  d'Orbigny's  '  Pal^ontologie  Fran^aise 
—Terrains  Oolithiques,*  1842-50;  D'Archiac,  *  Paleontologie  de  hi  France,'  1868,  and 
*  Pal^ntologie  Frau^aise,  continue  par  une  reunion  de  Paleontologistes — Terrain  Jurassique, ' 
in  course  of  publication  ;  Hebert,  *  Les  Mers  anciennes  et  leurs  Rivages,  dans  le  Bassin  de 
Paris,'  1857  (a  most  interesting  and  valuable  essay),  and  numerous  papers  in  Bull.  Soc.  OioL 
France  ;  Monographs  by  Loriol,  Cotteau,  Pellat,  Royer,  Tombeck  ;  Gosselet's  'Esquisse,*  cited 
ante,  p.  733  ;  J.  F.  Blake,  Q.  J,  OeoL  Soc,  1881,  p.  497,  gives  a  bibliography  for  N.  W.  France, 
and  Barrois  (Proc.  Geoi.  Assoc.)  gives  a  summary  of  results  for  the  Bonlonnais.  For  the 
last  named  district  consult  also  Pellat,  Bull.  Soc,  Oid.  France^  viii.  (1879) ;  Douville  et 
Rigaux,  op.  cit.  xix.  (1891)  p.  819.  Rigaux,  'Notice  Geologique  sur  le  Bas  Bonlonnais,' 
Boulogne-sur-mer,  1892. 

*  For  a  study  of  the  Tithonian  fauna  see  A.  Toucas,  Bull.  Soc.  OM.  France,  xviii. 
(1890)  p.  560. 


912  STHATIGRAPIIICAL  GEOLOGY         book  vi  part  ni 


associated  with  ammouites  closely  linked  with  Neoooniian  types.  In  the  Basse* 
Ceveuues,  the  limestouej^  attain  a  thickness  of  from  1200  to  1400  feet.  At  their 
base  lie  marls  and  marly  limestones  containing  AmmoniUs  nujurocephalut,  A. 
fraiisversariu4  and  A.  cardatus,  A  band  of  bluish  limestone  with  bituminons 
marls  (65  feet),  belonging  to  tlie  zone  of  A.  himainmalnSj  represents  the  Corallian. 
Sf)me  grey  limestones  (260  feet),  with  A.  polypU}rusj  contain  fossils  of  the  zone  of 
.1.  te/ntilobatHSf  equivalent  to  the  Sequaniau  stage.  These  are  succeeded  by  a 
massive  limestone  (330  feet)  with  Tcrehratulu  diphya  {Janitor)  and  Amm.  trait' 
Mitorii's,  and  this  by  a  comjjact  white  limestone  (500  -  650  feet)  with  Terdtratula 
inoracica  {Iiepdlini\  Cidaris  (jlandifentf  corals,  &c.  At  the  top  lie  some  lime- 
stones (200  feet)  with  Terebrattda  diphyuidei*  and  many  ammonites  {A,  OdyjMtt^ 
A.  privascnsinf  A.  fferriasensis^  &c.) 

V*.  Kimeridgian  =  Kimeridge  Clay,  diNided  in  central  and  northern  France  into 
the  following  sub-stages  in  ascending  onler :  1,  Sequaniau  or  Astartian  {Ostren 
dfUoidt'u^  zone  of  Anniwnite.H  tenuilobatus) ;  2,  I*terocerian  {Pteroctra  Oceania  zone 
of  Amm.  nmrtthicus) :  3,  Virgulian  {Ex.otjyra  vinjtda).  In  Normandy,  the  Coral- 
lian clays  with  Dicrras  arietinum  are  covered  by  other  clays  with  Ostrea  deitouien 
(Sequaniau),  aud  no<lular  limestone  with  Pteroc^nt  Oceani  (Pterocerian),  followed 
by  blue  clays  and  lumachelles  with  Exoffyra  rirgula  (Virgulian).  In  the  Pays  de 
Hray,  these  various  strata  are  330  to  400  feet  thick,  and  are  surmounted  by 
calcareous  marls,  sandstone  and  limestone  (115-160  feet)  containing  ihtrtu 
I'lito.lnniura,  Annniia  fn'ciyatUf  IlevikUluris  J/o/majini^  £cMnobrissus  Brodiei, 
Ostrett  hrnntrutanuy  and  representing  the  Bononian  sub-stage.  The  coast-sectiou 
near  Boulogne-sur-mer  exposes  a  series  of  clays,  sands,  and  sandstones  (180  feet), 
from  which  a  lar^e  series  of  characteristic  fossils  has  been  obtained,  and  which  as 
the  type  section  of  the  Bononian  beds,  indicate  a  local  littoral  deposit  in  the  up])er 
part  of  tlic  Kimeridge  Clay. 

hi    the    French  Ardennes,    the    Sequanian,   Pterocerian,   and    Vii^gulian    anb- 
stages  are  composed  of  a  succession  of  marls  ami  limestones  (500-560  feet),  the 
Se»|uaniaii  marls  and  lumachelles  l>eing  marke<l  by  Ostrea  (Mtoidea,  &c.,  the  Ptero- 
cerian limestones  by  Wtddheimui  hunieralis^  Ptenn'^ra  ponti,  &c.,and  the  Virgulian 
marls  by  immense  numlvers  of  JCxof/ynf  virgulo.     In  the  Meuse  and  Haute  Marne, 
a  gioup  of  compact  limestones,  more  than  500  feet  thick  (Calcaires  de  Barrois), 
with  Ai/imonifru  {Sfephanocertvi)  yiyos,  &c.,  represents  the  Bononian  sub-stage. 
Ill  Yonne,  the  Setjuanian  sub-stage  consi.sts  of  oolites  ami  contains  a  reef  of  coral  full 
of  bunches  of  St'jttofffnta,  Montlivaltia^  kc.     Towards  the  Jura,  this  sub-stage  (200 
tt'ct  tliick)  consists  of  limestones  aud  marls  {Asftirtf  niifiima)  :  the  Pterocerian  i.s 
well  (levelopcd,  aud  sliows  its  characteristic  fossils  ;  wliile  the  Bononian  comprises 
tlu'  so-called  "  P«»rllandiau"  limestones  of  the  .lura,  its  upper  part  becoming  the 
vt'llow  or  rtMl  unfossiliftMOUs   '*  Porllandian  dolomite."     In  the  department  of  the 
.lura,  the   IMeroceriaii   sub-stjige  contains  a  coral-reef,  more   than    J*00  feet  tliick. 
near  Saint  Claude,  and   farther  south  another  occurs  at  Oyonnax.     In  the  same 
rcf:ioii,  tlic  Viru'nlian   sub- stage,  composed  of  bituminous  shales  and  thin  litho- 
>s'ra]»lii«'   liincstoucs,  has  yielded  uumercms  fishes,  re])tiles,   and  abundant  cycads 
ainl  ferns.      The  position  of  these  be«is  is  fixed  by  the  occurrence  of  the  Jixw/yrtf 
rinjulo  below  them,  ami   of  the   Bononian  limestones  with   yerimca  and  Amnt. 
;/i)fi/s  above  them,      Kn)m  what  was  said  alK>ve  uiitler  the  Portlandian  stage,  it  will 
lie  seen  tliut   the  Kimeridgian  appears  in  a  totally  different  as{»ect  in  the  Medi- 
terranean ba>in,  bein-.;  there  <'ompose«l  of  thick  limestones  with  a  mixed  a.ssemblage 
of  ammonites,  ami  cliaracterist'd   in   the  liigher  i>arta   by  the  up{>e;irance  of  Tnr- 
liiatnlti  dijiln/ii. 

^.  Corallian,  divisible  into  ('/)  Argovian.  or  zone  of  sponges  smi  Annn.  canalicitla- 
///> ;  ;//)  (Ilyptieian,  or  zone  of  Olyptichiis  hi»'roy1yphicu3,  and  (c)  Diceratian,  or 
zone  of  l>io'riix  (tru-fitnun.  The  sub-stages  h  and  c  comprise  the  zone  of  Amm. 
I>iiininnn(ittis.  In  Noriiumdy.  the  stage  presents  a  lower  assise  (Trouville  oolite, 
or  zone  of  Amm.  MurtflU)  com|>osed  of  marly  and  oolitic  limestone  and  black 
flays  •  Kc/timJnisffus  .scufd/iis,  Triyoiiia  major,  kc.\  and  an  upper  coral-rag  with 
('iJtfits  ff,>i'i:/fiiim»i  and  a  «lark  marl  with  Kxtujyrn  nana  \  the  whole  passing 
laterally  into  clays  T  Havre).  In  the  Ardennes,  an  argillaceous  marl  (with  /*A<i- 
si<nnH>(  sfriiifti)  rejueseuts  the  Argovian  division,  and  is  surmounted  by  a  mass 
<.f  loral  limestone  MOO-PiO  feet).  In  Haute  Marne,  the  Corallian  betls  attain 
;i  tliickness  of  '.VM)  ftet,  but  are  mainly  formed  of  marls,  the  coral  beds  or  reefs 
<h\iinllin<,'  <l«»\vn  in  that  tlireetion.  South-westwards,  in  Burgundy,  massive  lime- 
>it<)nes  with  corals  rea]»pear,  with  lithograi>liii'  and  oolitic  limestones.     To  the  ea.st 


SECT,  ii  §  2  JURASSIC  SYSTEM  913 


also,  in  the  district  of  Be8aii9oii,  the  stage  is  represented  by  130  to  200  feet  of 
coral-limestone  with  compact  and  oolitic  bands,  and  sometimes  with  calcareous 
marls  that  abut  against  the  sides  of  what  were  formerly  coral-reefs.  Some  horizons 
in  the  Corallian  stage  are  marked  by  the  occurrence  of  remains  of  ferns  and  other 
land-plants  (Saint  Mihiel,  in  Lorraine  ;  Dept.  of  Indre). 
7.  Oxfordian,  divisible  into  («)  Callovian,  with  zones  of  Amm.  viacrocephaltUj  and 
A.  anceps,  and  {b)  Oxfordian,  with  zones  A,  Lamberti^  A,  Mariae^  A,  cordnt\is. 
This  stage  is  well  exposed  on  the  coast  of  Calvados,  between  TrouvUle  and  Dives, 
where  it  attains  a  thickness  of  330  feet,  and  is  divisible  into  a  lower  sub-group  of 
marls  (Dives)  with  Amm,  Lamberti,  a  middle  sub-group  of  clays  (Villiers)  with 
A.  Mariwy  and  an  upper  sub-group  of  clays  with  A.  cordatiis.  It  is  likewise  dis- 
played in  the  Boulonnais.  North-eastwanls,  in  the  Ardennes,  the  Callovian  sub- 
stage  appears  as  a  pyritous  clay  (25-30  feet)  with  oolitic  limonite,  the  Oxfordian 
as  a  series  of  clays,  marls,  ai^gillaceous  sandstone  (full  of  gelatinous  silica  and 
locally  known  as  t/aize)  and  oolitic  ironstone.  In  the  Cote-d'Or,  the  fossils  of  the 
Callovian  and  Oxfordian  beds  are  mingled  in  the  same  strata.  Round  Poitiers, 
the  Callovian  division  is  upwards  of  100  feet  thick.  Eastwards  it  dwindles  down 
towards  the  Jura,  but  is  recognisable  there  under  the  Oxfordian  pyritous  marls 
(330  feet). 

6.  Bathonian  (Grande  Oolithe)  may  be  divided  into  a  lower  sub-stage  (Vesulian) 
with  the  zone  of  Ostrea  acuminata  and  Atnvi.  /errtigineus^  and  an  upper  (Brad- 
fordien)  with  the  zones  of  Rhynchonella  decorata  and  Waldheimia  digona  {Amm, 
aspuloidea).  In  Normandy,  it  consists  of  (a)  a  lower  group  of  strata  which  at  one 
part  are  the  Port-en- Bessin  marls  (100  feet  or  more)  and  at  another,  the  famous 
Caen  stone,  so  long  used  as  a  building  material,  and  which  from  its  saurian  and 
other  remains  may  be  paralleled  with  the  Stonestield  Slate  ;  (6)  granular  limestone 
(Ranville),  bryozoau  limestone,  with  some  of  the  fossils  of  the  Bradford  Clay.  In 
the  Ardennes,  the  Fuller's  Earth  is  represented  by  some  sandy  limestones,  luma- 
chelles,  and  granular  limestone,  with  Ostrea  acuminata^  Amm,  Parkinson i^  Bdem- 
nites  (jiganteuSy  kc. ;  the  Great  Oolite  by  a  massive  limestone  (160-200  feet)  with 
Cardium  pts-bovis^  Purpura  minax,  followed  by  1  $0-180  feet  of  limestones,  with 
numerous  fossils  {Rkynchonella  decorata^  R.  elegantula,  Ostrea  flaheHoides^  &c.) 
The  limestones  are  replaced  eastwards  by  marly  and  sandy  beds.  In  the  C6te- 
<rOr,  the  stage  is  largely  developed,  and  is  divided  into  three  sub-stages :  (a) 
Lower  (115  feet),  limestones  and  marls  with  zones  of  Honwmya  gibbosoj  Tert- 
hratula  Mamlelslohi^  Pholadofnya  bucardium  ;  {h)  Middle  (196  feet),  white  lime- 
stones and  oolites  with  zone  of  Amm,  arhustigents.  Purpura  glabra  and 
echinoderms ;  (c)  Upper  (82  feet),  limestones  and  marls  with  Eudesia  cardium, 
Waldheimia  digona^  Pemastrea  PeUatif  Pentacrinus  Bumgnieri^  and  with  land- 
plants  in  one  of  the  zones.  ^ 

5.  Bajocian  (Oolithe  Inf^rieure)  is  well  developed  in  the  department  of  Calvados,  the 
name  of  the  stage  being  taken  from  Bayeux.  Its  thickness  is  60-80  feet,  and  it 
consists  of:  1,  Lower  limestone  {Amm.  Mnrchisonee)  \  2,  limestone  with  numerous 
ferruginous  oolites,  fossils  abundant  and  well  preserved  {Amm.  humphriesianusy 
A.  Sowerbyiy  A.  Parkinsoniy  &c.)  ;  3,  Upper  white  oolite  with  abundant  brachio- 
pods,  sponges  and  urchins  {Amm.  Parkinsonij  Terehratula  Phil^ipsi,  Stomechinus 
hig^ranvlarisy  kc.)  In  the  French  Ardennes,  the  stage  presents  a  lower  group  of 
marls  (32  feet)  with  Amm,  Murchisonw,  A.  Soiaerbyi\  &c.,  followed  by  an  upper 
limestone  (30-130  feet)  with  Amm.  Blagd^niy  A.  siibradiatus,  Belem.  giganteus, 
kc.  Towards  Lorraine,  this  limestone  becomes  charged  with  corals,  some  parts 
being  true  reefs.  North  of  Metz,  the  stage  is  mostly  limestone,  and  reaches  a 
thickness  of  330  feet.  In  Burgundy,  the  stage  is  chiefly  a  crinoidal  limestone 
(100  feet),  capping  boldly  the  Liassic  marls.  In  the  Jura,  it  attains  a  thickness  of 
upwards  of  300  feet,  and  consists  chiefly  of  limestone.  In  Southern  France,  it 
swells  out  to  great  proportions,  reaching  in  Provence  a  thickness  of  950  feet, 
where  it  consists  of  the  following  assises  in  ascending  ordc:  1,  Amm,  Murchisanw  ; 
2,  A.  Sauzei ;  3,  A.  humphriesianu^ ;  4,  A.  nioriensis. 

4.  Toarcian  (from  Tliouars  =  Upper  Lias).  In  Lorraine,  this  stage  (330-370  feet 
thick)  consists  of  a  lower  series  of  marls  fol1owe<l  by  sandstone  and  an  oolitic 
brown  ironstone  containing  Ammonites  opcUinus,  A.  insig^nis^  Belemnites  abbre- 


^  For  a  study  of  the  gasteropods  of  this  zone  in  France  see  M.  Cossmann,  Mem,  OioL 
Soc.  France  (3),  tome  iii.  No.  3  (1885). 

3  N 


9 1 4  STKA  TIGRA PHICAL  GEOLOGY         book  vi  pam  in 

vicUus,  This  ironstone  is  traceable  Irom  the  Ardk;he  to  Lnxemboui^.  In  the 
Ardennes,  the  stage  includes  a  lower  series  of  marls  and  clays  (300  feet)  with 
Amm.  serpentinuSf  a  middle  marl  containing  Amnu  radianSf  A.  b\/r<ms,  and  an 
upper  limonite  (Longwy)  vrith  Amm.  opalinusj  Oatrea  ferruginea,  Trigonia  novu. 
In  Yonne  and  Cdte-d'Or,  it  consists  of  the  following  members  in  ascending  order : 

1,  marls  with  Posidonia  and  lumachelle  with  Amm.  serperUinus  (15-30  feet)  ; 

2,  marls  with  A,  coinpLanatus  (26  feet) ;  3,  marls  with  Turbo  subduplieaius  (12- 
20  feet)  ;  4,  blue  marls  with  CanceUopkycua  liassicua  (25-30  feet).  Near  St. 
Amand,  Cher,  the  stage  consists  of  nearly  200  feet  of  marls  and  clays  with  seven 
recognisable  zones.  In  the  Haute  Mame,  it  is  nearly  as  thick.  In  the  Hhone 
basin,  it  consists  of  a  lower  group  of  limestones  with  Peeten  aequivalvis,  and  an 
upper  group  of  ferruginous  beds,  including  an  important  seam  of  oolitic  ironstone, 
and  containing  the  zones  of  Amm.  bi/rons  and  A.  opalinus.  In  Provence,  it 
reaches  a  thickness  of  950  feet,  and  in  this  region  the  whole  liassic  subdivisions 
attain  the  great  depth  of  2300  feet.  In  Normandy,  the  Toarcian  stage  is  only 
about  20  feet  thick,  but  shows  the  characteristic  ammonite  zones. 

3.  Liassian  (=  Middle  Lias  and  Lower  Lias,  in  part).  In  Lorraine,  where  this  stage 
reaches  a  thickness  of  230  to  260  feet  it  consists  of  the  following  three  assises  in 
ascending  order :  1,  limestones  {Amm^niUs  Davosi)  and  marls  ;  2,  marls  {A. 
margarUatus)  ;  3,  sandstones  (Qr^hma  regularis).  In  the  French  Ardennes,  it 
is  360  feet  thick,  and  comprises :  1,  sandy  clay  with  Amm^  planicosia,  Oryphva 
reyidaris,  Plicatula  spitiosa;  2,  marl  with  BdemniUs  davatus,  Amm.  eapricomus; 

3,  ferruginous  limestone  with  Amm.  apincUus,  Bel.  paxiUostis.  Westwards  this 
stage  becomes  almost  wholly  marly.  In  Yonne  and  COte-d'Or,  it  is  divisible  into 
three  assises,  in  the  following  ascending  order  :  1,  Belemnite  limestone  of  Venaiey 
(40  feet),  comprising  the  zones  of  (a)  Amm.  Valdani,  (6)  A.  venarensis,  (c)  A. 
HeTileyif  (d)  A.  Davori ;  2,  micaceous  and  pyritous  marls,  about  200  feet;  3, 
nodular  limestone  with  large  gryphites,  comprising  the  zones  of  (a)  Amm^  zeies, 
{b)  Peeten  xquivalvis,  (c)  Amm.  acanthus.  Near  St.  Amand,  Cher,  the  stage 
consists  of  nearly  300  feet  of  marls  and  marly  limestone  with  the  zones  of  (a) 
Qryphwa,  regularUf  (6)  Amm,  raricostatus^  (c)  A.  ibex,  (rf)  ^4.  DavoH,  («)  A. 
margaritaius^  (/)  A.  spinatiis.  In  the  Rhone  basin,  it  varies  up  to  340  feet  in 
thickness,  but  in  Pi*oveuce,  it  expands  to  nearly  900  feet,  the  lower  half  composed 
chiefly  of  limestones  and  the  upper  half  of  marls.  In  Normandy,  it  is  chiefly 
belemnite  limestone,  50  to  65  feet  thick. 

2.  Siuemurian  (  =  Lower  Lias).  Thi.s  stage  (Lias  a  grj'ph^es  arqu^es)  is  typically 
developed  at  Seniur,  COte-d'Or  (whence  its  name),  where  it  consists  of  nodular 
gryphite  limestone  with  marly  bands  (23-26  feet),  and  is  divisible  into  three 
zones,  whicli,  counting  from  below,  are  marked  respectively  by:  1,  Ajmnonitejs  roti- 
form  is  ;  2.  A.  Bucklandi ;  3,  ^1.  stelloris.  Near  St.  Amand,  Cher,  it  is  composed 
of  al>out  15  feet  of  marly  limestone,  which  represent  only  it«  upi>er  part.  In  the 
Haute  Marne  and  Jura,  it  is  a  limestone  with  curved  gryphites,  and  ranges  from 
15  to  25  feet  in  thickne.ss.  In  the  basin  of  the  Rhone  it  is  a  calcareous  formation, 
20  to  25  feet  thick,  containing  the  zones  of  Ammonites  Davidsoni^  A.  stellarisy 
A.  o.i:ynntusy  and  A.  planienst/i.  Farther  south,  it  swells  out  in  Provence  to  275 
feet,  and  is  separable  into  a  lower  group  with  Amm.  Buckiandij  and  a  higher 
with  Belevinites  aattua^  Amm.  hisulcalus.  In  Normandy,  it  is  about  100  feet 
thick,  and  comprises  clays  and  marly  gryphite  limestones  {Ammonites  bisulcatu*)^ 
surmounted  by  gryphite  limestones  and  clays  {Belannites  hrevis^  Waldheimia 
cor.) 

1.  Hettangiau  (  =  Infra- Lias),  marly  and  shelly  limestones  with  Amynonites planarbia, 
&c.  (corresponding  to  the  Angulatus  and  Plauorbis  zones  at  the  base  of  the  Lias), 
resting  conformably  on  the  sandstones,  marls,  and  bone-bed  of  the  Aviculu  contmia 
zone  or  Rhaetic  group.  In  Lorraine,  this  stage  is  only  13  feet  thick.  In  Luxembourg, 
the  lower  or  Planorbis  zone  is  composed  of  dark  clays  alternating  with  bands  of 
fetid  limestone  (10-40  feet).  The  upper  or  Angulatus  zone,  consisting  mostly  of 
sandstone  (200  feet),  is  well  seen  at  Hettauge,  whence  the  name.  This  zone 
becomes  less  sandy  as  it  advances  into  Belgium,  where  it  forms  the  Marne  de 
Janioigne.  The  Hettangian  stage  of  Burgundy  is  thin,  and  is  composed  of  a  lower 
Lumachelle  de  Bourgogne  {Ostrea  irregularis^  Cardinia  Listeria  Ammonites  Bur- 
gundiA^  and  an  upper  marly  limestone  known  as  *'Foie  de  Veau  "  {Ammonites 
Burgundiif,  A.  moreanus).  In  the  basin  of  the  Rhone,  the  Planorbis  zone  is 
about  40  feet  thick,  and  the  Angulatus  zone  20  to  26  feet.  In  Cotentin,  the 
stage  is  divisible  into  a  lower  sub-group  of  marls  [Mytilus  minutus,  Corbula 


SECT,  ii  §  2 


JURASSIC  SYSTEM 


915 


Ludcvica)  and  an  upper  sub- group  of  limestones  {Cardinia  condnna,  Pecten 
valoniensis). 

One  of  the  most  interesting  features  of  the  Lias  in  the  northern  or  Jura  part  of 
Switzerland  is  the  insect-beds  at  Schambelen  in  the  Canton  Aargau.  The  insects  are 
better  preserved  and  much  more  varied  than  in  the  English  Lias,  and  include 
representatives  of  Orthoptera,  Neuroptera,  Coleoptera  (upwards  of  100  species  of  beetles), 
Hymenoptera,  and  Hemiptera.  About  half  of  the  beetles  are  wood-eating  kinds,  so 
that  there  must  have  been  abundant  Woodlands  on  the  Swiss  dry  land  in  Liassic 
time.^ 

Oermany. — In  north-western  (Germany  the  subjoined  classification  of  the  Jurassic 
system  has  been  adopted  : ' — 


g 

a 

•-9 
5 


a 


feS 


'Porbeck  group  (Serpulit,  a  limestone  160  feet  thick,  and  Miinder  Mergel,  a 

series  of  red  and  gr^en  marls,  with  dolomite  and  gypsum,  at  least  1000 

feet  thick),  forming  a  transition  between  the  Purbeck  and  Portland  groups. 
Eimbeckhauser  Plattenkalke  and  zone  of  Amm.  gigasj  equivalent  Xo  the 

English  Portland  group  {Corbula^  Modiola^  Paludina^  Cyrtna), 
Kimeridge  group,  Upper,  with  Eocogyra  n'rgrwto  =  Virgulian  ;   Middle  or 

Pterocera  beds  (Pterocerian) ;  Lower  (Astartian,  Upper  Sequanian),  with 

Nerinaea  beds  and  zone  of  Terebratyla  humeralU,* 
Corallian,  with  Cidaria  florigemmay  corals,  Pecten  varians,  Ostrea  rasieUariSy 

Xerineea  visurgis, 
Oxfordian,  with  Oryphtea  dikUatay  Amm.  perarmaius.  A,  cordatua. 
Clays  with  Amm,  oniatus^  A,  Jason,  A.  Lamberti,  A.  anceps,  A.  aihUta= 

*'Omatus  clays."    This  stage  is  usually  included  by  German  geologists 

in  the  Middle  Jura. 


Upper 

20-100 

ft. 


Middle 
50  ft. 


Lower 

up  to  500 

ft. 


(Clays,  shales,  and  ferruginous  oolite  with  at  the  top  the  zone  of 
Amm,    {MacrocepJudUes)    macrocephalus,  equivalent  to  the 
Callovian  or  Kellaways  rock,  and  at  the  bottom  that  of  ^ mm. 
Parkinsoni. 
'^  * '  Bifurcatus-schichten  "  with  Amm.   {Coamoceras)  bifurcatus. 
These  "  Bifurcatus  beds,"  with  the  Hauptrogeustein  above 
them,  including  the  zones  of  Oppellia  fusca  and  0.  aspidoidea, 
form  the  Bathonian  stage.  ^ 
"  Coronatus-schichten,"   clays   with    Amm.    (Stephanoceras) 
humphrieaianuSf  A,  Blagdeni,  A,  Braiktnridgeiy  and  many 
corals  of  the  genera  MonUivaUia,  ThecoamUia,  Cladophylliay 
Isastrasa,  Conjuaastrseay  and  Thamnaatreea.^ 
Ostrea  limestone  with  Oatrea  Marahi^  0.  edid^formia,  Trigonia 

coatata. 

^  Clays  with  BeUmnUea  giganteus. 
'Shades,  sandstones,  and  ironstones,  with  Inoceramua  polyploctia, 

Amm.  {Harpoceraa)  Murchtaonte,  Pecten  peraonatua. 
Clays  and  shales  with  Amm.  {Harpoceraa)  opalinua,  A.  toru- 
loauay  Trigonia  navia. 


*  Heer,  *  Urwelt  der  Schweiz,'  p.  82. 

^  Heinr.  Credner,  OUr.  Jura  in  N.  W.  Deutachland,  1863.  See  also  the  works  of 
Oppel  and  Quenstedt  quoted  on  p.  897,  and  K.  von  Seebach's  Der  Hannoverache  Jura, 
1864.  Brauns*  Unter.  MUU,  und  Ober.  Jura,  1869,  1871,  1874.  0.  Fraas,  *Geognos- 
tische  Bcvschreibung  von  Wiirttemberg,  Baden  und  Hohenzollem,'  Stuttgart,  1882  ;  Th. 
Engel,  *  Geognostischer  Wegweiser  durch  Wiirttemberg,'  Stuttgart  (1883). 

»  Struckmann,  N.  Jahrb.  1881  (il)  p.  102. 

^  For  an  account  of  the  fauna  of  this  stage  in  the  upper  Rhenish  lowland  see  A.  0. 
Schlippe,  Ahhand.  Oeol.  SpecialkaH.  Elaaaa-Lothr.  IV.  Heft  iv.  (1888). 

'  G.  Meyer,  'Korallen  des  Doggers,*  Ahhand,  Oeoi.  Specialkart.  Elaaaa-Lothr.  IV.  Heft 
V.  (1888). 


916 


STRATIGRAPHICAL  GEOLOGY 


BOOK  VI  PABTm 


{ Grey  marls  with  ^-1  mm.  (Lytoeeras)  JurentU  (Jurensis-Mei^), 

Upper    J      A,  {ffarpoctras)  radians. 

30  ft.      J  Bituminous  shales  (Poddonien-Schiefer)  with  Amm.   lytheiuu, 
V.     A.  communis,  A.  btfronSj  Posidonia  Bronni, 
f  Clays    with    A  mm.    tpinatus,  A.   {Amaliheus)  margariiatus. 

Middle  Belemnites  paxiUosus. 

80-100   -^  Marls  and  limestones  with  Amm.  caprtcomut,  A.  Davod. 


3 

«J-{ 

hi 


ft. 


Lower 
100-115 


Dark  clays  and  fermginoos  marls  with  A.  brevitpina,  A.  Jammi, 

A.  ibex^  A.  Jamesoni,  TerebrattUa  numitmalis. 
Clays  with  Amm,  obtvsus  (Tumeri),  A.  oxynoius,  A.  rarieag- 

tatus  (Oxynotenlager). 
Oil  shales  and  Pentacrinns  beds  resting  on  gryphite  limestone 

with  Amm.   {Arielites)  Buddandi,  A.  Conybeari,  Orypksea 

arcuataj    Lima    gigantta^    Spiriferina     Waleolti    (Arieten- 

schichten). 
Sandstones  with  ^  mm.  an^</a^i/4  (Angulatenschichten),  Cardinia 

Listeri. 
Dark  clays,  sandy  layers,  and   limestone  with  Amm.  ploHoriis 

(psilonotus)  (Psilonotenkalk). 


In  lithological  characters  the  German  Lower  or  Black  Jnra  presents  numy  points 
of  resemblance  to  the  English  Lias.  Some  of  the  shales  in  the  upper  diviaioii  are  so 
bituminous  as  to  be  workable  for  mineral  oil.  With  the  generml  sacoeaBion  of 
organisms  also,  so  well  worked  out  by  Oppel,  Quenstedt,  and  others,  the  English 
Lias  has  been  found  to  agree  closely. 

The  Dogger  or  Brown  Jura  represents  the  Lower  Oolite  of  England  and  the 
Etages  l^jocicn  and  Bathonien  of  France.  Its  lower  division  consists  mainly  of  dark 
clays  and  shales,  passing  up  in  Swabia  into  brown  and  yellow  sandstones  with 
oolitic  ironstone.^  The  central  group  in  northern  Germany  differs  from  the  correspond- 
ing beds  in  England,  France,  and  southern  Germany  by  the  great  preponderance 
of  dark  clays  and  ironstone  nodules.  The  upper  group  consists  essentially  of  clays 
and  shales  with  bands  of  oolitic  ironstone,  thus  presenting  a  great  difference  to  the 
massive  calcareous  formation  on  the  same  platform  in  England  and  France. 

The  Malm,  or  Upper  or  White  Jura  coiTesponds  to  the  Middle  and  Upper 
Oolites  of  England,  from  the  base  of  the  Oxford  clay  upwards,  with  the  equivalent 
formations  in  France.  It  is  upwards  of  1000  feet  thick,  and  derives  its  name 
from  the  white  or  light  colour  of  its  rocks  contrasted  with  the  dark  tints  of 
the  Jurassic  strata  below.  It  consists  mainly  of  white  limestones  in  many 
varieties  ;  other  materials  are  dolomite  and  calcareous  marl.  Its  lower  (Oxfordian) 
group  is  essentially  calcareous,  but  with  some  of  the  fossils  which  occur  in 
the  Oxford  clay,  e.g.  Ammonites  oniatus  and  Gryphaea  dilatata.  The  massive 
limestones  with  Cidaris  florigcmma  are  the  equivalents  of  the  Corallian.  The 
Kimeridge  group  presents  at  its  base  beds  equivalent  to  part  of  the  Sequanian 
or  Astartian  sub-stage  of  France  [Astarte  suprffcorallina^  Xatica  glohosa,  kc),  with 
such  an  abundance  and  variety  of  the  gasteropod  genus  Nerinsea  that  the  beds  have 
been  named  the  *' Nerineen-Schichten."  Above  these  come  strata  with  Fteroecra 
Oceani  (Pterocerian),  marking  the  central  zone  of  the  Kimeridgian  stage.  Higher 
still  lie  compact  and  oolitic  limestones  with  Exogyra  virgula  (Viigulian).  At  the 
top  some  limestones  and  marly  clays  yield  Ammonites  giganteus  (Portlandian). 
The    most    important    member  of   the    German   Kimeridgian    stage  is  undoubtedly 


^  Yov  an  account  of  this  stage  see  J.  A.  Stuber,  Ahhandl.  Geol.  Sj)eeialkart.  Elsass-Lotkr. 
V.  ii.  (1893). 

-  For  a  detailed  strat {graphical  and  palaeontological  account  of  the  Lower  Dogger  of 
German  Lorraine   see  W.    Branco,  Abhand.   Geol.  Specialkart.  Elsass-Lothr.  IL  Heft  ii. 

{1879). 


SECT,  ii  §  2  JURASSIC  SYSTEM  917 

the  limestone  long  quarried  for  lithographic  stone  at  Solenhofen,  near  Munich. 
Its  excessive  fineness  of  grain  has  enabled  it  to  preserve  in  the  most  marvellous 
l)erfection  the  remains  of  a  remarkably  varied  and  abundant  fauna  both  of  the 
sea  and  land.  Besides  skeletons  of  fishes  {AspidorhynchuSf  LepidotuSy  Megalunts)^ 
cephalopoda  showing  casts  of  their  soft  parts,  crabs  with  every  part  of  the  Integument 
in  place,  and  other  denizens  of  the  water,  there  lie  the  relics  of  a  terrestrial  fauna 
washed  or  blown  into  the  neighbouring  shallow  lagoons — dragon-flies  with  the  lace- 
work  of  their  wings,  and  other  insects ;  the  entire  skeletons  of  Pterodactyle  and 
Rhamphorhynchus,  in  one  case  with  the  wing  membrane  preserved  (Figs.  899,  400,  401), 
and  the  remains  of  the  earliest  known  bird,  Archaopteryx  (pp.  893,  894).  iThe 
upper  Jurassic  series  is  well  developed  in  Hanover,  where  it  has  been  carefully  studied 
by  C.  Struckmann.  The  Portland  group  has  been  shown  by  him  to  contain  eighty-five 
species  of  fossils,  one-half  of  which  are  lamellibranchs,  and  to  include  the  characteristic 
ammonites  A.  gigcts,  portlandicus,  Oravesianus,  gigantcvs,^  The  German  Purbeck 
group  attains  an  enormous  development  in  Westphalia  (1650  feet),  where,  between 
limestones  full  of  Corbulc^  Paludinay  and  Cyclas,  pointing  to  fresh-water  deposition, 
there  occur  beds  of  gypsum  and  rock-salt. 

Alps. — The  Jurassic  system  in  the  Alps  is  developed  under  a  different  aspect  from  its 
varied  characters  in  central  and  western  Europe.  It  there  includes  massive  reddish 
limestones  or  marbles  like  those  of  the  Trias  of  the  same  region.  Indeed  it  would  seem 
that  the  pelagic  conditions  under  which  the  Triassic  limestones  were  deposited 
had  not  entirely  passed  away  when  the  Jurassic  formations  came  to  be  laid  down. 
The  Lias  is  well  represented  in  the  Alps  under  several  distinct  types.  At  the  western 
end  of  the  chain  in  the  region  north  and  south  of  Brian9on  it  consists  of  crystalline, 
often  brecciated  limestones  generally  full  of  lamellibranchs,  sometimes  of  corals.  In 
Dauj)hine  it  is  made  up  of  marly  non-crystalline  lime.stones  distinguished  by  the  number 
of  their  ammonites  and  belemnites,  and  sometimes  reaching,  according  to  Lory,  a  thickness 
of  more  than  6000  feet.  Southwards  in  Provence  the  limestones  are  especially  rich  in 
hrachiopods  and  crinoids.^  In  the  Tyrol  and  eastern  Alps  the  Lias  presents  still  other 
lithological  and  palieontological  characters.  A  distinguishing  feature  is  the  prominence 
of  red  and  variegated  marbles,  also  the  abundance  of  genera  of  ammonites  which  are  for 
the  most  part  feebly  represented  in  central  and  western  Europe,  some  of  the  conspicuous 
forms  being  species  of  PhylloceraSy  LytoceraSy  AmaJthenSy  fhn/noticerasy  ArietUeSy  Psilo- 
ceraSy  and  Schlothcimia.  At  Adneth,  in  Salzburg,  this  facies  has  been  long  studied. 
In  the  Hierlatz  Mountains  of  the  Salzkammergut  the  Lias  is  represented  by  massive 
white  and  pink  limestones  with  abundant  brachiopods.  Yet  with  these  calcareous 
deposits  there  are  also  developed  along  the  southern  borders  of  Bohemia  and  eastwards 
in  Hungary,  sandy  and  argillaceous  strata  containing  so  much  vegetation  as  to  afford  in 
some  places  beds  of  coal.'  The  Alpine  Lias,  in  spite  of  these  variations  of  character  and 
organic  contents,  shows  here  and  there  some  of  the  distinctive  ammonite  zones,  so  that 
it  can  be  placed  in  comparison  with  that  of  the  rest  of  Europe.  It  lies  conformably  on 
and  passes  down  into  the  Rhaetic  series. 

The  equivalents  of  the  English  Lower  Oolites  or  "Middle  Jura"  of  the  Continent 
have  been  detected  in  both  the  western  and  eastern  Alps,  but  are  not  well  developed 
there.  In  the  west,  where  they  are  about  1300  feet  thick,  they  consist  of  limestones, 
shales,  and  clays  with  calcareous  nodules,  which  form  regular  alternations.  Ammonites, 
especially  of  the  genera  Fhylloceras  and  LytoceraSy  alx)und,  together  with  Posidonia. 
The  zones  of  Amm.    {ffarpoceras)   Murchisomr,  A.    (Harpoceras)  concamfs,  A.  {Son- 

*  *  Der  Obere  Jura  der  Umgegend  von  Hanover,'  1878 ;  Paltieontolog,  Abhand. 
(Dames  u.  Kayzer)  I.  i.  (1882)  ;  ZeiUch.  Deutsch.  Geol.  Oes.  1887,  p.  82. 

2  Haug,  *Le8  Chaines  subalpines,'  B^Ul,  Carte  (/&>/.  France,  No.  21  (1891):  Lory, 
BviL  Soc.  Oetd.  France  (3),  ix. 

'  Neumayr,  Abhand.  k,  k\  Geof.  Reichaanst.  1879. 


918  STRATIGRAPHICAL  GEOLOGY         BOOKTiPAsrin 

ninia)  Soxcerhyi,  A.  {Sonninia)  Bomani,  A,  humphriesianus  {Cetlitoeras  wubeoromahim), 
A.  (Parkinsonia)  Parkinaoni,  and  A.  {Oppeilia) /k$cu$  hare  been  reoognued.' 

Tlie  Oxfordian  and  Corallian  divisions  of  the  Juraaric  system,  or  CaUoTian,  Oxlbrdiaii, 
and  Ser|uanian  formations  are  in  general  feebly  represented  in  the  Alpine  r^gum  ;  bat 
the  Upper  Oolites  or  Eimeridgian  and  Portlandian  series  attain  a  large  de¥elopnkNiL 
It  is  this  higher  part  of  the  system  which  in  the  Alps  specially  presents  the  Tltfaonian 
faciei  already  referred  to.  Above  the  zone  ofAmtJumites  {Oppeilia)  tenuUobaiuM  {A$pido- 
ceras  acantkieum)  comes  a  mass  of  strata  consisting  of  a  lower  groap  of  reddish  well- 
bedded  limestones  so  full  of  Terebratula  diphya  {janitor)  as  to  be  named  the  "  Diphya- 
limestone  "  ;  and  of  an  upper  thick-bedded  or  massive  light-coloared  limestone  (Stzam- 
berg  limestone,  from  Stramberg  in  Moravia).  Tlie  limestones  are  often  crowded  with 
cephalopoda,  of  which  a  large  number  of  species,  many  of  them  pecoliar,  have  been 
noticed  {Amm.  (Phylloeerxu) ptychoieus.  A,  volanensiSj  A.  hybonoiui,  A.  transilorius,  A. 
lithographieu8f  A.  ateraspis).  The  presence  of  some  of  these  in  the  Portlandian  rocks  of 
Germany  serves  to  place  all  these  Alpine  limestones  at  the  very  top  of  the  Jnra«ie 
.system.  About  a  dozen  species  of  fossils  pass  up  from  them  into  the  Cretaceous  lockiL 
The  shales  or  impure  shaly  limestones  are  sometimes  full  of  the  cnrions  cephslopod- 
appendages  known  as  Aptychus  (Aptychus-beds).  Some  of  the  more  massive  lime- 
.stones  are  true  coral-reefs.  Many  of  the  limestone  escarpments  of  the  Alps  (Hochge- 
birgskalk)  are  referable  to  the  Terebratula  diphya  beds.  In  some  places  they  are  over- 
lain by  the  •Di[)hyoides-beds  (with  Terebratula  diphyoides),  elsewhere  they  pass  insen- 
sibly upwards  into  the  so-called  Bianeone — a  white  compact  siliceous  limestone  contain- 
ing Cretaceous  cephalopods.  The  Diphya-limestone,  with  its  peculiar  fossils,  appears 
to  range  from  the  Carpathians  through  the  Alps  and  Apennines  (where  it  occurs  as  a 
marble)  into  Sicily. - 

Sweden. — The  coal-bearing  Rhsetic  series  developed  in  Scania  and  referred  to  on 
p.  870,  is  followed  by  a  series  of  marine  strata,  in  which  a  number  of  the  ammonite- 
zones  of  the  Lower  and  Middle  Lias  have  been  recognised  as  high  as  that  of  Awtm. 
margarUatus.^  Similar  strata  are  found  on  the  island  of  Bomholm.  These  Scandinavian 
deposits  and  those  in  the  north  of  Scotland  mark  the  northern  and  western  limits  of  the 
*\  Jurassic  system  in  Europe. 

•  Russia. — Jurassic  formations  spread  over  a  larger  area  in  Russia  than  in  any  other 

I  j)art  of  Eurojje,  for  they  sweep  northwards  over  a  vast  breadth  of  territory  to  the  White 

'"'.  Sea,   and  extend  eastwards  into  Asia.     Yet  in  this   wide  area  it  is  only  the  upper 

half  of  the  system  w^hich  appears.     The  Lias  and  the  stages  below  the  Callovian  are 
;  absent.     The  fauna  of  these  Russian  Jurassic  formations  is  so  peculiar,  and  for  a  long 

time  yielded  so  few  8i>ecies  found  elsewhere  in  Europe,  that  it  was  difficult  to  correlate 
these  rocks  with  those  of  better  known  regions.  More  sedulous  research,  however,  has 
now  in  large  measure  removed  this  difficulty,  and  shown  that  some  of  the  recognised 
life-zones  of  western  Europe  can  be  detected  in  Russia.*  At  the  bottom  lies  (1)  the 
Callovian  stage,  consisting  of  clays,  divided  into — a.  Lower  with  Amm.  {Cosmoeerat) 

'  Haug,  Buii.  Carl.  OS^fl.  France,  No.  21  (1891). 

'  In  the  voluminous  literature  of  this  subject  the  following  works  may  be  consulted : 
Op[>el,  Z.  Deutsch.  Geol.  <Jes.  xvii.  (1865)  535;  Neumayr,  Ahhandl.  Oeol.  RtichsanUaUt  ▼•; 
Zittel,  Pdliitrjit.  Mittheil.  Mus.  Bayer.  ;  Hubert,  Bull.  Soc.  Gtd.  France^  ii.  (2)  p.  148,  xL 
(3)  p.  400;  E.  W.  Benecke,  'Trias  und  Jura  in  den  Siidalpen,'  1866;  ' Geognostisch. 
Paliioutologische  Beitrjige,' 8vo,  Munich,  1868;  C.  Moesch,  'Jura  in  den  Alpen,  Ostsch- 
weiz,'  187'2  ;  E.  Fraas,  'Scenerie  der  Alpen.'  See  also  the  *  Jura-studien,'  &c.  of  Neumayr, 
already  cited  (p.  895),  and  the  papers  of  Favre,  Loriol,  Renevier,  and  others. 

^  J.  C.  MoU-rg,  Sxrrig,  (ieid.  UndersHkn,  Stockholm,  1888. 

^  Neumayr,  (Jetxjn.  PalaeonioL  BeUrdgey  1876,  vol.  it ;  Nikitin,  A>tf^«  Jahrb.  1886,  ii. 
p.  205;  Mtm.  Acad.  St.  Petersbounj,  1881  ;  Pavlow,  Bidl.  Soc.  OM,  Frunce,  xii.  (1884); 
Bull.  Soc.  yal.  Moscou,  1889,  1891. 


8BCT.  ii  §  2  JURASSIC  SYSTEM  919 

ealloviensiSt  A.  goioerianus;  b.  Middle  with  Amm.  (Cosmoceras)  J(U<m,  A,  {Stephano- 
ceras)  eoroncUtts;  c  Upper  with  Amm.  {Quenstedticeras)  Lamberti,  A.  (Cosmocercu) 
Duneani,  (2)  Oxfordian,  composed  of  dark  sandy  clays  and  divided  into — a.  Lower 
with  Amm.  {Cardioceras)  cordatus^  A.  {Card.)  vertebralis,  A.  {PerisphincUa) plieatUiaf 
A.  {Aspidoceras)  perarmcUus ;  b.  Upper  with  Amm.  {Cardioceras)  altemans,  A,  {Peri- 
sphinctes)  Martelli.  (3)  Yolgian,  consisting  of  green,  brown,  and  dark  sandstones  and 
sands.  The  lower  part  of  this  group  contains  Amm^  ( Perisphindes)  virgatus,  A .  {Perisph. ) 
Pallasij  BeUmnites  absoluiuSf  B.  magnificus,  Aucella  Pallasi,  A.  m^fsquensiSy  and  the 
higher  part  yields  Belemniles  mosquensiSj  HolcosUphanua  BlaJceiy  and  many  species  of  the 
lamellibranch  Aucella.  The  group  is  correlated  by  Pavlow  with  the  Portlandian  stage 
of  western  Europe.     At  the  top  a  number  of  species  pass  up  into  the  Neocomian  series.' 

North  America. — So  far  as  yet  known,  rocks  of  Jurassic  age  play  but  a  subordinate 
part  in  North  American  geology.  Perhaps  some  of  the  red  strata  of  the  Trias  belong  to 
this  division,  for  it  is  difficult,  owing  to  paucity  of  fossil  evidence,  to  draw  a  satisfactory 
line  between  the  two  systems.  Strata  containing  fossils  believed  to  represent  those  of 
the  European  Jurassic  series  have  been  met  with  in  recent  years  during  the  explorations 
in  the  western  domains  of  the  United  States.  They  occur  among  some  of  the  eastern 
ranges  of  the  Rocky  Mountains  (Colorado  ;  Black  Hills,  Dakota ;  Wind  River  Moun- 
tains ;  Uinta  Mountains ;  Wahsatch  range,  &c. ),  as  well  as  in  the  Sierra  Nevada, 
California,  and  other  localities  on  the  western  side  of  the  watershed.  They  have  been 
recognised  far  to  the  north,  beyond  the  great  region  of  Azoic  and  Palseozoic  rocks, 
in  the  Arctic  portion  of  the  continent  They  have  been  met  with  also  in  South 
America,  where  they  appear  to  range  far  southwards  into  the  Argentine  Republic*  The 
fossils  include  species  of  PerUacrinuSf  Morwtis,  GryphaM^  Trigoni4i,  Lima,  Ammonites 
{AmultheuSt  Arietites,  Cardioceras^  and  BeUmnites. 

The  American  Jurassic  rocks,  though  a  few  European  species  appear  to  occur  in  them, 
have  not  yet  been  satisfactorily  correlated  with  the  subdivisions  of  the  system  in  Europe. 
The  younger  members  of  the  series  are  probably  best  developed.  In  these  strata  as  ex- 
posed in  Wyoming,  Utah,  Dakota,  and  Colorado  great  discoveries  of  vertebrate  remains 
have  been  made.  Professor  Marsh  has  brought  to  light  from  the  upper  Jurassic  strata 
of  Colorado  the  remarkable  series  of  reptilian  forms  already  referred  to  which  have  given 
a  wholly  new  interest  and  importance  to  the  Jurassic  rocks  of  America.  Among  remains 
offish  {Ceratodus\  tortoises,  pterodactyles,  and  crocodilians,  he  has  recognised  the  bones 
of  herbivorous  deinosaurs  (AtlajUosaurus,  Brontosauni^f  Stegosaurusy  Morosaurus, 
Apatosaurus),  together  with  the  carnivorous  Crcosaurus  and  the  curious  ostrich-like 
Laosaurus.  With  this  rich  and  striking  reptilian  fauna  are  associated  the  remains  of 
many  genera  of  small  mammals  which  have  been  named  by  Professor  Marsh  Allodon^ 
Ctenacodmiy  Dryolestes,  StylacodonyAsthenodon,  Laodon,  Diplocynodoriy  Docodon  [Enneodon], 
MenacodoUy  Tinodon,  TriconodoHy  Pria^odoiif  Paurodan* 

Asia. — In  India,  as  already  stated,  the  upper  part  of  the  enormous  Gondwdna 
system  is  possibly  referable  to  the  Jurassic  period.  In  Cutch,  however,  a  marine  series 
of  strata  occurs  containing  a  representation  of  the  European  Jurassic  system  from  the 
Inferior  Oolite  up  to  the  Portland  group  inclusive.  These  rocks  attain  a  thickness  of 
6300  feet,  of  which  the  lower  half  is  chiefly  marine  and  the  upper  mainly  fresh-water. 
Among  the  zones  recognised  by  Stoliczka  were  those  of  Ammonites  macrocephultis,  A. 
ancepSt  and  A.  afhlefa  of  the  Kellaways  (Callovian)  group  ;  A.  Lamberti,  A.  cordatus,  A. 
transversarius  of  the  Oxford  clay  ;  A.  temdlobatus  of  the  Kimeridge  group.* 

1  Pavlow,  Bull.  Soc.  Nai.  Moscou,  1891. 

'  0.  Behrendsen  has  found  Lower  and  Middle  Lias,  and  higher  Jurassic  beds  on  the 
eastern  slopes  of  the  Argentine  Cordilleras.     Zeit.  Deutsch.  Oeol.  Oeaell,  xliii.  369  (1891). 

»  Marsh,  Amer.  Journ.  Set.  xv.  (1878)  p.  459  ;  xviil.  (1879)  pp.  60,  215,  396;  xx.  (1880) 
p.  235  ;  xxi.  (1881)  p.  511  ;  xxxiii.  (1887)  p.  237  ;  Oeol.  Mag.  (1887)  pp.  241,  289. 

•»  Medlicott  and  Blanford's  '  Geology  of  India,  *  p.  263.     Waagen,  PaUeont.  Indiea,  1876. 


i 


920  STRATIGRAPHICAL  GEOLOGY         book  vi  part  in 

Anstralasia. — The  existence  of  Jurassic  rocks  in  Queensland  and  western  Autialia 
I  has  been  demonstrated  by  the  discovery  of  recognisable  Jurassic  species  and  otbos 

'  closely  allied  to  known  Jurassic  forms.  ^    In  Queensland  above  the  Permo-Carboniferous 

I  rocks  comes   the    Burrum   formation,    a  great    series    of   coal -bearing    rocks,   with 

Sphenopteris^  Thinnfeldia,  AUthopUris,  TaniopUriitf  PodozamiUs,  OtazamUu,  BtUera, 
and  a  few  animal  remains,  including  s|)ecies  of  Carbicula  and  JRoeellaria,  This  groap 
is  followed  by  another  sandy  and  conglomeratic  series  with  abundant  remains  of  land- 
plants  and  workable  coals,  forming  the  valuable  Ipswich  formation.  From  thets 
strata  a  large  flora  has  been  collected,  together  with  cyprids,  coleoptera,  and  Unio. 
From  the  plant-remains  these  two  formations  have  l>een  grouped  as  Jara-Triaa.*  Traces 
of  Jurassic  rocks  have  been  found  in  New  Caledonia  and  the  northern  end  of  Kev 
Guinea. 

In  New  Zealand  a  thick  series  of  rocks  classed  as  Jurassic  is  subdivided  as  follows  :— 

Mataura  series,  estuariue,  with  terrestrial  plants  (8  species  known). 

Putakaka    HcrieH,   niarlstones   and    sandstoneu    passing  into  conglomerates,   and 

euclosing  plant-remains  and  irregular  seams  of  coal ;  marine  fossils  (11  sjiecies 

known)  of  Middle  Oolite  facies. 
Flag  Hill  series,  with  species  of  RhynchoneUa,  Terebratula,  ^Hferina^  &c. 
Catlin's  River  and  Bastion  series,  consisting  in  the  upper  part  of  conglomerates 

and  grits,  with  obHcure  plant-remains,  and  in  the  lower  part  of  sandstones. 

Fossils  abundant  (especially  ammonites),  and  aftbrding  means  for  defining 

horizons.     This  division  is  referred  to  the  Lias.' 

Section  ill.    Cretaceous. 

The  next  great  series  of  geological  formations  received  the  name  of 
Cretaceous  system,  from  the  fact  that,  in  north-western  £urope,  one  of 
its  most  important  members  is  a  thick  l)and  of  white  chalk  {creta).  It 
presents  very  considerable  lithological  and  jmlajontological  differences  as 
it  is  traced  over  the  world.      In  jwirticular,  the  white  chalk  is  almost 

*  wholly  confined  to  the  Anglo-Parisian  Iwisin  where  the  system  was  first 

studied.  Prokildy  no  contempjnineous  group  of  rocks  presents  more 
remarkiible  lociil  diflerences  than  the  Cretaceous  system  of  Europe. 
These    differences   are    the    records    of   an   increasing  diversity  of   geo- 

■j  graj)hical  conditions  in  the  history  of  the  Continent. 

il 

J  §  1.  General  Characters. 

Rocks. — In  the  European  area,  as  will  l>e  afterwards  ix>inted  out 
in  more  detail,  two  tolembly  distinct  areas  of  deposit  can  be  recognised, 
each  with  its  own  character  of  sedimentary  accumulations,  as  in  the 
case  of  the  Jurassic  svstem  alreiidv  described.  The  northern  tract 
includes  Britain,  the  lowlands  of  central  Europe  southwards  into  Silesia, 
I  Bohemia,  and  round  the  Ardennes  into  the  l)asin  of  the  Seine.     The 

j  southern  region  embraces  the  centre  and  south  of  France,  the  range  of 

^  Moore,  (^.  J.  UfU.  S,h:.  xxvi.  261.    W.  B.  Clarke,  np,  a'L  rxiii.  7.     R.  Etheridge  jun.. 
.  'Catuloj^'ue  of  Australian  Fossils/  1878. 

-  Jack  anil  EtheridKe,  *  Geology  and  Pahfontolog}'  of  Queensland'  (1892),  cliai>s.  xxiii.- 

XXX. 

■ 

•»  Hector's  *  Handbook  of  New  Zealau«l/  p.  31.     Compare  F.  W.  Bntton,  i^Mart.  Jovrn. 
Oeol.  SiK\  1885.  p.  204. 


; 


SECT,  iii  §  1  CRETACEOUS  SYSTEM  921 

the  Alps,  and  the  basin  of  the  Mediterranean  eastwards  into  Asia.  In 
the  northern  area,  which  appears  to  have  been  a  basin  in  great  measure 
shut  off  from  free  communication  with  the  Atlantic,  the  deposits  are 
largely  of  a  littoral  or  shallow-water  kind.  The  basement  beds,  usually 
sands  or  sandstones,  sometimes  conglomerates,  are  to  a  large  extent 
glauconitic  (greensand).  The  marked  diffusion  of  glauconite,  lx)th  in 
the  sandstones  and  marls,  is  one  of  the  distinctive  characters  of  this 
series  of  rocks.  Another  feature  is  the  abundance  of  soluble  silica 
(sponge-spicules)  more  particularly  in  the  formation  called  the  Upper 
Greensand,  and  in  the  Lower  Chalk  of  many  parts  of  the  south  and 
south-east  of  England  and  the  north  of  France.  In  Saxony  and  Bohemia, 
the  Cretaceous  system  consists  chiefly  of  massive  sandstones,  which 
appear  to  have  accumulated  in  a  gulf  along  the  southern  margin  of 
the  northern  basin.  Considerable  bands  of  clay,  occurring  on  different 
platforms  among  the  European  Cretaceous  rocks,  are  often  charged 
with  fossils,  sometimes  so  well  preserved  that  the  pearly  nacre  of  the 
shells  remains,  in  other  cases  encrusted  or  replaced  by  marcasite. 
Alternations  of  soft  sands,  clays,  and  shales,  usually  more  or  less 
glauconitic,  are  of  frequent  occurrence  in  the  lower  parts  of  the 
system  (Neocomian  and  older  Cenomanian).  The  calcareous  strata 
assume  sometimes  the  form  of  soft  marls,  which  pass  into  glauconitic 
clays,  on  the  one  hand,  and  into  white  chalk,  on  the  other.  The 
white  chalk  itself  is  a  pulverulent  limestone,  mainly  composed  of 
fragmentary  shells  and  foraminifera.  Its  upper  part  shows  layers 
of  flints,  which  are  irregular  lumps  of  dark -coloured,  somewhat 
impure  chalcedony,  disposed  for  the  most  part*  along  the  planes 
of  bedding,  but  sometimes  in  strings  and  veins  across  them.  The 
flints  frequently  enclose  silicified  fossils,  especially  sponges,  urchins, 
l)rachiopods,  &c.  (see  pp.  141,  495).  The  chalk,  in  some  places, 
becomes  a  hard  dull  limestone,  breaking  with  a  splintery  fracture. 
Nodular  phosphate  of  lime  or  phosphatic  chalk,  occurring  on  difl*erent 
horizons  in  the  system,  is  extensively  worked  as  a  source  of  artificial 
manure  in  the  Upper  Chalk  of  Belgium.^  It  has  been  found  also  in 
the  north  of  France,  and  at  Taplow,  near  Maidenhead,  in  England.* 

The  terrestrial  vegetation  of  the  period  has  in  diflferent  places 
been  aggregated  into  beds  of  coal.  These  occur  in  north-western 
Germany  among  the  Wealden  deposits,  where  they  are  mined  for  use  ; 
also  to  a  trifling  extent  in  the  Wealden  series  of  England ;  they  are 
like^nse  found  in  the  Cenomanian  series  of  Saxony  and  the  Senonian 
of  Magdeburg.  The  upper  Cretaceous  (Laramie)  rocks  of  the  Western 
Territories  of  the  United  States  consist  largely  of  sandstones  and 
conglomerates,  among  which  are  numerous  important  seams  of  coal. 
Beds  of  concretionary  brown  iron -ore  are  present  in  the  Cretaceous 
series  of  Hanover,  and  similar  deposits  were  once  worked  in  the 
English    Wealden   series.      In    the    southern    European    basin,    where 

^  Cornet,  i^uart.  Journ.  Oeol.   Soc,  xlii.  p.  325  ;  Kenard  et  Comet,  Bull,  Acad   R(/y, 
Belg,  xxi.  (1891)  p.  126. 

-  A.  Strahan,  Quart.  Jouni,  Oettl.  Sac.  rlvii.  (1891)  p.  356. 


922  STRATIGRAPHICAL  GEOLOGY         book  vi  fabt  m 

the  conditions  of  deposit  appear  to  have  been  more  those  of  an 
open  sea  freely  communicating  with  the  Atlantic,  the  most  noticeable 
feature  is  the  massiveness,  compactness,  and  persistence  of  the 
limestones  over  a  vast  area.  These  rocks,  often  crowded  with 
hippuritids,  from  their  extent  and  organic  contents,  indicate  that, 
during  Cretaceous  times,  the  Atlantic  stretched  across  the  south  of 
Europe  and  north  of  Africa,  far  into  the  heart  of  Asia,  and  may 
not  impossibly  have  been  connected  across  the  north  of  India  with 
the  Indian  Ocean. 

Life. — The  Cretaceous  system,  both  in  Eiux)pe  and  North  America, 
presents  successive  platforms  on  which  the  land -vegetation  of  the 
period  has  been  preserved,  though  most  of  the  strata  contain  only 
marine  organisms.  This  terrestrial  flora  possesses  a  great  interest^ 
for  it  includes  the  earliest  known  progenitors  of  the  abundant 
dicotyledonous  angiosperms  of  the  present  day.  In  Europe  during 
the  earlier  part  of  the  Cretaceous  period,  it  appears  to  have  closely 
resembled  the  vegetation  of  the  previous  ages,  for  the  same  genera 
of  ferns,  cycads,  and  conifers,  which  formed  the  Jurassic  woodlands, 
^  are   found   in   the   rocks.     Yet   that   angiosperms   must   have   already 

existed  is  made  certain  by  the  sudden  appearance  of  numerous  forms 
of  that  class,  at  the  base  of  the  Upper  Cretaceous  formations 
in  Saxony  and  Bohemia,  whence  forms  of  Acer,  Alnus,  Oredneria, 
CunningJuiiniifs,  Salix,  &c.,  have  been  obtained.  Still  more  varied  and 
abundant  is  the  dicotyledonous  flora  preserved  in  the  Upper  Cretaceous 
formations  in  Westphalia,  from  which  53  species  of  dicotyledonous 
plants  have  been  (Obtained,  belonging  to  the  genera  Populus,  Myrica, 
Qurrcm,  Finis,  Crednena,  Fibunmm,  Aralia,  Eumtt/pfm,  &c.,  besides 
algje,  ferns,  cycads,  conifers,  and  various  monocotyledons  (Fig.  410).^ 
Another  rich  CreUiceous  flora  is  found  in  the  corresponding  beds  at  Aix- 
la-Chapclle.  It  iiicliules  numerous  ferns  {Gleichenia,  Lygodium,  Daiimtes, 
Asphniium,  Pterklnlcimma),  conifers  {Sequoia,  Cv nninghamites),  Caulii^a, 
DryophijUvm,  MyricitphyUum,  Ficus,  LdurophyUum,  and  three  or  four  kinds 
of  screw-pine  {Pandamui).'^  The  prevalent  forms  which  give  so  modem 
an  aspect  to  this  flora,  and  which  occur  also  in  Westphalia,  are  ProUaces, 
many  of  them  l)eing  referred  to  genera  still  living  in  Australia  or  at 
the  Cape  of  (Tood  Hope.  These  interesting  fragments  indicate  that 
the  climato  of  Europe,  at  the  close  of  the  Cretaceous  period,  was 
doubtless  greatly  warmer  than  that  which  now  prevails,  and  nourished 
a  vegetation  like  that  of  some  parts  of  Australia  or  the  Cape.  Further 
information  has  been  afforded  regarding  the  extension  of  this  flora  by 
the  discovery  in  North  Greenland  of  a  remarkable  series  of  fossil- 
!  l)lants,  of  which  Heer  has  described  nearly  200  species,  including  more 

than   40  kinds  of  ferns,  with  club-mosses,  horsetail  reeds,  cycads  {Cycas, 

^  Hosius  unci  Von  der  Marck,  "Die  Flora  der  Westfalischen  Kreideformation/* 
Pnltiimtoffraphica^  xxvi.  (1880)  p.  125.  The  total  flora  describeti  by  these  obsenren  is 
made  up  of  85  species  from  the  Upper  and  20  species  from  the  Lower  Cretaceous  beds. 

;  2  X.  Lange,  Zdtsvh.  Dentsch.   Gcd.  fits.  1890,  i>.   G58  ;  and  H.  von  Dechen,  as  cited 

I  [H'Stea,  p.  954. 


1 

I  I 

1 


iigl 


CRETACEOUS  SYSTEM 


Podozamites,  Otoiomttes,  Zamiki),  conifers  {Baiera,  Oinkgo,  Juniperus, 
ThuyiUs,  Sequoia,  Dammara,  Pinus,  &c.),  monocotyledona  {Arando,  Polo- 
mogtion,  &c),  and  many  dicotyledons,  including  fomui  of  poplar,  myrica, 
oak,  fig,  walnut,  plane,  sassafras,  laurel,  cinnamon,  ivy,  aralia,  dogwood, 
magnolia,  eucalyptus,  ilex,  buckthorn,  cassia  and  others.' 

In  North  America,  also,  abundant  remains  of  a  similar  vegetation 
have  been  obtained  from  the  Cretaceous  rocks  of  the  Western  Terri- 
tories. The  Lanunie  group  of  strata  in  particular  has  yielded  a  remark- 
ably large  and  varied  flora.  Out  of  more  than  100  species  of  dicoty- 
ledonous angiosperms  there  found,  half  are  relat«d.to  still  living  American 


trees.  Among  them  are  species  of  oak,  willow,  beech,  plane,  poplar,  maple, 
hickory,  fig,  tulip-tree,  sassafras,  laurel,  cinnamon,  buckthorn,  together 
with  ferns,  American  palms  (sabal,  FbibeUoTia),  conifers,  and  cycada.^ 
The  "  Potomac  formation  "  of  Virginia  and  Maryland  has  a  special  interest 
from  its  age.  It  is  referred  with  some  probability  to  the  Neocomian 
period,  and  it  has  yielded  about  350  species  of  plants,  viz.  three  species 
of  (^juiseta,  139  ferns,  32cycada,andmorethan  100  conifers.     But  besides 


STRATIGRAPHICAL  GEOLOGY  book  vi  i-ASt in 


this  aasembUge,  which  is  distinctly  Mesozoic  in  character,  the  dt^iodU 
have  furnished  no  fewer  than  29  genera  and  75  species  of  angioBperma 
Of  these  higher  forms  of  vegetation  about  two-thirds  are  new,  and  the  more 
peculiar  fonns  seem  to  be  what  are  known  as  "  generalised  types,"  indicat- 
ing the  great  antiquity  of  the  flora.  But  among  the  genera  there  m 
found  Sassafras,  Finis,  Myrica,  Bombax,  and  Aralui.* 


The  known  Cretaceous  fauna  is  tolerably  extensive.     Foraminifoa 
HOW  reached,  an  importance  as  rock-builders  which  they  never  before 
attained.     Their  remains  are  abundant  iti  the  white  chalk  of  the  northern 
Eui-opean  basin,  and  some  of  the  hard  limestones  of  the  southern  basin 
_^  _  _,       are  mainly  composed  of  their  aggregated 
shells.     The  glauconite  grains  of  laany  of 
the  greenish  strata  are  the  internal  casts 
of  foraminiferous  shells  (see  pp.  456,  652). 
Some    of    the    more    frequent    genera    are 
GIobigeriHa,  OrUtotiiia,  Xiidosaria,  Textiiana, 
and  Rota! ia  (Fig.  -111).     Calcareous  sponges 
are  of  fi^eejuent  occurrence,  while  siliceous 
sjMHiges  must  have  swarmed  on   the  floor 
of  the  Cretaceous  seas,  for  their  siliceous 
spicules    are   abundant,   and    entire   indi- 
viduals are  not  uncommon.*    Characteristic 
genera  (Fig.  412)  are  F'enirkuUles,  Stphonia, 
Ccdoplychimn,  and  Conjndh.     The  formation 
of  flinU  has  been  referred  to  the  operation 
of    sponges.     Undoubtedly  these   animals 
voutritiiiitn.Li.curTeiiii.Yar.  tonuiiiii-    accreted  an  enormous  quantity  of  silica  from 
citiu,  siiiiiii  (».  (.jjg  „n[^jj.  of  tj,g  Cretaceous  sea,  and  though 

'  W.  M.  Fontaine,  'The  Potomac  or  Younger  Mesoioic;  Flora,'  Mviieg.  U.S.  Otol.  Svn. 
vol.  XV.  (188B).      See  also  0.  Feigtmiintel,  ZeitKh.  Itfutach.  Gtol.  Ga.  1888,  p.  27. 

'  Hee  oil  Sjionge  Kpiculea,  jiniKrH  by  Prof.  SoIIok,  Ann.  Mag.  A'at.  Hut.  aer.  G,  vi.  and 
niemoirs  by  Dr.  G.  J.  Hliide,  '  Fossil  Sponge  Spicules,'  Hunich,  1880;  'Cat  of  FoaUl 
SpoiiBes,  Brilisli  Muwiiiii,'  1883  :  Phil.  Trans,  vol.  rlxivi.  p.  403,  1886  ;  '  Britiab  Posiil 
SponKBK,'  Pal.  Hoc.  vol.  xl.  ili.  1887-88.  The  uponge  spicules  of  the  Upper  Cretactooi 
rocks  art  wry  genernlly  in  the  condition  of  ainoiiihous  or  colloid  silica ;  those  of  the  Lower 
Cretaceonii  are  fre-juently  of  crystalline  «i!icn. 


Bici.  iii  §  1 


CRETACEOUS  SYSTEM 


9S6 


the  flints  are  certainly  not  due  merely  to  their  action  alone,  amorphous 
silica  may  have  been  aggregated  by  a  process  of  chemical  elimination 
round  dead  sponges  or  other  organisms  (p.  495).  Molluska  and  urchins 
have  been  completely  silicificd  in  the  Chalk. 

On  the  whole,  corals  are  not  abundant  in  Cretaceous  deposits,  though 
they  occur  plentifully  in  the  so-called  coral  limestone  of  Faxoe.  They 
seem  to  have  been  chiefly  solitary  forms,  some  of  the  more  characteristic 
genera  being  Trofhocyathas,  Caryof^yliia,  Trochoi^nlia,PaTaS'milia,Micrabar,ia, 
and  Cyfhilite*  The  rugose  corals  so  abundant  among  Palteozoic  rocks  are 
no  ^  do  ibtfully  represented  by  the  1  ttlo  Ncocomian  ffoloeyatu  Sea- 
urch  na  are  conspicuous  among  the  fose  Is  of  the  Cretaceous  system.  A 
few  of  their  genera  are  also  Turassic  whil    a  not  inconsiderable  number 


vulgmtM,  LMkeKJ>, 


still  live  in  the  present  ocean.  One  of  the  most  striking  results  of  recent 
deep-sea  dredging  is  the  discovery  of  so  many  new  genera  of  echinoids, 
either  identical  with,  or  very  nearly  resembling,  those  of  the  Cretaceous 
period,  and  having  thus  an  unexpectedly  antique  character.*  Some  of  the 
moat  abundant  and  typical  Cretoceous  genera  (Pig.  413)  are  Anattckytes 
{Ediin'ieiirys\  Hdiuder,  Toxader,  Micraster,  Hemiaster,  Hemiptteoiles,  CardiasUr, 
Pyguras,  Echirwbrissiis  {NttdeolUei},  ErMnocoiiun  (GaltriUs),  DiAcoidta, 
Cyphosotaa,  Pievdodiadema,  SaUnia,  Cidari-<.  A  few  cnnoids  have  been 
met  with,  of  which  Bourguetierinvs  and  MarmpHes  of  the  Upper  Chalk 
are  characteristic. 

Polyzoa  abound  in  some  parts  of  the  system,  especially  in  the  upper 

formations  {C'ellaria,  Vincularia,  Membrunipm-a,  Mkropm-a,  lietejiora).     The 

brachiopods  (Fig.  414)  are  abundantly  represented  by  species  of  Tere- 

bratula  and  Rhyiii-hondla,  which  approach  in  form  to  still  living  species. 

'  A.  Agauiz.  "  Report  on  Echinoldea, "  fhalitiiner  Eipeditloii,  vol.  iii.  p.  26. 


STRATIORAPHICAL  GEOLOGY 


BOOK  TI  PASI  m 


Other  contemporaneous  genera  were  Crania  (numerous  spedes). 


Magas,   Terebraklla,   Lyra  (Ter^mrostm),   Tri^miosemus,  TerebraHtlina,  and 


Vig.  Hi CretaFmut  LiuielllbisDClin. 

",  K\w\r.-i  (OBlrra)  ci.luti.la.  Uiii.  ());  b,  Ohi™  vesicularis,  Lurn.  (i);  c.  Oitre*  r»riiuiU,  Um.  (H ; 
(J,  !*i-.iiaj  111-  (l.luis)  BpinMiiH,  DmIi,  (i) :  f,  Inucoraiiiiw  Cuvkri,  So*.  Cyoong  •[)«!.)  (J). 

Arifiojie.     Among  the  most  abundant  genera  of  lameUibraDchs  (Fig.  415) 


SECT,  iii  §  1  CRETACEOUS  SYSTEM  927 

are  Inoceramus,  Exofft/ra,  (ktrea,  Spondylus,  Lima,  PecUn,  Pema,  Modiola, 


Fig.  <IT.— CrtUeeuui  Cppliilopoilj. 

n,  TdirilitcicosUlub,  Lmu.  (J);  6,  Crioc«rM  Emerid,  Lti",  (1);  e.  D«ciilLt«  »iioEIH,  lam.^d) ; 

d.  Ammonites  (Aontboctru)  n>tboni>etn*L«,  BroDg.  (i) :  r,  AiDiDonitaa  Tirlui,  Soir.  (]). 

Trigonia,  Imicai-dia,  Cardiuin,  Feitus.     huxeramus  and  Exogyra  are  specially 


928  STRA  TIGRAPHICAL  GEOLOGY         book  vi  pabt  m 

characteristic,  but  still  more  so  is  the  family  of  Hippuritidss  or  Budisks. 
These  singular  forms  are  entirely  confined  to  the  Cretaceous  system: 
their  most  common  genera  (Fig.  416)  being  HippuriteSy  Eadiolites,  Sphse- 
rulUes,  Caprina,  M<moplewra^  and  Caprotina  (Bequienia),^  Hence,  according 
to  present  knowledge,  the  occurrence  of  hippuritids  in  a  limestone 
suffices  to  indicate  the  Cretaceous  age  of  the  rock.  The  most  common 
gasteropods  belong  to  the  genera  Adseotiella,  Turbo^  Solarium,  Trochus, 
Plenrotomariiiy  CerUhiuniy  Rostdlaria,  Apr/rrhau,  FusuSj  MitrOy  and  Murex. 
Cephalopods  must  have  swarmed  in  some  of  the  Cretaceous  seas  (Figs. 
417,  418,  419).  Their  remains  are  abundant  in  the  Anglo-Parisian  basin 
and  thence  eastwards,  but  are  comparatively  infrequent  in  the  southern 
Cretaceous  area.  To  the  geologist,  they  have  a  value  similar  to  those  of 
the  Jurassic  system,  as  distinct  species  are  believed  to  be  restricted  in  their 
range  to  particular  horizons,  which  have  by  their  means  been  identified 
from  district  to  district.  To  the  student  of  the  history  of  life,  they  have 
a  special  interest,  as  they  include  the  last  of  the  great  Mesozoic  tribes  of 
the  Ammonites  and  Belemnites.  These  organisms  continue  abundant  up 
to  the  top  of  the  Cretaceous  system,  and  then  disappear  from  the  European 
geological  record.-  Never  was  cephalopodous  life  so  varied  as  in  the 
Cretaceous  period,  just  before  its  decline.  It  included  some  old  Ammonite 
genera  such  as  Phylhcera.%  LyioceraSy  and  Haploceras,  some  of  which  had 
continued  even  from  Liassic  time,  together  with  new  genera,  some 
resembling  old  types  (SrMoenbachia\  others  which  now  appeared  for  the 
first  time.  Of  these  new  forms  Criocenis  (Fig.  417)  is  an  Ammonite  with 
the  coils  of  the  shell  not  contiguous.  Saiphites  and  Ancyloceras  have  the 
last  coil  straightened,  and  its  end  bent  into  a  crozier-like  shape  (Fig.  418). 
Ihx'jarasy  as  its  name  implies,  is  merely  bent  into  a  bow-like  fonn. 
Ifainife^i  is  a  long  tiipering  shell,  curved  round  hook-wise  upon  itself. 
In  rtyrhftrrms  the  long  tapering  shell  is  bent  once  and  the  tWo  parts 
'are  mutually  adherent.  Turiilites  (Fig.  417)  is  a  spirally  coiled  shell, 
and  Helirorn'fis  resembles  it,  but  has  the  coils  not  in  contact.  B(U'uliie^< 
(Fig.  4 1 7)  is  the  simplest  of  all  the  forms,  being  a  mere  straight-chambered 
shell  somewhat  like  the  ancient  Orthoceras.  These  forms,  in  numerous 
species,  are  almost  entirely  confined  to  the  Cretaceous  system,  at  the 
summit  of  which  they  disappear.  The  genus  Nautilus  is  found  not 
infrequently  in  Upper  Cretaceous  rocks.  Another  characteristic  cephal- 
opod  is  BelinnniteUa  (Fig.  419),  which  occurs  abundantly  in  the  higher 
parts   of  the   system.      The    Belemnites  are    more  particularly   charac- 

^  For  a  study  of  the  Rudistes,  see  the  Memoir  by  H.  Douvillo,  Mhn,  Soc.  G(ol.  France 
(3),  i.  (1890)  ;  ii.  (1892). 

-  No  abrupt  <lisapi»earauce  of  a  whole  widely-diffused  fauna  probably  ever  took  place. 
The  cessation  of  Ammonites  with  tlie  Cretaceous  system  in  Europe  can  only  mean  that  in  this 
area  tliere  intervened  between  the  deposition  of  the  Cretaceous  and  Tertiary  strata  a  long 
interval,  marked  by  such  jihysical  revolutions  as  to  extirpate  Ammonites  from  that  region. 
That  the  tribe  continued  elsewhere  to  live  on  into  Tertiary  time  appears  to  be  proved  by  the 
occurrence  of  some  Ammonite  remains  in  the  oldest  Tertiary  beds  of  California.  A.  Heilprio, 
'  Contributions  to  the  Tertiary  Geology  and  Palteontolog}'  of  the  United  States/  Philadelphia. 
1884,  p.  102. 


BKCT.  iii  g  1  CRETACEOUS  SYSTEM  929 

teriatic  of  Lower  CretaceouB  rocks,  and  belong  to  Zittel's  groups  of  the 
"  Bipartiti,"  "  Ckxiophorf,"  and  "  Dilatati." 

Vertebrate  remains  have  been   obtained  in  some  number  from    the 
Cretaceous  rocks.     Fish  are  represented  by  scattered  teeth,  scales,   or 


a,  Aneyloce™*  innlhi 


bones,  sometimes  by  more  entire  skeletons.     The  moat  frequent  genera 

are   Odoniaspi^,'-  Lamna,  Oxyrhina,  Ptychodus,  Hybodvs,  Mesodon  {Pt/rtwdut), 
lu  taagu  Dp  to  tiie  RopclUn  (Oligocene)  b«da.    A.  Bntot, 


930  STBATIGBAPBICAL  GEOLOGY  book  n  rAnm 

S^KTtjdut,  and  the  earliest  <A  the  t«leoneaii  tribes,  vUcb  iitdnde  the  rut 
majoritr  of  modern  fishes— iVotopijTjnM,  CiBniUktkm,  Bmdtodtu,  Strmtadmi, 
Bfry/i  fFig.  420),  Syllxmvt,  Portiuut,  ic 

B<;ptilian  life  has  not  been  so  abundantly  [Nneso^ed  in  the  Cretaceoos 
a«  in  the  .Jurassic  system,  nor  are  the  ioraa  so  nried.  In  the  Earopesn 
area  the  remains  of  Chelonians  of  several  genera  (CMoat,  Proltmifty  Pi»- 
lemyt)  have  been  recovered.  The  last  of  the  tribe  of  dnnoaars  died  out 
t^twards  the  close  <rf  the  Cretaceons  period.  Among  the  Cretaceoos 
forms  of  this  order  are  the  Megahtaurvf  and  CftioM^ntt,  which  Bnrrired 
frrjm  Jurassic  time ;  likewise  Ptltfrosanmt,  Polaeanikiu,  lyuamodom,  HyUro- 
tauruf,  UyptiUrpkoihn,  OmUAcjuii.  Of  these  Iffuanodon  is  the  most 
familiar  type  (Fig.  421).  Smne  of  its  teeth  and  bones  were  first  found 
in  the   Wealden  series  of  Sussex,  but  in  recent   years,  abnost  entire 

1^. 


skeletons  have  liecn  disinterred  from  the  ancient  alluvium  filling  up 
vallovH  of  the  Crotaceous  period  in  Belgium,  so  that  its  osteology  is  now 
well  kti'iwn.  Like  other  deinosaurs,  it  had  many  affinities  with  birds. 
PaliL'ontologists  have  differed  in  opinion  as  to  whether  it  walked  on  all 
fours  or  erect.  M.  Dollo,  who  has  had  the  advantage  of  working  out  the 
stmcturo  of  the  wonderfully  perfect  Belgium  specimens,  believes  that  the 
animal  moved  iiti  its  hind  legs,  which  are  disproportionately  longer  than 
the  fore  ones.  Its  powerful  tail  obiioiisly  sen"ed  as  an  organ  of  propul- 
sion ill  the  water,  and  likewise  to  balance  the  creature  as  it  walked.  Its 
stningc  fore-limbs,  armed  with  spurs  on  the  digits,  doubtless  enabled  it 
to  defend  itself  from  its  carnivorous  congeners  ;  it  was  itself  herbivorous.^ 
Among  Cretaceous  rocks  the  order  of  Lizards  is  represented  by  Comasaimif, 
'  Jluittcll'a  '  niustnitianji  of  the  Geologjr  of  Sussei,'  1S27.  For  recent  mdditioiu  to  oar 
kiiowlcil);c,  i.ee  Dollo,  Bull.  Mm.  Hog.  Belgiqur,  iL  (1883).  A«».  Set.  GM.  xiL  (1883} 
No.  6. 


8BCT.  iii  §  1  CRETACEOUS  SYSTEM  931 

Dolidutsaums,  and  Leiodon.  The  gigantic  Mosasaurus,  placed  among 
lacertiliana  by  Owen,  but  among  "  p)^honomorphB  "  by  Cope,  is  estimated 
to  have  had  a  length  of  76  feet,  and  was  furnished  with  fin-like  paddles, 
by  which  it  moved  through  the  water.  True  crocodiles  frequented  the 
rivers  of  the  period,  for  the  remains  of  several  genera  have  been 
recognised  (GoniephtAis,  Pholidosaurus,  Theriosuchvs).  The  ichthyosaurs 
and  plesiosaurs  were  still  represented  in  the  Cretaceous  seas  of  Europe. 
The  pterosaurs  likewiee  continued  to  be  inhabitants  of  the  land,  for  the 
bones  of  several  species  of  pterodactyle  have  been  found.  These  remains 
are  usually  met  with  in  scattered  bones,  only  found  at  rare  intervals  and 
wide  apart.  In  a  few  places,  however,  reptilian  remains  have  been  dis- 
interred in  such  numbers  from  local  deposits  as  to  show  how  much  more 
knowledge  may  yet  be  acquired  from  the  fortunate  discovery  of  other 
similar  accumulations.  One  of  the  most  remarkable  of  these  exceptional 
deposits  is  the  hard  clay  above  referred  to  as  filling  up  some  deep  valley- 
shaped  depressions  in  the  Carhoniferoua  rocks  near  Bernissart  in  Belgium, 


and  which  has  been  unexpectedly  encount«rGd  at  a  depth  of  more  than 
1000  feet  below  the  surface  iu  mining  for  coal.  These  precipitous  defiles 
were  evidently  valleys  in  Cretaceous  times,  in  which  fine  silt  accumulated, 
and  wherein  carcases  of  the  reptiles  of  the  times  were  quietly  covered  up 
and  preserved,  together  with  remains  of  the  river  chelonians  and  fishes, 
as  well  as  of  the  ferns  that  grew  on  the  clifTs  overhead.  These  deposits 
have  remained  undisturbed  under  the  deep  cover  of  later  rocks.^  Again, 
from  the  so-called  "  Cambridge  Greenaand  " — a  bed  about  1  foot  thick 
lying  at  the  base  of  the  Chalk  of  Cambridge,  and  largely  worked  for  the 
phosphate  of  lime  which  is  supplied  by  phosphatic  nodules  and  phosphated 
fossils — there  have  been  exhumed  the  remains  of  several  chelonians,  the 
great  deinosaur  AcartthophiolU,  several  species  of  Plesiosauras,  5  or  6  species 
of  Ichthyosaurus,  10  species  of  Pterodactylus — from  the  size  of  a  pigeon 
upwards,  one  of  them  having  a  spread  of  wing  amounting  to  35  feet — 3 
species  of  Moaasaurus,  a  crocodilian  (Polyptt/ehodtm),  and  some  others. 
From  the  same  limited  horizon  also  the  bones  of  at  least  two  species  of 
birds  have  been  obtained. 

In  recent  years  the  most  astonishing  additions  to  our  knowledge  of 
■  E.  DupoDt,  BuB.  Acad.  Any.  Belg.  2*  rir.  iItL  (1S78)  p.  387. 


STRATWRAPBWAL  GSOLOGY 


BOOE  TI  FAST  m 


ancient   reptilian   life    have  been  made  from    the  Cretaceoua    rocks  (A 
western  Xorth  America,  chiefly  by  Professors  Leidy,  Marsh,  and  Cope.' 


'  Leiily,  Smithaon.  Conlrih.  1865,  Mo.  192  ;  Rtp.  U.S.  Otot,  and  Otograph.  Survey  ^ 
Terriloriei,  voL  i.  (1873);  Cope,  JUp.  U.S.  Oeol.  and  Ga^tvpfu  Survey  of  TerHlonet, 
vol  ii.  (18''') !  ^"ter.  Ifaturalut,  1S7S  et  teq.  ;  Harsh,  Am^r.  Joam.  Seierux,  namtraiu 
[wpen  in  3rd  leriea,  vols.  L-xliL  (1892). 


SECT,  iii  §  1  CRETACEOUS  SYSTEM  933 

According  to  an  enumeration  made  a  few  years  ago  by  Cope,  but  which  is 
now  below  the  truth,  there  were  known  1 8  species  of  deinosaurs,  4  ptero- 
saurs, 14  crocodilians,  13  sauropterygians  or  sea-saurians,  48  testudinates 
(turtles,  &c.),  and  60  pythonomorphs  or  sea-serpents.  One  of  the  most 
extraordinary  of  reptilian  types  was  the  Discosaurus  or  Elasmosaurus — a 
huge  snake-like  form  40  feet  long,  with  slim  arrow-shaped  head  on  a 
swan-like  neck  rising  20  feet  out  of  the  water.-  This  formidable  sea- 
monster  "probably  often  swam  many  feet  below  the  surface,  raising 
the  head  to  the  distant  air  for  a  breath,  then  withdrawing  it  and  explor- 
ing the  depths  40  feet  below  without  altering  the  position  of  its  body. 
It  must  have  wandered  far  from  land,  and  that  many  kinds  of  fishes 
formed  its  food  is  shown  by  the  teeth  and  scales  found  in  the  position  of 
its  stomach "  (Cope).  The  real  rulers  of  the  American  Cretaceous 
waters  were  the  pythonomorphic  saurians  or  sea-serpents,  in  which 
group  Cope  includes  forms  like  MosasauruSy  whereof  more  than  40  species 
have  been  discovered.  Some  of  them  attained  a  length  of  75  feet  or 
more.  They  possessed  a  remarkable  elongation  of  form,  particularly  in 
the  tail ;  their  heads  were  large,  flat^  and  conic,  with  eyes  directed  partly 
upwards.  They  swam  by  means  of  two  pairs  of  paddles,  like  the 
flippers  of  the  whale,  and  the  eel-like  strokes  of  their  flattened  tail. 
Like  snakes,  they  had  four  rows  of  formidable  teeth  on  the  roof  of  the 
mouth,  which  served  as  weapons  for  seizing  their  prey.  But  the  most 
remarkable  feature  in  these  creatiu*es  was  the  unique  arrangement  for 
permitting  them  to  swallow  their  prey  entire,  in  the  manner  of  snakes. 
Each  half  of  the  lower  jaw  was  articulated  at  a  point  nearly  midway 
between  the  ear  and  the  chin,  so  as  greatly  to  widen  the  space  between 
the  jaws,  and  the  throat  must,  consequently,  have  been  loose  and  baggy 
like  a  pelican's.  The  deinosaurs  were  likewise  well  represented  on  the 
shores  of  the  American  waters.  Among  the  known  forms  are  Hadrosaurvs, 
a  kangaroo -like  creature  resembling  the  Iguanodoriy  and  about  28  feet 
long ;  Diclonius,  an  allied  form  with  a  bird-like  head  and  spatulate  beak, 
probably  frequenting  the  lakes  and  wading  there  for  succulent  vegetable 
food,  interesting  from  its  occurrence  in  the  Laramie  group  of  beds  at 
the  very  close  of  the  Cretaceous  series ;  and  Lxlaps,  which  probably  also 
walked  erect,  and  resembled  the  Megalosaurus,  Still  more  gigantic  was  the 
allied  Orniihotarsus,  which  is  supposed  to  have  had  a  length  of  35  feet. 
There  were  also  in  later  Cretaceous  time  strange  horned  creatures  such  as 
Ceratops  which,  attaining  a  length  of  25  or  30  feet,  had  a  massive  body, 
a  pair  of  large  and  powerful  horns,  and  a  peculiar  dermal  armour. 
Akin  to  it  were  various  deinosaurs  imited  in  the  genus  TriceratopSj  so 
named  from  the  third  rhinoceros-like  nasal  horn.  Some  of  their  skuUs 
exceeded  6  feet  in  length,  exclusive  of  the  homy  beak,  and  4  feet  in 
width,  with  horn -cores  about  3  feet  long.  Claosaurus  was  another 
gigantic  deinosaur  not  unlike  the  IguaiwdoUy  with  remarkably  small 
fore-limbs  compared  with  the  massive  hind  legs.^  Pterosaurs  have  like- 
wise been  obtained  characterised  by  an  absence  of  teeth  {PteranodarUs), 

'  MarRh,  on  Cretaceous  Deinosanrs,  op.  cit.  xxxvi.   (1888)  zxzviii.   xxxiz.    xli.   xlii. 
xliv.  xlv.  (1893). 


dS4  STRATIORAPBiaAL  OBOLOUY  booxtipakdi 

and  some  of  which  had  a  spread  id  wing  of  20  to  26  feet^  Antong  the 
ChelonianB  one  gigantic  species  is  supposed  to  have  measured  upwards  of 
16  feet  between  the  tips  of  the  flippers. 

The  remains  of  birds  have  be^  met  with  botli  in  Europe  and  in 


America  among  Cretaceous  rocks.     From   the   Cambridge   Greensand 
bones  of  at  least  two  species,  referred  to  the  genus  Snationtii,  have  been 


>  Marab,  on  American  CreUceoai  Pterodut^lea,  Amer.  Jman.  Sci.  L  (1871)  iii.  it 
lli.  Mi.  «»iL  (188*). 

*  For  this  reatontion  and  Fig.  123  I  am  indebted  to  the  klndaew  of  mj'  friend 
Profeisor  Harab. 


SECT,  iii  §  1  CRETACEOUS  SYSTEM  936 

obtained.  These  creatures  are  regarded  by  Professor  Seeley  as  having 
osteological  characters  that  place  them  with  the  existing  natatorial 
birds. ^  From  the  American  Cretaceous  rocks  nine  genera  and  twenty 
species,  represented  at  present  by  the  remains  of  about  120  individuals, 
have  been  obtained.  Among  these  by  far  the  most  remarkable  are  the 
Odontomithes,  or  toothed  birds,  from  the  Cretaceous  beds  of  Kansas. 
Professor  Marsh,  who  some  years  ago  described  these  wonderfully 
preserved  forms,  has  pointed  out  the  interesting  evidence  they  furnish  of 
a  reptilian  ancestry.^  In  the  most  important  and  indeed  unique  genus, 
named  by  him  Hespeiomis  (Fig.  422),  the  jaws  were  furnished  with  teeth 
implanted  in  a  common  alveolar  groove,  as  in  Ichthyosaurus ;  the  wings 
were  rudimentary  or  aborted,  so  that  locomotion  must  have  been  entirely 
performed  by  the  powerful  hind  limbs,  with  the  aid  of  a  broad,  flat, 
beaver -like  tail,  which  no  doubt  materially  helped  in  steering  the 
creature  through  the  water.  It  must  have  been  an  admirable  diver. 
Its  long  flexible  neck  and  powerful  toothed  jaws  would  enable  it  to  catch 
the  most  agile  fish,  while,  as  the  lower  jaws  were  united  in  front  only 
by  cartilage,  as  in  serpents,  and  had  on  each  side  a  joint  that  admitted 
of  some  motion,  it  had  the  power  of  swallowing  almost  any  size  of  prey. 
Heaperornis  regalis,  the  type  species,  must  have  measured  about  6  feet 
from  the  point  of  the  bill  to  the  tip  of  the  tail,  and  presented  some 
resemblance  to  an  ostrich.  Of  the  other  genera,  Ichthyomis  (Fig.  423) 
and  Apatomis  were  distinguished  by  some  types  of  structure  pointing 
backward  to  a  very  lowly  ancestry.  They  appear  to  have  been  small, 
tern -like  birds,  with  powerful  wings  but  small  legs  and  feet.  They 
possessed  reptile-like  skulls,  with  teeth  set  in  sockets,  but  their  vertebrae 
were  bi-concave,  like  those  of  fishes.  There  were  likewise  forms  which 
have  been  grouped  in  the  genera  Gracul^vtis,  Laoniis,  PaUestringa,  and 
Telmatomis,  Altogether  the  earliest  known  birds  present  characters  of 
strong  affinity  with  the  Deinosaurs  and  Pterodactyles.^ 

Though  mammalian  remains  had  long  been  known  to  occur  in  the 
Triassic  and  Jurassic  formations,  none  had  been  obtained  from  Cretaceous 
rocks,  and  this  absence  was  all  the  more  remarkable  from  the  great 
abundance  and  perfect  preservation  of  the  reptilian  forms  in  these  rocks. 
But  the  blank  has  now  been  filled  by  the  remarkable  discovery  in  the 
Upper  Cretaceous  rocks  of  Dakota  and  Wyoming  of  a  large  series  of 
jaws,  teeth,  and  different  parts  of  the  skeletons  of  small  mammals  belonging 
to  many  individuals,  and  including  not  a  few  genera  and  species.  They 
were  found  associated  with  remains  of  deinosaurs,  crocodiles,  turtles, 
ganoid  fishes,  and  invertebrate  fossils  indicating  brackish  or  fresh-water 
conditions.  The  mammalian  forms  show  close  affinities  to  the  Triassic 
and  Jurassic  types.  There  are  several  distinct  genera  of  small  marsu- 
pials, others  seem  to  be  allied  to  the  monotremes,  but  there  are  no 
carnivores,  rodents,  or  ungulates.      The  genera  proposed  for  them  by 

»  Q.  J.  Geol.  Soc.  1876,  p.  496. 

*  '  Odontornithes, '  being  vol.  i.  of  Memoirs  qf  PeaJbody  Museum  of  Yale  CoUege,  and 
also  vol.  vU.  of  Geol.  Explor,  iOth  Parallel,  "  Birds  with  Teeth,"  Hep,  U.S.  Oeol.  Surv, 
1881-82,  p.  45.  '  See  Marsh,  U,S.  Oeol.  Surv.  Report,  1881-82,  p.  86. 


936  STRATIGSAPHICAL  GEOLOGY  book  vi  pabtmd 

Profesaor  Marsh  are  Cimdomt/s,  Cimoloden,  Nanomys,  Dipriodan,  Tr^riodon, 
Selniaroilon,  Halalmi,  CampUymus,  Dryolealt*,  Dideiphops,  CiraolesUs,  Ptdiomys, 
Stagodrw,  Platarodon,  Oracodon,  and  Allatodon}  More  recently  the  diacoverj' 
of  a  single  small  tooth  in  the  Wealden  series  of  Haatings  is  the  first  ttsce 


of  mammnlian  life  yet  found  in  the  Cretaceous  formations  of  Europe.     The 
specimen    has    been    provisionally    referred    to    the     Purbeckiaii    genus 


Till'  Cretaceous  S3-9t«n3,  in  many  detached  areas,  coren  a  large  extent  of  Europe. 
From  llifi  soiitlL-west  of  Euglaiid  it  apreaJs  across  the  north  of  France,  up  to  the  baae  of 
'  Msrsli,  Amff.  Journ.  Sci.  xiiTiii.  (188B)  pp.  81,  177  ;  iliii.  (1892)  p.  249. 
'  A.  Smith  Woodward,  yalurr,  l\i.  (1891),  p.  164. 


SECT,  iii  §  2  CRETACEOUS  SYSTEM  937 

the  ancient  central  plateau  of  that  country.  Eastwards  it  ranges  beneath  the  Tertiary 
and  post-Tertiary  de{)osits  of  the  great  plain,  appearing  on  the  north  side  at  the  southern 
end  of  Scandinavia  and  in  Denmark,  on  the  south  side  in  Belgium  and  Hanover,  round 
the  flanks  of  the  Harz,  in  Bohemia  and  Poland,  eastwai*ds  into  Russia,  where  it  covers 
many  thousand  square  miles,  up  to  the  southern  end  of  the  Ural  chain.  To  the  south  of 
the  central  axis  in  France,  it  underlies  the  great  basin  of  the  Garonne,  flanks  the  chain 
of  the  Pyrenees  on  both  sides,  spreads  out  largely  over  the  eastern  side  of  the  Spanish 
tableland,  and  reappears  on  the  west  side  of  the  crystalline  axis  of  that  region  along 
the  coast  of  Portugal.  It  is  seen  at  intervals  along  the  north  and  south  fronts  of  the 
Alps,  extending  down  the  valley  of  the  Rhone  to  the  Mediterranean,  ranging  along  the 
chain  of  the  Apennines  into  Sicily  and  the  north  of  Africa,  and  widening  out  from  the 
eastern  shores  of  the  Adriatic  through  Greece,  and  along  the  northern  base  of  the 
Balkans  to  the  Black  Sea,  round  the  southern  shores  of  which  it  passes  in  its  progress 
into  Asia,  where  it  again  covers  an  enormous  area. 

A  series  of  rocks  covering  so  vast  an  extent  of  surface  must  needs  present  many 
difl'erences  of  type,  alike  in  their  lithological  characters  and  in  their  organic  contents. 
They  bring  before  us  the  records  of  a  time  when  a  continuous  sea  stretched  over  the 
centre  and  most  of  the  south  of  Europe,  covered  the  north  of  Africa,  and  swept  eastwards 
to  the  far  east  of  Asia.  There  were  doubtless  many  islands  and  ridges  in  this  wide 
expanse  of  water,  whereby  its  areas  of  deposit  and  biological  provinces  may  have  been 
more  or  less  defined.  Some  of  these  barriers  can  still  be  traced,  as  will  be  immediately 
pointed  out. 

While  there  is  suflicient  palaeontological  similarity  to  allow  a  general  parallelism  to 
be  drawn  among  the  Cretaceous  rocks  of  western  Europe,  there  are  yet  strongly  marked 
difl'erences  (pointing  to  very  distinct  conditions  of  life,  and  probably,  in  many  cases,  to 
disconnected  areas  of  deposit.  Having  regard  to  these  geographical  variations,  a 
distinct  northern  and  southern  province,  as  above  stated  (p.  920),  can  be  recognised  ; 
but  Giimbel  has  proposed  a  further  grouping  into  three  great  regions  :  (1)  the  northern 
province,  or  area  of  White  Chalk  with  Belemnitella,  comprising  England,  northern 
France,  Belgium,  Denmark,  Westphalia,  &c. ;  (2)  the  Hercynian  province,  or  area  of 
Exogyra  columha,  embracing  Bohemia,  Moravia,  Saxony,  Silesia,  and  Central  Bavaria  ; 
and  (3)  the  southern  province,  or  area  of  Hippurites,  including  the  regions  of  France 
south  of  the  basin  of  the  Seine,  the  Alps,  and  southern  Europe.^ 

Britain.^ — The  Purbeck  beds  bring  before  us  evidence  of  a  great  change  in  the 
geography  of  England  towards  the  close  of  the  Jurassic  period.  They  show  how  the 
floor  of  the  sea,  in  which  the  thick  and  varied  formations  of  that  period  were  deposited, 
came  to  be  gradually  elevated,  and  how  into  pools  of  fresh  and  brackish  water  the  leaves, 
insects,  and  small  marsupials  of  the  adjacent  land  were  washed  down.  These  evidences 
of  terrestrial  conditions  are  followed  in  the  same  region  by  a  vast  delta-formation,  that 
of  the  Weald,  which  accumulated  over  the  south  of  England,  while  marine  strata  were 
being  deposited  in  the  north.  Hence  two  ty[)es  of  Lower  Cretaceous  sedimentation 
occur,  one  where  the  strata  are  fluviatile  (Wealden),  the  other  where  they  are  marine 
(Neocoraian).  The  Upper  Cretaceous  groups,  extending  continuously  from  the  coasts  of 
Dorsetshire  to  those  of  Yorkshire,  show  that  the  diversities  of  sedimentation  in  Lower 
Cretaceous  time  were  eflaced  by  a  general  submergence  of  the  whole  area  beneath  the  sea 
in  which  the  Chalk  was  deposited.  Arranged  in  descending  order,  the  following  are  the 
subdivisions  of  the  English  Cretaceous  rocks  : — 

^  *  Geognost.  Besclireib.  Ostbayer.  Orenzgebirg.' 

'■*  Consult  Conybeare  and  Phillips,  *  Geology  of  England  and  Wales,'  1822;  Fitton,  Ann, 
Philos.  2nd  ser.  viii.  379  ;  Trans.  Oeol.  Soc.  2nd  ser.  iv.  103  ;  Dixon's  'Geology  of  Sussex,' 
edit.  T.  Rupert  Jones,  1878  ;  Phillips's  *  Geology  of  Oxford  and  the  Thames  Valley'  ;  H. 
B.  Woodward's  '  Geology  of  England  and  Wales, '.  2nd  edit.  Special  papers  on  the  English 
Cretaceous  formations  are  quoted  In  subsequent  footnotes. 


STRATIGRAPHICAL  GEOLOGY 


BOOK  TI  PABt  n 


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SECT,  iii  §  2  CRETACEOUS  SYSTEM  939 

Lower  Cretaceous  (Neocomian  ^). — Between  the  top  of  the  Jurassic  system  and 
the  strata  known  as  the  Gault,  there  occurs  an  important  series  of  deposits  to  which, 
from  their  great  development  in  the  neighbourhood  of  Neuchdtel  in  Switzerland,  the 
name  of  Neocomian  has  been  given.  This  series,  as  already  remarked,  is  represented  in 
England  by  two  distinct  types  of  strata.  In  the  southern  counties,  from  the  Isle  of  Purbeck 
to  the  coast  of  Kent,  there  occurs  a  thick  series  of  fresh-water  sands  and  clays  termed  the 
Wealden  series.  These  strata  pass  up  into  a  minor  marine  group  known  as  the  Lower 
Greensand,  in  which  some  of  the  characteristic  fossils  of  the  Upper  Neocomian  rocks 
occur.  The  Wealden  beds  of  England  therefore  form  a  fluviatile  equivalent  of  the  con- 
tinental Neocomian  formations,  while  the  Lower  Greensand  represents  the  later  marginal 
deposits  of  the  Neocomian  sea,  which  gradually  usurped  the  place  of  the  Wealden 
estuary.  The  second  type,  seen  in  the  tract  of  country  extending  from  Lincolnshire 
into  Yorkshire,  contains  the  deposits  of  deeper  water,  forming  the  westward  extension  of 
an  important  series  of  marine  formations  which  stretch  for  a  long  way  into  central  Europe. 

Neocomian.^ — The  marine  Neocomian  strata  of  England  are  well  exposed  on  the 
cliffs  of  the  Yorkshire  coast  at  Filey,  where  they  occur  in  an  argillaceous  deposit  long 
known  as  the"Speeton  Clay."  This  deposit  is  now  shown  to  contain  an  interesting 
continuous  section  of  marine  strata  from  the  Kimeridge  Clay  to  the  top  of  the  Lower 
Cretaceous,  or  even  into  the  Up))er  Cretaceous  series.  It  has  been  carefully  studied  by 
Mr.  Laniplugh  and  by  Professors  Pavlow  and  Nikitin,  by  whom  it  has  been  brought 
into  comparison  with  the  Neocomian  rocks  of  Russia.  The  lower  ])art  of  the  **  Speeton 
Clay  "  consists  of  hard  dark  bituminous  shales  with  large  septarian  nodules  and  many 
crushed  fossils.  Among  these  remains  there  occur  Belemnites  Chceni,  Ammonites  sp., 
Ling^ula  ovalisy  IHscina  latissima,  Oatrea  gibbosttf  Lucina  niinti9cula,  &c.  These  strata 
are  referred  to  the  higher  part  of  the  Kimeridge  Clay.  .They  are  succeeded  conformably 
by  the  ''zone  of  BelemniUs  UUeralia"  consisting  of  dark,  pale,  and  banded  clays  with 
the  fossils  mentioned  in  the  foregoing  table.  At  the  base  of  the  zone  lies  a  *'  coprolite 
bed,"  and  its  topis  taken  at  a  ''compound  nodular  bed"  rich  in  fossils  {Bel.  lateralis, 
Amm.  noricus,  A,  rotula,  Avicula  insBquivalviSj  Pecten  cinctuSf  kc )  The  total  thick- 
ness of  this  zone  is  about  34  feet  It  is  overlain  by  the  "zone  of  Belemnites  ja^mlum,'* 
consisting  likewise  of  various  dark  and  striped  clays  and  bands  of  nodules,  the  whole 
having  a  thickness  of  about  125  feet.  While  the  underlying  zone  has  obvious  Jurassic 
affinities,  this  zone  is  unmistakably  Lower  Cretaceous.  The  characteristic  belemnite 
ranges  through  120  feet  of  the  section  with  hardly  any  trace  of  another  species. 
Ammonites  noricus  occurs  in  the  lower  30  feet  of  the  zone,  and  is  succeeded  by  A. 
speetonensis.  An  interesting  paleeontological  feature  in  this  zone  is  the  occurrence  of 
abundant  tests  of  Echinospaiangxis  cordiformis,  a  highly  characteristic  Neocomian  type. 
The  "  zone  of  Belemnites  semieanaliculattis  (?) "  is  seldom  seen  in  complete  section,  owing 
to  the  slipping  of  the  cliffs  and  the  detritus  on  the  foreshore.  It  consists  of  dark  clays 
100  feet  thick  or  more.  Above  it  a  few  feet  of  mottled  green  and  yellow  clays  form  the 
top  of  the  Speeton  Clay.  These  strata  compose  the  zone  of  Belemnites  minimus,  and 
contain  also  B,  cUtenuatus,  B.  ultimus,  Inoceramus  concaUHcus,  J.  sulcalus,  kc.  Some 
of  their  fossils  are  found  in  the  Gault,  and  it  has  been  suggested  that  they  may 
represent  here  the  Lower  Gault,  while  the  Red  Chalk  above  may  be  the  equivalent  of  the 
Upper  Gault' 

^  Neocomian,  from  Neocomum,  the  old  name  of  Neuch&tel  in  Switzerland. 

»  Fitton,  Trans.  Geo.  Sac.  2nd.  ser.  iv.  (1837)  103  ;  Proc.  Oeol.  Sac.  iv.  pp.  198,  208  ; 
Q.  J.  GeoL  Sac.  i.  Consult  on  marine  Neocomian  type  Young  and  Bird,  '  Survey  of  the 
Yorkshire  Coast'  (1828),  2nd  edit.  pp.  68-64  ;  J.  Phillips,  'Geology  of  Yorkshire,'  p.  124. 
J.  Leckenby,  Geologist,  ii.  (1859)  p.  9.  Judd,  Q.  J.  Geol.  Soc.  xxiv.  (1868)  218  ;  xxvi. 
326  ;  xxvii.  207  ;  Ged.  Mag.  vii.  220  ;  C.  J.  A.  Meyer,  Q.  J.  Geol.  Soc.  xxviii.  243  ;  xxix. 
70.     A.  Strahan,  op.  cit.  xlii.  (1886)  p.  486  ;  Mem,  Geol.  Surv.  sheet  84. 

'  G.  W.  Lamplugh,  op.  ciL 


940  STRATIGRAPHICAL  GEOLOGY  book  vi  paw  m 

In  Lincolushire  the  marine  Neocomian  series  is  likewise  developed.  Riiung  to  the 
surface  from  beneath  the  Chalk,  the  highest  and  lowest  strata  are  chiefly  sand  and 
sandstone  ;  the  middle  portion  (Tealby  series)  clays  and  oolitic  ironstones.  According 
to  Mr.  Lamplugh,  the  Spilsby  Sandstone  and  the  Claxby  Ironstone  of  this  county, 
forming  the  base  of  the  Neocomian  series  and  resting  on  Upper  Kimeridge  sbftles,  are 
equivalents  of  the  zone  of  Belemnites  lateralis  at  Speeton.  Tlie  Tealby  Clay,  which  over- 
lies them,  is  regarded  as  representing  the  zone  of  B.  jacuhim,  the  Tealby  Limestone  the 
zone  of  B.  semicanaliculatus  (?),  while  the  Carstone  at  the  top  immediately  below  the 
Red  Chalk  is  placed  on  the  horizon  of  the  marls  with  B.  minimus,^  The  Carstone 
ranges  into  Norfolk,  and  perhaps  represents  the  entire  **  Lower  Greensand*'  of  central 
and  southern  England. 

Weald  en. — In  the  southern  counties  a  very  distinct  assemblage  of  strata  is  met 
with.'  It  consists  of  a  thick  series  of  fluviatile  deposits  termed  Wealden  (from  the 
Weald  of  Sussex  and  Kent,  where  it  is  best  developed),  surmounted  by  a  group  of  marine 
strata  ("Lower  Greensand"),  in  which  Upper  Neocomian  fossils  occur.  It  would 
appear  that  the  fresh-water  conditions  of  deposit,  which  began  in  the  south  of  England 
towards  the  close  of  the  Jurassic  period,  when  the  Purbeck  beds  were  laid  down,  cod- 
tinued  during  the  whole  of  the  long  interval  marked  by  the  Lower  and  Middle 
Neocomian  fonnations,  and  only  in  Upper  Neocomian  times  finally  merged  into  ordinaiy 
marine  sedimentation.  The  Wealden  series  has  a  thickness  of  over  2000  feet,  and  in 
Sussex  and  Kent  consists  of  the  following  subdivisions  in  descending  order  : — 

Weald  Clay 1000  feet. 

Hastings  Sand  group  composed  of — 

3.   Tunbridge  Wells  Sand  (with  Grinstead  Clay)  .         .     140  to  3«0    „ 

2.   Wadhurstaay 120  „  180    „ 

1.  Ashdown  Sand  (with  Fairliglit  Clays  in  lower  part)  400  or  600    ,, 

In  the  Isle  of  Wiglit  these  subdivisions  cannot  be  made  out,  and  the  total  visible 
thickness  of  strata  (sandstones,  sands,  clays,  and  shales)  is  only  about  half  of  what  can 
be  observed  on  the  mainland  farther  east,  but  the  base  of  the  series  cannot  be  seen. 
Westward  at  PnnficUl,  on  the  coast  of  Dorsetshire,  the  Wealden  strata  are  exposed  on  tiie 
shore,  and  are  there  estimated  to  be  from  1500  to  2000  feet  thick.  On  the  whole  the 
Wealden  scries  is  thickest  towards  the  west. 

The  sandy  and  clayey  sediments  composing  the  Wealden  scries  precisely  resemble 
the  deposits  of  a  modern  delta.  That  such  was  really  their  origin  is  borne  out  by  their 
organic  remains,  which  include  terrestrial  plants  {Equvtctumf  Spkcnoptcri^y  Alethoptrris, 
Thut/itf^'i,  cycads,  and  conifers),  fresh-water  shells  {Unio^  10  species  ;  Cyrena,  5  species  : 
Paludina^  Vicarya^  Mcfania,  &c.),  with  a  few  estuarine  or  marine  forms,  as  Ostrra, 
fh'jxpfra,  and  Mytihts^  and  ganoid  fishes  (Lepidotm)  like  the  gar  of  American  rivers. 
Among  the  spoils  of  the  land  floated  down  by  the  Wealden  river  were  the  carcases 
(if  huge  deinosaurian  reptiles  {Cetiosaunis,  Titaiwsaurus^  Jg^ianodon,  ffylxosaunts, 
Poltu^anfJiKs,  MegalafauruSj  Vectisaurus,  ffypsilophodon)^  long -necked  ])lesiosaurs,  and 
winged  pterodactyles.  The  deltoid  formation,  in  which  these  remains  Occur,  extends 
in  an  east  and  west  direction  for  at  least  200,  and  from  north  to  south  for  perhaps  100 
miles.  Hence  the  delta  may  have  been  nearly  20,000  square  miles  in  area.  It  has 
been  compared  with  that  of  the  Quorra  ;  in  reality,  liowever,  its  extent  must  have  l>een 
greater  than  its  present  visible  area,  for  it  has  suffered  from  denudation,  and  is  to  a  large 
extent  concealed  under  more  recent  formations.     The  river  probably  descended  from  the 

*  See  A.  J.  Jukes-Browne,  'Geology  of  East  Lincolnshire'  in  ^fem.  Oeol,  Sure,  sheet  84 
(1887)  ;  G.  W.  Laniplugh,  '  Argiles  de  Speeton,'  BulL  Soc.  Imp.  Nat.  Moscou  (1891). 

-  On  the  Wealden  or  fluviatile  type  consult,  besides  the  works  quoted  on  p.  937,  MantellV 
*  Fossils  of  the  South  Downs,'  4to,  1822  ;  Topley,  •  Geology  of  the  Weald,*  in  Mcni.  Ged. 
Surr.  8vo.  1875.  Bristow  and  Strahan,  'Geology  of  the  Isle  of  Wight,'  2nd  edit.  (1889),  in 
Afem.  Gfol.  Surv.  (list  of  Wealden  fossils,  p.  258). 


70  to  100  feet. 

76 

,,100 

»f 

80 

,,300 

It 

20 

„    60 

i> 

SECT,  iu  §  2  CRETACEOUS  SYSTEM  941 

north-west,  draining  a  wide  area,  of  which  the  existing  mountain  groups  of  Britain  are 
l)erhaps  merely  fragments. 

Lower  Green  sand. — The  Wealden  series  is  succeeded  conformably  by  the  group 
of  arenaceous  strata  which  has  long  been  known  under  the  awkward  name  of  *' Lower 
Greensand."  This  group  consists  mainly  of  yellow,  grey,  white,  and  green  sands,  but 
includes  also  beds  of  clay  and  bands  of  limestone  and  ironstone.  It  has  been  subdivided 
in  descending  order  as  under  :-^ 

Folkestone  beds  (Lower  Albian  in  the  upper  part) 

Hy"rLr}(^p'-){  :    :    :    :    :    :    : 

Atherfield  Clay  (Urgonian),  resting  on  Wealdeu 

These  strata  appear  to  represent  the  continental  series  up  into  the  base  of  the  Albian 
stage.  The  Atherfield  Clay  is  well  developed  at  Atherfield,  on  the  south  coast  of  the 
Isle  of  Wight.  It  contains  an  abundant  series  of  fossils,  among  which  are  Toxaater  coin- 
planatua,  TerebrcUula  sella,  Exogyra{Ostrea)  Coulonif  Ostrea  Leymeriei,  Pema  Mulleti,  Area 
Hauliniy  and  others  which  indicate  an  Urgonian  horizon  for  this  band.  ^  In  the  Hythe 
beds  are  found  Plicatula  placuiua,  Ammonites  Deshayesi,  A.  comueliantts,  Anq/loeeras 
gigaSy  A.  Hilsiif  BeleinnUea  sernicanaliculalus,  Crioceras  Boicerbankii.  Some  of  these 
fossils  are  found  also  in  the  Sandgate  beds,  while  the  upper  part  of  the  Folkestone  beds 
yields  likewise  Amm.  inamillaria.  The  Hythe  and  Sandgate  beds  may  therefore  repre- 
sent the  Aptian  stage,  while  the  Folkestone  subdivision  may  be  regarded  as  the  equiva- 
lent of  the  lower  part  of  the  Albian.* 

Of  the  total  assemblage  of  fossils  from  the  'M^ower  Greensand,"  about  300  in 
number,  only  18  or  20  per  cent  pass  up  into  the  Upper  Cretaceous  series.  This 
marked  paleeontological  break,  taken  in  connection  with  a  great  lithological  change, 
and  with  an  unconformability  which  in  Dorset  brings  the  Gault  directly  upon  the 
Kimeridge  Clay,  shows  that  a  definite  boundary  line  can  be  drawn  between  the  lower 
and  upper  parts  of  the  Cretaceous  system  in  England. 

Upper  Cretaceous. — Three  leading  lithological  groups  have  long  been  recognised 
as  constituting  the  Upper  Cretaceous  series  of  England.  First,  a  band  of  clay  ternied 
the  Gault ;  second,  a  variable  and  inconstant  group  of  sand  and  sandstones  called  the 
"  Upper  Greensand  "  ;  and,  third,  a  massive  calcareous  formation  chieHy  com|)osed  of 
white  chalk.  But  the  foreign  nomenclature,  founded  mainly  on  paltBontological  con- 
siderations, and  giveu  in  the  foregoing  table  (p.  938),  may  now  be  adopted,  as  it 
brings  the  English  Upper  Cretaceous  groups  into  recognisable  parallelism  with  their 
continental  equivalents. 

Gaul  t*  (Albian). — A  dark,  stiff,  blue,  sometimes  sandy  or  calcareous  clay,  with  layers 
of  pyritous  and  phosphatic  nodules  and  occasional  seams  of  green  sand.  It  varies  from 
100  to  more  than  300  feet  in  thickness,  forming  a  marked  line  of  boundary  between  the 
Upper  and  Lower  Cretaceous  rocks,  overlapping  the  latter  and  resting  sometimes  even  on 
the  Kimeridge  Clay.  One  of  the  best  sections  is  that  of  Copt  Point,  on  the  coast  near 
Folkestone,  where  the  following  subdivisions  have  been  established  by  Messrs.  De  Ranee 
and  Price  :  ** — 


^  For  a  list  of  the  fossils  of  the  Atherfield  Clay  and  other  members  of  the  Lower  Green* 
sand  in  the  Isle  of  Wight,  see  the  Qeol,  Surv.  Mem.  on  that  Island  cited  on  the  foregoing 
page. 

'  For  explanations  of  these  and  the  other  Cretaceous  stratigraphical  terms,  which  have 
been  chiefly  founded  on  the  names  of  continental  localities  or  districts  where  the  several 
subdivisions  are  especially  well  developed,  see  the  footnotes  on  the  succeeding  pages. 

^  **  Gault "  is  a  Cambridgeshire  provincial  name. 

*  C.  K  De  Ranee,  Qeol.  Mag.  v.  p.  163  ;  i.  (2)  p.  246  ;  F.  G.  H.  Price,  Q.  J.  OeoL 
Sac.  XXX.  p.  342  ;  'The  Gault,'  8vo,  London,  1879. 


942 


STRATIORAPHIGAL  GEOLOGY 


BOOK  YI  PABT  m 


cS 

C 


i 


Upper  Greensand. 

11.  Pole  grey  marly  clay  (56  ft.  3  in.),  characterised  by  Ammonites  (Sehldm- 
bachia)  rostrcttua  {ir^atus\  A.  Ooodhalli,  Ostrea  /tons,  Inoceramus 
Crispii, 
Hard  pale  marly  clay  (5  ft  1  in.),  with  Kingena  lima,  RosteUaria  maxima, 
Plicatula  pectinaides,  Pecten  ratUinianuSy  Pentacrinus  FSMoni,  ddaris 
gaultina. 
Pale  grey  marly  clay  (9  ft.  4^  in. ),  with  Inoceramus  sulcaius.  Ammonites  vari- 

coaus,  Phdadomya  ftArina,  Pleurot4>mar%a  Oihbsii,  Seaphites  aBqualis. 
Darker  clay,  with  two  lines  of  nodules  and  rolled  fossils  (9^  in.),  with  Am- 
monites cristaius,A.  Beudanti,  Ph^as  sanctx-crucis,  MytUus  Galliennei, 
Cucullsea  glabra,  Cyprina  quadrata. 

Dark  clay  (6  ft.  2  in.)  highly  fossiliferous,  with  Ammonites  auritus,  Nuc%Ua 

bivirgata,   N.   omatissima,   Aporrhais  Parkinsoni,  Fusus  indecisus, 

Pteroceras  bicarinaium. 
Dark  mottled  clay  (1   ft.),  Ammonites  denarius,  A.  comutus,  TSirrilites 

hugardiantbs,  NecrocarcintLs  Bechei. 
Dark  spotted  clay  (1  ft.  6  in.).  Ammonites  {Hoplites)  lautus,  Astarte  dupi- 

niana.  Solarium  moniliferum,  Phasianella  ervyana,  numeroiis  corals. 
Paler  clay  (4  in.)  Ammonites  Delaruei,  Natica  otdiqua,  Dentalium  deetts- 

satum,  Fusus  gaultinus. 
Light  fawn-coloured  clay,  *'  crab-bed  "  (4  ft.  6  in.)  with  numerous  carapaces 

of  crustaceans  {Paleeocorystes  Stokesii,  P.  Broderipii),  Pinna  t^raffona, 

Uamites  attenuatus. 
Dark  clay  marked  by  the  rich  colour  of  its  fossils  (4  ft.  3  in.),  Ammonites 

auritus,  Turrilites  elegans,  Ancyloceras  spinigerum,  Aporrhais ealearata, 

Fusus  itierianus,   Ceriihium   trimonile,   Corbula  gaultina,   PoUicipes 

rigidus. 
Dark  clay,  dark  greensand,  and  pyritous  nodules  (10  ft.  1  in.).  Ammonites 

interruptus,  Crioceras  astierianum,  Uamites  rolvndus. 
Lower  Greensand. 


10. 


9. 


8. 


7. 


6. 


5. 


p 
a 

O 

c 


i 


3. 


2. 


1. 


Mr.  Price  remarks  that,  out  of  240  species  of  fossils  collected  by  him  from  the  Ganlt 
only  39  are  common  to  the  lower  and  upper  divisions,  while  124  never  pass  up  from  the 
lower  and  59  ajipear  only  in  the  upper.  The  lower  Gault  seems  to  have  been  deposited 
in  a  sea  specially  favourable  to  the  spread  of  gasteropods,  of  which  46  species  occur 
in  that  division  of  the  formation.  Of  these  only  six  appear  to  have  survived  into  the 
period  of  the  upper  Gault,  where  they  are  associated  with  five  new  forms.  Of  the 
lamellibranch  fauna,  numbering  in  all  73  species,  39  are  confined  to  the  lower  division, 
four  arc  peculiar  to  the  passage-bed  (No.  8),  14  pass  up  into  the  upper  division,  where 
they  are  accompanied  by  16  new  forms.  About  46  jKjr  cent  of  the  Gault  fauna  pass  up 
into  the  upper  Greensand.^ 

Cenomanian.'-' — Under  the  name  of  Upper  Greensand  have  been  comprised  sandy 

^  Tlie  foraminifera  of  tlie  Gault  at  Folkestone,  with  reference  to  the  zones  here  given, 
have  been  described  by  F.  Chapman,  Jovm.  R.  Micros.  Soc.  1891,  p.  665  ;  1892,  pp.  321,  749. 

■•*  tVom  CaMiomanum,  the  old  Latin  name  of  the  town  Mans  in  the  department  of  Sarthe. 
The  old  lithological  subdivisions  of  the  English  Upi>er  Cretaceous  groups  have  been  found  to  be 
wanting  in  palceontological  precision,  and  are  gradually  being  supplanted  by  the  terms 
proposed  by  D'Orbigiiy,  which  have  long  been  in  use  in  France.  These  terms  are  here 
employed,  but  their  equivalents  in  the  old  nomenclature  will  be  understood  from  the 
table  on  p.  938.  To  M.  Hebert  geology  is  mainly  indebted  for  the  thorough  detailed 
study  and  classification  to  which  the  Upi>er  Cretaceous  formations  of  the  Anglo- Parisian 
basin  have  been  subjected.  In  1874  he  published  a  short  memoir,  in  which  the  Chalk  in 
Kent  was  sulxlivided  into  zones  equivalent  to  those  in  the  Paris  basin  {Bull.  Soc  GM. 
France,  1874,  p.  416).  Subsequently  the  same  task  was  taken  up  and  extended  over  the 
rest  of  the  English  Cretaceous  districts,  by  Dr.  Charles  Barrois  ('  Recherches  sur  le  Terrain 
Cretace  superieur  de  I'Angleterre  et  de  I'lrlande,'  Lille,  1876).  The  first  English  geologist 
who  appears  to  have  attempted  the  palxontological  subdivision  of  the  Chalk  was  Mr.  Caleb 


8BCT.  iii  §  2  CRETACEOUS  SYSTEM  943 


strata,  often  greenish  in  colour,  which  are  now  known  to  belong  to  different  horizons  of 
the  Cretaceous  series.  If  the  term  is  to  be  retained  at  all,  its  use  must  be  accompanied 
with  some  palseontological  indication  of  the  true  position  of  the  beds  to  which  it  is 
applied.  According  to  the  researches  of  Dr.  C.  Barrois,  the  English  Upper  Greensand, 
as  originally  defined  by  Berger,  Inglefield,  Webster,  Fitton,  and  others,  has  no  such 
distinct  assemblage  of  fossils  as  might  have  been  supposed  from  its  lithological 
characters,  but  appears  to  be  everywhere  divisible  into  two  groups :  a  lower  containing 
Ammonites  rostratus  {inJUUus),  and  an  upper  marked  by  Pecten  aaper.  These  strata  are 
well  developed  in  Devonshire  and  Somerset.  There  the  **Blackdown  beds"  below, 
linked  with  the  Gault  (of  which  Godwin-Austen  regarded  them  as  a  sandy  littoral 
representative)  contain  a  numerous  fauna,  including  AmmoniUa  OoodkaHi^  Hamitea 
alterruUuSf  Cytherea  parva,  Ven^is  aubmersa^  Area  glabra^  Trigonia  alse/ormiSf  Pecten 
laminosuSf  Janira  quinqueeostata^  J.   qucidricoatatay    J.    aequicostata^   Exogyra  conica 


Vermieularia  polygonalis ;  while  the  •*  Warminster  beds"  above  correspond  to  IfifcZT 
**zone  of  Holaster  rvodulosua"  of  M.  Hubert,  and  the  *'zone  of  Pecten  aaper'*  of  Dr.  ^^ 
Barrois,  and  contain  Ammonites  {Sehlonbachia)  varians,  A.  Mantelli,  A,  Coupei,  Belemnites 
nltimuSf  Pecten  aaper ^  Oatreafrona  (carinala\  Terebraiella  pectita,  Terebratula  biplieata, 
T.  squamoaa,  Ehynchonella  compreaaa,  JL  latiaaima,  PscvAJLodiadema  Michelini,  Peltaatea 
clathratttSf  Diacoidea  avhtumla,  &c.  A  tolerably  abundant  series  of  corals  has  been 
obtained  from  the  Devonshire  Upper  Greensand,  no  fewer  than  21  species  having  been 
described.^ 

The  so-called  Greensand  of  Cambridge  (pp.  931,  938),  a  thin  glauconitic  marl,  with 
phosphatic  nodules  and  numerous  (possibly  ice -borne)  erratic  blocks,  was  formerly 
classed  with  the  Upper  Greensand,  but  has  recently  been  shown  to  be  the  equivalent 
of  the  Glauconitic  Marl,  forming  really  the  base  of  the  Chalk  Marl  and  lying  uncon- 
formably  upon  the  Gault,  from  the  denudation  of  which  its  rolled  fossils  have  been 
derived.'* 

Lower  Chalk. — The  thick  calcareous  deposit  known  as  the  Chalk  is  classed  now  in 
three  chief  divisions — Lower,  Middle,  and  Upper.  Under  the  name  of  Lower  Chalk  are 
included  the  groups  of  the  Glauconitic  or  Chloritic  Marl,  the  Chalk  Marl,  and  the  Grey 
Chalk  up  to  the  top  of  the  zone  of  Belemnitella  plena  and  base  of  the  '*  Melbourne 
Rock." 

Glauconitic  {Chloritic)  Marl. — This  name  has  been  applied  to  a  local  white,  or  light 
yellow,  chalky  marl  lying  below  the  true  Chalk,  and  marked  by  the  occurrence  of  grains 
of  glauconite  (not  chlorite)  and  phosphatic  nodules.  It  varies  up  to  15  feet  in  thickness. 
Among  its  fossils  are  Ammonitea  laticlaviuay  A.  Coupei,  A,  Mantelli,  A.  variana,  Nautihia 
laRvigatuaf  Turrilitca  tuberculatua,  Solarium  omatumf  Plicatula  injiatay  Terebratula 
biplicata.     It  forms  the  base  of  the  Bolaater  aubgloboavs  zone. 

Cha^k  Marl  is  the  name  given  to  an  argillaceous  chalk  forming  with  the  chloritic 
marl,  where  the  latter  is  present,  the  base  of  the  true  Chalk  formation.  This  sub- 
division is  well  exposed  on  the  Folkestone  cliffs,  also  westward  in  the  Isle  of  Wight, 
where  a  thickness  of  upwards  of  100  feet  has  been  assigned  to  it  Among  its  charac- 
teristic fossils  are  Plocoacyphia  msBandrina,  Holaster  laevia  (var.  noduloaua)^  Hhynchonclla 
Martiniy  Inoceramus  striatiiaj  Lima  globoaa^  Plicatula  inflates,  Ammonitea  cenomanenaia, 

Evans  ('Sections  of  Chalk,'  Lewes,  8vo,  1870  ; /ar  the  Oeologista'  Aaaociation),  See  also 
W.  Whitaker,  *'  Geology  of  the  London  Basin  "  and  "Geology  of  London,*'  in  Oeol.  Survey 
MeiiioirSf  and  authors  there  cited.  A  tolerably  full  bibliography  will  be  found  in  Dr. 
Barrois'  volume. 

*  On  the  literature  of  the  Blackdown  beds,  see  W.  Downes,  Q.  J,  Geol.  Soc  xxxviii. 
(1882)  p.  75,  where  a  list  of  their  fossib  is  given.  The  corals  are  described  by  P.  Martin 
Duncan,  Q.  J,  Oeol.  Soc  xxxv.  p.  90. 

*  Jukes-Browne,  Q.  J.  Geol.  Soc.  xxxi.  p.  272,  xxxiii.  p.  485  ;  "Geology  of  Cambridge," 
Mem.  Geol,  Surr.  1881  ;  Geol.  Mag.  1877. 


944  STRATIGRAPHICAL  GEOLOGY  book  vi  part  m 

-     -    -  ■  -  _  , 

A,  falcatuSf  A.  MarUelli,  A,  nameiilaris,  A,  varians,   ScaphiUs  aequalis,    Turriliiei 
costatus. 

At  Hunstanton  in  Norfolk,  likewise  in  Lincolnshire  and  Yorkshire,  as  already 
(p.  939)  referred  to,  the  '*Red  Chalk" — a  ferruginous,  hard,  nodular  chalk  zone  (4  feet), 
lies  at  the  base  of  the  Chalk  and  rests  on  the  Upper  Keocomian  "Carstoney"  the  true 
Gault  being  there  absent,  although  it  occurs  a  few  miles  farther  south.  ^  Its  proper 
horizon  has  been  the  subject  of  much  discussion  ;  but  it  probably  belongs  to  the  Chalk 
Marl.  Bands  of  red  and  yellow  chalk  occur  in  the  lower  parts  of  the  Chalk  above  the 
horizon  of  the  "Red  Chalk"  in  Lincolnshire  and  Suffolk.' 

Grey  Chalk.  — The  lower  part  of  the  Chalk  has  generally  a  somewhat  greyish  tint, 
often  mottled  and  striped.     In  Bedfordshire  and  adjoining  counties  a  band  of  hard  grey 
sandy  chalk,  from  6  to  15  feet  thick,  containing  8  per  cent  of  silica  and  in  placet 
much  glauconite,  is  known  as  the  Tottemhoe  Stone,^  and  forms  the  base  of  the  Gr^ 
Chalk,  which  as  a  stage  comprises  the  palteontologieal  zones  of  HoltisUr  subglobonu 
and  BclcviniUlla  plena.     It  attains  its  fullest  development  along  the  shore -clifls  of 
Kent,  where  it  has  a  thickness  of  about  200  feet.     According  to  Mr.  F.  G.  H.  Price,*  it 
is  there  divisible  into  five  beds  or  sub-stages.     Of  these  the  lowest,  8  feet  thick  (  =  lower 
part  of  the  AmmofiiiUs  varians  zone),  contains  among  other  fossils  Diacoidea  aubucula^ 
Pecten  Bcaverij  Ammonites  varians;  the  second  bed  (11  feet)  contains  manj  fossils,  in* 
eluding  AmiTumiles  rothomagensiSf  A.  ManUlli,  A.  lewcsiensis  (  =  part  of  A.  tamitt 
zone) ;  the  third  bed  (2  feet  9  inches),  also  abundantly  fossiliferous,  contains  among 
other  forms  PcU-asUs  clathratusy  Hemiaster  Marrisiiy  Tcrebratula  rigida^  Rhynchondh 
mantelliaiui.  Ammonites  rothomagcnsis,  A.  varians  ;  this  and  the  two  underlying  beds  are 
regarded  as  comprising  the  zone  of  Ammonites  rothoniagensis  and  A.  varians  ;  the  fourth 
sub-stage,  or  zone  of  Ilolaster  subglobosus  (148  feet),  contains  among  its  most  character* 
istic  fossils  Discoidea  cylindrical  Holaster  m^bglobosus,  Goniaster  moaaieus,  and  in  its 
upper  part  Belemnitella  plena  ;  the  fifth  bed,  or  zone  of  Belemnitclla  piena^  consisting  of 
yellowish-white  gritty  chalk  (4  feet),  forms  a  well-defined  band  between  the  Grey  Chalk 
and  the  overlying  lower  sulxlivision  of  the  White  Chalk  (Turonian) ;  it  contains  few 
fossils,  among  which  arc  Belemnitella  plcna-j  Hippurites  (Radiolites)  Mort^yiii,  Ptychodui. 
In  Cambridgeshire  the  Chalk  Marl  is  covered  by  a  band  of  harder  stone  (Tottemhoe 
Stone),  passing  up  into  sandy  and   then  nearly  pure  white  chalk,  and  these  strata, 
equivalents  of  the  Chalk  Marl  and  Grey  Chalk,  are  probably  separated  by  a  (>alaeonto- 
logical  and  stratigraphical  break  from  the  next  overlying  (Turonian)  member  of  the 
series.'     According  to  the  original  classification  of  M.  Hebert,  this  zone  of  BeUmnileUa 
plena  is  placed  at  the  base  of  the  Turonian  grouj) ;  by  Dr.  Barrois  it  is  made  the  summit 
of  the  Ceuomanian.     The  latter  view  receives   support   from   traces  of  a  break  and 
deniulatiou  above  this  zone  in  England. 

Middle  Chalk,  Turonian.** — This  division  comprises  the  *'Lower  White  Chalk 
without  flints,"  and  is  marked  otf  at  the  base  by  a  band  of  hard  yellow  and  white 
nodular  chalk,  locally  known  in  Cambridgeshire  as  "rag,"  and  termed  by  geologists  the 
Melbourne  Rock.     It  is  about  8  or  10  feet  thick,  and  forms  a  convenient  baud  in  map- 

1  See  Whitaker,  Ged.  Mag.  1883,  p.  22 ;  Proc.  Oeol.  Assoc,  viii.  No.  3  (1888),  p.  133. 
This  author  gives  a  full  description  and  bibliography  of  the  Red  Chalk  in  Proc  Norwi(h 
Geol.  >Sof.  i.  part  vii.  (1883)  p.  212. 

■^  A.  J.  Jukes- Browne,  Geol.  Mag.  1887,  p.  24. 

^  For  the  list  of  fossils  of  this  bed  in  Norfolk  and  Suffolk  see  Jukes-Browne  and  W. 
Hill,  Quart.  Joum.  Geol.  Soc.  1887,  p.  575. 

*  Q.  J.  Ged.  Soc.  xxiii.  p.  436. 

''  A.  J.  Jukes-Browne,  Geol.  Mag.  1880,  p.  250.  See  also  the  same  author  in  "Greology 
of  the  Neighbourhood  of  Cambridge"  (Me7n.  Geol.  Surv.),  and  Qnart.  Joum,  Oeol.  Soc. 
1886,  p.  216  ;  1887,  p.  544. 

®  PYom  Touraiue,  where  the  marly  chalk  is  well  developed. 


SECT,  iii  §  2  CRETACEOUS  SYSTEM  945 

ping  out  tlie  subdivisions  of  the  Chalk.     It  contains  Ithynchoiiella  Cuvieri,  Terebratulina 
striata  J  Lioceramus  Cuvieri,  Spoftdylus  stricUuSy  Ammonites  peramplus,  &c.' 

The  White  Chalk  of  England  and  north«west  France  forms  one  of  the  most  con- 
spicuous members  of  the  great  Mesozoic  suite  of  deposits.  It  can  be  traced  from 
Flamborough  Head  in  Yorkshire  across  the  south-eastern  counties  to  the  coast  of  Dorset. 
Throughout  this  long  course,  its  western  edge  usually  rises  somewhat  abruptly  from  the 
])Iains  as  a  long  winding  escarpment,  which  from  a  distance  often  reminds  one  of  an  old 
coast-line.  The  upper  half  of  the  deposit  is  generally  distinguished  by  the  presence  of 
many  nodular  layers  of  flint.  With  the  exception  of  these  enclosures,  however,  the 
whole  formation  is  a  remarkably  pure  white  pulverulent  dull  limestone,  meagre  to  the 
touch,  and  soiling  the  fingers.  Composed  mainly  of  crumbled  foraminifera,  urchins, 
mollusks,  &c.,  it  must  have  been  accumulated  in  a  sea  tolerably  free  from  sediment,  like 
some  of  the  foramiuiferal  ooze  of  the  existing  sea-bed.  There  is,  however,  no  evidence 
that  the  depth  of  the  water  at  all  approached  that  of  the  abysses  in  which  the  present 
Atlantic  globigerina-ooze  is  being  laid  down.  Indeed,  the  character  of  the  foraminifera, 
and  the  variety  and  association  of  the  other  organic  remains,  are  not  like  those  which 
have  been  found  to  exist  now  on  the  deep  floor  of  the  Atlantic,  but  present  rather  the 
characters  of  a  shallow- water  fauna.^  Moreover,  the  researches  of  M.  Hebcrt  have  shown 
that  the  Chalk  is  not  simply  one  continuous  and  homogeneous  deposit,  but  contains 
evidence  of  considerable  oscillations,  and  even  perhaps  of  occasional  emersion  and 
denudation  of  the  sea-floor  on  which  it  was  laid  down.  The  same  observer  believed  that 
enormous  gaps  occur  in  the  Upper  Cretaceous  series  of  the  Anglo- Parisian  basin,  some 
of  which  are  to  be  supplied  from  the  centre  and  south  of  France  {postea^  p.  951). 

Following  the  modern  classification,  we  find  that  the  old  subdivision  of  *'  Chalk 
without  flints  "  agrees  on  the  whole  with  the  Turonian  section  of  the  system.  This 
division,  as  above  remarked,  appears  in  some  places  to  lie  unconforniably  upon  the 
members  below  it,  from  which  it  is  further  separated  by  a  marked  zoological  break. 
Nearly  all  the  Conomanian  species  now  disappear,  save  two  or  three  cosmopolitan  forms. 
The  echinoderms  and  brachiopods  are  entirely  replaced  by  new  species.''  Not  only  is 
tliu  base  of  the  Turonian  group  defined  by  a  stratigraphical  hiatus,  but  its  summit  is 
marked  by  the  "Nodular  Chalk"  of  Dover  and  the  hard  Chalk-rock,  which  appear  to 
indicate  another  stratigraphical  break  in  what  was  formerly  believed  to  be  an  uninter- 
rupted deposit  of  chalk.  The  three  Turonian  paleeontological  zones,  so  well  established 
in  France,  are  also  traceable  in  England.  As  exposed  in  the  splendid  Kent  clifls,  the 
base  of  the  English  beds  is  formed  by  a  well-marked  band  (32  feet)  of  hard  gritty  chalk, 
made  up  of  fragments  of  Inocerami  and  other  organisms.*  Fossils  are  here  scarce  ;  they 
include  Inoceramus  labiatus  (which  begins  here),  Rhynchonclla  Cuvierif  Echiiwconus 
subrotundusy  Cardiaster  pygmasus.  Above  this  basement  l)ed  lies  the  massive  Chalk 
without  flints,  full  of  fragments  of  Inoceramus  labiaiuSy  with  /.  Cuvierij  Tcrehratula 
semiglobosa,  Terebratulina  gracilis,  Echinoconus  subrotunduSy  &c.  The  lower  70  feet  or 
so  include  the  zone  of  Inoceramus  labiatus,  the  next  90  or  100  feet  that  of  Terebratulina 
gracilis^  and  the  upper  50  or  60  feet,  containing  layers  of  black  flints,  that  of  ffolaster 
planus.     At  the  top  comes  the  remarkably  constant  band  of  hard  cream-coloured  lime- 

^  W.  Hill  and  A.  J.  Jukes- Browne,  Quart.  Journ.  Oeol.  Soc.  1886,  p.  216  ;  op,  cit. 
1887,  p.  580. 

^  Dr.  J.  Gwyn  Jeflreys  shows  that  the  moUusca  of  the  Chalk  indicate  comparatively 
shallow-water  conditions  ;  Brit,  Assoc.  Rep,  1877,  Sees.  p.  79.  See  also  Nature,  3rd  July 
1884,  p.  215  ;  L.  Cayeux,  Ann,  Soc.  Giol.  Nordy  xix.  (1891)  pp.  95,  252.  For  a  general 
account  of  the  origin  of  the  Chalk,  with  special  reference  to  its  minuter  organisms,  see  T.  R. 
Jones,  Trans.  Hertford,  Nat,  Hist,  Soc,  iii.  part  5  (1885),  p.  143. 

'  Jukes-Browne,  Oeol.  Mag.  1880,  p.  250. 

■*  For  an  account  of  the  Middle  Chalk  of  Dover  see  W.  Hill,  Quart.  Journ,  Oeol.  Soc. 
1886,  p.  232. 

3  p 


946  STRATIGRAPHICAL  GEOLOGY         book  vi  part  in 

stone  known  as  the  **  Chalk  Rock,"  varying  from  a  few  inches  to  10  feet  in  thickneai. 
Its  upper  surface  is  generally  well  defined,  sometimes  even  suggestive  of  having  been 
eroded,  but  it  shades  down  into  the  Lower  Chalk. ^ 

Upper  Chalk,  ^enonidin^  {Upper  Chalk  with  flints). — This  massive  formation  is 
composed  of  white,  pulverulent,  and  usually  tolerably  pure  chalk,  with  scattered  flints, 
which,  being  arranged  in  the  lines  of  deposit,  serve  to  indicate  the  otherwise  indistinct 
stratification  of  the  mass.  It  has  been  generally  regarded  by  English  geologists  as  a 
single  formation,  with  great  uniformity  of  lithological  characters  and  fossil  contents. 
Mr.  Whitakcr,  however,  showed  that  distinct  lithological  platforms  occur  in  it,  and 
later  researches,  especially  by  MM.  Hubert  and  Barrois,  brought  to  light  in  it 
the  same  zones  that  occur  in  the  Paris  basin.  Of  these  the  lowest,  or  that  of  the 
^licrasters  (Broadstairs  and  St.  Margaret's  Chalk),  is  most  widely  spread,  the  others 
having  suffered  most  f;rom  denudation.  It  is  well  exposed  along  the  cliffs  of  Kent  at 
Dover,  and  also  in  the  Isle  of  Thanet.  At  Margate  its  thickness  has  been  ascertained 
by  boring  to  be  265  feet.  It  contains  two  zones,  in  the  lower  of  which  the  characterise 
urchin  is  Micraster  cor-testudinariitm,  while  in  the  upper  it  is  Ai.  eor-anguinum.  Near 
the  top  of  the  Micraster  group  of  beds  in  the  Isle  of  Thanet'  lies  a  remarkable  seam  of 
flint  about  three  or  four  inches  thick,  forming  a  nearly  continuous  floor,  which  has 
been  traced  southwards  at  the  top  of  the  cliffs  between  Deal  and  Dover.  Again,  on  the 
coast  of  Sussex,  what  may  be  nearly  the  same  horizon  in  the  Chalk  is  defined  by  a 
corresponding  band  of  massive  flattened  flints.  The  traces  of  emersion  and  erosion 
observed  by  M.  Hebert  in  the  Paris  Chalk  are  regarded  by  Dr.  Barrois  as  equally 
distinct  on  the  English  side  of  the  Channel,  in  the  form  of  surfaces  of  hardened  and 
corroded  chalk.  One  of  these  surfaces  marks  the  upper  limit  of  the  Micraster  group  on 
the  Sussex  coast,  where  it  consists  of  a  band  of  yellowish,  hardened,  and  corroded  chalk 
about  six  inches  thick,  containing  rolled  green-coated  nodules  of  chalk. ^  A  similar 
hardened,  corroded  band  forms  the  same  limit  in  the  Isle  of  Thanet.  Among  the 
fossils  of  the  Micraster  division  the  following  may  be  mentioned :  Mierastir  eor- 
iestudinariumy  M.  cor'anguinum^  Cidaris  clamgerOy  Echinoeorys  vulgaris,  Eehinoeonits 
conicvs^  Epiaster  gibbuSj  Terebratulina  gracilis^  Terebratula  semiglobosaf  Ostrea  vcstcvlaris, 
Inocera  mvs  in  vol  ui  its. 

The  middle  subdivision,  or  Margate  Chalk,  has  been  named  the  Marsupite  zone  by 
Dr.  Barrois,  from  the  abundance  of  these  crinoids.  It  attains  a  thickness  of  about  80 
feet  in  the  Isle  of  Thanet,  where  it  contains  few  or  no  flints,  and  upwards  of  400  feet  in 
the  Hampshire  basin,  where  flints  are  numerous.  Among  its  fossils  are  Porosphxria 
glohnlaris,  Bonrgueticrinus  elliptieuSy  MarsupiUs  aniatuSf  M.  MilUri,  Micraster  cor-angui- 
num,  Echijiocmws  coni^us^  Echinocorys  vulgaris^  Cidaris  clavigera^  C  sceptri/era,77i€cidinm 
JVethcreUi,  Terebratula  seimglobosa^  Rhynclwnella  plicaiilisy  Terehratulina  striata,  Spon- 
di/lus  {Lima)  spuwsus^  S.  dutempleanuSf  Peden  cretosus,  Ostrea  vesiattaris,  0.  JUppqpo- 
dium,  Iiioceramns  lingua  (and  several  others),  Bclemnitella  vera,  B,  Merceyi,  Ammonites 
leptophyllvs. 

The  highest  remaining  group,  or  Norwich  Chalk,  forms  the  Belemnitella  zone  so  well 
marked  in  northern  Europe.  It  attains  a  thickness  of  from  100  to  160  feet  in  the 
Hampshire  basin,  is  absent  from  that  of  London,  but  reappears  in  Norfolk,  where  it 
attains  its  greatest  development.  It  is  at  Norwich  a  w^hite  crumbling  chalk  with  layers 
of  black  flints.  Among  its  fossils  are  Parasmilia,  centralis,  Trochosmilia  laxa,  Cypho- 
soma  magnificvm^  Salenia  geo7netrica,  Echinocorys  vnlgaris,  Jthynchonella  octoplicata, 
It.  limbata,  Terebratula  camea,  T.  obesa,  Ostrea  lunata,  Belemnitella  mucronaia,  B. 
qiiadrata. 

^  Whitaker,  J/ip//i.  Geol.  Sure.  iv.  p.  46  ;  Jukes-Browne,  Oeol,  Mag.  1880,  p.  254.  A 
similar  band  occurs  in  Normandy.  '-^  From  Sens  in  the  department  of  Yonne. 

•''  F.  A.  Bedwell,  Geol.  Mag.  1874.  p.  16. 
^  liarrois,  *  Terrain  Cretace  de  TAngleterre,'  &c.  1876,  p.  21. 


SECT,  iii  §  2 


CRETACEOUS  SYSTEM 


947 


The  uppermost  division,  or  Danian,^  of  the  Continental  Chalk  appears  to  be  absent 
in  England,  unless  its  lower  portions  are  represented  by  some  of  the  uppermost  beds  of 
the  Norwich  Chalk. 

The  Cretaceous  system  is  sparingly  represented  in  Ireland  and  Scotland.  Under  the 
Tertiary  basaltic  plateau  of  Antrim,  there  lies  an  interesting  series  of  deposits  which  in 
lithological  aspect  differ  greatly  from  their  English  equivalents,  and  yet  from  their  fossil 
contents  can  be  satisfactorily  paralleled  with  the  latter.     They  are  thus  arranged :  ^ — 


Hard  white  limestone  65  to  200  feet 


>> 


»» 


13 

Glaiiconitic  (Chloritic) 
Chalk    ...     3 


) ) 


»» 


16 


zone  of  Belemnitella  mucro- 
nata. 
Marsupites. 


> » 


»» 


(Jlauconitic  (Cliloritic) 
sand  and  sandstone  .3    , , 

Grey  marls  and  yellow 

sandstones      .         •  3    ,, 
Glauconitic  sand         •  6    ,, 


16 


30 
10 


1 1 


it 


Micrasters. 

Holaster  planus. 
Terebratulina  gracilis. 


Holaster  subglobosus. 
Pecten  asper. 


In  the  west  of  Scotland,  also,  relics  of  the  same  type  of  Cretaceous  formations  have 
been  preserved  under  the  volcanic  plateaux  of  Mull  and  Morven.  They  contain  the 
following  subdivisions  in  descending  order  :' — 


'N^liite  marly  and  sandy  beds  with  thin  seams  of  lignite 
Hard  white  chalk  with  Belemnitella  tmicronata,  &c.  . 
Thick  white  sandstones  with  carbonaceous  matter 
Glauconitic  sands  and  shelly  limestones,  Pecten  asper ^  Exotjy^ra  arnica 
Janira  quinquecoettUOf  Nautilus  deslongchampsianns,  kc. 


•20  feet 
10 
100 


60 


)» 


France  and  Belgium.^ — The  Cretaceous  system  so  extensively  developed  in  western 
Europe  is  distributed  in  large  basins,  which,  on  the  whole,  correspond  with  those  of 
the  chief  rivers.  Thus  in  France,  there  are  the  basins  of  the  Seine  or  of  Paris,  of  the 
Loire  or  of  Touraine,  of  the  Rhone  or  of  Provence,  and  of  the  Garonne  or  of  Aquitania, 
including  all  the  area  up  to  the  slopes  of  the  Pyrenees.  In  most  cases,  these  areas 
present  such  lithological  and  paleeontological  differences  in  their  Cretaceous  rocks  as  to 
indicate  that  they  may  have  been  to  some  extent  even  in  Cretaceous '  times  distinct 
basins  of  deposit. 

A  twofold  subdivision  of  the  system  is  followed  in  France,  but  with  a  difference  of 
nomenclature  and  partly  also  of  arrangement  from  that  in  use  in  England,  as  shown  in 
the  subjoined  table : — 


*  So  named  from  its  development  in  Denmark. 

-  Barrels,  op.  cit.  p.  216.     R.  Tate,  Q,  J.  Oeol.  Soc.  xxi.  p.  15. 

^  Judd,  Q.  J,  Oeol.  Soc,  xxxiv.  p.  736. 

**  The  Cretaceous  system  has  been  the  subject  of  prolonged  study  by  the  geologists  of 
France,  and  has  given  rise  to  considerable  differences  of  nomenclature.  The  main  sub- 
divisions recognised  and  named  by  D'Orbigny  have  been  generally  adopted.  But  great 
diversity  of  opinion  exists  as  to  the  names  and  limits  of  the  lesser  groups.  There  has 
been  a  tendency  to  excessive  elaboration  of  subdivisions.  The  minor  sections  of  the  geo- 
logical record  must  always  be  of  but  local  significance,  and  it  is  to  be  regretted  when  they 
are  treated  as  of  any  higher  importance.  M.  Hebert  refrained  from  burdening  geological 
nomenclature  with  a  long  list  of  new  names  for  local  developments  of  strata,  contenting 
himself  with  employing  D'Orbigny's  names  for  the  formations  or  sections,  and  subdividing 
these  into  upper,  middle,  and  lower  stages.  Tlie  student  will  find  some  of  the  rival 
systems  of  classification  collected  by  Mr.  Davidson,  Ged.  Mag.  vi.  (1869). 


1 

.,.«,». 

Olnliv  p!K.Hlique. 

~»„ 

1 

Gatuiiiiiien.i- 
UnrolrtthtKll.' 

Cilislnil:«at«MaeIti«Mr. 
Ctmle  i  llgsltei  di  FQVmu. 

Llkaln  1  Buulltei  ilu  Ciilru- 
Hn.    Cmlo  ile  Ciply,  lUtv 

1 

(.■aminuieii.l 

CniB  rt^  Rphn*."' 

C>1mlr»  i  gnnili  nultiln. 
Manirt  et  g>1c.  A  if ';>^r»»  difi- 

St?-"Hr^""""^ 

Gr»s  A  ichinidn. 

1 

J 

i 
1 

rt 

1 

Auauiimi.,,.! 

d  gl*»  iuf.  <lr  MoTMi 

LiHerlcii.il 
L'ureolaniL'ii.r 

Miimea  ■  nucliralltni. 

Cuuch»f«Millffre  lie  Rouen. 

Cnilr    ElaneonlauM   a   rirlrir 
an  Bnj. 

CoLr.  4  Copr(»n  .idwroi  et  giT«  'Ir 
UundngoD. 

Hiil.leii  ju  AmiK.  i">l"»  {Vni-oniiicn).  0»t(r-  .l» 
Oilcirp,  in.,  A  Flf™t.i1f.. 

Giw  rt  culolm  Ik  l-U.™  i  Jsm. 

CuLcalrrl  i^nnlixmin J/cMrnwi. 

(.■.l«i™  i   &..;i*f(«    1-K.«f    rl  : 

l!,.iit.Tlvieii." 
(fi  iml^). 

Uamoi   Ft   cute,  i  Spauti- 
JlJn";  .K  tUulerive. 

'S  mil*  A  flu,™   nw- 
ni(«.,     BcUmnfla     piUiUi 

CHlcnln-' A   STnuuhm    SiuHrrl 
(.Vni;,..  tfrtid/i,.,,),    .Vr^T.nr 

CulMiM  i  (Woorra.   /liira;;  cl 

'  FiK  fuutooten  Mee  next  pigr. 


SECT,  iii  §  2  CRETACEOUS  SYSTEM  949 

From  this  table  it  will  be  perceived  haw  marked  a  lithological  difference  is  traceable 
between  the  Cretaceous  deposits  of  the  north  and  south  of  France.  The  northern  area 
indeed  is  linked  with  that  of  England,  and  was  evidently  a  part  of  the  same  great  basin 
in  which  the  English  Cretaceous  rocks  were  deposited.  But  in  the  south,  the  aspect  of 
the  rocks  is  entirely  changed,  and  with  this  change  there  is  so  marked  a  difference  in 
the  accompanying  organic  remains  as  to  indicate  clearly  the  separation  of  the  two 
regions  in  Cretaceous  times. 

Infra-cr6tac6. — Neocomian.** — This  division  is  well  seen  in  the  eastern  part  of 
the  Paris  basin.  The  lowest  dark  marl,  resting  irregularly  on  the  top  of  the  Portlandian 
series,  indicates  the  emersion  of  these  rocks  at  the  close  of  the  Jurassic  period.  It  is 
followed  by  ferruginous  sands,  calcareous  blue  marl,  spatangus-limestones,  and  yellow 
marls  (abounding  in  Toxaster  complancUus,  Exogyra  Cotilonif  Pteroeera  pelagi,  Amm, 
radiatxiSj  &c.),  the  whole  having  a  thickness  of  125  to  140  feet,  and  representing  chiefly 
the  upper  or  Hauterivian  sub-stage.  Much  more  important  is  the  development  of  the 
Neocomian  deposits  in  the  southern  half  of  France.  They  present  there  evidence  of 
<leoper  water  at  the  time  of  their  formation.  The  Neuchatel  tyi)e  (p.  954)  is  prolonged 
into  the  northern  part  of  Dauphine,  where  it  is  seen  in  a  group  of  limestones,  with  Exogyra 
Couloni,  &c.,  in  the  lower,  and  Toxaster  complaiuituSf  &c.,  in  the  upper  beds.  South  wants 
the  limestones  are  mostly  replaced  by  marls,  and  the  whole  at  Grenoble  reaches  a  thick- 
ness of  more  than  1600  feet,  resting  on  the  upper  Jurassic  limestones,  vdth  TerebrcUula 
diphyoides. 

Urgonian. — In  the  typical  district  of  the  lower  valley  of  the  Durance,  this  sub- 
division consists  of  massive  limestones  (1150  feet)  with  BckmnUcs  lotus,  B,  dilaiatus, 
in  the  lower  part;  Toxaster  complanaiua^  Exogyra  Couloni,  Janira  atava,  &c.,  in  the 
central  thickest  jwrtion  ;  and  Toxaster  ricordeaniis,  AncyloccraSf  Crioceras,  &c.,  in  the 
upjMjr  band.  The  Caprotina  limestone  of  Orgon  (whence  the  name  of  the  type  was 
taken)  is  a  massive  white  rock,  sometimes  1000  feet  thick,  marked  by  the  abundance  of 
its  hippuritids,  JReqitienia  {Caprotina)  ammonia^  R.  Lonsdalei,  R.  grypJwidcs^  gigantic 
forms  of  Xeruiaa,  and  corals.  In  the  northern  Cretaceous  basin,  the  Urgonian  stage 
ai)i>ears  as  a  series  of  sands  and  clays  which  in  Haute  Marne  are  from  60  to  80  feet 
thick,  and  contain  Toxaster  ricordeaniis,  &c. 

Aptian. — In  the  typical  district  round  Apt  in  Vaucluse,  this  stage  consists  of  a 
lower  group  of  blue  marls  (Marnos  de  Gargas),  with  Flicatula placunea,  Amm.  Kisus, 
A.  Dyfrcnoyiy  followed  by  a  marly  limestone  with  Ancyloceras  renauxianus,  Ostrea 
aquila.  These  beds  swell  out  in  the  Bedoule  to  a  thickness  of  650  feet.  One  of  their 
most  distinctive  characters  is  the  prominence  of  the  cephalopods  of  the  Ancyloceras 
{Crioccnts)  type.  In  northern  France  the  Aptian  stage  is  chiefly  clay,  with  Plicatula 
p/acunca,  P.  radiola,  hence  the  name  *'  Argile  ^  Plicatules, "  Near  St.  Dizier,  the  lower 
beds  are  characterised  by  Tercbratnla  sella,  Ostrea  aquila  ;  the  middle  by  Amm.  cor- 
niiflianus,  Ancyloceras  Matheroni  ;  the  upper  by  yiwiwi.  Nisus,  A.  Deshayesi. 

^  From  the  Haute  Garonne,  where  the  deposits  are  typically  developed. 
-  Well  seen  at  Maestricht.  ''  From  Champagne. 

^  From  Santonge.  '^  From  Angouleme. 

"  From  the  basin  of  the  Loire.  ^  From  the  Charente. 

^  From  Rouen  {Rothomagus).  ^  From  the  Department  of  the  Aube. 

*''  From  Apt  in  Vaucluse.  "  From  Orgon,  near  Aries. 

*2  From  Hauterive,  on  the  Lake  of  Neuch&tel  (see  p.  955). 
*'*  From  the  Ch&teau  de  Valengin,  near  Neuch4tel,  Switzerland  (see  p.  954). 
**  See  D'Archiac,  Mhn.  Soc.  G^L  France,  2«  s^r.  ii.  i>.  1 ;  Raulin,  op.  cit.  p.  219 ; 
Ebray,  Bull.  Soc.  G^ol,  France,  2®  s^r.  xvi.  p.  213  ;  xix.  p.  184  ;  Comuel,  BulL  Soc.  04ol, 
Francr,  2«  ser.  xvii.  p.  742  ;  3®  ser.  ii.  p.  371  ;  Heljcrt,  op.  cit.  2®  ser.  xxiv.  p.  323  ;  xxviii. 
p.  137  ;  xxix.  p.  394  ;  Coqnand,  op,  cit.  xxiii.  p;  661  ;  Rouville,  op,  cit.  xxix.  p.  728  ; 
Bleieher,  n^}.  cit.  3*^  s^'t.  ii.  p.  21  ;  Toucas,  op.  cit.  iv.  p.  315. 


950  STKATIGRAPHICAL  GEOLOGY         book  vi  part  m 


Albian.* — In  the  eastern  part  of  the  Paris  basin,  this  sta^  conaists  of  a  lower 
green  pyritous  sand}'  member  (Sables  verts),  30  feet  thick,  covered  by  an  upper  argil- 
laceous band  which  represents  the  English  Gault.  These  deposits  continue  the  English 
type  round  the  northern  and  eastern  margin  of  the  Paris  basin.  They  have  been  foosd 
also  in  deep  wells  around  Paris.  In  the  valley  of  the  Mease  and  in  the  Ardennes,  this 
stage  consists  of  three  subdivisions:  (1)  a  lower  green  sand  {Amm,  mamillaris),  with 
phosphatic  nodules  ;  (2)  a  brick  clay  with  Amm.  lantus^  A.  tubcrailatus ;  (3)  a  porous 
calcareous  and  argillaceous  sandstone  known  as  Gaize^  containing  a  large  percentage  of 
silica  soluble  in  alkali  {Amm.  inflatus^  &c.) 

The  English  type  of  strata  from  the  Weald  upwards  is  also  prolonged  into  France. 
Fresh -water  sands  and  clays  (with  Unio  and  Cyrena)^  found  above  the  Jurassic  series  in 
the  Boulonnais,  evidently  represent  the  Weald,  and  are  covered  by  dark  green  clays  and 
sands  (with  Ostrca  aquila),  which  are  doubtless  a  continuation  of  the  Folkestone  beds, 
and  by  a  thin  blue  clay  which  represents  the  Gault.  Again,  in  the  Pays  de  Bray,  to  the 
west  of  Beauvais,  certain  sands  and  clays  resting  on  the  Portlandian  strata  represent  the 
Wealdeu  scries,  and  are  followed  by  others  which  may  be  paralleled  with  the  Urgonian, 
Albian,  and  Gault.^ 

In  Belgium  the  Cretaceous  system  is  underlain  by  certain  clays,  sands,  and  other 
deposits  belonging  to  a  continental  period  of  older  date  than  the  submergence  of  that 
region  beneath  the  sea  in  which  were  deposited  the  uppermost  Neocomian  beds.  These 
scattered  continental  deposits  have  been  grouped  under  the  name  of  Aacheuian.'  That 
at  least  some  part  of  them  belongs  to  older  Neocomian  time,  and  may  be  coeval  with  the 
Weald,  may  be  inferred  from  the  remarkable  discovery  at  Bernissart,  already  alluded  to, 
where,  in  a  buried  system  of  Cretaceous  ravines,  the  reptilian  and  ichthyic  life  of  the 
time  has  been  well  preserved  {aiUc,  p.  931). 

Ck15taci^.. — The  Upper  Cretaceous  rocks  of  France  have  been  the  subject  of  prolonged 
and  detailed  study  by  the  geologists  of  that  country."*  The  northern  tracts  form  part  of 
the  Anglo-Parisian  basin,  in  which  the  upper  Cretaceous  rocks  of  Belgium  and  England 
were  laid  down.  The  same  pala>ontological  characters,  and  even  in  great  measure  the 
same  lithological  composition,  prevail  over  the  whole  of  that  wide  area,  which  belongs  to 
the  northern  Cretaceous  })rovince  of  Eurojic.  Apparently  only  during  the  early  part  of 
the  Cenomunian  period,  that  of  the  Rouen  Chalk,  did  the  Anglo- Parisian  basin  com- 
municate with  the  wider  waters  to  the  south,  which  were  bays  or  gulfs  freelj'  opening  to 
the  main  Atlantic.  In  these  tracts  a  notably  distinct  tyi)e  of  Cretaceous  deposits  was 
Ij  accumulated,  which,  being  tliat  of  the  main  ocean,  covers  a  much  larger  geographical 

area  and  contains  a  much  more  widely  diffused  fauna  than  are  presented  by  the  more 
ji  limited  and  isolated  northern  basin.     There  are  few  more  striking  contrasts  between 

*'  contemporaneously  formed  rocks  in  adjacent  areas  of  dei>osit  than  that  which  meets  the 

'  See,  besides  the  works  already  citod,  Barrois,  Jiu^i.  <Sf)r.  (w^ol.  Franc^j  2®  ser,  iii.  707  ; 
Ann.  Soc.  (h'ol.  du  Noni^  ii.  p.  1  ;  Renevier,  Bull.  S(h:.  (Jtd.  France,  2^  ser.  ii.  704. 

-  Wealdeu  deposits  have  been  described  as  occun'ing  even  as  far  south  as  the  province  of 

IjI  Santander,  Spain.     A.  Gonzalerz  de  Linares,  Anal.  Soc.  Esp.  Hist.  NcU.  vii.  487,  1878. 

'*  ^  On  the  Aacheuian  deposits  see  Dumont,  '  Terrains  Cretaces  et  Tertiaires  *  (edited  by  M. 

Mourlon,  1878),  vol.  i.  pp.  11-52. 

■*  Notably  by  MM.  Hebert,  Toucas,  Coquaud,  and  Cornuel.  As  already  stated,  consider- 
able differences  exist  among  French  and  Swiss  geologists  as  to  the  nomenclature  and  the  line> 
of  demarcation  between  the  upper  Cretaceous  formations,  arising  doubtless  in  great  part  from 
the  varying  aspect  of  the  rocks  themselves,  according  to  the  region  in  which  they  are  studied. 
I  have  followed  mainly  M.  Hebert,  whose  suggestive  memoirs  ought  to  be  carefully  read  by 
the  student.  See  especially  his  "  Oudulations  de  la  Craie  dans  le  Bassin  de  Paris,"  BnU.  Soc. 
Urol.  France  (2),  xxix.  (1872)  p.  446 ;  (3)  iii.  (1875)  p.  512  ;  and  Ann.  Sci.  06ol.  vii. 
(1876)  ;  "Description  du  Bassin  d'Uchaux,"  Ann.  Sci.  Oiol.  vi.  (1875)  ;  ** Terrain  Cretace 
des  Pyrenees,"  Bull.  S.k:  6'eW.  France  (2),  xxiv.  (1867)  p.  328  ;  (8).ix.  (1880)  p.  62. 


1 
'i 


SECT,  iii  §  2  CRETACEOUS  SYSTEM  951 

eye  of  the  traveller  who  crosses  from  the  basin  of  the  Seine  to  those  of  the  Loire  and 
Garonne.  In  the  north  of  Franoe  and  Belgiam,  soft  white  chalk  covers  wide  tracts, 
presenting  the  same  lithological  and  scenic  characters  as  in  England.  In  the  centre  and 
south  of  France,  the  soft  chalk  is  replaced  by  hard  limestone,  with  comparatively  few 
sandy  or  clayey  beds.  This  mass  of  limestone  attains  its  greatest  development  in  the 
southern  part  of  the  department  of  the  Durdogne,  where  it  is  said  to  be  about  800  feet 
thick.  The  lithological  differences,  however,  are  not  greater  than  those  of  the  fossils. 
In  the  north  of  France,  Belgium,  and  England,  the  singular  molluscan  family  of  the 
Hijfpuritidx  or  Itudistes  appears  only  occasionally  and  sporadically  in  the  Cretaceous 
rocks,  as  if  a  stray  individual  had  from  time  to  time  found  its  way  into  the  region,  but 
without  being  able  to  establish  a  colony  there.  In  the  south  of  France,  however,  the 
hippurites  occur  in  prodigious  quantity,  often  mainly  composing  the  limestones,  hence 
called  hippurite  limestone  (Rudisten-Kalk).  They  attained  a  great  size,  and  seem  to 
have  grown  on  extensive  banks,  like  our  modern  oyster.  They  appear  in  successive 
species  on  the  different  stages  of  the  Cretaceous  system,  and  can  be  used  for  marking 
palseontological  horizons,  as  the  cephalopods  are  employed  elsewhere.  But  while  these 
lamellibranchs  played  so  important  a  part  throughout  the  Cretaceous  period  in  the 
south  of  France,  the  numerous  ammonites  and  belemnites,  so  characteristic  of  the  Chalk 
in  the  Anglo- Parisian  basin,  were  comparatively  rare  there.  The  very  distinctive  tyi»e 
of  hippurite  limestone  has  so  much  wider  an  extension  than  the  northern  or  Chalk  ty|)e 
of  the  upper  Cretaceous  system  that  it  should  be  regarded  as  really  the  normal 
development.  It  ranges  through  the  Alps  into  Dalmatia,  and  round  the  great  Mediter- 
ranean basin  far  into  Asia. 

Cenomanian  (Craie  glauconieuse). — According  to  the  classification  of  M.  Hebert 
this  stage  is  composed  of  two  sub-stages  :  1st,  Lower  or  Rouen  Chalk,  equivalent  to  the 
Upper  Greensand  and  Grey  Chalk  of  England.  In  the  northern  region  of  France  and 
Belgium  this  sub-stage  consists  of  the  following  subdivisions  :  a,  a  lower  assise  of  giauco- 
nitic  beds  like  the  English  Upper  Greensand,  containing  Ammoniles  inflatus  below  and 
Pecicn  (isper  above  (Rothomagian  sub-stage) ;  6,  Middle  glauconitic  chalk  with  Turrilitcs 
iuberculcUuSt  Holastcr  carinatu8,  &c.,  probably  equivalent  to  the  English  Glauconitic 
Marl  and  Chalk  Marl ;  c.  Upper  hard,  somewhat  argillaceous,  giey  chalk  with  Holastcr 
suhglobosus  ;  the  threefold  subdivision  of  this  assise  already  given,  is  well  developed  in 
tiie  north  of  France  ;  d^  Calcareous  marls  with  Bekmnitclla  plena  (Carentonian  sub- 
stage).  2nd,  Upper  or  marine  sandstone ;  according  to  M.  Hebert  this  sub-stage  is 
wanting  in  the  northern  region  of  France,  England,  and  Belgium.  In  the  old  province 
of  Maine  it  consists  of  sands  and  marls  with  Anorthopygus  orbicularis^  Exogyra  {Ostrea) 
columba,  Trigonia^  and  Ostrea.  Farther  south  these  strata  are  replaced  by  limestones 
with  hippurites  {Caprina  adversa)^  which  extend  up  into  the  Pyrenees  and  eastwards 
across  the  Rhone  into  Provence.^ 

Turonian  (Craie  mameuse)." — This  stage  presents  a  very  different  facies  according 
to  the  part  of  the  country  where  it  is  examined.  In  the  northern  basin,  according  to 
M.  Hebert,  only  its  lower  portions  occur,  separated  by  a  notable  hiatus  from  the  base  of  the 
Senonian  stage,  and  consisting  of  marly  chalk  with  Inoceramus  lalnatuSf  I.  Broiigniariif 
Ammonites  nodosoides^  A.  peramplus,  Terehraiulina  gracilis  (Ligerian  sub-stage).  He 
placed  the  zone  of  Holaster plantts  at  the  base  of  the  Senonian  stage,  and  believed  that  in 
the  hiatus  between  it  and  the  Turonian  beds  below,  the  greater  part  of  the  Turonian 

^  See  a  memoir  on  the  Upper  Cretaceous  Rocks  of  the  basin  of  Uchaux  (Provence)  by 
Hebert  and  Toucas,  Ann.  Sciences  Oiol.  vL  (1875). 

"  For  a  review  and  parallelism  of  the  Turonian,  Senonian,  and  Danian  stages  in  the 
north  and  south  of  Europe  see  Toucas,  Bull.  Sac.  Oeol.  France,  3*"*  ser.  x.  (1882)  p.  154  ; 
xi.  p.  344  ;  xix.  p.  506  ;  for  a  general  description  of  the  formations  in  the  south-east  of 
France,  see  Fallot,  .Inn.  Sci.  Oiol.  xviii.  1,  1885,  and  Bvll.  Soc.  Oiol.  France  (3),  xiv. 
(1886)  p.  1. 


. 


952  STRATIGRAPHICAL  GEOLOGY         book  vi  pabt  m 


i 


stage  is  really  wanting  in  the  north.  On  the  other  hand.  Dr.  Barrens  and  othcfs  would 
rather  regard  the  zone  of  HoUuUr  planus  as  the  top  of  the  Taronian  stage  (Ao^oiimiaD 
sub-stage ;.  In  the  north  of  France,  as  in  England,  it  is  a  division  of  the  White  Chalk, 
containing  Ammonites  pcramplus^  ScaphiUs  Geinitzii^  Spomdylus  spimomis,  Imoearawums 
hiacquivalviSj  Terebralula  semigiobosa,  JBokuter  planus,  VetUricMlUes  wumiU/eruM,  kjc. 
t  Strata  with  Inoeeramus  labiatus,  marking  the  base  of  the  Taronian  stage,  can  be  traced 

I  through  the  south  and  south-east  of  France  into  Switzerland.     These  are  oTcrlain 

by  marls,  sandstones,  and  massive  limestones  with  Exogyra  eolumba  and  enormoiu 
numbers  of  hippurites  {Hippurites  eomuvaccinum,  Radiolites  eamu-ptuioriSj  &c.)  These 
hippnrite  limestones  sweep  across  the  centre  of  Europe  and  along  both  sides  of  the  great 
Mediterranean  basin  into  Asia,  forming  one  of  the  most  distinctive  landmarks  for  the 
Cretaceous  system. 

Senonian. — This  stage  is  most  fully  developed  in  the  northern  Iwsin,  whers  it 
consists  mainly  of  white  chalk  separable  into  the  two  divisions  of:  1st,  Micraster 
(Santonian)  sub-stage  composed  of  chalk  beds,  in  the  lower  of  which  Mieratier  emr- 
ffstiidirtariumf  and  in  the  npper  J/.  eor-aHguinum  is  the  prevalent  urchin.  The  same 
pala;ontological  facies  occurs  in  this  and  the  other  group  as  in  the  corresponding  strata 
of  England  already  described.  2nd,  Belemnitella  (Campanian)  sub-stage,  with  B. 
quadrcUa  in  a  lower  zone,  and  B,  niucronata  (Mendon  Chalk)  in  a  higher.  In  the  south 
and  south-east  of  France  the  corresponding  beds  consist  of  limestones,  sandstones,  and 
marls,  with  abundant  hippurites,  and  also  include  some  fresh-water  deposits  and  beds  of 
lignite. 

Da  Ilia  II. — This  subdivision  of  the  Cretaceous  system  is  specially  developed  in  the 
northern  basin.  In  the  Cotentin,  a  limestone  with  B(iculiUs  aneeps,  ScapkUes  eon- 
sfridns,  and  other  fossils  has  been  paralleled  with  the  Maestricht  Chalk  (Maestrichtian 
sub-stage).  In  the  iieighl)ourhood  of  Paris  and  in  the  department  of  Oise  and  Mame,  a 
rock  long  known  as  the  Pisolitic  Limestone  occurs  in  patches,  lying  nnconfonnably  on 
the  White  Chalk  (Garumnian  sub-stage).  The  long  interval  which  must  have  elapsed 
between  the  highest  Senonian  be<ls  and  this  limestone  is  indicated  not  only  by  the 
evidence  of  great  erosion  of  the  chalk  previous  to  the  dejwsit  of  the  limestone,  but  also 
by  the  marked  iMtla-ontological  break  between  the  two  rocks.  The  general  aspect  of 
the  fossils  resembles  that  of  the  older  Tertiary  formations,  but  among  them  are  some 
"^  J  undoubted  Cretaceous  species.     In  the  south-east  of  Belgium,  the  Danian  stage  is  well 

exposed,  resting  unconformably  on  a  denuded  surface  of  chalk.    In  Hainault,  it  consists 
of  successive  bands  of  yellowish  or  greyish  chalk,  between  some  of  which  there  are  sur- 
I  faces  of  denudation,  with  perforations  of  boring  mollusks,  so  that  it  contains  the  records 

,j  of  a  ])rolonged  period  (Chalk  of  St.   Vaast,  OlK)urg,  Xouvelles,  Spienne,  and  Ciply). 

Anion «^  the  fossils  are  Belemnitella  mHcronata^  Ba^nlites Faujasii,  Nautilus  Dckayi  (but 
no  Ammonites,  Jfamites,  or  Turrilites)^  Inoeeramus  Cuvicrif  Ostrea'^ /label/ i/ormis,  0, 
htteralia^  ().  vcsicvlnris^  Crania  i'/iiahergensis^  Terebratulina  striata,  Fissurirostra 
Pnlissil  (characteristic),  RatliolUcs  ciplyanus,  Eschara  several  species  and  in  great 
numbers,  Echin^Korys  vvlgaivs^  HolaMer  granulosus.  The  well-known  chalk  of 
ij'  Maestricht  is  equivalent  to  part  of  these  strata,  but  appears  to  embrace  also  a  higher 

*J  horizon   containing  Jlcmipneustes  striatct-radiatus^  Crania  ignabergensiSj   Tert^raiulina 

.  striata,  FissvriroHfra  ])ectini/(trjniSf  Ostrea  lunata,  0.  vesicularis,  Janira  quadrieosteUa, 

I  and  numerous  remains  of  3 fosasaur us  and  of  chelonians,  together  with  Valuta  fasciolaria, 

r  and  other  characteristically  Tertiary  genera  of  mollusks.^     Similar  strata  and  fossils 

5=  occur  at  Faxoe,  Denmark,  and  in  the  south  of  Sweden.^    The  terrestrial  flora  in  the 

highest  Cretaceous  series  at  Aix-la-Chapelle  has  been  already  referred  to  (p.  922). 

The  Danian  stage  is  likewise  represented  in  the  south  of  France  in  some  strongly 

^  Dumont,    *  Mem.    Terrains   Cretaces,'  &c.   1878  ;  Mourlon,   *  Gvol.  de   la   Belgique,' 
18S0. 

-  n<''bert.  Bull.  S>j€.  O'enl.  France  (3),  v.  645  ;  Lundgren,  op.  at.  x.  (1882)  p.  456. 


1 


SECT,  iii  §  2  CRETACEOUS  SYSTEM  963 

contrasted  forms.  Towards  the  west  it  consists  of  marly,  chloritic,  and  compact  lime- 
stones (about  650  feet  thick)  with  a  marine  fauna,  including  Nautilus  danicua, 
AnanchyteSf  Mtcraster  tercoisiSt  &c.  Eastwards,  however,  in  Provence  there  is  evidence 
of  a  gradual  shallowing  of  the  Upper  Cretaceous  sea  in  Cenomanian  and  Turonian 
time,  until  that  area  had  become  a  fluviatile  or  lacustrine  tract,  in  which  during  the 
later  stages  of  the  Cretaceous  period  a  mass  of  fresh-water  strata  more  that  2600  feet 
thick  was  accumulated.  This  enormous  development  of  strata  consists  of  limestones, 
marls,  and  lignites. 

€tonnany. — Tlie  Cretaceous  deposits  of  Germany,  Denmark,  and  the  south  of 
•Sweden  were  accumulated  in  the  same  northern  province  with  those  of  Britain,  the 
north  of  France,  and  Belgium,  for  they  present  on  the  whole  the  same  paleeontological 
succession,  and  even  to  a  considerable  extent  the  same  lithological  characters.  It  would 
appear  that  the  western  part  of  this  region  began  to  subside  before  the  eastern,  and 
attained  a  greater  amount  of  depression  beneath  the  sea.  In  proof  of  this  statement,  it 
may  be  mentioned  that  the  Neocomian  clays  of  the  north  of  England  extend  as  far  as 
the  Teutoburger  Wald,  but  are  absent  from  the  base  of  the  Cretaceous  system  in  Saxony 
and  Bohemia.  In  north-west  Germany,  Neocomian  strata,  under  the  name  of  Hils, 
appear  at  many  points  between  the  Isle  of  Heligoland  (where  representatives  of  part  of 
the  Speeton  Clay  and  the  Hunstanton  Red  Chalk  occur)  and  the  east  of  Brunswick, 
indicative  of  what  was,  doubtless,  originally  a  continuous  deposit.  In  Hanover,  they 
consist  of  a  lower  series  of  conglomerates  (Hils-conglomerat),  and  an  upper  group  of 
clays  (Hils-thon).  Appearing  on  the  flanks  of  the  hills  which  rise  out  of  the  great 
drift-covered  plains,  they  attain  their  completest  development  in  Brunswick,  where  they 
attain  a  total  thickness  of  450  feet,  and  consist  of  a  lower  group  of  limestone  and  sandy 
marls,  with  ToxasUr  comiplanatuSj  Exogyra  Couloni  {sinu(Ua\  Ammwiites  bidichotomuSf  A. 
a^ierianuSy  and  many  other  fossils  ;  a  middle  group  of  dark  blue  clays  with  Belemnitca 
bruiisviceiisiSy  Ammonites  Nisus,  Crioeeras  (Ancyloceras)  Emericij  Exogyra  Couloni 
{^inuatn),  &c.,  and  an  upper  group  of- dark  and  whitish  marly  clays  with  Ammonites 
Martini,  A.  Deshayesi^  A,  NisuSy  Belemnites  Eivaldiy  Toxoceras  royerianumy  Crioeeras y 
&c^  Below  the  Hils-thon  in  Westphalia,  the  Harz,  and  Hanover,  the  lower  parts  of 
the  true  marine  Neocomian  series  are  replaced  by  a  massive  fluviatile  formation  corre- 
sponding to  the  English  Wealden,  and  divisible  into  two  groups  :  1st,  Deister  sandstone 
(150  feet),  like  the  Hastings  Sand  of  England,  consisting  of  fine  light  yellow  or  grey 
sandstone  (forming  a  good  building  material),  dark  shales,  and  seams  of  coal  varying 
from  mere  partings  up  to  workable  seams  of  three,  and  even  more  than  six,  feet  in 
thickness.  These  strata  are  full  of  remains  of  terrestrial  vegetation  {Equisetumy  Bniera, 
OUundriiiiiimy  Lacoptcris,  Sagenopteris,  AnomozamileSy  Pterophylhimy  Podozamitrs,  and 
a  few  conifers),  also  shells  of  fresh -water  genera  {Cyrcna,  Pahidina)^  cyprids,  and 
remains  of  Lepidotvs  and  other  fishes ;  2nd,  Weald  Clay  (65  - 100  feet)  with  thin 
layers  of  sandy  limestone  {Cyrena^  UniOy  Paludina,  MelaniUy  CypriSy  Ac.)*  The  Gault 
(Aptiau  and  Albian)  of  north-western  Germany  contains  three  groups  of  strata.  The 
lowest  of  these  consists  of  blue  clays  with  Belemnites  brunsvicensiSy  Amm.  {Acanthocrraa) 
Mnrtiniy  A.  {Hoplites)  Deshayesiy  followed  by  white  marl  with  Belem.  Etcaldu  The 
middle  consists  of  a  lower  clay  with  the  zone  of  Amvumites  {Acanthoceras)  milletianus, 

*  A.  von  Stronibeck,  ZfiUch.  DexUsch.  Oeol.  Oes.  i.  p.  462  ;  xii.  20  ;  -^V.  Jahrh.  1855, 
pp.  159,  644  ;  Judd,  Q.  J,  Otd.  Soc.  xxvi.  p.  343  ;  Vacek,  Jahrb,  Oeol.  Reieksanst,  1880, 
p.  493. 

'•^  W.  Dunker,  *  Ueber  den  norddeutsch.  Wiilderthon,  u.  s.  w. ,'  Cassel,  1844  ;  Dunker 
and  Von- Meyer,  *  Monographic  der  norddeutsch.  Walderbildung,  u.  s.  w.,'  Brunswick, 
1846;  Heinrich  Credner,  *  Ueber  die  Gliederung  der  oberen  Jura  und  der  Wealdenbildung 
in  nordwestlichen  Deutschland,'  Prague,  1863  ;  C.  Stnickmann,  '  Die  Wealden-Bildungen 
der  Unigegend  von  Hannover,'  1880  ;  A.  Schenk  on  the  Wealden  Flora  of  North  Germany, 
Palteontographicay  xix.  xxiii. 


954  STRATIGRAPHICAL  GEOLOGY  book  vi  part  ra 

and  an  nppcr  clay  with  Amm.  {ffoplUen)  tardefurcatus.  The  highest  contains  at  its  bsw 
a  clay  with  Belemnites  minimus^  and  at  its  top  the  widely  diffused  and  characteristic 
**  Flammenmergcl" — a  pale  clay  with  dark  flame-like  streaks,  containing  the  zone  of 
Aminoiiitcs  (Sch/fjnbachia)  inJlaiuSy  Amm,  (HopUtes)  lauttts,  &c.'  In  the  Teutoboiger 
WaM  the  Gault  becomes  a  sandstone. 

The  Upper  Cretaceous  rocks  of  Germany  present  the  greatest  lithological  contrasts  to 
those  of  France  and  England,  yet  they  contain  so  large  a  proportion  of  the  same  fossils 
as  to  show  that  they  belong  to  the  same  i)eriod,  and  the  same  area  of  deposit.'  The 
Cenomanian  stage  consists  in  Hanover  of  earthy  limestones  and  marls  (Planer),  which 
traced  southward  are  replaced  in  Saxony  and  Bohemia  by  glauconitic  sandstones  (Untcr- 
Quader)  and  limestone  (Unter-Planerkalk).  The  lowest  parts  of  the  formation  in  the 
Saxon,  Bohemian,  and  Moravian  areas  are  marked  by  the  occurrence  in  them  of  days, 
shales,  and  even  thin  seams  of  coal  (Pflanzen -Quader),  containing  abundant  remains  of  a 
terrestrial  vegetation  which  possesses  great  interest,  as  it  contains  the  oldest  known 
forms  of  hard-wood  trees  (willow,  ash,  elm,  laurel,  &c.)  The  Turonian  beds,  traced 
eastwards,  from  their  chalky  and  marly  condition  in  the  Anglo- Parisian  Cretaceous 
basin,  change  in  character,  until  in  Saxony  and  Bohemia  they  consist  of  massive  sand- 
stones (Mittel-Quader)  with  limestones  and  marls  (Mittel-PlJEkuer).  In  these  strata,  the 
occurrence  of  such  fossils  as  Inoceramiis  lahicUus^  I,  Brongniartit  Ammonites  peramplv*, 
ScaphUes  Geinitzii^  Spondylus  {Lima)  spinosus^  Terebratula  semiglobosa,  &c.,  shows  their 
relation  to  the  Turonian  stage  of  the  west.  The  Scnonian'  stage  presents  a  yet  more 
extraordinary  variation  in  its  eastern  prolongation.  The  soft  upper  Chalk  of  England, 
France,  and  Belgium,  traced  into  Westphalia,  passes  into  sands,  sandstones,  and 
calcareous  marls,  the  sandy  strata  increasing  southwards  till  they  assume  the  gigantic 
dimensions  which  they  present  in  the  gorge  of  the  Elbe  and  throughout  the  picturesque 
region  known  as  Saxon  Switzerland  (Ober-Quader).  The  horizon  of  these  strata  is  well 
shown  by  such  fossils  as  BelemnUdla  quadratu,,  B,  mucroncUOf  Nautilus  dauieHs, 
Marsupites  nrnatvSy  BoufguHicrinus  ellipticus,  Crania  ignabergensis,  &c. 

At  Aix-la-Chapelle  an  exceedingly  interesting  development  of  Upper  Cretaceous 
rocks  is  exposed.  These  strata,  referable  to  the  Senonian  stage,  consist  of  a  lower 
group  of  sands  with  BchmnitHla  quad  rata  and  abundant  remains  of  terrestrial 
vegetation  (p.  92*2),^  and  an  upper  group  of  matl  and  marly  chalk  with  B^Umniiella 
viucroiinta^  Gryphsea  cesicHiaruH^  Crania  ignahcrgensis^  MusasauniSy  &c. 

Switzerland  and  the  Chain  of  the  Alps.^ — This  area  is  included  in  the 
southern  basin  of  deposit.  In  the  Jura,  and  especially  round  Neuch&tel,  the  Neoco- 
mian  bccls  are  typically  developed.  This  stage  and  its  two  sub-stages  have  received 
their  names  from  localities  in  that  region  where  they  are  best  seen  (pp.  948,  949).     (1) 

*  ^Vo/.  Mag.  vi.  (1869)  i».  261.  A.  von  Strombeck,  Zeitach.  Devtsch,  Oeol,  Ges.  xlii. 
(1890)  J..  557. 

-  On  the  tlistribution  of  the  Cephalopods  in  tlie  Upper  Cretaceous  rocks  of  nortb 
Germany,  see  C.  Schliiter,  Zeitsch.  Deutsch.  O'eol.  O'es.  xxviii.  p.  457  (see  Gtiol.  Mag.  1877, 
p.  169),  and  Paiteontitgraphica,  xxiv.  123-263,  1876.  For  the  Inocerami,  Zeitsch,  Deutuh. 
Geol.  Ges.  xxix.  p.  735. 

^  German  geologists  conmience  the  Senonian  with  the  zone  of  Belemnilella  qiwdraia,  the 
upj)er  Senonian  of  Hebert. 

*  For  a  list  of  these  plants  see  H.  von  Dechen,  *  Geol.  Paliiont.  tJbersicht  der  !^ein- 
I)rovinz,'  kc.  1884,  p.  427. 

•■'  Studer's  '  Geologie  der  Schweiz '  ;  Giimbel,  *  Geognostische  Beschreib.  Bayer. 
Alpen,'  vol.  i.  p.  517  et  .'teq.  :  *  Geognostische  Beschreib.  des  Ostbayer.  Grenzegebir]^'.* 
1868,  !>.  697  ;  Jules  Marcou,  Jftjn.  Soc.  Giol.  France  (2),  iii.  ;  P.  de  Loriol,  *  Invertebre.> 
de  r^tage  Neocomien  moyen  du  Mt.  Saleve,'  Geneva,  1861  ;  Renevier,  Bull,  Soc,  Ottil. 
France  (3),  iii.  ;  A.  Fa\Te,  ibid.  :  Von  Hauer's  *  Die  GeoI(^e  der  Oesterr.  Ungar. 
Monarch ie,'  1878,  j).  505  et  seq.     E.  Fraas,  'Scenerie  der  Alpen.' 


SECT,  iii  §  2  CRETACEOUS  SYSTEM  955 

Valenginian— a  group  of  limestones  and  marls  (130-260  feet)  with  Toxaster  Camptchei, 
Pygwnis  rostraittSy  Strombus  Sauiien  {NcUica  Leviathan)^  Cidaria  hirsuta,  BelemniUs 
pisUlliformiSf  B.  dilatcUuSf  Ammonilea  {Oxynoticeras)  gevrilianunif  &c.  ;  (2)  Hauterivian 
— a  mass  of  blue  marls  surmounted  by  yellowish  limestones,  the  whole  having  a  thick- 
ness that  varies  up  to  250  feet ;  ToxaaUr  complanatuSj  Exogyra  Coulonif  Janira  atava, 
PemaMulhtiy  NaiUUxis pseudo-eleganSj  Amm.  {Hoplites)  radiatua,  Amm.  {Holcostephanus) 
astierianus^  kc.  The  Aptian  and  Albian  stages  (Gault)  are  recognisable  in  a  thin  band 
of  greenish  sandstone  and  marls  which  have  long  been  known  for  their  numerous  fossils 
(Perte  du  Rhone,  St.  Croix). 

In  the  Alpine  region,  the  Neocomian  formation  is  represented  by  several  hundred 
feet  of  marls  and  limestones,  which  form  a  conspicuous  band  in  the  mountainous  range 
separating  Berne  from  Wallis,  and  thence  into  eastern  Switzerland  and  the  Austrian  Alps 
(Spatangenkalk).  Some  of  these  massive  limestones  are  full  of  hippurites  of  the 
Caprina  group  (Caprotinenkalk,  with  Requienia  {Caprotina)  Lonadaleiy  RadwlUea 
neocomiensis,  &c.),  others  abound  in  polyzoa  (Bryozoeukalk),  others  in  foraminifera 
(Orbitolitenkalk).  The  Aptian  and  Albian  stages  traceable  in  the  Swiss  Jura  can  also 
be  followed  into  the  Alps  of  Savoy.  In  the  Vorarlberg  and  Bavarian  Alps  their  place  is 
taken  by  calcareous  glauconite  beds  and  the  Turrilite  greensand  {T.  Bergeri) ;  but  in 
the  eastern  Alps  they  have  not  been  recognised.  The  lowest  portions  of  the  massive 
Caprotina  limestone  (Schrattenkalk)  are  believed  to  be  Neocomian,  but  the  higher  ]>arts 
ate  Upper  Cretaceous. 

One  of  the  most  remarkable  formations  of  the  Alpine  regions  is  the  enormous  mass 
of  sandstone  which,  under  the  name  of  Flysch  and  Vienna  Sandstone,  stretches  from 
the  south-west  of  Switzerland  through  the  northern  zone  of  the  mountains  to  the  plains 
of  the  Danube  at  Vienna.  Fossils  are  exceedingly  rare  in  this  rock,  the  most  frequent 
being  fucoids,  which  afford  no  clue  to  the  geological  age  of  their  enclosing  strata.  That 
the  older  portions  in  the  eastern  Alps  are  Cretaceous,  however,  is  indicated  by  the 
occurrence  in  them  of  occasional  Inocerami^  and  by  their  interstratiticatiou  with  tnie 
Neocomian  limestone  (Aptychenkalk).  The  definite  subdivisions  of  the  Anglo- Parisian 
Upper  Cretaceous  rocks  cannot  be  applied  to  the  structure  of  the  Alps,  where  the 
formations  are  of  a  massive  and  usually  calcareous  nature.  In  the  Vorarlberg,  they 
consist  of  massive  limestones  (Seewenkalk)  and  marls  (Seewenmergel),  with  Annnoniks 
Mantelli,  TurrUHes  costatits,  Inoceramus  strtatus^  Uolaster  carinaius^  &c.  In  the  north- 
eastern Alps,  they  present  the  remarkable  facies  of  the  Gosau  beds,  which  consist  of  a 
variable  and  locally  developed  group  of  marine  marls,  sandstones,  and  limestones,  with 
occasional  intercalations  of  coal-bearing  fresh-water  beds.  These  strata  rest  uuconform- 
ably  on  all  rocks  more  ancient  than  themselves,  even  on  older  Cretaceous  groups.  They 
have  yielded  about  500  species  of  fossils,  of  which  only  about  120  are  found  outside  the 
Alpine  region,  chiefly  in  Tnronian,  partly  in  Senonian  strata.  Much  discussion  and  a 
copious  literature  has  been  devoted  to  the  history  of  these  deposits.  ^  The  loosely  imbedded 
shells  suggested  a  Tertiary  age  for  the  strata  ;  but  their  banks  of  corals,  sheets  of 
orbitolite-  and  hippurite-limestone  and  beds  of  marl  with  Ammonites^  Inocerami  and 
other  truly  Cretaceous  forms,  have  left  no  doubt  as  to  their  really  Upper  Cretaceous 
age.  Among  their  subdivisions,  the  zone  of  Hippurites  comu-vaccinum  is  recognisable. 
From  some  lacustrine  beds  of  this  age,  near  Wiener  Neustadt,  a  large  collection  of  rep- 
tilian remains  has  been  obtained,  including  deinosaurs,  chelonians,  a  crocodile,  a  lizard, 

*  See  among  other  memoirs,  Sedgwick  and  Murchison,  Trans,  Oeol.  Soc.  2nd  ser.  iii.  ; 
Reuss,  Denkschrift.  A  had.  WieUf  vii.  1 ;  Sitzb.  Akad.  H'lVn,  xi.  882  ;  Stoliczka,  Sitzb. 
Akad.  Wien,  xxviii.  482  ;  liL  1  ;  Zekeli,  Abhandl  (Jed.  Beichsanst.  Wien,  i.  1  (Gastero- 
pods)  ;  F.  von  Hauer,  Sitzb.  Akad.  Wien,  liii.  800  (Cephalopods) ;  *  Paleeont.  Oesterreich,' 
i.  7  ;  '  Geologic,'  p.  516  ;  Zittel,  Denkschrift.  Akad.  Wien,  xxiv.  105  ;  xxv.  77  (Bivalves) ; 
Bunzel,  Abhandl.  Oeol.  Beichsanst.  v.  1  ;  Glimbel,  '  Geognostische  Beschreib.  Bayeriscb. 
Alpen,'  1861,  p.  517  et  seq.     Redlenbacker,  Abhandl.  Oeol.  Beichsanst.  v.  (Cephalopods). 


956  STRATIGRAPHICAL  GEOLOGY  book  vi  part  in 


and  a  pterodactyle — in  all  fourteen  genera  and  eighteen  species.^  ProbaUj  more  or  Um 
equivalent  to  the  Gosau  beds  are  the  massive  hippurite-limestones  and  certain  maris, 
containing  Belemnitdla  mucrtmatat  Eehinoearys  vulffaris,  &c,  of  the  Salzkammei|^t  and 
Bavarian  Alps.^  The  Upper  Cretaceous  rocks  of  the  south-eastern  Alpe  are  distingiualied 
by  their  hippurite-limestones  (Rudistenkalk)  with  shells  of  the  HippuriUs  and  RadiotiU* 
groups,  while  the  Lower  Cretaceous  limestones  are  marked  by  those  of  the  (Japrina  group. 
The}'  form  ranges  of  bare  white,  rocky,  treeless  mountains,  perforated  with  tunnela  and 
(lassages  (Dolinen,  p.  367).  In  the  southern  Alps  white  and  reddiah  limestones  (Scaglia) 
have  a  wide  extension. 

Basin  of  the  Meditezranoan. — The  southern  type  of  the  Cretaceous  system  attains 
a  great  deveIo[»ment  on  both  sides  of  the  Mediterranean  basin.  The  hipparite  {Oapro- 
Una)  limestones  of  Southern  France  and  the  Alps  are  prolonged  into  Italj  and  Greece, 
whence  they  range  into  Asia  Minor  and  into  Asia.'  Cretaceous  formations  of  the  same 
type  appear  likewise  in  Portugal,  Siiain,  and  Sicily,  and  cover  a  vast  area  in  the  north 
of  Africa.  In  the  desert  region  south  of  Algiers,  they  extend  as  wide  plateaux  with 
sinuous  lines  of  terraced  escarpments.^ 

Russia. — The  Cretaceous  formations,  which  are  well  developed  in  the  range  of  the 
Carpathian  mountains,  sink  below  the  Tertiary  deposits  in  the  plains  of  the  Dniester, 
and  rise  again  over  a  vast  region  drained  by  the  Donetz  and  the  Don.  They  have  been 
studied  in  central  and  eastern  Russia  by  the  officers  of  the  Russian  Geological  Surrey, 
who  have  pointed  out  the  remarkable  resemblance  between  their  organic  remains  aiid 
those  of  the  Anglo-French  region.  There  is  in  particular  a  close  parallelism  between 
them  and  the  English  Speeton  Clay  in  their  intimate  relationship  to  the  Jurassic 
system  below.  The  Volga  group  already  (p.  919)  referred  to  is  succeeded  by  typical 
Xeocomian  dei)osits,  which  are  well  developed  in  the  district  of  Simbirsk  along 
the  Volga,  where  they  consist  of  dark  clays  with  sandy  layers  and  pho8|^iatic 
concretions,  divisible  into  three  horizons.  The  lowest  of  these  yields  pyritous 
ammonites,  especially  Amm.  (Holcostephanus)  versicolor ^  A,  {BoUosi.)  invcmu,  also 
Bfhmniies  pscudopanderiatius,  Asiarte  porrteta.  The  middle  zone  contains  septaria 
enclosing  w4 mm.  {Holcost.)  Drcheniy  umhoiiaUiSj  progrcdicus^  fascicUofcUeaius^ditcofaicatus, 
Burbotij  Ifutc*:rainHs  avcella^  Rhynchonella  ohliterata.  The  highest  zone  is  almost  un- 
fossiliferous  near  Simbirsk,  but  its  lower  layers  yield  Pedfn  crassiUsta.  Deposits  of 
the  same  type  as  the  Anglo-French  Aptian  are  well  developetl  in  the  governments  of 
Simbirsk  and  Saratov,  and  are  characterised  by  Amm.  {HoplUcs)  Dcshayesi  and  A. 
( AmaJthnis)  hicurvatus.  The  Albian  or  Gault,  which  is  found  in  the  government  of 
Moscow,  and  may  eventually  be  traced  over  a  wide  area,  has  yielded  a  number  of 
ammonites,  especially  of  the  genus  HopUtes  {U.  dcntatus,  talitziamis^  BennettiMt 
Engrrsi,  Tethydis,  jachromcnsis,  DuUmplei^  Hapioceras  Beudanti).  This  stage  is  well 
developed  in  the  Caucasus,  Transcaucasia,  and  the  trans-Caspian  region.  In  the  chief 
Russian  Cretaceous  area  the  Cenomanian  stage  begins  with  dark  clay  closely  related  to 
the  underlying  Jurassic  series,  from  the  denudation  and  rearrangement  of  which  it  may 
have  been  derived.  The  clay  shades  upward  into  sandy,  glauconitic,  and  phosphatic 
deposits,  which  gradually  assume  the  condition  of  chalky  marls.     These  Cenomanian 

^  Seeley,  Q.  J.  Geol.  .Soc.  1881,  p.  620. 

-  See,  lor  tills  region,  Guinbel,  wlio  gives  a  table  of  correlations  for  the  European 
Cretaceous  rocks  with  those  of  Bavaria.  '  Geoguost.  Beschreib.  Ostbayer.  Grenzgeb.'  pp. 
700,  701. 

^  For  .111  account  of  Syrian  Cretaceous  fossils  see  R.  P.  ^^^litfield,  BtdL  Amtr,  Mhs.  SaL 
Hist.  iii.  (1891)  p.  381. 

*  Coquand,  *  Description  geol.  et  paleontol  de  la  region  sud  de  la  province  de  Con- 
-laiitiu,'  1S62;  RoUand  Bull.  Soc.  Gevl.  Fnmce  (3)  ix.  508  ;  Peron,  op,  ciL  p.  436  ;  this 
.-luthor  has  piihlishe<l  a  valuable  memoir  on  the  Geology*  of  Algeria,  with  a  full  bibliography, 
Ann.  Sciences  (rW.  1883  ;  Zittel,  *  Beitrage  zur  (Jeologie  der  Libyschen  Wiiste,*  1883w 


SECT,  iii  §  2  CRETACEOUS  SYSTEM  957 


Htrata  appear  to  have  a  wide  extent  at  the  base  of  the  Upper  Cretaceous  formations  of 
rentral  Russia.  They  contain  numerous  remains  of  fishes  {Ptychodns,  Lamiia^  Odont- 
aspis,  Otodns)  with  bones  of  ichthyosaurs  and  plesiosaurs.  Ammonites  arc  rare,  but 
Amm.  {Schl'&nboLckia)  varians  occurs,  also  Belemnitella  plena^  Exogyra  haliotidea^ 
E.  coniMy  Ostrea  hippopodiurtij  Janira  {Vola)  quiiiquecostcUa,  PecUn  laminosuSf  Rhyn- 
chonella  nvci/armiSf  &c.  Turonian  strata  have  likewise  been  found  over  a  wide  tract  in 
central  Russia.  The  lower  bands  with  Inoceramns  (/.  ritssieimSf  labiatuSj  Br(mgniarti, 
lohatus  aff.)  abundant,  Belemnitella  and  Ostrea  vesicniaris  are  of  constant  occurrence  in 
the  Cretaceous  region  of  central  Russia.  In  that  area,  however,  the  Senonian  and 
higher  Cretaceous  stages  are  not  well  developed,  though  they  assume  greater  importance 
in  the  southern  part  of  the  Empire.^ 

India. — The  hippurite  limestone  of  south-eastern  Europe  is  prolonged  into  Asia 
Minor,  and  occupies  a  vast  area  in  Persia.  It  has  been  detected  here  and  there  among 
the  Himalaya  Mountains  in  fragmentary  outliers.  Southward  of  these  marine  strata, 
there  apjKjars  to  have  existed  in  Cretaceous  times  a  wide  tract  of  land,  corresponding 
on  the  whole  with  the  present  area  of  the  Indian  i)eninsula,  but  not  improbably  stretch- 
ing south-westwards  so  as  to  unite  with  Africa.  On  the  south-eastern  side  of  this  area 
the  Cretaceous  sea  extended,  for  near  Trichiuopoly  and  Pondicherry  a  series  of  marine 
deposits  occurs,  corresponding  to  the  Euro()ean  Upper  Cretaceous  formations,  with  which 
it  has  16  per  cent  of  fossil  species  in  common.  Among  these  are  Amm.  {Acanthoc.) 
rhotoynagensiSf  A.  {Pachydiscics)  peramplus,  and  Jlhyiichmiella  compressa.  The  occurrence 
of  NaiUilua  danicus  in  the  higher  sands  of  Ninnyur  probably  shows  that  the  Cretaceous 
system  of  India  reaches  as  high  as  the  Dauian  stage.'''  Similar  strata  with  many  of  the 
same  fossils  appear  on  the  African  coast  in  Natal.  The  most  remarkable  episode  of 
Cretaceous  times  in  the  Indian  area  was  undoubtedly  the  colossal  outpouring  of  the 
Deccan  basalts  (p.  25d).  These  rocks,  lying  in  horizontal  or  nearly  hoiizontal  sheets,  attain 
a  vertical  thickness  of  from  4000  to  6000  feet  or  more.  They  cover  an  area  estimated  at 
200,000  S(|nare  miles,  though  their  limits  have  no  doubt  been  reduced  by  denudation. 
Their  oldest  portions  lie  slightly  uncohformably  on  Cenomanian  rocks,  and  in  some 
places  appear  to  be  regularly  interstratified  with  the  uppermost  Cretaceous  strata. 
The  occurrence  of  remains  of  fresh-water  mollusks,  land-plants,  and  insects,  both  in  the 
lowest  and  highest  parts  of  the  volcanic  series,  proves  that  the  lavas  must  have  been 
subaerial.     This  is  one  of  the  most  gigantic  outpourings  of  volcanic  matter  in  the  world.^ 

North  America. — The  Cretaceous  system  stretches  over  a  vast  poi*tion  of  the 
American  continent,  and  sometimes  reaches  an  enonnous  thickness.  Sparingly 
developed  in  the  eastern  States,  from  New  Jersey  into  South  Carolina,  it  there 
includes  the  younger  or  Neocomian  plant-bearing  strata  of  Virginia.  It  spreads  out 
over  a  wide  area  in  the  south,  stretching  round  the  end  of  the  long  Palseozoic  ridge 
from  Georgia  through  Alabama  and  Tennessee  to  the  Ohio ;  and  reappearing  from 
under  the  Tertiary  formations  on  the  west  side  of  the  Mississippi  over  a  large  space  in 
Texas  and  the  south-west.  Its  greatest  development  is  reached  in  the  Western  States 
and  Territories  of  the  Rocky  Mountain  region,  Wyoming,  Utah,  and  Colorado,  whence 
it  ranges  northward  into  British  America,  covering  thousands  of  square  taiiles  of  the 
prairie  country  between  Manitoba  and  the  Rocky  Mountains,  and  extending  westwards 
even  as  far  as  Queen  Charlotte  Islands,  where  it  is  well  developed.  It  has  a  prodigious 
northward  extension,  for  it  has  been  detected  in  Arctic  America  near  the  mouth  of  the 
Mackenzie  River,  and  in  northern  Greenland. 

The  Cretaceous  clays  and  groensand  marls  of  New  Jersey  have  yielded  a  tolerably 

^  Nikitiu,  *  Les  Vestiges  de  la  periode  Cr^tac^e  dans  la  Russie  centrale,'  Mem,  Com.  Oiol, 
Russe,  v.  No.  2  (1888)  p.  165. 

2  J.  Seunee,  Mem.  Soc.  O^ol.  France^  PalSant.  t.  ii.  fasc.  iii.  (1891)  p.  22. 

3  Medlicott  and  Blauford,  'Geology  of  India,'  see  ante,  pp.  259,  592.  The  Ui>per 
Cretaceous  fauna  of  India  is  described  in  PcUxontograpk.  Indioa,  ser.  xiv.  (1883). 


958  STRATIGRAPHICAL  GEOLOGY         book  vi  pabt  in 


aiiijile  molluscan  fauna,  comprising  species  of  TerebrcUida^  Terebralella,  Terehratulina, 
Ostrea,  GrypJisRa^  Exogyra,  Aiiomia,  FecUiiy  Amimum^  Spondylus,  Plieatula,  Mjftiliu, 
Modiolay  Inoccramvs,  Trigontat  Unio^  Cardila^  CrasscUella,  Cardium,  and  many  other 
genera.*  Towards  the  south  over  the  site  of  Texas,  the  Cretaceous  sea  appears  to  have 
Ijeen  dee]ier  and  clearer  than  elsewhere  in  the  American  region,  for  its  presence  is  recorded 
chiefly  by  limestones,  among  which  occur  abundant  hippurites  {Caproiina^  Caprina)  and 
foraminifera  {OrhitoUtes).'  Northwards  the  strata  are  chiefly  sandy,  and  present  alter- 
nations of  marine  and  terrestrial  conditions,  ])ointing  to  oscillations  which  especially 
affected  the  Rocky  ^lountain  and  western  regions.  The  greatest  development  of  the 
system  is  to  be  seen  in  the  nortli  of  Utah  and  in  Wyoming,  where  it  presents  a  continnons 
f^  series  of  deposits  imbroken  by  any  unconformability  for  a  thickness  of  from  11,000  to 

13,000  feet.     The  following  table  shows  the  character  of  these  deposits  in  descending 
onlcr  : — 

Laramie  (Lignitic)  group. — Buff  and  grey  sandstones,  with  bands  of  dark  clays 
nnd  numerous  coal-seams,  containing  abundant  terrestrial  vegetation  of  Terti- 
ary' types,  land  and  fresh- water  mollusks  {UniOj  Limntea,  Planorbis,  ffelij; 
Pupdy  &c.),  and  remains  of  fishes  {Beryx,  Lepidotus),  turtles  {Trionyx,  Emy»^ 
Compsemys)^  and  reptiles  {Crocodilus^  AgathaumuSy  &c.).  This  group  is  by 
some  geologists  placed  among  the  Tertiary  systems,  or  as  a  passage  series  between 
the  Cretaceous  and  Eocene  systems  (see  p.  982).  Thickness  in  Green  River  basin 
5000  feet. 

On  this  horizon  come  the  "Ceratops  l)ed9"  of  Wyoming,  3000  feet  thick, 
wliich  rest  directly  ui)on  the  Fox  Hills  group.     They  consist  of  alternating 
sandstones,  shales,  and  lignites,  and  are  remarkable  for  the  extraordinary  number 
aud  wonderful  preservation  of  the  deinosaurs,  mammals,  and  other  forms  which 
they  have  yielded.^ 
Fox  Hills  group.  — Grey,  rusty,  and  buff  sandstones,  with  numerous  beds  of  coal 
aud  interstratifications  containing  a  varied  assemblage  of  marine  shells  {Belenini' 
tella,   Xfiutihusy   AinmonHes^    BaculUes,   Mosasaurus^  &c.)     Thickness  on  the 
great  plains  1500  feet,  which  in  the  Green  River  basin  expands  to  from  3000  to 
4000  feet, 
(.'olorado  group.  — Calcareous  shales  and  clays  with  a  central  sandy  series,  and,  in 
tlie  Wahsatch  region,    seams  of  coal  as  well  as  fluviatile  and  marine  shells. 
Thickness  east  of  the  Rocky  Mountains  800  to  1000  feet,  but  westwards  in  the 
region  of  tlie  Uinta  and  Wahsatch  Mountains  2000  feet.     This  group  was  pro- 
j^osed  and  named  by  Haydeu  and  Clarence  King  to  include  the  following  sub- 
groups  in,  the   original   classification   of   Messrs.    Meek   and   Hayden   in   the 

fl  Missouri  region  : — 

Fort   Pierre  sub-group. — Carbonaceous  shales,  marls,  and  clays  {Inoeeramus 

|I  Jjiinihini,  lin.cuiites  <ny(ti'Ji,  ^ScaphiUs  7i(Hl«*suSf  AmmoniteSf  Ostrea  congesta^ 

Niobrara  sub-group. — Chalky  marls  and  bituminous  limestones  {BaculUes^ 

Inoceramus  th'fonnis^  I.  probtematicus^  Ostrea  congesta,  fish  remains). 

Fort  Benton  sub-group. — Shales,  clays,  and  limestones  {Shaphiles  loarrtnensUy 

AmmnnitfSy  Prionocydas  Ww^gari,  Ostrea  congesta). 

jj  Dakota  group,  composed  of  a  persistent  basal  conglomerate  (which  is  200  feet  thick 

i'  aud  very  coarse  in  the  Walisatch  region)  overlain  with  yellow  and  grey  mas.sive 

'.  sandstones,  sometimes  witli  clays  and  seams  of  coal  or  lignite  (dicotyledonous 

leaves  in  great  numbers,  ImtceramuSj  Cardium,  &c.)     Thickness  400  feet  and 
i  nj)\vanls.'* 


^ 


^  R.  P.  Whitfield,  Monogr.  U.S.  Geol.  Surv.  vol.  ix.  (1885). 

-  For  fossils  see  *  List  of  Invertebrate  Fossils  from  the  Cretaceous  Formations  of  Texas,' 
R.  T.  Hill,  Austin,  Texas,  1889. 

'  J.  B.  Hatcher,  Am^r,  Jouni.  /<c{.  xlv.  (1893)  p.  135. 

**  IIay»len's  Itepnrts  »/  (ieographical  ami  Geohtgical  Surveys  of  Westerj^  Territories; 
King's  ff'co/of/ica/  Heport  of  PJxplorathji  of  40th  Parallel,  vol.  i. ;  G.  H.  Eldridge.  Amer. 
.fntirn.  Si'i.  xxxviii.  (1889)  p.  313.     J.  J.  Stevenson,  Amer.  Geologist^  1889,  p.  391. 


SECT,  iii  §  2  CRETACEOUS  SYSTEM  969 

The  extraordinary  paheontological  richness  of  these  western  Cretaceous  deposits  has 
been  already  referred  to.  They  contain  the  earliest  dicotyledonous  plants  yet  found  on 
this  continent,  upwards  of  100  species  having  been  named,  of  which  one-half  were  allied 
to  living  American  forms.  Among  them  are  species  of  oak,  willow,  poplar,  beech,  elm, 
<logwood,  maple,  hickory,  fig,  cinnamon,  laurel,  smilax,  tulip-tree,  sassafras,  sequoia, 
American  palm  {S(ibal\  and  cycads.^  The  more  characteristic  marine  mollusca  are 
s|)ecies  of  Tcrebratula,  Ostrea^  Oiyphma,  Exogyra^  Inoceramus^  IlippiirUeSf  Jladiolites, 
AmmoniteSy  ScaphileSy  Hamites^  BaculiteSf  Belemnites,  AncyloceraSf  and  Turrilites,  Of 
the  fishes  of  the  Cretaceous  sea,  many  species  are  known,  comprising  large  predaceous 
representatives  of  modern  or  osseous  types  like  the  salmon  and  saury,  though  cestracionts 
and  ganoids  still  flourished.  But  the  most  remarkable  feature  in  the  organic  contents 
of  these  beds  is  the  extraordinary  number  and  variety  of  the  reptilian  remains,  to  which 
reference  has  been  already  made  (p.  981).  Some  of  the  early  tyj)es  of  toothed  birds 
also  have  been  obtained  from  the  same  important  strata  (p.  935). 

No  question  in  American  geology  has  given  rise  to  more  controversy  than  the  place 
which  should  be  assigned  to  the  Laramie  or  Lignitic  group,  whether  in  the  Cretaceous 
or  Tertiary  series.^  The  group  consists  mainly  of  lacustrine  strata,  with  occasional 
brackish -water  bands.  Somewhere  about  140  si)ecies  of  mollusks  have  been  obtained 
from  them  which  are  terrestrial  or  fresh -water  forms,  with  a  few  that  may  be  brackish - 
water.  They  include  numerous  species  of  Ostrea,  Anomia^  Unio^  Corbic2(/a,  Corbula, 
Limnma^  PlanorbiSf  Physa^  Helix,  Pupa,  Goniobasis,  Uydrobia,  and  Viviparus.^  The 
abundant  terrestrial  flora  resembles  in  many  respects  the  present  flora  of  North 
America.  A  few  of  the  plants  are  common  to  the  Middle  Tertiary  flora  of  Europe,  and 
a  number  of  them  have  been  met  with  in  the  Tertiary  beds  of  the  Arctic  regions. 
Some  of  the  seams  of  vegetable  matter  are  true  bituminous  coals  and  even  anthracites. 
According  to  Cope,  the  vertebrate  remains  of  the  Laramie  group  bind  it  indissolubly  to  the 
Mcsozoic  formations.  Lesquereux,  on  the  other  hand,  has  insisted  that  the  vegetation 
is  unequivocally  Tertiary.  The  former  opinion  has  been  maintained  by  Clarence  King, 
Mftrsh,  and  others ;  the  latter  by  Hayden  and  his  associates  in  the  Survey  of  the 
Western  Territories.  Cope,  admitting  the  force  of  the  evidence  furnished  by  the 
fossil  plants,  concludes  that  "there  is  no  alternative  but  to  accept  the  result  that  a 
Tertiary  flora  was  contemporaneous  with  a  Cretaceous  fauna,  establishing  an  unin- 
terrupted succession  of  life  across  what  is  generally  regarded  as  one  of  the  greatest 
breaks  in  geologic  time."  The  vegetation  had  apparently  advanced  more  than  the 
fauna  in  its  progress  towards  modem  types.*  The  Laramie  group  was  disturbed  along 
the  Rocky  Mountain  region  before  the  deposition  of  the  succeeding  Tertiary  formations, 
for  these  lie  unconformably  upon  it.  So  great  have  been  the  changes  in  some  regions, 
that  the  strata  have  assumed  the  character  of  hard  slates  like  those  of  Paheozoic  date, 
if  indeed  they  have  not  become  in  Cabfomia  thoroughly  crystalline  masses.  The  same 
mingled  marine  and  terrestrial  tyi)e  of  Cretaceous  rocks  can  be  followed  into  California, 
where  the  higher  parts  of  the  series  contain  beds  of  coal.  The  coast  ranges  are 
described   by  Whitney  as  largely  composed   of  Cretaceous   rocks,   usually  somewhat 

'  For  an  account  of  the  Laramie  Flora  see  L.  F.  Ward,  ^h  Ann.  Rep.  U.S.  Qeol. 
Surv.  1885,  p.  405.     BiiU.  U.S.  Geol.  Surv.  No.  37  (1887). 

-  For  a  rfstimi  of  the  progress  of  opinion  on  this  subject  see  Ward,  ^th  Ann,  Rep.  U.S. 
Ged.  Surv.  1885,  p.  406. 

'  C.  A.  White,  "  A  Review  of  the  Non-Marine  Fossil  Mollusca  of  North  America,"  Srd 
U.S.  Geol.  Survey  Report,  1883;  BuU.  U.S.  Ged,.  Surv.  No.  34,  1886.  See  the  same 
author^s  pai>er  on  the  mingling  of  an  ancient  fauna  and  modem  flora  in  these  deposits,  Amer. 
Jonrn.  Sci.  (3)  xxvi.  p.  120. 

"*  See  remarks  ante,  pp.  660,  668.  Neumayr  (X.  Jahrb.  1884,  i.  p.  74)  makes  a  comparison 
lietween  the  Laramie  group  and  the  inter-trappean  bods  of  the  Deccan. 


gsn 


STRATIQRAPHIVAL  GBOl 


metamurphic  »id  Bometimes  higbl;  bo.'  In  the  ftrnt-l 
the  Rotky  Mountains,  near  the  United  States  and  Can 
ooni;iarstively  undisturbed  and  the  coal  is  bituniiuoua  ; 
area  the  strata  are  greatly  contorted  and  the  eoal  is  there 
The  blending  of  marine  and  tfirrestrial  formations,  a 
Territories  of  the  American  Union,  can  be  traced  nort! 
Vancouver's  Island,  and  the  remote  Queen  Charlotte  groi 
thiekneBS  ot  the  series  of  strata.  The  section  at  Skidegal 
as  follows ;  •— 

Up)>er  shales   and    xauJstoneit.      (Few  fosails,  the  old 

iiiami  beiiiii  Iiioceramiu  problematicia) 

Conglomerates  and  saudatonee  (fngmauts  of  BtUmnilet) 

Lower  shales  aud  aaadstones  with  a  wortable  seuiii  of 

the  base  (fossils  abimdaat,  iacluding  species  of  Ammoi 

BtivmnUea,  Trigonia,  /nocfraiaut,  Oilrea,  f'nio,  Tei-rf 

Volcauic  agglomerates,  mndslones.  anrl  tiilfs,  with  bloc 

four  or  live  feet  in  diameter 

Lower  randstaniw,  Bome  tafaceous,  others  fosniliferous    . 


Reference  lias  already  ([i.  922)  been  made  to  the  n 
Ureenland.  Three  liorizonN  of  plant-bearing  beds  hare 
Konic  beds— dark  shales  resting  on  the  crjatallinc  rocks, 
to  be  a  Loner  Cretaceous  flora  ;  (i)  the  Atane  beds — gr 
{Upernivik.  Noursoab,  Disco,  fcc),  with  U|>per  Cretaceo 
clnys  Ij'lng  oil  the  AUne  beds.  Marine  fossils  found  in  i 
beds  likewise  serve  to  indicate  their  horiion.* 

AnaUalatia.  — Hepresentatives  of  the  Cretaceous  sy; 
Australia.  lu  Queensland  their  lower  member  (  "  Rot) 
estimated  to  cover  three-fourths  of  the  whole  of  the  col< 
found  in  some  districts  to  j>ass  down  conformably  into  the 
and  elsewhere  t«  lie  unooutormalily  on  ancient  schists.  ! 
yielded  imnierous  species  of  foraniintfcra,  brachiojHidd, 
culwiii.  Paten,  Auctlia,  Iiioceramua,  Piniiii,  Mijtiliia,  &i 
ammonites  ot  tlic  genera  Amalthevi,  Schliiiibachia,  Hap 
loecnis,  Crioccma,  and  Sa»tihti  :  likewise  fislies  of  the  gcii 
Bdonmlumiis,  aud  \-arious  iehthyosaurs  and  plesiosaurs. 
mations  arc  repi'cselitod  by  the  "  Desert  Sandstone,"  whic 
tbree-i|tiartcrs  of  the  colony.  It  lies  on  an  upturned  and  < 
Cretaceous  formations  and  contains  land-plants  and  a  ma 
choaclhi,  Ihtrca,  TeigonUi,  Bflfmnites).' 

Ill  New  Zealand  the  "Waijwra"  formation  of  Cantcrl 
Ui>iH.'r  Cretaceous  and  imssibly  some  of  the  older  Tertii 
massive  conglomerates  [Honietimea  6000  to  8000  feet  thick 
The   plants  include  dicoty 


>  G,  F.  Becker,  -Imri-.  Jimra.  Sd.  xxii.  (1886)  p.  348. 

-  {;.  M.  Dbwsou  in  Rrpart  «f  Pi'ognM  <•/  Gtol.  .Sc.r.  fo« 
xx.\viLi,  (1889)  p.  120  ;  op.  cit.  xitix.  (1890)  p.  180.  J.  I 
i.  parts  i.  ill.  iu  publications  of  tieal.  SvrTfg,  Ciiaa/in.  ; 
Oe'Jvijy  Hid  Rt>«»ire(i  qf  the  Rrgion  near  Ihr  49(A  i;,n 
iidaTji  Crjtnmisaioit,  1875  ;  Rriiort  im  Canadian  Pacific  Ra 

>  Ileer.  ■  Flora  Fosailis  Arctica,'  vi.  (1892). 

'  R.  L.  .lack  and  E.  EtheridKC.  'Geolog)'  of  Queensland,'  c 


PART  IV  CAINOZOIC  OR  TERTIARY  SYSTEMS  961 

branches  of  araucarians  and  leaves  and  twigs  of  Damnuira,  Among  the  shells  no 
cephalopods  uor  any  of  the  widespread  hip]>urite8  have  yet  been  found.  With  the  re- 
mains of  fishes  [OdontaspiSy  Lamna,  Hyhodus)  occur  numerous  saurian  bones,  which  have 
been  referred  to  species  of  PlcaiosauruSj  Mauisaurus^  Polycotylus^  kc.^  According  to  the 
work  of  the  Geological  Survey  Department  of  New  Zealand,  the  Cretaceous  system  con- 
sists of  a  lower  group  (500  feet)  of  green  and  grey  incoherent  sandstones,  in  which  beds 
of  bituminous  coal  occur  on  the  west  coast  (Lower  Greensand),  surmounted  by  a  mass  of 
strata  (2000  to  5000  feet)  which  appears  to  connect  the  Cretaceous  and  Tertiary  series. 
The  upper  part  of  the  group  (consisting  of  marls,  greensand,  limestone  and  chalk  with 
Hints)  is  thoroughly  marine  in  origin,  with  AncyloceraSj  BelemniteSy  Rosiellaria^ 
PlesioaaurtLSy  Leiodon,  kc.  The  lower  portion,  which  is  capped  by  a  black  grit  with 
marine  fossils,  contains  the  most  valuable  coal-de{)ositH  of  New  Zealand.  The  plants 
include  dicotyledonous  and  coniferous  forms  closely  allied  to  those  still  living  in  the 
country. - 

Part  IV.  Cainozoic  ok  Tertiary. 

The  close  of  the  Mesozoic  periods  was  marked  in  the  west  of  Europe 
by  great  geographical  changes,  during  which  the  floor  of  the  Cretaceous 
sea  was  raised  partly  into  land  and  partly  into  shallow  marine  and 
estuarine  waters.  These  events  must  have  occupied  a  vast  period,  so 
that,  when  sedimentation  once  more  became  continuous  in  the  region, 
the  organisms  of  Mesozoic  time  (save  low  forms  of  life)  had,  as  a  whole, 
disappeared  and  given  place  to  others  of  a  distinctly  more  modern  type. 
In  England,  the  interval  between  the  Cretaceous  and  the  next  geological 
period  represented  there  by  sedimentary  formations  is  marked  by  the 
abrupt  line  which  separates  the  top  of  the  Chalk  from  all  later  accumula- 
tions, and  by  the  evidence  that  the  Chalk  seems  to  have  been  in  some 
places  extensively  denuded  before  even  the  oldest  of  what  are  called  the 
Tertiary  formations  were  deposited  ujK)n  its  surface.  There  is  evidently 
here  a  considerable  gap  in  the  geological  record.  We  have  no  data  for 
ascertaining  what  was  the  general  march  of  events  in  the  south  of 
England  between  the  eras  chronicled  respectively  by  the  Upper  Chalk 
and  the  overlying  Thanet  beds.  So  marked  is  this  hiatus,  that  the  belief 
was  long  prevalent  that  between  the  records  of  Mesozoic  and  Cainozoic 
time  one  of  the  great  breaks  in  the  geological  history  of  the  globe 
intervenes. 

Here  and  there,  however,  in  the  continental  part  of  the  Anglo- 
Parisian  basin,  traces  of  some  of  the  missing  evidence  are  obtainable. 
Thus,  the  Maestricht  shelly  and  polyzoan  limestones,  with  a  conglome- 
ratic base,  contain  a  mingling  of  true  Cretaceous  organisms  with  others 
which  are  characteristic  of  the  older  Tertiary  formations.  The  common 
Upper  Chalk  crinoid,  Bourguetkrinwi  elHptiai,%  occurs  there  in  great  num- 
bers ;  also  Oatrea  ve.sicularuij  Baculites  Faujaaii,  Bekmnifella  murroiuUa^  and 
the  great  reptile  Mosasauras ;  but  associated  with  such  Tertiary  genera 

1  Etheritlge.  Q.  J.  Geol.  Sac.  xxviu.  183,  340  ;  Owen,  Oeof.  Mag,  vii.  49  ;  Hector, 
Trans.  New  Zealand  Inst.  vi.  p.  333;  Haast,  *  Geology  of  Canterbury  and  Westlaud,' 
p.  291  ;  liutton  and  Ulrich,  *  Geology  of  Otago,'  p.  44. 

-  Hector,  *  Hanclbook  of  New  Zealand,*  1883,  p.  29. 

3Q 


962  STBATIGRAPHICAL  GEOLOGY  book  vi 

as  Foluta,  FascioUma^  and  others.  At  Faxoe,  on  the  Danish  island  of 
Seeland,  the  uppermost  member  of  the  Cretaceous  system  (Danian)  con- 
tains, in  like  manner,  a  blending  of  well-known  Upper  Chalk  organisms 
with  the  Tertiary  genera  Cyprasa,  Oliva,  and  MUra,  In  the  neighbour- 
hood of  Paris  also,  and  in  scattered  patches  over  the  north  of  France,  the 
Pisolitic  Limestone,  formerly  classed  as  Tertiary,  has  been  found  to 
include  so  many  distinctively  Upper  Cretaceous  forms  as  to  lead  to  its 
being  relegated  to  the  top  of  the  Cretaceous  series,  from  which,  however, 
it  is  marked  off  by  the  decided  unconformability  already  described. 
These  fragmentary  deposits  are  interesting,  in  so  far  as  they  help  to  show 
that,  though  in  western  Europe  there  is  a  tolerably  abrupt  separation 
between  Cretaceous  and  Tertiary  deposits,  there  was  nevertheless  no- real 
break  between  the  two  periods.  The  one  merged  insensibly  into  the 
other ;  but  the  strata  which  would  have  served  as  the  chronicles  of  the 
intervening  ages  have  either  never  been  deposited  in  the  area  in  question, 
or  have  since  been  in  great  measure  destroyed.  In  southern  £urope, 
especially  in  the  south-eastern  Alps,  and  probably  in  other  parts  of  the 
Mediterranean  basin,  no  sharp  line  can  be  drawn  between  Cretaceous  and 
Eocene  rocks.  These  deposits  merge  into  each  other  in  such  a  way  as  to 
show  that  the  geographical  changes  of  the  western  region  did  not  extend 
into  the  south  and  south-east.  In  North  America,  also,  on  the  one  side 
(pp.  928,  959),  and  in  New  Zealand  on  the  other,  there  is  a  similar 
effacement  of  the  hard  and  fast  line  which  was  once  supposed  to  separate 
Mcsozoic  and  Tertiary  formations. 

The  name  Tertiary,  given  in  the  early  days  of  geology,  before  much 
was  known  regarding  fossils  and  their  history,  has  retained  its  hold  on 
the  literature  of  the  science.  It  is  often  replaced  by  the  terms  "  Cainozoic  " 
{rtceiit  life),  or  "  Neozoic  '*  {new  life),  which  express  the  great  fact  that  it 
is  in  the  series  of  strata  comprised  under  these  designations  that  most  recent 
species  and  genera  have  their  earliest  representatives.  Taking  as  the 
basis  of  classification  the  percentage  of  living  species  of  mollusca  found  by 
Deshayes  in  the  different  groups  of  the  Tertiary  series,  Lyell  proposed  a 
scheme  of  arrangement  which  has  been  generally  adopted.  The  older 
Tertiary  formations,  in  which  the  number  of  still  living  species  of  shells 
is  very  small,  he  named  Eocene  (dawn  of  the  recent),  including  under  that 
title  those  parts  of  the  Tertiary  series  of  the  London  and  Paris  basins 
wherein  the  proportion  of  existing  species  of  shells  was  only  3A  per  cent.^ 
The  middle  Tertiary  beds  in  the  valleys  of  the  Loire,  Garonne,  and  Dor- 
dognc,  containing  17  per  cent  of  living  species,  were  termed  Miocene 
{}('.<!>  recent),  that  is,  containing  a  minority  of  recent  forms.  The  younger 
Tertiary  formations  of  Italy  were  included  under  the  designation  Pliocene 
(ni'ne  recent),  because  they  contained  a  majority,  or  from  36  to  95  per 
cent,  of  living  species.  This  newest  series,  however,  was  further  sub- 
divided into  Older  Pliocene  (35  to  50  per  cent  of  living  species)  and 
Newer  Pliocene  (90  to  95  per  cent).  A  still  later  group  of  deposits  was 
termed   Pleistocene  {most  recent),  where  the  shells  all  belonged  to  living 

^  Sonie.i>aluoutologists,  however,  doubt  whether  any  older  Tertiary  species,  except  of 
foianiiuilera  or  otlier  lowly  organisms,  is  still  living. 


PART  IV  CAINOZOIG  OR  TERTIARY  SYSTEMS  963 

— — —  , 

species,  but  the  mammals  were  partly  extinct  forms.  This  classification, 
though  somewhat  artificial,  has,  with  various  modifications  and  amplifica- 
tions, been  adopted  for  the  Tertiary  groups,  not  of  Europe  only,  but  of 
the  whole  globe.  The  original  percentages,  however,  often  depending  oh 
local  accidents,  have  not  been  very  strictly  adhered  to.  The  most  impor- 
tant modification  of  the  terminology  in  Europe  has  been  the  insertion  of 
another  stage  or  group  termed  Oligocene,  proposed  by  Bey  rich,  to 
include  strata  that  were  formerly  classed  partly  as  Upper  Eocene  and 
partly  as  Lower  Miocene.^ 

Some  writers,  recognising  a  broad  distinction  between  the  older  and 
the  younger  Tertiary  deposits  of  Europe,  have  proposed  a  classification 
into  two  main  groups :  1st,  Eocene,  Older  Tertiary  or  Palaeogene,  including 
Eocene  and  Oligocene ;  and,  2nd,  Younger  Tertiary  or  Neogene,  com- 
prising Miocene  and  Pliocene.  This  subdivision  has  been  advocated  on 
the  ground  that,  while  the  older  deposits  indicate  a  tropical  climate,  and  con- 
tain only  a  very  few  living  species  of  organisms,  the  younger  groups  point 
to  a  climate  approaching  more  and  more  to  that  of  the  existing  Mediter- 
ranean basin,  while  the  majority  of  their  fossils  belong  to  living  species.^ 

The  Tertiary  periods  witnessed  the  development  of  the  present 
distribution  of  land  and  sea  and  the  upheaval  of  most  of  the  great 
mountain-chains  of  the  globe.  Some  of  the  most  colossal  disturbances 
of  the  terrestrial  crust,  of  which  any  record  remains,  took  place  during 
these  periods.  Not  only  was  the  floor  of  the  Cretaceous  sea  upraised 
into  low  lands,  with  lagoons,  estuaries,  and  lakes,  but  thrpughout  the 
heart  of  the  Old  World,  from  the  Pyrenees  to  Japan,  the  bed  of  the 
early  Tertiary  or  nummulitic  sea  was  upheiived  into  a  succession  of  giant 
mountains,  some  portions  of  that  sea-floor  now  standing  at  a  height  of 
at  least  16,500  feet  above  the  sea. 

During  Tertiary  time  also  there  was  an  abundant  manifestation  of 
volcanic  activity.  After  a  long  quiescence  during  the  succession  of 
Mesozoic  periods,  volcanoes  broke  forth  with  great  vigour  both  in  the 
Old  and  the  New  World.  Vast  floods  of  lava  were  poured  out,  and  a 
copious  variety  of  rocks  was  produced,  ranging  from  highly  basic  to 
rhyolites,  quartz-felsites,  and  granites. 

The  rocks  deposited  during  these  periods  are  distinguished  from 
those  of  earlier  times  by  increasingly  local  characters.  The  nummulitic 
limestone  of  the  older  Tertiary  groups  is  indeed  the  only  widespread 
massive  formation  which,  in  the  uniformity  of  its  lithological  and  palseon- 
tological  characters,  rivals  the  rocks  of  Mesozoic  and  Palaeozoic  time. 
As  a  rule.  Tertiary  deposits  are  loose  and  incoherent,  and  present  such 
local  variations,  alike  in  their  mineral  composition  and  organic  contents, 
as  to  show  that  they  were  mainly  accunmlated  in  detached  basins  of 
comparatively  limited  extent,  and  in  seas  so  shallow  as  to  be  apt  from 
time  to  time  to  be  filled  up  or  elevated,  and  to  become  in  consequence 

^  Boyd  Dawkins  has  proposed  to  use  the  fossil  niainmulia  as  a  basis  of  classification  {Q. 
J.  Geol.  Soc.  1880,  p.  379),  but  his  scheme  does  not  essentially  differ  from  that  in  common 
use  founded  on  molluscau  percentages. 

*  Homes,  Jahrh.  Geol,  Reichsanst.  1864,  p.  510. 


964  STRATIGRAPHICAL  GEOLOGY  book  vi  part  nr 


brackish  or  even  fresh.  ^  These  local  characters  are  increasingly  developed 
in  proportion  to  the  recentness  of  the  deposits. 

The  climate  during  Tertiary  time  underwent  in  the  northern  hemi- 
sphere some  remarkable  changes.  Judging  from  the  terrestrial  vegetation 
preserved  in  the  strata,  we  may  infer  that  in  England  the  climate  of  the 
oldest  Tertiary  periods  was  of  a  temperate  character,*  but  that  it 
became  during  Eocene  time  tropical  and  subtropical,  even  in  the  centre 
of  Europe  and  North  America.  It  then  gradually  grew  more  temperate, 
but  flowering  plants  and  shrubs  continued  to  live  even  far  within  the 
Arctic  circle,  where,  then  as  now,  unless  the  axis  of  the  earth  has  mean- 
while shifted,  there  must  have  been  six  sunless  months  every  year. 
Growing  still  cooler,  the  climate  passed  eventually  into  a  phase  of  extareme 
cold,  when  snow  and  ice  extended  from  the  Arctic  regions  far  south  into 
Europe  and  North  America.  Since  that  time,  the  cold  has  again  diminished, 
until  the  present  theimal  distribution  has  been  reached. 

With  such  changes  of  geography  and  climate,  the  plant  and  animal  life 
of  Tertiary  time,  as  might  have  been  anticipated,  is  found  to  have  been 
remarkably  varied.  Entering  upon  the  Tertiary  series  of  formations,  we 
find  ourselves  upon  the  threshold  of  the  modern  type  of  life.  The  ages 
when  lycopods,  ferns,  cycads,  and  yew-like  conifers  were  the  leading  forms 
of  vegetation,  have  passed  away,  and  that  of  the  dicotyledonous  angiosperms 
— the  hard-wood  trees  and  evergreens  of  to-day — now  succeeds  them,  but 
not  by  any  sudden  extinction  and  re-creation ;  for,  as  we  have  seen  (p. 
922),  some  of  these  trees  h^d  already  made  their  appearance  in  Cretaceous 
times.  The  hippurites,  inocerami,  ammonites,  belemnites,  baculites,  tur- 
rilites,  scaphites,  and  other  moUusks,  which  had  played  so  large  a  part  in 
the  molluscan  life  of  the  later  Secondary  periods,  now  cease.  The  great 
reptiles,  too,  which,  in  such  wonderful  variety  of  type,  were  the  dominant 
animals  of  the  earth's  surface,  alike  on  land  and  sea,  ever  since  the  com- 
mencement of  the  Lias,  now  waned  before  the  increase  of  the  mammalia, 
which  advanced  in  augmenting  diversity  of  type  until  they  reached  a 
maximum  in  variety  of  form  and  in  bulk  just  before  the  cold  epoch 
referred  to.  When  that  refrigeration  passed  away  and  the  climate  became 
milder,  the  extraordinary  development  of  mammalian  life  that  preceded 
it  is  found  to  have  disappeared  also,  being  only  feebly  represented  in  the 
living  fauna  at  the  head  of  which  man  has  taken  his  place. 

Section  i.  Eocene. 

§  1.  General    Characters. 

Rocks. — In  Europe  and  Asia  the  most  widely  distributed  deposit  of 
this  epoch  is  the  nummulitic  limestone,  which  extends  from,  the  Pyrenees 

^  The  peculiar  characters  of  tlie  Tertiary  rocks  of  the  Western  Territories  of  North 
America  are,  however,  displayed  over  areas  which  in  Europe  would  be  regarded  as 
enormous. 

-  J.  S.  Gardner  in  *  Geology  of  the  Isle  of  Wight,'  Mem.  Ged.  Sum,  1889,  p.  106. 


SECT,  i  §  1 


EOCENE  SVHTEM 


through  the  Alps,  Carpathians,  Caucasus,  Asia  Minor,  Northern  Africa, 
Persia,  Beloochistan,  and  the  Suleim&n  Mountains,  and  is  found  in  China 
iind  Japan.  It  attains  a  thickness  of  several  thousand  feet  In  some 
places  it  is  composed  mainly  of  foraminifera  (Nummuliles  and  other  genera) ; 
but  it  sometimes  includes  a  tolerably  abundant  marine  fauna.  Here  and 
there  it  has  assumed  a  compact  crystalline  marble-like  structure,  and  can 
then  hardly  be  distinguished  from  a  Mesozoic  or  even  Palaeozoic  rock. 
Enormous  masses  of  sandstone  occur  in  the  eastern  Alps  (Vienna  sand- 


stone, Flysch),  referred  partly  to  the  same  age,  but  seldom  containing 
any  fossils  save  fucoids  (p.  9.'^5).  The  most  familiar  European  type  of 
Eocene  deposits,  however,  is  that  of  the  Anglo-Parisian  and  Fmnco- 
Belgian  area,  where  are  found  numerous  thin  local  beds  of  usually 
soft  and  uncompacted  clay,  marl,  sand,  and  sandstone,  with  hard  and 
soft  hands  of  limestone,  containing  alternations  of  marine,  brackish,  and 
fresh-water  strata.  This  type  of  sedimentation  evidently  indicates  more 
local  and  shallower  basins  of  deposit  than  the  wide  Mediterranean 
sea,  which  stretched  across  the  heart  of  the  Old  World  in  early  Tertiary 

LiFK. — The  flora  of  Eocene  time  has  been  abundantly  preserved  on 
certain  horizons.  In  the  English  Eocene  groups,  a  succession  of  several 
<listinct  floras  has  been  observed,  those  of  the  London  Clay  and  Bagshot 
beds  being  particularly  rich.  The  plants  from  the  London  Clay  indicate 
a  warm  climate.'  They  include  species  of  CallUrii,  Solemslrobus,  Cupresst- 
Httrs,  Srquma,  Salisburia,  Agave,  Smiiax,  Amomum,  Nipa  (Fig,  424),  Mag- 
noiia,  A'elumbium,  Fleloria,  HighUa,  Sapindv.i,  Eucalyptus,  Cototuasta;  PrHnvn, 
Amygdalus,  Faboidea,  &c.  Proteaceoua  plants  like  the  living  Australian 
PHri^ila  and  Jmipogon  have  been  asserted  to  occur  in  the  Lower  Eocene 

'  Kltingahaumn,  Prw.  Roj,.  Sie,  nil.  (1879)  p.  388. 


906  STRATIGRAPHIGAL  GEOLOGY  book  vi  part  it 

vegetation,  but  their  occurrence  is  not  yet  proved ;  the  so-called  Petro- 
pkilvidex  is  now  regarded  as  an  alder  (Fig.  424).^  During  Middle  Eocene 
time  in  the  umbr^eous  forests  of  evergreen  trees — laurels,  cypresses, 
and  yewa — there  grew  species  of  ferns  (Lygodinm,  Jtfimivm,  Sue),  also 
of  many  of  oui*  familiar  trees  besides  those  just  mentioned,  such  as 
chestnuts,  beeches,  elms,  poplars,  hornbeams,  willows,  figs,  planes,  and 
maples.  The  subtropical  character  of  the  climate  was  shown  by  clnmpa 
of  Pandanus,  with  here  and  there  a  fan-palm  (Fig.  424)  or  feather-patm, 
a  tall  aroid  or  a  towering  cactus.' 

The  Eocene  fauna  of  western  and  central  Eiurope  presenta  similar 
evidence  of  tropical  or  subtropical  conditions.  Especially  characteristic  are 
foraminifera  of  the  genus  NummuUteit,  which  occur  in  prodigious  numbers 
in  the  niimmulite  limestone  (Fig.  425),  and  also  occupy  different  horizons 
in  the  English  and  French  Eocene  basins.     The  assemblage  of  moUusca  is 


very  large,  most  of  the  genera  being  still  living,  though  many  of  them 
arc  confined  to  the  warmer  seas  of  the  globe  (Figs.  426,  427).  Character- 
istic forms  are  Belmepia,  Nautil-us,  CanreUaria,  Fy.tus,  I'seudotiva,  Oliva, 
Vohila,  Cnnm,  Mifrn,  Cerithttim,  Mtlania,  Turritdh,  Boslellaria,  Pleurotoma, 
Oi/pnea,  Nalka,  Sraiu,  Corhuh,  Cijrnta,  Cylhfrea  (Merelrix),  Chama,  Lueina} 
Fish  remains  are  not  infrequent  in  some  of  the  clays,  chiefly  as  scattered 
teeth  (Fig.  42P)  and  otoliths.  The  living  tropical  siluroid  genus  An»s 
has  been  found  in  these  deposits.  Some  of  the  more  common  genera  are 
L'imiia,  Otloiiiuitpi^,  MylinbaUi>,  .4etobii/€^  f'ri.-'lU,  Pkyliodus.  The  Eocene 
reptiles  present  a  singular  contrast  to  those  of  Mesozoic  time.  They  con- 
sist larjiely  of  tortoises  and  turtles,  with  crocodiles  and  sea-snakes.  It  is 
sup^cstiic  to  fin  I  remiiiiB  of  siluroid  fish,  crocodiles,  and  chelonians,  pre- 
=iei  \  ed  m  de]  nnt'i  of  Focene  age,  for  the  assemblage  is  like  what  may  now 

r    's   (  iirJn«r  op   (  I    \     108. 

I  1  tfnrd  er  Brit  sh  Eocene  Flora,"  Palaoato^apA.  Snc  1879 ;  L.  Crie,  ■'  Rechercha 
irl  \  K^tatn  1e  10  est  U  In  Fraoce  h  I'Epoque  Tertiain,"  Ann.  Se^tacu  Oint.  ii. 
(!'•  ]  Htinfi  hausL  Prac  fl  *.  Sf,  j:xx.  (1880)  fi.  228  ;  Comle  de  S»port»,  'L«  Mood* 
I     II  titei    11  9  p  207 

'  ?nr  1  1   t  of  Br  1  <li  Eoc  ne  and  OUgocene  iiioUusca  consult  the  volame  by  R.  B. 
Nen(       one  of  tl  e  iene<i  of  Catalognes  iasned  by  the  British  HnnaiD. 


EOCENE  SYSTEM 


be  met  with  in  tropical  Beaa  of  the  present  time.     An  interesting  series 


^ 


Fig,  <M.— Eocne  Lamfllibtlnchi. 


iciiu  Hliuilaulii,  Dnh,  ; 


of  remains  of  birds  has  been  obtained  from  the  English  Eocene  beds. 
These  include  Argilhrnis  Umgipennis  (perhaps  representative  of,  but  larger 


— Eocfu*  GnHtsropoilB. 


B,  Fimi"  (C[«v«hthp»)  IrniKffiniH.  Band.  (]) ;  0,  Orllhium  <OHni«nllp)gt((»BUuin,  Idiii.  (A) ;  r.  MeUiil* 

.n.)tllnsU.  I)efr.  (j) ;  ./.  Volnl.  (V«mtlllth«)  clanU.  Sow.  {|> :  t.  I(«iUll.rl.{HlB,elliynMiit.ll., 

IM.  (D :  /,  Coiiiu  .IfiXT'lltax,  IlniK.  ()). 

■than,  the  moilerri  albatross),  DasmiiU,  and  Gantornin  (somewhat  akin  to  the 


968  STRATIGRAPHIGAL  GEOLOGY  book  vi  part  iv 

extinct  Dinmim  of  New  Zealand),  Uakyarnis  toliapicm,  IMhornis  vulturinvii, 
Maaonm  tanmqma,  Odaidoptei-yx  toliapicus  (a  bird  with  bony  tooth -like 
processes  to  its  large  beak).  From  the  upper  Eocene  beds  of  the  Paris 
basin  ten  species  of  birds  have  been  obtained,  including  forms  allied  to 
the  buzzard,  osprey,  hawk,  nuthatch,  quail,  pelican,  ibis,  flamingo,  and 
African  hornbill.^  But  the  most  notable  feature  in  the  palaeontology  of 
the  period  is  the  advent  of  some  of  the  numerous  mammalian  forms  for 
which  Tertiary  time  was  so  distinguished.  In  the  lower  Eocene  period 
appeared  the  primitive  carnivores  Arctoci/on  and  PalasmUdiSj  two  animals 
with  marsupial  affinities,  the  former  with  bear-like  teeth,  the  latter  with 
teeth  like  those  of  the  Tasmanian  dasyure  ;  also  the  tapir-like  Corifphodou  ; 
the  small  hog- like  Hyracotherium,  with  canine  teeth  like  those  of  the 
peccary,  and  a  form  intermediate  between  that  of  the  hog  and  the  hyrax. 
Middle  Eocene  time  was  distinguished  by  the  advent  of  a  group  of 
remarkable  tapir-like  animals  {Palie^iherinw,  PaJuphih/rium,  Lophudou,' 
Parhynolophus) ;  true  carnivores  {Pterodon  and  Provirerrci) ;  insectivores 
(Hfierofn/uHy  Mirrochtyms)  and  the  lemuroid  C(en(pifhenu%  the  earliest  re- 


FiK.  42S.— EfKMMie  Fish»'s. 
",  I^iiima  rlej^'uiis,  t^M)lh  of,  Ag.  (jj)  ;  h,  Odontaspis  (Otxxlus)  oblitjmis,  tiK>rh  of,  Ag.  (3). 

presentative  of  the  tribe  of  monkeys.  With  tlie  upper  Eocene  period. 
l)esi(les  the  abundant  older  ta])ir-like  forms,  there  came  others 
{Andiith('num\  which  presented  characters  intermediate  between  those  of 
the  tapiroid  Pala'otheres  and  the  true  Equidtt*.  They  were  about  the 
size  of  small  ponies,  had  three  toes  on  each  foot,  and  are  regardeil  as 
ancestors  of  the  horse.  Numerous  hog-like  animals  {Diplopus,  Hyffpohimna) 
mingled  with  herds  of  ancestral  hornless  forms  of  deer  and  antelopes 
(J)khobnit(',  Jjirhodftn,  Aniphifray/uhis).  Opossums  abounded.  Among  the 
carnivores  were  animals  resembling  wolves  {Cyaodon)^  foxes  {Amphicyou), 
and  wolverines  {Tylodou),  but  all  possessing  marsupial  affinities.  There 
appear  to  have  ])een  also  representatives  of  our  hedgehogs,  squirrels,  and 
bats.'^ 

'  Owen,  Q.  J.  Geol.  Soc.  1856,  1873,  1878,  1880;  Boyd  Dawkiiis,  'Early  xMan  iu 
Britaiu,'  \\.  33  ;    Milne  Eil wards,  *  Oiseaux  Fossiles,'  ii.  543. 

-  H.  Filliol,  Mem.  iieol.  Soc.  France  (3)  v.  No.  1  (1888). 

■*  (Jaudry.  *  Les  Eucliainements  du  Monde  Animal,'  p.  4  ;  Boyd  Dawkins,  *  Early  Man 
in  Britain,'  oliap.  ii. 


iS  1 


EOCENE  aVSTEM 


It  is  from  the  thick  Eocene  lactutrino  forraationa  of  the  western 
TerritorieB  of  the  United  States  that  the  most  important  additions  to  our 
knowledge  of  the  animals  of  early  Tertiary  time  have  been  made,  thanks 
to  the  admirable  and  untiring  labours,  Hrst  of  Leidy,  and  subsequently 
of  Marsh  at  Newhaven,  and  Cope  at  Philadelphia.  The  Laramie  group, 
ill  particular,  has  yielded  an  extraordinarily  abundant  and  varied  fauna, 
comprising  ophidians  {Coniophix),  true  lacertilians  (Ckamapn,  Iifuatiavun),  and 
gigantic  forma  of  deinoaaura  These  last-named  animals  are  of  peculiar 
interest,  inasmuch  as  they  show  that  just  before  the  final  extinction  of  the 
Bub-claas  to  which  they  belong  they  had  developed  into  many  highly 
specialized  tyjios  (OmUlumiimvii,  Cliuwaunu')}     The  herbivorous  ungiilata 


appear  to  have  formed  *a  chief  element  in  this  western  fauna.  They 
included  some  of  the  oldest  known  ancestors  of  the  horse,  with  four-toed 
feet,  and  even  in  one  form  {EiMppnt^)  mth  rudiments  of  a  fifth  toe ;  also 
various  hog-like  animals  (AV(yus,  Pamlufiii').  Some  of  the  most  peculiar 
forms  were  those  of  the  type  termed  Tillodont  by  Marsh,  presenting  a 
remarkable  union  of  the  characters  of  ungulates,  rodents,  and  carnivores, 
and  especially  striking  from  their  pair  of  long  incisor  teeth  (Till'ilkeriiim, 
Jiif/ii/ijKKlns,  Calamitloii).  This  author,  from  another  assemblage  of 
skulls  anil  bones  of  animals  about  as  large  an  a  fox,  has  proposed  to 
establish  a  separate  order  of  mammals,  that  of  the  Mesodactyla,  which  in 
his  opinion  stands  in  somewhat  the  same  relation  to  the  typical  Ungulates 
that  the  Tillodonw  tlo  to  Rodents.*  Still  more  extraordinary  were  the 
Deinocenita,  ranked  as  a  distinct  sub-owier,  possessing,  according  to 
Marsh,  the  size  of  elephants,  with  the  habits  of  rhinoceroses,  but  bearing 
a  pair  of  long  hom-like  prominences  on  the  snout,  another  pair  on  the 


970  STRATIGRAPHIf'AL  GEOLOGY  book  ti  fart  n- 

foreheail,  and  a  single  one  on  each  cheek  {Uinlaikerium,  Fig.  430,'  with 
the  forms  described  under  the  names  f)einorf,rat,  Tittoceras,  Fig.  431, 
Odotomm,  EobasUeus,  I/wnloplmdon).  With  these  animals  there  coexisted 
large  and  small  carnivores  and  some  lemuroid  monkeys. 


§   2.   Local   Development. 

BrilaiiL^— Eotircly  confined  to  the  aouth-eoatern  p&rt  of  Eng1»nd,'  the  Britiih 
Eoceno  strata  occuii;   two  Gynclinal    depressions    in    the    Chalk,   which,   owio;   lo 


iiulfltion,   liavp  lieoonie  lietathed  inti 

tiipsliire.     They  liave  been  arranged  it: 

Hiimpihire. 

S  I  Heodoii  Elill  or  Bnrton  ^ihIh. 
.-  \  Barton  Clay. 


the  two  Kell-deflned  basins  of  London  and 
in  the  snlijoined*tab)e  ; — 
I.ondt>n. 


torat        w      k     lly      ppl    I   by   Prof    V  rsh,     hos  H    ograph   on   the 

1    th     t  d    t   h     Id           It.     JT       I   S   il   I        TT       I  [1886). 

C      be            1  i  I  11  ps      a    logv     f  !■  g!    d      d  W  lea  PreBtwioh,  y.  J. 

1  i  Kd       d  F    bes      T    t    ry  FI  mariae  Fonna- 

n      fttgUJ/        (,o/*nt    1850     H    W    B    t  C    Reid,  and  A. 

r    W)    f  Ih    I  1      f  A\  gl  (     V«n   ff  rf    <a  n   3  d  *ditl  1889  ;  Whitaker, 

f    r       1           1/         O  J                 1889      Ph  !1  p«      (      logy  f  Oxford    and  tlie 


Ine    h 


Ins 


th     1 1     t  be  n  g  beds  of  Bovrj, 


SECT,  i  f  2  EOCENE  SYSTEM  971 

/lampihire.  Londmi. 

r  Part  of  LoWfr  Bagshot  SandB. 

i     London  CUl  (Bognor  bed.).  t?^""  '^'"J'^ 

I  ^1  Woolwich  .id  R^ilng  bed!  S^'""."* v       /n     ^-       k^. 

J    I  ■  Woolwich  and  Reading  beds. 

1.  Tbanet  Sanil. 

Lower  Eooenel— Tlie  Thanet  S*nd  '  at  the  base  of  the  London  batdn  consists  of 
pale  jcHow  and  greenish  lands,  soinetimea  clayey,  and  containing  at  their  bottom  a  thin, 
but  remarkably  conBlant,  layer  of  green.ooated  flinta  resting  directly  on  the  Chalk, 
According  to  Mr.  Whitaker,  it  is  doiibtlul  if  proof  of  actual  erosion  of  the  Chalk  on 
BnyH'iiere  be  seen  under  the  Tertiary  deposita  io  England,  and  he  stat«a  that  the 
Thanct  Sand  everywhere  lies  upon  an  eren  surface  of  Chalk  with  no  Tiaible  nnconform- 


i)liigein.  MmhtabpnlA)- 


ability,'  Professor  Phillips,  on  the  other  hand,  describes  the  Chalk  at  Beading  as 
having  been  "  literally  ground  do\rn  to  a  plane  or  undulated  surface,  as  it  is  this  day  on 
some  parts  of  the  Yorkshire  coast,"  and  having  likewise  been  abundantly  bored  by 
lithodomous  shells.*  The  Thanet  Sand  appears  to  have  been  formed  only  in  the  London 
basin  ;  at  least  it  has  not  been  recognised  at  the  liase  of  the  Eocene  series  in  Hamp- 
shire. It  has  yielded  numerous  organic  remains  in  East  Kent,  bnt  is  almost  unfosaili- 
ferous  farther  west.  Its  fossils  comprise  abont  70  known  species  (all  marine  except  a 
few  fragmenCa  of  terrestrial  vegetation).  Among  them  are  several  foraminifera,  numer- 
ons  lamellibranclis  {Jilaile  tenera,  Cyprina  xulellnrin  {pinnata),  Oalrta  billonKina, 
Cucultrta  decnaata  {crataatina),  Fholadomya  {intiiinia  I )  cvneala,  P.  Ktminrkii.  COrbula 
TtgulbUnnii,  &c.),  a  few  species  of  gasteropods  {yalka  infundibulHm  {gubdrprata), 
Apurrhaia  Soirerbii.  kc),  a  nautilus,  and  the  teeth,  si'ales,  and  bones  of  fishes 
{OdinUa^pix,  Pimidiii). 


'  PreMwich,  (?.  J.  Otd.  Sue.  viii.  (1852)  p,  237. 
'  'Geology  of  London,'  p.  107. 
'  'Geology  of  Oxford,'  p.  U2. 


972  STRATJGRAPHICAL  GEOLOGY  book  vi  part  iv 


The  Woolwich  ahd  Reading  Beds,'  or  '^Plastic  Clay"  of  the  older  ideolo- 
gists, consist  of  lenticular  sheets  of  plastic  clay,  loam,  sand,  and  pebble-beds,  so  variable 
in  character  and  thickness  over  the  Tertiary  districts  that  their  homotaxial  relations 
would  not  at  first  be  suspected.  One  type  (Reading)  presenting  unfossiliferoas  lenti- 
cular, mottled,  bright-coloured  clays,  >vith  sands,  sometimes  gravels,  and  even  sand- 
stones and  conglomerates,  occurs  throughout  the  Hampshire  basin  and  in  the  northern 
and  western  part  of  the  London  basin.  A  second  typo  (Woolwich),  found  in  W^est  Kent, 
Surrey,  and  Sussex,  from  Newhaven  to  Portsladc,  consists  of  light-coloured  sands  and 
grey  clays,  crowded  with  estuarine  shells.  A  third  type,  seen  in  East  Kent,  is  composed 
only  of  sands  containing  marine  fossils.  These  differences  in  lithological  and  palaeonto- 
logical  characters  serve  to  indicate  the  geographical  features  of  the  south-east  of 
England  at  the  time  of  deposit,  showing  in  particular  that  the  sea  of  the  Thanet  beds 
had  gradually  shallowed,  and  that  an  estuary  now  partly  extended  over  its  site.  The 
organic  remains  as  yet  obtained  from  this  group  amount  to  more  than  100  species. 
They  include  a  few  plants  of  terrestrial  growth,  such  as  Ficus  Forbesi,  ChrevilUa  Heeri, 
Lauru8  Hookerij  AroUia^  Lygodinm^  Liriodendrf>n,  Palmetto^  and  Flatanus — a  flora 
which,  containing  some  apparently  persistent  types,  has  a  temperate  facies.^  The 
lamellibranchs  are  partly  estuarine  or  fresh-water,  partly  marine  ;  characteristic  species 
being  Cyrcna  cnneifannU,  C.  cxtrd^ta,  and  C.  teiliiiellu.  Ontrca  heilovacina  forms  a 
thick  oyster-bed  at  the  base  of  the  series,  besides  occurring  throughout  the  group. 
Ostrea  tenera  is  likewise  abundant.  The  gasteropoda  include  a  similar  mixture  of 
marine  with  fluviatile  species  {Potamides  {(krUhivm)  funatuSt  Melania  inipiinata.  Melon- 
opsin  bucciiwidfis,  Neritimi  globulus^  Natica  infviid ibulumj  Pisania  {Fusus)  lata^  Vivipams 
{Paludina)  Icntiut,  Pianorbis  Ixi^igatiis,  Pitharella  Hiclnnannif  &c)  The  fish  are 
chiefly  sharks  (Odojitaspis).  Bones  of  turtles,  scutes  of  crocodiles,  and  remains  of 
gigantic  birds  {OasUymis)  have  been  found.  The  highest  organisms  are  bones  of 
mammalia,  including  the  Coryphod(nu 

The  B 1  a  c  k  h  e  a  t  h  or  0 1  d  h  a  v  e  n  B  e  d  s,^  at  the  base  of  the  London  Clay,  con- 
sist in  W.  Kent  almost  wholly  of  rolled  flint-pebbles  in  a  sandy  base,  which,  as  Mr. 
Whitakcr  suggests,  may  have  accumulated  as  a  bank  at  some  little  distance  from 
shore.  Tliougli  of  trifling  thickness  (20-40  feet),  they  have  yielded  upwards  of  15u 
species  of  fossils.  Traces  of  Ficvs,  Cinnaitutmvjnj  and  Coniferae  have  been  obtained  from 
them,  indicating  perhaps  a  more  subtropical  character  than  the  flora  of  the  beds  below, 
but  without  the  Australian  and  American  types  which  apj>ear  in  so  marked  a  manner  in 
the  later  Eocene  floras.^  The  organisms,  however,  are  chiefly  marine  and  partly 
estuarine  shells,  the  gasteropods  being  particularly  abundant  {Calyptrsa  trochiformitij 
Pi>t(tviuhs  {CcrHhiii'm)  funatvs,  Mt'laiiia  inquinnfa^  Nntico  infundibuhniu  Cartiivnir 
piinnsfr</ifns/%  PfxtnuruJus  tf.irhmiulitris^^  iic.) 

The  London  Clay'"'  is  a  deposit  of  stiff  brown  and  bluish-grey  clay,  with  layers 
of  septarian  noduli-s  of  argillaceous  limestone.  Its  bottom  beds,  commonly  consisting 
of  green  and  yellow  sands,  and  rounded  flint-pebbles,  sometimes  bound  by  a  calcareous 
cement  into  hard  tabular  masses,  form  in  the  London  basin  a  well-marked  horizon. 
The  London  Clay  is  tyj)ically  developed  in  that  basin,  attaining  its  maximum  thickness 
(500  feet)  in  the  south  of  Essex.  Its  representative  in  the  Hampshire  basin,  known  as 
the  '*  Bognor  Beds,"  and  exj>osetl  at  Bognor  on  the  Sussex  coast  and  at  Portsmouth, 
consists   of  clays,  sands,    and   calcareous   sandstones,  thus    differing   somewhat,  both 


'  Prestwich,  (^.  ./.  Oeol.  Sor.  x.  p.  75  :  Whitaker,  'Geology  of  London,'  p.  122. 
-  .1.  S.  Gardner,  *' British  Eocene  Flora,"  PolHimtwj.  Sot\  p.  29. 
■^  Wliitaker,  (^.  J.  OcU.  Soc-.  xxii.  (1866),  p.  412  ;  '(ieology  of  London,*  p.  214. 
^  J.  S.  Gar<lner,  op.  cit.  pp.  2,  10. 

^  Prestwicl),  Q.  J.  (ieoL  Soc,  vi.   p.   255  ;  x.  p.   435  ;  Whitaker,  *  Geology  of  London,' 
1>.  238. 


SECT,  i  §  2  EOCENE  SYSTEM  973 

lithologically  and  palseontologically,  from  the  typical  development  in  the  London 
basin.  The  London  Clay  has  yielded  a  long  and  varied  suite  of  organic  re- 
mains, that  point  to  its  having  been  laid  down  in  the  sea  beyond  the  mouth  of 
a  large  estuary,  into  which  abundant  relics  of  the  vegetation,  and  even  sometimes  of 
the  faunn,  of  the  adjacent  land  were  swept.  According  to  Prof.  T.  Rupert  Jones,  the 
depth  of  the  sea,  as  indicated  by  the  foraminifera  of  the  deposit,  may  have  been 
about  600  feet.  Professor  Prestwich  has  pointed  out  that  there  are  traces  of  the 
existence  of  jmlaeontological  zones  in  the  clay,  the  lowest  zone  indicating,  in  the  east 
of  the  area  of  deposit,  a  maximum  depth  of  water,  w^hile  a  progressive  shallowing  is 
shown  by  three  higher  zones,  the  uppermost  of  which  contains  the  greater  part  of'  the 
terrestrial  vegetation,  and  also  most  of  the  fish  and  reptilian  remains.  The  fossils  are 
mainly  marine  mollusca,  which,  taken  in  connection  with  the  flora,  indicate  that  the 
climate  was  somew^hat  tropical  in  character.  The  plants  include  the  fruits,  seeds,  or 
leaves  of  the  following,  among  other  genera,  the  fossils  having  been  mostly  obtained 
from  the  Isle  of  Sheppey :  Sequoia^  Finus^  CallUriSy  Salisburia ;  Musa,  Nipa,  ScUxil^ 
Chamxropa ;  QiveraiSf  .Liquidambar,  Laurus,  Nyasa^  Diospyrm^  Sj/mplocos,  Magnolia, 
Victtrriay  HiglUea,  Sapindtts,  Cupania^  Eugenia^  Euailyptus,  Amygdalus.^  Diatoms  are 
plentifully  diffused  through  the  London  Clay,  and  numerous  foraminifera  have  been 
found  by  washing  it.  Crustacea  abound  {XanthopstSj  Hoploparia).  Of  the  lamelli- 
branchs  some  of  the  most  usual  genera  are  Avicula,  Cardiuni,  Corhttla^  Leda,  Modtola, 
NiicnloL,  and  IHnna.  Gasteropods  are  the  prevalent  mollusks,  the  common  genera 
being  Pleurotoma  (45  species),  Fusiis  (15  species),  Cypraea,  Murex^  Natica^  Cassis 
{CassUlaria)^  Pyrtda,  and  Valuta.  The  cephalopods  are  represented  by  6  or  more 
species  of  NautUuSf  by  Belosepia  sepwideay  and  BeUyptera  Levesqnei.  Nearly  100  species 
of  fishes  occur  in  this  formation,  the  rays  {Myliobatcs,  14  species)  and  sharks  {Odontaspis^ 
Lamna^  &c.)  being  specially  numerous.  A  sword-fish  (Tetrapterus  prisciis),  and  a  saw- 
fish {Prisiis)  have  likewise  been  met  with.  The  reptiles  were  numerous,  and  markedly 
unlike,  as  a  whole,  to  those  of  Secondary  times.  Among  them  are  numerous  turtles 
and  tortoises  {Chelone,  10  species,  TrionyXy  1  species,  Platemys,  6  8|)ecies),  two  species 
of  crocodile,  and  a  sea-snake  {Palmophis  toliapicus)y  estimated  to  have  equalled  in  size  a 
living  Boa  conslrictar.  Remains  of  birds  have  also  been  met  with  {Lithomis  vuHurinuSf 
Ha/eyamis  toliapiciis,  Dasomis  londinensis,  Odontopteryx  toliapicus,  Argillornis  longi- 
pcnnis).  The  mammals  included  forms  resembling  the  tapirs  {Hyracotheriuiriy  Cory- 
ph4)don,  &c.),  an  opossum  {Didelphys\  and  a  bat.  The  carcases  of  these  animals  must 
have  been  borne  seawards  by  the  great  river  which  transported  so  much  of  the  vegetation 
of  the  neighbouring  land. 

Middle  Eocene. — In  the  London  basin  this  division  consists  chiefly  of  sands,  which 
are  comprised  in  the  two  sub-stages  of  the  lower  and  middle  "Bagshot  Beds."  The 
lower  of  these,  consisting  of  yellow,  siliceous,  unfossiliferous  sands,  with  irregular  light 
clayey  beds,  attains  a  thickness  of  about  100  to  150  feet  The  second  sub-stage,  or 
''Middle  Bagshot  Beds,"  is  made  up  of  sands  and  clays,  sometimes  50  or  60  feet  thick, 
containing  few  organic  remains,  among  which  are  bones  of  turtles  and  sharks,  with  a  few 
mollusks  {Cardita  acuticostay  C,  elegans,  C.  planicvstOy  C.  imbricaiay  CorbtUa  gallica,  C. 
Lavuirckiit  Ostrea  Jlabellula). 

In  the  Hampshire  basin,  the  Middle  Eocene  beds  attain  a  much  greater  development, 
being  not  less  than  660  feet  thick  at  the  west  end  of  the  Isle  of  Wight,  where  they 
consist  of  variously-coloured  unfossiliferous  sands  and  clays,  with  minor  beds  of  iron- 
stone and  plant-bearing  clays,  pointing  to  an  alternation  of  marine  and  estuarine 
conditions  of  deposit.'    On  the  mainland  at  Studland,  Poole,  and  Bournemouth,  the 

^  Ettingahausen  and  Gardner,  **  British  Eocene  Flora,"  PidtBontoffrapk,  Sac.  p.  12  ; 
Ettingshausen,  Proc.  Boy.  Sac  xxix.  (1879). 

*  'Geology  of  the  Isle  of  Wight'  in  Mem.  Geol.  Surv,  p.  109. 


974  STJiATIGRAPHICAL  GEOLOGY  book  in  partit 

same  beds  appear.  The  important  seriea  of  clays,  marls,  sands,  and  lignites,  upwards 
of  100  feet  thick,  known  as  the  Bracklesham  beds  from  their  occurrence  at  firacklesham, 
on  the  coast  of  Sussex,  has  yielded  a  large  series  of  marine  organisms.  Among  these 
are  the  iislies  Prist iSy  Odontaspis,  Lamnay  MyliobateSy  also  Palaophis,  and  the  moUnsks 
Bclosepia  sepioidcay  B.  Oweniiy  Cypraa  inflata,  C.  tuberculosa^  Marginella  ebumui,  M. 
omiliUay  Valuta  cremilatay  V.  spinosaj  V.  angusta,  V,  Branderi,  V.  q/thara,  V,  muri- 
citia,  Mitra  labrcdula,  Comis  deperditiis,  C.  Lamarchii,  PUurotoma  dentcUa,  P. 
textiliom,  Ptcronotiis  (Murex)  asper^  ClavalUhes  {Fvsus)  longmvuSt  TurrUella  imbriea- 
taria,  Ostrca  d</rsataj  0.  flabcllula,  Paextd'Cimusiwm,  {Pecten)  corw^iw,  P,  squamuki,  Lima 
expansaj  Spoiidylus  rarispiiuij  Avicvia  media.  Pinna  margarU€Uxa,Modiola  (Lithodamusf) 
DcsJuiycaiy  Area  biangula  {Branderi\  A.  interruptay  A.  pianicosta,  Limopsis  granulata^ 
NueiUa  minor,  NuaUana  (Leda)  galeottiana,  Cardiia  acuticostay  C.  elegans,  C.  imbricaia, 
C.  planicostay  Craasatella  grngnoncnsis,  Chama  calcarata,  C.  gigas,  Nummulites  IsBvigata, 
{N.  scabra)  Alveolina  fusiformis.^  The  Bracklesham  beds  reappear  to  a  small  extent, 
as  greenish  clayey  sands,  in  the  London  basin,  where  they  form  part  of  the  Middle 
Bagshot  beds. 

One  of  the  most  characteristic  features  of  the  English  Middle  Eocene  division  is  the 
abundant  terrestrial  flora  which  has  been  disinterred  especially  from  the  plant-beds  of 
Alum  Bay  and  Bournemouth.  It  is  remarkable  that  this  vegetation  is  apt  to  occur  in 
patches  or  *'  pockets  "  which  may  mark  the  sites  of  ))ools  into  which  it  was  blown  bj  wind 
or  transported  by  streams,  so  that  varied  though  it  be,  it  probably  affords  no  adequate 
picture  of  the  variety  of  the  flora  from  which  it  was  derived.  From  Alum  Bay,  in  the 
Isle  of  Wight,  according  to  Ettingshausen's  census,  no  fewer  than  116  genera  and  274 
species  belonging  to  63  families  have  been  obtained.'*'  A  feature  of  special  interest  in 
this  flora  is  to  be  found  in  the  fact  that  it  is  the  most  tropical  in  general  aspect  which 
has  yet  been  studied  in  the  northern  hemisphere.  This  character  is  particularly  indicated 
by  the  numbers  of  specieis  of  flg,  and  by  the  Artocarpeae,  Cinchonacea*,  Sapotaces, 
Ebenacese,  Biittneriaceie,  Bombace«e,  Sapindaceae,  Malpighiacese,  &c.  The  most  con- 
spicuous and  typical  forms  are  Ficus  Botocrbankii,  Aralia  primigenia,  Dyandra  acvtilcbay 
J).  Bunbiiryi,  Cassin  lingerie  and  the  fruits  of  Ctesalpina.  Many  of  the  dicotyledons 
belong  to  species  elsewhere  found  in  what  have  been  considered  to  be  Miocene  deposits. 
More  than  fifty  species  of  the  Alum  Bay  flora  are  found  also  in  those  of  Sotzka  and 
Hiiring  (p.  979),  while  a  lesser  number  occur  in  those  of.  Sezanne  (p.  976)  and  the 
Lignitic  series  of  Western  America.'  The  Bournemouth  beds  are  believed  to  be  rather 
liigher  in  the  series  than  those  of  Alum  Bay,  and  lie  immediately  below  the  Bracklesham 
beds.  None  of  the  prevailing  types  of  plants  are  found  in  them  that  occur  at  Alum 
Bay,  but  this  may  no  doubt  be  due  to  local  accidents  of  deposition.  The  Bourne- 
mouth flora  is  likewise  an  abundant  one,  and  suggests  a  com]>arison  of  its  climate  and 
forests  with  those  of  the  Malay  archipelago  and  tropical  America,*  The  celebrated 
liguitiferous  deposit  of  Bovey  Ti'acey  in  Devonshire  has  been  referred  by  Mr.  Gardner 
to  this  horizon.^     Crocodiles  still  haunted  the  waters,  for  their  bones  are  mingled  with 

*  See  Dixou's  'Geology  of  Sussex;'  Edwards  and  S.  Woo4l,  '* Monograph  of  Eocene 
MoUusca,"  Paltvontoyrajih.  Sac. 

-  Mr,  Gardner  suspects  that  in  this  estimate  species  from  other  localities  have  been 
included  with  those  from  Alum  Bay,  'Geology  of  the  Isle  of  Wight'  in  Mem.  Qeol,  Surr. 
p.  105. 

^  Ettiugsliausen,  Proc.  Hoy.  Soc.  1880,  p.  228.  See  J.  S.  Gardner,  Oeol,  Mag.  1877, 
p.  129  ;  Naturcy  vol.  xxi.  (1879)  181,  the  Monograph  on  Eocene  Flora  already  cited,  and 
•(Jeology  of  the  Isle  of  Wight '  in  Man.  Oeol.  Surv.  p.  104. 

*  J.  S.  Gardner,  Q.  J.  (Jeol.  Sih:.  xxxv.  (1879)  p.  209  ;  xxxviii.  (1882)  p.  1  ;  Proc. 
(Jeol.  Asaoc.  v.  p.  51  ;  viii.  p.  305  ;  Ucol.  Mag.  1882,  p.  470. 

^  Quart,  Joiirn.  0'e<il.  Si>c.  xxxv.  p.  227  ;  xxxviii.  p.  3.  For  an  account  of  this  dep0i»it 
and  its  tlora,  see  W.  Pengelly  and  0.  Heer,  rhil.  Trans.  1862. 


SECT,  i  §  2  EOCENE  SYSTEM  975 


those  of  sea-snakes  and  turtles,  and  with  tapiroid  and  other  older  Tertiary  ty|)es  of 
terrestrial  creatures.  The  occurrence  of  the  foraminiferal  genus  Nummulites  is  note- 
worthy. Though  not  common  in  England,  it  abounds,  as  already  stated,  in  the  Eocene 
deposits  of  central  and  eastern  Europe. 

Upper  Eocene. — The  highest  division  of  the  Eocene  strata  of  England,  according  to 
the  classification  here  followed,  includes  the  uppermost  part  of  the  Hampshire  series, 
which  has  long  been  known  as  the  *'  I^arton  Clay,"  with,  perhaps,  the  Upper  Bagshot 
Sand  of  the  London  basin.  The  Barton  Clay  does  not  occur  in  that  basin,  but  forms  an 
important  feature  in  Hampshire,  where,  on  the  cliffs  of  Hordwell,  Barton,  and  in  the 
Isle  of  Wight,  it  attains  a  thickness  of  300  feet.  It  consists  of  grey,  greenish,  and  brown 
clays,  with  bauds  of  sand,  and  has  long  been  well  known  for  the  abundance  and 
excellent  preservation  of  its  fossils,  chiefly  moUusks,  of  which  more  than  500  species 
have  been  collected,  but  including  also  fishes  {Lamna,  MyliobcUeSf  Aritis)  and  a  crocodile 
(Diploq/nodon).  The  following  list  includes  some  of  the  more  important  species  for  pur- 
poses of  comparison  with  equivalent  foreign  deposits  :  VoliUu  ludalrix,  V.  ambigiuiy  V. 
aihleta,  Conorbis  {Conns)  scabriculus,  C.  dormihrr,  Pleurotoma  rostreUa  (and  numerous 
other  species),  ClavalUhcs  {Fvsus)  longmvuSt  LeisUnna  pi/j^ts,  Ostrea  giganUcu,  O.  Jlabclluiaf 
Vulsella  deperdita,  Pedeii  recondittutt  Lima  compta,  L.  soror^  Avicula  mediae  Modiola 
seminuda,  M.  sulcata,  M.  tenuistriaia^  Area  append ievJataj  Axinsm  {Peetunciilus)  deleta-, 
Cardita  Davidsoniy  C.  stileata,  Crassalella  suicataj  Chama  squamosa,  Nummulites  elegans, 
N.  variolaria. 

In  the  London  basin  the  position  of  the  so-called  "  Upper  Bagshot  Sands  "  has  been 
the  subject  of  some  discussion,  there  being  no  marked  separation  between  them  and  the 
group  known  as  "  Middle  Bagshot."  They  consist  of  sands  with  ferruginous  concretions 
which  have  yielded  TurriUiia  imhricatai-ia,  Ostrea  flahellul a,  and  other  shells  found  in 
the  Barton  Clay. 

Above  the  Barton  Clay  and  forming  the  highest  member  of  the  Eocene  series  comes 
a  mass  of  unfossiliferous  or  sparingly  fossiliferous  sand,  from  140  to  200  feet  in  thickness, 
so  purely  siliceous  as  to  be  valuable  for  glass-making.  These  deposits  in  the  Isle  of 
Wight  are  immediately  covered  by  the  base  of  the  Oligocene  series.  They  have  been 
called  "  Upper  Bagshot,"  but  iis  they  probably  occupy  a  higher  horizon  than  the  true 
Upper  Bagshot  Sand  of  the  London  basin,  the  local  tenn  Hcadon  Hill  Sand  or  Barton 
Sand  is  more  convenient  for  them.^ 

It  is  probably  from  the  Bagshot  sands  that  the  great  majority  of  the  so-called 
'*  Grey  Wethers  "  or  "Druid  stones  "  of  the  south  of  England  have  been  derived,  which 
have  already  (p.  355)  been  referred  to. 

Northern  France  and  Belgium.' — The  anticline  of  the  Weald  which  separates  the 
basins  of  London  and  Hampshire  is  prolonged  into  the  Continent,  where  it  divides  the 
Tertiary  areas  of  Belgium  from  those  of  Northern  France.  There  is  so  much  general 
similarity  among  the  older  Tertiary  deposits  of  the  whole  area  traversed  by  this  fold  as 
to  indicate  a  probable  original  I'elation  as  parts  of  one  great  tract  of  sedimentation. 
Local  differences,  such  as  the  replacement  of  fresh-water  beds  in  one  region  by  marine 
beds  in  another,  together  with  occasional  gaps  in  the  record,  show  us  some  of  the 
geographical  conditions  and  oscillations  during  the  time  of  deposition.  The  following 
table  gives  the  general  grouping  and  correlation  of  the  Eocene  formations  iu  this 
region  : — 


u 


Marine  gypsum  of  Paris  basin.  Wemnieliau  sands  of  Belgium. 

Middle  sands  (Sables  Moyeus). 


1  C.  Rcid,  'Geology  of  the  Isle  of  Wight,'  Mem.  O'eoL  Surv.  p.  122. 

''^  For  a  couiparisou  of  the  Lower  Eocene  groups  of  Paris,  Belgium  and  England,  see 
Hebert,  Bull.  JSoc.  Uiol,  France  (3),  ii.  p.  27.  Prestwich  {/irit.  Assoc.  1882,  p.  638)  re- 
gards the  Sables  de  Bracheux  as  representing  only  the  lower  part  of  the  Woolwich  beds. 


976  STHATIGRAPHICAL  GEOLOGY  book  Ti  par  ir 


if 


Caillasses  or  Upper  Calcaire 

Grofisier  (fresh-water).  LackenUn  auidi. 

3  ~|  Middle  Calcaire  Grossier  (marine). 
^   V  Lower  Calcaire  Grossier  (fresh-water). 

.    ^  Sands  of  Guise  and  Soissons.  Paniselian  sands. 


Bmzellian  sands  and  sandstones. 


Plastic  clays  and  lignite.  v        •  j        j    i 

Limestones  of  RUly  and  Sezanne.  ^P^**."  «*"^  •^^  *^*y*- 

I,  Sands  of  Bracheux  and  Meudon  Marl.  Landenian  sands. 


Lower  Eocexe. — In  the  Paris  basin,  the  Sables  de,  Bracheux  form  an  excellent 
horizon,  which  corresponds  to  the  Thanet  Sand  of  England  and  Dumont*8  "Syst^e 
Landenien  "  in  Belgium.  Below  this  horizon,  there  occurs  in  the  Franco- Belgian  regiou 
a  lower  series  of  deposits  than  is  found  in  England. '  In  the  Paris  basin,  these  strata 
present  variable  and  local  characters.  They  include  the  Mames  de  Meudon,  remarkable 
for  containing  20  per  cent  of  carbonate  of  strontia ;  and  the  limestones  of  Rilly  and 
Sezanne — a  form  of  travertine  from  which  fresh-water  shells  and  a  rich  assemblage  of 
plants  have  been  obtained  {Cfiara,  Asplcniunif  Alsophylla,  JugiandiUs,  SastafiraSj 
He^fera,  &c.)  -  To  the  north  of  Paris,  the  Mames  de  Meudon  disappear,  and  their  place 
is  taken  by  the  Sables  de  Bracheux — greenish  glauconitic  sands  with  a  basement-band 
of  green-coated  flints  resting  generally  directly  on  the  Chalk.  This  sandy  member  of 
the  series,  traceable  as  a  definite  platform  through  the  Anglo-French  and  Belgian  area, 
contains  among  its  characteristic  fossils  Pholadomya  cunealaj  P.  Konindcii,  Cypruia 
Morrisii,  Cucullaea  crassiitina^  Pecten  breviauritus,  Psami/wbia  Edwardsii,  Oslrea  belio- 
vacinaj  Corbula  rcgulbkusis,  TurrUdlabcllovacuuiy  Xatica  dcshayesiana,  Voiula  eUpremu 
Higher  in  the  series  comes  the  ''  Argile  plasti«|ne  "  of  the  Paris  basin,  with  the  associated 
lignites  of  the  Soissonnais.  The  molluscan  fauna  of  these  strata  resembles  that  of  the 
Woolwich  and  Reading  beds.  But  a  break  seems  to  occur  in  the  series  at  this  point ;  for 
in  the  Paris  basin  no  representative  of  the  London  Clay  is  found.  The  lignites  of  the 
Soissonnais  are  covered  by  sands  (Sables  de  Cuise  or  du  Soissonnais)  containing,  among 
other  abundant  marine  organisms,  Numm\UUc8  planulaia,  TurrUdla  edUa,  T.  hybrida, 
Crassntdlu  prtfpinqun,  Liwina  stittamn/a  ;  they  are  regarded  as  the  equivalent  of  the 
lower  i)art  of  the  English  Bagshot  Sand,  and  form  the  highest  member  of  the  Lower 
Eocene  stages  of  the  Paris  basin. 

Ill  the  Belgian  area,  some  differences  are  presented  in  the  succession  of  sediments. 
The  strata  of  that  district  have  been  grouped  by  Dumout  into  a  series  of  "systemes." 
Tlie  most  ancient  Tertiary  deposit  of  the  west  of  EurojK^  api>ears  to  l>e  the  limestone  of 
Moiis  (Systeiiie  Montieii).  This  rock  lies  in  a  denuded  hollow  of  the  Chalk,  and  ha< 
been  found  by  boring  to  be  more  than  300  feet  thick.  It  consists  of  friable  and  compact 
limestone,  charged  with  a  remarkable  series  of  organic  remains.  Upwards  of  400  si)ecies 
of  fossils  Iiave  been  obtained  from  it,  including  marine,  fresh-water,  and  terrestrial 
shells.  Among  them  are  about  200  species  of  gasteropods,  about  125  lamellibranclis, 
and  fifty  polyzoa,  besides  numerous  foraminifers  {Quinqudociilina),  and  calcareous 
al^a*  {Dnctiifi)poray  Acicularia,  kc.)  Two  conspicuous  features  in  this  deposit  are  tin- 
extraordinary  proportion  of  its  new  and  peculiar  species,  and  the  resemblance  of  iU 
fauna,  especially  its  numerous  Cerithiums  and  Turritellas,  to  that  of  the  Middle  Eocene 
beds  of  Belgium  and  tlie  Paris  basin  rather  than  to  that  of  the  Lower  Eocene.  The 
Mens  limestone  has  thus  been  cited  as  an  illustration  of  Barrande's  doctrine  of  colonies.' 

Above  this  deposit  comes  the  "Systeme  Heersien,"  so  named  from  its  development 
at  lleers,  in  Limbourg.  With  a  total  depth  of  about  100  feet,  it  consists  of  (1)  a  lower 
ilivision  of  sandy  beds,  with  Cyprina  planata,  C.  Morristi,  Modiola  eiegans,  and  other 

^   Hehert,  Ann.  Scitmces  Gk>l.  iv.  (1873)  Art.  iv.  p.  14. 

-  Saporta,  Mem.  Soc.  (Mol.  France  (2)  viii.  ;   '  Le  Monde  des  Plantes,'  p.  212  ft  seq, 

^  Briart  anil  Cornet,  Mtni.  Couronn,  Acad.  Roy.  Belg.  xxxvL  (1870) ;  xxxvii.  (1878) ; 
xliii.  (1S80).  Mourlon,  '  Geol.  Belg.'  1880,  p.  192.  Hebert  {Ann,  Sciences  Q46L  iv.  1873, 
p.  15)  lias  noticed  an  affinity  to  the  uppermost  Cretaceous  fauna  of  Paris. 


SECT,  i  §  2 


EOCENE  SYSTEM 


977 


mariue  shells,  some  of  which  occur  in  the  Thanet  Sand  of  England  and  the  Sables  de 
Bracheux  ;  and  (2)  an  upper  division  of  marls,  containing,  besides  some  of  the  marine 
shells  found  in  the  lower  division,  numerous  remains  of  a  terrestrial  vegetation  {Osmunda 
eocenicaf  ChamaRcyparis  be/gica,  PoacUes  IcUissimus,  aud  species  of  Querciis,  Salhc^ 
Cinnanuyinum^  Lauriis,  Vihumuvi^  Hedera,  Aralia^  &c.)* 

The  **Syst^me  Landenien,"  corresponding  to  the  Thanet  and  Woolwich  and 
Reading  beds  of  England  and  the  Sables  de  Bracheux,  Argile  plastique,  and  Lignites 
du  Soissonnais  of  France,  is  divisible  into  two  stages :  1st,  Lower  marine  gravels, 
conglomerates,  sandstones,  marls,  &c.,  with  badly  preserved  fossils,  among  which  arc  Tur- 
ritella  bdlovaciTUif  Cucullaa  deciuscUa  {erassatina)^  Cardium  Edwardsi^  Cyprina  plaruiUi, 
Corbula  rcgulbiensis,  Pholadomya  Koninckii;  2nd,  Upper  fluvio-marine  sands,  sandstones, 
marls,  and  lignites  containing  Melania  inquinata,  Melanopsis  buccinoidcSf  CerUhiumfuiw- 
luniy  Ostrea  bellovaciiia-j  Cyrena  cunei/ormis,  with  leaves  and  stems  of  terrestrial  plants. 

The  **  Systeme  Ypr^sien  "  consists  of  a  great  series  of  clays  and  sands  answering 
generally  to  the  London  Clay,  but  not  represented  in  France.  It  is  divided  into  two 
stages  :  1st,  Lower  stiff  grey  or  brown  clay  (Argile  de  Flanders  on  d*Ypres),  sometimes 
becoming  sandy,  and  probably  an  eastward  extension  of  the  London  Clay.  The  break 
between  this  deposit  and  the  top  of  the  I^ndenian  beds  below  is  regarded  as  filled  up 
by  the  Oldhaven  beds  of  the  London  basin.  The  only  recorded  fossils  are  foraminifera 
agreeing  with  those  of  the  London  Clay.  2nd,  Upper  sands  with  occasional  lenticular 
intercalations  of  thin  greyish-green  clays,  with  abundant  fossils,  the  most  frequent-  of 
which  are  Nummulites  planulata  (forming  aggregated  masses),  Turriteila  edita,  T. 
hybnddf  Vermetus  bognorensiSf  PecUn  cameus,  Pedinicvlus  decussatus^  Lucina  sq^tamuia, 
Ditnipa  plana.  Out  of  72  species  of  mollusks,  45  are  found  also  in  the  Sables  de  Cuise 
and  20  in  the  London  Clay.* 

The  "Systeme  Paniselien,"  so  named  from  Mont  Panisel  near  Mous,  consists  chiefly 
of  sandy  deposits  not  markedly  fossiliferous,  but  containing  among  other  forms  Hostel- 
iarUt  Jissurclla,  Valuta  elevata,  Turriteila  Dixon i,  Cythcrea  avibiguu,  Lucina  squamula. 
Out  of  129  species  of  moUusca  found  in  this  deposit,  91  appear  in  the  Sables  de  Cuise, 
and  only  36  pass  up  into  the  Calcaire  Grossier.  Hence  the  Paniselian  beds  ai*e  placed  at 
the  top  of  the  Lower  Eocene  stages  of  Belgium. 

Middle  Eocene. — This  division  in  the  Paris  basin  is  formed  by  the  characteristic, 
prodigiously  fossiliferous  Calcaire  Grossier,  which  is  subdivided  as  under  :^ — 


eS 

5s 


o 

■mm      U, 


as 


Upper  sub- 
group with 
Cardium  ohli- 
quum  aud  Ce- 
rithiuni  den- 
tiadatuiu. 


Middle 
group 


8ub- 
with 
Lucina  »axo- 
rum  aud  Mi- 
liola. 

Lower  sub- 
group with 
Ceriihium  la- 
piduin  and 
Miliola. 


'  4.   Lime^itone    with    Cardium    obUquum    and    Cerithium 
Blainvilli. 

3.  Limestone  with  Cerithium    denticulatum    and  C,  cris- 

latum. 
'1.  Siliceous  limestone  witli  uudetermined  forms  of  Pota- 
mides. 
Ll.  Coral  limestone  (^y/oca>a*a). 

4.  Siliceous  limestone  with  parting  of  laminated  marl. 

3.  Limestone  in  small  thin  boards  with  Corbula  (Kochette). 
2.   Limestone  with  Miliola  aud  Lucina  saxorum  (Roche). 

1.  Siliceous    limestone  with  indeterminate  fossils  (Bancs 

francs). 

4.  Limestone  (doloniitic)  with  Afilida  (Cliquart). 
(  Green  marl     .... 

3. -|  Siliceous  limestone  in  two  beds 
\^  Green  marl     .... 

2.  Miliola  limestone  (doloniitic)  (Saint  Noni). 
1.  Siliceous  limestone  with  Potamides. 


-Blanc  vert. 


*  De  Saporta  and  Marion,  M^m.  Cour,  Acad.  Roij.  licltj.  xli.  (1878). 
^  Mourlon,  *Geol.  Belg.'  p.  211. 

3  Dollfus,  Bull.  Soc.  Geol.  France,  3«  ser.  vi.  (1878)  p.  269.     Compare  Michelet,  f*/>.  cit. 
2«  s«r.  xii.  p.  1336. 

\\  11 


978 


STRATIGRAPHICAL  GEOLOGY 


BOOK  VI  PABT  IT 


'  i). 


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GO 

V   o 

go 


>-l 

■3    . 

CS    •iM 

CO 

*-  2 

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Limestone  with  Luciiut  conceniricaf  Area  barbcUtUa,  Cardium  aviculartf 
Miliola^  &c. 

4.  Limestone  with  Orbitolites,  Fusus  bulbifonnisj  Volvaria  buUaidet,  Car- 

dium granulosuvif  Area  quadrilatera,  several  species  of  Urge  JFtustra 
or  Membranipora, 

3.  Limestone  with  Fahularia  and  terrestrial   vegetation  {OfbUoliU»  cam- 

plan  ata,  Chama  caicaraia,  Cardtia  imbricata^  &c.) 

2.  Mass  of  Miliola  limestone  {Turritella  imbricaiaria^  Chama  calcarata, 

Lucina  mutabUiSt  &c. ) 

1 .  Limestone  with  MUiola  and  Terdbratula  ( T,  bisinuata). 

5.  Glauconitic  calcaire  grossier  with  Cerithium  giganteum. 

4.  Glauconitic  calcareous  sand  with  Lenita  pateUaris. 

3.  Sandy  glauconitic  calcaire  grofisier  with  Cardium  porulofum. 

2.  Sandy  glauconitic  calcaire  grossier,  with  Nummulitea  lavigataf  N.  scabra, 

Ostrea  miUticostataf  0.  fiabeiluki,  Ditrupa  plana. 
1.  Glauconitic  sand,  sometimes  calcai^ous  and  indurated,  with  pebbles  of 
green  quartz,  shark's  teeth,  and  rolled  fragments  of  coraL 


Cm 


In  Belgium  the  Middle  Eocene  presents  a  different  aspect  from  that  of  Paris, 
approximating  rather  to  the  English  Type.  It  consists  of  (1)  a  lower  set  of  sandy  beds 
grouped  under  the  name  of  *'  Bruxellien,"  rich  in  fossils,  which,  however,  are  usually 
badly  preserved.  Among  the  forms  are  remains  of  terrestrial  vegetation  {Nxpa  Buriini), 
also  Paracyathiis  crassus^  Maretia  grignonensis,  Pyripora  cotUesta^  Ostrea  cymbula. 
Cardial  dcaissata,  Chama  calcarata^  Cardium  porulosum^  CerUhium  unisulcatum,  NcUica 
labellata^  Volutu  lincolaj  AncillaHa  bucciiuyidts^  Clavalithes  (Fnsus)  langa^us,  numerous 
remains  of  fishes,  especially  of  the  genera,  MyliobateSf  OdontaspiSf  Lamna,  OaUooerdo,  and 
various  reptiles,  including  species  of  Tricnyx  and  ChelonCy  with  Emys  Camperi,  Gari- 
alis  Dixoni,  and  Palseophis  typhsBus ;  (2)  a  group  of  sands  and  fossiliferous  calcareous 
sandstones  (''  Lackenien"),  made  up  of  Ditrupa  strangulata  and  Nummulitea  {N.  Imvi- 
gatGj  N,  smbra,  N.  H^berti^  N,  variolaria\  and  abounding  in  Anomia  suhlmvigata. 

Upper  Eocenk.— In  the  Paris  basin  this  subdivision  consists  of  the  following 
stages :  ^ — 

^Gypsum  with  nodules  of  silica  (menilite),  and  containing  marine  fossils 
{Cerithium  tricarinaium,  O.  pleurotoinoides^  Turntella  incerta). 

Yellow  marls  with  Lucina  inornaUi. 

Gypsum,  saccharoid  and  crystallized,  w^ith  brown  marls. 

Yellow,  brown,  and  greenish  marls,  with  Ph<tladomya  ludensis,  CroMaUlla 
Desmarestiy  kc. 

Green    sands   of    Monceaux    {Cerithium    Cordieri,    C.    tricarinaium^    Natiea 

parisiensis). 
Limestones  of  Saint   Ouen — a  marly  fresh-water  rock  20  to  26  feet  thick, 

composed  of  two  zones,  the  lower  full  of  Bythiniay  and  the  upper  abounding 

in  Limnnm. 
Sands  of  Mortefoutaine  {Atnc\da  De/rancei). 
Sands  and  sandstones  of  Beauchamp  {Cerithium  mtUabile,  C.  tuberculosum,  C. 

Boueiy  Melauia  hordacea,  M.  lactea,  Cyrena  deperdita^  Planorbis  nitidulus, 

Corbida  gailicaj  &c.) 
Sands,  &c.,  with  Nummulitea  variolaria,  Ostrea  dorsatUy  Cyrena  deperdita^ 

corals,  Lamna  degans^  Odontaspis  {OUtdus)  cbliquus^  &c. 

Northwards  in  the  Belgian  area,  near  Brussels,  the  highest  Eocene  strata 
consist  of  sands  and  calcareous  sandstones  ("  Wemmelien "),  separated  from  the 
similar  Lackeuiau  beds  below  by  a  gravel  full  of  Nummulites  variolaria.  Other 
common  fossils  are  Turbinolia  sulcata^  Corbula  pisum,  Cardita  sulcata,  Turritella  brevis, 
Clavalithes  {Fusiu^i)  longxvus. 

Receding  from  the  Paris  basin,  the  Eocene  deposits  assume  entirely  different 
characters  as  tliey  are  traced  into  the  west,  centre,  and  south  of  France.     According  to 


X 

>^ 

C 

— s 

!J2 


'  See  Dollfus,  op.  cit. 


SECT,  i  §  2  EOCENE  SYSTEM  979 

Vasseur's  detailed  researches,  a  long  irregular  arm  of  the  sea  penetrated  Brittany  in 
Eocene  times  from  where  the  Loire  now  enters  the  Atlantic,  while  the' north-western 
l>art  of  Vendee  was  likewise  submerged.  In  these  waters  a  series  of  limestones  and 
sands  was  deposited,  which  from  their  fossil  contents  appear  to  be  the  equivalents  of 
the  Calcaire  Grossier.  They  pass  up  into  lacustrine  and  brackish-water  beds  like  the 
corresponding  groups  at  Paris.*  In  the  south  of  France,  the  Eocene  rocks  chiefly 
present  the  nummulitic  facies  to  be  immediately  referred  to,  and  in  some  places  attain 
a  great  development,  as  near  Biarritz,  where  they  are  more  than  3000  feet  thick. 

Southern  Europe. — The  contrast  between  the  facies  of  the  Cretaceous  system  in 
north-western  and  in  southern  Europe  is  repeated  with  even  greater  distinctness  in  the 
Eocene  series  of  deposits.  From  the  Pyrenees  eastwards,  through  the  Alps  and 
Apennines  into  Greece  and  the  southern  side  of  the  Mediterranean  basin,  through  the 
Carpathian  Mountains  and  the  Balkan  into  Asia  Minor,  and  thence  through  Persia  and 
the  heart  of  Asia  to  the  shores  of  China  and  Japan,  a  scries  of  massive  limestones  has 
been  traced,  which,  from  the  abundance  of  their  characteristic  foraminifera,  have  been 
called  the  Nummulitic  Limestone.  Unlike  the  thin,  soft,  modern-looking,  undisturbed 
beds  of  the  Anglo-Parisian  area,  these  limestones  attain  a  depth  of  sometimes  several 
thousand  feet  of  hard,  compact,  sometimes  crystalline  rock,  passing  even  into  marble  ; 
and  they  have  been  folded  and  fractured  on  such  a  colossal  scale  that  their  strata  have 
been  heaved  up  into  lofty  mountain  crests  sometimes  10,000,  and  in  the  Himalaya  range 
more  than  16,000,  feet  above  the  sea.  With  the  limestones  is  associated  the  sandy 
series  known  as  Nummulite  Sandstone.  The  massive  unfossiliferous  Vienna  sandstone 
and  Flysch,  already  referred  to  as  probably  in  part  Cretaceous,  are  no  doubt  also  partly 
referable  to  Eocene  time.^  One  of  the  most  remarkable  features  of  these  Alpine  Eocene 
deposits  is  the  occun-ence  in  them  of  coarse  conglomerates  and  gigantic  erratics  of  various 
crystalline  rocks.  As  far  east  as  the  neighbourhood  of  Vienna,  and  westward  at  Bolgen 
near  Sonthofen  in  Bavaria,  near  Habkeren  and  in  other  places,  blocks  of  granite, 
granitite,  and  gneiss  occur  singly  or  in  groups  in  the  Eocene  strata.  These  travelled 
masses  appear  to  have  most  petrographical  resemblance,  not  to  any  Alpine  rocks 
now  visible,  but  to  rocks  in  southern  Bohemia.  Their  presence  may  jiossibly 
indicate  the  existence  of  glaciers  in  the  middle  of  Euroi)e  during  some  part  of  the 
Eocene  age.*  Another  interesting  Eocene  deposit  of  the  Alpine  region  is  the  coal- 
bearing  group  of  Haring,  in  the  Northern  Tyrol,  where  a  seam  of  coal  occurs  which, 
\nth  its  partings,  attains  a  thickness  of  32  feet. 

1  G.  Vasseur,  Ann.  Set.  0(ol,  xiii.  (1881).  Hel)ert,  Bull.  Soc.  Qeol.  France  (3)  x. 
(1882)  p.  364. 

-  The  history  of  the  Flysch  has  given  rise  to  some  discussion.  Th.  Fuchs,  for  instance, 
regarded  it  as  having  probably  been  derived  from  eruptive  discharges  such  as  those  of 
mud  volcanoes  {Sitz.  Akad.  WUn,  Ixxv.  1877,  p.  340  ;  Verh.  Geal.  Reichsansi.  1878, 
p.  135).  This  view  was  opposed  by  K.  M.  Paul,  wlio  looked  on  the  Flysch  as  a  normal 
sedimentary  formation  {Jahrb.  Geol.  Heichmnst.  1877,  p.  431  ;  Verh.  Ged.  Rtichaanst.  1878, 
p.  179).  By  some  geologists  the  rocks  have  been  regarded  as  a  deep-sea  de))osit,  by 
others  as  an  accumulation  in  shallow  water  (Renevier,  Arch.  Sci.  Phys,  Nat.  Oenevcu, 
(3)  xii.  1884,  p.  310).  See  also  Mantovani,  Neues  Jahrb.  1877  ;  Schardt  and  Favre, 
•  Description  Gdol.  des  Prealpes  du  Canton  de  Vaud,'  &c.  1887.  Kauffmann,  *  Description  de 
la  partie  nord-ouest  de  la  feuille  xii.  de  la  Carte  G^ol.  Suisse,'  1886.  F.  Sacco,  Btdl.  Soc. 
Beige  de  Gid.  iii.  (1889)  p.  153.  C.  Mayer-Eymar,  '  Versuch  einer  Classification  der  tertiar 
Gebilde  Europas,'  Verh.  Schweitz.  Natur/.  Ges.  1857. 

^  That  a  glacial  period  occurred  at  the  close  of  the  Cretaceous  period,  again  at  the  eud  of 
the  Eocene  aud  in  the  Miocene  (erratics  of  Superga,  near  Turin)  has  been  regarded  by  some 
geologists  as  probable  (A.  Vezian,  Rev.  Sci.  xi.  (1877)  p.  171  ;  Schardt,  'Etudes  G«k>logiqueH 
sur  le  pays  d'Enhaut  Vaudois,*  Bull.  Soc.  Vaud^  1884). 


980 


STRA  TIGRA  PHIGA  L  GEOLOG  Y 


BOOK  V'l  PAST  IV 


The  Numniulitic  series  has  been  divided  into  stages  in  different  regions  of  iu 
distribution,  and  attempts  have  been  made  by  means  of  the  included  fossils  to  parallel 
these  stages  in  a  general  way  with  the  subdivisions  in  the  Anglo-Parisian  basin.  But 
the  conditions  of  deposition  were  so  different  that  such  correlations  must  be  regarded 
as  only  wide  ai>proximations  to  the  truth.  In  the  Northern  Alps  (Bavaria,  &c.)  Giimbel 
arranges  tlie  Eocene  series  as  under : ' — 

Fly  sell  or  Vienna  sandstone  (Upper  Eocene),  including  younger  Nummulitic  beds 
and  Hiiring  beds. 

Lower  Nummulitic  group.  Kresseuberg  b^s — greenish  sandy  strata  abounding 
in  fossils,  which  on  the  whole  point  to  a  correspondence  with  the  Calcau'e 
Grossier. 

Burberg  beds — greensand  with  small  Nuumulites  and  Exogyra  Bt-angniarfi, 
answering  possibly  to  the  upper  part  of  the  lower  Eocene  beils  of  the  Anglo- 
Parisian  area. 

In  the  southern  and  south-eastern  Alps  the  Eocene  rocks  attain  a  much  larger 
development.     The  following  subdivisions  in  descending  order  have  been  recognised  :  - 


Cm  y 


'  Macigno  or  Tassollo,  having  the  usual  character  of  the  Vienna  sandstone. 

No  fossils  but  fucoids. 
^  FoSvSiliferous  calcareous  marls  and  shales,  and  thick  conglomerates. 
Chief  Numniulite  limestone,  containing  the  most  abundant  and  varied  de- 
velopment of  nummulites,  and  attaining  the  thickest  mass  and  widest 
geographical  range. 

•{  Borelis  (Alveolina)  limestone,  containing  numerous  large  foraminifera  of  the 
genus  Birrelis. 
Lower  Nummulitc  limestone,  with  small  nummulitei>,  and  in  many  places 
banks  of  corals. 

Upiier  Foraminiferal  limestone,  containing  also  intercalations  of  fresh- water 

beds  {Chara). 
Cosina  beds,  with  a  peculiar  fresh -water  fauna  {StrmnaUvpsis,  MeUinio, 

C  kit  I'd.,  kc.) 
Lower  Foraminiferal  limestone,  with  numerous  marine  raollusca  {Annmui, 

CtrUhium,  &c.),  and  occasional  beds  of  fresh -water  limestone  {Cfuira, 

Melanin^  &e. ) 


^ 


In  the  central  ])ai't  of  the  nortliern  ApenniHes  Professor  Sacco  n-gaixis  as  Eocene  a 
mass  of  strata  5500  feet  thick,  which  he  subdivides  as  follows-^ : — 


Bartouiun. 
100  metres. 


Parisian, 
1500  metres. 


Grey  marls  with  sandy  calcareous  layers  ;  numerous  fossils  {Zoophy- 
cn-f,  Lithothainniuniy  Nummulites  Tchihatchtffiy  N.  striata,  Orfn- 
tui(.h's  rculiansy  OjMrrculinUj  corals,  bryozoa,  crinoids,  &c.) 

'A  tliick  series  of  marly  and  shaly  limestones  (Flysch),  alternating 
with  sandstones  {llelminikoviei(  labyrinthica,  VhoiuiritKs  and 
other  fucoids).     Rooting  slates. 

Shales  and  sandstones  (Macigno). 

Sandy  greyish  and  brownish  marls  with  calcareous  sandy  beds 
{Lithothamnium,  Nwnmvlitcs  biarritzensi'S,  X.  Lavwrcki,  y. 
fxcasana,  AssUina  exponeas,  A.  ffiuniidosu,  Orbitoidesj  Opcrai- 
linay  Alveoluia,  corals,  echini,  crinoids,  tish-teeth,  &c.) 


Suessonian. 
100  metres. 


I  She 


lies  and  grey  and  brown  marls,  sandstones  and  limestones. 


'  •  Geognostische  Beschreib.  Bayerisch.  Alpen,'  1861,  p.  593  et  seq, 
-  Von  Hauer,  'Geologic,'  p.  569.  For  an  exhaustive  account  of  the  stratigraphy  and 
jiulfeoiitology  of  the  Liburnian  stage,  see  G.  Stache's  great  monograph,  'Die  U])urnische 
Stufe,'  Abhandl.  k.  k.  O'eul.  RcicJisanst.  xiii.  1889.  On  the  classification  of  the  older  Tertiary 
formations  of  Austria,  consult  Tietze,  Zeitsch.  JJei'tscb.  Oeol.  Ges.  xxxvi.  (1884)  p.  68  ; 
xxxviii.  (1886)  p.  26  ;  T.  Fuch.s,  op.  cit.  xxxvii.  (1885)  p.  131. 

•*  Prof.  Sacco  has  contributed  many  papers  on  this  subject.    See,  for  example.  Bull.  Sue. 
tweul.  France  (3)  xvii.  (1889)  p.  212. 


SECT,  i  §  2  EOCENE  SYSTEM  981 

To  the  Upi)er  Eocene  series  of  this  region  has  been  assigned  a  great  series  of  serpen- 
tines, gabbros,  diabases,  soda-potash  granites,  and  other  eruptive  rocks,  with  tuflfs  and 
conglomerates,  marking  copions  submarine  volcanic  activity.^ 

India,  ftc. — As  above  stated,  the  massive  Nummulitic  limestone  extends  through 
the  heart  of  the  Old  World,  and  enters  largely  into  the  structure  of  the  more  impoi-tant 
mountain  chains.  In  India  a  tolerably  copions  development  of  Eocene  rocks  has  been 
o])9erved,  but  it  is  not  quite  certain  where  their  upper  limit  should  be  drawn  to  place 
them  on  a  parallel  with  the  corresponding  groups  in  Europe.  The  following  sub- 
divisions in  descending  order  are  observed  in  Sind  :  - — 

Xari  group.  Sandstones  without  marine  fossils,  and  probably  of  firesh-water 
origin,  4000  to  6000  feet,  representing,  perhaps,  Upper  'Eocene  and  Oligocene 
or  Lower  Miocene  beds  of  Europe. 

Kasauli  and  Dagshai  groups  of  sub- Himalayas. 
Kirthar  group.     A  marine  limestone  formation  in  general,  but  passing  locally 
into  Randstones  and  shales.     The  upi>er  limestones  contain  Numm\dites  garan- 
seusiSf  N.  sublmvigata. 

Nummulitic  limestone  of  Sind,  Punjab,  Assam,  Burmah,  &c.     Subathu  of 
sub-Himalayas,  Indus  or  Shingo  beds  of  Western  Tibet. 
Uanikot  beds — sandstones,  shales,  clays  with  gypsum  and  lignite,  1500  to  2000 
feet  ;   abundant  marine  fauna,   including  NummuUUs  spxra^  N,   irregularis^ 
X.  Leyineriei. 

Lower  Nummulitic  group  of  Salt  Range. 

North  America.  — Tertiary  formations  of  marine  origin  extend  in  a  stri[)  of  low 
land  along  the  Atlantic  border  of  the  United  States  and  Mexico,  from  the  coast  of  New 
.lerscy  southward  into  Florida  and  round  the  margin  of  the  Gulf  of  Mexico,  whence 
they  run  up  the  valley  of  the  Mississippi  to  beyond  the  mouth  of  the  Ohio.  On  the 
western  seaboard  they  also  occur  in  the  coast  ranges  of  California  and  Oregon,  where 
they  sometimes  have  a  thickness  of  3000  or  4000  feet,  and  reach  a  height  of  3000  feet 
alK)ve  the  sea.  Over  the  Rocky  Mountain  region  Tertiary  strata  cover  an  extensive 
area,  but  are  chiefly  of  fresh -water  origin. 

In  the  States  bordering  the  Atlantic  and  Gulf  of  Mexico  the  oldest  Tertiary  de- 
lM)sit8  are  referred  to  the  Eocene  series,  and  in  some  places  (New  Jersey)  appear  to 
follow  conformably  on  the  Cretaceous  rocks.  They  have  been  subdivided  into  four 
groups,  which  in  the  state  of  Mississippi  are  well  develoi)ed,  \%'ith  the  following 
characters :  ^ — 

4.  Jackson  beds  ("White  Limestone"  of  Alabama),  white  and  blue  marls 
underlain  by  lignitic  clay  and  lignite  (80  feet)  with  Zextglodon  macrospondylus^ 
(^ardita.  planicosta^  Cardium  NicoHeti^  Leda  mtdtiiineata,  Corbida  bicarinataf 
Hnstellaria  vtlttta^  Valuta  dumom,  Mitra  dnmosa^  Conns  tortilis,  CyprsML  fents- 
fralisy  &c. 

3.  Claiborne  beds,  white  and  blue  marls,  and  sandy  beds  with  numerous  shells 
which  indicate  a  horizon  equivalent  to  that  of  part  of  the  Calcaire  Grossier  of 
the  Paris  basin. 

2.  Buhrstone  (Siliceous  Claiborne),  sandstones  and  siliceous  impure  limestones 
with  Claiborne  fossils  (400  feet  and  upwards). 

1.  Lignitic  sands  and  clays,  with  marine  fossils,  and  with  interstratified  lignites 
and  ])lant-remains  {Qtierais,  PopuluSf  FicuSf  LaurttSj  Persea^  Cormis,  Oleaj 
Rhamnus,  JditgnoHa^  &c.) 

Over  the  Rocky  Mountain  region  and  the  vast  plateaux  lying  to  the  east  of  that  range 
the  older  Tertiary  formations  consist  mainly  of  lacustrine  strata  of  great  thickness,  the 

*  C.  de  Stefani,  Boll,  Hioc.  Oed.  Ital,  viii.  fasc.  2  (1889)  ;  a  copious  list  of  previous 
writers  on  the  subject  will  be  found  in  this  pai)er. 

-  Medlicott  and  Blanford,  '  Geology  of  India,*  chap.  xix. 

^  A.  Heilprin,  *  Contributions  to  the  Tertiary  Geology  and  Palaeontology  of  the  United 
States,'  1884  ;  Proc.  Acml.  Philadelph.  1887. 


982  STRATIGRAPHICAL  GEOLOGY  book  vi  part  it 


extraordinary  richness  of  which  in  vertebrate  and  particularly  mammalian  remains, 
already  referred  to  (p.  969),  has  given  them  a  high  importance  in  geological  and 
palffiontological  history.  The  following  subdivisions  in  descending  order  were  estab- 
lished some  years  ago  : — 

4.  Uinta  group  (400  feet)  or  '*Diplacodon  beds." 

3.  Bridger  group  (5000  feet)  or  "  Deinoceras  beds." 

2.  Green  River  group  (2000  feet). 

1.  Wahsatch  (Vermilion  Creek)  group  (5000  feet). 

More  recent  researches  in  Colorado  and  elsewhere  have  somewhat  modified  this 
grouping.  In  the  Denver  region  the  so-called  ** Laramie"  series  (p.  958)  has  been  found  to 
consist  of  three  divisions  :  (1)  a  lower  member,  700  to  800  feet  thick,  conformable  with 
the  Cretaceous  Fox  Hills  group,  containing  productive  coal-seams  and  a  flora  and  fanna 
characteristic  of  the  Laramie  group  as  usually  understood  ;  (2)  a  middle  member,  called 
the  Arapahoe  group,  resting  on  the  first  unconformably,  with  a  conglomerate  at  its 
base,  containing  pebbles  of  the  underlying  formation  and  other  older  rocks  ;  (3) 
an  upper  member,  the  Denver  group,  1400  feet  thick,  unconformable  to  the  middle 
division,  and  largely  composed  of  the  debris  of  andesitic  lavas.  The  strong  unoon- 
fonnability  between  the  Laramie  beds  (No.  1)  and  the  Araj)ahoe  group  (No.  2)  is 
believed  to  mark  a  considerable  interval  of  time  between  the  highest  Cretaceous  and 
oldest  Tertiary  deposits  of  this  region.*  In  southern  Colorado  the  Eocene  strata  have 
been  described  as  7000  feet  thick,  resting  unconformably  on  the  Laramie  series.  The 
lowest  member  (Poison  Cafion),  3500  feet  thick,  and  the  next  division  (Cuchara),  300 
feet  thick,  are  classed  as  Lower  Eocene  ;  the  upper  (Huerfano),  8300  feet  thick,  is 
believed  to  be  equivalent  to  the  Bridger  group.' 

Australasia. — Tliough  vast  areas  in  this  region  are  covered  with  strata  which 
sometimes  attain  a  depth  of  several  hundred  feet,  containing  both  terrestrial  and  marine 
deposits,  and  which  are  referable  to  various  parts  of  Cainozoic  time,  no  satisfactory 
correlation  of  the  beds  with  European  equivalents  has  yet  been  made,  if,  indeed,  such  a 
correlation  is  at  all  probable  or  possible.  All  that  can  be  safely  affirmed  is  that  a 
succession  among  these  beds  can  be  traced  with  an  increasing  proportion  of  recent 
species  in  the  younger  parts  of  the  series.  Throughout  the  whole  of  eastern  Australia, 
including  most  of  New  Soutli  Wales  and  Queensland,  no  marine  Tertiary  fossils  have 
been  discovered.  In  the  south-west  of  New  South  Wales  and  in  Victoria,  previous  to 
the  eruption  of  basalt-sheets  and  tuffs,  an  extensive  series  of  conglomerates,  siliceous 
sandstones,  clays,  ironstones,  and  lignites  was  deposited  in  valleys  and  probably  lake- 
basins.  On  the  Dividing  Range  these  strata  rise  to  4000  feet  above  the  sea.  At 
Bacchus  Marsh  in  Victoria  and  elsewhere  they  have  yielded  leaves  of  Launm, 
Cinuanioiniunj  &c.,  some  of  which  closely  resemble  species  found  at  Oeningen.  The 
general  asjMJct  of  this  flora  is  rather  that  of  tropical  than  of  extra-tropical  Australia, 
and  this  indication  of  a  warmer  temperature  than  at  present  is  con'oborated  by  the 
occurrenc*^  of  coral-reefs  in  Tasmania  referred  to  the  Miocene  period.  Above  these 
plant -bearing  beds  which  have  been  regarded  as  Lower  Miocene  or  Upi)er  Eocene, 
marine  deposits  supposed  to  be  Middle  and  Upi)er  Miocene  occur  on  the  flanks  of  the 
Dividing  Range  of  New  South  Wales  up  to  heights  of  800  feet.  In  South  Australia 
and  Victoria  extensive  marine  accumulations  of  clay,  sand,  and  limestone,  often  under- 
lying widcispread  basalt- plateaux,  have  yielded  numerous  foraminifera,  especially  at 
Mount  Gambier  and  Murray  Flats  in  South  Australia  ;  40  species  of  corals,  which  are 
only  slightly  related  to  the  living  species  of  the  surrounding  seas,  but  include  three 

^  Whitman   Cross,  Amer.  Journ.    Sci.    xxxvii.    (1889)  p.    261;   xliv.   (1892)  p.   19; 
Proc.  Colorado  St-t',  Soc.  Oct.  1892. 

2  R.  C.  Hills,  Proc.  Colorado  Sci.  Soc.  iii.  (1888)  p.  148,  (1889)  p.  217  (1891). 


SECT,  ii  §  1  OLIOOCENE  SYSTEM  983 

European  Tertiary  species ;  ^  many  cchinodenns  and  polyzoa,  and  a  large  molluscan 
fauna,  in  which  the  genera  fFcUdheimict^  CucullaBo,  Fectunculits,  Trigonia^  CyprsBa, 
Fusus^  Haliotis,  Murez,  MUra,  Trivia,  Turritella^  VoluUij  &c.,  occur.  The  vertebrate 
organisms  consist  of  fishes  (including  the  world-wide  genera  Carcharodon,  Lanma,  Odon- 
taspiSf  Oxyrhina),  a  few  marsupials  (BetUmgia,  Notothcriunif  Phascolomys,  Sarcophilua), 
with  some  marine  mammalia  {Squalodon,  Arctocephalus).  In  South  Australia  the  older 
Tertiary  deposits  have  been  divided  by  Professor  Tate  into  four  groups,  which  in  ascending 
order  are :  (a)  Inferior  marine  beds,  chalk-rocks,  clays,  and  limestones  ;  (6)  Lower 
Murravian  sandstones  with  Zeuglodon,  Lovenia,  Magascllaf  Megalaster ;  (c)  Middle 
Murravian  limestones  and  sandstones,  with  an  abundant  and  varied  marine  fauna 
{Charcarodcnif  Lamna,  OdontaspiSt  Nassa,  Ancillaria,  Cassis,  Valuta,  Marginella, 
Mangelia,  Cerilhium,  Co7ius,  Cancellaria,  Natica,  Pecten,  Lima,  Spandyhis,  Nucula, 
Limopsis,  Chama,  Chione,  Rhynchonella,  Terebratulina,  Waldheimia,  Terehratula, 
EupatagiLS,  Deltoq/athtis,  &c.  ;  (d)  Upper  Murravian  oyster-beds  and  sandstones 
(Trigonia,  Pectunculus,  Tellina,  Mactra,  Clypeaster,  &c.) 

In  Tasmania  an  important  series  of  older  Tertiary  deposits  has  also  been  found. 
At  the  top,  leaf- beds,  lignites,  and  beds  with  marine  fossils  occur,  associated  with 
extensive  sheets  of  felspar-basalts  and  tuflDs.  Tlie  tuffs  have  yielded  Hypsiprimnus  and 
Phascolomys,  Next  comes  a  great  series  of  sandstones,  clajrs,  and  lignites,  varying 
from  400  to  1000  feet  in  thickness,  and  sometimes,  as  in  the  Launceston  basin,  covering 
an  area  of  at  least  600  square  miles.  This  series  encloses  a  rich  flora,  including  species 
of  oak,  elm,  beech,  laurel,  cinnamon,  and  araucaria,  with  fruits  of  proteaceous, 
sapindaceous,  and  coniferous  trees.  The  fresh -water  and  terrestrial  character  of  the 
deposits  is  further  confirmed  by  the  occurrence  in  them  of  Unio,  Helix,  Vitriiia, 
Buliimis,  &c.  The  third  group  in  descending  order  is  of  marine  origin,  and  is  well  seen 
at  Table  Cape.  It  consists  of  shelly  limestones,  calcareous  sandstones,  coral-rag  and 
pebbly  bands,  and  is  replete  with  fossils,  only  from  1  to  3  per  cent  of  the  shells 
belonging  to  existing  species.  Characteristic  forms  are  Valuta  anticiiigitlata,  Cassis 
sufflatus,  Cyprssa  ArcheH,  Aneillaria  mucronata,  Panopaea  Agnewi,  Waldhtimia 
garibaldiana,  Lovenia  Forbesi,  Cellepora  gambieretisisr 

In  New  Zealand  rocks  believed  to  be  referable  to  the  upper  part  of  the  Eocene  series 
are  mainly  composed  of  a  shelly  calcareous  sandstone  with  corals  and  polyzoa,  which  in 
its  lower  part  passes  occasionally  into  an  imperfect  numraulitic  limestone  (Nummulitic 
beds,  Hutchison's  Quarry  beds.  Mount  BrowTi  beds).  Volcanic  action  was  greatly 
developed  during  the  deposit  of  these  strata  in  both  islands.  I^ence  interbedded  lavas 
and  tuffs  are  frequent,  and  in  the  North  Island  the  calcareous  deposits  are  often  wholly 
replace^l  by  wide-spread  trachyte-flows  and  volcanic  breccias.^  _ 

Section  ii.   Oligocene. 

§  1.  General  Characters. 

The  term  "Oligocene"  was  proposed  in  1854  and  again  in  1858  by 
Professor  Beyrich  *  to  include  a  group  of  strata  distinct  from  the  Eocene 

1  Duncan,  Q.  J.  Oeol.  Soc.  1870,  p.  313.  See  also  the  papers  of  R.  Tate,  F.  M'Coy, 
J.  E.  Tennison  Woods,  R.  Etheridge  jun.,  F.  von  Miiller,  Ettingshausen,  and  R.  M. 
Johnston. 

2  Mr.  R.  M.  Johnston,  Registrar-General  at  Hobart,  Tasmania,  has  published  a  use- 
ful memoir  entitled,  "Observations  with  respect  to  the  Nature  and  Classification  of  the 
Tertiary  Rocks  of  Australasia  "  (1888),  with  references  to  the  principal  sources  of  information 
on  the  subject  of  Tasmanian  Tertiary  geology. 

'  Hector's  •  Handbook  of  New  Zealand,'  p.  28. 

*  Monatsbericht.  Akad.  Berlin,  1854,  pp.  640-666  ;  1858,  p.  51. 


»84  HTRATIGRAFHICAL  GEOLOGY  book  ti  paktit 

formations  of  Fntnce  and  Belgium,  and  which  Lyell  had  classed  as  "Older 
Miocene."  They  consist  partly  of  terrestrial,  partly  of  fresh- water  and 
brackish,  and  partly  of  marine  strata,  indicating  considerable  oeciUaticms 
of  level  in  the  European  area.  They  consequently  present  none  of  the 
massive  deep-water  characters  m  conspicuous  in   some  of   the  Eocene 


9iil)tli visions.  Among  other  geographical  changes  of  which  they  preserve 
the  chronicles  is  the  evidence  of  the  gradual  conversion  of  portions  of  the 
fiea-floor  over  the  heart  of  Europe  into  wide  lake-basins  in  which  thick 

lacustrine  deposits  were  aceumulated.  Some  of  these  lakes  did  not  attain 
their  fullest  ilevelopment  until  the  Miocene  [wriod. 


The  Oligocene  flora,  according  to  Heer,  is  composed  mainly  of  an 
o^'ergi'cen  vegetation,  and  has  characters  linking  it  with  the  living  tropical 
Honis  of  India  itnd  Australia  and  with  the  subtropical  flora  of  America. 
It  includes  some  ferns,  fan-palms,  and  feather-palms  (Sabal,  Phamdtfi), 
iinumlier  of  conifers  (.SV'/(W((i,  Fig.  -132,  &c.),  cinnamon-trees,  evei^reen  oaks, 
rustjird -apples,  gum-trees,  spimlle- trees,  oaks,  figs,  laurels,  willows,  vines, 
iind  pi-oteaceous  shnibs  {Itrijawira,  Dryaiulruidcx). 


SECT,  ii  5  I  OLIGOCENE  SYSTEM  885 

Among  the  mollusca  (Figs.  433,  434)  some  of  the  more  important 
genera  are  Onliea,  PfcUn,  Nuniia,  Girdium,  Mertlrix  {Cytherfa),  Cyreiia, 
VawfUaria,  Murer,  Fvsus,  Tyfihis,  Caains,  Pleumtoma,  Cmais,  Futula,  Cmthiam, 
Jlflanui,  Planorbis.'  Numerous  remains  of  birds  have  been  found  in  the 
lacustrine  beds  of  the  Department  of  the  ADier,  no  fewer  than  66  species 


Fl|(.  4S4.— Ollgocca^  Giul«ropudi. 

n.  )'l>n.>rbi>iciiouiplia1iu,  Suw.  0)1  b,  Tmbnliii  (Cerlthlnin)  pliati,  l^ni.  (i);  c.  PotniiililH 

clnrttu,  How.  (|);  <f,  l.lmna-.  longlw.la,  Uron^i.  (U 

hi'.'ing  been  described,  which  comprise  parroquete,  trogons,  flamingoes, 
ibises,  pelicans,  marabouts,  cranes,  secretary-birds,  eagles,  grouse,  and 
numerous  gallinaceous  birds — a  fauna  reminding  us  of  that  of  the  lakes 
in  Southern  Africa.*  The  mammalia  increase  in  variety  of  forms. 
According  to  Gaudry  the  following  chronological  sequence  of  appearances 
and  disappearances  during  the  Oligocene  period  have  been  noted  ;  ^— 


r.— St     G.'ran.i. 

>uy  (AlliCT),  Cul- 

ile  Bexace   ' 


Appearance  of  the  genera  J(Ai'rf.«friM  (I),  Tiipir,  Palsf- 
ehana,  shrew,  Pleaioaortj,  ilymr<uhnt,  mole,  musk  -n 


stead  bedi. 


PiiUeotherittBi. 
c-Uienun 


j  Appearance  of  the    geuera    Ciidnraillunnm,    Ilyrachiiis, 
I      kntetodon,   AHthraa<tl<eriiim,    Itactythfrium,    Chelim- 
.Miildle.— Calcairf     ilej      Uwrium,    TrtiffuIoHgitt,   l^'phiimrryj;   Hyxmatehiu  ID, 
Urie,  fcc.  I      tleUiaa,  ZhtMoUteriiiui,  TheTeulhtrium,  dog  (I),  oivet, 

I      muten,  Pttrictii,  Plfsi->galt,  jBlumgati,  JUiinJltiphHa, 
\     A'ecrolanur. 

1  nn...-  I         I  -      rAppeanince    of     the     genera     npomum,     Ourropotamui, 

i-ower.  wcuJtnne         Titpindut,     Aaoplot/iirium    {fig.  iSS).    Jiurythmum. 

Wi^of  Vaullua     8t  I       '^""""'^"'^  ^Anchihph«3,  Afothmilum,  OhoOuxrHii. 

^^iXelir  "'   pachyderm..      T],e    camivora    have    .till    partly 

i.ri,lKe  be.l».  1,     nianiupial  character.. 

'  For  n  list  of  British  Oligocene  molluKca,  Me   Mr.  K.  K  Newton's  volume  cited  on 

-  A.  iiilne  Edwanls,  'Oineaux  Foaailee  de  la  France,"  1867-11;  Boyd  Dawkina,  'Early 

11  in  BriUin,'  p.  54. 

'  '  Les  Enchainenieut.  du  Monde  Animal,'  1878,  p.  i. 


986  STRATIGRAPHICAL  GEOLOGY         book  vi  pabt  it 


§  2.  Local  Development. 

BritaixL — Oligocene  strata  are  confined  to  one  small  area  in  this  coontiy.  Thej 
occur  in  the  Hampshire  basin  and  Isle  of  Wight,  where  resting  conformably  npon  the 
top  of  the  Eocene  deposits,  they  consist  of  sands,  clays,  marls,  and  limestones,  in  thin- 
bedded  alternations.  They  were  accumulated  partly  in  the  sea,  partly  in  brackish,  and 
partly  in  fresh  water.     They  were  hence  named  by  Edward  Forbes  **  the  fluvio-marine 


)  t 


Fig.  4S5.— Anoplotherium  commune,  Cuv. 

series,"  and  were  divided  by  him  and  Mr.  Bristow  into  the  following  groups  in  descend- 
ing order : ' — 

Hamstead    Beds. — {h)    Marine   stage    with  CorbiUoy  Cytherea,  Osirea 

calUfera^  Vc^uta^  Natiatj  Cerithunn,  and  Mehmia       .  .  .        81  ft. 

{a)  Fresh -water,  eatuarine,  and  lagoon  stage,  with  t'wio,  Cyi^ena^ 
Cyda-'iy  Palvdhiffj  Hydrobia^  Melania,  Planorbis^  Cerithium  (rare), 
turtles,  crocodiles,  mainiDals,  leaves  and  seeds  .  .  .     225 

Bembridge  Beds. — {b)  Bembridge  marls — a  fresh -water,  estuarine,  and 
marine  series  of  cfays  and  marls,  with  Viviparus  {Palmlina)^  MeLnnia^ 
MdanopsiHy  Limnw(t,  Cyrena^  Cnio^  Ostrai,  Cytherea,  MytUuSy 
Xvcula  ........  70-120 

{(()    Bembridge    Limestone — full    of    fresh -water   shells    {Limnasaf 
PldiiorhiSj  &c. ),   and   sometimes   with    many    land -shells    {Bitliinu^, 
Achat ific,  HeliXy  &c.)     ......  15-25 

Osborne   Beds.  —  Marls,  clays,  shales,  and  limestones,  with  Limnsea^ 

Planarbis,  Palvdina^  Mduiiopsis^  Melania^  Chura^  &c.   .  .         80-110 

Headou  Beds. — (c)  Upper  stage,  consisting  of  fresh- water  clays,  marls, 
and  hands  of  limestone,  with  Potamomya,  Limnauiy  Cyreiia,  Unto, 
Potinindes,  Planorbis,  Palmli/ui^  Bidimiis^  &c.  .  .  .  40-60 

{b)  Middle  stage,  clays,  sands,  loams,  and  limestone,  with  brackish- 
water  and  marine  fossils  {Cerithium^  Pbtnorbis,  Lioinaeti,  Melania^ 
Xatica^  Neritinay  Ostrea^  Cyrena,  &c.)    ....         30-126 

{a)  Lower  stage,  marls,  clays,  sandstones,  and  tufaceous  limestones 
witli  fresh-  and  brackish-water  shells  {Limnsed,  Pahidina,  Planorbis, 
Cyreudy  Potamomyay  kc.)  .....         60-175 


»» 


»f 


i» 


j» 


>» 


1 1 


jl  A  large  number  of  the  marine  mollusca  of  the  Headon  Beds  range  downwards  into 

the  Barton  Clay,  but  about  half  are  peculiar  to  the  Oligocene  series.     Among  the  more 
abundant  forms  in  the  Isle  of  Wight  are  Cythcrea  incrassaUi,  Ostrea  velata,  O.flabellula, 


*  'Cleology  of  the  Isle  of  Wight,'  Mem,  Geol.  Survey y  2nd  edit  p.  124.  The  group- 
ing as  liere  given  has  Ijeen  slightly  modified  by  Mr.  C.  Reid  in  the  course  of  a  re-survey 
of  the  Isle  of  Wight.     Tlie  strata  were  formerly  reganled  as  Upper  Eocene. 


SECT,  ii  §  2  OLIGOCENE  SYSTEM  98' 


Nucula  headoneiisiSf  Cerithium  eoncavum,  Melanopsis  sitb/usiformis,  Buccinum  lahiaium^ 
Murex  sexderUatuSj  Nerita  aperta^  NerUina  concava^  Ancillaria  huccinoides^  Mdania 
muricata,  and  several  species  of  Cancellaria^  Naticn^  Pleurotoma,  and  FbZw/a,with  Balanus 
ungui/ormis.  The  estuarine  and  fresh-water  strata  are  marke<l  by  species  of  Potamomya 
and  Cyrcna,  while  the  purely  fresh-water  deposits  are  full  chiefly  of  Limnseids  belong- 
ing to  the  genera  Limnma  and  Planorbis,  L.  longiscata  and  P.  eiuymphalus  being  per- 
haps the  most  abundant  and  conspicuous  species  ;  Paludina  lenta  is  also  plentiful.  Mr. 
Reid  has  remarked  that  every  variation  in  the  salinity  of  the  water  seems  to  have 
affected  the  molluscan  fauna  of  the  estuary  in  which  these  deposits  were  accumulated. 
When  the  water  was  quite  fresh  the  pond  snails  flourished  in  abundance,  and  their 
remains  were  mingled  with  those  of  Unio  and  Helix,  The  gradual  inroad  of  salt  water 
is  marked  by  the  advent  of  Potanumiya,  Cyrniay  CeHthium  {Polamides)y  Melania,  and 
Melanopsis,  while  the  thoroughly  marine  fauna  with  volutes  and  cones  shows  when  the 
sea  had  entirely  replaced  the  fresh  water.  ^ 

The  Bembridge  Limestone,  one  of  the  most  conspicuous  members  of  the  Oligocene 
series  in  the  Isle  of  Wight,  is  a  remarkable  example  of  a  fresh-water  limestone,  full  of 
fresh- water  and  terrestrial  shells  and  nucules  of  Chara.  The  land-shells  comprise  tropical - 
looking  gigantic  species  of  Bulimus  and  Achatina.  An  interesting  feature  in  the  over- 
lying Bembridge  marls  is  the  occurrence  of  a  thin  band  from  two  inches  to  two  feet  in 
thickness  of  a  flne-grained  limestone  like  lithographic  stone,  containing  many  insect- 
remains  with  leaves  and  fresh-water  shells.  Some  twenty  genera  of  insects  have  been 
detected  in  it,  including  forms  of  coleoptera,-  hymenoptera,  lepidoptera,  diptera, 
neuroptera,  orthoptera,  and  hemiptera.* 

The  Hamstead  (formerly  Hempstead)  beds  form  an  interesting  close  to  the  Oligocene 
series.  They  consist  chiefly  of  fresh-water,  estuarine,  and  lagoon  deposits.  But  they 
pass  upward  into  a  group  of  marine  strata  of  which  only  about  30  feet  are  now 
visible.  Among  the  more  abundant  or  peculiar  of  the  shells  in  this  marine  band  the 
following  may  be  mentioned  :  Ostrea  cyathula,  0.  adlaUi  (both  peculiar),  Cytherea  Lycllii, 
Corhula  pisum,  C.  vectensis,  Cuma  Charlesicorthiy  Valuta  Bathieri,  Cerilhium  plicatum, 
C.  SedgwicHif  C.  inomatum,  Strehloccras,^ 

Considerable  interest  attaches  to  the  marine  band  forming  the  middle  division  of 
the  Headon  beds,  as  it  serves  for  a  basis  of  correlation  between  the  English  strata  and 
iheir  equivalents  on  the  Continent.  The  band,  so  well  seen  in  the  Isle  of  Wight, 
occurs  also  at  Brockenhurst  and  other  places  in  the  New  Forest.  It  has  yielded 
more  than  230  species  of  fossils,  almost  all  marine  mollusks,  but  including  also 
14  species  of  corals.  Of  these  organisms,  a  considerable  proportion  is  common  to  the 
Lower  Oligocene  of  France,  Belgium,  and  Germany,  and  22  species  are  found  in  the 
Upper  Bagshot  beds.* 

The  Oligocene  or  fluvio-marine  series  of  the  Hampshire  basin  has  likewise  yielded 
vertebrate  remains  such  as  characterise  the  corresponding  deposits  of  the  Continent. 
They  include  those  of  rays  {Myliohates),  snakes  {Pala^ryx\  crocodiles,  alligators,  turtles 
{Emys,  TrionyXy  numerous  species),  and  a  cetacean  {Balaiwptera) ;  while  from  the 
Bembridge  beds  have  come  the  bones  of  a  number  of  the  characteristic   mammals 

*  C.  Reid,  *  Geologj'  of  the  Isle  of  Wight,'  Mem.  Oeiil.  Survey,  p.  147. 

^  H.  Woodward,  Qiiart.  Joum.  Geol.  Sac.  xxxv.  p.  342.  C.  Reid,  'Geolog)'  of  the  Isle 
of  Wight,' p.  177. 

'  C.  Reid,  op.  cit.  p.  206. 

*  A.  von  Koenen,  Q.  J,  Oeol.  Soc.  xx.  (1864)  97.  Duncan,  op.  cit,  xxvi.  (1870)  p.  66. 
J.  W.  Judd,  op.  cit.  xxxvi.  (1880)  p.  137  ;  xxxviii.  (1882)  p.  461.  H.  Keeping  and  E.  B. 
Tawney,  op.  cit.  xxxvii.  (1881)  p.  85  ;  xxxix.  (1883)  p.  566.  E.  B.  Tawney,  Oed.  Mag. 
1883,  p.  157.  W.  Keeping,  Oeol.  Mag.  1883,  p.  428.  J.  W.  Elwes,  Brit.  Assoc.  1882. 
Sects,  p.  539. 


J>88  STRATIGRAFHICAL  GEOLOGY  book  n  past  it 

'  Aiirhi/ojtht'Sj  Aufhraetjihtrium,  AnopfciMerittm,  two  species,  FoimothcrimuL,  ox  or  mora 
species,  Chotropoiamvs,  £Hch4xloH),  The  top* of  the  flnvio-marine  series  in  the  Isle  of 
Wight  has  been  remored  in  denndfttion,  so  that  the  records  of  the  rest  of  the  OIi|90ceBe 
{leriod  have  there  entirely  disappeared. 

For  many  years  it  was  cnstomary  to  consider  as  Miocene  certain  plant -bearing  strata, 
of  which  a  small  detached  basin  occurs  at  Borey  Tracey,  Deronsbixe,  bat  which  are 
mainly  distributed  in  the  great  Tolcanic  plateaux  of  Antrim  and  the  west  of  Scotland. 
These  strata  have  since  been  regarded  as  equivalents  of  what  are  now  termed  Oligocaie 
formations  on  the  Continent.  At  the  Bovey  Tracey  locality,  which  is  not  more  than  80 
miles  from  the  Eocene  leaf- beds  of  Bournemouth  and  the  Isle  of  Wight,  a  small  bat 
interesting  group  of  sand,  clay,  and  lignite  beds,  from  200  to  300  feet  thick,  lies 
lietween  the  granite  of  Dartmoor  and  the  Greensand  hills,  in  what  was  evidently  the 
hollow  of  a  lake.  From  these  beds,  Heer  of  Zurich,  who  has  thrown  so  much  light  oa 
tlie  Tertiary  floras  of  both  the  Old  World  and  the  Xew,  described  abont  50  specks 
of  plants,  which,  in  his  opinion,  place  this  Devonshire  group  of  strata  on  the  same  geo- 
logical horizon  with  some  part  of  the  Molasse  or  Oligocene  (Lower  Miocene)  groups  of 
Switzerland.  Among  the  species  are  a  number  of  ferns  {Lastrxa  siiriaca^  PeeopUru 
{Offrninida)  lignitum^  kc)  ;  some   conifers,  particularly  Seqnoia  Conttsim,  the  matted 

•  iebris  of  which  forms  one  of  the  lignite  beds  ;  cinnamon-trees,  evergreen  oaks,  custard- 
apples,  eucalyptus,  spindle-trees,  a  few  grasses,  water-lilies,  and  a  palm  {P(»lmaintet\ 
leaves  of  oaks,  tigs,  laurels,  willows,  and  seeds  of  grapes  have  also  been  detected — the 
whole  vegetation  implying  a  subtropical  climate.*  More  recently,  however,  Mr.  Starkie 
(rardner  has  expressed  the  opinion  that  this  flora  is  on  the  same  horizon  as  that  of 
Bonmemonth,  that  is,  in  the  Middle  Eocene  group.^  If  this  view  were  established,  the 
volemic  rocks  of  the  north-west,  with  their  leaf-beds,  might  be  also  relegated  to  the 
Ko<?ene  period.  In  the  meantime,  however,  they  are  placed  in  the  Oligooene  series  as 
probable  equivalents  of  the  brown-coal  and  molasse  of  the  Continent. 

The  plateaux  of  Antrim,  Mull,  Skye,  and  adjacent  islands  are  composed  of  successive 
outp>ourings  of  basalt,  which  are  prolonged  through  the  Faroe  Islands  into  Iceland,  and 

♦  von  far  up  into  Arctic  Greenland.  In  Antrim,  where  the  great  basalt  sheets  attain  a 
thickness  of  1200  feet,  there  occurs  in  them  an  intercalated  band  abont  30  feet  thick, 
consisting  of  tuffs,  clays,  thin  conglomerate,  plsolitic  iron-ore  and  thin  lignites.  Some 
of  tliese  layers  are  full  of  leaves  and  fruits  of  terrestrial  plants,  with  occasional  insect- 
remains.  According  to  the  data  collected  by  a  Committee  of  the  British  Association, 
upwards  of  thirty  species  of  plants  have  been  obtained,  including  conifers  {CiipressinorylotL, 
Ti(jxmIIii„\^  Srquoia^  Finus)y  monocotyledons  {Ph irigTnUes,  PxiciteSy  Iris),  dicotyledons 
(Sn/t'j',  pnpuln'i,  Aliiii.^^  L'onjhm,  Qi'crcuSj  Fagns  {^\  P/ntanuSj Sass^ifras,  Acer,  Andrtntuda, 
Vihii  ran  illy  Arafia,  Xyaaa,  Magnolia,  Phamnus,  Jvglans,  &c.)*  In  the  west  of  Scotland 
the  volcanic  sheets  attain  still  greater  dimensions,  reaching  in  Mull  a  thickness  of  3000 
fret,  and  there  also  including  thin  tuff's,  leaf-l)eds,  and  coals.  In  Mull,  Skye,  and 
Antrim,  the  terraces  of  basalt,  with  occasional  comparatively  thin  bands  of  tuff,  form  a 
noble  example  of  the  extravasation  of  great  piles  of  lava  without  the  formation  of 
« entral  cones  or  the  discharge  of  much  fragmentary  matter  (p.  258).  They  have  been 
inva<led  by  huge  boss^-s  of  gabbro  and  of  various  granitoid  rocks,  which  send  veins  into 

»  Phil.  Trans.  1862. 

-  "British  Eocene  Flora,"  Palx^mt.  Soc.  1879,  p.  18.  See  also  Q.  J.  OwL  Soe.  ill 
I'.  Si.  The  great  uncertainty  in  the  correlation  of  dejwsits  by  means  of  land-plants  has 
been  already  referred  to  (pp.  660,  668,  959). 

'  W.  H.  Baily,  Brit.  Assr^.  1879,  Rep.  p.  162  ;  1880,  p.  107;  1881,  p.  152.  On  tlie 
north  <oast  of  Antrim,  jiear  Ballintoy,  a  band  of  tuff"  occurs  about  150  feet  thick.  But  iu 
Ireland,  a-«  in  Scotland,  the  tuffs  take  quite  a  subordinate  place  among  the  great  pfles 
of  basalt. 


SECT,  ii  §  2 


OLIGOCENE  SYSTEM 


989 


aud  alter  the  basalt.  They  are  likewise  traversed  by  veins  of  pitchstone,  but  more  espe- 
cially by  prodigious  numbers  of  basalt-dykes,  which  in  Scotland  have  a  prevalent 
W.N.W.  and  KS.E.  direction.  The  basalt-plain  was  channelled  by  rivers,  and  into  the 
ravines  thus  eroded  streams  of  pitchstone  made  their  way  (Scuir  of  Eigg),  whence 
it  is  evident  that  the  volcanic  eruptions  lasted  during  a  protracted  period.^ 

France. — In  the  Paris  basin,  where  a  perfect  upward  passage  is  traceable  from 
Eocene  into  Oligocene  beds,  the  latter  are  composed  of  the  following  subdivisions  :  - — 

Helix-limestone  of  the  Orleauais  {Hdxjc^  Plunorbia^  kc.)  Meulieres  de  Mont- 
morency— very  hard  siliceous,  cellular,  fossiliferous,  fresh-water  limestones 
employed  for  millstones  (Limrtma^  Bythiniaj  PlanorbiSf  Valvata^  Chara). 
This  deposit  is  replaced  towards  the  south  by  the  fresh-water  Calcaire  de 
la  Beauce,  which  iH  separable  into  a  higher  assise  (Molasse  du  G&tinais, 
sometimes  57  feet)  consisting  of  green  marl,  siliceous  sand,  and  calcareous 
sandstone  passing  into  limestones  {Helix  Morognesif  U,  aureliafius,  U. 
^  ■{  Tristanij  Planorbis  solidus^  LivmaM  Larlelij  Meiania  aquitanica^  &c.) ; 
and  a  lower,  composed  of  limestone  {Lhiineeti  Brangniartif  L.  cornea^  L. 
cylindrical  Helix  Ramondi^  Cydostanxa  antiquum^  Planorbis  cttrnUf  Pot- 
amides  Laniarckiy  &c,) 

Ores  de  Fontainebleau.  Sands,  and  hard  siliceous  sandstones.  At  the  top  of 
this  subdivision  there  occurs  at  Ormoy,  near  J^tampes,  and  elsewhere  a  band 
of  calcareous  marl  full  of  marine  fossils  {Cardila  Bazini,  Cytherea  in- 
crassatay  Lucina  Hiberti), 
'  Sables  de  Foutenay,  Jeurre  et  Morigny,  a  thick  accumulation  of  yellow  ferru- 
ginous, generally  unfossiliferous  sands,  covering  a  large  area  around  Paris, 
and  serving  as  a  foundation  for  most  of  the  new  military  forts  of  that 
locality.  The  *'  falun  de  Jeurre  "  contains  many  fossils  {Natica  crassatinu, 
Cerithiuvif  several  si)ecies,  Cytherea  incrassata^  Avicula  stainpiiiensis,  &c.) 

Oyster-njarls  with   Oatrea  longirostris,   0.  cyathiUay  and  Corhula  subpisnm. 
^  \      These  pass  into  the  Molasse  d'fitrechy  with  Cerithium  plicatumy  Meiania 
setnide^icssctta^  Cytlierea  incrasaata,  &c. 

Calcaire  de  la  Brie,  a  lacustrine  limestone  with  few  fossils,  Limnasa  cornea ^ 
PUtnorffia  cornu,  Chara^  &c. 

Green  marls  (Marnes  a  Gyrenes,  glaises  vertes),  consisting  of  an  upper  mass  of 
non -fossiliferous  clay,  and  a  lower  group  of  fossiliferous  laminated  marls 
[(^eriUiium  plicaiuniy  Paammobia  plana,  Cyrena  r/mr&ca). 

White  marls  (Marnes  de  Pantin)  with  Limnsea  strigom,  Plunarbia  planidatan, 
Nystia  Duchasteli. 

Supra -gypseous  blue  marls,  with  very  few  fossils  {Xystia  pliaUa). 

Lacustrine  gj'psuni  {Oyps  lacualre).  The  highest  and  most  important  gypsum 
^  ■{  bed  of  the  Paris  basin  (65  feet  thick  at  Montniartre),  with  a  remarkable 
prismatic  structure,  containing  skeletons  and  bones  of  mammals  {Paltfv 
therium,  Anajdotheriuniy  Xiphodon\  fragments  of  terrestrial  wood,  aud  a 
few  terrestrial  shells  {Hrlix,  Cyclostoma,  &c. )  Tliis  deposit  is  continuous 
with  the  marine  gypsum  underneath  it  (p.  978). 


Geographical  names  have  been  assigned  to  the  subdivisions  of  the  Oligocene  series  in 
France,  Belgium,  Switzerland,  and  North  Italy.  The  lowest  member  is  called  Tongrian, 
from  Tongres,  in  Linibourg.  Above  it  comes  the  Stampian,  so  named  from  6tam[)es, 
where  it  is  typically  develoi)ed.  The  uppermost  group  is  known  as  Aquitanian,  from  its 
well-marked  occurrence  in  Aquitania. 


1  Proc.Roy.  Soc.  Edin.  vi.  (1867)  p.  71  ;  Q.  J.  Ueul.  Soc.  xxvii.  (1871)  ]».  280  ;  Tran^. 
Roy.  Soc.  Ellin,  xxxv.  (1888)  p.  21  ;  Q.  J.  O'eol.  Site,  xlviii.  (1892),  Pres.  Address,  j). 
162.  Prof.  Judd  {op.  cit.  xxx.  (1874)  p.  220;  xlv.  (1889)  p.  187),  on  the  other  hand, 
believes  that  there  were  five  great  volcanic  cones  in  the  Western  Islands  whence  the  streams 
of  biisalt  flowed,  and  of  which  the  mountains  of  Mull,  Skye,  &c.,  are  the  degraded  ruins, 
and  he  regards  tlie  granitoid  rocks  as  older  than  the  others. 

2  Dollfus,    Bull.   Soc.    UM.    France,  3«  s<:r.    vi.    (1878)  p.   293.      The   separation  of 
an  Oligocene  series  in  the  Paris  basin  is  not  admitteil  by  many  eminent  French  geologists. 


990  STRA  TIGRA  PHICAL  GEOLOG  Y  book  ti  fab  it 


The  chief  area  of  Oligocene  strata  in  FniDce  lies  between  Paris  and  Orienm,  wkere, 
spreading  over  a  wide  extent  of  countrr,  they  hare  been  cut  down  by  the  stieaaM  so  at 
in  some  cases  to  reveal  the  Eocene  formations  below  thenu  The  next  area  in  impottanee 
lies  far  to  the  south-west  (Aquitania),  where  the  Lower  Oligocene  dirisMn  (Toagrian  of 
Belgium)  is  represented  by  a  thick  yellowish  marine  limestone  (Galeaire  a  Aiteries) 
Math.  CerUhium  plicatHin,  Trochns  Bneklaiuiij  XiUiea  craamtimtL,  Ac  Tlie  Aqutaniaa 
stage  is  represented  in  Languedoc  by  marine  marls  with  Ceritkiumi,  and  marine  candi> 
tions  are  indicated  by  the  corresponding  deposits  in  Provence. 

But  over  the  centre  and  south  of  France  marine  Oligocene  deposits  are  gienentlly 
absent,  their  place  being  taken  by  the  marls,  clays,  and  limestones  of  former  lakci, 
which  have  preserved  many  of  the  terrestrial  plants  and  animals  of  the  period. 
One  or  more  large  sheets  of  fresh  water  lay  in  the  heart  of  the  coontiy,  snnonnded 
by  slopes  clothed  with  a  tropical  flora.  In  these  basins,  a  series  of  maris  and  lime> 
stones  (1500  feet  thick  in  the  Limagne  d'Auvergne)  accumulated,  from  which  hare 
been  obtained  the  remains  of  nearly  100  species  of  mammals,  including  some  paheo- 
theres,  like  those  of  the  Paris  basin,  a  few  genera  found  also  in  the  Mainz  basia, 
crocodiles,  snakes,  numerous  birds,  and  relics  of  the  surrounding  land-v^etarion  of  the 
time.  This  water-basin  appears  to  have  been  destroyed  by  volcanic  explosions,  which 
afterwards  poured  out  the  great  sheets  of  lava,  and  formed  the  numerous  cooes  or  fm^/t 
so  conspicuous  on  the  plateau  of  Auvergne.  In  the  south  of  France,  the  Eocene  groups 
are  sometimes  surmounted  by  lacustrine  or  brackish-water  beds  that  point  to  the 
retirement  of  the  uummulitic  sea,  and  the  advent  of  those  more  terrestrial  and  shallow- 
water  conditions  in  which  the  Oligocene  de^tosits  were  accumulated.  In  Ptovenoe, 
lacustrine  beds  (PAt/sa,  Fianorbis,  Limnma,  BuIimuSf  kc.)  lie  immediately  upon  the 
Up]>er  Cretaceous  rocks.  At  Aix  these  beds  have  long  been  noted  for  their  abundant 
plants  {Callitris  Brongniarti,  WiddriAglonia  brachyphyllaut  Flabellaria  iamatumiM, 
Qnercus,  L<if'rHS,  Cinnamomvm),  insects  and  mammals  {Palmotktrimm^  XipkotUnL, 
Aii'jpfothfrivm.  Chceropo(aitn^s). 

A  singular  and  interesting  development  of  Oligocene  deposits  in  France,  Switzer- 
land, and  southern  Germany  is  foimd  where  they  have  filled  up  fissures  and  cavities  of 
oldtT.  es[H^cially  Upper  Jurassic,  limestones.  One  of  the  most  remarkable  of  these 
«)ccurrtuces  is  that  of  Quercy,  now  famous  for  the  large  number  of  remains  of  mammals 
which  have  been  found  there.  These  deposits  are  related  to  Tertiary  strata  in  their 
\-icinity,  and  never  occur  at  a  higher  altitude  than  these  strata.  They  consist  of  red 
•  lay  and  loam,  with  idsolitic  limonite,  becoming  more  phosphatic  towards  the  bottom, 
where  the  phosphate  of  lime  occurs  in  such  <|uantity  as  to  be  profitably  worked.  Among 
the  fossils  recovered  from  these  recesses  are  a  number  of  shells  {Cyc/ostotna,  LimnMa, 
Plaaoi-bis)  ami  sj>ecies  of  Palxotherium,  Anoplolhcrium,  Xiphodon^  Hyatatxiwn^ 
Caiiiotht:rinin,  Amphitraguius,  kc.  There  have  also  been  found  the  remains  of  a  lemur 
( X'Xtok III V. r  'I III iq k i«). * 

Belgium.- — The  succession  of  Oligocene  beds  in  this  country  differs  from  that  of 
France,  and  has  received  a  different  nomenclature,  as  follows  : — 

Upper. — Wanting. 
J        f  ^    /Wliite  sands  of  Bolderberg  {BoliUrian). 

'   =    I  Clay  of  Boom  and  Suada  clay  of  Bergh — upwards  of  40  species  of 
I  ^  ~|      fo^\\&y'mc\vidmf^yuculacompia{L^ialyeUiana)jCorhuUiguhpisum 
•    !    ^   i^    V     (="  Septarienthon  "  of  northern  Germany). 

-f  '!    ir  '\  .=   '  Cerithium  sands  of  Vieux  Jonc  (Klein  Spauwen)  and  Peetuneulux 
^   '    E"  !    5    ■       sands  of  Bergh. 

"^  \  2  "{  ^^"is  ^hiy*  '^^  fossils  in  this  clay  and  the  overlying  sands  are 
■g  !  fluvio* marine  {Cydostoma,  Succinea,  Pupa  ;  Piamorbis,  Limnaa, 
=    I      Serif ina  ;  Ceriihium,  Jleiania,  Bythinia,  Cyrena), 


Filhol,  Ann.  Set.  (ydil.  1876.  *  Mourlon,  *G«>1.  Belg.* 


SECT,  ii  §  2 


OLIGOCENE  SYSTEM 


991 


Sands  of  Neerepen. 
•I  Sands    of   Orimmertingen.      The    Tongrian    deposits  contain 
abundant  marine  fauna  =  the  Egein  beds  of  Grermany. 


an 


Germany.^ — In  northern  Germany,  while  true  Eocene  deposits  are  wanting,  the 
Oligocene  groups  are  well  developed  both  in  their  marine  and  fresh-water  facies,  and  it 
was  from  their  characters  in  that  region  that  Beyrich  proposed  for  them  the  term 
Oligocene.  They  occupy  large  more  or  leas  detached  areas  or  basins,  with  local 
lithological  and  palseontological  variations,  but  the  following  general  subdivisions  have 
been  established : — 

'  Marine  marls,  clays,  sands,  sparingly  distributed  (Doberg,  Hanover  ;  Wilhelms- 
hohe  ;  Mecklenburg-Schwerin),  with  Spatangus  Ubffmanni^  Tereln-ahUa 
grandiSy  Pecten  Janus,  P,  ctecusscUuSy  Area  Speyeri,  Nassa  pygmaa, 
Pleurotoma  subdenticuiata. 

Brown-coal  deposits  of  the  Lower  Rhine,'  &c.,  with  a  flora  of  less  tropical 
Indian  and  Australian  type,  and  more  allied  to  that  of  subtropical  North 
America  {Acer,  Cinnamomum,  Cupressinoxylon,  Jtiglans,  Nyssa,  Pinites, 
Quercus,  &c.)  Some  marine  beds  in  this  division  contain  Terehratula 
gi-andia,  Pecten  Janus,  P,  MOnsteri,  &c. 
f  Stettin  (Magdeburg)  sand  and  Septaria-clay  (iSf/>^«rie/iMow),  with  an  abundant 
marine  fauna  (Foraminifera,  Pecten  pennistus,  Leda  deshayesiana,  Nucula 
Chasteli,  Pleurotoma  scabra,  Axinus  ftUusus,  Fusus  Koninckii,  F.  mxUtisul- 
catus,  &c.)  These  beds  are  widely  distributed  in  north  Germany,  and  are 
usually  the  only  representatives  there  of  the  Middle  Oligocene  deposits.  In 
some  places,  however,  a  local  brown-coal  group  occurs  {Ainus  K^ersteini, 
Cinnanwmum  polyTnorphum,  Pnpulus  Zaddachi,  Taxtxiium  dubium). 
'E^eln  marine  beds  {Ostrea  verUilabrum,  Pecten  heUicostatus,  Leda  percvalis. 
Area  appendieulcUa,  Cardita  Dunkeri,  Cardium  Hausmanni,  Cytherea 
Solandri,  Cerithium  Imvum,  Pleurotoma  Beyrichi,  P.  stibconoidea,  Voluta 
decora,  Buccinum  buUutum,  kc,  and  corals  of  the  genera  Turbinolia^  Balawt- 
phyllia,  Caryophyllia,  Cyathina).^ 

Amber  beds  of  Konigsberg,  consisting  of  ligiiitiferous  sands  resting  on  marine 
glaucouitic  sands,  near  the  base  of  which  lies  a  band  coutaiuiug  abundant 
pieces  of  amber.  The  latter,  derived  from  several  species  of  conifers,  es^ieci- 
ally  Pinus  succini/era,  have  yielded  a  pleutiful  series,  estimated  at  about 
2000  species,  of  insects,  arachnids,  and  myriapods,  together  with  the 
fruits,  flowers,  seeds,  and  leaves  of  a  large  number  of  conifers  {Pinites,  Pinus, 
Abies,  Sequoia  Langsdorfii,  Widdringtonites,  Libocedrus,  Thuja,  Cupressus, 
Taxodium)  and  dicotyledons  {Quercus,  Castanea,  Fagus,  Myrica,  Polygonuvh 
Cinnanwmum,  Geranium,  Linum,  Acer,  Ilex,  Rhamnus,  Deutzia,  Proteacem, 
several  genera,  Andromeda,  kc.)*  The  sands  contain  Lower  Oligocene 
marine  mollusca,  sea-urchins,  &c. 

Lower  Brown-coal  series — sands,  sandstones,  conglomerates,  and  clays  with  inter- 
stratitied  varieties  of  brown-coal  (pitch-coal,  earthy  lignite,  pa]»er-coal,  wax- 
coal,  &c.),  a  single  mass  of  which  sometimes  attains  a  thickness  of  100  feet  or 
more.  These  strata  may  be  traced  intermittently  over  a  wide  area  of  northern 
Germany.  The  flora  of  the  brown-coal  is  largely  composed  of  conifers 
( Taxites,  Taxoxylon,  Cupressinoxylon,  Scquoiii,  kc. ),  but  also  with  Quercus, 
Laurus,  Cinnamomum,  Magnolia,  Dryandroides,  Ficus,  Sassafras,  Alnus, 
Acer,  Juglans,  Betula,  and  palms  {SabaL,  FlaJhellaria).  The  general  aspect  of 
this  flora  most  resembles  that  of  the  southern  states  of  North  America,  but 
with  relations  to  earlier  tropical  floras  having  Indian  and  Australian  affinities. 

*  Beyrich,  Monatsbericht,  Akad.  Berlin,  1854,  p.  640  ;  1858,  p.  51.  A.  von  Koenen, 
J^fitsch.  Deutsch.  Oed,  Qes.  xix.  (1867)  p.  23. 

*  For  a  popular  account  of  the  brown -coal  of  Germany  see  M.  VoUert,  *  Der  Braunkohlen- 
bergbau,'  Halle,  1889,  the  "  Festschrift  "  of  the  fourth  Deutsche  Bergmannstage  in  1889. 

^  For  detailed  descriptions  of  the  Lower  Oligocene  molluscan  fauna  of  north  Germany  see 
Prof.  A.  von  Koenen's  elaborate  monograph,  Ahhand.  Oeol,  Spedalkart,  Preuss.  x.  (1889-92). 

*  *  Flora  des  Bernsteins,'  vol.  L  on  the  conifene,  H.  R.  Goeppert,  1883 ;  vol  ii.  on  the 
dicotyledons,  Goeppert,  A.  Menge,  and  H.  Conwentz,  1886. 


0) 


992  STRATIGRAPHICAL  GEOLOGY  book  vi  part  iv 


III  the  Maiuz  basin  some  marine  sands,  clays,  and  marls  in  the  lower  part  of  it^ 
Tertiary  deposits  are  referretl  to  the  Oligoceue  series,  and  are  arranged  as  follows: — 

Cerithiuni  Beds. — Sandy  and  calcareous  strata  with  brackish-water  and  land-shells 

{Cerithium  plicatuiUf  Mytilus  Faujagi,  HeltJCj  &c.) 
Cyrena    marl    and    sand   {C^frena    semiHrUUa^    CerUhium  pUcatum,    C.    mar- 

garUacenmf  Penut  Sandbergeriy  kc. ) 
Septaria-clay  with  Z^eda  dtshayesiana. 
Marine  sand  of  Weinheim  with  Ostrea  caUifera^  Pectunculus  cbovaiH*^    Cftherta 

incrassata,  Natica  crasaaiina. 

Switzerland.* — Nowhere  in  Europe  do  Oli^ocene  strata  play  so  important  a  \vut 
in  the  scener}'  of  the  land,  or  present  on  the  whole  so  interesting  and  fall  a  pictnre  of  the 
state  of  the  continent  when  they  were  deposited,  as  in  Switzerland.  Rising  into  massire 
mountains,  as  iu  the  well-known  Rigi  and  Rossberg,  they  attain  a  thickness  of  several 
thousand  feet.  While  they  include  proofs  of  the  presence  of  the  sea,  they  have 
preserved  with  marvellous  perfection  a  large  number  of  the  plants  which  clothed  the 
Alps,  and  of  the  insects  which  flitted  through  the  woodlands.  They  form  part  of  a 
jijreat  series  of  deposits  which  have  been  termed  "  Molasse "  by  the  Swiss  geologists. 
The  Molasse  was  formerly  considered  to  be  entirely  Miocene.  The  lower  portions, 
however,  arc  now  placed  on  the  same  parallel  with  the  Oligocene  beds  of  the  regions 
lyin<;  to  the  north,  and  consist  of  the  following  subdivisions  : — 

Lower  Browu-coal  or  red  Molasse  (Aquitanian  stage) — the  most  massive  member 
of  the  Molasse,  consisting  of  red  sandstones,  marls,  and  conglomerates  (Kagel- 
tluc)  with  well-rounded  mutually  indented  pebbles,  resting  upon  variegated  reil 
marls.  It  contains  seams  of  lignite,  and  a  vast  abundance  of  terrestrial  v^^- 
tation. 

Lower  marine  Molasse  (Tongrian  stage) — sandstone  containing  marine  and  brackish- 
water  shells,  among  which  are  Ostrea  cyathula,  0.  longirostriSy  O.  call\fem^ 
Cyrena  semUtriata^  Cythereu  incrassaia^  Pectunculus  obovatusy  CerUhium  pfi- 
catxiitij  yaticu  crussatina.  This  division  is  well  developed  between  B&le  and 
Berue. 

By  far  the  larger  portion  of  tliese  strata  is  of  lacustrine  origin.  They  nmst  haw 
lieen  tbnued  in  a  large  lake,  the  area  of  which  probably  underwent  gradual  subsidence 
during  tlie  period  of  deposition,  until  in  Miocene  times  the  sea  once  more  overflowetl 
the  area.  We  may  form  some  idea  of  the  importance  of  the  lake  from  the  fact  that 
the  deposits  funned  in  its  waters  are  upwards  of  9000  feet  thick.  Thanks  to  the 
untiring  labours  of  Professor  Heer,  we  know  more  of  the  vegetation  of  the  mountain^ 
round  that  lake,  during  Oligocene  and  Miocene  time,  than  we  do  of  that  of  any  other 
ancient  geological  period.  The  woods  were  marked  by  the  predominance  of  an 
arborescent  subtropical  vegetation,  among  which  evergreen  forms  were  conspicuous,  the 
whole  having  a  decidedly  American  aspect.  Among  the  plants  were  palms  of  American 
type,  the  Californian  coniferous  genus  Sequoia j  alders,  birches,  figs,  laurels,  cinnamon- 
trees,  evergreen  oaks,  with  many  other  kinds. 

A  j)ortion  of  the  great  FJysch  formation  of  the  Alps  (which  has  been  already  referred 
to  as  partly  Cretiiceous,  partly  Eocene)  is  referred  to  the  Oligocene  series.  It  includes 
the  shales  of  Glarus,  long  known  for  their  fish-remains. 

Vienna  Basin.-*   -This  area  contains  a  typical  series  of  Tertiary  deposits,  sometimes 


^  Studer'a  'Geologic  der  Schweiz,'  vol.  ii.  ;  Heer's  *  Urwelt  dcr  Schweiz,'  1865  (au 
English  translation  of  which  by  Mr.  W.  S.  Dallas  appeared  in  1876);  *  Flora  Fossilis 
Helvetiie,'  li>54-5i)  ;  A.  Favre,  'Description  Geologique  du  Canton  de  Geneve,'  1880,  vol. 
i.  p.  09. 

-  Siiess,  'Der  Bodeu  von  Wieu,'  1860.  Th.  Fuchs,  ' Erlauterungen  zur  Geol.  Kaite  der 
Unjgebuiigen  Wiens,'  1873;  and  papers  in  Zeitsch.  Deuisch,  G&yl.  ihsd.  1877  (p.  6r»3) : 
Jahrfi.  Gad.  RcicKwu.st.  vols,  xviii.  et  se^.     Von  Hauer's  'Geologic.' 


SECT,  iii  §  1  MIOCENE  SYSTEM  993 

classed  together  as  *'  Neogene."  At  the  bottom  lies  an  incoustant  gi*oup  of  marls  aud 
sandstones  (A([uitanian  stage),  containing  occasional  seams  of  brown-coal  aud  fresh-water 
beds,  but  with  intercalations  of  marine  strata.  The  marine  layers  contain  Cerilhium 
plkatitnij  C.  iiiargarUaceunij  &c.  Tlie  brackish  and  fresh -water  beds  yield  Melania 
Eschrri  and  Cyreim  fiffnif/iria.  Among  the  vertebrates  are  Mastodon  angiifUidniSf  J/. 
tapiroideSy  Rhinoceros  sansaniensiSf  A mphicyon  iiU^nnedius,  AivchUherium  aurefiniiefise, 
and  numerous  turtles.  These  strata  have  suffered  from  the  upheaval  of  the  Alps,  and 
may  be  seen  sometimes  standing  on  end.  It  is  interesting  also  to  observe  that  the 
subterranean  movements  east  of  the  Alps  culminated  in  the  outpouring  of  enormous 
sheets  of  trachyte,  andesite,  propylite,  and  basalt  in  Hungary  and  along  the  flanks  of 
the  Carpathian  chain  into  Transylvania.  The  volcanic  action  appears  to  have  begun 
during  the  Aquitanian  stage,  but  continued  into  later  time.  Further  curious  changes 
in  physical  geography  are  revealed  by  the  other  '^  Neogene  "  de])osits  of  south-eastern 
£urope.  Thus  in  Croatia,  the  Miocene  marls,  with  their  abundant  land-plants,  insects, 
&c.,  contain  two  beds  of  sulphur  (the  up{)er  4  to  16  inches  thick,  the  under  10  to  15 
inches),  which  have  been  worked  at  Radoboj.  At  Hrastreigg,  Buchberg,  and  elsewhere, 
coal  is  worked  in  the  Aquitanian  stage  in  a  bed  sometimes  65  feet  thick.  In  Tran- 
sylvania, and  along  the  base  of  the  Carpathian  Mountains,  extensive  masses  of  rock-salt 
and  gj'psum  are  in terst ratified  in  the  **  Neogene  "  formations. 

Italy. — In  the  north  of  Italy  strata  assigned  to  the  Oligocene  series  attain  an 
enormous  develoiimcnt,  their  lotal  estimated  thickness  amounting  to  nearly  12,000  feet. 
They  dovetail  regularly  with  the  Eocene  below  and  the  Miocene  above,  and  are  thus 
grouped  by  Prof.  Sacco  in  the  central  part  of  the  northern  Apennines  : — 

Aquitanian  Staffe  ( "^  IP^**  thickness  of  grey  and  yellowish  sands  and  occaMional 
1000  metres       \      f^Y^^^  marls,  the  marly  character  increasing  northwards  aud 
(^     eastwards.     Fossils  scarce. 

Stain pian  Stage.    1  _,  ,  .  i         i  *  •  ui 

600  metres  f  ^^^V  ™*ri8  more  or  less  sandy  and  friable. 

(A  vast  series  of  sandy  marls,  sands,  conglomerates,  and  lenticles 
of  lignite,   with  frequent  nummulites  (\.   intenneiiia^   X. 
^ ^_.         Fichtelif  A\  striata),   Orbitoides,    fresh-water,   brackish,  and 

2000  metres.        1      marine  shells  (Ampullina  crassatina^  PotamideSt  (Jyrena  con- 

I      vtxoj  &c.),  Anthrncothcrium  mafjnumf  &c.     Sometimes  with 
1^     greyish  violet  marls. 

Sestian  Stage.        (  A  thin  band  of  sandy  marls  with  Numniiditcs  Fichteli,  N.  nutca, 
20  metres.  \      iV.  Boucheri,  Orbitoides,  IlderosUginUf  &c. 

North  America. — Overlying  the  Jaqkson  beds  referred  to  on  p.  981  a  conformable 
group  of  strata  known  as  the  '*  Vicksburg  beds"  (Orbitoitic)  occupies  a  narrow  band  in 
Alabama,  Mississippi,  and  Lonisiana,  covers  the  greater  \}a,vt  of  Florida,  and  extends 
into  Georgia  and  Texas.  These  strata  in  Mississippi  are  composed  of  a  lower  ferruginous 
rock  (Red  Bluff)  12  feet  thick,  aud  a  set  of  crystalline  limestones  and  blue  marls  (80  feet) 
resting  on  lignitic  clays  and  lignites  (20  feet).  Among  the  fossils  are  Ostrea  yigaiUcu^ 
Pecten  Ponlsoni,  Cardiuni  diversum^  CardiUi  p/anicosta,  PaiwpaML  oblongfUa^  Cyprxn 
lintcii,  Mitrd  mississippicnsiSy  Cassidaria  linteti,  Conus  sauridoiSj  Madrepora  mississippien.' 
sLs,  Flahfllum  IVailani,  Orbitoides  Mant4:ili,  The  last-named  fossil  is  specially  charac- 
teristic, and  is  found  also  in  the  West  Indies,  Malta,  and  the  Turco-Persian  frontier. 


Section  iii.  Miocene. 

§  1.  General  Characters. 

The  Euro])ean  Miocene  deposits  reveal  great  changes  in  the  geography 
of  the  Continent  as  compared  with  its  condition  in  earlier  Tertiary  time. 

3  s 


STRATIGRAFHICAL  GEOLOGY 


So  far  ae  yet  known,  Britain  aod  northern  Eorope  genHsUy,  asre  ui  are* 
over  the  eite  of  Schleswig-Hobtein  and  Friesland,  were  Uiid  dtiring  the 
Miocene  period ;  but  a  shallow  lea  extended  towards  the  aoath-eaat  and 
south,  covering  the  lowlands  of  Belgiiun  and  the  basin  trf  the  Lnre.  The 
Gulf  of  Gascon)-  then  swept  inland  over  the  vide  plains  of  tbe  Garonne, 
perhaps  even  connecting  the  Atlantic  with  the  Meditemmean  by  a  stnit 
running  along  the  northern  &ank  at  the  Pyreneea.  The  aea  washed  tbe 
northern  base  of  the  now  uplifted  Alps,  sending,  as  in  Oligocene  time,  a 
long  ann  into  the  \-alley  of  the  Rhine  as  far  as  the  site  of  Mainz,  which 
then  probably  stood  at  the  upper  end,  the  valley  draining  Bouthward 
instead  of  northward.  The  gradual  conversion  of  salt  into  Ivackish  and 
fresh  water  at  the  head  of  this  inlet  took  place  in  Miocene  time.  Fran 
the  Miocene  firth  of  the  Rhine,  a  sea-strait  ran  eastwards,  between  the 


base  of  the  Alps  itnd  the  line  of  the  Danube,  filling  up  the  broad  basin  of 
Vienna,  Bending  thence  an  arm  northwards  through  Moraria,  and  spread- 
ing far  and  wide  among  the  islands  of  south-eastern  Europe,  over  the 
regions  where  now  the  Black  Sea  and  Caspian  basins  remain  as  the  la£t 
relics  of  this  Tertiary  extension  of  the  ocean  across  southern  Eun^. 
The  Mediterranean  also  still  presented  a  far  larger  area  than  it  now 
possesses,  for  it  covere<l  much  of  the  present  lowlands  and  foot-hills  along 
its  northern  border,  and  some  of  its  important  islands  had  not  yet  appeared 
or  had  not  acquired  their  present  dimensions. 

Among  the  revolutions  of  the  time  not  the  least  important  in 
Euroi>ean  geography  was  the  continued  uprise  of  the  Alps  by  which  the 
Eocene  strata  had  been  so  convoluted  and  overthrown.  These  disturb- 
ances still  went  on  in  a  diminished  degree  in  Miocene  time-  One  of 
their  results  was  the  restoration  and  extension  of  the  wide  lake  or  chain 
of  lakes,  o^'e^  the  northern  or  molasse  region  of  Switzerland,  in  which  the 
red  niolasse  of  Oligocene  time  had  been  deposited.  The  lacustrine 
de{>osits  accumulated  there  have  presened  with  remarkable  fulness  a 
record  of  the  terrestrial  flora  and  fauna  of  the  time. 


BKTT.  iii  $  1  MIOCENE  SYSTEM  996 

The  flora  of  the  Miocene  period  (Figs.  436,  437)  indicates  a 
decidedly  eubtropical  climate  in  the  earlier  part  of  that  period  in  Europe, 
many  of  the  plants  having  their  nearest  modem  representatives  in  India 
and  Australia.'  Among  the  more  characteristic  genera  are  Sabal,  PhamicUes, 
Libocedrus,  Stquoia,  Myrica,  Qwrcua,  Ficus,  LaurUK,  Vintiamomum,  Daphne, 
Persaonia,  Banksia,  Dryandra,  Ciasus,  Magnolia,  Acer,  Ilex,  Rhamnvs,  Juglana, 


FJcuodecandollcwBd):  'I,  (Jiitrrui  illcoldu  (j). 


Ehw,  Myrtas,  Mimosa,  and  Acacia.  In  the  later  part  of  the  period,  the 
climate,  if  we  may  judge  from  the  character  of  the  flora,  had  become 
less  warm ;  for  as  the  palms  disappeared  there  came  the  flora  of  a 
more  temperate  type,  including  among  the  more  frequent  plants  species 
of  Glyptostrobus,  Betvla,  Populus,  Carpiuiis,  Ulmm,  Laura.*,  Ptr&ea,  Ilex, 
Podiigoniuvi,  and  PotanwgeUm.* 

The  fauna  points  to  somewhat  similar  climatal  conditions  in  Europe. 
There  occur  such  moUuscan  genera  as  Anallaria,  Buennum,  Cartullaria, 
Cassis,  Cffprten,  Milra,  Murex,  Pyrula,  Slrotiibwt,  Terebra,  Area,  Cardila, 
Cardium,  Cyilterea,  Mactra,  Ostrea,  Patwpiea,  Peclen,  Pectuncuivs,  Spondylvs, 
Tape.",  Telliiia,  &c.  (Fig,  438).  The  mammalian  forms  present  many 
pointe  of  contrast  with  those  of  older  Tertiary  time.  Huge  proboscideans 
now  take  a  foremost  place.  Among  the  more  important  generic  types  of 
the  time  are  the  coloesal  Mastodon  (Fig.  439)  and  Ikinotheriwn  (Fig.  440), 
the  latter  having  tusks  curving  downwards  from  the  lower  law.  With 
these  are  associated  Rhinoceros,  of  which  a  hornless  and  a  feebly  homed 
species  have  been  noted  ;  Anchithtrium,  a  small  horse-like  animal,  about  as 

■  Heer,  '  Unrclt  der  Scliw«iz ' ;  '  Flora  FomUi*  Helvelln-. ' 
■  Sapom,  '  Hands  dei  PUnl«i,'  p.  272. 


STRATIGRAPHICAL  GEOLOOY 


BOOC  TI  PART  IT 


big  as  a  sheep,  eurviving  from  earlier  Tertiary  time ;  Macrotherium,  a  huge 
ant-eater ;  DicToceras,  a  deer  allied  to  the  living  muntjak  of  eaatarn  Ads; 
Hyolh-eJiuin,  an  animal  nearly  related  to  the  hog.     A  number  of  living 


Kg.  48S,-lIluc«De  MnUiulu. 
o,  Psnopoo  FniijiislI,  Men.  de  U  Oroye  (» :  h,  Pwtunculua  glyolmfri.  {F.  pilwiw).  Ltnn.  (I) : 
c,  CinliU  aSnla,  DuJ.  :  d,  Tsp«8  gn^rla.  Pattscta.  ()). 

genera  likewise  made  their  entry  upon  the  scene,  Buch  as  the  bog, 
ottei-,  antelope,  beaver,  and  cat.  Some  of  the  most  formidable 
animals  were  the  sabre-toothed  tigers  {Maclmforhtx),  and  the  earliest  form 


KfHiuced  fttim 


VM.  C 


of  bear  {HiiA-narrtos).  The  Miocene  forests  were  also  tenanted  by  apes, 
of  which  several  genera  have  been  detected.  Of  these,  Pliofnlkecus  was 
pi-obably  allied  to  the  anthropoid  apes ;  l}ryopUhecus  (Fig,  441)  was  regardetl 

'   For  a  i-»st«ratioQ  of  M.  ameritanni,  see  MaRh,  A»ur.  JoBrn.  So.  iVn.  (1892)  p.  350. 


SECT,  iii  §  1 


MIOCENE  SYSTEM 


997 


by  Owen  as  allied  to  the  living  gibbons,  but  Gaudry  regards  it  as  an  anthro- 
poid form,  and  as  the  only  one  yet  found  fossil  which  can  be  compared 
with  man  ;  ^  Oreopithecus  is  supposed  to  have  had  affinities  with  the  anthro- 
poid apes,  macaques,  and  baboons ;  and  a  species  of  Cdobus  is  found  in 
Wurteraburg.* 

Among  the  discoveries  in  western  America,  which  have  thrown  so 


Fig.  440. — Deinotherlum  giganteoin,  Kaup.,  reduced. 

much  light  upon  the  history  of  vertebrate  life,  mention  should  be  made 
here  of  the  remarkable  assemblage  of  mammals  disinterred  from  the 
base  of  the  vast  lacustrine  Miocene  formations  on  the  eastern  flanks  of  the 
Rocky  Mountains.  The  Brontotheridse  or  Titanotheridae,  the  largest  of 
these  animals,  formed  a  distinct  family  more  nearly  allied  to  the  living 
rhinoceros  than  to  any  other  recent  form. 


Fig.  441.— Jaw  of  Dryopithecus  Fontani,  Oandry  (f). 

Considerable  uncertainty  must  be  admitted  to  rest  upon  the  correla- 
tion of  the  later  Tertiary  deposits  in  different  parts  of  Europe.  In  many 
cases,  their  stratigraphical  relations  are  too  obscure  to  furnish  any  clue, 
and  their  identification  has  therefore  to  be  made  by  means  of  fossil 
evidence.     But  this  evidence  is  occasionally  contradictory.     For  example, 

»  Menu  Soc.  OM,  France  (3),  i.  fasc.  L  (1890). 
*-'  Gaudry,  *  Les  Enchainements,'  p.  806  ;  Boyd  Dawkinv,  *  Early  Man  In  Britain/  p.  57. 


998  STRATIGRAPHICAL  GEOLOGY  book  vi  part  iv 


the  remarkable  mammalian  fauna  described  by  M.  Gaudry  from  Pikenni 
in  Attica  (postm,  p.  1019)  has  so  many  points  of  connection  with  the  recog- 
nised Miocene  fauna  of  other  European  localities,  that  this  observer  classed 
it  also  as  Miocene.  He  has  pointed  out,  however,  that  in  a  shell-bearing 
bed  underlying  the  ossiferous  deposit  of  Pikenni  some  characteristic 
Pliocene  species  of  marine  moUusca  occur.  Remembering  how  deceptiTe 
sometimes  is  the  chronological  evidence  of  terrestrial  faunas  and  floras, 
(ante,  pp.  660,  668)  we  may  here  take  marine  shells  as  our  guide,  and  place 
the  Pikermi  beds  in  the  Pliocene  series. 

§  2.  Local  Development 

France. — True  Miocene  deposits  are  not  known  to  occur  in  Britam.  In  France, 
however,  in  the  district  of  Touraine,  traversed  by  the  rivers  Loire,  Indre,  and  Cher, 
there  occurs  a  group  of  shelly  sands  and  marls,  which,  as  far  back  as  1833,  was 
selected  by  Lyell  as  the  type  of  his  Miocene  subdivision.  These  strata  occur  in  widely 
extended  but  isolated  patches,  rarely  more  than  50  feet  thick,  and  are  better  known 
as  ''  Faluns,"  having  long  been  used  as  a  fertilising  material  for  spreading  over  the  soiL 
They  present  the  characters  of  littoral  and  shallow-water  marine  deposits,  consisting 
sometimes  of  a  kind  of  coarse  breccia  of  shells,  shell-fragments,  corals,  polyzoa,  &c, 
occasionally  mixed  with  quartz-sand,  and  now  and  then  passing  into  a  more  compact 
calcareous  mass  or  even  into  limestone.  Along  a  line  that  may  have  been  near  the 
coast-line  of  the  ])eriod,  a  few  land  and  fresh-water  shells,  together  with  bones  of  terres- 
trial mammals,  are  found,  but,  with  these  exceptions,  the  fauna  is  throughout  marine. 
Among  the  fossils  are  numerous  corals,  and  upwards  of  300  species  of  mollusks,  of 
which  the  following  are  characteristic  :  Fholas  Dujardini^  Fentta  dOfthrata,  Osirea  eras- 
sissimaj  PccUn  atricUus^  Cardium  turonicum^  Cardita  affinis^  Troekua  incrasaaiu*, 
Cerithium  hitradtrUatum,  Turritella  Linnai,  T.  bicarinata,  Pleurotama  ivberctUonj 
with  species  of  Cyprxay  Conns y  Murex^  OHva,  Ancillaria^  and  Fasdolaria.  This  assem- 
blage of  shells  indicates  a  warmer  climate  than  that  of  southern  Europe  at  the  present 
time.  The  mammalian  bones  include  the  genera  Mastodon^  Rhinoceros^  ffippopotamvs, 
Chceropotamus,  deer,  &c.,  and  extinct  marine  forms  allied  to  the  morse,  sea-cow,  and 
dolphin.     Similar  falims,  perhaps  slightly  later  in  age,  are  found  in  Anjou  and  Maine. 

In  the  region  of  Bordeaux  and  the  plains  of  the  Garonne  southward  to  the  base  of  the 
Pyrenees,  a  large  area  is  overspread  with  Oligocene  deposits,  equivalents  of  the  youngo' 
Tertiary  series  of  the  Paris  basin.  Above  these  fresh-water  and  marine  beds  lie  patches 
of  faluns  like  those  of  Touraine,  containing  a  similar  assemblage  of  marine  fossils.  Other 
marine  de^wsits  of  Miocene  age  are  found  running  up  the  valley  of  the  Rhone.  But  in 
the  south  and  south-east  of  France  the  Miocene  strata  are  mainly  of  lacustrine  origin, 
sometimes  attaining  a  thickness  of  1000  feet,  as  in  the  important  series  of  limestones  and 
marls  of  Sausan  and  Siraorre,  whence  remains  of  numerous  interesting  mammalia  have 
been  obtained.  Among  these  remains  are  Deinoth^rium  giganteum,  Mastodon  angusiidcns^ 
M.  tiiplrouks,  M,  pyrcnaicu^.  Rhinoceros  ScMeicrmacheri,  R.  sansaniensis,  R.  brtJchypuSj 
Anchitherium  aurelianensey  AiUhracotherium  onoideum^  Amphicyon  ffigantetu,  Machai- 
rcHlvs  citltridenSy  Ilelladotherinm  Duvemoyi,  Dicroceras  eUganSy  and  several  apes  and 
monkeys  {PliopithecuSj  Dryopitheeus). 

The  Miocene  deposits  of  France,  though  scattered  in  isolated  patches,  have  been 
grouped  into  three  stages  in  the  following  ascending  order :  1st,  Lhangian — sands 
and  marls  (I'Orleanais,  Sologne,  &c.),  limestones  (Sansan,  Simorre) ;  2nd,  Helvetian — 
shelly  sands,  faluns  (Touraine,  Anjou,  Aquitaine)  ;  3rd,  Tortonian — marls  ¥rith  Hdix 
tit  roncHsis. 

Belgium. — In  this  country,  the  upper  Oligocene  strata  of  Germany  are  absent 
In  the  neighbourhood  of  Antwerp  certain  black,  grey,  or  greenish  glauoonitic  sands 


SBcr.  iii  §  2  MIOCENE  SYSTEM  999 


("  Black  Crag,"  Bolderian  and  AnyersiAii),  of  ^hich  the  palsontological  characters  were 
at  one  time  supposed  to  present  a  mingling  of  Miocene  and  Pliocene  affinities.  These 
deposits  were  accordingly  termed  by  some  geologists  Mio- pliocene.  They  consist  of 
gravelly  sandls  at  the  base,  containing  cetacean  bones  (Heteroeetus)^  fish-teeth,  Ostrea 
naviculariSf  PeeUn  Caillaudif  kc  They  are  followed  by  sands  with  Pectunculus 
glydmeris  (pilo8ua)t  and  these  by  sands  with  PanopsRa  Faujctsii  {Menardi),  More 
recent  research  has  shown  that  the  lower  part  of  this  series  of  deposits  is  Miocene,  and 
is  separated  by  a  break  and  erosion-line  from  the  superincumbent  Diestian  group  which 
is  referable  to  the  Pliocene  series. 

Oermany. — Certain  deposits  of  dark  clay  and  sand  spread  over  parts  of  the  north- 
west of  Germany  containing  Contia  Dvjardini,  C,  anted iluvianus,  Fususfestivus,  laocardia 
eoTf  Pectunculus  glydmeris  {pilosus)^  Limopsis  aurila,  &o.,  and  are  referred  to  the 
Miocene  formations.  These  are  doubtless  a  prolongation  of  the  Belgian  series.  Else- 
where the  deposits  referable  to  this  geological  period  are  lacustrine  or  fluviatile  in  origin, 
and  are  especially  marked  by  the  occurrence  in  them  of  brown-coals  which  are  worked. 

In  the  Mainz  Tertiary  basin  an  important  series  of  marine,  brackish,  and  fresh- 
water deposits  occurs,  which  has  been  arranged  by  Fridolin  Sandberger  as  follows  :  ^ — 

Pliocene — 

Uppermost  brown-coal. 

Bone-sand  of  Eppelsheim  (Deinotherium  sand),  see  p.  1017. 
Miocene — 

Clay,  sand,  &c.,  with  leaves. 

Limestone   with   LitarineUa   {ffydrolna)  acuta,    Helix  moguntina,   Planorbis^ 
Dreissena,  kc. 

Corbicula  beds  with  Corhicula  FaujasUf  Hydrobia  if\flata^  H.  acuta, 

Cerithium  limestone  and  land-snail  limestone. 

Sandstone  with  leaves  {Cinnamomum,  Sabal,  QuercuSf  Ulmua). 
Oligocene  (see  p.  992). 

The  lower  Miocene  beds  of  this  area  present  much  local  variation,  some  being  full  of 
terrestrial  plants,  some  containing  fresh-water,  and  others  brackish-water  and  marine 
shells,  indicating  the  final  shoaling  of  the  Oligocene  fjord  which  ran  down  the  upper 
valley  of  the  Rhine  as  far  as  Mainz.  Among  the  plants  are  species  of  Quercua,  Ulmus, 
PlanerOf  Cinnamomum,  Myrica,  Sabal,  &c.  The  land-snail  limestone  contains  numerous 
species  of  ffelix  and  Pupa,  with  Cycloatoma  and  Planorbis.  The  Cerithium  limestone 
contains  marine  or  estuarine  shells,  as  Pema,  Afytilus,  Ccrithium{C.  Rahtii,  C,plicatum\ 
Nerita.  Among  the  various  strata,  bones  of  some  of  the  terrestrial  mammals  of  the 
time  occur  {Aficrotherium,  Palmomeryx),  The  Litorinella  limestone,  the  most  extensive 
bed  in  the  series,  is  composed  of  limestone,  marl,  and  shale,  sometimes  made  up  of 
Hydrobia  (LiloHnella)  acuta,  in  other  places  of  Dreissena  {Tiehogonia,  Congeria)  Brardi, 
or  Afytilus  Faujasii.  Abundant  land  and  fresh-water  shells  also  occur.  Of  greater 
interest  are  the  mammalian  remains,  which  include  those  of  Deinotherium  gigantcum, 
PalsBotneryx,  Microtheriumf  and  Hipparum  {Hippotherium),  The  flora  of  the  higher 
parts  of  the  Miocene  series  includes  several  species  of  oak  and  beech,  also  varieties  of  ever- 
green oak,  magnolia,  acacia,  styrax,  fig,  vine,  cypress,  and  palm. 

Vieniui  Baiin.' — Overlying  the  Aquitanian  stage  (p.  993),  where  that  is  present,  in 
other  cases  resting  unconformably  upon  older  Tertiary  rocks,  come  the  younger  Tertiary 
or  Neogene  deposits  of  the  Vienna  basin — a  large  area  comprising  the  vast  depression 
between  the  foot  of  the  eastern  Alps  near  Vienna,  the  base  of  the  plateau  of  Bohemia 

^  '  Untersuchungen  iiber  das  Mainzer  Tertiarbecken, '  1853;  'Die  Conchylien  des 
Mainzer  Tertiarbeckens,'  1863. 

'  T.  FuchH,  Z.  Deutsch.  Oeol,  Oes.  1877,  p.  653  ;  Homes  and  Partsch,  *  Die  Fossil. 
Mollasken  Tertiiir.  Beckens,*  Wien,  1851-70  ;  Ettingshausen,  '  Die  Tertiarfioren  d.  Oesterr. 
Monarchic, '  1851  ;  Von  Hauer's  'Qeologie,'  p.  617. 


1000  STRATIGRAPHICAL  GEOLOGY  book  vi  pabt  nr 


and  Moravia,  and  the  western  slopes  of  the  Carpathians.     This  tract  communicated 
>vitli  tlie  open  Miocene  sea  by  various  openings  in  different  directions.     Its  Miocene 

deposits  are  composed  of  two  chief  divisions  or  stages  as  follows,  in  descending  order :— 

■ 

Sarniatian  or  Cerithinm  Stage. — Sandstones  pnsKing  into  sandy  Umestone» 
and  clays,  or  *  *  Tegel "  (the  local  name  for  a  calcareous  clay).  According  to  Fachs, 
the  following  subdivisions  occur  around  Vienna  : — 

Upper  Sarmatian  Tegel,  or  Muscheltegel — distinguishable  from  the  Hemals 
Tegel  below  by  an  abundance  of  shells  {Tapes  (fregaria,  Ervilia,  CttrdiuwL,  &c), 
295  feet. 

Cerithiuni-saiul — a  yellow,  abundantly  shell -bearing,  quartz-sand — the  main 
source  of  water-supply  at  Vienna,  where  it  is  sometimes  nearly  500  feet  thick. 

Hemals  Tegel — sand  and  gravel,  with  Cerithiuvi,  Rissoa,  Paludina,  remains 
of  turtles,  fish,  and  laud  plants. 

The  Sarmatian  stage  is  characterised  by  the  prodigious  number  of  individuals 
of  a  comparatively  small  number  of  species  of  shells,  of  which  some  of  the  most 
characteristic  forms  are  Tapes  gregaria  (Fig.  438),  Mactra  podoUca^  ErvUia 
pmiolica^  Cerithiumpictum^  C,  mtbiginosvm^  Bxiccinum  haccatum^  Troehtts  podoli- 
cus,  Afurex  sublsBvatus.  The  general  character  of  the  fauna  is  that  of  a  temperate 
climate,  and  is  strongly  contrasted  with  that  of  the  Mediterranean  stage  in  the 
absence  of  the  affinities  with  tropical  or  sub-tropical  forms,  and  even  with  those 
of  the  preseut  Mediterranean,  and  on  the  other  hand  in  some  curious  analogies 
with  the  living  fauna  of  the  Black  Sea.  Corals,  echinoderms,  bryozoa,  foraminifera 
are  absent  or  very  rare,  and  the  suggestion  has  been  made  that  the  change  of  the 
earlier  Mediterranean  fauna  into  that  of  the  Sarmatian  stage  points  to  a  gradual 
diminution  of  the  salinity  of  the  waters  of  the  Vienna  basin,  as  has  happened 
with  the  existiug  Black  Sea.  The  terrestrial  flora  is  characterised  by  some  plants 
that  survived  from  the  earlier  or  Mediterranean  stage  ;  but  palms  are  entirely 
absent,  and  the  American  element  in  the  flora  is  no  longer  surpassed  by  the 
preponderance  of  Asiatic  types. 
Mediterranean  or  Marine  Stage. — A  group  of  strata  varying  greatly  from 
place  to  place  in  petrographical  characters,  with  coiTesponding  differences  in  fossil 
contents.     Among  the  more  important  types  of  rock  the  following  may  be  named  : 

Leithakalk,  a  limestone  often  entirely  composed  of  organisms,  and  especially  of 
reef-building  corals,  also  bryozoa,  foraminifera,  echini  (large  clypeasters,  &c.),  large 
oysters  {Pecten  iatissimus  is  specially  characteristic),  boneij  of  mammals,  and 
sharks'  teeth.  Tlie  Leithakalk  passes  frequently  into  sandy  and  marly  beds,  and 
into  massive  conglomeratic  deposits  (Ijcithakalk-schotter  or  conglomerate). 

Tegel  of  Baden — tine  blue  clay,  richly  charged  with  shells,  esiiecially  gastero« 
po<ls  {Pleurotumay  Cancellaria^  Fusu/t,  &c.)  and  foraminifera. 

Marl  of  Gainfahreu,  Grinzing,  Nussdorf,  &c. — more  calcareous  than  the  Baden 
Tegel. 

Sand  of  Potzleinsdorf — a  fine  loose  sand  with  TeUina,  Psamnu^na,  and  many 
other  lamellibranchs. 

Sandstone  of  Sievering  with  many  lamellibranchs,  especially  pectens  and  oysters. 

These  various  strata  are  believed  to  represent  dift'erent  conditions  of  deposit  in 
the  area  of  the  Vienna  basin  during  the  time  of  the  Mediterranean  stage.  With 
them  are  grouped  certain  fresh-water  beiis  (brown -coals,  &c.),  found  along  the 
margin  of  the  basin,  which  are  supposed  to  mark  some  of  the  terrestrial  accumu- 
lations of  the  i>eriod. 

The  characteristically  marine  fauna  of  this  stage  is  a%imdant  and  varied.  It 
presents  as  a  whole  a  more  tropical  character  than  that  of  the  Sarmatian  stage 
above.  Of  its  niolluscan  genera  (of  which  more  than  1000  species  have  been 
described)  some  of  the  more  -characteristic  are  :  ConuSj  Oliva^  Cyprtea^  VolxtUt, 
Mitra,  (hufsis^  Stromhus^  Triton^  Murex\  Plciir(^toi)ut^  Cerithium,  Spondylus^ 
l*inno,  Pectunculus,  (Uirdita^  Venvs.  A  number  of  the  species  still  live  in  the 
Mediterranean,  or  in  the  seas  ofi"  the  West  Coast  of  Africa.  The  abundant  flora, 
with  its  various  kinds  of  palms,  had  also  a  tropical  aspect,  somewhat  like  those 
of  India  and  Australia. 

Switzerland.— Immediately  succeedinf^  the  strata  described  on  p.  992,  as  referable 
to  the  (Jligocene  series,  come  the  following  groups  in  descending  order  : — 

U[>})er  fresh-water  Molasse  and  brown-coal  (Oeningen  or  Tortonian  stage),  consisting 


SECT,  iii  $  2  MIOCENE  SYSTEM  '  1001 

of  saudstones,  marls,  and  limestones,  with  a  few  lignite-seams  and  fresh -water 
shells,  and  including  the  remarkable  group  of  plant-  and  insect-bearing  beds  of 
Oeuingen. 

Upper  marine  or  St.  Gallon  Molasse  (Helvetian  stage) — sandstones  and  calcareous 
conglomerates,  with  37  per  cent  of  living  species  of  shells,  which  are  to  be 
found  partly  in  the  Mediterranean,  and  partly  in  tropical  seas :  Peetunculus 
glycivicris  (pilosus),  Panopasa  Faujasii  (Menardi)^  Conus  ventricoaua^  &c. 

Lower  fresh-water  or  Grey  Molasse  (Lhangian  stage,  Mayencian) — sandstones  with 
abundant  remains  of  terrestrial  vegetation,  and  containing  also  an  intercalated 
marine  band'  with  Cerithium  liffnUarum,  Murex  plicatuSf  Venus  clathrata, 
Ostrea  crassianma^  &c. 

In  the  Oeningen  beds,  so  gently  have  the  leaves,  flowers,  and  fniits  fallen,  and  so 
well  have  they  been  preserved,  we  may  actually  trace  the  alternation  of  the  seasons 
by  the  succession  of  different  conditions  of  the  plants.  Selecting  482  of  those  plants 
which  admit  of  comparison,  Heer  remarks  that  131  might  be  referred  to  a  temperate, 
266  to  a  sub-tropical,  and  85  to  a  tropical  zone.  American  types  are  most  frequent 
among  them  ;  £uro|)ean  types  stand  next  in  number,  followed  in  order  of  abundance  by 
Asiatic,  African,  and  Australian.  Great  numbers  of  insects  (between  800  and  900 
species)  have  been  obtained  from  Oeningen.  Judging  from  the  proportions  of  species 
found  there,  the  total  insect  fauna  may  be  presumed  to  have  been  then  richer  in  some 
respects  than  it  now  is  in  any  part  of  Europe.  The  wood-beetles  were  specially  numer- 
ous and  large.  Nor  did  the  large  animals  of  the  land  escape  preservation  in  the  silt  of 
the  lake.  We  know,  from  bones  found  in  the  Molasse,  that  among  the  inhabitants  of 
that  land  were  species  of  tapir,  mastodon,  rhinoceros,  and  deer.  The  woods  were 
haunted  by  musk-deer,  apes,  opossums,  three-toed  horses,  and  some  of  the  strange,  long- 
extinct  Tertiary  ruminants,  akin  to  those  of  Eocene  times.  There  were  also  frogs, toads, 
lizards,,  snakes,  squirrels,  hares,  beavers,  and  a  number  of  small  carnivores.  On  the 
lake,  the  huge  Deinotherium  floated,  mooring  himself  perhaps  to  its  banks  by  the  two 
strong  tusks  in  his  under  jaw.  The  waters  were  likewise  tenanted  by  numerous  fishes, 
of  which  32  species  have  been  described  (all  save  one  referable  to  existing  genera), 
crocodiles,  and  chelonians. 

Italy.— The  enormous  Aquitanian  stage  of  Liguria  (p.  993)  is  followed  by  (1)  blue 
homogeneous  marine  marls,  reaching  a  depth  of  nearly  2000  feet  and  marked  by  the 
abundance  of  pteropods,  also  Ostrea  neglecta,  Caandaria  vulgaris^  and  Aturia  aturi.  This 
stage,  called  by  Mayer  **  Langhien,"  is  paralleled  with  that  of  Mainz.  It  is  surmounted 
by  (2)  the  Helvetian  stage  (3280  feet),  composed  of  three  divisions :  a  lower  (1000  to 
1 300  feet)  composed  of  shaly  marls  rich  in  Vaginella,  Cleodora,  kc.  ;  a  middle  (700  to 
750  feet)  consisting  of  yellowish  sandy  molasse  with  biyozoa,  Pecten  verUi/abrunij  Tere- 
bratula  laioceiiica^  &c. ;  and  an  upper  (more  than  300  feet)  composed  of  beds  of  conglom- 
erate and  nullipores,  \»ith  oysters,  pectens,  &c.  The  Tortonian  stage  (3)  is  made  up 
of  blue  marls  (650  feet),  forming  a  remarkably  constant  band,  with  a  profusion  of  Pleuro- 
iomarUi  and  species  of  Conus,  Naiica,  AncUlaria,  &c.^ 

Greenland.^ — One  of  the  most  remarkable  geological  discoveries  of  modern  times  has 
been  that  of  Tertiary  plant-beds  in  North  Greenland.  Heer  has  described  a  flora 
extending  at  least  up  to  70"*  N.  lat.,  containing  137  species,  of  which  46  are  found  also 
in  the  central  £uroi)ean  Miocene  basins.  More  than  half  of  the  plants  are  trees,  in- 
cluding 30  species  of  conifers  {Seqncia,  Thujopais,  Siiliaburia,  &c.),  besides  beeches,  oaks, 
planes,  poplai-s,  maples,  walnuts,  limes,  magnolias,  and  many  more.  These  plants  grew 
on  the  spot,  for  their  fruits  in  various  stages  of  growth  have  been  obtained  from  the 

1  C.  Mayer,  Bull.  Soc.  Ofol.  France  (3)  v.  p.  288.  F.  Sacco,  '  II  Bacino  Terziario  del 
Piemoute,'  Turin,  1889. 

2  Heer,  *'  Flora  Fossilis  Arctica,"  in  seven  vols.  1868-83  ;  Q.  J.  Oed.  Soc  1878.  p. 
66  ;  Nordenskiold,  (?fo^.  Mag,  iii.  (1876)  p.  207.  In  this  paper  sections,  with  lists  of  the 
plants  found  in  Spitzl>ergen,  are  given. 


1002  STRATIGRAPHICAL  GEOLOGY  book  VI  fart  it 


deposits.  From  Spitzbergen  (78"  56'  N.  lat)  136  species  of  fossil  plants  have  been  named 
by  Heer.  But  the  latest  English  Arctic  expedition  brought  to  light  a  bed  of  coal,  black 
and  lustrous  like  one  of  the  Palseozoic  fuels,  from  81**  45'  N.  lat.  It  is  from  25  to  30 
feet  thick,  and  is  covered  with  black  shales  and  sandstones  full  of  land-plants.  Heer 
notices  30  species,  12  of  which  had  already  been  found  in  the  Arctic  Miocene  zone.  As 
in  Spitzbergen,  the  conifers  are  most  numerous  (pines,  firs,  spruces,  and  cypresses),  bat 
there  occur  also  the  Arctic  poplar,  two  species  of  birch,  two  of  hazel,  an  elm,  and  a 
viburnum.  In  addition  to  these  terrestrial  trees  and  shrubs,  the  jacnstrine  waters  of 
the  time  bore  water-lilies,  while  their  banks  were  clothed  with  reeds  and  sedges.  When 
we  remember  that  this  vegetation  grew  luxuriantly  within  8**  15'  of  the  North  Pole,  in  a 
region  which  is  now  in  darkness  for  half  of  the  year,  and  almost  continaonsly  buried 
under  snow  and  ice,  we  can  realise  the  difficulty  of  the  problem  in  the  distribntion  of 
climate  which  these  facts  present  to  the  geologist. 

India.  — The  Oligocene  and  Miocene  deposits  of  Europe  have  not  been  satiafactorilj 
traced  in  Asia.  As  already  stated,  the  upper  part  of  the  massive  Nari  group  of  Sind 
may  represent  some  part  of  these  strata.  The  Nari  group  is  succeeded  in  the  same 
region  by  the  Gaj  group,  1000  to  1500  feet  thick,  chiefly  composed  of  marine  sands, 
shales,  clays  with  gypsum,  sandstones,  and  highly  fossiliferous  bands  of  limestone. 
The  commonest  fossils  are  Ostrea  multicodaiay  and  the  urchin  Breynia  carinata.  Some 
of  the  species  are  still  living,  and  the  whole  aspect  of  the  fauna  shows  it  to  be  later  than 
Eocene  time.  The  uppermost  beds,  are  clays  with  gypsum,  containing  estuarine  shells 
and  forming  a  passage  into  the  important  Manchhar  strata.  The  Manchbar  group 
of  Sind  consists  of  clays,  sandstones,  and  conglomerates,  sometimes  probably  10,000 
feet  thick,  divisible  into  two  sections,  of  which  the  lower  may  possibly  be  Miocene,  while 
the  upj)er  may  represent  the  Pliocene  Siwalik  beds  (p.  1020).  As  a  whole,  this  massive 
group  of  strata  is  singularly  un fossiliferous,  the  only  organisms  of  any  importance  yet 
found  in  it  being  mammalian  bones,  of  which  22  or  more  species  have  be^  recognised. 
All  of  these  occur  in  the  lower  section  of  the  group.  They  include  the  carnivore 
Amphiajon  palmindicus,  three  species  of  Mastodon,  one  of  Deirutiherium.,  two  of 
lihintK'^rns,  also  one  of  SuSy  Chalicoth^riuvi,  Anthrajcoiheriuviy  ffyopotamus,  HyofJierium, 
Dorcatheriiuii  (two),  Manis,  a  crocodile,  a  chelonian,  and  an  ophidian.* 

North  America.— Overlying  the  Eocene  formations  (p.  981),  and  following  in  a  general 
way  their  trend,  but  sometimes  with  a  slight  unconformability,  a  belt  of  marine  deposits, 
referred  to  the  Miocene  period,  runs  along  the  Atlantic  border  through  the  states  of  New 
Jersey,  Delaware,  Marj'land,  Virginia,  North  and  South  Carolina,  and  Georgia.  These 
strata  (**  Yorktown  ''  and  "Sumpter"  groups  of  Dana)  have  recently  been  classified  by 
A.  Heilprin  as  follows :  3,  Upper  or  Carolinian  (North  and  South  California,  Sumpter 
l)eds).  2,  Middle  or  Virginian  (Virginia  and  newer  group  in  Maryland  ;  Yorktown  beds, 
in  part)  ;  one  of  the  most  interesting  members  of  this  subdivision  is  the  "Richmond 
earth,"  a  diatomaceous  deposit,  sometimes  30  feet  thick,  lying  near  the  base  of  the 
group.  1,  Lower  or  Marylandian  (older  Miocene  deposits  of  Maryland  and  possibly 
lower  beds  in  Virginia  ;  Yorktown  beds,  in  part).*-^ 

Westward,  in  the  Upi)er  Missouri  region,  and  across  the  Rocky  Mountains  into  Utah 
and  adjacent  territories,  strata  assigned  to  the  same  geological  period  have  been  termed 
the  White  River  group.  They  were  laid  down  in  great  lakes,  and  attain  thickne-sses  of 
1000  to  2000  feet.  The  organic  remains  of  these  ancient  lakes,  so  well  studied  by  Leidy, 
Marsh,  and  Cope,  embrace  examples  of  three-toed  horses  (Anchithtrium^  Miohippus, 
Afesohippifs),  tapir-like  animals,  differing  from  those  of  the  older  Tertiary  strata 
[Lophiodmi)  ;  hogs  as  large  as  rhinoceroses  {Elolherium) ;  true  rhinoceroses  (BhinoccroSy 
JlfUCf'Kion,  JHccnith/'.rium),  huge  elephantoid  creatures  allied  to  the  Deinoceras  and 
tapir  {Bruntothcritim,  TUanotherium) ;  also  even-toed  ruminant  ungulates,  some  allied  to 

»  Medlicott  and  Blanford's  'Geology  of  India,'  p.  472. 
-  A.  Heilprin,  as  cited  on  p.  981. 


SECT,  iv  §  1  PLIOCENE  SYSTEM  1003 

the  hog  {Oreodonta)^  others  like  stags  {Leptomeryx)  and  camels  {Foihrotherium) ;  carnivores 
{CaniSf  Amphicyon  {Daphsmua),  Maehairodus^  ffyanodon),  several  of  which  are  gener- 
ically  identical  with  European  Tertiary  wolves,  lions,  and  bears.  Among  the  smaller 
forms  are  the  remains  of  the  earliest  known  beavers  {FalsBocastor), 

Aostralia. — Tertiary  deposits  are  extensively  developed  in  various  parts  of  the 
Australian  Continent.  In  Victoria  they  cover  nearly  half  of  the  colony,  and  are  there 
capable  of  subdivision  into  an  older  and  newer  series.  The  older  series  is  believed  to 
be  later  than  Eocene  and  to  be  possibly  of  Oligocene  or  older  Miocene  age.  It  con- 
sists principally  of  blue  or  grey  clays  with  septarian  nodules,  rich  in  fossils,  among 
which  gigantic  forms  of  Volutes  and  Cowries  are  conspicuous.  Later  than  these  clays 
are  certain  (Miocene)  deposits  indicating  marine,  lacustrine,  and  terrestrial  conditions, 
with  the  existence  of  contemporaneous  volcanic  activity  towards  the  end  of  the  series. 
The  marine  rocks  consist  mainly  of  calcareous  sandy  strata  and  limestones,  with 
CelUpora^  SpalanguSf  TerebrattUaf  kc.  The  lacustrine  deposits  are  clays  and  lignites,  and 
the  fluviatile  materials  consist  of  gravels  and  sands  which  are  often  auriferous.  Great 
sheets  of  basalt,  forming  the  older  volcanic  series,  have  been  poured  over  these  various 
accumulations,  which  are  sometimes  300  feet  thick.  A  large  series  of  plants,  mollusks, 
fishes,  and  marine  mammals  has  been  obtained  from  the  Miocene  series  of  Victoria.^ 

New  Zealaad. — Rocks  assigned  to  Miocene  time  in  New  Zealand  are  divisible  into  : 
1st,  A  lower  series,  consisting  of  calcareous  and  argillaceous  strata  widely  spread  over 
the  east  and  central  ])art  of  the  North  Island  and  both  sides  of  the  South  Island.  They 
can  be  traced  to  a  height  of  2500  feet  above  the  sea.  Marine  shells  abound  in  them, 
including  55  species  which  are  found  among  the  450  shells  that  now  live  in  the  adjacent 
seas.  Some  of  the  most  notable  fossils  are  DtrUalium  irregtUare,  Pleurotoma  awamoa- 
ensiSf  Conus  Trailli,  TurriUlla  gigantea,  Buccinum  Robinaonif  CuculUta  alta.  In  some 
places  thick  deposits  of  an  inferior  kind  of  brown-coal  occur  in  this  subdivision.  2nd, 
An  upper  series  composed  of  littoral  or  sub-littoral  accumulations  of  sand,  gravel,  and 
day.  They  have  yielded  120  recent  species  of  shells,  and  25  species  which  appear  now 
to  be  extinct.  Specially  characteristic  are  Ostrea  ingeiiHy  Murex  odagonvSy  Fumis  Irilan, 
StnUhiolaria  cingulata^  Chione  eusimiliSf  PecUn  gemmulatus.^ 


Section  iv.     Pliocene. 

§  1.  General   Characters. 

The  tendency  towards  local  and  variable  development,  which  is 
increasingly  observable  as  we  ascend  through  the  series  of  Tertiary 
deposits,  reaches  its  culmination  in  those  to  which  the  name  of  Plio- 
cene has  been  given.  The  only  European  area,  in  which  Pliocene  strata 
attain  any  considerable  dimensions  as  rock-masses,  is  in  the  basin  of  the 
Mediterranean,  especially  along  both  sides  of  the  Apennine  ct^ain  and  in 
Sicily.  In  that  region,  reaching  a  thickness  of  1 500  feet  or  more,  they 
were  accumulated  during  a  slow  depression  of  the  sea -bottom,  and 
their  growth  was  brought  to  an  end  by  the  subterranean  movements 
which  culminated  in  the  outbreak  of  Etna,  Vesuvius,  and  the  other  late 
Tertiary  Italian  volcanoes,  and  in  the  uprise  of  the  land  between  the 
base  of  the  Apennines  and  the  sea  on  either  side  of  the  peninsula.  Else- 
where the  marine  Pliocene  deposits  of  Europe,  local  in  extent  and  variable 

*  R.  A.   F.  Murray,  *  Geology  and   Physical  Geography  of  Victoria,'  1887.     M*Coy, 
*  Prodromus  of  Victorian  Paleontology.* 

'  Hector,  '  Handbook  on  New  Zealand,'  p.  27. 


1004  STRATIORAPHWAL  QEOLOOY  BOOK  vi  part  iv 

in  character,  reveal  the  beds  of  shallow  seas,  the  elevation  of  which  into 
land  completed  the  outlines  of  the  Continent  at  the  close  of  Tertiary 
time.  Thus  these  n'aters  covered  the  south  and  south-east  of  England, 
spreading  over  Belgium  and  a  small  part  of  northern  France,  but  leaving 
the  rest  of  northern  and  western  Europe  as  dry  land.  Here  and  there, 
in  south-eastern  Europe,  evidence  exists  of  the  gradual  isolation  of 
portions  of  the  sea  into  basins,  somewhat  like  those  of  the  Aralo-Caspian 
depression,  with  a  brackish  or  less  purely  marine  fauna.  In  some 
portions  of  these  basins,  however,  as  in  the  Karabhogas  Bay  of  the 
existing  Caspian  Sea,  such  concentration  of  the  water  took  place  as  to 
give  rise  to  extensive  accumulations  of  salt  and  gypsum.  In  a  few 
localities,  tluviatile  and  lacustrine  deposits  of  the  Pliocene  period  have 


been  preserved,  from  which  numerous  remains  of  terrestrial  vegetation 
and  mammals  have  been  obtained. 

The  Pliocene  flora  is  transitional  between  the  luxuriant  evergreen 
and  sub-tropical  vegetation  of  the  Miocene  period  and  that  of  modem 
Europe.  From  the  evidence  of  the  deposits  in  the  upper  part  of  the  valley 
of  the  Arnf),  above  Florence,  it  is  known  to  have  included  species  of 
pine,  oak,  evergreen -oak,  plum,  plane,  alder,  elm,  fig,  laurel,  maple, 
walnut,  birch,  buckthorn,  hickory,  sumach,  sarsaparilla,  sassafras,  cin- 
namon, glyptostrobus,  taxodium,  sequoia,  &c.'  The  researches  of  Count 
de  Saportii  have  shown  that  the  flora  of  Meximieux,  near  Lyons,  com- 
prised species  of  bamboo,  liquidambar,  rose-laurel,  tulip-tree,  maple,  ilex, 
glj'ptostrobus,  magnolia,  poplar,  willow,  and  other  familiat  trees.*     The 

'  Caudin,  '  Fi'uilli'S  (nasiles  rlu  In  ToHcniic';  Gaudin  and  Stroul  'Contributions  a  U 
h'lor,?  fosfiile  lt!ili,-imp '  ;  Ljell,  "Student's  Elements,'  4lh  edit.  \\  172. 

-  "  liecherclics  vat  les  Vegrtaux  foBsilei  de  Meximieux,"  ATthir.  Mia.  Lyon,  i.  (1875-76) 
and  his  ■  Monde  its  Pliintes,'  p.  314. 


vii 


PLIOCENE  SYSTEM 


1005 


forests  of  that  part  of  Europe  during  Pliocene  time  conjoined  some  of  the 
more  striking  characters  of  those  of  the  present  Canary  Islands,  of  North 
America,  and  of  Caucasian  and  eastern  Asia,  including  Japan.  There  is 
evidence,  however,  that  a  marked  refrigeration  of  climate  was  in  gradual 
progress,  during  which  the  plants,  such  as  the  palms,  especially  chaiac- 
teristic  of  warmer  latitudes,  one  by  one  retreated  from  the  European 


region,  or  lingered  only  on  its  southern  borders.  In  England,  towards 
the  end  of  the  Pliocene  period,  the  climate,  if  we  may  judge  of  it  from 
the  plants  preserved  in  the  Cromer  Forest^bed,  had  come  to-  be  very 
much  what  it  is  to-day.  Among  the  vegetable  remains  found  in  that 
deposit  are  those  of  many  of  the  familiar  forest  trees  still  living  in  the 
south-east  of  England.  Some  of  our  common  wild-flowers  and  wat«r- 
plants  had  now  made  their  appearance,  such  as  the  buttercup,  marsh- 


1006  STRATIGRAPHWAL  QEOLOGY  book  vi  fa«t  n 

marigold,  cbickweed,  milfoil,   mareeUil,  dock,  sorrel,  pondweed,  sedge, 

cotton-grass,  reed  and  royal  fem.^ 

The  fauna  of  the  Plioceoe  period  still  retained  a  nomber  fA  tfae  now 
extinct  types  of  earlier  time,  such  as  the  DeifuAerivoi 
and  MaUodon.  It  was  specially  charaeterued  also  by 
troops  of  rhinoceroses,  hippopotamuses,  and  elephants, 
the  Elephas  mendionaiis  being  a  distinctive  form ;  by 
large  herds  of  herbivoni,  including  numerous  forms  of 
gazelle,  antelope,  deer,  now  mostly  extinct,  and  types 
intermediate  between  still  living  genera.  Among  tbese 
were  some  colossal  ruminants,  including  a  species  of 
giraffe  and  the  extinct  giralTe-like  genera  HeiladoUterium 
and  Sfifnolherium,  as  well  as  other  types  met  with  among 
the  Siwalik  beds  of  India  {Siealherium,  Fig.  453,  Iframa- 
therium).  The  Eqiiidffi  were  represented  by  the  existing 
Equtis,  and  by  extinct  forms,  one  of  the  most  abundant 
of  which  was  Hipparion  (Fig  445),  like  a  small  ass  cm- 
qua^a,  with  three  toes  on  each  foot,  only  the  centisl 
FiK.  444,— Eifphu  one  actually  reaching  the  ground.  Besides  these  animab 
ni-TidioniLi.,  smu.  there  lived  also  variouB  apes  {Mese^tkeeug,  Fig.  4*6, 
n>»n     inu»r(      Oolic/wpUhfCus),  likewise  species  of  ox,  cat,  bear,  maclisi- 

rodus,  hywna,  fox,  viverra,  porcupine,  beaver,  hare,  and  mouse. 


— Hlpi"rioii  Bi»cH( 


The  advent  of  a  colder  period  is  well  shown  in  the  younger  Pliocene 
deposits  of  south-eastern  England,  where  a  number  of  northern  moUusks 
make   their  appearance.     The    proportion  of  northern  species  increases 

'  C.  Reiil,  -  Pliocene  Deposits  at  Brittiu,'  Mem.  Geol.  Shtt.  (1890)  pp.  188.  831 


PLIOCENE  SYSTEM 


rapidly  in  the  next  succeeding  or  Pleistocene  beds.     The  Pliocene  period, 
therefore,    embraces   the   long   interval   between   the   WMm   temperate 


climate  of  the  later  ages  of  Miocene  and  the  cold   Pleistocene  time. 
The  evidence  of  change  of  climate  derivable  from  the  English  Pliocene 


« 


— PllocaiK  Msrlpg 


i.pmu.(i).d. 


rD[>hoa  ■Dliquu 


marine  moUusca  may  be  grouped  as  in  the  subjoined  table,  which  shows 
the  gradual  extirpation  of  southern  and  advent  of  northern  forms  in  the 
long  interval  between  the  deposition  of  the  oldest  and  newest  Pliocene 
deposits.' 

'  C.  Bcid,  €p.  cil.  p.  U5. 


1006 


STRA  TIGRA  FHICA  L  GEOL'jG  Y 


HX>KTI  rAMS  IT 


Tocal 
Sperka. 

Arctic.      1 

f«*biMzaaffa£. 
0 

■«-^ 

W*rTV*u!Ti  Crag  . 

53 

5 

C-nill^-fonl  (."nss . 

»0 

I 

2 

14 

F.  a  \  io-  ruanoe  TYag 

112 

9 

« 

1* 

Re^l  f  rwr  of  Borton.  ite. 

119 

13 

2S 

S5 

K«<1  ('nil  of  Walton     . 

14ft 

2 

22 

50 

f'oraliine  Craz     . 

420 

1('» 

75 

1<» 

E 


j5  2.  Local   Development. 

Britain.*  — In  the  Pliocene  period,  after  a  long  {terioii  of  expasikre  as  a  land-sarfia, 
during;  »hi':h  a  contiouoos  and  oltimatelj  stufiendoiu  snbaerial  dcnodatkm  wa*  in  fto- 
grtriii,  Britain  underwent  a  gentle,  bnt  apparentlv  onlr  local,  sal«ideiiec;.  We  harv  no 
evi<leii'-e  of  the  extent  of  this  depression.  All  that  i^n  be  affinnoi  is  that  the  sonth- 
eastcrn  cf>uiities  of  England  began  to  snhnde,  and  on  the  submerged  siir£sc«  some  and- 
Vaiik.^i  and  »heily  de|N>»it4  were  laid  down,  rerr  much  as  similar  accnmnlations  now  take 
place  ou  the  l>ottom  of  the  Xorth  Sea.  These  formations,  termed  generallj  "  Cra^"  are 
follower  1  by  «»tuanue  and  fresh- water  strata,  the  whole  being  subdirided,  according  to  the 
pro[iortion  of  living  .species  of  shells,  into  the  following  gronps  in  descending  order  :— 


Base  of  the    I 
Pl'si'ito'.euc.  j 


Arctic  Fn»h-water  Bed  (with  SaJU  /^^arU.  Bttula  matuu  Jcc  t 
(  Leda  m^fdi*  Bed.  clashed  proTi»ionally  as  PUoccaie. 


N«;wer 

Plio'-ene 

(cold  tem- 

jjeratf ». 


,,       ^  ,    ,  ( Upper  FTe>h-water. 

(10to«0f«t|.       (Lo«r  F«»h-».t*r. 


k  GraveU  with 
f  ElepJut*  mer- 
t  idumalU  at 
)      Dewlish. 


OMer 

Pli'xrntr 
<  wanii  tern- 


I 


Wey bourn  CYag  (and  Cbillesford  Clay  h.  1  to  22  feci. 

rhii'.esford  Crag  (5  to  15  f«rt>- 

Norwich  CYa^  au«i  SrnJncularin  Crag  fS  to  10  ftet«.  \  -i  .-      f 

Pu-I  CraiT  of  Builey.  Ac.  C     L.  ,T    1 1** 

Waltou  <  ra^  i  Lower  Re«i  C^,  25  f«rt K  J       ^^^^^''^'^. 

St.  Erth  H^'is, 

roralllntr  Crair  i40  to  tK)  feet). 

I^enhain  h*ni>  \  I>iestian  i. 

Box->ton«rs    and    phojijihate   WIh    (with    derivative    early    Pliocene 

fo•'^il!»|. 


Oli>ki;  Vu*f  KNE. — The  de^iosits  of  this  age  prol>ably  at  one  time  extended  over  a  larpf 
jMirt  of  the  south  and  south-east  of  England,  but  they  have  been  reduced  by  denudatiou 
to  a  f«;w  widely  separate<l  jiatches,  the  largest  of  which,  around  Oxford  in  Suffolk,  does 
not  cov«-r  more  than  aV>out  ten  si|iiare  miles.  They  consist  chiefly  of  shelly  sands 
known  as  the  Corjilline  Crag  of  Suffolk,  but  a  small  outlier  of  fossiliferous  sand  occur 
on  the  e.lgH  df  th*-  North  Downs  at  Lenham,  and  other  ironstone  (latches,  probably  of  the 
same  agf,  rap  the  I)t»wn  as  far  as  Folkestone.  Far  to  the  west,  at  St.  Erth  in  Cornwall, 
an  is^jlat*-*!  d»'jKi>it  of  older  Plio<*ene  age  has  been  detected.  These  thin  and  scattered 
fra;;iii»rnts  convey  no  ad<.*<juate  conception  of  the  length  or  importance  of  the  geologi<*al 
perio^l  will)  h  they  repres**nt.  It  is  not  until  we  |wss  into  the  north  of  Italy  and  the 
basin  ot  tli»-  .Mediterranean  that  we  <liscover  the  Pliocene  system  to  he  represented  hy 

'  Pr«;Htwi.'h,  i^.  J.  O'U.  Soc.  ixrii.  ;  Lyell,  'Antiquity  of  Man,*  chap.  xii.  ;  Searks 
W.-rni.  -(ni;:  M'.llu.>oa,"  Palxi^nt.  .Stn:.  ;  H.  B.  Woodward,  "Geology  of  Norwich,'*  and  W. 
Wiiit;ikir.  -(Jeolofry  of  Ipswich,  A:c.,"  lx)th  in  M^m.  fiad.  Surety.  The  fullest  account  of 
th»'  -ut.jtct  will  l>e  found  in  the  monograph  by  C.  Reid,  already  cited,  on  the  'Pliocene 
Dej.o-il-  '.f  Britain,'  Mnn.  ^rVo/.  Surxxy,  1890. 


SECT,  iv  §  2 


PLIOCENE  SYSTEM 


1009 


thick  accumulations  of  upraised  marine  strata  comparable  in  extent  and  thickness  to 
some  of  the  antecedent  Tertiary  series. 

A  strongly  marked  break,  both  stratigraphical  and  palseontological,  separates  the 
Pliocene  deposits  of  Britain  from  all  older  formations.  They  lie  unconformably  on 
everything  older  than  themselves,  and  in  their  fossils  show  a  great  contrast  even  to 
those  of  the  Oligocene  series.  The  sub-tropical  plants  and  animals  of  older  Tertiary 
time  are  there  replaced  by  others  of  more  temperate  types,  though  still  pointing  to  a 
climate  rather  warmer  than  that  of  southern  England  at  the  present  time. 

A  conglomeratic  deposit  (Nodule  beds)  forms  the  base  of  the  Red  Crag,  and  appears 
generally  to  underlie  also  the  Coralline  Crag.  It  includes  fragments  of  various  rocks 
such  as  flints,  septaria,  sandstones,  quartz,  (^uartzite,  granite,  and  other  igneous 
materials,  together  with  a  miscellaneous  assortment  of  derivative  fossils,  including 
Jurassic  ammonites  and  brachiopods,  sharks'  teeth  and  other  fossils  from  the  London 
Clay,  the  teeth  of  many  land  mammals  (pig,  rhinoceros,  mastodon,  tapir,  deer, 
hipparion,  &c. ),  and  pieces  of  the  rib-bones  of  whales.  Many  of  these  organic  remains 
must  have  been  derived  from  some  older  Pliocene  deposit  which  has  otherwise  entirely 
disappeared.  They  have  been  to  a  large  extent  phosphatised,  and  hence  have  been 
extracted  as  a  source  of  phosphate  of  lime.  Among  the  contents  of  the  deposit  some  of 
the  most  interesting  and  important  are  rounded  pieces  of  brown  sandstone,  known  as 
**  box-stones,"  evidently  derived  from  the  denudation  of  a  single  horizon,  and  enclosing 
casts  of  marine  shells.  The  general  facies  of  the  assemblage  of  shells  obtained  from 
these  fragments  of  a  lost  formation  points  unmistakably  to  early  Pliocene  time.  At 
present  16  species  have  been  determined,  all  of  which  are  well-known  British  Pliocene 
forms,  except  two  which  occur  in  Continental  Pliocene  deposits.^ 

Coralline  Crag  (Bryozoan, White,  or  Suflblk  Crag)  consists  essentially  of  calcareous 
sands,  mainly  made  up  of  shells  and  bryozoa,  and  is  exposed  at  various  localities  in 
the  county  of  Suffolk.  According  to  the  census  of  Searles  Wood,  published  in  1882, 
the  number  of  mollusks  found  in  this  deposit  amounts  to  420  species,  of  which  251  or 
60  per  cent  are  still  living.  Some  of  the  genera  of  shells  give  a  southern  character 
to  the  fauna,  such  as  large  and  showy  species  of  Voluta,  Cassidaria,  GassiSy  FicuUiy 
HinniteSj  Chamay  Cardita,  and  Pholado7i),yay  likewise  Ovula,  Mitra,  Triton,  Vermetus, 
Kingicula,  Verticardia,  Coralliophaga,  and  Solecurtus.  Characteristic  species  are 
Cardita  carbis,  Cardita  senilis,  Limopsis  pygmasa,  Ringicuia  Iniccinea,  Valuta  Lainberti 
(Fig.  450),  Pyrula  reticulata,  Astarte  Omalii  (Fig.  449),  Pholadomya  histema,  Pecten 
upercularis,  Lingula  Dumortieri,  and  Terehratula  grandis.  Hardly  less  abundant  and 
varied  arc  the  bryozoa  or  **  Corallines,"  from  which  the  name  of  the  deposit  is  taken. 
No  fewer  than  118  species  have  been  named,  of  which  76,  or  about  64  per  cent,  appear  to 
be  extinct.  Specially  characteristic  and  peculiar  are  the  large  massive  forms  known  as 
Alveolaria  and  FasciculaHa  (Fig.  448).  There  are  three  species  of  corals  all  extinct. 
Of  the  16  species  of  echinoderms  at  present  known,  only  three  are  now  living.  Remains 
of  fishes  are  of  common  occurrence,  especially  in  the  form  of  gadoid  otoliths.  Teeth 
and  dermal  spines  of  the  skate  and  wolf-fish  are  met  with,  and  to  these  shell-eating 
fish  the  broken  condition  of  so  many  of  the  shells  may  probably  be  ascribed.  Traces 
of  one  of  the  larger  dolphins  have  been  found,  but  no  remains  of  any  of  the  contem- 
poraneous land  -  mammals,  though  a  few  drifted  land-shells  show  that  the  land  lay 
probably  at  no  great  distance.  The  Coralline  Crag  may  be  regarded  as  an  elevated 
shell-bank,  which  accumulated  on  the  floor  of  a  warm  sea  at  a  depth  of  from  40  to  60 
fathoms.  '^ 

Lenham  Beds,  Diestian. — On  the  edge  of  the  Chalk  Down  of  Kent  near  Len- 
ham,  patches  of  sand  are  found  capping  the  Chalk,  and  descending  into  pipes  on  its 
surface,  at  a  height  of  more  than  600  feet  above  the  sea,  and  other  similar  nests  of 
ferruginous  sands  are  met  with  along  the  downs  as  far  as'  Folkestone.     At  first  these 


^  C.  Reid,  op,  cit.  p.  6  seq. 


3t 


^  C.  Reid,  op.  dL  p.  19  seq. 


1010  STRATIGRAPHICAL  GEOLOGY  book  ti  rtasn 

ileponti  were  thought  to  be  portioiu  of  tha  base  of  the  TettwiT  tme^  bat  tbe  «taiT- 

reuce  of  apjiarvntly  Pliocene  shelU  in  tbem  led  to  ■  more  thorough  inTetfi^tioa  «f 

them,  with  the  result  that  they  have  becD  proved  to  be  of  tbe  aun«  age  aa  liiiiiUr 

ileposite  whieb  ca|i  the  hilla  on  the  other  ride  oF  tbe  Stnits  of  Dovar  from  BookpK 

into  Belgian  Flanden,  whence  they  stretch  northwards  as  a  wide  eoDtiniuMia  aheet  into 

Holland.      These  sands,  Iedowd  ma  Diestian,  have  )-ielded  t.t  Diest  and  Antwerp  a  large 

■Dsemblage  of  fossils,  which  prove   them   to  bs  of  older 

Plioceoe  age.      Of   the   Dieatian   foasU*  of  BoUand  and 

Belgium  so  large  a  proportion  has  been  detected,  t^nerally 

in  the  form  of  hollow  casts,  in  the  Lenbaoi  depoeCta  •■  to 

leave  no  doubt  of  the  geological  horizon  of  these  scattered 

fragments  of  &  fonnatioD.      Abont  67  apeciea  have  been 

obtMued  from  Lenham,  the  southern  character  of  which 

is   indicated    hy    the    genera    Ficuia   {Pyrula],   XexijAtm 

IFItoria),  Tri/oit,  and  Aricnla,  with  almndaiit  examples  of 

Ana  dilavii,  Cardium  papillaium,  and  CujnUaria  camari- 

iF  PoItzoou,     ''<*('•     1'  >*  interesting  to  notice  the  great  change  of  level 

FincldulKria  iimnttiiin  which  this  fragmentai]-    formation  serves   to   prove   sitiee 

M.  Edw.  ()).  older  Pliocene  time  in  tbe  south  of  England.     From  the 

general   character  of  the  bnoa  found  at   Lenham   it  is 

probable  that  the  shells  lived  in  a  depth  of  not  less  than  40  fathoms  of  water.     This 

vertical  amount,  added  to  the  present  height  of  the  deposit  above  tbe  sen,  gives  a 

minimum  of  860  ftet  of  nplift.' 

St.  Erth  Beds. — The  only  other  hagment  yet  known  of  older  Pliocene  forma- 
tions in  Britain  lies  far  to  the  weat  between  St.  Ives  and  Mount's  Bay  in  Cornwall, 
where  B  [>atoh  of  clay,  probably  less  than  a  quarter  of  a  sqnare  mile  in  area,  contained  in 
a  holluw  of  the  sUtes,  has  preserved  an  interesting  series  of  organic  remaiDii.  Among 
the  forms  which  connect  this  deposit  with  corresponding  strata  elsewhere  the  folloning 
may  be  mentioned  :  Cb^itnitzia  plitatula,  Colnmbtlla  milaUa,  Cuprma  atetlana,  E»H- 
mi^ne  terebellata,  Fumrella  coataHo,  Laevaa  aiiboperta,  Metampui  pyramidalit,  .Yarn 
rcUa-ta,  K'llua  niillepuiiclula,  Eingicvla  acuta,  Trixhus  noduli/enna,  Turrilella  I'ncnu- 
Kiln,  CnrdiUi  ncatcat-i,  Cardiuin  papillosvm,- 

Xk»'i:k  Pliui'Km:. — The  British  dejHisits  of  this  age  are,  so  far  as  we  know,  confined 
to  the  counties  of  Korfolk  and  SnOblk.  They  are  separatcU  by  a  considerable  breali 
from  the  older  series,  for  they  lie  on  an  eroded  surface  of  the  latter,  sad  pass  across  it  so 
as  to  rest  n)>o]i  the  Eocene  fummtions,  and  even  on  the  Chalk.  There  is  likewise  i 
marked  contrast  lictwecn  the  fauna  of  the  two  series.  The  newer  deposits  show  that 
the  IjL-eak  tnu^it  represent  a  long  perioil  of  geological  time,  during  which  a  great  change 
of  I 'li  mate  took  ]ilate  in  Kuropa,  for  the  southern  forma  are  now  found  to  have  generally 
disap]icar<'il,  and  to  have  been  replaced  by  northern  forms  that,  following  the  change 
of  tcmjK^raturc,  had  migrated  from  tbe  eokler  north. 

Ked  Cra)L — Uiiiler  this  nauie  is  classed  a  series  of  local  accumnlations  of  dark-red  or 
brown  ferruginous  shelly  sand,  which,  though  well  marked  off  from  the  Coralline 
Crag  !>elow,  is  less  definitely  separable  from  the  Xorwieh  Crag  above.  Judging  from  the 
variations  in  ils  fossil  contents,  geologists  have  inferred  that  some  portions  of  tbe  deposit 
are  olikr  than  others,  and  that  they  successively  overlap  each  other  as  they  are  followed 
nortliward.  The  oldest  part  is  believed  to  oerur  at  tbe  southern  end  of  the  area  at 
Walton,  ulierc  it  yields  a  fauna  closely  similar  to  that  of  the  Coralline  Crag.  This  portion 
is  lost  a  few  niiles  farther  north,  where  the  Red  Crag  of  Butley  appeata,  containing  many 
Arctic  luolluaua.  Iti  the  older  crag  of  Walton  the  advent  of  a  colder  climate  is  indi- 
cated by  the  appearance  of  the  northern  shells  Biiccinurii  glaciate  and  Trophon  aatiari- 

'  C.  lltid,  Of.  rii.  ,,(,.  -li  eS.  =  Ibiit  pp.  5»,  236. 


^52 


PLIOCENE  SYSTEM 


ion 


formit,  but  many  of  the  soathern  foniu  (till  linger,  such  m  Cer^iam  trilauaium, 
Chemniltia  intemodiUa,  Nana  limata,  Naiiea  tnillepundata,  Owia  tpeXta,  PlewroUmta 
hystrix,  TurrUella  iTierasaata,  Cardita  corbii,  Cythcrea  mdis,  and  Limoftit  pygtama. 
In  the  younger  part  of  the  Bed  Crag  the  proportion  of  northern  gtiells  greatly  increaBea. 
Among  theoi  are  Carwxllaria  viridttla,  Nalica  ocditaa,  Pleurotoma  p^framidalM,  P. 
lealarit,  jTropAon  icalariformii,  T.  Sarni,  Caniiain  gncidandiatm,  Leda  lanceolaia,  and 
Solen  gladiolus.  Characteristio  sheUa  of  tlie  Red  Crag  are  AcUeon  A'os,  CapuluaofJiguiu, 
Cerithium  tricinctum,  EuZimtJU  Urdxllala,  Nalica  kemielauaa,  Flewnloma  turrifera, 
Sealaria  funiculitl,  Tnxhui  dneroida,  AatarU  obliqnata,  Tellina  Bemdvni,  whitji  are 
■)l  extiucL     A  few  land  and  Ireih-water  moliuBke  have  been  met  with  in  the  deposit, 


Fig  *«.— PUoMne  Linii 
upeciM);  h. 


including  Ancyliis  lavuilrU,  Helir  kiapida,  Liinniea  paluatria,  Paladiim  media,  PUawrbia 
eomplaanius,  Papa  mvsairum,  Satcijua  patria,  and  Corlncula  fiuminalii. 

Norwicli  Crag  (Fluvio-marinB  or  Mammaliferous  Crag), — Ae  above  stated,  it  ie  im- 
possible to  draw  any  sharp  line  between  the  Red  and  the  Norwich  Craga.  They  prob- 
ably represent  varying  local  eonditiona  of  SBdimeritation  rather  than  different  agea  of 
deposit.  The  Norwich  Crag  consists  of  a  few  feet  of  shelly  Band  and  gravel,  containing, 
so  far  OS  known,  134  apeciea  of  shells,  of  which  16  i>er  cent  are  extinct.  About  20  of  the 
species  arc  land  or  liesh-water  sheUs.  The  name  of  "  Mammaliferous  "  was  given  from 
tie  large  number  of  bones,  cbietly  of  extinot  species  of  elephant,  recovered  from  this 
deposit.  The  mammalian  remains  comprise  both  land  and  marine  forma.  Of  the 
former  are  Lutnt  KavU,  Oaulla  anglica,  Cervut  camuloram,  Eq-u-us  SUnonii,  Mattodim 
arvernemia,    Elephaa  aidiqvas,    Arviaila  inUnnediia,    Trogontherivm   Cwntri.     The 


1012 


STBATIGRAPHICAL  GEOLOGY 


KTI  TAMIP 


Durine  animals  mclDde  Trirkediia  Bitxtnfi  and  Ixifkimmt  delfUa.  A  tern  riwiill  « 
«*-luba  bare  also  bcea  fooixl,  mch  ai  tlu  cod  and  pollack,  (laiiag  tfae  BMHBiea  tki 
roUuirinf!  are  chaTacteristie  tartaa  -.  J^ttiulima  mialia,  BfdrMa  \  i  af i  ini .  Tmrrildlt  oh 
•nxHit.  Tropi'rA  teaUiriformit,  LiOtnima  liBorta.  Jf^iiai  tialtM,  Xmcmlm  CtkUdim  {T^ 
449(.  Cin/iiM  <dmlt.  Om  intnnling  fodon  is  tiie  dniclai  mixtoic of  Bortksa ^pada 
of  sLttlj.  SQch  aa  IliifiieJumtila  pnttaa*,  ScaJana  ymlaadirm  (Kg.  450\,  J^Ba|M>a  aw 
nyiWi.  and  Atiarie  bortaiu  (Fig.  «49).  TbeH,  with  tbow  abore  BMBtiaacd,  woe  la* 
niDD«ra  of  tbe  great  ionsioii  of  Arctic  plauta  and  -"Jt"}*  vbicfa,  in  tbe  lii^aaia^  o 
the  Vnateniarj  agta,  came  loatlinid  into  Yaivjie,  with  tbe  aerae  iliaalii  of  Ite  M>A 

The  D|>per  |«n  of  the  Bed  Crag  somnimca  [lanii  i  into  ■  faaad.  aUtid  fn^  in  pc 
Tailing  mollnak  the  "  Scfnbicnlam  Crag.  "*  This  band,  vhiefa  ii  probaMr  a  eilinitatiw 
of  the  Nonrich  Crag  of  Norfolk,  is  aeen  at  Chillesfotd,  in  SuSbU,  to  |iaai  npvan 
withoni  a  break  into  the  CbitlMfnn]  Crag.' 

C  h  i  1 1  esfo  rd  C  rag.— rnder  this  name  ia  gmaped  a  local  M 


r,  TTOfibon  abliqints.  Hall.  (Fun* 


■«ini9  of  i^lay  ami  )iaD(k  of  shell>.  Some  of  these  sliells  I  Jfya  armariai  are  nprigtit  anc 
in  iL<:  ]4«ilioii  in  »hich  they  livM.  Northern  forms  are  still  mora  pnraiinent  het«,  whili 
a  nuni1>er  of  the  (.'ommon  Ked  Crag  forms  seem  to  hare  disap|>eared.  The  Eum*  con 
|irL-V'>  ISurriHam  fudolvin,  Hydn^ia  snhvmbUifBla,  ilelampHi  pynmidaiU.  Xatira  ia 
•■roMatn,  ,V.  rffiiVMi,  Farpvr^  lapillut,  KiH'/KvIa  renlfiivta.  TVwAm  (iinitrfiii,  Trvjiin 
nn'iq""'.  Anoiaia  tjAippiuHi,  Ailarit  lortalii,  Corditn  n-r^it,  Cardium  trnm/aadiraii 
C;'/"-.Ml  iilan'li.-'l.  L-da  laiKtolaUi,  Lorina  bontilia,  J/arfro  arrnala,  Xaeala  CtAha/diM, 
taaufsn  ni-rryjlro,  F/rttn  iipfrrnlarix.  T'i/ina  "i/"ina.  /^yneJu/ii/lIa  jmttaera. 

Weyliourii  CragandChillesford Clay.— At  ChiUesfonitb* Chilleaforf Crag  pafBB 
iDsen<'il<ly  upwards  into  the  Chillesford  Clay,  vhich  is  there  a  fine  micaecons  loam  « 
rlay  containing  a  fcT  shells  and  Hsh-vertcbrK.  Among  the  shells  of  this  deposit  ar 
B«fciH»Hi  iindatHm,  Parprra  lapillux,  A^arlf  {•/inprtsaa,  Cyprina  ulaHdita,  L»aiit 
toi-xila,  .Vbvh/ji  C-thoId-a,  X.  Unuii,  Tfllina  vbiiqiia.  Cantitim  gnmiandievm.  TtacM 
norllivar'ts  the  Chil1t«ford  Claj  appears  to  pass  into  the  deposit  knim-ii  as  the  Wejboan 
Crvg.  u'liich  U  a  band  of  laminalnl  green  and  blue  ilays  with  loamy  saod  fuU  of  marini 


'  ('.  Rtriil.  "p.  eit.  |>.  100.  For  an  arcount  of  the  vertehnte  fauna  of  then  deposits  » 
E.  T.  X*«toDs  monographs  on  "The  VertebraU  of  the  Forest  Bed  Series  of  N«folk  am 
SiilTolk"  {Ibbi   am!  "The  VertebraU  of  the  Pliocene  Deposiu  of  Britain"  in  Mtm.  Oal 


SECT,  iv  §  2  PLIOCENE  SYSTEM  1013 


shells,  well  seen  along  the  Norfolk  coast  to  the  west  of  Cromer.  This  member  of  the 
series  has  yielded  53  species  and  marked  varieties  of  marine  shells  {Tellina  halthicat 
specially  abundant,  Saxicava  arctica,  Ntunila  Cohholdia,  Mya  arenaria,  M.  trunccUa, 
Cyprina  islandicaf  Astarte  campressa,  A,  sulcata^  A.  barecUiSt  Turriiella  terebra^ 
Trophon  antiquus,  Purpura  lapilltu,  Pleurotoma  turricola,  LiUorina  liUorea^  Bucdnum 
undalum,  &c.),  of  which  five,  or  10 '6  per  cent,  are  extinct,  and  nine  species  are  Arctic 
forms. 

Forest-bed  Group.* — One  of  the  most  familiar  members  of  the  English  Pliocene 
series  is  that  to  which  the  name  of  the  **  Cromer  Forest-bed  "  has  been  g^ven.  It  occurs 
beneath  the  cliffs  of  boulder-clay  on  the  Norfolk  coast,  and  w^as  believed  to  mark  a 
former  land-surface,  with  the  stumps  of  trees  in  situ.  More  careful  study,  however,  has 
shown  that  the  stumps  have  all  been  transported  to  their  present  position,  and  lie  not 
on  an  old  soil,  but  in  an  estuarine  deposit.  It  is  now  agreed  that  the  group  of  strata 
known  as  the  Forest-bed  series  may  be  divided  into  three  groups,  an  upper  and  lower 
fresh -water  bed  separated  by  an  estuarine  layer.  The  general  character  of  the  strata 
comprised  in  this  member  of  the  Pliocene  series  is  shown  in  the  subjoined  table  : — 


^3 

o 

6 


Leda  inycUis  Bed  (p.  1014). 

Upper  Fresh-water  Bed,  consisting  of  sand  mixed  with  blue  clay  (2-7  feet)  and 

enclosing  twigs  and  shells  {Succinea  putris,  Cyclus  cornea^  Valvata  piscin- 

aliSf  Bythinia  UfUctculatOy  Piaidium  amnicumy  &c.) 
Forest-bed  (estuarine),  composed  of  laminated  clay  and  lignite,  alternating 

gravels  and  sands  with  pebbles,  cakes  of  peat,  branches  and  stumps  of  trees, 

and  mammalian  bones,  &c.  (ranging  up  to  more  than  20  feet  in  thickness). 
Lower  Fresh-water  Bed,  made  up  of  carbonaceous,  green,  clayey  silt  full  of 

seeds,  with  laminated  lignite  and  loam. 
Weybourn  Crag. 


The  vegetation  preserved  in  this  group  of  strata  embraces  at  least  66  species  of  flower- 
ing plants,  two  of  which,  the  water  chestnut  and  spruce  tir,  do  not  appear  to  have  belonged 
to  the  British  flora  since  the  Glacial  period  ;  the  others  are  nearly  all  still  living  in 
Norfolk.  The  variety  of  forest-trees  points  to  a  mild  and  moist  climate  ;  they  include 
the  maple,  sloe,  hawthorn,  cornel,  elm,  birch,  alder,  hornbeam,  hazel,  oak,  beech,  willow, 
yew,  pine,  and  spruce.  The  land  and  fresh -water  shells  number  68  species,  whereof 
five  appear  to  be  extinct  (Liinax  modioli/ormis,  Neviatura  ruiUoniana,  Paludiiia  glaci- 
alis,  P.  Tnedia^  Pisidium  astartoides)  and  five  no  longer  live  in  Britain  (including 
Hydrohia  Steiniiy  Valvata  fluviatilis,  Corbicula  fluminalis).  The  kno\ni  marine  shells  in 
the  Forest-bed  series  are  so  few  in  number  (19  species)  that  they  do  not  afford  a  satisfactory 
l)asis  for  comparison  with  other  parts  of  the  Pliocene  formations.  Sonic  of  them  may 
have  been  washed  out  of  the  Weybourn  Crag  below,  and  they  are  all  common  Weybourn 
Crag  fossils,  including  several  extinct  species  {Melampus  pyramidali%  Tellina  obliqua^ 
Nucula  Cobboldiw).  They  indicate  that  the  climate  of  the  time  when  they  lived  was 
probably  not  greatly  different  from  that  of  the  present  day.  Fourteen  species  of  fishes 
have  been  recognised  {Platax  Woodwardi,  cod,  and  tunny  among  marine  forms,  also 
perch,  pike,  barbel,  tench,  and  sturgeon  among  fluviatile  kinds).  The  fauna  also  in- 
cludes two  reptiles  {Tropidonotus  uatrixj  Pelias  bervs),  four  amphibians  (frogs  and 
tritons),  five  birds  (eagle-owl,  cormorant,  vrild  goose,  wild  duck,  shoveller  duck),  and 
fifty-nine  mammals.  These  last-named  fossils  give  the  Forest-bed  its  chief  geological 
interest.      They  include  a  few  marine  forms — seals,  whales,   walrus,  and  a  large  and 

1  On  this  group  see  Lyell,  Phil.  Mag.  3rd  ser.  xvi.  (1840)  p.  245,  and  his  'Antiquity 
of  Man  '  ;  Prestwich,  Quart.  Journ.  Oeol.  Soc.  xxvii.  (1871)  pp.  326,  452  ;  Geologist,  iv.  (1861) 
p.  68  ;  John  Gunn,  'Geology  of  Norfolk,'  1864  ;  C.  Reid,  Oeol.  Mag.  (2)  vol.  iv.  (1877)  p. 
300  ;  vii.  (1880)  p.  548  ;  'Geology  of  the  Country  around  Cromer'  in  Mem.  Geol.  Surv. 
1882  ;  '  Pliocene  Deposits  of  Britain  '  in  Mem.  Oeol.  Surv.  1890  ;  E.  T.  Newton's  monographs 
cited  on  the  previous  page. 


TUTi^  iaM^mi'.t.iiip  st  "mu.  juCi'fai.  fad  rrvv-aaaimui:  5i^iis.  aba 
*-Mr''0     i^'C^    '  f.-^njt,   C,  YiiJ^yiaL  Jff^BMs  Tvnas.    rHiM  yTiia 

Orrr-iJt  ':,r.it§jC9  .tad  luut  tcier  fOMom.  J'iippi>mi  ■■<'  ■■■■■ 
Tt^vx..'  tj  Z.  .it^^ttmu.  SSifiiMoaerm  4Brtms»m,  /lifyiiri  tamrasaL  IL  i 
ArT*'Ai.t   t.'-fC.jL  M*Jt  «)^7Meimi.  Camr  jfie*.    T  ijpinffiiii    ■■ 

^iu»  4^*134?^  ;r.t>f;&ia  iif  tsTTiai^*  lail  "Sim  JanrTfar  irfmRC  if  -as 
tif^m  VM  Li'Ojgr  t«ri  r^manbtfi  17  Lf»iL-    T^  iiii«c  uixnionc  ■> 

w.n:-r»nrA.      yf  t ji»  t-*-^  iiyrvH  m*  jft  -s^msc  lai*  iiBnii  mil  -vtM 

i^v.rji  \^ryn,  42ii  '.iLt  ilx  tc^citt  save  wBrrmgi  =r  aaj  gars  -if ' 

-.^  Tr-:i:ii  •..•.•*  trK  jcni^'.n.  3.  ^i*  lena  4^  fcr3u:^:m»  bin  a#ic  jm: 

m 

•..',r.^^>rt'.'*  ..-.vr:*!  '.f  tin*.     A=L'.g^  li*  *cAz.rj  .rrisis^s  :^  ici» 4fcosci  lie  5:9li:viac 

T'/p^/x  of//  ♦.  AtfjiT*^  ^rr-w-T-*,  Cjr*f.»w  -"'v.V.  '_''*7'?^"*.3  -Ji-wmdia^  IjriM  m,wu.y^ 
ii'ft  *r  .    ryirs.  J/y"."..*  td^'JI,  OtfVrl  *d-'\A.    T'"  *.a  M'\*Ava.     SxEie  •^f  w*L>«sK  «>«*'"-*    2* 

y>////  ;*  4.--  A.—  *.    *'«k:**  L-->t  kr.o'BT.  ill  ai^t  of  ti.*  :i:ii*rlTiix  fc-CBaiaoB*. 

V,  ;,'.:::;. iv'v  l:r/iu*»i  •■i:r.  :r-»t  '.*»:  d'srjTtVwL  It  cvyi.«i*:a  -i-f  sciffblae  Ioae=.  ^Iat.  4»i 
-jAr- J-  *»v;;.<«r':::,r:s  rr. '..•'»:  tr.An  t^o  f*itt  iLkk,  Ifk*  tl*  'i*i*:«C*  <A.  tzufeses:  fixMlL 
Va  'y.^:.**.  i :,'.!.>  i  r.-i-'r/'^r  'vf  nj'^uKa^.  with  ti*  'iirirf  Arrtx  t-ircfc  az>i  willoiw  Betmlm 
nn-n  ;»:.  ;  ,7V/  ,/  i-.'^rgrU.  Y\x.  4^1  — •  r^^ght^zi'^zk  '»h*^r*:n  tr«*  ««ii:  tobaTv  as  completeh' 
'i;-^;  T^^-ii.--'-;  %-  .:.  '.':.*-  Arctic  Iat.'!*.  It  may  ::.d:ja:*?  a  I<OTeriM  of  tempcrsmre  l^aboat 
'20  Fi'.r.  -••  4  :.!f*:r*riiCi«:  *.-  imAi  «  f*tw*«n  tL*:  5.>uth  of  Ecjduid  and  ibe  Xonh  Cape 
4*.  r:.*:  :r--.^..t  uy.  ^n^i  --;±c:*:nt  to  allow  the  «*as  to  be  blocked  with  k«  during  the 
•*;.'.*'.-.  ar.  \  *■'.  i!>.-jr  jflv^^ffrr^  to  form  in  tL^  hfllj  •ii^trict*,"*     Among  the  pLmls  a  few 

i'-V.-f-      ,*?.  '-,::. ■r  "srir.i'-'^a.i^s  <.f  >*etle*. 

\:.-  .  ..ty  ',f  Mi:i.    1-t  ^'lit.    1S^3    p.  216.     .>»  also  C.  Reid,  *PliooeDe  Deposits  of 
hr.r.  ...    :  .  :-j. 

-  •     1>.A,    p.    ./.  2  C.  Reid.  ^.  «/.  p.  19*. 


SECT,  iv  §  2  PLIOCENE  SYSTEM  1016 

Various  pebble-gravels  occur  in  different  parts  of  southern  England,  the  true  strati- 
graphical  position  of  which  is  still  undetermined.  They  are  generally  unfossiliferous. 
Some  parts  of  them  may  be  Pliocene.  In  the  south-west,  at  Dewlish  in  Dorset,  a 
deposit  of  sand  and  gravel  has  yielded  a  number  of  elephant  boues  and  teeth  referred  to 
Elephaa  mtridUmaliSj  and  pointing  to  an  Upper  Pliocene  age. 

Belgium  and  Holland. — The  sea  in  which  the  English  Pliocene  deposits  were  laid 
down  probably  extended  across  Belgium,  Hollaud,  and  the  extreme  north  of  France, 
but  no  trace  of  its  presence  has  yet  been  found  eastwards  in  Germany.  In  Belgium  the 
base  of  the  Pliocene  is  found  to  rest  with  a  strong  unconformability  on  all  older 
deposits,  even  on  the  Miocene  sands  (Bolderian  and  Anversian).  The  older  Pliocene 
group  consists  chiefly  of  sand,  and  has  been  named  Diestian  from  the  locality  where  it 
is  typically  developed.  At  Antwerp,  Utrecht,  and  other  places  it  has  yielded  a  large 
assemblage  of  fossils  (190  species),  all  of  which  save  22  occur  in  the  English  Cor- 
alline Crag  and  Lenham  beds.  This  horizon  may  be  paralleled  with  the  Plaisancian 
group  of  southern  France  and  Italy.  Above  the  Diestian  sands  comes  the  group  known 
as  Scaldesian,  which  is  likewise  made  up  mainly  of  sands  enclosing  a  fauna  closely 
resembling  that  of  •the  lower  part  of  the  English  Ked  Crag  (Walton  Crag).  The  higher 
groups  seen  in  England  have  not  yet  been  identified  by  means  of  fossils  in  Belgium  and 
Holland.  Yet  the  Pliocene  deposits  attain  in  these  countries  a  far  greater  thickness 
than  they  do  in  England.  At  Amsterdam,  for  example,  a  deep  boring  has  passed  through 
younger  Tertiary  strata  to  a  depth  of  1096  feet  below  sea-level,  and  yet  it  is  doubtful, 
according  to  Mr.  Reid,  whether  any  portion  of  this  great  thickness  is  so  old  as  the 
Diestian  group.  ^  Belgian  Pliocene  deposits,  of  which  the  precise  horizons  have  not  been 
determined,  have  yielded  a  large  number  of  bones  of  marine  mammalia,  including  seals, 
dolphins,  and  numerous  cetaceans,  as  well  as  remains  of  fishes  (Carcharodon^  Lamna, 
OxyrhiTuiy  &c.) 

France. — In  the  north  of  this  country  unfossiliferous  sands  which  cap  the  hills 
between  Boulogne  and  Calais  at  heights  of  400  or  500  feet,  and  stretch  eastwards  into 
French  Flanders,  are  believed  to  be  continuations  of  the  Lenham  and  Diestian  group. ^ 
In  central  France  younger  Pliocene  deposits  associated  with  the  volcanic  materials  of 
that  region  have  preserved  an  interesting  record  of  the  terrestrial  fauna  of  the  time. 
The  trachytic  conglomerate  of  Perrier  and  the  ossiferous  deposits  of  other  localities  in 
Auvergne  have  yielded  an  abundant  fauna,  in  which  the  apes  are  absent,  the  antelopes 
have  dwindled  in  size  and  number,  the  deer  have  grown  very  abundant,  tnie  elephants 
for  the  first  time  appear,  associated  with  a  species  of  hippopotamus,  nearly  if  not  quite 
identical  with  the  living  African  one,  two  kinds  of  hyaena,  and  the  hipparion  and 
machairodus  that  had  survived  from  earlier  times.  This  fauna  indicates  a  decided 
change  of  climate  to  a  more  temperate  character.  Among  the  volcanic  products  of 
Haute  Loire  remains  of  Mastodon  arvemensis^  Khinoceros  leptorhinnSy  Equua  Stenxmis, 
and  Machairodus  pliocanua  have  been  collected. 

Along  the  southern  coast  of  France,  marine  Pliocene  deposits  lying  unconformably 
on  every  series  older  than  themselves  bear  witness  to  the  elevation  of  that  region  since 
Pliocene  time,  some  of  the  beds  reaching  a  height  of  1150  feet  above  the  present  sea- 
level.  These  marine  strata  extend  for  some  distance  up  the  valley  of  the  Rhone, 
where  they  mark  the  final  deposits  of  the  sea  in  that  part  of  the  mainland  of  Europe. 
They  cap  the  plateaux  and  rise  towards  the  north  and  west,  indicating  a  maximum  of 
elevation  in  that  direction.  The  marls  of  Hauterives  (formerly  regarded  as  Miocene) 
are  remarkable  for  their  beds  of  coarse  conglomerate,  which  represent  some  of  the 
torrential  deiK>8its  swept  down  from  the  neighbouring  hills.  These  marls  contain  land 
and  fresh-water  shells. 

^  Op,  cit,  p.  220. 

*  C.  Reid,  op.  cii.  p.  50. 


1016 


STRATIGRAPHICAL  QEOLOOY 


BOOK  YI  PART  IT 


The  whole  scries  of  Pliocene  deposits  in  southern  France  has  been  divided  into  the 
following  groups.* 

r  Fresh- water  and  volcanic  groups  of  Auvergne,  &c.  (St.  Prest,  Perrier*), 
Arnusian.    -!      with  Elephas  meridumalis  in  the  younger  and  Mastodon  arvemtntU 
\     in  the  older  deposits. 

Sands  and  clays  of  fluviatile  or  lacustrine  origin,  with  a  few  shells 
( Cnio,  A  nodonta^  PlanorbiSf  Helix)  and  a  large  and  varied  assem- 
blage of  terrestrial  and  fluviatile  vertebrates  {Dolichopithectu, 
MachairoduSf  Caracal^  Hy^na^  Mastodon  arvemensisy  Hkino- 
Astian.  ^  ceros  leptorhinus,  Tajnrus  arvemensis,  Hipparioiif  HelardoSy 
Gasella,  Cenms,  &c.,  Montpellier,  Rousillon). 

Yellow  sands  with  Potamides  Basteroiiy  CerUhium  vulgatum^ 
Cojigeria,  Ostrea  cucuilatat  Pecten  benedictuSj  CardiuMy  Venus 
vudtilaindla . 

rSaudy  bine  micaceous  clays  (with  a  large  marine  fauna  (283  species) 
Plaisancian   I      comprising  Nassa  semistriataf  Mitra  striaiula,  Conus  pdagicus, 
(200-250    -|      Ceriihium    vtUgatum,    Cytherea    chione,    Pecten    penedictuA,   P. 
metres).      I      scabrelluSy  Ostrea  cucidlata). 

VLower  conglomerates  sometimes  80  feet  thick. 

Italy. — As  the  Pliocene  series  is  traced  eastwards  into  Italy  its  lacustrine  intercala- 
tions disappear  and  it  becomes  mainly  a  marine  formation,  which  is  so  amply  developed 
there  that  it  might  be  taken  as  typical  for  the  rest  of  Europe.  Along  both  sides  of  the 
chain  of  the  Apennines  it  forms  a  range  of  low  hills,  and  has  been  named  from  that 
circumstance  the  "  sub-Apennine  series."  In  the  Ligurian  region,  according  to  C. 
Mayer,  it  consists  of  the  following  groups  in  ascending  order  :  1,  Messinian  ( =  Zanclean 
of  Seguenz^),  composed  of  (a)  marls,  conglomerates,  and  molasse  (65  feet),  with  Cerithium 
pictum,  C.  ruhiginosiiiiiy  Venus  nuUtila/mella,  Pecten  cristatus,  Turritclia  communis,  T. 
subangiclata  ;  (6)  gypsiferous  marls,  limestones,  dolomites  (820  feet),  traceable  along  the 
range  of  the  Apennines  as  far  as  Girgenti  in  Sicily  by  its  well-known  gypsum  zone,  and 
containing  Turritella  suh(Uig\UaiUy  Natica  hclicinay  Pleurotoma  diviidicUa,  &c.  ;  {e) 
gravels  and  yellow  marls,  with  beds  of  lignite  (upwards  of  300  feet).  2,  Astian,  com- 
posed, at  the  foot  of  the  Ligurian  Apennines,  of  two  groups,  (a)  blue  marls  with 
Dentaliuiii  scxamjuhirct  Turritella  communis^  T.  tornatay  Marex  trunculuSy  Natica 
millcpunct^ta,  kc.  ;  {b)  yellow  sands  with  few  fossils  (300  feet  and  more).'  More 
recently  Professor  Sacco  has  estimated  the  whole  series  in  the  central  portion  of  the 
northern  Apennines  to  have  a  thickness  of  nearly  1500  feet,  which  he  groups  as  in  the 
subjoined  tabic  :  "*- 


Vilhifranchiau 
(100  metres). 

Astian 
(100  metres). 

Plaisancian 
(150  metres). 


Fliivio-lacustrine  alluvial  sands,  marls,  clays,  and  conglomerates, 
witli  shells  indicating  a  warm,  moist  climate,  Rhinoceros 
etru3CU.Hy  Mastodon  arvernensiSy  &c. 


Yellow  sands  and  gravels,   rich  in  littoral,  marine  or  estuarine 
fossils. 


( 

(Marls  and  sandy  clays  with  abundant  marine  fossils,  from  one- 
third  to  one-half  of  the  shells  belonging  to  living  species. 


^  FontannevS,  '  Etudes  Stratigrapli.  Paleont.  pour  servir  a  I'histoire  de  la  Periode, 
Terti.iire  dans  le  Bassin  <iu  Rh6ne,'  Paris,  1875-89  ;  Deperet,  Ann,  Sci.  Giol,  xvii.  (1885) ; 
Mem.  S()c.  Gi'ol.  Fnnicf,  I.  fascic.  1.  (1890). 

-  Potier,  BiUL  Soc.  (J^ol.  France,  vii.  (1879)  p.  937. 

^  C.  Mayer,  BvU.  Sh\  Geol.  France  (3),  v.  292. 

^  F.  Sacco,  'II  Bacino  Terziario  del  Piemonte,'  Milan,  1889.  See  also  De  Stefani, 
AUi.  Soc.  Tosr.  Sci,  Nat.  1876-84. 


SECT,  iv  §  2  PLIOCENE  SYSTEM  1017 


Measinian 
(100  metres). 


Sandy  and  clayey  marls  with  seams  of  gypsum  and  limestone 
marking  alternations  of  brackish- water  and  marine  conditions. 
The  shells  include  Drtissena,  Adacna,  Cyrena,  Neritodotita, 
MelaniOj  MelanopsU^  J/ydrotda^  &c.  Some  of  the  marls  are 
full  of  leaves  (TViM^rt,  Phmgmites,  Mi/rica,  Qiiercits,  Castanea, 
FaguSf  UlmuSy  Ficua,  Liquxdamhtir^  LiiuruSy  Sassafras, 
Cinnamomumf  Bhamnusj  &c. ) 


In  Sicily  a  similar  threefold  grouping  has  been  made  by  Seguenza,  who  has  traced 
the  same  arrangement  throughout  a  large  part  of  the  mainland.  The  lowest  group  is 
named  by  him  Zanclean,  and  consists  of  marls  and  light-coloured  limestones.  The 
Plaisancian  follows  in  a  group  of  blue  clays  or  marls,  while  the  succeeding  Astian  con- 
sists of  yellow  sands.  Of  these  stages  the  first  is  characterised  by  a  fauna  of  which 
nearly  y*^  are  peculiar  species,  and  only  85  out  of  504  species,  or  about  17  per  cent, 
belong  to  living  forms  which  are  nearly  all  found  in  the  Mediterranean.  Some  of  the 
common  species  of  the  deposit  are  Janira  flahcllifonnis,  TerebratiUina  captU-serpciUis, 
Rhyncliaiulla  hipartUay  DetUalium  triquelrum,  Liiiiopsis  aurUay  Lcda  dilatata,  Z.  striata^ 
Phill.,  Modiola  pliaseoluia.  Tropical  genera  are  well  represented  among  the  shells  of 
the  Italian  Pliocene  beds,  while  some  of  the  still  living  Mediterranean  genera  occur 
there  more  abundantly,  or  in  larger  forms  than  on  the  present  sea-bottom.  The  newer 
Pliocene  deposits  attain  in  Sicily  a  thickness  of  2000  feet  or  more,  rising  to  a  height  of 
nearly  4000  feet  above  the  present  sea-level,  and  covering  nearly  half  of  the  island.  To 
this  series,  though  possibly  it  should  be  regarded  as  Pleistocene,  is  assigned  a  yellowish 
limestone,  sometimes  remarkably  massive  and  compact,  and  700  or  800  feet  thick,  yet 
full  of  living  species  of  Mediterranean  shells,  some  of  which  even  retain  their  colour, 
and  a  part  of  their  animal  matter.  It  was  during  the  accumulation  of  the  Pliocene 
strata  that  the  history  of  Etna  began,  the  first  stages  being  submarine  eruptions,  which 
were  followed  by  the  piling-up  of  the  present  vast  sub-aerial  cone  upon  the  u|»raised 
Pliocene  sea-bottom. 

There  is  distinct  evidence  of  a  lowering  of  the  climate  of  southern  Europe  during 
the  deposition  of  the  Italian  Pliocene  series.  Not  only  did  many  of  the  distinctively 
southern  types  of  shells  gradually  disappear  from  the  Mediterranean,  but  others  of 
markedly  northern  character,  such  as  species  of  Aslartr,  took  their  place.  The  Italian 
Pliocene  dejwsits,  while  chiefly  of  marine  origin,  contain  also  among  their  higher  mem- 
bers lacustrine  or  fluviatile  strata,  in  which  remains  of  the  terrestrial  flora  and  fauna 
have  been  preserved.  In  the  upper  part  of  the  valley  of  the  Amo  an  accumulation  of 
lacustrine  beds  attains  a  depth  of  750  feet.  The  older  jwrtion  consists  of  blue  clays 
and  lignites,  with  the  abundant  vegetation  above  referred  to  (jk  1004).  The  upi)er  200 
feet  consist  of  sands  and  a  conglomerate  ('*sansino"),  and  have  yielded  remains  of  39 
species  of  mammals  including  Macacus  florefUinius,  Mastodon  arvcrnensis,  Elephas 
iiieridionnlisy  Rhinoceros  etruscus,  Hip])opotamiis  amphibius  (major),  Uyasna  (3  sp.)t  Felis 
(3  sp.),  Ursus  etritscuSy  Machairodus  (3  sp.),  Equiis  Strnonis,  Btts  etruscus,  Cervus  (5  sp.), 
PalsMryx,  Palg&orcas,  Castor,  HystriXy  Lepus  arvicola,^  These  strata  are  sometimes 
grouped  as  a  higher  zone  of  the  Pliocene  series  under  the  name  of  Arnusian.^ 

Germajiy. — The  absence  of  marine  Pliocene  formations  in  Germany  has  been  already 
referred  to.  Among  the  lacustrine  and  fluviatile  deposits  of  the  period,  however, 
numerous  remains  of  the  terrestrial  flom  and  fauna  have  Imjcu  preserved.  One  of 
the  most  celebrated  localities  for  the  discovery  of  these  remains  lies  in  the  Mainz  basin, 
where  at  Epjielsheim,  near  Worms,  above  the  Miocene  beds,  described  on  p.  999,  a  group 


1  C.  J.  Forsyth  Major,  Q.  J.  Oeol.  Soc.  ili.  (1885)  p.  1. 

'^  Mr.  C.  Reid  suggests  that  the  lignite  deposits  of  the  Val  d'Arno  (with  Tapirus)  may 
be  much  older  than  the  rest  of  the  lacustrine  strata  (with  Mastodon  and  Elephas).  A  large 
proportion  of  the  plants  in  them  is  extinct,  and  the  tapir  is  the  only  animal  whose  remains 
are  found  in  them.     They  may  possibly  be  even  Miocene. 


1018 


STRATIGRAPHIGAL  GEOLOGY 


BOOR  Vl  PART  I 


■!|. 


\\ 


of  sands  and  grayels  with  lignite  (Knochensand),  from  20  to  80  feet  thick,  has  yielded 
considerable  number  of  mammalian  bones.     Among  these  the  Deinoiherium  giganUm 
occurs,  showing  the  long  survival  of  this  animal  in  central  Europe ;    also  Madodo 
angustidensy  Rhinoceros  incmmis,  and  other  species,  Hippotherium  graeile,  several  specif 
of  SuSf  five  or  more  of  Cervus,  and  some  of  Felis. 

Interesting  collections  of  the  terrestrial  fauna  of  the  period  have  been  preserved  i 
the  calcareous  tuffs  of  mineral  springs  in  different  parts  of  Germany.  Besides  nmnei 
ous  remains  of  land -plants,  large  numbers  of  land  and  fresh -water  shells  have  bee 
obtained  from  these  deposits,  which  in  some  cases  \)o\xit  to  a  colder  climate  than  no^ 
exists.  In  the  Franconian  Alb,  for  instance,  the  occurrence  of  alpine  and  norther 
European  forms  of  land -shells  (PcUida  solaria,  Clausilia  densesiriaia,  C.  fiXogram 
Helix  vicina,  Pupa  pagodula,  Isihmia  costulata)  has  been  noted.  The  mammals  includ 
many  extinct  as  well  as  some  still  living  forms  {Elephas  antiquus.  Rhinoceros  Merkk 
Sus  scrofa,  Ccrv^cs  elaphus^  C.  capreolus.  Bos  primigeiiius,  Equvs  caballus,  Ursus  speltew. 
Mcles  vulgaris,  Hyscna  spelssa).^ 

Vienna  Basin. — In  consecutive  conformable  order  above  the  Miocene  strata  describe 
on  p.  1000,  come  the  highest  Tertiary  beds  of  this  area,  referred  to  the  Pliocene  perio*] 
and  known  by  the  name  of  the  **  Congerian  stage,"  from  the  abundance  in  them  c 
the  molluscan  genus  Congeria  {Dreissena)  (Fig.  449).  They  are  separable  into  tw 
tolerably  well-defined  zones,  which  in  descending  order  are  : — 

2.  Bel  vedere-Sch otter — a  coarse  conglomerate  or  gravel  of  quartz  and  other 
pebbles,  occasionally  yielding  bones  of  large  mammals  ;  Belvedere-sand — a 
yellow  micaceous  sand,  forming  the  lower  member  of  the  zone  and  containiog 
in  its  more  compact  portions  abundant  terrestrial  leaves.  These  strata  re- 
semble part  of  the  alluvia  of  a  large  river.  Their  name  is  taken  from  the 
Belvedere  in  Vienna,  where  they  are  well  developed. 

1.  Inzersdorf  Tegel — a  tolerably  pure  clay  reaching  a  depth  of  often  more  than 
300  feet.  This  deposit,  the  youngest  Tertiary  layer  that  is  widely  distributed 
over  the  Vienna  basin,  points  to  continued  and  general  submergence.  The 
facies  of  its  fossils,  however,  shows  that  the  water  no  longer  communicate 
freely  with  the  open  sea,  but  seems  rather  to  have  partaken  of  a  Caspian 
character.  Among  the  conspicuous  moUusks  are  Cofigeria  suhglobosa,  (J. 
Partschi,  C.  triangularis,  C.  spathuUUa,  C.  Czjzeki,  Cardium  carnuntinum,  C. 
(fpertum,  C.  conjungens,  Unio  aiavus,  U.  moravi^us,  Melanopsis  martininnaf 
M.  imj/rejisa,  M.  Hndobancnm^,  M.  BouH.  The  mammals  include  Mastodon 
longirostris,  M.  angustidens,  Deinotherium  giganteum,  Aceratherium  ineisi- 
uuvi,  Hippoth^riuifi  gracile,  antelope,  pig,  Machairodus  cultridens,  Uyama 
hijyparion inn.  The  flora  includes,  among  other  plants,  conifers  of  the  genera 
Glyptostrobus,  Sequoia,  and  Piyius,  also  s]>ecies  of  birch,  alder,  oak,  beech, 
chestnut,  honibeaiu,  liquidarabar,  plane,  willow,  poplar,  laurel,  cinnamon, 
buckthorn,  with  the  Asiatic  genus  Parrotia,  the  Australian  protcaceous  JJakea 
(Fig.  442),  and  the  extinct  tamarind-like  Podogonium. 

In  other  parts  of  the  Austro-Hungarian  emjure  interesting  evidence  exists  of  th 
gradual  uprise  of  the  sea-floor  during  later  Tertiary  time  and  the  isolation  of  detache 
areas  of  sea,  so  that  the  south-east  of  Europe  must  then  have  presented  some  resem 
bianco  to  the  great  Aralo-Caspian  depression  of  the  present  time.  The  Congerian  stag 
brings  before  us  the  picture  of  an  isolated  gulf  gradually  freshening,  like  the  moder 
Caspian,  by  the  inpouriug  of  rivers  ;  but  on  both  sides  of  the  Carpathian  range  ther 
were  bays  nearly  cut  off  from  the  main  body  of  water,  and  exposed  to  so  copious  a 
evaporation  without  counterbalancing  inflow  that  their  salt  was  de(>osited  over  th 
bottom.  Of  the  Trausylvanian  localities,  on  the  south  side  of  the  mountains,  the  ma< 
remarkable  is  Parajd,  where  a  mass  of  rock-salt  has  been  accumulated,  having 
maximum  of  7550  feet  in  length,  5576  feet  in  breadth,  and  590  feet  in  depth,  an 


i 


^  F.  von  Sandberger,  '  Land  und  Siisswasser  Conchylien  der  Vorwelt,'  1875,  p.  936 
Sitd>.  Bayer,  Akud.  xxiii.  (1893)  Heft  1  ;  Hellmaun,  PalaorUographica,  suppl. 


SECT,  iv  §  2 


PLIOCENE  SYSTEM 


1019 


estimated  to  contain  upwards  of  10,595  millions  of  cubic  feet.  On  the  northern  flank 
of  the  Carpathian  Mountains,  near  Cracow,  lie  the  famous  and  extensive  salt-works  of 
Wieliczka,  with  their  massive  beds  of  pure  and  impure  rock-salt,  gypsum,  and  anhydrite, 
some  of  the  strata  being  full  of  fossils  characteristic  of  the  upper  zones  of  the  Vienna 
basin. 

The  south-east  of  Europe,  during  later  Tertiary  time,  was  the  scene  of  abundant 
volcanic  action,  and  the  outpourings  of  trachyte,  rhyolite,  basalt,  and  tuflF  were  specially 
abundant  over  the  low  districts  to  the  south  of  the  Carpathian  chain. 

Greece. — A  remarkable  series  of  mammalian  remains  brought  to  light  from  certain 
hard  red  clays,  alternating  with  gravels  at  Pikermi,  in  Attica,  has  been  carefully  worked 
out  by  M.  Gaudry.i  xhe  list  includes  a  monkey  {Meaopithecus)  intermediate  between 
the  living  Semjwpilhecus  of  Asia  and  the  Macaques.  The  carnivores  are  represented 
by  Simocyon^  Mustela,  Promephilis,  IclUheriuytif — a  genus  allied  to  the  modem  civet — 
HijmnictiSf  Hyasnay  Machairodus^  and  several  species  of  Felis  ;  the  rodents  by  ffystrix, 


Fig.  451.— Helladotherinm  Duvernoyi,  Gaudry  (M- 

allied  to  the  common  porcupine  ;  the  edentates  by  the  gigantic  Ancylotherium  ;  the 
proboscideans  by  Mastodon  and  Deiiwtherium  ;  the  pachyderms  by  Rhinoceros  (several 
species),  Accraihcriumy  Leptodon^  Eijmarion,  and  a  gigantic  wild  boar  {Su8  erymanihius); 
the  ruminants  by  Cam^ilopardalis,  of  tiTe  same  size  as  the  living  giraffe,  ffelladotherium — 
a  form  between  the  giraffe  and  the  antelopes,  three  species  of  true  antelope — PalaotraguSy 
an  antelope-like  animal,  PalaoryXy  somewhat  like  the  living  African  gemsbok,  and 
PalsRoreas,  allied  to  the  African  eland  and  the  gazelles,' G'tfi^/to,  a  true  gazelle,  Dremo- 
theriuiriy  probably  a  hornless  ruminant  like  the  living  chevrotains.  A  few  remains  of 
birds  have  also  been  met  with,  including  a  Phasianus,  related  to  our  pheasant,  a  OcUluSf 
smaller  than  our  common  domestic  fowl,  a  Gf^rM^  closely  related  to  the  living  crane  ; 
also  bones  of  a  tui-tle  and  a  saurian  ( Varanus),  This  fauna  is  remarkable  for  the  extra- 
ordinary abundance  of  its  ruminants,  the  colossal  size  of  many  of  the  forms,  such  as  the 
giraffe  and  Helladoiherium^  the  singular  rarity  of  the  smaller  mammals,  the  marked 
African  facies  which  runs  through  the  whole  scries,  and  the  number  of  transitional 
types  which  it  contains.    Out  of  the  31  genera  of  mammals  which  have  been  obtained,  22 


^  *  Animaux  fossiles  et  Geologie  de  I'Attique,'  4to,  1862,  with  volume  of  plates  ;  BuU, 
Soc.  G^d.  France,  xiv.  (1886-86)  p.  288.  See  also  Roth  and  Wagner,  Ahhandi,  Bayer,  Akad, 
vii.  (1854);  T.  Fuchs,  Denksch.  Akad,  Wien,  xxxvii.  (1877)  2*  AbtheU,  p.  1  ;  BolL  Com, 
Oeol.  Ital.  ix.  (1878)  p.  110 ;  W.  T.  Blanford,  Address,  Qeol.  Sect.  Bnt.  Assoc,  1884.  W. 
Dames  {Zeilsch.  Deutsch.  Oeol.  Qes,  xxxvi.  1883,  p.  9)  has  added  a  species  of  Cervus  and  one 
of  Mus  to  the  previously  known  Pikermi  forms. 


1080 


STRATIGRAPHICAL  GEOLOGY  book  ti  pah  it 


•re  extinct.  Ths  Pik«nni  badi  lum  been  oImmiI  u  Upper  Hiooetie,  but  the  oeenimM 
of  4  cbaneteriaHc  m&rine  PliooetiB  «peciee  of  ■halls  below  them  (Arfot  hntdiditL, 
Spondylut  gadenput,  Ottna  lanullon,  0.  wuUita)  jiutifiee  tbeir  being  placed  in  •  later 
■tago  of  the  Teitiuy  aeriaa.  They  are  shown  by  Focha  to  form  put  of  the  Plioceoe 
aeries  of  Attica,  and  lie  in  the  higheat  part  of  thsC  series. 

Smmot. — In  an  iiregnlar  depoait  of  gnvela,  aandstones,  and  marU  in  the  iaUad  of 
Bunoa,  Dr.  Forsyth  Major  has  discovered  a  large  aMemblaga  of  rertabrste  nmaina  of 
an  age  aimilar  to  that  of  the  Pfkenni  atiata. 
Among  the  fossils  obtained  liy  him  «n  many 
of  the  aame  spedea  aa  are  found  at  the  GnA 
locality,  snch  as  PromepkitiM  Lorttti,  MiuUm 
pnUtaUim,  Xycyna  ChmnUt,  leHlkmum 
rabutlum,  I,  hipparioKimt,  ^luyloCAmiiM 
FenUliei,  MaHodoA  Fetittliei,  JOUiueervt 
pathygnaOivt,  Hipparion  vuditerrtaitwK,  Sus 
frgmaiiihiiu  ;  seven  antelopea,  PaUeortai  Hn- 
dermttyeri,  Ointlla  Irrerieonnt,  PtUmoryx  Pal- 
laHi,  and  two  otberi.  Beaidea  theae,  there 
are  aome  half-dozen  antelopea  of  African  Ijpea, 
and  true  edentate^  Oryiieniftu  Otautrgi, 
Paimvmanit  Naa,  a  new  genua  of  gigantic  niminantji,  SamaOiariuim,  belonging  to  the 
&inily  of  the  giraffes,  and  recalling  the  BttiadvUierium  of  Pikenni,  and  an  oatrich 
(Struthio  KaratheodorU).^ 

India.  —Not  lees  important  than  the  maMlTe  Fliooene  accumalationa  of  the  Mediter. 
rniean  basin,  are  thoee  which  have  been  (bnnd  in  Sind.  the  Pni^ab,  and  other  north- 


wostpni  traotH  of  India.  In  Hind,  the  noteworthy  fact  hsa  been  nude  out  by  the  Indian 
Geological  Survey  tJiat,  from  the  Upper  Cretaceoua  to  ths  Pliocene  beda,  the  whole  bdc- 
cessioii  of  strata,  with  some  trifling  local  exceptions,  is  conformable  and  continnons ; 
yvt  roiitains  evidence  of  allemationE  of  marine  and  terreatrial  condition^  the  latest 

'  Cinpt.  Tf«d.  31st  Dec.  1888  ;  18B1.  pp.  808,  708. 


SECT,  iv  §  2  PLIOCENE  SYSTEM  1021 

marine  intercalations  being  of  Miocene  date.  The  upper  division  of  the  Manchhar  group 
(p.  1002)  is  not  improbably  referable  to  the  Pliocene  period.  It  consists  of  clays,  sand- 
stones, and  conglomerate,  5000  feet  thick,  which  have  yielded  some  indeterminable 
fragmentary  bones.  Similar  strata  cover  a  vast  area  in  the  Punjab.  They  are 
admirably  exposed  in  the  long  range  of  hills  termed  the  Sub-Himalayas,  which  from 
the  Brahmaputra  to,  the  Jhelum,  a  distance  of  1500  miles,  flank  the  main  chain,  and 
consist  chiefly  of  soft  massive  sandstone,  disposed  in  two  parallel  lines  of  ridge, 
having  a  steep  southerly  face  and  a  more  gentle  northerly  slope,  and  separated  by  a 
broad  flat  valley.  These  strata,  with  an  aggregate  thickness  of  between  12,000  and 
15,000  feet,  contain  representatives  of  the  older  Tertiary  or  Nummulitic  series,  followed 
by  younger  Tertiary  deposits  which  are  classed  together  in  what  has  been  termed 
the  Siwalik  group.  This  group  is  of  fresh-water  origin,  for  its  included  organisms 
are  entirely  land  or  fresh -water  forms.  Its  component  clays,  sandstones,  and 
conglomerates  have  been  deposited  by  great  rivers,  which  appear  to  have  flowed  from 
the  Himalayan  chain  by  the  same  outlets  as  their  modem  representatives.  These 
deposits  vary  according  to  their  position  relatively  to  the  great  rivers.  They  have  lieen 
involved  in  the  last  colossal  movements  whereby  the  Himalayas  have  been  upheaved, 
yet  their  structure  shows  that  the  same  distribution  of  the  watercourses  has  been  main- 
tained as  existed  before  the  disturl>ance.  In  this  instance,  as  in  that  of  the  Green 
River  through  the  Uinta  range  in  western  America,  the  inference  seems  to  be  legitimate 
that  the  elevation  of  the  mountains  must  have  proceeded  so  slowly  that  the  erosion  by 
the  rivers  kept  pace  with  it,  and  the  positions  of  the  valleys  were  therefore  not  sensibly 
changed  (see  p.  1078). 

The  Siwalik  fauna  consists  partly  of  a  few  land  or  fresh-water  mollusks,  some,  if  not 
all,  of  which  are  identical  with  living  species  ;  but  chiefly  of  mammalia  ;  and  the  follow- 
ing list  comprises  the  vertebrate  fauna  so  far  as  at  present  known  : ' — 

Mammalia. — Primates. — Palaeopithecua,  1  sp.  ;  Ma^'acits^  2;  Cynocephalits,  2. 
Carnivora. — MustdcLy  1  ;  Mdlivora,  2  ;  Meiiivorodvn,  1  ;  Luira^  3  ;  Hyamodon, 
1  ;  Ursus,  1 ;  HymnardoSf  3  ;  Oanw,  1  ;  Amphici/on,  1  ;    Viverra,  2  ;  Hyeeiia^  5  ; 
Lepihymnay  1  ;  jEluropsis,  1  ;  jElurogale^  1  ;  /Ww,  5  ;  Marhairodtis^  2. 

Proboscidea. — Elephas,  6  {EueUphas,\  ;  Loxodouy  1 ;  SUgodony  4) ;  Mastodon,  7. 
Ungulata. — Chalicotheriumj  1 ;  Rhinoceros,  3 ;  Equiis,2;  Hipparion,  2 ;  Hip- 
popotarnast  2  ;  Tetraconodony  1  ;  Sus^  7  ;  Hippohyus,  2  ;  Sanitheriumy  1  ;  Mery- 
copotamusyd ;  CervusyA  ;  DorccUherium,  2  ;  Tragulusy  1 ;  PalaBomeryXy  1 ;  Brama- 
fhiTiujfij  1 ;  Hdladotherium  ('i)y  1  ;  Hydaspitheriumy  2  ;  Sivathtriumy  1 ;  Vishnu- 
therivmy  1  ;  Oiraffay  I  ;  AlctlaphuSy  1 ;  OazdUiy  1  ;  Oreas{1)y  1 ;  Palmoryx{l),  1  ; 
Leptohosy  2  ;  Bubalus,  4  ;  BisoUy  1 ;  Bosy  3  ;  Bucapray  1 ;  CaprUy  2 ;  Canulus,  2  ; 
BosdaphuSy  IlippotraguSy  Cobtis, 

Rotlentia. — RhyzomySy  1  ;  Hystrix,  1  ;  LepuSy  1. 
AvEs. — PhaUicrocomXy  1  ;  Leptoplilusy  1  ;  PelecanuSy  2  ;  MerguSy  1  ;  StriUhiOy  1. 
Reitilia. — Crocodilia. — CrocodiluSy  2  ;  Garialisy  5  ;  Rhamphosuchusy  1. 
Lacertilia. —  VaranuSy  1. 

Chelonia. — ColossochdySy  1  ;  Testudoy  2  ;  Bdlia,  2  ;  Dtimonitty  1  ;  BataguVy  1  ; 
Pangshuray  1  ;  EmydOy  4  ;  TrionyXy  1  ;  ClemmySy  7  ;  Chitniy  1. 
Pisces. — BagariuSy  1 ;  AriuSy  2;  RUoy  1  ;  ChrysichthySy  1  ;  Clarias  (?),  1  ;  Car- 
char  mlony  Carchftrias. 

In  this  list  there  is  considerable  resemblance  to  the  grouping  of  mammalia  in  the 
Pikermi  deposits  just  referred  to,  particularly  in  the  preponderance  of  large  animals, 
the  absence  or  rarity  of  the  smaller  forms  (rodents,  bats,  insectivores),  and  the  marked 
Miocene  aspect  of  certain  parts  of  the  fauna.     Mr.  Blaiiford  and  his  colleagues  of  the 

^  Falconer  and  Cautley,  '  Fauna  Antiqua  Sivalensis,'  1845-49.  Medlicott  and  Blanford, 
*  Geology  of  India,'  p.  577.  Blanfoi-d,  Brit.  Assoc.  1880,  p.  677  ;  Address,  Geol,  Sect,  Brit. 
Asmc.  1884.  Lydekker,  *  Palajontologia  ludica,'  ser.  x.  vols.  i.  ii.  iii.  Records  Oeol, 
Surv.  Indiity  1883,  p.  81  :  '  Cat.  Sewalik  Vert.  Ind.  Mus.'  1885-86,  and  Catalogues  of 
British  Museum. 


1022  STRATIQRAPHIGAL  GEOLOGY  book  vi 

Geological  Survey  of  India  have,  however,  shown  that,  though  usually  classed  as  Miooene, 
the  Siwalik  fauna  has  such  relations  to  Pliocene  and  recent  forms  as  are  found  in  no 
true  Miocene  fauna.  The  large  proportion  of  existing  genera  is  the  most  striking  featmv 
of  the  assemblage.  Twelve  of  the  genera  are  known  elsewhere,  7  are  Miooene  and 
Pliocene  ;  of  the  still  living  genera  9  range  back  in  Europe  to  Upper  Miocene  time,  10 
only  to  Pliocene,  while  6  are  only  known  elsewhere  as  living  forms  or  as  oocmring  in 
]X>st-Plioceue  beds.  The  large  preponderance  of  species  belonging  to  such  familiar 
genera  as  Maeacus,  Ur9U8,  Elephas,  JEqutis,  JUppopotamus,  Bos,  Hystrix,  Mellivora, 
AfeleSt  Ca}Yray  CameluSf  and  Jlhizomys,  give  the  whole  assemblage  a  singularly  modem 
aspect.  It  should  be  added  that,  of  the  six  or  seven  determinable  reptiles,  three  are 
now  living  in  northern  India ;  that  of  the  birds,  one  is  probably  identical  with  the 
living  ostrich,  and  that  all  the  knowii  land  and  fresh-water  shells,  with  one  possible 
exception,  are  of  existing  species.^ 

North  America. — It  appears  to  be  doubtful  whether  any  of  the  Tertiary  deposits  of 
the  Atlantic  boixler  can  be  referred  to  the  Pliocene  series.  They  seem  to  be  rather  older 
and  to  be  covered  directly  by  post-Pliocene  and  recent  accumulations.^  In  the  Uppo* 
Missouri  region,  the  White  River  group  (p.  1002)  is  overlain  by  other  fresh-water  beds, 
300  to  400  feet  thick  (Loup  River  group  of  Meek  and  Hayden,  or  Niobrara  group  of 
Marsh),  from  which  an  interesting  series  of  vertebrate  remains  has  been  obtained. 
Among  these,  are  those  of  an  eagle,  a  crane,  and  a  cormorant ;  a  tiger,  larger  than  that 
of  India,  an  elephant,  a  mastodon,  several  rhinoceroses,  the  oldest  known  camels 
{Procamelus,  Hanwcanielus),  equine  animals  of  the  genera  Protohippus,  Fliohijfpus, 
MerychippuSy  and  EqunSy  of  which  the  last  was  as  large  as  the  living  horse.  The 
remarkably  oriental  character  of  this  fauna  is  worthy  of  special  notice.  At  the  eastern 
base  of  the  Rocky  Mountains  in  Colorado  a  group  of  sandstones  (Denver  beds)  has 
3rielded  a  large  species  of  bison.  Again,  abundant  remains  of  AcercUherium  have 
recently  been  found  in  the  Pliohippus  beds  of  the  Upper  Pliocene  scries  of  Kansas.' 

Australia. — In  New  South  Wales,  during  what  are  supposed  to  correspond  ^*ith  the 
later  Miocene,  Pliocene,  and  Pleistocene  periods,  the  land  appears  to  have  been  gradually 
rising  and  to  have  been  exi)osed  to  prolonged  denudation  and,  in  the  Middle  Pliocene 
period,  to  great  volcanic  activity.  Hence  successive  fluviatile  terraces  were  formed  and 
erodcfl  in  the  valleys,  and  were  in  many  cases  buried  under  great  streams  of  lava.  It 
is  in  the.se  buried  river-beds  that  the  "deep-leads"  lie,  from  which  such  lai*go  quantities 
of  gold  are  obtained.  Tliey  have  preserved  with  wonderful  perfection  remains  of  the 
flora  and  fauna  of  the  period.  Among  the  plants  are  large  trunks,  branches,  and 
fruits  of  trees,  and  ferns.  With  these  are  associated  fresh -water  shells,  traces  of 
l)eetles,  and  bones  of  a  niunber  of  extinct  marsupials,  some  of  which  were  distinguished 
by  their  ^eat  size.  One  of  the  most  abundant  and  remarkable  of  these  creatures  was 
the  Dlprot(xlan,  which  attained  the  bulk  of  a  rhinoceros  or  hippopotamus.  Another  is 
the  Nolotherium,  probably  somewhat  like  a  large  tapir,  of  which  three  species  have 
been  named.  An  extinct  gigantic  kangaroo  {Macropus  Titan)  had  a  skull  twice  as  long 
as  that  of  the  largest  living  species.  There  were  also  wombats  (Phascolamys),  and  a 
marsui)ial  lion  {Thyhicoleo)^  with  the  marsupial  hyaena  {Thylacintis)^  and  Sarcophilvs 
or  "devil,"  which  still  live  in  Tasmania.  To  these  may  be  added  the  DromomU — a 
large  bird  rej)resented  now  by  the  emu.* 

In  Victoria  a  younger  Tertiary  series  overlies  the  older  volcanic  rocks  referred  to  on 
p.  1003,  and  is  likewise  associated  with  newer  volcanic  ejections.  It  includes  both 
marine  and  11  u  via  tile  deposits.  The  marine  group,  with  species  of  TVi^wiia,  Halioti$y 
Cerithium,   Waldheimia,  &c.,  is  found  up  to  heights  of  1000  feet  above  sea-level.     The 

1  Blauford,  Brit,  Assoc.  1880,  p.*  578,  and  1884,  Address. 

-  A.  Heil])rin,  as  cited  on  p.  981. 

^  Marsh,  A/ner.  Joum.  Sci.  xxxiv.  (1887)  p.  323. 

*  C.  S.  Wilkinson,  '  Notes  on  Geology  of  New  South  Wales,'  Sydney,  1882. 


PART  V  POST-TERTIARY  OR  QUATERNARY  1023 


fluviatile  deposits,  besides  auriferous  gravels,  include  also  beds  of  lignite  with  abundant 
remains  of  terrestrial  vegetation,  and  have  yielded  remains  of  Diprotodon,  Phascolomys, 
Thylacoleo,  MacropuSf  Frocqptodan,  DasyuruSy  HypsiprimnuSt  Cants  dingo^  &c.  Vast 
sheets  of  basaltic  and  doleritic  lavas  have  overspread  the  plains  and  filled  up  the 
Pliocene  river-beds.' 

In  Queensland  the  presence  of  Tertiary  rocks  is  inferred  rather  than  proved.  But 
from  the  similarity  of  the  volcanic  rocks  of  that  colony  to  those  of  Victoria  and  New 
South  Wales,  it  is  believed  that  the  older  and  newer  volcanic  groups  which  have  been 
established  are  likewise  of  Tertiary  age.^ 

New  Zealand. — Deposits  referable  to  the  Pliocene  division  of  the  geological  record 
play  an  important  part  in  the  geology  and  industrial  development  of  New  Zealand. 
According  to  Sir  J.  Hector,  they  belong  to  a  time  when  the  land  was  much  more  exten- 
sive than  it  now  is,  and  when  in  the  North  Island  volcanic  action  reached  its  greatest 
activity.  Some  of  the  beds  were  formed  on  the  sea-floor,  and  contain  in  abundance 
RoUlla  zealaiidicat  with  Dosinea  anuSt  Struthiolaria  Fraseri^  Buccinum  viaculcUum,  From 
70  to  90  per  cent  of  the  mollusca  are  of  still  living  species.  In  the  South  Island,  the 
Pliocene  strata  are  to  a  large  extent  unfossiliferous  gravels,  such  as  those  of  the  Canter- 
bury Plains  and  the  Monteri  Hills,  in  Nelson,  which  were  derived  from  the  moun- 
tainous interior.  That  considerable  terrestrial  disturbance  took  place  during  and 
subsequent  to  the  deposit  of  the  Pliocene  series  is  shown  by  the  disturbed  and  elevated 
positions  of  the  beds  in  some  places.  Here  and  there  the  marine  strata  have  been  raised 
to  a  height  of  300  feet  (near  Napier  to  more  than  2000  feet)  above  the  sea  without 
disturbance  of  their  horizontal  position  ;  but  elsewhere  they  have  been  completely  over- 
turned. The  economic  importance  of  these  deposits  arises  mainly  from  their  yielding 
the  richest  supplies  of  alluvial  gold.' 

Part  V.   Post-Tertiary  or  Quaternary. 

This  portion  of  the  Geological  Record  includes  the  various  superficial 
deposits  in  which  neariy  all  the  mollusca  are  of  still  living  species.  It  is 
usually  subdivided  into  two  series:  (1)  an  older  group  of  deposits  in 
which  many  of  the  mammals  are  of  extinct  species, — to  this  group  the 
names  Pleistocene,  Post-Pliocene,  and  Diluvial  have  been  given ;  and  (2) 
a  later  series,  wherein  the  mammals  are  all,  or  nearly  all,  of  still  living 
species,  to  which  the  names  Recent,  Alluvial,  and  Human  have  been 
assigned.  These  subdivisions,  however,  are  confessedly  very  artificial, 
and  it  is  often  exceedingly  difficult  to  draw  any  line  between  them.  The 
names  assigned  to  them  also  are  not  free  from  objection.  The  epithet 
"  human,"  for  example,  is  not  strictly  applicable  only  to  the  later  series 
of  deposits,  for  it  is  quite  certain  that  man  coexisted  with  the  fauna  of 
the  Pleistocene  series. 

In  Europe  and  North  America  a  tolerably  sharp  demarcation  can 
usually  be  made  between  the  Pliocene  formations  and  those  now  to  be 
described.  The  Crag  deposits  of  the  south-east  of  England,  as  we  have 
seen,  show  traces  of  a  gradual  lowering  of  the  temperature  during  later 

1  R.  a.  F.  Murray,  *  Geology  of  Victoria,' p.  113. 

^  These  volcanic  accumulations  are  extensive  aud  of  great  interest.  They  have  heen 
described  by  Mr.  R.   L.  Jack  in  the   'Geology  aud   Palreontology  of  Queensland/  chap. 

XXXV. 

'  Hector,  *  Handbook  of  New  Zealand/  p.  26  ;  Hutton,  Quart,  Jaum.  Oecl,  Soc.  1885, 
p.  211. 


1024  STRATIGRAPHICAL  GEOLOGY  book  vi  fast  i 


Pliocene  times,  and  the  same  fact  is  indicated  bj  the  liiocme  faiaa 
and  flora  on  the  Continent  even  in  the  Mediterranean  basin.  This  changi 
of  climate  continued  until  at  last  thoronghly  Arctic  coodhkms  prevailed 
under  which  the  oldest  of  the  Post-Tertiary  or  Pleistocene  deposits  wen 
accumulated  in  northern  and  central  Europe,  and  in  Canada  and  thi 
northern  part  of  the  United  States. 

It  is  hardlj  possible  to  arrange  the  Post-Tertiaiy  accumulations  in  i 
strict  chronological  order,  because  we  have  no  means  of  deciding;  in  mani 
cases,  their  relative  antiquity.  In  the  glaciated  regions  ol  the  nortben 
hemisphere  the  various  glacial  deposits  are  grouped  as  the  c^der  divisioi 
of  the  series  under  the  name  of  Pleistocene.  Above  them,  lie  yonngei 
accumulations  such  as  river-alluvia,  peat-mosses,  lake -bottoms,  cave 
deposits,  blown -sand,  raised  lacustrine  and  marine  terraces,  which 
merging  insensibly  into  those  of  the  present  day,  are  termed  Recent  oi 
Prehistoric. 

Section  L  neistoeene  or  GlaeiaL 

j$  1.  General  Characters. 

Under  the  name  of  the  Glacial  Period  or  Ice  Age,  a  remarkabk 
ge^^logical  episode  in  the  history  of  the  northern  hemisphere  is  denoted. 
The  Crag  deposits  (p.  1008)  afford  evidence  of  a  gradual  refrigeration  oj 
climate  at  the  close  of  the  Tertiary  ages.  This  change  of  temperatun 
affected  the  higher  latitudes  alike  of  the  Old  and  the  New  World.  Il 
reached  such  a  height  that  the  whole  of  the  north  of  Europe  was  bnrie<i 
imder  ice,  which,  filling  up  the  basins  of  the  Baltic  and  North  Sea 
spread  over  the  plains  even  as  far  south  as  close  to  the  site  of  London, 
and  in  Silesia  and  Gallicia  to  the  50th  parallel  of  latitude.  Beyond  the 
limits  reached  bv  the  northern  ice-sheet,  the  climate  was  so  arctic  that 
snow-fields  and  glaciers  spread  even  over  the  comparatively  low  hills  ol 
the  Lyonnais  and  Beaujolais  in  the  heart  of  France.  The  Alps  were 
loaded  ^nth  vast  snow-fields,  from  which  enormous  glaciers  descended 
into  the  plains,  overriding  ranges  of  minor  hills  on  their  way.  The 
Pyrenees  were  in  like  manner  covered,  while  snow-fields  and  glaciers 
extended  southwards  for  some  distance  over  the  Iberian  peninsula.  In 
North  America  also,  Canada  and  the  eastern  States  of  the  AmericaD 
Union  down  to  about  the  39th  parallel  of  north  latitude,  lay  under  the 
northern  ice-sheet. 

'  No  '.ection  of  K*^>logical  history  now  possesses  a  more  volaminoos  literature  thiin  tht 
(Jlacial  Perirxl,  e>pf<*ially  in  Britain  and  North  America.  For  general  information  th< 
>tu«ieiit  may  refer  to  Lyell's  *  Antiquity  of  Man/  J.  (Jt^ikie's  '  Great  Ice  Age,*  *  Prehistoric 
EurojH?,*  Ad«irt>s  to  (Jeolojnral  Section  of  British  A'»'««xMation,  18S9,  and  paper  in  Tntiu  Rotf. 
S'C.  3/in.  XXX vii.  part  i.  (1S1>3)  p.  127:  J.  Croll's  *  Climate  and  Time,*  *  I>i!ico59iottf  on 
I  '  Climate  and  (V.>rnolog>-' ;   A.    Penck,    *  Verglctscherung  der  Deutschen  Al}>en/   18S2  ;  J. 

Part>ch,  'Die  <;ietMher  der  Vorzeit  in  den  Karpathen,  &c/  1882;  A.  Falsan  and  E. 
Chantre.  '  Ancien**  (Jlaciers,  &o.,  de  la  partie  moyenne  du  Bassin  du  Rhone/  1879,  and  for 
detailed  de-  riptions,  to  the  (^tmrt.Jt'urn.  Off>l.  Sor.,  GfU.  Moij.^  Zeitseh.  DnttjrK  <y«^rf,  Cr*., 
Jiihrh.  l'r^ii.<A.  G^'A.  L/tndfjutnM.^  Am^,  Joum.  Sci^nre,  Annual  RepttriM  U.S.  Ufcl.  Sttrr.^ 
/iifU.  Am^rr.  tieU.  Soc.,  for  the  last  fifteen  or  twenty  years. 


SECT,  i  §  1  PLEISTOCENE  OR  GLACIAL    SERIES  1025 

The  effect  of  the  movement  of  the  ice  was  necessarily  to  remove  the 
soils  and  superficial  deposits  of  the  land-surface.  Hence,  in  the  areas  of 
country  so  affected,  the  ground  having  been  scraped  and  smoothed,  the 
glacial  accumulations  laid  down  upon  it  usually  rest  abruptly,  and  with- 
out any  connection,  on  older  rocks.  Considerable  local  differences  may 
be  observed  in  the  nature  and  succession  of  the  different  deposits  of  the 
glacial  period,  as  they  are  traced  from  district  to  district.  It  is  hardly 
}X)ssible  to  determine,  in  some  cases,  whether  certain  portions  of  the  series 
are  coeval,  or  belong  to  different  epochs.  But  the  following  leading  facts 
have  been  established.  First,  there  was  a  gradual  increase  of  the  cold, 
until  the  conditions  of  modern  North  Greenland  extended  as  far  south  as 
Middlesex,  Wafes,  the  south-west  of  Ireland,  and  50*  N.  lat.  in  central 
Europe,  and  about  39°  N.  lat.  in  eastern  America.  This  was  the  culmina- 
tion of  the  Ice  Age, — the  first  or  chief  period  of  glaciation.  Then  followed 
an  interval  or  interglacial  period,  during  which  the  climate  seems  to  have 
become  much  milder.  This  interlude  was  succeeded  by  another  cold 
j>eriod,  marked  by  a  renewed  augmentation  of  the  snow-fields  and  glaciers, 
— a  second  period  of  glaciation. 

It  has  been  maintained  by  some  observers  that  as  many  as  foiu*  ov 
five  distinct  epochs  of  cold  are  included  within  the  geological  interval 
represented  by  the  Pleistocene  deposits.  Other  writers  contend  for  the 
<3ssential  unity  of  the  glacial  period.  The  truth  \vill  probably  be  found 
to  lie  somewhere  between  the  extreme  views.  There  seems  to  be  demon- 
strable proof  that  there  was  at  least  one  interglacial  period.  There  may 
have  been  more  than  one  advance  of  the  northern  ice  into  temperate 
latitudes.  The  interval  of  milder  climate,  of  which  there  is  clear  proof, 
must  have  been  of  such  prolonged  duration  that  southern  types  of  plant 
and  animal  life  were  enabled  to  spread  northward  and  resume  their 
former  habitats.^  Eventually,  however,  and  no  doubt  very  gradually, 
after  intervals  of  increase  and  diminution,  the  ice  finally  retired  towards 
the  north,  and  with  it  went  the  Arctic  flora  and  fauna  that  had  peopled 
the  plains  of  Europe,  Canada,  and  New  England.  The  existing  snow- 
fields  and  glaciers  of  the  Pyrenees,  Switzerland,  and  Norway  are  remnants 
of  the  great  ice-sheets  of  the  glacial  period,  while  the  Arctic  plants  that 
people  the  mountains,  and  survive  in  scattered  colonies  on  the  lower 
grounds,  are  relics  of  the  northern  vegetation  that  covered  Europe  from 
Norway  to  Spain. 

The  general  succession  of  events  has  l)een  the  same  throughout  all  the 
European  region  north  of  the  Alps,  likewise  in  Canada,  Labrador,  and 
the  north-eastern  States,  though  of  course  with  local  modifications.  The 
following  summary  embodies  the  main  facts  in  the  history  of  the  Ice 
Age.     Some  local  details  are  given  in  subsequent  pages. 

Pre-glacial  Land-surfaces.  —  Here  and  there,  fragments  of 
the  land  over  which  the  ice -sheets  of  the*  glacial  period  settled  have 
escaped    the    general    extensive  ice -abrasion   of   that  ancient  terrestrial 

^  Those  who  wish  to  enter  into  this  debated  subject  will  find  it  discussed  from  oppoi^ite 
sitles  in  some  recent  papers  by  T.  C.  Chamberlin  and  G.  F.  Wright  in  the  Amer.  Journ. 
Sci.  (1892,  1893)  with  references  to  other  authorities. 

3  u 


1026 


HTRATIGRAFHIGAL  GEOLOGY  book  vi  pakt  v 


surface,  and  have  oven  retalDed  retica  of  the  forest  growth  that  covered 
them.  One  of  the  best-known  deposits  in  which  these  relics  have  been 
preserved  is  the  ao-called  "Forest  Bed"  (p.  1013).  Above  that  deposit,  as 
already  described  (p.  1014),  there  is  seen,  here  and  there,  on  the  Norfolk 
coast,  a  local  or  intermitteDt  bed  of  clay  containing  remains  of  Arctic 
plants  (Salix  jiolaris,  Betula  nana,  <&c.,  Fig.  454),  together  with  the  little 
marmot'like  rodent  Spermophiliis.  These  relics  of  a  terrestrial  ve^tation 
are  drifted  specimens,  but  they  cannot  have  travelled  far,  and  they  prob- 
ably represent  a  portion  of  the  Arctic  flora  which  had  already  found  its 
way  into  the  middle  of  England  before  the  advent  of  the  ice-sheet. 
Judging  from  the  present  distribution  of  the  same  plants,  we  may  infer 
that  the  climate  had  become  about  20°  colder  than  it  vaa  during  the 
time  represented  by  the  Forest  bed — a  difference  as  great  ae  that  between 
Norfolk  and  the  North  Cape  at  the  present  day,' 

The  Northern  Ice-sheet. — At  the  base  of   the  glacial   depoeits. 


the  solid  rocks  over  the  whole  of  northern  Eui-ope  and  America  present 
the  characteristic  smoothed  flowing  outlines  produced  by  the  grinding 
action  of  land-ice  (p.  4:?8).  The  rock-surfaces  that  look  away  from  the 
quarter  whence  the  ice  moved  are  usually  rough  and  weatherworn 
(Leeseite),  while  those  that  face  in  that  direction  (Stoss-seite)  are  all 
ice-worn.  Even  on  a  small  Ixiss  of  rock  or  along  the  side  of  a  hill,  it  is 
commonly  not  diflicult  to  tell  which  way  the  ice  flowed,  by  noting 
towai-ds  which  point  the  striffi  run  and  the  rough  faces  look.  Long 
exposed,  the  peculiar  ice-worn  surface  is  apt  to  be  effaced  by  the  disinte- 
grating action  of  the  weather,  though  it  retains  its  hold  with  extra- 
ordinary pertinacity.  Along  the  fjords  of  Norway  and  the  sea-lochs  of 
the  west  of  Scotland,  it  may  be  seen  slipping  into  the  water,  smooth, 
bare,  [xdished,  and  grooved",  as  if  the  ice  haid  only  recently  retreated. 
Inland,  where  a  protecting  cover  of  clay  or  other  superficial  deposit  has 

1  C.  Rei.l,   lltniioHlal  Setlifii,  Xo.  127  qf  Ocd.  Sarxty,  tm\  "Otology  of  the  CoanUr 
nroimil  Cromer  ■■  (ulieel  68  E,).  iu  Jfrsioiy*  o/ffwrf.  Sarctg,  1882. 


8BCT.  i  §  1  PLEISTOCENE  OR  GLACIAL  SERIES  1027 

been  newly  removed,  the  peculiar  ice -worn  surface  may  be  as  fresh  as 
that  by  the  side  of  a  modern  glacier. 

From  the  evidence  of  these  striated  rock -surfaces  and  the  scattered 
blocks  of  rock  that  were  transported  to  various  distances,  it  has  been 
ascertained  that  the  whole  of  northern  Europe  was  buried  under  one 
continuous  mantle  of  ice.  The  southern  edge  of  the  ice-sheet  must  have 
lain  to  the  south 'of  Ireland,  whence  it  passed  along  the  line  of  the 
Bristol  Channel,  and  thence  across  the  south  of  England,  keeping  to 
the  north  of  the  valley  of  the  Thames.  The  whole  of  the  North  Sea  was 
filled  with  ice  down  to  a  line  which  ran  somewhere  between  the  coast  of 
Essex  and  the  present  mouths  of  the  Rhine,  eastwards  along  the  base  of 
the  Westphalian  hills,  and  round  the  projecting  promontory  of  the  Harz, 
whence  it  swung  to  the  base  of  the  Thuringerwald  and  struck  eastwards 
across  Saxony,  keeping  to  the  north  of  the  Erz,  Riesen  and  Sudeten 
mountains ;  thence  across  Silesia,  Poland  and  Gallicia  by  way  of  Lemberg, 
and  circling  round  through  Russia  by  Kietf  and  Nijni  Novgorod  north- 
wards by  the  head  of  the  Dvina  to  the  Arctic  Ocean.  The  total  area  of 
Europe  thus  buried  under  ice  has  been  computed  to  have  been  not  less 
than  770,000  square  miles. 

Owing  mainly  to  the  direction  of  the  prevalent  moisture -bearing 
winds,  the  snowfall  was  greatest  towards  the  west  and  north-west,  and 
in  that  direction  the  ice-sheet  attained  its  greatest  thickness.  Over 
Scandinavia,  which  was  probably  entirely  buried  beneath  the  icy 
covering,  it  was  perhaps  between  6000  and  7000  feet  thick.  Thence  the 
sheet  spread  southwards,  gradually  diminishing  in  thickness.  But  from 
the  striae  left  by  it  on  the  Harz,  it  is  computed  to  have  been  at  least 
1470  feet  thick  where  it  abutted  on  that  ridge.  The  Scandinavian  ice 
joined  that  which  spread  over  Britain,  where  the  dimensions  of  the  sheet 
were  likewise  great.  Many  mountains  in  the  Scottish  Highlands  show 
marks  of  the  ice-sheet  at  heights  of  3000  feet  and  more.  If  to  this  depth 
we  add  that  of  the  deep  lakes  and  fjords  which  were  filled  with  ice,  we 
see  that  the  sheet  could  not  have  been  less  than  5000  feet  thick  in  the 
northern  parts  of  Britain. 

This  vast  icy  covering,  like  the  Arctic  and  Antarctic  ice-sheets  of  the 
present  day,  was  in  continual  motion,  slowly  draining  downwards  to 
lower  levels.  Towards  the  west,  its  edge  reached  the  sea,  as  in  Green- 
land now,  and  must  have  advanced  some  distance  along  the  sea -floor 
until  it  broke  off  into  bergs  that  floated  away  northward.  Towards 
the  south  and  east  it  ended  off  upon  land,  and  no  doubt  discharged  copious 
streams  of  glacier-water  over  the  ground  in  its  front.  In  North  America 
the  southern  edge  of  the  ice-sheet  is  sometimes  marked  by  a  "  terminal 
moraine '' — a  feature  well  displayed  from  Pennsylvania  to  Dakota. 

The  directions  of  movement  of  the  ice-sheets  can  be  followed  by  the 
evidence  (1st)  of  strisB  graven  on  the  rocks  over  which  the  ice  passed, 
and  (2nd)  of  transported  stones  ("erratic  blocks")  which  can  be  traced 
back  to  their  original  sources. 

In  Europe  the  great  centre  of  dispersion  for  the  ice-drainage  was  the 
table-land  of  Scandinavia.     As  shown  by  the  rock-strise  in  Sweden  and 


1028  STRATIGRAPHWAL  GEOLOGY  book  ti  part  v 


Xon^'ay,  the  ice  moved  off  that  area  northwards  and  north-eastwards  across 
northern  Finland  into  the  Arctic  Ocean ;  westwards  into  the  Atlantic 
Ocean,  south-westwards  into  the  basin  of  the  North  Sea;  southward, 
south-westward,  and  south-eastward  across  Denmark  and  the  low  plains 
of  Holland,  Germany,  and  Russia,  and  the  basins  of  the  Baltic,  Gulf  of 
Bothnia,  and  Gulf  of  Finland.  The  evidence  of  the  transported  stones 
coincides  \^4th  that  of  the  striation,  and  is  often  available  when  the  latter 
is  absent. 

United  with  the  Scandinavian  ice,  but  having  an  independent  system 
of  drainage,  was  the  ice-sheet  that  covered  nearly  the  whole  of  Britain. 
The  rock-striae  show  that  while  it  probably  buried  the  country  even 
over  its  highest  mountain-tops,  it  moved  outwArd  from  each  chief  mass 
of  high  ground.  Thus,  from  the  Scottish  Highlands,  which  were  the 
main  gathering  ground,  it  drained  northward  to  join  the  Norwegian  ice, 
and  move  with  it  in  a  north-westerly  direction  across  the  Orkney  and 
Shetland  Islands.  Westward  it  descended  into  the  Atlantic ;  eastwards 
into  the  basin  of  the  North  Sea,  to  merge  there  also  into  the  Scandinavian 
sheet  and  that  which  streamed  off  from  the  high  grounds  of  the  south  of 
Scotland,  and  to  move  as  one  vast  ice-field  in  a  south-south-west  direction 
across  the  north-east  and  east  of  England.  Southwards  it  flowed  into 
the  basin  of  the  Clyde  and  the  Irish  Sea,  to  unite  with  the  streams 
moving  from  the  south-west  of  Scotland  and  the  north-west  of  England 
and  Wales.  The  centre  of  Ireland  appears  also  to  have  been  an  area 
from  which  the  ice  moved  outwards,  passing  into  the  Atlantic  on  the 
one  side  and  joining  the  British  ice-fields  on  the  other. 

It  is  when  we  follow  the  direction  of  the  ice  striae,  and  see  how  they 
cross  important  hill  ranges,  that  we  can  best  realise  the  massiveness  of 
the  ice -sheet  and  its  resistless  movement.  As  it  slid  off  the  Scottish 
Highlands,  for  instance,  it  went  across  the  broad  plains  of  Perthshire, 
filling  them  up  to  a  depth  of  at  least  2000  feet,  and  passing  across  the 
range  of  the  Ochil  Hills,  which  at  a  distance  of  twelve  miles  runs 
parallel  with  the  Highlands,  and  reaches  a  height  of  2352  feet.  Moun- 
tains of  3000  feet  and  more,  with  lakes  at  their  feet,  600  feet  deep,  have 
been  well  ice-worn  from  top  to  bottom.  It  has  been  observed  that  the 
striae  along  the  lower  slopes  of  a  hill4)arrier  run  either  parallel  with  the 
trend  of  the  ground  or  slant  up '  obliquely,  while  those  on  the  summits 
may  cross  the  ridge  at  right  angles  to  its  course,  showing  a  differential 
movement  in  the  great  ice-sheet,  the  lower  parts,  as  in  a  river,  becoming 
embayed,  and  being  forced  to  move  in  a  direction  sometimes  even  at  a 
right  angle  to  that  of  the  general  advance.  On  the  lower  grounds,  also, 
the  strijB,  converging  from  different  sides,  unite  at  last  in  one  general 
trend  as  the  various  ice-sheets  must  have  done  when  they  descended 
from  the  high  grounds  on  either  side  and  coalesced  into  one  common 
mass.  This  is  well  seen  in  the  great  central  valley  of  Scotland.  Still 
more  marked  is  the  deflection  of  the  striae  in  the  basin  of  the  Momv 
Filth.  Northwards  they  are  deflected  in  a  N.N.W.  direction  across 
Caithness  and  the  Orkney  Islands,  pointing  to  the  influence  of  the 
Scandinavian  ice-sheet.     On  the  south  side  of  the  basin,  they  run  R  by 


SECT,  i  §  1  PLEISTOCENE  OR  GLACIAL  SEBIES  1029 


S.,  and  at  last  S.R,  on  the  north-east  of  Aberdeenshire,  showing  that  the 
ice  there  turned  southwards  into  the  North  Sea,  until  it  met  the  N.E. 
stream  from  KLincardineshire  and  the  valleys  of  the  Dee  and  Don,  with 
which  and  with  the  ice  from  Scandinavia  it  turned  southward  into  the 
basin  of  the  North  Sea.  The  great  mass  of  ice  which  crept  down  the 
basin  of  the  Firth  of  Clyde  was  joined  by  that  which  descended  from  the 
uplands  of  Carrick  and  Galloway,  and  the  united  stream  filled  up  the 
Irish  Sea  and  passed  over  the  north  of  Ireland.  At  that  time  England 
and  the  north-west  of  France  were  probably  united,  so  that  any  portion 
of  the  North  Sea  basin  not  invaded  by  land-ice  would  form  a  lake,  with 
its  outlet  by  the  hollow  through  which  the  Strait  of  Dover  has  since  been 
opened. 

When  this  glaciation  took  «place  the  terrestrial  surface  of  the  northern 
hemisphere  had  acquired  the  main  configuration  which  it  presents  to-day.. 
The  same  ranges  of  hills  and  lines  of  valley  which  now  serve  to  carry  off' 
the  rainfall  served  then  to  direct  the  results  of  the  snowfall  seawards. 
The  snow-sheds  of  the  Ice  Age  probably  corresponded  essentially  with  the 
water-sheds  of  the  present  day.  Yet  there  is  evidence  that  the  coinci- 
dence between  them  was  not  always  exact.  In  some  cases  the  snow  and 
ice  accumulated  to  so  much  greater  a  depth  on  one  side  of  a  ridge  than 
on  the  other  that  the  flow  actually  passed  across  the  ridge,  and  detritus 
was  carried  out  of  one  basin  into  another.  A  remarkable  instance  of 
this  kind  has  been  observed  in  the  north  of  Scotland,  where  so  thick 
was  the  ice-sheet  that  fragments  of  rock  from  the  centre  of  Sutherland 
have  been  carried  up  westward  across  the  main  water-parting  of  the 
country  and  have  been  dropped  on  the  western  side.^ 

In  North  America  also  abundant  evidence  is  afforded  of  a  northern 
ice-sheet  which  overrode  Canada  and  the  eastern  States  southwards  to 
about  the  39th  parallel  of  latitude  in  the  valley  of  the  Missouri.  So  ue 
details  regarding  the  area  which  it  covered  and  the  traces  it  has  left  of 
its  presence  are  given  at  p.  1050. 

Beyond  the  limits'  of  the  northern  ice-sheet,  the  European  continent 
noiu'ished  snow-fields  and  glaciers  wherever  the  ground  was  high  enough 
and  the  snowfall  heavy  enough  to  furnish  them.  As  already  mentioned, 
the  precipitation  of  moisture  during  the  Ice  Age,  as  at  present,  was 
greatest  towards  the  west,  and  consequently  in  the  western  tracts  the 
independent  snow-fields  and  glaciers  were  most  numerous  and  extensive. 
Even  at  the  present  time,  the  glaciers  of  the  western  part  of  the  Alpine 
chain  are  larger  than  those  farther  east.  At  the  time  of  the  northern 
ice-sheet  a  similar  local  difference  existed.  The  present  snow-fields  and 
glaciers  of  these  mountains,  large  though  they  are,  form  no  more  than 
the  mere  shrunken  remnants  of  the  great  mantle  of  snow  and  ice  which 
then  overspread  Switzerland.  In  the  Bernese  Oberland,  for  example, 
the  valleys  were  filled  to  the  brim  with  ice,  which,  moving  northwards, 
crossed  the  great  plain,  and  actually  overrode  a  part  of  the  Jura 
Mountains ;  for  huge  fragments  of  granite  and  other  rocks  from  the 
central  chain  of  the  Alps  are  found  high  an  the  slopes  of  that  range  of 

»  Peach  and  Home,  Brif.  Asuoc.  1892,  ]».  720. 


l^iZr,  iTkATTGRAFHirAL  GEoL/:*^T  mm^Tit^mr 


iKi^v.     H:*^  Rikooe  ^ataer  sweps.  wtascwArd  acnas  a^  ^ht 
rifi^-^s^.  ^A  TSkl^Ti.  and  fefs  iu  fji^infe  heapg  in.  «& 

Lv^/rrjfcL*.  hfinkJiykasA.  and   Auiqgiie   ♦!!:.    45'    S. 
fr.hrrr*  t^/rirjh^  rja  the  IKema  tabMu>i  ar 
ha^ir.  of   th«   Ivxcro  flat.   41' l     £a«tvard§ 

gSarI>^r  r<;Ii<3{  beawme  feantier  and  diBpiKar. 
^^ap  of  gia/^ien  viikh  tare  kft  beidod  ih>£K 
m^/raif:'^.  I>:^  extinkSfr^  vere  tkaie  of  the  Bfe^k  Forcsc 
ar«d  (^ikHAthiari*.  So  trace  of  ^acaSMo  hai^  been  dececud  im  tbe  IWflnw 
A  vimiUr  r«rlarkiii  >jecween  SMjmiaJl  and  ^a^datkio.  u  meemkit  m  Nonk 
Am^c^  brit  there  it  u  the  eastern  area  whkh  si^ifKcted  tke 
iee^h^^etn.  vhile  the  vectem  plateaux  and  BKAxntam-raB^Ba.  whiek  wi 
prr/r«iUy  then,  ax  nov,  eomparmtiTelT  arid,  had  onhr  Taflej-giaLMiK 

That  the  ice  in  iu  mareh  aerooB  the  land  striated  evcB  tke  kaideit 
roek.^  hr  Hiean^  of  the  and  and  stones  which  it  pfeMsed  against 
a  i^fxA  that,  to  .^>me  esoent  at  least,  the  teiieaiiial  sm^ce 
been  at  th»  time  ahraded  and  lowered  in  lertL     How  hr  this 
[^o^:eerie^L  or,  in  other  word%  hcpw  niiieh  of  the  nndoiibcedlT 
denu'lation  everywhere   visible  OTer  the  ^aciated  paitz  ol   Europe.  i$ 
attn^>«iuhie  Uj  the  actoal  work  of  land -ice,  is  a  proUem  which  bit 
never  Vie  even  approximately  solved.     There  seems  good  groand  for  the 
Ijelief  that  a  thick  cover  of  rotted  roek — the  result  of  ages  of  prvvioiis 
m^ffiensk]   wai^te — lav  over  the  surCaee,  and  that  the  '^gbdal  deposits' 
cori*in  in  crreat  roeaffore  of  this  material,  moved  and  reaaHrted  hr  ice  and 
water  'pp.  35 1 ,  431).     The  land,  as  above  remarked,  had  the  same  general 
feat/ire-?  of  mountain,  valley,  and  plain  as  it  has  now,  even  before  the  ice 
^ttif-il  rlown  ufjon  it.     But  the  prominences  reached   by  the  ice  were 
round *-<l  off  and    smoothed  over,  the  pre-glacial  soils  and  covering  of 
wf^ihftrfA  rock  were  in  large  measure  ground  up  and  pushed  away,  the 
valley-^  were  correspondingly  deejjened  and  widened,  and  the  plains  were 
-itrewn  with  ice-liome  debris.      It  is  obvious  that  the  infioence  of  the  moving 
\cjh-r\\f'j:U  has  >ieen  far  from  uniform  upon  the  rocks  exposed  to  it,  this 
variation  arising  from  the  differences  in  powers  of  resistance  of  the  rocks 
on  the  one  hanrl^  and  in  the  mass,  slope,  and  grinding  power  of  the  ice 
on  the  other.     Over  the  lowlands,  as  in  central  Scotland  and  much  <rf  the 
north  (^ierman  plain,  the  rocks  are  for  the  most  part  concealed  cmder  deep 
glacial  d'-bris.     But  in  the  more  undulating  hilly  ground,  particolarlT  in 
the  north  and  north-west,  the  ice  has  effected  the  most  extraordinary 
abrasion.     It  is  hardly  possible,  indeed,  to  describe  adequately  in  words 
the-?e  regions  of  mfist  intense  glaciation.      The  old  gneiss  of    NOTwaT 
and  .Sutherland.«hire,  for  example,  has  been  so  eroded,   smoothed,  and 
jiolished,  that  it  stands  up  in  endless  rounded  himmiocks,  many  of  them 
«till  srn^Kith  and  curved  like  dolphins'  backs,  with  little  pools,  tarns,  and 
larger   lakes    lying    l^etween    them.      Seen  from   a    hei^t  the  ground 
apf>c;ar^  like  a  billowy  sea  of  cold  grey  stone.     The  lakes,  each  lying  in 
a  hollow  of  erosion,  seem  scattered  broadcast  over  the  landscape.     So 
enduring  is  the  rcK-k,  that,  even  after  the  lapse  of  so  long  an  interval,  it 


sect:  i  ^  1  PLEISTOCENE  OR  GLACIAL  SERIES  1031 

retains  its  ice-worn  aspect  almost  as  unimpaired  as  if  the  work  of  the 
glacier  had  been  done  only  a  few  generations  since. ^  The  abundant 
smoothed  and  striated  rock-basin  lakes  of  the  northern  parts  of  Europe 
and  North  America  are  a  striking  evidence  of  ice-action  (pp.  430,  1086). 
The  phenomenon  of  "giants'  kettles,"  characteristic  of  glaciated  rock- 
surfaces  in  Sweden,  Silesia,  and  Switzerland  (p.  429),  is  another  mark  of 
the  same  process  of  erosion. 

Ice -crumpled  Kocks. — Not  only  has  the  general  surface  of  the 
land  been  abraded  by  the  ice-sheets,  but  here  and  there  more  yielding 
portions  of  the  rocks  have  been  broken  off  or  bent  l^ack,  or  corrugated 
by  the  pressure  of  the  advancing  ice.  Huge  blocks  300  yards  or  more 
in  length  have  been  bodily  displaced  and  launched  forward  on  glacial 
detritus.  Such  are  some  of  the  enormous  masses  of  chalk  displaced 
and  imbedded  in  the  drift  of  the  Cromer  cliffs,  and  the  transported 
sheets  of  Lincolnshire  Oolite  found  in  Leicestershire.^  The  laminae  of 
shales  or  slates  are  observed  to  be  pushed  over  or  crumpled  in  the 
<lirection  of  ice-movement.  Occasionally  tongues  of  the  glacial  detritus 
which  was  simultaneously  being  pressed  forward  under  the  ice  have 
been  intruded  into  cracks  in  the  strata,  so  as  to  resemble  veins  of 
eruptive  rock.^ 

Detritus  of  the  Ice-sheet — Boulder-clay — Till. — Underneath 
the  gi'eat  ice -sheet,  and  probably  partly  incorporated  in  the  lower 
j)ortion8  of  the  ice,*  there  accumulated  a  mass  of  earthy,  sandy,  and  stony 
matter  (till,  boulder-clay,  "gnindmorane,"  " moraine-prof onde,"  "older 
diluvium  ")  which,  pushed  along  and  ground  up,  was  the  material  where- 
with the  characteristic  flowing  outlines  and  smoothed  striated  surfaces 
were  produced.^  This  "  glacial  drift "  spreads  over  the  low  grounds 
that  were  biuied  under  the  northern  ice-sheet,  resting  usually  on  surfaces 
of  rock  that  have  been  worn  smooth,  disrupted,  or  crumpled  by  ice.  It 
is  not  spread  out,  however,  as  a  uniform  sheet,  but  varies  greatly  in 
thickness  and  in  irregularity  of  surface.     Especially  round   the  moun- 

*  Some  of  these  roches  moulonnSes  may  be  of  Palaeozoic  age  {Nature^  August  1880). 

'  Mr.  Fox  Strangways  has  noticed  one  such  sheet  near  Melton  which  measures  at  least 
300  yards  in  length  by  100  in  breadth,  but  may  extend  beneath  the  boulder-clay  to  a 
jO*eater  distance.  Report  of  Geological  Survey  of  the  United  Kingdom,  Science  and  Art 
RejHtrt  for  1892,  p.  249. 

'  On  the  disruption  of  the  Chalk  below  the  Till  of  Cromer  see  C.  Reid  on  Geology  of 
Cromer,  Mem.  Geof.  Surv.  1882.  For  analogous  phenomena  at  Moens  Klint,  off  the  coast 
of  Denmark,  see  Johnstrap,  Zcit.  Deutsch.  Qtcl.  Ges.  xxvi.  (1874)  p.  533.  Compare 
also  H.  CYedner,  op,  cit.  xxxii.  (1880)  p.  75.  F.  Wahnschaffe,  op.  cit,  xxxiv.  (1882) 
p.  562. 

**  Briickner,  Penck^s  Geoffraphische  Ahhandl,  Band  I.  Heft  1 . 

^  As  already  suggested,  the  materials  of  the  till  may  have  consisted  largely  of  a  layer  of 
decon]i>osed  rock  due  to  prolonged  pre-glacial  disintegration  (])p.  351,  431).  It  is  difficult 
to  exi>lain  by  any  known  glacial  operation  the  accumulation  of  such  deep  masses  of  detritus 
below  a  sheet  of  moving  land-ice.  Another  problem  is  presente<l  by  the  occasional  and 
sometimes  extensive  preservation  of  undisturbed  loose  pre-glacial  deposits  under  the  till. 
The  way  in  which  the  *'  Forest-bed  "  group  has  escaped  for  so  wide  a  space  under  the 
Cromer  cliffn,  with  their  proofs  of  enormous  ice  movement,  is  a  remarkable  example. 


1032  STKATIGRAPHICAL  GEOLOGY  book  vi  part  v 

tainous  centres  of  dispersion,  it  is  apt  to  occur  in  long  ridges  ("  drums," 
or  "  drumlins  "),  which  run  in  the  general  direction  of  the  rock-striation, 
that  is,  in  the  path  of  the  ice-movement.  It  may  be  traced  up  msaij 
valleys  into  the  mountains,  underlying  the  moraines  of  the  later  glacia- 
tion.  In  other  valleys,  it  has  been  removed  by  the  younger  glaciers.  In 
most  glaciated  countries  the  boulder-clay  is  not  one  continuous  deposit^ 
but  may  be  separated  into  two  or  more  distinct  formations,  which  lie  one 
on  the  other,  and  mark  distinct  and  successive  periods  of  time. 

In  those  areas  which  served  as  independent  centres  of  dispersion  fw 
the  ice-sheet,  boulder-clay  partakes  largely  of  the  local  character  of  the 
rocks  of  each  district  where  it  occurs.  Thus  in  Scotland,  the  clay  varies 
in  colour  and  composition  as  it  is  traced  from  district  to  district.  Over 
the  Carboniferous  rocks  it  is  dark,  over  the  Old  Red  Sandstones  it  is 
red,  over  the  Silurian  rocks  it  is  fawn-coloured.  The  material  of  the 
deposit  is  generally  an  earthy  or  stony  clay,  which  in  the  lower  parts  is 
often  exceedingly  compact  and  tenacious.  The  higher  portions  are 
frequently  loose  in  texture,  but  alternations  of  hard  tough  clay  and  more 
friable  material  may  be  met  with  in  the  same  deposit.  In  general, 
boulder-clay  is  unstratified,  its  materials  being  irregularly  and  tumultu- 
ously  heaped  together.  But  rude  traces  of  bedding  may  not  infrequently 
be  detected,  while  in  some  cases,  especially  in  the  higher  clays,  distinct 
stratification  may  be  observed. 

The  great  majority  of  the  stones  in  boulder-clay  are  of  local  origin, 
not  always  from  the  immediately  adjacent  rocks,  but  from  points 
within  a  distance  of  a  few  miles.  Evidence  of  transport  can  be  gathered 
from  the  stones,  for  they  are  found  in  almost  every  case  to  include  a  pro- 
portion of  fragments  which  have  come  from  a  distance.  The  direction 
of  transport  indicated  by  the  percentage  of  travelled  stones  agrees  with  the 
traces  of  ice-movement  as  shown  by  the  rock-striae.  Thus,  in  the  lower 
part  of  the  valley  of  the  Firth  of  Forth,  while  most  of  the  fragments 
are  from  the  surrounding  Carboniferous  rocks,  from  5  to  20  per  cent 
have  come  eastward  from  the  Old  Red  Sandstone  range  of  the  Ochil 
Hills — a  distance  of  25  or  30  miles — while  2  to  5  per  cent  are  pieces  of 
the  Highland  rocks,  which  must  have  come  from  high  grounds  at  least 
50  miles  to  the  north-west.  The  farther  the  stones  in  the  till  have 
travelled,  the  smaller  they  usually  are.  As  each  main  mass  of  elevated 
ground  seems  to  have  caused  the  ice  to  move  outward  from  it  for  a 
certain  distiince,  until  the  stream  coalesced  with  that  descending  from 
some  other  height,  the  bottom-moraine  or  boulder- clay,  as  it  was  pushed 
along,  would  doubtless  take  up  local  debris  by  the  way,  the  detritus  of 
each  district  becoming  more  and  more  ground  up  and  mixed,  until  of  the 
stones  from  remoter  regions  only  a  few  harder  fragments  would  be  left. 
In  cases  where  no  prominent  ridges  interrupted  the  march  of  the  ice- 
sheet,  and  where  the  ground  was  low  and  covered  with  soft  loose 
deposits,  blocks  of  hard  crystalline  rocks  might  continue  to  be  recognis- 
able far  from  their  source.  Thus  in  the  stony  clay  and  gravel  of  the 
plains  of  Northern  Germany  and  Holland,  besides  the  abundant  locally- 
derived   detritus,   fragments   occiu*  which   have  had  an  unquestionably 


SECT,  i  §  1  PLEISTOCENE  OR  GLACIAL  SERIES  1033 

northern  origin.  Some  of  the  rocks  of  Scandinavia,  Finland,  and  the 
Upper  Baltic  are  of  so  distinctive  a  kind  that  they  can  be  recognised  in 
small  pieces.  The  peculiar  syenite  of  Laurwig,  in  the  south  of  Norway, 
has  been  found  abundantly  in  the  drift  of  Denmark;  it  occurs  also 
in  that  of  Hamburg,  and  has  been  detected  even  in  the  boulder-clay 
of  the  Holdemess  cliffs  in  Yorkshire.  The  well-known  rhombenporphyr 
of  southern  Norway  has  likewise  been  recognised  at  Cromer  and  in 
Holdemess.  Fragments  of  the  Silurian  rocks  from  Gothland,  or  from  the 
Russian  islands  Dago  or  Oesel,  are  scattered  abundantly  through  the  drift 
of  the  North  German  plain,  and  have  been  met  with  as  far  as  the  north 
of  Holland.  Pieces  of  granite,  gneiss,  various  schists,  porph3nries,  and 
other  rocks,  probably  from  the  north  of  Eiu*ope,  occur  in  the  till  of 
Norfolk.^  These  transported  fragments  are  an  impressive  testimony  to 
the  movements  of  the  northern  ica  No  Scandina\'ian  blocks  have  been 
met  Avith  in  Scotland,  for  the  Scottish  ice  was  massive  enough  to  move 
out  into  the  basin  of  the  North  Sea,  until  it  met  the  northern  ice-sheet 
streaming  down  from  Scandinavia,  which  was  thereby  kept  from  reaching 
the  more  northerly  parts  of  England. 

The  stones  in  boulder-clay  have  a  characteristic  form  and  surface. 
They  are  usually  oblong,  have  one  or  more  flat  sides  or  "soles,"  are 
smoothed  or  polished,  and  have  their  edges  worn  round  (Fig.  160). 
Where  they  consist  of  a  fine-grained  enduring  rock,  they  are  almost 
invariably  striated,  the  striae  running  on  the  whole  with  the  long  axis  of 
the  stone,  though  one  set  of  scratches  may  be  seen  crossing  and  partially 
effacing  another,  which  would  necessarily  happen  as  the  stones  shifted 
their  position  under  the  ice.  These  markings  are  precisely  similar  to 
those  on  the  solid  rocks  underneath  the  boulder-clay,  and  have  manifestly 
been  produced  in  the  same  way  by  the  mutual  friction  of  rocks,  stones, 
and  grains  of  sand  as  the  whole  mass  of  debris  was  being  steadily  pushed 
on  in  one  general  direction. 

As  above  remarked,  boulder-clay  is  not  always  one  continuous  deposit. 
On  the  contrary,  when  a  sufficiently  large  extent  of  it  is  examined, 
evidence  can  commonly  be  found  of  two  distinct  divisions,  sometimes  even 
of  more  than  two.  These  are  separable  from  each  other  by  differences  of 
colour,  composition,  and  texture.  An  attentive  study  of  them  shows  that 
they  have  been  formed  successively  under  ice-sheets  moving  often  from 
different  directions  and  transporting  different  materials.  Their  limits  of 
distribution  also  vary,  the  lower  and  older  subdivisions  extending  farther 
south  and  spreading  over  a  wider  area  than  the  upper. 

Interglacial  Beds. — That  the  deposition  of  boulder-clay  in  Britain 
was  interrupted  by  milder  intervals,  when  the  ice,  imrtially  at  least, 
retreated  from  the  land  and  allowed  trees  and  other  vegetation  to  grow 
up  to  heights  of  800  or  900  feet  above  the  sea,  was  first  proved  by 

'  These  erratics,  from  their  petrographical  characters,  api>e«y  to  me  to  be  certainly  not 
from  Scotland.  Had  that  been  their  source  they  could  not  have  failed  to  be  accom- 
panied by  abundant  fragments  of  the  rocks  of  the  south  of  Scotland,  which  are  continuously 
absent.     See  V.  Madsen,  Quart.  Jmrni.  Oeitf.  Soc,  xlix.  (1893)  p.  114. 


1034  STRATIGRAPHICAL  GEOLOGY  book  vi  part  ▼ 

observations  at  Chapel  Hall,  Lanarkshire.^  During  the  thirty  yean 
which  have  intervened  since  these  observations  were  published,  a  hu*ge 
amount  of  additional  information  on  this  subject  has  been  collected  in  the 
British  Islands,  on  the  continent  of  Europe,  and  in  North  America.  The 
boulder-clays  are  now  well  known  to  be  split  up  with  inconstant  and 
local  stratifications  of  sand,  gravel  and  clay,  often  well  stratified,  pointing 
to  conditions  quite  distinct  from  those  under  which  ordinary  boulder^lay 
was  accumulated.  These  intercalations  have  been  recognised  as  bearing 
witness  to  intervals  when  the  ice  retired  from  some  districts  and  when 
ordinary  water -action  came  into  play  over  the  ground -moraine  thus 
exposed.  Much  controversy,  however,  has  arisen  as  to  the  chronologicsl 
value  to  be  assigned  to  these  intervals.  To  some  geologists  the  intercala- 
tions in  the  boulder-clay  appear  to  indicate  little  more  than  seasonal 
variations  in  the  limits  and  thickness  of  the  ice-sheets,  such  as  now  affect 
the  glaciers  of  Scandinavia  and  the  Alps.  To  others,  again,  they  furmsh 
proof  of  successive  interglacial  periods  by  which  the  long  Ice  Age  was 
broken  up.  Thus  Professor  James  Geikie,  recently  reviewing  the  whole 
evidence  on  the  subject,  has  come  to  the  conclusion  that  there  were  really 
five  glacial  intervals  embraced  within  what  is  called  the  Glacial  Period, 
separated  from  each  other  by  four  interglacial  periods  of  mild  tempera- 
ture.2 

Much  difticulty  in  forming  definite  conclusions  as  to  the  importance 
of  these  obvious  interruptions  in  the  deposition  of  the  boulder-clay 
arises  from  the  absence  of  continuous  sections  wherein  the  order  of 
succession  of  the  several  stages  of  the  glacial  history  can  be  demonstrated 
by  visible  relations  of  superposition.  A  section  at  one  locality  has  to  be 
correlated  with  another  at  a  greater  or  less  distance,  and  assumptions 
have  to  be  made  as  to  the  identity  or  difference  of  the  various  deposits. 
The  evidence  of  fossils  can  hardly  be  said  to  be  available,  for  it  is  so 
fragmentary  a.s  to  give  little  aid  in  determining  the  chronology  of  the 
(leix)sits  in  which  it  occurs. 

The  existence  of  two  distinct  deposits  of  boulder-clay,  with  an  inter- 
vening group  of  sands,  gravels,  clays,  and  peat-beds,  may  be  taken  to 
afford  good  proof  of  two  advances  and  retreats  of  the  ice-sheets,  with  an 
interval  of  non-glacial  conditions  between  them.  The  oldest  boulder-clay 
marks  the  greatest  extent  of  the  ice.  The  upper  boulder-clay  shows  that 
though  the  ice  on  returning  attained  huge  dimensions  and  formed  con- 
tinuous ice-sheets  over  much  of  northern  Europe,  it  did  not  descend  as  far  as 
at  first.  Yet  while  these  two  main  epochs  of  maximum  cold  can  be 
satisfactorily  established  there  appears  to  be  no  reason  to  doubt  that  each 
of  them  may  have  had  fluctuations  in  temperature  or  in  snowfall,  so  that 
the  ice-sheets  may  have  alternately  or  intermittently  advanced  and  retreated 
over  considerable  tracts  of  country.  The  ground-moraine,  when  thus  laid 
bare,  may  have  been  reassorted  by  water,  so  that  as  the  ice  once  more 
moved  forward,  it  here  and  there  pushed  its  detritus  over  the  aqueous 
deposits  of  the  milder  interval.     But  the  marked  contrast  between  the 

'  A.  G.  Trans.  (JcoL  Soc.  iilasgow,  vol.  i.  part  ii.  (1863). 
'  Ti-am.  Roy,  Soc.  Edin,  xxxvii.  part  i.  (1893)  p.  146. 


SECT,  i  §  I  FLEISTOCESE  OR  GLACIAL  SERIES  10311 

lower  and  upper  boulder-clay  in  composition  and  extent  shows  that  the 
interval  which  separated  them  was  probably  of  prolonged  duration.  We 
have  here  evidence  of  at  least  one  important  interglacial  period.  The 
occurrence  of  such  interludes  of  more  genial  climate  is  what  might  be 
expected  to  be  traceable  on  the  astronomical  theory  of  the  cause  of  the 
Ice  Age,  which  has  been  already  discussed  (p.  24).  The  deposits  which 
record  the  passage  of  an  intei^Iacial  period  consist  of  layers  of  sand  and 
gravel,  such  as,  over  a  wide  area  of  central  England,  separate  the  two 
boulder-clays,  also  deposits  of  clay  and  beds  of  peat  found  elsewhere  in 
a  similar  position.  To  this  age  also  have  been  assigned  the  older 
alluvial  terraces  which  have  been  preserved  chiefly  beyond  the  limits  of 
the  second  glaciation,  and  from  which  a  considerable  number  of  mam- 
malian remains  as  well  as  stone  implements  of  human  workmanship 
have  been  disinterred. 


During  interglacial  conditions  the  climate  in  the  northern  hemisphere 
was  probably  much  more  equable  and  mild  than  at  present,  with  a  higher 
mean  temperature  and  at  certain  intervals  a.  greater  precipitation  of 
moisture.'  From  the  general  aspect  of  the  flora  and  fauna  preserved  in 
interglacial  deposits  in  Britain  it  may  perhaps  be  inferred  that  there  was 
then  more  sunshine  than  now.  Mr.  Reid  suggests  that  the  scarcity  of 
thoroughly  aquatic  mollusks  and  of  fish  indicates  that  during  some  stages, 
at  least,  the  climate  was  dry  rather  than  moist.  As  a  re!^ult  of  more 
favourable  meteorological  conditions  vegetation  flourished  even  far  north 
where  it  can  now  hardly  exist.  The  frozen  tundras  of  Siberia  appear 
then  to  have  supported  forests  which  have  long  since  been  extirpated, 
the  present  northern  limit  of  living  trees  lying  far  to  the  southward. 
Indications  of  a  more  equable  and  milder  climate  are  likewise  supplied 
by  the  plant-remains  found  in  Pleistocene  tufas  of  different  parts  of 
■  J.  Croll,  Phil.  At«g.  1885.  p.  36. 


1036  STRATIGSAPHIVAL  GEOLOGY  BOOitviPJUffv 

Europe,  wliere  species  now  restricted  to  more  Boathem  countries  were 
then  able  to  flourish  together  with  those  which  are  still  native  there.' 

The  fauna  of  the  nortiiem  parts  of  our  hemisphere  was  then  an 
extraordinary  one.  It  was  marked  more  especially  hy  the  presence  (rf 
the  last  of  the  huge  pachyderms,  which  had  for  so  many  ages  been  the 
lords  of  the  European  forests  and  pastures.  The  hairy  mammoth  and 
woolly  rhinoceros  roamed  over  the  plains  of  Siberia  and  across  most,  if 
not  the  whole,  of  Europe.  These  animals  were  probably  driven  south- 
ward by  the  increasing  cold,  and  they  appear  to  have  survived  some  of 
the  advances  of  the  ice,  returning  into  their  former  haunts  when  a  lest 
wintry  climate  allowed  the  vegetation  on  which  they  browsed  once 
more  to  overspread  the  land.^  Some,  of  the  mammals  now  restricted  to 
the  far  north  likewise  found  their  way  into  countries  from  which  they 
liave  long  disappeared.  The  reindeer 
migrated  southwards  into  Switserland,* 
the  glutton  into  Auvergne,  while  the 
musk-sheep  and  Arctic  fox  travelled  txi- 
tainly  as  far  as  the  Pyrenees.  As  the 
climate  became  less  chilly,  animals  of  a 
more  southern  type  advanced  into  Europe: 
the  porcupine,  leopard,  African  lynx,  lion, 
striped  and  spotted  hyienas,  African  ele- 
phant, and  hippopotamus.  With  each 
ahfP[,(((ai™jiiKP«An/«.,  I)  tiricii-tBrti.,  osclllation  of  climate  there  would  be  a 
""■  ""  ■   ™''  corresponding  immigration  and  emigration 

of  northern  and  southern  types. 

Evidences  of  Submergence. — After  the  ice  had  attained  its 
greatest  development,  some  portions  of  north-western  Europe,  which  had 
|)erhaps  stood  at  a  higher  level  above  the  sea  than  they  have  done  since, 
began  to  subside.  The  ice-fields  were  carried  down  below  the  sea-level, 
where  they  broke  up  and  cumbered  the  sea  with  floating  bergs.  The 
heaps  of  loose  dt^bris  which  had  gathered  under  the  ice,  being  now 
exposed  to  waves,  ground-swell,  and  marine  currents,  were  thereby 
more  or  less  washed  down  and  reassorted.  Coast-ice,  no  doubt,  still 
foi-med  along  the  shores,  and  was  broken  up  into  moving  floes,  as 
happens  every  year  now  in  northern  Greenland.  The  proofs  of  this 
phase  of  the  long  glacial  period  are  contained  in  shell-bearing  sands, 
gntvels,  and  clays  which  overlie   the   coarse   older  till,  and  are  perhaps, 

'  Kalliorst.  Kiiater's  liotuiiiscli-  Jalirb.  1881,  p,  431  ;  C.  Bcbriiter,  'Die  Flora  der 
EiRzeil.'Ziiricli.  1SS3. 

''  Tie  ninniiiiotli  lived  in  tlie  neighbourhooil  of  the  eitinct  Tolcsnoes  of  central  Italy. 
wliicli  were  tlien  In  full  activily.  From  diacoveries  in  Fiulnnd,  it  h«s  been  inferred  that 
tbe  eitinctiou  of  this  animal  may  not  have  been  niiioh  bsfare  biatorical  timee.  A.  J. 
Molnigreu,  Or/r.  fiiuA:  Vet.  .5w,  Furk.  ivil.  p.  139.  Consult  Boyd  Dawkins  on  the  range 
of  tlie  maTnnioth  In  space  and  time :  Q.  J.  Gfol.  &■(.  xxxv.  (1879)  p.  138  ;  and  Howmlh. 
fifl.  Jlcff.  18S0;  'Tlie  Mammoth  and  the  Flood'  and  'The  Glacial  Nightmare.' 

'  On  t!ie  .lislributioD  of  .the  wiiideer  at  present  and  iu  older  time.  Me  C,  Stnirkmann, 
Zeihch.  llcil^d:  <le,>l.  tie,,  iiiii.  (1880)  p.  728. 


SECT,  i  §  1  PLEISTOCENE  OR  GLACIAL  SERIES  1037 


to  some  extent,  furnished  by  erratic  blocks.^  It  is  difficult  to  determine 
the  extent  of  the  submergence,  for,  when  the  land  rose,  the  more 
elevated  portions  continued  to  be  seats  of  glaciers,  which,  moving  over 
the  surface,  destroyed  the  deposits  that  would  otherwise  have  remained 
as  witnesses  of  the  presence  of  the  sea,  while  at  the  same  time  the  great 
bodies  of  water  discharged  from  the  retreating  glaciers  and  snow-fields 
must  have  done  much  to  reassort  the  detritus  on  the  surface  of  the  laud. 
From  the  evidence  of  marine  shells,  southern  Scandinavia  is  believed  to 
have  sunk  about  600  feet  below  its  present  level.  In  Britain  the  sub- 
mergence was  probably  not  less  than  500  feet.  If  indeed  we  take  the 
beds  of  marine  shells  which  have  been  found  in  North  Wales,  Cheshire, 
and  elsewhere  as  marking  actual  sea-bottoms,  the  depression  which  they 
would  then  indicate  must  have  been  at  least  1350  feet.  But  these  shelly 
deposits  are  probably  not  conclusive  proofs  of  submergence.^ 

That  ice  continued  to  float  about  in  these  waters  is  shown  by  the 
striated  stones  contained  in  the  fine  clays,  and  by  the  remarkably  con- 
torted structure  which  these  clays  occasionally  display.  Sections  may 
be  seen  (as  at  Cromer)  where,  upon  perfectly  undisturbed  horizontal 
strata  of  clay  and  sand,  other  similar  strata  have  been  violently  crumpled, 
while  horizontal  beds  lie  directly  upon  them.  These  contortions  may 
have  been  produced  by  the  horizontal  pressure  of  some  heavy  body 
moving  upon  the .  originally  flat  beds,  such  as  ice  in  the  form  of  an 
ice-sheet  or  of  large  stranding  masses  driven  aground  in  the  fjords  or 
shallow  waters  where  the  clays  accumulated ;  or  possibly,  in  some  cases, 
sheets  of  ice,  laden  with  stones  and  earth,  sank  and  were  covered  up 
with  sand  and  clay,  which,  on  the  subsequent  melting  of  the  ice,  would 
subside  irregularly.  Another  indication  of  the  presence  of  floating  ice 
is  furnished  by  large  scattered  boulders,  lying  on  the  stratified  sands  and 
gravels.  Though  these  blocks  probably  belong  as  a  rule  to  the  time  of 
the  chief  glaciation,  they  may  in  some  cases  have  been  shifted  about  by 
floating  ice  during  the  submergence. 

Second  Glaciation — Re-elevation — Raised  Beaches. — 
When  the  land  re-emerged  from  its  depression,  the  temperature  all  over 
central  and  northern  Europe  was  again  severe.  The  northern  ice-sheet 
once  more  advanced  southwards,  but  did  not  again  attain  nearly  the 
same  dimensions.  From  the  direction  of  the  strisB,  it  would  appear 
sometimes  to  have  moved  differently  from  its  previous  course,  occasion- 

'  For  an  accouut  of  the  dispersion  of  the  **  erratics  "  of  England  and  Wales,  see  Mackin- 
tosh, Q.  J.  Geol,  Soc.  XXXV.  (1879)  p.  425  ;  and  Reports  of  the  Committee  appointed  to 
investigate  this  subject  by  the  British  Association,  1872  et  seq.  For  those  of  Scotland 
much  information  has  been  gathered  by  the  Boulder  Committee  of  the  Royal  Society  of 
Edinburgh  ;  Proc.  Roy.  Soc.  JSdin.  1872-84.  Erratic  blocks  have  probably  in  the  vast 
majority  of  cases  been  dispersed  by  land-ice,  and  not  by  floating  ice. 

^  Mere  fragments  of  marine  shells  in  a  glacial  de]K>sit  need  not  prove  submergence  under 
the  sea  ;  for  they  may  have  been  pushed  up  from  the  sea-floor  by  moving  ice,  as  in  the  case 
of  the  shelly  till  of  the  west  of  Scotland,  Caithness,  Holderness,  and  Cromer.  How  far  this 
may  have  been  the  origin  of  the  shelly  deposits  found  at  high  levels  in  Britain  is  still  a 
<li»pute<l  question. 


1038  STRATIGRAFHICAL  GEOLOGY  book  vi  pakt  t 

ally  even  at  right  angles  to.  it.  In  the  basin  of  the  Baltic,  for  example, 
the  later  direction  of  the  ice-stream  appears  to  have  been  south-west- 
wards and  westwards.  Besides  the  evidence  of  this  direction  furnished 
by  striated  rock-surfaces,  abundant  fragments  of  the  fossiliferous  Silurian 
rocks  of  Gothland  are  strewn  over  the  Germanic  plain  even  as  far  as 
Holland.  There  seems  no  reason  to  doubt  that  during  this  second 
advance  of  the  ice  the  Scottish  and  Scandinavian  ice-sheets  were  again 
united  over  what  is  now  the  floor  of  the  North  Sea.  It  was  then  that 
the  upper  boulder-clay  of  Britain  was  formed.  The  glaciers  of  the  Alps 
once  more  marched  outwards  over  the  lower  grounds,  but  without  descend- 
ing so  far  as  before.  Their  limits  are  marked  by  an  inner  group  of 
moraines. 

From  its  second  maximum  the  ice-sheet  gradually  shrank  backward, 
though  probably  not  without  occasional  pauses  and  even  advances.  As 
it  retreated  from  the  lower  grounds  it  lost  the  aspect  of  a  continuous  ice- 
sheet,  and  when  it  reached  the  bases  of  the  mountains  it  eventually 
separated  into  valley-glaciers  radiating  from  each  principal  mass  of  high 
ground.  In  this  condition  also  there  was  probably  a  long  period  of 
oscillation,  the  glaciers  alternately  descending  and  shrinking  backward 
with  variations  in  the  seasons.  In  Britain  there  is  abundant  evidence  of 
this  stage  in  the  history  of  the  Ice  Age.  The  Scottish  Highlands,  being 
the  largest  area  of  high  ground  in  the  country,  was  the  chief  seat  of  the  ice. 
Not  only  did  every  group  of  mountains  nourish  its  own  glaciers ;  even 
small  islands,  such  as  Arran  and  Hoy,  had  their  snow-fields,  whence 
glaciers  crept  down  into  the  valleys  and  shed  their  moraines.  It  would 
appear  indeed  that  some  of  the  northern  glaciers  continued  to  reach  the 
sea-level  even  when  the  land  had  there  risen  to  near  or  quite  its  present 
elevation.  On  the  east  side  of  Sutherlandshire,  at  Brora,  and  on  the 
west  side  of  Ross-shire,  at  Loch  Torridon,  the  moraines  descend  to  the  50- 
feet  raised  beach ;  at  the  head  of  Loch  Eriboll,  they  come  down  to  the 
sea-level  and  even  extend  underneath  the  water,  showing  that  the  glacier 
at  the  head  of  that  fjord  actually  pushed  its  way  into  the  sea,  and  no 
doubt  calved  its  icebergs  there. 

Another  proof  of  the  magnitude  of  some  of  the  ice-streams  that  filled 
the  valleys  of  the  Scottish  Highlands  during  the  later  stages  of  the 
Glacial  Period  is  supplied  by  the  proofs  that  here  and  there  among  the 
loftier  or  broader  snow-fields  of  the  time  they  accumulated  in  front  of 
lateral  valleys,  the  drainage  of  which  was  in  consequence  ponded  back  and 
made  to  flow  out  in  an  opposite  direction  by  the  col  at  the  head  (p.  423). 
In  these  natural  reservoirs,  the  level  at  which  the  water  stood  for  a  time 
was  marked  by  a  horizontal  ledge  or  platform,  due  partly  to  erosion  of 
the  hillside,  but  chiefly  to  the  arrest  of  the  descending  debris  when  it 
entered  the  water.  The  famous  "  Parallel  Eoads  of  Glen  Roy  "  are  the 
most  familiar  examples.  In  some  instances,  as  at  Achnasheen  in  Ross- 
shire,  the  detritus  of  the  glacial  streams  was  arrested  and  spread  out  in 
broad  platforms  across  the  valleys. 

The  gradual  retreat  of  the  glaciers  towards  their  parent  snow-fields 
is    admirably   revealed   by  their   moraines,   perched   blocks,  and   rockes 


SECT,  i  §  1  PLEISTOCENE  OR  GLACIAL  SERIES  1039 

moutonii^es.  The  crescent-shaped  moraine-mounds  that  lie  one  hehind 
another  may  be  followed  up  a  glen,  until  they  finally  die  out  about  the 
head,  near  what  must  have  been  the  edge  of  the  snow-field.  The  highest 
mounds,  being  the  last  to  be  thrown  down,  are  often  singularly  fresh. 
They  frequently  enclose  pools  of  water,  which  have  not  yet  been  filled  up 
with  detritus  or  vegetation,  or  flat  peaty  bottoms  where  the  process  of 
filling  up  has  been  completed.  Huge  blocks  borne  from  the  crags  above 
them  are  strewn  over  these  heaps,  and  similar  erratics  perched  on  ice- 
worn  knolls  on  the  sides  of  the  valleys  mark  some  of  the  former  levels 
of  the  ice.  The  Scottish  Highlands,  the  southern  uplands  of  Scotland, 
the  hills  of  the  Lake  district  and  of  North  Wales  present  admirable 
examples  of  all  these  features. 

On  the  continent  of  Europe  also  similar  evidence  remains  of  the 
gradual  retreat  of  the  ice.  In  many  tracts  of  high  ground  glaciers  no 
longer  exist.  In  the  Vosges,  for  example,  they  have  long  since  vanished, 
but  fresh  moraines  remain  there  as  evidence  of  their  former  presence.  The 
Alpine  glaciers  are  the  lineal  descendants  of  those  which  filled  up  the 
valleys  and  buried  the  lowlands  of  Switzerland  and  the  Lyonnais. 

Before  the  retiring  ice-sheet  had  shrunk  into  mere  valley  glaciers, 
and  while  it  still  occupied  part  of  the  lower  ground,  there  would  doubtless 
be  a  copious  discharge  of  water  from  its  melting  front.  As  the  ice  had 
overridden  the  land  and  buried  its  minor  inequalities,  there  would  be 
great  diversity  in  the  level  of  the  bottom  of  the  ice,  and  consequently  the 
escaping  water  would  at  first  flow  with  little  relation  to  the  present 
main  drainage  lines.  Streams  of  water  might  be  let  loose  over  the 
plateaux  and  hilly  ridges  as  well  as  over  the  plains.  There  could 
hardly,  therefore,  fail  to  be  much  rearrangement  of  the  detritus  left  by 
the  ice.  Possibly  to  this  part  of  the  Ice  Age  and  to  this  kind  of  action 
we  should  attribute  the  masses  of  gravel  and  sand  which,  over  so  much 
of  northern  Europe,  rest  on  boulder-clay.  Among  these  accumulations 
are  the  sheets  of  coarse,  well-rounded  gravel  (plateau-gravel),  which, 
with  no  recognisable  relation  to  the  present  contours  of  the  ground,  are 
spread  over  the  plains  and  low  plateaux,  and  fill  up  many  valleys. 
These  gravels  rest  sometimes  on  boulder-clay,  sometimes  on  solid  rock, 
and  are  older  than  the  valley  alluvia.  They  have  evidently  not  been 
formed  by  any  ordinary  river-action,  nor  is  it  easy  to  see  how  the  sea 
can  have  been  concerned  in  their  formation.  They  are  well  developed 
in  Norfolk  and  adjacent  tracts  of  the  south-east  of  England,  where  they 
consist  mainly  of  well-rounded  flints  (cannon-shot  gravel). 

Still  more  remarkable  are  the  accumulations  of  sand  and  gravel  to 
which  the  name  of  **Kame  group"  has  been  given.  Covering  the 
lower  ground  in  a  sporadic  manner,  often  tolerably  thick  on  the  plains, 
these  deposits  rise  up  to  heights  of  1000  feet  or  more.  In  some  places, 
they  cannot  be  satisfactorily  separated  from  the  sands  and  gravels 
associated  with  the  boulder-clay,  in  others  they  seem  to  merge  into  the 
sandy  deposits  of  the  raised  beaches,  while  in  hilly  tracts  it  is  some- 
times hard  to  distinguish  between  them  and  true  moraine-stuff.  Their 
most  remarkable  mode  of  occurrence  is  when  they  assume  the  form  of 


1 040  STUA  TIGRA PHICAL  GEOLOQ  Y  book  vi  part  ▼ 


mounds  and  ridges,  which  run  across  valleys  and  plains,  along  hillsides, 
and  even  over  water-sheds.  Frequently  these  ridges  coalesce  so  as  to 
enclose  basin-shaped  hollows,  which  are  often  occupied  by  tarna.  Many 
of  the  most  marked  ridges  are  not  more  than  50  or  60  feet  in  diameter, 
sloping  up  to  the  crest,  which  may  be  20  or  30  feet  above  the  plain.  A 
single  ridge  may  occasionally  be  traced  in  a  slightly  sinuous  course  for 
many  miles,  as  in  the  case  of  the  famous  mound  which  runs  across  the 
centre  of  Ireland.  These  ridges,  known  in  Scotland  as  Kames,  in  Ireland 
as  Eskers,  and  in  Scandinavia  as  Osar,  consist  sometimes  of  coarse  gravel 
or  earthy  detritus,  but  more  usually  of  clean,  well-stratified  sand  and 
gravel,  the  stratification  towards  the  surface  corresponding  with  the 
external  slopes  of  the  ground,  in  such  a  manner  as  to  prove  that  the 
ridges  are  usually  original  forms  of  deposit,  rather  than  the  result  of  the 
irregular  erosion  of  a  general  bed  of  sand  and  gravel.  Some  writers 
have  compared  these  features  to  the  submarine  banks  formed  in  the 
pathway  of  tidal  currents  near  the  shore.  But  they  appear  rather  to  be 
of  terrestrial  origin,  due  in  some  way  to  the  melting  of  the  great  snow- 
fields  and  glaciers,  and  the  consequent  discharge  of  large  quantities  of 
water  over  the  country.  But  no  very  satisfactory  explanation  of  their 
mode  of  formation  has  yet  been  given. 

Over  the  tracts  from  which  the  ice-sheet  retired,  lakes  are  usually 
scattered  in  large  numbers.  Some  of  these  lie  in  ice-worn  basins  of 
rock.  Where  the  detritus  has  been  strewn  thickly  over  the  ground, 
however,  they  rest  in  hollows  of  the  clay,  earth,  sand,  or  gravel  The 
origin  of  these  depressions  in  the  drifts  cannot  be  found  in  any  denuding 
operation  since  the  ice  left.  They  are  obviously  original  features  of  the 
surface,  dating  back  to  the  time  when  the  various  drifts  were  laid  down. 
In  some  cases  they  may  be  due  to  irregular  deposition  of  the  detritus, 
as  where  successive  moraines  are  thrown  across  a  valley.  The  small 
pools  may  sometimes  have  been  originated  by  the  melting  of  portions  of 
ice  which  had  become  detached  from  the  main  mass,  and  were  surrounded 
))y  or  buried  under  detritus.  Many  small  rock-basins  may  have  had 
their  place  and  form  determined  by  that  })rolonged  deep  subaerial  rotting 
already  referred  to,  while  others  may  be  referable  to  underground  move- 
ments. But  the  glaciers,  in  smoothing  and  polishing  the  rocks,  wore 
them  down  unequally,  hollowing  them  into  rock-basins,  leaving  them 
in  prominent  smoothed  domes,  and  carrying  the  same  characteristic 
sculpture  over  all  the  durable  rocks  exposed  in  the  areas  of  intenser 
glaciation. 

The  uprise  of  the  land  in  Scandinavia  and  Britain  took  place  inter- 
ruptedly. During  its  progress  it  was  marked  by  long  pauses  when  the 
level  remained  unchanged,  when  the  waves  and  floating  ice  cut  ledges 
along  the  sea -margin,  and  when  sand  and  gravel  were  accumulated 
below  high -water  mark  in  sheltered  parts  of  the  coast-line.  These 
platforms  of  erosion  and  deposit  (raised  beaches)  form  conspicuous 
features  at  successive  heights  above  the  present  level  of  the  sea  (p.  285). 
The  coast  of  Scotland  is  fringed  with  a  succession  of  them  (Fig.  457). 
Those  .below  the  level  of  100  feet  above  the  sea  are  often  remarkably 


SECT,  i  g  1  PLEISTOOENE  OR  GLACIAL  SERIES  1041 

fresh.  The  100-feet  terrace  forms  a  wide  plateau  in  the  eBtuary  of  the 
Forth,  and  the  50-feet  terrace  is  as  conapicuoua  in  that  of  the  Clyde. 
In  Scandinavia,  especially  in  the  northern  parts  of  Norway,  the  euccesBire 
pauses  in  the  last  uprise  of  the  land  are  impressively  revealed  by  long 
lines  of  terraces  which  wind  around  the  hill-slopes  that  encircle  the 
fjords  (p.  287). 

The  records  of  the  closing  ages  of  the  long  and  varied  Glacial  Period 
merge  insensibly  into  those  of  later  geological  times.  It  is  obvious  tliat 
besides  the  elTect  of  a  general  change  of  climate  operating  over  the  whole 
of  the  northern  hemisphere,  we  must  remember  the  influence  which  the 
natural  features  of  different  countries  had  upon  the  cltmat«.  From  the 
plains,  the  ice  and  snow  would  retire  sooner  than  from  the  hills.  In  fact, 
we  may  regard  some  parts  of  Europe  as  still  retaining  the  conditions  of  the 
Glacial  Period,  though  id  diminished  intensity,  the  present  glaciers  of  the 
Alps  being,  as  above  remarked,  the  representatives  in  continuous  succession 


of  the  vaster  sheets  that  once  descended  into  the  lowlands  on  all  sides 
from  that  central  elevated  region.  And  even  where  the  ice  has  long 
since  disappeared,  there  remain,  in  the  living  plants  and  animals  of  the 
higher  and  colder  uplands,  witnesses  to  the  former  severity  of  the  climate. 
As  that  severity  lessened,  the  Arctic  vegetation,  that  hitherto  had  peopled 
all  the  lower  grounds  of  central  and  western  Europe,  was  driven  up  into 
the  hills  before  the  advance  of  plants  loving  a  milder  temperature,  which 
had  doubtless  been  natives  of  Europe  before  the  period  of  great  cold,  and 
which  were  now  enabled  to  reoccupy  the  sites  wlience  they  had  been 
banished.  On  the  higher  mountains,  where  the  climate  is  still  not  wholly 
uncongenial  for  tliem,  and  likewise  here  and  there  at  lower  levels,  colonies 
of  the  once  general  Arctic  flora  still  survive.  The  Arctic  animals  have 
also  been  mostly  driven  away  to  their  northern  homes,  or  have  become 
wholly  extinct.  But  the  remains  of  the  Arctic  plants  and  to  some  extent 
also  of  the  animals  occur  in  the  lacustrine  clays,  peat-mosses  and  other 
deposits  of  the  glacial  series,  even  down  into  the  heart  of  Europe. 

It  has  been  forcibly  pointed  out  by  Mr.  Wallace  that  the  present 
mammalian  fauna  of  the  globe  presents  everywhere  a  striking  contrast 
to  the  extraordinary  variety  and  great  size  of  the  mammals  of  the 
Tertiary  periods.  "  We  live,"  he  says,  "  in  a  zoologically  impoverished 
world,  from  which  all  the  largest,  and  fiercest,  and  strangest  forms  have 
3  X 


1042  STRATIGRAPHICAL  GEOLOGY  book  vi  party 

recently  disappeared."^  He  connects  this  remarkable  reduction  with 
the  refrigeration  of  climate  diiring  the  Glacial  Period.  The  change,  to 
whatever  cause  it  may  be  assigned,  is  certainly  remarkably  persistent 
in  the  Old  World  and  in  the  New,  and  not  merely  in  the  temperate  and 
northern  regions,  but  even  as  far  south  as  the  southern  slopes  of  the 
Himalaya  Mountains. 

§  2.  Local   Development. 

BritaiiL^ — Though  the  generalised  succession  of  phenomena  above  given  is  osoally 
observable,  some  variety  is  traceable  in  the  evidence  in  different  parts  of  the  British 
area.  In  Scotland,  where  the  ground  is  generally  more  elevated,  and  where  snow  and 
ice  were  most  abundant,  the  phenomena  of  glaciation  reached  their  maximum  develop- 
ment. In  the  high  grounds  of  England,  Wales,  and  Ireland  there  was  likewise 
extensive  accumulation  of  ice.  The  ice -worn  rocks  of  the  low  grounds  are  usually 
covered  with  boulder-clay,  which  in  Scotland  is  interstratified  with  beds  of  sand,  fine 
clay,  and  peat,  but  has  never  yielded  any  marine  organisms  except  near  the  coast, 
where  they  are  sometimes  common,  and  in  one  locality  in  Lanarkshire.  In  England, 
marine  shells,  usually  fragmentary,  occur  in  the  boulder-clays  both  in  the  eastern  and 
western  counties.  The  ice-sheet  no  doubt  passed  over  some  parts  of  the  sea-bottom, 
and  ground  up  the  shell-banks  that  happened  to  lie  in  its  way,  as  has  happened,  for 
example,  in  Caithness,  Holdcrness,  and  East  Anglia,  where  the  shells  in  the  boulder- 
clay  are  fragmentary,  and  sometimes  ice-striated.  The  **  Bridlington  Crag"  of  Yoik- 
shire,  according  to  Messrs.  Sorby,  Lamplugh,  and  Reid,  is  a  large  fragment  torn  from 
a  submarine  shell-clay,  and  imbedded  in  the  boulder-clay.'  With  the  exception  of 
such  marine  enclosures,  the  organic  contents  as  well  as  the  ph3r8ical  characters  of  the 
Scottish  Till  point  to  terrestiial  conditions  of  deposit  under  the  ice-sheet. 

The  depth,  extent,  and  movements  of  the  great  ice-sheet  which  covered  Britain  have 
already  been  referred  to.  The  proofs  of  the  former  presence  of  the  ice  are  scattered 
abundantly  over  the  country  north  of  a  line  drawn  from  the  Bristol  Channel  to  the 
estuary  of  tlie  Thames.  South  of  that  line  the  ground  is  free  from  boulder-clay,  though 
various  deposits,  jwssibly  of  contemporary  date,  serve  to  indicate  that  though  not  buried 
under  ice  this  soutlieru  fringe  of  England  had  its  own  glacial  conditions.*  Among  these 
is  the  "  Coombe-rock  "  of  Sussex — a  mass  of  unstratified  rubbish  which  has  been  referred 
by  Mr.  C.  Reid  to  the  action  of  hea\'y  summer  rains  at  a  time  when  the  ground  a  little 
below  the  surface  was  permanently  frozen.      In  the  glaciated  tract  one  of  the  most 

^  '  Geographical  Distribution  of  Animals,'  i.  p.  150.  Consult  also  Asa  Gray,  JVo/w/y, 
xix.  p.  327  (363). 

^  Besides  the  general  works  and  papers  already  cited,  the  following  special  papers  in  the 
Quarterhj  Journal  of  the  Geological  Society  may  be  consulted  :  WaleSy  Mackintosh,  1882, 
p.  184  ;  I.  W.  E.  David,  1883,  p.  39.  N,  \\\  England,  Mackintosh,  1879,  p.  425,  1880, 
p.  178  ;  T.  M.  Reade,  1874,  p.  27,  1883,  p.  83 ;  A.  Strahan,  1886,  p.  369.  S.E.  England, 
Searles  V.  Wood  jun.  1880,  p.  457,  1882,  p.  667  ;  A.  J.  Jukes-Browne,  1879,  p.  897, 
1883,  p.  596  ;  Rowe,  1887,  p.  351.  Scotland  (Long  Island),  J.  Geikie,  xxix.  (1873) ; 
xxxiv.  (1878);  (Shetlands)  Peach  and  Home,  1879,  p.  778;  (Orkneys)  1880,  p.  648; 
(Aberdeenshire)  T.  F.  Jamieson,  1882,  pp.  145,  160.  The  student  will  find  a  useful  digest 
of  the  literature  for  England  up  to  1887  in  Mr.  H.  B.  Woodward's  'Geology  of  England 
and  Wales.'  The  Memoirs  of  the  Geological  Survey  will  be  found  to  contain  much  local 
detail  on  this  subject. 

3  Lamplugh,  Quart.  Joum.  OeoL  Soc,  xl.  (1884)  p.  312.  C.  Reid,  *  Geology  of  Holder- 
ness  '  in  Mem.  Geol.  Surrey. 

*  C.  Reid,  Quart.  Joum.  Oeol.  Soc.  xliu.  (1887)  p.  364. 


PLEISTOCENE  OS  GLACIAL  SERIES 


1043 


atriking  features  in  showing  the  Qreralaad-like  maaaiveneM  of  the  ioe-aheet  is  furniBhed 
by  the  south  of  Ireland,  where  the  hills  of  Cork  and  Keny  have  baem  ground  amooth 
and  striated  down  to  the  sea,  and  even  under  sea-level,  detached  isletc  appearing  as 
well  ice-rounded  roe/ui  mautonrUet.  There  can  be  no  doubt  from  this  evidsnce  that 
even  in  the  south  of  Ireland  the  ice-sheet  continued  to  be  so  massive  that  it  went  out  to 
sea  as  a  great  wall  of  ice,  probably  breaking  off  there  in  icebergs. 

The  records  of  the  submersion  of  Britain  are  probably  very  incomplete.  If  we  rely 
only  on  the  evidence  of  untransported  marine  shells,  we  obtain  the  lowest  limit  of 
depression.  But,  as  above  remarked,  the  mere  presence  of  marine  shells  cannot  always 
be  accepted  as  conclusive.  Again,  the  renewed  ice  and  snow,  after  re-eleration,  may 
well  have  destroyed  most  of  the  shell-beds,  and  their  destruction  would  be  moat  oom- 
plete  where   the  suaw-fields  and  glaciers  were  most  erteuflive.      Bods  of  sand  and 


n,  Peclea  lilindlcDS,  HUIL  ()) 
liU.  OmeliD  (T.  calam, 
andSow.  ());  g.  Tropin 


gravel  with  recent  shells  hare  been  observed  on  Hoel  Tryfaen,  in  North  Wales,  at  a 
height  of  13S0  feet,  bat  ths  sheila  are  broken  and  show  such  a  cnrioua  commingling 
of  species  as  to  indicate  that  they  are  probably  not  realty  in  place.  In  Cheshire 
marine  shells  o<:cur  at  1200  feet.  In  Scotland  tliey  have  been  obtained  at  hU  feet  in 
the  boulder-clay  at  the  Lanarkshire  locality  already  referred  to  ;  but  the  Uyer  contain- 
ing them  may  have  been  transported  by  the  ice-sheet-  Subsequent  elevation  of  the 
loud  has  brought  up  within  tids-marks  some  of  the  clays  deposited  over  the  sea-Soar 
during  the  time  of  the  submei^ence.  In  the  Clyde  basin  and  in  some  of  the  western 
fjords,  these  clays  (Clyde  Beds)  are  flill  of  shells.  Comparing  the  species  with  thooe  of 
tiie  adjacent  seas,  wo  And  them  to  be  more  boreal  in  character ;  nearly  the  whole  of  the 
species  still  live  in  Scottish  seas,  though  a  few  are  eitremely  rare.  Some  of  the  more 
charocteristJc  northern  shells  in  these  deposits  are  PtOea  Ulandkui,  TtUina  lata{T. 
calcarea),  Leda  iTWKOta,  L.  lanceclata,  Toldia  arctica,  Stedfova  rugom,  Panopma  nor- 
vegiea,  Trephon  tcalariforna*  {T.  clatltrattu),  and  Ifatiea  daiaa  (Fig.  46B). 


STRATIQRAPHICAL  GEO. 


Of  the  later  stages  of  th«  GlaoikL  Period,  the  recon 
Britain,  allowance  being  mode  for  the  gnata  cold  and 
in  the  oorth  thau  in  the  south,  and  among  the  hills  thai 

In  Scotland  the  following  may  be  taken  as  the  aven 
mena  in  descending  order  : — 

Last  traces  of  glaciers,  small  monincs  at  the  toot  o 
mauntaia  groups.  The  glaciers,  do  donbt,  lingered 
mountains  of  the  nortb'West  (Highlands,  Gollowa! 
Loch  Skene,  Anan,  Hull,  Skye,  Harris,  Orkney,  Bhi 

Marine  terraces  [50  feet  and  higher).  Clay-beils  at  1 
Beds]  containing  northern  moUusks.  The  roarioe  t 
of  at  least  100  feet  beneath  the  present  level  of  t 
that  limit  the  submergence  reached  has  still  to  be  d( 

Large  moraines,  showing  that  glaciers  descended  to  the 
in  the  north-west  ot  Scotland.      Some  of  the  mor 

Erratic  blocks,  chiefly  transported  by  the  first  ice-sheet 
glaciers,  and  partly  by  floating  ice  during  the  period 

Sands  and  gravels^Kame  or  Eskat  series,  sonietiiaea 
isms,  sometimes  marine  shells. 

Upper  boulder-clay — rudely  stratified  clays  with  sandi 

Till  or  lower  boulder-clny  (bottom  moraine  of  the 
stratilied  clay,  varying  up  to  100  tset  or  more  in  thii 
Snely  laminated  clays,  layers  of  peat  and  terrattri 
mammoth  and  reindeer,  also  in  some  places  fragn 

clay.     Till  spreads  over  the  lower  grounds,  often 
ridges  or  dm  [as. 
Ice-worn  rock  surfaces. 


J 


.   Moraines  (North  Wales,  Lake  District,  tc.)  and  rai 

.  Upper  houldcr-eUy — a  stiff  atony  clay  or  luam  wit 
eolations  of  sand,  gravel,  or  silt.  It  occasionally 
iwssibly  does  not  come  sonth  ot  the  Wash. 

.  Middle  sands  and  gravels,  containing  marine  sliells. 
above  the  sea)  there  have  been  fonnd  Csthtrm 
Arcn  laiilea,  Te/lijia  boithica,  Cypriaa  i^andiea, 
shells  now  living  in  the  seas  around  Britain,  bu 
grouping  a  rather  colder  climate  than  the  pr 
abounds  in  some  gravels  which  underlie  the  upper 
Wash  it  it  found  in  similar  deposits  overlying  th< 
clay."  In  Ireland  marine  sbelL<  oF  living  Britis 
1300  feet  above  the  sea.  But,  for  the  reason  a 
gence  may  not  have  been  nearly  so  great  as  the 
might  lie  supposiil  to  indicate. 

.  Lower  boulder-clay — a  stilT  clayey  deposit  stuck  ] 
equivalent  to  tlie  till  of  Scotland.  On  the  east  c< 
Ijncoln  and  Norfolk)  it  contains  fragments  of  S 
ticular,  gneiss,  mica-schist,  quartzite,  granite,  nye 
pieces  of  red  and  lilack  flint,  probably  from  Den 
timvHtone  and  sandstone,  whicli  have  doubtles) 
Along  the  Norfolk  cliffs  it  presents  stratified 
sand,  wbir.h  have  been  extraordinarily  contorted, 
lower  boulder-clay  in  the  north  of  England  and 
parallel  ridges  or  drums  iu  the  prevalent  line  o 
mentioned,  the  "crag"  of  Bridlington,  Yorkshir 
su  old  marine  glacial  shell-bearing  clay,  torn  up  a 
clay  of  the  first  ice-aheel-     Its  shelb  are  strikii^l 


SECT,  i  §  2  PLEISTOCENE  OR  GLACIAL  SERIES  1046 


moraine  has  been  observed,  the  ground  to  the  south  of  the  ice-limit  being  free 
from  glaciation,  though  erratic  blocks,  probably  brought  by  drift-ice,  are  found 
on  the  Sussex  coast  The  Coombe-rock  has  been  already  referred  to  (p.  1042). 
Deep  superficial  accumulations  of  rotted  rock  occur  where  the  rock  has  decom- 
posed in  situ  in  the  southern  non -glaciated  region,  as  may  be  well  seen  over 
the  Palaeozoic  slates  and  granites  of  Devon  and  Cornwall.  In  the  non- 
glaciated  chalk  districts,  a  thick  cover  of  flints  and  red  earth  partly  represents 
the  insoluble  parts  of  the  chalk  that  remain  after  prolonged  subaerial  decay, 
but  from  the  frequent  presence  of  fragments  of  quartz,  which  does  not  occur 
in  the  chalk,  this  mantle  of  "clay  with  flints"  seems  to  indicate  also  a 
certain  amount  of  transport,  though  the  agent  by  which  this  was  effdcted  is 
not  obvious.  The  high  moorlands  of  eastern  Yorkshire  appear  to  have  risen 
as  an  insular  tract  above  the  ice-sheet ;  for  the  boulder-clay  advances  up  the 
valleys  that  indent  the  northern  face  of  the  Jurassic  table-land,  but  ceases 
about  a  height  of  800  feet,  and  the  table-land  itself  is  entirely  free  of  drift, 
but  its  rocks  are  much  decayed  at  the  surface. 

Scandinavia.^ — The  order  of  Pleistocene  phenomena  is  generally  the  same  here  as  in 
Britain.  The  snrface  of  the  country  has  been  everywhere  intensely  glaciated,  and,  as 
already  stated,  the  ice-strise  and  transported  stones  show  that  the  great  ice-sheet  prob- 
ably exceeded  3000  feet  in  thickness,  for  the  hills  are  ice-worn  for  more  than  5000  feet 
above  sea-level,  and  that  moving  outwards  from  the  axis  of  the  peninsula  it  passed  down 
the  western  Qords  into  the  Atlantic,  and  southwards  and  south-eastwards  into  the 
Baltic.  The  subsequent  partial  submergence  of  the  country  is  proved  by  numerous 
shell-bearing  clays.  The  fossils  in  the  higher  littoral  shell-beds  indicate  a  more  Arctic 
climate ;  they  include,  as  in  the  Scottish  glacial  clays,  great  ntimbers  of  thick-shelled 
varieties  of  Mya  truncata  and  Scueieava  rugosa;  also  Balanus  porceUvs,  B.  creTuUuSt 
Mytilus  ediilts,  Pecten  islandimUf  Buccinum  grcenlandicum,  Trophoji  sealari/ormis, 
{T.  clathrcUus),  NcUiea  clausa.  The  clays  of  deeper  water  contain  Leda  lanceolaUiy 
Yoldia  arctica,  Y,  intermedia,  Y.  pygmsuiy  Dentalium  ahyssorum,  &c.  The  fossiliferous 
deposits  of  lower  levels  point  to  a  climate  more  nearly  approaching  the  present,  for  the 
more  thoroughly  Arctic  species  disappear,  and  the  thick-shelled  varieties  of  Mya  and 
Saxicava  pass  into  the  usual  thin-shelled  kinds.  The  remarkable  terraces  that  fringe 
the  coast  of  Norway  from  the  southern  or  Christiania  region  to  the  North  Cape  mark 
pauses  in  the  re-elevation  of  the  land  (Fig.  78).  The  eastern  plains  of  Sweden  and  the 
lower  grounds  of  southern  Norway  are  covered  with  great  accumulations  of  sand  and 
gravel  (osar)  like  the  kames  of  Scotland  and  the  eskers  of  Ireland. 

Germany.^ — Since  the  year  1878  an  active  exploration  of  the  earlier  memorials  of 
the  glacial  period  has  been  carried  on  in  northern  Germany,  with  the  result  of  bringing 
out  more  clearly  the  evidence  for  the  prolongation  of  the  Scandinavian  and  Finland  ice 
across  the  Baltic  and  the  plains  of  Germany  even  into  Saxony.  The  limits  reached  by 
the  ice  are  approximately  fixed  by  the  line  to  which  northern  erratics  can  be  traced. 
Beneath  the  oldest  members  of  the  glacial  drifts,  deposits  are  found  in  a  fragmentary 
condition  containing  shells  now  living  only  in  southern  Europe,  such  as  Paludina 
dilumana  and  Corbicula  fluminalis.  Above  the  glaciated  rocks  comes  a  stiff, 
unstratified  clay,  with  ice -striated  blocks  of  northern  origin  —  the'  till  or  boulder- 
clay  (Geschiebelehra,   Blocklehm).     Two    distinct  boulder  -  clays  have  now  been  re- 

^  See  G.  de  Geer,  Zeitsch,  Deutsch,  Oeol,  Qts.  xxxvii.  (1885)  p.  177. 

^  There  is  now  an  ample  though  recent  literature  devoted  to  the  glacial  phenomena  of 
Germany.  The  volumes  of  the  ZeUsch,  Deutsch.  Geol.  Gesellscha/t  for  1879  and  subsequent 
years  contain  papers  by  G.  Berendt,  H.  Credner,  A.  Helland,  A.  Penck,  R.  Richter,  F. 
Noetling,  F.  Wahnschafle,  F.  E.  Geinitz,  F.  Schmidt,  &c.  See  also  the  Jahrb.  Preuss. 
Oeol.  Landesanstalt  for  1880  and  following  years  ;  the  Maps  and  Explanations  of  the  same 
Survey  for  the  neighbourhood  of  Berlin  (27  sheets)  and  the  memoirs  of  the  Geological  Survey 
of  Saxony. 


1046  STRATIGRAPHICAL  GEOLOGY  bookvipabty 


cognised — the  older  or  till  separated  by  interglacial  deposits  from  the  newer.  Terminal 
moraines  marking  the  limits  of  the  ice-sheet  have  been  found  in  the  form  of  Tamparti 
of  Scandinavian  blocks  and  gravel,  which  have  been  traced  for  many  miles  along  the 
coast-line  and  across  the  plains  of  northern  Germany.^  The  sources  of  the  varioos 
ice -streams  which  united  to  form  the  great  ice-sheet  that  crept  over  the  Germanic  plain 
are  well  shown  by  a  study  of  the  stones  in  the  moraine  material.  The  Scandiuaviaii 
rocks  are  found  towards  the  west  and  the  Finnish  towards  the  east  of  the  glaciated  area. 
Among  the  intercalated  materials  that  separate  the  two  boulder-clays  are  layers  of  peat, 
with  remains  of  pine,  fir,  aspen,  willow,  white  birch,  hazel,  hornbeam,  poplar,  holly,  oak, 
juniper,  ilex,  and  various  water-plants,  in  particular  a  water-lily  no  longer  living  in 
Europe.  With  this  vegetation  are  associated  remains  of  Elephas  anti^vs,  mammoth, 
rhinoceros,  elk,  megaceros,  reindeer,  musk-ox,  bison,  bear,  kc.  Some  of  the  interglacial 
deposits  are  of  marine  origin  on  the  lower  grounds  bordering  the  Baltic,  for  they  contain 
Cyprina  islandicoy  Yoldia  arctica,  Tellina  aoHdyZa,  kc  Among  the  youngest  glacial,  and 
probably  in  part  interglacial,  deposits  are  the  upper  sands  and  gravels  (Geschiebedeck- 
sand),  which  spread  over  wide  areas  of  the  Germanic  plain,  partly  as  a  more  or  less 
uniform  but  discontinuous  sheet,  and  partly  as  irregular  hillocks  and  ridges  strewn  with 
erratic  blocks,  and  enclosing  pools  of  water  and  peat-bogs.  These  mounds  and  ridges, 
with  their  accompanying  sheets  of  water,  form  a  conspicuous  feature  of  the  low  tract  of 
country  from  Schleswig  Holstein  eastwards  to  the  Vistula. 

In  some  of  the  mountain  groups  of  Germany  there  is  evidence  that  probably  at  the 
height  of  the  Ice  Age  glaciers  existed.  Reference  has  already  been  made  to  the  moraine 
mounds  of  the  Yosges^  and  Black  Forest,'  and  to  the  fact  that  the  glaciers  of  the 
western  hill -groups  were  more  extensive  than  those  to  the  east.  In  the  Carpathian 
range,  a  series  of  moraines,  sometimes  enclosing  lakes,  is  distributed  in  the  valleys  that 
radiate  from  the  Hohe  Tatra.^  On  both  sides  of  the  Riesengebirge,  moraines  occur.  At 
the  sources  of  the  Lomuitz,  on  the  southern  side,  they  enclose  two  lakes  at  the  foot  of 
high  recesses  and  cliffs.^  No  certain  traces  of  glaciers  appear  to  have  been  met  with  in 
the  eastern  part  of  the  Sudeten  range,  nor  in  the  Erzgebirge  or  Thuringerwald.  Farther 
north,  in  the  Harz,  mounds  of  detritus  which  resemble  moraines  have  been  referred  by 
Kayser  to  glacier-action.* 

France. — As  France  lay  to  the  south  of  the  northern  ice-sheet,  the  true  till  or 
boulder-clay  is  there  absent,  as  it  is  for  the  same  reason  from  the  south  of  England.  It 
is  consequently  difficult  to  decide  which  superficial  accumidations  arc  really  contem- 
porary with  those  termed  glacial  farther  north,  and  which  ought  to  be  grouped  as  of 
later  date.  The  ordinary  sedimentation  in  the  non -glaciated  area  not  having  been 
interrupted  by  the  invasion  of  the  ice-sheet,  deposits  of  pre-glacial,  glacial,  and  post-glacial 
time  naturally  pass  insensibly  into  each  other.  The  older  Pleistocene  deposits  (perhaps 
interglacial)  consist  of  fluviatile  gravels  and  clays  which,  in  their  composition,  belong 
to  the  drainage  systems  in  which  they  occur.  There  is  generally  no  evidence  of 
transport  from  a  great  distance,  though,  in  the  Champ  de  Mars  at  Paris,  blocks  of  sand- 
stone and  conglomerate  nearly  a  yard  long  sometimes  occur,  as  well  as  small  pieces 
of  the  granulite  of  the  Morvan.  Erratics  at  Calais  and  on  the  coast  of  Brittany  may 
also  have  been  carried  a  long  way.^    The  rivers,  however,  were  probably  much  larger 


^  G.  Berendt,  Jahrb,  Prexiss.  Oeol.  Landemnst,  1888,  p.  110  ;  K.  Keilhack,  op,  eU,  1889, 
p.  149. 

^  H.  Hogard,  *  Terrain  erratique  des  Vosges,*  1851. 

3  J.  Partsch,  *Gletscher  der  Vorzeit,'  1882,  p.  115. 

*  Ibid.  p.  9. 

^  Ibid.  p.  55. 

«  Lessen  and  Kayser,  Zeitsch.  Deutsch.  Qeol.  Ots,  xxxiii.  (1881). 

^  Ch.  Velain,  BxiU.  Soc.  OM,  France,  xiv.  (1886)  p.  669. 


8ECT.i§2  PLEISTOCENE  OR  GLACIAL  SERIES  1047 

during  some  part  of  the  Pleistocene  period  than  they  now  are,  and  the  transport  of  their 
stones  may  have  been  sometimes  effected  by  floating  ice.  They  have  left  their  ancient 
platforms  of  alluvium  in  successive  terraces  high  above  the  present  watercourses.  Each 
terrace  consists  generally  of  the  following  succession  of  deposits  in  ascending  order : 

(1)  A  lower  gravel  {gravier  tie  fond),  the  pebbles  of  which  are  coarsest  towards  the 
bottom  and  are  interstratified  with  layers  of  sand,  sometimes  inclined  and  contorted. 

(2)  Grey  sandy  loam  {sable  ffras).  (3)  The  foregoing  strata  are  covered  by  yellow  cal- 
careous loess  (p.  332),  or  with  an  overlying  dark  brown  loam  or  brick-earth.  The  upper 
exposed  parts  of  the  gravels  and  sands  are  commonly  well  oxidised,  and  present  a 
yellowish-brown  or  deep  reddish-brown  tint,  while  the  lower  portions  remain  more  or 
less  grey.  Hence  the  old  names  diluvium  gris  and  diluvium  rouge.  The  gravels  and 
brick-earths  have  yielded  terrestrial  and  fresh-water  shells,  most  of  which  are  of  still 
living  species,  and  numerous  mammalian  bones,  among  which  are  Rhinoceroa  aniiqui- 
ttUis  {tichorhintts),  R.  etruacus,  R.  leptorhinus,  Hippopotamus  amphihius,  Elephas  anti- 
quuSy  E.  primigeniuSf  wild  boar,  stag,  roe,  ibex,  Canadian  elk,  musk-sheep,  urus,  beaver, 
cave-bear,  wolf,  fox,  cave-hysena,  and  cave-lion.  Palteolithic  implements  found  in  the 
same  deposits  show  that  man  was  a  contemporary  of  these  animals  (see  p.  1057).^ 

It  is  in  the  centre  and  east  of  France  that  the  most  iinequivocal  signs  of  the  ice  of 
the  Glacial  Period  are  to  be  met  with.  The  mountain  groups  of  Auvergne,  which  even 
now  show  deep  rifts  of  snow  in  summer,  had  their  glaciers  whereby  moraine  heaps  and 
large  blocks  of  rock  were  strewn  over  the  valleys  ;  not  only  so,  but  there  is  evidence  in 
that  region  of  a  retreat  and  redescent  of  the  ice,  for  above  the  older  moraines  lie  inter- 
glacial  deposits  containing  abundant  remains  of  land -plants  with  bones  of  Elephas 
ineridionalis.  Rhinoceros  leptorhinus,  &c.,  the  whole  being  covered  by  newer  moraines.* 

The  much  lower  grounds  of  the  Lyonnais  and  Beaujolais  (rising  to  more  than  3000 
feet)  likewise  supported  independent  snow-fields.'  The  glacier  of  the  Rhone  and  its 
tributaries  at  the  time  of  the  maximum  glaciation  was  so  gigantic  as  to  fill  up  the 
hollow  of  the  Lake  of  Geneva  and  the  vast  plain  between  the  Bernese  Oberland  and  the 
Jura.  It  crossed  the  Jura  and  advanced  to  near  Be8an9on.  It  swept  down  the  valley 
below  Geneva,  and  then,  joined  by  its  tributaries,  spread  out  over  the  lower  hills  and 
plains  until  the  whole  region  from  Bourg  to  Grenoble  was  buried  under  ice.  The 
e\idence  of  this  great  extension  is  furnished  by  rock-striee,  transported  blocks  and 
moraine  stuff.  ^ 

Belginm. — The  Quaternary  deposits  of  this  country,  like  those  of  northern  France, 
belong  to  a  former  condition  of  the  present  river-basins.  In  the  higher  tracts,  they  are 
confined  to  the  valleys,  but  over  the  plains  they  spread  as  more  or  less  continuous 
sheets.  Thus,  in  the  valley  of  the  Meuse,  the  gravel-terraces  of  older  diluvium  on 
either  side  bear  witness  only  to  transport  within  the  drainage-basin  of  the  river,  though 
fragments  of  the  rocks  of  the  far  Vosges  may  be  detected  in  them.  The  gravels  are 
stratified,  and  are  generally  accompanied  by  an  upper  sandy  clay.  In  middle  Belgium, 
the  lower  diluvial  gravels  are  covered  by  a  yellow  loam  (Hesbayan),  probably  a  con- 
tinuation of  the  German  loess,  with  numerous  terrestrial  shells  {Succinea  oblonga.  Pupa 
muscorum.  Helix  hispida).     In  lower  Belgium,  this  loam  is  replaced  by  the  Campinian 

^  A  detailed  study  of  the  Quaternary  deposits  of  the  north  of  France  has  been  made  by 
J.  Ladri^re,  who  divides  them  into  three  stages,  each  marked  off  by  a  gravelly  layer  at  the 
base  and  terminating  above  in  a  loam  with  terrestrial  vegetation  and  fresh-water  and  terres- 
trial shells.  The  lowest  is  the  assise  with  Elephas  primigenius  and  Rhinoceros  tichorhinus, 
Ann.  Sac.  OSol.  Nord,  xviii.  (1890),  p.  98. 

*  Julien,   *  Des  Ph^nom^nes   glaciaires  dans  le   Plateau   central  de  la  France,'  1869. 
Rames,  Bull.  Soc.  OSol.  France,  1884. 

^  Falsan  and  Chantre,  *Anciens  Glaciers,'  ii.  p.  384. 

*  Falsan  and  Chantre,  op.  cit. 


1048  STRATIGRAPHICAL  GEOLOGY  book  vi party 

sands,  which  havo  been  observed  lying  upon  it.  The  Belgian  caverns  and  some  parts 
of  the  diluvium  have  yielded  a  large  number  of  mammalian  remains,  among  which 
there  is  the  same  commingling  of  types  from  cold  and  from  warm  latitudes  w 
observable  in  the  Pleistocene  beds  of  England  and  France.  Thus  the  Arctic  reindeer 
and  glutton  are  found  with  the  Alpine  chamois  and  marmot,  and  with  the  lion  and 
grizzly  bear. 

The  Alps.^ — Reference  has  already  been  made  to  the  vast  extension  of  the  Alpine 
glaciers  during  the  Ice  Age.  Evidence  of  this  extension  is  to  be  seen  both  among  the 
mountains  and  far  out  into  the  surrounding  regions.  On  the  sides  of  the  great  valleja, 
ice-striated  surfaces  and  transported  blocks  are  found  at  such  heights  as  to  show  that 
the  ico  must  have  been  in  some  places  3000  or  4000  feet  thicker  than  it  now  is.  The 
glacier  of  the  Aar,  for  instance,  which  was  a  comparatively  short  one,  being  tamed  aside 
by  and  merging  into  the  large  stream  of  the  Rhone  glacier  near  Berne,  attained .  such 
dimensions  as  not  only  to  fill  up  the  valley  now  occupied  by  the  Lakes  of  Than  sad 
Bricnz,  but  to  override  the  surrounding  hills.  The  marks  made  by  it  are  found  at  a 
height  of  930  metres  above  the  valley,  which  with  305  metres  for  the  depth  of  Lake 
Brienz  gives  a  depth  of  at  least  1235  metres  or  4000  feet  of  ice  moving  down  that 
valley.  Judging  from  the  evidence  of  the  heights  of  the  stranded  blocks,  the  slope 
of  this  glacier  varied  from  45  in  1000  in  its  upper  parts  to  not  more  than  2  in  1000 
towards  its  termination.'-'  From  the  variation  in  the  direction  of  the  striae,  as  well 
as  in  the  distribution  of  the  transported  blocks,  there  can  bo  little  doubt  that  the 
Alpine  glaciers  varied  from  time  to  time  in  relative  dimensions,  so  that  there  was 'a 
kind  of  struggle  between  them,  one  pushing  aside  another,  and  again  being  pushed 
aside  in  its  turn. 

Turning  to  the  regions  beyond  the  mountains,  we  find  that  proofs  of  glaciation  reach 
to  almost  incredible  distances.  The  Rhone  glacier  has  already  been  referred  to  as  over- 
whelming the  mountainous  and  hilly  intervening  country,  and  throwing  down  its  moraines 
with  blocks  of  the  characteristic  rocks  of  the  Yalais  where  Lyons  now  stands,  that  is, 
170  miles  in  direct  distance  from  where  the  present  glacier  ends.  The  same  ice-sheet, 
swelled  from  the  northern  side  of  the  Bernese  Oberland,  overflowed  the  lower  ridges  of 
the  Jura,  streaming  through  the  transverse  valleys,  even  as  far  as  Omans  near  Besan9on. 
Turning  north-eastward,  it  filled  up  the  great  valley  of  Switzerland,  and,  swollen  by  the 
tributary  glaciers  of  the  Aar,  the  Reuss,  and  the  Linth,  joined  the  vast  stream  of  the 
Rhine  glacier  above  Basle.  This  enormous  mer  de  glace  poured  over  the  Black  Forest 
and  down  the  valley  of  the  Danube  at  least  as  far  as  Sigmaringcn,  where  blocks  of  the 
rocks  of  the  Grisons  occur.  Eastward  it  was  joined  by  the  great  glacier  that  descended 
from  the  Swabian  and  Bavarian  Alps,  and  of  which  the  moraine-heaps  are  strewn  over 
the  lowlands  as  far  as  Munich.  The  Tyrolese  and  Carinthian  Alps  were  likewise  buried 
under  an  icy  covering  which  sent  a  huge  glacier  eastwards  down  the  valley  of  the  Drau. 
On  the  south  side  of  the  Alps,  the  glaciers  advanced  for  some  way  out  into  the  plains  of 
Lombardy,  where  they  threw  down  enormous  moraines,  which  sometimes  reach  a  height 
of  more  than  2000  feet  (Ivrea).  These  vast  accumulations,  to  which  there  is  no  parallel 
elsewhere  in  Europe,  rise  into  conspicuous  hills  and  crescent-shaped  ridges  round  the 
lower  ends  of  the  upper  Italian  lakes.  At  some  of  these  localities  the  moraine  stuff 
rests  on  marine  Pliocene  beds.     It  is  possible  that  the  glaciers  actually  reached  the  sea- 

^  Besides  the  works  of  Falsan  and  Chantre,  Peuck  and  Partsch,  cited  on  p.  1024,  the 
student  may  consult  Morlot,  Bib.  Univ.  1855  ;  Bull.  Soc  Vaud.  Sci,  Nat.  1858,  1860  ; 
Heer,  '  Urwelt  der  Schweiz '  ;  the  map  of  the  ancient  glaciers  of  the  north  side  of  the  Swiss 
Alps,  published  in  four  sheets  by  A.  Favre,  Geneva,  1884  ;  C.  W.  Giimbel,  Sitab,  Akad, 
U'lVw,  1872  ;  R.  Lepsius,  'Das  westliclie  Sud-Tirol,'  Berlin,  1878  ;  A.  Heim,  *Handbuch 
der  (Jletscherkunde,'  1885  ;  Baltzer,  Mittheil  Natvrf.  Ges.  Berne,  1887  ;  Reuevier,  Bull. 
Sin\  Hdc.  1887  ;  A.  Bohm,  Jahrh.  k.  k.  Ged.  Reichmnst.  xxxv.  (1885)  p.  429. 

2  A.  Favre,  Arch.  Ann.  Sci.  Phys.  Nat.  Oen^e,  xii.  1884. 


SECT,  i  §  2  PLEISTOCENE  OR  GLACIAL  SERIES  1049 

level. ^  There  appears  to  bo  no  doubt,  at  least,  that  they  descended  to  a  lower  level  on 
that  side  than  on  the  northern  side  of  the  Alps. 

By  tracing  the  distribution  of  the  transported  blocks,  the  movements  of  the  ancient 
glaciers  can  be  satisfactorily  followed.  These  blocks  are  not  dispersed  at  random  over 
the  glaciated  area.  Each  glacier  carried  the  blocks  of  its  own  basin,  and,  where  these 
are  of  a  peculiar  kind,  they  serve  as  an  excellent  guide  in  following  the  march  of  the 
ice.  Not  only  were  the  blocks  in  each  drainage  area  kept  separate  from  those  of  ad- 
joining basins,  but  those  on  the  left  sides  of  the  valleys  do  not,  except  along  the 
junction  lines,  mingle  with  those  of  the  right  sides.  As  a  rule,  the  blocks  lie  along  the 
slopes  of  the  valleys  rather  than  on  the  bottoms,  and  are  often  disposed  there  in  groups 
or  lines.  In  the  Arve  valley,  near  Sallanches,  for  example,  a  zone  comprising  several 
thousand  granitic  boulders  runs  for  a  distance  of  more  than  three  miles.  The  blocks 
of  Mouthey  have  long  been  famous.  On  the  flanks  of  the  Jura  near  Solothuni,  the 
boulders  of  Kiedholz,  stranded  there  by  the  ancient  Rhone  glacier,  still  number  228, 
though  they  have  been  reduceil  by  the  quarrying  operations  now  happily  interdicted 
(see  Figs.  151,  162,  153).* 

That  the  Ice  Age  in  the  Alps,  as  in  northern  Europe,  was  interrupted  by  at  least  one 
warmer  interglacial  period,  when  the  ice  retreating  from  the  valleys  allowed  an  abundant 
vegetation  to  flourish  there,  is  shown  by  the  lignites  of  Diimten  (Canton  Zurich), 
Utznach  (St.  Gall),  Hotting  (near  Innspruck),  and  several  other  places.  These  deposits 
can  here  and  there  be  seen  to  overlie  ancient  moraine  stuff ;  they  are  interstratified  with 
fluviatile  gravels  and  sands,  which  again  are  surmounted  with  scattered  erratic  blocks 
belonging  to  a  later  period  of  glaciation.  Among  these  interglacial  vegetable  accumu- 
lations Heer  recognised  several  pines  or  firs  {Pintis  abieSy  P.  gyltfestris,  P.  monlana), 
larch,  yew,  oak,  sycamore,  hazel,  mosses,  bog- bean,  bulrush,  raspberry,  and  Qalium 
palustre,  as  well  as  bog-mosses,  all  still  growing  in  the  surrounding  country.  With 
the  plants  there  occur  the  remains  of  Elephaa^  Rhinoceros  etruscus.  Bos  taurus,  var. 
primigenius  or  nrus,  red-deer,  cave-bear,  likewise  traces  of  fresh  •  water  shells  and 
insects,  chiefly  elytra  of  beetles. 

The  succession  of  main  events  in  the  history  of  the  Ice  Age  in  Switzerland  is  thus 
tabulated  :  ' — 

Post-glacial.     Ancient  lacustrine  terraces  (150  feet  above  present  level  of  Lake  of 

Geneva),  deltas,  and  river  gravels  with  Limnsea  stagruUis^  and  other  fresh-water 

shells,  bones  of  mammoth  (?). 
Second  extension  of  the  glaciers.     Erratic  blocks  and  terminal  moraines  of  Zurich, 

Baldegg,  Sempach,  Berne,  with  an  Arctic  flora  and  fauna. 
Interglacial  beds.     Gravels,  lignites,  and  clays  of  Utznach,  Diimten,  &c.,  covered 

by  the  moraine  stuff  of  the  second  glaciation  and  overlying  the  oldest  glacial 

deposits — Elephas  antiquu^^  Rhinoceros  leptorhinvs. 
First  glaciation.     Striated  blocks  found  under  the  interglacial  beds. 

Buuia. — A  vast  extent  of  Russia  was  buried  under  the  first  great  ice-sheet,  the 
southward  limits  of  which  across  the  country  have  already  been  stated  (p.  1027).  There 
appears  to  be  evidence  that  the  second  advance  of  the  ice  not  only  affected  the  western 
lowlands  that  were  covered  by  the  Baltic  glacier,  but  even  the  centre  of  the  country. 
Recently  proofs  have  been  obtained  of  an  interglacial  period  in  central  Russia  marked  by 
lacustrine  deposits  intercalated  between  glacial  clays.     They  have  yielded  an  abundant 

^  The  surface  of  the  Lago  di  Garda,  round  the  lower  end  of  which  glacier  moraines 
extend,  is  little  more  than  200  feet  above  the  sea-level. 

«  Favre,  Arch,  Sci,  Phys.  Nat,  Oenive,  xii.  (1884)  p.  399.  Penck  (*  Veigletscherung 
der  Dent8chen  Alpen ')  believes  that  he  can  trace  evidence  of  at  least  three  distinct  periods 
of  glaciation  in  the  Alps. 

'  Heer,  *  Urwelt  der  Schweiz. ' 


1060  STRATiaSAPEICAL  GEOLOGY  book  ti  fu 

flora,  inclndini;  sliUr,  birch,  hazel,  villotr,  fir,  wator-lUiea,  ukd  remaiiu  of  munin 
ftc' 

North  Amariok.'— The  general  BncwHsion  or  geolof^ul   change   in    Poat-Terl 

timt  appears  to  havo  been  broadly  th«  aame  oSl  over  the  Dorthem  ItetnUphBTs. 
North  AtDericD,  aa  in  Enrojie,  there  is  a  glaciated  and  non-glacUted  area  ;  but 
line  of  deniBTcation  between  them  haa  been  much  more  clearly  traced  on  the  wal 
aide  of  the  Atlantic.  The  glaciated  area  extending  over  Canada  and  the  sortb-Mi 
8tBt«B  preMQta  the  same  characteristic  features  as  in  the  Old  World.  The  rocki,  « 
thej  conld  receive  and  retain  the  ice -markings,  are  well  smoothed  and  striated. 
direction  o(  the  atrin  is  gensrally  louthiranl,  varying  to  soath^eaat  and  aoath- 
■ccording  to  the  form  of  tli*  gronnd.  The  great  thickneae  of  the  ice-aheet  ia  atriki 
shown  by  the  height  to  wliich  some  of  the  higher  elevations  are  polished  and  stri) 
Thus  the  Catskill  Hountaina,  rising  from  the  broad  plain  of  the  Hndaon,  have 
ground  smootli  and  striated  up  to  near  their  semmite,  or  about  3000  feet,  m  that 
ice  must  have  been  ofeven  greater  thickneaa  than  that  The  White  Honntadna  an 
iTorii  even  at  a  height  of  SBOO  feet  0.  M.  Dawson  has  found  glaciated  surbc* 
British  CoUimbia  7000  feet  above  the  sea.* 

Aa  in  £uro[ic,  the  glacial  deposits  increaao  in  thickneaa  and  variety  from  aont 
north,  spreading  across  Canada,  over  a  considerable  area  of  the  norUi-emstem  Si 
and  rising  to  a  height  of  5600  feet  among  the  White  Mountains.  From  the  eviden 
the  rock-etrite  and  the  dispersion  of  boulders,  it  appears  that,  though  the  glad 
region  was  buried  under  one  deep  continuous  tner  de  glaet  like  that  of  GAenland  at 
present  time,  moving  steadily  down  from  the  north,  there  were  conaiderabie  varial 
in  the  liirecUon  of  motion,  mainly,  no  doubt,  owing  to  inequalities  in  the  gei 
slope  of  the  ground  uudomeath.  Nothing,  however,  is  more  atriking  than 
apparent  indifference  with  which  the  ice  streamed  onward,  nndeflected  evon  by  OM 
erable  ridges  and  hilU,  The  line  of  the  southern  margin  of  the  ice  can  ttill  be  foUf 
by  tracing  the  limits  to  which  the  drift  deposits  extend  southwards.  From  this  evid 
we  learn  that  the  ice-aheet  ended  off  in  a  sinuous  line,  protruding  in  great  tongue 
promontories  and  retiring  into  deep  and  wide  bays.  In  the  eastern  states,  the  sout 
limit  of  the  glacialeil  region  is  marked  by  one  of  the  most  extraordinary  glacial  acci 
lations  yet  known,  aurl  to  which  in  F,uro[>e  there  ia  no  rival.  It  consists  of  a  b 
irregular  band  of  conFused  heaps  of  drift,  or  more  strictly  of  two  such  banda,  irhich  ai 
times  unite  into  one  broad  belt  and  sometimes  separate  wide  enongh  to  allow  an  iab 
of  twenty  or  thirty  miles  between  them,  each  being  from  one  to  six  miles  in  breadth 
rising  Keveral  hundred  feet  above  the  surrounding  country.  The  surface  of  these  ti 
presents  a  characteristic  hummocky  sapect,  rising  into  eones,  domes,  and  conS 
ridgi's,  ami  sinking  into  basin-shaped  or  other  irregularly-formed  depressions,  like 
kames  or  osar  of  Europe.  The  upper  J>art  of  the  material  composing  -the  ri 
generally  ronaists  of  assorted  and  stratified  gravel  and  sand,  the  stratification  b 
irregular  and  discordant,  but  inclined  on  the  whole  towards  the  south.     Below  t 

1  N.  Krisi'htarowilsch,  Bull.  &'C.  Imp.  Ifat.  J/iwrou,  No.  4  (ISM).  On  the  gIscUti< 
the  Tlrah  see  Nikitjn,  Xeufi  Jn/irh.  1888,  i.  p.  172. 

'  See  J.  D.  Whitaey,  "Climatic  Changes  oflalrr  Geological  Times,"  JTmi.  JTiu.  0>a 
Zifii.  Hiirvard,  vol.  vii.  1882  ;  and  papers  by  J,  D.  Dans,  T.  C.  Cbamberlin,  B.  D.  E 
bury,  W.  Uphani,  George  M.  Dawson,  H.  Carvill  Lewis,  G.  F.  Wright,  and  others  in  A 
Joum.  Sel.,  American  GtclogUt,  Canadian  Naiiiraliit,  Canadian  Jovmal,  Ann.  Jlq 
o^r.-S:  (/to/.  Sanvy.  and  Canadian  Qml.  SHnfH,  Second  Oeol.  Airr.  o/ Pmnt^lta 
J.  W.  Dawaou,  'Acadian  Ceolr^y,'  187S  ;  '  Handbook  of  Canadian  Geology,'  1886  ;  Q 
Dawson.  Tfipx.  Kn/,.  s-K.  Canada,  viii.  sect,  iv.  (1890)  p.  25 ;  O.  F.  Wright, '  Btan  an^ 
Glacial  Period."  'The  Ice  Age  in  America.' 

'  Geol.  Mag.  1889,  p.  3S1  ;  see  also  W.  Uphsm,  Appalaehia,  v.  (1SB9)  pL  291. 


SECT,  i  §  2  PLEISTOCENE  OR  GLACIAL  SERIES  1051 

rearranged  materials  is  a  boulder-drift — a  mixtnre  of  clay,  sand,  and  gravel,  with  boolders 
of  all  sizes,  up  to  blocks  many  tons  in  weight  and  often  striated.  Though  some- 
times indistinguishable  from  ordinary  till,  it  presents  as  a  rule  a  greater  preponderance 
of  stones  than  in  typical  till,  but  contains  also  fine  stratified  intercalations.  A  large 
proportion  of  the  material  of  the  ridges  has  been  derived  from  rocks  lying  immediately 
to  the  north,  and  the  nature  of  the  ingredients  constantly  varies  with  the  changing 
geological  structure  of  the  ground.  There  is  also  always  present  a  greater  or  less 
amount  of  detritus  representing  rocks  that  lie  along  the  line  of  drift-movement  for  500 
miles  or  more  to  the  north.  The  band  of  drift-hills  lies  sometimes  on  an  ascending, 
sometimes  on  a  descending  slope,  crosses  narrow  mountain  ridges  and  forms  embank- 
ments across  valleys,  showing  such  a  disregard  of  the  topography  as  to  prove  that  it 
cannot  have  been  a  shore-line,  and  has  not  been  laid  down  with  reference  to  the  present 
drainage  system  of  the  land.^ 

To  this  remarkable  belt  of  prominent  hummocky  ground  the  name  of  ''terminal 
moraine  "  has  been  given  by  the  American  geologists  who  have  so  successfully  traced 
its  distribution  and  investigated  its  structure.  The  conditions,  however,  under  which 
the  drift  rampart  in  question  was  formed  certainly  differed  widely  from  those  that 
determine  an  ordinary  terminal  moraine.  The  constituent  materials  can  hardly  have 
travelled  on  the  surface  of  the  ice,  but  must  rather  have  lain  underneath  it  or  have  been 
pushed  forward  in  front  of  it.  But  the  mode  of  formation  is  a  problem  which  has 
not  yet  been  satisfactorily  solved. 

There  seems  good  reason  to  believe  that  there  are  at  least  two  "  terminal  moraines  " 
belonging  to  two  distinct  and  perhaps  widely  separated  epochs  in  the  Ice  Age.  The 
most  southerly  and  therefore  oldest  of  them  begins  on  the  Atlantic  border  off  the  south- 
eastern coast  of  Massachusetts,  where  it  is  partially  submerged.  Rising  above  the  level 
of  the  sea  in  Nantucket  Island,  Martha's  Vineyard,  No  Man's  Island  and  Block  Island, 
it  is  prolonged  into  Long  Island,  of  which  it  forms  the  back -bone,  and  where  it  reaches 
heights  of  200  to  nearly  400  feet.  A  second  or  later  and  less  prominent  line  of  drift-hills 
runs  along  the  north  shore  of  Long  Island,  and  is  prolonged  by  Fisher's  Island  into  the 
southern  edge  of  the  State  of  Rhode  Island,  whence,  striking  out  again  to  sea,  it  forms 
the  chain  of  the  Elizabeth  Islands,  passes  thence  into  the  State  of  Massachusetts,  and 
runs  nearly  east  and  west  through  the  peninsula  of  Cape  Cod.  The  distance  between 
these  two  bands  of  hummocky  ridge  varies  from  five  to  thirty  miles.  From  the 
western  end  of  Long  Island  the  moraine  passes  across  Staaten  Island  and  the  northern 
part  of  New  Jersey,  enters  Pennsylvania  a  little  north  of  Easton,  and  follows  a  sinuous 
north-westerly  course  across  that  State  and  for  some  miles  into  the  State  of  New  York, 
where,  forming  a  deep  indentation,  it  wheels  round  in  a  south-westerly  direction,  re- 
enters Pennsylvania,  and  passes  into  Ohio.  Throughout  this  long  line,  the  moraine 
coincides  with  the  southern  limit  of  the  drift  and  of  rock-striation,  though  in  western 
Pennsylvania,  in  front  of  the  ridge,  scattered  northern  boulders  are  found  over  a  strip  of 
ground  which  gradually  increases  south-westwards  to  a  breadth  of  five  miles.'  Beyond 
central  Ohio,  however,  the  drift  extends  far  to  the  south.  Taking  its  limits  as  probably 
marking  the  extreme  boundary  of  the  ice-sheet  (then  at  its  largest),  we  find  that  it  goes 
southwards,  perhaps  nearly  as  far  as  the  junction  of  the  Ohio  with  the  Mississippi, 
sweeping  westwards  into  Kansas,  and  then  probably  turning  northwards  through 
Nebraska  and  Dakota,  but  keeping  to  the  west  of  the  Missouri  River. 

The  inner  or  second  terminal  moraine  is  well  developed  in  the  southern  part  of  the 
State  of  New  York,  lying  well  to  the  north  of  the  first  moraine,  and  much  more  irregu- 

*  H.  C.  Lewis,  "Report  on  the  Terminal  Moraine,"  Second  Oed.  Surv.  Pennsylvania, 
Z,  1884,  p.  45,  with  Preface  by  J.  P.  Lesley. 

'  This  strip  of  ground,  called  by  the  late  Prof.  H.  C.  Lewis  the  "  fringe,"  widens  out 
south-westwards,  as  stated  abo^e,  to  a  breadth  of  five  miles,  in  which,  though  there  are  no 
rock-strise  or  drift,  scattered  northern  boulders  occur.     Op,  cU.  p.  201. 


1052  STRATIGRAPHIOAL  GEOLOGY  book  ti  pai 

larly  dtstributcd.  8outh-w«8twardg  the  tiro  arries  of  raDipkits  unite  at  the  aliBrp 
of  the  older  ridge  just  meDtioaed.  and  continue  as  one  into  the  centre  of  Ohio. 
janction  probably  indicates  that  the  eoutham  edfie  o(  the  ice  at  the  time  of  the  ae 
moraine,  though  generallj  keeping  to  the  north  of  its  previous  limit,  reachmi  its  fa 
extent  in  north-western  Pennsylvania,  and  united  ita  debris  with  that  left  at  the 
of  tlio  greatest  eitcnsion  of  the  ice-sheet.  From  the  middle  of  Ohio,  the  7011 
moraine  pursucB  an  eitraordinarily  sinuous  Course.  One  of  its  most  remarkabls  li 
encloses  the  southern  lislf  of  Lake  Michigan,  which  was  the  bed  of  a  K^eat  tong 
ice  moving  from  the  north.  Immediately  to  the  west  of  this  loop  there  Ilea  an  exlffi 
driftless  ares  in  Wisconsin  and  Minnesota.  The  course  of  the  moraine  beats  dis 
witness  to  the  independent  direction  of  flow  of  the  united  glaciers  that  constitntM 
great  ice-slieet.  It  sweeps  in  vast  indentations  and  promontories  acrosa  Wiwxr 
Miimesota,  and  Iowa,  forming  probably  tbe  most  eitensivs  moraine  in  the  world, 
strikes  north- westward  through  Dakota  for  at  least  100  miles  into  the  British  PoBsen 
wliere  its  further  course  has  been  partially  traced.  The  known  portion  of  tbe  mo 
thus  extends  with  a  wonderful  persistence  of  character  for  3000  miles,  reaching  a 
two-thinls  of  the  breadth  of  the  continent.' 

In  the  non -glaciated  regions  evidence  of  the  presence  and  influence  of  the  ioen 
is  probably  furnished  by  liigb  allurial  terraces,  which  could  not  have  been  fanned  n 
tiip  present  conditions  of  drainage.  From  this  kind  of  evidence  it  is  beliered  that  1 
the  ice -sheet  crossed  the  Ohio  River  near  Cincinnati,  it  ponded  back  the  dnina, 
the  entire  water-basin  of  East  Kentucky,  south-east  Ohio,  West  Virginia,  and  irei 
Pennsylrania,  up  to  a  height  of  perhaps  1000  feet,  forming  a  lake  at  that  leTel.*  % 
indications  of  a  lake,  caused  by  an  ice-dam  ponding  back  the  drainage,  are  fonnd  ai 
head  of  tlie  Red  River  in  Minnesota.'  The  largest  sheet  of  fteah  water  which  has  lei 
records  in  that  region  has  been  called  "  Lake  Agassii."  It  occapied  the  basin  of 
Red  River  of  the  Noi'th  and  Lake  Winnipeg.  It  is  computed  to  have  covered  an  an 
110.000  square  miles,  thus  exceeding  the  total  area  of  the  five  great  existing  Isk 
Sujierior  (31,200),  Michigan  (22,450).  Huron  with  Geor^an  Bay  (33,800),  Erie  (K 
Ontario  (7240),  wliich  have  a  united  area  of  94,651)  square  miles.'  Many  other  "  gt. 
lakes,"  which  no  longer  exist  because  their  ice -barriers  have  disappeared,  have 
foimd  scattered  over  Canada."  ^ 

Tlie  de]>osit8  left  by  the  ice-sheet  within  tbe  limits  of  the  terminal  moraini 
resemble  those  of  Europe  that  no  special  description  of  them  is  required.  Tlie  lo 
of  them,  resting  on  ice-worn  mcks,  is  a  stilT.  unstratilied  boulder-drift  or  till,  ta 
polished  and  striated  stones.  Occasional  intercalations  of  sand  and  clay,  whic 
Portland,  in  tlaine,  have  yielded  many  existing  s)>ecies  of  marine  organisms,  an 
some  places  land-plants  and  fresh-water  shells,  separate  the  lower  from  an  upper  bon 
clay,  which  is  looHer,  and  more  gravelly  and  sandy  than  the  older  depoeit,  coni 
larger  rough  and  anf^lar  blocks,  and  has  acquireil  a  yellow  tint  from  the  oiid 
influence  of  surface  waters.    The  boulders  vary  up  to  10  feet  (sometimes  even  40  fee 

'  T.  C.  Chaniberliu,  '■preliminary  Paiwr  on  the  Terminal  Moraine,"  Third  Ann. 
f'.S.  (Jeiil.  Svrven,  18S3.  Every  stuitent  of  glacial  geology  ought  to  make  himself  fan 
with  this  atlniirable  summary.  Consult  also  G.  M,  Daw»ou.  'Report  on  4(lth  Parall 
F.  Wnhnaehaffe,  ZtilMh.  Vrul^Ji.  Oiol.  Qa.  18P2,  p.  107  ;  J.  B.  Tyrrell  (BiUL  Oevl. 
AmtT.  i.  (1890)  p.  395)  describes  the  terminal  moraines  in  Manitoba  and  the  adjacent  1 
toriesof  N.W.  Canaila. 

"  H.  C.  I*wi3.  "  Report  on  the  Terminal  Moraine,"  cited  on  p.  1061. 

"  W.  Uphnni,  I'Tor-..  Amer.  Aaaoc.  xiiii.  (1883)  p.  21i. 

'  For  a  full  account  of  this  vanished  lake  (now  re[>re9ented  only  by  scattered  theei 
water  in  the  liollows  of  itn  basin),  with  its  terraces,  dunes,  deltas,  and  other  features,  sa 
Uphum,  Rep.  0;-'J.  Swrn.  Canada,  vol.  iv.  for  1888-89. 

■''  W.  U[ihani,  Bull.  Geal.  Soc.  Amer.  ii.  (1891)  p.  243. 


SECT,  i  §  2  PLEISTOCENE  OR  GLACIAL  SERIES  1063 

diameter,  and  have  seldom  travelled  more  than  20  miles.  The  boulder-clays  over  wide 
areas  are  distributed  in  lenticular  hills  or  drums  from  a  few  hundred  feet  to  a  mile  in 
length,  from  25  to  200  feet  high,  and  with  a  persistent  smoothness  of  outline  and  rounded 
tops.^  As  in  Europe,  the  longer  axes  of  these  drums  is  generally  parallel  with  that  of 
the  striation  of  the  underlying  rooks. 

At  the  height  of  the  Ice  Age  there  were  large  glaciers  in  the  Rocky  Mountains,  of 
which  the  small  glaciers  found  some  years  ago  among  the  Wind  River  Mountains  in 
Wyoming  are  some  of  the  last  lingering  relics.^  But  though  the  ice  filled  up  the  valleys 
to  a  depth  of  1600  feet  or  more,  and  transported  vast  quantities  of  detritus  which  now 
remains  in  prominent  moraines  and  scattered  boulders,  it  never  advanced  into  the  plateau 
of  the  prairie  country  to  the  east.  Whether  or  not  the  glaciers  at  the  north  end  of  the 
Rocky  Mountains  merged  into  and  were  turned  aside  by  the  southward-moving  ice-sheet 
has  still  to  be  ascertained.  Even  far  to  the  west,  the  Sierra  Nevada  nourished  an 
important  group  of  glaciers.^ 

The  loose  deposits  or  drifts  overlying  the  lower  unstratified  boulder-clay  belong  to 
the  period  of  the  melting  of  the  great  ice-sheets,  when  large  bodies  of  water,  discharged 
across  the  land,  levelled  down  the  heaps  of  detritus  that  had  formed  below  or  in  the 
under  part  of  the  ice.  This  remodelled  drift  has  been  called  the  *'  Champlain  group."  * 
Lower  portions  are  sometimes  unstratified  or  very  rudely  stratified,  while  the  upper  parts 
are  more  or  less  perfectly  stratified.  Towards  the  eastern  coasts,  and  along  the  valleys 
penetrating  from  the  sea  into  the  land,  these  stratified  beds  are  of  marine  origin,  and 
prove  that  during  the  Champlain  period  there  was  a  depression  of  the  eastern  part  of 
Canada  and  the  United  States  beneath  the  sea,  increasing  in  amount  northwards  from  a 
few  feet  in  the  south  of  New  England  to  more  than  500  feet  in  Labrador.  The  marine 
accumulations  are  well  developed  in  eastern  Canada,  where  the  drift-deposits  show  the 
following  subdivisions : — 

Post-glacial  accumulations. 

Saxicava  sand  and  gravel,  often  with  transported  boulders  (Upper  Boulder  dejiosits, 

St.  Maurice  and  Sorel  Sands).     Shallow-water  boreal  fauna,  Saxiaiva  rugoaa, 

bones  of  whales,  &c. 
Upi>er  Leda  clay  (and  probably  *'Sangeen  clay"  of  inland) ;  clay  and  sandy  clay 

with  numerous  marine  shells,  which  are  the  same  as  those  now  Uving  in  northern 

part  of  Gulf  of  St.   Lawrence ;  also  in  some  districts  fresh-water  shells  and 

plants.  ^ 
Lower  Leda  clay,  fine  clay,  often  laminated,  with  a  few  large  travelled  boulders 

(probably  equivalent  to   **Erie  Clay"  of  inland;   *' Champlain  Clay,"  Lower 

Shell -sand  of  Beauport)  ;  contains  Leda  arctica,  Tellina  grwnlandica  ;  probably 

deposited  in  cohl  ice-laden  water. 
Boulder-clay  or  Till ;  in  the  Lower  St.  Lawrence  region  contains  a  few  Arctic  shells, 

but  farther  inland  is  unfossiliferous. 
Peaty  beds,  marking  pre-glacial  land-surfaces.^ 

The  Leda  clays  rise  to  a  height  of  600  feet  above  the  sea.  On  the  banks  of  the 
Ottawa,  in  Gloucester,  they  contain  nodules  which  have  been  formed  r6und  organic 

1  W.  Upham,  Proc.  Bost,  Soc.  Nat.  Hist,  xxiv.  (1889)  p.  228.  See  on  Till,  W.  0.  Crosby, 
op.  cit.  XXV.  (1890)  p.  115. 

2  F.  V.  Hay  den's  Twelfth  Report,  U.S.  Oeol.  and  Geo*/.  Survey  of  the  Territories. 

'  J.  Leconte,  Amer.  Joum.  Sci.  (3)  ix.  (1875)  p.  126.  See  Amer.  Naturalist^  1880,  for 
a  paper  on  the  ancient  glaciers  of  the  Rocky  Mountains. 

*  See  J.  D.  Dana,  Amer.  Joum.  Sci.  x.  (1875)  p.  168,  xxvi.  (1883)  xxvii.  (1884)  ; 
Winchell,  op.  cU.  xi.  (1876)  p.  225. 

*  For  a  list  of  Canadian  Pleistocene  plants  see  D.  P.  Penhallow,  B%Ul.  Oeol.  Soc. 
Amer.  i.  (1890)  p.  321. 

*  J.  W.  Dawson,  Supplement  to  'Acadian  Geology,'  1878;  Canadian  NatureUist,  vi. 
(1871) ;  Geol.  Ma//.  1883,  p.  Ill  ;  Bull.  Geol.  Soc.  Amer.  i.  (1890)  p.  311. 


1084  STRATIOBAPHICAL  OEOLOGY  book  vi  P4» 

bodies,  particularly  tbe  fiah  Mallotut  mllMut  or  capeliog  of  the  Lower  St.  I^win 
Sir  J.  W.  DawBOii  alBO  obtained  numcroiw  reluaiDB  of  terreBtrial  manh-plalitii,  gm 

caricvs,  mosses,  and  algn>-  This  writer  states  that  about  100  species  of  manne  is 
tel)rBt«9  have  been  obtained  from  the  clays  of  the  St.  I^wreoce  vallej.  All  en 
four  or  live  specius  in  the  alder  part  of  the  deposits  are  shells  of  the  borsal  or  Ai 
regions  of  tlie  Atlantic ;  and  about  half  are  found  also  in  the  glacikl  e\a,f»  of  Brit 
The  great  majority  are  now  living  in  the  Gulf  of  St.  Lawrence  aud  on  ncighbon 
coasts,  especially  off  Labrador.' 

Terraces  of  marine  origin  occur  both  on  the  coast  and  far  inland.  On  the  coai 
Maine  they  appear  at  heights  of  150  to  900  feet,  round  l^e  Cbamplain  Mt  leM 
higli  as  300  feet,  and  at  Montreal  neariy  SOO  feet  abore  the  present  level  of  tbe  i 
In  the  abaetico  of  organic  remains,  however,  it  is  not  always  possible  to  diatingi 
between  terraces  of  marine  origin  msrking  former  ses-margias,  and  those  left  by 
retirement  of  rivers  and  lakes.  In  the  Bay  of  Fundy  evidence  has  been  cited 
Dawson  to  prove  subsidence,  for  be  baa  observed  tbere  a  submerged  forest  of  pine 
beech  lying  25  feet  below  high-water  mark." 

Inland,  the  stratified  parts  of  the  "  Champlain  group  "  have  been  Kccnmulated  on 
sides  of  rivers,  and  present  in  great  perfection  the  terrace  character  already  (p.  ', 
described.  Tlie  successive  platforms  or  terracea  mark  the  diminution  of  the  ctre* 
They  may  be  connected  also  with  an  intermittent  nprise  of  the  land,  and  an  I 
analogous  to  sea-terraces  or  raised  beaches.  Each  uplift  that  increued  the  decUrit 
the  rivers  would  augment  tbcir  rate  of  Sow,  and  consequently  tbeir  scour,  so  that  I 
would  be  unable  to  reach  their  old  Hood-plains.  Such  evidences  of  diminution 
almost  universal  among  the  valleys  in  the  drift-covered  parts  oF  North  America,  ai 
the  similar  regions  of  Eurojie.  Sometimes  four  or  five  platforms,  the  highest  bi 
100  ffot  or  more  above  the  present  level  of  tbe  river,  may  be  seen  rising  abore  c 
other,  as  In  the  wetl-kuown  example  of  the  Connecticut  Valley. 

The  terraces  are  not,  however,  confined  to  river-valleys,  but  may  be  tiBced  lo 
many  lakes.  Tiius,  in  the  basin  of  Lake  Huron,  <lcposila  of  fine  sand  and  clay  cont 
ing  frcsli-water  sbella  rise  to  a  height  of  4,0  feet  or  more  above  the  present  level  of 
water,  and  run  hack  from  the  shore  sometimes  for  '20  miles.  RegnUr  terraces,  cort«apc 
ing  to  former  water-levels  of  the  lake,  run  for  miles  along  the  shores  at  heights  of  1 
150,  and  200  feet.  Sliingle  Iteaches  and  mounds  or  ridges,  exactly  like  tliose  noi 
course  of  formation  along  the  exposed  shores  of  Lake  Huron,  can  be  recognised  at  hei| 
of  60,  70,  anil  100  feet.  Uufossiliferous  terracea  occur  abundantly  on  the  margii 
Lake  Su[>erior.  At  one  point  mentioned  by  Logan,  no  fewer  than  seven  of  these  anci 
beaches  occur  at  inti^rvala  np  to  a  Iieight  of  331  feet  above  the  present  level  of  the  la 
The  great  abundance  of  terraces  of  fluviatile,  lacustrine,  and  marine  origin  led,  as  aire 
stated,  to  the  use  of  the  term  "Terrace  epoch"  to  designate  the  time  when  these 
niarkable  topographical  features  were  produced.  Tlie  caose  of  the  former  higher  le 
of  the  watiT  is  a  diOicult  problem.  In  some  cases  it  has  doubtless  arisen  from  di 
formed  by  tongues  of  ice  during  the  retreat  of  the  ice-aheet. 

India.  —  There  is  abundant  evidence  that  at  a  late  geological  jieriod  glac 
descended  from  the  southern  alopes  of  the  Himalaya  Mountains  to  a  height  of  less  t 
3000  feet  above  the  present  sea-level.  Large  moraines  are  found  in  many  valleyi 
Sikkirn  am!  eastern  Nepal  between  7000  and  SOOO  feet,  and  ereu  down  to  5000  I 
above  sea-level.  In  the  western  Himalayas  perched  blocks  are  found  at  3000  feet,  : 
in  the  Upper  I'uBJaubvery  large  erratics  have  been  observed  at  still  lower  elevatii 

'  DawMon,  'Acadian  Geology,'  p.  7B. 

■  On  ti-rmcflS  at  lake  Ontario  see  Aaicr.  Journ.  Sci.  (3)  iiiv.  p.  iOS. 
=  'AcmliauOeolr>g)-,'p,  28. 

*  Logan,  'Geology  ot  Canada,'  p.  SIO.  Consult  also  tbe  paper  by  Gilbert  on  L 
Sbores  cited  a,.le.  p.  407. 


SECT,  ii  §  1    REGENT,  POST-QLAGIAL  OR  HUMAN  PERIOD  1055 

No  traces  of  glaciation  have  been  detected  in  southern  India.  Besides  the  physical 
evidence  of  refrigeration,  the  present  facies  iind  distribution  of  the  flora  and  fauna  on 
the  south  side  of  the  Himalaya  chain  suggest  the  influence  of  a  former  cold  period.^ 

Australasia. — The  present  glaciers  of  the  New  Zealand  Alps  had  a  much  greater 
extension  at  a  recent  geological  period.     According  to  Sir  J.  Haast  they  descended  into 
the  plains,  and,  on  the  west  side  of  the  island,  probably  advanced  into  the  sea,  for  along 
that  coast-line  their  moraines  now  reach  the  sea-margin  ;  huge  erratics  stand  up  among 
the  waves,  and  the  surf  breaks  far  outside  the  shore -line,  probably  upon  a  seaward 
extension  of  the  moraines.'    Captain  Hutton,  however,  points  out  that  there  is  no 
evidence  from  the  fauna  of  any  general  and  serious  refrigeration  of  the  climate  during  this 
glacier  period.^    He  believes  that  the  principal  part  of  the  sub-tropical  flora  and  fauna 
of  New  Zealand  was  introduced  before  the  Miocene  period,  and  has  flourished  ever  since, 
and  that  any  serious  diminution  of  the  temperature  of  the  islands  would  have  ex- 
terminated all  but  the  more  cold-loving  species  of  plants  and  animals.     He  maintains 
that  the  cause  of  the  former  greater  extension  of  the  glaciers  is  to  be  sought  in  the  fact, 
of  which  there  are  other  independent  proofs,  that  the  land  then  stood  at  a  far  higher 
level  than  it  does  at  present,  an  additional  3000  to  4000  feet  being  estimated  to  suffice 
for  restoring  the  glaciers  to  their  former  maximum  size.     He  likewise  adduces  grounds 
for  believing  that  the  glacier  epoch  (which  he  declines  to  regard  as  a  gkicial  epoch)  in 
New  Zealand  dates  back  to  a  much  earlier  time  than  the  Ice  Age  of  the  northern 
hemisphere,  probably  to  the  Pliocene  period. 

To  the  Upper  Pliocene  and  Pleistocene  periods  are  assigned  the  wide  terraced 
gravel-banks  and  alluvial  flats  which  occur  in  the  main  valleys  of  Australia,  and  the 
great  alluvial  plains  which  in  some  of  the  colonies  form  such  marked  features.  These 
deposits  vary  up  to  300  feet  in  depth,  and  are  a  great  storehouse  of  alluvial  gold. 
They  may  possibly  indicate  that  a  greater  rainfall  was  concerned  in  their  formation  than 
now  characterises  the  same  regions.  If  the  glaciers  of  New  Zealand  advanced  into  the 
sea,  and  the  great  Antarctic  ice-sheet  ever  crept  north  towards  the  Australian  shores, 
during  some  part  of  this  cold  period,  the  rainfall  may  have  been  so  augmented  that  the 
rivers  spread  out  far  beyond  the  limits  within  which  they  are  now  confined.  Evidence 
indeed  has  been  adduced  in  favour  of  true  glaciation  in  the  Australian  Alps.  What  are 
described  as  ice-worn  surfaces  have  been  observed  on  Mount  Cobboras  at  elevations  of 
between  4000  and  6000  feet,  and  on  Mount  Kosciusko  in  New  South  Wales.  Erratic 
blocks  and  moraines  arc  likewise  cited.  ^ 


Section  IL  Recent,  Post-glacial  or  Human  Period. 

§  1.  General  Characters. 

The  long  succession  of  Pleistocene  ages  shaded  without  abrupt  change 
of  any  kind  into  what  is  termed  the  Human  or  Recent  Period.^     The  Ice 

1  Medlicott  and  Blanford,  *  Geology  of  India,'  p.  586. 

^  *  Geology  of  Canterbury  and  Westland,*  p.  371.  This  however,  as  above  stated,  is  not 
admitted  by  Captain  Hutton  {N.  Zealand  Joum.  Sci.  1884). 

'  *  Geology  of  Otago,*  p.  83.  See  for  a  fuller  statement  of  his  views  on  this  subject  his 
address  on  the  Origin  of  the  Fauna  and  Flora  of  New  Zealand,  N,  Zealand  Joum,  Sci. 
(1884);  also  Proc  Linn.  Soc  N.S.  WaleSj  x.  part  3. 

*  J.  Stirling,  Trans.  Ray.  Soc.  Vict.  1884,  p.  23  ;  Nature,  xxxv.  (1886)  p.  182  ;  Dr. 
Ton  Lendenfeld,  Proc.  Linn.  Soc.  N.S.  Wales,  1885,  p.  45. 

*  See  for  general  information  Lyell's  *  Antiquity  of  Man,*  Lubbock's  *  Prehistoric  Times,' 
Evans's  *  Ancient  Stone  Implements,*  Boyd  Dawkins's  *  Cave  Hunting  '  and  *  Early  Man  in 
Britain,'  J.  Geikie's  'Prehistoric  Europe.* 


1056  STRATIGRAPHICAL  GEOLOGY  book  vi  fast  t 


Age,  or  Glacial  Period,  may  indeed  be  said  still  to  exist  in  Europe.  The 
snow -fields  and  glaciers  have  disappeared  from  Britain,  France,  the 
Yosges,  and  the  Harz,  but  they  still  linger  among  the  Pyrenees,  remain 
in  larger  mass  among  the  Alps,  and  spread  over  wide  areas  in  northern 
Scandinavia.  This  dovetailing  or  overlapping  of  geological  periods  hu 
been  the  rule  from  the  beginning  of  time,  the  apparently  abrupt 
transitions  in  the  geological  record  being  due  to  imperfections  in  the 
chronicle. 

The  last  of  the  long  series  of  geological  periods  may  be  subdivided  into 
subordinate  sections  as  follows : — 

Historic,  up  to  the  present  tinie. 

/  Iron,  Bronze,  and  later  Stone. 
Prehistoric -|  Neolithic 

V  Paleolithic 

The  Human  Period  is  above  all  distinguished  by  the  presence  and 
influence  of  man.  It  is  dif^Kcult  to  determine  how  far  back  the  limit  of 
the  period  should  be  placed.  The  question  has  often  been  asked  whether 
man  was  coeval  with  the  Ice  Age.  To  give  an  answer,  we  must  know 
within  what  limits  the  term  Ice  Age  is  used,  and  to  what  particular 
country  or  district  the  question  refers.  For  it  is  evident  that  even  to-day 
man  is  contemporary  with  the  Ice  Age  in  the  Alpine  valleys  and  in 
Finmark.  There  can  be  no  doubt  that  he  inhabited  £urope  after  the 
greatest  extension  of  the  ice.  He  not  improbably  migrated  with  the 
animals  that  came  from  warmer  climates  into  this  continent  during  inter- 
glacial  conditions.  But  that  he  remained  when  the  climate  again  became 
cold  enough  to  freeze  the  rivers  and  permit  an  Arctic  fauna  to  roam  far 
south  into  Europe  is  proved  by  the  abundance  of  his  flint  implements  in 
the  thick  river-gravels,  into  which  they  no  doubt  often  fell  through  holw 
in  the  ice  as  he  was  fishing. 

The  proofs  of  the  existence  of  man  in  former  geological  periods  are 
not  to  be  expected  in  the  occurrence  of  his  own  bodily  remains,  as  in  the 
case  of  other  animals.     His  bones  are  indeed  now  and  then  to  be  found, 
but  in  the  vast  majority  of  cases  his  former  presence  is  revealed  by  the 
implements  he  hius  left   behind  him,   formed  of  stone,  metal,   or  bone. 
Many  years  ago  the  archaeologists  of  Denmark,  adopting  the  phraseolog\* 
of  the  Latin  poets,  classified  the  early  traces  of   man    in    three    great 
divisions — the  Stone  Age,  Bronze  Age,  and  Iron  Age.     There  can  be  no 
doubt  that,  on  the  whole,  this  has  been  the  general  order  of  succession  in 
P^urope,  where  men  used  stone  and  l>one  before  they  had  discovered  the 
use  of  metid,  and  learnt  how  to  obtain  bronze  before  they  knew  anything 
of  the  metallurgy  of  iron.     Nevertheless,  the  use  of  stone  long  survived 
the  introduction  of  bronze  and  iron.     In   fact,   in  European    coimtries 
where  metal  has  been  known  for  many  centuries,  there  are  districts  where 
stone  implements,  iire  still  employed,  or  where  they  were  in  use  until 
(juite  recently.     It  is  obvious  also  that,  as  there  are  still  barbarous  tribes 
unacquainted  with  the  fabrication  of  metal,  the  Stone  Age  is  not  yet 
extinct  in  some  i)arts  of  the  world.     In  this  instance,  we  again  see  how 
geologiciil  i)eri(xls  run  into  each  other.     The  material  or  shape  of  the 


S  1     RECENT,  POST-OLACIAL  OR  HUMAN  PERIOD 


1067 


implement  cannot  therefore  be  always  a  very  sutisfactory  proof  of 
antiquity.  We  muat  judge  of  it  by  the  circumstances  under  which  it  was 
found.  From  the  fact  that  in  north-western  Europe  the  ruder  kinds  of 
atone  weapons  (Fig.  459)  occur  in  what  are  certainly  the  older  deposits, 
white  others  of  more  highly  finished  workmanship  (Figs.  462,  463)  are 
found  in  later  accumulations,  the  Stone  Age  has  been  subdivided  into  an 
early  or  Paleeolithic  and  a  later  or  Neolithic  epoch.  There  can  be  no 
doubt,  however,  that  the  latter  was  in  great  measure  coeval  with  the  age 
of  bronze,  and  even,  to  some  extent,  with  that  of  iron.' 

The  deposits  which  contain  the  history  of  the  Human  Period  are  river- 


alluvia,  brick-earth,  cavern-loam,  calcareous  tufa,  locas,  lake-bottoms,  peat- 
mosses, sand-dunes,  and  other  superficial  accumulations. 

Pal,EOLITHic.* — Under  this  term  arc  included  those  deposits  which 

'  The  stuileiit  may  profitably  coDiult  Sii  Artbur  Mitcliell'*  '  Post  in  the  Prcmnt.'  ISSO. 
for  the  warniiitpt  il  cDDtains  as  to  tbe  dangei  of  il«ciiliiig  ujiou  the  nntiquity  of  an  iniplGineDt 
merely  from  itn  ruileuesn. 

'  This  term  has  been  further  subdivided  into  minor  MClions  according  to  the  degree  of 
"  finish  "  iu  the  instruments  and  their  preaomcd  clironological  order.  Thus,  depo«ils  con- 
tainiug  the  very  nide  type  of  worked  flinU  founrl  at  Chelle,*  near  Poris  and  at  St.  Aeiieul 
hare  been  called  CheUtan  or  Aeheuiiai:.  Those  with  implements  like  the  scraper*  of 
Hnustier  (Dordogne)  hare  been  named  Movalcrion.  Tlioae  where  the  Hints  have  been  more 
deftly  worked,  like  the  iniplomenta  fonnd  at  Solulri  in  Bargimdy,  have  been  called  Sotiiirian  ; 
while  those  whicb  eontsia  well.flnLihed  implemepts  associated  with  carved  bone  aod  ivor]', 
u  at  the  caves  of  La  Madelaine  (Piirigord),  have  been  called  ilngdnlrniax.  But  this 
dassiRcatiou  does  not  rest  on  the  evideocs  of  soperposition,  and  is  probably  of  little  chrono- 
3y 


1068  STRATIGRAPHICAL  GEOLOGY  book  vi  part  v 

have  yielded  rudely -worked  flints  of  human  workmanship  associated 
with  the  remains  of  mammalia,  some  of  which  are  extinct,  while  others 
no  longer  live  where  their  remains  have  been  obtained.  An  association 
of  the  same  mammalian  remains  under  similar  conditions,  but  without 
traces  of  man,  may  be  assigned  to  the  same  geological  period,  and  be 
included  in  the  Palaeolithic  senes.  A  satisfactory  chronological  classifi- 
cation of  the  deposits  containing  the  first  relics  of  man  is  perhaps  unat- 
tainable, for  these  deposits  occur  in  detached  areas  and  offer  no  means 
of  determining  their  physical  sequence.  To  assert  that  a  brick-earth  is 
older  than  a  cavern-breccia,  because  it  contains  som'e  bones  which  the 
latter  does  not,  or  fails  to  show  some  which  the  latter  does  yield,  is  too 
often  a  conclusion  drawn  because  it  agrees  with  preconceptions. 

River- Alluvia. — Above  the  present  levels  of  the  rivers,  there  lie 
platforms  or  terraces  of  alluvium,  sometimes  up  to  a  height  of  80  or  100 
feet  These  deposits  are  fragments  of  the  river-gravels  and  loams  laid 
down  when  the  streams  flowed  at  these  elevations,  and  therefore  after 
the  excavation  of  the  valleys.  The  subsequent  action  of  the  running 
water  has  been  to  clear  out  much  of  the  old  alluvial  material  then 
accumulated,  so  as  to  leave  the  valleys  widened  and  deepened  to  their 
present  form.  Kiver-action  is  at  the  best  but  slow.  To  erode  the 
valleys  to  so  great  a  depth  beneath  the  level  of  the  upper  alluvia,  must 
have  demanded  a  period  of  many  centuries.  There  can  therefore  be  no 
doubt  of  the  high  antiquity  of  these  deposits.  They  have  .  yielded  the 
remains  of  many  mammals,  some  of  them  extinct  {Elephas  aniiquuSi 
Hippopotamus  aviphibitis,  Bhirioceros  megarhinus  (MerckU),  together  with 
flint-flakes  made  by  man.  From  the  nature  and  structure  of  some  of  the 
high-level  gravels  there  can  be  little  doubt  that  they  were  formed  at  a  time 
when  the  rivers,  then  possibly  larger  than  now,  were  liable  to  be  frozen 
and  to  be  obstructed  by  accumulations  of  ice.  We  are  thus  able  to 
connect  the  deposits  of  the  Human  Period  with  some  of  the  later  phases 
of  the  Ice  Age  in  the  west  of  Europe. 

Brick-Earths. — In  some  regions  that  have  not  been  below  the  sea 
for  a  long  period,  a  variable  accumulation  of  loam  has  been  formed  on  the 
surface  from  the  decomposition  of  the  rocks  in  situ,  aided  by  the  drifting 
of  fine  particles  by  wind  and  the  gentle  washing  action  of  rain  and 
occasionally  of  streams.  Some  of  these  brick-earths  or  loams  are  of 
high  antiquity,  for  they  have  been  buried  under  flu  via  tile  deposits 
which  must  have  been  laid  down  when  the  rivers  flowed  far  above  their 
present  levels.  They  have  yielded  traces  of  man  associated  with  bones 
of  extinct  mammals. 

Cavern  Deposits. — Most  calcareous  districts  abound  in  under- 
ground tunnels  and  caverns  which  have  been  dissolved  by  the  passage 
of  water  from  the  surface  (p.  367).  Where  these  cavities  have  com- 
municated with  the  outer  surface,  terrestrial  animals,  including  man 
himself,  have  made  use  of  them  as  places  of  retreat,  or  have  fallen  or 
been  washed  into  them.     The  floors  of  some  of  them  are  covered  with  a 

logical  value,  though  some  weight  may  be  attached  to  the  presence  of  different  mammals 
with  the  different  types  of  instrument. 


SECT,  ii  §  1     RECENT,  POST-GLACIAL  OR  HUMAN  PERIOD  1069 

reddish  or  brownish  loam  or  cave-earth,  resulting  either  from  the  in- 
soluble residue  of  the  rock  left  behind  by  the  water  that  dissolved  out 
the  caverns,  or  from  the  deposit  of  silt  carried  by  the  water  which  in 
some  cases  has  certainly  flowed  through  them.  Very  commonly  a 
deposit  of  stalagmite  has  formed  from  the  drip  of  the  roof  above  the 
cave-earth.  Hence  any  organic  remains  which  may  have  found  their 
way  to  these  floors  have  been  sealed  up  and  admirably  preserved. 

Calcareous  Tufas. — The  deposits  of  calcareous  springs  have  in 
various  parts  of  Europe  preserved  remains  of  the  flora  and  fauna  con- 
temporaneous with  the  early  human  inhabitants  of  the  Continent. 
Among  the  more  celebrated  of  these  deposits  are  those  of  Cannstadt  in 
Wiirtemburg,  which  have  yielded  specimens  of  twenty-nine  species  of 
plants,  consisting  of  oaks,  poplars,  maples,  walnuts  and  other  trees  still 
living  in  the  surrounding  coimtry,  but  with  the  remains  of  the  extinct 
mammoth ;  and  of  La  Celle,  near  Moret,  in  the  valley  of  the  Seine. 

Loess. — The  physical  characters  and  probable  seolian  origin  of  this 
remarkable  deposit  having  been  already  mentioned  (p.  332),  we  may  now 
consider  it  in  reference  to  its  place  in  geological  history.  In  central 
Europe  it  covers  a  wide  area.  Beginning  on  the  French  coast  at  San- 
gatte, it  sweeps  eastward  across  the  north  of  France  and  Belgium  (Hes- 
bayan  loam),  filling  up  the  lower  depressions  of  the  Ardennes,  passing 
far  up  the  valleys  of  the  Rhine  and  its  tributaries,  the  Ncckar,  Main  and 
Lahr ;  likewise  those  of  the  Elbe  above  Meissen,  the  Weser,  Mulde,  and 
Saale,  the  Upper  Oder  and  the  Vistula.  Spreading  across  Upper  Silesia, 
it  sweeps  eastward  over  the  plains  of  Poland  and  southern  Russia,  where 
it  forms  the  substratum  of  the  Tschernosem  or  black-earth.  It  extends 
into  Bohemia,  Moravia,  Hungary,  Grallicia,  Transylvania  and  Roumania, 
sweeping  far  up  into  the  Carpathians,  where  it  reaches  heights  of  2000 
and,  it  is  said,  even  4000  or  5000  feet  above  the  sea.  It  has  not  been 
observed  on  the  low  Germanic  plains  south  of  the  Baltic,  nor  south  of 
central  France  and  the  Alpine  chain.  Though  thickest  in  the  valleys 
(100  feet  or  more),  it  is  not  confined  to  them,  but  spreads  over  the 
plateaux  and  rises  far  up  the  flanks  of  the  uplands.  Near  its  edge, 
where  it  abuts  against  higher  ground,  it  contains  layers  or  patches  of 
angular  debris,  but  elsewhere  it  preserves  a  remarkable  uniformity  of 
texture. 

The  loess  is  sometimes  found  resting  on  gravels  containing  remains  of 
the  mammoth.  It  may  be  observed  to  shade  ofl*  into  more  recent  alluvial 
accumulations.  It  is  probably  not  all  of  one  age,  having  been  deposited 
during  a  prolonged  period  and  at  many  diflerent  altitudes.  The  older 
portions  may  not  impossibly  belong  to  the  later  part  of  the  Glacial  Period. 
Though  on  the  whole  not  rich  in  fossils,  the  loess  has  yielded  a  peculiar 
fauna,  which  singularly  confirms  Richthofen*s  view  that  the  deposit  was  a 
subaerial  one.  In  the  first  place,  the  shells  found  in  it  are  almbst  with- 
out exception  of  terrestrial  species.  Out  of  211,968  specimens  from  the 
loess  of  the  Rhine,  Braun  found  only  one  brackish  and  three  fresh-water 
forms,  Limiixa  and  Planorhis,  of  which  there  were  only  32  specimens  in 
all.     Of  the  rest,  there  were  98,502  examples  of  two  species  of  Swccinea, 


1060  STRATIGRAPHICAL  GEOLOGY  book  \i  past  v 

an  amphibious  genus,  and  113,434  specimens  belonging  to  25  species 
of  HeliXy  Pupa,  Clausilia,  Bulimus,  Limax,  and  Vitrina — unquestionable 
terrestrial  fonns.^  It  is  worthy  of  note  that  Helices  and  Suedneas 
abound  at  present  in  the  steppe-regions  of  central  Asia,  and  that  many 
of  the  species  of  loess  mollusks  are  now  living  in  east  Russia,  south- 
west Siberia,  and  on  the  prairies  of  the  Little  Missouri  in  North 
America.  2 

From  various  parts  of  the  Eiiropean  loess,  Dr.  Xehring  has  described 
a  remarkable  assemblage  of  animals,  which  included  a  jerboa  {Ahuiaga 
jaculus),  mannots  (Spermophilus,  several  species),  Arctomys  hobaCy  tailless 
hare  {Lagomys  pusillm),  numerous  species  of  Arvicoloy  Cricdus  frumentarius, 
C.  phmis,  porcupine  {Hystrix  hirsuiirostris),  wild  horses,  and  antelopes 
{Antilope  saiga).  This  fauna,  excepting  some  extinct  or  extirpated 
species,  is  identical  with  that  which  now  lives  in  the  south-east 
European  and  south-west  Siberian  steppes.^  Besides  these  distinctively 
step[)e  animals  the  loess  contains  numerous  remains  of  the  mammoth 
and  woolly  rhinoceros,  likewise  bones  of  the  musk-sheep,  hare,  wolf, 
stoat,  &c.  It  has  also  yielded  flint  implements  of  Palseolithic  types. 
The  bones  of  man  himself  were  claimed  many  years  ago  by  Ami  Boa^ 
to  have  been  found  in  the  loess,  and  his  opinion  has  been  in  some 
measure  strengthened  by  more  recent  observations. 

The  origin  of  the  loess  is  a  problem  which  has  given  rise  to  much 
discussion-  It  has  been  regarded  by  some  writers  as  the  dep>osit  of  a 
vast  series  of  lakes ;  by  others  as  the  mud  left  by  swollen  rivers  dis- 
charged from  melting  ice-fields ;  by  others  as  a  sediment  washed  over 
the  surface  of  the  land  by  an  abimdant  rainfall.  The  remarkably 
unstratified  character  of  the  loess  as  a  whole,  its  uniformity  in  fineness 
of  giaiii,  the  general  absence  of  coarse  fragments,  except  along  its 
margin,  where  they  might  be  expected,  its  singular  independence  of  the 
underlying  contour  of  the  groiuid,  and  the  almost  total  absence  in  it  of 
fiuviatile  or  lacustrine  shells,  seem  to  prove  conclusively  that  it  cannot 
have  been  laid  down  ]>v  rivers  or  lakes.  On  the  other  hand,  its  internal 
composition,  the  thoroughly  oxidised  condition  of  its  ferruginous  con- 
stituent, its  distribution,  and  the  striking  character  of  its  enclosed 
orgjinic  remains,  point  to  its  having  l)een  accumulated  in  the  open  air, 
probal)ly  in  circumstances  similar  to  those  which  now  prevail  in  the  dry 
stej)])e  regions  of  the  globe.  It  appears  to  mark  some  arid  interval  after 
the  height  of  the  Glacial  Period  had  passed  away,  when,  whilst  the 
climate  still  remained  cold  and  the  Arctic  fauna  had  not  entirely  retreated 
to  the  north,  a  series  of  grassy  and  dusty  steppes  swept  across  the  heart 
of  Kurope  and  Asia.** 

^  Zt'ltscli.  far  die  tjcadmint,  Xatunnss.  xl.  p.  45,  as  quoted  by  H.  H.  Howorth,  Gfd. 
M<n/.  188li,  p.  14. 

-  A.  Nehrintr,  O'tol.  Mrnj.  1883,  p.  57  ;  Xeifes  Jdhrh.  1889,  p.  66. 

^  Neliring,  ojk  cit.  p.  51,  where  a  reference  to  this  author's  numerous  memoini  on  the 
Bubjfct  will  Ix'  found. 

"*  The  views  propounded  by  Richthofen  for  the  loess  of  China  and  applied  by  Nehring 
to  that  of  Euroj»e  have  been  widely  adopted  by  geologists  (see,  for  example,  T.  P.  Jamieson, 


SECT,  ii  §  1     RECENT,  POST-GLACIAL  OR  HUMAN  PERIOD  1061 


False olithic  Fauna. — The  mammalian  remains  found  in  Palseo- 
lithic  deposits  are  remarkable  for  a  mixture  of  forms  from  warmer  and 
colder  latitudes  similar  to  that  already  noted  among  the  interglacial 
beds.  It  has  been  inferred,  indeed,  that  the  Palaeolithic  gravels  are 
themselves  referable  to  interglacial  conditions.  On  the  one  hand,  we 
meet  with  a  number  of  species  of  warmer  habitat,  as  the  lion,  hysena, 
hippopotamus,  lynx,  leopai;d,  and  caffer  cat ;  and,  in  the  loess,  the 
assemblage  of  forms  above  referred  to  as  that  which  still  characterises 
the  warm  dry  steppes  of  south-eastern  Europe  and  southern  Siberia. 
But,  on  the  other  hand,  a  large  number  of  the  forms  are  northern,  such 
iis  the  glutton  {G^ilo  luscus), 
Arctic  fox  {Cants  lagopus),  rein- 
deer (Cervus  tarandus),  Alpine 
hare  (Lepus  variabilis),  Norwegian 
lemming  ( Myodes  torquatiis),  Arctic 
lemming  (M.  lemmus,  M,  obeiisis), 
marmot  (Ardomys  marniotta), 
Russian  vole  (Arvicola  ratticeps), 
musk -sheep  (Ovibos  moschatus), 
snowy-owl  (Stryx  iiydea).  There 
is  likewise  a  proportion  of  now 
wholly  extinct  animals,  which 
include  the  Irish  elk  (Cervus 
gigantem  or  Megaceros  hibernictis), 
Elephas  primigenius  (mammoth), 
E.  aiiiiqam,  Rhinoceros  megarhinuSy 
R.  antiquitatis  (tichorhinus)  (woolly 
rhinoceros),  R,  leptorhinus,  and 
cave-bear  (Ursus  spdxus).  The 
Palreolithic     fauna     has      been 

divided  into  three  sections, ^ch  ^.^  ^_^,,,,,,  of  Reindeer  (A)  found  at  Bilney  Moor. 
supposed    to    correspond   with    a^  East  Dereham,  Norfolk. 

distinct  period  of  time  :   1st,  the 

Ago  of  Elephas  antiquuSy  with  which  species  are  associated  Rhinoceros  inega- 
rhinus  (Merckii)  and  Hippopotamus  ampkibitLs  {major),  2nd,  The  Age  of  the 
mammoth,  with  the  woolly  rhinoceros,  cave-bear  and  cave-hyaena.  3rd, 
The  Age  of  the  reindeer,  when  that  animal  passed  in  great  numbers  across 
central  Europe.  But,  as  already  stated,  such  subdivisions  are  admittedly 
artificial,  and  should  only  be  used  as  provisional  aids  in  the  comparison  of 
deposits  which  cannot  be  tested  by  the  law  of  superposition. 

That  man  was  contemporary  with  these  various  extinct  animals  is 
proved  by  the  frequent  occurrence  of  undoubtedly  human  implements, 
formed    of   roughly    chipped    flints,    &c.,    associated    with    their   bones. 

Ocol.  Ma4j.  1890,  p.  70).  But  they  have  not  been  universally  received,  some  geologists 
contending  that  water  in  different  ways  has  been  concerned  in  the  formation  of  the  loess. 
See  J.  Geikie,  *  Prehistoric  Europe,'  ]).  244  ;  Rep.  Brit.  Assoc.  1889  ;  Address  to  Geol.  Sect.; 
Wahuschaffe,  Zeitsch.  Devtsch.  Ofj>l.  Oes.  xxxviii.  (1886)  p.  583  ;  F.  Sacco,  Bull.  Soe. 
G6ol.  France,  xvi.  (1887)  p.  229. 


STRATIORAPHWAL  GEOLOOY 


BOOK  VI  FAK  \ 


Much  more  rarely,  portioiu  of  bmnan  skeletoiu  hsve  been  rBcorcred 
from  the  same  deposits.     The  men  <d  the  time  appMT  to  h>Te  onpad 


in  rock-shelter?  and  caves,  and  to  have  lived  by  fishing  and  by  hunting 
the  reindeer  bison  horse  mammoth  rhinoceros,  cave-bew,  and  othw 
animals.     That  they  verB  not  without  some  kind  of  onltnra  !■  ihown 


8KOT.  ii  §  1     RECENT,  POST-GLACIAL  OB  HUMAN  PERIOD  1063 

by  the  vigorous  incised  sketches  and  carving  which  they  have  left 
behind  on  reindeer  antlers,  mammoth  tusks  (Fig.  461),  and  other  bones, 
depicting  the  animals  with  which  they  were  daily  familiar.  Some  of 
these  drawings  are  especially  valuable,  as  they  represent  forms  of  life 
long  ago  extinct,  such  as  the  mammoth  and  cave-bear.  The  men  who 
in  Palffiolithic  time  inhabited  the  caves  of  Europe  must  have  had  much 
similarity,  if  not  actual  kinship,  to  the  modern  Eskimos. 

Nkolttuic. — The  deposits  whence  the  history  of  Neolithic  man  is 
compiled  must  vary  widely  in  age.  Some  of  them  were  no  doubt 
contemporaneous  with  parts  of  the  Paleeolithic  series,  others  with 
the  Bronze  and  Iron  series.  They  consist  of  cavern  deposits,  alluvial 
accumulations,  peat-mosses,  lake-bottoms,  pile-dwellings,  and  shell-mounds. 


Fig.  MI.— NsoUthic  atoae  Implt 


The  list  of  mammals,  &c,  inhabiting  Europe  during  Neolithic  is 
distinguished  from  that  of  Palteolithic  time  by  the  absence  of  the 
mammoth,  woolly  rhinoceros,  and  other  extinct  types,  which  appear 
to  have  meanwhile  died  out  in  Europe.  The  only  form  now  extinct 
which  appears  to  have  survived  into  Neolithic  time  was  the  Irish  elk, 
which  may  have  continued  to  live  until  a  comparatively  late  date.^  The 
general  assemblage  of  animals  was  probably  much  what  it  has  been 
during  the  period  of  history,  but  with  a  few  forma  which  have  dis- 
appeared from  most  of  Europe  either  within  or  shortly  before  the 
historic  period,  such  as  the  reindeer,  elk,  unis,  grizzly  bear,  brown  bear, 
wolf,  wild  boiir,  and  beaver.  But  besides  these  wild  animals  there  are 
remains  of  domesticated  forms  introduced  by  the  race  which  supplanted 
the  Palaeolithic  tribes.  These  are  the  dog,  horse,  sheep,  goat,  shorthorn, 
and  hog.  It  is  noteworthy  that  these  domestic  forms  were  not  parts  of 
the  in<ligenous  fauna  of  Europe.  They  appear  at  once  in  the  Neolithic 
>  Otol.  Mag.  18S1,  p.  3G4  :  Mature,  xx«i  p.  246. 


STRATTGEAPHICAL  GEOLOGY 


BOOK  VI  PART  T 


deposits,  leading  to  the  inference  that  they  were  introduced  by  the 
human  tribes  which  now  migrated,  probably  from  Central  Asia,  into 
the  European  continent  These  tribes  were  likewise  acquainted  with 
agricniture,  for  several  kinds  of  grain,  as  well  as  seeds  of  fruita,  have 
been  found  in  their  lake-dwellings ;  and  the  deduction  has  been  drawn 
from  these  remains  that  the  plants  must  have  been  brought  from 
southern  EurO{>e  or  Asia.  The  arts  of  spinning,  weaving  and  pottery- 
making  were  also  known  to  these  people.  Human  skeletons  and  bones 
belonging  to  this  age  have  been  met  with  abundantly  in  barrows  and 
peat-mosses,  and  indicate  that  Neolithic  man  was  of  small  stature,  with  a 
long  or  oval  skull. 


II  l.ikr  nvrUlDKB :  < 


Ihti  bistorj  of  tht  Hionze  and  Iron  Ages  in  Europe  is  told  in  great 
fuhie'w  but  I  (.longs  more  fittingly  to  the  domain  of  the  archieologist, 
who  (.lanus  is  hia  piopcr  field  of  research  the  history  of  man  upon  the 
glolit  The  ri.ni  iins  from  which  the  record  of  these  ages  is  compiled  arc 
objLtts  of  Inimin  manuficture,  graves,  cairns,  sculptured  stones,  &c, 
and  their  rihtne  lUtts  have  in  most  cases  to  be  decided,  not  upon 
geologii  i\,  but  upon  irchieological  grounds.  When  the  sequence  of 
human  ilIks  t  m  Ix  shown  by  the  order  in  which  they  have  lieen 
successneh  eutomlx-d  the  inquiry  is  strictly  geological,  and  the 
rcAsonni^,  11  is  logital  ind  trustworthy  as  in  the  case  of  any  other 
kind  if  f(v„iK  \\hc(c,  on  the  other  hand,  as  so  often  happens,  the 
question  of  iiitiqmtj    has  to  bo  decided  solely  by  relative  finish  and 


SECT.ii  §  2     RECENT,  POST-GLACIAL  OR  HUMAN  PERIOD  1066 

artistic  character  of  workmanship,  it  must  be  left  to  the  experienced 
antiquary. 

•  

§  2.  Local   Development 

A  few  examples  of  the  nature  of  the  deposits  of  the  Palaeolithic  and  Neolithic  series 
will  safhce  to  show  the  general  character  of  the  evidence  which  they  supply. 

Britain. — Paleolithic  deposits  arc  absent  from  the  north  of  England  and  from 
Scotland.  They  occur  in  the  south  of  England,  and  notably  in  the  valley  of  the 
Thames.  In  that  district,  a  series  of  brick-earths  with  intercalated  bands  of  river- 
gravel,  having  a  united  thickness  of  more  than  25  feet,  is  overlain  with  a  remarkable 
bed  of  clay,  loam,  and  gravel  ("trail"),  three  feet  or  more  in  thickness,  which  in  its 
contorted  bedding  and  large  angular  blocks  probably  bears  witness  to  its  having  been 
accumulated  during  a  time  of  floating  ice.  The  strata  below  this  presumably  glacial 
deposit  have  yielded  a  remarkable  number  of  mammalian  bones,  among  which  have 
been  found  undoubted  human  implements  of  chipped  flint  The  species  include 
Rhinoceros  Icptorhinus,  R,  antiquitatis  {tichorhinus),  R.  mtgarhinuSj  Eleplias  anliqutiSf 
E.  primigenius,  Cervus  gigaiUeus  {Megaoeros  hibernicus),  Felis  ko,  Hyfena  crocuta^  Ursus 
ferox^  U.  arctoSf  Ovibos  inoschattis^  Hippopotamus  amphibius  {major)^  and  present 
another  example  of  the  mingling  of  northern  with  southern,  and  of  extinct  with  still 
living  forms,  as  well  as  of  species  which  have  long  disappeared  from  Britain  with  others 
still  indigenous.  Other  ancient  alluvia,  far  above  the  present  levels  of  the  rivers,  have 
likewise  furnished  similar  evidence  that  man  continued  to  be  the  contemporary  in 
England  of  the  northern  rhinoceros  and  mammoth,  the  reindeer,  grizzly  bear,  brown 
bear,  Irish  elk,  hippopotamus,  lion,  and  hyena. 

The  caverns  iu  the  Devonian,  Carboniferous,  and  Magncsiau  limestones  of  England 
have  yielded  abundant  relics  of  the  same  prehistoric  fauna,  with  associated  traces  of 
Paleolithic  man.  In  some  of  these  places,  the  lowest  deposit  on  the  floor  contains  rude 
flint  implements  of  the  same  type  as  those  found  in  the  oldest  river- gravels,  while 
others  of  a  more  finished  kind  occur  in  overlying  deposits,  whence  the  inference  has 
been  drawn  tliat  the  caverns  were  first  tenanted  by  a  savage  race  of  extreme  rudeness, 
and  afterwards  by  men  who  had  made  some  advance  in  the  arts  of  life.  The  association 
of  bones  shows  that  when  man  had  for  a  time  retired,  some  of  these  caves  became  hyena 
dens.  Hyeua  bones  in  great  numbers  have  been  found  in  them  (remains  of  no  fewer 
than  300  individuals  were  taken  out  of  the  Kirkdale  cave),  with  abundant  gnawed 
bones  of  other  animals  on  which  the  hyenas  preyed,  and  quantities  of  their  excrement. 
Holes  in  the  limestone  opening  to  the  surface  (sinks,  swallow -holes)  have  likewise 
become  receptacles  for  the  remains  of  many  generations  of  animals  which  fell  into  them 
by  accident,  or  crawled  into  them  to  die.  In  a  fissure  of  the  limestone  near  Castleton, 
Derbyshire,  from  a  space  measuring  only  25  by  18  feet,  no  fewer  than  6800  bones, 
teeth,  or  fragments  of  bone  were  obtained,  chiefly  bison  and  reindeer,  with  bears,  wolves, 
foxes,  and  hares.  ^ 

France.  —  it  was  in  the  valley  of  the  Soninie,  near  Abbeville,  that  the  first 
observations  were  made  which  led  the  way  to  the  recognition  of  the  high  antiquity  of 
man  upon  the  earth.  That  valley  has  been  eroded  out  of  the  Chalk,  which  rises  to  a 
height  of  from  200  to  300  feet  above  the  modern  river.  Along  its  sides,  far  above  the 
present  alluvial  plain,  are  ancient  terraces  of  gravel  and  loam,  formed  at  a  time  when 
the  river  flowed  at  higher  levels.  The  lower  terrace  of  gravel,  with  a  covering  of 
flood-loam,  ranges  from  20  to  40  feet  in  thickness,  while  the  higher  bed  is  about  30 
feet.  Since  their  formation,  the  Somme  has  eroded  its  channel  down  to  its  present 
bottom,  and  may  have  also  diminished  in  volume,  while  the  terraces  have,  during  the 

^  Boyd  Dawkins,  'Early  Man  in  Britain,'  }\  188.  llie  reindeer  has  yet  not  been 
found  in  such  abundance  in  the  English  caverns  as  in  those  of  Southern  France. 


1066  STRATIGRAPHICAL  GEOLOGY  BOOKViPAEry 


interval,  here  and  there  suffered  from  denudation.  Flint  implements  have  heen 
obtained  from  both  terraces,  and  in  great  numbers,  associated  with  bones  of  mammoth, 
rhinoceros  and  other  extinct  mammals  (p.  1047). 

The  CHYerns  of  the  Dordogne  and  other  regions  of  the  south  of  France  have  yielded 
abundant  and  varied  evidence  of  the  coexistence  of  man  with  the  reindeer  and  other 
animals  either  wholly  extinct  or  no  longer  indigenous.  So  numerous  in  particular  are 
the  reindeer  remains,  and  so  intimate  the  association  of  traces  of  man  with  them,  that 
the  term  "  Reindeer  period  "  has  been  proposed  for  the  section  of  prehistoric  time  to 
which  these  interesting  relics  belong.  The  art  displayed  in  the  implements  found  in 
the  caverns  appears  to  indicate  a  considerable  advance  on  that  of  the  chipped  flints  of  the 
Somme.  Some  of  the  pictures  of  reindeer  and  mammoths,  incised  on  bones  of  these 
animals,  are  singularly  spirited  (Fig.  461). 

Germany.  —  From  various  caverns,  particularly  in  the  dolomite  of  Franconis 
(Muggendorf,  Gailenreuth)  and  in  the  Devonian  limestone  of  Westphalia  and  Rhine- 
land,  remains  of  extinct  mammals  have  been  obtained,  sometimes  in  great  nomben, 
including  cave-bear  (of  which  the  remains  of  800  individuals  have  been  taken  out  of  the 
Gailenreuth  cave),  hyaena,  lion,  rhinoceros,  and  others.  From  the  cavern  of  Hohlefels 
in  Swabia  remains  of  elephants,  rhinoceroses,  reindeer,  antelopes,  horses,  cave-bears  and 
other  animals  have  been  found,  together  with  interesting  proofs  of  the  contemporaneity 
of  man,  in  the  form  of  rude  flint  implements,  axes  of  bone,  or  teeth  and  bones  which  he 
had  bored  through,  or  split  open  for  their  marrow.  At  Schussenried  in  the  Swalnan 
Saulgau,  not  far  from  the  Lake  of  Constance,  beneath  a  deposit  of  calcareous  tufa 
enclosing  land-shells,  thei*e  is  a  peaty  bed  containing  Arctic  and  Alpine  mosses,  together 
with  abundant  remains  of  reindeer,  also  bones  of  the  glutton,  Arctic  fox,  brown  fox, 
polar  bear,  horse,  &c.  While  this  truly  Arctic  assemblage  of  animals  lived  near  the 
foot  of  the  Alps,  man  also  was  their  contemporary,  as  is  shown  by  the  presence,  in  the 
same  deposit,  of  his  flint  implements,  stones  that  have  been  blackened  by  fire,  bones  of 
the  reindeer  and  horse  that  have  been  broken  open  for  their  marrow,  needles  of  wood 
and  bone,  and  balls  of  red  pigment  probably  used  for  painting  his  body.* 

Switzerland.  — The  lakes  of  Switzerland,  as  well  as  those  of  most  other  countries  in 
Europe,  have  yielded  in  considerable  numbers  the  relics  of  Neolithic  man.  Dwellings 
construct<3d  of  piles  were  built  in  the  water  out  of  arrow-shot  from  the  shore.  Partly 
from  destruction  by  lire,  j)artly  from  successive  reconstructions,  the  bottom  of  the  water 
at  these  j)laces  is  strewn  with  a  thick  accumulatidn  of  di^bris,  from  which  vast  numbers 
of  relics  of  the  old  population  have  been  recovered,  revealing  much  of  their  mode  of  life.* 
Some  of  these  settlements  probably  date  far  back  beyond  the  beginning  of  the  historic 
period.  Others  belong  to  the  Bronze,  and  to  the  Iron  Age.  But  the  same  site  would 
no  doul)t  be  used  for  many  generations,  so  that  successive  layei*s  of  relics  of  progressively 
later  age  would  be  deposited  on  the  lake-bottom.  It  is  believed  that  in  some  cases  the 
lacustrine  dwellings  were  still  used  in  the  first  century  of  our  era. 

Denmark. — The  shell-mounds  {Kjokken-modiiing)^  from  3  to  10  feet  high,  and  some- 
times 1000  feet  long,  heaped  up  on  various  parts  of  the  Danish  coast-line,  mark  settle- 
ments of  the  Neolithic  age.  They  are  made  up  of  refuse,  chiefly  shells  of  mussels, 
cockles,  oysters,  and  i>eriwinkles,  mingled  with  bones  of  the  heiTing,  cod,  eel,  flounder, 
great  auk,  wild  duck,  goose,  wild  swan,  capercailzie,  stag,  roe,  wild  boar,  urus,  lynx, 
wolf,  wild  cat,  bear,  seal,  porpoise,  dog,  &c.,  with  human  tools  of  stone,  bone,  horn,  or 
wood,  frafjments  of  rude  i>ottery,  charcoal,  and  cinders. 

The  Danish  j)cat-mosses  have  likewise  furnished  relics  of  the  early  human  races  in 
that  region.  They  are  from  20  to  30  feet  thick,  the  lower  portion  containing  remains 
of  Scotch  fir  {Pinus  sylvestri^)  and  Neolithic  implements.  This  tree  has  never  been 
indigenous  in   the  country  within  the  historic  period.     A  higher  layer  of  the  peat 

^  0.  Fraas,  Arch iv  far  Anthropologies  Brunswick,  1867. 
'^  Keller's  '  I-.ake  Dwellings  of  Switzerland.' 


SECT,  ii  §  2     RECENT,  POST-QLACIAL  OR  HUMAN  PERIOD  1067 

contains  remains  of  the  common  oak  with  bronze  implements,  while  at  the  top  come 
the  beech-tree  and  weapons  of  iron.^ 

North  America. — Prehistoric  deposits  are  essentially  the  same  on  both  sides  of  the 
Atlantic.  In  North  America,  as  in  Europe,  no  very  definite  lines  can  be  drawn  within 
which  they  should  be  confined.  They  cannot  be  sharply  separated  from  the  Champlain 
series  on  the  one  hand,  nor  from  modem  accumulations  on  the  other.  Besides  the 
marshes,  peat-bogs,  and  other  organic  deposits  which  belong  to  an  early  period  in  the 
human  occupation  of  America,  some  of  the  younger  alluvia  of  the  river-valleys  and 
lakes  can  no  doubt  claim  a  high  antiquity,  though  they  have  not  supplied  the  same 
copious  evidence  of  early  man  which  gives  so  much  interest  to  the  corresponding 
£uro{)ean  formations.  From  the  peat-bogs  of  the  eastern  States,  and  from  the  older 
alluvium  of  the  Missouri  River,  the  remains  of  the  gigantic  mastodon  have  been  obtained. 
There  have  likewise  been  found  bones  of  reindeer,  elk,  bison,  beaver,  horse  (six  speciea), 
lion  and  bear  ;  while  southwards  those  of  extinct  sloths  {Mylodon^  Megatherium)  make 
their  appearance.  In  California,  from  the  deep  auriferous  gravels  remains  of  mastodon 
and  other  extinct  animals  have  been  met  with,  also  human  bones,  stone  sx>ear-heads, 
mortars  and  other  implements.  Prof.  Whitney  has  described  the  famous  Calaveras 
skull  as  occurring  at  a  depth  of  120  feet  in  undisturbed  gravel  which  is  covered  with  a 
sheet  of  basalt^  Heaps  of  shells  of  edible  species,  like  those  of  Denmark,  occur  on 
the  coasts  of  Nova  Scotia,  Maine,  &o.  The  large  mounds  of  artificial  origin  in  the 
Mississippi  valley  have  excited  much  attention.  The  early  archseology  of  these  regions 
is  full  of  interest. 

In  South  America,  the  loams  of  the  Pampas  have  furnished  abundant  remains  of 
horses,  tapirs,  lamas,  mastodons,  wolves,  panthers,  with  gigantic  extinct  sloths  and 
amiadillos  {Megatherium,  Olyptodon),^ 

Australasia. — No  line  can  be  drawn  in  this  region  between  accumulations  of  the 
present  time  and  those  which  have  been  called  Pleistocene.  The  modem  alluvia  have 
been  formed  under  similar  conditions  to  those  under  which  the  older  alluvia  were  laid 
down,  though  possibly  with  some  differences  of  climate.  In  New  South  Wales,  ossifer- 
ous caverns  contain  bones  of  the  extinct  marsupial  animals  mentioned  on  p.  1022, 
mingled  with  those  of  some  of  the  species  which  are  still  living  in  the  same  places. 
In  one  locality  in  the  same  colony,  in  sinking  a  well,  teeth  of  crocodiles  were  found  with 
bones  of  Diprotodan,  kc.  No  human  remains  have  yet  been  found  associated  with 
those  of  the  extinct  animals  ;  but  a  stone  hatchet  was  taken  out  of  alluvium  at  a  depth 
of  14  feet* 

In  New  Zealand,  the  most  interesting  featiire  in  the  younger  geological  accumula- 
tions is  the  presence  of  the  bones  of  the  large  bird  Dinomis,  which  has  become  extinct 
since  the  Maoris  peopled  the  islands.  The  evidences  of  the  human  occupation  of  the 
country  are  confined  to  the  surface-soil,  shelter-caves,  and  sand-dunes.' 

*  See  Steenstrup  on  "  Kjokken  Moddinger"  ;  Nathorst,  Nature,  1889,  p.  453. 

^  Mem.  Mus.  Compar.  Zool.  Harvard,  vi  (1880).  But  the  age  of  this  relic  is  the  subject 
of  dispute.  The  evidence  adduced  in  support  of  the  great  antiquity  of  man  in  America,  and 
his  contemporaneity  with  the  Mastodon  and  other  extinct  animals,  is  summarised  by  the 
Manjuis  de  Nadaillac  in  his  'L'Amerique  Prehistorique  *  (translated  by  N.  d'Anvers,  1885). 

^  See  Florentino  Ameghino,  *  La  Antiquedad  del  Hombre  en  el  Plata, '  where  a  good 
account  of  the  Pampas  country  will  be  found. 

*  C.  S.  Wilkinson,  'Notes  on  Geology  of  New  South  Wales,  1882,'  p.  59. 

*  Hector,  '  Handbook  of  New  Zealand,'  p.  25. 


BOOK  VII. 

PHYSIOGRAPHICAL  GEOLOGY. 

An  investigation  of  the  geological  history  of  a  country  involves  two 
distinct  lines  of  inquiry.  We  may  first  consider  the  nature  and  arrange- 
ment of  the  rocks  that  underlie  the  surface,  with  a  view  to  ascertain 
from  them  the  successive  changes  in  physical  geography  and  in  plant  and 
animal  life  which  they  chronicle.  But  besides  the  story  of  the  rocks,  we 
may  tiy  to  trace  that  of  the  surface  itself — the  origin  and  vicissitudes  of 
the  mountains  and  plains,  valleys  and  ravines,  peaks,  passes,  and  lake- 
basins  which  have  been  formed  out  of  the  rocks.  The  two  inquiries 
traced  backward  merge  into  each  other ;  but  they  become  more  and  more 
distinct  as  they  are  pursued  towards  later  times.  It  is  obvious,  for 
instance,  that  a  mass  of  marine  limestone  which  rises  into  groups  of  hills, 
trenched  by  river-gorges  and  traversed  by  valleys,  presents  two  sharply 
contrasted  pictures  to  the  mind.  Looked  at  from  the  side  of  its  origin, 
the  rock  brings  before  us  a  sea-bottom  over  which  the  relics  of  generations 
of  a  luxuriant  marine  calcareous  fauna  accumulated.  We  may  be  able  to 
trace  every  bed,  to  mark  with  precision  its  organic  contents,  and  to 
esta])lish  the  zoological  succession  of  which  these  superimposed  sea- 
bottoms  are  the  records.  But  we  mtiy  be  quite  unable  to  explain  how 
such  sea-formed  limestone  came  to  stand  as  it  now  does,  here  towering 
into  hills  and  there  sinking  into  valleys.  The  rocks  and  their  contents 
form  one  subject  of  study  ;  the  history  of  their  present  scenery  forms 
another. 

'I  The  branch  of  geological  inquiry  which  deals  with  the  evolution  of 
the  existing  contours  of  the  dry  land  is  termed  Physiographical  Geolog}'. 
To  be  a])le  to  pursue  it  profitably,  some  acquaintance  with  all  the  other 
branches  of  the  science  is  requisite.  Hence  its  consideration  has  been 
reserved  for  this  final  division  of  the  present  work ;  but  only  a  rapid 
summary  can  be  attempted  here. 

At  the  outset  one  or  two  fundamental  facts  may  be  stated.  It  is 
evident  that  the  materials  of  the  greater  part  of  the  dry  land  have  been 
laid  down  upon  the  floor  of  the  sea.  That  they  now  not  only  rise  above 
the  sea-level,  but  sweep  upwards  into  the  crests  of  lofty  mountains,  can 


BK.  VII      ORIGIN  OF  THE  MATERIALS  THAT  FORM  LAND         1069 


only  be  explained  by  displacement.  Thus  the  land  owes  its  existence 
mainly  to  upheaval  of  the  terrestrial  crust,  though  it  may  have  been  to 
some  extent  increased  and  diminished  by  other  causes  (ante,  pp.  282,  292). 
The  same  sedimentary  materials  which  demonstrate  the  fact  of  displacement, 
aftbrd  an  indication  of  its  nature  and  amount.  Having  been  laid  down 
in  wide  sheets  on  the  sea-bottom,  they  must  have  been  originally,  on  the 
whole,  level  or  at  least  only  gently  inclined.  Any  serious  departure 
from  this  original  position  must  therefore  be  the  effect  of  displacement, 
so  that  stratification  fonns  a  kind  of  datum-line  from  which  such  effects 
may  be  measured. 

Further,  it  is  not  less  apparent  that  sedimentary  rocks,  besides  having 
suffered  from  disturbance  of  the  crust,  have  undergone  extensive  denuda- 
tion. Even  in  tracts  where  they  remain  horizontal,  they  have  been 
carved  into  wide  valleys.  Their  detached  outliers  stand  out  upon  the 
plains  as  memorials  of  what  has  been  removed.  Where,  on  the  other 
hand,  they  have  been  thrown  into  inclined  positions,  the  truncation  of 
their  strata  at  the  surface  points  to  the  same  universal  degradation. 
Here,  again,  the  lines  of  stratification  may  be  used  as  datum -lines  to 
measure  approximately  the  amount  of  rock  which  has  been  worn  awav. 

While,  therefore,  it  is  true  that,  taken  as  a  whole,  the  dry  land  of 
the  globe  owes  its  existence  to  upheaval,  it  is  not  less  true  that  its 
present  contours  are  due  largely  to  erosion.  These  two  antagonistic 
forms  of  geological  energy  have  been  at  work  from  the  earliest  times,  and 
the  existing  land  with  all  its  varied  scenery  is  the  result  of  their  combined 
operation.  Each  has  had  its  own  characteristic  task.  Upheaval  has,  as 
it  were,  raised  the  rough  block  of  marble,  but  erosion  has  carved  that 
block  into  the  graceful  statue. 

The  very  rocks  of  which  the  land  is  built  up  bear  witness  to  this 
intimate  co-operation  of  hypogene  and  epigene  agency.  The  younger 
stratified  formations  have  been  to  a  large  extent  derived  from  the  waste 
of  the  older,  the  same  mineral  ingredients  being  used  over  and  over 
again.  This  could  not  have  happened  but  for  repeated  uplifts,  whereby 
the  sedimentary  accumulations  of  the  sea-floor  were  brought  within  reach 
of  the  denuding  agents.  Moreover,  the  internal  characters  of  these 
formations  point  unmistakably  to  deposition  in  comparatively  shallow 
water.  Their  abundant  intercalations  of  fine  and  coarse  materials,  their 
constant  variety  of  mineral  composition,  their  sun-cracks,  ripple-marks, 
rain-pittings,  and  worm-tracks,  their  numerous  unconformabilities  and 
traces  of  terrestrial  surfaces,  together  with  the  prevalent  facies  of  their 
organic  contents,  combine  to  demonstrate  that  the  main  mass  of  the 
sedimentary  rocks  of  the  earth's  cnist  was  accumulated  close  to  land,  and 
that  no  trace  of  really  abysmal  deposits  is  to  be  found  among  them. 
From  these  considerations  we  are  led  up  to  the  conclusion  that  the 
present  continental  areas  must  have  been  terrestrial  regions  of  the  earth's 
surface  from  a  remote  geological  period.  Subject  to  repeated  oscillations, 
so  that  one  tract  after  another  has  disappeared  and  reappeared  from 
beneath  the  sea,  the  continents,  though  constantly  varying  in  shape  and 
size,  have  yet,  I  believe,  maintained  their  individuality.     We  may  infer. 


1070  PHYSIOGRAPHICAL  OEOLOQT  booi 

likewise,  that  the  existing  ocean-basins  have  probably  always  been  the 
great  depressions  of  the  earth's  surface.^ 

Geologists  are  now  generally  agreed  that  it  is  mainly  to  the  effects  of 
the  secular  contraction  of  our  planet  that  the  deformations  and  disloca- 
tions of  the  terrestrial  crust  are  to  be  traced.     The  cool  outer  shell  has 

■ 

sunk  down  upon  the  more  rapidly  contracting  hot  nucleus,  and  the 
enormous  lateral  compression  thereby  produced  has  thrown  the  crust  into 
undulations,  and  even  into  the  most  complicated  corrugations.^  Hence, 
in  the  places  where  the  crust  has  yielded  •  to  the  pressure,  it  must  have 
been  thickened,  being  folded  or  pushed  over  itself,  or  being  perhaps 
thrown  into  double  bulges,  one  portion  of  which  rises  into  the  air,  while 
the  corresponding  portion  descends  into  the  interior.  Mr.  Fisher  believes 
that  this  downward  bulging  of  the  lighter  materials  of  the  crust  into  a 
heavier  substratum  underneath  the  gre^t  mountain-uplifts  of  the  sur&ce 
is  indicated  by  the  observed  diminution  in  the  normal  rate  of  augmen- 
tation of  earth -temperature  beneath  mountains,^  and  by  the  lessened 
deflection  of  the  plumb-line  in  the  same  regions. 

The  close  connection  between  upheaval  and  denudation  on  the  one 
hand  and  depression  and  deposition  on  the  other  has  often  been  remarked, 
and  striking  examples  of  it  have  been  gathered  from  all  parts  of  the 
world.  It  is  a  familiar  fact  that  along  the  central  and  highest  parts 
of  a  mountain  chain,  the  oldest  strata  have  been  laid  bare  after  the 
removal  of  an  enormous  thickness  of  later  deposits.  The  same  region 
still  remains  high  ground,  even  after  prolonged  denudation.  Again,  in 
areas  where  thick  accumulations  of  sedimentary  material  have  taken 
place,  there  has  always  been  contemporaneous  subsidence.  So  close  and 
constant  is  this  relationship,  as  to  have  suggested  the  belief  that 
denudation  by  unloading  the  crust  allows  it  to  rise,  while  deposition  bv 
loading  it  causes  it  to  sink  {amie,  p.  295).* 

It  is  evident  that  in  the  results  of  terrestrial  contraction  on  the 
surface  of  the  whole  planet,  subsidence  must  always  have  been  in  excess 

^  See  J.  D.  Daua,  Amer.  Journ.  Sci.  (2)  ii.  (1846)  p.  352;  "Geology"  in  *  Wilkes' 
Exploriug  Ex[)edition,'  1849;  Amer.  Journ.' Sci  (2)  xxii.  (1856);  'Manual  of  Geology,' 
1863,  2ud  edit.  1874,  3rd  edit.  1880;  Darwin,  'Origin  of  Species,'  1st  edit.  p.  343;  L. 
Agassiz,  Bull.  Mus.  Comp.  Zool.  1869,  vol.  i.  No.  13  ;  J.  D.  Whitney,  Mem.  Jius,  Comp. 
Zool.  J/an-ard,  vii.  No.  2,  p.  210.  See  also  Proc.  Roy.  Ocograph.  Soc.  new  ser.  i.  (1879) 
p.  422.  The  contrary  view  that  land  and  sea  have  continually  changed  places  over  the 
surface  of  the  globe  was  licld  by  Lyell,  and  is  still  maintained  by  some  geologists.  For  a 
statement  of  geological  evidence  in  favour  of  this  interchange  of  terrestrial  and  marine 
areas  the  student  may  consult  the  memoirs  of  the  late  Professor  Neumayr,  cited  on  p.  895. 

-  The  Rev.  0.  Fisher  in  his  'Physics  of  the  Earth's  Crust,'  maintains  that  the  secular 
contraction  of  a  solid  globe  through  mere  cooling  will  not  account  for  the  obseired 
plienomeua.     See  nnfe,  p.  56. 

•*  The  rate  observed  in  the  Mont  Cenis  and  Mont  St.  Gothard  tunnels  was  about  1^  Fahr. 
for  every  100  feet,  or  only  about  half  the  usual  rate. 

•*  This  belief  has  been  forcibly  urged  by  American  geologists  who  have  studied  the 
structure  of  the  Western  Territories.  See  especially  the  geological  Reports  of  Mr.  Clarence 
King,  Major  Powell,  and  Captain  Button ;  also  Mr.  T.  Mellard  Reade's  *  Origin  of  Mountain- 
Riuiges,'  and  Phil.  Mag.  June  1891. 


VII  TERRESTRIAL  FEATURES  DUE  TO  DISTURBANCE       1071 

of  upheaval — that  in  fact  upheaval  has  only  occurred  locally  over  areas 
where  portions  of  the  crust  have  been  ridged  up  by  the  enormous 
tangential  thrust  of  adjacent  subsiding  regions.  The  tracts  which  have 
thus  been,  as  it  were,  squeezed  out  under  the  strain  of  contraction  have 
been  weaker  parts  of  the  crust,  and  have  usually  been  made  use  of  again 
and  again  during  geological  time.  They  form  the  terrestrial  regions  of 
the  earth's  surface.  Thus,  the  continents  as  we  now  find  them  are  the 
result  of  many  successive  uplifts,  corresponding  probably  to  concomitant 
depressions  of  the  ocean  bed.  In  the  long  process  of  contraction,  the 
earth  has  not  contracted  uniformly  and  equably.  There  have  been,  no 
doubt,  vast  periods  during  which  no  appreciable  or  only  excessively 
gradual  movements  took  place;  but  there  have  probably  also  been 
intervals  when  the  accumulated  strain  on  the  crust  found  relief  in  more 
or  less  rapid  collapse. 

The  general  result  of  such  terrestrial  disturbances  has  been  to  throw 
the  crust  of  the  earth  into  wave-like  undulations.  In  some  cases,  a  wide 
area  has  been  upheaved  as  a  broad  low  arch,  with  little  disturbance  of 
the  original  level  stratification  of  its  component  rocks.  More  usually, 
the  undulations  have  been  impressed  as  more  sensible  deformations  of 
the  crust,  varying  in  magnitude  from  the  gentlest  appreciable  roll  up  to 
mountainous  crests  of  complicated  plication,  inversion,  and  fracture. 
As  a  rule,  the  undulations  have  been  linear,  but  their  direction  has 
varied  from  time  to  time,  having  been  determined  at  right  angles,  or 
approximately  so,  to  the  trend  of  the  lateral  pressure  that  produced 
thera.  As  the  crust  has  thickened,  and  in  consequence  of  the  structure 
imparted  to  it  by  successive  subsidences,  certain  tracts  even  of  the  land 
have  acquired  more  or  less  immobility,  and  have  served  as  buttresses 
against  which  surrounding  areas  have  been  pressed  and  dislocated  by  sub- 
sequent movements.  Suess  has  pointed  out  various  areas  of  the  earth's 
surface,  named  by  him  "  Horsts,"  which  seem  to  have  served  this  purpose 
in  the  general  rupture  and  subsidence  of  the  terrestrial  crust 

Considered  with  reference  to  their  mode  of  production,  the  leading 
contours  of  a  land-surface  may  be  grouped  as  follows :  1 .  Those  which 
are  due  more  or  less  directly  to  distiu-bance  of  the  crust.  2.  Those 
which  have  been  formed  by  volcanic  action.  3.  Those  which  are  the 
result  of  denudation.^ 

1.  Terrestrial  Features  due  more  or  less  directly  to  Dis- 
turbance of  the  Crust. — In  some  regions,  large  areas  of  stratified 
rocks  have  been  raised  up  with  so  little  trace  of  curvature,  that  they 
seem  to  the  eye  to  extend  in  horizontal  sheets  as  wide  plains  or  table- 
lands. If,  however,  these  areas  can  be  followed  sufficiently  far,  the  flat 
strata  are  eventually  found  to  curve  down  slowly  or  rapidly,  or  to  be 
truncated  by  dislocations.  In  an  elevated  region  of  this  kind,  the 
general  level  of  the  ground  corresponds,  on  the  whole,  with  the  planes 
of  stratification  of  the  rocks.     Vast  regions  of  Western  America,  where 

*  For  a  sketch  of  the  physiography  of  the  British   Isles  see  Mature,  xxix.  (1884)  pp. 
325,  347,  396,  419,  442. 


1072  PHYSIOGRAPHICAL  GEOLOGY  booi 

Cretaceous  and  later  strata  extend  in  nearly  horizontal  sheets  for 
thousands  of  square  miles  at  heights  of  4000  feet  or  more  above  the  sea, 
may  be  taken  as  illustrations  of  this  structure. 

As  a  rule,  curvature  is  more  or  less  distinctly  traceable  in  every 
region  of  uplifted  rocks.  Various  types  of  flexure  may  be  noticed,  of 
which  the  following  are  some  of  the  more  important : — 

((f)  Monodinal  Flexures  (p.  538). — These  occur  most  markedly  in 
broad  plateau-regions  and  on  the  flanks  of  large  broad  uplifts,  as  in  tie 
ta])le-lands  of  Utah,  Wyoming,  <&c.  They  are  frequently  replaced  by 
faults,  of  which  indeed  they  may  be  regarded  as  an  incipient  stage 
(p.  551). 

(h)  Symmeirkul  Flexures,  where  the  strata  are  inclined  on  the  two  sides 
of  the  axis  at  the  same  or  nearly  the  same  angle,  may  be  low  gentle  un- 
dulations, or  may  increase  in  steepness  till  they  become  short  sharp  curves. 
Admirable  illustrations  of  different  degrees  of  inclination  may  be  seen  in 
the  ranges  of  the  Jum^  (Fig.  464)  and  the  Appalachians  (Fig.  246),  where 
the  influence  of  this  structure  of  the  rocks  on  external  scenery  may  be 
instructively  studied.  In  many  instances,  each  anticline  forms  a  long 
ridge,  and  each  syncline  runs  as  a  corresponding  and  parallel  valley.  It 
will  usually  be  observed,  however,  that  the  surface  of  the  ground  does 

S.E.  N.W. 

()knsin«;en.  Ballsthai-  MCnster.  RAifEUX. 


Fig.  4iV4.— Symmetrical  Flexures  of  Swiss  Jura 
(the  ridges  cuincidiiig  with  auticliuen  ami  the  valleys  with  synclines). 

not  strictly  conform,  for  more  than  a  short  distance,  to  the  sui-facc  of  any 
one  ]>ed  ;  but  that,  on  the  contrary,  it  passes  across  the  edges  of  succes- 
sive beds,  as  in  Fig.  464.  This  relation — so  striking  a  proof  of  the 
extent  to  which  the  surface  of  the  land  has  suffered  from  denudation — 
may  be  followed  through  successive  phases  until  the  original  superficial 
contours  are  exactly  reversed,  the  ridges  running  along  the  lines  of 
syncline  and  the  valleys  along  the  lines  of  anticline  (Figs.  244,  245). 
Among  the  older  rocks  of  the  earth's  crust  which  have  been  exposed 
alike  to  curvature  and  prolonged  denudation,  this*  reversal  may  be  con- 
sidered to  be  the  rule  rather  than  the  exception.  The  tension  of  curvature 
may  occasionally  have  produced  an  actual  rupture  of  the  crest  of  an 
anticline  along  which  the  denuding  agents  would  effectively  work. 

The  Uinta  ff/pe  is  a  variety  of  this  structure  seen  to  great  perfection 
in  the  Uinta  Mountains  of  Wyoming  and  Utah.  It  consists  of  a  broad 
flattened  flexure  from  which  the  strata  descend  steeply  or  vertically  into 
the  low  grounds,  where  they  quickly  resume  their  horizontality.  In 
the  Uinta  Mountains,  the  flat  arch  has  a  length  of  upwards  of  150  and  u 
breadth  of  about  50  miles,  and  exposes  a  vast  deeply  trenched  plateau 

^  On  tlie  geolopy  of  the  Jura  see  C.  Clerc,  '  Le  Jura,'  Paris,  1888  ;  G.  Boyer,  *  Remanjue-^: 
sur  r()ro^rai)hie  ties  Monts  Jura,'  Besan^oii,  1888  ;  and  the  older  work  of  Thurmaus. 
'Esquisses  Orographiques  de  la  Chaine  du  Jura,'  1852. 


VII 


MOUNTAIN  FLEXURES 


1073 


with  an  average  height  of  10,000  to  11,000  feet  above  the  sea,  and 
jOOO  to  6000  feet  above  the  plains  on  either  sida  This  elevated  region 
consists  of  nearly  level  ancient  Palaeozoic  rocks,  which  plunge  below 
the    Secondary  and    Tertiary   deposits    that    have    been    tilted    by    the 


FiK.  405.— UintH  Tyi>e  of  Flexur«. 
»i,  l*aliPozoic  rockN  ;  h,  Mesozoic  ;  c,  Tertiary  ;  y,  fault. 

Uplift  (Fig.  465).  Powell  believes  that  a  depth  of  not  less  than  three 
and  a  half  miles  of  strata  has  been  removed  by  denudation  from  the  top 
of  the  arch.^  In  some  places,  the  line  of  maximum  flexure  at  the  side  of 
the  uplift  has  given  way,  and  the  resulting  fault  has  at  one  point  a  ver- 
tical displacement  estimated  by  him  at  20,000  feet. 

Another  variety  of  more  complex  structure  may  be  termed  the  Park 
type^    from    its    singularly    clear   development    in    the    Park    region    of 


-■.■?.:*.tLtvT 


Fig.  4t}«J.— Park  Tyi)p  oi  Flexure, 
a,  Crystalline  rockn  ;  />,  MeM<)Zi)ic  rocks. 


Colorado.  In  this  type,  an  axis  of  ancient  crystalline  rocks — granites, 
gneisses,  &c. — has  been  as  it  were  pushed  through  the  flexure,  or  the 
younger  strata  have  been  bent  sharply  over  it,  so  that  after  vast  denuda- 
tion their  truncated  ends  stand  up  vertically  along  the  flanks  of  the 
uplifted  nucleus  of  older  rocks  (Fig.  466). 

There  may  be  only  one  dominant  flexure,  as  in  the  case  of  the  Uinta 
Mountains,  the  long  axial  line  of  which  is  truncated  at  the  ends  by  lines 
of  flexure  nearly  at  right  angles  to  it.     More  usually,  numerous  folds 


Chaux  1)1 

DoMKIEF. 


St.  ('l-Afl)K. 


Vaij*erine. 


Near  Lake 
Geneva. 


i»a  V  i^ V  V 


Fig.  467. —.Section  acroHa  Western  Part  of  Jura  MountainM. 
(After  P.  CliotTat,  jjAnn,  A.  Heiui,  '  Mechauisni.  Gebirgsb."  pi.  xiii.) 

« 

run  approximately  parallel  to  each  other,  as  in  the  Jura  and  Appalachian 
chains.  Not  infrequently,  some  of  them  die  out  or  coalesce.  Their 
axes  are  seldom  perfectly  straight  lines. 

{r)  Umyminftrical  Flexures^  where  one  side  of  the  fold  is  much  steeper 

'  *  Geology  of  Uinta  Mountains,'  p.  201.  There  is  in  this  work  a  suggestive  discussion 
on  types  of  niouutuin  structure.  See  also  Clarence  King's  '  Report  on  Gtjology  of  40th 
Parallel,'  vol.  i. 

3  z 


^ 

I 
t 


1074 


PHYSIOGRAPHICAL  GEOL 


i.J 


than  the  other,  but  where  tbey  are  atill  incline< 
occur  in  tracts  of  considerable  disturbance.  Th 
from  the  area  of  maximum  movement,  and  are  i 
they  approach  it,  until  the  flexures  becom* 
examples  of  thia  structure  are  presented  by  the  . 
'  Appalachian  chain.  In  these  tracts,  it  is  obser 
as  the  flexures  increase  in  angle  of  inclination 
and  closer  together;  while,  on  the  other  banc 
symmetrical  forms,  they  become  broader,  flatty 
they  disappear  (Figs.  246,  467). 

(d)  Reversed  FUxuret,  where  the  strata  have  t 
a  it'ay  that  on  both  sides  of  the  axis  of  curvatui 
direction,  occur  chiefly  in  districts  of  the  mos 
as  a  great  mountain  chain  like  the  Al[)8.  The 
for  the  most  part  towards  the  region  of  maxim 
flexures  are  often  so  rapid  that  after  denudation 
the  strata  are  isoclinal,  or  appear  to  be  dipping  i 
(p.  540).  A  gradation  can  be  traced  through  th 
of  flexure.  The  inverted  or  reversed  type  is  fou 
(if  the  crust  has  been  greatest.  Away  from  thi 
turbance,  the  folds  pass  into  the  unsymmetrical  I 
lessening  slopes  into  the  symmetrical,  finally  wii 
into  the  plains.  If  we  bisect  the  flexures  in  a  s( 
region  we  find  that  the  lines  of  bisection  or  " 
in  the  symmetrical  folds,  and  gradually  incline  t> 
ground  at  lessening  angles.' 

Fractures  not  infrequently  occur  along  the  aj 
inverted  flexures,  the  strata  having  snapped  undi 
one  side  (in  the  case  of  inverted  flexures,  usuallj 
been  pushed  over  the  other,  sonietimes  with  a 
several  thousand  feet,  or  a  horizontal  thrust  of  bi 
or  parallel  to  the  axes  of  plication,  and  there' 
general  strike,  that  the  great  faults  of  a  plicated 
dislocations  are  more  easily  traced  among  low  | 
mountains.  One  of  the  most  remarkable  and  im 
for  example,  is  that  which  bounds  the  southern  ( 
field  (p.  835).  It  can  be  traced  across  Belgium, 
Boiilonnais,  and  may  not  improbably  run  ben 
Tertiary  rocks  of  the  south  of  England.  The  ex 
of  the  north-west  of  Scotland  (pp.  625,  706) 
gigantic  horizontal  displacement.  It  is  a  remarki 
have  a  vertical  throw  of  many  thousands  of  feet 
effect  upon  the  surface.  The  great  Belgian  fault 
of  the  Meuse  and  other  northerly  flowing  stream 
marked  in  the  Meuse  valley  that  no  one  woi 
from  any  peculiarity  in  the  general  form  of  t 
experienced  geologist,  until  he  had  learned  the 
'  H.  D.  Rogers,  Tmns.  Ray.  Sue  Edia.  : 


vn  ALPINE  TYPE  OF  MOUNTAIN-STRUCTURE  1075 

would  scarcely  detect  any  fault  at  all.  The  Scottish  thrustrplanes  are 
eroded  like  ordinary  junction-planes  between  strata,  and  produce  no  more 
effect  than  these  do  on  the  topography  (see  Figs.  311,  334). 

In  some  regions  of  intense  disturbance,  such  as  the  Alps,  the  rocks 
have  been  plicated  rather  than  fractiu*ed.  The  folds  have  been  so  com- 
pressed that  their  opposite  limbs  often  lie  parallel  to  each  other  at  a  high 
inclination.  In  other  regions,  such  as  the  north-west  of  Scotland,  where 
the  gigantic  pressiu^e  has  encountered  the  resistance  of  a  "  horst "  or  solid 
buttress  of  immovable  material,  the  rocks  have  been  ruptured  by 
innumerable  thrust-planes  and  faults,  and  have  been  driven  over  each 
other  in  a  kind  of  imbricated  structure  (p.  624). 

{e)  Alpine  Type  of  Mountain-Structure}— ^\t  is  along  a  great  mountain 
chain  like  the  Alps  that  the  most  colossal  crumplings  of  the  terrestrial 
crust  are  to  be  seen.  In  approaching  such  a  chain,  one  or  more  minor 
ridges  may  be  observed  running  on  the  whole  parallel  with  it,  as  the 
heights  of  the  Jura  flank  the  north  side  of  the  Alps,  and  the  sub- 
Himalayan  hills  follow  the  southern  base  of  the  Himalayas.  On  the 
outer  side  of  these  ridges,  the  strata  may  be  flat  or  gently  inclined.  At 
first  they  undulate  in  broad  gentle  folds ;  but  traced  towards  the 
mountains  these  folds  become  sharper  and  closer,  their  shorter  sides 
fronting  the  plains,  their  longer  slopes  dipping  in  the  opposite  direction. 
This  inward  dip  is  often  traceable  along  the  flanks  of  the  main  chain  of 
mountains,  younger  rocks  seeming  to  underlie  others  of  much  older  date. 
Along  the  north  front  of  the  Alps,  for  instance,  the  red  molasse  is  over- 
lain by  Eocene  and  older  formations.  The  inversions  increase  in  magni- 
tude till  they  reach  such  colossal  dimensions  as  the  double  fold  of  the 
Glarnisch,  where  Triassic,  Jurassic,  and  Cretaceous  rocks  have  been 
thrown  over  above  the  Eocene  flysch  and  nummulitic  limestone  (p.  539). 
In  such  vast  crumplings  it  may  happen  that  portions  of  older  strata  are 
caught  in  the  folds  of  later  formations,  and  some  care  may  be  required 
to  discriminate  the  enclosure  from  the  rocks  of  which  it  appears  to  form 
an  integral  and  original  part.  Some  of  the  recorded  examples  of  fossils 
of  an  older  zone  occurring  by  themselves  in  a  much  younger  group  of 
plicated  rocks  may  be  thus  accounted  for. 

The  inward  dip  and  consequent  inversion  traceable  towards  the  centre 
of  a  mountain  chain  lead  up  to  the  fan-shaped  structure  (p.  541),  where 
the  oldest  rocks  of  a  series  occupy  the  centre  and  overlie  younger  masses 
which  plunge  steeply  under  them.  Classical  examples  of  this  structure 
occur  in  the  Alps  (Mont  Blanc,  Fig.   250,  St.  Grothard),  where  crystalline 

^  For  recent  information  on  the  internal  structure  of  the  Alpine  chain  see  especially  the 
maps,  sections,  and  explanatory  memoirs  by  Renevier,  Helm,  A.  Baltzer,  £.  Favre,  K.  J. 
Kaufmann,  C.  Moesch,  H.  Schardt,  A.  Gutzwiller,  and  others  in  the  BeitrUge  xur  Oeol.  KarU 
der  Schweiz]  also  Fritz  Freeh,  "Die  Eamiachen  Alpen,"  Abhand.  Naiwf,  Ots,  HaUe^ 
xriii.  (Heft  i.)  1892  ;  2^cagna  on  the  Oraian  Alps,  Boll.  Com.  Oeol.  Ital.  ser.  iii.  vol.  iii. 
(1892)  p.  175;  consult  also  Heim's  *Mechanismus  der  Gebirgsbildung '  ;  Suess,  'Antlitz 
der  Erde '  and  *  Entstehung  der  Alpen ' ;  A.  Favre,  *  Recherches  G^ol.  dans  les  parties  de  la 
Savoie  dn  Pi^mont  et  de  la  Suisse  voisines  du  Mont  Blanc,'  1867,  and  'Description  Geol. 
Canton  Geneve,'  1880. 


1076  PHYSWGRAPHICAL  GEOLOGY  book 


rocks  such  as  granite,  gneiss,  and  schist,  the  oldest  masses  of  the  chain, 
have  been  ridged  up  into  the  central  and  highest  peaks.  Along  these 
tracts,  denudation  has  been  of  course  enormous,  for  the  appearance  of  the 
granitic  rocks  at  the  surface  has  been  brought  about,  not  necessarily  by 
actual  extrusion  into  the  air,  but  more  probably  by  prolonged  erosion, 
which  in  these  higher  regions,  where  many  forms  of  sub-aerial  waste  reach 
their  most  vigorous  phase,  has  removed  the  vast  overarching  cover  of 
younger  rocks  under  which  the  crystalline  nucleus  doubtless  lay  buried. 

With  the  crumpling  and  fracture  of  rocks  in  mountain-making,  the 
hot  springs  must  be  connected,  which  so  frequently  arise  along  the  flanks 
of  a  mountain  chain.  A  further  relation  is  to  be  traced  between  these 
movements  and  the  opening  ^f  volcanic  vents  either  along  the  chain  or 
parallel  to  it,  as  in  the  Andes  and  other  prominent  ridges  of  the  crust. 
Elevation,  by  diminishing  the  pressure  on  the  parts  beneath  the  upraised 
tracts,  may  permit  them  to  assume  a  liquid  condition  and  to  rise  within 
reach  of  the  surface,  when,  driven  upwards  by  the  expansion  of  super- 
heated vapours,  they  are  ejected  in  the  form  of  lava  or  ashes.  Mr.  Fisher 
supposes  that  the  lower  half  of  the  double  bulge  of  the  crust  in  a  mountain, 
by  being  depressed  into  a  lower  region,  may  be  melted  off,  giving  rise  to 
siliceous  lavas  which  may  rise  before  the  deeper  basaltic  magma  begins  to 
be  erupted. 

A  mountain-chain  may  be  the  result  of  one  movement,  but  probably 
in  most  cases  is  due  to  a  long  succession  of  such  movements.  Formed 
on  a  line  of  weakness  in  the  crust,  it  has  again  and  again  given  relief 
from  the  strain  of  compression  by  undergoing  fresh  crumpling  and 
upheaval.  The  successive  stages  of  uplift  are  usually  not  difficult  to 
trace.  The  chief  guide  is  supplied  by  unconformability  (p.  641).  Let 
us  .suppose,   for  example,   that  a  mountain  range  (Fig.   468)  consists  of 

* 

b  a  b 

KiK.  4<18.  —.Section  sliowing  two  iK;rio<ls  of  Upheaval. 

upraised  Lower  Silurian  rocks  (a),  upon  the  u})turned  and  denuded  edges 
of  which  the  Carboniferous  Limestone  (b  l>)  lies  transgressively.  The 
original  upheaval  of  that  range  must  have  taken  place  between  the 
Lower  Sihirian  and  the  Carboniferous  Limestone  periods.  If,  in  follow- 
ing the  range  along  its  course,  we  found  the  Carboniferous  Limestone  also 
higlily  inclined  and  covered  unconformahly  by  the  Upper  Coal-measures 
(/•  /•),  we  should  know  that  a  second  uplift  of  that  portion  of  the  ground 
had  taken  place  between  the  time  of  the  Limestone  and  that  of  the 
Upper  Coal-measures.  Moreover,  as  the  Coal-measures  were  laid  down 
at  or  l>elow  the  sea-level,  a  third  uplift  has  subsequently  occurred 
whereby  they  were  raised  into  dry  land.  By  this  simple  and  obvious 
kind  of  evidence,  the  relative  ages  of  different  mountain  chains  may  be 
compared.       In  most  great  mountain  chains,   however,   the  rocks  have 


VII  GEOLOGICAL  HISTORY  OF  THE  ALPS  1077 

been  so  intensely  crumpled,  and  even  inverted,  that  much  labour  may  be 
required  before  their  true  relations  can  be  determined. 

The  Alps  offer  an  instructive  example  of  a  great  mountain  system 
formed  by  repeated  movements  during  a  long  succession  of  geological 
l)eriods.  The  central  portions  of  the  chain  consist  of  gneiss,  schists, 
granite,  and  other  crystalline  rocks,  partly  referable  to  the  pre-Cambrian 
series,  but  some  of  which  are  metamorphosed  Palaeozoic,  Secondary,  and 
even  older  Tertiary  deposits  (p.  622).  It  would  appear  that  the  first 
outlines  of  the  Alps  were  traced  out  even  in  pre-Gambrian  times,  and  that 
after  submergence,  and  the  deposit  of  Palaeozoic  formations  along  their 
flanks,  if  not  over  most  of  their  site,  they  were  re-elevated  into  land. 
From  the  relations  of  the  Mesozoic  rocks  to  each  other,  we  may  infer 
that  several  renewed  uplifts,  after  successive  denudations,  took  place 
before  the  beginning  of  Tertiary  times;  but  without  any  general  and 
extensive  plication.  A  large  part  of  the  range  was  certainly  submerged 
during  the  Eocene  period  imder  the  waters  of  that  wide  sea  which 
spread  across  the  centre  of  the  Old  World,  and  in  which  the  nummulitic 
limestone  and  flysch  were  deposited.  But  after  that  period  the  grand 
upheaval  took  place  to  which  the  present  magnitude  of  the  mountains  is 
chiefly  due.  The  older  Tertiary  rocks,  previously  horizontal  under  the 
sea,  were  raised  up  into  mountain-ridges  more  than  11,000  feet  above 
the  sea-level,  and,  together  with  the  older  formations  of  the  chain,  were 
crumpled,  dislocated,  and  inverted.  So  intense  was  the  compression 
and  shearing  to  which  clays  and  sands  were  subjected,  that  they  were 
converted  into  hard  crystalline  rocks.  It  is  strange  to  reflect  that  the 
enduring  materials  out  of  which  so  many  of  the  mountains,  cliffs,  and 
pinnacles  of  the  Alps  have  been  formed  are  of  no  higher  geological 
antiquity  than  the  London  Clay  and  other  soft  Eocene  deposits  of  the 
south  of  England  and  the  north  of  France  and  Belgium.  At  a  later 
stage  of  Tertiary  time,  renewed  disturbance  led  to  the  destruction  of 
the  lakes  in  which  the  molasse  had  accumulated,  and  their  thick  sedi- 
ments were  thrust  up  into  large  broken  mountain  masses,  such  as  the 
Rigi,  Rossberg,  and  other  prominent  heights  along  the  northern  flank 
of  the  Alps.  Since  that  great  movement,  no  paroxysm  seems  to  have 
affected  the  Alpine  region  except  the  earthquakes,  which  from  time  to 
time  show  the  process  of  mountain-making  to  be  only  suspended  or  still 
slowly  in  progress. 

The  gradual  evolution  of  a  continent  during  a  long  succession  of 
geological  periods  has  been  admirably  worked  out  for  Europe  by  Suess 
and  Neumayr,  and  for  North  America  by  Dana,  Dawson,  Dutton,  Gilbert, 
Hayden,  King,  Newberry,  Powell,  and  others.  The  general  character  of 
the  structure  of  the  American  continent  is  extreme  simplicity,  as  com- 
l>ared  with  that  of  the  Old  World.  In  the  Rocky  Mountain  region,  for 
example,  while  the  Palaeozoic  formations  lie  unconformably  upon  pre- 
Cambrian  gneiss,  there  is,  according  to  King,  a  regular  conformable 
sequence  from  the  lowest  Palaeozoic  to  the  Jurassic  rocks.  During  the 
enormous  interval  of  time  represented  by  these  massive  formations,  what 
is  now  the  axis  of  the  continent  remained  undistiu*bed  save  by  a  gentle 


1078  PHYSIOGRAPHICAL  GEOLOGY  book 

and  protracted  subsidence.  In  the  great  depression  thus  produced,  all 
the  Palaeozoic  and  a  great  part  of  the  Mesozoic  rocks  were  accumulated. 
At  the  close  of  the  Jurassic  period,  the  first  great  upheavals  took  place. 
Two  lofty  ranges  of  mountains — the  Sierra  Nevada  (now  with  summits 
more  than  14,000  feet  high)  and  the  Wahsatch — 400  miles  apart^  were 
pushed  up  from  the  great  subsiding  area.  These  movements  were 
followed  by  a  prolonged  subsidence,  during  which  Cretaceous  sediments 
accumulated  over  the  Rocky  Mountain  region  to  a  depth  of  9000  feet  or 
more.  Then  came  another  vast  uplift,  whereby  the  Cretaceous  sedimente 
were  elevated  into  the  crests  of  the  mountains,  and  a  parallel  coast- 
range  was  formed  fronting  the  Pacific.  Intense  metamorphism  of  the 
Cretaceous  rocks  is  stated  to  have  taken  place.  The  Rocky  Mountains, 
with  the  elevated  table-land  from  which  they  rise,  now  permanently 
raised  above  the  sea,  were  gradually  elevated  to  their  present  height. 
Vast  lakes  existed  among  them,  in  which,  as  in  the  Tertiary  basins  of 
the  Alps,  enormous  masses  of  sediment  accumulated.  The  slopes  of  the 
land  were  clothed  with  an  abundant  vegetation,  in  which  we  may  trace 
the  ancestors  of  many  of  the  living  trees  of  North  America.  One  of  the 
most  striking  features  in  the  later  phases  of  this  history  was  the  out- 
pouring of  great  floods  of  trachyte,  basalt,  and  other  lavas  from  many 
points  and  fissures  over  a  vast  space  of  the  Rocky  Mountains  and  the 
tracts  lying  to  the  west.  In  the  Snake  River  region  alone  the  basalts 
have  a  depth  of  700  to  1000  feet,  over  an  area  300  miles  in  breadth. 

These  examples  show  that  the  elevation  of  mountains,  like  that  of 
continents,  has  been  occasional,  and  perhaps  sometimes  paroxysmal. 
Long  intervals  elapsed,  when  a  slow  subsidence  took  place,  but  at  last 
a  point  was  reached  when  the  descending  crust,  unable  any  longer  to 
withstand  the  accumulated  lateral  pressure,  was  forced  to  find  relief  by 
rising  into  mountain  ridges.  With  this  effort  the  elevatory  movements 
ceased.  They  were  followed  either  by  a  stationary  period,  or  more 
usually  by  a  renewal  of  the  gradual  depression,  until  eventually  relief 
was  again  obtained  by  upheaval,  sometimes  along  new  lines,  but  often 
on  those  which  had  previously  been  used.  The  intricate  crumpling  and 
gigantic  inversions  of  a  great  mountain-chain  naturally  suggest  that  the 
movements  which  caused  these  disturbances  of  the  strata  were  sudden 
and  violent.  And  this  inference  may  often,  if  not  generally,  be  correct. 
It  is  not  so  easy,  however,  to  demonstrate  that  a  disturbance  was  rapid 
as  to  prove  that  it  must  have  been  slow.  That  some  uplifts  resulting 
in  the  rise  of  important  mountain  ranges  have  been  almost  insensibly 
brought  about,  can  be  shown  from  the  operation  of  rivers  in  the  regions 
aflbcted.  Thus  the  rise  of  the  Uinta  Mountains  has  been  so  quiet,  that 
the  Green  River,  which  flowed  across  the  site  of  the  range,  has  not  been 
deflected,  but  has  actually  been  able  to  deepen  its  canon  as  fast  as  the 
mountains    have  been    pushed    upward.^      The    Pliocene   accumulations 

'  Powell's  "  Geology  of  the  Uinta  Mountains,"  in  the  Reports  of  U.S.  Gfograpkual  and 
(rcoloijiad  Survey^  Rocky  Mountain  Region,  1876.  The  same  conclusion  is  drawn  by 
Gilbert  from  the  structure  of  the  Wahsatch  Mountains.     See  his  admirable  essay  on  "  Land 


VII  TERRESTRIAL  FEATURES  DUE  TO  DENUDATION        1079 

along  the  southern  flanks  of  the  Himalayas  show  that  the  rivers  still  run 
in  the  same  lines  as  they  occupied  before  the  last  gigantic  upheaval  of  the 
chain  (p.  1021).^  A  similar  conclusion  has  been  drawn  from  the  river- 
valleys  in  the  Elburz  Mountains,  Persia.- 

2.  Terrestrial  Features  due  to  Volcanic  Action. — The  two 
types  of  volcanic  eruptions  described  in  Book  III.  Part  L  give  rise  to  two 
very  distinct  types  of  scenery.  The  ordinary  volcanic  vent  leads  to  the 
piling  up  of  a  conical  mass  of  erupted  materials  round  the  orifice.  In  its 
simplest  form,  the  cone  is  of  small  size,  and  has  been  formed  by  the 
discharges  from  a  single  funnel,  like  many  of  the  tuff*  and  cinder-cones  of 
Auvergne,  the  Eifel,  and  the  Bay  of  Naples.  Every  degree  of  divergence 
from  this  simplicity  may  be  traced,  however,  till  we  reach  a  colossal 
mountain  like  Etna,  wherein,  though  the  conical  form  is  still  retained, 
eruptions  have  proceeded  from  so  many  lateral  vents  that  the  main  cone 
is  loaded  with  minor  volcanic  hills.  Denudation  as  well  as  explosion 
comes  into  play ;  deep  and  wide  valleys,  worn  down  the  slopes,  serve  as 
channels  for  successive  floods  of  lava  or  of  water  and  volcanic  mud.  On 
the  other  hand,  the  type  of  fissure-eruption  in  which  the  lava,  instead  of 
issuing  from  a  central  vent,  has  flowed  out  from  minor  vents  along  the 
lines  of  many  parallel  or  connected  fissures,  leads  to  the  formation  of 
wide  lava-plains  composed  of  successive  level  sheets  of  lava.  By  subse- 
quent denudation,  these  plains  are  trenched  by  valleys,  and,  along  their 
margin,  are  cut  into  escarpments  with  isolated  blocks  or  outliers.  Thus 
they  become  great  plateaux  or  table -lands  like  those  of  north-west 
Europe,  the  Deccan  and  Abyssinia  (pp.  258,  592). 

The  forms  assumed  by  volcanic  masses  of  older  Tertiary  and  still 
earlier  geological  date  are  in  the  main  due  not  to  their  original  contoiuis, 
but  to  denudation.  The  rocks,  being  commonly  harder  than  those 
among  which  they  lie,  stand  out  prominently,  and  often,  in  course  oi 
time  and  in  virtue  of  their  mode  of  weathering,  assume  a  conical  form, 
which,  however,  has  obviously  no  relation  to  that  of  the  original  volcano. 
Eminences  formed  after  the  type  of  the  Henry  Mountains  (p.  571)  owe 
their  dome -shape  to  the  subterranean  effusion  of  erupted  lava,  but  the 
superficial  irregularities  of  contour  in  the  domes  must  be  ascribed  to 
denudation. 

3.  Terrestrial  Features  due  to  Denudation.  —  The  general 
results  of  denudation  have  been  discussed  in  Book  III.  Part  H.  Sect.  ii. 
Every  portion  of  the  land,  as  soon  as  it  rises  above  the  sea -level,  is 
attacked  by  denuding  agents.  Hence  the  older  a  terrestrial  surface,  the 
more  may  it  be  expected  to  show  the  results  of  the  operation  of  these 
agents.  We  have  already  seen  how  comparatively  rapid  are  the  pro- 
cesses of  subaerial  waste  (p.  465).  It  is  accordingly  evident  that  the 
present  contours  of  the  land  cannot  be  expected  to  reveal  any  trace 
whatever  of  the  early  terrestrial  surfaces  of  the  globe.     The  most  recent 

Sculpture,"  in  his  ''Geology  of  the  Henry  Mountains,"  published  in  the  same  series  of 
Reports,  1877. 

^  Medlicott  and  Blanford,  'Geology  of  India,'  p.  670. 

2  E.  Tietze,  Jnhrb,  Oeol,  Reichmnat,  xxviii.  (1878)  p.  581. 


1080  PHYSIOGRAPHIC AL  GEOLOGY  book 


mountain  chains  and  volcanoes  may,  indeed,  retain  more  or  less  markedly 
their  original  superficial  outlines ;  but  these  must  be  more  and  more 
effaced  in  proportion  to  their  geological  antiquity. 

The  fundamental  law  in  the  erosion  of  the  terrestrial  surfaces  is  that 
harder  rocks  resist  decay  more,  while  softer  rocks  resist  it  less.  The 
former  consequently  are  left  projecting,  while  the  latter  are  worn  down. 
The  terms  "  hard  "  and  "  soft "  are  used  here  in  the  sense  of  being  less 
easily  and  more  easily  abraded,  though  every  rock  suflfers  in  some 
measure.  If,  therefore,  a  perfectly  level  surface,  composed  of  ix)cks 
exceedingly  unequal  in  power  of  resistance,  were  to  be  raised  above  the 
sea,  and  to  be  exposed  to  the  action  of  weathering,  it  would  eventually 
be  carved  into  a  system  of  ridges  and  valleys.  The  eminences  would  be 
mainly  determined  by  the  position  of  the  harder  rocks,  the  depressions 
by  the  site  of  the  softer.  Every  region  of  Mesozoic  or  Palaeozoic  rocks 
affords  ample  illustration  of  this  result.  The  hills  and  prominent  ridges 
are  found  to  be  where  they  are,  not  so  much  because  they  have  there 
been  more  upheaved,  but  because  they  are  composed  of  more  durable 
materials,  or  because,  by  the  disposition  of  the  original  drainage-lines, 
they  have  been  less  eroded  than  the  valleys. 

In  this  marvellous  process  of  land-sculpture,  we  have  to  consider,  on 
the  one  hand,  the  agents  and  combinations  of  agents  which  are  at  work, 
and  on  the  other,  the  varying  powera  of  resistance  arising  from  declivity, 
composition,  and  structure  of  the  materials  on  which  these  agents  act. 
The  forces  or  conditions  required  in  denudation  —  air,  aridity,  rain, 
springs,  frost,  rivers,  glaciers,  the  sea,  plant  and  animal  life — have  been 
described  in  Book  III.  Part  II.  Every  country  and  climate  must 
o])viouslv  have  its  own  combination  of  erosive  activities.  The  decjiv  of 
the  surface  in  Egypt  or  Arizona  arises  from  a  different  group  of  forces 
from  that  which  can  be  seen  in  the  west  of  Europe  or  in  New  England. 

In  tracing  the  sculpture  of  the  land,  we  are  soon  led  to  perceive  the 
powerful  influence  of  the  angle  of  slope  of  the  ground  upon  the 
rate  of  erosion.  This  rate  decreases  as  the  angle  lessens,  till  on  level 
plains  it  reaches  its  minimum.  Other  things  being  equal,  a  steep  mountain 
ridge  will  be  more  deej)ly  eroded  than  one  of  the  same  elevation  whicli 
rises  gradually  out  of  the  plains.  Hence  the  declivity  of  the  ground,  at 
its  first  elevation  into  land,  must  have  had  an  important  bearing  upon 
the  subsequent  erosion  of  the  slopes.  It  is  important  to  observe  that  the 
depressions  into  which  the  first  rain  gathered  on  the  surface  of  the  newly 
upraised  land  would,  in  most  cases,  become  the  pemianent  lines  of  drainage. 
They  would  be  continually  deepened  as  the  water  coursed  in  them,  so  that, 
unless  where  subterranean  disturbance  c^me  into  play,  or  where  the 
channels  were  obstructed  by  landslips,  volcanic  ejections,  or  otherwise, 
the  streams  would  be  unable  to  quit  the  channels  they  had  once  chosen. 
The  permanence  of  drain  age -lines  is  one  of  the  most  remarkable 
features  in  the  geological  history  of  the  continents.  The  main  valleys 
of  a  country  are  usually  among  the  oldest  p<'irts  of  its  topography.  As 
they  are  widened  and  deepened,  the  ground  between  them  may  be  left 
])rojecting  into  high  ridges  and  even  into  prominent  isolated  hills. 


vn    INFLUENCE  OF  GEOLOGICAL  STRUCTURE  ON  SCENERY    lOfil 


A  chief  element  in  the  progresa  of  land  sculpture  ib  geological 
Htructure — the  character,  arrangement,  and  composition  of  the  rocks, 
and  the  manner  m  which  each  vanety  yields  to  the  attacks  of  the  de- 
nuding agents  Besides  the  general  relations  of  tlic  so-called  hard  rocks 
to  resulting  prominences,  and  of  soft  rocks  to  depressions,  the  bnuuler 


geotectonic  characters  have  had  a  <]ominaiit  influence  upon  the  evolution 
of  terrestrial  contours  As  illustrations  of  this  influence,  reference  may 
be  made  to  the  marked  difference  between  the  scenery  of  districts  com- 
posed of  stratified  sedimentary  rocks,  and  that  of  areas  of  massive 
eruptive  rocks,  such  as  granite.     In  the  former  case,  I>eddinjr  and  joints 


1082  PHYSIOGRAPHICAL  GEOLOGY  book 

furnish  divisional  lines,  the  guiding  influence  of  which  upon  the  external 
forms  of  the  mountains  is  everywhere  traceable.  In  the  case  of  eruptive 
masses,  the  rock  is  split  open  along  joints  only,  which  mainly  determine 
the  shapes  of  crest,  cliff,  and  corry. 

Bedding  produces  a  distinct  type  of  scenery  which  can  be  traced 
from  the  sides  of  a  mere  brook  up  into  tall  sea-clififs  or  into  lofty 
mountain  -  groups.  Moreover,  much  of  the  ultimate  character  of  the 
scenery  depends  upon  whether  the  strata  have  been  left  undisturbed ; 
for  the  position  of  the  bedding,  whether  flat,  inclined,  vertical,  or 
contorted,  largely  determines  the  nature  of  the  surfoce.  The  most 
characteristic  scenery  formed  by  stratified  rocks  is  undoubtedly  where 
the  bedding  is  horizontal,  or  nearly  so,  and  the  strata  are  massive.  A 
mount^iin  constructed  of  such  materials  appears  as  a  colossal  pyramid, 
the  level  bars  of  stratification  looking  like  gigantic  courses  of  masonry. 
Joints  and  faults  traversing  the  bedding  allow  it  to  be  cleft  into  blocks 
and  deep  chasms  that  heighten  the  resemblance  to  ruined  architecture. 
Probably  the  most  marvellous  illustrations  of  these  results  are  to  be  found 
in  the  Western  Territories  of  the  United  States.  The  vast  table-lands  of 
the  River  Colorado,  in  particular,  offer  a  singularly  impressive  picture  of 
the  effects  of  mere  subaerial  erosion  on  undisturbed  and  nearly  level 
strata  (see  Frontispiece).  Systems  of  stream-coiuises  and  valleys,  river 
gorges,  unexampled  elsewhere  in  the  world  for  depth  and  length,  vast 
winding  lines  of  escarpment,  like  ranges  of  sea -cliffs,  terraced  slopes 
rising  from  plateau  to  plateau,  huge  buttresses  and  solitary  stacks 
standing  like  islands  out  of  the  plains,  great  mountain  masses  towering 
into  picturesque  peaks  and  pinnacles,  cleft  by  innumerable  gullies,  yet 
everywhere  marked  by  the  parallel  bars  of  the  horizontal  strata  out 
of  which  they  have  been  carved  —  these  are  the  orderly  symmetrical 
characteristics  of  a  country  where  the  scenery  is  due  entirely  to  the 
action  of  subaerial  agents  and  the  varying  resistance  of  level  or  little 
disturbed  stratified  rocks. 

On  the  other  hand,  where  stratified  rocks  have  been  subjected  to 
plications  and  fractures,  their  characteristic  features  may  be  gradually 
almost  lost  among  £hose  of  the  crystalline  masses  which  under  these 
circumstances  are  so  often  found  to  have  been  forced  through  them  (see 
Fig.  252).  The  Alps  may  be  cited  as  a  well-known  example  of  this  kind 
of  scenery.  The  whole  geological  aspect  of  these  mountains  is  suggestive 
of  former  intense  commotion.  Yet  on  every  side  are  to  be  seen  proofs  of 
the  most  enormous  denudation.  Twisted  and  crumpled,  the  solid  sheets 
of  limestone  may  be  seen  as  it  were  to  writhe  from  the  base  to  the  summit 
of  a  mountain,  yet  they  present  everywhere  their  truncated  ends  to  the 
air,  and  from  these  ends  it  is  easy  to  see  that  a  vast  amount  of  material 
has  been  worn  away.  Apart  altogether  from  what  may  have  been  the 
shape  of  the  ground  immediately  after  the  upheaval  of  the  chain,  there 
is  evidence  on  every  side  of  gigantic  denudation.  The  subaerial  forces 
that  have  been  at  work  upon  the  Alpine  surface  ever  since  it  first  appeared 
have  dug  out  the  valleys,  sometimes  acting  in  original  depressions,  some- 
times eroding  hollows  down  the  slopes.     Moreover  they  have  planed  down 


VII    INFLUENCE  OF  GEOLOGICAL  fiTRUCTURE  ON  SCENERY   1083 

the  flexures,  excavated  lake-baains,  scjirpt?<l  tiie  mountain  sides  into  did' 
and  nrjuc,  notched  and  furrowed  the  ridges,  splintered  the  create  into 
chasm  and  atgttiile,  until  no  part  of  the  original  aurfaeo  now  remaina  in 
sight  And  thus  the  Alps  remain  a  marvolloiis  monument  of  stupendous 
earth-throes,  followed  by  prolonged  and  gigantic  denudation. 


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In  massive  rocks,  the  structure -lines  are  those  of  joints  alone,  and 
according  to  the  direction  of  the  intersecting  jointe  the  trend  nnd  shape 
of  the  ridges  are  determined.  The  importance  of  Toek-joinla,  not  only  in 
details  of  acencry,  but  even  in  some  of  the  main  featuros  «f  the  mniintain 
outlines  of  massive  rocks,  is  hardly  at  first  credible.     It  is  along  these 


PHYSIOQRAPHICAL  GEOLOGY 


VII  MOUNTAINS^  HILLS,   TABLE-LANDS  1085 

divisional  lines  that  the  rain  has  filtered,  and  the  springs  have  risen,  and 
the  frost  wedges  have  been  driven.  On  the  bare  scarps  of  a  high  mountain, 
where  the  inner  structure  of  the  mass  is  laid  open,  the  system  of  joints  is 
seen  to  have  determined  the  lines  of  crest,  the  vertical  walls  of  cliff  and 
precipice,  the  forms  of  buttress  and  recess,  the  position  of  cleft  and  chasm, 
the  outline  of  spire  and  pinnacle.  On  the  lower  slopes,  even  under  the 
tapestry  of  verdure  which  nature  delights  to  hang  where  she  can  over  her 
naked  rocks,  we  may  detect  the  same  pervading  influence  of  the  joints 
upon  the  forms  assumed  by  ravines  and  crags.  Each  kind  of  eruptive 
rock  has  its  own  system  of  joints,  and  these  in  large  measure  determine 
its  characteristic  type  of  scenery. 

A  few  of  the  more  important  features  of  the  land  may  be  briefly 
noticed  here  in  their  relation  to  this  branch  of  geology.  In  the  physio- 
graphy of  any  region,  mountains  are  the  dominant  features  (p.  40). 
A  true  mountain-chain  consists  of  rocks  that  have  been  crumpled  and 
pushed  up  in  the  manner  already  described.  But  ranges  of  hills,  almost 
mountainous  in  their  bulk,  may  be  formed  by  the  gradual  erosion  of 
valleys  out  of  a  mass  of  original  high  ground.  In  this  way,  some  ancient 
table-lands  have  been  so  channelled  that  they  now  consist  of  massive 
rugged  hills,  either  isolated  or  connected  along  the  flanks.  Eminences 
detached  by  erosion  from  the  masses  of  rock  whereof  they  once  formed 
a  part,  have  been  tenned  hills  of  drcumdenudaiian.  Their  isolation  may 
either  be  due  to  the  action  of  streams  working  round  them,  apart  alto- 
gether from  geological  structure,  or  to  their  more  resisting  constitution, 
which  has  enabled  them  to  remain  prominent  during  the  general  degrada- 
tion of  the  whole  surface. 

Table-lands  (p.  43)  may  sometimes  arise  from  the  abrasion  of  hard 
rocks  and  the  production  of  a  level  plain  by  the  action  of  the  sea,  or 
rather  of  that  action  combined  with  the  previous  degradation  of  the  land 
by  subaerial  waste  (p.  470).     Such  a  form  of  surface  may  be  termed  a 
table-land  of  eiosion.     Notable  examples  are  to  be  seen  in  the  extensive 
"fjelds"  or  elevated  plateaux  of   Scandinavia,  many  of  which,   rising 
above  the  snow-line,  form  the  gathering-ground  of  glaciers  that  descend 
almost  to  the  sea -level.     Fragments  of  a  similar  table -land   may   be 
recognised  among  the  Grampian  Mountains  of  Scotland.     But  most  of 
the  great  table-lands  of  the  globe  seem  to  be  platforms  of  little  disturbed 
strata,  either  sedimentary  or  volcanic,  which  have  been  upraised  bodily 
to  a  considerable  elevation.     These  may  be  termed  tahk-lands  of  deposit. 
But,   whatsoever  its  mode  of  origin,  the  plateau  undergoes  a  gradual 
transformation  under  continued  denudation.     No  sooner  are  the  rocks 
raised  above  the  sea,  than  they  are  attacked    by  ninning  water,  and 
begin  to  be  hollowed  out  into  systems  of  valleys.     As  the  valleys  sink, 
the   platforms  between   them    grow    into    narrower   and    more    definite 
ridges,   until  eventually  the  level  table-land  is  converted  into  a  com- 
plicated  network   of   hills  and  valleys,   wherein,   nevertheless,  the  key 
to  the  whole  arrangement  is  furnished  by  a  knowledge  of  the  disposition 
and  effects  of  the  flow  of  water.     The  examples  of  this  process  brought 
to  light  in  Colorado,  Wyoming,  Nevada,  and  the  other  Western  Terri- 


1086  PHYSIOGRAFHICAL  GEOLOGY  book 

tories,  by  Newberry,  King,  Hayden,  Powell,  Gilbert,  Dutton,  and  other 
explorers,  are  among  the  most  striking  moniiments  of  geological  opera- 
tions in  the  world.  The  erosion  of  the  ancient  table-lands  of  Scandinavia 
and  Scotland,  and  their  conversion  into  systems  of  hilly  ridges  and 
valleys,  convey  less  impressive  but  still  instructive  evidence  of  the 
efficacy  of  subaerial  waste. 

Watersheds  are  of  course  at  first  determined  by  the  form  of 
the  earliest  terrestrial  surface.  But  they  are  less  permanent  than  the 
watercourses  that  diverge  from  them.  Where  a  watershed  lies  sym- 
metrically along  the  centre  of  a  country  or  continent,  with  an  equal 
declivity  and  rainfall  on  either  side,  and  an  identity  of  geological 
structure,  it  will  be  permanent,  because  the  erosion  on  each  slope  pro- 
ceeds at  the  same  rate.  But  such  a  combination  of  circumstances  can 
happen  rarely,  save  on  a  small  and  local  scale.  As  a  rule,  watersheds 
lie  on  one  side  of  the  centre  of  a  country  or  continent,  and  the  declivity 
is  steeper  on  the  side  nearest  the  sea.  Hence,  apart  from  any  influence 
from  difference  of  geological  structure,  the  tendency  of  erosion,  by 
wearing  the  steep  slope  more  than  the  gentle  one,  is  to  carry  the 
watershed  backward  nearer  to  the  true  centre  of  the  region,  especially 
at  the  heads  of  valleys.  Of  course  this  is  an  extremely  slow  process ; 
but  it  must  be  admitted  to  be  one  of  real  efficacy  in  the  vast  periods 
(luring  which  denudation  has  continued.  Excellent  illustrations  of 
its  progress,  as  well  as  of  many  other  features  of  land-sculpture,  may 
often  be  instructively  studied  on  clay-banks  exposed  to  the  influence 
of  rain.^ 

The  crests  of  mountains  are  watersheds  of  the  sharpest  type,  where 
erosion  has  worked  backward  upon  a  steep  slope  on  either  side.  Their 
forms  are  mainly  dependent  upon  structure,  and  especially  upon  systems 
of  joints.  It  will  often  be  observed  that  the  general  trend  of  a  crest 
coincides  with  that  of  one  set  of  joints,  and  that  the  bastions,  recesses, 
and  peaks  have  been  determined  by  the  intersection  of  another  set.  If 
the  rock  is  uniform  in  structure,  and  the  declivity  equal  in  angle  on 
either  side,  a  crest  may  retain  its  position ;  but  as  one  side  is  usually 
considerably  steeper  than  the  other,  the  crest  advances  at  the  expense  of 
the  top  of  the  gentler  declivity.  But,  under  any  circumstances,  it  is 
continually  lowered  in  level,  for  it  may  be  regarded  as  the  part  of  a 
mountain  where  the  rate  of  subaerial  denudation  reaches  a  maximum. 
An  ordinary  cliff  is  attacked  only  in  front,  but  a  crest  has  two  fronts, 
and  is  further  splintered  along  its  summit.  Nowhere  can  the  guiding 
influence  of  geological  structure  be  more  conspicuously  seen  than  in  the 
array  of  spires,  buttresses,  gullies,  and  other  striking  outlines  which  a 
mountain  crest  assumes. 

Valleys  are  mainly  due  to  erosion,  guided  either  by  original  de- 
pressions of  the  ground,  or  by  geological   structure,  or  by  both.-     Their 

^  See  on  this  subject  Mr.  Gilbert's  suggestive  remarks  in  the  Essay  on  *  Land  Sculpture  * 
already  cited  (p.  934).  See  also  Nature^  xxix.  (1884)  p.  325,  where  the  history  of  the 
watersheds  of  the  British  Isles  is  traced. 

-  The  student  should  read  the  suggestive  essay  by  the  late  J.  B.  Jukes  {Qtiart,  Joum. 


vu  ORIGIN  OF  LAKES  1087 

contours  depend  partly  on  the  structure  and  composition  of  the  rocks, 
and  partly  on  the  relative  potency  of  the  different  denuding  agents. 
Where  the  influence  of  air,  rain,  frost  and  general  subaerial  weathering 
has  beeb  slight,  and  the  streams,  supplied  from  distant  sources,  have 
had  suflScient  declivity,  deep,  narrow,  precipitous  ravines  or  gorges  have 
been  excavated.  The  canons  of  the  Colorado  are  a  magnificent  example 
of  this  result  (Fig.  471).  Where,  on  the  other  hand,  ordinary  atmo- 
spheric action  has  been  more  rapid,  the  sides  of  the  river  channels  have 
been  attacked,  and  open  sloping  glens  and  valleys  have  been  hollowed 
out.  A  gorge  or  defile  is  usually  due  to  the  action  of  a  waterfall,  which, 
beginning  with  some  abrupt  declivity  or  precipice  in  the  course  of  the 
river  when  it  first  commenced  to  flow,  or  caused  by  some  hard  rock 
crossing  the  channel,  has  eaten  its  way  backward,  as  already  explained 
(p.  388). 

A  pass  is  a  portion  of  a  watershed  which  has  been  cut  down  by 
the  erosion  of  two  valleys,  the  heads  of  which  adjoin  on  opposite  sides 
of  a  ridge.  Each  valley  is  cut  backward  until  the  intervening  ridge  is 
demolished.  Most  passes  no  doubt  lie  in  original  but  subsequently 
deepened  depressions  between  adjoining  mountains.  The  continued 
degradation  of  a  crest  may  obviously  give  rise  to  a  pass. 

Lakes  may  have  been  formed  in  several  ways.  1.  By  subterranean 
movements,  as,  for  example,  in  mountain  -  making  and  in  volcanic 
explosions.  The  subsidence  of  the  central  part  of  a  mountain  system 
might  conceivably  depress  the  heads  of  the  valleys  below  the  level 
of  portions  farther  from  the  sources  of  the  stream.  Or  the  elevation 
of  the  lower  parts  of  the  valleys  might  cause  an  accumulation  of  water 
in  their  upper  parts.  Or  each  lake-basin  might  be  supposed  to  be  due  to 
a  special  subsidence.  But  these  hollows,  unless  continually  deepened  by 
subsequent  movements  of  a  similar  nature,  would  be  filled  up  by  the 
sediment  continually  washed  into  them  from  the  adjoining  slopes.  The 
numerous  lakes  in  such  a  mountain  system  as  the  Alps  cannot  be  due 
merely  to  subterranean  movements,  unless  we  suppose  the  upheaval  of 
the  mountains  to  have*  been  quite  recent,  or  that  subsidence  must  take 
place  continuously  or  periodically  below  each  independent  basin.  But 
there  is  evidence  that  the  Alpine  uplift  is  not  of  such  recent  date,  while 
the  idea  of  perpetuating  lakes  by  continued  local  subsidence  would  demand, 
not  in  the  Alps  merely,  but  all  over  the  northern  hemisphere,  where 
lakes  are  so  abundant,  an  amount  of  subterranean  movement  of  which,  if 
it  really  existed,  there  would  assuredly  be  plenty  of  other  evidence. 
2.  By  irregularities  in  the  deposition  of  superficial  accumulations  prior 
to  the  elevation  of  the  land,  or,  in  the  northern  parts  of  Europe  and 
America,  during  the  disappearance  of  the  ice-sheet.  The  numerous  tarns 
and  lakes  enclosed  within  mounds  and  ridges  of  drift-clay  and  gravel  are 

examples.     3.  By  the  accumulation  of  a  barrier  across  the  channel  of 

• 

Oeol,  Soc  xviii  (1862)  p.  378,  which  was  the  first  attempt  to  work  out  the  history  of  the 
excayation  of  a  yaUey  system  in  reference  to  the  geological  history'  of  the  ground.  See 
also  Penck,  Xfues  Jahrb,  1890,  p.  165  ;  E.  Tietze,  Jahrb.  OeoL  Reichsanst.  xxxviii.  (1888) 
p.  633. 


1088  PHYSIOGKAFHICAL  GEOLOGY  book 


a  stream  and  the  cousequent  ponding  back  of  the  water.  This  may  be 
done,  for  instance,  by  a  landslip,  by  a  lava- stream,  by  the  advance  of 
a  glacier  across  a  valley,  or  by  the  throwing  up  of  a  bank  by  the  sea 
across  the  mouth  of  a  river.  4.  By  erosion.  Water  keeping  stones  in 
gyration  can  dig  out  pot-holes  in  the  bed  of  a  river,  or  on  the  sea-shore. 
Unequal  subaerial  weathering  may  cause  rocks  to  rot  much  more  deeply 
in  some  places  than  in  others,  so  that,  on  the  removal  of  the  rotted 
material,  the  surface  of  the  solid  rock  might  be  full  of  depressions.  But 
the  only  known  agent  capable  of  excavating  such  hollows  as  might  fomi 
rock-basin  lakes  is  glacier-ice  (p.  427).  It  is  a  remarkable  fact,  of  which 
the  significance  may  now  be  seen,  that  the  innumerable  lake-basins  of  the 
northern  hemisphere  lie  on  surfaces  of  intensely  ice -worn  rock.  The 
striie  can  be  seen  on  the  smoothed  rock-surfaces  slipping  into  the  water 
on  all  sides.  These  striae  were  produced  by  ice  moving  over  the  rock. 
If  the  ice  could,  as  the  striae  prove,  descend  into  the  rock-basins  and 
mount  up  the  farther  side,  smoothing  and  stria  ting  the  rock  as  it  went, 
it  could,  to  a  certain  depth  at  least,  erode  basins. 

In  the  general  subaerial  denudation  of  a  country,  innumerable  minor 
features  are  worked  out  as  the  structure  of  the  rocks  controls  the  opera- 
tions of  the  eroding  agents.  Thus,  among  undisturbed  or  gently  inclined 
strata,  a  hard  bed  resting  upon  others  of  a  softer  kind  is  apt  to  form 
along  its  outcrop  a  line  of  cliff  or  escarpment.  Though  a  long  range  of 
such  cliffs  resembles  a  coast  that  has  been  worn  by  the  sea,  it  may  be 
entirely  due  to  mere  atmospheric  waste.  Again,  the  more  resisting 
portions  of  a  rock  may  be  seen  projecting  as  crags  or  knolls.  An 
igneous  mass  will  stand  out  as  a  bold  hill  from  amidst  the  more  decom- 
{)osable  strata  through  which  it  has  risen.  These  features,  often  so 
marked  on  the  lower  grounds,  attain  their  most  conspicuous  develoj)- 
ment  among  the  higher  and  barer  parts  of  the  mountains,  where 
subaerial  disintegration  is  most  rapid.  The  torrents  tear  out  deep 
gullies  from  the  sides  of  the  declivities.  Corries  or  cirques,  if  not 
originally  scooped  out  by  converging  streamlets  (their  mode  of  formation 
is  a  somewhat  difficult  problem),  are  at  least  enlarged  by  this  action,  and 
their  naked  precipices  are  kept  bare  and  steep  by  the  wedging  off  of 
successive  slices  of  rock  along  lines  of  joint.  Harder  bands  of  rock 
project  as  massive  ribs  down  the  slopes,  shoot  up  into  prominent  peaks, 
or,  with  the  combined  influence  of  joints  and  faults,  give  to  the  summits 
the  notched  saw-like  outlines  they  so  often  present. 

The  materials  worn  from  the  surface  of  the  higher  are  spreiid  out 
over  the  lower  grounds.  We  have  already  traced  how  streams  at  once 
begin  to  drop  their  freight  of  sediment  when,  by  the  lessening  of  their 
declivity,  their  carrying  power  is  diminished  (p.  393).  The  great 
plains  of  the  earth's  surface  are  due  to  this  deposit  of  gravel,  sand,  and 
loam.  They  are  thus  monuments  at  once  of  the  destructive  and 
reproductive  {)rocesses  which  have  been  in  progress  unceasingly  since  the 
first  land  rose  above  the  sea  and  the  first  shower  of  rain  fell.  Every 
p(?bble  and  particle  of  the  soil  of  the  plains,  once  a  portion  of  the  distant 
mountains,  has  travelled  slowly  and  fitfully  downward.     Again  and  again 


vn  TERRESTRIAL  PLAINS  1089 

have  these  materials  been  shifted,  ever  moving  seaward.  For  centuries, 
perhaps,  they  have  taken  their  share  in  the  fertility  of  the  plains  and 
have  ministered  to  the  nurture  of  flower  and  tree,  of  the  bird  of  the  air, 
the  beast  of  the  field,  and  of  man  himself.  But  their  destiny  is  still  the 
great  ocean.  In  that  bourne  alone  can  they  find  undisturbed  repose,  and 
there,  slowly  accumulating  in  massive  beds,  they  will  remain  until,  in 
the  course  of  ages,  renewed  upheaval  shall  raise  them  into  future  land, 
and  thereby  enable  them  once  more  to  pass  through  a  similar  cycle  of 
change. 


4  A 


LIST  OF  AUTHOES  QUOTED  OR  EEFERRED  TO. 


Abbadib,  a.  o',  271 

Abbot,  M.  L.,  373,  383,  384,  399,  462 

Abich,  145,  235,  239,  328,  411 

Adams,  A.  Leith,  649 

Adhemar,  J.,  21 

Adie,  A.  J.,  299 

Agassiz,  A.,  33,  285,  404,  438,  442,  452, 

457,  485,  486,  490,  491,  492,  493,  666, 

668,  925 
Agassiz,  L.,  417,  798 
Airy,  G.  B.,  46,  437 
Aitken,  J.,  340 
Alberti,  F.  von,  858 
Allen,  J.  A.,  410 
Allen,  O.  D.,  411 
Allport,  S.,  162,  170,  176,  597,  606,  606, 

631,  710 
Ameghino,  F.,  1067 
Anderson,  J.,  798 
Anderson,  T.,  202 
Anderssen,  N.  J..  336 
Andrews,  T.,  68,  171 
Angelin,  N.  P.,  782,  766 
Angell,  A.,  379 

o 

Angstrom,  A.  J.,  11 
Ansted,  D.,  399,  402 
Aoust,  T.  Virlet  d',  808 
Arago,  D.  F.  J.,  24 
Arohiac,  E.  J.  A.  d',  911,  949 
Arends,  F.,  292 
Armstrong,  G.  F.,  32 
Artigues,  H.,  288 
Ashbumer,  C.,  235 
Aughey,  S.,  473 
AveUne,  W.  T.,  846,  864 

Babbaob,  C,  284,  295,  450 

Bachmann,  I.,  430 

Backstrom,  H.,  159 

Bader,  H.,  413 

Baer,  K.  E.  von,  15,  411 

Bailie,  J.  B.,  46 

Baily,  F.,  46 

Baily,  W.  H.,  988 

Bakewell,  R.,  389 

Ball,  R.  a,  23,  29 


Ball,  v.,  253,  806 

Baltzer,  A.,  221,  370,  371,  624,  1048, 

Barrande,    J.,    664,    714,    719,    726, 

734,  740,  744,  772,  773,  778 
Barrois,  C,  165,  180,  181,  261,  287, 

567,  607,  684,  694,  714,  715,   733, 

771,  788,  836,  942,  943,  944,  946, 

950 
Barrow,  G.,  627,  708 
Barus,  C,  56,  381 
Bateman,  J.  H.,  374 
Baumert,  F.  M.,  341 
Baumgarten,  — ,  383 
Beardniore,  N.,  373,  395 
Becke,  F.,  182,  183,  185,  186 
Becker,  A.,  304 
Becker,   G.   F.,   97,    113,    169,    242, 

629,  631,  960 
Bed  well,  F.  A.,  946 
Behrendseu,  0.,  919 
Behrens,  II.,  88,  165 
Bell,  T.  H.,  866 
Belt,T.,  350,  353,  729 
Benecke,  E.  W.,   849,  850,  868,  869, 

871,  911,  918 
Bennie,  J.,  670 
Berendt,  G.,  1045,  1046 
Berger,  J.  F.,  942 
Bergeron,  J.,  734,  771,  851 
Berthier,  P.,  75,  175 
Bertrand,  E.,  851 

Bertrand,  M.,  262,  263,  541,  551,  884 
Bessel,  F.  W.,  13 
Bevan,  E.  J.,  650 
Beyrich,  E,  778,  782,  983,  991 
Bigsby,  J.  J.,  739 
BUlings,  K,  720,  735 
Binney,  E.,  824 
Bird,  J.,  93D 
Bischof,  G.,  96,  147,  152,  196,   198, 

302,  304,  310,  322,  361,  365,  378, 

412,  441,  453,  631 
Bittner,  A.,  628,  871,  872,  876 
Blake,  J.  F.,  709,  710,  897,  908,  909, 

911 
Blake,  W.  P.,  880,  836 


1075. 
731, 

356, 
770, 
947» 


314, 


870, 


836 


201, 
388, 


910, 


1092 


TEXT-BOOK  OF  GEOLOGY 


Blanford,  H.  F.,  89,  809 
Blanford,  W.  T.,  241, 259, 403, 658, 660, 661, 
680,  717,  776,  809,  854,  877,  919,  957, 
981,  1002,  1019,  1021,  1022,  1055, 1079 

Bleasdell,  W.,  416 

Bleicher,  Dr.,  949 

Boose,  H.  U.,  187,  605 

Bobierre,  — ,  478 

Bohm,  A.,  1048 

Bois,  P.  du,  404 

Boltou,  H.  C,  87,  472 

Bonney,  T.   6.,  173,  529,  580,  596,  606, 
622,  624,  698,  709,  711,  725 

Boricky,  K,  88,  166,  170 

Borrell,  L.,  370 

Bou6,  A.,  80,  1060 

Boulay,  M.,  835 

Boule,  M.,  219 

Bourne,  J.  C,  485,  492 

Boyer,  G.,  1072 

Bozzi,  L.,  838 

Brady,  H.  B..  673,  810 

Braithwaite,  F.,  872 

Braaco,  W.,  667,  916 

Braun,  — ,  67 

Braun,  D.,  915,  1059 

Braims,  R,  86 

Bravais,  A.,  287 

Breislak,  S.,  228,  230,  262 

Breiteulohner,  J.  J.,  379 

Breon,  R.,  202 

Brewster,  D.,  69,  110,  111,  810,  845 

Brezina,  A.,  10 

Briart,  A.,  868,  976 

Bristow,  H.  W.,  451,  867,  940,  970,  986 

Brodie,  P.  B.,  867,  899 

Broeck,  E.  van  den,  352,  521 

Bnipger,  W.  (\.  158,  ir»9,  179,  430,  666, 
582,  608,  621,  622,  719,  731,  732,  766, 
769 

Bronguiart,  A.,  172,  814 

Brongniart,  C,  746,  820 

Brooks,  T.  B.,  716 

Brown,  C.  B.,  396 

J.  C,  476 

R.,  515 

Th.,  829 

Browne,  G.  F.,  359 

Bruckner,  E.,  1031 

Brims,  H.,  35 

Brush,  G.  J.,  89 

Bucli,  L.  von,  167,  191,  198,  234,  241, 
321,  322,  895 

Buchan,  A.,  327,  405 

Buchanau,  J.  Y.,  34,  35,  37,  38,  68,  106, 
405,  454,  456,  458,  469,  484 

Biicking,  H.,  869 

Bucklaiid,  W.,  376 

Buck-man,  S.  S.,  897,  904 

Buddie,  J.,  504 

BuU,  G.,  239 

Bunsen,  R.  W.,  11,  234,  237,  239,  269,  341 

Biinzel,  E.,  955 

Burbauk,  L.  S.,  351 


Burton,  F.  It,  867 
Burton,  W.  K.,  213 
Bu88,  £.,  870 
Butler,  A.  G.,  886 

Cadbll,  H.  M.,  818,  625,  699 

Cailletet,  L.,  307 

Call,  R.  K,  409 

Callaway.  C,  625,  698,  709,  710,  711,  729 

Camerlander,  C.  von,  837 

Candolle,  C.  de,  496,  507 

Caralp,  J.,  734,  771 

Carlini,  F.,  46 

Carll,  J.,  235 

Carpenter,  W.  B.,  411,  434,  486,  452,  694 

Carret,  J.,  476 

Carruthers,  W.,  666,  793 

Cathrein,  A.,  134,  618 

Cautley,  P.  T.,  1021 

Cavendish,  H.  H.,  46 

Cayeux,  L.,  140,  946 

C^nne,  — ,  872,  393 

Chamberlin,  T.  C,  716,  1025,  1050,  1051, 

1052 
Chambers,  R,  287 
Champemowne,  A.,  778,  788,  784 
Chandellon,  J.  T.  P.,  383 
Chandler,  C.  F.,  89 
Chantre,  E.,  1024,  1047 
Chapman,  F.,  942 
Charbonelle,  — ,111 
Chatard,  T.  M.,  410 
Chatelier,  H.  le,  838 
Chester,  F.  D.,  169 
Choffat,  P.,  262,  1073 
Cliolsy,  A,,  330,  334,  359 
Chree,  C,  14 
Christison,  R.,  405 
Church,  J.  A.,  641 
Ciahli,  A.,  438 
Clarke,  A.  R,  13 
Clarke,  W.  B.,  776,  790,  839,  920 
Clerc,  C,  1072 
Clough,  C.  T.,  576,  625 
Coan,  T.,  205,  206,  220 
Coaz,  J.,  416 
Cochrane,  C,  300 
Cohen,  E.,  10,  114,  171,  205,  258 
Cohn,  P.,  482 

Cole,  G.  A.,  80,  89,  109,  161,  171,  747 
Colladon,  D.,  407 
Collet,  R,  512 
Comstock,  T.  B.,  236,  477 
Constant  Prevost,  — ,  241,  250 
Conwentz,  H.,  991 
Conybeare,  W.,  370,  603,  826,  970 
Cooile,  J.,  451 
Cook,  Captain,  418 
Cope,  E.  D.,  668,  846,  931,  932,  938,  959, 

969,  1002 
Coppincer,  R  W.,  364 
Coquand,  H.,  629,  949,  950,  956 
Cordier,  L.,  264 
Comeliusseu,  0.  A.,  711 


LIST  OF  A  UTHORS 


1093 


Cornet,  F.  L.,  868,  921,  976 

Cornet,  J.,  142,  494 

Cornish,  V.,  122,  484 

Coma,  A..  46 

Cornnel,  J.,  949,  950 

Cossmann,  M.,  913 

Cotta,  B.  von,  96,  631 

Cotteau,  G.,  911 

Cottlier,  — ,  340 

Cox,  S.  H.,  840 

Credner,  Heinrich,  915,  953 

Credner,  Hermann,  272,  578,  631,  687,  714, 

846,  1031,  1045 
Crid,  L.,  966 
CroU,  J.,  16,  17,  20,  24,  29,  58,  283,  418, 

436,  441,  460,  1024,  1035 
Cromarty,  Earl  of,  480 
Crosby,  W.  0.,  623,  527,  1053 
Cross,  C.  F.,  660 

Cross,  Whitman,  101,  112,  168,  982 
Cunningham,  R  Hay,  625,  698 
Cashing,  H.  P.,  417,  420 
Cuttell,  F.  G.,  90 
Cuvier,  F.,  377 
Czemy,  F.,  327,  329 

Dahll,  T.,  711,  769 

Dakyns,  J.  R.,  105,  288,  564,  567 

Dalimier,  P.,  770 

Dalmar,  K.,  606 

Dames,  W.,  737,  893,  917,  1019 

Damon,  R.,  910 

Dana,  £.  S.,  205,  413 

Dana,  J.  D.,  56,  125,  140,  191,  199,  205, 
207,  217,  220,  223,  226,  227,  230,  246, 
256,  264,  290,  320,  396,  485,  486,  570, 
572,  596,  628,  738,  1050,  1053,  1070 

Darwin,  C,  191,  201,  234,  253,  255,  276, 
288,  290,  352,  473,  485,  486,  490,  547, 
630,  660,  661,  665,  666,  1070 

Darwin,  G.  H.,  9,  18,  21,  23,  48,  66,  57, 
68,  271,  293,  607 

Dathe,  E.,  170,  173,  183,  186 

Daubeny,  C,  191,  239 

Daubr^e,  A.,  10,  68,  89,  132,  211,  240, 
266,  298,  301,  305,  306,  307,  308,  309, 
310,  314,  318,  323,  337,  356,366,377, 
385,  427,  499,  523,  527,  631 

Dausse,  — ,  379,  395,  408 

David,  T.  W.  K,  839,  1042 

Davidson,  T.,  743,  756,  865,  897,  947 

Davies,  D.  C,  141,  846 

DaviB,  J.  W.,  824,  867 

Davison,  C,  279,  460,  474 

Dawkiiis,  W.  B.,  366,  867,  963,  968,  985, 
997,  1036,  1055,  1065 

Dawson,  G.  M.,  960,  10,50,  1052 

Dawson,  J.  W.,  521,  649,  694,  793,  803, 
821,  1050,  1053,  1054,  1077 

Day,  H.,  899 

Debray,  H.,  480 

Dechen,  U.  von,  244,  836,  849,  922,  954 

De  la  Beche,  H.  T.,  9,  43,  250,  273,  286, 
287,  290,  317,  370,  399,  427,  602,  604, 


511,  515,  516,  518,  619,.  526,  546,  560, 

666,  569,  605,  636,  637,  638,  798,  826, 

827,  846,  866 
Delafond,  F.,  851 
Delaire,  A.,  374 
Delaunay,  C,  54 
Delesse,  A.,  298,  301,  302,  304,  305,  306, 

438,  631 
Delfortrie,  R,  288,  292 
Delgado,  J.  F.  N.,  771 
Denisou,  W.  T.,  649 
Dep^ret,  C,  1016 
De  Ranee,  C.  E.,  941 
Descloiseaux,  A.,  237 
Deshayes,  G.  P.,  962 
Desmarest,  M.,  300 
Desor,  R,  336,  359 

Deville,  C.  Sainte-Claire,  195,  225,  301,  310 
Dewalque,  G.,  733 
Dewar,  J.,  12 
Dick,  A.  B.,  93,  129 
Dick  Lauder,  T.,  372,  381 
Dieffenbach,  F.,  271 
Dieulafait,  L.,  458,  868 
Diller,  J.  S.,  173,  328,  583 
Dittmar,  A.  von,  869 
Dittmar,  W..  36,  37,  38 
Dixon,  F.,  974 

Doelter,  C,  252,  302,  303,  322 
Dolfuss,  A.,  260 
Dollfus,  G.,  977,  989 
Dollo,  L.,  930 
Dolomieu,  D.  de,  301 
Domeyko,  — ,  234 
Doss,  B.,  165 
Douglas,  ,1.  N.,  457 
Douville,  H.,  911,  928 
Downes,  W.,  943 
Drasche,  R.  von,  243,  253,  629 
Drew,  F.,  394,  408 
Drygalski,  R  von,  283,  418,  419 
Dudgeon,  P.,  346 
Dufour,  C,  420 
Dulk,  L.,  15 
Duraont,  A.,  179,  619,  620,  733,  769,  950, 

952,  976 
Duncan,  P.  M.,  867,  943,  983,  987 
Dunker,  R,  15 
Dunker,  W.,  953 

Dupont,  R,  58,  140,  779,  804,  835,  931 
Dupr^,  A.,  148 
Durham,  W.,  381 
Durocher,  J.,  61,  269,  631 
Dutton,  C.  R,  130,  137,  168,  204,  205,  222, 

229,  256,  258,  264,  279,  391,  394,  407, 

538,  584,  592,  1070,  1077 
Dwight,  Dr.,  628 

Ebray,  T.,  563,  949 
Eccles,  J.,  328 
Eck,  H.,  850 
Edwards,  F.  R,  974 
Egerton,  P.  de  G.  M.,  867 
Ehrenberg,  C.  G.,  68,  146,  837 


1094 


TEXT-BOOK  OF  GEOLOGY 


Eichstadt,  F.,  137,  170 

Elderhorst,  G.  W.,  89 

Eldridge,  G.  H.,  958 

jftlie  de  Beaumont,  80,  241,  308,  810,  321, 

322,  335,  336,  353,  399,  402,  403 
Ellis,  W.,  205 
Elwes,  J.  W.,  987 
Enjiel,  Th.,  915 
Erdmaim,  E.,  291,  870 
Etheridge,  R.,  730,  747,  783,  864,866,  881, 

897,  899,  907,  909,  910,  961 
Etheridge,   R.,  junr.,   737,  776,   839,   840, 

854,  878,  920,  960,  983 
Ettingsliauseii,  Baron  C.  von,  965,  966,  973, 

974,  983 
Evans,  C,  943 

Sir  J.,  19,  372,  1055 

J.  W.,  86 

Everest,  R.,  383 
Everett,  J.  D.,  50,  51 


Fabrt,  B.,  366 

Falb,  R.,  270 

Falconer,  H.,  894,  1021 

Fallot,  K,  951 

Falsan,  A.,  1024,  1047 

Favre,  A.,  317,  622,  623,  918,   954,  979, 

992,  1075 
Favre,  E.,  540,  1075 
Fayol,  H.,  377,  500,  502,  514,  808,  886 
Feistmantel,  C,  821,  837 
Feistmantel,  O.,  840,  854,  878,  924 
Ferrel,  W.,  15 
Fielden,  Captain,  17 
Filhol,  II.,  'MS,  990 
Fisclier-Benzon.  R.  von,  480 
Fisher,  0.,  20,  35,  48,  54,  56,  57,  259,  266, 

292,  293,  304,  316,  451,  523,  645,  1070, 

1076 
Fitton,  W.  H.,  939,  943 
Fleming,  J.,  791,  793 
Fletcher,  L.,  10 
Flight,  W.,  10,  ^^ 
Fdhr,  G.  F.,  1«56 
Fondouce,  C.  de,  330 
Fontaine,  W.  M.,  855,  879,  924 
Fontiinnes,  F.,  1016 
Forbes.  C,  648 

1).,  53,  56,  213,  216,  250,  304 

E.,  252,  290,  970,  986 

J.  D.,   51,  284,  417,  418,  419,  425, 

426,  428 
Jolm,  G05 


Forchliammer,  G.,  36,  336,  477,  483,  649 

Forel,  F.  A.,  404,  407,  420,  422,  507 

Forster,  W.,  631 

Forsyth,  T.  D.,  336 

Forsyth -Major,  C.  J.,  1017,  1020 

Foster,  C.  le  Neve,  352 

Foullon,  Baron  von,  623,  629 

Fouqui:',  F.,  65,  68,  71,  72,  73,  74,  75,  86, 
87,  89,  108,  109,  114,  115,  119,  120,125, 
155,   191,  194,  195,  196,  197,  201,  216, 


223,  225,  226,  242,  245,  252,  258,  279, 

801,  802,  303,  304,  810,  676 
Fowler,  J.,  346 
Fox,  H.,  783 

Fox-Strangways,  C,  897,  899,  905,  1081 
Fraas,  E,,  918,  954 
Fraas,  0.,  863,  873,  915,  1066 
Freeh,  F.,  771,  787,  788,  1075 
Fr^my,  E.,  143 
Fresenius,  C.  R.,  362 
Freshfield,  W.  D.,  431 
Friedel,  C,  301 
Fritsch,  A.,  821,  837,  846 
Fritsch.  K.  von,  252 
Friih,  J.  J.,  143,  478 
Fuchs,  C.  W.  C,  191,  270,  606 

E.,  336 

T.,  350,  478,  669,  979,  980,  992, 

999,  1019,  1020 
Fulcher,  L.  W.,  202 

Gardiner,  Miss  M.  J.,  606 

Gardner,  J.  S.,  964,  966,  970,  972,  978, 

974,  988 
Garrigou,  F.,  715 
Gaspard,  A.,  292 
Gatta,  M.,  271 
Gaudin,  C.  T.,  1004 
Gaudry,  A.,  658,  667,  668,  678,  846,  968, 

985,  997,  998,  1019 
Geer,  G.  de,  1045 
Geikie,  J.,  478,  1024,  1084,   1042,    1055, 

1061 
Geinitz,  E.,  274 

F.  E.,  1045 

H.  B.,  821,  836,  842,  848 

Gemmellaro,  G.  G.,  852 

Genth,  F.  A.,  62 

Georgi,  C.  de,  350 

Geyler,  H.  T.,  68 

Gibson,  J.,  459 

Gilbert,  Dr.,  372 

GUbert,  G.  K.,  15,  286,  330,  394,  898,  407, 

408,  409,  415,  523,  526,  538,  571,  1054, 

1077,  1078,  1086 
Girard,  T.,  292 
Giulio,  C.  J.,  46 
Gobel,  A.,  411 
Godwin- Austen,   R.  A.  C,  352,  455,  783, 

791,  836,  943 
Gooch,  F.  A.,  163,  363 
Goodchild,  J.  G.,  344,  472,  864 
Goppert,  H.  R.,  843,  991 
Gosselet,  J.,  620,  733,  769,  786,  797,  811, 

835,  911 
Grad,  C,  148,  288,  292 
Graeve,  — ,  374 

Grand'  Eury,  C,  808,  821,  886,  843 
Gray,  Asa,  1042 
Greaves,  C,  373 

Green,  A.  H.,  536,  824,  839,  855 
Green,  W.  L.,  205,  488 
Gregory,  J.  W.,  695 
Gresley,  W.  S.,  806 


LIST  OF  A  UTHORS 


1095 


Griesbach,  C.  L.,  877 

Griffith,  R.,  831 

Grodtleck,  A.  von,  165, 631 

Groger,  F.,  281 

Grothe,  Mr.,  442 

GrubenmanD,  — ,  624 

Gruner,  E.  L.,  142,  420 

Guerard,  A.,  383,  384,  402 

Guillier,  A.,  783 

Guiscanli,  G.,  201 

Gilmbel  C.  W.,  71,  122,  138,  162,  164, 
173,  182,  239,  316,  328,  482,  602,  631, 
714,  807,  868,  871,  954,  955,  956,  980, 
1048 

Gunn,  J.,  1013 

Guun,  W.,  625,  699 

Guppy,  H.  B.,  285,  332,  384,  485,  492 

Gutbier,  — ,  848 

Guthrie,  F.,308,  309 

Gutzwiller,  A.,  1076 

Haast,  J.,  288,  717,  1055 

Haeckel,  E.,  457 

Hagge,  — ,  618 

Hague,  A.,   161,  166,  167,  168,  169,  171, 

260,  736 
HalK  Sir  James,  89,  115,  297,  300,  301,  305, 

317 

J.  (Albany),  735,  776 

T.  M.,  783 

Hamilton,  W.,  213 

Hammerschmidt,  F.,  297 

Hardraan,  E.,  141,  495 

Harker,  A.,  161,  314,  316,  819,  523,  645, 

567,  584,  605,  606,  748,  750 
Harkness,  R,  321,  725,  749,  750,  762,  848 
Harper,  A.  P.,  427 
Harrison,  J.  B.,  485,  493 

J.  T.,  455 

W.  J.,  867 

Hartley,  C,  383,  401,  403 

Hartley,  W.  N.,  69,  110,  111 

Hartsoeker,  — ,  383 

Hartung,  — ,  241 

Haschert,  — ,  433 

Hatch,  F.  H.,  159,  162,  166,  182,  666,  828 

Hatcher,  J.  B.,  958 

Hauer,  Ritter,  F.   von,  871,  954,  955,  980, 

992,  999 
Haug,  — ,  917,  918 

Haughton,  S.,  58,  61,  157,  316,  441    • 
Hautefeuille,  P.,  310 
Hawes,  G.  W.,  579,  608 
Hayden.  F.   V.,   236,   355,  394,  958,  969, 

1053,  1077 
Hayes,  C.  W.,  417 
Heaphy,  C,  588 
Heath,  D.  D.,  20,  283 
Hdbert,  E.,  279,  678,  727,  733,  868,  870, 

911,  918,  942,  943,  944,  946,  946,  947 

950,  951,  952,  975,  976,  979 
Hector,  J.,  717,  777,  791,  840,  878,  920, 

961,  983,  1003,  1023,  1067 
Heddle,  M.  F.,  687 


Heer,  O.,  17,  623,  803,  888,  881,  915,  922, 

960,  974,  984,  988,  992,  995, 1001, 1002, 

1048,  1049 
Heilprin,  A.,  485,  928,  981,  1002,  1022 
Heim,  A.,  314,  317,  347,  370,  397,  417, 636, 

540,  543,  547,  550,  616,  624,  1048,  1076 
Helland,  A.,  202,  211,  222,  256,  417,  431, 

432,  1045 
Hellraann,  — ,  1018 
Helmersen,  Count  von,  49,  410 
Helniholtz,  H.  von,  418 
Hendei'son,  E.,  202 
Heunessy,  H.,  54 
Henry,  — ,  601 
Henslow,  J.  S.,  603 
Henwood,  J.  W.,  445,  681,  641 
Herdman,  W.,  477 
Herman,  D.,  116 
Herschel,  A.,  52 

J.,  13,  20,  24,  29,  39,  436 

W.,  8 

Hibbert,  S.,  244,  328 

Hicks,   H.,  141,  625,  699,  709,  710,  719, 

723,  725,  726,  728,  729,  740,  747 
Higgin,  G.,  384 
Hildebrandsson,  Prof.,  405 
Hilgard,  E.  W.,  333 
Hill,  E.,  19,  56,  711 

J.  B.,  708 

R.  T.,  958 

W.,  944,  946 

Hills,  R.  C,  982 

Hind,  H.  Y.,  416,  439,  450 

Hind,  Wheelton,  833 

Hin<le,  G.  J.,  141,  721,  722,  740,  742,  826, 

924 
Hiuxman,  L.,  625,  699 
Hirschwald,  J.,  322 

Hise,  0.  R.  van,  110,  132,  696,  715.  716 
Hitchcock,  C.  H.,  316 
Hobson,  B.,  827 
Hochstetter,  F.  von,  173,  255 
Hofer,  H.,  272,  276 
Hoff,  E,  A.  von,  191,  461 
Hoffmann,  F.,  228,  234 
Hogard,  H.,  1046 
Hogbom,  A.  G.,  288 
Holl,  H.,  711 
Holland,  W.  J.,  260 
Holmes,  T.  V.,  283,  846 
Holmes,  W.  H.,  1084 
Homan,  C.  H.,  711 
Hondaille,  F.,  352 
Hopkins,  W.,  54,  56,  314,  316,  418 
Home,  J.,   550,  606,  625,  627,  690,   699, 

705,  728,  827,  1029,  1042 
Homer,  L.,  383 
Homes,  R.,  281,  322,  667,  968 
Homes,  — ,  999 
Hosins,  A.,  922 
Howard,  T.,  383 
Howell,  H.  H.,  846 
Howorth,  H.  H.,  418,  1036,  1060 


1096 


TEXT-BOOK  OF  GEOLOGY 


Hudleston,  W.  H.,  698,  897,  904,  908 

Huggins,  W.,  11,  12 

Hughes,  T.  M*K.,  58,  709,  726,  747,  749, 

750 
Hull,  E.,  141,  191,  412,  495,  621,  669,  778, 

802,  825,  827,  832,  846,  848,  864,  897 
Humboldt,  A.  von,  24,  39,  191,  222,  242, 

274,  328,  337 
Humphreys,  A.  A.,  373,  383,  384,  399,  462 
Humpidge,  T.  S.,  60 
Hunt,  A.  R.,  438,  445,  451,  507 
Hunt,  T.  S.,  32,  36,  141,  145,  173,  175, 

310,  350,  351,  381,  412,  472,  514,  621, 

613,  622,  641,  650,  682 
Hussack,  E.,  302,  303 
Hutton,    Captain   F.    W.,   260,   717,  840, 

878,  920,  961,  1023,  1055 

James,  7,  178,  353,  384 

W.,  814 

Huxley,  T.  H.,  658,  863 
Hyatt,  A.,  663,  667 
Hyland,  J.  S.,  182 

iDDHfcs,  J.  P.,  97,  98,  101,  105,  154,  161, 
163,  166,  167,  168,  169,  260,  529 

Imhoff,  O.  E.,  407 

Inglefield,  Commander,  943 

Inostranzeff,  A.,  631 

Irvine,  R.,  442,  450,  456,  459,  482,  484, 
487,  492,  494,  495 

Irving,  A.,  848,  864 

Irving,  R.  D.,  110,  132,  631,  683,  696,  716 

Ives,  Lieut.,  391 

Jack,  R.  L.,  717,  840,  854,  920,  960,  1023 

Jaekel,  O.,  742 

James,  Sir  II.,  46 

Jamieson,  T.  F.,  295,  1042 

Janko,  J.,  403 

Jannettaz,  E.,  52,  106,  314,  629,  630,  652 

Jaussen,  J.,  196 

JefiFieys,  J.  Gwyn,  285,  945 

Jennings,  C.  V.,  747 

Johnson,  W.  H.,  332 

Johnston,  J.  F.  W.,  352 

Johnston,  R.  M. ,  983 

Johnston-Lavis,  H.  J.,  191,  195,  198,  200, 

217,  223.  213,  249,  262 
Johnstrup,  Prof.,  1031 
Jokelv,  J.,  714 

Jones',  T.  R.,  478,  649,  673,  945,  973 
Jordan,  J.  B.,  90 
Judd,  J.  W.,  110,  169,  171,  191,  199,  202, 

225,  319,   570,  865,  897,  899,  906,  939, 

947,  953,  987,  989 
Jukes,  J.  B.,  96,  526,  527,  536,  568,  570, 

631,  778,  798,  831 
Jukes  -  Browne,  A.  J.,  80,  485,  493,940, 

943,  914,  945,  946,  1042 
Julien,  A.,  265,  809,  1047 
Julien,  A.  A.,  79,  343,  347,  472,  473,  483, 

484,  495 
Jung,  — ,  ^9> 
Junghuhu,  F.,  49,  197,  200 


Kalkowsky,  E.,  184,  185,  578,  619,  714 

Kant,  E.,  8,  14 

Karpinsky,  A.,  863 

Katzer,  F.,  714,  735,  772 

Kauffmann,  K.  J.,  979,  1075 

Kayser,  E.,  179,  330,  698,  606,  610,  737, 

772,  778,  779,  783,  785,  786,  790,  917, 

1046 
Keeping,  H.,  747,  987 
Keeping,  W.,  987 
Keilhack,  K-,  202,  1046 
Keilhau,  B.  M.,  711 
Keller,  F.,  1066 
Kelvin,  Lord  (Sir  W.  Thomson),  12.  18,  23, 

49,  50,  53,  54,  55,  56,  68,  229,  283 
Kendal,  P.  F.,  122,  484 
Kenngott,  A.,  96 
Keyserling,  A.  von,  353,  359,  766,  788,  842, 

852 
Kidston,  R.,  794,  814,  819,  833 
Kinahan,  G.  H.,  335,  451,  454,  802,  831 
Kindler,  A.,  473 
King,  Clarence,  156,  160,   166,  167,  168, 

409,  413,  417,  592,  958,  959,  1070,  1078, 

1077 
King,  W.,  314,  318,  527,  596,  694,  846 
Kirchhoff,  11 

Kirkby,  J.  W.,  673,  829,  833,  846 
Kjerulf,  T.,  179,  208,  287,  512,  608,  711, 

732,  766,  769 
Klein,  D.,  86 
Klemeut,  — ,  88 
Klemra,  G.,  127 
Klockmann,  F.,  15 
Kloos,  J.  H.,  368 
Kluge,  E.,  206,  207,  277 
Kobel,  F.  von,  89 
Koch,  C,  138 

G.  A.,  381,  393,  394,  496 

K.,  620 

M.,  165 

Koeneu,  A.  von,  782,  987,  991 
Kolb,  J.,  480 

Koninck,  L.  G.  de,  776,  834,  839 
Krasnopolsky,  A.,  853 
Krenner,  J.  A.,  359 
Krischtafowitsch,  N.,  1050 
Kuhn,  J.,  120 
Kyle,  J.,  378 

Lacrois,  a.,  173 
Lacroix,  — ,  89,  108 
Ladriere,  J.,  1047 
Lagorio,  A.  E.,  262 
Laloy,  R.,  363 
Lamarck,  J.  B.  de,  665,  667 
Lamplugh,  G.  W.,  910,  938,  939,  940,  1042 
Lang,  0.,  125,  169 
Lange,  T.,  922 
Langenbeck,  R.,  290 
Lankester,  E.  Ray,  795 
Laplace,  A.  de,  8,  17,  48 
Lap]>arent,  A.  de,  39,  40,  96,  283,  311,  460, 
727,  770,  808,  911 


LIST  OF  A  UTHORS 


1097 


Lapworth,  C,  625,  678,  698,  710,  711,  728, 

729,  732,  738,  741,  747,  748,  749,  756, 

763,  764 
Lartet,  E.,  413 
Lasaiilx,  A.  von,  68,  96,  108,  165,  179,  191, 

192,  217,  229,  239,  272,  276,  837,  342 
Laube,  G.  C,  272 
Laube,  G.  L.,  871 
Laval,  — ,  336 
Lavaleye,  A.  de,  292 
Lavergne,  M.  de,  476 
Lawes,  J.  B.,  372 
Lawson,  A.  C,  616,  685,  688,   689,  715, 

716 
Layard,  A.  H.,  332 
Lebesconte,  P.,  733,  770 
Leblanc,  F.,  195 
Lebour,  G.  A.,  52,  576 
Leckenby,  J.,  939 

Leconte,  J.,  56,  256,  258,  483,  1053 
Lecoq,  H.,  21«,  241 
Ledoiix,  E.,  68 
Lees,  F.  A.,  824 
Legendre,  — ,  48 
Lehnmnn,  J.,  156,  176,  181,  182,  185,  187, 

280,  571,  616,  617,  629,  631,  688,  714 
Lehraanu,  R.,  288 
Leidy,  J.,  932,  969,  1002 
Leipoldt,  G.,  39 
Lemberg,  J.,  604 
Lendeufeld,  Dr.  von,  1055 
Lenk,  H.,  260  • 

Lepsius,  R.,  870,  1048 
Lesley,  J.  P.,  390,  716,  1051 
Lesquereux,  L.,  740,  959 
Lesseps,  F.  tie,  413 
Lewis,  H.  Carvill,  1050,  1051,  1052 
Liais,  E.,  350 
Limur,  Comte  de,  607 
Linares,  A.  Gonzalerz  de,  950 
Lindley,  J.,  814 
Lindstroni,  G.,  746,  768,  870 
Link,  H.  F.,  496 
Linnaeus,  291 

Linnarsson,  J.  G.  0.,  719,  732,  766 
Liveing,  Prof.,  12 
Livingstone,  D.,  329,  382 
Lobley,  J.  L.,  195 
Lock,  W.  G.,  256 
Lockyer,  J.  N.,  9,  11,  12 
Logan,  W.  E.,  142,  519,  650,  684,  692,  694, 

698,  715,  775,  803,  807,  1054 
Login,  T.,  379,  383 
Lohest,  M.,  796 
Lonibardini,  E.,  383,  395 
Longe,  F.  D.,  883 
Lonsdale,  W.,  778,  783 
Lorenz,  V.,  350 
Loretz,  H.,  152,  734 
Lorie,  J.,  290,  292,  335,  480 
Lorieux,  E.,  834 
Loriol,  P.  de,  911,  918,  954 
Lory,  C,  186,  314,  640,  616,  622,  623,  624, 

917 


Lessen,  K.  A.,  74,  125,  159,  165,  169,  179, 
181,  184,  186,  697,  606.  618,  620,  787, 
1046 

Lotti,  B.,  628,  629 

Ldwl,  — ,  264 

Lubbock,  Sir  J.,  1055 

Lucas,  J.,  372 

Lundgreri,  B.,  732,  766,  870,  871,  952 

Lycett,  J.,  884,  897 

Lydekker,  R.,  1021 

Lyell,  C,  24,  29,  222,  226,  241,  245,  250, 
284,  292,  366,  370,  390,  597,  660,  807, 
962,  984,  1004,  1008,  1013,  1014,  1024, 
1055,  1070 

Macculloch,  J.,  96,  179,  478,  625,  698 

Mackenzie,  G.,  202 

Mackintosh,  C,  1042 

Maclaren,  C,  287,  536,  572,  829 

Madsen,  V.,  1033 

Malaise,  C,  769,  770 

Malcolmson,  J.  G.,  241 

Malherbe,  R.,  363 

Mallet,  F.  R.,  239,  253 

Mallet,   R.,   52,   191,   197,  244,  266,   270, 

271,  275,  298,  304,  442,  529,  584 
Malnigren,  A.  J.,  1036 
Maufredi,  — ,  461 
Mantell,  G.,  930,  940 
Mantovani,  — ,  979 
Marck,  W.  von  der,  922 
Marcou,  J.,  390,  646,  954 
Margerie,  K  de,  536,  547 
Marion,  A.  F.,  977 
Marr,  J.  K,  418,  567,  605,  606,  725,  732, 

734,  749,  750,  763,  766,  772,  773 
Marsh,  G.  P.,  336,  496 
Marsh,  0.  C,  316,  653,  657,  667,  863,  890, 

892,  893,  919,  932,  933,  934,  935,  936, 

959,  969,  970,  996,  1002,  1022 
Martel,  — ,  369 
Martins,  C,  403 
Marvin,  C,  235 
Mascart,  E.,  340 
Maskelyne,  N.,  46 
Matthew,  G.  F.,  720,  735,  737 
Maury,  Cai»tain,  337,  436 
Maw,  G.,  756 
Mayer,  C,  1001,  1016 
Mayer-Evmar,  C,  979 
M'Coy,  Prof.,  776,  983 
Medlicott,  H.   B.,  241,  259,  403,  717,  776, 

854,    877,   919,   957,    981,   1002,    1021, 

1055,  1079 
Meek,  F.  B.,  958 
Melllss,  J.  C,  255 
Meneghini,  G.,  735,  775 
Menge,  A.,  991 
Mercalli,  G.,  191,  192,  195,  202,  206,  222, 

250,  262,  271,  279 
Merill,  F.  J.  H.,  455 
Meunier,  S.,  10,  89 
Meyer,  C.  J.  A.,  939 
G.,  868,  915 


1098 


TEXT-BOOK  OF  GEOLOGY 


Meyer,  H.  von,  953 
0..  152 


Miall,  L.,  821 

Michelet,  977 

Michel-Levy,  M.,  65,  68,  71,  72,  73,  74,  75, 
86,  87,  89,  99,  101,  108,  109,  114,  115, 
155,  119,  120,  125,  156,  157,  170,  173, 
186,  219,  225,  265,  301,  302,  303,  304, 
310,  576,  579,  604,  605,  6l6,  622,  714 

Milch,  L.,  182,  838 

Miller,  Hugh,  285,  798 

H.  (Geol.  Surv.),  396,  827  ' 

W".  A.    11    12 

MUne  (Home),  D.,  270 

Milne,  J.,  205,  213,  242,  253,  259,  260,  271, 
272,  273,  276,  280,  281 

Milne  Edwards,  A.,  985 

Mitchell,  A.,  1057 

Mitscherlich,  E.,  75,  301 

Moberg,  J.  C,  918 

Moberg,  K.  A.,  714 

Mobius,  K.,  695 

Moesch,  C,  918,  1075 

Moesta,  A.  F.,  602 

Mohl,  H.,  156,  170 

Mohn,  H.,  288 

Mohr,  C.  F.,  13 

Moissan,  H.,  301 

Mojsisovics,  E.  von,  322,  350.  368,  476,  592, 
852,  871,  872,  874,  875,  876,  877 

Monaco,  Prince  of,  434 

Montcssus  de  Ballore,  F.  de,  279 

Moutserrut,  E.  de,  260 

Moore,  C,  636,^67,  920 

Moore,  Commander,  434 

Morgan,  C.  Lloyd,  710,  728 

Morlot,  A.  von,  1048 

Morris,  J.,  897 

Morton,  G.  H.,  824 

Morton,  H.,  36 

Mosely,  Canon,  418 

Moseley,  Prof.,  265 

Mouret,  G.,  836,  851 

Mourlon,  M.,  733,  770,  835,  950,  952,  976, 
977,  990 

Mousson,  A.,  417 

Miiller,  A.,  622 

F.  von,  983 

F.  K,  608 

Dr.  Max,  ^.^ii 

Murchison,  R.  I.,  353,  359,  625,  680,  698, 
699,  725.  726,  737,  746,  747,  748,  752, 
753,  754,  757,  758,  760,  766,  768,  783, 
788,  789,  797,  800,  836,  842,  846,  848, 
852,  955 

Murray,  Alexander,  684,  692,  715,  735 

John,  455 

J.    [ChaUenger   Expedition),  40,   125, 

216,  340,  374,  405,  441,  450,  451,  454, 
456,  457,  458,  459,  460,  482,  484,  485, 
487,  490,  492,  494,  495 

R.   A.    F.,    717,    791,    854,     1003, 


1023 
MuschketoflF,  J.,  239 


Nadaillac,  Mar(|ui8  de,  1067 

Nansen,  F.,  418 

Nason,  H.  B.,  89 

Nathorst,  A.  G.,  291,  350,  695,  721,  782, 

740,  766,  769,  870,  871,  1036,  1067 
Naiimann,  C,  14,  172,  621 
Naumann,  E.,  260,  330 
Nehring,  A.,  1060 
Nelson,  R.  J.,  128,  337,  481 
Neumayr,  M.,  23,  350,  628,  655,  667,  877, 

895,  896,  917,  918,  959,  1070,  1077 
Newberry,  J.  S.,  235,  391,  521,  641,  807. 

879,  1077 
Newcombe,  S.,  29,  441 
Newton,  E.  T.,  863,  1012,  1013 

I.,  13 

R.  B.,  966,  985 

Nicholson,  H.  A.,  749,  750,  762 

Nicol,  J.,  625,  698 

Nicol,  W.,  108,  110 

Nikitin,  Prof.,  838,  918,  939;  957 

Niles,  W.  H.,  312 

Noetling,  F.,  1045 

Nolan,  J.,  802 

Nordenskiold,  A.  E.,  68,  171,  217,  288/338 

418,  455,  876,  1001 

Oehlert,  — ,  787 

Ogilvie,  MissM.,  871,872 

Oldham,  R.  D.,  840 

Oldham,  T.,  276 

Ollech,  Dr.,  478,  484 

Oppel,  A.,  897,  911,  915,  916,  918 

Orbigny,  A.  d',  911,  942,  947 

O'Reilly.  J.  P.,  279,  530 

Oshorn,  H.  F..  969 

Owen,   R.,  863,  893,  894,  897,   931,   968 

997 
Oxenham,  E.  L.,  395 

Palgravk,  W.  G.,  336 

Pander,  C.  H.,  744 

Parona,  C.  F.,  838 

Parran,  A.,  336 

Partsch,  J.,  999,  1024,  1046 

Partsch,  P.,  10 

Passarge,  S.,  869 

Paul,  B.  H.,  36,  361,  362 

Paul,  K.  M.,  979 

Pavlow,  A.,  910,  918,  919,  938,  939 

Payer,  J.,  438 

Peach,  B.  N.,550,  625,  627,  699,  705,  72g 

730,  746,  750,  762,  794,  1029,  1042 
Peacock,  R.  A.,  292 
Peale,  A.  C,  236 
Peligot,  E.,  341 
Pellat,  E.,  911 
Penck,  A.,  283,  1024 
Penck,  F.,  134,  137 
Peuf^elly,  W.,  974 
Penhallow,  D.  P.,  793,  1053 
Penning,  W.  H.,  80,  513 
Penrose,  R.  A.  F.,  142,  494 
Percy,  J.,  144,  147 


LIST  OF  A  UTHORS 


1099 


Peron,  A.,  956 

Perrey,  A.,  270 

Pescliel,  O.,  496 

Petrie,  W.  Flinders,  329 

Pettersen,  K.,  288,  711,  769 

Pettersson,  0.,  35,  148,  438 

Pfaff,  F.,  13,  48,  270,  280,  283,  300,  306, 

307,  344,  418 
Phillips,  J.,  58,  195,  621,  545,  711,  750, 

756,  824,  881,  885,  887,  892,  897,  904, 

939,  970,  971 
J.     A.,    114,   127,    128,    131,   138, 

156,  159,  165,  473,  566,  569,  605,  631, 

640,  641 
W.,  970 


Phipson,  T.  L.,  68 

Pickwell,  R.,  446 

Pidgeon,  D.,  290 

Pierre,  J.  J.,  341 

Pilar,  S.,  279 

Plantamour,  E.,  271 

Plattner,  C.  F.,  89 

Playfair,  J.,  13,  46,  282,  384,  444,  460 

Poey,  A.,  206 

PoiseuUle,  J.  L.  M.,  306 

Pokoruy,  A.,  478 

Pomel,  A.,  336 

Portlock,  J.  E.,  752 

Posepny,  F.,  184 

Potier,  1016 

Poussin,  De  la  Vallee,  71,  110,   112,  165, 

184   308 
Powell,  J.  W.,  391,  538,  550,  1070,  1073, 

1077,  1078 
Pozzi,  G.,  203 
Pratt,  J.  H.,  20,  47,  283 
Prestwirh,  J.,  50,  264,  287,  356,  358,  374, 

378,  451,  824,  971,  972,  973,  975,  1008, 

1013 
Price,  F.  G.  H.,  941,  944 
Puillon-Boblaye,  607 
Pulligny,  J.  de,  433 
Punipelly,  R.,  333,  351,  716 

QuENSTEDT,  F.  A.,  897,  915,  916 

Rae,  Dr.,  415 

Raisin,  Miss,  161 

Rames,  J.  B.,  1047 

Rammelsberg,  C,  10 

Ramoud,  — ,  328 

Ramsay,  A.  C,  149,  367,  404,  430,  547, 
662,  675,  709,  725,  729,  747,  748,  752, 
791,  797,  802,  843,  846,  847,  850 

Ramsay,  W.,  381 

Randall,  J.,  760 

Ranyard,  A.  C,  68 

Raulin,  V.,  949 

Reade,  T.  Mellard,  68,  292,  299,  800,  379, 
436,  449,  462,  464,  1042,  1070 

Reclus,  E.,  367,  402,  476 

Redman,  J.  B.,  446,  451 

Redteubacher,  A.,  956 

Reich,  F.,  46 


Reid,  C,  151,  331,  352,  446,  970,  975,  986, 

987,  1006, 1007, 1008,  1009,  1010,  1012, 

1013,  1014,  1016,  1017,  1026,  1031, 

1035,  1042 
Reid,  Dr.,  417 

Rein,  J.  J.,  128,  481,  485,  490 
Reinach,  A.  von,  851 
Renard,  A.,  71,  88,  110,  112,  125,  134, 

141,  142,  152,  165,  184,  216,  308,  464, 

457,  458,  459,  494,  619,  921 
Renault,  B.,  808,  814,  851 
Rendu,  Bishop,  417 
Renevier,  E.,  314,  640,  624,  674,  680,  918, 

950,  954,  979,  1048,  1075 
Rennie,  R.,  478 
Retgers,  J.  W.,  86 
Reusch,  H.,  181,  316,  320,  430,  446,  466, 

616,  621,  711,  712,  769,  802 
Reuss,  A.  E.,  955 
Reyer,  E.,    125,  135,   137,  191,  222,  247, 

255,  265,  266,  269,  570,  587,  608 
Ricco,  A.,  250 
Richter,  E.,  417 
Richter,  R.,  1045 
Richthofen,  F.  von,  62,  80,   100,  160,  161, 

168,  256,  263,  329,  332,  333,  336,  352, 
538,  717,  719,  720,  776,  790,  839,  871, 
1060 

Riedl,  E.,  371 

Rigaux,  E.,  836,  911 

Rink,  H.,  418,  426 

Roberts,  G.,  760 

Rol)erts-Au8ten,  W.  C,  316 

Robertson,  A.  C,  239 

Robertson,  J.  R  M.,  840 

Rodie,  E.,  48,  851 

Rodwell,  G.  F.,  192 

Roemer,  F.  von,  695,  782,  787,  868 

Rogers,  H.  D.,  307,  399,  485,  1073 

Rogers,  W.  B.,  307 

Rohon,  J.  v.,  745 

Rohrbach,  C.  E.  M.,  261 

Rolland,  G.,  336,  359,  956 

Rolleston,  G.,  473,  496,  497 

Roscoe,  H.  E.,  11,  32,  362 

Rose,  G.,  10,   73,  75,  122,  139,  152,  301, 

328 
Rosenbusch,  IL,  69,  80,  89,  97,  99,   108, 

117,  118,  119,  125,  154,  156,  160,  162, 

169,  184,  607,  608,  617,  618,  687,  720 
Ross,  J.  G.,  442 

Rossi,  M.  S.  di,  271,  273 

Roth,  J.,  69,  96,  152,  341,  344,  345,  362, 

363,  377,  378,  411,  412,   471,  484,  681, 

652 
J.  R.,  1019 

Rothpletz,  A.,  184,  248,  314,  370,  616 

Rouville,  P.  de,  771 

Rowe,  A.  W.,  1042 

Rowney,  Prof.,  596,  694 

Rover,  E.,  911 

Rudler,  F.  W. ,  695 

Ruskin,  J.,  42 


1100 


TEXT-BOOK  OF  GEOLOGY 


Russell,   I.   C,   332,  333,  335,   336,  351, 

378,  394,  407,  409,  417,  792 
Russell,  R.,  287,  824 
Rutley,  F.,  109,  115,  162,  328,  711,   748, 

827 
Rutot,  A.,  521,  929 

Sacco,  F.,  979,  980,  993,  1001,  1016,  1061 

St.  Hilaire,  J.  G.,  496 

Salisbury,  R.  D.,  1050 

Salter,  J.  W.,  725,  726,  728,  783 

Saiidlierger,  — ,  237 

Saudberger,  F.  von,  619,  631,  638,  641,  786, 

869,  999,  1018 
Sandler,  C,  288 
Sautesson,  H.,  183 
Saporta,  Comte  de,  740,  966,  976,  977,  995 

1004 
Sarasin,  E.,  301 
Sars,  M.,  285 
Sauer,  A.,  181,  333 
Saussure,  H.  B.  de,  300,  328,  417 
Sauvage,  H.  E.,  S86 
Scacchi,  Prof.,  225 
Schardt,  H.,  540,  679,  1075 
Schaiiroth,  V.,  482 
Scheerer,  C,  186,  308 
Scheuck,  A.,  618,  933,  953 
Schinz-Gessner,  H.,  478 
Schleideu,  E.,  229 
Schlippe,  A.  0.,  915 
Schliiter,  C,  954 
Schmelck,  L.,  36,  38 
Schmidt,  C,  650 
F.,   719,   727,    732,   766,    768,  775, 

1045 
J.    F.   J.,   191,    195,  202,   208,   270, 

281 
Schorlemmer,  Prof.,  32,  362 
Schroter,  C,  1036 
Schumacher,  E.,  333,  869 
Scoresbv,  W.,  437 
Scott,  K.  H.,  212 
Soott  Russell,  R.,  437 
Scrope,  G.  P.,  191,  194,  202,  205,  219,  222, 

240,  241,  269,  324,  529,  587 
Scudder,  S.,  794,  820,  821,  886,  899 
Sedgwick,  A.,  545,  547,  603,680,  719,  725, 

72G,  729,  738,  746,   748,  749,  750,   753, 

754,  762,  783,  846,  955 
SeelKich,  K.  vou,  244,  255,  260,   272,  276, 

279,915 
Seehmd,  F.,  420 
Seeley,  H.  G.,  893,  93.5,  956 
Segueuza,  G.,  1016,  1017 
Sekiya,  Prof.,  272 
Selwyu,  A.  C.  R.,  717,  775,  776 
Semichon,  L.,  352 
SemiKT.  C,  253,  485,  490 
Senft,  F.,  96,  143,  146,  343,  366,  478 
Serpieri,  f^.,  273 
Seniles,  J.,  957 
Seward,  A.  C,  655 
Sexe,  S.  A.,  287,  430 


Shaler,  N.  S.,  56,  293,  455,  480,  481 

Sharpe,  D.,  314,  316,  783 

Shone,  W.,  290 

Siemens,  — ,  196 

Siemens,  C.  W.,  11,  461 

Silvestri,  C,  192,  208 

Sirodot,  S.,  478 

Sismonda,  A.,  622 

Siteusky,  F.,  480    " 

Sjogren,  H.,  235,  239 

Skey,  W.,  298 

Sluiter,  C.  P.,  485,  492 

Smith,  Angus,  32,  33,  341,  342 

Smith,  W.,  656,  665,  680,  879,  897,  898, 

906 
Smyth,  Brough,  641 
Sollas,  W.  J.,  131,  141,  156,  398,  472,  484, 

488,  495,  722,  924 
Sonstadt,  E.,  86 
Sorby,  H.  C,  69,  78,  108,  110,  111,  115. 

121,  122,  127,  129,  132,  134,  138,  139, 

140,  147,  150,  176,  184,  302,  307,  308, 

314,  815,  322,  324,  366,  385,  484,  499, 

507,  575,  599,  605,  1042 
Spallanzani,  L.,  202,  228,  262 
Spittell,  — ,  383 
Spratt,  Admiral,  252,  401 
Spring,  W.,  143,  307,  311 
Stache,  G.,  623,  787,  838,  839,  852,  871, 

980 
Stangeland,  G.  E.,  480 
Stanley,  H.  M.,  329 
Staptf,  F.,  50,  407,  540,  547 
Stecher,  Dr.,  571,  572,  575,  576,  601,  609 
Steele,  A.,  478 

Steenstrup,  K.  J.  T.,  68,  1067 
Stefani,  C.  de,  204,  981,  1016 
Stelzuer,  A.,  173,  186 
Stevenson,  D.,  373,  376,  380,  405,  443 

J.  J.,  841,  958 

T.,  380,  437,  438,  442,  443,  444 

Stiefeflsand,  — ,  383 

Stiffe,  A.  W.,  239 

Stirling,  J.,  1055 

Stoliczka,  F.,  919,  955 

Stoppani,  A.,  350,  869 

Strachey,  R.,  212 

Strahan,  A.,  141,  312,  494,  824,  921,  939, 

940,  970,  1042 
Strahan,  C,  336 
Strickland,  H.  E.,  867 
Strombeck,  A.  von,  953,  954 
Strozzi,  C,  1004 

Strnckmann,  C,  915,  917,  953,  1036 
Stuber,  J.  A.,  916 
Studer,  B.,  3.59,  954,  992 
Stur,    D.,  623,    806,   816,    833,   837,   838, 

871 
Suess,  E.,  21,  265,  281,  283,  284.  290.  292. 

852,  871,  876,  992,  1071,  1075,  1077 
Sullivan,  \V.  K.,  67,  69,  77,  80 
Surell,  — ,  372,  383,  393 
Svedniark,  E.,  170 
Sveuonius,  F.,  769 


LIST  OF  A  UTHORS 


1101 


Swanston,  W.,  752 
Sweet,  E.  T.,  716 
Symonds,  W.  S.,  760,  798 
Symons,  G.  J.,  373 
Szabo,  J.,  88,  166 

Tait,  p.  G.,  9,  12,  23 

Tarainelli,  T.,  279 

Tate,  G.,  827 

Tate,  R.,  839,  897,  899,  947,  983 

Tawney,  E.  B.,  867,  987 

Tchihatcbef,  P.  von,  336,  359 

Teall,  J.  J.  H.,  105,  108,   129,   182,  564, 

567,  576,  616,  617,  788 
Teller,  A.,  876 
Teller,  F.,  628 
Templeton,  J.,  478 
Termier,  P.,  219,  624,  838 
Theuius,  G.,  478 
Thirion,  ~,  111 
Thomas,  A.  P.  W.,  260 
Thomson,  James,  376,  388,  418,  529 

William.     See  Kelvin,  Lord 

Wyville,  28,  33,  128,  337,  354,  434, 

481 
Thoreld,  A.  F.,  146,  407 
Thoroddsen,  H.,  202,  256 
Thorpe,  T.  E.,  36 
Thoulet,  J.,  830,  380,  381 
Thurmaun,  — ,  1072 
Thury,  Prof.,  359 
Tietze,  E.,  130,  333,  338,  360,  368,  980, 

1079 
Tissandier,  G.,  68 
Tizzard,  Captain,  451 
Tombeck,  —,911 
Toramasi,  A.,  838 

Topley,  W.,  335,  352,  517,  576,  679,  940 
Torell,  0.,  732,  746 
Tomebohm,  A.  E.,  621,  711,  713,  769 
Tornoe,  H.,  36,  38 
Tornquist,  S.,  733,  766 
Totten,  Colonel,  292,  299 
Toucas,  R.,  911,  950,  951 
Toula,  F.,  623,  871 
Touniaire,  J.,  240 
Traquair,  R.  H.,  796,  833 
Trautschold,  H.,  145,  284,  333 
Tresca,  H.,  316 
Tristram,  Canon,  336 
Tromelin,  G.  de,  733,  770 
Trotter,  Coiitts,  418 
Tsohermak,  G.,  10,  72,  74,  77,  95, 173,  183, 

265,  266 
Tschernyschew,  T.,  788,  838,  853 
Tullberg,  S.  A.,  731,  732,  768 
Turner,  H.  W.,  407 
Twisden,  J.  F.,  19 
Tylor,  A.,  460 
Tyndall,  J.,  33,  314,  417 
Tyrrell,  J.  B.,  1052 

Ulrich,  G.  H.  F.,  718,  961 
Uuger,  F. ,  496,  607,  608 


Upham,   W.,   286,  407,   716,   1050,  1052, 

1053 
Ussher,  W.  A.  E,,  287,  336,  783,  786,  826, 

864,  865,  866 

Vacek,  M.,  540,  624,  629,  953 

Vasseur,  G.,  979 

\7'lain,  C,  191,  219,  225,  248,  253,  850, 

1046 
Verbeek,  R.,  212 
Verneuil,  E.  P.  de,  239,  368,  859,  629,  766, 

788,  842,  852 
Verril,  A.  E.,  485 
Vezian,  A.,  979 
Viguier,  Prof.,  416 
Vogel,  H.  W.,  12 
Vogelsang,  H.,  98,  116,  128,  126,  187,  241, 

302 
Vogt,  C,  893 
Vogt,  J.  H.  L.,  97 
Volger,  0.,  270 
Vollert    M.    991 
Vom  Rath,'G.,  67,  69,  161,  217,  285,  338, 

602 

Waaqen,  W.,  809,  853,  877,  919 
Wadsworth,  M.  E.,  56,  169,  173,  696,  696, 

716 
Wagner,  J.  A.,  1019 
Wjihner,  F.,  272 
Wahnschaffe,   F.,   333,   1031,  1045,   1062, 

1061 
Walcott,  C.  D.,  694,  720,  727,  736,  736, 

737,  744,  775 
Walker,  — ,  444 
Walker,  G.  B.,  840 
Wallace,  A.  R.,  29,  291,  660,  1041 

W.,  131,347 

W.,  631 

Waller,  T.  H.,  711 

Walleraut,  F.,  851 

Wallich,  Dr.,  495 

Waltershausen,  S.  von,  191,  192,  199,  206, 

209,  211,  217,  222,  229,  269 
Walther,  J.,  330,  335,  477,  485,  529,  839 
Wanklvn,  J.  A.,  378 
Ward,  J.  C,  112,  137,  156,  306,  605,  747, 

749,  750 

L.  F. ,  740,  923,  969 

T.,  367 

Wariugton,  R.,  342,  362 

Watt,  Ciregorv,  301 

Watts,  W.  L.,  256 

Watt.s,  W.  W.,  748 

Weber,  E.,  185 

Webster,  T.,  943 

Weed,  W.  H.,  481,  483 

Weiss,  C.  E.,  814,  816,  836,  849 

Weiss,  E.,  669,  868 

Werner,  A.  G.,  178,  680,  865 

Wervecke,  L.  van,  849,  850,  869 

Wethered,  E.,  150 

Wevprecht,  K.,  438 

Wharton,  J.  C,  213,  485,  492 


1102 


TEXT-BOOK  OF  GEOLOGY 


Wheeler,  G.,  409 

Whidborne,  G.  F.,  890 

Whitaker,   W.,   352,  366,   449,   451,    468, 

865,  943,  944,  946,  970,  971,  972,  1008 
White,  C.  A.,  415,  959 
White,  I.  C,  855 
Whiteaves,  J.  F.,  796,  960 
Whitfield,  J.  E.,  363 
Whitfield,  R.  P.,  746,  956,  958 
Whitney,  J.  D.,  200,  629,  695,  696,   716, 

959,  1050,  1067,  1070 
Whittelegge,  T.,  475 
Whymper,    E.,   202,   206,  214,    242,   260, 

418 
Wichmann,  A.,  134,  181,  183,  328 
Wiebel,  K.  W.  M.,  446 
Wilkes,  C,  205,  246 
Wilkiusou,  C.  S.,  776,  839,  854,  878,  1022, 

1067 
Williams,  G.   H.,   169,  182,  596,  615,  616, 

617,  618,  628,  631,  696 

G.  J.,  747 

H.  S.,  841 

Williamson.  W.  C,  814 
Wilson,  E.,  846,  867 

G.,  383 

J.  M.,  318 

Winchell,  A.,  716,  1053 
Winchell,  N.  H.,  390,  716 
Wing,  A.,  628 
Winkler,  E.  C,  336 
Witham,  H.,  108 
Woeikotf,  A.,  384 


Wohrmann,  S.  F.  von,  873 

Wolf,  T.,  195,  198,  202,  208,  214,  232 

Wolff,  J.  E.,  616 

Wood,  H.,  410 

Searles,  974,  1008,  1009 

. S.  v.,  jun.,  1042 

Woods,  J.  E.  Tennison,  983 
Woodward,  A.  S.,  878,  936 

H.,  821,  987 

H.  B.,  841,  846,  864,  866,  867.  897, 

898,  1008,  1042 

R.  S.,  35,  282,  283 


Wright,  A.  W.,  10 

C.  E.,  716 

G.  F.,  417,  1025,1050 

T.,  867,  897,  899 

Wiirtenberger,  L.,  391 
Wynne,  A-  B.,  628,  776 

YoDNG,  A.  A,,  132 

George,  939 

J.,  671 

Zaccagna,  D.,  628,  629,  1075 

Zay,  — ,  371 

Zeiller,  R,,  814,  834,  836 

Zekeli,  F.,  955 

Zincken,  J.  C.  L.,  606 

Zirkel,  F.,  69,  96,  108,  110,  112,  117,  134, 
153,  156,  160,  163,  165,  166,  167,  169, 
170,  172,  173,  176,  202,  302,  338,  603, 
607,  619 

Zittel,  K.  H.,  695,  918,  955,  956 


INDEX 


An  asterisk  aii<iched  to  a  number  denotes  that  a  figure  of  the  subject  will  be  found  on  the 
page  indicated,  Oenera  and  species  of  fossils  are  printed  in  italics.  A  single  reference 
only  is  given  to  each  main  division  of  the  Geological  Record  in  which  a  genus  is 
mentioned. 


**  Aa  "  form  of  lava-streams,  217 

Aachenian,  950 

Aar  glacier,  erosion  by,  432 

former  size  of,  1048 

Abies,  991 

Absorption-spectrum,  11«  12 

"  Abtheilung"  in  stratigraphy,  678 

Abysmal  deposits,  457,  648,  650 

Abyssinia,  volcanic  plateau  of,  258 

Acacia^  995 

Acanthocerasj  927* 

Acanthocladia^  844 

Aeanthodes,  795*,  796,  830,  845 

Acanthopholisy  931 

Acanthospongia,  748 

Acer,  922,  988,  995 

AceratheriujUy  1018 

Acerocare,  731 

Acervularia,  742,  757*,  780 

Achatina,  986 

Acheulian  deposits,  1057 

Adiyrodon,  894 

AcurularicL,  976 

Acid,  uses  of,  in  rock  determination,  87 

acetic,  87 

l%K>crenic,  471 

citric,  87  ;  use  of,  in  field-work,  81 

crenic,  471 

humic,  471 

hydrochloric,  81 

hydrofluoric,  87 

hydrofluosilicic,  88 

nitric,  88 

oi^anic,  action  of,  146,  343,  458,  471 

ulmic,  471 

Acid  series  of  massive  works,  156 ;  gradation 
of,  into  basic,  105,  225, 262,  269,  564,  676 
AeUlaspis,  741*,  743,  781 
Acotherulum,  985 
Aeroculia,  744.  781 
Acrodut,  862,  886 


Acrolepis^  845 

Acrosalenia,  883 

Acrostichites,  859 

Acrothde,  732 

AcroiretUy  725,  743 

ActsLon,  1011 

ActAonella,  928 

Actitonina,  906 

Actinoceras,  754 

ActinocrinuSy  742,  811 

Actinodon,  846 

Actiuolite,  74 

Actinolite-schist,  182,  686 

Actinnpht/llunif  759 

Actinostrinna,  785 

Adacna,  1017 

Adnpij/,  985 

Adinole,  183 

Adnanites,  845,  852 

Adriatic,  detrital  deposits  in,  395,  402 

.£chmixius,  886 

jEger,  860,  885 

jfiglina,  741*,  742 

jEgoccras,  874,  884,  900*,  902* 

JUlurogale,  985,  1021 

jEluropsis,  1021 

iEolian  dej^sits,  333 ;  rocks,  126,  128 

Aerolites,  10 

iEtites,  147 

Aetobates,  966 

Aetosaurusy  863 

Africa,  active  volcanoes  of,  260  ;  Carbonifer- 
ous rocks  in,  839  ;  Permian,  855  ;  Trias, 
956  ;  Cretaceous,  956,  957 

Agate,  artificial  colouring  of,  306 

Agat/iaumas,  958 

Agathicercu,  862 

Agave,  965 

Agelacrinites,  749 

Agglomerate,  volcanic,  137,  201 

Agglomerated  structure,  103 


1104 


TEXT-BOOK  OF  GEOLOGY 


^ 


Aggregation,  state  of,  in  rocks,  105 

Agnostusy  722* 

Agnotozoic  rocks,  684 

Af/rauios,  724 

Agriculture,  geological  effects  of,  496 

"  Aignes-niortes,"  388,  403 

Air  absorbs  little  radiant  heat,  26  ;  effects 
of  compression  and  expansion  of,  in  marine 
erosion,  443  {see  aho  under  Atmosphere) 

Alactaga,  1060 

Alaria,  904 

Alaska,  glaciers  of,  417,  420 

Alhertia,  859 

Albian,  938,  941,  948,  950,  953,  955,  956 

Albite,  72 

Albitisation,  618 

AlrcUiphnSy  1021 

Alces,  1014 

Alder,  fossil,  966,  1004 

Aleclo,  883 

Alef haptens,  815*,  816,  850,  876,  880 

Alga?,  geological  action  of,  476,  477,  482, 
483,  860,  872,  874 

Algonkian,  715,  716 

Allacodon,  936 

Ai lotion,  919 

Allogenic,  65 

AUorisma,  844,  854 

Allotrioinorphic,  64,  109,  118 

Alluvia,  Paheolithic,  1058 

Alluvium,  333  ;  deposition  of,  393,  404 

Alraesakra  group,  713 

Alnus,  922,  988,  1005* 

Alpine  type  of  mountain-structure,  1075 

Alps,  relative  bulk  of,  39  ;  fjord  lakes  of, 
'29\  ;  crumpling  of,  317,  540*  ;  earth- 
l.ilhirs  of,  355  ;  alluvia  from,  393,  394, 
395  ;  snow-line  in,  410  ;  glaciers  of,  417, 
419,420*,  4'Jl*  ;  former glaciation  of,  426, 
1021,  1038,  1048;  glacier  -  moulins  of, 
429  ;  inverted  rocks  of,  539,  540*  ;  meta- 
morpliisni  in,  622,  623,  627^  628,  629, 
774  ;  a^'e  of  schists  of,  624,  774 

jire-Canibriau  rocks  in,  714  ;  Silurian, 

774  ;  Devonian,  787  ;  Carboniferous,  622, 
838  ;  Perniiau,  845,  852  ;  Trias,  871, 
873;  .Jurassic  rocks,  917;  Cretaceous, 
951  ;  Kocene,  979,  980  ;  molasse  of,  992; 
]M).st-01ig()cenc  elevation  of,  993,  994; 
Pleistocene  glaciation  of,  1024,  1038, 
1018  ;  jiresent  glaciers  of,  rei)reseut  those 
of  Pleistocene  time,  1041,  1048  ;  history 
of,  1077  ;  cause  qf  characteristic  scenei-y 
of,  1082 

AlHophnlla,  976 

Alteration  of  rocks  [see  under  Metamorphisra 
nnd  Weatlieriufj) 

Alum,  origin  of,  135 

slate,  135,  188,  739 

Pay,  leaf-beds  of,  970,  974 

Alumina,  62 

Aluminium,  61,  69 

Alvf'nhirla,  1009 

Alvcolina.,  974 


AlveolUes,  749,  780,  810 
Amallheias,  884,  900*,  902*,  907* 
Amazon,  terraces  of  the,  396  ;  seaward  ex- 
tension of  sediment  of,  404,  452  ;  minoBl 
matter  dissolved  in,  462 
Amber,  651 

beds  of  Konigsberg,  991 

AnMotfieriumy  894 
Amblypterus,  845 
Amhonychia,  744,  745*,  761* 
America,  active  volcanoes  of,  260 

Central,  volcanoes  of,   197,  214,  216; 

oscillations  of,  291 

North,  estimated  mean  height  of,  39 ; 


extent  of  coast-line  of,  45  ;  fjords  of,  291 ; 
deserU  of,  329,  336  ;  weathering  in,  350; 
earth  -  pillars  of,  355  ;  buttes  and  bad 
lands  of,  356 ;  caftons  of,  389*,  391,  392*, 
1084*  ;  alluvial  fans  of,  394* ;  river- 
terraces  of,  396*;  coast -bars  of,  399; 
vanished  lakes  of,  407,  409,  1052  ;  salt- 
lakes  of,  408  ;  frozen  rivers  and  lakes  of^ 
415  ;  salt-marshes  of,  455 
—  pre-Cambrian  rocks  of,  715  ;  Cambrian, 
735  ;  Silurian,  775  ;  Devonian,  789 ;  Cw- 
boniferous,  840 ;  Pennian,  855 ;  Trias, 
878  ;  Jurassic,  919  ;  Cretaceons,  957  ; 
Eocene,  981  ;  Oligocene,  993  ;  Miocene, 
1002  ;  Pliocene,  1022  ;  glaciation  of, 
1029,  1050;  post-glacial  deposiU in,  1067; 
physiographical  evolution  of,  1077 

South,  estimated  mean  height  of,  39 ; 


extent  of  coast-line  of,  45  ;  volcanoes  of 
{see  Andes)  ;  earthquakes  of,  274,  279  ; 
ui>heaval  of,  288  ;  Trias  of,  878  ;  Jurassic 
of,  919;  prehistoric  deposits  and  extinct 
mammalia  of  the  Pauipas,  1067 

Ammonia,  molybtlate  of,  in  testing  rocks, 
88 

Ammonites^  877 

Ammonites,  900*,  902*,  903*,  904*,  907*, 
927*  ;  as  tyin.*- fossils,  657  ;  early  types 
of,  815,  852  ;  abundance  of,  in  Junu»sic 
time,  884:  separation  of,  into  families  and 
genera,  884  ;  disappearance  of,  928 

A  i/i  man  ites  nca  n  th  us-zoue,  914 

altemans-zowe,  919 

(inc^ps-zoue,  913,  919 

(infjuhUifs-zone,  899,  914,  916  • 

arbnstifferus-zoue,  913 

(ispulcndcji-7.oue,  913 

(Kst u'lianioi-zon^i,  938,  953,  955 

■ nnritius-zom:,  938 

hi/rons-zoua,  914 

hi/trrcaliis-zoue,  915 

hi  mummai  US  zona  J  912 

hisu/i-afif-s-zonti,  914 

liurklandi-zoim,  899,  914,  916 

calhrv{en,sis-zone,  907,  918 

coinmunis-zone,  899 

ronr^rKA'-zone,  917 

c or datus -zone,  908,  919 

caronat  US-zone,  915,  919 

crUtatiiS'Zoutf  938 


INDEX 


1105 


Ammonites  Dandsoni-zone^  914 

Z>aiYKt-zoiie,  914 

/errugiruus -zonef  913 

giganteus'Zouet  909,  917 

gigas-zoiiey  911,  912,  915 

Henleyi'uoxii,  899,  914 

kuviphriesianus-zonet  904,  913 

ibex-zone,  899,  915,  916 

inJUitus  {roslnUus)    zoue,   938,    943, 

948,  950,  954 

Janiejtoni'Zonef  899,  916 

.  Jason-zoney  908,  919 

juraisis-zouef  903,  916     • 

Lamberti-zone,  913,  918,  919 

laui  US-zone,  938,  948,  950,  954 

macTocephalus-zone,  912,  915,  919 

•  mamiliaris-zonef  948,  950 

inargaritaius-zxynty  899,  914,  918 

Mariw-zonef  913 

Martelli-zone,  912 

milletianus-zone,  948,  958 

Murehisonw-zone,  904,  913,  917 

nwrtemis-zoney  913 

Musus-zone,  899,  916 

opaliniis-zoney  903,  913,  914 

ornatus-zone,  915 

ox'^/to^M^-zoDe,  899,  914,  916 

—  Parkinsoni-zoney  904,  913,  915 

perarniatus-zoney  908 

planicosta-zone,  914 

planorbis-zoue,  899,  914,  916 

plimtilis-zonef  908,  919 

•  portiatuiic US-zone f  911 

■  psihiiotus-zouef  916 

•  raricostatus-uone,  899,  916,  916 

rostratus  (in/latus)  zone,  938,  943 

rothoniagensiszonef  938,  944 

TV^j/onnw-zone,  914 

Sauzei-zone,  913 

serpentinus-zone,  899,  914 

a?/imi/i«-zone,  899,  914,  916 

Stella  nS'Zone,  914 

Unuilohat US-zone,  911,  912,  918,  91 

Turneri-zone,  899,  916 

r«/t^7a'-zoue,  914 

varians-zone,  938,  943,  944 

vennrensis-zone,  914 

virgat  us -zone,  919 

2g/e*-zone,  914 

Ammosaurus,  863 
^iTiomu;/}^  965 
Amorphous,  65 
Amphibia,  fossil,  821,  845 
Amphibole,  74 
Amphibole-trachyte,  166 
Amphibolites,  18*2 
Amphibolite-schist,  182 
Amphicyon,  968,  993,  998,  1021 
Amphilestes,  893 
Amphimeryx,  985 
Amphion,  743 
AmphipeUis,  803 
Amphipora,  785 
Amphispongia,  740 


Amphistegina,  809 

Amphitheriuin,  893 

Amphitragulus,  968,  990 

Amphitylus,  893 

Amplexus,  810 

Ampullina,  993 

Ampyx,  741*,  743 

^4mujrium,  958 

Amygdaloidal  structure,  102*,  104,  227     , 

Amygda/us,  965 

Analcime,  77 

Analysis,  chemical,  87. 

Ananchytes,  925* 

Anarcestes,  782 

Anatifopsis,  742 

Anatina,  907 

Anchilophus,  985 

Anchippodus,  969 

Anchisaurus,  863 

Anchitherium,  968,  993,  995 

Anchor- ice,  415,  439 

Anciilarui,  978,  987,  995 

Ancyloceras,  907,  928,  929* 

Ancylotherium,  1019 

Ancylu^,  1011 

Andahisite,  76  ;   in  contact- metamorph Ism,. 

605,  607  ;  in  regional  metaraorphiflm,  627 
Andalusite-schist,  607  nnfl 

Andes,  volcanoes  of,  197,  202,  206,  213,  214, 

232,  234,  247 
Andesine,  72 

Andesite,  167  ;  passage  of,  into  basalt,  171 
Andrarumskalk,  731 
Andromeda,  988 
Angelina,  729 

Angiosperms,  first  api)earance  of,  922,  954 
Angoumian,  938,  948,  952 
Anhydrite,    79,    152 ;    conversion   of,   into 

gypsum,  298,  345 
Animals,  geological  inferences  from  distribu- 
tion of,  290  ;  destrudtive  action  of,  473  ; 

conservative  influence  of,  476  ;   deposits 

formed  by,  484  ;  geograpliical  distribution 

of,  660 
Animikie  series,  716 
Anisotropic  substances,  94,  115 
Annelids,  fossil,  721,  723*,  742  ;  fossilisa- 

tion  of,  652 
Annulana,  740,  815*,  816 
AnodonOi,  790,  802,  1016 
Anoinia,  959,  9/8,  1012 
Anomocare,  724 
Anomodout  reptiles,  846,  863 
Anamopteris,  859,  870 
Anomoza mites,  880,  953 
Anoplophora,  862 
Anoplothcrium,  985 
Anopoleuus,  724 

Anorthite,  72  ;  in  meteorites,  10 
Anorthopygus,  951 
Antarctic  re-^'ious,  laud-ice  of,  418  ;  icebergs 

of,  440,  441* 
Antelope,  fossil,  1060  ;  ancestral  forms  of, 

968,  996,  1019 


4  B 


1106 


TEXT-BOOK  OF  GEOLOGY 


Anthodon,  863 

AntholUhus,  817,  819* 

Anthophyllite,  74 

Anthracite,  144,  322 

Anthracite-slate,  135 

A  nthracomyay  819 

Anthracopleraj  819 

Anthracopiipa,  821 

AnthracosauritSy  821 

Anihracosia,  806,  811,  819,  853 

AnOiracotheriumy  985,  998 

AnlhrapaUemon^  812 

Anticlines,  538,  539*  ;  effects  of  faults  on, 

554 
Anversian,  999,  1015 
Apatite,  79  ;  test  for,  88 
ApatosauruSj  919 
Apeunine  chain,  Eocene  in,  980;  Oligocene 

in,  993  ;  Pliocene  in,  1003,  1016 
Apes,  fossil,  996,  1006 
Aphanite,  166 
Aphanitic  structure,  98 
Aphelion,  16,  25 
Aphylliles,  782 
Apiocrmus,  883 
Aplite,  158*,  159 
Apocrenic  acid,  471 
Apophyses  of  granite,  580 
Aporrhais,  928,  971 
Aptian,  938,  941,  948,  919,  953,  955 
AptychopisiSy  742 
Aptychus-beds,  918,  955 
Aqueous  rocks,  124 
Aquitanian  stage,  989,  992,  993 
Aquo-igneous  fusion,  308 
Arachnids,  fossil,  746,  762*,  794,  820* 
Arachtiophylhan^  769 
Aragonite,   78,   122,   138,   139,   650  ;    com- 

parative  instability  of,  484 
Aral,  Sea  of,  410,  411 
Aralia,  922,  972,  988 
Ararat  Mount,  243  ;  effects  of  lightning  on, 

328 
Araucaria,  881 

Araucarioxylom  818,  850,  869 
Araucnrites,  850,  905 
Arbroath  Flags,  797,  799 
Arc  of  meridian,  measured,  13 
Area.  844,  906,  974,  995,  1010,  1044 
Arce^teif,  845,  862 
*'  Archiean  "  rocks,  680,  684 
ArchnocidariSy  811 
Archwocyathus,  722,  730,  740 
Archivodiscus,  809 
Archwopteris,  785 
Archwopferyx,  893,  894* 
ArchfcoptilvSy  820 
Archegosaurus,  846 
Archimedej>\  841 
Archiulus,  820 
ArcJwdus,  745 

Arctic  fresh-water  bed  (Cromer),  1014 
flora   of    Europe,    history    of,    1025, 

1041 


Arctic  glaciers,  417,  420,  432,  439,  453  ;  ice- 
bergs, 440*,  453 

shells  in  Pleistocene  deposits,  1008, 

1013 
ArctocepfudiLA,  983 
ArctoofOfij  968 
ArcUmys,  1060 

Ardennes,  metamorphism  in,  619 
Ardwell  group,  764 
Arenicolites,  723*,  742 
Arenig  group,  746,  747 
ArethusinOy  664 
Arfvedsonite,  74 
Argillaceous  composition,  104 
Argillite,  135,  179 
ArgiUomis,  967 
ArgwpCj  926 
Argovian,  912 

Aridity,  consequences  of^  329 
ArietUes,  884,  900* 
Ariondlus,  732 
Aristozoe,  724,  742 
Anus,  966,  1021 
Arkose,  132 

Armorican  Sandstone,  771 
Amo,  Pliocene  deposits  of  the,  1017 
Arnusian,  1016,  1017,  1018 
ArpaditeSy  877 
Artesian  wells,  358 
ArthrophycuSy  740 
Arthropitus,  821,  843 
Arih/roatigma,  793 
Artinsk  group,  853 
Artisiay  822 
ArundOy  923 
Arvicoloy  1011,  1060 
"Arvonian,"  710 
Asaphu3y  729,  741*,  743 
Ascension  Island,  34,  201,  260 
Asche  (Zechstein),  842,  849 
Ascocerasj  744 
Ash,  volcanic,  136,  199 
Ash -tree,  fossil,  954 
Ashgill  shales,  749,  750 
Ashprington  volcanic  group,  784 
Asia,  estimated  average  height  of,  39 ;  extent 

of  coast- line  of,  45 ;  active  volcanoes  of, 

260 
pre- Cambrian   rocks   in,    717 ;    Cam- 

briau,    737  ;    Silurian,    776  ;     Devonian, 

790  ;  Carboniferous,  839  ;  Permian,  853 ; 

Trias,    877  ;   Jurassic,    919  ;    Cretaceous, 

956  ;  Eocene,  981 
Asphalt,  145,  602 
Aspidocerasy  884,  907*,  918 
AspidorhyTichuSy  908 
AsjilenitcSy  869 
Aspleniuniy  922,  966,  976 
"  Assise  "  in  stratigraphy,  678 
Assyria,  dust-growth  on  sites  in,  332 
Astartey  854,  883,  887*,  971.  1009,  1011*, 

1044 
Astartian  sub-stage,  909,  912,  915 
AsteroccUamiteSy  816 


INDEX 


1107 


AsteroUpiSy  745,  775,  796 

AsteropecteUf  781 

Asterophylliies,  815*,  816,  843 

AsthenodoTiy  919 

Astian  group,  1016,  1017 

Astreeospongiay  740 

Astrocceniay  900 

Astronomy  aud  geology,  8 

Astrapecteriy  903 

Aatylospongia^  741 

Atherfield  clay,  941 

^/Ayrw,  781,  811,  853,  861 

Atlantic  Ocean,  depth  and  form  of  bottom 
of,  34  ;  volcanoes  of,  260 

AtlantosauruSy  892 

Atmosphere,  currents  of,  16,  326  ;  geological 
relations  of,  31  ;  present  composition  of, 
32,  61  ;  primeval  composition  of,  35, 
809  ;  geological  action  of,  325  ;  move- 
ments of,  327  ;  destructive  action  of,  ibid,; 
reproductive  action  of,  331  ;  action  of 
plants  and  animals  on  the,  471 

Atmospheric  pressure,  326,  327  ;  influence 
of,  on  volcanic  action,  205  ;  influence  of, 
on  water-level,  339,  404,  438,  437 

AtoUs,  487*,  490 

Atractites,  862 

Atrypa,  743,  745*,  781 

Aturia,  1001 

Aucdla,  883 

AttchenaspiSy  744 

Augengneiss,  186 

Augite,  74,  95 ;  in  meteorites,  10 ;  con- 
verted into  hornblende,  703 

Augite-granite,  159 

Augite-porphyry,  170 

Augite-rock,  181 

Augite-achist,  181 

Augite-syenite,  164 

Augite-trachyte,  166 

AulacoceraSf  862 

AulacopteriSy  823 

Auhphyllum^  810 

Aulostegesy  844 

Australia,  pre -Cambrian  rocks  of,  717  ; 
Cambrian,  737  ;  Silurian,  776 ;  Devonian, 
790  ;  Carlwniferous,  839 ;  Permian,  854 ; 
Trias,  877  ;  Jurassic,  920  ;  Cretaceous, 
960  ;  Eocene,  982  ;  Miocene,  1003  ;  Plio- 
cene, 1022 ;  Pleistocene,  1055 ;  recent 
deposits  in,  1067 

Ausweichungsclivage,  543 

Authigenic,  65 

Auvergne,  203,  219,  220*,  229,  231,  240, 
243,  244,  245*,  246,  263,  264,  990, 
1047 

Avalanches,  416  ;  influence  of  forests  on, 
476 

Avicula,  844,  862,  868*,  883,  978,  989, 
1010 

AvicuUi-cantorta  zone,  867,  869 

Avicul4ipeden,  781,  810*,  811,  862 

Axin»i,  975 

AxinuH,  844*,  991 


Aymestry  Limestone,  763,  768 
Azoic  rocks,  680,  684 
Azores,  34 

Bactrjtss,  782 

Baculites,  927*,  928 

Bagariusy  1021 

Baggy  group,  784 

Bagshot  Sands,  970,  973,  976 

Baieray  879,  928 

Baikal,  Lake,  seals  in,  410 

Bairdia,  812,  869 

Bajocian,  903,  906,  913 

Baked  shale,  136 

BakeveUia,  844* 

Bala  group,  746,  748 

Balitnoptera,  987 

BcUanaphyllia,  991 

Balanus,  987,  1045 

Baltic  Sea,  increasing  salinity  of,  36 ;  ground- 
ice  in,  439 

Bamboo,  fossil,  1004 

Banded  structure,  100,  686 

Bandschiefer,  179,  606 

Banksiaf  995 

Bannisdale  Flags,  763 

Barbados,  upraised  oceanic  deposits  of,  494 

Barium,  61 

Barnacles,  protective  influence  of,  476 

Barometer,  indications  of  the,  327 

Barr  Limestone,  753 

BarrandeocrinuSf  768 

Barrandiay  743 

Barren  Island,  253 

Barrier-reefs,  489* 

Bars  of  rivers,  398  ;  on  coasts,  399,  464 

Barton  Clay,  970,  975  ;  Sands,  970,  976 

Bartonian,  980 

Barytes,  79 

Basalt,  170,  222 ;  vitreous,  171 ;  artificial, 
302  ;  weathering  of,  81,  348 

Basalt-glass,  171 

Basaltic  (columnar)  structure,  172,  348,  690 

Basic  massive  rocks,  169  ;  gradation  of,  into 
acid,  225,  262,  269,  664 

Basset,  633 

Bastite,  76 

Batagur.  1021 

Bath  Oolite,  898 

Bathonian,  905,  913 

Bats,  fossil,  968 

Bavaria,  pre-Cambrian  rocks  of,  714  ;  Per- 
mian, 849  ;  Trias,  873 

Beaches,  Raised,  20,  286,  286*,  287*,  1040, 
1041*,  1054 

Beania,  880 

Bear,  fossil,  1006 

Beaver,  fossil,  996,  1006 ;  geological  action 
of,  474 

Bed  or  stratum,  600,  678 

Bedded  structure,  104 

Bedding,  forms  of,  498 ;  false,  601* ;  irregu- 
larities of,  604 ;  influence  of,  on  scenery, 
1081 


1108 


TEXT-BOOK  OF  GEOLOGY 


Beech,  fossil,  928,  966 

Beetles,  fossil,  820.  886,  915 

Belemnitdla,  928,  930* 

viucronata-zon^,  938,  946,  947 

plena-zone^  938,  944 

Belemnites,  early  forms  of,  862 

Behmnites,  884,  888*,  928 

jacvlum-zone^  938,  939 

lateralis-zone,  938,  939 

minimus-zone^  938,  939 

seMicanaliculatus{i)-zone^  938, 939, 948 

Belgium,  subsidence  of,  292  ;  peat  mosses 
of,  480  ;  Cambrian  rocks  of,  733  ;  Silur- 
ian, 770  ;  Devonian,  786  ;  Carboniferous, 
834  ;  Cretaceous,  947  ;  Eocene,  975  ;  Oli- 
gocene,  990 ;  Miocene,  998 ;  Pliocene, 
1010,  1015  ;  Pleistocene,  1047 

BelinuruSy  802 

BeUerophon,  724*,  725,  743*,  744,  781, 
811,  852 

Bellerophon  Limestone  (Permian),  852,  874 

Bdlia,  1021 

BelUnurus,  812 

Beloceras,  782 

Belodon,  864 

Belonites,  116 

BeUynorhynchiis,  878 

Belonostomus,  960 

Bdoptera,  973 

Belosepia,  966 

Belotntthis,  884 

Bembridge  Beds,  986,  987 

Beneckeia,  869 

Bengal,  Bay  of,  volcanoes  of,  253 

Benmttites,  880 

Bermuda,  dunes  of,  128,  336 ;  mangrove 
swamps  of,  481 

Bn-y.r,  930,  931*,  958 

Bettongvr,  983 

Betula,  991,  995,  1014,  1026* 

Bei/richio,  742,  812,  819 

Biancoue,  918 

Biotite,  73 

Biotite-traohyte,  166 

Birch,  fossil/ 1004 

Birds,  fossil,  893,  894*,  934*,  935,  936*, 
967,  985  ;  supposed  Triassic,  864,  879 

Birkhill  shales,  765 

Bimn,  1014 

Bison-wallows,  477 

Bitter  Lakes  of  Egypt,  413 

spar,  78 

Bituminous  odour  of  rocks,  107 

Black  as  a  colour  of  rocks,  106 

Blnok-baiid  ironstone,  147 

Black  Crag  of  Antwerp,  999 

Blackdown  Beds,  938,  943 

Blackheath  Beds,  972 

Black  Se.i,  delta  in,  403 

"  Blake,"  Three  Cruises  of,  33 

Blastoids,  811 

Bleaoliing  action  of  organic  acids,  472 

by  intrusive  rocks,  598 

Blocks,  volcanic,  136,  200 


Blood-rain,  337 

Blow-holes  made  by  sea,  444 

Blow-pipe  tests,  88 

Blown  sand,  128 

Blue  as  a  colour  of  rocks,  106 

Bognor  Beds,  972 

Bogs,  478 

Bog-iron,  70,  146,  483 

Boghead  fuel,  851 

Bohemia,  bogs  of,  480 ;  volcanic  pheno- 
mena, 262  ;  pre-Cambrian  rocks  of,  714  ; 
Cambrian,  734  ;  Silurian  plants  of,  740 ; 
Silurian  rocks,  772  ;  Carboniferous,  837 ; 
Permian,  846,  850 

Bohnerz,  146,  153 

Bojan  gneiss,  714 

Bolderian,  990,  999,  1015 

Bolodon,  894 

Bomftax,  924 

Bombs  (volcanic),  136,  200* 

Bone  beds,  142,  744,  759,  825,  867 

breccia,  142 

caves,  647 

Bonneville  Lake,  409 

Bononian,  911,  912 

Boracic  acid  af"  volcanoes,  196,  234 

Borax  lakes,  408 

Bore  in  estuaries,  433 

Boreal  is  bank,  767 

Boric  acid,  in  contact-metamorphism,  610 

Boricky's  method  of  analysis,  88 

Borkholm-zone,  767 

Boniia,  819 

Borrowdale  volcanic  series,  749 

Borscale,  68 

Bos,  1021 

Bostlaphus,  1021 

Bosses,  564  ;  of  granite,  565  ;  of  diorite,  etc., 
571  ;  connected  with  volcanic  action,  569, 
573  ;  converted  into  schist,  573 

Bothriolepis,  790,  796 

Both  riospon di/licn,  909 

Bottom-moraine,  425 

Boulder  beds,  510 

Boulder-clay,  133,  431,  1031,  1042 

Bourbon,  Isle  of,  219,  243,  253 

Bourr/ueticnnus,  925 

Bournemouth,  Eocene  flora  of,  970,  974 

Bovey  Tracey  plant-beds,  9S8 

Box-stoues  (hiocene),  1008,  1009 

Bracheux,  sands  of,  976 

Brachiopods,  fossil.  724,*  725 

BracJit/mctopus,  812 

Brarhi//jhi///iim,  881 

Brarhiftrcma,  906 

Brackleshani  Beds,  970 

Bradford  Clay,  898,  905,  906,  913 

Bradfordian,  913 

Brahmaputra,  delta  of,  403* 

Bniynafhen'um,  1006 

Bra  n  rh  iosa  u  ru.s,  846 

Brathay  P'lags,  763 

Brazil,  dei)th  of  weathering  in,  350 

Brazilian  current,  28  / 


INDEX 


1109 


Breakers,  436,  443 

Breaks  iu  succession  of  organic  remains,  662, 

675,  677 
Breccia,  130  ;  volcanic,  136 
Brecciated    conglomerate,    130  ;    structure, 

103,  635 
Brettelkohle,  837 
Breynia,  1002 
Brick-clay,  133 

Brick-earth,  128,  352;  Palaeolithic,  1058 
Bridger  group,  982 
Bridlington  Crag,  1042,  1044 
Brienz,  Lake  of,  397 
Brine  springs,  362 
Britain,  submarine  plateau  of,  469* 
volcanic  phenomena  of,  200,  258,  261, 

592,  692,  705,  720,  739,  747,  748,  750, 

764,  765,  779,  783,  784,  793,  799,  827. 

828,  847,  848,  988 

pre-Cnmbrian  rocks  of,  698  ;  Cambrian, 


725  ;  Silurian,  746  ;  Devonian,  783  ;  Old 
Red  Sandstone,  797  ;  Carboniferous,  824 ; 
Permian,  846 ;  Triassic,  864 ;  Jurassic, 
897  ;  Cretaceous,  937  ;  Eocene,  970 ;  Oli- 
gocene,  986  ;  Pliocene,  1008  ;  Pleistocene, 
1025,  1042  ;  post-glacial,  1065 

British  Association,  underground  tempera- 
ture, Committee  of,  50 

Brittany,  contact- metamorphism  in,  607 

Brockram,  847,  865 

Brodiay  820 

Bronteus,  743,  780* 

Brontosaurus,  892 

Brontotheridje,  997 

Broniotherium,  1002 

Bronze  Age,  1056,  1064 

Bronzite,  75  ;  in  meteorites,  10 

Browgill  Beds,  763 

Brown  as  a  colour  of  rocks,  106 

Brown  coal,  143  ;  of  Germany,  991 

iron-ore,  153 

Bnixellian,  976,  978 

Bubal  us,  1021 

Bucaprctt  1021 

Buccinum,  909,  987,  995,  1010,  1045 

Bucheustein  Beds,  873,  874 

BuMandia,  880 

Buckthorn,. fossil,  923,  1004 

Budleigh  Salterton  pebbles,  865 

Buhrstone,  131,  981 

Bidimus,  983,  986      ' 

BumastiM,  755 

Bunter  (Trias),  864,  870,  874 

Burdie  House  Limestone,  829 

Burlington  group  (U.S.  Carboniferous),  841 

Burnot  conglomerate,  787 

Bulhotrophis,  740 

Buttes  and  bad  lands  of  North  America,  356 

Byssacanihus^  783 

Bythiniaj  978 

CadurcotueriuMj  985 
Caen  Stone,  913 
CadsaJpina^  974 


Caffer  cat,  fossil,  1061 
Caillasses,  976,  977 
Cainotheriurih,  985 

Cainozoic,  denned,  680,  962  ;  systems,  961 ' 
Caithness  Flags,  797,  800 
Calabria,  earthquakes  of,  272,  273,  274,  276 
Calamites,  793,  816,  843,  875 
CalnmocktdiiSt  816 
Calamodendron^  816,  843 
Calamodoriy  969 
Calamophycusy  740 
CaJamophyllia^  883 
Ca/amostachys,  816 
Calathium,  730 
Calcaire  grossier,  976,  977 
Calcaphanite,  170 

Calcareous  composition,  104  ;  deposits,  365, 
454,  455,  457,  482,  484,  485,  492 

detritus,  disintegration  of,  122    . 

fragmental    rocks  of  organic  origin. 


138 


484 


organisms,  proportion  of,  in  sea-water. 


rocks,  weathering  of,  350 

springs,  362 

Calceda,  779,  782* 

Calceola  group,  786 

Calciferous  Sandstone  series,  825 

Calcination  by  eruptive  rocks,  COO 

Calcite,  77,  84,  122,  139 ;  variations  in 
solubility  of,  according  to  crystalline  con- 
dition, 347  ;  solubility  of;  362  ;  compara- 
tive  durability  of,  484,  651  ;  in  fossilisa- 
tion,  651 

Calcium,  61,  63 

Calcium-carbonate,  63,  66,  77,  87,  122,  149, 
360,  362,  365 

Calcium-sulphate,  78 

Calc-mica-schist,  184,  185 

Calc-sinter,  150,  366,  482 

California,  metamorphosed  Cretaceous  rocks 
of,  628 ;  metamorphosed  Jurassic  rocks  of, 
629 

CaUipteridium,  822,  855 

Callipteris,  843 

t\aiUrb,  965,  990 

Calhpristodus^  829 

Callovian,  907,  913,  915,  918,  919 

Calymene,  730,  741*,  743,  781 

Cidyplrapo,  972 

Canmrophoria^  781,  844 

Cambrian  system,  719  ;  base  of,  680,  697  ; 
rocks  of,  720  ;  volcanic  action  in,  ibid.  ; 
life  of,  ibul. ;  plants  of,  721 ;  in  Britain, 
725  ;  limits  of,  726 ;  in  Scotland,  727,  730 ; 
fossils  of,  found  in  Silurian  system,  730  ; 
in  Ireland,  731  ;  in  Continental  Europe, 
ibid.;  in  Scandinavia,  ibid.  ;  in  Central 
Europe,  733  ;  in  North  America,  735  ;  in 
South  America,  737  ;  in  China,  ibid.  ;  in 
India,  ifnd.  ;  in  Australia,  ibid. 

Camel^pardaliSj  1019 

Camels,  ancestry  of  the,  668 


1110 


TEXT-BOOK  OF  GEOLOGY 


Camdits,  1021 

Campanian,  938,  948,  952 

Campanile t  967* 

Campiuian  Sands,  1047 

Camptomu^t  936 

Camptopteris,  871 

CamptosauruSt  909 

Canada,  frozen  rivers  and  lakes  of,  415  ;  pre- 
Cambrian  rocks  of,  692,  716  ;  Cambrian, 
735  ;  SUurian,  775  ;  Devonian,  789  ;  Old 
Red  Sandstone,  803  ;  Carboniferous,  821, 
840  ;  lYias,  878  ;  Creteceous,  957  ;  glaci- 
ation  of,  1024,  1050 

Caiicellaria,  966,  985,  995,  1011 

CanceUophycus,  914 

CaniSy  1003 

Cafions,  origin  of,  391,  1084* 

Capra,  1021 

Caprinay  928 

CaprotinUy  927* 

Capulusy  781,  1011 

CarabxiSy  888* 

Caracal  y  1016 

Caradoc  group,  746,  748 

Carbon  in  earth's  crust,  61,  63,  67 

Carbon-dioxide,  32,  37,  63,  64,  196,  233, 
234  ;  increases  solvent  power  of  water, 
307,  310  ;  in  rain,  341  ;  in  spring-water, 
360 

Carbonaceous  composition,  104 

deposits,  142 

rocks,  metamorphism  of,  622 

Carbonas  (mineral  veins),  639 

Carbonates,  63,  77,  124 ;  alkaline,  influence 
of,  in  rocks,  310,  360  ;  formation  of,  344, 
364 

Carbonic  acid  {see  Carbon-dioxide) 

Carboniferous  Limestone,  825,  826  ;  fauna 
of,  801 

Slate,  831 

system,   804  ;  basins  of,  ibid.  ;  rocks 

of,  ibid.  ;  climate  indicated  by,  809  ;  life 
of,  ibid.  ;  subdivision  of,  by  plants,  821 ; 
in  Europe,  824  ;  in  Britain,  ibid.  ;  in  Con- 
tinental Europe,  834 ;  in  France  and 
Belgium,  iitid.  ;  in  Germany,  836  ;  in 
Boliemia,  837  ;  in  the  Alps  and  Italy, 
838;  in  Russia,  i6iV/. ;  inSpitzbergen,t6i</.; 
in  Africa,  839  ;  in  Ajsia,  ibid.  ;  in  Austral- 
asia, ibid.;  in  North  America,  840  ;  meta- 
morphism of,  622,  838 

Cardmriasy  1021 

Carcharodmiy  983,  1015 

CardiasteTy  925,  945 

Cardinia,  854,  883 

Cardiocarpiis,  817,  819* 

Cardiocera^y  919 

Cardiodariy  906 

Cardiola,  744,  761*,  781 

CardiUiy  862,  973,  989,  996,  996*,  1009 

Cardium.,  862,  868*,  883,  887*,  927,  967*, 

985,  995,  1010,  1044 
Carentonian,  938,  948,  951 
Carinthian  stage,  873,  S75 


CariophylliOy  991 

Camallite,  79,  149,  850 

Camiola,  subterranean  caverns  of,  368,  369 

Carolinian  group,  1002 

Carpathian  mountains,  old  glaciers  of,  1046 

CarpinuSy  995 

CarpclitheSy  851 

Carrara,  altered  Trias  of,  629,  871 

Carstone,  940,  944 

CaryocariSy  742 

Caryophylliay  925 

Caspian  Sea,  area  of,  411  ;  composition  of 
water  of,  ibid,  ;  depth  of,  402  ;  dunes  of, 
336 

Cassia,  fossU,  923,  974 

Cassian  beds,  873,  875 

Cassiandla,  862,  868* 

Cassuluriay  973,  993,  1001.  1009 

Cassisy  973,  985,  995,  1009 

Castanea,  991,  1017 

CastoTy  1014 

Cat,  fossil,  996,  1006 

Catskill  Red  Sandstone,  789 

CatuniSy  886 

CaidineOy  922 

Caulopterisy  790,  793,  822,  843,  859 

Cave-bear,  1061 

Cavernous  structure,  102 

Caverns,  formation  of,  by  underground  water, 
367  ;  phosphatic  deposits  in,  494  ;  preser- 
vation of  organic  remains  in,  647  ;  Palieo- 
lithic  and  Neolithic  deposits  in,  1058, 
1065 

Caves,  on  sea-coasts,  as  proofs  of  upheaval, 
284 

CebochceruSy  985 

Cellariay  925 

Cellcporay  983,  1003 

Cellular  structure,  102 

Cellulose,  650 

Cement-stone,  150 

Cement-stone  group,  827,  828 

Cementation  of  rocks,  31 1 

Cementing  materials  of  sedimentary  rocks. 

127,131 
Cenomanian,  938,  942,  948,  951,  953,  956 
Cei)hulaspi^,  744,  795* 
Cejyhalograptus,  754 

Cephalopods,  evolution  of  the,   667  ;  reach 
their  highest  development  in   Cretaceous 
time,  928 
Ceratiocarisy  729,  742,  757*,  812 
Ceratitesy  861*,  862 
Ceratodusy  796,  862 
Ceratopsy  933 
Ceratops  Beds,  958 
Ceratopygey  731 
Ceratopyge  limestone,  768 
Cerioporay  811 
CeHtcUa,  907 
Ceriihiumy  862,  884,  928,  966,  967*,  985,* 

999,  1011 
Cerithium  stage  (Miocene),  1000 
Cervusy  1011- 


INDEX 


1111 


Cetioaaurus,  892,  930 

OuBtetes,  742,  810 

Chalcedony,  65,  69 

Chalicotherium,  985,  1002,  1021 

Chalk,    82,    140;    pbosphatic,    142,   494; 

absorbent  power  of,  366  ;  marmarosia  of, 

602 

Grey,  944 

Nodular,  945 

Red,  939,  944,  953 

Upper,  Middle,  and  Lower,  938,  943 

Chalk-marl,  938,  943 

rock,  938,  945,  946 

Challenger  Exj>edition,  reports  of,  33,   35, 

36,  37  ;   results  of,  404,  452,  458,  455, 

457,  458*,  459*,  650 
Chalybeate  waters,  362,  366 
Chalybite,  78 
Chajna,  966,  974,  1009 
ChamsBcyparis^  977 
Chamaerops^  973 
Chamops,  969 

Champlain  group,  1053,  1054 
Chara  forms  calc-siuter,    482 ;  fossil,  976, 

984* 
Cham  wood  Forest,  rocks  of,  711 
ChasTnopSj  743 
Chazy  group,  775 
Cheiracanthu3^  796 
Ckeirodusy  819*,  820 
CheiroUpis,  796,  879 
Cheirotherium,  866 
CTieirurus,  729,  743,  781 
Chellean  deposits,  1057 
Chelone,  930,  973 
Chemical  analysis  in  geology,  64,  87 

synthesis,  64,  89 

transformation,   heat    produced    by, 

298 
Chemistry  of  rocks,  124 
ChemnUzia,  844,  862,  901,  1010 
Chemung  group,  789 
Chert,  141,   154,  805,  826  ;  pre-Cambrian, 

693  ;   with  radiolaria  in  older  Pabeozoic 

rocks,  708,  751 
Chesil  Bank,  451 

Chester  group  (U.S.  Carboniferous),  841 
Chestnut-trees,  fossil,  966 
Chiastolite,  76 

slate,  179 

Chillesford  Crag,  1006,  1012 
Chimborazo,  glaciers  of,  418 
Chinai  action  of  wind  in,  329  ;  pre-Cambrian 

rocks  of,  717  ;  Cambrian,  737  ;  Silurian, 

776 
— —  clay,  77,  133 
qiione,  983,  1003 
Chitin,  650 
IhUon,  844 
Chitra,  1021 
Chlorides  in  sea  water,  36  ;  in  the  air,  83 ; 

in  rocks,  79  ;  at  volcanoes,  196,  228  ;  in 

springs,  361,  362  ;  in  salt  lakes,  411 
hlorine,  61 


Chlorite,  77,  365 

rocks,  183 

^schist,  183,  188 

Chloritic  Marl,  938,  943 

ChloritLsation,  618 

Chloritoid,  77 

Chlorophseite,  77 

Choeropotamu^y  985,  998 

Choke-damp,  322 

Chondres  ot  cosmic  dust,  457,  458* 

Chondrites,  733,  740,  759 

ChMietes,  743,  7^81,  811,  854 

ChorisioceraSy  875 

Christianite,   formed   in  abysmal   deposits, 

458 
Chromite,  71  ;  in  meteorites,  10 
Chronology  in  geology  determined  by  fossils, 

655,   658,    675 ;    relative   value   of  pre- 

Cambrian,  697 
Chrysichthys,  1021 
Chudleigb  limestone,  784 
Cidaris,  860,  875,  883*,  925 
Cinwlestes,  936 
Cimdichthysy  930 
Cinwlodon,  936 
Cimolcmys,  936 
Cincinnati  group,  776 
Cinder-cones,  244 

Cinnamomum,  923*,  972,  984,  994*,  1004 
Ciply,  Craie  de,  948 
Cipolino,  151 

Circumdenudation,  hills  of,  1083 
Cirques,  origin  of,  1088 
Cirri  pedes,  fossil,  742 
Cissusj  995 

Citric  acid  as  a  mineral  solvent,  87,  472 
Civet,  fossil,  985 
Cladiscites,  862 
CladiscuSf  822 
CladodM,  812 
Cladophlebis,  879 
Cladyodon,  863 
Claiborne  Beds,  981 
Claosaurus,  933,  969 
Clarias,  1021 
Clastic  rocks,  126  ;   determination  of,  84  ; 

structure,  103,  121,  122* 
CUUhraria,  880 
CUUhrograptuSf  751 
Clathropteris,  859,  899 
Clausilia,  1018 
Clavalithes,  967*  ' 
Clay,  definition  of,  133  ;  origin  of,  77,  132  ; 

absorbent  i>ower  of,  306 
Clays,  red  and  grey,  of  deep  sea,  457 
Clay-ironstone,  78,  147,  153,  806 

rocks,  132,  133 

slate,  134,  179,  188,  314,  319  ;  meU- 

morpbism  of,  610,    619  ;   microlites  and 

crystals  in,  619 
Claxby  Ironstone,  940 
Cleat  of  coal,  525 
Cleavage,  due  to   pressure,  312  ;   examples 

of,    313*,    315 ;    experiments*  in,    814  ; 


1112 


TEXT-BOOK  OF  GEOLOGY 


origin  of,  314 ;  compared  with  jointing, 
527  ;  relation  of,  to  foliation,  546  ;  strain- 
slip,  543  * 

Cleaved  structure,  103 

Cleidophorvsj  744,  745 

CleithrolepUj  877 

CiemmySf  1021 

Cleodora,  1001 

ClepsydropSf  846 

aiff  debris,  127 

Climacammi/uif  809 

ClinutctH/raptus,  722,  741 

Climate  in  its  geological  relations,  23  ;  indi- 
cated by  organisms,  654  ;  in  the  Carbon- 
iferous period,  809  ;  in  Jurassic  time,  895 ; 
indications  of  changes  of,  during  Teartiary 
and  post -Tertiary  time,  964,  965,  966, 
972,  973,  974,  992,  995,  998,  1000,  1002, 
1005,  1006,  1009, 1010, 1013, 1014, 1015, 
1017,  1018,  1023 

Climatiu.%  800 

Clinkstone,  166 

Clinochlore,  77 

Clinometer,  631 

Clinton  group,  775 

Cliona,  754 

Clisioph  ijU  a  m,  810 

Clonoyraptu^,  747 

Clouds,  formation  of,  340 

Clyde  Beds,  1043 

Cli/in^yiiuy  781 

Clypeaster,  983 

iUypeva,  883 

Coal,  143*  ;  chemistry  of,  822  ;  columnar, 
599  ;  effects  of  depression  upon,  297 

Old  Red  Saudstoue,  800  ;  Carbonifer- 
ous, S06  ;  Permian,  842  ;  Triassic,  869, 
870;  Jurassic,  905,  917;  Cretaceous,  921, 
953,  954,  955,  958,  959,  960,  961  ; 
Eocene,  979  ;  Oligocene,  991,  993  ;  Mio- 
cene, 999,  1002 

Coal -basins,  origin  of,  804 

Coal-measures,  825,  832 

Coal-seams,  channels  in,  504*,  505*  ;  associ- 
ated with  tireclay,  514  ;  persistence  of,  516  ; 
joints  of,  525  ;  alteration  of,  by  igneous 
rocks,  588,  589,  600  ;  mode  of  occurrence 
of,  806  ;  origin  of,  806  ;  flora  of,  814 

Coast -barriers  of  detritus,  399,  454 

Coast-lines,  44  ;  in  relation  to  depth  of  sea, 
469 

Cobleiizieu,  787 

Ci}f)HS,  1021 

Coccolite,  74 

Coccosteus,  782,  795*,  796 

Cochliodus,  812 

Cochloceras,  872 

Cod,  fossil,  1012 

Cwlnster,  781 

Coelenterates,  fossilisation  of,  651 

Codoplychinm,  924 

Cctnites,  756 

Ccenoifropt  hs,  748 

CanopUhectfSj  968 


Coking  by  eruptive  rocks,  60O 

Coldweil  Beds,  763 

Coleoptera,  fossil,  820 

Colloid  condition  of  minerals,  65 

Colonies,  Barrande's  doctrine  of,  778,  976 

Colorado    River,    average    sloi>e    of,    876; 

caCons  of,  391,  1084 
Colorado  group,  958 

Coloration  produced  by  eruptive  rocks,  598 
ColossochelySy  1021  ', 
Colour  of  rocks,  106 
Cdumbella,  1010 

Columnar  structure,  104,  300,  527,  590 
Comby  structure  (mineral  veins),  635 
Comley  Sandstone,  727 
ComoseriSj  882* 

Compact  structure,  97,  99,  103 
Composition  of  rocks,  104 
Compression,  effects  of,  311,  527,  614 
Compseniys^  958 
Compsognathus^  892 
Concretionary  structure,  66,  103,  510,  618*, 

1053 
Condros,  Psammites  de,  786 
Condrusien,  786 
Cones,    volcanic,    192 ;   structure   of,   210 ; 

origin  and   growth   of,    216,    240,   242; 

types  of,  244 

alluvial,  393 

Conformability,  641 

Congeria,  1011*,  1018 

Congerian  stage,  1018 

Conglomerate,  130;  schistose,  181 ;  volcaoic, 

136,  201  ;  as  evidence  of  shore-lines,  610; 

associated    with    sandstone,    515  ;    local 

character  of,  515  ;  may  belong  to  different 

horizons  along   the   same  outcrop,    616 ; 

deformation  of,  314  ;   metamorphism  of, 

626 
Conglomeratic  structure,  103 
Conifers,  fossil,  793,  818* 
Coniophis,  969 
Coniosaurus^  930 

Coniston    Flags,   763  ;    Grits,  ibid.  ;  Lime- 
stone, 749 
CoTWCfirdiiim,  810*,  811 
CimocepfuditeSy  724 
Conocoryphey  722*,  724 
Conodonts,  744,  745 
Conor  bis,  975 

Consolidation  due  to  pressure,  311,  SJ2 
Contactschiefer,  179,  606 
Contemporaneous  igneous  rocks,  561,^80 
Continents,  form  and  grouping  of  the,  38; 

of  ancient  origin,  38,  459  ;  permauenc'»of, 

296,  650,  1069 
Contortion  of  rocks,  317,  1072,  1075  ;  &A 

false  bedding,  502  ;  and  metamorphism 

681 
Contraction,  effects  of  terrestrial,  264,  1070 

of  rocks,  526 

Canidaria,  724*,  725,  744,  798,  812* 

Coniis,  966,  967*,  985,  998,  1016 

Cooling,  influence  of,  on  lava,  225  ;  on  under- 


INDEX 


1113 


ground  rocks,  292,  300  ;  of  the  earth,  53, 
1070 

Coombe-rock  of  Sussex,  1042 

Copper-slate,  850 

Copper-ores,  diffusion  of,  842,  849,  853 

Copperas  (iron  vitriol),  in  spring  water,  362 

Coprolitic  nodules  and  beds,  142,  646 

Coquina,  485 

Coral-mud,  456,  486 

Coral-reefs,  485  ;  upraised,  284  ;  as  evidence 
of  subsidence,  290,  488,  492  ;  destruction 
of,  by  boring  shells,  474  ;  growth  of,  485  ; 
distribution  of,  486  ;  oolitic  structure  pro- 
duced at,  ibid.  ;  interstratitication  of  vol- 
canic detritus  at,  487  ;  connection  with 
volcanic  ishinds,  490  ;  Darwin's  theory  of, 
290,  488-f 

Coral-rock^^e,  486,  804 

Corallian,'^8,  908,  912,  916,  916,  918 

Coralline  Crag,  1008,  1009 

ConUliophngn^  1009 

Corals,  fossil,  722,  742,  749,  779,  784,  790, 
804,  810,  844,  882,  900,  908,  925 

Corbicula,  959,  999,  1011,  1044 

Corbi,t,  907 

rorbula,  875,  959,  966,  967*,  986 

Cordnit^s,  816,  843 

Cordierite,  76 

Cornbrash,  898,  901,  905,  906 

Corniferous  Limestone,  790 

Cornstone,  150 

Comubianite,  187,  605 

Comulitesy  760 

Cornus,  981 

Corries,  origin  of,  1088 

Corsite,  165 

Corundum,  69 

Coryil^UiSj  888* 

Corylus,  988 

Corynella,  924 

Cori/pht)don,  968 

Coseguina,  eruption  of,  214,  216 

Coseisiuic  lines,  274 

Cosmic  dust,  68,  342,  457,  458* 

rosmorera.%  884,  904*,  907* 

Cotham  Stone,  867 

Cotomasferf  965 

Cotopaxi,  195,  202,  206,  213,  214,  231,  232, 
242 

Country-rock,  633 

Couserauite,  76 

Coutchiching  rocks,  716 

Crag,  1008,  1023 

Crainjopsi%  812 

Crania,  743,  745*,  906,  926 

Crassatelln,  958,  974 

Crater-lakes,  240 

Craters,  volcanit-,  192,  243 

Cray-fish,  burrowing  habits  of,  474 

Credneria,  922 

Crematopteris,  859 

Crenic  acid,  471 

Creosaurus,  919 

Crests  of  mountains,  decay  of,  1086 


Cretaceous  system,  920  ;  rocks  of,  ibid. ; 
flora  of,  922 ;  fauna  of,  924  ;  valleys  of, 
in  Carboniferoiu  rocks,  931  ;  local  de- 
velopment of,  936  ;  provinces  indicated 
by,  920,  937  ;  in  Britain,  937  ;  in  France 
and  Belgium,  947  ;  in  Germany,  953 ;  in 
Switzerland  and  the  Cliain  of  the  Alps, 
954  ;  in  the  basin  of  the  Mediterranean, 
956  ;  in  Russia,  ibid, ;  in  India,  957  ;  in 
North  America,  ibid.;  in  Australasia,  960 ; 
metamorphism  of,  628 

Crevasses,  419 

CrictiHs,  1060 

Criuoidal  limestone,  140 

Crinoids,  fossil,  722,  742,  780,  811,  869 

Crioceras,  927*,  928 

CrLsfellaria,  900,  924* 

Crocodiles,  earliest  forms  of,  864,  887,  931, 
933 

CroaHlilm,  958,  1021 

CrossojwdiOf  764 

Vrossopterygidw,  fossil,  796 

Crota/ocrinuSf  756 

Crumpling  of  rocks,  541 

Crushing,  heat  produced  by,  267  ;  effects  of, 
on  rocks,  616,  626,  690,  703*,  704* 

Crust  of  earth,  7,  46,  56  ;  composition  of,  60 

Cnistacca,  fossil,  722*,  723 

Cniziana,  723 

Cryolite,  61,  79 

VryphttHs,  781 

Cryptocaris,  742 

Cryptoclastic  structure,  103 

Cryptocrystalline  structure,  97 

Cryptodraco,  909 

CryptoyraptuSy  748 

CryptomeriteSj  905 

Crystalline  parts  of  rocks,  109 

structure,  64,  97 

Crystallisation,  experiments  in,  300,  302,307, 
309,  310,  311 

CVystallites,  64,  115,  301 

Crystals,  corrosion  of,  in  rocks,  109  ;  dif- 
ferent stages  of  formation  of,  155  ;  enclosed 
within  crystals,  114*  ;  secondary  enlarge- 
ments of,  110,  132 

Ctenacanthus,  782,  313*,  820 

('tenacodon,  919 

Ctenodonta,  724»,  725,  744 

Cienodus,  812,  820 

CtenophyUwn,  879 

Ctenoptychius,  812,  820 

Cuboides  Beds,  786 

Cuadiaa,  781,  782»,  971,  994,  1003 

Cuise,  sands  of,  976 

Culm,  821,  826,  837 

Cuma,  987 

CunninghamiteSf  922 

CupaniUj  973 

Cnpressiniies,  965 

Vupre^tinoxylon,  988 

Cupressocrinidtef  780 

Cupressus,  991 

CupulariOf  1010 


1114 


TEXT-BOOK  OF  GEOLOGY 


Cnrrent-bedding,  501*  ;  deceptive,  in  schis- 
tose rocks,  184 

Currents  of  the  ocean,  338,  839,  434 

CurtonotuSf  781 

Curvature  of  rocks,  536 

Custard-apples,  fossil,  984 

Cyatkaspis,  798 

Cyathaxoniat  74 2 j 

Cyaiheites,  887 

CytUhina,  991 

CyatkocrinidsB,  780,  811 

Cyathocrinusy  742,  749,  809*,  844 

CycUhophoniy  906 

Cyatfwphyllum,  742,  780,  807*,  810 

Cybele,  743 

GycadeostrobuSf  880 

Cycadmocarpusy  880 

Cycadites,  879,  880 

Oycadoideaf  880 

CycadospadiXy  880 

Cycads,  Age  of,  860  ;  first  appearance  of, 
844  ;  great  development  of,  859 

Oycas,  922 

Cydas,  986 

Cydodadia,  816 

OydognathiiSj  731 

CydolUes,  926 

Cydolobus,  852 

Cydonemay  744 

Cyclones,  effects  of,  331 

Cydopterisy  793,  816,  859,  877 

dydosUgma,  802,  823 

Cydostoma^  989,  999 

Cynocephalusy  1021 

Cynodoriy  968 

Cynodracon^  863 

Cyphaspisy  743,  757* 

Cyphosoma,,  925 

Cyprnay  966,  993,  995,  1010 

Cypress-swanipa,  807 

Cypricardia,  901 

Cypricardiniay  785 

CypridelUna,  812 

CypHdimiy  780*,  781 

**Cypridinen-schiefer,"  781,  784,  785,  786 

Cyprinay  884,  971,  1012,  1044 

Cyj[^m,  911,  953 

Cyren^T,  901,  910,  953,  966  967*,  985,  1017 

Oyr^w,  781 

Cyrtinri,  785,  861 

Cyrtoceras,  729,  744,  748,  781,  812,  844 

Cyrtograptus,  741 

Cyrtoplev rittSy  875 

Cyrtothecay  728 

Cystideans,  722,  742,  781,  811  ;  as  type 
fossils,  657 

Cystip/ii/Uum,  780 

CytJiere,  749,  812 

Cytherea,  966,  984*,  985,  995,  1010,  1044 

Dachstein  Limestone,  873 
Dacite,  167 
Dacrythenunij  985 
Dactyloidites,  722 


Daclyloporay  976 

Dadoxylofiy  793,  818,  847 

DakosaurttSy  909 

Dakota  group,  958 

Dala  Sandstone,  713 

Dalmanitesy  743,  781 

Dalmatia,  subsidence  of,  292 

Dalmatinusy  874 

"  Dalradian  "  series  of  Scotland,  627,  708 

Dalsland  group,  713 

Dalveen  group,  765 

Daminaray  878,  923 

Damonia,  1021 

Damourite,  74 

Damuda  Beds,  661,  854,  877 

DaneeiiUy  922 

Danian,  938,  947,  948,  952,  962 

Danube,  mineral  water  dissolved  in,  879  ; 
sediment  suspended  in,  383 ;  delta  of, 
403,  area  of,  462  ;  amount  of  rock  re- 
moved by,  462 

DaandlOy  862 

Dapediusy  862,  886 

DaphfenvSy  1003 

Daphncy  995 

DaraditeSy  852 

Dasomis,  967 

Dasyceps,  847 

Dasyurusy  1023 

DawsondUiy  821 

Dawsoniay  846 

Davidiay  725 

DavidsonellUy  737 

Dead  Sea,  composition  of  water  of,  411,  412 

Deccan  Traps,  259,  957 

Deer,  ancestral  forms  of,  968,  996 

Deformation  of  rocks,  314,  543,  615,'6S9 ; 
exaggerated  views  of  effects  of,  615,  690 

Deinocerata,  969,  970* 

Deinosaurs,  863,  890*,  892,  930,  933,  969 

DeinoUieriumy  995,  997*,  1006 

Deister  Sandstone,  953 

Dejection,  cones  of,  393 

Delessite,  77,  365 

Delphinusy  1012 

Deltas,  origin  of,  397,  400,  401 ;  in  the  sea, 
400  ;  entombment  of  organic  remains  in, 
647 

Deltocyathusy  983 

Denbighshire  grits,  753,  762 

Dendrerpetony  846 

Dendritic  forms,  71 

DeiidrocriniLSy  722 

Dendropvpay  821 

Denmark,  peat-mosses  of,  479,  480,  1066  ; 
shell-mounds  of,  1066 

Densities,  planetary,  8 

Density  of  solid  and  melted  bodies,  56 

Dentalinay  900 

Dentalxuv^  1003,  1045 

Denudation,  subaerial,  460  ;  marine,  466  ; 
relation  of,  to  movements  of  the  earth's 
crust,  295,  467,  1070;  effects  of,  659, 
560*,  565,   666,   568,  626,   1069;    pre- 


INDEX 


1115 


Cambrian,  692  ;  and  deposition,  460,  470, 
692  ;  and  upheaval,  283,  293  ;  terrestrial 
features  due  to,  1079 

Deoxidation,  343,  364,  472 

Deposition,  inequalities  of,  504  ;  relation  of, 
to  movements  of  the  earth's  crust,  295, 
467,  1070;  and  denudation,  460,  470, 
692  ;  and  depression,  283,  293 

Depression,  terrestrial  {see  Subsidence) 

Derbyia,  854 

Desert  Sandstone,  960 

Deserts,  330,  334,  336 

Desmosite,  179,  606 

Detritus,  117 

DeiUzia,  991 

Devillien,  733 

Devitritication,  98,  100,  115.  117,  119,  120, 
121,  161,  162,  163,  224,  230,  301,  307, 
309,  310,  345,  575*,  576 

Devonian  system,  777  ;  rocks  of,  778  ;  life 
of,  779 ;  in  Britain,  783  ;  in.  Central 
Europe,  785  ;  in  Russia,  788  ;  in  North 
America,  789 ;  in  Asia,  790  ;  in  Austral- 
asia, 790 

Dew,  impurities  in,  342 

Diabase,  170 

Diabase-aphanite,  170 

Diaclase,  523 

Diallage,  75 

Diamond,  67  ;  in  meteorites,  10 

Diastopora,  883,  906 

Diastrome,  499 

Diatom-earth,  141,  481,  1002 

Dicdlog^rajjtuSf  741 

Diceras,  912 

Diceratheriumy  1002 

Diceratian  sub-stage,  912 

Dichobune,  968 

Dichoilon,  968,  988 

Dichograptus,  747 

Dichroism,  95 

Dichroite,  76 

Didonius,  933 

Dicotyledons,  earliest,  793,  922,  954  ;  final 
predominance  of,  964 

Dicranographis^  739* 

Dicroceras^  996 

DictyocariSj  742 

Dicti/ogmptus,  722,  729 

Dictyonema,  722,  729 

Dictyoneuray  820 

I>iciyopyget  878 

Dictyoxylojiy  823 

Dicynodon,  863 

Dicynodont  reptiles,  863 

Didelphopsy  936 

Didelphys,  973 

Didyinaspis^  798 

Dufyinites,  862 

DidyvKXiraptuSy  739*,  741 

Diestiau  stage,  999,  1009,  1015 

JHkelocf'phaltiSj  724 

Diluvial  formations  (see  Pleistocene) 

**Dimetian,"  710 


Dinwrphodonj  891 

DimarphograptuSt  763 

DinarUeSf  876 

Dingle  Beds,  802 

Dinichthys,  790,  796 

Dioonites,  878,  880 

Diopside,  74  ;  in  meteorites,  10 

Diorite,  165  ;  bosses  of,  571 ;  contact-meta- 

morphism  by,  572  ;    conversion  of,    into 

schist,  573,  627 
Diorite-schist,  182,  572,  627 
IHospyroSj  973 

Dip  of  strata,  531 ;  qufi-qud-versal,  533 
Dip-faults,  552 
Dip-joints,  525 
Diphya  Limestone,  918 
Diphyoides  beds,  918 
DiplocanthuSj  796 
IHplocynodoTiy  919,  975 
Diplograptus,  739*,  741 
DiploporUy  874 
DiplopteruSy  796 
DiplopitSy  968 
DiplospondyluSf  846 
Dipnodon,  936 
Diprotodon,  1022 
Dipteronotus,  866 
Dipterus,  795* 
Dipyre,  76 
Dipyre-slate,  179 
Dirt  beds,  654,  910 

Discina,  723*,  743,  745*,  811,  819,  909,  939 
IHsciiiocaris,  742 
JHscites,  812* 
Disccfidea,  925,  944 
DiscoJiauruAf  933 
Disintegration  of  rocks  in  sitVy   351,  431, 

1030  (see  under  Weathering) 
Dislocation  of  rocks,     318,  547  {see  under 

Faults) 
Dislocation-metamorphism,  596 
Dithyrocaris,  812 
Ditroite,  164 
Ditrupa,  901,  977 
DocodoHj  919 
Dog,  fossil,  985 ;   introduction  of  domestic, 

1063 
Dogger  (Jurassic),  898,  905,  916 

Bank,  origin  of,  455^ 

Dogwood,  fossil,  923 

Dolerite,  169  ;    weathering  of,  348  ;    bosses 

of,  571  ;   melting  down  of  contact  rocks 

by,  ibid. 
Dolgelly  Slates,  729 
DolichosauruSf  931 
DolichosonMj  846 
Dolicopithecus,  1006,  1016 
Dolinas,  367,  956 
Dolomite,  78, 151,  805  ;  weathering  of,  844, 

349  ;  formation  of,  412 
Dolomitic  Conglomerate,  865 
Dolomitisation,  321,  322,  618,  805 
Domite,  166 
Dorcatherium,  1002,  1021 


1116 


TEXT-BOOK  OF  GEOLOGY 


Dormouse,  fossil,  985 

DorycordaiteSy  834 

Dorypyge,  737 

Dosinea,  1023 

Douariienez,  Phyllades  de,  733 

Downtoii  Castle  Sandstone,  753,  760 

Drainage,  effects  of,  496 

Drainage-lines,  permanence  of,  1080 

Dreis.fcna,  999 

Drift-wood,    transport  of,    by  rivers,  401  ; 

marine  accumulations  of,  455 
Droinatherium,  864 
Dromornisy  1022 
Dromotherivm,  985,  1019 
Druid-stones,  355 
Drums  or  <lrunjlins,  1032,  1053 
Drusy  cavities,  66,  102,  109,  635 
Dryamira,  974,  984,  995 
Dryandroulesy  984 
Dryolestes,  919,  936 
Dryophyllnmy  922 
J>ry()pUh€cus,  996,  997* 
''Dry  way"  analysis,  89 
Dudley  Limestone,  753,  756 
Dufton  shales,  750 

Dunes,  128,  33 1;  protected  by  vegetation,  475 
Dunite,  173,  183 
Dunstone,  321,  827 
Dura  Den  beds,  797,  801 
Durance,  sediment  in  the,  383 
Durness  Limestone,  699,  728,  730 
Dust  in  the  air,  32  ;  growth  of,  on  the  surface 

of  the  land,  331  ;  volcanic,  213 
Dust-storms,  332,  337 
Dya-s,  841 
Dykes,  209*.  210*,  220,  233,  258,  577,  582*, 

587*,  989 
Dyiianiiciil  metnmorphism,  596 
Dwarfed  organisms,  evidence  of,  654 
Dwyka  Conglomerate,  855 

EAaLE-STOXES,   147 

Eartli,  crust  of,  7 ;  relations  of,  in  Solar 
system,  8  ;  density  of,  9,  45  ;  form  and 
size  of,  13  :  distribution  of  sea  and  land 
OH,  14  ;  earliest  surface  of,  14  ;  move- 
ments of,  15  ;  axis  of,  16,  17  ;  changes  of 
centre  of  j:fravity  of,  20 ;  eccentricity  of 
orbit  of,  IG,  24*;  crust  of,  46,  54,  60  ; 
interior  of,  47,  53,  56  ;  internal  heat  of, 
48  ;  rigidity  of,  54,  57  ;  age  of,  58 ; 
sources  of  energy  in,  189  ;  origin  of  sur- 
face features  of,  293,  1068  ;  contraction 
of,  59,  294,  1070 

Eartliquake.s,  270  ;  amplitude  of  earth- 
movements  in,  271  ;  velocity  of,  272  ; 
duration  of,  273  ;  iuHuenced  by  geological 
structure.  ?//<>/. ;  sometimes  arise  from 
volcanic  action,  207  ;  extent  of  country 
affected  by,  274 ;  geological  effects  of, 
276  ;  distribution  of,  279  ;  origin  of,  280, 
369  ;  jointing  of  rocks  referred  to,  527  ; 
sandstone  dykes  produced^  by,  582  ;  de- 
struction of  life  by,  648 


Earth -pillars,  355 

Earth-worms,  geological  action  of,  352, '353, 
473 

Eatonia,  768 

EccidiomphaluSy  748 

Echini,  embryonic  development  of,  667 

Echinids,    Cretaceous    aspect    of    deep-sea 

forms  of,  925 
EchinobrissHS,  883,  925 
KchinoconuSy  925* 
Echinocorys,  925* 
Echinoderms,  fossilisation  of,  652  ;  maximum 

development  of,  810 
Echinoids,  early  jiredominance  of,  883 
EchinospatanguSy  939 
EchinoxphscriieSy  742 
Ecliptic,  change  in  obliquity  of,  1 7 
Eclogite,  182 
Eilmmidia,  811,  844 
Efflorescence  products,  338 
Egeln,  Oligocene  beds  of,  991 
Eifel,  volcanoes  of,  197,  201,  213,  234,  240, 

244,  251,  587 
Eifelien,  786 
Elajolite,  73 
El*olite-syenite,  164 
KiasmosavruSy  933 
Elbe,  discharge  of,  374  ;  influence  of  man 

on,   374  ;    mineral    matter    dissolved  in, 

378  ;  sediment  suspended  in,  383 
Elements,  chemical,  60,  Ql 
Klephasy  1006*,  1036 
KUphas  antiquuSy  Age  of,  1061 
Elevation,  at  volcanoes,  231,  251  ;  by  earth- 

(|uakes,  278  {see  Upheaval) 
Elevation-crater  theory,  226,  241 
Elgin  Sandstone,  863 
Ely  in  1(1,  863 
Elk,  Irish,   480,  1061  ;   final  extinction  of 

1063 
Ellip.^ncephdus,  722*,  724 
Elm,  fossil,  954,  966,  1004 
Ehynichthysy  829 
ElotheriHniy  1002 
Elton   Lake,  composition  of  water  of,  411, 

412 
Eluvium,  333 
El  van,  159,  579 
Embryonic  development^  666 
Enii)vreumatic  odour  of  rocks,  107 
Emyda,  1021 
Emys,  958,  987 
Ennlhdtelys,  909 
EnaUorniSy  934 
Enaliosaurs,  888,  889* 
KnalJocnnuSy  768 
Eiichodusy  930 
Encrinite  Limestone,  140 
Encrinxirus,  743 
EncrbuiSy  860,  861* 
EndoceraSy  748 
Endomorph,  65,  69 
Endolhyniy  809 
Energy,  sources  of  geological,  189,  190 


INDEX 


1117 


Enstatite,  75,  302  ;  in  meteorites,  10 

Enstatite-dolerite,  170 

Entelodon,  985 

Bntmnis,  728,  742,  780*,  781 

EiUomoceras,  877 

EobasiUuSy  970 

Eocene,  defined,  962 

system,  general  characters,  964  ;    flora 

of,  965  ;  fauna  of,  966  ;  in  Britain,  970 ; 

in   Northern  France  and  Belgium,  975  ; 

in  Southern  Euroi>e,  979  ;  erratic  blocks 

of,   ibid.  ;    in  the   Alps,  980 ;    in    Italy, 

ibid.  ;  in  India,  981  ;   in  North  America, 

ibid.  ;  in  Australasia,  982 
£ohippv.%  668,  969 
Eohyus,  969 
£osauru8j  840 
JSoscorpius,  820* 
Eozoic  rocks,  680 
Eozoon,  694 
Epiaster,  946 
Epicampodoiij  877 
Ephemera,  fossil,  794 
Epidiorite,    165,  182  ;    metamorphic  origin 

of,  618 

schist,  182 

Epidosite,  183 
Epidote,  76 

rocks,  183 

schist,  183 

Epidotisation,  618 
Epigene  action,  190,  325 
Eppelsheim,  bone-sand  of,  999,  1017 
Epsomites,  316 
Equatorial  current,  339 

diameter  of  earth,  13 

Equinoxes,  precession  of,  16,  30 
Equisetites,  844 
Equisetum,  859*,  880 
Equxts,  1006,  1014 
Erbray  Limestone,  788 
Erie  Lake,  area  of,  1052 
£rinnf/Sj  722 

rrosion,    contemi)oraneous,  506  ;    of  land, 
fundamental    law   of,    1080 ;    conditions 
'^       governing,    ^ul.  ;    influence   of  angle  of 

slope  on  rate  of,  ibuL  ;    j)ermanence   of 

drainage  lines  in,  iUd. 
Erratic  blocks,  128,   425,   1031,    1037  ;    of 

Carboniferous  age,  805 
Eruptions,    volcanic,    206,    207,    210,   255 

(see  vnder  Volcanic) 
Eruptive  rocks,  154,  559 
Enu'lufy  1000 
Eri/ma,  901 
Eryim^  885 

Escarpments,  origin  of,  1088 
Esino  Limestone,  873 
Eskers,  1040 

EsthcrUr,  780*,  801,  812,  819,  860,  861* 
Estuaries,  turbidity  of,  398  ;  deposits  of,  ibid. 
Eth  ill ophyllu in,  722 
Etna,  volcanic  geology  of,  192,   193*,   194, 

196,  200,  203,  206,  208,  209,  211,  217, 


220,  222,  226,  228,  229,  230,  231,  244, 
248*,  249  ;  date  of  appearance  of,  1017 

EucalyptocriniiSj  742 

Eucalyptus,  922,  965 

Euchirosa a rus,  846 

Euclculia,  742 

Eudea,  860 

Ettffenia,  973 

Ewjnathns,  862,  901 

Eukeraspi^y  744 

Eidiniene,  1010 

Euoniphcdus,  744,  761*,  781,  811*,  863 

Enpata/juSy  983 

Euphoberio,  820 

Euphotide,  169- 

Eurite,  160  ;  schistose,  186 

Euritic  structure,  118 

Europe,  estimated  mean  height  of,  89  ; 
extent  of  coast  of,  45  ;  volcanoes  of,  260 
{see  under  Britain,  France,  etc.) 

EurycarCy  731 

Eurynotusy  813 

Eurjpterids,  occurrence  of,  743,  755,  780*, 
796,  812 

Eurytheriuviy  985 

Evaporation  and  river-discharge,  373 

Evolution,  geological  progress  of,  660,  665, 
668  ;  evidence  of  pre-Cambrian,  697 

Exofjyray  883,  887*,  926*,  927 

Erosiplionitesy  758 

ExiKjrience,  duration  of  human,  190 

Explosion-lakes,  240 

Exjilosions,  volcanic,  211,  219,  229 

Exsulans-kalk,  731 

Extinction,  angles  of,  hi  microscopic  investi- 
gation, 95 

Extrucrinus,  882* 

Exudation-visen,  99 

Fahoidea,  965 

F(ibiilariHy  978 

Facies,  i»alajontological,  674 

Faijusy  988,  1017 

Faidbands,  640 

Fairy  stones,  512* 

Fakes,  131 

False-bedding,  501* 

Faluns  of  Touraine,  998 

Famennien,  786 

Fan-sha|>ed  structure,  541*,  1075 

FascictiUo'ia,  1009,  1010* 

Fascuilanffy  998 

Fassaite,  74 

Faults,  547*  ;  nature  of,  818,  548*  ;  throw 
of,  549*,  550  ;  hade  of,  549  ;  origin  of, 
550  ;  normal,  318,  550  ;  reversed,  ibid.  ; 
tlinist-planes,  551  ;  dip  and  strike, 
552,  554*  ;  heave  of,  553*  ;  effects  of,  on 
anticlines  and  synclines,  554,  555*  ;  dying 
out  of,  555  ;  groups  of,  556*  ;  step-,  556*  ; 
trough-,  557*  ;  origin  of,  318  ;  detection 
an<l  tracing  of,  557  ;  and  dykes,  683  ;  in 
mountain  structure,  1074 

Fault-rock,  130 


1118 


TEXT'BOOK  OF  GEOLOGY 


Pauna,  preservation  of  remains  of  terrestrial, 

646  ;  evolution  of,  660,  668 
FavosUe^,  742,  780,  810 
Faxoe,  highest  Cretaceous  rocks  of,  962 
Feel  of  rocks,  107,  183 
Felch  Mountain  series,  716 
Feli^,  1018 

Felsite  (Felstone),  82,  161,  164 
Felsitoid  rocks,  183 
Felsophyre,  98 

Felspars,  71,  302  ;  decomposition  of,  344 
Felspar-amphibolite,  182 
Felspathic  composition,  104 
FenestcUa,  729,  743.  811,  844 
Ferns,  fossil,  793,  814 
Ferric  oxide,  63 
Ferrite,  123 
Ferrous  carbonate,  66,  78,  362 

oxide,  64  ;  oxidation  of,  343 

sulphate,  362 

Ferruginous  cement,  131 

deposits,  146 

Fetid  limestone,  150 

r  odour  of  rocks,  107 

Fibrolite,  76 

Fibrous  structure,  103 

Ficula,  1009 

Ficus,  922,  923*,  972,  995*.  1017 

Fig,  fossil,  923*,  966,  984,  1004 

Filamentous  structure  (mineral  veins),  635 

Filaments  in  crystals,  114 

Fire-clay,  133  ;  association  with  coal,  514, 

806 
Fire-marble,  139 
Fire-wells,  234 
Firn,  417 

Firths,  origin  of,  291 
Fishes,  causes  of  mortality  among  marine, 

649,  801,  877  ;  shells  broken  by,  1009 
fossil,  744,  759,  782,  795*,  796,  812, 

813*,  820,  845*,  877,  886,  929,  931* 
earliest  teleostean,  930 


Fish-excrement,  deposits  formed  of,  485 

Fissility,  kinds  of,  500 

Fissurella,  1010 

Fissures,  volcanic,    192,   208,   219  ;  caused 

by  earthquakes,  276  ;  in  rocks,  547 
Fissure-eruptions,  192,  211,  222,  255,  988, 

1079 
Fissurir astray  952 

Fjelds  of  Norway,  an  old  tableland,  44 
Fjords  of  Norway,  as  evidence  of  subsidence, 

291 
Flabdlnria,  923,  990 
Flahellmiu  993 
Flagstone,  131 
Flammenmergel,  953 
Flat-works,  639 
Fleckschiefer,  179,  606 
Flexures,  various  types  of  terrestrial,  1072 
Flint,  141,  154,  493,  921 
Flinty  structure,  102,  106 
Floe-ice,  439,  450 
Floods,  372,  373,381,382,  385,  395,  415, 416 


Flood-plains,  395  ;  heightened  by  filtering 
action  of  plants,  475 

Flora,  preservation  of  remains  of  terrestrial, 
646  ;  comparative  rate  of  evolution  of, 
660,  668,  959  ;  earliest  terrestrial,  740 

Flow-structure,  100,  120* 

Flucan,  634 

Fluor-spar  (Fluorite),  61 

Fluorides,  79 

Fluorine,  61  ;  at  volcanoes,  196  ;  influence 
of  on  precipitates,  310 

Flustra,  978 

Fluxion-structure,  100,  120* 

Flysch,  955,  965,  979,  980,  992 

Foliated  structure,  103,  106 

Foliation,  cause  of,  568,  604,  610,  614,  615, 
630  ;  artificial  imitation  of,  324 

and  cleavage,  546,  619  ;  and  thrust- 
planes,  551,  619,  701,  703*,  704*;  and 
bedding,  619 

Footprints  in  rocks,  509* 

Foraminifera,  protective  influence  of^  477 ; 
deposits  formed  by,  492 

Foraminiferal  ooze,  139,  140* 

Fordilla,  725 

Foreland  Grits,  784 

Forests,  geological  influence  of,  473,  475, 
476,  496  ;  submerged,  289*,  654,  1054 

Forest-Bed  group  of  Cromer,  1008,  1018 

Forest  Marble,  905,  906 

"Formation,"  definition  of,  678 

Formations,  geological,  674 

Fossil,  definition  of  term,  645 

Fossils,  nature  and  uses  of,  2,  522,  645, 
653  ;  evidence  of  cleavage  from,  315  ;* 
stratigraphical  value  of,  661  ;  show 
changes  in  physical  geography,  653; 
fix  geological  chronology,  655  ;  typical,  in 
stratigraphy,  657  ;  may  prove  inversion, 
ibid.;  prove  the  relative  chronological  value 
of  unconformabilities,  661 ;  subdivision  of 
the  Geological  Record  by,  664  ;  collecting  of, 
347,  669  ;  earliest  known,  694  ;  dwarfed 
forms  of,  830,  850  ;  weathering  of,  670,  671 

Fossilisation,  650,  651 

Fox,  Arctic,  fossil,  1036,  1061 

fossil,  1006,  1014 

Fox  Hills  group,  958 

Foyaite,  164 

Fracture,  105  ;  eff'ects  of,  in  rocks,  311,  317 

of  rocks,  82,  83,  84 

Fragmeutal  rocks,  126  ;  of  organic  origin, 
138  ;  of  volcanic  origin,  135,  199 

Fragmental  structure,  103 

Fragmentenkalk  of  Scania,  731 

France,  ancient  volcanoes  of,  261  {see 
Auvergne)  ;  raised  beaches  of,  287  ;  sub- 
sidence of  coast  of,  290 ;  peat-mosses  of, 
480  ;  metamorphism  in,  629 
pre -Cambrian  rocks  of,  714  ;  Cam- 
brian, 733 ;  Silurian,  770 ;  Devonian, 
786  ;  Carboniferous,  834 ;  Permian,  851  ; 
Trias,  868 ;  Jurassic,  910 ;  Cretaceous, 
947  ;  Eocene,  975  ;  Oligocene,  989  ;  Mio- 


INDEX 


1119 


cene,  998  ;  Pliocene,  1015  ;  in  the  Pleis- 
tocene period,  1024,  1080,  1046  ;  in 
Post-glacial  time,  1065 

Frasnien,  786 

Freestone,  131 

Fresh -water,  destructive  effects  of,  in  the 
sea,  649 

Freshets,  372,  373,  381,  382,  385,  416,  416 

Friable  texture,  106 

Friendly  Islands,  34 

Fringing  coral-reefs,  488 

Frondicularia^  900 

Frost,  346,  413  ;  influence  of,  on  rivers, 
382;  effects  of,  on  soils  and  rocks,  414,  527; 
on  shores,  649 

Fruchtschiefer,  179,  607 

Fuci,  protective  influence  of,  476,  477  ; 
peat  formed  from,  478 

**Fucoid  Bed"  (Upper  Ludlow),  759 

Fulgurites,  328 

Fuller's  earth,  133  "^ 

Fuller's  Earth  (Jurassic),  898,  905,  913  '^ 

Fumaroles,  194,  195,  228 

•*  Fundamental  complex,"  715 

Fundy  Bay,  tides  in,  433 

Fusion,  experiments  in,  300 ;  caused  by 
lightning,  328 

Fusion -point  lowered  by  the  presence  of 
water,  308 

Fusuiina,  809,  852 

Fusulindl<i,  839 

Fusus,  928,  966,  967*,  985,  999,  1012* 

Gabbro,  169  ;  schistose,  182 ;  native  iron 

in,  68 
Gaize,  913,  950 
Gala  group,  764 
OaleocerdOy  978 
OaUHtes,  925* 
OaJetkylax,  985 
Galium,  1049 
OaUiis,  1019 
Oangamopteris,  854 
Ganges,  periodic  rise  of  the,  372  ;  infusoria 

in  water  of,   381  ;    sediment  carried  by, 

383  ;  delta  of,  403*  ;  area  of,  462  ;  amount 

of  material  removed  by,  ibid, 
Gangue,  634 
Gannister,  133 

Beds,  825,  838 

Oanodusy  906 
Garbenschiefer,  179 
GarialU,  978,  1021 
Garnet,  76  ;  fusion  of,  304 

rocks,  182 

Garumnien,  948,  952 

Gases  in  the  air,  32  ;  at  volcanoes,  198,  233  ; 

in  rain,  341  ;  in  springs,  360 
Gas-cavities  in  crystals,  110 
Gas-eruptions,  234,  238,  240 
Gas-springs,  234 
Gas-spurts,  510 
Gash -veins,  639 
Gasp^  sandstones,  808 


Qastomis^  967 

Qaatrioceras,  852 

Oaudryina,  924* 

Gault,  938,  941,  953,  956 

Gaylussite,  413 

Gazella,  1011 

Gedinuien,  787 

Oeikia,  863 

Odidium,  740 

GdocuSy  985 

Genesee  group,  789 

Geneva,  Lake  of,  398,  404,  405,  407,  408 

Geognosy,  31 

Geography,  geological,  895 

Geological  Congress,  International,  678 

Greological  Record,  3,  674  ;  imperfection  of, 
661,  677  ;  subdivisions  of,  678 

Geological  Society  of  London,  influence  of,  on 
progress  of  geology,  7 

Geological  structure,  influence  of,  on  marine 
erosion,  447 ;  on  topography,  1081 

Geological  Survey  of  Great  Britain,  work  of, 
in  N.W.  Scotland,  625,  699,  702 

Geology,  definition  o^  1  ;  wide  basis  of,  1, 
2  ;  special  domain  of,  28  ;  based  on  study 
of  present  economy  of  nature,  8  ;  oni- 
formitariauism  in,  3 ;  cosmical,  4,  7  ; 
geognostical,  4,  31  ;  dynamical,  4,  189  ; 
geotectouic  or  structural,  4,  498  ;  palteon- 
tological,  5,  645  ;  stratigraphical,  5,  674  ; 
physiographical,  5,  1068 

Gephyroceras.,  782 

Geranium,  991 

Germany,  pre -Cambrian  rocks  in,  714  ; 
Cambrian,  734  ;  Silurian,  774  ;  Devonian, 
786  ;  Carboniferous,  836,  837  ;  Permian, 
848  ;  Trias,  868  ;  Jurassic,  915  ;  CreU- 
ceous,  953  ;  Oligocene,  991 ;  Miocene, 
999;  Pliocene,  1017  ;  glaciation  of,  1027  ; 
Post-glacial  dei>osits  in,  1066 

GermUia,  862,  883 

Geyserite,  153,  235,  237 

Geysei-s,  235,  236*,  363,  367 

Giants'  ketUes,  429,  430*,  1031 

GigantosauTus,  909 

Gingkoy  844,  923 

Giraffe,  fossil,  1019,  1021 

GirvaneUa,  151 

Givet,  Calcaire  de,  785 

Givetien,  786 

Glacial  deposits,  in  Britain,  1042  ;  in 
Scandinavia,  1045  ;  in  Germany,  iWrf.  ; 
in  France,  1046  ;  in  Belgium,  1047  ;  in 
the  Alps,  1048 ;  in  Russia,  1049 ;  in 
North  America,  1050  ;  in  India,  1054  ; 
in  Australasia,  1055 

Glacial  period,  succession  of  events  in,  1025 ; 
interglacial  episodes,  1025,  1049  ;  traces 
of  pre-glacial  land -surfaces,  1025  ;  traces 
of  the  northern  ice-sheet,  1026  ;  snowfall 
greatest  in  Europe  towards  the  west,  1027, 
1029  ;  thickness  and  movements  of  the 
ice,  1027  ;  identity  of  general  confignn- 
tion  of  the  pre-glacial  surface  with  that  of 


1120 


TEXT-BOOK  OF  GEOLOGY 


present  time,  1029  ;  fracture  and  cramp- 
ling  of  rocks  by  the  ice,  1031  ;  detritus 
of  the  ice-sheets,  1031  ;  boulder-clay  or 
till,  ibid. ;  iuterglacial  beds,  1033  ; 
remarkable  fauna,  1036  ;  evidences  of 
submergence,  1036,  1043  ;  second  glacia- 
tion,  1037  ;  re -elevation  and  raised 
beaches,  1037,  1040;  latest  valley -glaciers, 
1038  ;  relics  of  the  melting  ice,  kames, 
1040  ;  glacial  lakes,  ibid. ;  closing  stages 
of  the  perioil,  1041  ;  effects  of  the  cold  on 
the  mammalian  fauna  of  the  northern 
hemisphere,  ihid. 

Glacial  periojjv*  v  Idfiliiie  of  successive,  23 
(see  lce^«r6fion) 

Glacieres,  359 

Glacier-ice,  148 

Glaciers,  417  ;  motion  of,  ibid',;  of  the  first 
order,  420*  ;  of  the  second  order,  422  ; 
re-cemented,  422*  ;  ice-falls  from,  423  ; 
lakes  formed  by,  423,  1030,  1087  :  trans- 
port by,  423 ;  erosion  by,  427,  1026, 
1032  ;  supposed  evidence  of,  in  ancient 
geological  formations  (s€«  Ice -action)  ; 
former  greater  size  of,  1048 

Glass,  formation  of  natural,  64  ;  in  rocks, 
114,  120,  301  ;  production  of,  by  fusion 
of  rocks,  301  ;  contraction  of,  in  becoming 
lithoid,  304  ;  devitrification  of,  by  heated 
water,  307,  309  («ec  vnder  Devitrification) ; 
devitrification  of,  by  weathering,  345; 
value  of,  as  test  of  eruptive  character  of 
rocks,  563  ;  occurrence  of,  in  dykes,  584 

Glass-inclusions  in  crystals,  113 

Glassy,  defined,  64 

stnicture,  100,  155,  562 

Glauconitc,  77  ;  in  marine  deposits,  456, 
921  ;  as  a  petrifying  agent,  652 

Glauconite  Saml,  745,  767 

Glaueonitic  Marl,  938,  943 

Sandstone,  131 

Olauconoiiie,  749,  811 

Glaucopliane,  74 

Glaucophane-scliist,  182 

Gleirhenlo,  922 

Glengariir  Grits,  802 

Gleukiln  Shales,  751 

Glon^iferina,  860,  921* 

Globiireiiiia  ooze,  456 

Globulites  115 

Olossocernii,  768 

Glo8soiiroptit,s,  748 

Oh^sopferL<,  8:J9,  844   859 

Glossozinniffs,  J577,  880 

Glutton,  fo.>sil,  1014,  1036,  1061 

Gb/plmii,  1»01 

Glifptam',  721) 

Glyplaapin^  775 

G/f/plichtts,  912 

Glyj)ticiaii  sub-stage   912 

Gbjptocrin  us,  742 

GlypUulcnJi'on,  740 

Glyplolit  II L  a  5,  8  0 1 

Glyptolepis,  796 


Glyptostrobus,  995,  1004*,  1005* 

Gneiss,  176*,  185,  188  ;  igneous  origin  of 
some,  186,  615,  687, 688, 689,  700 ;  Unded 
structure  of,  685  ;  associated  claiitic  rocks 
of,  686,  692,  704;  absence  of  strati- 
graphical  subdivisions  in,  686,  691  ;  re- 
garded as  part  of  the  original  crust  of  the 
globe,  687  ;  analogy  of,  with  structure  of 
intrusive  sills,  177,  687,  688,  689,  701 ; 
supposed  sedimentary  origin  of,  688 ; 
gradation  of,  into  granite,  688*';  mechanical 
deformation  of,  177,  186,  615,  689,  690, 
691,  700,  702  ;  differences  of  age  in,  689, 
691  ;  production  of,  by  granitisation,  605. 
690  ;  systems  of  dykes  in,  691  ;  possible 
association  of,  with  volcanic  action,  691 ; 
graphite  in,  695,  704  ;  pegmatit«  veins 
of,  700* 

Gneiss,  Fundamental,  682 

Gneiss-mica-schist,  185 

Goat,  introduction  of,  1063 

Gobi,  desert  of,  336 

Gomphoceras,  781 

Gondwana  system,  854,  877 

Goiiutster,  903 

Goniatiies,  781,  812*,  852 
'^GomobaniSf  959 

GoniofflyptvSj  877 

Goniomya^  884 

Goninpholis,  887,  931 

Goniopliora^  744,  760,  761* 

Gonioptcris^  855 

Gopher,  geo](^ical  action  of,  474 

Gordoniaj  863 

Gossan,  68 

Gosau  beds,  955 

Graculu  vus,  935 

Graham's  Island,  250,  254 

Granimysiu^  758,  781 

Granite,  1 56  ;  traces  of  glassy  base  in,  159  ; 
absorbent  |>o\ver  of,  306  ;  weathering  of, 
348,  349*  ;  jointji  of,  528 ;  intrusive 
nature  of,  561  ;  eruptive  bosses  of,  565  : 
depth  of  consolidation  of,  112,  565  ;  tem- 
perature of  consolidation  of,  308,  597  ; 
witle  range  of  geoloj^ical  ago  of,  565 ; 
enclosed  substances  in,  566  ;  concretionary 
or  globular,  ibid.;  variations  in  texture  of. 
567  ;  effects  of  pressure  on,  ibid.;  relation 
of,  to  contiguous  rocks,  ibid.;  contact-meta- 
morphism  by,  568,  578,  598,  605  ;  con- 
nection of,  with  volcanic  rocks,  569  ;  neck- 
like  forms  of,  570  ;  supposed  metamorphic 
origin  of,  ibiil.;  eruptive  nature  of,  iijid.: 
laminar  structure  in,  571  ;  veins  of,  578  ; 
impregnation  by,  579  ;  apophyses  of,  580 ; 
pegmatite  veins  of,  581* 

Granitic  structure,  97,  98 

Granitisation,  571,  579,  604,  618,  690 

Granitite,  159 

Granitoid  structure,  118,  155 

Granophyre,  98,  158,  160 

Granophyric  structure,  119 

Granular  structure,  99,  155 


INDEX 


1121 


Granulite,  159,  186,  188 

Granulitic  structure,  99,  119,  187 

Granulitisation  of  rocks,  61 G,  690,  700,  703 

Graphic  structure,  98,  158*,  682 

Graphite,  67,  146,  623  ;  in  Laurentian 
gneiss,  696 

Graphite-schist,  origin  of,  622,  623 

Graptolites,  as  type-fossils,  657,  741  ;  Cam- 
brian, 722;  Silurian,  739*,  741;  Devonian 
779 

Graptolitic  Mudstones,  763 

Gravel  and  sand  rocks,  126 

Greece,  metamorphic  rocks  of,  628  ;  Cretace- 
ous, 956  ;  Eocene,  979  ;  Pliocene,  1019 

Green  as  a  colour  of  rocks,  106 

Greenland,  native  iron  of,  68  ;  sinking  of, 
291  ;  effects  of  frost  in,  414  ;  glaciers  of, 
417,  420,  432  ;  ice-sheet  of,  418,  431  ; 
former  glaciation  of,  426 ;  climate  of,  in 
Cretaceous  time,  922,  960  ;  Miocene  de- 
posits of,  1001 

Green  Mountains,  metamorphism  in,  628 

Green  River  group,  982 

Greensand,  Cambridge,  938,  943 

Lower,  938,  941 

Upper,  938,  942 

Greenstone,  165,  169  ;  bosses  of,  571 

Greisen,  169 

Gres  Armoricain,  733,  771 

Gres  bigarr6,  870 

Oresslya,  883 

GrevUleay  972 

Guano,  142,  494 

Grey  as  a  colour  of  rocks,  106 

Greywacke,  132 

Greywacke-slate,  135 

"Grey  Wethers,"  131,  355,  975 

QHjffithides,  812 

Grit,  131 

Gritty  structure,  103 

Groden  Sandstone,  852,  874 

Ground-ice,  415,  439 

Ground-moraine,  425,  431,  1031 

Ground -swell,  436 

Group  in  stratigraphy,  678 

Oru8,  1019 

Oryphaea,  883,  885* 

Gryphite  Limestone,  884 

Gulf-Stream,  27,  28 ;  influence  of,  on  climate, 
441  ;  transport  of  silt  by,  452 

Gulo,  1014,  1061 

Gum-trees,  fossil,  984 

Guttenstein  Limestone,  873 

Oymnograptusy  768 

Gypseous  composition,  104 

Gypsum,  67,  68,  79,  82,  152,  234,  306,  344, 
843,  848,  849,  1002,  1016,  1019 

precipitated    from    sea  -  water,    411, 

412 ;    decomposition   of,    344 ;    solution 
of,  ibid. 

of  Paris  basin,  978,  989 


Gyracanthus,  820 
Gyroceras,  781,  841,  845 
Gyrodus,  886 


Gyrolepis,  862 
Gyro2M>rdla,  860 
Gyroptichius^  796 

Hade  of  faults,  549 

JUhdromuruSf  933 

Haematite,  70,  153 

Haggis-rock  group,  765 

Hail,  geological  action  of,  416 

Ilakea,  1004* 

Haley omis^  968 

llaliotis,  983,  1022 

Haliserites,  779 

Halleflinta,  183       ' 

llalodon,  936 

HaloniOy  816 

HaJonUs,  862 

Ualy»ite8^  742 

Hamilton  group,  789 

HamUes,  928,  929* 

Hammer,  shape  of  geological,  81 

Hamstead  (Hempstead)  Beds,  986 

Hangman  Grits,  784 

Haploc^raSy  928 

Hardness  of  minerals,  table  of,  82 

Hare,  fossil,  1006, 1060;  Alpine,  fossil,  1061 

Haring,  Eocene  coal  of,  979 

Harlech  group,  727 

Harpes,  743,  781 

Harpoceras,  884,  903*,  904* 

Hartfell  Shales,  751 

Harz,  contact-metamorphism  of,  606 

Hastings  Sand,  938,  940 

Haughtoniaj  731 

Hauterivien,  948,  949,  964 

Hauyne,  76 

Haujoie-andesite,  168 

Hauyne-trachyte,  167 

Hawaii  (Sandwich  Islands),  volcanic  phe- 
nomena of,  197,  206.  207,  216,  217,  220, 
221,  223*,  226,  227,  229,  230,  246*,  264, 
256,  265,  487 

Hawick  group.  764 

Hawkesbur}'  Beds,  877 

Hay  Fell  Flags,  763 

"  Heatl "  of  Southern  England,  362 

Headon  Beds,  986 

Hill  Sands,  970 

Heat,  effects  of,  on  rocks,  292,  297  ;  pro- 
duced by  chemical  transformation,  298  ; 
produced  by  rock-crushing,  ibid.  ;  due  to 
intrusion  of  igneous  rock,  299  ;  expands 
rocks,  ibid.  ;  increases  solvent  power  of 
water,  307 

Heave  of  faults,  553 

Heckla-Hook  formation,  803 
Uedera,  976 
Hedgehogs,  fossil,  968 
Heersien,  976 
Hdarctos,  1016 
Helderberg  group,  790 
UdiarUhctster^  781 
Helicoc^a^,  928 
HdicntomtL,  744 


4  C 


1122 


TEXT-BOOK  OF  GEOLOGY 


Hdiolites,  749,  780 

Helix,  958,  983,  986,  998 

HeUadothenum,  998,  1006,  1019* 

Helvetian  stage,  998,  1001 

HemiaspiSf  743 
^emidster,  925 

ffemicidaria,  883 

HemicosmiteSf  742 

Hemi-crystalline,  118*,  119,  155 

HemipedinOj  883 

HemipneusteSf  925 

Hemiptera,  fossil,  820 

Hemiptychina^  854 

Hempstead  Beds  (see  Hamstead  Beds) 

HemcliUa,  877 

ffercoceraSf  781 

Herculaneum,  volcanic  phenomena  at,  197, 
232 

Hercynian  gneiss,  714 

Hercynite,  71 

Hesl»yan  loam,  1047 

ffesperomiSf  934* 

ffeterocetus,  999 

HtterohyuSy  968 

ffeteroporot  906 

HeterosteginOj  993 

Hettangian,  914 

Hickory,  fossil,  923,  1004 

High -neater  mark,  433 

ffightea,  965 

Hils,  953 

Himalayas,  snow -line  in  the,  416  ;  Creta- 
ceous rocks  in,  957  ;  slow  upheaval  of, 
1021,  1078 

Hinnitts,  862,  907,  1009 

ffipparion,  999,  1006* 

Hippohyus^  1021 

ffippopodium,  883,  885* 

Hippopotamus,  998,  1014,  1036,  1061 

HippotheHum,  999,  1018 

Hippothoa,  743,  907 

Hippotragus,  1021 

Hippurite  Limestone,  951,  956,  957 

HippuHtes,  927* 

Hippuritids,  typically  Cretaceous  fossils, 
928,  951 

Hirnaut  Limestone,  748 

Histialenruij  731 

Historic  period,  1056 

Hoaug  Ho,  alluvial  deposits  of,  395  ;  area 
of,  462  ;  amount  of  material  removed  by, 
ibid. 

Hoar-frost,  impurities  in,  342 

Hog,  fossil,  996  ;  intro<iuction  of  domesti- 
cated, 1063 

Hol^-spis,  796 

Holaster,  925 

planus-zone,  938,  945,  947,  951 

sitbfffobosus-zone,  938,  944,  947 

Holcostephanus,  919 

Holectypus,  907 

Holland,  subsidence  of,  290,  292  ;  dunes  of, 
335 ;  deltoi<l  accumulations  of,  402 ; 
Diestian  beds  of,  1010,  1015 


Hollies  Limestone,  754 

Holocrystalline   structure,    97,    118*,   155, 

156,  157* 
Holocystis,  925 
HolopaeOf  744 
HolopeUa,  744,  854 
Hol^tychius,  745,  775,  783,  796 
Holothuridee,  discovery  of^  in  Carboniferous 

system,  673 
Hamalonotus,  743,  767*,  780* 
Homocamelusj  1022 
Hojtwmya,  913 
HomosteitSy  796 
Homotaxis,  658 
Hone-stone,  135 
Hopl^>paria,  973 
Horderley  Sandstone,  748 
Horizon  in  stratigraphy,  678 
Hornbeam,  fossil,  966 
Hornblende,  74,  95  ;  in  meteorites,  10 
Hombleude-andesite,  167 
Hornblende-granite,  159 
Hornblende-rocks,  182,  188 
Hornblende-schist,  182,  188,  626 
Hornstone,  154,  606 
Horny  texture,  102 
Horse,   ancestral  forms  of,  667,  968,  969, 

1001,  1002 

fossil,   1060 ;   introduction  of  domes- 
ticated. 1063 

Horsts,  1071 

Hudson  River  group,  775 

Human  period,  deposits  of,  1023,  1055 

Humic  acid,  471 

Humus,     formation     of,     321  ;     geological 

action  of,  342,  360,  472,  477,  483 
Hundsrllckien,  787 
Huron  Lake,   area  of,    1052  ;    terraces  of, 

1054 
Huronian,  692.  715,  716 
Hysemoschus,  985 
Hymiia,  1006,  1036,  1061 
Hymnardos,  996,1021 
Hyxnictis^  1019 
Hymnodon,  985,  1003,  1021 
Hyalite,  due  to  action  of  humus  acids,  483 
Hyhodus,  862,  886,  929 
Hydaspiiherium,  1021 
Hydration  of  minerals,  345 
Hydraulic  limestone,  149 

pressure,    influence    of,     in     marine 

erosion,  444 

Hydrobia,  959,  986,  999,  1012 
Hydrocarbons  at  volcanoes,  196 
Hydrocephalus,  734 

Hydrochloric  acid  at  volcanoes,  195,  233 
Hydrofluoric  acid,  87 

Hydrogen  in  earth's  crust,  61 ;  in  meteorites, 
sun,  nebulae,  10, 11,  12;  at  volcanoes,  196 
Hydro-mica-schist,  184 
Hydrothernial  action^  308 
Hylseosaurus,  930 
Hylonormts,  846 
Hyloplesiojiy  846 


INDEX 


1123 


HymenocariSf  724* 
HyUithellus,  725 
ffyolithes,  725,  744 
Hyopotamus,  968,  985,  1002 
Hyotheriuin,  996,  1002 
Hyperite,  169 
Hyperodajyedon,  862 
HypersthcDe,  75 
Hypersthene-andesite,  168 
Hyperstliene-gabbro,  169 
Hypersthenite,  169' 
Hypocrystalliue,  119 
Hypogene  action,  190,  562 

changes  in  rocks,  296 

HypsUopIiodon,  930,  940 
Hypsijyrimnus,  983,  1023 
ffyrachiusy  985 
Hyrwodo?!,  1002 
ffyracotheriumj  968 
Hystrix,  1017,  1060 
Hythe  Beds,  941 

Ice,  148,  413  ;  effects  of,  on  climate,  25,  26  ; 
on  earth-temperature,  49  ;  expansiye  force 
of,  414  ;  on  rivers  and  lakes,  ibid.  ;  shear- 
structure  of,  418,  419  ;  erosive  action  of, 
427,  1026  ;  on  the  sea,  origin  and  action 
of,  438  ;  erosion  by,  449  ;  transport  by, 
453 

Ice-action,  supposed,  in  Old  Red  Sandstone 
time,  802  ;  in  Carboniferous  time,  805, 
809  ;  in  Permian  time,  843,  847,  849,  854  ; 
in  Triassic  time,  877  ;  in  Cretaceous  time, 
943,  979;  in  Eocene  time,  979*;  in 
Pleistocene  time,  1024 

Ice  Ajje,  history  of  the,  1024  {see  Glacial 
Period) 

Icebergs,  422*,  439,  440*,  450,  453 

Ice-cap,  418  ;  effects  of,  on  earth's  centre  of 
gravity,  20,  283,  286 

Ice-caves,  359 

Ice-falls,  418*,  422* 

Ice-foot,  439,  450,  453 

Ice -sheets,  417,  418,  1026,  1050;  sup- 
posed subsidence  caused  by,  293  note 

Iceland,  volcanoes  of,  202,  208,  210,  211, 
216,  217,  222,  229,  234,  235,  237,  249, 
256,  262 

Ichthyodorulites,  813* 

Ichthyosaurs,  as  type  -  fossils,  657  ;  early 
forms  of,  863,  931 

Ichihyomuru^,  864,  888,  889* 

Ictithertum,  1019 

Idiomorphic,  64,  109,  118 
^  Idocrase,  76 

^Igneous  rocks,  1 24,  559  ;  metamorphism  of, 
597,  611 

IguanavuSj  969 

IgiMnod(m,  909,  930,  932* 

Ilex,  fossil,  923,  991,  995,  1004 

IlfracomlHj  Slates,  784 

lUtenopsis,  747 

Illwnu^,  741*,  743 

llmenite,  70,  618 


Impervious,  defined,  357 

Implements,  Palaeolithic,  1057*  ;  Neolithic, 
1063*,  1064 

Inclination  of  rocks,  531 

Inclusions  in  minerals,  69 

Incoherent  aggr^ation,  106 

Indertsch  Lake,  composition  of  water  of, 
411 

India,  coast-bars  of,  399  ;  volcanic  plateau 
of,  259  ;  pre -Cambrian  rocks  of,  717  ; 
Cambrian,  737  ;  Silurian,  776  ;  Permian, 
853  ;  Trias,  877  ;  Jurassic,  919  ;  Cre- 
taceous, 957 ;  Eocene,  981  ;  Miocene, 
1002  ;  Pliocene,  1020  ;  former  extension 
of  glaciers  in,  1054 

Induration  by  eruptive  rocks,  584,  598  ;  by 
exposure,  345 

Infiltration,  effects  of,  122,  123,  364,  365, 
454,  486,  492 

Infra-Lias,  867,  914 

Infra-littoral  deposits,  455 

Infusorial  earth,  141,  481 

Inaceramus,  926*,  927 

Inoceramus  labiattts-zouet  938,  945 

Insect-beds,  899 

Insects,  destructive  action  of,  475 

fossU,  746,  794,  820,  886,  888*,  899, 

910,  915,  917,  987,  1001 
fossilisation  of,  645 


Interbedded  igneous  rocks,  561,  589 

Interglacial  periods,  29,  1025,  1033,  1049 

Intermediate  Massive  rocks,  163 

Intersertal  structure,  168 

Intrusive  rocks,  560,  563  ;  proposed  chrono- 
logical arrangement  of,  125,  563 ;  law 
determining  forms  assumed  by  masses  of, 
564  ;  melting  of  contact  rocks  by,  571, 
572,  609  ;  alteration  of,  by  carbonaceous 
materials,  601 

Intumescens-beds,  786 

Inversion  of  rocks,  539,  540*,  1075  ;  proved 
by  fossils,  657 

Iodine  at  volcanoes,  196 

lolite,  76 

Ireland,  sea-action  on  coast  of,  444,  445  ; 
bogs  of,  480 ;  granite  of,  568 ;  pre- 
Cambrian  rocks  of,  708  ;  Lower  Silurian, 
752  ;  Upper  Silurian,  765  ;  -Old  Red  Sand- 
stone, 802 ;  Carboniferous  litoieatOM^  831 ; 
Trias,  866  ;  Lias,  901  ;  Qretaoewllb  947  ; 
Tertiary  volcanic  series,  988 ;  glatlttion  of, 
1043,  1044 

Iris,  988 

Iron  Age,  1056,  1064 

Iron,  alloyed  with  nickel  in  meteorites,  10  ; 
as  a  colouring  matter,  106,  131  ;  in  earth's 
crust,  61,  63,  68,  69  ;  native,  68,  458 

Iron -carbonate,  66,  78,  147 

Iron-chloride,  228  ;  at  volcanoes,  196 

Iron-ore,  dcpasits  of,  146,  152,  367,  712  ; 
oolitic,  147,  151, 153  ;  Carboniferous,  806  ; 
Jurassic,  898,  904  ;  Cretaceous,  921 

-oxides,  69  ;  sublimed,  196  ;  dissolved 

and  removed  by  humous  acids,  472 


1124 


TEXr-BOOK  OF  GEOLOGY 


Iron-pan  under  soils,  367 

Iron-sulphate,  362 

Iron-sulphide,  79  ;   gives  rise  to  chalybeate 

springs,  362 ;   in  marine  deposits,  455  ; 

concretions  of,  512 
Iron,  titaniferous,  70,  94,  61 8 
Irrawaddy,  sediment  in  the,  383 
Isastrma,  882*,  883 
IschaditMt  741 
Ischia,  203 
IsehnacanthuSy  796 
JachyoduSj  908 
laocardia,  884,  927,  999 
Isoclinal  folds,  540 

Isogeothermal  lines,  49,  50,  295,  297,  307 
IsopogoUj  965 

Isothermal  lines,  cause  of  divergence  of,  440 
Isotropic  substances,  94,  115 
Isthmiay  1018 
Itacoluniite,  180 
Italy,  coast-deposits  of,  402  ;  Cambrian  rocks 

of,  735  ;    Silurian,   774 ;    Carboniferous, 

838  ;    Permian,   852 ;    Trias,  629,   871  ; 

Jurassic,  918  ;  Cretaceous,  956  ;  Eocene, 

980  ;    Oligocene,    993  ;    Miocene,    1001  ; 

Pliocene,  1008,  1016 
Itfer  beds,  767 
lulus,  820 
Ivy,  fossil,  923 

Jackson  beds,  981 

Jade,  182 

Janira,  943,  1017 

Japan,  volcanpes  of,  205,  206,  213,  215 

Jasper,  154  '    " 

Java,  volcanoes  of,  197,  200,  233,  234, 
243 

Jaws,  frequency  of  lower,  as  fossils,  647 

Jerboa,  fossil,  1060 

Jewe  zone,  767 

Jointed  structure,  104 

Joints  of  rocks,  318  ;  influence  of,  on  wave- 
action,  446  ;  described,  523  ;  in  stratified 
rocks,  5*J4*  ;  intersection  of,  ibid.;  dip  and 
strike  joints,  525  ;  cause  of,  526  ;  resem- 
blance of,  to  faults,  547  ;  influence  of,  on 
scenery,  1082 

Joly's  spring  balance,  85 

Jtirden  beds,  767 

Jorullo,  228 

JngVtndite^t  976 

JugUnis,  923*,  988,  995 

Juniperus,  923 

Jupiter,  density  of  planet,  0 

Jura,  White,  915,  916 

Brown,  915,  916 

Mountains,  flexures  of,  1072*,  1073* 

Jurassic  system,  879  ;  general  characters, 
ihid.  ;  flora  of,  880  ;  fauna  of,  881  ;  dis- 
tribution of,  81*5  ;  climate  and  homoiozoic 
belts  of,  896  ;  in  Britain,  897  ;  in  France 
and  the  Jura,  910  ;  in  Switzerland,  915  ; 
in  Germany,  ibid.;  in  the  Alps,  917  ;  in 
*  Sweden,  918  ;  in  Russia,  ibid. ;  in  North 


America,    919 ;    in   Asia,  ibid. ;   in  Aus- 
tralasia, 920  ;  metamorphism  of,  629 

Juvavian  stage,  873j 

Juvavian  Triassic  province,  872 

Kames,  1038 

Kaministiquia  series,  716 

Kampecaris,  794 

Kaolin,  77,  133,  349 

Karharbari  beds,  877 

Karoo  beds,  839,  855,  863,  878 

Karrenfelder,  347 

Kaysena,  784 

Keewatin  series,  716 

Kellaways  Rock,  898,  907 

Kentucky,  Mammoth  Cave  of,  368 

Keratophyre,  162 

Kersantite,  164 

Keokuk  group,  841 

Keuper  (Trias),  864,  869 

Keupermergel,  869 

Keweenawan,  716 

Kieselguhr,  69 

Kieserite,  850 

Kilauea,  see  Hawaii 

Killas,  783 

Kiltorcan  beds,  802 

Kimberley  Shales,  855 

Kimeridgian,  898,  908,  912,  915,  916,  919 

Kinderhook  group,  841 

KingenOy  942 

Kinzigite,  182 

Kirkby  Moor  Flags,  763 

Kirkbya,  812 

Klein's  solution,  86 

Knorruiy  785,  816 

Knoten-schiefer,  179,  605,  607 

Knotted  schist,  179 

Kohleukeuper,  869 

Koninckdla,  883,  884* 

Kaninchina,  875 

Kcissen  beds,  873,  876 

Krakatoa,  207,  212,  214 

Kramenzelkalk,  786 

Kuckers  shale,  767 

Kugel-diorite,  101*,  165 

Kupferschiefer,  842,  849 

Kurile  Islands,  volcanoes  of,  253 

Kurtodon,  894 

Kutorgina,  725 

Kyanite,    76  ;    in    contact -metamorphism, 

708 
Kyanite  rock,  182 

Labrador  porphyry,  170 
Labradorite,  72  ;  in  meteorites,  10 
Labyrinthodonts,  821,  846,  862,  887 
Laccolites,  571 
Lacertilians,  fossil,  969 
Lackenian,  976 
Lacoptcris,  953 
Lacuna,  1010 
Lalaps,  933 
Lageim,  809 


INDEX 


1125 


Lago  Maggiore,  405 

LagomySf  1060 

Lagoon  barriers,  398 

Lahontan,  Lake,  409.  413 

Lake  Agassiz,  an  extinct  glacial  lake,  286, 
1062 

Lake  District  (England),  granite  of,  505 

Lake-dwellings,  Neolithic,  1066 

Lake-ore,  146 

Lake  terraces,  286,  409*,  413,  423,  1038 

Lakes,  Great  (North  America),  areas  of,  1032 

Lakes,  volcanic,  229,  240 ;  affected  by 
earthquakes.  277  ;  wave  -  action  in, 
339 ;  effects  of  atmospheric  pressure 
on,  339,  404  ;  deposits  in^  397  ;  filling 
up  of,  by  streams,  ibid. ;  distribution  of, 
404  ;  temperature  of,  405 ;  geological 
functions  of,  ibid, ;  equalise  temperature, 
ibid.;  regulate  drainage,  ibid.;  filter  rivers, 
385,  397,  405  ;  waves  in,  406  ;  vanished, 
inNorth  America,  286,  407, 409,413, 1052; 
chemical  deposits  in,  407  ;  organic  deposits 
in,  146,  407,  483  ;  recent  origin  of,  408  ; 
saline,  ibid.;  frozen,  414;  formed  by 
beavers,  474  ;  entombment  of  organic  re- 
mains in,  647  ;  characteristic  fauna  and 
flora  of,  647  ;  former  existence  of,  proved 
by  fossils,  654 ;  glacial,  1038,  1040, 1052 ; 
terraces  of,  1054  ;  origin  of,  1087 

Lamellibranchs,  fossil,  725 

Laminae,  498 

Laminated  structure,  104,  498 

Lamna,  929,  966,  968*,  1015 

Lamprophyre,  164 

Land,  origin  of  general  arrangement  of,  14  ; 
attraction  of,  on  ocean,  21,  34  ;  area  of, 
38  ;  average  height  of,  39  ;  greatest  height 
of,  40  ;  contours  of,  ibid. ;  evidence  of 
proximity  of,  456,  654 ;  materials  of, 
generally  formed  under  the  sea,  1068  ; 
origin  of  surface  contours  of,  1070  ;  in- 
fluence of  subterranean  agents  on  topo- 
graphy of,  1071  ;  influence  of  denudation 
in  topography  of,  1079  ;  fundamental  law 
in  erosion  of,  1080  ;  conditions  governing 
denudation  of,  1080 

Land-plants,  stratigraphical  correlation  by 
means  of,  660,  668,  959,  988 

Land-shells,  earliest  forms  of,  821  ;  Pleisto- 
cene northern  forms  of,  1018 

Land-surfaces  shown  by  fossils,  653 

Landenian,  976,  977 

Landslips,  ordinary  origin  of,  370*  ;  effects 
of,  on  rivers,  382  ;  caused  by  earthquakes, 
280 

Lcuxion^  919 

Laopteryx^  893 

Laornisy  935 

Laosaurus,  919 

LapiUi,  volcanic,  136,  199 

Laramie  flora,  923 

^oup,  669,  958,  969,  982 

Lasiograptidse,  749 

Lasirtea^  988 


Laterite,  133 

Laurel,  fossil,  923,  954.  984,  1004 

Laurentian  rocks,  684  ;  supposed  origin  of, 
from  fusion  of  ancient  sediments,  688  ; 
occurrence  of,  in  Canada,  715,  716 

Laurophyllunu  922 

Laurus,  972,  995,  1017 

Lava,  saturated  with  water-substanca,  194  ; 
characters  of,  198  ;  varying  liquidity  of, 
215,  222  ;  streams  of,  217  ;  outflow  of, 
218  ;  hydrostetic  pressure  of,  209,  219, 
220  ;  fountains  of.  220,  223  ;  rate  of  flow 
of,  221  ;  crystallisation  of,  224  ;  occur- 
rence of,  in  crust  of  the  earth,  589,  591  ; 
gradation  of  acid  into  basic,  225  ;  tempera- 
ture of,  225 ;  fusion  and  sublimation 
effected  by,  226,  230 ;  inclination  and 
thickness  of  streams  of,  226  ;  structure  of 
streams  of,  227  ;  tunnels  and  caverns  in, 
221,  227 ;  vapours  and  sublimates  of, 
228  ;  slow  cooling  of^  ibid. ;  effects  of,  on 
superficial  water,  229  ;  overlying  snow, 
230  ;  weathering  of,  231  ;  cones  of,  245  ; 
subaerial  and  submarine,  253 ;  subter- 
ranean iqjection  of,  299,  559 ;  more 
coarsely  crystalline  when  consolidated 
within  the  crust,  559 

Layer,  seam  or  bed,  678 

Leaia,  812 

Le4la,  811,  901.  973,  990,  1011,  1048* 

Leda-clays  of  Canada,  1053 

Leda-myalia  bed,  1008,  1014 

Ledbury  Shales,  758,  760 

Lee-seite  in  glacial  erosion,  1026 

Legnonotus,  867 

Leiodon,  930 

LeistomOf  975 

Lemming,  fossil,  1061 

Lenham  beds,  1008,  1009 

Lenitaj  978 

Leoben,  graphitic  schists  of,  623 

Leopard,  fossil,  1036,  1061 

Leperditia,  724,  742,  812 

LepidasUr,  742 

Lepidodendra,  793,  814,  816,  817*,  844  ;  as 
type-fossils,  657 

Lepidolite,  74 

I^pulophioiosy  816 

Lepidophyllunu  822 

IjepidopteriSf  859 

Lepidt)8trobus,  816,  817* 

LepiiiotosauruSf  845 

Lepidotusy  862,  886,  958 

Leptiena,  743,  761*,  781,  852,  883,  884* 

Lepthymna,  1021 

Leptobos,  1021 

Leptoclase,  523 

LeptodomuSj  811 

Leptodiniy  1019 

LeptograptidsBy  749 

Leptalepis,  886 

LeptotneryXy  1003 

LeptamytuSf  722 

Leptophieum,  793 


1126 


TEXT-BOOK  OF  GEOLOGY 


LeptoptUua,  1021 

Leptynite,  186,  188 

Lepus,  1017 

Lettenkoble,  869 

Leucite,  73 

Leucite-andesite,  168 

Leucite  basalt,  172 

Leucite-phonolite,  167 

Leucite-trachyte,  167 

Leucitite,  173 

Leucoxene,  71,  618 

Level  course  in  mining,  535 

Lewisian  gneiss,  624,  625*,  699 

Lhangian  stage,  998,  1001 

Lherzolite,  173  » 

Lias,  898,  914,  916,  917  ;  life-zones  of,  665 

Liassian,  914 

Libocedrus,  991,  995 

Libumian  stage,  980 

Lichas,  743,  781 

Lichens,  protective  influence  of,  475 

Life,  plant  and  animal,  in  its  geological 
relations,  471  ;  influence  of  man  on  dis- 
tribution of,  497  ;  preservation  of  records 
of  former,  646,  675,  677  ;  traces  of  pre- 
Cambrian,  694  ;  variations  in  progress  of 
plant  and  animal,  660,  665,  668 

Ligerian,  938,  948,  951 

Light,  reflected,  transmitted,  and  polarised, 
94 

Lightning,  effects  of,  328 

Lignilites,  316 

Lignite,  143,  144,  322 

Lignitic  group,  958 

Lima.  854,  862,  883,  885*,  926*,  927 

Limax,  1013 

Limburgite,  173 

Lime,  carlwnate  of  («*!<•  Calcite,  Aragonite), 
influence  of,  in  natural  waters,  412 ; 
natural  precipitation  of,  in  saline  water, 
412,  413  ;  in  sea  water,  37,  484  ;  source 
of,  for  shells  of  marine  organisms,  484 

Lime-phosphate  in  fossilisation,  650 

Lime-sulphate  {see  Gypsum,  Anhydrite)  in 
Kea-water,  transformed  into  carbonate  by 
marine  organisms,  484,  495 

Limestone,  63,  82, 139,  149  ;  origin  of,  804  ; 
tests  for,  87  ;  formed  at  mouth  of  Rhone, 
453  ;  formed  by  nullipores,  453,  477  ; 
formed  of  shells,  calcareous  sand,  etc., 
454,  484,  492 ;  formed  of  coral,  486  ; 
associated  with  shale  or  clay,  515  ;  rela- 
tive persistence  of,  ibid.  ;  fossils  peculiar 
to,  813;  formed  by  algae,  860,  872; 
solution  of,  344,  349;  weathering  of,  81, 
346,  350  ;  insoluble  residue  of,  350 

acquired  crystalline  structure  of,  122, 

138  ;  artificially  converted  into  marble, 
300  ;  mamiarosis  of,  320,  584 

Limestone,  Carboniferous,  distribution  and 
origin  of,  517,  522 

Limestone  Shale  (Lower  Carboniferous),  825, 
826 

Limnsa,  910,  958,  978,  985*,  1011 


LimnerpeUm,  846 

limonite,  70,  153 

Limcpns,  974,  983,  999,  1009,  1017 

Limpets,  protect  shore  rocks,  477 

Lingula,  725,   743,  745*,  761*,  811,  819, 

848,  869,  939,  1009 
Lingula  flags,  727,  728 
LinguUUa,  724*,  725 
Lingulina,  839 
LinguiocariSf  729 
Linnarssonia,  725 
Linum,  991 
Lion,  fossil,  1036,  1061 
LiostrctcuXf  737 
Lipari  Ishinds,  202,  205,  206,  215,  221.  224, 

233,  234,  243 
Liparite,  160 

Liquid  in  cavities  of  crystals,  110 
Liquidambar,  973,  994*,  1004 
Liriodendrony  972 
Lithoclase,  523 
Lithoid  structure,  97 
Lithological  characters  as  a  basis  for  gronp- 

ing  strata,  522  ;  as  evidence  of  geologic^ 

age,  655,  658,  692,  698 
Lithology,  60 
Lithophyse,  100 
LiUwmis,  968 
Lithosphere  of  the  globe,  38 
Lithostrotum,  807*,  810 
Liihothamniumy  980 
Lit(ynnellay  999 
Littoral   deposits,    preservation   of   organic 

remains  in,  648 
Littarina,  1012 
LituiUs,  730,  744,  761* 
Llanboris  group,  727,  728 
Llandeilo  group,  746,  747 
Llandovery  group,  746,  753,  754 
Llanvim  group,  747 
Loam,  133,  352 

Lob-worms,  transference  of  silt  by,  474 
LobUe.%  862 
Lodes,  633 

Loess,  133,  332,  352,  1059 
Loganoffmptus.  747 
Lirnchopteris,  822,  879 
London  Clay,  971,  972 
Longmyudian  rocks,  710 
Ijongulites,  115 
LonsdaletGy  810 
Lophiodon,  968,  1002 
Lophiomeryx,  985 
Lmripes,  854 
Losspuppeu,  512 
LoveniiTy  983 
Low-water  mark,  433 
Ijoxodnn.  1021 
lA>Xf>lophodotiy  970 
Loxoiumay  821 

Loxouevia,  758,  781,  811,  862 
Lucina,  781,  854,  907,  939,  966,  967*,  1012 
Ludlow  group,  746,  753,  757 
Luidia,  901 


INDEX 


1127 


Lumachelle,  139 

LuDZ  Sandstone,  873 

Lustre  of  rocks,  107 

Lustre-mottling,  107 

LiUra,  1011 

Luirictis,  985 

Lyckholm-zone,  767 

LycopoditeSj  822 

Lycopods,  fossil,  740,  816 

Lycosaurus^  863 

LychysencL,  1020 

Lydian  stone,  154,  180 

Lyginodendron^  823 

Lygodium,  922,  965 

Lynton  group,  784 

Lynx,  fossil,  1036,  1061 

Lyra,  926 

Lyrodesma,  754 

Lytoceras,  872,  884,  903*,  904*,  928 

Lyttonia,  854 

Maabs  or  crater-lakes,  240 

Macacus,  1017,  1021 

Maccalubas  or  mud-volcanoes,  238 

Machairodus,  996,  1006,  1020* 

Macigno,  980 

Maclurea,  624,  730,  744 

Macrocephalite^,  915 

MacrocheUus,  781,  811,  862 

Macromerioriy  846 

Macroraerite,  98 

Macropetcdichthys,  790 

Macropus,  1022 

MacrorniSy  968 

Macroscopic  characters  of  rocks,  80,  81.  96 

Macrostachys,  816,  822 

MiLcroUeniopteris,  877 

Mcunrotherium,  996 

Mactra,  983,  995 

MadreporOy  993 

Maentwrog  Flags,  729 

Maestrichtien,  948,  952,  961 

Magas,  926 

Magasdla,  983 

Magdalenian  deposits,  1057 

Magma,  differentiation  of  acid  and  basic  con- 
stituents in,  225,-  262,  269,  564 

Magma-basalt,  173 

Magnesiau  limestone,  151,  847 ;  concre- 
tionary structure  of,  510 

silicates,  weathering  of,  345 

Magnesium,  61,  63 

Magnesium  -  chloride,  149;  in  lakes,  408; 
influence  of,  in  formation  of  dolomite, 
321,  412 

Magnetic  analysis  of  rocks,  86,  108 

Magnetite,  70,  153  ;  in  meteorites,  10 

Magnolia,  fossil,  923,  965,  988,  995*,  1004 

Mainz  basin,  992,  999,  1017 

Malacolite,  74 

Malacolite-rock,  181 

Mcdlotusy  concretions  around,  1054 

Malm,  915,  916 

Mammalia,  value  of,  as  fossils,  653,  657  ; 


earliest   types  of,  864,  893,   895*,   919, 
935 

Mammaliferous  Crag,  1011 

Mammoth,    1036,    1037*,  *  1060,  1062*  ; 
preservation  of  carcases  of,  646 

Age  of,  1061 

Man  as  a  geological  agent,  495  ;  geological 
evidence  of  existence  of,  646  ;  influence  of, 
on  flow  of  rivers,  374  ;  antiquity  of,  1056, 
1065  ;  evidence  for  the  presence  of,  1056  ; 
earliest  artistic  efforts  of,  1062  ;  Paleo- 
lithic, akin  to  Eskimo,  1063 

Manchhar  group,  1002,  1021 

Manganese,  61,  63  ;  oxides  of,  71,  343  ;  de- 
posits of,  on  sea-floor,  456,  457,  458,  469*, 
495 

Mangdia,  983 

Mangrove-swamps,  476,  481,  807 

Manis,  1002 

MarUellia,  880 

Maple,  fossil,  923,  966,  1004 

Marble,  151*,  602 ;  artificial  production 
of,  300  ;  weathering  of,  344 

Marcasite,  79,  135;  as  a  i)etritying  medium, 
652 

Marcellus  group,  789 

Maretia,  978 

Margarodite,  74 

Marginella,  974,  983 

MarginulinOf  900 

Marine  denudation,  466 

Marl,  82,  139,  484 

Marl-slate,  842,  847,  848 

Marmarosis,  320,  584,  602,  618 

Marmot,  fossil,  1060,  1061 

Mames  irises,  870 

Marquette  district,  metamorphism  in,  628 

series,  716 

Mars,  density  of  planet,  9 

Marsh -gas  at  volcanoes,  196 

Marsupial  mammals,  fossil,  864,  893,  895*, 
919,  935,  968 

Marsupiocrinusy  756 

Marsupite-zones,  938,  946,  947 

Marsupitea,  925 

Marten,  fossil,  985,  1014 

Marylandian  group,  1002 

Masonry  alteration  of,  by  hot  springs,  307, 
365 

Massive  rocks,  126,  154  ;  proposed  chrono- 
logical classification  of,  156 ;  joints  of, 
527 

Massive  stnicture,  104 

MastodoHy  993,  995,  996*,  1006 

Mastodonsaurus,  862 
.  Mauisaurusy  961 

Mauna  Loa  {set  Hawaii) 

May  Hill  Sandstone,  748,  752,  758,  754 

Mayencian,  1001 

Mechanical  analysis  of  rocks,  86 

deformation  of  rocks,  314,  543,  615 

Medina  group,  775 

Mediterranean  Province  (Trias),  872 

Mediterranean  Sea,  rise  of  coast-line  ot  287  ; 


1129 


TEXTBOOK  OF  GEOVjGY 


<l<<:ptL  of  TaT«-»ct»&  in,  4^ ;   cuue  of 

bl;:<%esfr  of,  4^2 

)fe<i:>rrkn'«a£  cta^   MiDcene  .  1000 

Moi:^ue.  fon-il,  722 

M'.^odxrtA.  ICrtl  ;  in  boes  4*0 
M'tool^jjrpU^  7  "59 

Mty^JidUMv*.  820 
Jf^j^j-^jm,  7*1.  7S2*.  M2 
JV>s7a^>MM4,  76% 
Mc^akmn*  Limestone.  76S 
J/<^^HnM,  690*.  692,  930 
Mfsiyu^rnM,  SrA,  917 

Me^jpkfUm,  S22 

Utfpucofiic  chanctcn  of  rocks,  SO,  SI.  96 

Mctoaite.  76 

M<lamf»u,  1010 

Mtlaiurpeffmy  S46 

Jielamia.  953.  966,  967*.  9S^  1017 

Mtlafu*f»HM,  972.  9S6,  1017 

MeUphTTe,  172 

Melbonrne  Bock.  93S,  944 

Jtfe/^,  1016 

Meliiite-bualt.  173 

jr<//iV/ra.  1021 

MtHir^/rfjfUm,  1021 

M*h,niUJi.  611 

Mfrlt»r<l  rock*,  density  of.  56 

M^rlrmz  of  rock-   bv  ercptire  mas«e(.  571. 

Mf^i'JrraH'jPff't,  &25.  97^ 
Mfraao.ar.it*-,  70 

Mene%-:an  ^rroup,  7:i^.  729 

M*rnori-iL«r  «ii»tri:t.  m^tamorphi^^m  in.  62^ 

Mer.  UiiD**  •<r>-.  716 

Merc  jr> ,  «i»-L«i:y  of  pUnet,  9 

Mrne-»  ill  Oie-ihini',  367 

Mrr'j.iA,  1021 

Meh'i:.'in.  arc  of,  mea^^Tinnl^  13 

JZ/rf.*//.  7*5 

J// n'../^.'.V/,  743 

J/- /■'-••'/■»/.  7ol* 

Merj-tor.'.ata.  ar>pi*irance  of.  743.  755 

J/,  r  y  .   '".  ipt  ..'A.     1022 

M' rj- .  ,m  ','1,    /.«,    ITrJl 

J/r.v '■"-..'/-./..,  7'>5*,  796 

M^-l-i'irtyli,   C^rlf* 

J/'  .■«  •  / .  /. .   *  ^  o .   1^  2t> 

JAv /<.y7-'..  10«j2 

J/'  V  ■'/;■ «,  ""JO 

J/-'  /..'/,.    ,,  'i0*)6.  1»hj7* 


61 


llMSziu  err-..  1016.  i:':7 
M«:lA£2:«sjic  =<«cas« 
MiCftcrksis,  594 
MeiiII-:>>is  ir  catia's 

McC*3::::-niie  rock*.  Ii4u  175 

M«CULorp«i»=^  319  :  rjiief  :t 
tioas  in  zs*  ^5f  li*  t-era.  5>jr5  : 
kinds   oC  596  :  ^xal  597  ;  r*rnrn-^]. 
616  :  tb«or>s  o^.  613 


—  of  -x-n;^-  2:^.  *W3l  5«7,  572.  576. 
5^4.  56%  569.  597.  627.  7*6  :  ssrcoc*  oC 
6'>3.  706 :  relatkv  ol  to  rnsBom^  4SS.  70^ 

—  retdocal.    611  :    — --*- 
fr.   614,   615:   =:i 
in.  61 7  :  aiOcti  o>ier   zoek^,   46v ; 


tp^p;«  ot  619.  701,  7<k5.  707: 
of.  to  coc.tv.t-BoecaC'Or^GiBK.  611.  627. 
706  :  laTcf  than  CaaLbnia  matai  Sfhoiu 
peno-is,  626.  7'  6,  71 S.  717.  730.  7«. 
776:  fo*:  -  DtTv^ta.  619.  766:  po<- 
Carbonifcrost.  636:   |  itr  Ti  awi  .    67L 


9(59:  port- 


<kpaEXA.  66,  iSi'i 


676  :  poft- 

965 
MetAfonuroiiA.  596 
Metaia&sis.  596 

Metrorvr  iicn  in  deep 

Meteoriies.  10.  12.  66 

MethTkMis.  596 

Meadon  marL  976 

Men«e.  sediment  in  iIk,  3e3 

MitfvUtic  116 

Miascite.  164 

Mica,  73.  64 

Mica-phyllite.  179 

Mica.-i'«ai:.E::i:c.  131 

Mica-s-rh:*-..  177*.  179.  16*:.*.  164.  166 

M;oa-*Teiii:c.  164 

M:  a-:rap,  164 

Mi  a:-eO-s  conif<:«»itior:u  105 

:u*tr*.  73.  107 

Mi-.^asisatioQ.  617 

Mi-'MinUK  610 

Mi.Li^nn.   Lkke.  am  of.   1052:   dace«  '^f. 

336 
Miri-^rifzin,  732 
Micr''r.».Jr"K  925 
.Vj>-''^'- -zour.  935.  946.  947 
J^i'-r-.^ '.A iji,  646 
ytirnKfy^f  ruf,  96  6 
M:-:roclir.e.  72 

M::ro-ory«*tAliine  ^tnl■?::3*,  97 
M'C'i'^vji,  722* 
Mi.ToftrKitio  structure.  119 
Miorocranite,  160 
Mi:r->rranitic  strucrrine,  96.  116 
Mi.rojraiialiiit'  *tnxc:are.  119 
.Vi'V  .Vrf/^.  664 
Miorolite*.  64.  114.  115.  116 
Mi-.roli!ic  rock>,  155 
M!.;n..mcrite,  96 
Micrr-pr^Tualitie   stractnre.   96*.    119.    156. 

6l^'» 
Mi..To-fi«rrthite.  166 


INDEX 


1129 


Micropho/iSf  863 

Micropora^  925 

Microscope,  petrographical,  93 

Microscopic  investigation,  89 

characters  of  rocks,  108,  109,  117 

Microspherulitic  structure,  120 

Micro-syenite,  164 

MicrotheriuMi  999 

Midford  Sand,  898,  903 

Miliola,  977 

Millericrinust  906 

Millipedes,  fossil,  820 

Millstone  Gnt,  522,  825,  832 

MimoceraSf  782 

Mimosa,  995 

Mine,  deepest  in  Britain,  51 

Minerals,  chief  rock-forming,  64  ;  essential 
or  accessory,  65  ;  original  and  secondary, 
ibid. ;  formed  on  sea- floor,  458  ;  production 
of,  in  contact  -  metamorphisro,  603  ;  se- 
quence of,  in  contact-metamorphism,  627  ; 
in  regional  uetamorphism,  618 ;  pro- 
duction of  new,  323 ;  artificial  production 
of,  309  ;  supposed  sequence  of,  in  schists, 
682 

Mineralising  agents,  310,  311 

Mineral  veins,  547,  633 

Minette,  164 

Miocene,  defined,  962 

Miocene  formations,  general  characters  of, 
993 ;  flora  of,  994 ;  fauna,  995  ;  in 
France,  998  ;  in  Belgium,  ibid.  ;  in  (Ger- 
many, 999  ;  in  the  Vienna  basin,  ibid,  ;  in 
Switzerland,  1000 ;  in  Italy,  1001  ;  in 
Greenland  and  Spitsbergen,  iJbid, ;  in 
India,  1002 ;  in  North  America,  ibid,  ; 
in  Australia,  1003 

MiohippuSj  1002 

Mississi]>pi,  area  of  basin  of,  462  ;  discharge 
of  the,  373  ;  mineral  matter  dissolved  in, 
379,  464  ;  rafts  of,  381  ;  sediment  trans- 
ported by,  384  ;  recession  of  falls  of,  390  ; 
delta  of,  399, 401,  402 ;  amount  of  material 
removed  by,  462 

Missouri,  a\o\)e  of,  376 

Mitra,  928,  966,  993,  995,  1009 

Modiola,  811,  883,  886*,  927,  973,  1017 

Modiolopsis,  725,  744,  761* 

Moel  Tryfaen,  shell  beds  of,  1043 

Mofettes,  195 

"Moine-schists,"  625*,  707 

Molasse  of  Switzerland,  992,  1000 

Mole,  geological  action  of,  474  ;  fossil,  985 

Molhisca,  fossilisation  of,  652  ;  marine,  as  a 
basis  of  stratigraphical  classification,  652, 
962,  998  ;  pulmoniferous,  earliest  forms 
of,  795,  821 

**  Moiiian  system,"  709 

Monkeys,  fossil.  968,  970 

Monoclines,  538*,  1072 

Mimog^raptuSy  739*,  741 

MonophyUite.%  876 

MonvpleurUy  928 

Monoiis,  862 


Monotremes,  fossil,  935 

Mons  limestone,  976 

Mont  Blanc,  glaciers  of,  419,  420*  ;  erratics 
from,  425  ;  fan-shaped  structure  in,  541*, 
1075 

Monte  Nuovo,  201,  212 

Mmiticulipora,  749  ^ 

ManUivaltia,  882*,  883 

Monzonite,  164,  604 

Moon,  attraction  of,  16,  21 

Moorband  pan,  146,  366 

Moor  Rock,  832,  833 

Moraines,  formation  of,  424*,  1039 ;  crescent 
form  of,  417  ;  terminal,  of  ice-sheets,  1027, 
1046, 1050 

Moraine-stuflF,  127,  417,  423,  424* 

Moroaaurtcs,  919 

Mortar,  150  ;  weathering  of,  in  towns,  341 

Morte  Slates,  784 

MoadsauruSj  931 

Moselle,  transport  of  gravel  along  bed  of, 
380  ;  gorge  of,  387* 

Mosses,  deposits  formed  by,  478  ;  calc-sinter 
formed  by,  482 

Mountain-chains,  elevation  of,  during  Tertiary 
time,  963 

Mountain- Limestone,  826 

Mountains,  relative  bulk  of^  89  ;  kinds  of, 
40,  1083 ;  structure  of,  1071 ;  formation 
of,  gives  rise  to  hot  springs  and  volcanoes, 
1076  ;  successive  upheavals  of,  ibid,  ;  his- 
tory of,  illustrated  by  the  Alps  and  Rocky 
Mountains,  1077 ;  slow  uprise  of,  shown 
by  river-courses,  1021,  1078  ;  of  volcanic 
origin,  1079 

Mourlonia,  854 

Mouse,  fossil,  1006 

Mousterian  deposits,  1057 

Moya,  a  volcanic  mud,  232 

Mud,  132  ;  green,  of  sea-floor,  455  ;  volcanic, 
197,  232 

Mud-lava,  197,  232 

Mud-lumps,  399 

Mud-volcanoes,  238,  245 

Mudstone,  82,  133 

Mummification  of  organisms,  651 

Munenosaurus,  908 

Murchiaonia,  624,  744,  781,  844,  862 

Murex,  928,  973,  985,  995 

Mu8,  1014 

Musa,  973 

Muschelkalk,  869,  874 

Muscovite,  73 

Musk-rat,  fossil,  985 

Musk-sheep,  fossil,  1036*,  1060 

Mussels,  protective  influence  of,  477 

Mufitela^  1014 

Mya,  1012,  1045 

Myacites,  870,  884 

Afyalinaf  785 

Mylu)bcU€3,  974,  987 

Mylonitic  structure,  100,  180 

MyoffoJ^j  1014 

Myophoria,  854,  862 


1130 


TEXT-BOOK  OF  GEOLOGY 


Myriapods,  foMil,  794,  820 
Myrica,  922,  991,  995,  1017 
MyrieophyUum,  922 
MyrioUpia^  877 
Myrtus,  995 
MyKiTQchne,  985 
MystriosauruJt,  887 

MytUiu,   754,  848,   88«*,  980,  999,  1012, 
1045 

Nagelflcb,  992 

Xanomys,  936 

Saosaurus,  S46 

Naphtha,  145,  238 

Napoleonite,  101*,  165 

yassa,  983,  991,  1010 

yaiirM,    811,    85-2,   884,    887*,    966,   986, 

1001,  1010,  1048* 
Naticella,  874 
yaticfrpsis,  862 
Natrolite,  77 
Natron  lakes,  408 
NautUus,  730,   744,  812*,  844,  861*,  862, 

907.  928,  930*,  966 
Nebake,  spectra  of,  12 
Nebolar  hypothesui,  8,  14 
Necks,  volcanic,  255,  584 
Secrocarcinus^  942 
NeeroUmuT,  985 
Neetotelwn,  851 
Negative  crystals,  110 
Nelumhium,  965 
Nemacanthus,  867 
Nematophycus,  740,  793 
NemaU*ptychiiiSt  829 
NtfihoLus,  737 
Neolwlus  beds,  737 
Neoconiian,   938,  939,  948,  949,  953,  954, 

955,  956  - 
Neogene,  963,  993,  999 
Neolithic  deposits,  1063  ;    fauna  of,  ihid. 

man    and    his    characteristics   in,   ibid. 

implements  in,  1064*  ;  in  Britain,  1065 

in  France,  1066  ;  in  Germany,  ibid.  ;  in 

Switzerland,  iftid.  ;  in  Denmark,  ibid. ;  in 

North  America,  1067 
Neozoic  defined,  680,  962 
Nepheline,  73  ;  testing  for,  88  ;  crystallises 

easily,  302 
Nepheline-andesite,  168 
Nepheliue-V>a8alt,  172 
Nepheline-dolerite,  170 
Nepheline-syenite,  164 
Nepheline-trachyte,  166 
Nephrite,  182 
Nephr(AiiA,  862 
Neptune,  densitv  of  planet,  9 
Nercites,  733,  742 
Nerinaa^  904 
Nerineen-Schicht«n,  916 
Nerita,  887*,  986,  999 
Neritirw.,  972,  986 
Neritodonta,  1017 
Nesifuretus,  729 


I 


Neuroptera,  fo»il,  794,  820,  88«,  SS8*,89» 

yeuroptendium,  859 

yeuMtictmiunu,  864 

Nevadite,  161 

Nere,  148,  417 

New  Hebrides,  nplicaTal  oC  285 

New  R«d  3iarL  864 

New  R«d  Sandstone,  858 

New  South  Wales,  fossils  <ji  Lower  Coal 
measixres  of,  66l  :  Carbonifettms  system 
in,  839;  Permian,  854;  Trias,  877; 
Eocene,  982;  PUocene,  1022;  KbctxA 
deposits,  1067 

New  Zealand,  hot  springs  of,  238  ;  Tolcanoes 
of,  260,  262;  earthquakes  of,  272  ;  raised 
beaches  of,  288  ;  Qords  of;  291 ;  former 
larger  size  of  glaciers  of,  427 ;  pre-Oun- 
brian  rocks  of,  717  ;  Silurian,  777  :  De- 
vonian, 791  ;  Carboniferous,  840  :  Trias. 
878  ;  Jurassic,  920 ;  Cretaceous,  960  ; 
Eocene,  983  ;  Miocene,  1003  ;  Pliocene. 
1023  ;  fonner  greater  extension  of  glaciers 
in,  1055  ;  recent  deposits  in.  1067 

Niagara  River,  filtered  by  Lake  Erie,  386 ; 
gorge  of,  389 ;  pre-^cial  channel  of, 
ibid.  ;  rate  of  recession  of  Falls  d,  390 

Nicol  prisms,  use  of,  94 

yidulUeSy  741 

Nile,  rise  and  fall  of,  371 :  average  slope  <^ 
376 ;  infusoria  in,  351  ;  sediment  in. 
384;  delU  of,  401,  403;  amount  of 
mineral  matter  dissolved  in,  464 

yiUtania,  859,  880 

yioU,  729 

yipa,  965* 

Nipigon  series,  716  ^ 

Nitric  acid  in  rain,  341 

Nitrogen  in  air,  32  ;  in  rain,  341  :  free,  at 
volcanoes,  196 

yodosaria,  924 

yoggerathia,  823 

yoggerathiopsis,  839 

Nomenclature,  stratigraphical,  679 

None  stage,  873,  874 

Norite,  169 

yvrites,  874 

North  Sea,  floor  of,  455  ;  possible  conver- 
sion of,  into  a  lake  during  the  Glacial 
period,  1029 

Northampton  Sand,  898,  904 

Norway,  raised  beaches  of,  284.  287*.  288  ; 
Qords  of,  291  ;  snow-line  in,  416 ;  glaciers 
of,  419*,  420,  421*,  432  ;  giants*  kettles 
of,  429  ;  contact-metaraorphism  in,  608  ; 
r^onal  metamorphlsm  in,  621  k»k 
Scandinavia) 

yorvsegian  yorth  Atlantic  Expedition,  33, 
36,  37,  38 

Norwich  Crag,  1011 

Nosean,  75 

Nosean-andesite,  168 

Nosean-trachj'te,  167 

yothosaurus,  864 

yot^Mhenum,  983,  1022 


INDEX 


1131 


Notothyris,  854 

Novaculite,  135 

Nucleospira^  771 

Nucida,  811,  862,  885»,  973,  985,  1011* 

Nuculana^  974 

NuUipores,  geological  influence  of,  476,  477, 

482 
Nummulites,  965,  966 
Numraulitic  Limestone,  963,  965,  966*,  979 
NuUtion,  16 
Nyrania^  846 
NysM,  973,  988 
Nystia,  989 

Oak,  fossil,  923,  984,  1004 ;  evergreen, 
fossil,  1004 

OboUlla,  723*,  725,  747 

Obolus,  732 

Obsidian,  162 

Occluded  gases,  10 

Ocean,  area  of  primeval,  14  ;  currents  of, 
deflected,  27  ;  present  area  of,  33  ;  cubic 
contents  of,  34  ;  density  of  water  of, 
35  ;  composition  of,  ibid. ;  movements  of, 
432 ;  tides,  ibid. ;  currents,  434  ;  distri- 
bution of  temperature  in,  ibid.  ;  nature 
of  floor  of,  435  ;  cause  of  circulation  of, 
436  ;  waves  and  ground-swell  of,  ibid.  ; 
geological  work  of,  440  ;  affiects  climate, 
ibid.  :  erosive  power  of,  441 ;  transporting 
power  of,  450 ;  currents  of,  diffuse  food 
of  protozoa,  452 ;  general  conservative 
influence  of,  470  ;  permanence  of  area  of, 
296,  650,  1070 

Ocean-basins,  antiquity  of,  459  ;  probable 
permanence  of,  296,  660,  1070 

Ocean -currents,  338 

Oceanic  circulation,  theories  of,  436 

Ochre,  70 

Octotamusy  970 

Odontaspis,  929,  966,  968* 

OdorUopteris,  816,  843,  877 

OdontopteryXy  968 

OdoTUomUh^,  935 

OdontosauruSy  870 

Oeningen  stage,  1000 

Oesel  zone,  767 

Ogygia,  729,  741*,  743 

Oil-shale,  145 

Oil-wells,  145,  235 

Oldhaven  Beds,  971,  972 

Old  Red  Sandstone,  791  ;  geographical 
changes  attendant  on  deposition  of,  777  ; 
rocks  of,  792  ;  life  of,  793  ;  volcanoes  of, 
203,  204,  261,  593,  799  ;  in  Britain,  797  ; 
in  Norway,  802 ;  in  Spitzbergen,  803  ; 
in  North  America,  ibid. 

Oldhamia,  721,  723* 

Oldhamina,  854 

OUa,  981 

OUandridium,  880,  953 

Olendlus,  624,  656,  721*,  724 

Olenellus-group,  725,  727,  732 

Olenellus-zone,  conformable  strata  beneath. 


697  ;  in  Scotland,  706,  780  ;  in  England, 
710  ;  position  of,  718,  727,  728 

Olenidian  group,  725,  727,  728,  731 

OUnoides,  724 

Olenus,  722*,  724 

Oligocene,  proposed  by  Beyrich,  963  ;  gen- 
eral characters  of  system,  983  ;  flora,  984  ; 
fauna,  985  ;  in  Britain,  986  ;  in  France, 
989 ;  in  Belgium,  990  ;  in  Germany, 
991  ;  in  Switzerland,  992  ;  in  the  Vienna 
basin,  992 ;  .in  Italy,  993 ;  in  North 
America,  ibid. 

Oligoclase,  72 

Olivay  966,  998 

Olivine,  75,  173,  174*,  365  ;  in  meteorites, 
10 

Oli  vine-diabase,  170 

Olivine-dolerite,  170 

Olivine-free-dolerite,  170 

Olivine-gabbro,  169 

Olivine-rocks  (schistose),  183 

OmoMzuruSf  909 

Omphacite,  75 

Omphyma,  742,  757* 

Onchusy  744 

Onondago  Limestone,  790 

Salt  group,  775 

Ontario,  Lake,  area  of,  1052  ;  terraces  of, 
1054  ;  unequal  elevation  of  terraces  of, 
286 

Onychodusy  790 

Oolite,  104,  150  ;  formation  of,  in  salt-lakes, 
413 

Oolite,  Great,  898,  905,  913 

Inferior,  898,  903,  913 

Oolites,  Lower,  898,  901 

Oolitic  formations,  879,  898 

structure,   104,   147,  150,  151*,  486, 

805 

Ooze,  139,  456,  492,  498 

Opacite,  123 

Opal,  65,  69 

Operculina,  980 

OphiderpUon,  821,  846 

Ophidians,  fossil,  969 

Ophileta,  624,  727,  744 

Opkioglyphaf  901 

Ophite,  120,  170 

Ophitic  structure,  119*,  120,  155 

OphtfuUmosaunf^j 

Opossums,  fossil,  968,  985 

C^peUia,  884 

Opponitz  Limestone,  873 

Oracodon,  936 

Orfncida,  734 

Orbicular  structure,  101* 

Orbit,  eccentricity  of  earth's,  16,  24 

OrbitoUleSy  980,  993 

Orbitoitic  group,  993 

Orbitolinay  924 

Orbitolile^y  978 

Orbulifia,  860 

"Ordovician,"  738 

Ore  deposits,  631 


li:S2 


TEXT-iyyjK  'jF  GEjDa^T 


'^% 


OsBtij   i^'aU,  urZxA  'JL,  UX,  »!,  &t.  471, 
iri  iZX.  4*4 

rx:2.  ^xn  whI.  ^(f(^;  Acssim  <v^  ol   ic»- 

«&.  <*^^i,  *!•>;  ^jTOkj  a 
f^/z.  47^.  477 

f/fJUff^  742 

OrtAu.  4'it4,  7^*,   7±S.   743*,   745*, 

Ml,  "i^ 

fMMf^^aerajL,  424, 7W*.  72S,  74J*,  744, 741*. 

7%1,  «2*,  M4.  642 
OrtiKnmtite  fi»f<loftft   iSeaadottTu  ,  747, 

74* 
Oitfao>:«niitlt«t  M  type-fnmah,  457 
OrtV^Uiie,  71  ;  dc^ay  oC,  114 
OrtlK<Uiie'p<>rp]iji7,  144 
fHtX//t^Aa,  744.  741* 

Ortb</f/'>rra.  fc/wil,  7&4,  %20 
ffri/mxa,  ^11 

6»ar,  1040 
0»V/rrie  B«i*,  5f^4 
0*mv«////,  1^77.  &•% 

04</-//,  ^71.  %*•';*,  J5«57*.  Sr24*.  &->;,  9«7^ 

5»^4*,  $^«j.v.  5f^5 
Oupiri  nerval.  878 
f/t/ff-AraJt,  ^77 

ffOn/imit.*ji,  ne/y    fcfti)   923 
OtUrr,  f-/^5il.  if>;,  1014 
Otlrdit/i,  77 

Ottw-si'frr  Be^ls  ^:i7 

Outcrop,  533 

Ov^-rla;..  618 

/>r<W  10J4 

'>'''////,  1009 

Ow],  ^riv**y.  fo-sil,  1061 

Ox,  fo.-j],  ]00»> 

Oxford  (J'AiUh,  ^98,  907 

Oxfopli-'tn,  ^^9^,  907.  913,  915,  916,  918,  919 

Oxi'latioij,  3J3,  345,  364 


•>ijiit«-  -t?-  Ii4 
aaa-  *4l,  44»J:  5r 

5^.  ii?X  :* 


OncanL 


:  «i«ir«  7 


/'«dl^/>r<ft^  •77 
/*«aK^i<«|Mju  747 


Fm/»Lf^ii«rt.  74"/-  75* 

FKsdac  Ocsa^  ^cii  «£.  34 

*•  Pi3«!>eb:ie  '  Larir-iCRaaEdL  217 

F^siMorm^  721*.  725.  74»*.  744.  745' 

P^tlmmje^,  742 

P^iMdkimm/k,  754,  •ll 
PmLmti^Kfim^  7^ 
PaJM^atiimm,  744 
PmlMocmrU,  5l2 

9^ 

742 

F«lmacory$ia,  »42 
pfaiMf^mmifvm^  ^12 
PmljMdturMa.  75^ 
FnlMrAmM,  745 


F9lMtikatUsri0L,  S44 

105*  :    br>rk  -  carifi^    i 
1059  :   Ic***   in,  »'/< 


fiT^j  o£.  1«:«41 


Xrhf*A  of  TTiJin  i^  i^Mf.  ;  ia  Brnaza.  1065  ; 

in  France,  i^^i.  /  in  G«nzsazT.  1064 
P»ljK>l::hic    iiLii'ltexx^eaU,    1057*  : 

1«>32* 
PaiatotMiAiii^  1020 
PolM>/w<rryx^  l«j21 
PalMr/itiu:ii0.  9^8 
P^xistonvuru*,  845*.  844 
PaUfontina,  884 
FxlabouvAofrj.  445 
Palaon^fris^  985 
PaiMf/phU,  973 
PaUoph/m^y^,  744,  742*,  794 
PaJ.3tophy<iu^  740 
Palaeopikrite,  173 
PalmopitJucuM.  1021 
PaUopirrig,  785,  793,  823 
PaJ»ortas,  1019 
PaUioryx^  1017 
Pol»o*auruM^  863 
Poi£t'>»irtn.  846 
PaUotherium,  968,  969*,  9S5 
PaUofraffus,  1019 
Pnlft'jzamia,  906 
Palaeozoic  defined,  680 


INDEX 


1133 


Palaeozoic  rocks,  718 

PaltfTj/x,  987 

Paia'slringa,  935 

Palagonite,  172 

Palagonite-tuflF,  137*,  138 

PcUaplotherinm,  968 

Paleschara,  743 

Palmetto,  972 

Palms,  fossil,  923,  966,  984 

Paludina,  910,  953,  972,  986,  1011,  1045 

Pauama  Isthmus,  marine  fauna  on  two  sides 

of,  291 
Panchet  series,  661,  854,  877 
Pandaniis,  922,  966 
Pangshura,  1021 
Pan -ice,  450 

Panidiomorphic,  118,  119 
Paulselian,  976,  977 

Panopxa,  983,  993,  995,  996*,  1007*,  1043 
Pantelleria,  166 
Parabolina,  731 
Paraclase,  523 
ParacyathuSy  978 
ParadoxideSf  722*,  728  ;  supposed  descent 

of,  656 
Paradoxides  group,  725,  727,  731 
Paragouite,  74 
Paragonite-schist,  185 
Parahyus,  969 
Parallel  Roads,  423,  1038 
Paramorphism,  74,  364 
ParaproroniteSj  852 
Parasmiliay  925 
PareiasavruSf  863 
ParexuSf  800 
Parisian  stage,  980 
Parka,  793 
Parrotia,  1018 
Partnach  Beds,  873 
Passes,  origin  of,  1087 
Patella,  906 
Patula,  1018 
Paurodon,  919 
Pea-grit,  150 
Pearlstone,  161 
Peat,  142,  144  ;  effect  of  pressure  on,  312  ; 

marine,  478  ;    growth  of,  ibid.  ;  rate  of 

growth  of,  480 
Peat-mosses,  331,  478,  479*  ;  entombment 

of  organic  remains  in,  647  ;  human  relics 

in,  1066  ;  successive  vegetation  in,  ibid. 
Pebbly  structure,  103 
"Pebidian,"  710,  727,  728 
Pecnpteris,  816,  843,  859,  880*,  988 
Pectai,  852,  862,  868*,  883,  927, 
Pecten  a^per-zoue,  938,  948.  947,  976,  985, 

995,  1009,  1043* 
Pectunculus,  972,  995,  996 
Ptuiiomya,  936 
Pegmatite,   158  ;    veins,   700* ;    in   granite, 

580,  581* 
Pegmatitic  structure,  98,  119 
Pegniatoitl  structure,  155 
Pelagic  deposits,  457 


PeUcanus,  1021 

**  Pele's  Hair,"  223 

Peli/ts,  1013 

Pelites,  132 

Pelitic  structure,  103 

Pelobatochelys,  909 

Pdoroaaurus,  930  * 

Peitastes,  944 

Peltocaris,  742,  748 

Peltura,  731 

Pcmphyx,  860 

Pen»;us,  860 

Penarth  beds,  864,  867 

Pennant  grit,  833 

Pennine,  77 

Pentacrinus,  875,  882* 

Pentamerm,  743,  755*,  781 

Pentamerus  beds,  743,  753 

Pentland  Firth,  tides  in,  434,  447 

Pentremites,  811 

Peperino,  138,  587 

Ptralestes,  894 

Peramus,  894 

Perched  blocks,  128,  425 

Peridot,  75 

Peridotite,  173  ;  of  crj-stalline  schists,  183 

Periechocrinvs,  756 

Perihelion,  16,  25 

Perimorphs,  65,  67,  69 

Perisphindea,  884 

Periite,  161 

Perlitic  structure,  101*,  120*,  161,  530 

Permian  system,  841  ;  rocks  of,  842  ;  life  of, 
843  ;  volcanoes  of,  201,  203,  204,  261, 
847,  seq. ;  in  Britain,  846  ;  in  Germany, 
848  ;  in  Bohemia,  850 ;  in  the  Vosges, 
ibid.  ;  in  France,  851  ;  in  the  Iberian 
peninsula,  852  ;  in  tlie  Alps,  ibid. ;  in 
Russia,  ibid.  ;  in  Asia,  853  ;  in  Australia, 
854  ;  in  Africa,  855  ;  in  North  America, 
ibid.  ;  in  Spitzbergen,  856 

Permo-Carl>oniferous  rocks,  842,  854 

Pmia,  909,  927,  992,  999 

Pernostrea,  913 

Peronella,  860 

Persfionin  995 

Perse^t,  981,  995 

Persistent  tyj)es  of  oi^anisms,  667 

Perthite,  70 

Peru,  proofs  of  uprise  of,  285 

Petalodus,  812 

Petalograjytvs,  765 

Petraia,  742,  757*,  784 

Petrifaction,  process  of,  364,  651 

Petrifying  agents,  78,  364,  378,  651 

Petrography  (Petrology),  60 

Petroleum,  145,  235,  363,  602 

PetrophUa,  965 

Petrophry^ne,  863 

Petrophylloidcs,  965*,  966 

Petrosiliceous  rocks,  155 

structure,  119 

Pence,  881 

Phacopa,  743,  757*,  780* 


1134 


TEXT-BOOK  OF  GEOLOGY 


PhalacrocoraXy  1021 

Phanerocrystalline  structure,  97 

Phaneropleuron,  796 

Phascolomys,  983,  1022 

Phascolotherium,  893,  895* 

Pheuganocaris^  768 

PhasianeUh,  854,  912,  942 

PhasianuSj  1019 

Fhenocrysts,  98,  155 

Philippine  Islands,  volcanoes  of,  253 

PhiUipmstraea,  780,  810 

PhiUipsia,  812 

PhUbopteris,  880,  905 

Phlogopite,  74 

Phoenicites,  984,  995 

Pholadamya,  884,  971,  977,  1009 

Pkolas,  998 

Pholidtrpeton,  821 

Pholidqphorus,  867,  886 

Pholichsaurusj  931 

Phonolite,  166 

Phortis,  1010 

Phosphates,  79,  124 

Phosphatic  deposits,  141,  494,  513,  921, 
990,  1009 

Phosphorite,  141 

Phosphorus,  61 

PhragmiUs,  988,  1017 

Phragvioceras^  744,  761* 

Phtonite,  141,  805,  826 

Phyllades  de  St  Ld,  714 

Phyllite,  134,  179  ;  relation  of,  to  clay- 
slate  and  mica-schist,  314,  319 

Phylloceras,  872,  884,  903*,  928 

Phyllodu^,  966 

PhyUogruptiis,  11%  739* 

Phyllopods,  fossil,  724,  742,  758,  780 

PhyUolh4:ca,  839,  854,  877 

Phym,  910,  959,  990 

Physiographical  geology,  5,  1068 

Phytosaurus,  863 

Piceites,  851 

Pickwell  Down  group,  784 

Picotite,  71 

Piesociase,  523 

Pikrite,  173 

Pilocertis,  624 

Pilton  group,  784 

PirutcUcs,  782 

Pinacocercifff  862 

Pine,  fossil,  1004 

Pinites,  818,  881,  991 

Pinna,  841,  883,  973,  1000 

Pinus,  923,  973,  988 

Pipe -clay,  133 

Pisania,  972 

Pisuitum,  1013 

Pmtfius,  971 

Pisolite,  150 

Pisolitic  limestone,  952,  962 

structure,  104,  150 

Pistiicite,  76 

Pistacite-rock,  183 

Pitchstone,  163 


PUharella,  972 

Placer-works,  632 

Placoderms  (fishes),  744 

Placoparia,  743 

Plagiaulax,  894,  895*.  936 

Plagioclase,  71 

Plains,   44  ;   ratio   of,   to  Tmlleys,  465 ;  of 
marine  denudation,  468  ;  oirigin  of;  lc!jB8 

Plaisancian  stage,  1015,  1016,  1017 

Planer,  954 

Planeroy  999 

Plane-tree,  fossil,  923,  966,  1004 

Planets,  origin  of,  8 

PlanoliUs,  723 

Planorbis,  910,  958,  972,  985,  986*,  999, 
1011 

Plants,  geological  inferences  afforded  by, 
291  ;  destructive  action  of;  471  ;  con- 
servative action  of,  475  ;  reprodactive  in- 
fluence of,  477  ;  calc-sinter  formed  by, 
482  ;  comparative  rate  of  evolation  of 
terrestrial,  660,  668  ;  geographical  distri- 
bution of,  660 

Planularia,  900 

PlasmoporcL,  769 

Plastic  aay,  972 

Plasticity  of  earth's  interior,  57 

Platacodan,  936 

Platanusy  972,  988,  1005* 

PUUax,  1013 

Plateau-gravel,  1038 

Plateaux,  43 

Platemys,  930,  973 

PlateosauruSy  863 

Plate  River,  sediment  in,  384 ;  mineral  matter 
dissolved  in,  462 

Plattelkohle,  837 

Platycerasy  725 

Platycrinus,  811 

Platyschisma,  744,  854 

Platysdenitcs,  732 

Plaiysomus,  845* 

Plectrodus,  744 

Pleistocene,  defined,  962 

Pleistocene  deposits,  1023 ;  general  characters, 
1024  ;  in  Britain,  1042  ;  in  Scandinavia, 

1045  ;    in   Germany,    ibid. ;   in   France, 

1046  ;  in  Belgium,  1047  ;  in  the  Alps, 
1048 ;  in  Russia,  1049  ;  in  North 
America,  1050 ;  in  India,  1054  ;  in 
Australasia,  1055 

Pleochroism,  95 

Pleonaste,  71 

Plesiarctomys,  985 

Plesictis,  985 

Planogale,  985 

Plesiosaurs  as  ty|>e-fossils,   657  ;  forms  of, 

863 
PUsiosaurus,  888,  889*,  931 
PUsiosortx,  985 
Pkuracnnthus,  820,   851 
PUurocysfites,  742 
Pleurodictyum^  779 
PUurograptuSj  748,  751 


INDEX 


1135 


Pleuromya,  901 

PleuronatUUus,  876 

Pleuromuraf  851 

Pleurotmna,  966,  985,  998,  1010 

Pleurotomariay   624,   726,  744,  781,  811*, 

844,  884,  887*,  928,  1001 
Plication  of  rocks,  317,  1072,  1075 

and  metamorphism,  681 

Plicatula,  901,  943 

Pliocene,  defined,  962 

Pliocene  formations,  general  characters  of, 

1003  ;   flora  of,  1004  ;   fauna,  1006  ;  in 

Britain,  1008  ;  in  Belgium  and  Holland, 

1014  ;  in  France,  iWd. ;  in  Italy,  1016  ; 

in  Germany,  1017  ;  in  the  Vienna  basin, 

1018  ;  in  Greece,  1019  ;  in  Samoa,  1020  ; 

in  India,  ibid.  ;  in  {iArth  America,  1022 ; 

in  Australia,  ibid. 
Pliopithecus,  996,  1022 
Pliosaurus,  890 
Plocamiunif  740 
Plocoscyphia^  943 
Plum,  fossil,  1004 
Plutonia,  722*,  724 
Plutonic,  definition  of,  160 

action,  190,  560,  663 

Plymouth  limestone,  784 

Po,  sediment  in  the,  383  ;  plains  of,  396, 

463  ;  delta  of,  395,  402 ;  area  of,  462  ; 

amount  of  material  removed  by,  ibid. 
Poaciies,  977,  988 
Ptxiogonium^  995,  1018 
PodozamUes,  860,  880,  923 
Po^broiherium,  1003 
Poikilitic,  841 
Polacanthus,  930 
Polar  diameter  of  earth,  13 
Pollack,  fossil,  1012 
Pdlicipes,  942 
Polycodia,  844 
Polycotyhis,  960 
Polygonum^  991 
Potymorphina^  900 
PdyjKyra,  811,  844 
Pdypterus,  796 
Polyptychodofij  931 
PomtUograptus^  742 
Pom(>eii,  volcanic  phenomena  at,  197 
Popanoceras,  845 
Poplar,  fossil,  923,  966,  1004 
Populus,  922,  988,  996,  1005* 
Porambonitea,  743,  745* 
Porcelain-clay,  133 
Porcdlia,  781    884 
Porcellanite,  135 

Porcupine,  fossil,  1006,  1036,  1060 
Porosphteriat  946 
Porous  structure,  102 
Porphyric  structure,  119 
Porpbyrite,  168 

Porphyritic  structure,  97,  98,  99*,  155 
Porphyroid,  98.  184 
Portage  group,  789 
Portheiu,  930 


Portland  Oolites,  898,  908 

Portlandian,  898,  908,  909,  911,  915,  919 

Posidonia,  914 

Posidonien-Schiefer,  916 

Posidorumya,  811,  883,  885* 

Post- Pliocene  {see  Pleistocene) 

Post-Tertiary  formations,  1023 

Pot-clay,  133 

ffnthi>*rn,,386,  429 

Potamides,  972,  985* 

Potamogeton,  923,  996 

PotamomyOt  986 

Potassium,  61,  63 

Potassium-chloride,  149 

Poteriocrinu^y  811 

Pothocites,  816,  819 

Potomac  formation,  923 

Potstone,  183 

Powder  of  rocks,  examination  of,  86,  87 

Prairie-dog,  geological  action  of,  474 

Prearcturus,  798 

Pre-Cambrian  rocks,  680 ;  sediments  and 
volcanic  masses  of,  681,  692 ;  homotazis 
of,  680 ;  liability  of,  to  alteration,  681 ; 
conversion  of,  into  schists,  ihid, ;  nomen- 
clature of,  683 ;  oldest  gneisses  and  schists 
of,  685  ;  sameness  of  lithological  characters 
of,  ibid.  ;  banded  structure  in,  ibid.  ;  sedi- 
mentation of,  692  ;  limestones,  cherts,  and 
ironstones  of,  693  ;  graphite  of,  695  ;  vol- 
canic masses  in,  692,  696,  710  ;  traces 
of  life  in,  694  ;  metamorphism  of,  696  ; 
chronological  value  of,  697  ;  thickness  of, 
ibid. ;  of  Britain,  698  ;  of  Scandinavia, 
711  ;  of  Finland  and  Russia,  713 

Pre-Cambrian  topography,  692,  706 

Precession,  16,  30 

Prehistoric  Period,  1066 

Prehnite,  77 

PrepecopteriSy  822 

Present,  the  key  to  the  Past,  3 

Pressure,  effects  of,  47,  143 ;  increases 
chemical  action,  307  ;  produces  consolida- 
tion, 311,  312  ;  promotes  crystallisation, 
ibid. ;  produces  schistose  structure,  567 

Presticichia,  812 

Priacodoriy  919 

Pribram  Shales,  714 

Primary  rocks,  680 

PrimUia,  742,  749 

Primitive  rocks,  684 

Primordial  zone,  719,  726,  731,  734,  772 

PrionocyclciSy  958 

Prismatic  (columnar)  structure,  104  ;  artifi- 
cial production  of,  300 ;  examples  of, 
528,  529,  530*;  induced  by  eruptive 
rocks,  599 

PristiograptuSf  742 

Pri^tiji,  966 

PristisomuSy  878 

Procamdusy  1022 

Produciusy  781,  810*,  811,  844* 

ProittiSy  743,  781,  812 

Prolecanitesy  782 


IXX^ 


T£JrT'^.':±  'if  yJ.-'^y-r 


/'-  '^/UM-ul^  f  ".    Til 

p-*v;«i.»>er   i-^I.  >*±.  Kt.  »*4u  W-I 
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t'mmmf/^,  >T<-  >**,  lOVi 
J'$^,<^/l//f^/t,  770 
/'♦<  v//>'//'/*x///  *  #  // .   i-'i  4 

/■';<-.<:  ///////» <///»/y '  «>,  r  7  ^ 

F«*:  .'\rt'.i.^*r\.:.»..  ^-/,  ^7.  7*,  'X4 

/'»<  '/////*  *  y » ////  /'*//.   r  1^2 

I'tiJofthyt'fi',  7*'».  71^-i,  7i*J* 

/'trro.n/fji,  7t4,  7i*^ 
Inftvltplrinima,  'ATI 

/*t*  ronotu/f,  1*7  I 
l*t^roj,h>/H>,n,,  '•n,  ^51«,   **<«0 
I'l/rnifhir^   '•*J\ 
l't*:ro<;iiimn..  >»I*0*,  I<31,  933 

/Urrnfh^rn,    7H 

rf^rifffof''^,  743,   780*,   71^^> 
I 'I,  I,, ^1, 'I  If  n^   1\?, 

J'fLlnf,l,i//l„i,i,   H*',() 
I'tj/r.kiOM,   Hf,'2 


P*T.tr:t»*nk.itltm^  "itL  ~ 

/"«*  -♦.-..we  i«10 
?"im:>';>t.    I'd  :    '?«■!:. 

/"«:»»f.  ril  ie^  W*L  W»-  IiEl 

/v->*#.-T. :  .#:?* 

FTT^SfWt.    y.tLiag-TiiffrjniiTrpBiiaiL 

771  :  I^v-xLido.  7?»?  : 
Ffr\f0.'^-.  >7* 
PTt-j*.  7>  :  &*  & 

pTTidi^oie.  101,  141 

Pyrnrxsait.  74,  75.  ^I^;  etHivtsrsiia  it  -"~i 

Pyr>x«K.aaitts5fc  I'57 
FtToxcbe-^EncBlite.  149* 
Pyr-^KBit-rogki,  161 
FjrriKfOut.  **> 
/Vr«:a,  S'73.  S'S^  KO* 

Qtai^ee.  >54 

Q:ii-<j:Li-T*-raaI  dijv  SS3.  5i^ 

<^^iAnz.  •>>,  r;i.   1^4  :   ibeM-.-rbes:  zcw-er  :•"'. 

Qr:arLz-Ar.-ioric.  lo7 

Qaartz-diiljaA^,  170 

QoArtz-dionMr,  Ifio 

QiiArtx-j«oq.brrT.  ICO 

Qoaru-rock-,  17'.* 

QoartZ'Scbi-n,  17& 

Quartz-tnuhj-te.  1»>0 

QuartziUr,  132.  1S<:«*  ;  origin  of,  319 

Qaartzl<A*-|»orphyry,  144 

QuartzoM:  coni{io»ition,  104 

Quateniary  formations,  1023 
'•    Qiieeiwbrrr}'  grits,  764 

,  Queen.HlaDd,  pre  -  Cambrian  rocks  of^  717: 
Penuo-Carboniferotts.  840.  854  ;  Jnntssic. 
920  ;  Cretaceous,  960  ;  Tcrtiarj-,  1023 

(^unuftt^'U ir^roji^  919 

(^uerciis,  922,  923*,  973,  988,  995*,  1017 

Quercy,  Olieocene  deposits  of,  990 

Quinquel'tculina,  976 
I 

I    Rabbits,  geological  action  of,  474 
I    KadiatioD-spectrum,  11 


INDEX 


1137 


Radiolaria,  earliest  remains  of,  694 ;  SilnriaD, 

740,  783,  note 
Radiolarian  ooze,  141,  493* 
RadioliUs,  928 
Raibl  beds,  873,  875 
Raikiill  beds,  767 
Rain,  composition  of,  32,  341,  342 ;  chemical 

action  of,  341  ;  action  of,  in  weathering, 

346  ;  mechanical  action  of,  353  • 

Rainfall,   effects   of    forests   on,   473,  476 ; 

influence  of  variations  of,  on  sediment  in 

rivers,  382  ;  relation  of,  to  river  discharge, 

464  ;  man's  influence  on,  496 
Rain-prints  in  rocks,  508 
Rain-wash,  128,  352 
Raised  beaches,  285,  286*,  287* 
Rake-veins,  639 
Randanite,  69 
Raphistomaj  744 
Rapids  of  rivers,  386,  392 
Rapilli,  136,  199 
RastHtes,  739*,  741 
Rats,  burrowing  habits  of,  474 
Rauchwadke,  842,  849 
Ravines,  origin  of,  391 
Recent  or  post-glacial  period,  1028,  1055 
Receplacu/ites,  741,  779 
Recoaro  Limestone,  873,  874 
Red  as  a  colour  of  rocks,  106 
Red  Crag,  1008,  1010 
Redonia,  743*,  744        ^ 
Reduction  by  organic  matter,  343,  360,  456, 

472 
Regur,  or  Black  Soil  of  India,  133,  477 
Reitliug  Limestone,  873 
Reindeer,  in  the  glacial   period,   1036  ;  in 

post-glacial  time,  1061*  ;  Age  of,  1061, 

1066 
Rcinop/eurideSf  743 
Reiisseleridi  781 
Reptiles,  Age  of,  887 
Requiaiia^  927* 
Resin,  fossil,  645 
Resinous  lustre,  107 

structure,  100 

Rctepora,  743,  925 

Reti()(fraptus,  747 

lictiditesy  741 

Retzia,  856,  861 

Reunion  Isle  i^set  Bourbon) 

Reviuien,  733 

Revolution  of  earth,  16 

RhahlocentSy  862 

Rhabilophora,  distribution  of  the,  741 

RJwbilnphyUUi,  908 

RhacnpteriSf  839 

Rfuu/iiiacnnfhus,  796 

Rlutdinichthys,  820 

Rhretic  group,  864,  867,  869,  873,  876 

JVuimnus,  981,  988,  995,  1017 

R/iamphocephalus,  890* 

RlMmphorhynchus,  891*,  893*,  894* 

Rhamphosuchuti,  1021 

Rhine,   mineral   matter   dissolved   in,    378, 


379  ;  transport  of  gravel  along  bottom  of, 

380  ;  proportion  of  sediment  in,  888  ; 
cause  of  milky  tint  of,  385 ;  gorge  of, 
388 ;  shifting  of  course  of,  at  Schaffhausen, 
391  ;  marine  delta  of,  402 

Rhinoceros,  985,  993,  995, 1014, 1036,  1060 

RhinoIqphuSf  985 

RhizocortUlium,  870 

Rhizodxuiy  812,  813* 

RhizomySf  1021 

Rhodanien,  948 

Rhodea,  823 

RhodocriuHSf  811 

Rhomboporay  811 

Rhone,  rise  of,  372 ;  salts  dissolved  in, 
379  ;  sediment  in,  383 ;  transport  of 
sand  on  bed  of,  384  ;  filtered  by  Lake  of 
Geneva,  386,  398,  406  ;  marine  delta  of, 
401,  402 ;  limestone  formed  at  mouth  of, 
453  ;  area  of  basin  of,  462  ;  amount  of 
material  removed  by,  ibid.  ;  glacier  of,  in 
Pleistocene  time,  1030,  1047,  1048 

Rhus,  995* 

Rhynchocephalons  reptiles,  846,  862 

Rhynchorieila,  743,  745,  761*,  781,  811, 
861,  883*,  925,  926*,  1007* 

RhynchosaurnSf  862 

Rhyolite,  160 

Rhyolite-glass,  162 

RiSeiria,  747 

Riders  (mineral-veins),  635 

Riebeckite,  74 

Rill-marks,  508 

Rimdla,  967* 

Ringicula,  1009 

Ripidolite,  77 

Ripple-marks,  335,  507,  607* 

Rissoa,  1000 

Rifa,  1021 

Rivers,  influence  of  earth's  rotation  on  flow 
of,    15 ;    sources   of,    371  ;    inflnence  of 
drought  on.  372  ;  discharge  of,  373,  462 ; 
influence  of  man  on,  374,  496,  497  ;  flow 
of,  375  ;  average  slope  and  rate  of  flow  of, 
ibid. ;  affected  by  upheaval  and  subsidence, 
377,  397  ;  chemical  action  of,  377,  462  ; 
composition  of  water  of,  377  ;  mechanical 
action  of,   379  ;    transporting  pow<^r  of, 
ibid.  ;    influence   of  ice   on,   882,    414  ; 
varying  effect  of  rainfall  on,  882 ;  pro- 
portion of  sediment  in,  383  ;  transport  of 
sediment    on    beds   of,   384 ;    excavating 
power   of,    ibid. ;    serpentine   curves   of, 
387  ;  shifting  of  channels  of,  by  glacial 
action,  391  ;  reproductive  action  of,  393  ; 
former  greater  volume  of,  397  ;  Influence 
of  terrestrial  movements  on  flow  of,  ibid. ; 
relation  of,  to  lakes,  ibid. ;    influence  of 
melted  snow  on,  416  ;  amount  of  material 
reriioved  by,  462  ;  slow  rate  of  erosion  by, 
1021,  1078  ;  Palaeolithic  alluvia  of,  1058 
River-gorges,  origin  of,  391 
River- terraces,  396*,  1047,  1058,  1054 
Robulina,  909 


4  D 


1139 


TEXT-BOOK  or  GEOLOGY 


Eockft,  tL'tfnaal  ruMtuice  oi;  ^ :  de&ntj 
of,  in  iolki  asid  Ohelud  ftatb,  S6 ;  dcccr- 
ci&Ation  c<  ^J ;  iMcbaakal  aaalTiidi  of, 
S<{ ;  ezax&iAAlion  of  powder  of,  d»«l.  / 
^/E.«ix,i«al  a&al j*u  Q^,  87  ;  fyntKcsu,  8i9 ; 
u.y.T4>»r»f4c  ittT«ftti;^tMia  ol^  iS«i.;  OMfB- 
^ftyye:  cLanwrt«n  of.  S^;  stnactimi  of^ 
i)!^. ;  c«ii.f«ositwa  of,  104  ;  graditkmt  in 
cotaif^iliou  of,  105  ;  fUte  of  ac7r««ftSioB 
ol  i^/i.  ;  fnctsre  of,  Md. ;  coloor 
ukd  Icftn:  of,  104 ;  feel  ftod  nnell  of, 
107 ;  mtcro*copk  chaneter  ol^  lOS ; 
tbicroftcr^^ic  tVtwtnU  dr  109 ;  micn>> 
iKOf^ic  fftroctnra  of^  117  ;  clairifica- 
tioD  of,  123;  UpitfAu,  124,  154,  559; 
aq^eooA,  124  ;  metamorpluc,  124,  126 ; 
fttratifted,  t7/M/.  /  anstntiiied,  124  ;  ledi- 
rcenUrr,  126 ;  fngmenUl,  126,  1S4 : 
cry  stall  i&e  AtntiiWs'i,  14%  ;  nuHiTe.  154  ; 
eff^rcu  of  beat  on,  297,  299 ;  contnct  in 
fAMibg  from  glaety  to  litboid  state,  304  ; 
nnirerMl  (VMcnoe  of  water  in,  306 ;  ab- 
•r/rbent  power  of,  Odd.;  solTent  power  of 
w*t«r  io,  307  ;  minor  mptnres  of,  311. 
31%;  cleara^e  of^  312;  defonnatioQ  of, 
314. 543  ;  plication  of,  317, 536  ;  jointing 
ot  31S.  523  ;  metamorpbinn  of,  319,  595, 
611,  6?0  ;  aadergroiind  water  in,  356; 
alteration  of,  by  andergroand  water,  364  ; 
inclination  of,  531  ;  emptiTe,  in  earth's 
crojit,  559 

Rock->jafiiDH,  349,  350*,  351  ;  scooped  out 
by  icH,  430 

Ro^:k-':niMiirig.  heat  evolved  by,  29S 

Ro^k-':r^-«uL  61* 

m 

Rock-oil,  145 

Fuyk-^alt,  14S 

Kockiii;?-fltori«r«,  349 

Ro'.ky  Mountains,   fonii  of.   39  ;   stmcturc 

aii'l  upheaval  of,  1077,  1078 
Ko^erist*:in,  150.  ^70 
Kohrljach's  solution,  86 
Rrxibng  hlate.  135 
Root.**,  jreolojncal  action  of,  473 
Ros^V>*rg,  fall  of,  till 
Rf^UVaria,  ^'1%.  966,  967* 
Rfftalia,  ifiii 
RoUlion  of  earth,  15,  21,  22  ;  effects  of,  on 

flow  of  rivers  15  ;  effects  of,  on  ocean - 

currents,  'i''j9 
HoUl/a,  102:5 
Roth    Triav,  ^70 
R^^thliegende,  ^49 
Rothoniatrian,  93S,  948,  951 
iiotten-stone,  IjO 
Rul^IIan,  7:5 
Rurli,ten-Kalk,  i-.vl,  956 

RujM-lian,  1*1*0 

liuptures  of  r«'x;k.i,  311 

Ru-iia,   tundra-s  and   black   earth  of,  352  ; 


06  «2:  C 

i-       "■♦-♦  -    Sf-— *^*       'JSJt     — 'T  . 

7^%  :  1arv:»iiierr»i.  *A» 

OK-r,  l'>49 
bCtij.?,  WSf,  ji3. 


rrtce 


1^:  P^cac 


^'>^-,  9i55*.  9i4.  9Sa  »* 
Sabal.  Jcissil,  9:55 
S»bMe!^  xaoycas,  975 
Saar^immima.  740,  ?«1^ 
SaorLar^x-i,  11*,  151 

Sjgmaria^  740,  7*5 
•^^mt^tfiL  575 

Sahara,  fia&<i-wutes  o^.  $36 

Sahlitev  74 

St.  AntLonr  Fall^.  on 

of,  -90 
St.  Ca!4ian  beds,  S73.  *74.  *75 
St.   David'i,  FiTppoaed  p*e-Caai»ia=.  r>d 

of,  710 
St.  Erth  bed<,  1010 
St.  Helena,  34,  255.  260 
St.  LAvrence  River,  fiitercd  bj  Lake  C^^an 

3^7  :    ice  on,  415 ;    mx&cnl  KaUcr  di 

j-jlved  in,  462 
St.  LAais  grocp   U.S.  Carboaiferocs  ^  $41 
Sl  Paul  Uland,  34^252,  253*.  2W*,  255 
Sal  ammoniac  at  Tokanoea,  196,  225 
^loiia,  925 

Saliferoos  compontkm,  106 
Sali^bmria,  965,  973,  1001 
Stlij:.  922.  977,  9SS,  1005*.  1014,  1»>26* 
Saln.ien,  733 

Sa'.^^  or  ifi  ad -volcanoes.  23S 
Salt,  common.  79  ;  depo'it^  od  14^.  15:1  41 

737,  739,  775,  7^9,  >43.  ^4?.  s4&.  ^i 

853,  ^59,  566.  ?69,  %70,  993,  1004,  101 

1019 
Salt  Lake  of  Uub,  40?,  411 
Salt-n.ar>he<,  454 
Salt  Range  of  Punjab,  Cambrian  rocks.  7oi 

Silurian,  776  ;  Permian,  853  :  Trias.  6 
Salt-water,  destructive  effects  of,  on  bra^rki 

water  onranisms,  649  ;    influence  of. 

dej-osit  of  Merilment.   3S1,    450;    aolve 

action  of,  3S,  441,  442,  491 
SalUrfJln.  69^,  725 
Samos  Piic»cene  deposits  of.  1020 
!iavn,th^riii,iu  1C»06,  1020 
Sand,   128  ;  abrading  effects  ot   driven  1 

wind,  330  ;  finer  kinds  of,  escape  tritoi 

tion  in  rivers,   3S5 ;    heavy  minerals 

ancient  origin  in,  129,  705 
San«l  volcanic,  136,  199 
Sand-river»,  Livingstone  on,  3S2 
Sandhills,  334 
SaniL-tone,    131,  1S5  ;  weathering  of.  3  J 

chanceil      into     quartzite,      610,      61 

col  11  lunar,  599*  ;  crystallised,  132 
Sandstone-dykes,  552,590 


INDEX 


1139 


Sandwich  Islands  {see  Hawaii) 

SanyuiTwlaria^  785 

SarujuinoliteSy  811 

Saiiidine,  72 

Sanitherium^  1021 

Sansan,  mammaliferous  deposits  of,  998 

Santonian,  938,  948,  952 

Santorin,  volcanic  phenomena  of,  195,  196, 
197,  200,  201,  207,  210,  211,  216,  223, 
226,  231,  245,  251*,  252 

Saone,  rise  of  the,  372 

Sapindus,  965 

Saportxa,  855 

SarcophUus,  983,  1022 

Sarmatian  stage,  1000 

Sarsaparilla,  fossil,  1004 

Sarsen-stones,  355 

Siio,  734 

^safrus,  923*,  988,  1004 

Satellites,  origin  of,  8,  9  note 

Saturn,  rings  of,  8  ;  density  of,  9 

SauHchthys,  862 

Sauropterygians,  933 

Sdurosternan,  863 

Saussurite,  73 

Saassuritization,  618 

Stxicnva,  1013,  1043* 

Saxicavous  shells,  474 

Saxon  Switzerland,  954 

Scaglia,  956 

Scald,  966 

Scalaria,  1011,  1012* 

Scaldesian  group,  1014 

Scania,  subsidence  of,  288,  291  ;  Cambrian 
rocks  of,  731  ;  Silurian,  768 

Scandinavia,  upheaval  of,  288  ;  subsidence 
of,  291  ;  snow-line  in,  416  ;  glaciers  of, 
419*  ;  metaraorphism  in,  621,  713,  769  ; 
pre- Cambrian  rocks  of,  711  ;  Cambrian, 
731  ;  Silurian,  767  ;  Old  Red  Sand- 
stone, 802  ;  Trias,  870  ;  Jurassic,  918  ; 
Cretaceous,  953 ;  glaciation  of,  during 
Glacial  period,  1027,  1030,  1045;  dis- 
persion of  erratics  from,  1033 ;  sub- 
mergence of,  1037 

Scapliaspis,  744,  758,  796 

Scaphites,  928,  929* 

Scaphognaihus,  890* 

Scapolites,  76 

Scaur  limestone,  825 

ScelidosauTUs^  901 

Scendla^  725 

Scenery,  influence  of  weathering  on,  349 

Schalstein,  138,  779 

Schiller-fels,  169 

Schiller-spar,  75 

Schist,  definition  of,  103, 175,  178  ;  derived 
from  eruptive  rocks,  573  ;  characters  and 
origin  of,  611,  625  ;  supposed  antiquity 
of,  613  ;  most  ancient,  681,  682 

Schist,  spotted,  605,  607 

Schistose  rocks,  126,  175,  611  ;  joints  of, 
530 

structure,     103,     175,    176*,    177  ; 


artificial  production  of,  309,  323  ;  origin 
of,  615 

Schizodus,  811,  844 

Schizograptus,  747 

SchizolepiSf  852 

Schizaneura,  854,  859 

SchizopteriSj  850 

Schlerndolomite,  873  • 

Schleswig-Holstein,  bogs  of,  480 

Schlanhtichia,  928,  938 

Schlotheimia,  917 

Schmidtia,  732 

Schori-rock,  129,  184 

Schorl-schist,  184 

Schrattenkalk,  955 

Schotter,  130 

ScolUkus,  723,  742 

Scoriaceous  structure,  102 

Scorpions,  fossil,  746,  762*,  794,  820* 

Scotland,  Tertiary  volcanoes  of,  200,  258, 
261,  592  ;  inverted  Silurian  rocks  of,  539  ; 
temperature  of  lakes  in,  405  ;  force  of 
waves  on  coasts  of,  436,  443,  444,  445, 
447,  448*  ;  persistence  of  thin  limestones 
in,  515  ;  volcanic  dykes  of,  582  ;  necks 
of,  586;  granites  «of,  569,  570;  contact- 
metamorphism  in,  606  ;  regional  meta- 
morphism  in,  624,  698-708 ;  pre-Cambrian 
rocks  of,  698 ;  Cambrian,  730 ;  Lower 
Silurian,  750  ;  Upper  Silurian,  763 ;  Old 
Red  Sandstone,  798  ;  Old'  Red  Sandstone 
volcanoes  of,  203,  204,  261,  693,  799  ; 
Carboniferous  limestone  series,  827  ;  Mill- 
stone grit,  832  ;  Coal  -  measures,  833  ; 
Carboniferous  volcanoes  of,  201,  204,  246, 
261,  585,  586*,  587,  588*,  691,  592,  594  ; 
Permian,  848  ;  Permian  volcanoes  of,  201, 
203,  204,  261  ;  Trias,  866  ;  Lias,  901  ; 
Oolites,  909 ;  Cretoceous,  947  ;  Tertiary 
volcanic  series,  988  ;  glaciation  of,  1027, 
1028,  1030,  1044  ;  submergence  of,  1037 

Srrobiciilaria,  1012 

Sea  {see  Ocean),  density  of,  35  ;  composition 
of,  ibid.  ;  transport  of  sediment  to,  408  ; 
tides  of,  432;  currents  of,  338,  434; 
distribution  of  temperature  in,  434  ;  con- 
ditions  of  deposit  of  sediment  on  floor  of, 
435,  451  ;  circulation  of,  436  ;  waves  and 
ground -swell  of,  ibid. ;  geological  work  of, 
440 ;  influence  of,  on  climate,  ibid. ;  erosion 
by,  441  ;  solvent  action  of  water  of,  88, 
441,  442,  491  ;  transporting  power  of, 
450  ;  deposition  of  sediment  on  floor  of, 
435,  451,  452,  454  ;  chemical  deposits 
from  evaporation  of  water  of,  412, 453,*492 ; 
preservation  of  organic  remains  in  deposits 
of,  648  ;  destruction  of  marine  life  by,  in 
storms,  ihid. ;  poisonous  efl'ects  of  fresh 
water  in,  649  ;  effects  of  earthquakes  on, 
278 

Sea-bottoms,  evidence  of,  654 

Sea-dust,  337 

Sea-level,  determination  of,  34  ;  variations 
of,  282 


1140 


TEXT-BOOK  OF  GEOLOGY 


Sea-serpeuts,  fossil,  933 

Sea- water,  solvent  action  of,  88,  441,  442, 

491,  648 
Sea- weeds,  geological  action  of,  476, 477, 482, 

805 
Seals  in  inland  seas,  410 
Seam  or  stratum,  500,  678 
Secondarjf  minerals,  66 
Secondary  Rocks,  680  ;  described,  856 
Section  in  stratigraphy  defined,  678 
Sections,  exaggerated,  in  geology,  42 
Secretionary  structure,  104 
Sedimentary  deposits  as  measures  of  geo- 
logical time,  58 

rocks,  124,  125,  499 

Sedimentation  as  an   indication   of  former 

physical     conditions,     499,     500,     513  ; 

natural    cycle    of,    521  ;    pre-Cambrlan, 

692 
Seeleyn,  846 
Seewenkalk,  955 
Segregated  structure,  99 
Segregation-veins,  66,  99,  157,  578,  580 
Seine,  rise  of  the,  372  ;  discharge  of,  374  ; 

terraces  of,  396 
Seismic  vertical,  275 
Selenacitdony  936 
SfinionotuSj  862 
Semi -metallic  lustre,  107 
Semi-opal,  69 

Senonian,  938,  946,  947,  948.  952,  954 
Sepia,  884 

Sei)tarian  structure,  104,  147*,  511 
Septarienthon,  911 
Septa^tr^'a,  901 

Sequanian  sub-stage,  912,  915,  918 
Sequoia,  1)22,  965.  984*,  993,  1004 
Serpentine,  74,  77,  82,  173,  182,  183,  365 
Serpentinization,  018 
Serpula,  798,  901  ;  protective  influence  of, 

47ei 
Serjufiiics,  741»,  811 
Sericite,  74 

Sericite-pliyllito,  179,  185 
Sericite-soliist,  ISf) 
SericitKition,  617 
Series  iu  stratiprraphy  defined,  678 
Sestian  stagf,  99-'] 
Severn,  discharge  of,  374  ;  estuarine  deposits 

of,  308 
St'zanne,  liniestones  of,  870 
Shale.  134  ;  relative  persisten«'e  of,  515 
Shallow-wuttM- deposition,  proofs  of,  501-510, 

1009 

Shaly  structure,  1(»4 

Shannon,  average  slope  of,  370 

Shear-structure,  310,  418,  419,  544,  026 

Shec]),  introduction  of,  1003 

Sheets,  intrusive,  209,  233,  573,  590  ;  varia- 
tions in  composition  of,  570  ;  etfet^ts  of, 
if>i(i.;  connected  with  volcanic  action,  ibid. 

Shell-iuarl,  139 

Shell-iiiouiids  (Kjokken-ini)«hiiug\  1000 

Shell-sand,  139 


Shetland,  force  of  waves  at,  443';  glaciation 
of.  1027 

Shineton  shales,  727 

Shingle,  129 

Shore-deposits  454,  648 

Shorthorn,  introduction  of,  1063 

Shrew,  fossil,  985 

Sibiriie^,  876 

Sicily,  sulphur  deposits  of,  1016  ;  thickness 
of  Pliocene  groups  in,  1017 

Siderite,  66,  78,  153 ;  as  a  petrifying 
medium,  652 

Siderolites,  10 

Siemi  Nevada,  old  glaciers  of,  1053 ;  up- 
heaval of,  1078 

Sigiilnrid,  740,  793,  816,  817*,  843  ;  as  a 
type  fossil,  657 

Silica  (silicic  acid),  62,  64,  65,  66,  68,  69. 
493,  650  ;  in  river  water,  378  ;  dissolved 
by  humus  acids,  472,  483  ;  whence  ob- 
tained by  marine  plants  and  animals, 
450,  482,  494;  introduction  of,  iu  contact- 
nietamorphiam,  610 ;  as  a  petrifying 
medium,  651  ;  soluble,  in  rocks,  921,  924 

Silicates,  62,  71,  124 ;  crystallisation  of,  on 
sea-door,  459,  495 

Siliceous  composition,  104 

deposits,  141,  153,  235,  481,  493,  512 

schist,  180 

Silicification,  651 

Silicon  in  earth's  cnist,  61,  62 

Sillimanite,  76  ;  in  contact-metamorjihism, 
708 

Sills,  209,  233,  578,  590  ;  variations  in 
coniiK)sitionof, 576;  effects  of,  ibid.;  con- 
nected with  volcanoes,  ibi<i. ;  examples  of, 
574,  577 

Silurian,  Primordial  (sar  Primordial  Zone)       / 

Silurian  system,  737;  rocks  of,  738;  lifeV" 
of,  739;  plants,  740  ;  animals,  ibit/.  ;  of 
Britain.  7-16  ;  of  Baltic,  Russia,  and  Scan- 
dinavia, 766  ;  of  Western  Europe,  769  ; 
of  Central  and  Southern  Europe,  772 ;  of 
North  America,  775  ;  of  Asia,  776  ;  of 
Australia,  ibid. 

Sivwci/on,  1019 

Siniorre,  niainmaliferous  deposiU^  of,  99S 

Simo^:<(unts,  804 

Sineniin'ian,  914 

Sinisian  formation,  737 

Sinks,  367 

Sinter,  calcareous,  150,  366,  482 

siliceous,  09,  153,  235,  237,  367,  483 

Sipboufo,  924* 

SipJtoiuitrHn,  743 

Sirocco-dust,  337 

Sivath>-rioui,  1000.  1020* 

Siwalik  groul^  1021 

Skeletons,  ^os^ilisation  of,  050 

Skelgill  beds,  703 

Skiddaw  slates,  749 

Slag,  201 

Slaggy  structure,  102 

Slate,  134 


INDEX 


1141 


Slickensides,  526,  647 

Slides,  preparation  of  microscopic,  90 

tSfimoniat  743 

Slyne,  525 

Sniaragtlite,  74 

Siinltix,  965 

Suake  River,  lava-fields  of,  257* 

Snow,  influence  of,  on  climate,  26  ;  dust 
carried  down  by,  337  ;  formation  of,  416 ; 
geological  action  of,  ibid. 

Snowfall,  greatest  in  Europe  towards  the 
west  in  the  Glacial  period,  1027,  1029 

Snow-ice,  148 

Snow-line,  416 

Soda-aniphiboles,  74 

Soda-lakes,  413 

Soda-trachyte,  166 

Sodium,  61,  63  ;  sj^ctrum  of,  11 

Sodium-carl>onate,  native  (trona),  240  ;  in 
lakes,  408  ;  influence  of,  in  precipitation 
of  lime-salts,  412 

Sodium-chloride,  79,  148;  in  sea-water,  86; 
at  volcanoes,  196,  228  ;  in  rain,  841  ;  in 
air,  342 ;  in  saline  lakes,  408*,  412;  pre- 
cipitated by  magnesium  chloride,  412 

Soffioni,  233 

Soil,  128,  331,  477  ;  formation  of,  351  ; 
varieties  of,  352 ;  removal  and  renewal 
of,  353 

Soil-cap,  movement  of,  354,  414,  532 

Soissons,  sands  of,  976 

Solarium^  862,  928,  942 

Si^laster,  903 

MfYurtiis,  1009 

.S(>/y^w?/«.  844 

Solent  1011 

Solenhofen  limestone,  890,  893,  91 7 

A^}lenopieuraf  724 

Sofenosirobifs,  965 

SolfaUra,  194,  195,  203,  233 

Soliditication,  contraction  in,  56 

Solomon  Isles,  upheaval  of,  285 

Solution  by  surface  waters,  344 

Solutions,  use  of  heavy,  86 

Solutrian  deposits,  1057 

Solva  group,  728 

.*>««/« iVi,  917 

Sonsta<lt's  solution,  86 

.<*>rrx,  1014 

S*ni)erhya,  909 

Spain,  Cambrian  rocks  of,  734  ;  Silurian, 
771  ;  metamorphosed  Trias  of,  629 

Spalocothenvmy  894 

Sparagmite,  132,  713 

S/HirmiaSy  846 

Spars,  634 

Spatangenkalk,  955 

SiKitannHS,  991,  1003 

Spathic  iron  ore,  78,  153 

Species,  diffusion  of,  660  ;  non-reappearance 
of,  675 

Specific  gjavity  of  rocks,  85,  108 

Spectroscoi>e,  applications  of,  11 

Speeton  Clay,  910,  938,  939,  953,  956 


SpermophiluSy  1026,  1060 

SphmrexochiiSf  743 

SphsBTodus^  930 

Spfueronites,  742 

Sphaerosiderite,  147,  153 

Sphferogponffi/if  748 

SphgRrulUes,  928 

SphagodvSj  744 

Sphenacanthn^j  813* 

Sphene,  70,  76,  618 

iSphenonchuJt,  866 

Sphenophyllum,  740,  816,  850 

SphennpterU,  793,  814*,  816,  850,  869,  877, 
880* 

Sphenozamites,  878,  880 

Spherulitic  structure,  100*,  119* 

Spiders,  fossil,  820 

Spilosite,  179,  606 

Spilsby  sandstone,  940 

Spindle-trees,  fossil,  984 

Spinels,  71 

Spirifer,  743,  781,  782*,  810*,  811,  844 

Spiriferina,  854,  861,  883,  884* 

Spiriffera,  781,  852 

SpirocyathnSj  722 

SpirorbiSf  811 

Spitzbergen,  action  of  frost  in,  414  ;  recent 
uprise  of,  284,  288;  "Heckla-Hook " 
group  of,  803  ;  Carboniferous  rocks  of, 
838  ;  Permian,  856  ;  Trias,  876 ;  Jurassic 
flora  of,  881  ;  Miocene  flora  of,  1002 

Splendent  lustre,  107 

Splintery  fracture,  106 

Spondylus,  926*,  927,  995 

Sponges,  supply  silica  to  marine  deposits, 
493  ;  fossil,  722,  740,  860,  924 

SporcuioceraSf  782 

Springs,  influence  of  volcanic  eruptions  on, 
207  ;  hot,  49,  235,  359,  363,  1076  ;  in- 
fluence of  earthquakes  on,  277  ;  give  rise 
to  deceptive  appearance  of  subsidence, 
289  ;  fonnation  of,  357*;  temperature 
of,  359,  363 ;  chemical  action  of,  360  ; 
kinds  of,  301  ;  mineral,  362  ;  calcareous, 
ibid.  :  ferruginous  or  chalybeate,  ibid.  ; 
medicinal,  363  ;  preservation  of  organic 
remains  in  deposit^  of,  648 

Sprudelstein,  150 

Stiu<ilod4m,  983 

Squirrels,  fossil,  968 

Stachannularuij  816 

Stache^ceraSy  845 

Stage  in  stratigraphy,  678 

Stagodon,  936 

StaffonolepiSy  864 

Stalactite,  150,  365* 

Stalagmite,  150,  365,  647,  1059 

Stampian  stage,  989,  993 

Star-fishes,  fossil,  722,  742,  749,  781 

Stars,  spectra  of,  12 

Sfauria,  742 

Staurocephnlus,  751 

Staurolite  in  contact-metamorphism,  708 

Staurolite-slate,  179 


IU± 


TEXT'B^yjK  OF  GErLj^T 


SK^eaxn  ar.  TM'^sujtsL    :«,   I^T.  215.    S>- 
±2:Z,  rJK.  i:iT  :  *^Jiftaz  ysm^  oC  »e 

^jft-.'VTAr-tJi.  •'r'z 
/9>jZAjefA'.  J*  'X 

jft«».f>:^MC#  .'-M.   •••7 
3¥jtMy/f^/' 1 .  •** 

f!l!^r3^j^<"r,tT .  •17 

-«i7»v?nyi.  ioL  ?:<,  •!7*-  •!•*,  543 

Stil>>r>,  7*".  77 
Struck.  U>  r^c^ifc*.  7^ 

«e/»*:<&^M  .SU^»r.  ^^.  *&5,  »5.  >0< 
HCorzf.*.  <iingin  of,  327  ;  4cstnaetk«  of  Ii£k 
hj,  ih.  til*  **»,  ^* 

^>«W"««':t«  tn  jAmmlL  eriMbioB,  1028 

Htranir«^;f  ii«ic<iV>B*r,  &1* 
^r^i!3f«//- /»/!>>/«    raiK<l  beach**,.  20,  285 
HtnU  or  WU.  500.  67S  ;  alternatiofu  and 
at^ViCiAtioii.*.  of,   513  :  reUtire  |*r*ist«iM* 

of.  ^A't  :  ir.fl3*rt.'^  of  atteaoation  of.  on 
ai/[>are;-*.  'iij*.  517  ;  t:rne  r*:prw«&t**i  by. 
SI"!  ;  chr''iiolojncaiI  ralae  of  interraU  be- 
tw*-*:rj,  5'JO  ;  ten-ary  AGcc*::*«on  of.  521. 
^•1  :  ifToriiH  of.  522  ;  order  of  superpwi- 
lion  of.  52-5  ;  joifit-  of.  i^*V/. 

.Stratifjcation  an*!  it-»  ac/^:oni[janimeiiU.  4&S 

Stratitj^d  ro'ks  VZl^'J^ 

!»tni'tnr*r,  104,  4&S 

Htrati^^raphical  (ffrol^^ej',  ♦574 

Streak^l  structure,  100 

Htr'tp'^floii,  819*,  820 
Sfrepiorh;/nch,M^  781,  811,  S52 
Stretchiu;,',  effects  of,  on  rocks,  615 
Htriation   by  glacier- ice,    429 ;    Ijy  slicken- 

Hides,  526,  547 
Hl-nrMoiuiinia^  743 
Strike,  534,  537 
Strike-faults,  552 
Strike-joiiitH,  525 
iUriiiffttrfpUahiJi,  781,  782* 
Striiigooephalu-i  lime»tone,  785,  786 
airfrnu/tfi/Mfra^  779 
StromntopHiM,  980 
f^romfj^pfft^j,,  742 
Stroiij>x>li,  202,  205,  206,  215 


scranr.ar^arTiuxaa^  2.  Ifcynfiin  war'''    174 

MrwtU'ma^  7*^  *4rfe* 

itrt90.i,s^a.  7'}C 

Arff^ssmtr^r^  7*4,  741*.  7TI*-  7^I_  •si 

<r~.fK<]ii.  447    "rirfTi«*nin>  iiL  ol  aiq»a|Eag<tT. 

I.  .iT^ii 

scyj' 1  r.'^i^»  '«  .  '.  ^ . ,. 
•fmjtr'i-*,  74-*.  7i'? 

'dtg.y.t^TT.  JIlfniSlt.1t  -jfi 

erxi-AL.  447 

Snfcf^arj>:ay  «xa3L:u»  aL  70 

Sabmarisifr  TOurazoei.  14> 
S«lnerc«*i  f-^reicj^  ^9>\  434^  1464 
SatKaiecr:^.  ±?1,  ±^^  :  pvw&^  oC  3Sd,  514  : 
<a!=ie»  cf.  29±.   $>4  :   a:  vaicMmic  Wfik 
231.   24*>.  24-X  244.   *&?  :   proic^-^i  by 
canl'^-zakei.  27? ;  fram.  xanierccosid  lolc- 
tioQ,  3«7 
ctfce&arrfcr  ikadk 


fonzLa:k>as.   ^!^ ;  peneral^r 
aphearaL  107*/ 
—  aM  dMMKiiao,  2S3. 


e€ 


±a.  w*.  SOS, 

1070 

Sab«Kl,  12S,  351*,  353 

Sab-*tai^,  d«±aitkxi  of,  575 
I    Sntff%Uujf.  744 
I    SVo'V'f..'.  &90,  1011 
[   Suess-jnian  sta^re.  9S*> 
1   Sncz  CanaL  «aiifrrocis  detosil*  E*ar.  413 

I     SMiOT-tUp"-'-!,   811 

Snlf  Late*.  7S  :  nrdactkm  oC,  344 
SolpLi.le*,  79  :  vcath^iufi:  of,  %\Z  :  r*.ir:o«*i 

froKj  j'.ilpbates.  45*5,  849 

of  iron,  70,  79.  455 

Salphar.  61,  63,  67,  344.  993.  1016,  1019  ; 

at  volcanoes.  196,  228,  233 
Sclpharette«i   bydrog^en,  67,    344,   3*53  :    at 

volcanoes,  195,  2:J3 
Sulphuric  aciii,  6^i,  64  ;  in  nin,  341  :  pro- 
duce* i  from  sulphides,  343  ;  at  volcanoes 

196 
Sulphurous  acid  at  volcanoes,  195 

odour  of  rocks.  107 

Sumach,  fossil,  1004 

Sumbawa,  eruption  of.  215,  216 

Sumpter  beds  (Miocene),  1002 

Sun,  den.>ity  of,  9 ;  composition  of,  11.  12  ; 

influence  of,  on  earth,  21 
Sun -cracks  in  strata,  508,  509* 
Sunlight,  eflfects  of,  on  minerals,  327 
Sunshine,     influence     of,     in     weathering, 

346 
Superior,   Lake,  405,  406  ;  area  of,    1052  ; 

terraces  of^  1054 


INDEX 


1143 


Superposition,  order  of,  523  ;  law  of,  666, 
674 

Sus,  1002,  1014 

Swallow-holes,  367 

Sweden,  upheaval  of,  288  ;  subsidence  of, 
291  (see  under  Scandinavia) 

Switzerland,  ice-barriers  in  rivers  of,  382  ; 
-lakes  of,  397,  398,  404,  405;  river- 
deposits  of,  397,  406  ;  glaciers  of,  419  ; 
erratic  blocks  of,  425  ;  giants'  kettles  of, 
429  ;  contorted  rocks  in,  540,  541  ;  re- 
gional metamorphisni  in,  622  ;  pre-Cam- 
briau  rocks  of,  714 ;  Carboniferous,  623, 
838  ;  Trias,  871  ;  Jurassic  rocks,  624,  915, 
917  ;  Cretaceous,  954  ;  Eocene,  979  ; 
Oligocene,  992  ;  Miocene,  1000  ;  glaciation 
of,  1029,  1039;  post-glacial  records  in, 
1066 

Syenite,  163 

Syllsemu^j  930 

SymphysuruSj  769 

Symplocosj  973 

Synclines,  538,  539*  ;  effects  of  faults  on, 
554* 

Sj/nocladia^  344 

Syringodendroiiy  822 

Syrin^oporOj  742 

"System,"  definition  of,  678 

Szabo's  ilame- reactions,  88 

Tablelands,  42,  1084 

Tachylyte,  171 

Txniopteris,  843,  859,  880* 

Talc,  74,  77 

Talc-rocks,  183 

Talc-schist,  183,  188 

Talcose- schists,  origin  of,  686 

Talchir  group,  854,  877 

Talpa,  1014 

Tancredia,  906 

Tangle,  protective  influence  Of)  476 

Tanne  Greywacke,  787 

TapinocephcUus,  863 

Tapes,  995,  966* 

Tapirulu^j  985 

Tajyirus,  985,  1016 

Tar,  mineral,  145 

Tarannon  Shales,  753,  754 

Tasmania,  Tertiary  deposits  in,  983 

Tassello,  980 

Taunus,  metamorphism  in  the,  620 

Taunusien,  787 

Taxites,  905,  991 

TaxocrinuSj  742 

Tiucodium,  988,  1004 

Taxoxyltm,  991 

"Tchernayzem  "  (Tchernosem)or  black  earth 

of  Russia,  133,  478 
Tealby  clay,  940 
Tegel,  1000 
Teleosaurus,  887 
Telerpeton,  862 

Tellina,  983,  995  1011,  1043* 
Tdmatomis,  935 


Temttof/raptus,  749 

Temperature,  zone  of  invariable,  49  ;  as  an 
indication  of  the  age  of  intrusive  rocks, 
49  ;  irregularities  in  downward  increment 
of,  51  ;  effects  of  changes  of,  on  surface- 
rocks,  328,  346 

Teneriffe,  247*,  254*,  262 

Tenorite  at  volcanoes,  196 

Tension,  effects  of,  311 

TentacuLites,  744,  781 

Tephrite,  168 

TercUosaums,  863 

Terehray  995 

Terdyralia,  985*     . 

Terehratellay  906,  926 

TerebratxUa,  743,  810*,  811,  848,  861,  883*, 
925,  926*,  978,  991,  1009 

Terebratulimt,  926,  1017 

Terebratulina-gracUis  zone,  988,  945 

Terehrirostra,  926 

Terra  rossa,  350 

Terrace-Epoch,  396,  1054 

Terraces,  of  lakes,  406,  409*,  1054  ;  of 
rivers,  395,  1054,  1058,  1065  ;  marine 
{see  Raised  Be-aches) 

Terrigenous  sediment  on  sea-floor,  452,  454, 
648 

Tertiary  systems,  961 

Tertiary  time,  geographical  changes  in,  963, 
979,  1003,  1010,  1017,  1018,  1023  ; 
changes  of  climate  during,  964  {see  under 
Climate)  ;  plant  and  animal  life  of,  964 

TcstudOj  1021 

Tetraconodon,  1021 

Tetracns,  985 

Tetradiumj  742 

Tetragraptus,  739*,  741 

Teirapt^ruSf  973 

Teudopsisy  884 

Tfjctilariay  924 

ThalassoceraSy  845 

Thames,  tlischarge  of,  878,  374  ;  average 
slope  of,  376 ;  mineral  matter  dissolved  in, 
378 

Tfuimnastrapa,  883 

Thanet  Sand,  971 

ThaujnaiopteriSf  871 

Theca,  723*,  725,  744 

Thecia,  742 

Thecidium,  901,  926,  946 

ThecodontosauruSj  863 

Thecosmilia,  883 

Theiodusy  744 

Theriodont  reptile^*,  863 

Theriosuchus,  910,  931 

Thermal  springs,  235,  359 

Therutherium,  985 

Thinnfeidiaj  875 

Thinolite,  413 

Tholeite,  168 

Thracia,  910 

Throw  of  faults,  549,  551 

Thrust-planes,  541,  550,  625*,  701*,  703*, 
1074 


1144 


TEXT-BOOK  OF  GEOLOGY 


Thuja,  991,  1017 

Thujopnis^  1001 

Thursius,  790 

ThuyiUs,  881,  923 

Thy  Uu-i  nils,  1022 

Thylwyho,  1022 

TiW.  tnrbidity  of,  402 

Tidal  wave,  influence  ot,  on  earth's  rotation, 

233 ;    influence   of    form   of    shores   on, 

434 
Tides,  iuflneuj^e  of,  on  rivers,  398  ;  amplitude 
-  of,  432,  433  ;  effects  of,  on  transport  of 
"'  «iediment,  451 
Tideless  seas,  432 
Ti(/i/iiO'.i.  733 
TitjrisurhuSy  863 
Tilestoues,  753,  760 
Till,  133,  431,  1031 
Tillodont^,  969 
TiUnthentnn,  969 
Time,  measures  of  geological,  58,  518  ;  classi- 

tication  of  rocks  according  to,  125 
Timtcerag,  970,  971* 
TiniKio/i,  919 
TirrAiUs,  873 
TiUnic  iron,  70,  618 
Titanite,  76 
Tif/rni'ifrruru.'i,  940 
TitauotherifL-e,  997 
Tifanofheriiim,  1002 
Tithoiiian  stage,  911,  918 
Toad.stone,  827 
Toarcian  stage,  913 
Toninian  sta^'e.  989,  991,  992,  993 
TQiijnie,  adhesion  of  rocks  to  the,  I07 
7'"/"//'wv'/YAs',  7S2 
Torquay  Limestone,  784 
Torrent-,    average    sloj»e    of,    376  :    erosive 

action  of,  303 
Torridonian  ro<-ks  of  Scotland,  624,  625*  ; 

de.>.:ril>ed,  699,  705 
ToT-i  of  granite,  349* 
Tor>iou,  effects  of,  in  rooks,  31>*.  527 
Tortonian  sta^'e,  998,  1000,  1001 
Totlernboe  Stone,  944 
Touraine,  Miocene  dejwsits  of,  998 
Tourmaline,  76.  129,  131 
Touriualine-<n"anite,  159 
Tourmaline-schist,  184 
TnynMiyr,  9*25 
Tn.rH,:r,is,  928,  929* 
Trorhi/rrrns,  862 

TnirJii/iltriiUi^   760 

Trrichyte,  16r>,  222 

Ti-acliytoi.l,  119,  155 

Ti'ich;li>iii,   722 

Trade-winds  15,  28 

Trd'jiih'hynM.  9S5 

Trn.n'liis,  1021 

Transition  rork^^.  6S0,  726,  737 

Tra]>-^Tanulitt'.  160 

Tra.vs,  137,  107 

Travertine,  150,  366 

Trechviny.i^  9S5 


Trees,  dnrabilitT  of  stems  of,  518  ;  fossils  in 
trauks  of,  519 

Tremadoc  slates,  727,  728,  729 

TrtMoto^turifs,  862 

Tremolite,  74 

Trenton  gronp,  775 

Trttoceras^  754 

Trvteanthod^n,  894,  895* 

Triassic  system,  858  ;  flora  of,  859  ;  fanna 
of,  860;  in  Britjun,  8«4 ;  in  Central 
Europe,  868  ;  in  German r,  iUd.  ;  in  the 
Vosges,  870 ;  in  Scandinavia,  ibid.  ;  in  the 
Alps,  871  ;  in  Spitsbergen,  876 ;  in  Asia, 

877  ;  in  Australia,  ibid. ;  in  New  Zealand, 

878  ;  in  Africa,  ibid.  ;  in  North  America, 
ihid.  ;  metamorphism  of,  629 

Trireratf*j>s,  933 

Trichechus,  1012 

Trichites,  116 

Trichr'^rapiiu,  750 

Triconodon,  894,  895*,  919 

Tridymite,  69 

THffonia,  874,  884,  886*,  887*,  927,  1022 

Trigonocarpus^  818 

Trigonodns,  869 

Tri'jonfigraptHS^  750 

Trigono9tinv9,  926 

Trilobites,    721*,    7-22*,   723,   741*,    757*, 

780*,  812  ;  as  typefoessils,  657  ;  earliest 

traces  of,  694,  723,  742 
Triiner^'Ua,  768 
TfiHucUuSj  741*,  743 
Trionyr,  958,  987,  1021 
TripUsin,  743,  745* 

Tripoli  powder  (Tripolite),  69,  141,  481 
TripriixluH,  936 
Tristan  d'Acunha,  34 
Tristichoptfrns,  796 
Triton,  1000,  10o9 
Tricin,  983 
Trochfimminn,  809 
Trr^'hiUnhilk,  869 
Ti-'ihi^'i/stiUji,  734 
Tnihii.'i,    744,   761*,  901,    928.   990,    99S, 

1010 
Tnx-iH'i/athu.'i,  925 
Tr<K't»sinllia,  925 
Troctolite,  169 
Tr'-gonthtrifim,  1011  . 
Troua,  240 

Tmphon,  1007*.  1012*,  1043* 
Tropin  fotiot  us,  1013 
Tropih:s,  862 
Trough- faulty  557 
Tsien-Tang-Kiang,  bore  in,  434 
Tiif/icaulis,  S49 
Tueilian  group,  827 
Tufa,    150  :    precipitation   of,  in  salt-lakes, 

413  ;  of  Palaeolithic  age,  1059 
Tuffs,  135,  137,  197,  201,  244,   253,   593; 

value  of,  as  evidence  of  volcanic  explosions, 

593 
Tuff- cones,  244 
Tulip-tree,  fossil,  923,  1004 


INDEX 


1145 


Tundras  of  Siberia,  352,  410,  478 
Turhimlia,  978,  991 
Turhiy,  748,  844,  873,  901,  928 
Turf,  conservative  influence  of,  475 
Turonian,  938,  944,  948,  951,  954,  957 
Tnrrilepas,  742 
Turn'lites,  927*,  928 
Turrilite-greensand,  955 
Turritdla,  862,  901,  966,  998,  1010,  1012 
Turtles,  earliest  forms  of,  887 
Tylodon,  968 

Types,  persistent,  in  the  organic  world,  653 
Type-fossils,  657 
Typhisy  985 

Tyrol,  Trias  of,  871,  873  ;  volcanic  rocks  of, 
604,  876 

Uinta  group,  982 

Mountains,  structure  of,  1072 

Cintatherium,  970*,  971 

UUmunniu,  847 

Ulmic  acids,  343,  471 

Uhmis,  995,  1017 

Uloilendrmi,  816 

Ultra-basic  rocks,  173,  681 

Uncites,  781,  782* 

Unconforniability,  510*,  518*,  641,  675, 
697 

Unctuous  feel  of  rocks,  107,  183 

Uudercliff,  origin  of,  370 

Unjjulates,  fossil,  969 

Ungulite  grit  of  Russia,  732 

Uniformity  in  geological  causation,  3 

rnioy  853,  878,  901,  953,  983,  986,  1018 

Cniondloy  878 

United  States,  volcanic  phenomena  of,  203, 
204,  226,  235,  244  ;  pre-Cambrian  rocks 
of,  715  ;  Cambrian,  735  ;  Silurian,  775 
Devonian,  789  ;  Old  Red  Sandstone,  803 
Carboniferous,  840  ;  Permian,  855 
Trias,  878  ;  Jurassic,  919  ;  Cretaceous 
957  ;  Eocene,  981  ;  Oligocene,  993 
Miocene,  1002  ;  Pliocene,  1022  ;  glacia- 
tiou  of,  1029,  1050 

Unstratified  rocks,  124 

structure,  104 

Ui»heaval,  281,  284  ;  proofs  of,  283  ;  in- 
riueuce  of,  on  river-action,  397,  1054 ; 
causes  of,  282,  292,  304  ;  supposed  to 
arise  from  denudation,  283,  293  ;  effected 
locally  by  conversion  of  anhydrite  into 
g\psura,  345,  503 

Uralite,  74 

Uralitisation,  617 

Uranus,  density  of  planet,  9 

Ui-oster,  901 

Urgneiss,  682 

Urgouian,  938,  941,  948,  949 

Uriconian  rocks,  710 

Untcordtflusy  821,  846 

Ci'suK,  ioi  t 

Urns,  1049 

Utah,  Great  Salt  Lake  of,  408,  411 

Utica  group,  775 


Valkngixien,  948,  954 

Valleys,  longitudinal,  40  ;  transverse,  41  ; 
rate  of  excavation  of,  466  ;  antiquity  of, 
1080  ;  origin  of,  1086 

VcUvata,  910,  989,  1013 

Vaiuulina,  809 

Vancouver  Island,  Cretaceous  rocks  of,  960 

Vapours,  volcanic,  193,  209,  228,  233 

Varanvs,  1019 

Variolite,  170 

VcctisauruSf  940 

Vegetation,  terrestrial,  transport  and  deposit 
of  by  sea,  455,  457 

Veins  and  dykes,  677  ;  contemporaneous. 
99,  578,  580,  581*  ;  segregation,  99,  577, 
580;  intrusive,  577;  of  granite,^  158*, 
159,  578  ;  of  lava,  209 

Veins  mineral,  633  ;  variations  in  breadth, 
ibid.  ;  structure  and  contents,  634  ;  suc- 
cessive infilling  of,  635  ;  pebbles  and 
shells  in,  636  ;  connection  with  faults, 
ibid.  :  relation  to  surrounding  rocks,  638  ; 
decomposition  and  recomposition  in,  iind. ; 
origin  of,  640 

Vein -quartz,  154 

Vein-stones,  634 

Vcntncidites,  924* 

Vents,  volcanic,  255,  584,  828  ;  frequent 
independence  of  lines  of  fault,  585 

Venics,  927,  998,  1016 

Venus,  density  of  planet,  9 

Vennetus,  977,  1009 

Vermilin,  811,  901 

Vermilion  series,  716 

Verrucano,  838,  852 

Vertcbraria^  854 

Vertebrata,  fossilisation  of,  650  ;  first  traces 
of,  744 

VeHicellit4Ui,  860 

V'erticordia,  1009 

Vesicular  structure,  102,  198,  227 

Vesulian  sub-stage,  913 

Vesuvianite,  76 

Vesuvius,  volcanic  phenomena  of,  195,  196, 
197,  200,  201,  202,  203,  205,  206,  207, 
208,  209,  211,  212*,  213,  214,  215,  217, 
218*,  220,  221,  223,  224,  225,  226,  227, 
228,  229,  230,  231,  232,  243,  244,  249*, 
250 

Vibunium,  922,  977,  988 

Vkarifay  940 

Vicksburg  beds,  993 

Victoria y  965 

Victoria  {see  Australasia) 

Vienna  sandstone,  955,  965,  979 

Tertiary  basin,  992,  999,  1018 

Villafranchiiui  group,  1016 

Vi,icuhriay  811,  925 

Vines,  fossil,  984 

Virginian  grouj),  1002 

Virgulian  sub-stage,  908.  912.  915 

Viridite,  123 

Viscosity  of  earth's  interior,  54,  56 

Vishnutherium,  1021 


1146 


TEXTBOOK  OF  GEODjGY 


Vitrecms  denned.  €4 

lujt/e,  107 

Atracture.  IOOl  155 

Vitreocj  a<rid  rocks,  162 

rir<rr?i,  1006 

VirianiU,  7i»,  652 

rtn>iru4,  &5&.  972,  9S« 

Volhr/rtMlA,  IZ'2 

Volcacello,  L»le  of,  243*.  249 

Volcanic  adioiu  l&l,  «02 ;  ftiUs  oC,  203 ; 
conoectioD  of,  vith  faults,  204  ;  influence 
of  atmoEarpheric  pressure;  on,  205  ;  supposed 
connection  of,  with  bun -spots,  206  ;  pdurox- 
jsnlSd  phase  of^  207  ;  prodnces  earth- 
quakes, 207,  279  ;  gires  rise  to  fiasores. 
20%  ;  influence  of  gases  and  raponrs  in, 
209.  22>;.  233,  240  ;  geological  hisUnyof, 
260  ;  cauMiS  ot  263  ;  sabtcrranean  phases 
of,  560,  569.  576  ;  materials  for  hUtory  of, 
562.  591  ;  saljundeDce  connected  with, 
5^%  ;  quiescence  of^  in  Mesozoic  tinte,  in 
Europe,  591  ;  destractire  effects  of,  on 
marine  life,  649 ;  connected  with  nionn* 
taiu  -  making,  1076,  1078  ;  terrestrial 
features  due  to,  1079 

blockij,  136 

breccia,  130 

chimney,  effects  of  dosing,  233 

cones,  240 

deposits,  organic  remains  preserred  in. 

648 


—  eruptions,  pre  -  Cambrian,  692,  696 ; 
Torridonian,  705  ;  Cambrian,  720,  727  ; 
Silurian,  739,  747,  74S,  750.  764,  765. 
770,  772  ;  Devonian,  779,  783,  784,  788, 
7Cn  ;  Old  Red  Sandstone,  793,  799,  801, 
802  ;  Carboniferous,  805.  826,  827,  828, 
821^  8.'50,  832,  840  ;  Permian,  842,  847, 
84  S,  849, 850,  851, 852 ;  general  absence  of, 
from  Mesozoic  formations,  857  ;  Triaiisic, 
874,  S76  ;  CYetaceous,  957,  960  ;  Tertiarv, 
963  ;  l-xKjene,  981,  982,  983  ;  Oligocene, 
98%,  990,  993  ;  Miocene,  1003  ;  Pliocene, 
100.%  1015,  1017,  1019,  1022,  1023; 
Pleistocene,  1036 

—  frasmental  rocks,  135,  563 

—  i.slands  and  coral-reefs,  490 

—  necks,  255 
products,  191 


Volcano.  Lsland  of,  206,  221,  224,  233, 
234,  245*.  255 

Volcanoes,  as  i>roofs  of  earth's  internal  heat, 
47,  57  :  de.scril>ed,  191  ;  parts  of,  192  ; 
active,  dormant,  and  extinct,  202;  ordinary 
j»hase  of,  204  ;  conditions  of  eruption  of, 
205  ;  ]>eriodicity  of  eruptions  of,  206, 
207,  210 ;  influence  on  spring,  207 ; 
hyilrostatic  j)re.s.snrc  of  lava-column  in, 
209,  219,  220  ;  explosions  of,  211,  219  ; 
.showers  of  dust  and  stones  at,  213  ;  lava- 
streams  from,  217 ;  structure  of,  239 ; 
without  craters,  243  ;  cones  of,  192,  216, 


24»),    242,   244  ; 
graplii^  S£«i 
259:    (^e-CksLbrsas.    t» 
'ief^h  <4  socrce  gL,  2«7  : 
masjiTev  255 
Vr,l«,  fowil,  1«1 

Vo!ga,  aTcra«e  ft3-:pe  oC  374 

Volgias.  919,  95* 

VtAkwkammieL,  823 

VoUzia^  844,  859,  8«0* 

VUuta,  952,  9««.  «7*.  >Sc.  10l».  1012* 

VoiulaJuAc^  S>67* 

Voiraria^  978 

Vosges    contact -ibctaaDor^iLiiaa    m^    407 ; 

ancient  glaciers  ai,  lOM,  10(39 
Vo^giin,  870 
Vraconnien,  948 
YuUdbi,  975 

Waaqesoceilas,  ^5 

Wacke,133 

Wad,  71 

Wahsatch  group,  982 

Wairca  series,  878 

Waichia,  821.  843,  8«6.  S81 

WaidknmifL,  785,  h^  983,  1022 

Walker^s  specific  grsTitT  ^**^**-^,  S5 

Walnut,  fossil,  923,  1004 

Warminster  beds,  938,  943 

Water,  vapour  of,  in  air,  32,  340 ;  ccaapOB- 
tion  of^  61  ;  presesce  ol,  is  eutii''s  crmH, 
64  ;  influence  oC,  in  rokantic  actioB,  193, 
197,  215,  219,  223,  22«,  227,  2«5,  309; 
critical  point  of.  194, 309 ;  expeiiaaents  on 
heated,  305,  309  ;  presence  ol  in  all  rock& 
306 :  solvent  power  of,  on  rocks,  3%i7,  343  : 
suspends  solidification  of  rorks..  308  : 
lowers  the  fusion  point  <rf  boditSv  ih^i. ; 
surface  action  of^  339 ;  form.*  of,  340 : 
circulation  of,  ibid, ;  nnder^roand.  356 : 
soft  and  hard,  360  ;  influence  of^  in 
dolomitization,  321  ;  expands  in  freezing, 
414 

Waterfalls,  origin  of,  388,  390 

Watersheds,  1085 

Water-gas,  193,  209,  215,  219,  226,  265 

Water-ice,  148 

Water-level,  changes  of,  339,  404,  437 

Water-Lime  group,  775 

Water-stones,  864 

Waves,  generation  of,  339,  436  ;  height  and 
force  of,  436,  443  ;  depth  of  influence  of, 
438,  451,  455 

Wealden,  938,  940,  953 

Weathering,  indicated  by  cffenrescence  with 
acid,  345,  365  ;  description  of,  345  ;  Taria- 
tions  in  rate  and  character  of,  346  ;  zone 
of,  472  ;  of  fossils,  670.  671  ;  frequency 
of,  81,  595  ;  depth  of  layer  of,  82  ;  gives 
a  clue  to  composition  of  rocks,  ihitl.  ;  ex- 
amples of,  69,  70,  71,  72,  73,  74,  75,  76, 
77,  78.  79,  80,  87,  110,  123,  133,  159, 
160,  174,  231,  343,  344,  365,  530  ;  imiu- 
tion  of  efliects  of,  672 


INDEX 


1147 


Welding  of  rocks  by  pressure,  312 
Welleiikalk,  869 
Wells,  357  ;  Artesian,  368 
Wemraelian,  975,  978 
Wengen  beds,  873,  874 
Wenlock  group,  746,  753,  764 
Werfen  beds,  873 
Wesenberg  zone,  767 
West  Indies,  upheaval  among,  284 
"  Wet  way  "  analysis,  89 
Wetterstein  Limestone,  873 
Weybouru  Crag,  1008,  1012 
Whet-slate,  135,  619 
Wliin  sill,  575 

White,  as  a  colour  of  rocks,  106 
White  Lias,  864,  867 
White  River  group  (Miocene),  1002,  1022 
Wliite  trap,  601 
Whitjkldia,  757 
Wianamatta  beds,  878 
Wichita  beds  (Texas),  865 
Widdringtonia^  990 
Widdringtonites^  991 
Wieda-shales,  787 
Wiliiam^onia,  880 

Willow,  fossil,  923,  954,  966,  984,  1004 
Wind,  velocity  and  pressure  of,  327  ;  effects 
of,  329  ;  transport  of  dust  by,  331,  334, 

337  ;  diffusion  of  plants  and  animals  by, 

338  ;  influence  of,  on  water-level,  339,  405 
Wolf,  fossil,  1014,  1060 

Wood,  composition  of,  144  ;  conversion  of, 

into  lignite,  322 
Wood -opal,  69 

Woolhojie  limestone,  753,  755 
Woolwich  and  Reading  beds,  971,  972 
Worms,  geological  action  of,  352,  363,  473 


Xantuopsis,  973 
XenodiscuSf  854 
Xenophoray  1010 
Xiphodotij  985 
Xylobim,  820 

Yakutsk,  frozen  soil  at,  49 

Yangtse,  sediment  in  the,  384  ;  rise  of  bed 

of,  395 
Yellow,  as  a  colour  of  rocks,  106 
Yellowstone  Park,  235 
Yew,  fossil,  966 
Yddia,  1043 

Yoredale  Group,  825,  827 
Yorktown  beds  (Miocene),  1002 
Ypresian,  976,  977 

Zamia,  877,  905 

Zamiostrobus,  879,  880 

Zamites,  860,  880,  923 

Zanclean  group,  1016,  1017 

ZanclodoTij  863 

ZaphrentU,  742,  807*,  810 

Zechstein,  842,  849 

Zeolites,  76  ;  formed  in  Roman  bricks  by 
warm  springs,  307  ;  as  proofs  of  altera- 
tion, 365 ;  formed  in  abysmal  deposits, 
458 

Zcugl(xJon^  981 

Zircon,  76,  129,  131,  705 

Zircon-syenite,  164 

Zirknitz  Lake,  368 

Zoisite,  76 

Zones,  palaeontological,  644,  678 

ZoniUs,  821 

Zoophycus,  980 

ZygosauruSf  846 


THE   END 


Printed fy  R.  &  R.  Clark,  Edwburgh 


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clearness  and  conciseness.  The  woodcuts  are  good,  and  the  statistical  information,  admirably  tabulated, 
is  neither  more  nor  less  than  is  necessary." 

MAPS  &  MAP  DRAWING.    By  W.  A.  Elderton.  Pott  Svo.    is. 

MORXIjVG  post. — "May  be  recommended  as  a  useful  book  to  those  students  who  have  an 
elementary  knowledge  of  geometry,  and  the  use  of  the  more  common  mathematical  instruments.  ...  A 
careful  perusal  of  Mr.  Klderton's  book  will  remove  many  of  the  diflllculties  experienced  by  beginners,  and 
help  them  to  retain  in  their  memories  the  form  of  the  countries  they  have  to  delineate." 

THE  ELEMENTARY  SCHOOL  ATLAS.  With  24  Maps 
in  Colours,  specially  designed  to  illustrate  all  Elementary  Text-Books 
of  Geography.     By  John  Bartholomew,  F.R.G.S.     4to.      is. 

GUAR niAX.—''*'Yhe  .shilling  Elementary  School  Atlas,  which  we  have  received  from  Messfs. 
Macmillan  and  Co.,  is  almost  a  model  of  judicious  selection  and  arrangement." 

MACMILLAN    AND  CO.,     LONDON. 


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Text-book  of  geology, 

Stanford  University  Libraries 


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